rfb_13199_for_info_only_4-supplemental_geotechnical_evaluation.pdf

SUPPLEMENTAL

GEOTECHNICAL EVALUATION

COLE GRADE ROAD IMPROVEMENTS

VALLEY CENTER, CALIFORNIA

ORACLE PROJECT NO. 1003046

PREPARED FOR:

County of San Diego

Department of Public Works, Materials Lab 5555 Overland Avenue, Suite 5101

San Diego, California 92123

PREPARED BY:

Ninyo & Moore

Geotechnical and Environmental Sciences Consultants 5710 Ruffin Road

San Diego, California 92123

June 12, 2014 Project No. 106898008

Cole Grade Road Improvements June 12, 2014 Valley Center, California Project No. 106898008

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TABLE OF CONTENTS

Page

1. INTRODUCTION

2. SCOPE OF SERVICES

3. SITE AND PROJECT DESCRIPTION

4. FIELD EXPLORATIONS AND LABORATORY TESTING

4.1. Geologic Reconnaissance

4.2. Seismic Refraction Surveys

4.3. Exploratory Borings

4.4. Laboratory Testing

5. GEOLOGY AND SUBSURFACE CONDITIONS

5.1. Regional Geologic Setting

5.2. Site Geology

5.2.1. Fill

5.2.2. Topsoil/Residual Soils

5.2.3. Alluvium

5.2.4. Granitic Rock

5.3. Groundwater

5.4. Faulting and Seismicity

5.4.1. Ground Motion

5.4.2. Ground Surface Rupture

5.4.3. Liquefaction and Seismically Induced Settlement

5.4.4. Landsliding and Rock Slope Stability

6. CONCLUSIONS

7. RECOMMENDATIONS

7.1. General

7.2. Earthwork

7.2.1. Site Preparation

7.2.2. Excavation Characteristics

7.2.3. Temporary Excavations

7.2.4. Treatment of Existing Fill, Topsoil, Alluvium and Residual Soils

7.2.5. Materials for Fill

7.2.6. Compacted Fill

7.2.7. Drainage

7.3. Fill Placement on Sloping Ground

7.4. Slopes

7.5. Cut Slopes in Bedrock

7.6. Cantilever Retaining Walls

7.6.1. Shallow Footings

7.6.2. Lateral Resistance

7.6.3. Lateral Earth Pressures

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7.6.4. Static Settlement

7.7. Soil Nail Retaining Walls

7.7.1. Materials

7.7.2. Nail Testing

7.7.3. Shotcrete Cover

7.7.4. Construction Considerations

7.8. Seismic Design Parameters

7.9. Corrosion

7.10. Concrete

7.11. Pre-Construction Conference

7.12. Plan Review and Construction Observation

8. LIMITATIONS

9. REFERENCES

Tables Table 1 – Approximate Locations of Seismic Refraction Surveys Table 2 – Seismic Design Factors

Figures Figure 1 – Project Location Figure 2 – Exploration Locations Figure 3 – Geology Figure 4 – Geologic Cross Section A-A’ Figure 5 – Fault Locations Figure 6 – Lateral Earth Pressures for Yielding Retaining Walls

Appendices Appendix A – Boring Logs Appendix B – Laboratory Testing Appendix C – Borings Logs Previously Prepared by Ninyo & Moore (2012) Appendix D – Seismic Refraction Survey (Southwest Geophysics, Inc.)

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1. INTRODUCTION

In accordance with your request, we have performed a supplemental geotechnical evaluation for the proposed improvements along Cole Grade Road in Valley Center, California (Figure 1). This report presents the results of our field exploration and laboratory testing, our conclusions regarding the geotechnical conditions at the site, and our recommendations for the design and earthwork con-struction of this project. The intent of this report is to supplement the findings, conclusions, and recommendations contained in our previous report for the project (Ninyo & Moore, 2012). This report’s focus is to evaluate and provide geotechnical recommendations for soil nail retaining walls and cut slopes that are being considered along portions of the Cole Grade Road alignment. The recommendations contained herein supersede those contained in our previous report (Ninyo &

Moore, 2012), where the recommendations in the two reports conflict with each other.

2. SCOPE OF SERVICES

The scope of services for this study included the following:

Reviewing of background information including available geologic and geotechnical reports, utility maps, project plans, topographic maps, geologic maps, fault maps, and historical stereo-scopic aerial photographs.

Siting and staking of the proposed exploratory boring locations for clearance by Under-ground Service Alert (USA).

Obtaining boring permits from the County of San Diego, Department of Environmental Health.

Performing a geologic reconnaissance of the project to evaluate exposed existing conditions.

Manually excavating, logging, and sampling three hand-auger borings to a depth of up to approximately 3 feet below the existing ground surface, where refusal to our excavating equipment was encountered.

Drilling, logging, and sampling five hollow-stem auger borings in the areas specified by the County of San Diego, Department of Public Works. Bulk and relatively undisturbed drive samples of soil were collected at selected intervals from the borings and transported to our in-house geotechnical laboratory for testing.

Performing five seismic refraction surveys.

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Performing laboratory testing including gradation analysis, in-situ moisture content and dry density, grain size analyses, shear strength, and soil corrosivity.

Compiling and performing engineering analysis of data.

Preparing this supplemental report presenting our findings, conclusions, and recommenda-tions regarding the geotechnical design and construction of the project.

3. SITE AND PROJECT DESCRIPTION

The project area is relatively hilly and generally descends to the south. Elevations range from approximately 1,420 feet mean sea level (MSL) at the south end of Cole Grade Road to 1628 feet

MSL at the intersection of Cole Grade Road and Cool Valley Road.

We understand that an approximately 2.6-mile section of Cole Grade Road will be widened to pro-vide a center turn lane and bike lanes along with a graded shoulder. The improvements include retaining structures at four locations along the roadway (approximately at Stations 50+00, 55+50, 101+00, and 127+50). Three of the retaining walls will be designed to support cut slopes, currently from near vertical to approximately 1:1 (horizontal to vertical), ascending from the roadway, and one retaining wall will be designed to support new fill over a slope descending from the roadway. We un-derstand that as an alternative to the proposed retaining walls, graded cut slopes in conjunction with additional right-of-way acquisition and/or soil nail walls are being considered.

4. FIELD EXPLORATIONS AND LABORATORY TESTING

Our field explorations at the site were conducted on March 14, 2014, and on May 1, 5, 6, and 19, 2014.

The explorations consisted of geologic reconnaissance, excavation of three hand auger borings, drilling, logging, and sampling of five soil borings using a truck-mounted drill rig, and seismic refraction surveys. Figure 2 indicates the approximate locations of the mapped cross section, bor-ings, and seismic surveys. Boring logs from our recent subsurface exploration are presented in

Appendix A, while those from Ninyo & Moore’s previous subsurface exploration (2012) are pre-sented in Appendix C.

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4.1. Geologic Reconnaissance

Our geologic reconnaissance consisted of observation of the surficial geology of the site and the excavation of three hand auger borings (HA-1, HA-2, and HA-3) on March 14, 2014.

The hand auger borings were drilled at the locations shown on Figures 2 and 4 because the

County of San Diego desired geotechnical information at that location, which is located in an environmentally sensitive area (which precluded the use of a truck mounted drill rig).

Geologic Cross section A-A’ (Figure 4) depicts the locations of the three hand auger borings and the observed geologic units encountered.

4.2. Seismic Refraction Surveys

Five seismic refraction surveys were performed on May 1 and 19, 2014 adjacent to the bridge abutments where Keys Creek crosses the Cole Grade Road Alignment. The locations of the seismic refraction surveys are summarized in Table 1. The results of the seismic re-fraction surveys are included in Appendix D.

Table 1 – Approximate Locations of Seismic Refraction Surveys

Location of North End of Survey Location of South End of Survey Seismic Refraction

Survey Station Offset from Centerline* Station Offset from Centerline*

SL-1 139+35 20 feet west 138+10 20 feet west SL-2 137+00 35 feet east 135+75 35 feet east SL-3 135+40 30 feet east 134+15 35 feet east SL-4 137+05 20 feet west 136+05 20 feet west SL-5 135+55 20 feet west 134+80 20 feet west

Note:

*Centerline of existing Cole Grade Road

4.3. Exploratory Borings

Five exploratory borings (B-1 through B-5) were drilled on May 5 and 6, 2014 using a truck-mounted drill rig equipped with hollow stem augers. The borings were drilled to depths of up to approximately 28.5 feet below existing grades.

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4.4. Laboratory Testing

Geotechnical laboratory testing of representative soil materials obtained during our subsur-face exploration included in-situ moisture content and dry density, gradation, direct shear testing, and soil corrosivity (i.e., electrical resistivity, pH, chloride content, and sulfate con-tent). The laboratory tests were performed in our in-house laboratory by Ninyo & Moore’s laboratory technicians. Results of the in-situ moisture content and dry density tests are shown at the corresponding sample depths on the boring logs in Appendix A. The results of the other laboratory tests performed are presented in Appendix B.

5. GEOLOGY AND SUBSURFACE CONDITIONS

Our findings regarding regional and site geology, including geologic structure, groundwater con-ditions, excavatability, faulting and seismicity, seismic hazards, and landslides and rockslides at the subject site are provided in the following sections.

5.1. Regional Geologic Setting

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

Geomorphic Province. This geomorphic province encompasses an area that extends ap-proximately 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 portion of the province in San Diego

County that includes the project area consists generally of Cretaceous-age Cole Grade to-nalite, as shown in Figure 3.

The Peninsular Ranges Province is traversed by a group of sub-parallel faults and fault zones trending roughly northwest. Several of these faults are considered active faults. The Elsinore, San Jacinto, and San Andreas faults are active fault systems located east of the project area and the Coronado Bank, San Diego Trough, San Clemente, and Rose Canyon faults are active faults located west of the project area. The Julian segment of the Elsinore Fault, the nearest active fault

106898008 R.doc 5 system, has been mapped approximately 7 miles northeast of the project site a shown in Fig-ure 5. 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

Per the regional geologic mapping, the site is predominantly underlain by the Cretaceous gran-itic rock. However, geologic units encountered at the site also include fill, residual soils/topsoil, and alluvium that mantle the granitic rock. Generalized descriptions of the earth units encoun-tered 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.

5.2.1. Fill

Fill was observed in borings B-4 and B-5 to approximate depths of between 1 and 1.5 feet bgs. Fill was also encountered in our hand auger borings HA-1 and HA-2 to approximate depths of 3 feet and 1.5 feet bgs, respectively. From our borings, the fill consisted of fine-to medium-grained, medium dense to dense silty sand with varying amounts of gravel and cobbles. Documentation regarding placement of the fill was not provided.

5.2.2. Topsoil/Residual Soils

Topsoil/residual soils were observed in borings B-1, B-2 and B-3 to depths of up to ap-proximately 2 feet (bgs). As encountered in our exploratory borings, the topsoil/residual soils consisted of fine- to medium-grained, loose silty sand with organic material.

5.2.3. Alluvium

Alluvium was observed in our hand auger boring HA-3 to a depth of approximately

2 feet bgs. The alluvium consisted of loose, poorly graded sand with silt.

5.2.4. Granitic Rock

As noted previously, granitic rock mapped as Cole Grade tonalite underlies the project site. The granitic rock is exposed in slopes adjacent to the roadway and was observed in our borings. This material generally consists of generally massive, fine- to medium-

106898008 R.doc 6 grained, weathered rock. Less weathered corestones, which are expected to be resistant to excavation by conventional methods, were observed at numerous locations. We also observed widely spaced dikes and veins of more resistant material within the granitic rock. As noted, the bedrock was generally massive, however fractures were observed and the orientation of fractures were generally N30°W/25°SW. Although not perva-sively fractured, intersecting joints and fractures were noted. Refusal of the drilling rig was encountered on the granitic rock in our borings B-1 through B-5 at approximate depths of between 13 and 28.5 feet bgs. Practical refusal was encountered in our hand auger boring HA-3 at a depth of 2 feet bgs.

5.3. Groundwater

Groundwater was not encountered in our exploratory borings. As the project is in an area of bedrock, groundwater can be expected to occur within joints or fractures and may not occur at a specific elevation. Groundwater can also be expected in areas along drainage channels such as Keys Creek. Fluctuations in the groundwater level and perched water conditions may occur due to variations in ground surface topography, subsurface geologic conditions and structure, rainfall, irrigation, and other factors.

5.4. Faulting and Seismicity

Like the majority of southern California, the project area is considered to be seismically ac-tive. Based on our review of the referenced geologic maps and stereoscopic aerial photographs, as well as on our geologic field mapping, the subject site is not underlain by known active or potentially active faults (i.e., faults that exhibit evidence of ground dis-placement in the last 11,000 years and 2,000,000 years, respectively). However, the site is located in a seismically active area and the potential for strong ground motion is considered significant during the design life of the proposed structures.

In general, hazards associated with seismic activity include strong ground motion, ground surface rupture, liquefaction, and seismically induced settlement. These hazards are dis-cussed in the following sections.

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5.4.1. Ground Motion

The 2013 California Building Code (CBC) recommends that the design of structures be based on spectral response accelerations in the direction of maximum horizontal response (5 percent damped) having a 1 percent probability of collapse in 50 years. Such spectral response accelerations represent the Risk-Targeted Maximum Considered

Earthquake (MCER) ground motion. The horizontal peak ground acceleration (PGA) that corresponds to the MCER for the site was calculated as 0.59g using the United States

Geological Survey (USGS, 2013) seismic design tool (web-based). The mapped and design PGA were estimated to be 0.57g and 0.38g, respectively, using the USGS (2013) calculator and the American Society of Civil Engineers (ASCE) 7-10 Standard.

5.4.2. Ground Surface Rupture

Based on our review of the referenced literature and our field evaluation, no active faults are known to cross the project vicinity. Therefore, the potential for ground rupture due to faulting at the site is not a design consideration. However, lurching or cracking of the ground surface as a result of nearby seismic events is possible.

5.4.3. Liquefaction and Seismically Induced Settlement

Liquefaction of cohesionless soils can be caused by strong vibratory motion due to earthquakes. Research and historical data indicate that loose granular soils and non-plastic silts that are saturated by a relatively shallow groundwater table are susceptible to liquefaction. Since the site is underlain by bedrock materials at relatively shallow depths, it is our opinion that the potential for liquefaction and seismically induced set-tlement is not a design consideration at the site.

5.4.4. Landsliding and Rock Slope Stability

Based on our review of published geologic literature and aerial photographs and our subsur-face exploration, no deep-seated landslides or related features underlie or are adjacent to the site. However, erosion of the existing cut slopes, shallow rockfall deposits, and boulders

(corestones) were observed along the existing slopes and adjacent to the site. Due to the

106898008 R.doc 8 steep slopes that exist along the alignment and variable weathering of in-situ materials, the potential for rock falls to impact the site is considered high.

6. CONCLUSIONS

Based on our review of the referenced background data, subsurface exploration, and laboratory test-ing, it is our opinion that construction of the proposed roadway improvement project is feasible from a geotechnical standpoint provided the recommendations presented in this report are incorporated into the design and construction of the project. In general, the following conclusions were made:

The project site is underlain by fill, topsoil, residual soils, alluvium and granitic bedrock ma-terials. Fill, topsoil, residual soils and alluvium generally consist of loose to dense, silty sands. The bedrock was weathered but was generally observed to range from moderately hard to very hard. Corestones are present in the bedrock.

Rip rap was encountered in the bottom of the drainage channel in the vicinity of our hand auger borings HA-1, HA-2, and HA-3.

Groundwater was not encountered during our field evaluation. However, groundwater can be expected to occur within joints or fractures and may not occur at a specific elevation.

Groundwater can also be expected in areas along drainage channels such as Keys Creek. Fluc-tuations in the groundwater level and perched water conditions may occur due to variations in ground surface topography, subsurface geologic conditions and structure, rainfall, irrigation, and other factors.

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.

Areas of adverse jointing and fracturing may be encountered during cuts for the planned slopes.

Such areas may be potentially unstable and could require the use of shoring or other methods of stabilization.

Much of the site excavations along the roadway alignment are anticipated to generate significant quantities of oversize rock. Disposal or crushing of this material should be anticipated.

Results of our seismic refraction traverses (Appendix D) indicate that the depth to relatively competent bedrock materials is variable where Cole Grade Road crosses Keys Creek. Surficial materials in the vicinity of the seismic traverses are expected to be comprised of alluvium, top-soil, and/or fill. Review of the traverses also suggests that corestones may be present within the upper, more highly weathered portion of the granitic rock and/or surficial materials.

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The existing fill, topsoil, and near-surface alluvial soils are generally loose to medium dense, and are not considered to be adequate for the support of new structural fill embankments in their present state.

The potential for rock falls to affect the site is considered high. Protection of the new road-way from rock falls is recommended. Typical methods to protect the roadway from rock falls are listed below in Section 7.5.

Based on Caltrans and FHWA criteria, the site is considered to be corrosive.

7. RECOMMENDATIONS

Based on our understanding of the project, the following recommendations are provided for the design and construction of the cut slopes and retaining structures that are being considered for use at the proposed project. The proposed site improvements should be constructed in accor-dance with the requirements of the applicable governing agencies.

7.1. General

As noted above in Section 3 of this report, improvements associated with the proposed widening of

Cole Grade Road include retaining structures at four locations along the roadway (approximately at

Stations 50+00, 55+50, 101+00, and 127+50). Three of the retaining structures will be designed to support cut slopes, currently from near vertical to approximately 1:1 (horizontal to vertical), ascend-ing from the roadway, and one retaining wall will be designed to support new fill over a slope descending from the roadway. Geotechnical recommendations for cantilever retaining walls are presented in our previous report (Ninyo & Moore, 2012) and are reiterated in Section 7.6 of this re-port. We understand that as an alternative to the proposed retaining walls, graded cut slopes in conjunction with additional right-of-way acquisition and/or soil nail walls are being considered.

Based on our supplemental geotechnical evaluation, we consider the excavation of 1:1 (hori-zontal to vertical) cut slopes and the construction of soil nail walls within granitic bedrock materials to both be viable methods to accommodate the proposed widening of Cole Grade

Road in the vicinity of Stations 50+00, 101+00, and 127+50. Selection of the appropriate method for use at the site should be based on the potential benefits, as well as the potential construction considerations and life-cycle costs associated with each.

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The construction of cut slopes within the granitic rock may result in lower construction costs.

However, the presence of corestones and more resistant, less weathered zones within the gran-itic rock may require the use of special excavation methods, such as heavy ripping, rock breaking, and/or blasting. The intersection of joints and other discontinuities within the rock may increase the potential for rock falls to occur. In addition, the cut slopes will require more right-of-way than the near-vertical retaining structures that were recommended in our previous report (Ninyo & Moore, 2012) or soil nail walls that are also being considered. Maintenance of slope areas (including repair of eroded areas, maintaining of surface drainage along tops and toes of slopes, clearing of loose/raveling materials and other debris, and potential installa-tion of rock fences along the toes of slopes) is another consideration. The evaluation of long-term life-cycle costs is outside the established scope of this report.

Like the other retaining structures described in our previous report (Ninyo & Moore, 2012), a soil nail wall can be expected to result in near-vertical cuts, which could potentially require less right-of-way than the cut slope (depending on the design length of the soil nails). The soil nail wall, which includes a shotcrete facing, can result in less exposed rock face and thus a lower po-tential for rock falls than a cut slope within these materials. In addition, less frequent maintenance can be anticipated for the soil nail walls, as opposed to cut slopes. However, soil nail walls can be expected to have higher costs associated with their design and construction versus those for cut slopes. Like cut slopes, the presence of corestones and less weathered zones within the granitic rock could necessitate the use of such special excavating methods as rock breaking, heavy ripping and/or blasting. In addition, drilling holes (for soil nails) through corestones and/or more resistant weathered zones could require the use of special drilling meth-ods such as percussion drilling and coring in order to achieve design bonded length for the nails.

In light of the above information, we recommend that the County of San Diego perform a cost-benefit analysis in the selection of whether cut slopes within the granitic bedrock or soil nail walls (or other retaining structures) is more appropriate for the Cole Grade Road widen-ing project. The cost-benefit analysis should be based on life-cycle cost considerations.

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7.2. Earthwork

In general, earthwork should be performed in accordance with the Caltrans Standard Speci-fications (Caltrans, 2010a) and the recommendations presented in this report. Ninyo &

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

7.2.1. Site Preparation

Site preparation should begin with the removal of existing improvements, vegetation, utility lines, asphalt, concrete, and other deleterious debris from areas to be graded. Tree stumps and roots should be removed to such a depth that organic material is generally not present. Clearing and grubbing should extend to the outside of the proposed excava-tion and fill areas. Debris and unsuitable material (e.g., oversize and organic materials) generated during clearing and grubbing should be removed from areas to be graded and disposed of per Section 16-104 Removal and Disposal of Materials of the Caltrans

Standard Specifications (Caltrans, 2010a).

7.2.2. Excavation Characteristics

As described in the boring logs in Appendix A, portions of the roadway where cut slopes and soil nail walls are being considered are underlain by granitic bedrock. Excavations will encounter corestones and hard rock, which will make such excavations difficult and require special construction considerations, including heavy ripping, rock breaking, and/or blasting. A specialty contractor with significant experience in rock excavation may be utilized to achieve the desired construction. In addition, as noted, caving of fill, topsoil, and alluvium should be anticipated during performance of excavations in these materials due to the loose condition and granular nature of the on-site soils.

Excavations should be made under the observation of a registered Geotechnical Engi-neer, Certified Engineering Geologist, or their designee.

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7.2.3. Temporary Excavations

Temporary excavations into the bedrock materials may encounter intersecting joints and other discontinuities, which will result in blocks of rock that may be susceptible to falling. A registered Geotechnical Engineer, Certified Engineering Geologist, or their designee should be contacted to evaluate the exposed joints, corestones, and rockfall potential during con-struction. The contractor should take precautionary measures to mitigate the potential for rockfall during construction. The precautionary measures may include, but are not limited to, reinforced shotcrete coverings, wire mesh and cable netting, setbacks, or debris barriers.

For temporary excavations, we recommend that the following Occupational Safety and

Health Administration (OSHA) soil classifications be used:

Fill, Topsoil, Alluvium and Residual Soil Type C Granitic Bedrock Type A

Upon making the excavations, the soil classifications and excavation performance should be evaluated in the field in accordance with OSHA regulations. Temporary exca-vations should be constructed in accordance with OSHA recommendations. For trench or other excavations, OSHA requirements regarding personnel safety should be met us-ing appropriate shoring (including trench boxes) or by laying back the slopes to no steeper than 1.5:1 (horizontal to vertical) in fill and alluvium and 0.75:1 (horizontal to vertical) in bedrock. Temporary excavations that encounter seepage may be shored or stabilized by placing sandbags or gravel along the base of the seepage zone. Excava-tions encountering seepage should be evaluated on a case-by-case basis. On-site safety of personnel is the responsibility of the contractor.

Shoring systems for excavations may be designed using the lateral earth pressure pa-rameters provided in our previous report (Ninyo & Moore, 2012). The lateral earth pressures contained therein (including those imposed by surcharge loads) should be evaluated by a structural engineer for the design of the shoring systems.

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7.2.4. Treatment of Existing Fill, Topsoil, Alluvium and Residual Soils

As noted, the existing fill, topsoil, alluvium and residual soils are loose to medium dense and are not considered adequate for structural support in their present condition.

Where new fill slopes will be constructed (such as where the existing bridge approaches will be expanded at Cole Grade Road’s crossings with Keys Creek), we recommend that the existing fill and alluvial soils be removed to the underlying bedrock and replaced with compacted fill. Results of seismic refraction traverses conducted adjacent to where

Keys Creek crosses the Cole Grade Road alignment (Appendix D) indicate that remov-als in these areas could be somewhat variable, ranging from less than 5 feet (along seismic line SL-3) to 15 feet or more (in the vicinity of seismic lines Sl-1, SL-2, SL-4 and SL-5) . The actual depths of removals will depend on the project layout and actual depths of the surficial materials encountered during excavations. The extent and depths of removals should be evaluated by Ninyo & Moore or the County’s designated repre-sentative in the field based on the materials exposed. The existing fill may be reused as engineered compacted fill if it meets the criteria described below.

7.2.5. 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 mate-rial 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

(2006b). 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 represen-tative prior to filling or importing.

bmehtlan Highlight

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7.2.6. 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 contents generally near the optimum moisture content. The scarified materials should then be compacted to 90 percent relative compaction as evaluated by California Test (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 (2006b), 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 inclines 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.

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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.

7.2.7. Drainage

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

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

Civil Engineer.

7.3. Fill Placement on Sloping Ground

Fills constructed on sloping ground having an inclination steeper than 6:1 (horizontal to verti-cal) should be keyed and benched into competent materials underlying loose soils. Keys should generally be 15 feet in width or greater and extent at least 3 feet into the competent ma-terial. The actual width of the toe keys and extent of removal of any existing loose surficial soil or other native materials should be evaluated by Ninyo & Moore or the County’s desig-nated representative in the field during construction. In addition, toe key excavations should be observed by Ninyo & Moore or the County’s designated representative prior to placing fill.

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7.4. Slopes

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

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

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

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

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

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

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7.5. Cut Slopes in Bedrock

The global stability of the cut slopes into bedrock excavations is contingent on the shear strength of the rock mass. Based on the observations made in the field, the materials encountered in the borings, and our evaluation of field and laboratory data, global failure of the proposed 1:1 (horizontal to verti-cal) cut slopes is not a design concern. However, intersecting joint patterns and various degrees of weathering were observed in the existing rock materials. The stability of individual rock blocks within the cut slope will be governed by the discontinuities within the rock mass.

The potential for rockfall will be a design and construction consideration and should be evaluated during construction. If adverse discontinuity orientations are encountered, the po-tential rockfall hazard should be mitigated with protective measures. The extent of the protection will be based on the consequence and potential cost of damage to the roadway, or life safety if workers or motorists will be present. Possible mitigation alternatives may in-clude but are not limited to, reinforced shotcrete coverings, wire mesh and cable netting, setbacks, or construction of debris barriers. A specialty contractor or manufacturer can pro-vide specific mitigation measures appropriate for the project.

In light of the above, we recommend that a Certified Engineering Geologist observe the exca-vation of the slope and map the exposed slope face. This will help to document adversely-oriented discontinuities or other geologic factors that could affect the stability of the slope. At that time, recommendations to mitigate the observed conditions can be made.

7.6. Cantilever Retaining Walls

Although details are not provided on the plans by the County (County of San Diego, 2012), we understand that cantilever retaining walls are being considered in four locations (in the vicinity of Stations 50+00, 55+50, 101+00, and 127+50). The proposed retaining walls may be supported on shallow, spread footings bearing on engineered fill compacted in accor-dance with the recommendations presented in this report or on bedrock. Foundations should be designed in accordance with structural considerations and the following recommenda-tions. In addition, requirements of the appropriate governing jurisdictions and applicable building codes should be considered in the design of the structures.

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7.6.1. Shallow Footings

The proposed retaining walls may be supported on conventional shallow, spread foot-ings. Footings should extend 18 inches or more below the lowest adjacent finished grade and have a width of 24 inches or greater. Footings for the proposed retaining walls, as described and bearing on compacted fill or bedrock materials, may be de-signed using an allowable bearing capacity of 3,000 per square foot (psf). The footings should be setback from the toe or top of slopes in accordance with Figure 1808.7.1 of the CBC (CBC, 2010). The spread footings should be reinforced in accordance with the recommendations of the project structural engineer.

7.6.2. Lateral Resistance

For resistance of footings to lateral loads, we recommend an allowable passive pressure of 400 psf per foot of depth up to 4,000 psf. This value assumes that the ground is hori-zontal for a distance of 10 feet, or three times the height generating the passive pressure, whichever is greater. We recommend the upper 1 foot of soil not protected by pavement be neglected when calculating passive resistance. For frictional resistance to lateral loads, we recommend that a coefficient of friction of 0.35 be used between soil and con-crete. The allowable lateral resistance can be taken as the sum of the frictional and passive resistance provided the passive resistance does not exceed one-half of the total allowable resistance. The passive resistance values may be increased by one-third when considering loads of short duration such as wind or seismic forces.

7.6.3. Lateral Earth Pressures

Recommendations for lateral earth pressures to be used in design of the yielding retain-ing walls are provided on Figure 6. The values presented are based on non-expansive and free-draining (granular) backfill conditions. The quality and consistency of the backfill should be checked prior to its use behind the retaining walls.

Measures should be taken to reduce the potential build-up of moisture behind the retaining walls. Drainage design should include free-draining backfill materials and perforated drains according to the Caltrans Standard Plans (Caltrans, 2010b). Solid outlet pipes should be

106898008 R.doc 19 connected to the perforated drains and then routed to a suitable area for discharge of accu-mulated water. The portions of retaining walls supporting backfill should be coated with waterproofing compound or covered with a similar material to reduce the potential for infil-tration of moisture through the walls. It is the responsibility of the project structural engineer and/or the retaining wall contractor to provide specifications for waterproofing materials and suitable methods of application.

7.6.4. Static Settlement

We estimate that the proposed retaining walls, designed and constructed as recommended herein, will undergo total settlement on the order of 1 inch. Differential settlement on the order of 0.5 inches over a horizontal span of 40 feet should be expected.

7.7. Soil Nail Retaining Walls

It is our understanding that the County of San Diego is considering the use of soil nail walls as an alternative to cut slopes. Preliminary soil nail recommendations for the subject walls are presented below. These recommendations were developed based on the subsurface informa-tion obtained from our exploratory borings (B-1 through B-5) and surface data gathered from our geologic reconnaissance of the site. These soil nail recommendations are also based on our understanding of the general wall profiles and sections along the existing road alignment. We assume that no additional fill will be placed above the top of the soil nail walls. In the event that additional fill materials are to be placed and the retained heights are to be increased, the following recommendations will need to be revisited and refined, as needed.

Based on our understanding of the proposed wall configuration, we anticipate that up to 10 rows of nails will be installed. This is based on a typical nail spacing of 4 feet on center in both the horizontal and vertical directions. An ultimate unit pullout resistance of 3,700 pounds per square foot (psf) and an allowable unit pullout resistance of 1,850 psf is estimated for the bonded por-tion of the nail in the bedrock materials comprising of granitic rock. This pullout resistance is based on Table 3.10 of the Federal Highway Administration Soil Nail Design Guidelines

(FHWA, 2003) and assumes rotary drilling methods and gravity grouting of the soil nails. We

106898008 R.doc 20 recommend that soil nails be inclined at an angle of 15 degrees below the horizontal plane with a minimum grout diameter of 6 inches. A nail bar diameter of 1 inch is recommended for design.

For preliminary design purposes, we recommend that soil nails be designed using an un-bonded length of 10 feet and a bonded length of 10 feet or more into the bedrock materials of the granitic rock. The actual bonded length should be determined by the wall designer based on anticipated loading considerations. The design bond length may be revised based on the pullout testing of the nails during construction. Design of the soil nail walls should be performed in accordance with FHWA Soil Nail Wall Design Guidelines (FHWA, 2003) and should utilize minimum factors of safety presented on Table 5.3 of the FHWA Guidelines.

In the event the actual nail spacing chosen for the project is different than that recommended herein, the recommendations presented above should be reevaluated and modified as needed. The nail lengths may also need modification if underground utilities are present near the retained areas. While preparing the soil nail layout profile, special care should be taken in limiting the non-anchored portion of the wall (i.e., the top, bottom and the edge segments of the wall) to less than the nail spacing (i.e., less than 4 feet in this case).

7.7.1. Materials

The nail bars should conform to ASTM A615 - Grade 75 or ASTM A722 - Grade 150. The soil nails should be epoxy coated in accordance with ASTM A775. The epoxy coat should have a minimum thickness of approximately 0.03-inch, and should be electro-statically ap-plied. If potentially aggressive ground conditions (i.e., low electrical resistivity) are encountered, the use of encapsulation should be considered. Encapsulation should be achieved by grouting the steel bar inside a corrugated HDPE or PVC sleeve. A neat cement grout containing admixtures to control water bleed from the grout should be used to fill the annular space between the bar and the sleeve. The cement grout for the nail should consist of either neat cement or a sand-cement mixture with a minimum three-day compressive strength of 1,500 pounds per square inch (psi) and a minimum 28-day compressive strength of 3,000 psi per ASTM C109. The cement should conform to ASTM C150, Type II/V Port-land cement. Fine aggregate for the grout mix should comply with ASTM C33. Water used

106898008 R.doc 21 for grout should potable, clean and free from substances deleterious to concrete and steel.

Testing of nail grout during construction should be performed at a frequency no less than one test for every 50 cubic yards of grout placed or once a week, whichever occurs first.

7.7.2. Nail Testing

The soil nails should be tested during construction to evaluate the design assumptions and the nail capacities. The contractor should provide equipment and instrumentation needed to check the adequacy of the nails. A dial gauge capable of measuring displace-ments to 0.01-inch precision should be used to measure the nail movement. A hydraulic jack and pump should be used to apply the test load, and the jack and a calibrated pres-sure gauge should be used to measure the load. The standard testing procedures typically consist of the following methods:

Verification Test – These tests are typically performed on a limited number of sacri-ficial nails to check that 1) the design test load (DTL) may be safely carried,

2) effective bond length corresponds to the design requirements, and 3) the residual movement is within tolerable range. The verification test consists of a single cycle of incremental loading to a maximum test load of 200 percent of the DTL in accor-dance with the guideline presented in Appendix B1 of FHWA Manual for Design and Construction Monitoring of Soil Nail Walls (Publication No. FHWA-SA-96- 069R, dated October 1998). The verification test should be conducted on at least three soil nails within the wall. The location of the verification test nails will be provided by the County’s designated representative once the soil nail wall profile is developed. The sacrificial nails used for verification tests should not be used as production nails.

Proof Test – These tests are typically performed on about 5 percent of the produc-tion nails in each row to check that the load-deflection behavior of the production nail is consistent with the specified acceptance criteria. The proof test consists of a single cycle of incremental loading to a maximum test load of 150 percent of the DTL in accordance with the guideline presented in Appendix B1 of the aforemen-tioned FHWA Manual (1998). The location of the proof test nails will be evaluated by Ninyo & Moore or the County’s designated representative once the soil nail wall profile is developed.

The verification and proof test schedules for the nails, including the acceptance criteria and repair mechanism of failed test nails, should be developed by the contractor utiliz-ing his experience on similar projects, the nail design/testing recommendations

106898008 R.doc 22 presented here, and the FHWA (1998) guidelines. In general, the acceptance criteria for the tested nails should be based on the following aspects.

For verification tests, a total creep movement of less than 2 millimeters (mm) per log cycle of time between the 6- and 60-minute readings is measured during creep testing and the creep rate is linear or decreasing throughout the creep test load hold period.

For proof tests, a total creep movement of less than 1 mm is measured between the 1- and 10-minute readings or a total creep movement of less than 2 mm is measured between the 6- and 60-minute readings and the creep rate is linear or decreasing throughout the creep test load hold period.

The total measured movement at the maximum test load exceeds 80 percent of the theoretical elastic elongation of the test nail unbonded length.

A pullout failure does not occur at the maximum test load. Pullout failure is defined as the load at which attempts to further increase the test load result in continued…