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Preliminary Geotechnical Engineering and Geologic ReviewHoldrege & Kull Nevada City • Yuba City • Truckee • Chico www.HoldregeandKull.com PRELIMINARY GEOTECHNICAL ENGINEERING AND GEOLOGIC REVIEW for Truckee River Legacy Trail – Phase IV Truckee, Nevada and Placer Counties, California Prepared for: Mark Thomas & Company 7300 Folsom Boulevard, Suite 203 Sacramento, California Prepared by: Holdrege & Kull 10775 Pioneer Trail, Suite 213 Truckee, California 96161 Project No. 42169-01 September 20, 2016 (530) 587-5156 • FAX (530) 587-5196 • E-mail: handk@HandK.net • 10775 Pioneer Trail, Suite 213 • Truckee, CA 96161 • A California Corporation Project No. 42169-01 September 20, 2016 Mark Thomas & Company 7300 Folsom Boulevard, Suite 203 Sacramento, California 95826 Attention: Garry Horton Reference: Truckee River Legacy Trail – Phase 4 Truckee, Nevada and Placer Counties, California Subject: Preliminary Geotechnical Engineering and Geologic Review This report presents the results of Holdrege & Kull’s (H&K’s) preliminary geotechnical engineering and geologic review for the proposed Truckee River Legacy Trail – Phase 4 project to be constructed in Truckee, Nevada and Placer Counties, California. The proposed project will include constructing approximately 2.3 miles of a Class I bikeway and recreation trail between the Truckee Regional Park and State Route (SR) 89. Appurtenant construction will include temporary and permanent erosion control features and a pedestrian bridge to span the Truckee River near the west trail terminus adjacent to West River Street. This report is based on previous geotechnical investigations performed in the site area, review of geologic maps and literature covering the project area, and H&K’s experience in the site area. A subsurface investigation must be performed prior to construction in order to confirm the assumed subsurface conditions used to prepare this report. Steep slopes are located adjacent to the central and northeast portions of the trail. Based on our surface reconnaissance and previous studies completed by others at the Truckee Springs property, the steep slopes are likely subject to avalanche and rockfall hazards. An avalanche occurred on the steep slopes above the Truckee Springs property in 1982. Avalanche runout zones have been mapped on the Truckee Springs property. Large volcanic boulders were observed on the ground surface near the base of the steep slopes during our surface reconnaissance. We have provided possible mitigation measures in the following report to help reduce potential hazards associated with avalanches and rockfall. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 2 Holdrege & Kull We anticipate that bridge abutments will be founded on coarse granular glacial outwash or river alluvium that should provide adequate support for conventional spread foundations. These materials are not expected to be susceptible to potential liquefaction or excessive settlement. Based on the results of our site reconnaissance and a review of available subsurface information, H&K’s professional opinion is that the site is suitable for the proposed trail and bridge crossing using conventional earthwork grading and foundation construction techniques. No highly compressible or potentially expansive soil conditions or potentially liquefiable deposits are expected at the site. Specific recommendations regarding the geotechnical aspects of project design and construction are presented in the following report. We appreciate the opportunity of providing our services for this project. Please contact us if you have any questions regarding this report. Sincerely, Holdrege & Kull Prepared By: Reviewed By: Pamela J. Raynak, P.G. John K. Hudson, P.E., C.E.G. Senior Geologist Principal copies: Electronic copies to Garry Horton, Mark Thomas & Company, ghorton@markthomas.com and Jessica Thompson, Town of Truckee, jthompson@townoftruckee.com s:Project Data\42100-42199\42169-01 TRLT Phase 4/42169-01 TRLT Phase 4 Geotechnical and Geologic Review.docx TABLE OF CONTENTS 1. INTRODUCTION ................................................................................................................. 1  1.1 Purpose............................................................................................................................ 1  1.2 Scope of Services ............................................................................................................ 1  1.3 Site Description ................................................................................................................ 2  1.4 Proposed Project ............................................................................................................. 2  2. LITERATURE REVIEW ....................................................................................................... 4  2.1 Site Soil ............................................................................................................................ 4  2.2 Site Geology .................................................................................................................... 4  2.3 Slope Stability and Rockfall Hazards ............................................................................... 6  2.4 Avalanche Hazard ............................................................................................................ 6  2.5 Previous Investigations .................................................................................................... 7  3. SEISMICITY AND FAULTING .......................................................................................... 10  3.1 Regional Seismicity ........................................................................................................ 10  3.1.1 Western Nevada Zone ............................................................................................ 10  3.1.2 Other Seismic Sources ............................................................................................ 10  3.1.3 Historic Seismicity ................................................................................................... 11  3.2 Regional Faulting ........................................................................................................... 12  3.3 Secondary Seismic Hazards .......................................................................................... 14  4. ANTICIPATED SUBSURFACE CONDITIONS ................................................................. 16  4.1 Existing Fill ..................................................................................................................... 16  4.1 Alluvial Deposits ............................................................................................................. 16  4.1 Glacial Outwash Deposits .............................................................................................. 16  4.2 Talus and Volcanic Rock ............................................................................................... 17  4.3 Groundwater .................................................................................................................. 17  5. PRELIMINARY CONCLUSIONS ...................................................................................... 18  6. PRELIMINARY RECOMMENDATIONS ........................................................................... 20  6.1 Grading .......................................................................................................................... 20  6.1.1 Clearing and Grubbing ............................................................................................ 20  6.1.2 Preparation for Fill Placement ................................................................................. 21  6.1.3 Fill Placement .......................................................................................................... 21  6.1.4 Cut/Fill Slope Grading ............................................................................................. 22  6.1.5 Best Management Practices and Erosion Control ................................................... 23  6.1.6 Surface Water Drainage .......................................................................................... 24  6.1.7 Water Quality Protection ......................................................................................... 24  6.1.7 Plan Review and Construction Monitoring .............................................................. 25  6.2 Preliminary Structural Improvement Design Criteria ...................................................... 25  6.2.1 Preliminary Bridge Abutment Foundations .............................................................. 25  6.2.2 Pavement Design .................................................................................................... 25  7. LIMITATIONS .................................................................................................................... 26  8. REFERENCES .................................................................................................................. 28  FIGURES Figure 1 – Site Vicinity Map Figure 2 – Site Plan Figure 3 – Geologic Map APPENDICES Appendix A Proposal Appendix B Important Information About This Geotechnical Engineering Report Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 1 Holdrege & Kull 1. INTRODUCTION This report presents the results of Holdrege & Kull’s (H&K’s) preliminary geotechnical engineering and geologic review for the proposed Truckee River Legacy Trail – Phase 4 project to be constructed in Truckee, Nevada and Placer Counties, California. H&K prepared this report in general accordance with our January 25, 2016 proposal for the project. A copy of the proposal is included as Appendix A of this report. For your review, Appendix B contains a document prepared by the Geoprofessional Business Association (GBA) entitled Important Information About This Geotechnical-Engineering Report. This document summarizes the general limitations, responsibilities, and use of geotechnical engineering reports. 1.1 Purpose The purpose of this preliminary review is to provide general geotechnical and geologic information to be considered during the planning and design of the project. H&K’s evaluation addresses the general soil and groundwater conditions at the project site, with emphasis on how the conditions are expected to affect the proposed construction. This report also considers potential geologic hazards including faulting and seismicity, slope instability, and other secondary seismic hazards. The preliminary recommendations contained in this report should not be extrapolated to other areas or used for other developments. A more detailed geotechnical investigation must be performed prior to design and construction. 1.2 Scope of Services To prepare this report H&K performed the following scope of services:  Review of previous geotechnical engineering and geologic reports prepared by H&K and other consultants near the project area.  Review of available geologic and seismicity maps and literature covering the project area;  Geologic surface reconnaissance of the project area;  Engineering analyses to develop preliminary geotechnical engineering recommendations for project planning and design; and  Preparation of this preliminary engineering report. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 2 Holdrege & Kull 1.3 Site Description The project area is located on the eastern side of the Sierra Nevada Mountain Range and south of downtown Truckee, California (see Figure1). Topography in the project area is characterized as moderately steep to steep mountainous terrain and gently sloping glacial outwash valleys and alluvial terraces. Vegetation in the project area consists of conifer trees, shrubs, and riparian brush and grasses. According to the 1992 edition of the Truckee, California 7.5-minute quadrangle map published by the United States Geological Survey (USGS); the project area comprises a portion of Sections 14, 15, 16 and 21, Township 17 North, Range 16 East. In general, most of the proposed trail is situated on gentle slopes adjacent to the Truckee River. The northeast terminus adjacent to Brockway Road portion of the proposed trail will traverse gently sloping terrain associated glacial outwash deposits. The north portion of the trail will traverse moderately to steeply sloping terrain associated with volcanic flows. The central and southwest portions of the trail will traverse gently sloping terrain adjacent to the Truckee River. The northeast portion of the trail has been modified by existing development. The approximate location of the project is shown on Figure 1, Site Vicinity Map. A plan view of the proposed project is shown on Figure 2, Site Plan. The proposed trail will be a continuation of Phase 3 of the Truckee River Legacy Trail that will begin at the Truckee Regional Park. The trail will continue southwest across Brockway Road at the intersection with Palisades Drive and travel westward along the south side of Brockway Road. The trail will veer south through the Hilltop Master Plan area near Cottonwood Restaurant and traverse lands owned by the Truckee Donner Public Utilities District, Truckee Springs LLC, United States Forest Service, and State of California. The trail will cross the Truckee River along a pedestrian bridge and may enter into Placer County, depending on the selected bridge location. The trail will terminate at West River Street, near the confluence of Donner Creek and the Truckee River. 1.4 Proposed Project Information about the proposed project was obtained from H&K’s site visits, conversations with Garry Horton of Mark Thomas & Associates, and review of preliminary plans prepared by Mark Thomas & Associates, dated August 2016. The proposed project will involve constructing an approximately 2.3 mile long Class 1 paved bikeway and recreation trail. The trail will be 10 feet wide with 2 foot wide unpaved shoulders. A pedestrian bridge will be needed where the trail crosses the Truckee River. Several alternatives are currently under consideration for the trail (see Figure 2). Appurtenant construction will include temporary and permanent erosion control features, surface drainage improvements, and Low Impact Development (LID) techniques. H&K anticipates the pedestrian bridge will be supported by conventional Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 3 Holdrege & Kull shallow footings in areas outside existing flood plains. Bridge foundations may require deepened footings if footings will be located inside existing flood plains. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 4 Holdrege & Kull 2. LITERATURE REVIEW H&K reviewed available geologic and soil literature in order to evaluate geologic, seismic, and anticipated subsurface conditions at the project site. The following section of this report incorporates geologic features observed during H&K’s site reconnaissance and literature review. 2.1 Site Soil Soil information throughout the project area was researched by accessing the Natural Resources Conservation Service (NRCS) web soil survey (http://websoilsurvey.nrcs.usda.gov). Based on our review of the NRCS web site, several different soil types are mapped across the project including units from the Inville-Riverwash-Aquolls, Kyburz-Trojan, Martis-Euer, and Rubble Land-Rock Outcrop Series. Soil throughout most of the project area is mapped as Inville-Riverwash-Aquolls (EWB), 2 to 5 percent slopes. This soil unit typically forms on glacial outwash and river terraces, is relatively thick (54 inches), well drained, and has a moderately high to high permeability rate. The Kyburz-Trojan Series soil unit (FUE) is mapped near the northeast corner of the project area near the eastern trail terminus. This soil unit typically forms on mountain slopes, has an average thickness of 34 to 60 inches, is well drained, and has a moderately high permeability rate. The Martis-Euer soil unit (MEB) is mapped in the southwest corner of the project near the western trail terminus. This soil typically forms on glacial outwash plains, has an average thickness of 67 inches, is well drained, and has a moderately high permeability rate. The Rubble Land-Rock Outcrop soil unit (SUG) is mapped along sloping terrain near the northeast and southwest portions of the project area. This soil unit typically forms on tallus slopes and composed of 60 percent rubble land and 30 percent rock outcrop. 2.2 Site Geology The geology of the eastern Sierra Nevada is composed primarily of Cretaceous age intrusive granitic rocks and Late Tertiary age (Pliocene) basaltic andesite and pyroclastic volcanic rocks. In the project area, late Pleistocene aged volcanic rocks primarily composed of Bald Mountain basalt, dominate the terrain. Following the Miocene and Pliocene volcanic activity era, glaciation dominated the geology of the area during the Quaternary epoch. Three generations of glaciation (Donner Lake, Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 5 Holdrege & Kull Tahoe, and Tioga) are characterized by terrace deposits within the Truckee area, including the project site. Glacial deposits within the project area include Tioga and Donner aged outwash deposits. Alluvial deposits represent deposition from more recent fluvial processes and occur along the banks of the Truckee River. To help evaluate geology in the project area, the following maps and literature were reviewed:  Geologic Map of Part of Eastern Placer County, Northern Sierra Nevada, California. Prepared by David S. Harwood, G. Reid Fisher, and Richard E. Hanson, 2014, California Geological Survey, Map Sheet 61.  Geologic Map of the North Lake Tahoe-Donner Pass Region, Northern Sierra Nevada, California, prepared by Arthur Gibbs Sylvester, William S. Wise, Jordan T. Hastings, and Lorre A. Moyer, 2012, California Geological Survey, Map Sheet 60.  Geologic Map of the Lake Tahoe Basin, California and Nevada, by G.J. Saucedo, California Geological Survey, 2005.  Pleistocene History of the Truckee Area, North of Lake Tahoe, California, by Peter W. Birkeland, Stanford University Ph.D. Thesis, 1962. The geologic maps indicate that several different stratigraphic units underlie the project area, including alluvial deposits, glacial outwash, talus deposits, and volcanic rock. Holocene aged alluvial deposits are mapped on the southeast bank of the Truckee River near the southwest end of the project area. The alluvium typically consists of silt, sand, gravel and cobbles deposited by fluvial processes. Tioga aged glacial outwash deposits are mapped along the banks of the Truckee River throughout much of the project area and generally consist of dense silt, sand, gravel, cobbles, and boulders. Donner aged glacial outwash deposits are mapped in the relatively level area near the northeast trail terminus. Quaternary aged talus deposits are mapped along the slopes southeast of the Truckee River below and adjacent to volcanic rock outcrops. Talus deposits are mapped near the northeast, central, and southwest ends of the proposed trail. Pleistocene aged volcanic rock composed of olivine latite is mapped along a prominent ridgeline above and south of the proposed trail alignment. The ridgeline generally consists of an olivine latite flow derived from Bald Mountain, south of the project area. H&K completed a surface reconnaissance at the site in August 2016. The ground surface near the northeast end of the trail has been modified by grading and likely contains existing fill. Unpaved access roads containing brush were observed below Cottonwood Restaurant and sloping sections of the proposed trail alignment. Alluvial Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 6 Holdrege & Kull deposits consisting of loose to medium dense granular soil types are located adjacent to the Truckee River in the area of one proposed bridge crossing near the southwest trail terminus. Glacial outwash deposits are present along the banks of the Truckee River throughout most of the trail alignment and near the northeast trail terminus. The glacial till deposits consist of silt, sand, gravel, cobbles, and boulders. Talus deposits were observed along the flanks of the steep slopes above and south of the Truckee River. Scattered large volcanic boulders were observed on the ground surface near the base of the talus slopes. Volcanic rock is present near the top of the prominent ridgeline above and south of the project area. The volcanic rock consists of olivine latite, is weathered at the surface, and moderately to widely fractured. A geologic map presenting the results of our surface reconnaissance is presented as Figure 3, Geologic Map. 2.3 Slope Stability and Rockfall Hazards Slope instability includes landslides, avalanches, debris flows, and rockfall. Much of the trail alignment is located adjacent to steep slopes that may be subject to landslides, avalanches, debris flows and rockfall. No landslides or debris flows are located along the planned trail alignment. Several small landslide deposits are mapped near the north end of the Truckee River Canyon, approximately 500 feet south of the west trail terminus. The potential for landslides is low due to the relatively competent nature of volcanic rock exposed above the trail alignment and competent nature of adjacent materials. Debris flows typically occur in steep gullies with weak rock or soil material at the source area and are triggered by large storm events. The proposed trail is not located in nor does it cross a steep gully with a source area of weak rock or soil material at the source area; therefore, we anticipate that debris flow hazards are low. Rockfall is the process where rock fragments detach and fall through bouncing, rolling, or sliding until they are deposited. The potential for damage due to rockfall is a relatively rare and unpredictable event. The planned trail alignment traverses across and is located near the base of talus slopes that may be prone to rockfall. Volcanic boulders were observed on the ground surface near the base of talus slopes. In the event of a forest fire, the risk of rock fall and debris flow may increase. 2.4 Avalanche Hazard As previously stated above, the project area is located adjacent to steep slopes that are subject to avalanches. To help evaluate potential avalanche hazards in the project area, we reviewed an avalanche hazard study prepared by Dick Penniman, Avalanche Specialist, dated August 1998 and we observed the slopes in the trail area. The avalanche hazard study was prepared for the Truckee Springs Subdivision located near the northeast portion of the proposed project. The report contains maps that show the location and estimated runout zones of avalanche paths generated from the steep slopes located adjacent to the project. Based on the avalanche hazard study report, an avalanche occurred on the slopes above the Truckee Springs Subdivision in January Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 7 Holdrege & Kull 1982. The runout zone of the 1982 avalanche extended onto the Truckee Springs property. A portion of the proposed trail crosses low, moderate, and high potential avalanche runout zones, as shown on Figure 2. To help evaluate avalanche hazards on steep slopes adjacent to the project area, H&K observed steep slopes near the project area and developed slope profiles along six slopes, including previously mapped avalanche areas. Several locations above the proposed south alignment have slope profiles that could potentially develop landslides and are similar to the 1982 slide path. Slopes up to approximately 35 degrees are present above the most southern alternative alignment between approximate Stations 103 and 168. Based on this preliminary evaluation, it appears that potential avalanche hazards are present on the slopes above the proposed trail between approximate Stations 103 to 168. Snow avalanches in the site area are relatively infrequent events that would only occur during or within about 24 hours after unusually large snow storms. Mitigation of avalanche hazards for the trail could include avoidance of avalanche paths by re- aligning the trail location; active control, such as at alpine ski areas; anchoring or modification of the snow in the source area; construction of protective structures, such as snow sheds; and/or warnings, such as signs. Based on H&K’s understanding of the project and the potential avalanche hazard, we recommend a warning system and public education program to alert pedestrians to the potential hazard during large snow storm events. 2.5 Previous Investigations To help evaluate subsurface conditions in the project area, we reviewed the following investigative reports prepared by H&K and others at nearby properties:  Geotechnical Engineering Report for Proposed Truckee Retail Center, 10040 Palisades Drive, Truckee, California, prepared by H&K, dated January 14, 2013 (H&K 2013).  Geotechnical Engineering Report for Hilltop Senior Living Cottages Project, Truckee, California, prepared by H&K, dated December 8, 2009 (H&K 2009a).  Geotechnical Engineering Report for Hilltop Senior Living Lodge Project, Truckee, California, prepared by H&K, dated December 8, 2009 (H&K 2009b).  Geotechnical Investigation, Truckee Springs Project, Truckee, California, prepared by Black Eagle Consulting, Inc., dated October 11, 2012 (BECI 2012). Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 8 Holdrege & Kull  Geotechnical Engineering Report, Truckee River Pedestrian Bridge, Truckee, California, prepared by H&K, dated November 29, 2007 (H&K 2007). Truckee Retail Center The proposed Truckee Retail Center is located at the intersection of Brockway Road and Palisades Drive, near the northeast corner of the project. An unnamed creek traverses the south portion of this property in a general west to east direction. An inventoried freshwater wetland has been established by the United States Fish and Wildlife Service (USFWS) along the south portion of the property. H&K excavated eight test pits across the site to depths up to 10 feet below the ground surface (bgs). The subsurface conditions encountered in the test pits consisted of existing fill, medium dense to dense native granular and soft to very stiff native fine-grained soil types. The existing fill consisted of about 8 inches of loose to medium dense granular soil containing some deleterious material (wood, concrete, asphalt, and metal). Fat clay soil was encountered near the southwest corner of the proposed building area. Groundwater was encountered at depths ranging from 3 to 9 feet bgs. H&K recommended removal of existing fill and fat clay soil in areas that would support foundations, retaining structures, and pavements. H&K also recommended drainage improvements to protect wetland hydrology and seasonal saturation of near-surface soil (H&K 2013). Based on the subsurface conditions encountered at this property, it is likely underlain by glacial outwash deposits. Hilltop Senior Living Cottages and Lodge The Hilltop Senior Living Cottages and Lodge project is located upslope and southwest of the Truckee Retail Center in the Hilltop Master Plan area. H&K previously completed a subsurface investigation at this property in 2004, the results of which were incorporated into the Senior Living Cottages and Lodge reports (H&K 2004). In 2009, H&K excavated 15 test pits across the Cottages and Lodge project areas to depths ranging from 4.5 to 20.5 feet bgs. Subsurface conditions encountered in the test pits consisted of existing fill, medium dense to dense granular soil, very stiff to hard fine- grained soil, cobbles and boulders, and volcanic rock. The existing fill generally consisted of loose to medium dense granular soil. Groundwater was not encountered. Based on the subsurface conditions encountered at this property, it is likely underlain by glacial outwash, Prosser Creek Alluvium, and volcanic rock. Truckee Springs In 2012, Black Eagle Consulting, Inc. (BECI) completed a subsurface investigation at the Truckee Springs property located near the northwest central area of the trail alignment. BECI excavated ten test pits to depths of 1.5 to 8 feet bgs. Subsurface conditions encountered in the test pits consisted of loose to very dense granular soil types with cobbles and boulders. Groundwater was not encountered. Based on the Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 9 Holdrege & Kull subsurface conditions encountered in the BECI test pits, it is likely that this site is underlain by glacial outwash deposits. Truckee River Pedestrian Bridge The Truckee River Pedestrian Bridge project is located at the former Nevada County Corporation Yard off of West River Street, near the central portion of the site. The Town of Truckee Redevelopment Agency was considering a 190-foot pedestrian bridge crossing without supporting piers or columns. H&K excavated eight test pits to depths ranging from 3 to 14.5 feet bgs (H&K 2007). Approximately 15 to 25 feet of loose existing fill containing debris was encountered at the proposed north bridge abutment. Subsurface conditions encountered at the south bridge abutment consisted of loose to very dense granular soil containing cobbles and boulders. Groundwater was encountered at a depth of 6.5 feet bgs. Based on the subsurface conditions encountered, it is likely that this property is underlain by glacial outwash deposits. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 10 Holdrege & Kull 3. SEISMICITY AND FAULTING 3.1 Regional Seismicity Similar to nearly all of California, the project site is located in a potentially active seismic area. The site has experienced moderate ground shaking due to historic earthquakes. We reviewed California Geological Survey (CGS) Open File Report 96-08, Probabilistic Seismic Hazard Assessment for the State of California, and the on-line revisions and California Geological Survey updates to the report, 2002 California Fault Parameters. The document categorizes faults as Type A, B, or C. Type A faults are capable of producing large magnitude events, and have a high rate of slip. Type C faults are not capable of producing large magnitude earthquakes, and have a relatively low slip rate. Type B faults are all other type faults. The CGS report indicates only B and C type faults are within 100 kilometers of the subject site. 3.1.1 Western Nevada Zone According to the California Geological Survey Fault Parameters Map (2002), the project site is located within the Western Nevada Seismic Zone. The Western Nevada Zone is composed of a poorly defined system of strike slip and dip slip faults within the eastern portion of the Sierra Nevada and the western portion of Nevada. The 2002 California Geological Survey earthquake catalog categorizes the Western Nevada Zone as an approximately 150-mile long shear zone with the hazard derived from an areal source, rather than from a single fault. The fault system is designated as Type C, with a low rate of slip and low rate of recurrence. 3.1.2 Other Seismic Sources The California Geological Survey earthquake catalog (2002) identifies other potential seismic sources including the faults noted below.  The Mohawk Valley Fault Zone, located north of the Western Nevada Zone about 15 miles northwest of the site, is designated as a Type C shear zone with the hazard distributed over the area of the zone. The fault zone includes the zone roughly between the Town of Truckee and Lake Almanor.  The Genoa fault is a Type B east dipping, normal fault located approximately 23 miles southeast of the site. The Genoa fault has produced up to 50 feet of displacement within the last 2000 years.  The Antelope Valley fault zone is a series of northwest trending, east dipping normal faults and fault splays located near Topaz Lake approximately 65 miles southeast of the site. The Antelope Valley fault is designated as a Type B fault zone. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 11 Holdrege & Kull  The Honey Lake fault zone is located approximately 43 miles northeast of the site, and is located in eastern California and northwestern Nevada. The Honey Lake Fault Zone is characterized as normal, east dipping faults with a strike slip component of displacement. The Honey Lake fault zone is designated as a Type B fault zone.  The West Tahoe- Dollar Point fault (WTDPF) is located approximately 4 miles southeast of the site. Based on recent information (Brothers, et. al, 2009 and Seitz 2015), the fault has a mapped length of 45 kilometers, the fault slip rate is 0.4 to 0.8 mm/year, and the fault is capable of large earthquakes. The fault is a Type B fault and is included in the Western Nevada Zone. The slip rate and earthquake potential along the fault is comparable to the nearby Genoa Fault.  The Polaris fault is located in Martis Valley near Truckee, approximately 2.5 miles east of the site. The United States Army Corps of Engineers (USACE) completed high-resolution bare-earth airborne Light Detection and Ranging (LiDAR) imagery in 2009 surrounding the Martis Creek Dam located near Truckee, California. The results of the study indicated that an active fault (now named the Polaris Fault) travels beneath the Martis Creek Dam. The fault is estimated to be capable of producing a Richter magnitude 6.4 to 6.9 earthquake. In addition, the study estimated a slip rate of 0.4±0.1 mm/yr for the Polaris Fault, making it a significant seismic hazard to the region and speculation that it may be an extension of the WTDPF.  The Dog Valley fault is located approximately 4.5 miles northwest of the site and extends from Dog Valley to Donner Lake (less than 25 miles in length). The Dog Valley fault was the source of the 1966 magnitude 6.0 Truckee earthquake and is a northeast-trending, strike-slip fault. This fault is a Type B fault and considered part of the Western Nevada Zone. 3.1.3 Historic Seismicity Several earthquakes have occurred since 1850 which have produced noticeable ground shaking in the site vicinity. We reviewed available online documents and reports in our files for information about local effects.  An earthquake with magnitude 6.0 on the Dog Valley fault, located near Stampede Reservoir approximately 4.5 miles northeast of the site, produced noticeable shaking and ground rupture in 1966. The displacement was left lateral strike-slip with a secondary vertical component. Structural damage included damage to two concrete dams, highways, railroads, and flumes, and minor structural damage to buildings. Highway damage included cracks in bridge abutments, settlement of engineered fill, landslides, slumps, and rock fall. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 12 Holdrege & Kull  In 1959, a 5.8 magnitude earthquake was centered approximately 3 miles northeast of the town of Loyalton, approximately 25 miles north of the subject site.  Several historic earthquakes have occurred near the California-Nevada border near Verdi, Nevada, approximately 17 miles northeast of the subject site. In 1948, a magnitude 6.0 earthquake occurred in Dog Valley just north of Verdi, approximately 25 miles northeast of the site. The event resulted in structural damage to nearly every building in Verdi and Floriston, damage to power and telephone lines, and incidences of rockfall along Highway 40. In 1914, a magnitude 6.0 event centered in the Verdi area resulted in damage to brick buildings.  A series of earthquakes in 1868 and 1869 centered near the Virginia Range shook western Nevada and eastern California, and resulted in surface ruptures and structural damage to brick buildings.  In 1860, an estimated magnitude 7.0 earthquake occurred on the Olinghouse fault near Pyramid Lake that was felt as far away as Yreka and San Francisco, and resulted in rock fall and surface ruptures.  An estimated 6.2 earthquake shook the areas near the California/Nevada border in 1857, caused ground rupture and structural damage to brick buildings.  In 1852, one of the first earthquakes on record was reported near Stillwater, east of Carson City approximately 90 miles east of the site. The estimated magnitude 7.3 event resulted in collapsed river banks and ground rupture.  More recently (April 2008), the Mogul-Somersett sequence of earthquakes, which included about 5,000 earthquakes up to M4.7, occurred in the Verdi, Nevada area, about 17 miles (27 km) northeast of the site. Some of these earthquakes were felt in the Truckee-Tahoe area. 3.2 Regional Faulting The project is located in a tectonically active area with faults trending near or through the site. To evaluate the location of mapped faults relative to the project site, we reviewed the following maps and reports:  Fault Evaluation Report FER 261, The West Tahoe Fault in the Emerald Bay and Echo Lake Quadrangles, El Dorado County, California, prepared by Gordon Seitz, California Geological Survey, November 16, 2015. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 13 Holdrege & Kull  Geologic Map of Part of Eastern Placer County, Northern Sierra Nevada, California. Prepared by David S. Harwood, G. Reid Fisher, and Richard E. Hanson, 2014, California Geological Survey, Map Sheet 61.  Geologic Map of the North Lake Tahoe-Donner Pass Region, Northern Sierra Nevada, California, prepared by Arthur Gibbs Sylvester, William S. Wise, Jordan T. Hastings, and Lorre A. Moyer, 2012, California Geological Survey, Map Sheet 60.  LiDAR-Assisted Identification of an Active Fault near Truckee, California, by L.E. Hunter, J.F. Howle, R.S. Rose, and G.W. Bawden, Bulletin of Seismological Society of America, Volume 101, No. 3, pp 1162-1181, June 2011.  Fault Activity Map of California; by Charles W. Jennings and William A. Bryant, California Geological Survey, 2010.  New Constraints on Deformation, Slip Rate, and Timing of the Most Recent Earthquake on the West Tahoe-Dollar Point Fault, Lake Tahoe Basin, California, by Daniel S. Brothers, et. al., Bulletin of Seismological Society of America, Volume 99, No. 2A, pp 499-519, April 2009.  60 k.y. record of extension across the western boundary of the Basin and Range province: Estimate of slip rates from offset shoreline terraces and a catastrophic slide beneath Lake Tahoe, by G.M. Kent, et. al., Geology, volume 34, no. 1, p 365-368, May 2005.  Geological Map of the Lake Tahoe Basin, California and Nevada, compiled by George J. Saucedo, California Geological Survey, 2005;  Geologic Map of the Chico Quadrangle, California, by G.J. Saucedo and D.L. Wagner, California Division of Mines and Geology, 1992.  Pleistocene History of the Truckee Area, North of Lake Tahoe, California, by Peter W. Birkeland, Stanford University Ph.D. Thesis, 1962; The potential risk of fault rupture is based on the concept of recency and recurrence. The more recently a particular fault has ruptured, the more likely it will rupture again. The California Geological Survey (2010) defines an “active fault” as one that has had surface displacement within the past 11,000 years (Holocene). Potentially active faults are defined as those that have ruptured between 11,000 and 1.6 million years before the present (Quaternary). Faults are generally considered inactive if there is no evidence of displacement during the Quaternary. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 14 Holdrege & Kull The referenced geologic maps show several active and potentially active faults located near the project site, including the Dog Valley Fault (active, approximately 4.5 miles northwest), a group of unnamed faults southeast of Truckee (potential active, crossing the northeast portion of the proposed trail; and approximately 1.2 miles southeast and 1.5 miles south), the Polaris Fault (active, approximately 2.5 miles east), the West Tahoe – Dollar Point Fault (WTDPF, active, approximately 4 miles southeast), and the North Tahoe Fault (active, approximately 13 miles southeast). The Genoa Fault trends in a north-south direction approximately 23 miles southeast of the site and is capable of very large earthquakes. Earthquakes associated with these faults may cause strong ground shaking at the project site. The potential hazard associated with active earthquake faults involves surface rupture and strong ground motion. Saucedo (2005) shows an unnamed fault trending across the northeast portion of the project area. The unnamed fault trends in a general northwest to southeast direction, is relatively short (about 2.3 miles long) and is shown as concealed (dotted) beneath Prosser Creek alluvium and glacial outwash as it crosses the site. The geologic map prepared by Sylvester et al (2012) shows a near east-west trending unnamed fault crossing the northeast portion of the project area, near the base of the volcanic ridgeline. This fault is relatively short (approximately 4,000 feet long) and shown as dipping to the north. The hazard associated with strong ground motion is dependent on the magnitude of the source earthquake, which is related to the size of the fault (length and depth). The mapped unnamed faults are less than one mile to about two miles long and in a relative sense are not capable of producing large earthquakes. Earthquakes on larger regional faults in the area, such as the West Tahoe – Dollar Point fault, would likely result in higher ground motion at the site than earthquakes on the unnamed fault passing near the project site. We reviewed the “Digital Images of Official Maps of Alquist Priolo Earthquake Fault Zones of California, Northern California Region”, which describes active faults and fault zones (activity within 11,000 years), as part of the Alquist-Priolo Earthquake Fault Zoning Act. The document and the on-line update indicate the site is not located within an Alquist-Priolo active fault zone. The proposed project does not involve construction of habitable structures. Therefore, further investigation of potentially active faults is not warranted at this time. 3.3 Secondary Seismic Hazards Secondary seismic hazards include liquefaction, lateral spreading, and seismically induced slope instability and rock fall. Liquefaction is a phenomenon where loose, saturated, granular soil deposits lose a significant portion of their shear strength due to excess pore water pressure buildup. Cyclic loading, such as an earthquake, typically causes the increase in pore water pressure and subsequent liquefaction. Based on the results of our preliminary site assessment, we anticipate that near-surface soil across Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 15 Holdrege & Kull much of the trail alignment will consist of medium dense to dense granular soil types. The sloping portions of the site are likely underlain by little to no soil overlying near surface rock. These soil profiles will have a low potential for liquefaction. Lateral spreading is the lateral movement of soil resulting from liquefaction of subadjacent materials. Since we anticipate that there is a low potential for liquefaction of soil at the site, the potential for lateral spreading to occur is also considered low. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 16 Holdrege & Kull 4. ANTICIPATED SUBSURFACE CONDITIONS The anticipated subsurface conditions are based on our literature review, a site visit by an engineer and geologist, and our experience in the project area. We have developed the following discussion and conclusions based on the geologic units that will likely underlie the proposed trail. Figure 3, Geologic Map, shows the approximate geologic contacts within the site area. 4.1 Existing Fill Due to the previously developed nature near the northeast trail terminus, we suspect that existing fill is present in this area. Based on our previous subsurface exploration completed at the Truckee Retail Center near the northeast trail terminus, a relatively thin layer (approximately 8 inches) of fill was encountered. In addition, a large stockpile containing boulders is located near the center of the Truckee Retail property. We anticipate the fill will consist of loose to medium dense granular soil types with varying amounts of cobbles. The slopes west and south of Cottonwood Restaurant were formerly used as ski runs. Remnants of old ski lifts remain in this area. Scattered areas containing shards of glass, ceramic and metal were observed on the ground surface west of Cottonwood Restaurant. Due to the historic uses of the Cottonwood Restaurant property, areas of buried debris (shards of ceramic, glass, and metal) may be encountered during grading of the proposed trail. 4.1 Alluvial Deposits An isolated area containing alluvium was observed during our surface reconnaissance in the location of one bridge crossing near the southwest trail terminus. Alluvial deposits generally consist of loose to medium dense coarse sand and gravel with varying amounts of cobbles and boulders. Prosser Creek Alluvium is located near the northeast portion of the trail. This geologic unit consists of dense silty sand with gravel (SM), cobbles and boulders. These materials should provide suitable support for the proposed trail subgrade and pavement sections. Excavations should be possible will conventional earthmoving equipment. An excavator with a thumb attachment will increase the ease of boulder removal. Reuse of near-surface material for engineered fill will be possible provided all over-sized material is removed. On-site processing (screening) may be required. 4.1 Glacial Outwash Deposits Glacial outwash deposits generally consist of medium dense to dense coarse sand and gravel with varying amounts of cobbles and boulders. Outwash deposits are located near the northeast terminus and within much of the trail alignment adjacent to the Truckee River and southwest terminus. It is likely that bridge locations would be Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 17 Holdrege & Kull founded on glacial outwash material. Near surface soil in these areas will likely consist of medium dense to dense silty sand (SM) and poorly sorted sand (SP) with varying amounts of gravel, cobbles, and boulders. The glacial outwash was deposited ina relatively high energy depositional environment, resulting in graded coarse material that should not be prone to liquefaction. These materials should provide suitable support for the trail subgrade and pavement sections. Excavations should be possible with conventional earthmoving equipment. An excavator with a thumb attachment will increase the ease of boulder removal. Reuse of near-surface material for engineered fill will be possible provided all over-sized material is removed. On-site processing (screening) may be required. 4.2 Talus and Volcanic Rock Talus deposits and volcanic rock are located on the slopes near the northeast portion of the proposed trail and on the slopes above the central and southwest trail segments and terminus. Rock will likely be encountered during trail construction that traverses moderate to steep slopes in the northeast portion of the trail between Cottonwood Restaurant and the Truckee Springs property. The talus may be subject to instability and may require support through retaining walls or other engineering structures to help support trail. The volcanic rock is strong, moderately to widely fractured, and weathered at the surface. Excavation conditions in volcanic rock may be difficult and will likely require hydraulic hammers or spot blasting depending on the depth of excavation. Rockfall hazards should be considered for excavations that extend into talus deposits. Due to the over-size rock, near-surface material may not be suitable for reuse as engineered fill. Construction of pavement sections over coarse talus rock should include a sub-base of coarse gravel to fill the void spaces prior to placement of aggregate base. 4.3 Groundwater Fluctuations in soil moisture content and groundwater levels should be anticipated depending on precipitation, runoff conditions and other factors. In trail sections adjacent to the Truckee River, (central and southwest portions), near-surface groundwater should be anticipated. During our surface reconnaissance, we observed ponded water at several locations adjacent to the Truckee River. In addition, a relatively wet area was observed at the southwest end of South River Street near the northeast corner of the project area. Based on our experience in the project area, seasonal saturation of near- surface soil should be anticipated, especially during and immediately after seasonal snowmelt. In the remaining portions of the project, it is unlikely that groundwater will be encountered and should not affect the planned trail. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 18 Holdrege & Kull 5. PRELIMINARY CONCLUSIONS The following conclusions are based on our literature review, site visit, and experience in the project area. Subsurface exploration must be performed prior to construction to confirm site subsurface conditions used to provide conclusions and recommendations in this report. 1. Based on our findings, anticipated soil/rock conditions will consist of medium dense to dense granular soil types of low plasticity and near-surface rock that should provide suitable support for the proposed trail subgrade and pavement sections and bridge abutments. No severe soil, groundwater, or geologic constraints were observed that would preclude construction as generally planned. 2. Steep slopes with talus are located within and adjacent to portions of the trail that are subject to natural hazards such as rockfall and avalanches. Avalanche runout zones were identified by others on a portion of the Truckee Springs property located near the northeast portion of the trail. Avalanches and rockfall present hazards to human life and possible damage to the trail. Possible mitigation measures to reduce the risk of avalanches and rockfall include avoidance of high hazard areas, active control, retaining structures and fences, and signage. H&K understands that the trail may be plowed during the winter season for recreational access. H&K anticipates installing warning signs and developing a public education program to help mitigate potential avalanche hazards along the trail. Further evaluation of potential avalanche and rockfall hazards is recommended during a design level investigation to help develop appropriate mitigation measures. 3. Areas of near surface rock and a significant amount of boulders and over-sized material will likely be encountered during excavations for the trail and bridge foundations. Excavations that extend into rock will be difficult. A large track- mounted excavator equipped with a ripper tooth or hydraulic hammer, or spot blasting may be required. With the exception of organic surface soil, the site soil is generally suitable for reuse as structural fill; however, processing to remove oversized material will likely be necessary. 4. A potentially active fault is mapped crossing the northeast portion of the proposed trail (Saucedo, 2005). The proposed project does not involve construction of habitable structures. Therefore, further investigation of potentially active faults is not warranted at this time. The proposed pedestrian bridge should be designed in accordance with current codes and standards to help reduce potential hazards associated with surface rupture and ground motion. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 19 Holdrege & Kull 5. Due to existing development and our previous experience in the project area, it is likely that existing fill is present near the northeast trail terminus and northeast portion of the trail. Due to the potential for excessive settlement and soft unstable soil, the existing fill may not be suitable for support of pavement sections and retaining structures. We have provided recommendations for removing, and if necessary, replacing the existing fill with compacted structural fill. The existing fill will likely be suitable for reuse as structural fill; however, processing to remove some oversize material may be necessary. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 20 Holdrege & Kull 6. PRELIMINARY RECOMMENDATIONS The following preliminary recommendations are based on our understanding of the project as currently proposed, our field observations, preliminary engineering analysis, and our experience in the project area. A subsurface investigation must be performed prior to or concurrent with construction in order to confirm the assumed subsurface conditions used to prepare this report. 6.1 Grading The following sections present our preliminary recommendations for site clearing and grubbing, preparation for and placement of fill material, cut/fill slope grading, best management practices and erosion control, surface water drainage, water quality protection, plan review, and construction monitoring. 6.1.1 Clearing and Grubbing Areas proposed for fill placement should be cleared and grubbed of vegetation and other deleterious materials. Existing vegetation, organic topsoil, and any debris should be stripped and stockpiled outside the construction limits. Based on our experience in the area, we expect that the average depth of stripping will vary across the site and will likely be greater in low lying areas than on steeper slopes. Organic surface soil may be stockpiled for future use in landscape areas, but is not suitable for use as structural fill. We anticipate that the actual depth of stripping will vary across the site and may be greater in wooded areas. Man-made debris or any other onsite excavations should be overexcavated to underlying, competent material and replaced with compacted structural fill. Grubbing may be required where concentrations of organic soil or tree roots are encountered during site grading. If encountered, all existing fill should be removed in areas that will support foundation elements, retaining structures, and pavement sections. The existing fill should either be replaced with compacted structural fill or improvements may be founded directly on properly prepared underlying native soil/rock. The existing fill material will likely be suitable for re-use as engineered fill material provided any rock exceeding 8 inches in maximum dimension and all organic or deleterious material are removed prior to placement. Preparation of the subgrade exposed by over-excavation and requirements for engineered fill should be in accordance with recommendations provided below. All rocks greater than 8 inches in greatest dimension (oversized rock) may be used in landscape areas, rock faced slopes, or removed from the site. Oversized rock should not be placed in fill without prior approval by the project geotechnical engineer. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 21 Holdrege & Kull 6.1.2 Preparation for Fill Placement Prior to fill placement all man-made debris, areas of existing fill, or other deleterious material should be removed to expose non-expansive native soil as discussed in the previous section. Where fill placement is planned, the near-surface soil should be scarified to a depth of about 12 inches below the existing ground surface or to competent material and then uniformly moisture conditioned to within 2 percent of the ASTM D1557 optimum moisture content. Areas to receive fill should be compacted with appropriate compaction equipment to at least 90 percent of the maximum dry density per ASTM D1557, and proof rolled with a loaded, tandem-axle truck under the observation of a representative of Holdrege & Kull. Any areas that exhibit pumping or rutting should be overexcavated and replaced with compacted fill placed according to the recommendations below. 6.1.3 Fill Placement Material used for fill construction should consist of uncontaminated non-expansive native soil or approved import soil. Native engineered fill should be nearly free of organic debris, with a liquid limit less than 40, a plasticity index less than 15, 100 percent passing the 8-inch sieve, and less than 30 percent passing the No. 200 sieve. In general, near surface, on-site soil will likely be suitable for re-use in a fill provided all oversized material is removed prior to placement and compaction. Rock used in fill should be broken into fragments no larger than 8 inches in diameter. Rocks larger than 8 inches are considered oversized material and should be stockpiled for offhaul, later use in rock faced slopes, or placement in landscape areas. Imported fill material should consist of predominately granular soil, non-expansive, and free of deleterious or organic material. Import material that is proposed for use onsite should be submitted to Holdrege & Kull for approval and laboratory analysis at least 72 hours prior to import. If site grading is performed during periods of wet weather, near-surface site soil may be significantly above optimum moisture content. These conditions could hamper equipment maneuverability and efforts to compact fill materials to the recommended compaction criteria. Fill material may require drying to facilitate placement and compaction, particularly during or following the wet season or spring snowmelt. Suitable compaction results may be difficult to obtain without processing the soil (e.g., discing during favorable weather, covering stockpiles during periods of precipitation, etc.). Fill should be uniformly moisture conditioned to within 2 percent of optimum moisture content and placed in maximum 8-inch thick, loose lifts (layers) prior to compacting. Fill should be compacted to at least of 90 percent of the maximum dry density per ASTM Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 22 Holdrege & Kull D1557. The upper 8 inches of fill in paved areas should be compacted to at least 95 percent of the maximum dry density per ASTM D1557. Moisture content, dry density, and relative compaction of fill should be evaluated by our firm at regular intervals during fill placement. The earthwork contractor should assist our representative by preparing test pads with the onsite earth moving equipment. Fill material with more than 30 percent rock larger than ¾-inch is not testable using conventional compaction testing equipment. We recommend that a procedural approach, or method specification, be used for quality assurance during rock fill placement rather than a specified relative compaction. The procedural requirements will depend on the equipment used, as well as the nature of the fill material, and will need to be determined by the geotechnical engineer on site. Based on our experience in the area, we anticipate that the procedural specification will require a minimum of six passes with a Cat 563 or similar, self-propelled vibratory compactor to compact a maximum 8-inch thick loose lift. Processing or screening of the fill may be required to remove rocks larger than 8-inches in maximum dimension. Continuous observation by a representative of Holdrege & Kull will be required during fill placement to confirm that procedural specifications have been met. 6.1.4 Cut/Fill Slope Grading Permanent cut and fill slopes at the subject site should be stable at inclinations up to 2H:1V; however, we recommend re-vegetating or armoring all cut/fill slopes to reduce the potential for erosion. Steeper slopes may be possible at the site provided slopes are protected from excessive erosion using rock slope protection or similar slope reinforcement. Slopes steeper than 2H:1V should be evaluated on a case by case basis. Fill should be placed in horizontal lifts to the lines and grades shown on the project plans. Slopes should be constructed by overbuilding the slope face and then cutting it back to the design slope gradient. Fill slopes should not be constructed or extended horizontally by placing soil on an existing slope face and/or compacted by track walking. Equipment width keyways and benches should be provided where fill is placed on side- slopes with gradients steeper than 5H:1V. Benching must extend through loose surface soil into suitable material, and be performed at intervals such that no loose soil is left beneath the fill. Holdrege & Kull should observe keyways and benches prior to fill placement. The upper two to three feet of cut slopes should be rounded into the existing terrain above the slope to remove loose material and produce a contoured transition from cut face to natural ground. Scaling to remove unstable cobbles and boulders may be necessary. Fill slopes should be compacted as recommended for the placement of engineered fill. The upper 4 to 8 inches may be scarified to help promote revegetation. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 23 Holdrege & Kull 6.1.5 Best Management Practices and Erosion Control Based on our site observations and experience in the area, site soil will be moderately to highly susceptible to erosion, particularly on steep, unprotected slopes. Best management practices (BMPs) should be incorporated into the design and construction of this project. A reference regarding appropriate BMPs is the “Erosion and Sediment Control Guidelines for Developing Areas of the Sierra Foothills and Mountains”, prepared by the High Sierra Resource Conversation and Development Council, 1991. The California Regional Water Quality Control Board, Lahontan Region, Best Management Practices Plan is another source of BMPs. Erosion and sediment control measures can be categorized as temporary or permanent. Temporary measures should be installed to provide short-term protection until the permanent measures are installed and effective. Covering all exposed soil with gravel or wood chip mulch is highly effective in preventing erosion. Temporary erosion control structures generally are designed to slow runoff velocity and intercept suspended sediment to prevent sediment discharge from the construction area while allowing runoff to continue down gradient. Typical temporary measures include properly installed silt fences, straw bales, wattle-sediment logs, water bars, detention basins, channel linings, and inlet protection. Following completion of construction and planting/seeding, temporary erosion control measures may be left in place, possibly for a complete growing season. Temporary erosion control measures require regular inspection and maintenance. The selection and sizing of a sediment barrier is dependent on slope angle, slope length, and soil type. Sediment barriers should be installed down gradient and at the edges of all disturbed areas and around topsoil and spoil piles where necessary. Sediment barriers should be placed as needed on slope contours, within small drainages, and in gently sloping swales. Berms, waterbars and ditches should be used to divert or channel storm water runoff away from sensitive, disturbed or construction areas. Waterbars are intended to slow water traveling down a disturbed slope and divert water off disturbed soil into adjacent stable often well-vegetated areas. Where possible, interceptor ditches and waterbars should take advantage of existing terrain and vegetation to divert runoff before it reaches slopes and disturbed areas. Waterbars should be constructed above and within disturbed areas. The spacing for temporary waterbars should be as needed to divert water off the disturbed areas. Waterbars should be located adjacent to non-erodible (vegetated or rocky) receiving areas. If stable receiving areas are not present, flow energy dissipaters or “J-hook” shaped silt fences should be positioned at the waterbar outlet. In highly erodible soils, waterbar ditches should be protected by temporary lining or by decreasing waterbar spacing and length of flow line slopes. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 24 Holdrege & Kull Permanent erosion and sediment control measures may include rock slope protection (RSP), rock lined ditches and inlet/outlet protection, rock energy dissipaters, infiltration/detention basins, and vegetation. All areas disturbed by construction should be revegetated, and existing vegetation should be protected and undisturbed where possible. Revegetation should consist of native brush and grass species. Slope faces should be temporarily protected against erosion resulting from direct rain impact and melting snow using the methods described above until permanent vegetation can be established. Surface water drainage should not be directed to flow over slope faces. Interceptor (brow) ditches should be considered at the tops of slopes in order to collect and divert runoff which otherwise would flow over the slope face. The intercepted water should be discharged into natural drainage courses or into other collection and disposal structures. 6.1.6 Surface Water Drainage Based on our past experience with geotechnical investigations in the project vicinity, there is potential for seasonal saturation of near-surface soil. Due to the anticipated low permeability and shallow depth of near-surface rock, especially within the northeast portion of the proposed trail, site soil will likely have poor infiltration capabilities and groundwater may develop above on-site rock. We recommend the project civil engineer in conjunction with the project geotechnical engineer develop appropriate measures to capture, detain, and manage surface water runoff. 6.1.7 Water Quality Protection To help protect water quality and habitat trail design and construction should use low impact development (LID) techniques. LID is a storm water management and land development strategy applied at the parcel scale that emphasizes conservation and use of on-site features integrated with engineered and small scale hydrologic controls to more closely mimic the natural hydrologic function. In general, surface water along the proposed trail should not be collected and discharged at points. The trail should slope to one side or be crowned so that all runoff should be continuously infiltrated at the shoulder of the trail. Water should not be collected in ditches or curbs to be discharged at points. LID strategy mimics natural drainage as much as possible. Vegetation at the side of the trail should be protected to help infiltrate and filter surface water runoff, where possible. Infiltration gravel provides retention and infiltration of surface water runoff, which helps reduce runoff volume and peak flow rates, and disconnects the flow path that would otherwise concentrate drainage. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 25 Holdrege & Kull 6.1.7 Plan Review and Construction Monitoring Construction monitoring includes review of plans and specifications and observation of onsite activities during construction as described below. We should review final grading and foundation plans prior to construction to evaluate whether our recommendations have been implemented and to provide additional and/or modified recommendations, if necessary. We also recommend that our firm be retained to provide construction monitoring and testing services during site grading and foundation installation to observe subsurface conditions with respect to our engineering recommendations. 6.2 Preliminary Structural Improvement Design Criteria The following sections provide preliminary design criteria for foundations and seismic design. Site specific subsurface exploration must be performed prior to preparation of design level drawings and specifications. 6.2.1 Preliminary Bridge Abutment Foundations Depending on loads and bridge abutments located outside the flood plain of the Truckee River, we anticipate that conventional shallow foundations will be suitable for support of the proposed bridge abutments. If final design loads are high or foundations will be constructed within the flood plain of the Truckee River, deep foundations may be required to support bridge abutments. We have provided preliminary design criteria for conventional shallow foundations below. Deep foundation design criteria can be provided as project plans develop. Foundations should be embedded a minimum of 24 inches below the lowest adjacent finish grade for frost protection and confinement. Reinforcing steel requirements for foundations should be provided by the project structural engineer. Foundations founded in competent, undisturbed native soil may be designed using allowable bearing capacities between 3,000 and 5,000 pounds per square foot (psf) for dead plus live loads. Allowable bearing pressures may be increased by 33 percent for transient loading such as wind or seismic loads. If water is present during concrete placement, concrete should be placed into the footing excavation using tremie methods. Concrete should displace water in the excavation and not mix with unintended water. Holdrege & Kull should observe footing excavations prior to reinforcing steel and concrete placement. 6.2.2 Pavement Design Site soil should provide adequate support for the trail asphalt concrete (AC) pavement. Based on the anticipated traffic, soil, and environmental conditions at the site, we Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 26 Holdrege & Kull recommend a minimum pavement section of 3 inches of asphalt concrete (AC) on 6 inches of Class II aggregate base (AB). A graded sub-base or non-woven filter fabric, such as Mirafi 160N or equivalent, should be placed between the AB and underlying angular rock or native soil. Due to the potential for excessive erosion, we do not recommend sand material or decomposed granite for shoulders backing on sloping trail segments. Frequent surface drainage and infiltration will help reduce excessive erosion. Specific recommendations can be provided in a design level report. Based on our experience in the Tahoe-Truckee area, environmental factors, such as freeze-thaw cycles and thermal cracking will usually govern the life of asphalt concrete (AC) pavements. Thermal cracking of asphalt pavement allows more water to enter the pavement section, which promotes deterioration and increases maintenance costs. Due to the long and narrow nature of the proposed trail, it will be subject to transverse cracking. Thicker pavement sections are not as susceptible to cracking as thinner sections. Due to the low anticipated traffic loads, pavements should be designed with environmental considerations and regular maintenance should be performed. The upper 6 inches of native soil should be compacted to at least of 95 percent of the maximum dry density per ASTM D1557 prior to placing aggregate baserock. Aggregate baserock should also be compacted to a minimum of 95 percent. Subgrade and AB dry density should be evaluated by Holdrege & Kull. In addition to field density tests, subgrade should be proof rolled under the observation of Holdrege & Kull prior to baserock placement. To improve pavement performance and lifespan, we recommend promoting drainage of the pavement subgrade. Drainage can be accomplished through roadway layout and design. A representative of Holdrege & Kull should evaluate pavement subgrade at the time of construction and provide location-specific recommendations for pavement drainage. Pavement subgrade should be graded and prepared such that water drains from beneath pavement section and away from the trail. 7. LIMITATIONS The recommendations in this report are preliminary in nature. Actual subsurface conditions may vary from those described above. A full geotechnical investigation must be performed prior to construction. This report is only valid if Holdrege & Kull performs a subsurface exploration prior to or at the time of construction. Our professional services were performed consistent with the generally accepted geotechnical engineering principles and practices employed in the site area at the time the report was prepared. No warranty, express or implied, is intended. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 27 Holdrege & Kull Our scope of services did not include evaluating the project site for the presence of hazardous materials or petroleum products. Although we did not observe evidence of hazardous materials or petroleum products at the time of our site visit, project personnel should take necessary precautions should hazardous materials be encountered during construction. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 28 Holdrege & Kull 8. REFERENCES Birkeland, Peter W., 1962, Pleistocene History of the Truckee Area, North of Lake Tahoe, California, Stanford University Ph.D. Thesis. Black Eagle Consulting, Inc., October 11, 2012, Geotechnical Investigation, Truckee Springs Project, Truckee, California. Project No. 1583-01-1. 36 Pages. Brothers, Daniel S., et. al., April 2009, New Constraints on Deformation, Slip Rate, and Timing of the Most Recent Earthquake on the West Tahoe-Dollar Point Fault, Lake Tahoe Basin, California, Bulletin of Seismological Society of America, Vol. 99, No. 2A, pp. 499-519. California Department of Conservation, Division of Mines and Geology, 1996, Probabilistic Seismic Hazard Assessment for the State of California, Open-File Report 96-08, prepared in conjunction with the United States Geological Survey. California Geological Survey, 2016, on-line revisions to 2002 California Fault Parameters and interactive map. Hart, Earl W., Revised 1992, Fault-Rupture Hazard Zones in California, Alquist-Priolo Special Studies Zones Act of 1972 with Index to Special Studies Zones Maps, California Department of Conservation, Division of Mines and Geology, Special Publication 42. Harwood, David S., G. Reid Fisher, and Richard E. Hanson, 2014, Geologic Map of Part of Eastern Placer County, Northern Sierra Nevada, California, California Geological Survey, Map Sheet 61. Holdrege & Kull, January 15, 2004, Geotechnical Engineering Report for Hilltop Master Plan, Truckee, California. Project No. 40377-01. 22 Pages. Holdrege & Kull, November 29, 2007, Geotechnical Engineering Report, Truckee River Pedestrian Bridge, Truckee, California. Project No. 41013-01. 21 Pages. Holdrege & Kull, December 8, 2009, Geotechnical Engineering Report for Hilltop Senior Living Cottages Project, Truckee, California. Project No. 41370-01. 24 Pages. Holdrege & Kull, December 8, 2009, Geotechnical Engineering Report for Hilltop Senior Living Lodge Project, Truckee, California. Project No. 41370-01. 23 Pages. Holdrege & Kull, January 14, 2013, Geotechnical Engineering Report for Proposed Truckee Retail Center, 10040 Palisades Drive, Truckee, California. Project No. 41687- 02. 23 Pages. Project No. 42169-01 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 September 20, 2016 Page 29 Holdrege & Kull Hunter, J.F. Howle, R.S. Rose, and G.W. Bawden, June 2011, LiDAR-Assisted Identification of an Active Fault near Truckee, California, Bulletin of Seismological Society of America, Volume 101, No. 3, pp 1162-1181. Jennings, Charles W. and William A. Bryant, 2010, Fault Activity Map of California, California Geological Survey, Geologic Data Map No. 6. Kent, G.M., et. al., May 2005, 60 k.y. record of extension across the western boundary of the Basin and Range province: Estimate of slip rates from offset shoreline terraces and a catastrophic slide beneath Lake Tahoe, Geology Magazine, volume 34, no. 1, p 365-368. National Resources Conservation Service, 20156 Web Soil Survey, http://websoilsurvey.nrcs.usda.gov. Penniman, Dick, September 1998, Avalanche Hazard Study, APN 19-30-12, Truckee, California. 15 Pages. Saucedo, George J., 2005, Geologic Map of the Lake Tahoe Basin, California and Nevada, California Geological Survey, Regional Geologic Map Series, Map No. 4. Saucedo, G. J. and D. L. Wagner, 1992, Geologic Map of the Chico Quadrangle, California Department of Conservation, Division of Mines and Geology, Regional Geologic Map Series, Map No. 7A. Seitz, Gordon, 2015, Fault Evaluation Report FER 261, The West Tahoe Fault in the Emerald Bay and Echo lake Quadrangles, California Geological Survey, 29 Pages, 3 Plates. United States Geological Survey, 1992, Truckee Quadrangle, 7.5 minute series. FIGURES Figure 1 Site Vicinity Map Figure 2 Site Plan Figure 3 Geologic Map Holdrege & Kull APPENDIX A PROPOSAL Holdrege & Kull APPENDIX B IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL ENGINEERING REPORT (Included with permission of GBA, Copyright 2016) Geotechnical-Engineering Report Important Information about This Subsurface problems are a principal cause of construction delays, cost overruns, claims, and disputes. While you cannot eliminate all such risks, you can manage them. The following information is provided to help. The Geoprofessional Business Association (GBA) has prepared this advisory to help you – assumedly a client representative – interpret and apply this geotechnical-engineering report as effectively as possible. In that way, clients can benefit from a lowered exposure to the subsurface problems that, for decades, have been a principal cause of construction delays, cost overruns, claims, and disputes. If you have questions or want more information about any of the issues discussed below, contact your GBA-member geotechnical engineer. Active involvement in the Geoprofessional Business Association exposes geotechnical engineers to a wide array of risk-confrontation techniques that can be of genuine benefit for everyone involved with a construction project. Geotechnical-Engineering Services Are Performed for Specific Purposes, Persons, and Projects Geotechnical engineers structure their services to meet the specific needs of their clients. A geotechnical-engineering study conducted for a given civil engineer will not likely meet the needs of a civil- works constructor or even a different civil engineer. Because each geotechnical-engineering study is unique, each geotechnical- engineering report is unique, prepared solely for the client. Those who rely on a geotechnical-engineering report prepared for a different client can be seriously misled. No one except authorized client representatives should rely on this geotechnical-engineering report without first conferring with the geotechnical engineer who prepared it. And no one – not even you – should apply this report for any purpose or project except the one originally contemplated. Read this Report in Full Costly problems have occurred because those relying on a geotechnical- engineering report did not read it in its entirety. Do not rely on an executive summary. Do not read selected elements only. Read this report in full. You Need to Inform Your Geotechnical Engineer about Change Your geotechnical engineer considered unique, project-specific factors when designing the study behind this report and developing the confirmation-dependent recommendations the report conveys. A few typical factors include: • the client’s goals, objectives, budget, schedule, and risk-management preferences; • the general nature of the structure involved, its size, configuration, and performance criteria; • the structure’s location and orientation on the site; and • other planned or existing site improvements, such as retaining walls, access roads, parking lots, and underground utilities. Typical changes that could erode the reliability of this report include those that affect: • the site’s size or shape; • the function of the proposed structure, as when it’s changed from a parking garage to an office building, or from a light-industrial plant to a refrigerated warehouse; • the elevation, configuration, location, orientation, or weight of the proposed structure; • the composition of the design team; or • project ownership. As a general rule, always inform your geotechnical engineer of project changes – even minor ones – and request an assessment of their impact. The geotechnical engineer who prepared this report cannot accept responsibility or liability for problems that arise because the geotechnical engineer was not informed about developments the engineer otherwise would have considered. This Report May Not Be Reliable Do not rely on this report if your geotechnical engineer prepared it: • for a different client; • for a different project; • for a different site (that may or may not include all or a portion of the original site); or • before important events occurred at the site or adjacent to it; e.g., man-made events like construction or environmental remediation, or natural events like floods, droughts, earthquakes, or groundwater fluctuations. Note, too, that it could be unwise to rely on a geotechnical-engineering report whose reliability may have been affected by the passage of time, because of factors like changed subsurface conditions; new or modified codes, standards, or regulations; or new techniques or tools. If your geotechnical engineer has not indicated an “apply-by” date on the report, ask what it should be, and, in general, if you are the least bit uncertain about the continued reliability of this report, contact your geotechnical engineer before applying it. A minor amount of additional testing or analysis – if any is required at all – could prevent major problems. Most of the “Findings” Related in This Report Are Professional Opinions Before construction begins, geotechnical engineers explore a site’s subsurface through various sampling and testing procedures. Geotechnical engineers can observe actual subsurface conditions only at those specific locations where sampling and testing were performed. The data derived from that sampling and testing were reviewed by your geotechnical engineer, who then applied professional judgment to form opinions about subsurface conditions throughout the site. Actual sitewide-subsurface conditions may differ – maybe significantly – from those indicated in this report. Confront that risk by retaining your geotechnical engineer to serve on the design team from project start to project finish, so the individual can provide informed guidance quickly, whenever needed. This Report’s Recommendations Are Confirmation-Dependent The recommendations included in this report – including any options or alternatives – are confirmation-dependent. In other words, they are not final, because the geotechnical engineer who developed them relied heavily on judgment and opinion to do so. Your geotechnical engineer can finalize the recommendations only after observing actual subsurface conditions revealed during construction. If through observation your geotechnical engineer confirms that the conditions assumed to exist actually do exist, the recommendations can be relied upon, assuming no other changes have occurred. The geotechnical engineer who prepared this report cannot assume responsibility or liability for confirmation- dependent recommendations if you fail to retain that engineer to perform construction observation. This Report Could Be Misinterpreted Other design professionals’ misinterpretation of geotechnical- engineering reports has resulted in costly problems. Confront that risk by having your geotechnical engineer serve as a full-time member of the design team, to: • confer with other design-team members, • help develop specifications, • review pertinent elements of other design professionals’ plans and specifications, and • be on hand quickly whenever geotechnical-engineering guidance is needed. You should also confront the risk of constructors misinterpreting this report. Do so by retaining your geotechnical engineer to participate in prebid and preconstruction conferences and to perform construction observation. Give Constructors a Complete Report and Guidance Some owners and design professionals mistakenly believe they can shift unanticipated-subsurface-conditions liability to constructors by limiting the information they provide for bid preparation. To help prevent the costly, contentious problems this practice has caused, include the complete geotechnical-engineering report, along with any attachments or appendices, with your contract documents, but be certain to note conspicuously that you’ve included the material for informational purposes only. To avoid misunderstanding, you may also want to note that “informational purposes” means constructors have no right to rely on the interpretations, opinions, conclusions, or recommendations in the report, but they may rely on the factual data relative to the specific times, locations, and depths/elevations referenced. Be certain that constructors know they may learn about specific project requirements, including options selected from the report, only from the design drawings and specifications. Remind constructors that they may perform their own studies if they want to, and be sure to allow enough time to permit them to do so. Only then might you be in a position to give constructors the information available to you, while requiring them to at least share some of the financial responsibilities stemming from unanticipated conditions. Conducting prebid and preconstruction conferences can also be valuable in this respect. Read Responsibility Provisions Closely Some client representatives, design professionals, and constructors do not realize that geotechnical engineering is far less exact than other engineering disciplines. That lack of understanding has nurtured unrealistic expectations that have resulted in disappointments, delays, cost overruns, claims, and disputes. To confront that risk, geotechnical engineers commonly include explanatory provisions in their reports. Sometimes labeled “limitations,” many of these provisions indicate where geotechnical engineers’ responsibilities begin and end, to help others recognize their own responsibilities and risks. Read these provisions closely. Ask questions. Your geotechnical engineer should respond fully and frankly. Geoenvironmental Concerns Are Not Covered The personnel, equipment, and techniques used to perform an environmental study – e.g., a “phase-one” or “phase-two” environmental site assessment – differ significantly from those used to perform a geotechnical-engineering study. For that reason, a geotechnical- engineering report does not usually relate any environmental findings, conclusions, or recommendations; e.g., about the likelihood of encountering underground storage tanks or regulated contaminants. Unanticipated subsurface environmental problems have led to project failures. If you have not yet obtained your own environmental information, ask your geotechnical consultant for risk-management guidance. As a general rule, do not rely on an environmental report prepared for a different client, site, or project, or that is more than six months old. Obtain Professional Assistance to Deal with Moisture Infiltration and Mold While your geotechnical engineer may have addressed groundwater, water infiltration, or similar issues in this report, none of the engineer’s services were designed, conducted, or intended to prevent uncontrolled migration of moisture – including water vapor – from the soil through building slabs and walls and into the building interior, where it can cause mold growth and material-performance deficiencies. Accordingly, proper implementation of the geotechnical engineer’s recommendations will not of itself be sufficient to prevent moisture infiltration. Confront the risk of moisture infiltration by including building-envelope or mold specialists on the design team. Geotechnical engineers are not building- envelope or mold specialists. Copyright 2016 by Geoprofessional Business Association (GBA). Duplication, reproduction, or copying of this document, in whole or in part, by any means whatsoever, is strictly prohibited, except with GBA’s specific written permission. Excerpting, quoting, or otherwise extracting wording from this document is permitted only with the express written permission of GBA, and only for purposes of scholarly research or book review. Only members of GBA may use this document or its wording as a complement to or as an element of a report of any kind. Any other firm, individual, or other entity that so uses this document without being a GBA member could be committing negligent Telephone: 301/565-2733 e-mail: info@geoprofessional.org www.geoprofessional.org