2015-03-06_Geotechnical_Report_Final.pdf

CONVICT LAKE ROAD

CA FLAP CR4S07(1)

FINAL GEOTECHNICAL REPORT

Report # CA-FX-4S07-15-01

Geotechnical Section

Technical Services Branch

March, 2015

March 2015

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Table of Contents

SECTION ONE--INTRODUCTION

1.1 SITE CONDITIONS

1.1.1 Segement 1: Station 1+00 to 116+00

1.1.2 Segment 2: Station 116+80 To 114+74.06

1.1.3 Option X: Convict Lake Trail Station 200+00 To 217+01.97

1.2 PROPOSED CONSTRUCTION

1.3 OBJECTIVE & SCOPE

SECTION TWO--GEOLOGY AND SEISMICITY

2.1 GEOLOGY

2.1.1 Regional Geology

2.1.2 Site Geology

2.2 GEOLOGIC HAZARDS

2.3 SEISMICITY

2.4 SEISMIC DESIGN PARAMETERS

SECTION THREE—FIELD AND SUBSURFACE INVESTIGATIONS

3.1 EXISTING TIMBER RETAINING WALLS

3.2 SUBSURFACE EXPLORATION

3.3 SUMMARY OF SUBSURFACE EXPLORATION

3.3.1 Wall Site 1: B-101 – Station 131+20

3.3.2 Wall Site 2: B-102 & B-103- Stations 132+40 & 134+40

3.3.3 Wall Site 3: B-104 & B-105- Stations 139+25 & 141+05

3.4 LABORATORY TESTING

SECTION FOUR--ANALYSIS AND RECOMMENDATIONS

4.1 RETAINING WALL REPLACEMENT

4.1.1 Persumpptive Material Parameters

4.1.2 Factors of Safety Employed in wall Design

4.1.3 Rockery design: Wall Site 1

4.1.4 GRS Wall Design: Wall Site 2 & 3

4.1.5 Rockery Facing: Wall Sites 2 & 3

4.1.6 Slope Stability

4.2 SLOPE SCALING

4.3 ROADWAY ALIGNMENT SHIFTS

4.4 EMBANKMENT RECOMMENDATIONS

4.4.1 Cut Slopes

4.4.2 Fill Slopes

4.4.3 Shoulder Stabilization

4.4.4 Subexcavation

4.5 CORROSION

4.6 SHRINK/ SWELL RECOMMENDATIONS

4.7 REHABILITATION OF CONVICT LAKE TRAIL

4.8 SPECIFICATIONS

4.9 LIMITIATIONS

SECTION FIVE -- REFERENCES

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TABLES

TABLE 1: Fault Data

TABLE 2: Summary of Seismic Parameters Corrected for Site Class C

TABLE 3: Generalized Geometry of Existing Timber Retaining Walls

TABLE 4: Summary of Borings

TABLE 5: Proposed Wall Geometry & Recommended Retaining Methods

TABLE 6: Presumptive Material Parameters for Design of Rockery and Rockery-Faced GRS

Walls

TABLE 7: Factors of Safety for Static and Seismic Design

TABLE 8: Recommended Geometry for Rockery at Wall Site 1

TABLE 9: GRS Wall Design Summary

TABLE 10: Rockery Facing Geometry

TABLE 11: Summary of Rockery Faced GRS Wall Slope Stability

TABLE 12: Estimated Scaling Effort and Quantity

TABLE 13: Estimated Subexcavation Locations and Volume

FIGURES

FIGURE 1: Project Location and Site Map

FIGURE 2: Option X Location and Site Map

FIGURE 3: USGS Geologic Map of the Convict Lake Road

FIGURE 4: Schematic of Rockery and Notation for Recommended Geometry Layout at Wall

Site 1

FIGURE 5: Schematic of Rockery-Faced GRS Wall and Notation for Recommended Geometry

Layout at Wall Sites 2 & 3.

APPENDICES

APPENDIX A: Field exploration

APPENDIX B: Laboratory Test Results

APPENDIX C: Slope Scaling

APPENDIX D: Slope Stability

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SECTION ONE--INTRODUCTION

This report presents the geotechnical engineering study and recommendations pertaining to retaining wall replacement, rockfall, proposed alignment, erodible embankments, proposed cut slopes, subexcavation, soil corrosion potential, and the soils shrink/ swell potential on the

Convict Lake Road, CA FLAP CR 4S07(1) project. This report also contains recommendations for the rehabilitation of Convict Lake Trail.

The Convict Lake Road project is 2.72-miles long and begins just off US-395 South (4.5 miles southeast of the intersection of US-395 South and CA-203 East). The project begins at an approximate elevation of 7100 feet and climbs to a maximum elevation of 7590 feet along the route. The roadway is the primary access route to Convict Lake and Convict Lake Resort in Inyo

National Forest. Figure 1 displays the project location and site map.

FIGURE 1:- Project Location and Site Map

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1.1 SITE CONDITIONS

1.1.1 SEGEMENT 1: STATION 1+00 TO 116+00

From Station 1+00 just southwest of Highway 395 the roadway is relatively flat with rolling terrain ascending gently upward with minimal cuts and fills to approximate Station 40+00. From

Station 40+00 to the end of Segment 1 at Station 116+80, the roadway encounters several cut slopes extending up to 40 feet high with an average slope angle of 35-degrees. The slopes are comprised of unconsolidated glacial and alluvial material and are known to produce rockfall that fill the ditches and occasionally reach the roadway requiring regular maintenance. Within the upper portion of Segment 1, embankment slopes are over-steepened but appear generally stable due to well established vegetation. However, in localized areas primarily near Station 112+00, the roadway drainage is poor and causing erosional rivulets to encroach on the roadway prism.

The subgrade materials within this segment are comprised of large boulders in loose sand and gravel matrix. The roadway width within this segment is between 20 to 22 feet.

1.1.2 SEGMENT 2: STATION 116+80 TO 114+74.06

At Station 116+80 the roadway crosses over Convict Creek by way of a single-span bridge which did not present any obvious visual signs of distress within the abutment or foundations.

Up-station from the bridge the cut slopes are generally stable, less than 15-feet in height with slope angles ranging between 27 and 32 degrees. Parking pullouts on the right of the centerline are supported by a series of three treated timber retaining walls. The walls range in length from

95 to 270 feet in length with maximum heights of 4.7 to 7.7 feet tall. Section Three of this report discusses the existing condition of these walls and provides rationale and recommendations for their replacement. The asphalt surface averages 22 feet in width but up to 37 feet at the pullout locations above the timber retaining walls.

1.1.3 OPTION X: CONVICT LAKE TRAIL STATION 200+00 TO 217+01.97

Included in the proposed construction is the rehabilitation of 1700 feet of the Convict Lake Trail located near the end of the alignment as shown in Figure 2. The trails asphalt pavement varies in width from 5 to 8 feet. The trails width narrows in many locations due to erosion where years of public foot-traffic have created “social trails” deviating from the main trail. These erosional features are up to 4 feet in height, near-vertical and have resulted in up to 5 feet of lost trail width.

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FIGURE 2:- Option X Location and Site Map

1.2 PROPOSED CONSTRUCTION

The Federal Highway Administration (FHWA) Central Federal Lands Highway Division

(CFLHD), in cooperation with the Inyo National Forest, Bureau of Land Management and Mono

County is proposing a roadway 3R construction treatment (restoration, resurfacing and rehabilitation) including the retrofit of a pedestrian walkway to Convict Creek Bridge and minor amounts of roadway widening, grading, drainage improvements and slope scaling. As well as, the replacement of the timber retaining walls and the rehabilitation of Convict Lake Trail adjacent to Convict Lake.

1.3 OBJECTIVE & SCOPE

The objectives of this study were to develop recommendations concerning the retaining wall replacement, slope scaling, roadway alignment shifts, shoulder stabilization, proposed cut slopes, subexcavation, corrosion protection, and shrink/swell identified during the 30 percent field review. Recommendations for the rehabilitation of the Convict Lake Trail are also included in the scope of this study. In accomplishing these objectives, the scope of work included visual observations, field exploration and sampling, correlation of available data, and engineering analysis and recommendations. Specifically, the field exploration included site reconnaissance, subsurface exploration, material sampling and lab testing of representative materials conducted by members of CFLHD geotechnical engineering team. The interpretation of laboratory testing, correlation of available data, and engineering analysis was conducted by geotechnical personnel from CFLHD.

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SECTION TWO--GEOLOGY AND SEISMICITY

2.1 GEOLOGY

2.1.1 REGIONAL GEOLOGY

Convict Lake Road is located on eastern edge of the Sierra Nevada geologic province near the transitional zone with the Basin and Range geologic province to the east. The Sierra Nevada is comprised of a single large westward tilted granitic batholith, predominantly of Jurassic and

Cretaceous age characterized by steep slopes and granitic rock. A batholith is an intrusive igneous rock that is formed by magma cooling slowly at depth. This is the result of a subduction zone along California’s coast wherein the Pacific plate sank under the less dense North American plate, heating the subducted plate at depth and producing magma that ascended through the overlying lithosphere. These granitic rocks are primarily granite, granodiorite, quartz diorite and monzonite of various degrees of weathering.

During the last 10 million years, at least four periods of glacial advance have coated the mountains in a thick mantle of ice. Today, a few small glaciers remain representing the southern-most glaciers in North America. The extensive history of glaciation within the range coupled with the erosion resistant nature of the granitic rocks that make up most of the Sierra Nevada have created a landscape of hanging valleys, towering waterfalls, craggy peaks, alpine lakes and gigantic glacial canyons.

Rain and winter snows combined with the steep character of the landscape create an environment that includes massive movements of sediment and rapid erosion. The mountains are being removed by erosion almost as quickly as they grow. This erosion has created and deposited sediments thousands of feet thick on the floor of the nearby valleys.

2.1.2 SITE GEOLOGY

Convict Lake Road is located on the eastern piedmont of the Sierra Nevada Mountain between

Mammoth Lakes and Bishop and west of the Owens River. The road continues southwesterly into the Sierra Nevada. The soils are comprised mainly of Pleistocene age glacial moraines consisting of cobbles and large boulders of granitic origin in a sand and gravel matrix (Qom, Qym) and Quaternary age alluvial soils consisting of silty sand with varying amounts of gravels, cobbles and boulders (Qf, Qal, Qt). The underlying bedrock likely consists of late Cretaceous age granodiorite and quartz monzonite with varying degrees of weathering ranging from moderately to slightly weathered (Kcp, Krv, Kwc, and Kam) and sedimentary rocks of Ordovician or Silurian age (Sous, SOuh, SOis, SOih, SOls, SOlh, SOmm, and Occ, Ocq). These units are displayed on Figure

3 USGS Geologic Map of the Convict Lake Road.

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Figure 3: USGS Geologic Map of the Convict Lake Road

2.2 GEOLOGIC HAZARDS

Geologic hazards exist from both the natural environment of the project sites and from existing and proposed construction of the roadway. Geologic hazards that exist in the vicinity of the project areas are primarily rockfalls, sloughing of highly erodible granular soils, seismic events and debris flows. Rockfall, in particular, is likely to occur during construction along the route.

Section four of this study discuss the rockfall along the route and rockfall mitigation alternatives in detail.

2.3 SEISMICITY

The Convict Lake Road Project is located in an active seismic region. The closest active faults to the project site are listed in Table 1, Fault Data. The major fault zones within a 50-mile range to the project location are the, Hilton Creek, Long Valley, Round Valley, Hartley Springs, Black

Mountain, Wheeler Crest, Silver Lake, White Mountains Fault, Zone Montgomery, Fish Slough, Owens Valley, Fish Lake Valley, and other Unnamed Faults in Volcanic Tablelands. These faults are strike-slip to normal with slip-rates of 0.2 to 5 millimeters per year. The overall trend of the faults within 50 miles of the project location is Northwest by Southeast with a dip directions varying from East, West to vertical. From the USGS geologic map of the area, the faults that likely cross within the project limits are the Hilton Creek and Long Valley Fault Zones

Table 1 displays the fault data and the Maximum Moment Magnitude of each of these faults.

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Table 1: Fault Data

Fault Name

Distance From the

Center of the

Project (Miles)

Approximate

Station Fault

Crosses

Roadway (2)

Maximum

Moment

Magnitude

(Mmax)

Age

(years)

Slip Rate mm per year

Hilton Creek Fault

Zone (1) (2)

0.3 1+00 to 35+00 6.7

< 150 1-5

3.8 < 15,000

Long Valley Fault Zone (1)

3.8 80+00 to

100+00 Unknown

< 15,000 Unknown

6.8 < 130,000

Round Valley Fault

Zone

7.1 NA 6.8 < 15,000 1-5

Hartley Springs Fault

Zone

9.0 NA 6.6 < 15,000 0.2-1

Black Mountain Fault 13.4 NA 6.4 < 130,000 Unknown

Wheeler Crest Fault 15.6 NA 6.5 < 130,000 Unknown

Silver Lake Fault 16.2

NA Unknown < 130,000

0.2-1

16.6 < 15,000

Unnamed Faults in

Volcanic Tablelands

17.6 NA Unknown < 150 0.2-1

White Mountains Fault

Zone Montgomery

Section

25.8 NA 7.1 < 15,000 0.2-1

Fish Slough Fault 26.6 NA 6.6 < 15,000 0.2-1

Owens Valley Fault

Zone Keough Hot

Springs Section

31.5 NA 7.4 to 7.6 < 15,000 1-5

White Mountains Fault

Zone Central Section

30.2

NA 7.1

< 150 0.2-1

32.2 < 15,000

Fish Lake Valley Fault

Zone Leidy Creek

Section

36.1 NA Unknown < 15,000 1-5

Owens Valley Fault

Zone 1872 Rupture

Section

40.2 NA 7.4 < 150 1-5

White Mountains Fault

Zone Inyo Section

41.7 NA 7.1 < 15,000 0.2-1

Notes:

(1) These fault zones are drawn on Figure 3- Geologic Map of the Convict Lake Road Project

(2) Fault used in Probabilistic Spectrum

2.4 SEISMIC DESIGN PARAMETERS

Recommended seismic response parameters for the Convict Lake Road design are based on the

California Department of Transportation (Caltrans) ARS On-Line, and represents horizontal peak ground acceleration (PGA) with 5 percent probability of exceedance in 50 years

(approximately 975-year return period). The 975-year return period uniform hazard spectrum for the Convict Lake Road Project site, located at 37.5911º N latitude and 118.8521º W longitudinal, was obtained for the probabilistic horizontal acceleration values corresponding to specific peak ground acceleration (PGA) and the spectral coefficients, namely the short- and long- period ground acceleration (Ss and S1 respectively) and corrected for the soil profile at the wall sites.

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Ground motions caused by earthquakes are influenced not only by the distance from the fault planes, but also by the geology and soils found at the site. Subsurface profiles similar to the ones within the project, with soil layers overlaying bedrock that significantly differ in stiffness and density, will have amplified ground motions and a resonant period governed by the layer thickness and shear wave velocity of the materials. The damage potential of strong ground motions with respect to a structure is typically affected by the period of the strong earthquake motion and the resonant period of both the soil and the structure. Based on the subsurface profile at the wall sites, the average time-weighted shear wave velocity for the top 100 feet (VS100) of subsurface materials was estimated between 1,200 and 2,500 feet per second. Therefore, the site is classified as Site Class C according to the soils profile types specified in Table B.1 in

Appendix B of the Caltrans Seismic Design Criteria of June 2006, Version 1.4.

A seismic hazard analysis to establish ground motions for seismic design was conducted. The recommended spectral acceleration coefficient values for probabilistic design with a return period of 975 years were calculated using the On-Line program provided with the Caltrans ARS

On-line Seismic Design Criteria adapted from the Caltrans “Seismic Design Criteria”; version

1.7 dated April 2013 and is summarized in Table 2, Summary of Seismic Parameters Corrected for Site Class C.

Table 2: Summary of Seismic Parameters Corrected for Site Class C

Seismic Parameters

Horizontal Peak Ground Acceleration, (As) 0.485g

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 1.209g

Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.466g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.00

Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.00

Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.20

Based on the long acceleration coefficient SD1 value of 0.466 calculated as Fv.S1, the alignment is assigned to seismic hazard Zone 3 according to Table 3.10.6-1 in AASHTO. Seismic hazard zones reflect the variation in seismic risk in different regions needing different requirements for design as depicted in Table 4.7.4.3.1-1 in AASHTO.

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SECTION THREE—FIELD AND SUBSURFACE INVESTIGATIONS

Field observations and investigation to assess roadway and slope conditions along the route were performed during the 30 percent design site visit conducted on April 23 rd

, 2014. The investigation involved a reconnaissance of the existing timber retaining walls, rockfall-prone slopes, embankments and roadway surface as well as proposed cut and fill slope locations. A subsurface investigation was conducted from May 13 th

-15 th

, 2014. During the subsurface investigation, five borings were advanced within the backfill of the existing retaining walls.

3.1 EXISTING TIMBER RETAINING WALLS

The three existing retaining walls located along the proposed alignment are constructed of 8 x 8-inch treated timber vertical posts (spaced 8-10 feet on center) with lagging. The walls range from

95 to 270 feet in length and are 4.7 to 7.7 feet tall including an estimated one foot of embedment at the wall’s toe. Horizontal metal tiebacks were installed through the vertical timbers at the base and near the top of the wall extending into the wall backfill to an unknown distance. A concrete footing extending to an unknown depth is exposed at Wall 3 and is buried approximately 8-inches from the surface in another location. It is uncertain if this footing exists on Walls 1 and 2. Table 3 depicts the generalized geometry of each wall.

Table 3: Generalized Geometry of Existing Timber Retaining Walls

Wall Site Approximate Roadway Centerline

Stationing Length (ft)

Max Height

(ft)

Average height

(ft)

1 130+68 RT to 131+63 RT 95 4.7 4.6

2 132+21 RT to 134+91 RT 270 6.8 5.0

3 138+81 RT to 141+30 RT 249 7.7 6.4

The foreslope below the walls is heavily vegetated and relatively flat for 2 to 6-feet then steepens to a maximum estimated 1V: 2H slope before reaching Convict Lake below. A parking area and pedestrian railing are located above each wall. No as-built drawings were available that might clarify the standard of work employed during their design and construction.

In general, the walls do not appear to be experiencing stability concerns related to bearing, sliding or overtopping failure. However, in addition to some apparent retrofitting/ replacement of vertical timbers and other elements, the wall elements are showing signs of age specifically:

Several timber elements have rotted primarily at the base of the wall.

Several tie rods are no longer under tension. Due to the unknown extent or connection of these tie rods it is difficult to determine if re-tensioning will provide the original retention capacity.

Trees and tree roots are actively growing out of the face of the wall between the lagging forcing openings in the face where wall backfill has been lost.

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Consolidation of the wall backfill has led to pavement distress and poor surface drainage.

This may be indicative of poor compaction, wall deflection, backfill volume loss (loss of material out the face of wall), gap-graded backfill material (boulders), tree roots displacing the backfill as they grow, or combination of these factors.

Poor surface drainage conditions allow water to pond along the back face of the walls, infiltrate into the wall backfill, and concentrate flow down the wall face leading to erosion at the toe of the wall.

The pedestrian railing is heavily damaged in many areas.

3.2 SUBSURFACE EXPLORATION

The subsurface exploration program for the wall sites was completed May 13 th to the 15 th

, 2014 consisting of five total borings, all within the backfill of the existing retaining walls. The borings were conducted to characterize the subsurface conditions the retaining wall locations.

All boring were advanced to practical auger refusal. Water was not encountered at the time of drilling. Drilling operations were advanced using hollow stem augers conducting Standard

Penetration Tests (SPT’s). All of the borings were logged by a CFLHD engineering geologist on site. Representative soil samples were classified in the field and transported to the CFLHD

Materials Laboratory for testing. Details of the field exploration program and the boring logs are included in Appendix A. A summary of the boring locations and depths are shown in

Table 4, Summary of Borings.

Table 4: Summary of Borings

BORING

WALL

NUMBER

BORING APPROX STATION &

OFFSET (1)

APPROXIMATE

GROUND ELEVATION

(ft)

BORING DEPTH (ft)

B-101 Wall 1 131+20 15.0 ft RT 7619.0 4.0

B-102 Wall 2

132+40 15.0 ft RT 7620.0 2.5

B-103 134+40 12.0 ft RT 7620.5 4.0

B-104 Wall 3

139+25 16.5 ft RT 7610.0 3.5

B-105 141+05 16.0 ft RT 7607.5 2.5

Notes:

(1) See Appendix A for boring location layout.

3.3 SUMMARY OF SUBSURFACE EXPLORATION

Field test results were recorded on the field logs at the time of the geotechnical investigation.

The data sheets and logs contain information concerning the boring methods; samples attempted and recovered; indications of the presence of various materials such as gravel, cobbles and boulders, organic matter, water levels, and loss of water circulation. Also included in the boring logs are laboratory test results and visual interpretations by the field personnel of the conditions based on the performance of the drilling equipment and samples/cuttings brought to the surface.

Specific site descriptions are based on field observations, subsurface characterization information, and may vary in material type and strength with distance away from test boring

CA FLAP CR4S07 (1) CONVICT LAKE ROAD Page 10 locations. This variability is due to topography and land forming process.

Groundwater was not encountered at the time of drilling. Fluctuations in the ground water level due to seasonal precipitation and climatic effects are, however, expected. The following is a detailed description of materials encountered at each structure location.

3.3.1 WALL SITE 1: B-101 – STATION 131+20

Wall #1, located between mainline Station 130+71.84 to 131+66.26 right (Wall Stations

10+00.00 to 11+00.56). Borings B-101 was advanced at station 130+71 to refusal at a depth of 4 feet.

Boring-101 located at Station 131+20 and approximately 15 feet to the right of the proposed centerline and was advanced to refusal using continuous flight augers to a total depth of 4 feet from a starting elevation of 7619.0 feet. The subsurface materials encountered were 0.1 feet of asphalt pavement overtop of a moist, dark brown SILTY SAND with GRAVELS and

COBBLES to a depth of 2.5 feet. From 2.5 feet to 4 feet COBBLES and BOUDLERS were encountered. Very hard drilling conditions were noted during the drilling process. Standard

Penetration Tests recorded in the COBBBLES and BOULDERS were 10 blows for 0 inches

(sample refusal). The SPT blow counts are considered unreliable due to the presence of gravels, cobbles and boulders. Groundwater was not encountered at the time of drilling.

3.3.2 WALL SITE 2: B-102 & B-103- STATIONS 132+40 & 134+40

Wall #2, located between mainline Station 132+24.30 to 134+94.47 right (Wall Stations

20+00.00 to 23+00.17). Borings B-102 and B-103 were advanced at this location to refusal at a depth of 2.5 feet and 4 feet respectively.

Boring-102 located at Station 132+40 and approximately 15 feet to the right of the proposed feet from a starting elevation of 7620.0 feet. The subsurface materials encountered were 0.1 feet of asphalt pavement overtop of a moist, dark brown SILTY SAND with GRAVELS and

COBBLES to a depth of 2.5 feet. Standard Penetration Tests recorded were 10 blows for 0 inches (sample refusal). The SPT blow counts are considered unreliable due to the presence of gravels, cobbles and boulders. Groundwater was not encountered at the time of drilling.

Boring-103 located at Station 134+40 and approximately 12 feet to the right of the proposed feet from a starting elevation of 7620.5 feet. The subsurface materials encountered were 0.1 feet of asphalt pavement overtop of a moist, dark brown SILTY SAND with GRAVELS and

COBBLES to a depth of 2.5 feet. From 2.5 feet to 4 feet COBBLES and BOUDLERS were encountered. Very hard drilling conditions were noted during the drilling process. Standard

Penetration Tests recorded in the COBBBLES and BOULDERS were 10 blows for 0 inches

(sample refusal). The SPT blow counts are considered unreliable due to the presence of gravels, cobbles and boulders. Groundwater was not encountered at the time of drilling.

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3.3.3 WALL SITE 3: B-104 & B-105- STATIONS 139+25 & 141+05

Wall #3, located between mainline Station 138+84.99 to 141+33.53 right (Wall Stations

30+00.00 to 32+98.96). Borings B-104 and B-105 were advanced at this location to refusal at a depth of 3.5 feet and 2.5 feet respectively.

Boring-104 located at Station 139+25 and approximately 16.5 feet to the right of the proposed centerline and was advanced to refusal using continuous flight augers to a total depth of 3.5 feet from a starting elevation of 7610.0 feet. The subsurface materials encountered were 0.1 feet of asphalt pavement overtop of a moist, dark brown SILTY

SAND with GRAVELS and COBBLES to a depth of 2.5 feet. From 2.5 feet to 3.5 feet

COBBLES and BOUDLERS were encountered. Very hard drilling conditions were noted during the drilling process. Standard Penetration Tests recorded in the COBBBLES and

BOULDERS were 10 blows for 0 inches (sample refusal). The SPT blow counts are considered unreliable due to the presence of gravels, cobbles and boulders. Groundwater was not encountered at the time of drilling.

Boring-105 located at Station 141+05 and approximately 16 feet to the right of the proposed centerline and was advanced to refusal using continuous flight augers to a total depth of 2.5 feet from a starting elevation of 7607.5 feet. The subsurface materials encountered were 0.1 feet of asphalt pavement overtop of a moist, dark brown SILTY SAND with GRAVELS and

COBBLES to a depth of 2.5 feet where auger refusal was achieved. Standard Penetration

Tests recorded were 10 blows for 0 inches (sample refusal). The SPT blow counts are considered unreliable due to the presence of gravels, cobbles and boulders. Groundwater was not encountered at the time of drilling.

3.4 LABORATORY TESTING

Laboratory testing was conducted on combined samples obtained from B-101 through B-105 indicated the material is non-plastic and classifies as SM in the USCS as an A-1-b (0) in the

AASHTO system. Electrochemical tests were conducted on the same combined representative sample to determine soil corrosivity and potential detrimental effects on concrete, buried metals and geosynthetics. Testing for resistivity and pH were performed in general accordance with

AASHTO T 288 and T 289, respectively. A discussion of corrosion requirements, electrochemical test results, and recommendations for corrosion restrictions is presented in

Section 4.7, Corrosion Potential. Laboratory test procedures and results, along with grain size curves are presented in Appendix B, Laboratory Test Results.

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SECTION FOUR--ANALYSIS AND RECOMMENDATIONS

This section provides general interpretation of site investigations and gives recommendations for the retaining wall replacement, slope scaling, roadway alignment shifts, embankments, proposed cut slopes, subexcavation, corrosion protection, shrink/swell, rehabilitation of the Convict Lake

Trail and special contract requirements.

4.1 RETAINING WALL REPLACEMENT

Given their current condition, it is recommended that the existing timber retaining walls be replaced with a Rockery at Wall Site 1 and a Rockery-Faced Geosynthetic Reinforced Soil

(GRS) Wall for Wall Sites 2 & 3 as shown in Table 5 below. These wall types were chosen to minimize construction disturbance area and the aesthetic appeal of a Rockery façade.

Table 5: Proposed Wall Geometry & Recommended Retaining Methods

Wall Site Stationing Along Proposed Alignment Length (ft) Max Height

(ft)

Retaining

Method

1 130+72 RT to 131+66 RT 94 5 Standard

Rockery Wall

2 132+24 RT to 134+94 RT 270 10 Rockery-Faced

GRS Wall

3 138+85 RT to 141+34 RT 249 8 Rockery-Faced

GRS Wall

4.1.1 PERSUMPPTIVE MATERIAL PARAMETERS

Using the data obtained from the subsurface investigation, the on-site soils were given presumptive material parameters using Table 5-15 from the FHWA Publication “Soils and

Foundations”. These parameters were employed during the design of the Standard Rockery Wall and Rockery-Faced GRS Walls. The rock for the Standard Rockery and the Rockery Facing are assumed to be imported during construction. The material parameters used in design are presented in Table 6.

Table 6: Presumptive Material Parameters for Design of Rockery and Rockery-Faced GRS Walls

Notes: (1) Cohesion of Rockery rock was artificially inflated to simulate assumed rigidity of rock face.

4.1.2 FACTORS OF SAFETY EMPLOYED IN WALL DESIGN

Material Type Unit Weight, γ pcf.

Friction Angle, (deg)

Cohesion Intercept, c (psf)

Existing Embankment (SM) 132 36 50

Wall Backfill (SM) 132 34 0

Rock for Standard Rockery Wall & Rockery Facing 150 40 500(1)

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The design of the Standard Rockery and the Rockery Faced GRS Walls was checked for adequate factors of safety against sliding, overturning, and bearing capacity. Global slope stability is discussed in Section 4.1.6. The recommended factors of safety for static and seismic design are presented in Table 7 below:

Table 7:- Factors of Safety for Static and Seismic Design

Mode of Failure Static Factor of Safety Seismic Factor of Safety

Sliding 1.5 1.1

Overturning 2.0 1.5

Bearing 3.0 2.0

4.1.3 ROCKERY DESIGN: WALL SITE 1

Rockeries are earth retaining structures constructed of large interlocking, dry-stacked rocks typically placed without mortar or reinforcement. These structures rely on the weight, size, shape, and interface friction of the individual rocks to resist earth pressures and provide overall stability. Rockeries have been recently utilized on many projects as context sensitive solutions preserving scenic, aesthetic, historic and environmental resources.

The Rockery for Wall Site 1 was designed in accordance with the guidelines provided in the

Central Federal Lands publication, “Rockery Design and Construction Guidelines”. The

Rockeries were designed as gravity retaining walls and a surcharge load of 150 pounds per square foot to simulate low volume traffic loading. Static and seismic lateral earth pressures were applied to the back of the rockery; however, passive earth pressure was neglected at the toe of the wall. Table 8 presents the recommended geometry for the Rockery at Wall Site 1 and Figure

4 displays the relevant dimensions.

Table 8: Recommended Geometry for Rockery at Wall Site 1

H

Maximum

Height (ft)

B

Minimum Base

Width (ft)

D

Embedment

Depth at Toe

(ft) ß

Backslope

Angle

Wall Face

Batter

(V:H) φ Back Cut

Inclination

(V:H)

5 3.5 1.5 0 4:1 8:1

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Figure 4: Schematic of Rockery and Notation for Recommended Geometry Layout at Wall Site 1

4.1.4 GRS WALL DESIGN: WALL SITE 2 & 3

GRS consists of tightly-spaced geosynthetic reinforcements that influence the fundamental interaction of the soil particles that promotes confining stresses within the soil creating a soil-geosynthetic “composite”. The reinforcement spacing within a GRS system is limited to 12 inches and usually ranges from 4 to 12 inches to acquire significant benefits of soil-reinforcement interaction. Although reinforced soil-composite performance diminishes with increased geosynthetic reinforcement spacing, other contributing factors including the maximum aggregate size and friction angle are also of importance and should be considered in the design. Tightly-spaced reinforcements not only resist tensile stresses developed within the soil, but have significant impact on the strength and behavior of GRS performance through the following added benefits:

(1) Generate apparent cohesion in a granular fill,

(2) Enhances compaction induced stresses,

(3) Restrains lateral deformation in the soil,

(4) Increases soil ductility and,

(5) Reduces migration of fines.

The AASHTO load resistance factored design (LRFD) guidelines provided in “FHWA

Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide, January

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2011” were implemented using the presumptive material parameters for design of the GRS Wall.

Since the LRFD resistance factors for the GRS-IBS are not fully developed due to lack of statistical information available for this system, the LRFD method has been fit to the allowable stress design (ASD) solution. A unit weight, friction angle and cohesion intercept of 132 pounds per cubic foot, 34 degrees, and 0 pounds per square foot respectively was used as conservative material parameters for the GRS Wall Backfill. Recommended GRS Wall geometry and reinforcement layout is presented in Table 9; Figure 5 displays the relevant dimensions.

Table 9: GRS Wall Design Summary

Notes:

(1) Minimum reinforcement is 8 feet at all GRS Wall locations.

(2) Re-embedment of 3 feet is required after wrapping wall face. Therefore; including the 1-foot spacing between lifts the total required reinforcement length per lift is 12 feet.

4.1.5 ROCKERY FACING: WALL SITES 2 & 3

The Rockery facing of the GRS Walls at Sites 2 & 3 is an aesthetic façade to provide long-term protection of wrapped GRS face. The GRS wall is an internally stable mass with little to no lateral earth pressures anticipated on the Rockery facing. However, the rockery base width should have sufficient geometry to stand alone without collapse. Table 10 presents the recommended rockery facing geometry; Figure 5 displays the relevant dimensions.

Table 10: Rockery Facing Geometry

(1)

Maximum

Height (ft)

B

Minimum Base

Width (ft)

D

Embedment

Depth at Toe (ft) ß

Backslope Angle

(degrees)

Face Batter

(V:H)

Inclination

At Back of Drain

Rock

(V:H)

4 2.5 1.5 0 4:1 8:1

6 3.0 1.5 0 4:1 8:1

8 3.0 1.5 0 4:1 8:1

10 3.5 1.5 0 4:1 8:1

Notes:

(1) Based on GRS Wall Height

(2) Maximum height of Rockery Facing to match GRS Slope height

(3) Inclination at the back of the drain rock is the same as the GRS slope ratio from Table 9.

Wall Geometry Reinforcement Requirements

Applied Traffic

Surcharge Load

(psf)

Wrapped

Wall

Face

Ratio

(V:H)

H

Maximum Wall

Height (ft)

Sv

Maximum

Vertical

Spacing (in)

L (1)

Horizontal

Length (ft)

Re-embedment

Length From

Wrapped Wall

Face (ft)

Tensile

Strength at

2% Strain

(lb/ft)

8:1 10 12 8 3 1370 150

CA FLAP CR4S07 (1) CONVICT LAKE ROAD Page 16

Figure 5: Schematic of Rockery-Faced GRS Wall and Notation for Recommended Geometry

Layout at Wall Sites 2 & 3.

4.1.6 SLOPE STABILITY

Cross sections at the proposed Standard Rockery and Rockery-Faced GRS Wall sites were based on survey obtained at the 30 percent design stage of the project. Cross sections were evaluated where the slope height was at its maximum and/or where the slope in front of the wall site was the steepest. According to Caltrans document “Bridge Design Aids 3-8” (April 2013) minor distortion of the GRS mass is considered acceptable under seismic loading. The document recommends that at least one-third of the site maximum acceleration be applied during the analysis and design of an MSE wall. As an MSE wall is similar in design to a GRS wall; the reduced seismic loading is applicable to the wall design.

Slope stability analyses were performed using Slide 6.0 to verify reinforcement lengths for a minimum static F.S. of 1.3 in static conditions and 1.1 under seismic loading. The Bishop and

Janbu methods of slices were used with the isotropic soil parameters presented in Table 7 as well as 150 pounds per cubic foot rockery unit weight. In the long term static analysis a 250 pounds per square foot surcharge traffic load was included for the roadway in addition to the 150 pounds per square foot surcharge parking load surcharge. The results of the global stability analysis are presented in Table 11. Illustrations of the stability analysis are presented in Appendix D.

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Table 11: Summary of Rockery Faced GRS Wall Slope Stability

Wall Site Retaining

Method

Maximum

Wall

Height (ft)

Reinforcement

Length (ft)

Rockery Base

Rock Width (ft)

Maximum Slope

Angle in Front of

Wall (degrees)

Calculated Factor

Of Safety

Static Seismic

1 Rockery 5 N/A 3.5 14 2.38 1.63

2 & 3

Rockery-

Faced

GRS

Wall

10 8 3.5 27 1.65 1.21

4.2 SLOPE SCALING

Slope scaling is the removal of loose, broken, or partially detached rock and debris from the slope using manual methods, or excavation equipment. Manual, or “hand-scaling”, methods commonly involve 3 man crews consisting of experienced scalers skilled at systematically removing loose, isolated rockfall hazards with a variety of specialized hand tools, manual and pneumatic jacks. Scaling crews typically work off ropes, but may occasionally work from crane baskets or lifts, and always require spotters on the roadway to continually survey the slope for unsafe working conditions. Slope excavation, or “machine-scaling”, methods commonly involve excavators using buckets to pry rock from the slope, hydraulic hammers to bring down larger unstable rock masses, and/or heavy chains to brush large quantities of loose debris from the slope. Machine scaling uses similar crew sizes and clean-up/haul equipment as hand-scaling, and also relies on roadway spotters to ensure operation safety. Hand-scaling and machine-scaling may be done in tandem on raveling slopes where high, localized rockfall exists outside the reach of common excavators (30-40 ft). For high slopes with substantial rockfall hazard, long-reach excavators are available.

Visual field investigations revealed that rockfall events occurring within the constructed (or engineered) limits along the Convict Lake Road corridor originate from unconsolidated slopes.

During original construction of the alignment, these slopes were cut at an over-steepened angle of 34 to 38 degrees.

Four locations have been identified for rockfall scaling treatment. Table 12 presents the estimated crew hours and volume of material the scaling operation will produce within each station range. The table also provides an approximate location where the scaling will be performed on each slope, and the anticipated scaling effort (“Light” to “Heavy”). Appendix C photographically depicts the areas of scaling interest that the scaling operation should address.

The highlighted areas may be adjusted as they are general locations of the proposed scaling operation. All proposed scaling locations should be discussed on-site during the scaling pre-work meeting with a member of the CFLHD Geotechnical team.

It is recommended that scaling be accomplished by means of hand tools only, unless the contractor can successfully demonstrate the use of machinery (long-reach excavators), to the satisfaction of the CFLHD Geotechnical engineer. Incorporating machinery could easily loosen additional rock within the unconsolidated slopes that was not intended to be removed. Equipment

CA FLAP CR4S07 (1) CONVICT LAKE ROAD Page 18 used during the scaling operation and ditch clean-up work should be sufficiently armored for the operator’s safety.

Table 12: Estimated Scaling Effort and Quantity

Station to Station Crew

Hours

Estimated

Scaling Volume

(CUYD)

Anticipated Scaling Effort & Particular Location

On Each Slope (2)

For info only

61+00 - 62+00 8 20 Light scaling at slope brow and intermittent boulders on slope.

64+00 - 68+00 8 20 Light scaling at slope brow and intermittent boulders on slope.

112+50 - 116+00 8 40 Light scaling at slope brow and intermittent boulders on slope.

Notes:

(1): Stations, quantities and measurements presented are approximate and may be adjusted during the pre-scaling meeting and/or during scaling operations after mutual agreements between the contractor and the FHWA.

Key attributes of slope scaling include:

For slopes with moderate to low volumes of loose rock, scaling crews can quickly dress slopes with minimal delays to traffic. For slopes with large rockfall volumes, large localized rock masses to be removed, dangerous rockfall exposures high on the slope, or requiring slope crest excavation, roadway closures may be required.

Scaling cannot be expected to remove the entire loose rock hazard from the slope. Hand-scalers will generally do a better job of removing small, loose rock to ensure safe overhead conditions as work progresses downslope. Machine-scaling operations typically remove larger volumes of larger rock from the slope, but tend to leave small slope debris behind.

“Chaining” or “brushing” by dragging chains over the slope can be effective in removing small rock debris.

Hand scaling should not progress downslope if stable conditions cannot be obtained within the upper reaches of the slope. If unstable conditions cannot be remedied, the scaling operation should be abandoned or machine scaled after discussions with a member of the

CFLHD Geotechnical staff.

Scaling results are proportional to the skill and experience of the scalers. Experienced crews cost more, but typically work faster and produce higher quality results (buying years of slope performance in many cases).

Unsupervised scaling may turn into small mining operations if expected results are not well communicated, resulting in substantial over-excavation and quantity and schedule cost overruns.

Scaling is not a permanent solution to rockfall. Scaling frequency is dependent on many factors, including the erosional/weathering durability of the slope, extent and persistence of structural features producing loose rock, and presence of other effective mitigation measures such as catchment ditches or barriers.

CA FLAP CR4S07 (1) CONVICT LAKE ROAD Page 19

Effective scaling programs require clearly communicated expectations during contracting (pre-bid on-site meetings and special contact specifications), and supervised operations by knowledgeable construction management staff. Scaling measures for each slope identified should be evaluated early in the scaling program for continuity of work products.

4.3 ROADWAY ALIGNMENT SHIFTS

At all rockfall locations catchment ditch widths are limited to non-existent, with frequent rockfall reaching the roadway paved surface. To increase ditch width and thereby rockfall catchment capacity; two measures can be implemented: (1) perform cut slope excavation to move the slope further from the roadway, or (2) realigning the roadway further from the existing cutslope.

Fortunately, at the rockfall locations it is not feasible to shift the roadway away from the cutslope to create the maximum possible inboard ditch catchment area without requiring an outboard retaining structure for additional roadway bench width. It is therefore recommended to shift the proposed alignment left from the rockfall-prone slopes within the station ranges highlighted in

Table 12 to gain inboard ditch width and increase rockfall containment capacity.

4.4 EMBANKMENT RECOMMENDATIONS

4.4.1 CUT SLOPES

No cut slopes were proposed on the project at the time of this report. However; if it becomes necessary to cut an existing slope it is recommended the maximum constructed slope ratios for proposed permanent cut slopes are 1V:2H in soil or unconsolidated material (block in matrix).

The recommended slope ratio is dependent on the nature of the observed material and the existing stability of the slope. During the excavation of cut slopes on the project large boulders will be encountered. Care should be taken to avoid over excavation by removing (or destabilizing) large boulders found to be crossing the proposed finished slope “neat line”.

4.4.2 FILL SLOPES

A slope ratio of 1V:2H is recommended as the maximum slope ratio for all proposed permanent fill slopes.

4.4.3 SHOULDER STABILIZATION

In the shoulder erosion areas from approximate station 115+00 to 116+50 Left, a shoulder stabilization method constructed of mechanically placed riprap (rock inlay) according to Section

252 of the SCR’s and 705-2 of the FP-03 is recommended. The rock should be placed to match the surrounding slopes but should not exceed a 1V:1.5H slope face. The rock should bench into the slope with vertical steps not to exceed 4 feet and minimum bench widths of 4 feet.

4.4.4 SUBEXCAVATION

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Subexcavation may be required in several areas along the roadway. These areas can be identified by wheel rutting and/ or alligator cracking in the pavement surface. A minimum of 2 feet of subexcavation is recommended for the areas denoted in Table 13. This will entail excavation and re-compaction or complete replacement at the discretion on the CO of 2 feet material below the pavements structural section. The bottom of the excavation should be lined with separation geotextile Type IV-E.

Table 13: Estimated Subexcavation Locations and Volume

Station to Station Side Estimated Volume

(CUYD)

62+50 - 63+00 LT 44

96+50 - 96+75 RT 22

109+15 - 109+20 LT 4

117+40 - 117+60 RT 18

118+50 - 119+00 RT 44

Notes: (1) Assumes 12-foot lane width by 2-foot depth for the station range provided.

4.5 CORROSION

The corrosion testing conducted on the soils show electrical resistivity parameters of more than

3000 ohm-cm and a pH between 6 and 9. This indicates minimally corrosive ground conditions for buried steel culverts. The CFLHD Culvert Pipe Selection Criteria rates the corrosion level

(CR) at “CR-1” which does not preclude the use of any type of culvert material.

4.6 SHRINK/ SWELL RECOMMENDATIONS

The proposed Convict Lake Road will require cuts in slopes comprised of sands and gravels with cobbles and boulders. Many of the fill slopes are overlain with a veneer of loose, uncompacted granular side-cast material with cobbles and boulders up to several feet thick.

For the purpose of estimating shrink/swell values, sands and gravels with cobbles and boulders are assumed to be the predominate material located in permanent cuts and excavations for shoulder stabilization below the roadway. The recommended shrink/swell factors are based on a combination of standard tabled values for common materials in the FLH Technical Guidance

Manual (2006) and experience with other CFLHD projects in similar materials.

The estimated shrink/swell values for the sands and gravels with cobbles and boulders of the cut slopes are assumed to Shrink 5% (Shrink Factor 0.95).

4.7 REHABILITATION OF CONVICT LAKE TRAIL

To regain trail width lost due to the surface erosion from social trails it is recommended that 8x8-

CA FLAP CR4S07 (1) CONVICT LAKE ROAD Page 21 inch treated timbers be used to stabilize the embankment. These timbers should be installed and anchored using rebar according to the drawings provided by the USFS and included the contract documents.

4.8 SPECIFICATIONS

Special provisions were developed to be consistent with geotechnical recommendations stated above and should be incorporated into the special contract requirements (SCR). The following

SCR’s were added for this project:

Section 252 – Rockery & Special Rock Embankment

Section 623 – Special Labor Slope Scaling

Section 704 – Backfill Material

4.9 LIMITIATIONS

The recommendations in this report are based on the data obtained from visual and subsurface investigations, field review, and laboratory test results. The results of these explorations and tests represent conditions at the specific locations indicated. Subsurface variations across the site are likely and may not become evident until excavation is performed. The Analysis and

Recommendations sections in this report include interpretations and recommendations developed by the Government in the process of preparing the design. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgment of the

Contractor.

CA FLAP CR4S07 (1) CONVICT LAKE ROAD Page 22

SECTION FIVE -- REFERENCES

American Association of State Highway and Transportation Officials (AASHTO), 2012, LRFD

BRIDGE DESIGN SPECIFICATIONS, 5th Edition.

California Department of Transportation (Caltrans), ARS Online (v2.3.06) http://dap3.dot.ca.gov/ARS_Online/

California Department of Transportation (Caltrans), ARS Online Appendix B.1 http://www.dot.ca.gov/hq/esc/earthquake_engineering/sdc/

California Department of Transportation (Caltrans), 2013, “Bridge Design Aids 3-8”, dated April

2013.

California Department of Transportation (Caltrans), 2013, “Seismic Design Criteria Version

1.7”, dated April 2013.

Federal Highway Administration (FHWA), 2013, “Composite Behavior of Geosynthetic

Reinforced Mass”, Publication FHWA-HRT-10-077, dated July 2013.

Federal Highway Administration (FHWA), 2011, “Geosynthetic Reinforced Soil Integrated

Bridge System Interim Implementation Guide”, Publication FHWA-NHI-05-094, dated

January 2011.

Federal Highway Administration (FHWA) Central Federal Lands Highway Division (CFLHD), 2006, “Rockery Design and Construction Guidelines”, Publication FHWA-CFL/TD-06-

006, Dated November 2006.

Federal Lands Highway (FLH) “Project Development and Design Manual” (PDDM), FHWA, 2008, dated March.

Federal Lands Highways (FLH), 2006, “Technical Guidance Manual (TGM)”, a geotechnical supplement to the FLH Project Development and Design Manual.

RocScience Geotechnical Software, Computer Program, SLIDE with SLIDE INTERPRET

Version 6.033, Copyright 2015

Samtani, Naresh C. and Nowatzki, Edward A., 2006, “Soils and Foundation Reference Manual

Volumes I & II”, Publication FHWA NHI-06-088 and FHWA NHI-06-089, dated

December 2006.

United States Geological Survey (USGS), 1953-1955, “Geologic Map and Sections of the Mount

Morrison and Part of the Mount Abbot Quadrangles, Mono and Fresno Counties, California”, Professional Paper 385 Plate 1, Scale 1:62,500.

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