rfb_13199_for_info_only_13-draft_geotechnical_report.pdf

DRAFT GEOTECHNCIAL REPORT

Cole Grade Road Improvements

Valley Center, California

County Contract No. 557947 Task Order No. 20

Oracle Project No. 1003046

Prepared for:

Mr. E. David Gasaway County of San Diego

Department of Public Works Materials Lab

5252 Kearny Villa Way San Diego, CA 92123

Prepared by:

WSP USA Environment & Infrastructure, Inc.

9177 Sky Park Court

San Diego, California 92123 (858)-278-3600

August 16, 2024

WSP Project No. 5015-18-0024.20 https://www.wsp.com/

DRAFT GEOTECHNICAL REPORT

Valley Center, California

August 16, 2024

WSP Project No. 5015-18-0024.20

This report was prepared by the staff of WSP USA Environment & Infrastructure, Inc., under the supervision of the engineer and geologist whose seals and signatures appear hereon.

The findings, recommendations, specifications, or professional opinions are presented within the limits described by the client, in accordance with generally accepted professional engineering and geologic practice. No warranty is expressed or implied.

Mike Farr, C.E.G. #1938 Principal Engineering Geologist

Hari Ponnaboyina, GE #3108 Associate Geotechnical Engineer

Cole Grade Road Improvements, Project No. 5015-18-0024.20

TABLE OF CONTENTS

Page

TABLE OF CONTENTS ............................................................................................................. I

TABLES .................................................................................................................................... II

APPENDICES .......................................................................................................................... II

1.0 PROJECT DESCRIPTION AND SCOPE OF SERVICES

2.0 SITE DESCRIPTION

3.0 DATA REVIEW

4.0 FIELD INVESTIGATION

5.0 LABORATORY TESTING

6.0 REGIONAL GEOLOGIC AND TECTONIC SETTING

7.0 SITE GEOLOGY

7.1 GENERAL

7.2 GRANITIC BEDROCK

7.3 SLOPEWASH/RESIDUAL SOIL

7.4 ARTIFICIAL FILL

7.5 GROUNDWATER

8.0 GEOLOGIC HAZARDS

8.1 FAULT HAZARD

8.2 GROUNDSHAKING

8.3 LANDSLIDE AND ROCK FALL HAZARD

8.4 LIQUEFACTION AND LATERAL SPREADING

8.5 FLOOD HAZARD

9.0 RIPPABILITY

9.1 SOLDIER PILE RETAINING WALL

9.2 CUT SLOPES

10.0 PRELIMINARY RECOMMENDATIONS

10.1 SOLDIER PILE RETAINING WALL

10.2 LATERAL EARTH PRESSURE

10.3 SEISMIC LATERAL EARTH PRESSURE

10.4 DRAINAGE

10.5 DESIGN OF SOLDIER PILES

10.6 LAGGING

10.7 ANCHOR DESIGN

10.8 ANCHOR INSTALLATION

10.9 ANCHOR TESTING

10.14 TEMPORARY CUT SLOPES

10.15 COMPACTION

10.16 MATERIAL FOR FILL

11.0 CLOSURE

12.0 REFERENCES

TABLE OF CONTENTS

(Continued)

TABLES

Table 1 Range of Depth of Geologic Units

Table 2 Tieback Recommendations

PLATES

Plate 1 Site Location Map Plate 2 Geotechnical Map Plates 3-5 Geologic Cross Sections Plate 6 Geology Map Plate 7 Fault Map

APPENDICES

Appendix A Field Exploration/Boring Logs Appendix B Laboratory TestingTAble Appendix C Seismic Refraction Survey

Cole Grade Road Improvements, Project No. 5015-18-0024.20 1

DRAFT GEOTECHNICAL REPORT

Valley Center, California

WSP USA Environment and Infrastructure, Inc. (WSP) is pleased to present our draft geotechnical report for the subject project.

1.0 PROJECT DESCRIPTION AND SCOPE OF SERVICES

We understand the overall project will consist of the widening of Cole Grade Road, extending from Fruitvale Road on the south to Oak Glen Road on the north, in Valley Center, California. The main purpose of the widening will be to increase roadway safety by constructing a center left turn lane. Geotechnical evaluations for the project were previously completed by Ninyo and Moore

(2012, 2014).

To assist in the preparation of this report, we were provided with preliminary plans and profiles for the planned street widening, including suggested boring locations. Our scope of services was limited to a proposed soldier pile wall location and seven locations where existing cut slopes will be reconfigured to accommodate the proposed street widening. Our scope of services did not include acceptance of previous geotechnical work performed by others or becoming the geotechnical engineer of record for the overall project.

Our geotechnical study was performed in general accordance with our proposal dated March 7, 2024, as authorized by County of San Diego Department of Public Works (County) Task Order

No. 20. Our scope of services generally consisted of:

• Pre-drilling activities, including permitting, subsurface utility screening, and logistics.

• Field investigation program including two exploratory borings in the area of the proposed soldier pile wall and seismic refraction lines at eight locations. The eight locations include the location of the proposed soldier pile wall and 7 locations selected by the County where existing cut slopes will be reconfigured.

• Laboratory testing.

• Preparation of this report presenting the findings of our study.

The two exploratory borings and one of the seismic refraction lines were performed to obtain subsurface information to be used in developing geotechnical recommendations for design of the proposed soldier pile wall. The other seven seismic refraction lines were performed to obtain subsurface information about the granitic rock underlying the seven selected cut slopes to be reconfigured. A particular focus of the seismic lines was evaluation of the rippability characteristics of the granitic rock.

Services related to environmental assessment, characterization, and/or remediation were not part of the requested scope of work, and investigations or analyses of potential environmental contaminants have not been conducted or presented as part of this investigation.

Cole Grade Road Improvements, Project No. 5015-18-0024.20 2

2.0 SITE DESCRIPTION

The overall project site consists of Cole Grade Road, extending from Fruitvale Road on the south to Oak Glen Road on the north, in Valley Center, California. Cole Grade Road is a two-lane, moderately traveled, paved roadway providing access between Valley Center and rural communities to the north. The roadway is divided by a double yellow line, without a left turn lane.

Paved and unpaved shoulders of variable widths extend from the outside edges of the travel lanes to the right-of-way boundaries on either side of the roadway. Improvements within the right-of-way include power lines and communications lines on both sides of the roadway.

The study area covered by this report is limited to the following locations specified by the County:

• A proposed soldier pile wall with a maximum height of about 22 feet, to be located at the eastern right-of-way boundary of Cole Grade Road, extending from about 200 feet to 830 feet south of Cool Valley Road (approximate Stations 123+00 to 129+30).

• Seven existing cut slope locations chosen by the County to be reconfigured and extended further to the east or west to accommodate the proposed road widening.

The existing cut slope at the proposed soldier pile wall location has a length of about 700 feet and a maximum height of about 22 feet, with slope gradients ranging from about 0.5:1 horizontal:vertical (h:v) to 1:1 h:v. Most of the seven cut slopes where the seismic lines were performed have maximum heights on the order of 10 to 20 feet, but are higher than 20 feet in a few limited locations. These slopes exhibit variable gradients generally ranging from about 0.5:1 h:v to 1:1 h:v.

3.0 DATA REVIEW

Before initiating the field exploration program, WSP reviewed readily available documents relevant to the study area. This included review of preliminary project drawings, geologic maps, geologic literature, and topographic maps. We also reviewed previous geotechnical reports for the overall road widening project prepared by Ninyo and Moore (2012, 2014). Information obtained from our data review is incorporated into the findings presented in this report.

4.0 FIELD INVESTIGATION

The field investigation included predrilling activities, two exploratory borings, and eight seismic refraction lines. A brief description of the field investigation is provided below, while a more detailed description and logs of the exploratory borings are provided in Appendix A.

4.1 Pre-Drilling Activities

Pre-drilling activities included review of relevant documents, permitting, and subsurface utility screening.

4.2 Exploratory Borings

Two exploratory borings extending to depths of 30 to 31 feet were drilled at the approximate locations indicated on Plate 2. The exploratory borings were drilled with a track-mounted Fraste drill rig operated by Pacific Drilling of San Diego, California.

Cole Grade Road Improvements, Project No. 5015-18-0024.20 3

4.3 Seismic Refraction Survey

Eight seismic refraction lines were performed by our geophysical subconsultant Atlas Technical

Consultants (Atlas) at the locations indicated on Plates 1 and 6. The seismic refraction survey methodology and results are presented in a report prepared by Atlas, included herein as Appendix

C. One seismic line (SL-4) was performed along the alignment of the proposed soldier pile wall.

The other seven lines were performed at locations selected by the County where existing cut slopes will be reconfigured as part of the widening of Cole Grade Road.

The seismic refraction survey performed by Southwest Geophysics provides tomographic images of the estimated geologic conditions based on subsurface velocity models. The tomographic images depict layers and/or zones of varying P-wave (compression wave) velocity that generally increase with increasing depth. The P-wave velocities can be correlated to rippability (and to some degree hardness) based in part upon published information from the 2018 Caterpillar

Performance Handbook. The categories of rippability difficulty presented in Table 1 of the Atlas seismic survey report (see Appendix C) are based on a Caterpillar D-9 bulldozer using a single shank ripper. Difficulty of ripping is typically higher for trenching operations.

4.4 Cut Slope Mapping

Geologic conditions exposed in the existing cut slope where the soldier pile wall will be constructed were mapped by a WSP registered engineering geologist. The primary focus of the mapping was the degree of weathering of the exposed granitic rock and the presence of potentially non-rippable rock.

5.0 LABORATORY TESTING

All laboratory testing was performed in-house by WSP. Tests performed included in-situ moisture content and dry density, direct shear, maximum density/optimum moisture, unconfined compressive strength, and collapse potential. Testing was performed in accordance with ASTM and/or California Department of Transportation (Caltrans) methods. Test results and a more detailed description of the laboratory testing program are presented in Appendix B. A summary of the laboratory test results is presented on Plate B1 in Appendix B.

6.0 REGIONAL GEOLOGIC AND TECTONIC SETTING

The project site is located within the Peninsular Ranges Geomorphic Province of California. The

Peninsular Ranges province extends from the north end of the Los Angeles Basin on the north into Baja California on the south. In San Diego County, the province is characterized by foothills and mountains in the east and the coastal plain in the westernmost part of the province. The geology of the subject site and surrounding area is depicted on Plate 6.

The project site is located within the foothill zone of the Peninsular Ranges Geomorphic Province, near the boundary with the coastal plain zone to the west. The foothill and mountain zones in the eastern part of the county generally range from about 40 to 50 miles in combined width. They are primarily underlain by Cretaceous-age granitic rock associated with the Southern California

Batholith, and Jurassic- to Cretaceous-age, mildly metamorphosed volcanic rocks. The foothill and mountain zones also include a number of isolated sedimentary roof pendants, remnants of the pre-batholithic meta-sedimentary cover into which the granitic batholith was intruded. The coastal plain in San Diego County ranges from less than 1 mile to about 14 miles wide. The

Cole Grade Road Improvements, Project No. 5015-18-0024.20 4 coastal plain is underlain at depth by Cretaceous- to Jurassic-age igneous and metamorphic basement rock. The basement rock is overlain by a series of relatively flat-lying and undeformed sedimentary deposits that generally thicken toward the west. These sedimentary deposits range from Cretaceous sedimentary rocks to relatively unconsolidated Holocene alluvium.

The tectonic setting of Southern California is characterized by a series of Quaternary-age fault zones typically consisting of several en echelon faults striking in a north to northwest direction.

These faults are part of the San Andreas fault system, a series of primarily right-lateral, strike-slip faults that form the boundary between the North American and Pacific tectonic plates.

Approximately 50 mm/year of right lateral shear is thought to be accommodated across the parallel fault zones associated with the fault system. In Southern California, the San Andreas fault system encompasses an approximately 150-mile-wide zone extending from Imperial Valley on the east to offshore of San Diego on the west. Major active faults in the eastern part of the fault system include the San Andreas, San Jacinto, and Elsinore fault. Major faults in the western part of the fault system include the Rose Canyon, Coronado Bank, San Diego Trough, and San

Clemente faults. The Julian segment of the Elsinore Fault, the nearest active fault system, has been mapped approximately 7 miles northeast of the project site. Regional faulting is depicted on Plate 7.

7.0 SITE GEOLOGY

7.1 General

The study area is underlain by granitic bedrock associated with the Southern California Batholith and undifferentiated slopewash/residual soils. Geologic cross sections depicting geologic conditions underlying the proposed soldier pile wall area are presented on Plates 3 through 5.

Plate 3 (Cross Section A-A’) depicts bedrock features exposed in the existing cut slope where the wall will be constructed. Plates 4 and 5 (Cross Section B-B’ and C-C’, respectively) depict interpretations of subsurface conditions along east-west oriented cross sections through Borings

1 and 2, respectively. The geology of the general vicinity of the site is shown on Plate 6. Detailed descriptions of the rock and soil materials encountered in our exploratory borings our presented in the boring logs in Appendix A, while a summary is provided in Sections 7.2 through 7.5 below.

A summary of the depths of each geologic unit at each boring location is provided in Table 1 below.

7.2 Granitic Bedrock

Granitic bedrock associated with the Southern California Batholith underlies the entire study area.

Granitic rock is exposed in all of the existing cut slopes included in the study area, and granitic boulders outcrop sporadically on adjacent natural terrain. In most areas, the granitic rock is covered by a relatively thin mantle of slopewash and/or residual soil.

The granitic rock encountered in our borings has been assigned to the Cretaceous-age Tonalite of Cole Grade (Kennedy and Tan, 2005, see Plate 6). Tonalite is an intrusive igneous rock similar to granite but containing more plagioclase feldspar and less potassium feldspar than granite. The primary mineral constituents of these rocks are plagioclase feldspar and quartz, with subordinate amounts of biotite and hornblende, and minor to trace amounts of other minerals.

Cole Grade Road Improvements, Project No. 5015-18-0024.20 5

The tonalite encountered in exploratory borings B1 and B2 is massive (non-foliated), medium- to coarse-grained, fresh (relatively unweathered) to intensely weathered, and very strong to extremely weak. Color is most commonly medium dark gray where fresh , grading to various shades of yellowish-brown to reddish-brown with increasing degrees of weathering. In Boring B1, highly to intensely weathered tonalite extended to a depth of about 21 feet, underlain by moderately to highly weathered tonalite extending to the maximum exploration depth of 31 feet.

In Boring B2, highly to intensely weathered tonalite extended to a depth of about 15 feet, underlain by moderately to highly weathered tonalite to a depth of 24.5, underlain by fresh tonalite extending to the maximum exploration depth of 30 feet.

The granitic rock observed in the existing cut slope at the proposed soldier pile wall location is generally similar to that observed in the rock core borings as described above (see Cross Section

A-A’, Plate 3). Weathering of the tonalite exposed in the cut slope is highly variable, with the degree of weathering increasing with increasing depth overall. Highly to intensely weathered tonalite is exposed in much of the slope, while moderately to highly weathered tonalite is exposed in the lower portion of the slope in some areas. Localized zones or large boulders (“floaters”) of fresh to slightly weathered tonalite are also exposed in the cut slope, and scattered outcrops of fresh to slightly weathered tonalite are present along the top of the slope. Significant jointing observed in the soldier beam wall cut slope is limited to a steeply-dipping joint set around Station

126+60. These joints generally dip 70 to 85 degrees toward the southwest and northeast. The highly to intensely weathered rock, and most of the moderately to highly weathered rock, is generally lacking in well-developed jointing.

Our scope of services did not include subsurface exploration or geologic mapping of the 7 cut slopes to be reconfigured where seismic refraction lines were performed. Cursory observations of these slopes, made in the course of site reconnaissance and seismic refraction line operations, indicate that these cut slopes are generally underlain by granitic rock similar to that exposed in the soldier pile wall cut slope. This rock generally consists of moderately to intensely weathered granitic rock (most likely tonalite) with localized zones and/or floaters of fresh to slightly weathered rock.

7.3 Slopewash/Residual Soil

A relatively thin mantle of undifferentiated slopewash/residual soil is present in most of the study area overlying the granitic rock. As observed in the exploratory borings and exposures in the upper portions of the existing cut slopes, these deposits are generally reddish-brown, dry to damp, and medium dense, and are comprised of highly variable clay/silt/sand mixtures. Significant amounts of clay are present in the slopewash/residual soil in some areas but largely absent in others. The slopewash/residual soils extend to depths ranging from about 1 to 3 feet in most areas but are absent in a few localized areas.

7.4 Artificial Fill

No significant artificial fill soils were identified in the exploratory borings, or the soldier pile wall cut slope, but pockets of artificial fill soils could be present locally.

Cole Grade Road Improvements, Project No. 5015-18-0024.20 6

Table 1 – Range of Depth Geologic Units

Boring Depth Range Geologic Unit

B1

0’-4’ Slopewash/Residual Soil

4’-21’ Highly to Intensely Weathered Tonalite

21’+ Moderately to Highly Weathered Tonalite

B2

0’-2.5’ Slopewash/Residual Soil

2.5’-15’ Highly to Intensely Weathered Tonalite

15’-24.5’ Moderately to Highly Weathered Tonalite

24.5’+ Fresh Tonalite

7.5 Groundwater

Groundwater or water seepage were not encountered in our exploratory borings, and are not expected to be a factor within the study area either during or after construction for the proposed street widening.

8.0 GEOLOGIC HAZARDS

8.1 Fault Hazard

No active or potentially active faults are known to traverse the vicinity of the study area, and the study area is not located within a State of California designated Earthquake Fault Rupture Hazard

Zone. Active and potentially active faults are faults that exhibit evidence of ground displacement in the last 11,000 years and 2,000,000 years, respectively. The Julian segment of the Elsinore Fault, the nearest active fault system, has been mapped approximately 7 miles northeast of the proposed soldier pile wall location.

8.2 Groundshaking

The study area is located within the seismically active Southern California region and is likely to be subjected to significant seismic shaking during the design life of the project. Recommendations for the design of the proposed soldier pile wall with respect to seismic ground shaking are provided in Section 10.3.

8.3 Landslide and Rock Fall Hazard

There are no known landslides within or near the study area, and no evidence of landslides or deep-seated slope instability were noted in our investigation. However, shallow erosion of existing cut slopes, shallow rockfall deposits, and dislodged individual boulders were observed within the study area. Due to the steep slopes along the alignment and the variable weathering of the materials, the potential for surficial slope failures and localized rock falls is considered high.

8.4 Liquefaction and Lateral Spreading

The term liquefaction describes a phenomenon in which loose, saturated, granular soil deposits lose shear strength and mobilize as a result of increased pore water pressure induced by strong ground shaking during an earthquake. Structures founded on or above potentially liquefiable soil may experience settling (both total and differential) and loss of foundation support. The factors known to influence liquefaction potential include soil type, grain size, relative density, confining pressure, depth to groundwater, and the intensity and duration of ground shaking. Soils most susceptible to liquefaction are saturated, loose sandy soils, and some silts.

Cole Grade Road Improvements, Project No. 5015-18-0024.20 7

Lateral spreading is a form of liquefaction-induced ground failure. When an underlying soil layer liquefies, blocks of overlying surficial soil displace downslope or towards a sloping surface or unsupported “free face”. The lateral displacement typically ranges from a few inches to several feet and can cause significant damage to structures.

The study area is underlain by dense granitic rock with a relatively thin mantle of slopewash and/or residual soil. These materials are not considered to be susceptible to liquefaction or lateral spreading.

8.5 Flood Hazard

The study area is not within a Federal Emergency Management Agency (FEMA) 100-year floodplain. The overall risk of flooding with the study area is considered to be minimal.

9.0 RIPPABILITY

9.1 Soldier Pile Retaining Wall

The rippability of the granitic bedrock (tonalite) underlying the proposed soldier pile wall location is expected to vary depending on a number of factors including rock strength, degree of weathering, and joint spacing/orientation. Overall anticipated rippability characteristics can generally be correlated to degree of weathering as follows:

• Slopewash/residual soil and intensely weathered rock – easily rippable

• Highly weathered rock – rippable with varying degrees of difficulty

• Moderately weathered rock – rippable with difficulty, blasting or rock breaking possibly required.

• Fresh to slightly weathered rock – generally non-rippable

Based on subsurface information obtained from or study, including slope-face mapping, exploratory borings, and seismic refraction lines, the proposed soldier pile wall location is underlain by a thin mantle of surficial slopewash/residual soil and granitic bedrock consisting of tonalite (see Plates 3 through 5). About 1 to 3 feet of slopewash/residual soils form a surficial layer overlying the granitic rock in most areas of the existing cut slope. The degree of weathering in the underlying tonalite is highly variable, but generally decreases with increasing depth. Highly to intensely weathered tonalite is exposed in the upper portion of the cut slope throughout its entire length, extending to depths of approximately 2 to 10 feet from the top of the slope. In portions of the cut slope, highly to intensely weathered tonalite extends to the base of the slope

(street grade), so the maximum depth of this material is unknown. Moderately to highly weathered tonalite is exposed in the lower portion of the slope in many areas, extending upward to as little as 2 feet below the top of the slope in some areas. Localized floaters of fresh to slightly weathered tonalite are exposed within the cut slope and at the top of the cut slope, primarily between approximate Stations 126+80 and 128+00.

The granitic rock underlying the proposed soldier pile wall is generally expected to range from easily rippable to rippable with varying degrees of difficulty, with some non-rippable zones or floaters. Excavation for the soldier pile wall and foundations is expected to produce oversize material. Moderately weathered rock could include material that is rippable with a moderate to high degree of difficulty and rock within the lower end of the non-rippable range. Large zones of non-rippable rock would typically require blasting to excavate, whereas other techniques such as

Cole Grade Road Improvements, Project No. 5015-18-0024.20 8 pneumatic rock breaking or expansive grouts might be suitable for excavating localized zones of non-rippable material within generally rippable rock masses.

9.2 Cut Slopes

Based on seismic survey results and cursory slope face observations, the reconfiguration of the

7 cut slope locations included in our study area is expected to encounter granitic rock generally similar to that described above for the soldier pile wall location. The majority of the granitic rock at these locations is expected to be rippable with varying degrees of difficulty, but non-rippable floaters and zones of marginally-rippable to non-rippable rock should also be expected.

10.0 PRELIMINARY RECOMMENDATIONS

10.1 Soldier Pile Retaining Wall

It is our understanding that a permanent soldier pile and lagging retaining wall will be constructed at the eastern right-of-way boundary between approximate stations 123+00 and 129+30 to accommodate the widening of Cole Grade Road. The soldier pile wall will most likely be designed as a cantilevered wall without intermediate bracing as currently planned, but alternately may be designed as a braced wall supported with one row of tieback anchors. Braced walls are usually more rigid with smaller lateral deflections, and therefore earth pressures are usually closer to at-rest pressure conditions, as opposed to active pressure conditions for cantilever walls. Preliminary recommendations for cantilevered and braced soldier pile and lagging walls are provided in the following sections.

10.2 Lateral Earth Pressure

For design of a cantilevered retaining wall, where the surface behind the top of the wall is level, it can be assumed that drained soils will exert a lateral active earth pressure equal to that developed by a fluid with a density of 35 pounds per cubic foot. For the design of braced walls, it may be assumed that drained soils will exert a lateral at-rest earth pressure equal to that developed by a fluid with a density of 50 pounds per cubic foot. In addition to the recommended earth pressure, the wall should be designed to resist any applicable surcharges due to foundation, storage, or traffic loads (if any); however, surcharge pressures are not anticipated at this time.

10.3 Seismic Lateral Earth Pressure

In addition to the above-mentioned lateral earth pressures, retaining walls should be designed to support a seismic active pressure. The recommended seismic active pressure distribution on the wall may be taken as that developed by a fluid with a density of 25 pounds per cubic foot. The seismic earth pressure can be combined with the active earth pressure to estimate the total static plus seismic earth pressure imposed on the wall. For braced wall, the wall should be designed for the greater of the at-rest pressure and combined active plus seismic earth pressure.

10.4 Drainage

A soldier pile and lagging wall is considered a permeable wall because of the joints between the lagging boards. As such, permeable drainage material and perforated drainpipes behind the wall are generally not considered necessary. If shotcrete lagging is used, we recommend that the retaining wall be provided with a drain or weepholes. The drain should include a 4-inch-diameter perforated pipe placed with perforations down at the base of the wall. The pipe should be sloped at least 2 inches in 100 feet and surrounded by filter gravel. The filter gravel should meet the requirements of Class 2 Permeable Material as defined in the current State of California, Cole Grade Road Improvements, Project No. 5015-18-0024.20 9

Department of Transportation, Standard Specifications. If Class 2 Permeable Material is not available, ¾-inch crushed rock or gravel separated from the on-site soil by an appropriate filter fabric can be used. The crushed rock or gravel should have less than 5% passing a No. 200 sieve.

10.5 Design of Soldier Piles

For the design of soldier piles spaced at least two diameters on centers, the allowable lateral bearing value (passive value) of the rock below the level of excavation may be assumed to be

900 pounds per square foot per foot of depth, up to a maximum of 9,000 pounds per square foot.

To develop the full lateral value, provisions should be taken to assure firm contact between the piles and the undisturbed rock. The concrete placed in the soldier pile excavations may be a lean-mix concrete. However, the concrete used in that portion of the soldier pile which is below the planned excavated level should be of sufficient strength to adequately transfer the imposed loads to the surrounding rock.

The frictional resistance between the piles and the retained earth may be used in resisting the downward component of the anchor load. The coefficient of friction between the piles and the retained earth may be taken as 0.4. This value is based on the assumption that uniform full bearing will be developed between the steel soldier beam and the lean-mix concrete and between the lean-mix concrete and the retained earth. In addition, provided that the portion of the soldier piles below the excavated level is backfilled with structural concrete, the piles below the excavated level may be used to resist downward loads.

For resisting the downward loads, the frictional resistance between the concrete piles and the rock below the excavated level may be taken to be 500 pounds per square foot. If the bottom of the pile excavation is cleaned for loose cuttings and verified, an allowable bearing capacity of

5,000 pounds per square foot may be used for weathered rock beneath the pile tip to resist the downward load. The downward load resistance is preliminary based on an assumption of toe length of about 10 feet. For greater toe embedment, the downward load resistance would be greater; we should be consulted regarding locations with a need for higher-than-usual downward load.

10.6 Lagging

Continuous lagging will be required between the soldier piles. The soldier piles and anchors should be designed for the full anticipated lateral pressure. However, the pressure on the lagging will be less due to the arching effect of the retained earth. We recommend that the lagging be designed for a triangular earth pressure distribution (triangular in plan view), with a maximum value of 400 pounds per square foot at the midpoint between soldier piles, and a minimum value of 0 pounds per square foot at the soldier piles. The lagging pressure may be taken as being uniform as a function of depth.

10.7 Anchor Design

Tied-back anchors may be used to resist lateral loads. Either conventional or post-grouted anchors may be used. For design purposes, it may be assumed that the active wedge adjacent to the wall is defined by a plane drawn at 30 degrees with the vertical through the bottom of the excavation. Table 2 below describes the minimum distance that anchors should extend beyond the potential active wedge. The anchors should extend to a greater length if necessary to develop the desired capacities. We should review the anchor lengths when the final locations of the wall

Cole Grade Road Improvements, Project No. 5015-18-0024.20 10 and shored heights have been established. Anchors may require a greater length, depending on the depth of excavation to develop the desired capacities, therefore, some significant encroachment outside the property limits should be anticipated.

The capacities of anchors should be determined by testing of the initial anchors as outlined in a following section. For design purposes, we estimate that drilled friction (also known as gravity-grouted) anchors will develop average friction values as presented in Table 2 below. For post-grouted (also known as pressure-grouted) anchors, it may be estimated that the anchors will develop an average friction of three times the friction values presented below.

Table 2 - Tieback Recommendations for 20-foot Excavation

Depth from retained ground surface to anchor at soldier pile (feet)

Minimum length of anchor beyond active wedge

(feet)

Average friction along anchor length

(psf)

0 to 10 10 400

10+ 10 700

The capacities of anchors should be determined by testing of the initial anchors as outlined in a following section. Only the frictional resistance developed beyond the active wedge will be effective in resisting lateral loads. If the anchors are spaced at least 6 feet on centers, no reduction in the capacity of the anchors needs to be considered due to group action.

10.8 Anchor Installation

The anchors may be installed at angles of 15 to 40 degrees below the horizontal. Caving of the anchor holes may occur, and provisions should be made to minimize such caving. The anchors should be filled with concrete placed by pumping from the tip out, and the concrete should extend from the tip of the anchor to the active wedge. If there is significant caving of the anchor shaft, we suggest that the portion of the anchor shaft within the active wedge be backfilled with sand before testing the anchor. This portion of the shaft should be filled tightly and flush with the face of the excavation. The sand backfill may contain a small amount of cement to allow the sand to be placed by pumping. For post-grouted anchors of 8 inches in diameter or less, the anchor may be filled with concrete to the surface of the wall.

10.9 Anchor Testing

At least four anchors should be tested for 24-hour 200% tests. In addition to the anchors designated for 24-hour tests, 10% of the remaining anchors should be selected for quick 200% tests. The purpose of the 200% tests is to verify the friction value assumed in design. The anchors should be tested to develop twice the assumed friction value. Where satisfactory tests are not achieved on the initial anchors, the anchor diameter and/or length should be increased until satisfactory test results are obtained.

The total deflection during the 24-hour 200% tests should not exceed 12 inches during loading;

the anchor deflection should not exceed 0.75 inch during the 24-hour period, measured after the

200% test load is applied. If the anchor movement after the 200% load has been applied for 12 hours is less than 0.5 inch, and the movement over the previous 4 hours has been less than 0.1 inch, the test may be terminated.

Cole Grade Road Improvements, Project No. 5015-18-0024.20 11

For the quick 200% tests, the 200% test load should be maintained for 30 minutes. The total deflection of the anchor during the 200% quick test should not exceed 12 inches; the deflection after the 200% test load has been applied should not exceed 0.25 inch during the 30-minute period. Where satisfactory tests are not achieved on the initial anchors, the anchor diameter and/or length should be increased until satisfactory test results are obtained.

All of the production anchors should be pretested to at least 150% of the design load; the total deflection during the tests should not exceed 12 inches. The rate of creep under the 150% test should not exceed 0.1 inch over a 15-minute period for the anchor to be approved for the design loading.

After a satisfactory test, each production anchor should be locked-off at the design load. The locked-off load should be verified by rechecking the load in the anchor. If the locked-off load varies by more than 10% from the design load, the load should be reset until the anchor is locked-off within 10% of the design load.

The installation of the anchors and the testing of the completed anchors should be observed by our firm. Permanent anchors should be provided with necessary corrosion protection in accordance with PTI Manual DC35.1-14: Recommendations for Prestressed Rock and Soil

Anchors.

10.10 Deflection

It is difficult to accurately predict the amount of deflection of a shored embankment. It should be realized, however, that some deflection will occur. We estimate that this deflection could be on the order of ½ to 1 inch at the top of the shored embankment. If greater than the estimated deflection occurs during construction, additional bracing may be necessary to minimize settlement of any adjacent structures and utilities. If desired to reduce the deflection of the wall, a greater design lateral earth pressure could be used in the wall design, and a deflection analysis of the wall could be performed.

10.11 Global Slope Stability

Slope stability calculations indicate that the global stability of the retained earth outside the soldier pile wall will have an adequate factor of safety with respect both static and seismic loading conditions.

10.12 Monitoring

Some means of monitoring the performance of the wall system is recommended. The monitoring should consist of periodic surveying of the lateral and vertical locations of the tops of all the soldier piles. We will be pleased to discuss this further with the design team when the design of the wall system has been finalized.

10.13 Cantilevered Retaining Walls with Shallow Foundations

It is our understanding that the proposed retaining wall will most likely be a soldier beam and lagging wall per the preceding sections of this report, but the possibility of constructing a cantilevered retaining wall with shallow foundations has not been ruled out. For design of a cantilevered retaining wall with shallow foundations, where the surface of the backfill is level, it can be assumed that drained soils will exert a lateral active earth pressure equal to that developed by a fluid with a density of 35 pounds per cubic foot. In addition to the recommended earth

Cole Grade Road Improvements, Project No. 5015-18-0024.20 12 pressure, the wall should be designed to resist any applicable surcharges due to foundation, storage, or traffic loads (if any); however, surcharge pressures are not anticipated at this time.

In addition to the above-mentioned lateral earth pressures, retaining walls should be designed to support a seismic active pressure. The recommended seismic active pressure distribution on the wall may be taken as that developed by a fluid with a density of 25 pounds per cubic foot.

Cantilevered retaining walls with shallow foundations should be provided with weepholes spaced every 10 to 15 feet, or a drainage system consisting of a suitable drainage board placed against the back of the wall and a drain at the base of the wall. The drain at the base of the wall should include a 4-inch-diameter continuous perforated pipe placed with perforations down at the base of the wall. The pipe should be sloped at least 2 inches in 100 feet and surrounded by at least 4 inches of ¾-inch crushed rock or gravel surrounded by an appropriate filter fabric. The crushed rock or gravel should have less than 5% passing a No. 200 sieve. In addition, the retaining wall should be damp-proofed or waterproofed.

Conventional spread/continuous footings embedded at least 2 feet below the lowest adjacent grade and underlain by undisturbed rock may be designed to impose a net dead-plus-live load pressure of 5,000 pounds per square foot. A one-third increase in the above bearing value may be used for wind or seismic loads. The recommended bearing value is a net value, and the weight of concrete in the footings may be taken as 50 pounds per cubic foot; the weight of backfill may be neglected when determining the downward loads. We estimate the settlement of the proposed retaining wall, supported on conventional spread/continuous footings in the manner recommended, will be on the order of 1/2 inch or less. Differential settlement is estimated to be less than 1/2 inch in 50 feet.

Lateral loads may be resisted by earth friction and by the passive resistance of the earth. A coefficient of friction of 0.4 may be used between the footings and the supporting weathered bedrock. The passive resistance of the weathered bedrock may be assumed to be equal to the pressure developed by a fluid with a density of 450 pounds per cubic foot. A 1/3 increase in the passive value may be used when considering wind or seismic loads. The passive resistance and the frictional resistance of the earth may be combined without reduction in determining the total lateral resistance.

10.14 Temporary Cut Slopes

Temporary cut slopes are likely to be required if a cantilevered wall with shallow foundations is constructed rather than a soldier pile wall. All excavations should comply with applicable local, state, and federal safety regulations including the current OSHA Excavation and Trench Safety

Standards. Construction site safety is the responsibility of the contractor, who should also be responsible for the means, methods, and sequencing of construction operations. The surficial slopewash/residual soils generally expected to extend to depths of about 1 to 3 feet below the top of the cut slope should be considered as Type B according to OSHA guidelines, while the variably weathered tonalite expected to be exposed in the rest of the temporary slopes can be assumed as OSHA Type A material. The contractor’s competent person should evaluate the temporary slopes on a daily basis as required by OSHA guidelines.

Cole Grade Road Improvements, Project No. 5015-18-0024.20 13

10.15 Compaction

Any required fill should be placed in loose lifts not more than 8-inches-thick and compacted. The fill should be compacted to at least 90% of the maximum density obtainable by the ASTM

Designation D1557 method of compaction. The moisture content of coarse-grained fill at the time of compaction should vary no more than 2% below or above optimum moisture content. The moisture content of fine-grained fill at the time of compaction should be between 2% and 4% above optimum moisture content.

In lieu of compacted fill, it may be practical to use controlled low strength (CLSM) slurry rather than attempting to compact fill in narrow trenches or excavations. The CLSM slurry should have a minimum compressive strength of 100 pounds per square inch (psi) and meet the requirements of County of San Diego.

10.16 Material for Fill

The on-site soils and excavated bedrock material, less any debris or organic matter, are generally considered suitable for use in any required fills. However, any residual soils with a high clay content and an expansion index greater than 35 should be removed from the site or mixed with non-expansive soils to achieve an expansion index of less than 35. Cobbles larger than 4 inches in diameter should not be used in the fill. Any required import material should consist of relatively non-expansive soil with an expansion index of less than 35. The imported materials should contain sufficient fines (at least 15% passing the No. 200 sieve) to be relatively impermeable and result in a stable subgrade when compacted. All proposed import materials should be approved by personnel of our firm prior to being placed at the site.

11.0 CLOSURE

The conclusions, recommendations, and opinions presented here are: (1) based upon our evaluation and interpretation of the limited data obtained from our field and laboratory programs;

(2) based upon an interpolation of soil and rock conditions between and beyond the borings; (3) are subject to confirmation of the actual conditions encountered during construction; and, (4) are based upon the assumption that sufficient observation and testing will be provided during construction.

Cole Grade Road Improvements, Project No. 5015-18-0024.20 14

12.0 REFERENCES

Jennings, C.W., and Bryant, W.D., 2010, Fault Activity Map of California, California Geologic Survey Data Map No. 6.

Kennedy, M.P., and Tan, S S., 2005, Geologic Map of the Oceanside 30’ x 60’ Quadrangle, California.

Ninyo & Moore, October 2, 2012, Geotechnical Evaluation, Cole Grade Road Improvements, Valley Center, California, Oracle Project 1003046, N&M Project No. 106898005.

Ninyo & Moore, June 12, 2014, Supplemental Geotechnical Evaluation, Cole Grade Road Improvements, Valley Center, California, Oracle Project 1003046, N&M Project No. 106898005.

PLATES

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Base: ESRI, 2024.

SITE LOCATION MAP

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Cool Valley Road

Miller Road

Bernabeo Court

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Cross Section A-A' (Map of Cut Slope Face)

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9177 Sky Park Court San Diego, CA 92123 Phone (858) 278-3600 wsp.com

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Variable Clay/Silt/Sand Highly to Intensely Weathered Tonalite Moderately to Highly Weathered Tonalite

Residual Soil Kcg1 Kcg2

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Geologic Units

Kennedy, M.P., Tan, S.S., Bovard, K.R., Alvarez, R.M., Watson, M.J., and Gutierrez, C.I., 2007, Geologic map of the Oceanside 30' x 60' quadrangle and adjacent areas, California, California Geological Survey, Regional Geologic Map RGM-2, 1:100,000, spatial data.

Unit - Description (Age)

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Environment & Infrastructure Inc.

9177 Sky Park Court San Diego, CA 92123 Phone (858) 278-3600 wsp.com

Date Exported: 8/14/2024 12:50 PM

Contacts:

Mzu - Metasedimentary and metavolcanic rocks, undivided (Mesozoic)

Kc…