Geotech_Report.pdf

PAVEMENT DESIGN AND GEOTECHNICAL REPORT

CA FLAP NEV 40(1)

Donner Pass Road Improvements

Nevada and Placer Counties, California

Yeh Project No.: 217-306

February 22, 2019

Prepared for:

Muller Engineering Company, Inc.

777 S. Wadsworth Boulevard 4-100

Lakewood, Colorado 80226 Attn: Karen Fuhr, P.E.

Prepared by:

Yeh and Associates, Inc.

391 Front Street, Suite D

Grover Beach, California 93433 Phone: 805-781-9590

391 Front Street, Suite D Grover Beach, CA 93433

(805) 481-9590 www.yeh-eng.com

Colorado California

Denver | Colorado Springs | Durango | Glenwood Springs | Grand Junction | Greeley Grover Beach | Ventura

PAVEMENT DESIGN AND GEOTECHNICAL REPORT

CA FLAP NEV 40(1)

Donner Pass Road Improvements

Nevada and Placer Counties, California

Yeh Project No.:217-306

February 22, 2019

Prepared by:

Jonathan D. Blanchard, G.E. John D. Duffy, C.E.G.

Geotechnical Engineer Senior Engineering Geologist

Jamie L. Cravens Staff Engineer

Pavement Engineering: Independent Reviewer:

Robert F. LaForce, P.E. T. Samuel Holder, G.E.

Senior Materials Manager Senior Project Engineer http://www.yeh-eng.com/

Pavement Design and Geotechnical Report Project No. 217-306 Donner Pass Road Improvements February 22, 2019 i

TABLE OF CONTENTS

1. PURPOSE AND SCOPE OF STUDY

2. PROJECT UNDERSTANDING

3. SITE CONDITIONS

3.1 GEOLOGIC SETTING

3.2 TERRAIN AND VEGETATION

3.3 CLIMATE

3.4 EXISTING PAVEMENT SURFACE CONDITION

4. GEOTECHNICAL INVESTIGATION

4.1 GEOLOGIC RECONNAISSANCE

4.2 GROUND SURFACE PROFILES

4.3 DRILLING

4.4 LABORATORY TESTING

4.5 PREVIOUS STUDIES

4.6 DEFLECTION TESTING

5. SUBSURFACE CONDITIONS

5.1 EXISTING PAVEMENT STRUCTURAL SECTION

5.2 SUBGRADE CONDITIONS

5.3 GUARDRAIL BORINGS

5.4 GROUNDWATER

6. GEOTECHNICAL ANALYSES AND EVALUATION

6.1 SEISMIC DATA

6.2 SLOPE STABILITY

7. CONCLUSIONS AND 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 DESIGN

7.3 GEOGRID-REINFORCED MECHANICALLY STABILIZED EARTH (MSE) WALLS

7.4 GUARDRAIL REPLACEMENTS

ii

7.5 CUT SLOPE DESIGN

7.6 ROCKERY WALL DESIGN, STATION 261+22 TO 262+36

7.7 PAVEMENT DESIGN

7.7.1 DESIGN TRAFFIC VOLUME

7.7.2 SUBGRADE CONDITIONS

7.7.3 STRUCTURAL SECTION

7.7.4 BINDER AND MIX RECOMMENDATIONS

7.7.5 SUBGRADE STABILIZATION

7.7.6 PAVEMENT MAINTENANCE

7.8 CORROSION CONSIDERATIONS

8. NOTES TO CONSTRUCTION

9. MATERIAL SOURCES

9.1 PAVEMENT AND AGGREGATE

9.2 GENERAL BORROW

10. LIMITATIONS

11. REFERENCES

List of Figures

FIGURE 1: PROJECT LOCATION MAP

FIGURE 2: TYPICAL PAVEMENT DISTRESS

FIGURE 3: TYPICAL SLOPE WIDENING

List of Tables

TABLE 1: SUMMARY OF AVERAGE BACKCALCULATED LAYER MODULI IN KSI

TABLE 2: SUMMARY OF EXISTING PAVEMENT THICKNESSES

TABLE 3: SUBGRADE SOIL PROPERTIES TEST SUMMARY 1F

TABLE 4: SUMMARY OF LABORATORY TESTS

TABLE 5: SUMMARY OF SLOPE STABILITY ANALYSES

TABLE 6: SUMMARY OF EMBANKMENT WIDENINGS

TABLE 7: SUMMARY OF ROCK CUT RECOMMENDATIONS

TABLE 8: SUMMARY OF EXISTING PAVEMENT AND SUBGRADE CONDITIONS

TABLE 9: SUMMARY OF PAVEMENT STRUCTURAL SECTION RECOMMENDATIONS

iii

List of Plates

Plates 1.1 to 1.5 – Boring Location Maps

Plate 2 - Regional Geologic Map

List of Appendices

Page

Appendix A – Boring Logs

Boring Record Legend ................................................................................................................... A-1

Legend for Rock Classification ...................................................................................................... A-2

Boring Logs for 17P-01 to 16 ............................................................................................... A-3 to 18

Boring Logs for 18R-01 to 18R-05 ..................................................................................... A-19 to 23

Boring Location and Rock Core Photos ............................................................................. A-24 to 31

Holdrege & Krull (2013) Summary of Coring and Sampling .............................................. A-32 to 61

Appendix B – Results of Laboratory Testing

Summary of Laboratory Results .................................................................................................... B-1

Atterberg Limits ............................................................................................................................ B-2

Percent Passing No. 200 Sieve ...................................................................................................... B-3

Mechanical Sieve Analysis .................................................................................................... B-4 to 6 pH and Resistivity .......................................................................................................................... B-7

Proctor Compaction ...................................................................................................................... B-8

Corrosivity Test Summary (for chloride and sulfate) .................................................................... B-9

R-value Test ....................................................................................................................... B-10 to 16

Uniaxial Compressive Strength Tests ................................................................................ B-17 to 21

Direct Shear Test ......................................................................................................................... B-22

Appendix C – Pavement Deflection Testing

Dynatest FWD Field Testing Plan ........................................................................................... C-1 to 4

Backcalculated Moduli Values and Plots for Segment P-3, CTL Group ............................... C-5 and 6

Plotted FWD Deflections for Segments N-1, N-2, N-4 and N-5........................................... C-7 and 8

Appendix D – Pavement Design Calculations

Pavement Design Calculations ............................................................................................ D-1 and 2

Appendix E – Slope and Wall Calculations

SLIDE Output ........................................................................................................................ E-1 to 24

MSE Wall Calculations ........................................................................................................ E-25 to 32

Rockery Calculations .......................................................................................................... E-33 to 36

1. PURPOSE AND SCOPE OF STUDY

Yeh and Associates was retained by Muller Engineering to provide geotechnical recommendations for the design of roadway improvements and pavement rehabilitation as part of the Federal Highway

Administration, Central Federal Lands Highway Division (FHWA-CFLHD) project for Donner Pass Road in Nevada and Placer Counties, California. The project location begins at the intersection of U.S.

Interstate 80 and Donner Pass Road and extends to the western Truckee Town Limits. The location of the site is shown in Figure 1. Donner Pass Road, previously the historic US Highway 40, provides access to the hamlet of Soda Springs, ski resorts, the Pacific Crest Trail, recreational rock-climbing sites, and other attractions. The existing roadway is typically two lanes paved with asphalt concrete pavement with a posted speed limit of 35 miles per hour. Approximately 3.4 miles of the pavement is underlain by Portland Concrete Cement (PCC) pavement. Left turn channelization and a center turn lane are provided at the intersection with Soda Springs Road, approximately ¾ miles east of Interstate

Highway 80 (I-80).

The scope of our services was subsequently modified to include recommendations for the design of rock cuts for an uphill bicycle climbing lane on the east side of Donner Pass Road, a rockery wall, and for embankment widenings and steepened slopes that will allow for guardrail replacement and reconstruction of the eastbound shoulder of Donner Road in selected areas (Muller 2018).

Figure 1: Project Location Map

The geotechnical evaluation consisted of a program of project coordination, data review, field exploration, laboratory testing, and engineering analyses as a basis for providing the recommendations in this report. This report provides recommendations for rehabilitation of the pavement and geotechnical considerations for the design of the bicycle climbing lanes. Yeh is also preparing plans and specifications for specific aspects of the project. The plans that are being prepared by Yeh and will be submitted for inclusion in the construction documents separate from this report include:

• Sinkhole Patching Detail (1 sheet)

• Rockery Detail, Rockery Wall Layout (2 sheets)

• Rock Reinforcement Details (2 sheets)

• Rock Cut Plan (23 sheets)

• Reinforced Soil Slope Detail, Reinforced Soil Slope Facing Detail (3 sheets)

2. PROJECT UNDERSTANDING

The project consists of the design of the rehabilitation and addition of safety improvements to approximately 6.54 miles of Donner Pass Road pavement (as shown on Plates 1.1 to 1.5). The layout and scope of the improvements are shown on the Muller (2018) plans and described in the Scoping

Report (Muller 2015).

The roadway will remain on essentially the same horizontal and vertical alignment as the existing facility. The design will address safety issues for bicyclists by the addition of 4-foot shoulders to both sides on the west side of the pass where feasible, improvement of access to recreational and historic sites along the alignment, as well the addition of a climbing lane for bicycles on the east side of

Donner Pass.

• Hard rock borders portions of the road adjacent to the existing uphill travel lane. Excavation into this hard rock will be needed on the east side of Donner Pass (Stations 311+00 to 440+90) to accommodate the bicycle climbing lane. Excavation for the climbing lane will involve presplitting, blasting, hydraulic splitters, scaling and stabilization with rock bolts, rock dowels, and shear pins to remove a limited amount of rock from the base of existing rock slopes. Yeh is preparing the plans and specifications for the rock excavation.

• A rockery wall is proposed along the eastbound shoulder of Donner Pass Road (approximately

Stations261+22 to 262+36, right) above the Sugar Bowl Ski Resort parking lot to support the shoulder widening. The rockery will have a maximum height of approximately 8 feet. Yeh is preparing the plans for the rockery.

• The eastbound shoulder of Donner Pass Road will be widened and reconstructed to remove and replace existing non-standard guardrail on the east side of Donner Pass Road

(approximately station 312+00 to 396+25) and allow for the general widening of Donner Pass

Road. Yeh is providing recommendations for widening the existing embankment using conventional grading for slopes of 1v:2h or 1v:1.5h (vertical to horizontal). Yeh is also preparing plans and providing recommendations for near-vertical, geogrid reinforced, mechanically stabilized earth (MSE) walls with shotcrete facing. The MSE walls will be provided along portions of the widening that are above existing rock fill embankment slopes that are steeper than 1v:2h. The existing rock fill embankment slopes range from approximately 20 to 70 feet in height and are constructed over relatively steep mountain sides.

3. SITE CONDITIONS

3.1 GEOLOGIC SETTING

The regional geology in the site vicinity as mapped by Sylvester, Wises, Hastings and Moyer (2012) is shown on Plate 2 - Regional Geologic Map. The project is located within the Sierra Nevada geologic and geomorphic province, which extends from the Mojave Desert in southern California to the

Cascade Mountains in northern California. The project location is on the crest of the Sierra Nevada

Mountains in eastern/central California. The Sierra Nevada Mountains are derived from the Sierra

Nevada Batholith, a collection of granitic plutons that formed during the Triassic and Jurassic Periods.

Approximately 25 million years ago, during the Oligocene Epoch, tectonic uplift caused the Sierra

Nevada Mountains to rise. This uplift continues to the present, creating earthquakes in active fault zones throughout the High Sierra Mountains.

The project site is in the northwestern extent of the tectonically active Tahoe-Truckee graben.

Donner Summit is on the western boundary of the graben, approximately 1 mile west of the Tahoe

Sierra Frontal Fault Zone (see Plate 2). The Donner Summit area is characterized by late Miocene and

Pliocene age lava flows, domes and dikes, and volcaniclastic rocks amid Cretaceous age granitic intrusions. Quaternary age glaciers impacted the region, depositing till and drift throughout the graben. At the project location, west of Donner Summit, the geology along Donner Pass Road includes Quaternary age colluvium and glacial drift overlying bedrock units of Miocene age rhyolitic tuff and Cretaceous age granitic rocks. East of the summit the geology is composed of predominantly

Quaternary age Glacial Till overlying Cretaceous age granitic rocks.

3.2 TERRAIN AND VEGETATION

The project site is in the mountainous terrain of the Sierra Nevada. The existing road surface ranges from approximately elevation (el.) 6,700 feet at the intersection with I-80, to el. 7,100 feet at Donner

Summit, descending to approximately el. 6,000 feet in 2.5 miles at the Truckee Town Limits.

West of Donner Summit, Donner Pass Road is constructed in a side hill cut/fill fashion on the southern flank of Boreal Ridge overlooking Summit Valley and Van Norden Lake and meadow.

Through this section the road climbs gradually to the east through the forested mountainside with gentle curves and tangent sections. The vegetation on the mountainside is mixed pine and fir forest with associated understory vegetation. As the road nears the summit and the geology changes to granitic rock, the vegetation becomes sub-alpine with stunted pines, and low growing plants such as

Huckleberry Oak and Starved Daisy.

East of Donner Summit to the project limits at Truckee, the roadway drops steeply with tight curves cut into the exposed granitic rock. The vegetation below the granites is similar to west of the summit area and the vegetation returns to a mixed forest near the town of Truckee. The existing rock slopes have been cut along the westbound side of the road, and at a through cut near the top of the pass

(~Station 307+00). The existing cuts on the rock slopes are commonly about 20 to 40 feet high, except for a cut exceeding approximately 75 feet in height at the base of prominent rock slope (~Station

393+00).

3.3 CLIMATE

The climate data (www.usclimatedata.com) for Truckee, California (located approximately 8 miles east of the Donner Pass Road area, reports an average annual rainfall of 22 inches and 97 inches of snow. Most of the rainfall and snow is reported to occur between November and March. The reported monthly average temperature ranges from a low of 11 degrees for January to a high of 85 degrees in July.

3.4 EXISTING PAVEMENT SURFACE CONDITION

The exposed pavement surface along Donner Pass Road consists of asphalt concrete. The overall roadway is in poor to fair condition. Photos of typical pavement distress are presented in Figure 2.

The pavement has high severity reflective transverse cracks where the asphalt overlies Portland cement concrete pavement. Longitudinal moderate to high severity reflective cracks were also observed presumably along the edge of the Portland cement concrete, and along the centerline cold joint nearer to Truckee where PCC pavements are not present. This degradation includes low to high severity transverse, longitudinal and reflective cracking with base failures. Moderate to high severity edge cracking and breakage is common in areas where the edge of pavement is bordered by an http://www.usclimatedata.com/ unlined ditch at the base of rock slopes, or where off tracking occurs at the edge of pavement.

Moderate severity longitudinal and block cracking was also observed, particularly in the bends below the summit bridge, and on the grade just above the town of Truckee.

4. GEOTECHNICAL INVESTIGATION

The geotechnical investigation consisted of a program of geologic reconnaissance, drilling and sampling, laboratory testing, deflection testing, and review of previous data available for the site. The locations of the borings, pavement thicknesses encountered, and previous coring locations are shown on Plate 1.1 through 1.5. The logs of the explorations are presented in Appendix A, laboratory test results are presented in Appendix B, and deflection test data are presented in Appendix C.

Memoranda describing the Field Investigation Plan and a summary of the field work performed were submitted by Yeh (2017a, 2017b).

4.1 GEOLOGIC RECONNAISSANCE

Field reconnaissance was performed on September 27, 2017, and as part of the 30 percent field review meeting during the period of July 9 through 11, 2018. The reconnaissance was mainly performed to visually observe the existing rock cuts and to assess the need for scaling to remove loose rock. Yeh geologists also reviewed the cuts relative to existing catchment and the

Figure 2: Typical Pavement Distress

Top left: Sta. 190+10; Top Right: Sta. 240+00 Bottom left: Sta. 345+00; Bottom Right: Sta. 322+00 approximately available width to accommodate the proposed uphill bike climbing lane on the eastern side of Donner Pass. Conditions and observations, particularly relative to the climbing lane, were summarized in the Yeh (2017c) memorandum and in the conclusions and recommendations of this report.

4.2 GROUND SURFACE PROFILES

Ground surface profiles were estimated along selected cross sections to supplement survey data, map the geologic and site conditions on the slopes below the planned guardrail replacements on the east portion of Donner Pass, and to observe the surface conditions along the existing slopes and rock fill embankments. The field notes and estimated ground surface profiles for the 30 field sections that were taken by Yeh on November 18 and 19, 2018 were transmitted to Muller in our memorandum of

November 28, 2018 (Yeh 2018). The sections were taken to estimate the ground surface profile at selected locations, as well as to supplement the survey data for the project in accordance with our scope of services and provide information need for our subsequent slope evaluation of these sites.

The sections were taken using tape and compass, clinometers, range fingers, and by pacing and siting to estimate the approximate ground surface profile along section lines downslope of the existing guardrail.

4.3 DRILLING

The drilling was performed during the periods of October 4 and 5, 2017; December 12, 2018; and

December 20, 2018. Drilling services were provided by Taber Drilling, Inc. of West Sacramento, California and Woodward Drilling of Rio Vista, California. Statewide Safety Inc, of Sacramento, California provided traffic control and flagging for the drilling. The drilling was performed under encroachment permits with Nevada County and Placer County. Well permits were obtained for the

December 2018 borings from Nevada County.

The October 2017 phase of drilling was performed by Taber as part of the pavement evaluation. The

December 2018 drilling was performed as a modification to our scope of services to provide additional subsurface information for the design of the guardrail replacement planned on the east side of Donner Pass, and required multiple cancelations and delays due to poor weather conditions

(snow) as well as a drill rig breakdown. The Woodward drill rig broke down on the first boring on

December 12 and they were unable to complete the drilling.

Pavement Borings. Taber drilled sixteen pavement borings (17P-01 to 17P-16) along the alignment using a trailer-mounted CME 45 drilling rig equipped with 6-inch continuous flight augers. The borings were advanced through the existing asphalt pavement and into the subgrade to a typical depth of 5.5 feet below the pavement surface, unless refusal to drilling on bedrock or boulders limited the depth of the borings. Eight of the 16 borings required coring through PCC pavement encountered directly below the asphalt pavement. Coring the PCC pavement required switching from the auger to a core barrel and running water to cool the coring bit. After drilling through the PCC pavement, the core barrel was removed, and was replaced with the auger and the drilling and sampling continued. A Yeh geologist measured the thicknesses of the existing asphalt pavement and base course materials at the boring locations, logged the subgrade conditions encountered, and secured samples for subsequent laboratory testing.

The pavement borings were sampled using drive samplers and by collecting bulk samples from the auger flights. Drive samples were collected using a modified California or standard penetration test

(SPT) split spoon sampler at typical 5-foot intervals. The SPT sampler has 2-inch outside diameter, 1-

3/8-inch inside diameter and is equipped for but was used without liners. The modified California sampler has a 3-inch outside diameter, 2-3/8-inch inside diameter and was used with 1-inch high brass liners. The drive samples were collected using a 140-pound automatic trip hammer in accordance with ASTM D1586, Standard Penetration Test, procedures. Drive samples were typically taken at depths of approximately 2 and 4 feet below the road surface. The sampling depth, selected lab test results, recorded blow counts (N-values) for the driven samples, and a description of subsurface conditions encountered are presented on the boring logs in Appendix A.

Guardrail Slope Borings. Woodward drilled boring 18R-01 using a Mobile B-57 truck-mounted drill rig equipped with 6-inch hollow stem auger and HQ rock coring equipment. The clutch failed on the rig during drilling and Woodward demobilized from the site on December 12, 2018. Taber completed the remaining borings (18R-02 to 05) in the guardrail areas using a CME C-75 truck-mounted drill rig equipped with 6-inch solid stem auger and HQ rock coring equipment. The borings were not sampled with drive samples due to the presence of hard bedrock and boulders within the rockfill. Coring was used to advance the borings starting at approximately 2 to 3 feet below the existing pavement surface. The drilling was canceled on December 21 due to snow fall and road conditions. The borings were backfilled with bentonite chips and patched with rapid set concrete.

Rock coring was performed in rock encountered 18R-01 to 05 once drive sampling met practical refusal. Coring was performed using an HQ continuous rock coring system with diamond bit core-barrels retrieved on a wire-line system. The HQ core barrel drilled an approximately 3.4-inch diameter hole and collected 2.5-inch diameter core samples. Core was collected in typical 5-foot runs and recovered from the hole on the wire-line system. The percent recovery and the rock quality designation (RQD) were measured and reported on the logs for each run. Samples for laboratory tests were taken from the recovered core. Photographs of the core samples are included in

Appendix A.

4.4 LABORATORY TESTING

The results of laboratory tests are presented in Appendix B. Laboratory tests were performed on selected samples recovered from the field exploration program. Tests for moisture content, unit weight, Atterberg limits, percent passing the 200 sieve, particle size distribution by sieve analysis, pH and resistivity were performed at our laboratory in Grover Beach, California. Tests for soluble sulfates and soluble chlorides were performed by Cooper Testing Labs in Palo Alto, California. R-value tests were performed by the NV5 Labs in Ventura, California. Testing was performed in accordance with

American Society for Testing and Materials (ASTM) and American Association of State Highway and

Transportation Officials (AASHTO) methods as noted on the lab reports. After the completion of the laboratory testing, the field descriptions were confirmed or modified as necessary and boring logs were prepared.

4.5 PREVIOUS STUDIES

Previous coring and deflection testing was performed along the portions of the alignment that are within Nevada County by Nichols (2013a, 2013b). Holdrege & Krull (2013) performed pavement coring as input to the pavement study being performed by Nichols at that time. The Holdrege & Krull

(2017) report for the coring is presented with boring log information collected during this study in

Appendix A. The locations of the cores are shown on Plates 1.1 through 1.5. A summary plot for those deflection data are presented with the deflection data collected for this study in Appendix D.

4.6 DEFLECTION TESTING

Deflection testing was conducted on September 26, 2017 in general accordance with the Dynatest testing plan. The testing plan was submitted with the Yeh (2017a) Field Investigation Plan memorandum and is included in Appendix C of this report. The testing was performed by Dynatest of

Ventura, California. Dynatest used a Model 800E Falling Weight Deflectometer (FWD) to measure the load-response characteristics of the pavement layers and subgrade in accordance with ASTM D4695

“Standard Guide for General Pavement Deflection Measurements.” Tests were performed in the outside wheel path of the eastbound and westbound lanes at a typical spacing of approximately

0.1 mile. The FWD test procedure utilized three load levels acting on a buffered, 12-inch-diameter plate to simulate wheel loading. Geophones and an on-board computer were used to measure the surface deflection of the pavement at various intervals as the FWD strikes the pavement. Nine deflection sensors were set at 6 to 12 inches apart for this project. Sensor No. 1 was located directly under the load and Sensor No. 9 was located 6 feet along the wheel path from the point of the load application. A total of 112 data points were collected. Summary plots and the deflection test data are presented in Appendix C.

The FWD data was backcalculated by CTL Group, working for Yeh, following the procedures outlined in ASTM D5858 “Guide for Calculating In-Situ Equivalent Elastic Moduli of Pavement Materials Using

Elastic Layer Theory.” The layered elastic model was prepared and analyzed using the computer program Flexible Pavement System Design FPS-21. The moduli are an indication of the strength of the pavement layers and subgrade. This information provides an indication of the structural capacity of the pavement structure to carry loading. The Percent Average Error per Sensor (PAES) is also calculated to give an indication of the accuracy and reliability of the backcalculated layer moduli. It is desirable to have PAES less than 2 percent. PAES less than 5 percent is acceptable and PAES greater than 5 percent should be used with caution. The average backcalculated moduli in kips per square inch (ksi) for the pavement layers in the Placer County segment and subgrade (using both directions) are as follows:

Table 1: Summary of Average Backcalculated Layer Moduli in ksi

Segment AC Layer PCC Layer AB Layer Subgrade

P-3 680 248 57 19

There are different performance criteria utilized in pavement design. For flexible pavements, the primary performance criteria are the pavement serviceability (ride quality), fatigue cracking and rutting. The mechanistic model utilized for estimating the performance life due to fatigue cracking was the following Asphalt Institute equation corresponding to 20 percent fatigue cracking.

Nf = 0.0796 (t)-3.291 (E1)-0.854 where:

Nf = Loads to Failure (ESALs) t = Tensile Strain (in/in) at the bottom of the AC layer

E1 = AC Modulus (psi)

The mechanistic model utilized for estimating the performance life due to rutting was the following

Asphalt Institute equation for an 0.5-inch rutting depth.

Nd = 1.365 X 10-9 (v)-4.447 where:

Nd = Loads to Failure (ESALs) v = Vertical Compressive Strain (in/in) at the top of the subgrade

The FPS-21 analysis calculates the critical strains due to a 9,000-pound loading. The estimated remaining life for all existing sections of Donner Pass Road exceeded 1,000,000 ESALs for fatigue cracking and rutting, far more than the required design life based on the current ADT of 1,409 and design 215,000 ESALs. The pavement design for the project can therefore focus on strategies to rehabilitate the existing pavement with emphasis on reducing the high severity of reflective and transverse cracks that impacts the existing roadway.

5. SUBSURFACE CONDITIONS

5.1 EXISTING PAVEMENT STRUCTURAL SECTION

The existing pavement thicknesses encountered in the borings and previous core data are summarized on Plates 1.1 to 1.5 in Table 2. The segments designated in Table 2 correspond to those referenced in the Preliminary Pavement Section Recommendations memorandum (Yeh 2018) and the

Scoping Report (Muller 2015). The pavement structural section encountered in drill holes 17p-

1,3,4,5,6,7,8 and 10 consisted of approximately 4 to 9 inches of asphalt over approximately 6 to 8 inches of PCC. Approximately 2 to 8 inches of aggregate base (AB) coarse was encountered below the

PCC. The remainder of the borings did not encounter PCC. Where PCC was absent, borings encountered approximately 4 to 18 inches of asphalt pavement over either no aggregate base or to a thickness ranging from approximately 4 to 8 inches. The approximate locations of the drill holes for the borings are shown on the Boring Location Plan and noted on the logs in Appendix A.

Table 2: Summary of Existing Pavement Thicknesses

Boring No. Segment Station

Directi on

Asphalt Thickness

(inches)

PCC

Thickness

(inches)

Base Thickness

(inches) Subgrade

AASHTO0F

1 Comment

C-1

N-1

107+80 EB 11.75 0 -- ABC No gap observed below AC

C-2 107+82 WB 12 0 -- ABC

Upper 8" intact; 1.5" below appeared to be base rock over 2.5" of AC

C-3 110+85 EB 13 0 -- ABC

Upper 4" intact; 9" below crumbled during coring

C-4

N-2

111+70 EB 7 7 -- Silty Sand

17P-01 113+02 EB 5 7 0

Silty Sand

A-4 No base

C-5 115+40 WB 3.75 6 -- ABC --

C-6 116+90 WB 5 7.25 -- ABC --

C-7 119+65 EB 7 7 -- ABC --

C-8 137+50 WB 4.5 0 -- ABC --

17P-02 141+23 WB 5 0 13

Silty Sand w/ Gravel

A-1-b 3” AC below base

C-9 154+10 EB 7 7 -- Silty Sand --

C-10 164+15 WB -- -- -- -- --

17P-03 170+75 EB 10 7 6

Silty Sand w/ Gravel

A-2-4

1 AASHTO symbol may be based on manual classification of soil. ABC is noted where pavement cores were terminated on aggregate base course material.

Boring No. Segment Station

Directi on

Asphalt Thickness

(inches)

PCC

Thickness

(inches)

Base Thickness

(inches) Subgrade

AASHTO0F

1 Comment

C-11 181+70 WB 4 7 -- Silty Sand

AC was observed below sandy gravel and was resistant to coring, stopped at 15"

C-12 EB 90+00 4 * 7 aggregate base appeared to be a weak layer of "chip seal" between

17P-04 191+10 WB 9 6 6

Silty Sand w/ Gravel

A-1-b

Base is well graded sand with gravel

(SW)

C-12 194+05 EB 4 7 -- ABC

Appeared to be a weak layer of "chip seal" between the upper AC and the concrete below

C-13 201+95 WB -- -- -- -- --

C-14 202+75 EB 5.5 7 -- ABC

Appeared to be a weak layer (~1" thick) of "chip seal" between the upper AC and the concrete below.

Fabric at 1.5" below surface.

C-15 205+10 EB 5.5 7 -- Silty Sand

Appeared to be a weak layer (~1" thick) of "chip seal" between the upper AC and the concrete below.

C-16 208+30 EB 4.5 7 -- ABC

Appeared to be a weak layer (~1.5" thick) of "chip seal" between the upper AC and the concrete below.

17P-05 215+02 WB 4 8 6

Silty Sand

A-2-4

C-17 220+90 EB -- -- -- -- --

C-18 223+85 WB 3.5 6.5 -- ABC

Appeared to be a vertical cold joint in concrete

17P-06

P-3

238+60 EB 4 6 6

Silty Sand

A-2

17P-07 244+80 WB 4 6 8

Silty Sand

A-4

17P-08 254+30 EB 4 6 8

Silty Sand

A-4

17P-09 263+46 WB 12 -- 6

Silty Sand

A-2 No PCC

17P-10 272+25 EB 4 6 8

Silty Sand

A-4

C-19 282+85 WB 6 9 -- Silty Sand --

C-20

N-4

290+90 WB 12 -- -- ABC

The upper 4" of AC remained intact, the lower 8" crumbled during coring

C-21 292+05 EB 12 -- 2 ABC

Concrete observed below the aggregate at the base (possibly observed bedrock)

C-22 293+15 EB -- -- -- -- No core data reported

Boring No. Segment Station

Directi on

Asphalt Thickness

(inches)

PCC

Thickness

(inches)

Base Thickness

(inches) Subgrade

AASHTO0F

1 Comment

C-23

N-5a

304+30 EB 6.5 6.75 -- ABC

AC broke into 1" layers above the concrete

C-24 304+95 WB 13 -- -- ABC

Appeared to be a weak layer (~1" thick) of "chip seal" between upper AC and lower section of AC

C-25 305+45 EB 9.75 -- -- ABC --

C-26 306+70 WB -- -- -- -- No core data reported

18R-01 312+50 EB 10 -- --

Silty Sand w/ gravel

A-4 No PCC

17P-11 314+60 EB 7 -- 6

Silty Sand w/ Gravel

A-4 No PCC

C-27 316+80 EB 8 -- -- ABC --

C-28 324+80 EB 7.75 -- -- ABC --

C-31 330+75 WB 12 -- -- ABC --

17P-12 331+00 WB 4 -- 12

Silty Gravel w/

Sand A-1-b

No PCC

C-32 338+00 WB -- -- -- -- --

17P-13

N-5b

343+60 EB 13 -- 6

Silty Gravel w/

Sand A-1-b

No PCC

C-33 346+00 EB 5.5 -- -- --

Base has smell of petroleum product

C-29 349+75 WB 9 -- -- ABC

Lower 5" of AC section very weak and separated into multiple layers

C-34 359+10 WB 15 -- -- --

Upper 5" of AC intact;

remainder of AC crumbled during coring operations

C-30 361+75 EB 5 -- -- ABC --

C-35 373+50 EB 10 -- -- ABC --

18R-03 374+33 EB 14 -- 6

Cobbles and boulders

Rock Fill. No PCC. No base.

17P-14 382+30 WB 18 -- --

Well Graded Sand w/ Silt and Gravel

A-2

No base

C-36 385+05 WB -- -- -- -- --

Boring No. Segment Station

Directi on

Asphalt Thickness

(inches)

PCC

Thickness

(inches)

Base Thickness

(inches) Subgrade

AASHTO0F

1 Comment

18R-02 369+00 EB 12 -- --

Cobbles and gravel

Rock Fill. No PCC. No base.

18R-04 387+00 EB 13 -- 7

Cobbles and boulders

Rock Fill. No PCC.

18R-05 395+70 EB 14 -- 6

Cobbles and Silty

Sand w/gravel

No PCC.

17P-15 405+80 EB 19 -- --

Silty Sand w/ Gravel

A-1-a No base

17P-16 438+55 WB 11 -- 9

Well Graded Sand w/ Gravel

A-2

5.2 SUBGRADE CONDITIONS

The subgrade encountered along the alignment reflected the geology on the west and east sides of

Donner Summit (See Plate 2). Table 3 presents a summary of selected laboratory test results performed on subgrade samples that were generally taken within the upper 5 feet of the borings.

Table 3: Subgrade Soil Properties Test Summary 1F

Location Borings

Dry Unit Wt. (pcf) and Moisture

Particle Size

Analyses

(%G, %S, %F)

Atterberg Limits

(LL, PI)

Proctor

(dmax, wopt) R-value

Common USCS and AASHTO

Classifications

West of Summit:

Sta. 102+80 to 295+00

17P-01 through 17P-10 d: 86-117 wo: 7-27%

0-38 G 46-65 S 12-46 F

34-36 LL

3-6 PI

-- 28-73

Silty SAND (SM) Silty SAND w/ gravel (SM)

Clayey SAND (SC) A-1-b, A-2, A-4

East of Summit:

Sta. 295+00 to 415+00

17P-11 through 17P-16 d: 77-126 wo: 6-11%

7-51 G 23-83 S 10-23 F

-- -- 63-72

Silty SAND w/ gravel (SM) Silty GRAVEL w/ sand (GM) Decomposed Granite, RX

A-1-a, A-1-b, A-2

The west section is generally between the I-80 and Sugar Bowl Road (Stations 102+80 to 295+00).

Bedrock and soil exposed existing cut slopes was predominantly glacial drift (Qd) overlying bedrock.

The bedrock is mapped as predominantly granodiorite (hbg). Relatively hard granitic rock is

2 Geotechnical properties are noted for dry unit weight (d) and moisture content (wo); particle size as percent gravel (G), sand size (S) and fines content (F); Atterberg liquid limit (LL) and plasticity index (PI); shear strength (S) in kips per square foot measured by pocket penetrometer (pp), torvane (tv) or unconsolidated undrained (uu) tests; friction angle () or cohesion (c) in kips per square foot measured from direct shear (ds) or consolidated undrained (cu) tests, uniaxial compressive strength of rock (a).

commonly exposed in cut slopes along the westbound lane between approximately Stations 170+00 and 210+00. The subgrade encountered to the west of the summit was predominately loose to dense silty sand with varying amounts of fine gravel, except for clayey sand and lower R-value material encountered in 17P-08 near Station 254+34. The lower R-value material appears to be associated with subgrade and existing fill derived from weathering volcanic tuff that is exposed in the adjacent road cut between about Stations 228+00 and 281+00.

The east section is generally between Sugar Bowl Road and the Truckee Town Line (near Station

445+00). Granite outcrops, through cuts, and relatively tall sidehill cuts in rock are common through this section. The eastbound side of the road is commonly on rockfill embankment. Borings commonly encountered refusal to augering above the target 5-foot depth on bedrock or boulders within the embankment fill. The subgrade soil east of the summit predominantly consisted of silty sand and sand with gravel derived from excavation of the granitic bedrock.

5.3 GUARDRAIL BORINGS

Borings 18R-01 to 18R-05 were drilled along the eastbound lane of Donner Pass Road, east of Donner

Summit where guardrail replacements and reinforced soil slopes are planned. The borings generally encountered rockfill that was underlain by granitic bedrock where the fill was penetrated by the borings. The rock fill appears to be derived from hard granitic rock that was generated from the cut slopes along the uphill side of Donner Pass Road. Additionally, grab samples were taken from the existing fill neat Station 262+00 and from selected rock cuts for strength testing. The results of the testing are summarized in Table 4 below.

The fill was generally composed of cobble to boulder sized rock that was sometimes within a soil matrix composed of silty sand with gravel. The rock fill encountered in borings, and observed while taking measurements for the ground surface profiles, contained boulders 5 to 20 feet in size near the toe of embankment slopes, and voids that ranged up to 15 feet deep and from a few inches to several feet in height. Granitic bedrock was encountered below the fill in each of the borings at depths ranging from approximately 4.5 to 14 feet below the pavement drilled in the eastbound lane of

Donner Pass Road.

Table 4: Summary of Laboratory Tests

Location Borings Dry Unit Wt.

(pcf)

Proctor

(dmax, wopt) Strength USCS Classifications

Sta. 262+00 Grab from fill −− 115.9 10.0%

= 41 c = 0.18 ksf

Silty SAND (SM)

Rock Sample S-1, Sta. 312+20 S-2, Sta. 385+00

S-3, 343+00 d: 162-169 -- 10,000 – 20,000 psi Granite, RX

5.4 GROUNDWATER

Groundwater was not encountered in any of the borings to the maximum depths explored, about 5 feet below the road surface. Water can be present where drainages and culverts cross road. The alignment is within higher elevations that receive snowfall. Wet soil and seeps are likely to be present during periods of snow melt and spring thawing. Soil moisture and groundwater conditions are likely to vary seasonally and in response to local weather and particularly from runoff from the exposed granitic outcrops on the east side of the summit.

6. GEOTECHNICAL ANALYSES AND EVALUATION

The design and recommendations for mechanically stabilized earth (MSE) walls included slope stability analyses considering static and pseudostatic (earthquake) loads. Estimations of the earthquake magnitude and corresponding peak ground acceleration for the design earthquake were used in conjunction with the slope stability analysis to estimate the seismic displacement of a slope.

The methods and analyses are summarized in the following sections.

6.1 SEISMIC DATA

Seismic data were estimated using the Caltrans website ARS Online (v2.3.09, accessed on October 24, 2018). These data were used to estimate a peak ground acceleration and earthquake magnitude for the design earthquake used for the slope stability analysis. A typical shear wave velocity of 760 meters per second was estimated for the site based on review of the site geology. The site coordinates were estimated near the center of the project as latitude 39.3226 and longitude -

120.3141. ARS Online uses a California fault database to estimate peak ground and spectral accelerations for a site with specified coordinates. The deterministic earthquake is estimated using fault parameters in the database to estimate the ground acceleration attenuated from each fault to the site. The probabilistic earthquake uses the fault database to estimate the ground acceleration at the site that would occur from a seismic event having a 5 percent probability of exceedance in 50 years. The design earthquake is the greater of the estimated maximum deterministic and probabilistic events.

The design earthquake for the site is a Magnitude 6.7 event occurring on the Dog Valley fault with an estimated peak ground acceleration of 0.32g. The Dog Valley fault is mapped trending away from the site approximately 3 miles northeast of the easterly project limits.

6.2 SLOPE STABILITY

Slope stability analyses were performed to provide a basis for recommending the minimum reinforcement lengths, strengths and spacing for the design of the geogrid-reinforced mechanically stabilized earth (MSE) walls in this report. 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. SLIDE was used to estimate the reinforcement needed to provide the minimum factors of safety for slope stability. The output from SLIDE is presented in

Appendix E.

Subsurface Conditions. In 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 ground surface profile was estimated from topography and cross sections provided by Muller (2018a), and extension of the ground surface profile along selected cross section by Yeh (2018d). The subsurface profile was estimated mainly using the boring information for borings drilled in the existing roadway, as well as field reconnaissance and mapping of the existing slopes. The profile boundaries and geotechnical properties assigned to the various geologic units is shown on the output in Appendix E.

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) containing structures, and a resistance factor of 0.75 (factor of safety of at least 1.3) for slopes not supporting structures. Vertical reinforced soil slopes were evaluated using a resistance factor of 0.65. Slope stability analyses were also performed considering an equivalent uniform traffic load of 240 pounds per square foot.

Earthquake loads were evaluated using criteria presented in the California Geologic Survey (2008)

Special Publication 117A – Guidelines for Evaluating and Mitigating Seismic Hazards in California. A screening analysis was performed to evaluate whether the slope being analyzed is vulnerable to significant displacement in response to the design earthquake. The screening analysis was performed per SP117A for a tolerable slope displacement of about 2 inches corresponding to a horizontal pseudostatic coefficient of 0.15 for the design earthquake. The seismic displacement of the reinforced soil slope was considered tolerable when the estimated factor of safety from the pseudostatic analysis met or exceeded 1.0.

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. SLIDE was used to check that the estimated factor of safety for the existing slope exceeded 1.0 before evaluating the reinforced soil slope.

Retaining Wall Calculations. The final configuration of the MSE was checked for sliding and overturning considering both static and seismic earth pressures and traffic surcharge forces. The analysis included a check that the seismic displacement of the wall did not exceed 2 inches when considering dynamic earth pressures for the design earthquake. The calculations for the MSE Walls are also presented in Appendix E.

Summary of Analyses. The length, spacing, or strength of the reinforcement was varied until a configuration was estimated that would satisfy the stability criteria. If the reinforcing for the new slope forced the estimated critical failure into the slope below the reinforced portion of the slope, no additional reinforcing or analyses were performed. The estimated factor of safety for the unreinforced portion of the slope always exceeded the factor of safety estimate for the existing slope, and the estimated factor of safety for potential failure surfaces through the reinforced portion of the slope always exceeded 1.5. Recommendations for the design of the geogrid-reinforced MSE walls are presented in the subsequent sections of this report. The plotted outputs for the existing and reinforced slopes considering static and pseudostatic loads are included in Appendix E. Table 5 summarizes the approximate slope geometry for the existing slope condition, the estimated factor of safety for the various slope conditions considered, and the estimated seismic displacement of the slope based on the screening analysis.

Table 5: Summary of Slope Stability Analyses

Station

Existing Slope Geogrid Reinforced MSE

Comment Height (feet)

Slope (degrees)

Factor of Safety (static)

Factor of Safety

(Static)

Seismic Displacement (M6.7, 0.32g)

312+50 42 40 1.27 1.39 <2 inches Critical surface on slope below MSE

369+50 55 37 typ. 1.50 1.55 < 2 inches Critical surface on slope below MSE

374+25 16 28 1.98 1.71 < 2 inches --

382+25 17 45 1.42 1.56 < 2 inches --

386+00 42 45 typ. 1.40 1.61 < 2inches Critical surface on slope below MSE

387+00 52 40 1.39 1.41 < 2 inches Critical surface on slope below MSE

393+25 50 39 1.25 1.32 < 2 inches --

395+75 26 28, 40 1.06 1.60 < 2 inches --

7. CONCLUSIONS AND RECOMMENDATIONS

7.1 EARTHWORK - GENERAL

7.1.1 GENERAL GRADING

The recommendations of this report assume that pavement and fill materials will be placed in general accordance with Standard Specifications FP-14. Compliance with those specifications is needed for the recommendations in this report to remain applicable. 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 full depth reclamation (FDR) material 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 silty sand and hard bedrock. Drive samples had dry unit weights that ranged from approximately 86 to 127 pounds per cubic foot. It is recognized that disturbance to drive samples occurs due to compaction of samples during driving and loosening of soils when recovering the sampler from the drill hole and during trimming in the laboratory.

Differences in the soil density likely occur in areas where the existing pavement subgrade and embankment fill materials were compacted, topsoil and softer soil is present near the existing bridge crossings, or from inherent differences in soil type and consistency.

The estimated earthwork factors 2F

3 do not account for losses for removal of vegetation or unsuitable material. An average earthwork factor of no less than 1 can be used considering the pulverization and compaction of the existing structural section. An average earthwork factor of 0.85 can be used for areas where the subgrade will be compacted outside the limits of the existing structural section.

An average earthwork factor of 0.95 can be used for on-site borrow obtained from road cuts 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 will be vulnerable to erosion. Surface drainage should be maintained similar to existing conditions and without concentrated flows or runoff. 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 diversion drainage should be provided to carry water to the base of slopes and reduce the potential

3 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.

for erosion. Energy dissipation devices should be provided at the outlet of pipes and along ditches to reduce the potential for erosion where there are concentrated flows of runoff.

Erosion can occur where drainage can run uncontrolled along the edge of pavement, such as at the base of cut slopes. Care should be taken that widening of the road does not block or prevent the flow of surface drainage that is provided by existing ditches. The edge of the existing pavement is commonly undercut by erosion. Curbs or lined swales can be provided along the edge of pavement to help reduce the potential for erosion control as shown on the typical detail in Figure 3 in the following section.

7.2 EMBANKMENT DESIGN

Fill will generally be placed along the eastbound shoulder to accommodate the road widening, rebuild the shoulder in areas where existing non-standard guardrail will be replaced, and repair areas where the existing pavement was undercut by erosion (Yeh 2018b) along the downhill side…