Geotechnical.pdf

GEOTECHNICAL REPORT

Sierra Vista Lane

Carson City, Nevada

NV FLAP 100 (1)

Yeh Project No.: 216-406

August 30, 2017

Prepared for:

Muller Engineering Company, Inc.

777 South Wadsworth Boulevard, Suite 4-100 Lakewood, Colorado, 80226

Prepared by:

Yeh and Associates, Inc.

391 Front Street, Suite D

Grover Beach, California 93433

Phone: 805-481-9590

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G l e n w o o d S p r i n g s , C O • G r a n d J u n c t i o n , C O • D u r a n g o , C O • C o l o r a d o S p r i n g s , C O w w w . y e h - e n g . c o m

GEOTECHNICAL REPORT

Sierra Vista Lane

Carson City, Nevada

NV FLAP 100 (1)

Yeh Project No.: 216-406

August 30, 2017

Prepared by:

Judd J. King, G.E. (CA), P.E. (NV) Jonathan D. Blanchard G.E. (CA) Senior Geotechnical Engineer Principal Geotechnical Engineer

Independent Reviewer:

Robert F. LaForce, P.E.

Senior Project Engineer

Geotechnical Report Yeh Project No. 216-406 Sierra Vista Lane August 30, 2017 i

TABLE OF CONTENTS

1. PURPOSE AND SCOPE OF STUDY

2. PROJECT UNDERSTANDING

3. FIELD EXPLORATION AND TESTING

3.1 DRILLING

3.2 LABORATORY TESTING

4. SITE CONDITIONS

4.1 GEOLOGIC SETTING

4.2 TERRAIN AND VEGETATION

4.3 CLIMATE

4.4 EXISTING PAVEMENT SURFACE CONDITION

4.5 UPSLOPE SIDE OF ROAD

4.6 DOWNSLOPE SIDE OF ROAD

4.6.1 STATION 1062+00 TO 1064+50

4.6.2 STATION 1103+00 TO 1110+00

4.6.3 STATION 1167+80 TO 1176+00

5. SUBSURFACE CONDITIONS

5.1 PAVEMENT AND SUBGRADE

5.2 ERODED DOWNSLOPE AREAS

6. GEOTECHNICAL ANALYSES AND EVALUATION

6.1 SEISMIC DATA

6.2 SLOPE STABILITY

7. GEOTECHNICAL RECOMMENDATIONS

7.1 EARTHWORK - GENERAL

7.1.1 GENERAL GRADING

7.1.2 EARTHWORK FACTORS

7.1.3 EROSION AND SLOPE DRAINAGE

7.2 EMBANKMENT SLOPE DESIGN

ii

7.2.1 DESIGN OF SLOPE RESTORATION AND EMBANKMENT REPAIRS

7.3 CUT SLOPE DESIGN

7.4 SUBEXCAVATION

7.5 PAVEMENT DESIGN RECOMMENDATIONS

7.5.1 DESIGN TRAFFIC VOLUME AND SUBGRADE

7.5.2 STRUCTURAL SECTION

7.5.3 BINDER AND MIX RECOMMENDATIONS

7.5.4 PAVEMENT MAINTENANCE

7.5.5 PARKING AREAS

7.6 CORROSION CONSIDERATIONS

8. MATERIAL SOURCES

9. NOTES TO CONSTRUCTION

10. LIMITATIONS

11. REFERENCES

List of Figures

FIGURE 1: PROJECT LOCATION MAP

FIGURE 2: REGIONAL GEOLOGIC MAP (AFTER BINGLER (1977) AND GARSIDE AND RIGBY (2009))

FIGURE 3: TYPICAL SURFACE PAVEMENT CONDITIONS ALONG ROAD

FIGURE 4: CUT SLOPE BETWEEN STATIONS 1167+85 TO 1175+00

FIGURE 5: EROSIONAL GULLY AT STATION 1062+50

FIGURE 6: STATION 1103+00 TO 1110+00

FIGURE 7: EMBANKMENT SLOPE AND RIP-RAP REVETMENT STATION 1168+80 TO 1176+00

FIGURE 8: FILL PLACEMENT ON EXISTING SLOPES

FIGURE 9: TYPICAL SUBEXCAVATION

FIGURE 10: SUMMARY OF STRUCTURAL SECTION RECOMMENDATIONS

List of Tables

TABLE 1: EXISTING PAVEMENT CONDITIONS ENCOUNTERED

TABLE 2: SUBGRADE SOIL PROPERTIES TEST SUMMARY

TABLE 3: SOIL PROPERTIES TEST SUMMARY

iii

TABLE 4: SUMMARY OF AREAS OF SLOPE INSTABILITY AND EMBANKMENT REPAIR

List of Plates

Plate Station 1167+85 to 1175+00 Boulder Removal and Catchment Restoration Plan Sheet: Embankment Benching Details

List of Appendices

Page Appendix A – Boring Logs Boring Location Plans ......................................................................................................................... A-1 to 12 Boring Record Legends .................................................................................................................. A-13 and 14 Boring Logs 16P-01 to 08 for Pavement ........................................................................................... A-15 to 22 Borings Logs 16S-01 to 04 for Stability ............................................................................................. A-23 to 29 Photos of Drill Rig Setups ................................................................................................................. A-30 to 32 Appendix B – Results of Laboratory Testing Summary of Laboratory Test Results .......................................................................................................... B-1 Atterberg Limits ........................................................................................................................................... B-2 Percent Passing No. 200 Sieve (%gravel, sand, fines) ................................................................................. B-3 Mechanical Sieve Analysis, Summary of Results .................................................................................. B-4 to 5 Mechanical Sieves Analysis, Gradation Charts ................................................................................... B-6 to 10 pH and Resistivity ..................................................................................................................................... B-11 Proctor Compaction ......................................................................................................................... B-12 to 13 Consolidated Undrained Triaxial Test .............................................................................................. B-14 to 21 Resistance “R” Value Test ................................................................................................................ B-22 to 26 Appendix C – Slope Stability Plots Slope Stability Plots .............................................................................................................................. C-1 to 8 Appendix D – Pavement Design Calculations AASHTO (1993) Spreadsheet Calculations ........................................................................................... D-1 to 2

1. PURPOSE AND SCOPE OF STUDY

Yeh and Associates, Inc., as a subconsultant to Muller Engineering Company of Lakewood, Colorado, was retained by the Federal

Highway Administration (FHWA), Central

Federal Lands Highway Division (CFL) to provide geotechnical and pavement recommendations for the design of improvements to 2.5 miles of Sierra Vista Lane in Carson City, Nevada. Sierra Vista Lane is located at the southeast limits of Carson City and runs north-south along the east side of the

Carson River. The road provides access for recreational use to the Pine Nut Mountain

Range, the Carson River, and rural residential home sites located along the roadway. The project location is shown on Figure 1. Note that some maps identify the roadway as Mexican Dam Road; however, the roadway is now referred to as Sierra Vista Lane as referenced in this report.

The geotechnical evaluation consisted of a program of project coordination, reviewing project information, field exploration, laboratory testing, and engineering analyses as a basis for providing the recommendations in this report. This report provides recommendations for reconstruction and minor widening of the road with some modifications to the vertical profile, drainage improvements, pavement structural section thicknesses, earthwork factors, slope design, and grading.

2. PROJECT UNDERSTANDING

The project limits are within the section of Sierra Vista Lane between its intersection with Rio

Vista Lane north to its intersection with South Pinion Hills Road. This project is to design the reconstruction (4R) of the 2.5-mile segment of Sierra Vista Lane to improve mobility and access to the Carson River, Federal Lands, and residential properties. The project understanding is based on the Muller (2015) scoping report, Muller (2017) plans and participation in project team meetings.

Figure 1: Project Location Map

The existing roadway is two lanes (one in each direction) with asphalt pavement. The paved road width is approximately 17.5 to 26 feet with gravel shoulders. The existing roadway is constructed on a combination of cut and fill. Most of the existing embankments are 5 to 30 feet high and have typical side slope inclinations of about 1v:1.5h or flatter. Cut slopes vary from 5 to

30 feet tall and have inclinations of 1:1 to essentially vertical. The slopes are sparsely vegetated and exhibit surficial erosion.

The roadway will be designed on essentially the same horizontal alignment as the existing facility. Adjustments to the roadway alignment will be made to improve drainage and the super elevation where needed. The improvements will generally consist of the following:

• The new roadway will have a paved width of 22 to 26.5 feet. The new road width will require widening of the roadway, embankment fills and cuts in some areas (as discussed in this report).

• Changes to the roadway profile will be mostly near existing site grades with localized cuts and fills to improve the road drainage or super elevation. Cuts and fills are generally less than approximately 2 to 4 feet.

• Drainage improvements will consist of improving cross drainage across the road, removing impediments to surface drainage along the road shoulder, replacing and adding new culverts, channelizing drainage, and placing riprap along existing drainages and for bank stabilization along the river

• Two existing parking areas near Station 1127+00 will be improved to accommodate various vehicle types including trailers. Improvements to parking areas include minor grading to improve drainage and placing gravel for parking areas, and a new access road to Parking Area No. 3.

• New signage, conforming to intersections, and erosion control along stream channels and drainages is also planned.

3. FIELD EXPLORATION AND TESTING

3.1 DRILLING

Drilling services were provided by Andresen Drilling, Inc. of Reno, Nevada. RoadSafe, Inc. from

Reno, Nevada provided traffic control and flagging for the drilling. Andresen used a truck-mounted CME 55 drill rig equipped with hollow stem augers to advance twelve (12) borings during the period of October 3 through 5, 2016: eight pavement borings and four borings in the river bank areas. The pavement borings (series 16P-01 to 08) were drilled using 8-inch auger to a typical depth of 5.5 feet below the road surface. The borings along the river bank (series 16S-

01 to 04) were drilled using 6-inch auger to depths ranging from 17 to 41 feet below the road surface. Memoranda describing the field exploration program and findings were submitted by

Yeh (2016a, 2016b). The approximate boring locations of the borings plotted on the Muller

(2016) plans are shown in Appendix A. The logs of the borings are presented in Appendix A.

Photos of the boring locations are presented with the logs in Appendix A.

The borings were sampled by collecting bulk samples of the pavement and subgrade materials from the auger flights, and by driving either a 2-inch standard penetration test (SPT) split spoon sampler or a 3-inch modified California split spoon sampler into the soil at various depths during drilling. Drive samples were typically taken at depths of approximately 1.5 and 4 feet below the road surface. Sampling below 4 feet was performed at 5-foot intervals. The modified California sampler contained 1-inch high brass liners. The SPT sampler was used without liners. The samplers were driven into the ground at the bottom of the boring using a 140-pound safety hammer powered by a rope and cathead type winch system in accordance with ASTM 1586, the

Standard Penetration Test. Yeh measured the thickness of the existing pavement and any base course materials encountered at each P-series boring location, collected and secured bulk and drive samples for subsequent laboratory testing, recorded blow counts (N-values) for drive samples, and prepared a log of subsurface conditions encountered.

Upon completion, the borings were backfilled with auger cuttings and topped with asphalt cold patch. Groundwater was encountered in boreholes, 16S-01 and 16S-02 at a depth of 21 feet and 16S-03 and 16S-04 at depths of 15.5 and 15.0 feet, respectively. These borings were backfilled with cuttings to the level of the groundwater and then sealed with about 1 foot of bentonite chips and then cuttings to the surface. The borings were capped with asphalt cold patch in the roadway areas and with soil cuttings where borings were drilled beyond the edge of pavement.

3.2 LABORATORY TESTING

Laboratory testing was performed on selected samples recovered from the field exploration program. Tests for classification and resistivity were performed at the Yeh office in Grover

Beach, California. Strength tests were performed by the Geo-E lab at the California Polytechnic

State University, Civil Engineering Department in San Luis Obispo, California. NV5 of Ventura, California performed R-value Testing and a gradation for the AC grindings that were tested.

Tests for moisture content, unit weight, Atterberg limits, percent passing the 200 sieve, particle size distribution by mechanical sieve analysis, pH and resistivity, compaction relationships between dry unit weight and moisture content using the modified Proctor test, consolidated undrained triaxial compressive strength with pore-pressure measurements, and R-Value were performed as part of this program. The results of laboratory tests are presented in Appendix B.

Testing was performed in accordance with applicable procedures of American Association of

State Highway and Transportation Officials (AASHTO) or ASTM. The soil descriptions were confirmed or modified as necessary on the boring logs based on the test results and further review of selected samples in the laboratory.

4. SITE CONDITIONS

4.1 GEOLOGIC SETTING

The regional geology in the site vicinity as mapped by Bingler (1977) and Garside and Rigby

(2009) is shown on Figure 2: Regional Geologic Map. The project is located within the Basin and Range geologic and geomorphic province, which spans from the Sierra Nevada to the

Colorado Plateau and from southern Idaho into the state of Sonora, Mexico. The province is characterized by a series of north-south trending linear mountain ranges paired with low lying intervening valleys.

The topography is a result of extensional tectonics in the east-west direction that caused thinning of the earth’s crust followed by normal faulting that created the uplifted mountain ranges and down dropped basins. The mountain ranges are commonly flanked by a continuous apron of alluvial sediments known as a bajada that eroded from the highlands and is formed by coalescing alluvial fans. The transitional area between the mountain ranges and the bajada is known as a pediment and is characterized as an erosional bedrock surface often covered by a thin veneer of sediment.

Sierra Vista Lane lies adjacent to the Carson River in an endorheic1 basin draining toward the

Carson Sink to the north. The bedrock geology exposed at the southern portion of the project site is mapped as microdiorite (Jmd), which is largely concealed by floodplain and terrace

1 An endorheic drainage is a closed basin that has inflows but no outflow. In the Basin and Range Province, endorheic drainages dominate and often terminate in lakes or sinks.

deposits of the Carson River (Qf. Qot/Qt), alluvial fan and pediment deposits (Qpa/QTg) and artificial fill (Af) at the site.

Figure 2: Regional Geologic Map (after Bingler (1977) and Garside and Rigby (2009))

4.2 TERRAIN AND VEGETATION

Sierra Vista Lane provides access to recreational lands southeast of Carson City. Most of the road is bordered by gently rolling hillsides, steep river bluffs and sloping terrain. The road borders the east side of the Carson River between about Stations 1101+00 to 1110+00, near the middle of the project, and between about Stations 1170+00 and 1175+00 near the north end of the project. The Carson river flows north and has water year-round. The road is constructed with a typical cut and fill cross section where it lies adjacent to the river and along adjacent hills to the east, and near adjacent grades across the flatter portions of alignment. Cut and fill slopes are generally less than about 30 feet in height, but are near the base of steep hillsides or above the banks of the river and small valleys. Ephemeral2 drainages cross the roadway either through culverts or run uncontrolled over the roadway surface. Vegetation consists of chaparral, sparse grasslands with pockets of trees. The riparian corridor along the river is commonly vegetated with a combination of mature trees, brush, and willow.

4.3 CLIMATE

The climate data (www.usclimatedata.com) for Carson City, Nevada reports an average annual rainfall of 9.63 inches. The majority of the rainfall is reported to occur between November and

March. The reported monthly average temperature ranges from a low of 22 degrees for

December and January to a high of 90 degrees in July.

The estimated frost depth in the site vicinity is reported to be 5 to 10 inches. The distress to the existing pavement does not appear to have been specifically impacted by freeze-thaw conditions or frost heave, groundwater is not present near the subgrade elevation, and the recommended structural section will exceed the existing pavement condition. No special recommendations are provided to address freeze-thaw conditions.

4.4 EXISTING PAVEMENT

SURFACE CONDITION

Photos showing the typical surface pavement conditions observed along Sierra Vista

Lane are presented on Figure

3. The exposed pavement surface along Sierra Vista

Lane consisted of asphalt concrete with a posted speed limit of 25 miles per hour.

The pavement was in poor to fair condition. The pavement generally had a raveled pavement surface;

2 Ephemeral: Term used to describe drainages that are usually dry that will fill with water during brief episodes of heavy rainfall.

Figure 3: Typical Surface Pavement Conditions along Road Top left to right: Sta. 1180+90, Sta. 1134+25 Bottom left to right: Sta. 1148+50, Sta. 1145+00 low to highly developed block, fatigue and edge cracking; and localized but highly developed fatigue cracking, raveling, patches and deteriorating potholes.

4.5 UPSLOPE SIDE OF ROAD

The existing slopes along the uphill side of the road vary from 1:1.5 to vertical. Exposures in the cut are generally comprised of terrace deposits in the northern section of the road and pediment/alluvial fan deposits in the remaining portion. One bedrock exposure was observed between Stations 1109+00 to 1110+00 where bedrock outcrops along the river and is exposed in the cut slope above the road and on the slope below the road. Soil exposed in existing cut slopes consisted of silty sand, poorly graded sand with gravel, and silty sand with gravel.

Materials were slightly to moderately cemented. Intermittent 4- to 8-foot wide catchment zones were present at the bottom of the slopes. The catchment zones were mostly filled with eroded material that was beginning to encroach onto the roadway in some areas or has reduced roadway widths in some areas.

The existing cut slope along the northbound shoulder of the road between Stations 1167+75 to

1175+00 typically consists of a 10- to 20-foot high near-vertical cut with a natural slope above the cut that continues at about a 100 percent grade to the top of the adjacent mesa.

Subrounded boulders, estimated to range from 1 to 3 feet in size were observed on the slope.

Figure 4 presents photographs of the cut slope conditions in this area.

Figure 4: Cut slope between Stations 1167+85 to 1175+00

4.6 DOWNSLOPE SIDE OF ROAD

Two areas (between approximately Stations 1103+00 to 1110+00 and Stations 1168+80 to

1176+00) were investigated for slope instability. The existing slopes below the roadway in these areas have been subject to surficial erosion from uncontrolled stormwater runoff flowing across the road. Yeh also reviewed the condition of an embankment in the vicinity of Station 1062+50 where the slope below the road has been eroded and a gully has formed adjacent to the roadway. No evidence of undercutting of these slopes from erosion along the Carson River was observed during our site visit. The predominant cause of the erosion and instability of the slopes below road is attributed to uncontrolled drainage running over these slopes that has resulted in rills, erosional gullies and reduced roadway width. These areas are described in detail in the following sections of this report.

4.6.1 STATION 1062+00 TO 1064+50

An embankment was constructed to cross a gully at this location where there is a sharp turn in the road. A storm drain inlet is located on the upslope side of the road. An erosional gully and instability of the embankment was observed at this location as seen in Figure 5. The instability does not appear to have impacted the road bench or pavement, and appears limited to instability of the berm and uncontrolled side-cast fill that has been placed along the downslope

Figure 5: Erosional gully at Station 1062+50 side of the road. The Muller (2017) cross sections show that the embankment slope along the downhill side of the road is approximately 30 feet high, and the inlet to culvert is about 10 feet below the road on the uphill side. The slopes of the embankment along the downslope side of the road are about 1:1.3 to 1:1.5.

A 2- to 3-foot high earth berm is present along the southbound side of the road at this location.

The berm appears to have been constructed in association with dumping fill material on the slope below the road. The downhill side of the fill has settled and tension cracks have formed along the top of the berm as a result of instability of the fill. The gully shown on Figure 5 likely formed in association with stormwater that overflowed the road and onto the slope. The resulting gully has an approximately 13-foot vertical drop off the side of the existing road. The head cut of the gully is about 1 to 2 feet from the existing edge of pavement.

4.6.2 STATION 1103+00 TO 1110+00

The southbound side of road is on embankment. The Muller (2017) cross sections show that slope of the embankment is up to approximately 16 -feet high. The embankment slopes have a typical inclination of 1:1.75, with areas that have eroded to approximately 1:1.25 to 0.75:1. The toe of the existing embankment appears to catch the natural grade at about 10 to 20 feet east from the edge of the active river channel (as observed in October 2016). The soil exposed on the slope was comprised of slightly cemented silty sand with gravel and poorly graded sand with silt and gravel. The soil contained some cobbles and a few boulders up to about 1.5 feet across. Typical conditions at this location are shown in Figure 6.

The area on the east/upslope side of the road is a low-lying fluvial plain that has two to three shallow drainage channels. Colluvium and evidence of debris flows were observed within this plain and along the existing slope on the

Figure 6: Station 1103+00 to 1110+00 upslope side of the road. The slope along the northbound shoulder of the road is about 5-feet high and cut into these deposits at some locations. Stormwater appears to flow through these natural channels and across the road. Uncontrolled flow from these drainages appears to have caused mud/debris flows, erosion, rills, steepening and undercutting of the subject embankment on the downslope side of the road. Intermittent earth berms are present along the southbound shoulder of the road. The berms appear to channelize stormwater and concentrate drainage water until the flow eventually erodes the berms when water can then run uncontrolled over the top of the slope.

4.6.3 STATION 1167+80 TO 1176+00

The Muller (2017) cross sections show that the slope below the road is approximately 25 feet high. The slope has a typical inclination of approximately 1:1.5, with localized areas of erosion that have steepened the slope to about 0.2:1 to 0.75:1. An existing rip-rap revetment is present along the base of this slope from below the edge of water at the Carson River to about 6 feet above the water level at the time of our visit in October 2016. The rip-rap is estimated to be composed of 2,000- to 4,000-pound rock (typically about 4 feet in size). The revetment extends from a bridge on Carson River Road over the Carson River upstream of the project limits to

Figure 7: Embankment slope and rip-rap revetment Station 1168+80 to 1176+00 about Station 1168+80, a total length of about 700 feet. Typical surface conditions of this slope and the existing rip-rap revetment are shown in Figure 7.

Soil exposed on the existing slope consisted of slightly to moderately cemented silty sand with gravel, silty sand, and poorly graded sand with silt and gravel. Surface water running over the slope appears to have caused erosion, rills, gullies, instability, and steepening of the slope and a reduction in roadway width. Intermittent berms are present along the river side of the Sierra

Vista Lane in this area. These berms appear to channelize stormwater and concentrate the surface water flow until the berms erode and water can then flow freely over the top of the existing slope. Vegetation consisting of willows, brush, and trees were present along the river banks. We looked for and did not observe evidence of global slope instability of the slope that has impacted the existing road bench, such as is typically manifested as longitudinal or arcuate cracks in the pavement surface.

5. SUBSURFACE CONDITIONS

5.1 PAVEMENT AND SUBGRADE

The subgrade soil encountered in the pavement borings consisted of materials associated with the existing pavement structural section for Sierra Vista Lane overlying units of artificial fill, pediment/alluvial fan deposits, and terrace deposits. The approximate locations of the drill holes for the pavement borings are shown on the Boring Location Plan and noted on the logs in

Appendix A. A description of the units encountered are summarized below.

Existing Pavement and Subgrade Soil. The existing structural section generally consisted of variable thicknesses of a double-chip seal surface treatment overlying reclaimed aggregate base course (AC Grindings). Hot mix asphalt (HMA) pavement was present at selected locations as noted in Table 1. The base materials were generally underlain by artificial fill and pediment/alluvial fan deposits composed of medium dense to very dense silty sand, silty sand with gravel, poorly-graded sand with silt, and well-graded sand with clay and gravel subgrade soils. Table 1 presents a summary of the pavement thicknesses and subgrade conditions encountered in each of the borings:

Table 1: Existing Pavement Conditions Encountered

Boring No. Station

Pavement* Thickness (inches)

Base (AC Grindings**) Thickness (inches)

Subgrade soil and AASHTO3 Comment

16P-01 1180+90 1 3 Silty SAND with gravel A-2

16S-01 1171+00 1 4 Well-graded SAND with clay and gravel

A-1-b

16S-02 1170+00 1 4 Silty SAND with gravel

16P-02 1159+50 1 4 Silty SAND with gravel A-1-b

16P-04 1145+00 1 4 Silty SAND with gravel A-1-b

16P-03 1129+20 1 2.5 Silty SAND A-2

Loose subgrade conditions

16P-05 1118+45 1 3.5 Poorly graded SAND with silt

16S-03 1107+00 -- -- Silty SAND with gravel

16S-04 1105+75 -- -- Silty SAND with gravel A-1-b

16P-06 1082+75 1 3.5 Silty SAND A-2

16P-07 1063+50 2 inches HMA 3 Silty SAND

A-2

16P-08 1054+50 1 5.5 Poorly graded SAND with silt

* Pavement surface was a chip seal unless otherwise noted.

** Full-depth reclamation from previous project.

Subgrade Conditions. The pavement subgrade encountered below the structural section generally consisted of medium dense to dense silty sand, silty sand with gravel, poorly-graded sand with silt, and well-graded sand with clay and gravel. The subgrade consisted of a mixture of artificial fill and native soil deposits comprised of pediment, alluvial fan and terrace deposits.

Table 2 presents a summary of selected laboratory test results performed on subgrade samples that were generally taken within the upper 4 feet of borings.

3 AASHTO symbol may be based on manual classification of soil.

Table 2: Subgrade Soil Properties Test Summary4

Subgrade Unit Location

Dry Unit Wt. (pcf) and moisture

Particle Size

Analyses (%G, %S, %F)

Proctor γdmax, wopt

R-value

Common USCS and AASHTO

Classifications

Artificial Fill (Af)

16P-01, 7, 8

16S-01 to 4 γd: 106-wo: 6-9%

3-24 G 64-85 S 12-18 F

123.8 pcf at 10.0%

Silty SAND with gravel (SM), Well-graded Sand with clay and gravel (SW-SC), Poorly graded SAND with silt (SP-SM), A-2, A-1-b Pediment/

Alluvial Fan

Deposits (Qpa)

16P-02 to 6 γd: 107-wo: 4- 10%

4-31 G 34-84 S 12-34 F

127.9 pcf at 9.8% 59, 76

Silty SAND with gravel (SM), Silty Sand (SM), Well-graded Sand with clay and gravel (SW-SC), Poorly graded SAND with silt (SP-SM), A- 2, A-1-b

5.2 ERODED DOWNSLOPE AREAS

The S-series borings (16S-01 through 4) were drilled to assess subsurface conditions at the two areas of potential slope instability. The subsurface conditions encountered in these borings consisted of artificial fill, pediment, alluvial fan, and terrace deposits. The following and Table 3 present a description of the geologic units and a summary of selected laboratory test results associated with the various units. The following provides a description of the geologic units that were encountered.

Artificial fill. Artificial fill (Af) was encountered in borings 16P-01, 16P-07, 16P-08, and 16S-01 through 04. The fill generally consisted of existing embankment fill that was likely placed in association with the construction of Sierra Vista Lane. At most of the boring locations, the fill appears to have been placed along the downslope side of the road as part of cut and fill construction and may pinch out at the transition to a cut above the road. Fill was encountered to depths of approximately 4.5 to 9.5 feet below the road surface where encountered along the southbound shoulder of Sierra Vista Lane. Fill was not encountered in borings 16P-02 through

06, and likely pinches out at the ends of embankment fill placed across drainages. The fill was predominantly composed of medium dense to very dense silty sand with gravel and well-graded sand with clay and gravel, silty sand, and poorly graded sand with silt. The fill also contained varying amounts of cobbles and boulders. 16P-07 encountered refusal on a boulder at a depth

4 Geotechnical properties are noted for dry unit weight (γd) and moisture content (wo); particle size as percent gravel (G), sand size (S) and fines content (F); maximum dry density and optimum moisture content (Proctor), and R-value.

of about 3 feet below the road surface. The gravel content of the fill varied as noted on the boring logs. The artificial fill was underlain by either pediment/alluvial fan or terrace deposits.

Pediment/Alluvial Fan Deposits. Pediment and/or alluvial fan deposits (Qpa) are mapped at the ground surface along much of the roadway alignment. The deposits exposed in cuts along the road and Carson River are composed of sand with varied concentrations and layers of gravel, cobbles, clay and some boulders. These deposits were predominantly encountered in borings south of about Station 1167+00, and below either a layer of artificial fill and/or the existing pavement section. Pediment/alluvial fan deposits encountered in the borings consisted of medium dense to dense silty sand with gravel, silty sand, poorly graded sand with silt and gravel, clayey gravel, and well-graded sand. The deposits were relatively loose in Boring 16P-

03 from 1.5 to 4 feet and in Borings 16S-03 and 4 between about 6.5 and 15.5 feet. Bedrock was encountered below the pediment/alluvial fan deposits were penetrated in Borings 16S-03 and 04 at depths of 15.5 and 17.5 feet, respectively.

Terrace Deposits. Terrace deposits (Qot) were encountered below artificial fill at depths of 4.5 to 9.5 feet in borings 16S-01 and 2. The terrace deposits were encountered to the maximum depths of these borings, up to 41 feet below the road surface. This unit predominantly consisted of dense to very dense poorly graded sand with clay and silt, clayey sand, well-graded sand with gravel, well-graded gravel with sand, silty sand and hard clay and sandy clay with varying amounts and lenses and layers of gravel, clayey gravel and cobbles. A layer of very stiff fat clay was encountered between 29.5 to 34.5 feet in boring 16S-01.

Bedrock. Igneous Bedrock was encountered in borings 16S-03 and 04 at depths of 15 to 18 feet, and was exposed in outcrops. The rock was classified as hard, microdiorite (Jmd). The microdiorite was moderately weathered and fractured where exposed on the cut slope between approximately Stations 1109+00 to 1110+00. Similar rock was also observed in outcrops along the river bank in this vicinity. The roadway in this area appears to be cut into the bedrock and is likely present at the roadway subgrade. Borings 16S-03 and 04 were terminated within the bedrock at depths of 17 and 25 feet below the road surface because of hard drilling conditions.

Table 3: Soil Properties Test Summary5

Geologic Unit

Locations Encountered

Predominant material description

Dry Unit Wt. (pcf) and moisture

Internal Angle of Friction, φ’cu

(degrees)

Cohesion, c’cu (ksf)

Artificial Fill (Af) 16S-01 to 04

Very dense to medium dense silty sand, poorly graded sand, and well-graded sand with varying amounts of silt, clay and gravel γd: 106-121 wo: 6-9% 34 - 40 0 - 0.1

Pediment/Alluvi al Fan Deposits

(Qpa)

16S-03 and 04

Very loose silty sand over stiff to medium clay γd: 90 -116 wo: 5-19% 31 0.2

Terrace Deposits

(Qot)

16S-01 and 02

Dense to very dense poorly to well-graded sand with clay and gravel, silty sand, and well-graded gravel with sand with a layer of very stiff fat clay γd: 89-116 wo: 7-32% 37 0.7

Igneous Rock (Microdiorite –

Jmd)

16S-03 and 04

Hard, slightly to moderately weathered microdiorite -- -- --

6. GEOTECHNICAL ANALYSES AND EVALUATION

6.1 SEISMIC DATA

Seismic data were estimated for the geotechnical evaluation in accordance with AASHTO

Bridge Design Manual Section 3.10.2.1 using USGS earthquake hazards program U.S. Seismic

Design Maps (2015) and the United States Geologic Survey 2008 Interactive Deaggregations website accessed on December 14, 2016. These data were used to estimate the peak ground acceleration and earthquake magnitude for the design earthquake used in the slope stability analysis. A typical shear wave velocity of 1,200 feet per second (365 meters per second) was estimated for the site based on review of the site geology and information collected from the borings. The site coordinates were estimated near the center of the project as latitude 39.1328 and longitude -119.7057. The USGS design maps use a fault database to estimate peak ground and spectral accelerations for a site with specified coordinates. The design earthquake

5 Geotechnical properties are noted for dry unit weight (γd) and moisture content (wo); friction angle (φ) or cohesion (c) in kips per square foot measured from direct shear (ds) or consolidated undrained (cu) tests.

for the site has an estimated peak ground acceleration of 0.7g and resulting from a magnitude

6.9 earthquake, most likely occurring on Prison Hill fault mapped about 5 miles west of the site.

6.2 SLOPE STABILITY

Analyses were performed to evaluate the stability of the existing slopes below the roadway between Stations 1168+80 to 1176+00 and Stations 1103+00 to 1110+00. The bases of these slopes are bordered by the Carson River. As previously discussed, uncontrolled surface drainage running over the road and onto these slopes appears to have resulted in erosion and steepening of the slopes. We looked for and did not observe evidence of global instability of the slopes. The analyses were performed to evaluate the stability of the existing slopes in these areas, and as a basis for providing recommendations.

Yeh used the computer program SLIDE 7.0, developed by Rocscience, to evaluate the stability of the existing and proposed slopes under static and pseudostatic (earthquake) conditions.

These analyses are limited to evaluating the slopes between the existing roadway and the

Carson River. The stability of the steep cut slopes along the northbound side of the road were not analyzed as part of our scope of work. The output from SLIDE is presented in Appendix C.

Subsurface Conditions. For use with SLIDE, the user defines the surface and subsurface profile boundaries, groundwater conditions, the type of analysis to be performed, the layout and strength of any slope reinforcement, boundary loads, and the unit weight and strength of the soil and rock materials included in the analysis. The subsurface profile was estimated mainly using the boring information for borings drilled in the existing roadway, field reconnaissance and mapping of the existing slopes and the river banks, and field measurements. Subsurface profiles were extrapolated from points of exploration or observation to complete the profile used in the analysis. Profile boundaries and geotechnical properties assigned to the various geologic units are shown on the plotted output in Appendix C.

Slope Stability Criteria. Slope stability criteria are defined by the ASHTO LRFD Bridge Design

Specifications. SLIDE reports a single factor of safety for slope stability. The resistance factor for slope stability is therefore the inverse of the reported factor of safety. AASHTO defines slope stability as having a resistance factor of 0.65 (factor of safety of at least 1.5) for slopes supporting or containing structures, and a resistance factor of 0.75 (factor of safety of at least

1.3) for slopes not supporting structures. Slope stability analyses were performed considering an equivalent uniform traffic load of 240 pounds per square foot applied over the traveled way.

Earthquake loads were evaluated using screening analysis procedures for seismic slopes stability developed by Stewart, Blake and Hollingsworth (2002) and are also described in the

California Geologic Survey (2008) Special Publication 117A – Guidelines for Evaluating and

Mitigating Seismic Hazards in California. The screening analysis was performed to evaluate whether the slope being analyzed is vulnerable to significant displacement in response to the design earthquake discussed in Section 6.1. The screening analysis was performed per

SP117A for a tolerable slope displacement of about 6 inches corresponding to a horizontal pseudostatic coefficient (kh) of 0.2 for the design earthquake. The seismic displacement of the slope was considered tolerable when the estimated resistance factor and/or factor of safety from the pseudostatic analysis met or exceeded 1.0, which is applicable for the extreme limit state.

Strength Characterization for Existing Slope Condition. The slope stability model for each location analyzed was checked by evaluating the existing slope condition. The existing slope was typically evaluated by assigning strength values, profile boundaries and slope geometry based on the results of the geotechnical investigation and laboratory and then checked against observations of the existing slope.

Laboratory triaxial shear tests performed on a saturated sample of the existing fill material had a cohesion of approximately 0 psf. The existing slopes adjacent to the road are steep, commonly stand vertical, and therefore appear to have some level of cementation that allows these slopes to stand vertical and without indications of distress to the roadway because of slope instability.

The laboratory test results show that the saturated specimens are essentially cohesionless when saturated, and the test results are consistent with the severe erosion and steeply gullied slopes that were observed in the areas that were analyzed.

Analysis of existing slopes was used to estimate a value of cohesion that corresponded to the observed stability of the vertical slopes at their existing moisture content. The cohesion value in the analysis was varied to provide a stable slope condition corresponding to the height of vertical slopes observed at the site and using the soil friction angle estimated from laboratory testing. A cohesion of 100 pounds per square foot was estimated and selected for subsequent analysis of the existing slope conditions.

Conclusions. The estimated factors of safety for the existing slope conditions exceed the minimum of 1.3 needed to satisfy stable slope conditions. The screening analysis for the existing slope had an estimated factor of safety greater than 1.0 for the design earthquake corresponding to a seismic slope displacement of about 6 inches. The stability of the slopes appears to be dependent on the cohesion and cementation of the soil that can be destroyed by water. The slopes are therefore vulnerable to weakening, instability and severe erosion due to saturation and uncontrolled runoff over these slopes as appears to have been occurring.

Recommendations for the design of reconstructed soil slopes are presented in the subsequent sections of this report. The plotted outputs for the existing slope conditions, proposed widening and grading the existing embankment slopes near Stations 1173+50 to 1175+00, and the final slope geometry considering static and pseudostatic loads are included in Appendix C for the various scenarios considered.

7. GEOTECHNICAL RECOMMENDATIONS

The recommendations of this report assume that grading, paving and fill materials will be placed and constructed in general accordance with Standard Specification FP-14. Compliance with those specifications is needed for the recommendations in this report to remain applicable.

7.1 EARTHWORK - GENERAL

7.1.1 GENERAL GRADING

All site grading should conform to the applicable sections of FP-14. Fill materials should not contain organic matter or other deleterious material. On-site soil removed from excavations should be suitable for use as borrow for general embankment fill and non-critical areas of construction. Existing base or excess FDR should be reviewed by the Engineer during construction prior to reuse as shoulder backing material or fill. Excess fill or cut material should be hauled offsite for proper disposal, unless an onsite location has been specifically identified for placement of excess fill on the project plans. Temporary excavations should conform to

OSHA requirements.

7.1.2 EARTHWORK FACTORS

The subgrade generally consisted of medium dense to very dense silty sand, silty sand with gravel, poorly-graded sand with silt, and well-graded sand with clay and gravel. Drive samples had dry unit weights that ranged from approximately 107 to 121 pcf. The maximum dry unit weight of the subgrade estimated from Proctor tests was approximately 123 to 128 pcf.

It is recognized that disturbance to drive samples occurs due to compaction of samples during driving, loosening of the soil when recovering the sampler from the boring and during trimming in the laboratory. Differences in the in-situ soil density likely occur in areas where the existing pavement subgrade and embankment fill materials were compacted as opposed to where the roadway was constructed in alluvium or terrace deposits.

The estimated earthwork factors6 do not account for losses for removal of vegetation or unsuitable material. An average earthwork factor of 1 can be used considering the pulverization and compaction of the existing structural section. An average earthwork factor of 0.91 can be used for areas where the subgrade will be compacted outside the limits of the existing structural section or in areas where subexcavation is recommended. An average earthwork factor of 0.96 can be used for on-site borrow obtained from road cuts, catchment areas or deeper excavations in soil. The earthwork factors are approximate and should be checked and adjusted as-needed as the grading work progresses.

7.1.3 EROSION AND SLOPE DRAINAGE

Graded slopes are vulnerable to erosion. Surface drainage should be maintained and concentrated flows of runoff should be controlled to reduce erosion. Topsoil, erosion control matting (if needed) and appropriate vegetation should be placed on graded slopes to reduce the potential for erosion. Maintenance and repairs to gullied or eroded areas should be performed promptly while vegetation is being established. Concentrated flows of runoff should not be allowed to run over slopes. Overside drains, curbing, and drainage inlets should be used to divert drainage water to the base of slopes where needed. Riprap and energy dissipation devices should be installed at all outlets of culverts.

7.2 EMBANKMENT SLOPE DESIGN

Graded fill slopes should generally be designed to inclinations of 1:1.5 or flatter unless otherwise recommended in this report. Figure 8 presents the recommended detail for placing fill on existing slopes. A plan sheet with details for specific embankment slope restorations are attached with this report. Additional recommendations for the restoration areas are presented in the following sections of this report. Embankment fill should generally be keyed and benched into the existing slope where fill will be placed on existing grades of 1:4 or steeper. The fill

6 The earthwork factor represents the ratio of the existing in-situ unit weight of the soil before construction to the unit weight of the same soil after compaction. For example, an earthwork factor of 0.95 represents a 5 percent loss in the volume of soil following compaction. An earthwork factor of 1.05 represents a 5 percent gain in volume of the soil or pavement following compaction due to bulking of a relatively dense material.

should be initiated from a base key excavated at the toe of the new embankment. The base should be at least 2 feet deep and be excavated into firm and stable ground. The new embankment can then be keyed and benched into the existing slope as the fill is placed and such that at least the outer 5 feet of the existing slope is removed.

7.2.1 DESIGN OF SLOPE RESTORATION AND EMBANKMENT REPAIRS

Table 4 presents a summary of areas where Sierra Vista Lane has been impacted by erosion and recommendations for improving surface drainage and restoration of those slopes. Drainage improvements and grading is recommended to help stabilize these slopes. The table summarizes the type of distress that has impacted the existing road and the recommendations for those areas. A detailed discussion, photos, and evaluation of these slopes is presented in

Section 4.6 of this report. Details summarizing the grading recommendations for these slopes are presented on the attached Embankment Benching plan.

Figure 8: Fill Placement on Existing Slopes

Table 4: Summary of Areas of Slope Instability and Embankment Repair

Boring

Approximate Station Limits Existing Condition Recommendation

16P-07 1062+00 to 1064+50

The southbound side of the road is along an approximately 20 to 30-foot high embankment slope.

Surface water running over the top of this slope has eroded a deep gully adjacent to the southbound edge of pavement.

The gully has a 13-foot high vertical head cut just off the existing edge of pavement. Fill placed below the roadway in this area appears to be loose, has tension cracks and is unstable.

Correct surface drainage and provide overside drains or culverts to prevent concentrated flows of runoff from flowing over the top of the slope.

The existing deep gully, loose fill, and eroded soil should be removed and the slope should be reconstructed with properly compacted fill. The finished grade of the slope should have an inclination of 1:1.5 or flatter.

The new fill should be keyed and benched into the existing slope as shown on the Embankment Benching plan.

The vertical portion of the existing slope should be flattened to at least 1:1 prior to placing fill to provide a transition between the new fill and existing slope.

Erosion control measures should be provided for slopes following grading. If needed, a reinforced soil slope could be provided to allow for a steeper fill slope inclination. See Plate 2 for details.

16S-03 and 04

1103+00 to 1110+00

Debris flows and runoff from valley on east side of road is impacting the existing alignment.

The drainage enters the roadway from the top of an approximately 5-foot high cut slope into the toe of the fan and debris flow deposits above the road level.

Surface drainage can flow over the cut slope, run across the road and appears to have impacted the approximately 15-foot high slope on southbound side of road.

Maintenance of this area appears to have consisted of placing loose fill to fill eroded areas, maintain the slope and prevent undermining of the pavement.

Pavement distress that could be attributed to erosion of the slope was not observed.

Correct drainage and provide overside drains or culverts to prevent concentrated flows of runoff from flowing over the top of the slope.

The existing loose fill placed along the southbound side of the road should be removed and the existing embankment slope cut back to an inclination of 1:1.5 measured from the toe of the existing embankment.

Boring

Approximate Station Limits Existing Condition Recommendation

16S-01 and 02

1171+00 to 1176+00

The Muller (2017) plans show that the widening will be performed on the existing road bench and by regrading the slope below the road through about Station 1173+50.

The downslope side of the roadway is constructed on approximately 25-foot high embankment fill. The slope on southbound side of road along Carson River has been subject to surface erosion that has encroached into the existing road.

Correct drainage and provide overside drains or culverts to prevent concentrated flows of runoff from flowing over the top of the slope.

Remove loose and eroded material.

Place compacted fill for the embankment widening by keying and benching into the existing slope per limits shown on Plate 2. Keying and benching should remove at least the outer 5 feet of the existing slope.

The finished slope should be graded to an inclination of 1:1.5.

7.3 CUT SLOPE DESIGN

Graded cut slopes should generally be designed to an inclination of 1:1 or flatter. Plate 1 shows a detail for removing selected boulders from existing slopes above the road and for restoring catchment along the shoulder of Sierra Vista Lane. The boulders are approximately 1 to 3 feet in size and should be able to be removed using hand-scaling and excavation techniques without the use of equipment operating on the slopes above the road.