rfb_13199_for_info_only_6-geotechnical_design_report_keys_creek.pdf

GEOTECHNICAL DESIGN REPORT

KEYS CREEK TRIBUTARY CROSSING

COLE GRADE ROAD

VALLEY CENTER, CALIFORNIA

ORACLE PROJECT NO. 1003046

PREPARED FOR:

County of San Diego

Department of Public Works, Materials Lab 5252 Kearny Villa Way

San Diego, California 92123

PREPARED BY:

Ninyo & Moore

Geotechnical and Environmental Sciences Consultants 5710 Ruffin Road

San Diego, California 92123

May 8, 2015 Project No. 106898010

May 8, 2015 Project No. 106898010

Mr. E. David Gasaway County of San Diego Department of Public Works, Materials Lab 5252 Kearny Villa Way San Diego, California 92123

Subject: Geotechnical Design Report Keys Creek Tributary Crossing Cole Grade Road Valley Center, California

Oracle Project No. 1003046

Dear Mr. Gasaway:

In accordance with your request and authorization, we have prepared a Geotechnical Design Re-port for the proposed Cole Grade Road overcrossing at Key Creek in Valley Center, California.

This report summarizes our findings and conclusions regarding the geologic and geotechnical conditions at the site and provides recommendations for the proposed project.

As an integral part of our role as the geotechnical engineer-of-record, we request the opportunity to review the construction plans before they go to bid and to provide follow-up construction ob-servation and testing services.

We appreciate the opportunity to be of service on this project.

Respectfully submitted, NINYO & MOORE

William Morrison, PE, GE Senior Engineer

Ronald D. Hallum, PG, CEG Chief Geologist

LLB/MAC/WRM/RDH/

Distribution: (1) Addressee

Cole Grade Road May 8, 2015 Valley Center, California Project No. 106898010

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TABLE OF CONTENTS

Page

1. INTRODUCTION

2. SCOPE OF SERVICES

3. PERTINENT REPORTS AND EVALUATIONS

4. EXISTING FACILITIES AND PROPOSED IMPROVEMENTS

4.1. Cole Grade Road

4.2. Keys Creek Tributary Crossing

4.3. Existing Developments

4.4. Proposed Bridge Improvements

5. PHYSICAL SETTING

5.1. Climatic Conditions

5.2. Topography and Drainage

5.3. Man-Made and Natural Features of Engineering and Construction Significance

5.4. Regional Geology

6. GEOTECHNICAL EXPLORATION

6.1. Drilling, Coring and Sampling

6.2. Exploration Notes

7. GEOTECHNICAL TESTING

7.1. In-Situ Testing

7.2. Laboratory Testing

8. GEOTECHNICAL CONDITIONS

8.1. Site Geology

8.1.1. Lithology

8.1.2. Existing Slope Stability

8.2. Subsurface Soil and Geologic Conditions

8.2.1. Pavement Section

8.2.2. Fill

8.2.3. Alluvium

8.2.4. Granitic Rock

8.3. Water

8.3.1. Surface Water

8.3.2. Groundwater

8.4. Faulting and Site Seismicity

8.4.1. Shaking and Strong Ground Motion

8.4.2. Surface Fault Rupture

8.4.3. Liquefaction and Seismically Induced Settlement

8.4.4. Lateral Spreading

8.4.5. Slope Stability

8.4.6. Tsunamis and Seiches

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9. GEOTECHNICAL ANALYSIS AND DESIGN

9.1. Earthwork

9.1.1. Rippability

9.1.2. EMBANKMENTS

9.1.3. Grading Factors

9.2. Slope Stability

9.3. Drainage

9.4. Corrosion

9.5. Structure Foundations

9.6. Pavement Structural Section Design

9.6.1. Design R-Value

9.6.2. Design Recommendations

10. MATERIAL SOURCES

10.1. Off-Site Material

11. MATERIAL DISPOSAL

12. CONSTRUCTION CONSIDERATIONS

12.1. Construction Advisories

12.2. Construction Considerations that Influence Design

12.3. Construction Considerations that Influence Specifications

12.4. Construction Monitoring

12.5. Hazardous Waste Considerations

12.6. Differing Site Conditions

13. RECOMMENDATIONS AND SPECIFICATIONS

14. CONSTRUCTION OBSERVATION

15. LIMITATIONS

16. REFERENCES

Tables Table 1 – Laboratory Testing Table 2 – Seismic Parameters for Maximum Moment Magnitude Earthquakes Table 3 – Corrosivity Test Results Table 4 – R-value Test Results Table 5 – Pavement Structural Sections

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Figures Figure 1 – Site Location Figure 2 – Boring Location Figure 3 – Regional Geology Figure 4 – Fault Location Figure 5 – Peak Acceleration Contour Figure 6 – Site Acceleration Response Spectrum Curve Figure 7 – Lateral Earth Pressures for Temporary Cantilevered Shoring Figure 8 – Lateral Earth Pressures for Braced Excavation (Granular Soil)

Appendices Appendix A – Log of Test Borings Appendix B – Laboratory Testing Appendix C – Analyses and Calculations Appendix D – As-Built Plans

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1. INTRODUCTION

In accordance with your request and authorization, we have performed a geotechnical evaluation for the proposed Cole Grade Road overcrossing at Keys Creek in Valley Center, California (Fig-ure 1). We previously prepared a geotechnical evaluation and a supplemental geotechnical evaluation within a portion of Cole Grade Road where improvements/widening are planned. The results of the previous geotechnical evaluations are presented in our reports dated October 2, 2012 and June 12, 2014 (Ninyo & Moore, 2012; 2014).

The purpose of our geotechnical evaluation was to evaluate the subsurface soil and geologic conditions underlying the project site, and to provide design recommendations in accordance with California Department of Transportation (Caltrans) guidelines (Caltrans, 2006c) for the pro-posed improvements. This Geotechnical Design Report (GDR) presents our findings, conclusions, and geotechnical recommendations regarding the subject project. A separate Foun-dation Report (FR) will also be submitted in connection with the design and construction of the proposed bridge at the site.

2. SCOPE OF SERVICES

Our report was prepared in general accordance with Caltrans Guidelines for Preparing Geotechnical

Design Reports (Caltrans, 2006c). The scope of our geotechnical services included the following:

Review of our previous reports, published geologic maps, faulting and seismic hazard re-ports, topographic maps, stereoscopic aerial photographs.

Acquisition of County of San Diego, Department of Environmental Health (DEH) boring permit and a County of San Diego encroachment permit.

Preparation of a traffic control plan.

Performing a geologic reconnaissance of the site to mark-out the boring locations and noti-fying Underground Service Alert (USA) for utility clearance.

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Drilling, logging and sampling of five (5) borings to depths up to approximately 50 feet within the proposed footprint of the bridge, using both truck-mounted and track-mounted drilling rigs equipped with hollow stem augers. The lower 5 to 10 feet of four of the borings were cored using NQ rock coring equipment. Due to the highly weathered nature of the rock encountered at the bottom of boring B-4, along with the inability to recover core samples from the other 4 borings (where the rock was observed to be less weathered than in B-4), coring was not performed in this boring. A representative of Ninyo & Moore logged the sub-surface conditions exposed in the borings and collected bulk and relatively undisturbed soil and rock samples for laboratory testing. The borings were backfilled in accordance the with County of San Diego DEH requirements.

Laboratory testing of select soil samples to evaluate the geotechnical properties of the sub-surface material including in-situ moisture content, grain size distribution, Atterberg Limits, consolidation, direct shear, expansion index, corrosivity, and R-value.

Compilation of the field and laboratory data and engineering analysis.

Preparation of this Geotechnical Design Report in general accordance with Caltrans guidelines.

3. PERTINENT REPORTS AND EVALUATIONS

As part of our study, we have reviewed our previously prepared geotechnical report and supple-mental geotechnical report dated October 2, 2012 and June 14, 2014, respectively (Ninyo &

Moore, 2012; 2014), as well as geological and geotechnical publications and documents, and

Caltrans publications, including as-built plans for the Key Creek Tributary overcrossing.

4. EXISTING FACILITIES AND PROPOSED IMPROVEMENTS

4.1. Cole Grade Road

The project site is located along Cole Grade Road at the Keys Creek crossing approximately

500 feet north of the Cool Valley Road intersection in the Valley Center area of San Diego

County, California. Cole Grade Road is a two-lane roadway that extends from State Route

76 south to its terminus at the intersection with Valley Center Road. In the vicinity of the

Keys Creek crossing, Cole Grade Road is oriented roughly north-south. The existing drain-age within Keys Creek generally trends to the west towards Keys Canyon and then flows into the San Luis Rey River.

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4.2. Keys Creek Tributary Crossing

The current crossing at Keys Creek is comprised of a double 8-foot by 8-foot reinforced concrete box culvert. Details regarding the as-built construction of the bridge are present-ed in Appendix D. Embankments up to approximately 15 feet in thickness at present serve as approaches to the north and south of the existing crossing. The east and west sides of the embankments are sloped with estimated inclinations on the order of 1.5:1 (horizontal to vertical). Vegetation at the crossing and its vicinity generally consist of moderate to thick growths of grass, brush, and weeds, with mature trees being situated along the toe of the embankment. The coordinates near the center of the existing bridge are approximately

33.2685 degrees north latitude and approximately -117.0235 degrees west longitude

(Google Earth, 2015).

4.3. Existing Developments

Development in the vicinity of the proposed overcrossing generally consists of agricultural and residential properties.

4.4. Proposed Bridge Improvements

We understand that a new bridge is planned as a part of the proposed widening of an approxi-mately 2.6-mile section of Cole Grade Road that will include center turn lanes and bike lanes with a graded shoulder. Ninyo & Moore has previously evaluated the geotechnical conditions along the proposed widened portion of the road (Ninyo & Moore, 2012; 2014). Based on our conversations with County of San Diego representatives, we understand that the new bridge will be supported by either shallow foundations bearing upon granitic rock or cast-in-drilled-hole

(CIDH) foundations that will be rock-socketed into granitic rock that has been mapped as under-lying the site (Kennedy and Tan, 2005). Given the encountered depth to the granitic bedrock, we consider the use of CIDH foundations to be the more preferred foundation option for bridge support from a geotechnical engineering standpoint. However, our foundation report (Ninyo &

Moore, 2015) includes geotechnical recommendations for shallow foundation design, so that the

County of San Diego can evaluate the feasibility of each alternative foundation system.

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Details regarding the proposed construction of the bridge are not known by Ninyo & Moore at this time. Based on our communications with Mr. Samson Engeda of the County of

San Diego, we understand that the bridge design will utilize a short seat abutment. We also understand that the new bridge will have a greater span than the existing box culverts. The existing approach embankments will be widened to accommodate the widened road section and excavated as needed to accommodate the new bridge deck.

5. PHYSICAL SETTING

The following sections describe the climatic, topographic, drainage, and geological aspects of the project.

5.1. Climatic Conditions

Our review of climatic data for the project area indicates that the annual average temperature is approximately 65 degrees Fahrenheit (IDcide, 2015). The average low temperature is ap-proximately 42 degrees Fahrenheit in December and the average high temperature is approximately 89 degrees Fahrenheit in August. Precipitation data indicate that the mean annual rainfall is approximately 15 inches with the precipitation occurring predominantly during the period of January through March.

5.2. Topography and Drainage

The ground elevation in the vicinity of proposed improvements varies from approximately

1420 feet above mean sea level (MSL), within portions of the Keys Creek channel located to the west of the crossing, to 1434 feet above MSL, within the existing travel lanes of Cole

Grade Road at the crossing. The east and west sides of the existing embankments are sloped with estimated inclinations on the order of 1.5:1 (horizontal to vertical). Vegetation at the crossing and its vicinity generally consist of moderate to thick growths of grass, brush, and weeds, with mature trees being situated along the toe of the embankment. As previously dis-cussed, Keys Creek drains west to Keys Canyon and then flows into the San Luis Rey River.

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5.3. Man-Made and Natural Features of Engineering and Construction Significance

Significant existing man-made and natural features within the project area include the following:

The existing 8 feet by 8 feet reinforced concrete box culvert.

Fill and embankment slopes at and adjacent to the abutments at Keys Creek Crossing.

5.4. Regional Geology

The project is situated in the northern San Diego County section of the Peninsular Ranges Geo-morphic Province. This geomorphic province encompasses an area that extends approximately

900 miles from the Transverse Ranges and the Los Angeles Basin south to the southern tip of

Baja California (Harden, 1998; Norris and Webb, 1990). The province varies in width from ap-proximately 30 to 100 miles. In general, the province consists of rugged mountains underlain by

Jurassic metavolcanic and metasedimentary rocks, and Cretaceous igneous rocks of the southern

California batholith. The portion of the province in San Diego County that includes the project area consists generally of Cretaceous-age Cole Grade tonalite (Figure 3).

The Peninsular Ranges Province is traversed by a group of sub-parallel faults and fault zones trending roughly northwest. Several of these faults are considered active faults. The Elsinore, San Jacinto, and San Andreas faults are active fault systems located east of the project area and the Coronado Bank, San Diego Trough, San Clemente, and Rose Canyon faults are active faults located west of the project area. The Julian segment of the Elsinore Fault, the nearest active fault system, has been mapped approximately 5 miles northeast of the project site (Figure 4). Major tectonic activity associated with these and other faults within this regional tectonic framework consists primarily of right-lateral, strike-slip movement.

6. GEOTECHNICAL EXPLORATION

Our subsurface evaluation in the area of the existing bridge was conducted between Decem-ber 29, 2014 and January 21, 2015. The exploration activities for the proposed bridge included the drilling, sampling, and logging of five exploratory borings. Rock cores were advanced in the

106898010 R Geo Design final.doc 6 lower 5 to 10 feet of the borings. The boring locations and the boring logs are presented of the

Log Of Test Boring (LOTB) sheet (Appendix A).

6.1. Drilling, Coring and Sampling

The exploratory borings were advanced to depths ranging from 26.5 to 40.6 feet below the ground surface using truck-mounted and track-mounted drill rigs equipped with hollow-stem augers. Rock cores were advanced in borings B-1, B-2, B-3, and B-5 to depths ranging from approximately 30 to 50 feet using NQ coring equipment; however, no samples were recov-ered from the cores after several attempts due to the highly weathered nature of the rock within the explored depths. At boring location B-4, granitic rock was encountered at a depth of 30½ feet and extended to the total depth explored. Due to the highly weathered nature of the rock encountered at the bottom of boring B-4, along with the inability to recover core samples from the other 4 borings (where the rock was observed to be less weathered than in

B-4), coring was not attempted in this boring.

A representative of Ninyo and Moore logged the subsurface conditions exposed in the bor-ings and collected Modified California and Standard Penetration Test (SPT) samples for laboratory testing. The Modified California drive sampler had an external diameter of ap-proximately 3 inches and was lined with six, 1-inch-long, thin-walled brass rings with inside diameters of approximately 2½ inches in accordance with the ASTM International (ASTM)

Test Method D 3550. The sampler barrel was driven into the ground with an automatic hammer weighing 140 pounds and falling through a height of 30 inches.

The SPT samples were collected in accordance with ASTM D 1586 using a split-spoon drive sampler with an external diameter of approximately 2 inches. The sampler barrel was driven into the ground with an automatic hammer weighing 140 pounds and falling through a height of 30 inches. The samples were removed from the split-spoon barrel, sealed in plastic bags, and transported to our laboratory for testing.

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6.2. Exploration Notes

The subsurface materials within the project area are anticipated to range from easily excavatable surficial soils to competent granitic bedrock material that may require considerable effort

(i.e., heavy ripping, rock breaking, or blasting) to reach the desired depths of embedment for the foundation elements. Groundwater was encountered in borings B-1 through B-5 at approximate depths of between 14 and 21 feet. The presence of relatively granular fill and alluvial soils and groundwater may cause caving in unsupported excavations or drilled holes.

7. GEOTECHNICAL TESTING

The following sections describe the geotechnical testing that was performed during our subsur-face evaluation of the project area.

7.1. In-Situ Testing

In-situ testing consisted of SPTs performed during the drilling of the hollow-stem auger ex-ploratory borings in accordance with ASTM D 1586. Our testing also included counting of blow counts during the driving of modified California Samplers during our subsurface ex-ploration. The SPT blow counts, the blow counts needed to advance the modified California sampler, and the penetration depths are noted on the LOTB sheets in Appendix A.

7.2. Laboratory Testing

Geotechnical laboratory testing was performed on representative samples to evaluate in-situ moisture content, grain size distribution, Atterberg Limits, consolidation, direct shear, ex-pansion index, corrosivity, and R-value. The results of the moisture content and dry density tests are recorded on the LOTB sheets (Appendix A). The other laboratory test results are presented in Appendix B.

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Table 1 – Laboratory Testing

Number of Tests

Type of Test Test Designation

21 In-situ Moisture Content and Dry Density ASTM D 2937 2 Atterberg Limits ASTM D 4318 1 Expansion Index ASTM D 4829 3 Consolidation Test ASTM D 2435 7 Sieve Analysis ASTM D422 5 Direct Shear ASTM D 3080 2 R-Value CT 301 2 Corrosivity CT 643, CT 417, CT 422

Notes:

ASTM – American Society for Testing and Materials CT – California Test

8. GEOTECHNICAL CONDITIONS

Per the regional geologic mapping (Kennedy and Tan, 2005), the site is dominantly underlain by the Cretaceous-age Cole Grade Tonalite (referred to herein as granitic rock). Surficial units ob-served at the site include fills and alluvium. Generalized descriptions of the earth units encountered during our field mapping and discussions relative to site geology, subsurface soil conditions, groundwater and surface water, faulting, and seismicity are provided below.

8.1. Site Geology

As noted above, the site has been mapped as being dominantly underlain by the Cretaceous-age Cole Grade tonalite. Surficial units at the site include fills and alluvium. Generalized de-scriptions of the earth units encountered during our field mapping and subsurface exploration are provided below. Additional descriptions of the subsurface units are provided on the boring logs in Appendix B.

8.1.1. Lithology

The site has been mapped as being underlain by the Cretaceous-age Cole Grade tonalite.

As encountered, this granitic rock generally consisted of grayish and yellowish brown, moist to wet, weathered coarse-grained granitic rock. In the vicinity of the project site, this granitic rock is mantled by varying thicknesses of alluvium and fill.

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8.1.2. Existing Slope Stability

Although the site is mapped as being generally susceptible to landslides (Tan and

Giffen, 1995), the crossing site is relatively flat-lying and no evidence of landslides were observed during our geologic reconnaissance. The potential for slope instability due to landslides, mudslides, or rockfall is, therefore, not a design consideration for the crossing site.

8.2. Subsurface Soil and Geologic Conditions

The following sections describe subsurface soil and geologic conditions encountered in our exploratory borings.

8.2.1. Pavement Section

Borings B-1 and B-2 were advanced through existing pavements within Cole Grade

Road. The pavement section consisted of approximately 11 to 13 inches of asphalt con-crete. No aggregate base was observed beneath the asphalt concrete in either boring.

8.2.2. Fill

Fill was encountered in borings B-1 and B-2 under the pavement section to depths of about 8 to 11 feet. Fill was encountered in the remaining borings from the ground sur-face to depths of about 9 to 14 feet. As encountered, these fill materials generally consisted of moist, loose to medium dense silty sand and well-graded sand. An apparent oil odor was detected in the upper 4 inches of fill in borings B-1 and B-2. We consider the odor to be related to a tack coat layer that likely was placed as part of previous roadway construction.

8.2.3. Alluvium

Alluvium was encountered in the borings beneath the fill to depths of 15 to 36 feet.

As encountered, the alluvium consisted of very loose to medium dense clayey sand, loose to medium dense poorly-graded sand with silt, and very loose to medium dense silty sand.

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8.2.4. Granitic Rock

As noted, the site is underlain by the Cretaceous-age Cole Grade Tonalite (referred to as granitic rock on our boring logs and herein). Granitic rock was encountered in each of our borings underlying the alluvium and extended to the total depths explored. As en-countered, the granitic rock generally consisted of grayish and yellowish brown, moist to wet, weathered coarse-grained granitic rock.

8.3. Water

The following sections describe the surface water conditions, including surface drainage pat-terns, scour and erosion, as well as the groundwater conditions.

8.3.1. Surface Water

Surface water was generally not observed during our field exploration; however, some areas of stagnant water were observed adjacent to the existing box culvert structure.

Based on our review of documents, surface water is typically directed to drainage sys-tems which outlet outside of the project area.

8.3.1.1. Drainage Patterns

The existing drainage within Keys Creek generally trends to the west and continues westward within Keys Canyon and then flows into the San Luis Rey River.

8.3.1.2. Scour

The proposed improvements are anticipated to handle much of the surface runoff during periods of precipitation. Uncontrolled surface flow is anticipated to be gen-erally limited to landscaped embankment slopes. A hydrology or hydraulics report was not available for our review at the time of our evaluation. Based on the infor-mation obtained during our site evaluation and the nature of the soils encountered, we consider it likely that the channel bottom and embankments could be subject to scour during episodes of flooding and/or high flow. As such, we recommend that

106898010 R Geo Design final.doc 11 consideration be given to the installation of measures to protect the channel bottom and embankment against scour. Such measures can include the placement of riprap over geotextile, articulating concrete block, or grout-filled mats along the proposed embankments, or full-channel armor across the bridge opening (if the hydrolo-gy/hydraulics report call for such measures).

8.3.1.3. Erosion

The fill and alluvial soils encountered on site can be considered erodible, although erodibility is not anticipated to be a constraint to project development, provided runoff is adequately controlled. The granitic bedrock encountered is considered to be generally resistant to erosion.

8.3.2. Groundwater

Groundwater was encountered in borings B-1 through B-5 during drilling at depths of about 14 to 21 feet. However, fluctuations in the groundwater levels may occur due to variations in precipitation, ground surface topography, subsurface stratification, local ir-rigation, and other factors which may not have been evident at the time of our field evaluation and groundwater may rise to or be encountered at a higher level than ob-served in our borings.

8.4. Faulting and Site Seismicity

The subject site is located in a seismically active area, as is the majority of southern Califor-nia. Based on our background review of the referenced geologic maps and stereoscopic aerial photographs as well as or geologic field mapping, the subject site is not underlain by known active or potentially active faults (i.e., faults that have exhibited evidence of ground displacement in the last 11,000 years and 2,000,000 years, respectively). However, the site is located in a seismically active region and the potential for strong ground motion is consid-ered significant during the design life of the proposed bridge construction and associated improvements.

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Table 2 presents a list of selected known active faults in the area, approximate fault-to-site distances, type of fault displacement, and the Maximum Moment Magnitude (MMax) earth-quake associated with each fault. The MMax is defined as the largest earthquake that is considered to be reasonably capable of occurring under the conditions of the presently known geological framework.

Table 2 – Seismic Parameters for Maximum Moment Magnitude Earthquakes

Fault Name Fault I.D.

Number 1

Site Distances 2 (km)

Fault Type 2

Maximum Moment

Magnitude Earthquake 2

(MMax)

Rx Rjb Rrup

Elsinore Fault Zone (Julian section)

390 7.8 7.8 7.8 SS 7.7

Elsinore Fault Zone (Temecula section)

378 5.5 7.8 7.8 SS 7.7

Elsinore Fault Zone (Glen Ivy section) rev

365 13.9 45.7 45.7 SS 7.7

Notes:

1 Caltrans Deterministic Fault Database, 2007b 2 Caltrans ARS Online Web Tool, 2009 Rx – Perpendicular distance from the site to the surface projection of the top of the fault Rjb – Distance from the site to the closest surface projection of the fault plane Rrup – Closest distance from the site to any point on the fault plane SS – Strike Slip

The principal seismic hazards at the subject site are ground motion, surface fault rupture po-tential, liquefaction, lateral spreading, and seismic slope stability. A brief description of these hazards and the potential for their occurrences are discussed below.

8.4.1. Shaking and Strong Ground Motion

Considering the proximity of the site to active faults capable of producing a maximum moment magnitude of 6.0 or more, the project area has a high potential for experiencing strong ground motion. Based on the Caltrans Acceleration Response Spectra (ARS)

(Caltrans, 2009) and the Caltrans Deterministic Peak Ground Acceleration (PGA) Map

(Caltrans, 2007a), the design seismic event with respect to the proposed improvements is an earthquake associated with the Julian section of the Elsinore Fault Zone. The Cal-trans Deterministic PGA Map (Caltrans, 2007a) indicates that the bridge site is located

106898010 R Geo Design final.doc 13 in an area within the 0.2g peak ground acceleration contour (Figure 5). Based on our probabilistic evaluation using the Caltrans ARS (Caltrans, 2009), it is our opinion that a design peak ground acceleration of 0.392g is appropriate for the site. The design ARS curve evaluated for the site is presented on Figure 6.

We performed deaggregation of the probabilistic ground motion at the site using the

USGS interactive webpage (web address https://geohazards.usgs.gov/deaggint/2008/), which estimates the modal magnitude for a given probabilistic seismic ground motion.

Results of our seismic hazard deaggregation yielded a modal magnitude of 7.57, with a modal distance of 8.2 km.

8.4.2. Surface Fault Rupture

Surface fault rupture is generally caused by relative displacement across a fault during an earthquake. No active or potentially active faults are known to underlie the project site; therefore, the potential for surface fault rupture is considered to be low. The poten-tial for lurching or cracking of the surface as a result of nearby or distant seismic events is also considered low.

8.4.3. Liquefaction and Seismically Induced Settlement

Liquefaction is the phenomenon in which loosely deposited, saturated granular soils (locat-ed below the water table) with clay contents (particles less than 0.005 mm) of less than

15 percent, liquid limit of less than 35 percent, and natural moisture content greater than

90 percent of the liquid limit undergo rapid loss of shear strength due to development of ex-cess pore pressure during strong earthquake-induced ground shaking. Ground shaking of sufficient duration results in the loss of grain-to-grain contact due to rapid rise in pore water pressure, and eventually causes the soil to behave as a fluid for a short period of time. Liq-uefaction is known generally to occur in saturated or near-saturated cohesionless soils at depths generally shallower than 50 feet below grade. Factors known to influence liquefac-tion potential include composition and thickness of soil layers, grain size, relative density, groundwater level, degree of saturation, and both intensity and duration of ground shaking.

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The liquefaction potential at the project site was evaluated using the groundwater depth, laboratory test results, our evaluation of the site ground motion (described above), and our experience in the site vicinity. The liquefaction analysis was based on the National Cen-ter for Earthquake Engineering Research (NCEER) procedure (Youd, et al., 2001) using the computer program LiquefyPro (CivilTech Software, 2007). Our evaluation assumes granu-lar alluvial soils located below the groundwater level (assumed to be at the elevation of the box culvert’s invert) are potentially liquefiable between depths of approximately 14 and 31 feet below the existing grades. From this, we estimate that dynamic settlement of up to approximately 3½ inches could occur as the result of a major nearby seismic event

(Appendix C), using the method proposed by Tokimatsu and Seed (1987). Differential dynamic settlement of approximately 1¾ inches over a horizontal distance of approxi-mately 30 feet should be anticipated.

8.4.4. Lateral Spreading

Lateral spreading of ground surface during an earthquake usually takes place along weak shear zones that have formed within a liquefiable soil layer. Lateral spreading has generally been observed to take place in the direction of a free-face (i.e., retaining wall, slope, channel) but has also been observed to a lesser extent on ground surfaces with very gentle slopes. An empirical model developed by Bartlett and Youd (1995, revised

2002) is typically used to predict the amount of horizontal ground displacement within the site. For a site located in proximity to a free-face, the amount of lateral ground dis-placement is strongly correlated with the distance of the site from the free-face. Other factors such as earthquake magnitude, distance from the earthquake epicenter, thickness of the liquefiable layers, and the fines content and particle sizes of the liquefiable layers also affect the amount of lateral ground displacement. Based on the thickness of poten-tially liquefiable soil layers underlying the bridge, the potential for seismically induced lateral spreading is generally considered high at the site.

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Because the site is situated directly adjacent to Keys Creek, Bartlett and Youd’s empiri-cal method (1995, revised 2002) is not considered applicable. As such, common practice is to model the lateral spreading directly adjacent to a free face as a slope sta-bility model, where the slide plane can be expected to develop within the liquefied layer. The shear strength of the liquefied layer is considered to possess a residual shear strength, which can be estimated using the procedure recommended by Kramer and

Wang (2010). Utilizing the peak ground acceleration as the seismic coefficient, a pseu-dostatic slope stability analysis is performed to determine the free face’s factor of safety, as well as its yield acceleration (ky). When a pseudostatic factor of safety of less than

1.0 is calculated, a Newmark-type displacement analysis is then performed to estimate the amount of lateral spread.

A two-dimensional stability analysis program, GSTABL7 (Gregory, 2003), was used for this purpose. Our evaluation utilized search routines that incorporates Janbu’s

Method of Slices to define the most critical failure surface within potential block fail-ures, and Modified Bishop’s Method of Slices to define the most critical failure surface for potential rotational failures. When a pseudo-static analysis is performed on a single failure surface (determined to be the critical failure surface during the software’s search routine), the software conducts a simplified Newmark sliding block analysis (New-mark, 1964) to calculate the probable peak ground deformation of the slope. The software calculates the yield acceleration for the slope, which is incorporated into the

Newmark Analysis subroutine to calculate the probable displacement.

For our analysis, the earth materials were assigned with homogeneous, isotropic proper-ties. The shear strength parameters for the non-liquefiable fill and alluvial materials were selected based on our laboratory direct shear testing and our experience with simi-lar materials. The residual shear strength of the liquefiable materials was estimated in accordance with Kramer and Wang’s method (2010). The relevant shear strength pa-rameters used in our stability calculations for the different material types are presented in the attached calculations.

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We anticipate that the bridge deck will be able to provide added resistance to lateral spreading. This additional resistance can be expected to approach the mobilized passive soil forces acting on the abutment.

Utilizing the design PGA of 0.39g (as described above in the discussion of our seismic shaking and strong ground motion evaluation), our pseudo-static analysis (Appendix C) in-dicated a factor of safety of less than 1.0. The Newmark deformation analysis performed by the GSTABL7 software indicates a lateral deformation on the order of 2½ inches.

8.4.5. Slope Stability

Although the site is mapped as being generally susceptible to landslides (Tan and Giff-en, 1995), the crossing site is relatively flat-lying and no evidence of landslides were observed during our geologic reconnaissance. The potential for slope instability due to landslides, mudslides, or rockfall is, therefore, not a design consideration for the cross-ing site, provided construction is performed in accordance with the recommendations contained herein.

8.4.6. Tsunamis and Seiches

Tsunamis are long seismic sea waves (long compared to ocean depth) generated by sud-den movements of the sea floor caused by submarine earthquakes, landslides, or volcanic activity. Seiches are waves generated in a large, enclosed body of water. Based on our review of the literature, the project site is not located within an area considered susceptible to tsunamis or seiche inundation. Therefore, damage due to tsunamis or seiches is not a design consideration.

9. GEOTECHNICAL ANALYSIS AND DESIGN

Our geotechnical analyses for the improvements addressed in this GDR included settlement characteristics of the underlying soils and proposed embankments, the corrosive potential of the on-site soils, and recommendations for the pavement structural sections. Our analyses and rec-ommendations are based on the conditions encountered and the anticipated earthwork.

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9.1. Earthwork

In general, earthwork should be performed in accordance with the Caltrans Standard Speci-fications (Caltrans, 2006a) and the recommendations presented in this report. Ninyo &

Moore or the County’s designated representative should be contacted for questions regarding the recommendations or guidelines presented herein.

9.1.1. Rippability

The excavations along the proposed improvements are anticipated to be in fill, alluvium and granitic rock. Granitic rock was encountered beneath the alluvial soils to the total depth explored. Variations in these subsurface conditions encountered may exist. The fill and alluvial soils may be considered to be generally rippable with earthmoving equipment in good working order. While we were able to advance the hollow stem auger drilling equipment to depths of up to 50 feet, oversized materials and “corestones” may be en-countered. While not encountered during our subsurface exploration, there is a likelihood of encountering loosely placed cobbles and boulders in the existing fill materials during earthwork. We also note that there is a potential for encountering rock fills that may have been placed within the existing fill soils at the site. Excavation in areas of relatively hard granitic rock may require the use of heavier equipment or more innovative excavation techniques. Such techniques could include air hammer attachments to excavators, single shank rippers attached to heavy-duty earthwork equipment and blasting.

9.1.2. EMBANKMENTS

The following sections describe earthwork associated with the construction of the pro-posed embankments. Recommendations regarding site preparation, remedial grading

(overexcavation) and fill placement are provided. Embankment work should be per-formed in accordance with Section 19 of the Caltrans Standard Specifications (2006a).

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9.1.2.1. Site Preparation

Prior to performing site excavations, the surface areas should be cleared of existing vegetation, surface obstructions, and other deleterious materials. Existing utilities within the project limits should be re-routed or protected from damage by construc-tion activities. Vegetation and debris from the clearing operations should be disposed of at a legal dumpsite away from the project area. Obstructions that ex-tend below the finish grade, if any, should be removed and the resulting holes filled with compacted fill.

9.1.2.2. Remedial Earthwork

The existing fill and alluvium are loose to medium dense and are not considered ade-quate for structural support in their present condition. Where new fill embankments will be constructed (such as where the existing bridge approaches will be expanded at

Keys Creek Crossing), we recommend that the existing fill and upper portions of the alluvial soils be removed and replaced with compacted fill. Results of our subsurface exploration, along with our previous geotechnical evaluations (Ninyo & Moore, 2012;

2014) indicate that removals in these areas could be somewhat variable, but are antici-pated to be on the order of 7 feet. The actual depths of removals will depend on the project layout and actual depths of the surficial materials encountered during excava-tions. For preliminary purposes, the lateral extent of these removals should be 10 feet beyond the limits of the new embankment. The extent and depths of removals should be evaluated by Ninyo & Moore or the County’s designated representative in the field based on the materials exposed. The existing fill may be reused as engineered com-pacted fill if it meets the criteria described below.

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In addition to the compressible surficial fill and alluvial soils, unsuitable soils as defined in Section 19 of the Caltrans Standard Specifications (2006a) should be removed to expose competent material as evaluated by the project geotechnical en-gineer. Additionally, removals of unsuitable soils should be in accordance with

Section 19 of Caltrans Standard Specifications (2006a).

9.1.2.3. Excavation Characteristics

We anticipate that excavation in the fill and alluvium present on site may be ac-complished with grading equipment in good operating condition. Based on the results of our subsurface exploration, we expect that the near surface soils will con-sist of predominantly silty sands. However, bedrock was encountered in our borings at depths of 15 to 30.5 feet. Thus, the contractor should be prepared to take appro-priate measures to address the presence of such materials in the excavation. Such measures could include difficult ripping, rock breaking, and/or blasting.

As noted above, there is a likelihood of encountering loosely-placed cobbles and boulders within the existing fills. Rock fills may also be encountered within the ex-isting fill materials. Consequently, oversize materials may be generated which may necessitate offsite disposal.

9.1.2.4. Temporary Excavations and Shoring

Temporary excavations into the bedrock materials may encounter intersecting joints and other discontinuities, which will result in blocks of rock that may be susceptible to falling. A registered Geotechnical Engineer, Certified Engineering Geologist, or their designee should be contacted to evaluate the exposed joints and corestones during con-struction. For temporary excavations, we recommend that the following Occupational

Safety and Health Administration (OSHA) soil classifications be used:

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Fill and Alluvium Type C Granitic Bedrock Type A

Upon making the excavations, the soil classifications and excavation performance should be evaluated in the field in accordance with OSHA regulations. Temporary excavations should be constructed in accordance with OSHA recommendations. For trench or other excavations, OSHA requirements regarding personnel safety should be met using appropriate shoring (including trench boxes) or by laying back the slopes to no steeper than 1.5:1 (horizontal to vertical) in fill and alluvium and

0.75:1 (horizontal to vertical) in bedrock. Temporary excavations that encounter seepage may be shored or stabilized by placing sandbags or gravel along the base of the seepage zone. Excavations encountering seepage should be evaluated on a case-by-case basis. On-site safety of personnel is the responsibility of the contractor.

Shoring systems for excavations may be designed using the lateral earth pressure pa-rameters presented on Figures 7 and 8. These lateral earth pressures should be evaluated by a structural engineer for the design of the shoring systems. These design earth pressures assume that spoils from the excavations, or other surcharge loads, will not be placed above the excavations within a 1:1 plane extending up and back from the base of the excavation. For bracing subjected to surcharge loads, such as soil stockpiles or construction materials/equipment, an additional horizontal uniform pres-sure of 0.40q may be applied to the full height of the excavation, where “q” is the surcharge pressure.

9.1.2.5. Fill Material

In general, fill material should not contain rocks or lumps over approximately 6 inches in diameter, and not more than approximately 30 percent larger than ¾ inch. Materials to be utilized as structural backfill should meet the requirements of Section 19 of Cal-trans Standard Specifications (2006a). Oversize materials, if encountered, should be separated from material to be used for compacted fill and removed from the site.

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Imported fill material should generally be granular soils with a low expansion po-tential (i.e., an expansion index of 50 or less as evaluated by the ASTM

International [ASTM] Test Method D 4829). Import material should also be non-corrosive in accordance with the current version of the Caltrans corrosion guide-lines (Caltrans, 2012b). Materials for use as fill should be evaluated by Ninyo &

Moore or the County’s designated representative prior to filling or importing.

9.1.2.6. Fill Placement and Compaction

Prior to placement of compacted fill, the contractor should request an evaluation of the exposed ground surface by Ninyo & Moore or the County’s designated repre-sentative. Unless otherwise recommended, the exposed ground surface should then be scarified to a depth of approximately 8 inches and watered or dried, as needed, to achieve moisture contents generally near the optimum moisture content. The scarified materials should then be compacted to 90 percent relative compaction as evaluated by CT 216.

Compacted fill should be placed in horizontal lifts of approximately 8 inches in loose thickness. Prior to compaction, each lift should be watered or dried as needed to achieve a moisture content generally near the laboratory optimum, mixed, and then compacted by mechanical methods to 90 percent relative compaction as evalu-ated by CT 216. Successive lifts should be treated in a like manner until the desired finished grades are achieved.

As noted in Section 19-5 of the Caltrans Standard Specifications (2006a), fill should be compacted to 95 percent relative compaction as evaluated by CT 216 in areas that include (but are not necessarily limited to) the following:

0.5 feet below the grading plane for the width between the outer edges of the shoulders of the road way.

2.5 feet below the finished grade for the width of the traveled way plus 3 feet on each side.

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The full width and depth of the embankment within 150 feet of each bridge abut-ment (except for the outer 5 feet measured horizontally from the embankment side slope). The 150-foot limit is measured horizontally from the bridge abutment, ei-ther parallel to or concentric with the roadway centerline.

Prior to placement of additional compacted fill material following a delay in the grading operations, the exposed surface of previously compacted fill should be prepared to receive fill. Preparation may include scarification, moisture condi-tioning, and recompaction.

The evaluation of compaction by the geotechnical consultant should not be consid-ered to preclude any requirements for observation or approval by governing agencies. It is the contractor's responsibility to notify the appropriate governing agency when project areas are ready for observation, and to provide reasonable time for that review.

9.1.3. Grading Factors

Based on our subsurface evaluation and experience, with similar materials in the project area, a grading factor of 1.05 to 1.1 (i.e., 5 to 10 percent bulking) may be used for the granitic rock encountered. For other existing materials, including fill and alluvium, a grading factor of 0.85 (i.e., percent shrinkage) may be used.

9.2. Slope Stability

Unless otherwise recommended by Ninyo & Moore and approved by the County, fill slopes should not be constructed steeper than 2:1 (horizontal to vertical) and cut slopes in granitic rock should not be steeper than 1:1. Cut slopes in fill or alluvium should not be constructed steeper than 2:1 (horizontal to vertical). If excavations for cut slopes expose loose, cohesion-less, significantly fractured, or otherwise unsuitable materials, overexcavation of the unsuitable material and replacement with a compacted stabilization fill should be evaluated.

Compaction of the face of fill slopes should be performed by backrolling at intervals of

4 feet or less in vertical slope height or as dictated by the capability of the available equip-ment, whichever is less. Fill slopes should be backrolled utilizing a sheepsfoot-type roller.

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Care should be taken in maintaining the desired moisture conditions and/or reestablishing them, as needed, prior to backrolling. The placement, moisture conditioning, and compac-tion of fill slope materials should be done in accordance with the recommendations presented in the Fill Placement and Compaction section of this report.

Compacted fill slopes should be overbuilt and cut back to grade, exposing firm compacted fill. The actual amount of overbuilding may vary as field conditions dictate. If the desired results are not achieved, the existing slopes should be overexcavated and reconstructed in accordance with the recommendations of Ninyo & Moore or the County’s designated repre-sentative. The degree of overbuilding may be increased until the desired compacted slope face condition is achieved. Care should be taken by the contractor to provide mechanical compaction as close to the outer edge of the overbuilt slope surface as practical.

Site runoff should not be permitted to flow over the tops of slopes. Positive drainage should be established away from the slopes. This may be accomplished by incorporating brow ditches placed at the top of the slopes to divert surface runoff away from the slope face where drainage devices are not otherwise available. The project plans and specifications should contain design features and construction requirements to reduce the potential for ero-sion of the on-site soils both during and after construction.

9.3. Drainage

Roadway and slope drainage should be directed such that runoff water is diverted away from slopes and structures to suitable discharge areas by nonerodible devices (e.g., gutters, down-spouts, concrete swales, etc.). Positive drainage may be accomplished by providing drainage away from the structures at a gradient of 2 percent or steeper for a distance of 5 feet or more out-side the structure, and further maintained by a graded swale leading to an appropriate outlet, in accordance with the recommendations of the project civil engineer and/or landscape architect.

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Surface drainage on the site should be provided so that water is not permitted to pond. A gradient of 2 percent or steeper should be maintained and drainage patterns should be estab-lished to divert and remove water from the site to appropriate outlets. Best management practice (BMP) design for site drainage should be evaluated by the Project Civil Engineer.

Drainage for the proposed improvements should be in accordance with Chapter 800 of the

Caltra…