Amec Foster Wheeler_Denver_32781004_Geotechnical Report_022117.pdf

GEOTECHNICAL REPORT

Ruth Zenia Road and Van Duzen Road Trinity County, California

CA FLAP TRI CR502(1) & 511(1)

Prepared by:

Amec Foster Wheeler Environment & Infrastructure, Inc.

180 Grand Avenue, Suite 1100 Oakland, California 94612

February 2017

Project No. 32781004

Amec Foster Wheeler

CA FLAP TRI CR502(1) & 511(1) i

TABLE OF CONTENTS

Page

1.0 INTRODUCTION

1.1 PROJECT DESCRIPTION

1.2 SCOPE OF WORK

2.0 GEOLOGIC AND GEOGRAPHIC SETTING

2.1 REGIONAL GEOLOGIC SETTING

2.2 LOCAL GEOLOGIC AND PHYSICAL SETTING

2.3 SEISMIC SETTING

2.4 CLIMATIC SETTING

3.0 EXISTING GEOTEHNICAL INFORMATION

4.0 EXPLORATION PROGRAM

4.1 FIELD RECONNAISSANCE

4.1.1 Segment 1 – Van Duzen Road

4.1.2 Segment 2 – Van Duzen Road

4.1.3 Segment 3 – Van Duzen Road

4.1.4 Segment 4 – Ruth-Zenia Road

4.1.5 Segment 5 – Ruth-Zenia Road

4.2 SUBSURFACE EXPLORATION PROGRAM

4.3 LABORATORY TESTING

5.0 SUBSURFACE CONDITIONS

5.1 MATERIALS

5.2 GROUNDWATER CONDITIONS AND EVIDENCE OF SPRINGS

6.0 SITE GEOTECHNICAL AND GEOLOGIC HAZARDS

6.1 LANDSLIDES

6.2 SEISMIC CONSIDERATIONS

6.2.1 Seismic Design Parameters

6.2.2 Liquefaction

6.2.3 Seismic Compression of Unsaturated Soils

6.3 SOIL CORROSIVITY

7.0 RECOMMENDATIONS

7.1 RECOMMENDATIONS FOR EARTH RETAINING STRUCTURES

7.1.1 MSE Walls

7.1.1.1 Bearing Capacity

7.1.1.2 Lateral Earth Pressures

7.1.1.3 Lateral Sliding and Overturning Resistance

7.1.1.4 Global Stability

7.2 GENERAL EARTHWORK

7.2.1 Permanent Cut Slopes

7.2.2 Fills and Backfills

7.3 PAVEMENT

7.3.1 Existing Flexible Pavement Sections

7.3.1.1 Thickness and Quality of Existing Asphalt

7.3.1.2 Quality of Existing Pavement Subgrade Soil

TABLE OF CONTENTS

(Continued) ii CA FLAP TRI CR502(1) & 511(1)

7.3.2 New Flexible Pavement Sections

7.3.2.1 Segment 1

7.3.2.2 Segment 2

7.3.2.3 Segment 3

7.3.2.4 Segment 4

7.3.2.5 Segment 5

7.3.3 Materials and Compaction

7.3.4 Geotextiles

7.3.4.1 Separation

7.3.4.2 Erosion

7.3.4.3 Subgrade Improvement

8.0 GENERAL LIMITATIONS AND BASIS FOR RECOMMENDATIONS

9.0 REFERENCES

FIGURES

Figure 1 Regional Location Map Figure 2 Site Location Plan Figure 3a Field Exploration Location – Segments 1 and 2 Figure 3b Field Exploration Location – Segment 3 Figure 3c Field Exploration Location – Segment 4 Figure 3d Field Exploration Location – Segment 5 Figure 4 Regional Geologic Map

APPENDICES

Appendix A Field Explorations Appendix B Laboratory Tests Appendix C Global Stability Results Appendix D Field Photos

CA FLAP TRI CR502(1) & 511(1) 1

GEOTECHNICAL REPORT

Ruth-Zenia Road and Van Duzen Road

Trinity County, California

1.0 INTRODUCTION

Amec Foster Wheeler has prepared this geotechnical report in accordance with our understanding of the scope of work dated April 13, 2016 as presented to the Federal Highway

Administration (FHWA), Central Federal Lands Highway Division (CFLHD) and other stakeholders. This report was prepared in support of the overall planning and design of the proposed improvements to Van Duzen Road (County Road, CR 511) and Ruth-Zenia Road

(CR 502), located in Trinity County, California.

Within this report, we present the results of our field investigation including field reconnaissance, field investigation, and laboratory testing; and the results of our engineering analyses and evaluations including a summary of the regional geology, local site setting, seismic and geologic hazards, and other considerations. We conclude with discussion of recommendations to support design and construction of the proposed improvements to Van

Duzen Road and Ruth-Zenia Road.

1.1 PROJECT DESCRIPTION

The proposed project covers select sections of Van Duzen Road (County Road, CR 511) and

Ruth-Zenia Road (CR 502) in Trinity County, California. A map showing the project location is shown on Figure 1. The project includes total reconstruction using reclaimed asphalt pavement (RAP) mixed with roadway aggregate to form the base coarse.

Geotechnical services were requested to support design of the following proposed improvements:

Segment 1 (S1) - Van Duzen Road from project station 100+00 to 105+07:

Widening of road to 22 feet;

Installation of a new 239 foot retaining wall;

Correction of super elevation; and

Replacement of pavement.

2 CA FLAP TRI CR502(1) & 511(1)

Segment 2 (S2) - Van Duzen Road from project station 200+00 to 218+50:

Widening of road to 22 feet;

Installation of a new 62 foot retaining wall;

Installation of a new 160 foot-long retaining wall;

Replacement of two culverts; and

Replacement of pavement.

Segment 3 (S3) - Van Duzen Road from project station 3001+20 to 3154+94:

Replacement of five culverts; and

Replacement of pavement.

Segment 4 (S4) – Ruth-Zenia Road from project station 400+00 to 461+25:

Widening of road to 22 feet;

Replacement of four culverts; and

Replacement of pavement.

Segment 5 (S5) – Ruth-Zenia Road from project station 500+00 to 526+04:

Removal of debris in roadside ditches;

Installation of 1,000 linear-feet of subdrain;

Replacement of existing 36 inch-diameter, 85 feet-long culvert;

Replacement of pavement; and

Installation of a 750-foot storm sewer.

The purpose of the geotechnical services provided and the recommendations discussed in this report was to provide supporting information and analyses regarding material properties, unstable slopes, and other recommendations and information to the design team to support their design and reconstruction of portions of Van Duzen Road and Ruth-Zenia Road as identified above and discussed below.

1.2 SCOPE OF WORK

The scope of work (SOW) was developed in consultation with the FHWA CFLHD, and consisted of the following:

Field reconnaissance including geotechnical field studies, laboratory testing, and geologic hazards survey;

Evaluation of the regional and local site setting including geology, climate, and other factors;

Evaluation of geologic, seismic, and geotechnical hazards; and

CA FLAP TRI CR502(1) & 511(1) 3

Developing recommendations to support design and construction for specified structures including retaining walls, cut slopes, utilities and pavements.

2.0 GEOLOGIC AND GEOGRAPHIC SETTING

The regional geologic setting of the project area is discussed in this Section.

2.1 REGIONAL GEOLOGIC SETTING

The project site lies in the northern reach of the California Coast Ranges Province, bordered by the Klamath Mountains Province on the east and the Pacific Ocean on the west. The Coast

Range Province is underlain by Mesozoic and Cenozoic rocks of the Franciscan Complex, Coast Range Ophiolite, and Great Valley Sequence. These structural units represent different settings and sequences of the history of ocean basin deposition, and subsequent subduction, forearc deposition and volcanism, and uplift, erosion, and emplacement/accretion of these units to the North American Plate along boundary faults of the Pacific and North American plate margin.

The Franciscan Complex is subdivided into three fault-bounded belts, extending generally northwest-southeast, and including the Eastern, Central, and Coastal Belts. Rocks exposed across the project site are mapped as the Central Belt, and include a mélange with interleaved blocks of folded and broken sequences of sandstone and shale turbidites (McLaughlin and others, 2000). The Central Belt was accreted to the Eastern Belt along the Grogan-Red

Mountain fault zone, which extends northwest-southeast along Ruth Lake on the east side of the project area (Figure 4). The Pine Butte fault is considered to be part of the structural boundary of the Central and Eastern Belts; it also trends northwest through the area west of

Ruth Lake, and passing along the top of the ridge just east of Section 4. The Mule Ridge fault extends northwest-south east in the area west of Segments 1, 2, 3, and 5 (Figure 4). The

Coastal Belt lies west of the project area, and was accreted to the Central Belt along the northwest-southeast trending Coastal Belt Thrust (McLaughlin and others, 2000).

The matrix of the mélange is dominantly sheared argillite (shale) and metamorphosed (meta) tuff. Blocks or slabs of chert, limestone, metabasalt, meta-igneous rocks, eclogite, amphibolite, schist, phyllite, and serpentinite also are present within the sandstone and shale of the Central

Belt. The basalt ranges in age from late Jurassic to Middle Cretaceous (about 190 to 93 millions of years before present [Ma]). The sandstone and shale represent pre-subduction ocean basin deposits that have been subducted, deformed, and interleaved with forearc basin and volcanic arc deposits in an accretionary wedge above the subduction zone during the late

Jurassic to Eocene (about 40 Ma), forming the mélange and intercalated blocks of the Central

Belt. Uplift and accretion of the Central Belt was largely complete by about 40 Ma, as the locus of subduction stepped westward to the Coastal Belt. Although no clear evidence for post-

Eocene (younger than about 40 Ma) activity of the Grogan-Red Mountain-Pine Butte fault zone

4 CA FLAP TRI CR502(1) & 511(1)

or the Mule Ridge fault has been identified, it is possible that post-Eocene or Quaternary displacement has occurred along some of these faults (McLaughlin and others, 2000).

2.2 LOCAL GEOLOGIC AND PHYSICAL SETTING

The local geologic and physical setting for the project is summarized below. Van Duzen Road extends generally northwest-southeast along the bottom and lower slopes of the valley of the

Van Duzen River. Ruth Zenia Road extends northward and eastward in the project area, eastward across a broad valley at the headwaters of the Van Duzen River and then northward up the ridge across the drainage divide to the Mad River Valley and Ruth Lake just east of

Segment 4.

In the project area, the primary bedrock units exposed along Van Duzen and Ruth-Zenia roads include mélange of locally tuffaceous argillite (cm1 on Figure 4) and meta-graywache, meta argillite, and other undivided sandstone and shale deposits of the Yolla Bolly terrane (ybh, ybi, and yb on Figure 4). Localized blocks of chert, basalt, and other rocks occur within the mélange and metagraywache. Quaternary (post-2.6 Ma) fluvial terrace deposits are present along the base and margins of the river valleys, and are dominantly sand, gravel, cobbles and boulders, with some local deposits of fine-grained materials.

Quaternary landslide deposits of varying size from tens of meters up to several kilometers in width and length are mapped within the mélange and meta-graywache bedrock (Figure 4).

Numerous active landslides were observed on natural and modified slopes and in road embankments throughout the project area. There are landslides mapped along several of the proposed roadway improvement areas. In most cases, the landslides are potentially large.

Although we were generally unable to identify specific evidence of active sliding during the field investigation performed for this study, future sliding and corresponding distress to adjacent roadways, retaining walls, culverts, etc., is possible. The roadway improvements planned are not substantial enough to mitigate the occurrence of these generally large landslides.

Further details of the setting and conditions of each road segment are provided in Section 4.

2.3 SEISMIC SETTING

The project site in southern Trinity County lies with the transition zone between the transform plate boundary in California, and the subduction-dominated plate boundary in northernmost

California, Oregon, Washington, and Vancouver, B.C. As this region lies along the active plate boundaries between the North American, Pacific, and Gorda plates, there are many active faults that may generate large earthquakes. Earthquakes on various faults across the region may cause strong ground shaking across the project area, with potential for seismic geologic hazards of landsliding and slope failures, liquefaction and related deformation, and other

CA FLAP TRI CR502(1) & 511(1) 5

ground settlement occurring from the ground shaking. This ground deformation that may result in damage to roads and other structures in the project area.

The major sources for large earthquakes that may cause ground shaking and ground deformation in the area are noted below. The San Andreas fault zone extends north-northwest and northwest through the offshore region southwest of the site, with a closest approach of about 36 miles to the onshore extension at Point Arena. The seismic potential of the San

Andreas fault is well known, with the 1906 M7.9 earthquake having ruptured over a distance of about 280 miles from San Juan Bautista to Shelter Cove, and possibly further offshore.

The northern end of the northwest-trending Bartlett Springs fault zone lies about 3 miles south of the project area. The Bartlett Springs fault zone is one of the major north-northwest trending right-lateral strike slip fault zones that accommodate part of transform motion between the

North American and Pacific Plates in northwestern California. The Little Salmon and Mad

River fault zones lie northwest and north-northwest of the project site, and are reverse faults that accommodate compressional motion resulting from subduction of the Gorda Plate beneath the North American Plate north of Cape Mendocino. These faults are classified as

Holocene active by the California Geological Survey (Jennings and Bryant, 2010).

Other potentially active faults near the project site include the Garberville-Briceland fault zone and the Eatons Rough (Mule Creek) fault. These faults lie southwest and west of the project site, respectively. All of these faults are included as potential seismic sources in the most recent assessment of fault sources that may cause earthquakes in California (Uniform

California Earthquake Rupture Forecast [UCERF 3]; Field and others, 2013).

The Eatons Rough Fault (including the Mule Ridge fault) is of particular concern because it passes directly west of Van Duzen and Ruth-Zenia Roads (Figure 4). As noted in Section 2.1, no clear evidence for late Quaternary displacement has been identified on the Mule Ridge fault, but as it extends between the Holocene active Bartlett Springs and Mad River fault zones, it is the likely structure by which slip on the Bartlett Springs fault zone is transferred to the Mad River fault zone. As the Mule Ridge fault passes close to portions of Van Duzen and

Ruth-Zenia Roads, rupture of this fault may result is significant ground shaking and ground deformation within the project area.

Other types of earthquake sources include the Mendocino Fault Zone (MFZ), which extends westward from offshore of Cape Mendocino, and the Cascadia Subduction Zone (CSZ), which extends from offshore of northern California to offshore of Vancouver, British Columbia.

Transform earthquakes of up to M7.2 have occurred offshore on the MFZ, and plate interface earthquakes of up to M9 have occurred along the CSZ. Earthquakes of up to M7 or larger also may occur on offshore faults in the Gorda Plate north of the MFZ.

6 CA FLAP TRI CR502(1) & 511(1)

Projects Segments 1, 2 and 3 generally lie 1.7 miles west of the Pine Butte fault and 1.7 miles east of the Mule Ridge fault zone. Segment 4 is as close as approximately 400 feet from the

Pine Butte fault. Segment 5 is as close as approximately 700 feet from the Mule Ridge fault zone. No known faults cross the project segments (Figure 4).

2.4 CLIMATIC SETTING

The National Weather Service has had a cooperative weather station in Weaverville since

1894. Based on those records, average January temperatures are a maximum of 47.2 °F and a minimum of 27.4 °F. July temperatures are a maximum of 94.1 °F and a minimum of 49.1 °F.

There are an average of 77.3 days with highs of 90 °F (32 °C) or higher an average of 126.8 days with lows of 32 °F (0 °C) or lower. The record high temperature was 116 °F on August 4, 1932, and the record low temperature was -10 °F on December 9, 1972. Average annual precipitation is 35.28 inches. There are an average of 83 days annually with measurable precipitation. The wettest year was 1970 with 53.77 inches and the dryest year was 1932 with

20.19 inches. The most precipitation in one month was 20.86 inches in December 2005. The most precipitation in 24 hours was 5.50 inches on January 4, 1982. The area may experience snow coverage and freezing temperatures during the winter season. Average annual snowfall is 22.5 inches. The most snowfall in one month was 75.3 inches in January 1950. Frost occurs commonly in December through February, less often in March or November.

3.0 EXISTING GEOTEHNICAL INFORMATION

Representatives of Amec Foster Wheeler attempted to identify existing geotechnical information relevant to the project segments (Van Duzen Road and Ruth-Zenia Road). No existing geotechnical information was obtained from Client or Trinity County.

4.0 EXPLORATION PROGRAM

Amec Foster Wheeler performed an exploration and testing program to support design and construction of proposed improvements to Van Duzen Road and Ruth-Zenia Road. The exploration and testing program was designed to develop an understanding of the geologic setting and geotechnical conditions at the various locations of interest as identified in the

Scoping Trip Report prepared for FHWA CFLHD, prepared by Amec Foster Wheeler, dated

July 1, 2015. The exploration program included site reconnaissance, geotechnical explorations

(soil borings) and geotechnical laboratory testing. The results of the exploration and testing program are discussed below.

4.1 FIELD RECONNAISSANCE

Initial reconnaissance was completed by Amec Foster Wheeler and documented as part of the

Scoping Trip Report, which documents initial findings and potential areas requiring improvements. On May 27 and 28, 2016, Donald Wells, CEG, senior engineering geologist with Amec Foster Wheeler, performed limited reconnaissance, including layout of possible soil

CA FLAP TRI CR502(1) & 511(1) 7

boring investigation locations for planning, including utility notifications. Observations from the reconnaissance are discussed below.

4.1.1 Segment 1 – Van Duzen Road

Segment 1 travels along a narrow bridge on an embankment extending over a creek that drains eastward to the Van Duzen River. The road has a relatively sharp bend (~270 degrees) across this small valley, passing southwest downslope toward the creek and southeast upslope on the opposite side of the small valley. Mapping by McLaughlin and others (2000) shows that the bedrock in this area is Yolla Bolly graywache (ybh), and that the hillslope and ridge area north and northwest of the bridge is a large landslide complex extending over an area of about a square mile (Figure 4). Fluvial terrace deposits are present along the creek downstream of the bridge, but the creek is incised to bedrock at the bridge.

From the north, Van Duzen Road extends southwestward down a cut-slope across the north valley margin, transitioning to embankment fill over the creek, and extending southeast along a cut slope on the south valley margin. The embankment appears to be composed of boulder and rock fill up to about 3 feet in diameter, and the creek flows through a corrugated steel culvert passing through the bottom of the embankment. Fractured graywache bedrock was present along in the cut slope on the south valley margin and probable massive landslide deposits were present in the cut-slope on the north valley margin. These deposits contain sand, gravel, cobbles and boulders, and appear to be the landslide (Qls) mapped north and west of the bridge (Figure 4). The materials forming the cut-slope appear to be stable and we did not observe any evidence for recent slope deformation along the hillslope.

The road surface is rough and damaged on the north side of the bridge, but the damage appears related to traffic wear and settlement of the road subgrade rather than to any potential slope movement. Although we did not observe any evidence for recent slope movements in the landslide deposits, the cut-slope area north of the bridge may be susceptible to slope movement in the future. The rock fill embankment over the creek is covered by heavy grass, but appears to be stable with no obvious signs of settlement or slope deformation.

4.1.2 Segment 2 – Van Duzen Road

Segment 2 includes a short reach of Van Duzen Road that extends along the lower portion of the eastern slope above the valley bottom. The road is cut into the front edge of older fluvial terrace deposits along the lower part of the hillslope. This older fluvial terrace is the highest of at least three fluvial terraces exposed along the Van Duzen River. At the northern end of the segment, the terrace is very narrow, and the road is cut through the back edge of the terrace, with a limited amount of fluvial deposits overlying bedrock. The terrace deposits consist of a mixture of sand, gravel, and cobbles. The slope above the road is underlain by graywache

(ybh) and mélange (cm1) bedrock extending to the ridgeline (Figure 4). The terrace is wider to

8 CA FLAP TRI CR502(1) & 511(1)

the south, and the front edge of the terrace, where the road passes, is located far from the bedrock extending upslope from the back edge of the terrace.

The front edge of the older terrace extends about 6 to 10 feet down from the road to a younger terrace (Qal), and about 10 to 15 feet above to the surface of the older terrace above the road.

There may be some fill along parts of the west half of the road. The lower slope is covered by brush and thick grasses. At the south end of the segment, the cut above the road is very steep to subvertical, is about 3 to 10 feet high; and shows a continuous exposure of well-consolidated sand and gravel with some cobbles. The road climbs slightly up the front of the older terrace to the south, such that the road is near the surface of the terrace at the south end of this segment. Although very steep to subvertical, this cut slope in the front edge of the terrace appears quite stable, with minor raveling of materials.

4.1.3 Segment 3 – Van Duzen Road

Segment 3 extends for 3 miles along the bottom of the Van Duzen River valley and along the bottom of an abandoned valley east of the current location of the Van Duzen River (Figure 2).

Most of this segment of the road lies along the youngest fluvial terrace/modern floodplain

(Qal), with the southern-most reach of this segment passing up a low ridge underlain by meta-graywache (ybh), and in a few places, underlain by older fluvial terrace deposits (Figure 4).

The young fluvial terrace deposits include unconsolidated sand, gravel, and cobbles deposited along the floodplain; the older terrace deposits are of similar composition, but are well consolidated and indurated.

The road passes along the surface of the youngest floodplain, however, some fill is present in areas where former small creeks have been placed in culverts to pass under the road, and in some places, the road lies on a low embankment constructed along the floodplain. Cut-slopes were present only in a few locations along hillslopes, and exposed older terrace deposits or meta-graywache. Some damage was observed to the road surface, but it appeared related to settlement of the road subgrade and traffic wear. Although we did not observe any significant slope failures, there are steep slopes adjacent to and below limited portions of this road segment, and existing slides are difficult to identify because of the heavy vegetation along the slopes.

4.1.4 Segment 4 – Ruth-Zenia Road

Segment 4 extends along the upper portion of the slope on the east side of Hettenshaw Valley at the headwaters of the Van Duzen River. Ruth-Zenia Road extends east and northeast up the west side of the ridge, and continues over the ridge and down to Ruth Lake (Figure 2). The hillslope is characterized by broad areas of swales and hummocky topography that may represent individual or contiguous large, shallow landslide complexes. Alternatively, the

CA FLAP TRI CR502(1) & 511(1) 9

irregular topography may result from differential erosion of varying bedrock materials within the mélange that underlies the west-facing slopes above Hettenshaw Valley.

Much of Ruth-Zenia Road along this slope is on a cut-fill bench along the hillside, with a moderate to gentle slope (1V:1H to 1V:2H) extending 5 to 20 feet vertically above and below the road. The materials along the hillslope are only exposed in few small cuts, and consisted of clayey-sandy soil with some gravel. These materials likely represent mixed mélange and or shallow landslide deposits of mélange materials. The road is damaged and depressed in a number of areas, apparently due to settlement of the wedge of fill along the outer (downslope) half of road. We did not observe any obvious signs of deformation or sliding of the slopes above or below the road, but these slopes are covered by high grass, and any deformation would be distributed and difficult to identify.

4.1.5 Segment 5 – Ruth-Zenia Road

Segment 5 is a short reach of road extending along a extended cut-fill pad on a steep south-facing slope (Figure 2). This steep slope is underlain by metagrawache (ybh) (Figure 4), and the slope has been extensively modified by construction and repair activities. At the eastern part of this segment, the fill pad is slightly wider than the road, and the underlying wedge of fill extends about 50 feet to 100 feet downslope from the road to Bar Creek. The cut-slope extends about 50 to 100 feet above the road, and the natural slope extends higher to the ridgetop. The cut slope above the road has been modified (benched with drains installed) to control drainage and to mitigate potential slope failures. There were no obvious signs of instability or landslides along this slope above the road. Both the upper and lower slopes along the road are steep, on the order of 1V:1H.

There are several culverts that carry runoff from the base of the cut-slope under the road and down the face of the fill wedge towards the creek. Two of these culverts are broken near the top of the fill wedge, with subsequent rapid erosion of, and incision into, the face of fill wedge by water spilling from the broken culverts. Recent repairs in this area include replacement of one damaged culvert, but it appears that at least one additional culvert has not been repaired and that rapid erosion of the fill wedge may occur in the area.

There is one large area of subsidence along the outer lane of the road, perhaps as much as two feet over one hundred feet laterally along the eastern segment of the road. There was no obvious deformation of the face of the fill wedge below the area of subsidence, but the slope is covered with high grass such that it is unclear whether there is any movement of the fill wedge below the road. This subsidence appears to result from deformation of the fill wedge below the road. Smaller amounts of subsidence, as shown by cracking along the outer lane of the road, occur in several other areas along the eastern part of Segment 5. This deformation also appears to result from subsidence of the road subgrade or underlying fill wedge.

10 CA FLAP TRI CR502(1) & 511(1)

The western part of Segment 5 is located on a wide cut-fill pad that extends up to 80 south from the road, and about 30 to 80 feet down to the base of the fill wedge. The cut-slope is 20 to 50 feet high along the western part of Segment 5. There is ongoing slumping along the face of the cut-slope in this area, resulting in debris collecting along the edge of the road at the base of the cut slope. The sloughing of the face of the cut slope appears to extend to the top of the cut slope.

4.2 SUBSURFACE EXPLORATION PROGRAM

Subsurface explorations conducted for this study consisted of 11 soil borings. Drilling was performed between June 6 and June 8, 2016 by Taber Drilling, of West Sacramento, California. Borings were advanced to depths indicated below using a CME-55 truck rig and hollow stem auger methods. Boring locations and sample depths were selected in consideration of the proposed roadway improvements. Locations of the borings are presented in Figure 2, and Figures 3a through 3d.

The date drilled and total depth for each boring is summarized in Table 4.2.1 below.

Table 4.2.1

Summary of Drilled Dates and Depths

Segment Date Drilled Boring Total Depth

(ft)

Segment 1 6/6/16 S1-1 35.8

Segment 2 6/6/16

S2-1 14.2

S2-2 15.3

6/7/16 S2-3 16.3

Segment 3 6/7/16

S3-1 6.5

S3-2 6.5

S3-3 6.5

Segment 4 6/7/16 S4-1 11.5

S4-2 11.5

Segment 5 6/8/16 S5-1 26.5

S5-2 11.5

In general, in-situ testing was performed and representative samples were collected for every

5 vertical feet. The locations of the soil borings are shown on Figures 3a, 3b, 3c, and 3d.

Samples were generally collected using Modified California Drive Samplers (inside diameter

(I.D.) of 2.375, outside diameter (O.D.) of 3.0 inches). Bulk samples were obtained from the upper 5 feet in all borings.

The results of the field investigation program including the logs of the borings (LOTB) are presented in Appendix A. Note that soils and soil descriptions are according to the uniform soil classification system (USCS). Sample locations and types are indicated on the boring logs

CA FLAP TRI CR502(1) & 511(1) 11

along with sample number, an indication of the laboratory testing completed on the sample, and in many cases, the corresponding laboratory result.

4.3 LABORATORY TESTING

Samples were generally collected at specific areas of interest. Geotechnical laboratory tests were performed on selected samples obtained from the borings to aid in the classification of the soils and to evaluate the pertinent engineering properties of the subsurface soils. The following tests were performed in general accordance with standards of the American

Association of State Highway and Transportation Officials (AASHTO):

Moisture Content (AASHTO T265)

Sieve Analysis (AASHTO T11&T27)

Liquid Limit Test (AASHTO T89)

Plastic Limit Test (AASHTO T90)

Minimum Resistivity (AASHTO T288) pH (AASHTO T289)

Chloride (AASHTO T291)

Sulfate Tests (AASHTO T290)

Shrink-Swell /Expansion Pressure (AASHTO T258)

R-Value (AASHTO T190)

Direct Shear (AASHTO T236)

Triaxial UU Testing (AASHTO T296)

Details of the geotechnical laboratory testing program, and test results, are presented in

Appendix B.

5.0 SUBSURFACE CONDITIONS

Subsurface conditions were characterized based on the results of the field and laboratory testing discussed above.

5.1 MATERIALS

The existing asphalt was observed to range from 2 to 5 inches thick at the explorations performed for this study, with the asphalt condition ranging from poor to fair. Detailed observations of the existing asphalt are discussed in Section 7.4. Beneath the asphalt colluvial and alluvial deposits and fill underlie most of the road alignments, which mostly consisted of clayey sand with gravel, clayey gravel with sand, or lean clay with sand. These deposits are likely to have a low permeability (i.e. poorly draining). No aggregate base materials were encountered below the asphalt.

12 CA FLAP TRI CR502(1) & 511(1)

5.2 GROUNDWATER CONDITIONS AND EVIDENCE OF SPRINGS

Groundwater was not encountered during drilling of any of the soil borings performed for this study. We note that seasonal changes, and freezing of the subgrade may result in perched water at shallower depths for portions of the year, and drainage is recommended for road surfaces to reduce the risk of frost heave and roadway deterioration due to inadequate drainage.

In addition some locations are near active streams or seepage features. Specifically at S5-1, a spring was noted on the hillside uphill of the road; and at S3-1 a nearby streambed, dry at the time of drilling, was noted to be located near the road with a bottom channel elevation about 4 to 6 feet below the paved surface of the road. Note that borings were completed in the early summer, and that we anticipate higher water tables and additional seepage features at other times of year.

6.0 SITE GEOTECHNICAL AND GEOLOGIC HAZARDS

6.1 LANDSLIDES

As discussed in Section 2, there are landslides mapped along several of the proposed roadway improvement areas. In most cases, the landslides are potentially large. Although we were generally unable to identify specific evidence of active sliding during the field investigation performed for this study, future sliding and corresponding distress to adjacent roadways, retaining walls, culverts, etc., is possible. The roadway improvements planned are not substantial enough to mitigate the occurrence of these generally large landslides.

Temporary and permanent excavations into existing hillsides may be required for the construction of the proposed roadway improvements. Structural orientations must be considered in determining the slope of proposed cuts. In general, temporary slope inclinations shall not be excavated steeper than the dip of the foliations of the rock. When present, unfavorable jointing and fracturing could also adversely affect the stability of cut slopes in rock.

A qualified engineering geologist shall observe all rock cut slopes during construction to evaluate the potential influence of faults, fractures, bedding, foliation, etc., on slope stability.

The stability of existing slopes was not evaluated as part of this study. No new permanent slopes are currently planned. If the project is modified to include new cut slopes, the stability of those cuts will need to be evaluated.

6.2 SEISMIC CONSIDERATIONS

Faults and seismicity of the region are discussed in Section 2.3. The site(s) of the proposed road improvements are expected to be subject to strong ground shaking.

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6.2.1 Seismic Design Parameters

Based on Seismic Design Maps from AASHTO Guide Specifications for LRFD Seismic Bridge

Design (2009), the following design parameters were determined.

Table 6.2.1

Summary of Seismic Design Parameters

Location Site

Classification

PGA

(g)

Ss (g)

S1 (g)

Segment 1 D 0.430 1.000 0.444

Segment 2 D 0.445 1.026 0.457

Segment 3 D 0.457 1.047 0.469

Segment 4 D 0.425 0.981 0.434

Segment 5 D 0.509 1..141 0.515

6.2.2 Liquefaction

Liquefaction is not expected to pose a hazard to the proposed roadway improvements due to the clayey nature of the subsurface materials and lack of groundwater observed in the soils explored.

6.2.3 Seismic Compression of Unsaturated Soils

Based on data collected during the field investigation, the existing embankments consist of generally loose fill. There is visual evidence that the existing embankments have experienced some degree of settlement which has caused depressions and cracking in discrete pavement locations. During future strong ground shaking at the site(s), the existing embankments may experience additional compression, which may be expressed as settlement or distress at the pavement, or retaining wall, surface(s).

6.3 SOIL CORROSIVITY

Laboratory tests were performed on a representative sample from each segment to evaluate the corrosion potential of the existing near surface soils. Laboratory results are presented in

Appendix B.

For underground structure elements and utility lines, a soil is considered to be corrosive if one or more of the following conditions exist for the representative soil and/or water samples taken at the site:

pH is less than 5 or greater than 10;

Chloride concentration is greater than 100 parts per million; and

Sulfate concentration is greater than 200 parts per million or greater.

14 CA FLAP TRI CR502(1) & 511(1)

Table 6.3.1

Summary of Corrosion Results for Structures

Location Boring / Sample Depth pH ≤ 5 or ≥?

Chloride ≥ 100 ppm?

Sulfate ≥ 200 ppm?

Segment 1 (representing 0.2 miles)

S1-1 / 0 to 5 feet no no no

Segment 2 (representing 0.4 miles)

S2-1 / 0 to 5 feet no no no

Segment 3 (representing 3 miles)

S3-1 / 0 to 5 feet no no no

Segment 4 (representing 1.1 miles)

S4-2 / 0 to 5 feet no no no

Segment 5 (representing 0.5 miles)

S5-1 / 0 to 5 feet no no no

For fill materials to be used in the construction of MSE systems, the backfill materials shall conform to these additional considerations include a minimum resistivity value and more stringent chloride restrictions, as noted in the following:

pH is less than 5 or greater than 10

Chloride concentration is greater than 100 parts per million

Sulfate concentration is greater than 200 parts per million or greater

Resistivity is less than 3,000 ohm-cm

Table 6.3.2

Summary of Corrosion Results for MSE Walls

Location Boring /

Sample Depth pH ≤ 5 or

Chloride ≥ 100 ppm?

Sulfate ≥ 200 ppm?

Resistivity ≤ 3,000 ohm-cm

Segment 1 (representing 0.2 miles)

S1-1 / 0 to 5 feet no no no no

Segment 2 (representing 0.4 miles)

S2-1 / 0 to 5 feet no no no no

Based on the combined results of resistivity, pH, chloride, and sulfate measurements on near-surface samples, the soils are assumed to be non-corrosive in project segments 1 and 2 where MSE walls are planned. Based on these results, no protective coating for metallic reinforcements are required.

The observations of existing subgrade materials are based on relatively small samples (about

0.5 ft3 each) recovered at the discrete soil boring locations. Actual subgrade soils encountered during construction of the road improvements may be different than what is indicated above. In

CA FLAP TRI CR502(1) & 511(1) 15

order to verify conformity to project requirements, periodic corrosion testing of blended on-site materials is recommended. Approval for any imported fill materials will require an evaluation of corrosion potential prior to use on-site.

7.0 RECOMMENDATIONS

Section 7 presents recommendations to support design and construction of new retaining walls, general earthwork, new utilities, and new pavements anticipated for the new roadway improvements.

Design and construction will generally conform to latest edition of FHWA Standard

Specifications for Construction of Roads and Bridges on Federal Highway Projects, 2014 including applicable Standard Drawings and CFLHD Drawings.

7.1 RECOMMENDATIONS FOR EARTH RETAINING STRUCTURES

There are discrete areas where retaining structures on the order of 12 feet high, or less, are planned to support road widening and/or realignment. MSE walls are an appropriate solution.

During discussions with the Designers, they determined that a Reinforced Soil Slope (RSS) or cutting into existing hillslopes to avoid the need for a MSE wall would have an adverse effect to the environmental conditions that exist in these locations.

7.1.1 MSE Walls

MSE walls are usually provided as a proprietary system that is designed by the supplier on a design-build basis, with the internal stability and overturning of the wall being evaluated by the supplier. The external stability (including bearing capacity, global stability, and settlement) is normally evaluated by the geotechnical engineer. The design will be performed in accordance with the AASHTO LRFD Section 10.11 and FHWA-NHI-00-043.

The MSE walls will have a minimum embedment below the lowest adjacent finished grade meeting the requirements shown in Section 11.10.2.2 of AASHTO LRFD Bridge Design

Specifications (2011b). In general, the MSE embedment will extend below the lowest adjacent grade by at least 0.1 times the maximum height of the wall or 2 feet, whichever is greater. Wall base and leveling pads will be founded in competent native soils or structural backfill compacted to 95% of its maximum dry density per AASHTO T180. To ensure a uniform subgrade, a minimum of 8-inches below the proposed leveling pads and wall base is recommended to be scarified and re-compacted to 95% relative compaction if the wall is founded on native soils. We recommend a minimum horizontal berm width of 4 feet in front of

MSE walls founded on slopes (if applicable; a drainage ditch adjacent to the toe of the MSE wall is considered a slope). Where the width of the berm will be less than 4 feet, the top of the leveling course will be lowered to 1 foot below the elevation of the bottom of the adjacent drainage ditch. In order to minimize differential settlement and excessive outward tilting of

16 CA FLAP TRI CR502(1) & 511(1)

walls, walls shall be proportioned so that the base pressure on soil under the footing is as nearly uniform (within 10 percent) as practical under the long term loading.

The overlying unsaturated soils may also undergo some minor amount of densification and settlement due to the ground motions. The overall settlement of unsaturated layers could be on the order of 1 to 3 inches. The distribution of settlement is difficult to predict, as is the amount of differential settlement.

7.1.1.1 Bearing Capacity

The allowable bearing pressure for the foundations will be controlled by either the bearing capacity of the soil or the estimated settlement for the service loads. Allowable bearing pressures for the foundations are presented below.

The walls will have a minimum embedment depth (Df) of 2 feet below the lowest adjacent grade. Effective widths for MSE embankments will be calculated as a fraction of the width of the reinforced material or footing as determined by the approach described in AASHTO BDS

Article 10.6.1.3 to account for eccentricity caused by horizontal earth pressures acting on the back side of the reinforced material. The horizontal to vertical force ratio (H/V), comprises the horizontal earth pressure forces due to surcharge and backfill divided by the vertical forces due to surcharge and retained soil mass weight. If the H/V ratio is larger than 0.54 a shear key will be required for the retaining wall. Nominal bearing capacities listed below may be assumed by the MSE wall designer. Bearing capacity values can be linearly interpolated between the given Df and effective width values.

Table 7.1.1

Recommended Bearing Capacity for MSE Walls Strength Limit State

Effective Width (ft)

Bearing Capacity for MSE Walls Strength Limit State (psf)

Df = 2 ft Df = 4 ft Nominal Nominal

4 8,600 14,250 6 10,350 15,600 8 12,200 17,200 12 15,900 20,700

CA FLAP TRI CR502(1) & 511(1) 17

The following resistance factor (“φ”), taken from AASHTO LRFD BDS with Caltrans

Amendments (Table 11.5.7-1 for MSE walls) are recommended for calculating the factored resistances for the strength limit state:

Bearing Resistance Factor for MSE Walls: φ = 0.65

The values in the table below are recommended for the nominal bearing capacity for the service limit state or other settlement based evaluations of bearing capacity, so as to limit the immediate local settlement of the walls, relative to the surrounding fill, to less than the listed allowable settlement values.

Table 7.1.2

Recommended Bearing Capacity for MSE Walls Settlement

Allowable Immediate Settlement Beneath Foundation (inches)

Bearing Capacity for MSE Walls based on Settlement (psf)

Df = 2 ft Nominal

1 1,500 2 3,000 3 4,500

For the service and extreme limit states a resistance factor of 1.0 is recommended for all foundations.

7.1.1.2 Lateral Earth Pressures

For preliminary evaluation of loading from the fill, we recommend the following parameters be used:

Select granular backfill: 34 degrees

Active earth pressure against retaining wall or back of reinforced fill with level backfill: 37 pounds per cubic foot

Adequate drainage is recommended to relieve hydrostatic forces from building up behind the walls. We recommend that retaining walls be designed for a maximum horizontal deformation of 1/2-inch.

We recommend a coefficient of active lateral pressure of 0.30 for any surcharge earth pressures acting against the wall.

18 CA FLAP TRI CR502(1) & 511(1)

7.1.1.3 Lateral Sliding and Overturning Resistance

Resistance to lateral loads for the MSE walls will come from frictional resistance between the bottom of the fill and the underlying soils using an ultimate friction coefficient of 0.6 with a resistance factor of 1.0 for the strength, service and extreme limit states.

We evaluated the sliding and overturning resistance of the proposed MSE section at critical locations in Segment 1 and Segment 2. Locations that were thought to be critical (i.e., tallest sections, steepest slopes, and/or lowest foundation strengths) along the length of the proposed MSE walls were evaluated. Engineering properties for materials used in the analyses is based on field and laboratory data from the closest geotechnical borings from this study, and where necessary, were supplemented with our experience modeling similar soils. A surcharge was applied in the static and seismic slope stability analysis to account for traffic loading.

The computed static factor of safety against sliding and overturning for Segment 1 and 2 MSE walls exceeds 2.0, which is acceptable. The factor of safety is likely higher in areas where the height of the MSE wall is lower than 12 feet as modeled at Station 100+03 or lower than 10 feet as modeled at Station 200+85.

The computed seismic factor of safety against sliding exceeds 1.1 and seismic factor of safety against overturning exceeds 1.5 for an 8 feet wide MSE wall only when the height of the wall is

5 feet or less. To achieve a computed seismic factor of safety against sliding exceeding 1.1 and seismic factor of safety against overturning exceeding 1.5 for a 12 feet high MSE wall, the corresponding width of the MSE wall needs to be about 17 feet. To achieve a computed seismic factor of safety against sliding exceeding 1.1 and seismic factor of safety against overturning exceeding 1.5 for a 15 feet high MSE wall, the corresponding width of the MSE wall needs to be about 20 feet.

For MSE walls higher than 5 feet, the width needs to extend beyond 8 feet, or additional measures will be required to resist lateral forces. Additional measures could include a shear key or CIDH foundations.

7.1.1.4 Global Stability

We evaluated the global stability of the proposed MSE section at critical locations in Segment

1 and Segment 2. Locations that were thought to be critical (i.e., tallest sections, steepest slopes, and/or lowest foundation strengths) along the length of the proposed MSE walls were evaluated. Stability evaluations were performed using the slope stability program Slope/W

(Geostudio, 2012). Topography for the analyses was based on survey data in the vicinity of the section as indicated in sheets C1 and C2 of the project plans dated December 16, 2016.

Slopes were extrapolated as necessary to complete the models. Stratigraphy for the analyses was based on the closest geotechnical borings from this study and may be approximate.

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Engineering properties for materials used in the analyses is based on field and laboratory data from the closest geotechnical borings from this study, and where necessary, were supplemented with our experience modeling similar soils. A surcharge was applied in the static and seismic slope stability analysis to account for traffic loading.

The static slope stability results for MSE walls are summarized below, and printouts are included in Appendix C as Figures C-2, C-5, C-8, and C-11.

Table 7.1.1.4.1

Summary of Static Slope Stability Analyses

Location Static Factor of Safety before MSE Static Factor of Safety with MSE

Segment 1 (Approximate Station 103+00)

1.48 Fig C-2

1.39 Fig C-5

Segment 2 (Approximate Station 200+85)

1.54 Fig C-8

1.46 Fig C-11

In both segments, adding the MSE wall and additional road width, slightly reduced the global static factor of safety. In both cases, the critical failure surface passes below the MSE reinforcement zone and the reduced factor of safety corresponds directly with the increased mass of the MSE wall and width of the road prism.

The static factor of safety for Segment 2 (with MSE wall) is about 1.5, which is acceptable. The factor of safety is likely higher is areas where the height of the MSE wall is lower than 10 feet as modeled at Station 200+85. The static factor of safety for Segment 1 (with MSE wall) is about 1.4, which is slightly lower than the recommended value, 1.5. The factor of safety is likely higher is areas where the height of the MSE wall is lower than 12 feet as modeled at

Station 100+03.

The seismic slope stability results for MSE walls are summarized below, and printouts are included in Appendix C as Figures C-3, C-6, C-9, and C-12. The applied seismic coefficient, kh, was 0.43 for Segment 1 and 0.45 for Segment 2.

20 CA FLAP TRI CR502(1) & 511(1)

Table 7.1.1.4.2

Summary of Seismic Slope Stability Analyses

Location

Seismic Factor of Safety before MSE

Seismic Factor of Safety with MSE

Segment 1 (Approximate Station 103+00)

1.18 Fig C-3

1.13 Fig C-6

Segment 2 (Approximate Station 200+85)

1.53 Fig C-9

1.46 Fig C-12

The seismic factor of safety for Segment 2 (with MSE wall) is significantly higher than 1.0, which is acceptable. The factor of safety is likely higher is areas where the height of the MSE wall is lower than 10 feet as modeled at Station 200+85. The seismic factor of safety for

Segment 1 (with MSE wall) is slightly higher than 1.1. The factor of safety is likely higher is areas where the height of the MSE wall is lower than 12 feet as modeled at Station 100+03.

7.2 GENERAL EARTHWORK

Grading at the site will consist of excavating for new retaining wall foundations, excavating for placement of utility lines, excavating existing materials from high areas at the site and placing fill in low areas, and preparing the pavement subgrade. Earthwork will generally conform to

Division 200 of the latest edition of FHWA Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, 2014, including applicable Standard Drawings and

CFLHD Detail Drawings.

7.2.1 Permanent Cut Slopes

Based on the current project drawings, dated December 16, 2016, there are no permanent cuts slopes planned for the project. The permanent cut slope, previously identified in the

Scoping Report at Segment 2, has been eliminated. If this or other permanent cut slopes are added to the project, we recommend that Amec Foster Wheeler evaluate the stability of the proposed cut slope with respect to existing and proposed topography and corresponding interpreted stratigraphy.

7.2.2 Fills and Backfills

Fill and backfill shall conform to the requirements in FHWA Standard Specifications for

Construction of Roads and Bridges on Federal Highway Projects, 2014, including applicable

Standard Drawings and CFLHD Detail Drawings.

Based on the soil borings, the exposed subgrade soil is expected to be firm. Observations of existing pavement by Designers suggest the following areas of potentially soft subgrade:

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