rfb_13199_for_info_only_5-foundation_report_keyes_creek.pdf

FOUNDATION 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, PE County of San Diego Department of Public Works, Materials Lab 5252 Kearny Villa Way San Diego, California 92123

Subject: Foundation Report Key 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 Foundation Report for the proposed Cole Grade Road overcrossing at Keys Creek in Valley Center, California. This report summarizes our findings and conclusions regarding the geologic and geotechnical conditions at the site and provides foundation recommendations for the project.

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

Sincerely, NINYO & MOORE

William Morrison, PE, GE Senior Engineer

Ronald D. Hallum PG, CEG Chief Geologist

LLB/MAC/WRM/RDH/gg

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. PROJECT DESCRIPTION

4. SUBSURFACE EVALUATION AND LABORATORY TESTING

4.1. Drilling, Coring and Sampling

4.2. Laboratory Testing

5. GEOLOGY AND SUBSURFACE CONDITIONS

5.1. Regional Geology

5.2. Site Geology

5.2.1. Pavement Section

5.2.2. Fill

5.2.3. Alluvium

5.2.4. Granitic Rock

5.3. Groundwater

5.4. Scour Evaluation

5.5. Corrosion Evaluation

5.6. Geologic Constraints

5.6.1. Faulting and Seismicity

5.6.2. Strong Ground Motion

5.6.3. Surface Fault Rupture

5.6.4. Liquefaction

5.6.5. Lateral Spreading

5.6.6. Slope Stability

6. AS-BUILT FOUNDATION DATA

7. CONCLUSIONS

8. RECOMMENDATIONS

8.1. Approach Embankments

8.1.1. Treatment of Existing Fill, Topsoil, Alluvium

8.1.2. Materials for Fill

8.1.3. Compacted Fill

8.2. Foundations

8.3. Shallow Foundation

8.4. Deep Foundations

8.5. Abutments

9. CONSTRUCTION CONSIDERATIONS

9.1. Excavation

9.2. Temporary Slopes

9.3. Groundwater

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9.4. Other Considerations

10. CONSTRUCTION OBSERVATION

11. LIMITATIONS

12. REFERENCES

Tables Table 1 – Seismic Parameters for Maximum Moment Magnitude Earthquakes Table 2 – Spread Footing Data Table Table 3 – Input Parameters for Pile Capacity Analysis - North Abutment Static Conditions Table 4 – Input Parameters for Pile Capacity Analysis - North Abutment Seismic Conditions ...21 Table 5 – Input Parameters for Pile Capacity Analysis - South Abutment Static Conditions Table 6 – Input Parameters for Pile Capacity Analysis - South Abutment Seismic Conditions ...22 Table 7 – Summary of Axial Load Capacities for Single Pile Table 8 – North Abutment 16-Inch Diameter CIDH Pile Lateral Load Capacity Table 9 – South Abutment 16-Inch Diameter CIDH Pile Lateral Load Capacity Table 10 – North Abutment 24-Inch Diameter CIDH Pile Lateral Load Capacity Table 11 – South Abutment 24-Inch Diameter CIDH Pile Lateral Load Capacity Table 12 – North Abutment 36-Inch Diameter CIDH Pile Lateral Load Capacity Table 13 – South Abutment 36-Inch Diameter CIDH Pile Lateral Load Capacity

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

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 prepared this Foundation Report

(FR) for the proposed Cole Grade Road overcrossing at Keys Creek in Valley Center, California

(Figure 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 this FR was to assess the geologic conditions and geotechnical foundation is-sues at the proposed overcrossing location based on our subsurface evaluation and to provide foundation recommendations. This FR presents our findings, conclusions, and recommenda-tions regarding the geotechnical aspects pertinent to the proposed bridge. A separate geotechnical design report (GDR) will be submitted in connection with the grading of the pro-posed embankments and the construction of the proposed improvements.

2. SCOPE OF SERVICES

Our report was prepared in general accordance with Section 3, Foundation Reports, from the

Caltrans guidelines for Foundation Report Preparation for Bridges (Caltrans, 2009a). 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 Foundation Report in general accordance with Caltrans guidelines.

3. PROJECT DESCRIPTION

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 Coun-ty, 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 cross-ing, Cole Grade Road is oriented roughly north-south. Keys Creek drains toward the west.

The current crossing at Keys Creek is comprised of a double 8-foot by 8-foot reinforced concrete box culvert. Embankments up to approximately 15 feet in thickness at present serve as approach-es 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.

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We understand that a new bridge over the crossing is planned as a part of the proposed widening of an approximately 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 con-versations 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, which has been mapped as underlying the site (Kennedy and Tan, 2005). 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.

4. SUBSURFACE EVALUATION AND LABORATORY TESTING

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 lower 5 to 10 feet in four of the borings. The boring locations and the boring logs are presented on the log of test borings (LOTB) sheet (Appendix A).

4.1. 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

106898010 R Foundation final.doc 4 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.

4.2. 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.

5. GEOLOGY AND SUBSURFACE CONDITIONS

The following sections describe geologic, soil, and groundwater conditions at the site.

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5.1. Regional Geology

The project is situated in the northern San Diego County section of the Peninsular Ranges

Geomorphic Province. This geomorphic province encompasses an area that extends approx-imately 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 approximately 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 bedrock underlying the portion of the province in San Diego County that includes the project area consists generally of Creta-ceous-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 pro-ject 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 pro-ject 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.

5.2. Site Geology

The site is mapped as being primarily underlain by the Cretaceous-age Cole Grade Tonalite

(Kennedy and Tan, 2005). These materials are referred to herein as granitic rock. Other geo-logic units observed at the site include fills and alluvium. Generalized descriptions 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 A.

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5.2.1. Pavement Section

Borings B-1 and B-2 were advanced through the 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 in either boring.

5.2.2. Fill

Fill was encountered in borings B-1 and B-2 under the pavement section to depths of 8 to 11 feet. Fill was also encountered in borings B-3, B-4, and B-5 from the ground sur-face to depths of 9 to 14 feet. As encountered, these materials generally consisted of moist, loose to medium dense silty sand and well-graded sand. A strong oil odor was de-tected in the upper 4 inches of fill in borings B-1 and B-2. The odor is likely due to a tack coat layer that was placed as part of previous roadway construction.

5.2.3. Alluvium

Alluvium was encountered in each of 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. These materials were observed to be moist to wet during our subsur-face exploration.

5.2.4. Granitic Rock

As noted, the site is predominantly underlain by the Cretaceous-age Cole Grade Tonalite

(referred to as granitic rock on our boring logs). 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.

5.3. 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

106898010 R Foundation final.doc 7 precipitation, ground surface topography, subsurface stratification, local irrigation, 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 observed in our borings.

5.4. Scour Evaluation

The proposed improvements are anticipated to handle much of the surface runoff during pe-riods of precipitation. Uncontrolled surface flow is anticipated to be generally limited to landscaped embankment slopes. A hydrology or hydraulics report was not available for our review at the time our evaluation. Based on the information obtained during our site evalua-tion 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 consideration be given to the installation of measures to protect the channel bottom and embankment against scour. Such measures can include the place-ment 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).

5.5. Corrosion Evaluation

Laboratory testing was performed on two representative samples of the site soils to evaluate pH, electrical resistivity, water-soluble chloride content, and water-soluble sulfate content.

The soil pH and electrical resistivity tests were performed in general accordance with Cali-fornia Test Method (CT) 643. Chloride content tests were performed in general accordance with CT 422. Sulfate testing was performed in general accordance with CT 417. The labora-tory test results are presented in Appendix B.

The soil pH ranged from approximately 7.2 to 7.6, and the electrical resistivity was approxi-mately 5,000 ohm-centimeters. The chloride contents of the samples were approximately 105 and 135 parts per million (ppm). The sulfate content of the samples ranged from approximate-ly 0.0090 percent (90 ppm) to 0.0120 percent (120 ppm). Based on the laboratory test results and Caltrans criteria, the project site is not classified as a corrosive site, which is defined as

106898010 R Foundation final.doc 8 having earth materials with more than 500 ppm chlorides, more than 0.20 percent sulfates, a pH of 5.5 or less or an electrical resistivity of less than 1,000 ohm-centimeters. If corrosion-susceptible improvements are planned on site, we recommend that a corrosion engineer be consulted for further evaluation and recommendations, as necessary.

5.6. Geologic Constraints

Geologic constraints that may have an impact on the proposed improvements are described in the following sections.

5.6.1. Faulting and Seismicity

The subject site is located in a seismically active area, as is the majority of southern

California. Based on our background review of the referenced geologic maps and stere-oscopic 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 ev-idence 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 poten-tial for strong ground motion is considered significant during the design life of the proposed bridge construction and associated improvements.

Table 1 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) earthquake 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 present-ly known geological framework.

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Table 1 – 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, 2009b 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 potential, liquefaction, and seismic slope stability. A brief description of these hazards and the potential for their occurrences are discussed below.

5.6.2. Strong Ground Motion

Considering the proximity of the site to active faults capable of producing a maximum moment magnitude earthquake (MMax) of 6.0 or more, the project area has a high potential for experiencing strong ground motion. The MMax is defined as the largest earthquake that is considered to be reasonably capable of occurring under the conditions of the presently known geologic conditions. Based on the Caltrans Acceleration Response Spectra (ARS)

(Caltrans, 2009b) 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 Caltrans

Deterministic PGA Map (Caltrans, 2007a) indicates that the bridge site is located in an ar-ea within the 0.2g peak ground acceleration contour (Figure 5). Based on our probabilistic evaluation using the Caltrans ARS (Caltrans, 2009b), it is our opinion that a design peak

106898010 R Foundation final.doc 10 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.

5.6.3. 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.

5.6.4. Liquefaction

Liquefaction is the phenomenon in which loosely deposited, saturated granular soils

(located 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 great-er than 90 percent of the liquid limit undergo rapid loss of shear strength due to development of excess 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. Liquefaction 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 liquefaction 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

Center for Earthquake Engineering Research (NCEER) procedure (Youd, et al., 2001) using the computer program LiquefyPro (CivilTech Software, 2007). Our evaluation assumes granular alluvial soils located below the groundwater level (assumed to be at the eleva-tion of the box culvert 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 ma-jor nearby seismic event (Appendix C), using the method proposed by Tokimatsu and

Seed (1987). Differential dynamic settlement of approximately 1¾ inches over a hori-zontal distance of approximately 30 feet should be anticipated.

5.6.5. 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 failures, and

Modified Bishop’s Method of Slices to define the most critical failure surface for poten-tial 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 rou-tine), the software conducts a simplified Newmark sliding block analysis (Newmark, 1964) to calculate the probable peak ground deformation of the slope. The software cal-culates 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 0.392g (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.

5.6.6. 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.

6. AS-BUILT FOUNDATION DATA

The existing structure consists of an approximately 8-feet by 8-feet double reinforced concrete box culvert with abutments that was constructed in about 1962. The box culvert structure was constructed to replace a previously existing bridge. Reinforced concrete wing walls were also constructed at the openings of the box culvert. Riprap, grass, and shrubs were observed lining the slopes on the north and south ends of the box culvert. The as-built plans for the existing bridge are included in Appendix D.

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7. CONCLUSIONS

Based on the results of our geotechnical evaluation, it is our opinion that the Keys Creek Tribu-tary Overcrossing project is feasible from a geotechnical standpoint provided that the recommendations presented in this report are incorporated into the design and construction. Fac-tors which may affect the construction of the proposed project include:

The site is underlain by surficial soils consisting of fill and alluvial soils overlying weath-ered Cole Grade Tonalite. The fill and alluvial soils are not considered to be suitable for foundation support for the bridge structure. Due to the encountered depth to the Cole Grade Tonalite, recommendations are provided herein for the bridge to be founded on cast-in-drilled-hole (CIDH) foundations bearing on the Tonalite.

Based on the results of minimum resistivity tests performed on selected samples from the site, and in accordance with Caltrans guidelines, the project site is not considered to be corrosive.

Excavations within the fill and alluvial soils should be feasible with conventional heavy duty earthmoving equipment in good working condition. While not encountered during our sub-surface 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.

Areas of granitic bedrock will be encountered requiring heavier equipment or more innova-tive excavation techniques.

Difficult ripping, rock breaking, and/or blasting should be anticipated for excavations in the bedrock. Excavations will encounter corestones and hard to very hard rock, which will make such excavations difficult.

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 level may occur due to variations in precipitation, ground surface topography, subsurface stratification, local irrigation, 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 observed in our borings. Excavations ex-tending below the ground water level can be expected to require dewatering.

Deep foundations for support of bridge structures will likely extend below the water table.

The contractor should take appropriate measures during construction to reduce the potential for sloughing of the drilled holes, including the use of casing and/or drilling mud, and placement of the concrete utilizing tremie methods.

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The subject site is not located within a State of California Earthquake Fault Zone and, based on our review of published geologic maps, there are no known mapped active faults underly-ing the site. Therefore, the potential for surface fault rupture at the site is considered low.

There is a potential for seismically induced liquefaction of site soils. Seismic settlements on the order of 3½ inches and displacement due to lateral spreading of approximately 2½ inches are estimated as a result of a design seismic event.

Site improvements should be designed for a peak ground acceleration of 0.392g.

8. RECOMMENDATIONS

Based on the results of our subsurface evaluation and our understanding of the proposed construc-tion, we present the following geotechnical recommendations relative to the design and construction of the proposed bridge improvements. We should note that the proposed construction should also be performed in accordance with the requirements of the applicable governing agencies.

8.1. Approach Embankments

A separate GDR will be prepared with recommendations for grading of the Keys Creek Tribu-tary Overcrossing and associated improvements. It is our understanding that the western embankment will be widened as part of the proposed improvements. Approach embankments should be constructed in accordance with Caltrans Memo to Designers 5-3 (Caltrans 1992), the recommendations presented in this report, as well as in the project specifications.

8.1.1. Treatment of Existing Fill, Topsoil, Alluvium

As noted previously, the existing fill and alluvium are loose to medium dense and are not considered adequate 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 por-tions 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 anticipated to be on the order of 7 feet. If shallow foundations are selected for

106898010 R Foundation final.doc 16 support of the proposed bridge, we recommend that the removals extend to weathered granitic rock. The actual depths of removals will depend on the project layout and actual depths of the surficial materials encountered during excavations. For preliminary pur-poses, 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 ex-posed. The existing fill may be reused as engineered compacted fill if it meets the criteria described below.

8.1.2. Materials for Fill

On-site soils with an organic content of less than approximately 3 percent by volume (or

1 percent by weight) are suitable for reuse as compacted 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 19-3 of Caltrans Standard

Specifications (2006a). Oversize materials, if encountered, should be separated from material to be used for compacted fill and removed from the site.

Imported fill material should generally be granular soils with a low expansion potential

(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 guidelines (Caltrans, 2012). Materials for use as fill should be evaluated by Ninyo & Moore or the County’s designated repre-sentative prior to filling or importing

8.1.3. Compacted Fill

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 representative.

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

106898010 R Foundation final.doc 17 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 evaluated by CT 216. Succes-sive 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.

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.

The entire depth of fill to be placed beneath retaining wall footings (without pile foundations. The lateral limits of this fill are established by inclined planes sloping

1.5 : 1 (horizontal to vertical) out and down from lines 1 foot outside the bottom edges of the footing.

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 conditioning, and recompaction.

The evaluation of compaction by the geotechnical consultant should not be considered 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.

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Embankment slopes built in accordance with these recommendations at an inclination of 2:1 (horizontal to vertical) or flatter should be stable against both deep-seated and surficial failures. To reduce the potential for erosion, we recommend that slopes be planted with drought-tolerant vegetation as soon as practicable after grading. Abutment slopes beneath the bridges may be paved to reduce the potential for erosion.

8.2. Foundations

It is our understanding that the new bridge structure will be supported on either shallow foun-dations or cast-in-drilled-hole (CIDH) foundations that will be rock-socketed within competent granitic rock at the abutment locations. Based on our subsurface exploration and review of the log of test borings for the existing bridge structure, we anticipate the granitic rock is up to approximately 35 feet below the elevation of the existing roadway surface. 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, we are including geotechnical recommendations for shallow foundation design, so the feasibility of each alternative foundation system can be evaluated.

As noted earlier, groundwater was encountered a depths of 14 to 21 feet below the ground sur-face. The potential for possible reduction in bearing capacity due to fluctuation of the groundwater table in foundation soils is a design consideration. Based on information provid-ed to us by the County of San Diego, we understand that the preliminary design loads for each of the two abutments will be 1,080 kips.

8.3. Shallow Foundation

It is our understanding that the new bridge structure may be supported on shallow, spread footings founded on granitic rock at the abutment locations. The spread footings were evalu-ated in accordance with Section 4.0 of the Caltrans Bridge Design Specifications (2004).

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The permissible gross contact stress in Working Stress Design (WSD) and the permissible net contact stress in Load Resistance Factor Design (LRFD) were calculated using the prin-ciple of general shear failure of foundation material subjected to a footing of finite shape and loading. These contact stress values are equivalent to un-factored nominal bearing resistance or ultimate bearing capacity of the footings. The design contact stress and bearing resistance values for the bridge footings are summarized in Table 2 for various combination of footing dimensions and embedment depths. In addition, the elastic settlements under the service loads for abutments based on Caltrans Bridge Design Specifications (Caltrans, 2004) are al-so included in Table 2.

Table 2 – Spread Footing Data Table

Support Location

(1)

Footing Size (ft)

Bottom of

Footing Elevati on (ft)

Minimum Footing

Embedm ent Depth

(ft)

WSD (LRFD

Service-I Limit State Load Combination)

LRFD

Settlement Under Service

Load (in) (4)

Service Strength b = 0.45

Extreme b = 1.0

B’ (2)

L’ (2)

Permissible Gross

Contact Stress (ksf)

Allowable Gross

Contact Stress (ksf) (3)

Permissi ble Net Contact Stress (ksf)

Factore d Gross Nominal Bearing Resistan ce (ksf)

Factored Gross

Nominal Bearing

Resistance (ksf)

A1 8.0 90.0 1396 5 24.0 8.0 24.0 10.8 24.0 1.0 A2 8.0 90.0 1404 5 24.0 8.0 24.0 10.8 24.0 1.0

Notes:

(1) A1 – Abutment 1 (North Abutment); A2 – Abutment 2 (South Abutment);

(2) B’ – Effective footing width; L’ – Effective footing length (Based on Caltrans Bridge Design Specifications Section 4.4.7.1.1.1)

(3) A factor of safety of 3 was used.

(4) Elastic Settlement (Based on Caltrans Bridge Design Specifications Equation 4.4.7.2.2-1)

A coefficient of friction of 0.38 should be used between the horizontal surface of the spread footings and the underlying foundation material in order to evaluate resistance against slid-ing. A passive resistance value of 360 pounds per square foot (psf) per foot of depth should be used in evaluating the lateral resistance of the structural footings. The allowable lateral resistance should be taken as either the sum of 50 percent of the frictional resistance and

50 percent of the passive resistance, or 100 percent of the frictional resistance only, or

100 percent of the passive resistance only, whichever is lower.

106898010 R Foundation final.doc 20

Based on our evaluation of the bearing capacity of the spread footings, we anticipate that settlement of the foundation material (consisting of granitic rock) should not exceed 1 inch.

We estimate a differential settlement of about ½ inch for the footings over a horizontal span of 20 feet. The foundation settlement should take place relatively instantaneously during construction of the footings.

8.4. Deep Foundations

It is our understanding that the new bridge structure may be supported on cast-in-drilled-hole (CIDH) foundations that will be rock-socketed within competent granitic rock at the abutment locations. Based on our subsurface exploration and review of the log of test borings for the existing bridge structure, we anticipate the granitic rock is up to approximately 35 feet below the elevation of the existing roadway surface. As noted earlier, groundwater was en-countered a depths of 14 to 21 feet below the ground surface. The potential for possible reduction in bearing capacity due to fluctuation of the groundwater table in foundation soils is a design consideration. Based on information provided to us by the County of San Diego, we understand that the preliminary design loads for each of the two abutments will be 1,080 kips.

Our evaluation considers the use of Caltrans Standard 16-inch and 24-inch CIDH piles, as well as 36-inch CIDH piles for foundation support of the proposed bridge abutments. Pile capacity versus pile length curves were generated (Appendix C) for each of the CIDH pile sizes for each abutment. The capacities were evaluated using the computer program AllPile

(CivilTech, 2009) based on the CIDH diameters and the soil properties, described below. The pile capacities calculated for each CIDH pile are based on both end-bearing and frictional ca-pacity assuming the pile is socketed within the Cole Grade Tonalite. The curves depict the allowable vertical load capacity based on a socketed length of zero feet (pile resting on granit-ic rock contact) to 20 feet. The calculated pile capacities include the downdrag forces due to liquefaction of the alluvium during a design seismic event. A pile spacing of three pile diameters on center or more should be maintained. The soil layer boundaries and the respective soil properties used in our evaluation of pile capacities under both static and liquefied conditions are summarized in Tables 3 through 6 below.

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Table 3 – Input Parameters for Pile Capacity Analysis - North Abutment Static Conditions

Material

Depth to Layer

Bottom (ft)

General Soil Type*

Buoyant Unit Weight

(pcf)

Friction Angle

(degrees)

Cohesion (psf) k (pci) ε50

Fill 9 Silty Sand 115 29 120 25 ---

Alluvium 14 Silty Sand 110 30 140 90 ---

Alluvium (Below Ground Water)

25 Silty Sand 47.6 30 140 20 ---

Alluvium (Below Ground Water)

35 Silty Sand 47.6 30 140 60 ---

Granitic Rock --- Coarse- Grained

Igneous Rock

62.6 42 140 1,100 ---

Notes:

ft = feet pcf = pounds per cubic feet psf = pounds per square foot pci = pounds per cubic inch *Sand (Reese) options should be used for Silty Sand layers in LPILE program.

Table 4 – Input Parameters for Pile Capacity Analysis - North Abutment Seismic Conditions

Material

Depth to Layer

Bottom (ft)

General Soil Type*

Buoyant Unit Weight

(pcf)

Friction Angle

(degrees)

Cohesion (psf) k (pci) ε50

Fill 9 Silty Sand 115 29 120 25 ---

Alluvium 14 Silty Sand 110 30 140 90 ---

Alluvium (Below Ground Water)

25 Soft Clay 47.6 0 120 0.1 0.05

Alluvium (Below Ground Water)

35 Soft Clay 47.6 0 200 0.1 0.05

Granitic Rock --- Coarse- Grained

Igneous Rock

62.6 42 140 1,100 ---

Notes:

ft = feet pcf = pounds per cubic feet psf = pounds per square foot

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Table 5 – Input Parameters for Pile Capacity Analysis - South Abutment Static Conditions

Material

Depth to Layer

Bottom (ft)

General Soil Type*

Buoyant Unit Weight

(pcf)

Friction Angle

(degrees)

Cohesion (psf) k (pci) ε50

Fill 9 Silty Sand 115 29 120 25 ---

Alluvium 14 Silty Sand 110 30 140 90 ---

Alluvium (Below Ground Water)

22 Silty Sand 47.6 30 140 20 ---

Alluvium (Below Ground Water)

30 Silty Sand 47.6 30 140 60 ---

Granitic Rock --- Coarse- Grained

Igneous Rock

62.6 42 140 1,100 ---

Notes:

ft = feet pcf = pounds per cubic feet psf = pounds per square foot

Table 6 – Input Parameters for Pile Capacity Analysis - South Abutment Seismic Conditions

Material

Depth to Layer

Bottom (ft)

General Soil Type*

Buoyant Unit Weight

(pcf)

Friction Angle

(degrees)

Cohesion (psf) k (pci) ε50

Fill 9 Silty Sand 115 29 120 25 ---

Alluvium 14 Silty Sand 110 30 140 90 ---

Alluvium (Below Ground Water)

22 Soft Clay 47.6 0 120 0.1 0.05

Alluvium (Below Ground Water)

30 Soft Clay 47.6 0 200 0.1 0.05

Granitic Rock --- Coarse- Grained

Igneous Rock

62.6 42 140 1,100 ---

Notes:

ft = feet pcf = pounds per cubic feet psf = pounds per square foot

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Several of the granular layers of alluvium located below the groundwater are expected to liquefy to a depth of approximately 35 feet below the surface should the design seismic event occur. The calculated pile capacities include the downdrag forces of the alluvium due to liquefaction during the design seis-mic event. The pile tip should be embedded in granitic bedrock. The pile design loads and recommended lengths were analyzed using the computer program AllPile (CivilTech, 2009). Table 7 presents the recommended pile tip elevations, design loads and nominal resistances assuming an em-bedment depth of 5 feet or more.

Table 7 – Summary of Axial Load Capacities for Single Pile

Location Pile

Type

Design Load 1 (kips)

Nominal Resistance Elevation

Compression (kips)

Tension 2

(kips) Cut Off

Elevation

Design Tip Elevation3

(feet)

Specified Tip Eleva-tion (feet)

Abutment 1 (North)

16-inch

CIDH

90 180 90 N/A 1391 1391

Abutment 1 (North)

24-inch

CIDH

140 280 140 N/A 1391 1391

Abutment 1 (North)

36-inch

CIDH

200 400 200 N/A 1391 1391

Abutment 2 (South)

16-inch

CIDH

90 180 90 N/A 1399 1399

Abutment 2 (South)

24-inch

CIDH

140 280 140 N/A 1399 1399

Abutment 2 (South)

36-inch

CIDH

200 400 200 N/A 1399 1399

Notes:

1 = A static factor of safety of 2.0 was used in obtaining the design load from nominal resistance in compression.

2 = A static factor of safety of 1.5 was used in obtaining the design load from nominal resistance in tension.

3 = The design tip elevations are based on an average ground surface elevation of feet MSL.

Lateral load capacities for the proposed piles were evaluated for 1/4- and 1/2- deflection at the pile head assuming a free-head condition. The lateral pile capacity estimates presented in Tables 8 through 13 are based on the soil properties presented in Tables 3 and 5 for the static condition.