CA.FLAP.SR36(13).State_Road_36_Humboldt_County.Final_Geotech.2016.pdf

U.S. Department of Transportation

Federal Highway Administration

STATE ROAD 36

HUMBOLDT COUNTY

CA FLAP SR36(13)

FINAL GEOTECHNICAL REPORT

Report # CA-FX-0036-16-04

Geotechnical Services Branch

May 2016

U.S. DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

CENTRAL FEDERAL LANDS HIGHWAY DIVISION

LAKEWOOD, COLORADO

i CA FLAP SR36(13)

Table of Contents

SECTION ONE -- INTRODUCTION

1.1 OBJECTIVE AND SCOPE

1.2 PROPOSED CONSTRUCTION

1.3 SITE CONDITIONS

SECTION TWO -- GEOLOGY AND SEISMICITY

2.1 GEOLOGY

2.2 GEOLOGIC HAZARDS

2.3 SEISMICITY and SEISMIC DESIGN PARAMETERS

SECTION THREE – FIELD EXPLORATION AND LABORATORY TESTING

3.1 PRELIMINARY GEOTECHNICAL INVESTIGATION

3.2 GEOTECHNICAL DATA REPORT

3.3 GEOPHYSICAL INVESTIGATION

SECTION FOUR -- ANALYSIS AND RECOMMENDATIONS

4.1 GEOTECHNICAL PROFILE

4.1.1 Subsurface Material Parameters:

4.1.2 Interpreted Ground Water and Soil Moisture Conditions:

4.2 LANDSLIDES

4.3 EMBANKMENTS

4.3.1 Reinforced Soil Slopes and Deep Patch

4.3.2 MSE Walls

4.3.3 Minor Structures

4.4 PERMANENT CUT SLOPES AND CREST ROUNDING

4.4.1 Anchored Slope Protection

4.4.2 Rockfall

4.5 DRAINAGE RECOMMENDATIONS

4.6 SHRINK/SWELL ANALYSIS

4.7 CORROSION POTENTIAL

4.8 CONSTRUCTION CONSIDERATIONS

4.9 SPECIFICATIONS

4.10 LIMITATIONS

SECTION FIVE--REFERENCES

ii CA FLAP SR36(13)

TABLES

TABLE 1:- Seismic Parameters for Soil Profile Type D TABLE 2:- Estimated Material Parameters for Design TABLE 3:- Summary of Landslide Stability Analysis TABLE 4:- Summary of Rock Embankment TABLE 5:- Summary of Proposed RSS and Deep Patch TABLE 6:- Summary of MSE Wall Reinforced Foundation TABLE 7:- Resistance Factors, Culvert Headwalls and Wingwalls TABLE 8:- Lateral Earth Pressure Coefficients, Culvert Headwalls and Wingwalls TABLE 9:- Cut Slope Recommendations TABLE 10:- Preliminary Anchored Slope Protection, Site 1 TABLE 11:- Preliminary Anchored Slope Protection, Site 2 TABLE 12:- Preliminary Anchored Slope Protection, Site 3 TABLE 13:- Horizontal Drain Summary TABLE 14:- Underdrain Summary TABLE 15:- Summary of Shrink/Swell by Location

FIGURES

FIGURE 1:- Vicinity Map FIGURE 2:- Project Location Map FIGURE 3:- Geologic Map FIGURE 4:- Geologic Site Map FIGURE 5:- Regional Fault Map FIGURE 6-18:- Geologic Investigation Map

APPENDICES

APPENDIX A - Instrumentation Program APPENDIX B - Analysis APPENDIX C - Slope Stability

1 CA FLAP SR36(13)

SECTION ONE -- INTRODUCTION

1.1 OBJECTIVE AND SCOPE

This report presents the results of the geotechnical study conducted for the 4R realignment design of the proposed State Road 36 Humboldt County, CA FLAP SR 36(13) project. The project is located in Humboldt County, maintained by Caltrans, and provides access to the Six Rivers National Forest and serves as a link across California’s Coastal Range between U.S. 101 and Interstate 5. The route provides access to recreation, timber and grazing, and serves other local needs including routes for school bus, mail delivery, and commercial supply traffic.

The project begins at approximately Humboldt County Post Mile (HUM PM) 36.1 (approximate Station 1903+00) and ends at HUM PM 40.5 (approximate Station 2109+00). The proposed length is 3.9 miles; 0.5 miles shorter than the existing alignment. The general location of the SR36 Humboldt County Project with respect to major topographic features and existing facilities are shown on the Vicinity Map and Project Location Map in the Figures section of this report.

This report summarizes the relevant findings of three previous investigation reports and provides geotechnical recommendations pertaining to the 95 percent design plans dated October 2014.1 Pertinent field and laboratory information used in the development of the final recommendations were previously presented in the following reports. These reports must be read in conjunction with this report:

U.S. Department of Transportation, Federal Highway Administration, Central Federal Lands Highway Division, Geotechnical Services Branch, “State Road 36 Humboldt County, CA FLAP SR36 (13), Preliminary Geotechnical Recommendations, Report # CA-FX-0036-13-01”, dated July 2013.

Kleinfelder, “Geotechnical Data Report, State Road 36, Humboldt County, California, CA FLAP SR 36(13)”, Dated January 23, 2014.

NORCAL Geophysical Consultants, Inc., “Geophysical Investigation, State Road 36, CA FLAP SR36(13), Humboldt County, California”, Dated January 30, 2014

The Preliminary Geotechnical Recommendations report was conducted in general accordance with the initial plans dated February 5, 2013. The Geotechnical Data Report (GDR) and Geophysical Investigation were conducted with respect to the 15 percent design plans dated August 20132. Alignment adjustments were completed by CFLHD roadway design engineers between the 15, 70, and 95 percent design plans. Figures 6 through 18, Geological Investigation Map, presents the corrected station locations of the various field investigations with respect to the 95 percent design plans alignment. This report takes precedence over possible discrepancies between the data reports.

1 U.S. Department of Transportation Federal Highway Administration Plans for Proposed CA FLAP SR 36(13) State Route 36, Humboldt County, Length 3.9 miles. Preliminary 95% Plans Dated October 2015.

2 U.S. Department of Transportation Federal Highway Administration Plans for Proposed CA FLAP SR 36(13) State Route 36, Humboldt County, Length 3.9 miles. Preliminary 15% Plans Dated August 2013.

2 CA FLAP SR36(13)

1.2 PROPOSED CONSTRUCTION

Proposed roadway reconstruction includes roadway widening to a consistent 24 foot width, realignments to improve roadway geometry, utility relocation, drainage improvements, new embankments, cut slopes, and right of way (ROW) acquisition. The existing and proposed alignment traverses several known landslides of various severity and depths and private land with limited ROW. Cut slope and embankment fill treatments are proposed to limit impacts.

Proposed roadway widening includes common embankment fill slopes, reinforced soil slopes (RSS), and Mechanically Stabilized Earth (MSE) walls. Embankment fills require geosynthetic back drains, rock bed drain channel fill, horizontal drains, and rock shear keys to resist seepage and improve stability. Cut slopes, in soil and bedrock, may require stabilization improvements such as rock inlays, buttresses, draped mesh, and anchored mesh to resist shallow mass movement. Complete mitigation of deep and shallow landslides that exist within the project limits is not feasible within the scope of this project; however, the roadway design concepts attempted to limit the impacts of movements at or below current levels. Deep patch techniques were also incorporated to control or minimize roadway failures in the case of land movement.

1.3 SITE CONDITIONS

SR36 crosses California’s Coastal Range linking U.S. 101 near Fortuna on the coast to Interstate 5 at Red Bluff in the interior. The project site is located near Dinsmore and traverses relatively steep north and west facing slopes above the Van Duzen River at elevations between 2,300 and 2,900 feet. The project location is in a moderately wooded area with occasional residences requiring a number of paved and unpaved roads to exit the highway within the project limits.

Private land owners occupy the majority of properties on both sides of the roadway. Both overhead and underground utilities run along the highway.

Based on weather station data at Forest Glenn, located 18 miles to the southeast at an elevation of 2,343 feet, the average annual precipitation from 1971 to 2000 is about 65 inches, with extremes ranging from 30 inches to 120 inches. Roughly 80 percent of the precipitation is anticipated to occur from November to March. Numerous perennial and seasonal springs are observed throughout the project, with increased flow during the winter and spring seasons. The average air temperature is approximately 52 degrees Fahrenheit (F), with average monthly extremes of 27.5 degrees F in January to 90.2 degrees F in July.

The existing roadway consists of an alternating one to two lane travel way with variable width, sharp curves, no shoulders, limited guardrail, and intermittent center stripe. Generally, the roadway was constructed utilizing steep cut and fill shoulder treatments with limited to no ditch width. Cuts in the project area generally range from 5 to 50 feet in height and have slope ratios that range from 1:1.5 (V:H) to nearly vertical. Fills have a maximum height of approximately 40 feet with slope ratios up to 1:1. Existing cut and fill slopes are performing poorly with slumps and potouts at many locations requiring buttresses, re-grading, and enhanced drainage. In general, slope failures appear to occur in areas of seeps, springs, and poor surface water flow. In

3 CA FLAP SR36(13)

addition, much of the alignment is constructed on active and dormant landslides. Landslide events and continued creep of subgrade soils have damaged the roadway, hindering traffic, and requiring significant repairs. Maintenance of the roadway requires constant attention, with sections of the roadway requiring paving several times a year.

Only one structure was identified within the project limits, a soldier pile and tieback wall near the end of the project area, at PM 40.35 (approximate Station 2105+00). The wall was constructed in 2009 and appears to have effectively stabilized the roadway. The wall is approximately 140 feet in length and consists of two rows of tiebacks. The proposed alignment avoids impacts to this structure.

4 CA FLAP SR36(13)

SECTION TWO -- GEOLOGY AND SEISMICITY

2.1 GEOLOGY

The project site crosses the boundary of the Central and Eastern belt of the Franciscan Complex in the Coast Range Geologic Province. The Central belt is the middle of three broad belts that make up the Franciscan Complex that become younger to the west. The Central belt consists predominately of mélange characterized by interlayered slabs and blocks of more coherent broken and folded sequences of sandstone and shale turbidites (McLaughlin and others, 2000).

The Eastern belt consists primarily of metasedimentary and metavolcanic rocks. The Garberville Quadrangle shows the project corridor crossing an alternating pattern of three generally northwest-southeast trending units referred to as the Broken Formation and Mélange of the Central Belt, and the Metagraywacke of Hammerhorn Ridge of the Eastern Belt. These Cretaceous to late Jurassic age units appear to be fault bounded. In addition, small and large landslide features are anticipated through the site as discussed in the following Geologic Hazards section. Figures 3 through 5, illustrate the geology at the project site.

The Broken Formation is mapped from the start of the project at PM 36.1(approximate Station 1903+00) to the Burr Creek crossing at PM 37.6 (approximate Station 1969+00) where it is bounded by an unnamed fault and the Mélange unit. The Broken formation generally consists of bedded to massive metasandstone and meta-argillite. Bedrock in this part of the project is observed near the ground surface and the terrain exhibits sharp-crested topography with well incised sidehill drainages.

The Mélange extends from Burr Creek to approximately the Mule Ridge Fault Zone near PM

39.0 (approximate Station 2035+00). The Mélange formation is characterized as a sheared, locally tuffaceous, meta-argillite. Dormant and active landslides are predominately observed overlaying this formation, exhibiting rounded, poorly incised, lumpy, and irregular topography.

The Mélange unit is also identified at the very end of the project area at the existing soldier pile and tieback wall.

The Metagraywacke of Hammerhorn Ridge is interpreted to extend from the Mule Ridge Fault Zone to near the end of the project at PM 40.31 (approximate Station 2103+00). This unit is primarily composed of coherent quartzo-feldspacthic metagraywacke with abundant basal layers of metachert. This unit is generally observed near the ground surface with sharp-crested topography and steep colluvial slopes. However, from about PM 39.2 (approximate Station 2043+00) to PM 39.5 (approximate Station 2060+00) is appears to be overlain by thick deposits of dormant and active landslide debris.

2.2 GEOLOGIC HAZARDS

Geologic hazards exist from both the natural setting of the project site and from existing and proposed roadway construction practices. Primary geologic hazards that exist in the vicinity of the project area are landslides, slope instability at cut and fill slope locations, rock fall, flash

5 CA FLAP SR36(13)

floods, and seismic events. Neither geologic maps nor site observations indicate that the site is an area of naturally occurring asbestos.

Active and dormant landslides, are encountered along the majority of the alignment from PM

37.35 (approximate Station 1956+00) to PM 39.50 (approximate Station 2059+00). These landslides are assumed to originate from the weathered and sheared Mélange formation and activated from perched and high groundwater. The landslides as a whole extend from the ridgeline at approximate elevation of 4,000 feet to the Van Duzen River at about 2,000 feet. The landslides generally appear to be large complex geometry rotational slides.

The large scale landslides appear to move slowly within this region with an estimated creep rate of about 1 foot annually; however, accelerated movement should be anticipated during periods of high groundwater. Due to the large mass of the slides, movement will be most noticeable when smaller lobes of the slide mass activate and move rapidly in relation to the larger slide.

Mitigation of the larger slide mass is not likely feasible; therefore, mitigating and/or avoiding the smaller active lobes of the slides is the primary focus of design. A high level of maintenance is likely to persist with the new roadway.

Cut and fill slope instability is observed throughout the existing roadway and is likely to continue after construction. In general, slumps and potouts appear to occur in oversteepened slopes with seeps, springs, and poor surface water flow. Slumps and potouts have occurred at many cut and fill locations requiring buttresses, re-grading, and enhanced drainage.

Active rockfall was observed at PM 37.05 (approximate Station 1940+00), PM 39.80 (approximate Station 2080+00), and PM 40.30 (approximate Station 2101+00), resulting from, both, boulders dislodging from oversteepened colluvial soil slopes and from lose blocks that exist within the closely jointed rock mass outcrops. Rockfall is primarily anticipated to originate from existing and proposed cut slopes and likely to occur during construction. The risk is particularly high during high precipitation seasons, specifically winter and early spring months.

Flash Floods from the Van Duzen River have historically encroached on the existing roadway from approximate PM 40.00 to PM 40.20 (approximate Stations 2089+00 to 2099+00), during large storm events. Burr Creek and other numerous side drainages are likely to flood in large and smaller storm events. See the hydraulic recommendations report prepared by the CFLHD Hydraulic discipline for full analysis and results.

2.3 SEISMICITY and SEISMIC DESIGN PARAMETERS

There are several known large active faults within the vicinity of the project site. According to the "Caltrans 2007 Fault Database" the fault controlling ground shaking at this site is the Cascadia Subduction Zone; it is located 40 miles to the west and is capable of generating an earthquake with a maximum magnitude value of 8.3. The next influential faults are the Big Lagoon- Bald Mountain and Trinidad (alt1) faults; located 22.0 miles and 24.5 miles, respectively, to the northwest. Both of these faults are part of the Mad River Fault Zone and are capable of generating a 7.5 magnitude earthquake. The closest known active fault is the reverse

6 CA FLAP SR36(13)

style Little Salmon-Yager Fault located approximately 4 miles west of the project, capable of generating an earthquake with a magnitude of 7.0. Another regional fault of interest is the Bartlett Springs fault; located 19 miles to the southeast and capable of generating a 7.2 magnitude earthquake. Figure 5, shows the mapped locations of regional active faults listed in the Caltrans 2007 Fault Database.

There are two mapped faults, not listed in the Caltrans 2007 Fault Database, that cross the alignment within the project area. The unnamed fault at Burr Creek and the Mule Ridge Fault Zone (Garberville Quadrangle, McLaughlin and others, 2000). The unnamed fault at Burr Creek is mapped as east-west trending along the approximate alignment of Burr Creek and is estimated to cross the roadway at station 1969+00. The Mule Ridge Fault Zone is northwest-southeast trending and interpreted to cross the roadway alignment between stations 2030+00 and 2040+00.

Recommended seismic response parameters are based on the California Department of Transportation (Caltrans) ARS On-Line, and represents a probabilistic horizontal peak ground acceleration (PGA) with 5 percent probability of exceedance in 50 years (1000-year return period). The middle of the project was selected as a representative location for the probabilistic horizontal acceleration values for analysis corresponding to 40.4866º N latitude and -123.6423º W longitudinal. Based on the borings and geophysical investigation, the average time-weighted shear wave velocity for the top 100 feet (VS100) of subsurface materials was estimated between 600 and 1,200 feet per second. Therefore, the site is classified as Soil Profile Type D according to Table B.1 in Appendix B of the Caltrans Seismic Design Criteria dated 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 web-based tool, Caltrans ARS Online, version v2.3.06 , updated November 23, 2013. Due to the high seismic parameters, seismic design is anticipated to control slope stability analysis. Results are summarized in Table 1.

TABLE 1:- Seismic Parameters for Soil Profile Type D

Horizontal Peak Ground Acceleration, (As) 0.529 g Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 1.218 g Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.602 g

7 CA FLAP SR36(13)

SECTION THREE – FIELD EXPLORATION AND LABORATORY TESTING

Field and laboratory investigations are presented in the Preliminary Geotechnical Recommendations report dated July 20133, GDR dated January 23, 20144, and the Geophysical Investigation report dated January 30, 20145. A summary of the scope of field work completed for the three data reports is summarized herein. In addition, surface survey points were installed in September 2013 and monitored through July 2015, between Station 2043+00 and 2060+00, to record landslide movement. Figures 6 through 18 present survey points, subsurface borings, and geophyiscal investigation locations with respect to the 95 percent design plans. The results of the instrumentation monitoring are contained in Appendix A.

3.1 PRELIMINARY GEOTECHNICAL INVESTIGATION

CFLHD geotechnical personnel and Taber Drilling conducted the initial subsurface investigation during the weeks of March 4 through the 25, 2013, to characterize and instrument suspected landslide locations. The subsurface exploration program consisted of drilling a total of 20 borings; 10 inclinometers labeled SI-1 through SI-10 with adjacent piezometers labeled P-1 through P-10. Subsurface conditions were logged and representative samples were transported to the CFLHD Materials Laboratory in Lakewood, CO, for testing.

Select soil and rock core samples were tested to classify materials into similar geologic groups and to determine material properties for use in geological engineering design. The testing program included index test and geotechnical engineering property tests. Index tests included moisture content, specific gravity, grain size distribution including particle size analysis, Atterburg Limits, Resistivity, pH, sulfate content, chloride content, and dry density tests.

Strength tests on soil included consolidated undrained triaxial compression test and one-dimensional consolidation using incremental loading. Strength test in rock included unconfined compressive strength (UCS).

3.2 GEOTECHNICAL DATA REPORT

An additional field exploration program consisting of 65 subsurface borings was conducted by Kleinfelder and drilled by Pitcher Drilling Company between September 24, 2013 and November 1, 2013. Of these borings, 57 (designated B-101 through B-159) were drilled along the project alignment and eight (designated P-101 through P-108) where drilled at a potential wetland mitigation site and had piezometers installed. In addition, Time Domain Reflectometry (TDR) cables were installed in 7 alignment borings. A Kleinfelder staff engineer or geologist logged the borings by visually identifying and field classifying soils in general accordance with

3 U.S. Department of Transportation, Federal Highway Administration, Central Federal Lands Highway Division, Geotechnical Services Branch, “State Road 36 Humboldt County, CA FLAP SR36(13), Preliminary Geotechnical Recommendations, Report # CA-FX-0036-13-01”, dated July 2013.

4 Kleinfelder, “Geotechnical Data Report, State Road 36, Humboldt County, California, CA FLAP SR 36(13)”, Dated January 23, 2014.

5 NORCAL Geophysical Consultants, Inc., “Geophysical Investigation, State Road 36, CA FLAP SR36(13), Humboldt County, California”, Dated January 30, 2014

8 CA FLAP SR36(13)

ASTM International Standard D 2488 and the Unified Soil Classification System (USCS). Upon completion of laboratory testing, soil classifications were further evaluated in general accordance with ASTM D 2487. The potential wetland mitigation site investigated was not determined to be a favorable site; therefore piezometer monitoring results are not included in this report.

3.3 GEOPHYSICAL INVESTIGATION

A geophysical investigation consisting of 21 lines of geophysical data at 12 locations throughout the project was conducted by NORCAL Geophysical Consultants, Inc. of Cotati, California. Of these lines, 17 lines obtained seismic refraction data, 3 obtained electrical resistivity data, and 1 line utilized Multi-channel Analysis of Surface Waves (MASW) techniques. Data collected in the field was processed until a 2-D model was computed. Seismic refraction was used to characterize the thickness of overburden, depth to rock, and apparent rippability of rock based on measured rock velocities. The electrical resistivity and MASW methods were used to further characterize overburden materials at suspected locations of landslides and faulting.

9 CA FLAP SR36(13)

SECTION FOUR -- ANALYSIS AND RECOMMENDATIONS

The rehabilitation and reconstruction of CA SR 36 consists of extensive embankment work including retaining walls, reinforced soil slopes, permanent cut slopes, utility relocation, and drainage improvements. In addition, temporary cut slopes, shoring, and barriers are required to maintain traffic through construction.

Subsurface characterization and geotechnical profiles were developed for use in the design of structures and embankments. Recommendations regarding landslides, Reinforced Soil Slopes (RSS), Mechanically Stabilized Earth (MSE) walls, minor structures, cut slopes, subsurface drainage, material shrink/swell, corrosion potential, and construction considerations are discussed in the following sections. All foundations are in accordance with the American Association of State Highway and Transportation Officials (AASHTO) probabilistic Load and Resistance Factor Design (LRFD), 7th edition, 20146.

4.1 GEOTECHNICAL PROFILE

Geotechnical profiles were developed from the data collected during the subsurface investigations, results of laboratory testing, site observations, back-analysis modeling of existing conditions, and known empirical strength characteristics of like materials. Geotechnical profiles consist of estimated subsurface material parameters, anticipated design geomaterials, and interpreted ground water and soil moisture conditions.

4.1.1 Subsurface Material Parameters

Subsurface conditions encountered can be simplified as existing roadway embankment fill, native overburden soil, and rock. Overburden soils generally consisted of alluvium, colluvium, landslide deposits, and residual soils from the in-place weathering of bedrock. Bedrock generally consists of three geologic units of the Franciscan Complex as discussed in the Geology section. The description contained herein provides a general classification of materials for clarifying chosen material parameters used in design. Refer to the boring logs for a, location specific, detailed description of subsurface conditions.

Embankment fill encountered is predominantly coarse grained; typically poorly graded gravel to clayey gravel with variable percentages of fines, sand, gravel, and cobble/boulders. The average depth of fill along the mainline alignment is approximately 5 to 6 feet however fills of over 10 feet were encountered in multiple borings. Asphalt concrete along the existing roadway was typically 6 to 12 inches thick with a total pavement section of approximately 18 inches (asphalt concrete and aggregate base). Thicker sections of asphalt concrete were encountered at borings B-123 and B-124, approximately 3 to 4 feet; and at borings B-151 and B-151a, approximately 10 to 12 feet thick. Fill soils encountered were primarily dry to moist and medium dense. Locally moist to wet, porous, and/or loose, fill materials were also encountered. Occasionally roots and

6 American Association of State Highway and Transportation Officials (AASHTO), 2014, AASHTO LFRD Bridge Design Specifications, 7th Edition

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wood fragments were encountered in the fill material or at the transition from fill to native material.

Native soils encountered during the subsurface investigations consisted of alluvium, colluvium, active/dormant landslide deposits, and residual soil. Similar to the fill material, the native soil encountered was predominately coarse grained, poorly graded gravel to clayey and silty gravel as well as clayey sand and silty sand with varying amounts of gravel content. Fine grained, cohesive material was normally encountered in the near surface soils (for example, lean clay and low plasticity silt was observed in the upper 10 to 15 feet of borings B-105 through B 109).

Areas of observed slope instability also contained higher percentages of lean and fat clay. The contact between the colluvial soil, residual soil, and bedrock was generally indistinguishable;

therefore, an interpretation of contacts between units and variable material parameters were used in design, as shown in Table 2.

The Franciscan Complex geologic units which underlie the site are described as the Broken formation, a mélange unit, and the Metagraywacke of Hammerhorn Ridge (McLaughlin and others (2009)). Franciscan Complex sandstone, meta-sandstone, meta-shale, and meta-graywacke bedrock was encountered during the exploration. The meta-graywacke encountered along the alignment is similar in composition and appearance to the meta-sandstone encountered.

Graywackes are considered a variety of sandstone and considering their similarity, the rock types were not differentiated on the boring logs. The bedrock encountered is generally moderately to slightly weathered, extremely weak to weak, intensely fractured and pervasively sheared.

However, strong, high RQD, bedrock blocks were also encountered, as shown on the boring logs. The average depth to rock encountered during the subsurface exploration was approximately 30 to 35 feet below the roadway with a general range from 8 to 60 feet; however bedrock is observed in many of the existing cut slopes near or at the ground surface.

Native and engineered geo-material parameters estimated for design are based on field investigations, results of laboratory testing, and known empirical strength characteristics of like materials. In addition, slope stability and infinite slope analysis was conducted from observations of site conditions to calibrate estimated material values of the overburden. A range of selected design values for soil unit weight (γ), internal friction angle (), and cohesion intercept (c) are shown in Table 2. Local specific design parameters were determined for anticipated application and conditions on a site-by-site basis as presented in the analysis and slope stability contained in Appendices B and C.

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TABLE 2:- Estimated Material Parameters for Design

Source Material Unit

Weight γ (pcf)

Cohesion c (psf)

Friction Angle (deg)

Analysis Application

Existing On-Site

Materials

Fill (GC or better)

115 -120 50-100 34

Native Geomaterial

Stability Analysis

Colluvium

(SC-GC)

120 -130 100-200 34-36

Landslide Debris*

120-135 0-1000 18-38

Residual Soil and Bedrock (R1 or less)

125-135 100-300 36-38

Bedrock (R1 or better)

135 500 38

Processed On-Site

Backfill Material

(704.03(d)) 130 0-50 36

RSS, Deep Patch, Retained Materials

Unclassified Borrow (704.06)

125 50 30 Embankment

Imported Material

Select Granular Backfill (704.10)

130 0 34 MSE Walls, Reinforced Foundations

Special Rock Embankment/

Rock for Buttress

140 0 45

Channel Fill, Rock

Embankment, Rock for Buttress

* See Site Specific Stability Models for Landslide Material Parameters

4.1.2 Interpreted Ground Water and Soil Moisture Conditions:

Groundwater conditions were difficult to detect during the subsurface investigation due to low permeability of encountered material and required drilling methods, such as mud rotary; however site observations indicate localized perched or concentrated horizontal groundwater flow that vary significantly within short distances and the volume of groundwater flow may fluctuate between seasons. Standpipe piezometers installed during the initial, preliminary investigation and drilling observations indicate groundwater to range from just below the surface to approximately 60 feet deep. Average estimated pieziometric surfaces were included in stability analysis where high soil moisture conditions are anticipated to control design. This is considered conservative as these conditions are not anticipated to be continuous.

Springs and seeps in the native slopes, cut slopes, and roadway inboard ditch have been observed year round and correspond to areas of embankment and pavement distress. Many of these springs have been claimed by local residence for irrigation water. However, very few formal

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drinking water wells have been established within the project boundary and residences typically rely on other sources for fresh water. These are an interpreted result of poor well performance due to a low permeability and variable aquifer combined with soil mass movement. Controlling slope movement by lowering groundwater levels with drainage is imperative to construct and maintain proposed embankments. Flows from groundwater and seeps are anticipated to be significant in some locations during construction requiring dewatering/water diversion and special slope mitigation methods to control the movement rates and prevent failures.

4.2 LANDSLIDES

Landslides observed in the project area are likely large, slow moving, complex geometry rotational slides, as described in the geologic hazard section above. The landslide debris observed at the roadway alignment generally consists of thick deposits of clayey sand with gravel (SC) to clay with gravel (CL) containing 30 to 60 percent fines with low to medium plasticity. Distinct slide planes were not detected during the subsurface investigation; however, inclinometer and TDR results indicated a primary distinct slide plane at depth, with secondary movement at various shallower locations. Preliminary observations and measurements suggest the movement is highly reliant on groundwater elevation within the soil mass. Details and monitoring results of the instrumentation program can be found in Appendix A.

Two active landslides were identified from approximate Station 1969+50 to 1975+00, referred to as the Burr Creek Slide, and from approximate Station 2044+50 to 2059+00, referred to as the Windy Point Slide. The Burr Creek Slide referenced in this report is the most active southern edge of a larger slide mass that is believed to extend to Station 1999+00; however, evidence of recent deep seated movement was not observed during the duration of the investigation. Like the Burr Creek Slide complex, Windy Point slide is observed to move at different rates and depths with the most active movement at the western edge of the slide complex.

The Burr Creek Slide is composed of debris originating from meta-shale bedrock of the Mélange formation and has a primary direction of movement to the southwest toward Burr Creek.

Numerous tension cracks, shear scarps, and leaning or fallen trees are observed though the slide mass on both sides of the roadway. The slide is most noticeably active along the over-steepened slopes adjacent to Burr Creek. Roadway pavement distress requires extensive maintenance and the steep grade of alignment in this area is attributed to slide movement. Instrumentation indicates slide movement at approximately 65 to 73 feet below the roadway elevation with groundwater being variable over a short distance from 15 to 40 feet deep. Instrumented construction monitoring by the contactor for slide movement is recommended during the Burr Creek culvert replacement excavations as discussed in more detail in the following construction consideration section.

The Windy Point Slide debris is predominately composed of granular material but contains significant potions of medium to highly plastic clays. The slide has a northerly direction of movement extending to the Van Duzen River. Numerous tension cracks, shear scarps, and leaning or fallen trees are observed though the slide mass on both sides of the roadway. The

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slide is most noticeably active at the western edge of the slide (station 2059+00) were maintenance efforts have resulted in a large asphalt embankment. Homeowners within the Windy Point Slide have reported that structures are likely to experience foundation distress if jacking and maintenance are not performed every two years. Poor roadway geometry in this area is attributed to slide movement. Instrumentation indicates movement approximately 45 to 140 feet below the roadway elevation with groundwater being variable from 10 to 40 feet deep.

Surface survey points monitored from September 2013 through July 2015 indicate an average annual creep rate of up to 16 inches per year. Creep rates are anticipated to increase with increased precipitation.

Approximate extents of the Burr Creek and Windy Point slides mapped during the preliminary investigation can be found on the Bore Location figures in the Preliminary Geotechnical Recommendations report. In addition to the proposed embankment stability analysis, the Burr Creek and Windy Point slides, along with a shallower slide located from approximate Station 2027+50 to 2030+00 were evaluated over a 500 to 1500 foot long cross sections as a comparison of existing to proposed alignment condition. The analysis suggests that the proposed alignment with standard embankment drainage increases the Factor of Safety slightly relative to the existing condition. To improve conditions further, horizontal drains are recommended in landslide areas as described in the following horizontal drain section. A summary of the landslide stability analysis is presented in Table 3 with stability program output in Appendix C.

TABLE 3:- Summary of Landslide Stability Analysis

Critical Section Station/Slide Designation

Factors of Safety

Existing Proposed*

1972+00/Burr Creek Slide 1.00 1.02

2029+25 1.00 1.12

2058+00/Windy Point Slide 1.00 1.03

* Proposed does not include recommended horizontal drains

Although the monitoring time for design of this project is relatively short for drawing conclusions concerning slide movement, it is recommended that a combination of reducing the driving force and a maintenance approach will likely have the highest cost/benefit mitigation ratio. In general, avoidance and increase resisting forces are not feasible mitigation techniques due to the large scale/deep seated nature of the known landslides. Increasing the internal strength is likely to provide limited benefit without significant costs. The no action alternative is not recommended as landslide movement of the new roadway will increased maintenance efforts.

Recommendations for reducing the driving force consists of a favorable alignment already incorporated in the 95 percent design plans and the installation of extensive horizontal drain systems. Reinforced embankments and widened roadway bench discussed in the following sections aim to maintain or reduce the required maintenance to current levels by limiting the areas requiring maintenance to the slide boundaries.

14 CA FLAP SR36(13)

4.3 EMBANKMENTS

Roadway reconstruction requires embankment construction associated with roadway realignment, widening, driveway approaches, and culvert replacement. It is recommended that proposed embankments be constructed at 1V:2H slopes or flatter to maintain slope stability and facilitate vegetation growth. As an alternative, embankments can be steepened when mitigation treatments such as reinforced soil slopes (RSS) or retaining walls are applied. In general, RSS can be constructed at 1V:1.5H with proper seeding and erosion control and steeper with slope armoring such as rock inlay or revetment mattress. Retaining walls and RSS are discussed later in this section.

It is recommended that embankments be benched into existing slopes, contain drainage systems on every bench, and the toe of embankments be constructed with special rock embankment at locations shown in Table 4. The rock embankment is intended to provide a capillary break and resist seepage forces from seasonally saturated, low permeability soil and improve constructability. Rock embankment dimensions are determined from stability analysis and depend on slope geometry, foundation material, and groundwater conditions.

TABLE 4:- Summary of Rock Embankment

Embankment Location Station to Station

Side

Rock Embankment Geometry Top of Rock

Embankment Elevation

(ft)

Height of Rock Embankment Above Toe of

Slope (ft)

Key Base

Width (ft)

Key Base

Depth (ft)

Additional Mitigation

1917+00 to 1919+50 LT 2585 -- 5 5 Channel Fill 1930+60 to 1934+00 LT 2700 -- 5 5 Channel Fill 1934+00 to 1937+46 LT 2745 -- 5 5 Channel Fill 1943+18 to 1945+05 LT 2860 -- 5 5 1950+50 to 1953+10 LT 2904 -- 5 5

1955+75 to 1958+25 LT 2880 -- 10 5 Horizontal

Drains

1962+50 to 1962+75 LT -- 5 6 5 Shoulder

Stabilization 1967+50 to 1969+74 LT 2740 -- 10 5

1970+00 to 1973+75 LT -- 5 10 5 Horizontal

Drains 1974+45 to 1975+80 LT 2730 -- 5 5

1982+95 to 1985+75 LT -- 5 10 5 Horizontal

Drains 1998+65 to 2001+13 LT 2580 -- 5 5 Channel Fill 2001+45 to 2003+25 LT 2578 -- 5 5 2020+35 to 2023+30 LT 2480 -- 5 5

2027+50 to 2030+00 LT 2490 -- 10 5 Horizontal

Drains

2051+00 to 2051+50 LT 2520 -- 10 5 Horizontal

Drains

2054+50 to 2058+75 LT 2510 -- 10 5 Horizontal

Drains

2084+10 to 2084+30 LT -- 5 6 5 Shoulder

Stabilization

2093+64 to 2094+00 LT -- 5 6 5 Shoulder

Stabilization

15 CA FLAP SR36(13)

Settlement of foundation soils of proposed embankments are estimated based on SHANSEP empirical theory that correlates the shear stress, stress history, and in-situ pressures of the soils.

Total stress (applied stress) analysis under consolidated undrained conditions, for short term stability, is considered the most critical case. Foundation materials are considered granular and clay layers observed in the boring logs are anticipated to be non-continuous. Assuming that laboratory tests conducted adequately represent the conditions in the field with respect to material parameters and changes in pore pressure, total elastic settlement is anticipated to be less than 4 inches. Elastic settlement is expected to occur during construction and secondary consolidation is estimated to be negligible; therefore, construction staging or settlement mitigation is not anticipated. Structures are not anticipated to be adjacent to proposed embankments; therefore, instrumentation for settlement monitoring is not required.

Relatively thick pockets of undocumented fill should be anticipated in some areas due to the nature of the original road construction. Groundwater may be encountered within the embankment and culvert foundations. Dewatering and temporary stabilization methods may be required during construction.

4.3.1 Reinforced Soil Slopes and Deep Patch

The proposed alignment requires the construction of over-steepened slopes requiring reinforcements for stability. Reinforced Soil Slopes (RSS) were selected over conventional embankments constructed of flatter slope ratios to lessen right of way and environmental impacts. Deep patch subexcavation is also recommended in areas of existing embankment distress and across active landslides where removal and replacement of embankments are not feasible.

Analyses of the RSS were performed using RESSA, Version 3.0, and Slide, Version 6.0 (SLIDE), two dimensional, limit equilibrium computer programs from ADAMA Engineering and RocScience, respectively. The Simplified Bishop method of slices was used with isotropic soil parameters. The analysis conducted for RSS design consisted of external, compound, and internal stability in accordance with AASHTO “LRFD Bridge Design Specifications” (7th edition), and FHWA Publication NHI-10-024 and NHI-10-025, Volumes 1 and 2, respectively, entitled “Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes” dated November 2009. Analysis incorporated rock embankment and drainage recommendations contained herein.

Preliminary RSS analyses was performed following Shmertmann, et al.chart (1987) to develop minimum trial reinforcement lengths and spacing’s at the top and bottom of the proposed RSS.

The trial length and spacing were then modeled in RESSA and SLIDE to determine final embedment length, spacing, and tensile strength required of the geosynthetic reinforcement for a desired minimum static factor of safety of 1.30 and a seismic factor of safety check exceeding

1.0 as the proposed embankments do not impact an adjacent structure; in accordance with the Caltrans Geotechnical Manual. A 250 pound per square foot traffic surcharge was applied in the long-term static stability analysis.

16 CA FLAP SR36(13)

In general, based on the aforementioned Shmertmann design chart a minimum reinforcement embedment length of 0.5 and 0.2 times the height is recommended as the base and top length, respectively. A multi zone approach was used for slopes exceeding 35 to 40 feet in height to optimize reinforcement distribution and strength required. Summary tables of the preliminary design (trial length) and final RSS design results and RESSA output for the most critical cross section cases are included in Appendix B. The ten most critical global stability surfaces determined in SLIDE for the most critical cross section cases are presented in Appendix C. A Summary of the recommended RSS and Deep Patch design by location are shown in Table 5.

17 CA FLAP SR36(13)

TABLE 5:- Summary of Proposed RSS and Deep Patch

Station to Station Side Slope Ratio (V:H)

Reinforcement Zone 1 Reinforcement Zone 2 Reinforcement Zone 3 Geometry Remarks Height

(H1) Length

(ft) Spacing

(Sv) Strength

(Tal) Height (H2)

Length (ft)

Spacing (Sv)

Strength (Tal)

Height (H3)

Length (ft)

Spacing (Sv)

Strength (Tal)

1917+00 to 1919+50 LT 1:1.5 <30 20 5 1750 30-60 35 5 2350 >60 45 3 2350

1930+60 to 1934+00 LT 1:1.5 <30 20 5 1750 30-60 35 5 2350 >60 45 3 2350 Continuous Embankment 1934+00 to 1937+46 LT 1:1.6 <30 20 5 1750 30-60 35 5 2350 >60 45 3 2350

1943+18 to 1945+05 LT 1:1.5 <20 15 5 1750 20-40 25 5 1750 -- -- -- --

1950+50 to 1953+10 LT 1:1.5 <25 15 5 1750 -- -- -- -- -- -- -- --

1955+75 to 1958+25 LT 1:1 <20 15 3 1750 -- -- -- -- -- -- -- --

1959+25 to 1960+85 LT 1:1 <10 15 3 1750 -- -- -- -- -- -- -- --

1967+50 to 1970+00 LT 1:1.5 <25 20 5 1750 25-40 30 5 2350 40-65 40 3 2350 Continuous Embankment 1970+00 to 1973+75 LT 1:1.5 <25 20 5 1750 25-50 30 5 2350 -- -- -- --

1974+45 to 1975+80 LT 1:1.5 <25 20 5 1750 -- -- -- -- -- -- -- --

1979+28 to 1982+31 LT 1:1.5 <35 20 5 1750 -- -- -- -- -- -- -- --

1982+95 to 1985+75 LT 1:1.5 <35 20 5 1750 -- -- -- -- -- -- -- --

1999+40 to 2001+13 LT 1:1.5 <20 15 5 1750 20-40 25 5 1750 -- -- -- --

2001+45 to 2003+25 LT 1:1.5 <25 15 5 1750 -- -- -- -- -- -- -- --

2020+35 to 2023+30 LT 1:1.5 <25 15 5 1750 -- -- -- -- -- -- -- --

2027+50 to 2030+00 FW 1:2 6 50 1 325 -- -- -- -- -- -- -- -- Full Width

Patch

2032+00 to 2034+00 LT 1:1 6 25 1 325 -- -- -- -- -- -- -- -- Deep Patch Left Lane

2043+50 to 2054+50 FW 1:1.5 6 80 1 325 -- -- -- -- -- -- -- -- Full Width

Patch

2054+50 to 2058+75 LT 1:1.5 <40 25 5 1750 -- -- -- -- -- -- -- --

2058+75 to 2059+15 FW 1:1.5 6 60 1 325 -- -- -- -- -- -- -- --

2082+50 to 2083+75 LT 1:1 3 25 1 325 -- -- -- -- -- -- -- -- Deep Patch Left Lane

18 CA FLAP SR36(13)

A secondary reinforcement woven geotextile, with a nominal long-term strength of 325 pound per foot, is recommended for face confinement with a maximum 1 foot spacing and 4 foot embedment length. On-site materials are generally suitable for RSS fill but should be limited to A-2 soils or better with a maximum liquid limit of 40 percent and maximum plasticity index of 20 to achieve estimated design strength parameters and to limit deformations. Geocomposite sheet drains, with 30 percent minimum coverage, should be installed on each lift of embankment benching and outlet at low points and every 100 feet. Stagger outlet points between rows.

Imported topsoil, seeding, and rolled erosion control product or hydraulic soil admixtures and mulching designed for the site conditions are imperative for vegetation and long term performance of RSS.

4.3.2 MSE Walls

The proposed SR36 alignment consists of widening the roadway and correcting un-conforming curves as it transverses the steep mountainous valley requiring 3 MSE walls; Site 1 (Station 2013+57 to 2015+70), Site 2 (2060+42 to 2065+28), and Site 3 (2069+12 to 2079+50). MSE walls tolerate larger differential settlement than other wall alternatives and were selected as the most economical due to relatively shallow bedrock, steep slopes, rapid construction, and consistent wall heights above 10 feet. Temporary shoring and bedrock excavation are anticipated to be required to construct the MSE walls at this site. Large boulders are anticipated within the excavation zone for MSE walls.

Preliminary external design including slope stability, bearing resistance, overturning, sliding, and eccentricity was performed to evaluate the proposed wall geometry. The final MSE wall bearing and internal stability should be designed and submitted by the contractor. Slope stability was performed using MSEW, Version 3.0, and Slide, Version 6.0, two dimensional, limit equilibrium computer programs from ADAMA Engineering and RocScience, respectively. Analysis was conducted in accordance with AASHTO “LRFD Bridge Design Specifications” (5th edition), and FHWA Publication NHI-10-024 and NHI-10-025, Volumes 1 and 2, respectively, entitled “Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes” dated November 2009.

Proposed MSE walls were evaluated based on the existing site conditions and available subsurface information. Cross sections were evaluated where the wall height was at its maximum and/or where the slope in front of the fill wall was the steepest. A traffic surcharge of 250 psf was modeled in the analyses. For seismic design, the most critical slip surface was reanalyzed using pseudostatic seismic coefficients. The horizontal seismic coefficient (Kh) of

0.176 g is applied in the MSE wall stability analysis, representing the inclusion of static horizontal force; this is equal to 1/3 the peak seismic ground acceleration coefficient (As). The vertical seismic component (Kv) is assumed to be zero for conservative results. The results of the most critical cross section cases for the recommended design are shown in Appendix B and C.

For the preliminary external design, bearing capacity was determined to control. Based on the results of the analyses, the proposed MSE wall system will meet the required acceptance criteria

19 CA FLAP SR36(13)

provided that the wall system is constructed with a minimum horizontal setback from the slope at the wall toe of 6 feet and a minimum reinforcement length equal to 70 percent of the wall height (0.7H) or 8 feet, whichever is greater. In addition, walls with bearing pressures that exceed bearing resistances calculated with the 6 foot setback require a reinforced foundation to improve bearing resistance and reduce excessive deformation of the wall face. Reinforced foundations should consist of select granular backfill with three layers of woven geotextile equally spaced at 8 inches. Provide reinforced foundations at the locations and dimensions shown in Table 6.

Wrap the face with a 3 foot lap length and 3 inch separation between the lap and next layer of geotextile. The heal of the reinforced foundation should extend to within 2 feet of the heal of the wall. The required minimum nominal long-term strength of the reinforcement geotextile is 550 lb/ft.

TABLE 6:- Summary of MSE Wall Reinforced Foundation

Station to Station Wall

Height, H (ft)

Depth, D (inch)

Number of

Layers

Extent in front of the wall, X (ft)

2014+20 to 2015+08 48 40 5 8.00

2069+75 to 2071+12 18 24 3 3.50

2076+07 to 2078+57 16 40 5 3.00

In evaluating sliding stability, thrust was calculated by adding factored loads from the retained backfill. The maximum driving force effect for the sliding limit state was determined by applying the maximum load factor of 1.5.