Geotechnical Investigation Report_LumsdenMP9.17.pdf

GEOTECHNICAL INVESTIGATION

Lumsden Road Repair

Milepost 9.17

Tuolumne County, California

Prepared For:

USDA-FS CSA Pacific Southwest 1323 Club Drive

Vallejo, California 94592

Prepared By:

Divis Consulting, Inc. &

Langan CA, Inc.

135 Main Street, Suite 1500

San Francisco, California 94105

Marcos Pinheiro, PE, GE

Senior Associate - Divis Consulting

Patrick Drumm, PG, CEG

Certified Engineering Geologist – Divis Consulting

Scott A. Walker, PE, GE

Principal - Langan

16 July 2025

750703601

Geotechnical Investigation Report 16 July 2025

Lumsden Road Repair, Milepost 9.17 750703601

Tuolumne County, California Page i

TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 SCOPE OF SERVICES

3.0 FIELD EXPLORATION AND LABORATORY TESTING

3.1 DPTs/Test Pits

3.2 Laboratory Testing

3.3 Seismic Refraction Survey

3.4 Review of Landslide and Seismic Hazards Maps

3.5 Reconnaissance Engineering Geologic Mapping

4.0 REGIONAL AND SITE GEOLOGY

5.0 SITE AND SUBSURFACE CONDITIONS

6.0 GEOLOGIC HAZARDS

6.1 Landslides

6.2 Faults

7.0 DISCUSSION AND CONCLUSIONS

7.1 Slope Stability

7.1.1 Back Analysis of Landslide Failures

7.1.2 Evaluation of Slope Stabilization System

7.1.3 Recommended Repair and Slope Stability Considerations

7.2 Site Grading

8.0 RECOMMENDATIONS

8.1 Site Preparation and Earthwork

8.1.1 Excavations and Cut Slopes

8.2 Reinforced Soil Slope (RSS)

8.3 Site Drainage

8.4 Alternate Stabilization System Soldier Pile Retaining Wall

9.0 ADDITIONAL GEOTECHNICAL SERVICES PRIOR TO AND DURING

CONSTRUCTION

10.0 LIMITATIONS

REFERENCES

FIGURES

APPENDICES

750703601.01 Geotechnical Investigation Report_LumsdenMP9.17

Tuolumne County, California Page ii

LIST OF FIGURES

Figure 1 Site Location Map

Figure 2 Site Plan

Figure 3 Engineering Geologic Map

LIST OF APPENDICES

Appendix A Logs of Test Pit, DPTs, and Seismic Refraction Survey

Appendix B Laboratory Test Data

Appendix C Slope Stability Evaluations

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GEOTECHNICAL INVESTIGATION REPORT

Lumsden Road Repair, Milepost 9.17

Tuolumne County, California

1.0 INTRODUCTION

This report presents the results of the geotechnical and geologic evaluations performed by

Langan CA, Inc. (Langan) and Divis Consulting, Inc. for the Lumsden Road repair at Milepost (MP)

9.17 project. The site is located in the Groveland Ranger District of the Stanislaus National Forest within Tuolumne County on Forest Route 1N10, Lumsden Road, at about coordinates

37°50'4.38"N, 120° 4'53.06"W. The site location is shown on Figure 1.

The nearest town is Groveland, California. Forest Route 1N10 is a crushed aggregate surfaced road that runs parallel to the Tuolumne River. The roadway grade near MP 9.17 is about 380 feet above the typical water level of the river and the area is predominately vegetated with scrub brushes and grasses. It serves as an access road for recreation sites and is used heavily during the summer months. We understand storms and wildfires of increasing severity are causing significant damage, requiring repairs at a greater frequency.

During a heavy atmospheric river storm event, the site at MP 9.17 was subjected to heavy rain.

A combination of the soils (deeply weathered bedrock) being saturated and potential surface water flow combined to an ultimate slope failure on the cut slope above of the road. This failure resulted in soil and rock debris flowing onto the road and then caused a fill slope failure below the road. The resulting slope conditions following the sliding event include a combination of soil debris on top of the roadway, as well as a loss in roadway section/width where material on outboard edge of the roadway slid downslope.

The USDA Forest Service contracted Divis Consulting Inc. (Divis) to evaluate and geologically map the road failure site, identify repair alternatives, and design the repair of the road through contract number 140F0822D0143/123970225F0002. The Divis team includes resources from

DJ&A, P.C. and Langan CA, Inc. to support the analysis and design of the project.

2.0 SCOPE OF SERVICES

Our scope of services were outlined in the Architect/Engineer (A/E) Statement of Work document titled Lumsden Road Repair Design, prepared by the Pacific Southwest Region of the USDA

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Forest Service. Our scope included reviewing available subsurface information, review of the

Detailed Damage Inspection Reports (DDIR), performing a site geologic reconnaissance, preforming a geotechnical investigation, and performing laboratory testing to further evaluate the subsurface conditions at the site, performing engineering studies including slope stability analysis, and developing conclusions and recommendations regarding:

• soil, bedrock, and groundwater conditions at the site

• site seismicity

• geologic hazards, including evaluation of the stability of existing and proposed slopes under static and seismic loading conditions

• the alignment and configuration of a slope stabilization system

• design criteria for the proposed slope stabilization system, including lateral earth pressures under static and seismic conditions

• temporary construction conditions and sequencing

• site preparation

• earthwork, including site grading and criteria for fill quality, placement, and compaction

• erosion control measures.

3.0 FIELD EXPLORATION AND LABORATORY TESTING

To evaluate subsurface conditions, the team performed three dynamic penetrometer tests

(DPTs) and excavated and logged one representative test pit. The approximate locations of the

DPTs and test pit are shown on Figure 2. We submitted soil samples for laboratory testing and completed engineering geologic mapping, as described in the remainder of this section. In addition, our team retained a geophysical testing consultant to perform a seismic refraction evaluation to evaluate the depth to and character of the subsurface materials.

3.1 DPTs/Test Pits

One test pit, designated as TP-1, was excavated by Down to Earth Construction from Groveland, California on 21 January 2025 at the location shown on Figure 2. The test pit dimensions were

10 feet long, 2 feet wide, and 6 feet deep. Our geotechnical engineer logged the test pit to document the soil types exposed, which is described in Section 5.0. Plan views and sections of the test pits are presented in Appendix A, Figure A-1, and soil was logged in accordance with the classification system shown on Figure A-2. Representative samples of the material encountered at the site were collected for laboratory testing. Samples taken for laboratory strength testing

Tuolumne County, California Page 3 and dry density were collected by pushing/driving in stainless steel liners to limit the disturbance as much as possible.

We performed three DPTs at the locations shown on Figure 2. The DPTs were used to quantitatively evaluate the relative density of the soil adjacent to and below the existing footings.

The penetrometer consists of a 1.4-inch cone attached to 1.1-inch-diameter rods that are driven with a 35-pound hammer falling 15 inches. The number of blows required to advance the probe a vertical distance of 10 centimeters was recorded as blow counts. The blows used to drive the cone were converted to standard penetration test (SPT) N-values and used in our engineering analyses.

Each DPT was advanced to depths ranging from approximately 4.7 to 14.8 feet bgs. The DPT results are presented in Appendix A.

3.2 Laboratory Testing

The soil and rock samples from the field exploration program were re-examined in the office by

Divis to confirm classifications, and representative samples of soil were selected for laboratory testing. Samples were tested to measure moisture content, max dry density, gradation, Atterberg limits, and shear strength. The samples collected in stainless steel liners for strength testing were extruded and tested using the standard direct shear method per ASTM D3080 at increasing normal (confining) load to determine their strength properties over a range of stress states. Results of the laboratory tests are included on the boring logs and in Appendix B.

3.3 Seismic Refraction Survey

A seismic refraction survey was performed at the site by Terracon Geophysical Services (Line

SR-2). The length of the survey was approximately 125 feet and located along the alignment of the buried roadway segment affected by the landslide. The seismic refraction survey imaged the subsurface conditions for a maximum depth of approximately 25 feet below the existing ground surface. The survey suggests that the subsurface materials within the affected roadway section are relatively uniform in at least the upper 20 feet with a compression wave (p-wave) velocity of

2,000 feet per second. This velocity if consistent with a dense to very dense soil. The seismic velocity increases with depth under the affected roadway and also to the southeast of the affected roadway segment, indicating stronger or less weathered materials at these depths/locations. . The results of the seismic refraction survey are also presented in Appendix A.

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3.4 Review of Landslide and Seismic Hazards Maps

According to published geologic maps, there are no landslides identified within or adjacent to the site (Wills and others, 2022). The site and surrounding areas have not yet been included as part of the ongoing liquefaction and earthquake-induced landslide mapping by the State of California

Seismic Hazards Mapping Act of 1990. Therefore, no Seismic Hazard Zone Maps for Earthquake-

Induced Landslides are available for the site.

The site in not within or adjacent to a State of California Earthquake Fault Zone as designated for

Holocene-active faults (last 11,700 years). The closest major faults to the site include the northwest-trending Calaveras Shoo Fly Thrust and the northwest-trending Sonora Fault located approximately 1.7 miles to the northeast and approximately 4.3 miles to the southwest. The

Sonora Fault is part of the Foothills Fault System that includes a series of subparallel northwest-trending fault that define the western base of the Sierra Nevada Mountains (Hart and Bryant, 1997; Jennings, and Bryant, 2010; Wagner and others, 1991; and Wills and others, 2022).

3.5 Reconnaissance Engineering Geologic Mapping

The limits of the slope failure above the roadway are pronounced by a relatively steep upper scarp and lateral margins generally measuring between 3 and 5 feet high (see Figure 2). The upper portion of the landslide is approximately 40 feet wide while the lower portion of the landslide near the roadway is approximately 100 feet wide. There are pockets of displaced materials within the failure area along with several erosional channels exposing intact bedrock. The slope failure is predominately a debris flow type of landslide. The bulk of the landslide materials have mounded up along roadway alignment burying the old road surface.

Some of the landslide debris has continued over the downslope edge of the roadway and into a broad swale. The is evidence of past slope failures along the downslope side of the same roadway section. We observed a lateral margin of a previous slope failure below the affected roadway during our geologic reconnaissance.

There was no prominent geologic structure identified within the bedrock exposed with the landslide scar or within the adjacent road cuts. We mapped several smaller failures within the road cuts along the upslope side of the roadway to the east of the larger landslide. These smaller failures appear to have occurred within the same bedrock.

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Listed below is a brief description of the geologic features and units encountered during geologic reconnaissance.

Qdf: Denotes a debris flow scar above the roadway with scattered pockets of displaced materials and erosional channels dissecting intact bedrock.

Mzg: Predominately gray granodiorite in a deeply weathered and friable condition (P-Wave

Velocity 2,000 feet/second), referred to as either saprolite or decomposed granite. There are scattered moderately weathered and moderately hard core blocks within the friable materials exposed in road cuts along the east side of the landslide (P-Wave Velocity

3,000 feet/second).

4.0 REGIONAL AND SITE GEOLOGY

The site is within the Sierra Nevada geomorphic province of eastern California, a region dominated by multiple metamorphosed exotic terrains of Mesozoic and Paleozoic age accreted to the west coast of North America hundreds of millions of years ago by plate tectonics. There are ancient continuous faults that cross the region, especially along the western foothills, which are not considered to have been active in the Quaternary Period or within the last 1.6 million years. These rocks have since been intruded by younger granitic plutons that characterize the central portion and east flank of the mountain range. The area has undergone significant tectonic uplift and a variety of subsequent mass wasting processes creating the present day denuded and rugged landscape (Bateman and Wahrhaftig, 1966; Clark, 1964: Jennings and Bryant, 2010;

Norris and Webb, 1976; Turner and Ransome, 1897; Wagner and others, 1991; and Wills and others, 2022).

The site is along the southern slope of the west-draining Tuolumne River valley located north of

State Highway 120. The mountains within the site area are predominately supported by metasedimentary rocks assigned to the Calaveras Complex of Mesozoic or Paleozoic age. The

Calaveras Complex rocks represent deep ocean sediments that were originally deposited to the west. These rocks have been intruded by felsic (mainly granodiorite composition) sills and dikes and related plutonic rocks throughout the region. There have been no faults mapped through or adjacent to the site. No significant surficial deposits and no landslides have been previously mapped within the site area (Wills and others, 2022).

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5.0 SITE AND SUBSURFACE CONDITIONS

The failure of the steep cut slope above the roadway reveals granitic rocks that are in a deeply weathered and friable condition (saprolite or decomposed granite). Harder core rocks are exposed within portions of the deeply weathered rock mass. Although not exposed in any of the nearby roadcuts, rocky debris distinctly from the metasedimentary rocks caps the top of the slope failure.

Past failures have occurred along this segment of the road as is evident by relatively old ground scars downslope of the road (see Figure 2). These past failures were likely repaired/mitigated by the placement of additional artificial fill along the downslope side of the roadway.

One test pit was excavated through the slide mass at an orientation roughly parallel to the roadway alignment. The test pit dimensions were 10 feet long, 2 feet wide, and 6 feet deep.

Materials exposed within the test pit include:

• 0 to 4.7 feet deep – Slope debris: A combination of displaced clayey sand, silty sand with gravel, and clayey gravel with sand. The materials are moist to wet, weak and somewhat intermixed with a chaotic appearance. Pockets of wood debris and roots are pervasive, wood is relatively fresh (i.e. not significantly oxidized).

• 4.7 to 5.1 feet deep – Buried roadway section: A layer of gray silty sand with gravel is present below the landslide debris. The materials are wet and relatively compact. Some roots are present and heavily oxidized.

• 5.1 to 6 feet deep - Saprolite: A yellow-orange with gray-black modeling soil was encountered beneath the buried roadway section. The materials consisted of deeply weathered granitic rocks, mainly granodiorite, that had been altered to a clayey gravel with sand consistency. Based on visual observations of the nearby road cuts and slopes, we conclude this material is in-place, but has weathered to a soil-like consistency

(saprolite). Based on the dynamic cone penetration test results conducted adjacent to the test pit, this material becomes more competent (increased in relative density) with depth suggesting less weathered granodiorite below the roadway.

These conditions are shown on the Test Pit Log, see Figure A-1.

During our field investigation, we observed limited to no seepage in the sidewalls of the test pit, and no ponded or perched water at the site. However, water can travel in fractures in the rock

Tuolumne County, California Page 7 and could be perched on top of the bedrock or fine grained soil layers. These are not real groundwater levels, but water should be expected at the site during periods of persistent rainfall.

6.0 GEOLOGIC HAZARDS

6.1 Landslides

According to published geologic maps, there are no landslides identified within or adjacent to the site (Wills and others, 2022). Observations from our geologic reconnaissance suggests that there have been previous failures at least along this segment of the roadway. We did not observe any evidence that the previous failure had been property mitigated with an engineered solution. The deeply weathered granodiorite is prone to erosion and debris flow landslides.

6.2 Faults

The site in not within an area of known high seismicity. The closest major fault to the site is the northwest-trending Calaveras Shoo Fly Thrust located approximately 1.7 miles to the northeast, which has low seismicity and classified as a Pre-Quaternary fault (older than 1.6 million years).

The northwest-trending Sonora Fault is located approximately 4.3 miles to the southwest. The

Sonora Fault is part of the Foothills Fault System that includes a series of subparallel northwest-trending fault that define the western base of the Sierra Nevada Mountains. Most of the faults within the Foothills Fault System are considered to be Pre-Quaternary faults. However, some of the faults within the system have been shown to have Late Quaternary displacement during the past 700,000 years (Hart and Bryant, 1997; Jennings, and Bryant, 2010; Wagner and others, 1991; and Wills and others, 2022).

7.0 DISCUSSION AND CONCLUSIONS

We conclude the slopes should be stabilized, drained, and the roadway regraded.

On the basis of the results of our investigations and geotechnical evaluations, we conclude the most appropriate stabilization measures would include the installation of a geo-structural system consisting of reinforced soil slope (or mechanically stabilized earth (MSE) slope). Factors we considered during our studies included adequate stability of the slopes (factors of safety) during static and earthquake conditions, impacts of groundwater, and soil and rock physical properties.

These and other geotechnical issues are discussed in the remainder of this section.

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7.1 Slope Stability

To evaluate the slope stability onsite and develop recommendations for appropriate road repair and slope stabilization measures, we performed slope stability analyses using the computer program Slope/W (GeoStudio 2024). Slope/W is a fully-integrated slope stability analysis program.

We used Morgenstern-Price’s Method to search for the most critical surfaces and to calculate the factor of safety of the slopes against failure. The factor of safety against slope failure is the ratio of the resistance to sliding to the driving forces. The higher the factor of safety, the more resistance the slope has to failure. Representative output files from our slope stability analysis are provided in Appendix C.

7.1.1 Back Analysis of Landslide Failures

The engineering properties of the landslide debris, roadway fill, and saprolite/DG materials used in our slope stability models were developed based on the results of our field exploration, laboratory testing results, and a back analysis performed in Slope/W. The back analysis was performed on a section through Station 11+10 which is a relatively steep cross section immediately adjacent to the failure surface. This section is shown on Figures C-1 and C-2. The results of this analysis were used to calculate the strength in the deposits which would produce a factor of safety near 1.0 for a failure plane similar to the geometry observed on site.

The use of back analyses was critical for this section, as the heavy gravel content in the slope debris and historic roadway fill precluded laboratory shear strength testing on these materials. In addition, the samples of saprolite were obtained using a driving tool, which may disrupt materials with limited cohesion. The soil layering was based on the results of our field exploration program and the seismic velocity profiles. Groundwater was assumed at about one foot below the ground surface. However, due to the low permeability of the in-place saprolite, the effects of groundwater were only attributed to the soils in the upper 3 to 4 feet below the ground surface.

The results of our back-analysis of the landslide failure indicate the material strengths that would produce failure planes similar to those observed onsite at the site, with factors of safety just below 1.0 are presented in Table 1. The static factor of safety in the back-analysis upslope condition is slightly below 1.0 (Figure C-02); however, we note that no upslope failure has been observed at this actual location. Using the same soil strengths, the lower native slopes have factors of safety slightly over 1.0. In addition, the back-calculated internal friction for the intact saprolite (DG) was similar to the results of direct shear laboratory testing at nearby sites

(Lumsden Road MP 10.21, but slightly lower than the direct shear laboratory test results from

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DG taken from a sample at MP 9.17), therefore, we note the modeled frictional strength is representatively conservative and appropriate for modeling the DG as a single unit.

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TABLE 1

Soil Properties Used in Slope Stability Analyses

Material

Description

Total

Unit Weight

(pcf)

Cohesion, c’

(psf)

Internal Friction, ’ (degrees)

Fill (saturated/unsaturated) 120 100 34

Slope Debris

(saturated/unsaturated

110 100 32

Decomposed Granite (DG, saturated/unsaturated)

120 100 35.5

Reinforced Fill* 125 100 35.5

Intact Bedrock – Modeled as impenetrable in this analysis

Notes: *the engineering parameters for Reinforced Fill is based on estimated properties following compaction as recommended in Section 8.2.

7.1.2 Evaluation of Slope Stabilization System

We then evaluated the cross section for the road repair and proposed slope stabilization system consisting of a reinforced soil slope (RSS) below the road at Station 11+40, through the critical section along the landslide. Prior to modeling the repair, we checked the factors of safety of the current slope condition for both static and seismic conditions using the properties from the back analysis. The results are presented in Table 2 and show the slope in its current condition is marginally stable under static conditions. However, this does not include the weight of the repaired roadway section or the vehicular surcharge.

We also performed a stability evaluation under seismic shaking conditions using the pseudo-static approach, where a horizontal acceleration is applied to the static slope stability model. For this analysis, we used a horizontal seismic coefficient (Kh) of 0.11g, which corresponds to ½ of the PGAM mapped value consistent with the guidelines suggested by Hynes-Griffin, et al. (1984) for seismic response of slopes. Site coefficients and spectral response to acceleration parameters were determine for the site using USGS/NEHRP tools based on a seismic Site Class

C (very-dense soil or soft rock) per ASCE 7-22.

We then installed reinforcing and engineered fill of various configurations within the computer program to develop a slope stabilization design to adequately stabilize the slide and allow for the reconstruction of the roadway. Ultimately, we selected an RSS section consisting of a ¾:1

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(horizontal to vertical) for the outside face and back-cut slope. The resulting reinforced fill is approximately 16 feet high at Station 11+40 and the minimum reinforcing/grid lengths are 12 feet long. Wire welded reinforcement mat was added to the model in rows at a vertical spacing of 2 feet within the RSS.

The internal stability of the RSS was checked per guidelines in FHWA-NHI-00-043. The welded wire mats were assumed to have a minimum yield strength of 65 kips per square inch and minimum ultimate tensile strength of 2,800 pounds per foot (lb/ft). The source material for the engineered fill between the reinforcing was assumed to be from onsite soil (saprolite), which will be compacted adequately to achieve the design friction angle of 35.5 degrees (based on the results of the undisturbed direct shear laboratory data (Appendix B). The internal stability check was to confirm that welded wire mat was strong enough and a length of 12 feet was long enough to resist the pullout forces in the reinforced slope.

The global stability was then checked using SlopeW and the same soil properties and groundwater level used in the back-analysis was also used in this section, except the RSS was assumed to be backdrained with the piezometric line following the back of the RSS.

The proposed road was modeled with a vertical, uniform surcharge of 250 pounds per square foot to model the loading on the roadway surface. The global stability of the repair and RSS was performed both under static and seismic conditions. The seismic condition checked the stability using a horizontal acceleration of 0.11g, as discussed above. The results are presented in Table

2 and sections are shown on Figures C-3 through C-13. We note that though the factor of safety does not change for the upper slopes after RSS repair, some smoothing/rounding of the upper slope was also considered.

TABLE 2

Results of Slope Stability at Repair Section

Section 11+40 Slope

Stability Run

Static Factor of Safety

Shallow Raveling Type

Failure

Static Factor of

Safety

Deeper/Global

Surfaces Type

Failure

Seismic Factor of Safety

(kh = 0.11g)

Current Condition -

Upper Slopes 1.0 1.2 0.9

Current Condition –

Slope Below Roadway 1.3 1.3 1.1

RSS Repair - Upper

Slopes 1.0 1.2 0.9 (above roadway)

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RSS Repair - Lower

Repaired Section NA 1.4 1.1

We note that the slopes above the road are over steepened and may be prone to raveling type instability, especially during periods of heavy rainfall. However, the stabilized lower slope should generally be resistant to erosion or displacement from soil deposition on the roadway surface;

the stability modeling includes an areal load of 250 psf surcharge representing vehicular loading.

If the upper shallow slide impacts the road in the future then the vehicular loading will be replaced as a soil load until such time that the roadway is cleared.

7.1.3 Recommended Repair and Slope Stability Considerations

In developing the targeted repair alternative, a selective sensitivity evaluation was performed. It included varying the wight of the existing soil materials as well as changes in the internal friction angle of up to 2.5 degrees. The changes in the soil weight/density had negligible impact on the results or the options for repair. Conversely, changes in the internal friction angle of the

DG/saprolite materials resulted in corresponding changes in the factors of safety, but then resulted in potentially unrealistic conditions (i.e. factors of safety significantly below 1.0 or much higher than 1.0 for slopes on slopes that appear to be moderately stable from on our reconnaissance). We therefore concluded the values for internal friction in the DG/Saprolite were appropriate as initially modeled. We also evaluated if adding stronger (i.e. higher internal friction angle import) material would benefit the RSS design. Ultimately, the critical slope stability surfaces do not engage with the RSS grid system except at the base of the RSS, and improving the internal friction angle had negligible impact on the factors of safety of the repaired road section.

Lastly, as part of our evaluation we extended the RSS grid length in order to obtain a static factor of safety greater than the target value of 1.3. To obtain a Factor of Safety significantly higher than

1.3, the new RSS grid lengths would need to be significantly longer than 12 feet , which would begin to require cutting into the upper slope on the opposite side of the roadway. This approach does not seem constructable or an appropriate design alternative. As a reminder, the slopes below Lumsden Roadway and further down the slope have a factor of safety of about 1.2.

In conclusion, based on our analysis, the repair option presented allows for the reconstruction of the roadway, while maintaining a reasonable factor of safety through the roadway section. In addition, the seismic factor of safety is significantly improved. The selected RSS approach, with a reinforced length of 12 feet at the critical section, appears to meet the design intent: stabilize

Tuolumne County, California Page 13 and repair the existing roadway section to reestablish the serviceability of the road, and is the most appropriate given considerations for cost, suitability, and resiliency.

However, the upper slope section remains mostly unchanged, as there is only minor regrading of this slope planned to remove loose or over steepened debris. This upper slope is therefore still marginally stable for shallow raveling type failures, or may subject to erosion, above the repaired roadway section. This condition is similar to the surrounding areas along Lumsden Road.

7.2 Site Grading

Because of the uneven topography at the site, some areas will require fill placement while other areas will require cuts to reach final grades and repaired slope configurations in conformance with the final grading plan included in the project civil drawings. In addition to the final grades, temporary cuts and fills will be needed during construction to create level working platforms for site preparation and installation of the stabilization system. Recommendations related to site preparation and temporary construction conditions are discussed in Section 8. Considering the soil types present, we anticipate that grading at the site can be achieved using conventional earth moving equipment.

Cuts will be required on existing slopes for site preparation to create level, safe working pads on the slopes. In addition, excavations will be required to install the RSS and the drainage. We anticipate the cuts will expose decomposed granite and more intact rock. Cuts should be graded in accordance with our recommendations in Section 8.1.

8.0 RECOMMENDATIONS

Our recommendations for site preparation, earthwork, slope stabilization, drainage, and other geotechnical aspects of the project are presented in the following sections of this report.

8.1 Site Preparation and Earthwork

The soil encountered at the site will likely consist of clayey sand (AASHTO M 145 Soil Class A-2-

6), silty sand with gravel (A-2-4), and clayey gravel with sand (A-1-b), which is susceptible to disturbance and strength loss under construction equipment loads. Care should be taken to not disturb the on-site site soil/rock except where it is required for removal for the slope repair. The onsite materials generated during grading can generally be used as general engineered fill and engineered fill for the RSS, with the exception of the buried wood and organic materials.

Recommendations for fill placement, fill slopes, excavations and cuts slopes are below.

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All fill, (consisting of on-site clayey sand, silty sand with gravel, and clayey gravel with sand, or other granular import fill) should be moisture-conditioned to above optimum moisture content, placed in horizontal lifts not exceeding eight inches in loose thickness, and compacted to at least

95 percent relative compaction.1 Field testing of compacted fill should be completed in accordance with the field operating procedure in AASHTO T310.

Any imported fill brought to the site should be: granular in nature (sand or gravel), free of organic matter, contain no rocks or lumps larger than three inches in greatest dimension; have a liquid limit less than 40 and plasticity index less than 10, have low corrosion potential2, and be approved by the geotechnical engineer of record (GER). Imported fill must meet the requirement for

AASHTO M 145 Soil Class A-1, A-2-4, and A-2-5 or AASHTO M43 No. 57 or No. 67 aggregate.

The GER should approve all sources of import fill, if used, at least three days before use at the site.

Fills used as backfill to reach final grades should be keyed and benched into the slopes as detailed in Section 8.2 All fill should be placed on benches in horizontal lifts; effective compaction cannot be achieved on sloped surfaces. In general, the fill placed at the project will be reinforced, and constructed as recommended in Section 8.2. However, areas of localized fill may also be required. Typical fill slopes should be no steeper than 1.5:1 (horizontal to vertical).

To limit the concentration of surface water on slopes or within the RSS, areas upslope of the fill slope should be graded to drain away from new slopes where possible. Rock fills, if using FHWA

Class 2 or Class 3 rock (per Section 705), can be sloped as steep as 1:1, provided the rock fill makes up the entire fill section being placed. Rock fill should be separated from the on-site materials using filter fabric (Mirafi 140NC or equivalent).

The finished face of new cut and soil fill slopes should be planted with vegetation and covered by an erosion control blanket or netting to reduce the potential for surface erosion until vegetation can be established.

8.1.1 Excavations and Cut Slopes

We anticipate the soil and bedrock encountered at the site can be excavated with conventional earth-moving equipment such as loaders, backhoes, and drill rigs. However, specialized

1 Relative compaction refers to the in-place dry density of soil expressed as a percentage of the maximum dry density of the same material, as determined by the ASTM D1557 or AASHTO T180 laboratory compaction procedure.

2 Low corrosion potential is defined as a minimum resistivity of 2,000 ohms-cm and maximum sulfate and chloride concentrations of 250 parts per million.

Tuolumne County, California Page 15 equipment, such as long-reach excavators, or installation of a temporary roadway may be appropriate for completing the cut in the challenging topographic conditions at the site.

The stability of temporary cut slopes is a function of many factors, including soil density and type, presence of perched water, slope height and inclination, and duration. The final backslope stability of temporary slope back cuts is the responsibility of the contractor and should be based on their assessment of the conditions exposed during construction. The topography of the site may require benching back into the existing slope so that the excavation for the slope repair can be completed. Excavations that will be deeper than five feet and will be entered by workers should be shored or sloped in accordance with CAL-OSHA standards (29 CFR Part 1926). The contractor should be familiar with the most recent OSHA Trench and Excavation Safety standards.

For planning, based on the existing slope configurations in the intact decomposed granite materials, temporary excavations can be planned to be as step as ¾:1 (horizontal to vertical). We recommend that the RSS installation be performed during the summer months and temporary slopes should be protected from surface water runoff. Permanent cut slopes should match the existing site grades where applicable. In general, we recommend permanent cut surfaces should be no steeper than 1:1.

We recommend that the RSS installation be performed during the summer months and temporary slopes should be protected from surface water runoff.

8.2 Reinforced Soil Slope (RSS)

As discussed previously; to stabilize the slope below the road repair, a reinforced soil slope (RSS) structure could be constructed at this site to support the roadway across the landslide area.

Based on our field reconnaissance observations and topographic information obtained by Langan, the location of the new about 15-foot-wide roadway as specified in the USFS plan and the addition of 4-foot-wide shoulder, we estimate that a maximum RSS height of about 16 feet will be need.

This is based on the recommended RSS surface slope of ¾:1 (horizontal to vertical).

The RSS should be embedded into the intact (non-displaced) decomposed granite unit or more intact rock blocks. The global stability analysis shows that an RSS slope that is up to 16 feet high and 12 feet wide has an improved factor of safety over the existing or adjacent conditions, see

Table 2. The welded wire mat should have a minimum length of 12 feet and spaced vertically every two feet. The welded wire mesh, or equivalent, should have a minimum ultimate tensile

Tuolumne County, California Page 16 strength of 2,800 pounds per foot (lb/ft). Towards the edges of the RSS slope, where slope heights are less than 10 feet, the welded wire mat length can be reduced to 10 feet long.

The RSS should be embedded a minimum of 3 feet below the outboard toe of the slope and found in competent in-place saprolite (DG). Based on the surveyed topography of the slope below the repair section, 3 feet of vertical embedment provides at least 4 feet of horizontal cover at the toe of the RSS.

Proper embedment is crucial to minimize potential damage to the wall resulting from future erosion or movement of the slope below. Loose or disturbed material should be over excavated and replaced with reinforced engineered fill.

We expect that in some areas the excavation for the lowermost reinforcing mat may result in a somewhat irregular and or a fractured bottom. If this condition exists, an approximate 6-inch-thick layer of 4-inch-diameter to ¾-inch-diameter open-graded crushed rock (No. 1 to No. 4 aggregate per AASHTO M 43) be placed at the base of the excavation. This material should be compacted to a dense, unyielding condition to establish a uniform bearing surface for placement of the lowermost wire mat.

Based on the materials present at the site, the majority of the excavated material would make a suitable backfill between the layers of RSS reinforcement. If import material is used, the material should meet the requirements outlined in Section 8.1. The RSS backfill should be placed and compacted as recommended in Section 8.1.

To achieve the recommended outboard slope angle of ¾:1, the facing of the RSS may consist of wrapped geotextile fabric facing, or to mitigate the hazards associated with potential future forest fires, the surface should consist of welded wire mesh system. To eliminate the use of flammable materials in the surface materials, the welded wire mesh baskets can be filled with 3-inch-diameter crushed rock (ballast rock), classified as No. 1 aggregate per AASHTO M 43, placed similar to a gabion facing element. The 3-inch-diameter crushed rock should be placed for the soil anywhere within 3 feet of the outside edge of the wire mesh baskets. Beyond 3 feet, the engineered fill should be utilized. The engineered fill should be separated from the 3-inch-diameter crushed rock using filter fabric (Mirafi 140NC or equivalent), or by a gradation of rock materials engineered to filter these materials.

Tuolumne County, California Page 17

8.3 Site Drainage

Proper drainage will be required to maintain the integrity of the stabilization system. A continuous subsurface drain should be constructed along the backside of the RSS to collect groundwater and prevent the buildup of hydrostatic pressure. The RSS can be backdrained using prefabricated drainage panels spaced every 5 feet. These panels should be at least 2 feet wide and conduct the water to an approved collection system at the base of the wall; drainage panels should cover at least 35 percent of the wall area. Alternatively, gravel backdrains can be used, which consist of ¾-inch-diameter drain rock (No. 57 aggregate per AASHTO M 43) wrapped in filter fabric

(Mirafi 140NC or equivalent) or Caltrans Class 2 permeable material [Caltrans Standard

Specifications Section 68-2.02F(3)]. The tops of the drainage panels or gravel backdrain should be embedded at last 2 feet below the ground surface to avoid collecting surface water flow.

Water within the gravel should be collected with a continuous 4-inch-diameter perforated

Schedule 40 or SDR 35 PVC pipe or equivalent, placed within the bottom portion of the gravel drain, with perforations facing down. The perforated pipe should be surrounded by at least 6 inches of Caltrans Class 2 permeable material or ¾-inch-diameter (No. 57) crushed rock wrapped in filter fabric (Mirafi 140NC or equivalent). The perforated pipe should be inclined to drain at a minimum gradient of at least one percent to a centralized location where the water can be collected in a solid collector pipe. The collector pipe should discharge onto an energy dissipater at an appropriate discharge location (where subsequent infiltration would not pose a slope stability hazard). Where practical, the drain discharge should be located downslope of the RSS system and field located to discharge into an area that is more resistant to erosion (for example into the granitic dike materials) The subdrains should be provided with cleanouts to allow for periodic cleaning and maintenance.

The subdrain outlet pipe should transition from PVC to a fire resistant material (such as galvanized steel), at a distance of at least 5 feet from the point where the pipe meets the face of slope.

Surface water may be directed over the RSS, provided water is not concentrated and the surface of the RSS is designed to be protected from erosion (e.g. gravel filled welded wire baskets);

however, water should not be allowed to pond near or above the new slopes. We recommend any culvert outlets include design of energy dissipaters and FSSS Class 2 rip-rap rock (per Section

705) at the outlet to limit the potential for erosion.

Tuolumne County, California Page 18

8.4 Alternate Stabilization System Soldier Pile Retaining Wall

The existing road geometry at this site also renders it a suitable candidate for a soldier pile wall.

Piles will be drilled and set in concrete into the subgrade below the fill slope failure. Where the soldier beams extend above grade, they may be lagged with timber or precast concrete lagging between the piles to retain the roadway fill material. Drainage systems will be considered to prevent hydrostatic loading against the wall. Typical pile spacing would be on the order of 8 feet on center. The retained height for this system would be on the order of 3 to 8 feet at the edge of roadway. Typical embedment depths into the competent deeply weathered rock in steep terrain would be on the order of 1.5 to 2 times the retained height. Challenges for this construction type is the potential for unknown subgrade conditions that may present challenges for this type of construction include the requirement to use relatively heavy soldier beams that can act in cantilever and unknown strength of materials within the embedment length of the piles.

9.0 ADDITIONAL GEOTECHNICAL SERVICES PRIOR TO AND DURING CONSTRUCTION

The GER should review the project plans and specifications to check their conformance with the intent of our recommendations. During construction, the GER should provide on-site observation and testing during site preparation, grading, placement and compaction of the RSS elements and fill, and installation of subdrains. These observations will the GER to compare actual with anticipated subsurface conditions and to check that the contractor's work conforms to the geotechnical aspects of the plans and specifications.

10.0 LIMITATIONS

The conclusions and recommendations presented in this report apply to the site and to construction conditions as we have described them. They are the result of limited engineering studies and our interpretations of the geotechnical conditions existing at the site at the time of investigation. Actual subsurface conditions may vary. If any variations or undesirable conditions are encountered during construction, or if the proposed construction will differ from that described in this report, Langan/Divis should be notified to make supplemental recommendations, as necessary.

Our firm has prepared this report for the exclusive use of DJ&A PC, USFS, and members of the design team for this project in accordance with the generally accepted geotechnical engineering practice as it exists in the site area at the time of our study. No warranty is expressed or implied.

The recommendations provided in this report are based on the assumption that an adequate

Tuolumne County, California Page 19 program of tests and observations will be conducted by our firm during the construction phase in order to evaluate compliance with our recommendations.

REFERENCES

2014 Working Group on California Earthquake Probabilities (2015). “UCERF3: A new earthquake forecast for California’s complex fault system.” U.S. Geological Survey 2015–3009, 6 p., http://dx.doi.org/10.3133/fs20153009.

ASCE/SEI 7-16 (2016). Minimum Design Loads for Buildings and Other Structures.

Bateman, P.C., and Wahrhaftig, C. (1966). “Geology of the Sierra Nevada.” in Bailey, E.H., ed., Geology of Northern California. California Geological Survey Bulletin 190, p.

107-172.

California Building Standards Commission (CSBC), 2016 and 2022 California Building

Codes.

California Division of Mines and Geology (1996). “Probabilistic seismic hazard assessment for the State of California.” DMG Open-File Report 96-08.

California Geological Survey (2008). “Guidelines for Evaluating and Mitigating Seismic

Hazards in California.” Special Publication 117A.

Cao, T., Bryant W. A., Rowshandel, B., Branum, D. and Wills, C.J. (2003). “The Revised

2002 California Probabilistic Seismic Hazard Maps June 2003,” California Geological

Survey.

Clark, L.D. (1964). “Stratigraphy and Structure of Part of the Western Sierra Nevada, Metamorphic Belt, California.” U.S. Geological Survey Professional Paper 410.

Duncan et al. (2014), “Soil Strength and Slope Stability, 2nd Edition”

Forest Service Supplemental Specifications taken from FHWA (2024). Standard

Specifications for the Construction of Roads and Bridges on Federal Highway Projects

(FP-14). U.S. Department of Transportation, Federal Highway Administration.

Hart, E.W., and Bryant, W.A. (1997). “Fault-Rupture Hazard Zones in California.”

California Geological Survey Special Publication 42.

Hynes-Griffin, M.E., and Franklin, A.G. (1984). “Rationalizing the Seismic Coefficient

Method.” Department of the Army, US Army Corps of Engineers, Waterways

Experiment Station. Miscellaneous Paper GL-84-13, July.

Jennings, C.W., and Bryant, W.A. (2010), “Fault Activity Map of California.” California

Geological Survey Geologic Data Map No. 6.

http://dx.doi.org/10.3133/fs20153009

McFarland, F. S., Lienkaemper, J. J. and Caskey, S. J. (2009, recent revision 2016), “Data From Theodolite Measurements on San Francisco Bay Region Faults, California,”

USGS Open File Report-2009 1119 v.8.

REFERENCES

(Continued)

Norris, R.M., and Webb, R.W. (1976). “Geology of California.” John Wiley & Sons.

Southern California Earthquake Center (SCEC) (2002). “Recommended Procedures for

Implementation of DMG Special Publication 117, Guidelines for Analyzing and

Mitigating Liquefaction in California.”

Turner, H.W., and Ransome, F.L. (1897). “Geologic Atlas of the United States, Sonora

Folio, California.” U.S. Geological Survey Folio 41.

Wagner, D.L., Bortugno, E.J., and McJunkin, R.D. (1991). “Geologic Map of the

San Francisco-San Jose Quadrangle.” California Geological Survey Regional Geologic

Map Series Map No. 5A.

Wesnousky, S. G. (1986). “Earthquakes, quaternary faults, and seismic hazards in

California.” Journal of Geophysical Research, 91(1312).

Wills, C.J., O’Neal, M.D., Holland, P.J., Parrish, B.M., Key, E.L., and Delattre, M.P.

(2022). “Preliminary Geologic Map of the Oakdale 30’ x 60’ Quadrangle, California.”

California Geological Survey Preliminary Geologic Map 22-09.

Working Group on California Earthquake Probabilities (WGCEP) (2003). “Summary of

Earthquake Probabilities in the San Francisco Bay Region: 2002 to 2031.” Open File

Report 03-214.

Working Group on California Earthquake Probabilities (WGCEP) (2008). “The Uniform California

Earthquake Rupture Forecast, Version 2.” Open File Report 2007-1437.

FIGURES

1 mi

N

N Image © 2025 Airbus

Image © 2025 Airbus

Image © 2025 Airbus

135 Main Street, Suite 1500 San Francisco, CA 94105

T: 415.955.5200 F: 415.955.5201 www.langan.com

La ng an

Langan Engineering and Environmental Services, Inc. 1SLOPE STABILITY

ANALYSIS 12

SITE LOCATION MAP

(MP 9.17)

NFS LUMSDEN ROAD

REPAIR

731703601

3/7/2025 Langan CA, Inc.

135 Main Street, Suite 1500 San Francisco, CA 94105

T: 415-955-5200 F: 415-955-5201 www.langan.com

JDO

PDB/SAW

STANISLAUS NATIONAL FOREST

TUOLUMNE COUNTY CALIFORNIA 20

SITE

DPT-1

DPT-2

DPT-3

TP-1

Field Sample: DG

Filename: C:\bms\langan-pw-01\d0677758\FG01-750703601-B-SP0103.dwg Date: 3/5/2025 Time: 11:59 User: agekas Style Table: Langan.stb Layout: FIRURE 2_9.17

Approximate scale

0 30 Feet

La ng an

Langan CA, Inc.

135 Main Street, Suite 1500 San Francisco, CA 94105

T: 415.955.5200 F: 415.955.5201 www.langan.com

DPT-1 Approximate location of cone penetrometer test by Langan, February 2025

Approximate location of test pit by Langan, February 2025

EXPLANATION

Reference:

Filename: C:\bms\langan-pw-01\d0677758\FG01-750703601-B-SP0103.dwg Date: 3/11/2025 Time: 11:22 User: agekas Style Table: Langan.stb Layout: FIRURE 3 Geology_9.17

Approximate scale

0 30 Feet

Reference:

La ng an

Langan CA, Inc.

135 Main Street, Suite 1500 San Francisco, CA 94105

T: 415.955.5200 F: 415.955.5201 www.langan.com

Predominately debris flow evacuation scar exposing Mzg with scattered pockets of displaced materials

Predominately Mesozic age coranodiorite characterized by deep weathered and friable rock with scattered moderately weathed and moderately strong core blocks

Prominant erasional channel

Approximate geologic contact

Approximate debris flow path

Laseral margin of older slope…