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Supplement No. 1 Premliminary Geotechnical Engineering and Geologic Review Letter (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 November 13, 2017 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: Supplement No. 1 Preliminary Geotechnical Engineering and Geologic Review This letter presents geologic and geotechnical engineering conclusions and preliminary recommendations for soft trails associated with the Truckee River Legacy Trail – Phase 4. Our Preliminary Geotechnical Engineering and Geologic Review Report for the Truckee River Legacy Trail – Phase 4 was dated September 20, 2016. The purpose of this supplement letter is to include a soft trail portion to the overall project that was not included in our previous report. The proposed soft trails will connect the Ponderosa Palisades residential community to the new Class I Legacy Trail and connect with existing soft trails in the central portion of the project area. A small area of soft trails is also planned near the northeast corner of the project. The scope of our services included surface reconnaissance within the soft trail area not observed during our previous investigation, engineering analysis, and preparation of this supplement letter. Project Description The Truckee River Legacy Trail – Phase 4 project will include construction of approximately 2.3 miles of Class I paved multi-use trail. The Class I trail will continue southwest from the Legacy Trail at the Regional Park, 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 on a pedestrian bridge and may enter into Placer County, depending on the selected bridge location. The trail will connect to the existing Mousehole Class I trail at West River Street. Project No. 42169-01 Supplement No. 1 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 November 13, 2017 Page 2 Holdrege & Kull We understand that a portion of the overall trail project will include construction of soft trails. The location of the proposed soft trails will be near the central portion of the project, climbing up a steep slope to the Ponderosa Palisades subdivision, and northeast corner of the project area near the west end of Southwest River Street. Figure Supplement No. 1 – 3 presents a Geologic Map of the project area. We anticipate the soft trails will be approximately 2 feet in width and will be constructed using hand tools and small equipment. Slope inclinations range up to about 45 degrees along or near the planned trail to Ponderosa Palisades. This area has been previously identified to have snow avalanche potential. However, the planned soft trail will climb the slope on the west side of a ravine or canyon where slope inclinations are less than 45 degrees. The planned soft trail in this area of the site do not cross under avalanche paths. However, the soft trail planned near the northeast corner of the site crosses under a previously identified avalanche path. 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 underlie the proposed soft trail area. Figure Supplement No. 1 - 3, Geologic Map, shows the geologic units within the site area. Glacial Outwash Deposits Glacial outwash deposits consist of medium dense to dense coarse sand and gravel with varying amounts of cobbles and boulders. Outwash deposits are located along the Truckee River near the north terminus of the soft trail where it will intersect with the new Class I trail in the central portion of the project area. Outwash deposits are also located at the north end of the soft trail planned near the west end of Southwest River Street. Near surface soil in these areas will likely consist of medium dense to dense silty sand (SM) with varying amounts of gravel, cobbles, and boulders. The glacial outwash was deposited in a relatively high energy depositional environment, resulting in graded coarse material. These materials should provide suitable support for the proposed soft trails. Excavations should be possible with hand equipment. However, areas of the trail may encounter large cobbles and boulders that may be difficult to excavate with hand equipment. Project No. 42169-01 Supplement No. 1 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 November 13, 2017 Page 3 Holdrege & Kull Talus and Volcanic Rock The majority of the proposed soft trails will cross areas underlain by talus and volcanic rock of the Bald Mountain Basalt. Rock will likely be encountered along the planned soft trail construction to the Ponderosa Palisades subdivision. Talus will also likely be encountered along soft trails that climb the moderate to steep slopes between the Ponderosa Palisades subdivision and the Truckee River. The talus may be subject to instability and may require support through retaining walls or other engineering structures to help support trails. The volcanic rock is strong, moderately to widely fractured, and slightly weathered at the surface. The talus rock is strong durable material that should be suitable for low rockery walls and rock slope protection. Excavation conditions in talus and volcanic rock may be difficult. Rockfall hazards should be considered for excavations that extend into talus deposits. Construction of soft trails over coarse talus rock should include a sub-base of coarse gravel to fill the void spaces within the talus. 4.3 Groundwater We did not observe wet low lying areas or water seepage from steep slopes during our reconnaissance. However, fluctuations in soil moisture content and groundwater levels should be anticipated depending on precipitation, runoff conditions and other factors. In the proposed trail section adjacent to the Truckee River, (northern terminus of proposed soft trail in the central portion of the site), near-surface groundwater should be anticipated in low lying areas, especially during and immediately after seasonal snowmelt. The area of the proposed Truckee Springs development, west of the existing Southwest River Street, is wet. This area of the site contains a perennial spring and a small pond that retains water year-round. The planned soft trail in this area will likely cross wet areas. Wet areas should be avoided where possible. Trail design for water and/or wet area crossing should involve hard scape surfaces such as rock cobble stone pavement and/or small bridges. Preliminary Conclusions and Recommendations The following conclusions and preliminary recommendations are based on our literature review, site visit, and experience in the project area. 1. 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 soft trails. No severe soil, such as highly plastic clay, Project No. 42169-01 Supplement No. 1 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 November 13, 2017 Page 4 Holdrege & Kull 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 soft trails that are subject to natural hazards such as rockfall and avalanches. Avalanches and rockfall present hazards to human life and possible damage to the trail. However, based on our site reconnaissance, the planned soft trail in the center of the project do not cross under or through avalanche paths. The soft trail near the northeast corner of the site will likely cross a previously identified avalanche path. We identified potential avalanche zones that could run out onto the main trail in our previous preliminary report. 3. We anticipate that much of the soft trail alignments will be located in areas with surface talus and volcanic rock. Excavations that extend into rock will be difficult. We anticipate that soft trails in area of near-surface or at surface rock and talus will be constructed by around the rocks. Hand placing large rocks along the outside edge of the trail sections and filling voids with imported soil may be necessary for trail construction in this area of the proposed project. 4. The northern terminus of soft trail planned near the center or the site and near the northeast corner of the site will likely encounter glacial outwash deposits. Excavations into glacial outwash deposits will likely encounter cobbles and boulders. 5. Temporary and permanent Best Management Practices (BMPs) should be employed during and after soft trail construction to prevent erosion and sediment discharge. Trail design should include frequent grade reversals, out sloping, and rolling dips to helpr educe erosion along the trail and provide for sustainable trails. Limitations 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 Supplement No. 1 Preliminary Geotechnical Engineering and Geologic Review for TRLT – Phase 4 November 13, 2017 Page 5 Holdrege & Kull The limitations presented in our preliminary report dated September 20, 2016, are applicable to this supplement letter. Closing We appreciate the opportunity to provide continuing assistance on this project. If you have any questions regarding this letter, please contact the undersigned. Sincerely, Holdrege & Kull Prepared By: Reviewed By: Pamela J. Raynak, P.G. John K. Hudson, P.E., C.E.G. Senior Geologist Principal Attachments: Figure Supplement No. 1 - 3, Geologic Map Electronic copies: Garry Horton, Mark Thomas & Company, ghorton@markthomas.com Jessica Thompson, Town of Truckee, jthompson@townoftruckee.com 11/13/2017 11/13/2017