HomeMy Public PortalAboutSupplement No. 1 Premliminary Geotechnical Engineering and Geologic Review Letter
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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
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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
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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
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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
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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