00-31-32-01_1-3-B-16_2008_05_23_Smugglers_Gulch_Geotech_Report_Ninyo Moore.pdf

US Army Corps of Engineers L os Angeles District

Department of Homeland Security Customs and Border Protection San Diego Sector

Area V – Smuggler’s Gulch Border Infrastructure System

Contract No.: W912 BV-07-D-2018/CQ01

Geological and Geotechnical Considerations

Prepared By:

Ninyo & Moore Geotechnical and Environmental Sciences Consultants 5710 Ruffin Road San Diego, California 92123

23 May 2008

GEOLOGICAL AND

GEOTECHNICAL CONSIDERATIONS

SMUGGLER’S GULCH BORDER FENCE

SAN DIEGO COUNTY, CALIFORNIA

PREPARED FOR:

HNTB/ Kiewit

PREPARED BY:

Ninyo & Moore

Geotechnical and Environmental Sciences Consultants 5710 Ruffin Road

San Diego, California 92123

May 23, 2008 Project No. 106327001

Smuggler’s Gulch Border Fence May 23, 2008 San Diego County, California Project No. 106327001

1. FIELD EXPLORATION AND LABORATORY TESTING

Our field exploration of the subject site included reconnaissance-level geologic mapping and ob-servation and a subsurface evaluation. These activities were conducted between March 25 and April 11, 2008. The subsurface evaluation consisted of soil borings, test pits, and geophysical surveys. The approximate locations of the subsurface evaluation are indicated on Figure 2.

As noted our subsurface evaluations included soil borings, test pits, and geophysical surveys (seismic refraction). The specifics of the subsurface evaluations are presented in Table 1.

Table 1 - Summary of Subsurface Explorations

Type of Evaluation General Location Number of Explorations

Soil Borings Bottom of Smuggler’s Gulch 4 Soil Borings Goat Canyon 2

Test Pits Smuggler’s Gulch (includes the mesa east of Smuggler’s Gulch)

Test Pits Goat Canyon 6 Test Pits Spooners Mesa 11 Geophysical Surveys Bottom of Smuggler’s Gulch 4

The subsurface exploration locations were selected based on the results of our background geo-technical review and field reconnaissance. Prior to commencing the subsurface exploration and to obtain clearance for the potential presence of existing utilities, the selected subsurface explorations were staked and the United States Border Patrol and Underground Service Alert were notified. The soils borings were drilled to approximate depths of 32½ to 61½ feet below adjacent grades. The test pits were excavated to approximate depths of 4 to 16 feet below adjacent grades. Detailed logs of the borings and test pits are presented in Appendix I.

The geophysical surveys were performed using a 12-channel, digital seismograph with a 10-pound hammer as the energy source. The results of the seismic refraction traverses are presented in Appendix II.

Samples of the earth material encountered were collected from our subsurface explorations and returned to our in-house geotechnical laboratory for analysis. Geotechnical laboratory testing in-cluded in-situ dry density and moisture content, grain size analysis, Proctor density, shear strength, R-value, and corrosivity. Laboratory test results are presented in Appendix III.

2. GEOLOGY AND SUBSURFACE CONDITIONS

The following sections present our findings relative to regional geology, site geology, and groundwater.

2.1. Regional Geologic Setting

The project study area is situated in the western portion of the Peninsular Ranges geomor-phic province of southern California. This geomorphic province encompasses an area that extends 125 miles from the Transverse Ranges and the Los Angeles Basin, south to the Mexican border, and beyond another 795 miles to the tip of Baja California (Norris and Webb, 1990; Harden, 1998). The geomorphic province varies in width from 30 to 100 miles, most of which is characterized by northwest trending mountain ranges separated by subpar-allel fault zones. In general, the Peninsular Ranges are underlain by Jurassic-age metavolcanic and metasedimentary rocks and by Cretaceous-age igneous rocks of the south-ern California batholith. The westernmost portion of the province in San Diego County, including the project area, generally consists of uplifted and incised Upper Cretaceous-, Ter-tiary-, and Quaternary-age sedimentary rocks.

The Peninsular Ranges are traversed by several major active faults. The Whittier-Elsinore, San Jacinto, and the San Andreas faults are major active fault systems located northeast of the site and the Agua Blanca-Coronado Bank, San Clemente, and Rose Canyon faults are ac-tive faults located to the west-southwest. Major tectonic activity associated with these and other faults within this regional tectonic framework is right-lateral, strike-slip movement.

These faults, as well as other faults in the region, have the potential for generating strong ground motions at the project site. Further discussion of faulting relative to the site is pro-vided in the Faulting and Seismicity section of this report.

2.2. Site Geology

Geologic units encountered during our subsurface evaluation include fill, alluvium, top-soil/colluvium, landslide deposits, and sedimentary earth material of the Pleistocene-age Lindavista Formation and the Pliocene-Pleistocene-age San Diego Formation. Generalized descriptions of the earth units encountered are provided below. Following this discussion presented in Table 2 below are the specifics of our findings at each of the general site loca-tions.

2.2.1. Undocumented Fill

Undocumented fill (hereafter fill) was encountered in several of our subsurface explora-tions. The fill is generally associated with roadways and structures within and adjacent to Smuggler’s Gulch and Goat Canyon and as surficial deposits within the drainages of those canyons. As encountered in our subsurface explorations, the fill generally con-sisted of brown to dark brown, dry to damp, loose to medium dense, clayey sand, gravelly sand, and sandy gravel.

2.2.2. Topsoil/Colluvium

Topsoil/colluvium was encountered in several of our subsurface explorations. As en-countered, the topsoil/colluvium generally consisted of dry to moist, loose to medium dense, silty sand, gravelly sand, and sandy gravel. The topsoil/colluvium is generally thicker along the lower elevations of the slopes and grades into alluvium near the bot-toms of the canyons.

2.2.3. Alluvium

As encountered in our explorations, alluvium generally consisted of moist, loose to me-dium dense gravelly sand and sandy gravel with locally abundant cobble- to boulder-sized clasts.

2.2.4. Landslide Deposits

An ancient landslide has been mapped on the eastern side of Smuggler’s Gulch (Ken-nedy and Tan, 1977; Kennedy and Tan, 2005; Kleinfelder, 1999; Kleinfelder, 2006).

The mapped landslide underlies the proposed roadway from roughly Station 126+00 to Station 137+50. The landslide also extends approximately 1,000 feet north of the pro-posed construction and extends to the south roughly 1,000 feet into Mexico. Based on our review of previous mapping and aerial photographs, the landslide is a composite feature that has displaced several formational blocks of San Diego and Lindavista For-mations to the west through a combination of rotational and translational failures. The basal failure surface of the landslide, developed within the San Diego Formation, has not been observed due to access and drilling limitations and due to the dense cobble conglomerate nature of major portions of the slide deposits. The toe of the landslide is currently buried by the alluvial soils within Smuggler’s Gulch. The approximate contact between the landslide debris, alluvial soils and formational soils are presented on the figures contained in Appendix I. The stability of the landslide is discussed herein.

2.2.5. Lindavista Formation

The Lindavista Formation has been recently redesignated as Very Old Paralic Deposits (Kennedy and Tan, 2005). However, to remain consistent with previous design work, the older Lindavista Formation terminology will be used throughout. As encountered in our explorations, earth materials of the Pleistocene-age Lindavista Formation were en-countered in our subsurface explorations and as outcrops in the higher elevations of the site. This material generally caps the mesas and hills in the Smuggler’s Gulch area. As encountered, materials of the Lindavista Formation generally consist of red-brown to light brown, massive to thickly bedded, weakly to well cemented fine- to medium-grained silty sandstone, fine sandy siltstone, and sandy pebble to cobble conglomerate.

2.2.6. San Diego Formation

Earth materials of the Pliocene-Pleistocene-age San Diego Formation were encountered in our subsurface explorations and as outcrops. It unconformably underlies the Lindav-ista Formation beneath the mesas and underlying the alluvium in the canyons. As encountered, the San Diego Formation generally consists of light reddish brown and yellow-brown, weakly cemented, finely-bedded to massive, silty fine-grained sandstone and weakly to moderately cemented sandy cobble conglomerate with sandstone inter-beds.

Table 2 - Findings of Subsurface Explorations

General Location Generalized Findings

Fills: Encountered to depths of greater than 11 feet below existing grades Topsoil/Colluvium: Encountered to depths of approximately 6 feet below existing grades Alluvium: Encountered to depths of approximately 46 feet below existing grades Landslide Deposits: Encountered at depths as shallow as 0 feet in the eastern wall of Smuggler’s Gulch and estimated to extend to depths of roughly 25 feet below existing base of Smuggler’s Gulch

Smuggler’s Gulch

San Diego Formation: Encountered at depths as shallow as 3 feet below existing grades Fills: Encountered in isolated areas to depths of greater than 5 feet below existing grades Topsoil/Colluvium: Encountered to depths of up to approximately 3 feet below existing grades Landslide Deposits: Encountered at depths as shallow as 1.5 feet below existing grades

Mesa East of Smuggler’s Gulch

Lindavista Formation: Encountered at depths as shallow as 1.5 feet below existing grades Fills: Encountered to depths of greater than 13 feet below existing grades Topsoil/Colluvium: Encountered to depths of up to approximately 3 feet below existing grades Alluvium: Encountered to depths of approximately 30 feet below existing grades

Goat Canyon

San Diego Formation: Encountered at depths as shallow as 3 feet below existing grades Fills: Encountered in isolated areas to depths of greater than 5 feet below existing grades Topsoil/Colluvium: Encountered to depths of up to approximately 3 feet below existing grades Spooners Mesa

Lindavista Formation: Encountered at depths as shallow as 1.5 feet below existing grades

2.3. Groundwater

Groundwater or seepage was encountered in our subsurface explorations within Smuggler’s Gulch. Groundwater was not encountered in our borings or test pits within Goat Canyon or on Spooner Mesa.

Groundwater including seepage was encountered in our test pits excavated within the active Smuggler’s Gulch drainage at depths of roughly 12 feet below existing grades. Groundwater was also encountered within our borings at approximately 20 feet below existing grade.

Fluctuations in the groundwater level may occur due to variations in ground surface topog-raphy, subsurface geologic conditions and structure, rainfall, irrigation, and other factors.

3. GEOTECHNICAL ANALYSIS AND DESIGN RECOMMENDATIONS

Our geotechnical analyses for the improvements addresses the settlement characteristics of the underlying soils and proposed embankments, evaluation of the stability of the proposed cut and fill slopes, design criteria for the reinforced concrete box culvert and recommendations for pavement structural sections. Our analyses and recommendations are based on the conditions encountered and the anticipated earthwork.

3.1. Earthwork

Earthwork for the project will include excavations to achieve grade and generate materials for fills, fill placement to achieve planned grades, removal of compressible surficial soil to reduce ground settlement, remedial grading to improve slope stability, the excavation and backfilling of trenches for storm drain systems and water lines, and the placement of pave-ment sections. The earthwork should be conducted in accordance with the requirements of governing jurisdictions and applicable codes.

3.1.1. Rippability

The excavations along the proposed Border Fence alignment are anticipated to encounter sedimentary materials of the San Diego Formation and Linda Vista Formation, alluvium, and surficial soils. Alluvium and surficial soils may be considered generally excavatable and the sedimentary formational material may be considered generally rippable with grading equipment in good working order. Zones or concretions of strongly cemented sandstone or gritstone may be encountered which may require single shank ripping.

Blasting is not anticipated on this project. Excavations within the surficial soils should be relatively easy to perform.

3.1.2. Embankments

The following sections describe earthwork associated with the construction of proposed embankments along the subject segment. Recommendations regarding site preparation, remedial grading (over-excavation), subsurface drainage, fill placement and compaction are provided.

3.1.2.1. Site Preparation

Prior to placement of fill associated with the construction of the proposed im-provements, the surface area within the embankment footprint should be cleared of any surface obstructions and stripped of brush and vegetation. Obstructions, which extend below finish grade, if present, should be removed and the resulting holes filled with compacted soil.

Subsequent to clearing and grubbing operations, removal of loose or soft surficial soils (including topsoil, undocumented fill, colluvium, and alluvium) should be per-formed to expose competent material as evaluated by Ninyo & Moore’s field representative. Site preparation of ground surfaces that are to receive fill should in-clude scarification, moisture conditioning, and compaction of the exposed surfaces in accordance with the recommendations presented herein. The scarification and compaction should extend to a depth that includes any loose/soft, dry materials as indicated by our field representative, but in no case to a depth less than approxi-mately 6 inches.

Removal and recompaction below the reinforced box culvert and associated retain-ing walls should be made so to achieve a minimum of a 3 foot thick uniform compacted fill mat below the foundations. Existing alluvium should be removed to a depth of 3 feet below existing grade or 3 feet below foundation elevation, which-ever is greater, and replaced with compacted fill. Compaction should be to 95 percent of ASTM D1557. These excavations shall be observed by a Geotechnical Engineer and where adverse soil conditions are encountered, recommendations made during construction.

3.1.2.2. Materials for Fill

The proposed improvements will include construction of roadway embankments with a reinforced concrete box culvert constructed perpendicular to the alignment at or near existing grades. The heights of the new embankments will be up to ap-proximately 130 feet. Fill material for the construction of these embankments will be derived from excavation of cut areas and removal of alluvial/colluvial deposits.

Fill soils should be free of trash, debris, roots, vegetation, organics, or other delete-rious materials. Rocks larger than 4 feet in diameter should be broken into smaller pieces or placed as oversized materials following our recommendations presented in the Oversized Material section. Materials for use as fill should be evaluated by Ninyo & Moore’s representative prior to filling.

The surfical soils may contain some expansive materials. Expansive materials, if encountered, should be placed in the lower portion of embankments or placed in non-structural fill areas. During placement of expansive soils in the lower portions of the embankment, efforts should be made to mix expansive soils with sandier ma-terials.

3.1.2.3. Compacted Fill

Prior to placement of compacted fill, the contractor should request an evaluation of the exposed ground surface by Ninyo & Moore. Unless otherwise recommended, the exposed ground surface should then be scarified to a depth of approximately 6 inches and watered or dried, as needed, to achieve moisture contents above the laboratory optimum. The scarified materials should then be compacted to a compac-tion of 95 percent as evaluated by American Society for Testing and Materials (ASTM) Test Method D 1557. The evaluation of compaction by the geotechnical consultant should not be considered to preclude requirements for observation or ap-proval by governing agencies. It is the contractor's responsibility to notify Ninyo &

Moore and the appropriate governing agency when project areas are ready for ob-servation, and to provide reasonable time for that review.

Fill materials should be moisture conditioned to generally above the laboratory op-timum moisture content prior to placement. The optimum moisture content will vary with material type and other factors. Moisture conditioning of fill soils should be generally consistent within the soil mass. Prior to placement of additional com-pacted fill material following a delay in the grading operations, the exposed surface of previously compacted fill should be prepared to receive fill. Preparation may in-clude scarification, moisture conditioning, and recompaction as outlined above.

Compacted fill should be placed in uniform lifts of up to 12 inches in loose thick-ness. Prior to compaction, each lift should be watered or dried as needed to achieve a moisture content generally above the laboratory optimum, mixed, and then com-pacted by mechanical methods to a relative compaction of 95 percent or higher as evaluated by ASTM D 1557. Successive lifts should be treated in a like manner un-til the desired finished grades are achieved.

The outer five feet of fill slopes for the Smugglers Gulch embankment should be compacted to 90 percent or higher as evaluated by ASTM D1557. The face of the slope should be compacted at four foot intervals as the depth of fill increases. Track walking and trimming of the slope face should be performed after completion of the embankment fill.

3.1.2.4. Oversized Material

Oversized material should be placed in accordance with the following recommenda-tions. Rocks approximately 8 inches in dimension and greater may be utilized within the compacted fill, provided they are placed in such a manner that nesting of rock does not occur.

Where oversized rock (greater than 8 inches in dimension) or similar irreducible material is generated during grading, it is recommended, where practical, to utilize the material in the crushing operation to generate materials to be utilized as base material in the pavement sections, or placed within the central core of the embank-ment as outlined below.

Rocks or similar irreducible materials greater than 8 inches but less than 4 feet in dimension generated during grading may be placed in windrows and capped with finer materials in accordance with the recommendations of the geotechnical con-sultant, the approval of the governing agencies. Selected native granular soil (Sand Equivalent of 30 or higher) should be placed and flooded over and around the wind-rowed rock such that voids are filled. Windrows of oversized materials should be staggered so that successive windrows of oversized materials are not in the same vertical plane. Rocks greater than 4 feet in dimension should be broken down to 2 feet or smaller before placement, or they should be disposed of off site.

3.1.3. Concrete Box Culvert Settlement

To reduce the magnitude of the anticipated box culvert settlement, we recommend ap-proximately 3 foot removal and recompaction of the underlying alluvial soils beneath the bottom of the box culvert. The underlying fill mat should be compacted to 95 percent relative compaction. Where the proposed box culvert is within a 1:1 (horizontal to verti-cal) projection from the backfill of the existing drainage channel, the removal elevation within the drainage channel should be extended horizontally beneath the box culvert such that a uniform fill thickness is established below the box culvert and extending 8 feet be-yond the limit of the box. The excavations should be observed by Ninyo & Moore to evaluate the conditions exposed. If adverse conditions are exposed, additional removal of materials may be required. Settlements of the box culvert are anticipated to be on the or-der of 4 inches along the center section and ½ inch along the outer edges. Adverse effects of settlement may be mitigated by constructing the box culvert in sections with flexible joints and placing the box culvert on a camber fill (higher relative elevation toward the middle of the embankment and lower elevation at the edges).

3.1.4. Embankment Settlement

The proposed roadway embankments will be underlain by colluvium, alluvium, and for-mational material. For the relatively large embankment being placed over colluvium or alluvium, settlement of the ground surface is expected to be generally on the order of 8 inches. Approximately one-third of this settlement is attributed to colluvium or alluvium, and the remaining two-thirds is attributed to the embankment fill settling under its own weight. Where embankments will be founded on formational material, surface settlement is expected to be on the order of 2 inches.

To reduce the potential for ground settlement for the design life performance of the im-provements, particularly where the alignment is underlain by saturated alluvium, we recommend removing the loose/soft compressible soil and replacing it with compacted fill.

Soils subject to collapse consist predominately of sand and silt size particles arranged in a loose honeycomb structure held together by cementing agents. The cementing agents lose strength when wetted, resulting in a relatively sudden collapse of the soil. The surficial soils on site, including the colluvium and alluvium, consist generally of stiff clay or silt and loose to medium dense clayey sand. Material with abundant clay constituents in gen-eral are not considered subject to hydro-collapse. Our field and laboratory results indicate that settlement due to hydro-collapse is unlikely.

3.1.5. Shrinkage/Bulking of Onsite Soils

The development of the subject segment of the Border Fence will involve extensive cuts and fills (i.e., subgrade excavation and embankment construction). Based on our subsur-face exploration and experience with similar materials in the project area, we recommend an average shrinkage factor of 0.85 or 15 percent to be applied to colluvial/alluvial de-posits. Average bulking factors of 1.00 and 1.05 are recommended for Lindavista and San Diego Formations, respectively. This would indicate that there would be no shrinkage for Lindavista Formation and 5 percent for San Diego Formation. Further, these factors can vary by 10% to 20% from the recommended values.

3.2. Slope Stability

Slope stability analyses for the fill and cut slopes planned for this alignment were performed using the two-dimensional stability analysis program, GSTABL7 (Gregory, 2003). The geo-logic conditions exposed during our exploration were evaluated under static and pseudo-static conditions. A seismic coefficient of 0.15g was used for pseudo-static analysis.

Critical failure surfaces were generated using the “CIRCLE” searching algorithm in the GSTABL7 program for Bishop’s analysis and Spencer’s general limit equilibrium analysis methods. Shear strength parameters used in our analyses were based on laboratory testing, our experience with similar materials and professional judgment. Slope stability analyses are shown in Appendix IV.

3.2.1. Cut Slopes

The proposed cut slopes will be up to about 100 feet high and have inclinations of ap-proximately 1½:1 (horizontal to vertical) or flatter. Our stability analyses of cut slopes indicate that the proposed slopes will generally have calculated factors of safety of 1.5 or higher under static conditions, and 1.1 or higher under pseudo-static conditions.

Our evaluation of the proposed cut slopes was based on the explorations conducted to date and our experience in the project area. We recommend that the slope excavations be observed by Ninyo & Moore’s field geologist for evidence of weak planes, friable sand lenses, high plasticity claystones, or other geologic conditions that may adversely affect the stability of the excavated slopes. Should additional adverse conditions be encountered during grading, the slope stability should be re-evaluated by Ninyo & Moore to assess the appropriate remedial actions. Buttress or stability fills, as needed, should be evaluated during grading.

3.2.2. Fill Slopes

The proposed roadway embankment fill slopes will be up to about 130 feet high at ap-proximate inclinations of 2:1 (horizontal to vertical) or flatter. Our analyses of deep-seated stability of the fill slopes indicate that the proposed embankment slopes, will have calculated factors of safety of 1.5 or higher and 1.1 or higher for static and pseudo-static conditions, respectively.

3.2.3. Surficial Slope Stability

The excavations will likely encounter primarily sedimentary formational materials such as the San Diego Formation and Linda Vista Formation. Lesser amounts of surficial soils, including topsoil, fill, alluvium, and colluvium will also be encountered during the exca-vations.

Surfical stability was calculated assuming infinite slope conditions with the upper 3 feet of material saturated. A stability analysis was also performed to evaluate a condition where a thin layer of colluvium or topsoil may be left in place at the top of a cut slope.

Our analyses indicated that the excavated slopes will have calculated factors of safety greater than 1.5 for surficial stability.

Slope face materials will require periodic maintenance. Wetting and drying of the slope face is expected to reduce the natural cohesion in the soil, resulting in a greater potential for erosion. Seepage along continuous beds may result in surficial slope failures. We rec-ommend that excavated slopes be evaluated by Ninyo & Moore’s field geologist during grading. Slopes exposing seepage or conditions conducive to seepage (allu-vium/colluvium, channels, claystone, or siltstone bedding exhibiting out-of-slope inclinations) should be evaluated for their potential for surficial failures. Recommenda-tions regarding the placement of stabilizing fills where surficial failures may occur should be provided based on the conditions observed. Alternatively, in lieu of the aforemen-tioned recommendations, a landscape architect may be consulted to provide plans to reduce the erosion potential of the slope face and increase surficial stability. Slopes should be protected from erosion by planting as soon as practical after the completion of grading. Erosion control and planting should be in accordance with project specifications.

3.3. Surface Drainage

Surface water should be diverted away from the top of slopes and should not be allowed to pond at the toe of slopes. Positive drainage should be established at the toe of all slopes and surface water should be diverted off site by means of appropriate erosion-reducing devices.

Runoff should not be allowed to flow over the top of slopes. Slope ditches, sometimes called surface, brow, interception, or slope protection ditches, should be provided at the top of cuts where drainage will be directed from natural slopes towards the roadway. When the ditch grade exceeds a 4:1 (horizontal to vertical) slope, a down drain is advisable.

Benches should be constructed on the faces of the fill slopes as specified in the California Building Code at vertical intervals of 30 feet. These benches should be sloped at 2 percent or steeper with drainage directed into concrete brow ditches.

3.4. Reinforced Retaining Wall and Box Culvert Design

Cantilevered retaining walls may be designed using an allowable bearing capacity of 2,000 pounds per square foot (psf) plus 200 psf for each additional foot of width beyond a 5-foot width up to a maximum capacity of 3,000 psf. A compacted unit weight of 140 pounds per cubic foot for retained and fill materials with internal friction angle of 31 degrees may be used for de-sign. An active pressure represented by an equivalent fluid unit weight of 47 pounds per cubic foot (pcf) for level backfill and a passive resistance of 300 pounds per square foot per foot of depth are recommended. Active pressures for 2:1 and 2.8:1 sloping backfills may be taken as 70 pcf and 58 pcf respectively. A coefficient of friction of 0.35 may be used for con-crete in contact with soil and 0.60 may be used for soil to soil friction. Thickness and reinforcement of the mat foundation should be in accordance with the recommendations of the project structural engineer.

The reinforced box culvert may be designed using a compacted unit weight of 140 pcf for vertical design earth loads. Lateral design pressure range may be taken as 100 pcf maximum and 35 pcf minimum. Settlement of the culvert is anticipated as discussed in Section 3.1.3.

Thickness and reinforcement of the mat foundation should be in accordance with the recommen-dations of the project structural engineer.

3.5 MSE Wall Design

Mechanically Stabilized Embankment (MSE) walls may be designed using a friction angle of 31 degrees and a unit weight of 130 pounds per cubic foot for retained, reinforced and foundation soils. The recommended strength parameters were obtained from laboratory test results.

3.5.1 Remedial Grading for MSE Wall Construction

Based on our subsurface evaluation of the existing soils within Goat Canyon, we recom-mend that the existing alluvial materials underlying the proposed Mechanically Stabilized Earth (MSE) retaining walls be removed to a depth of 3 feet below the proposed founda-tion bottom elevation. The overexcavated materials should then be replaced with compacted fill materials to the proposed foundation subgrade.

The removal limits should extend a horizontal distance of 5 feet beyond the limits of the retaining wall structure (i.e., the wall and the reinforced backfill zone) or the approximate depth of removal, whichever is greater. The depth and extent of the removals should be evaluated in the field by Ninyo & Moore.

In addition to the removal and replacement of the existing fills, we recommend that the exposed remedial excavation subgrade be scarified to a depth of 6 inches. After the soil has been scarified, it should be moisture conditioned to generally above the optimum moisture content and recompacted to 95 percent of the modified Proctor dry density as evaluated by ASTM D 1557.

The retained, reinforced and foundation fill materials should be moisture conditioned generally above the optimum moisture content and compacted to 95 percent of the modi-fied Proctor dry density as evaluated by the American Society for Testing and Materials (ASTM) Test Method D 1557.

3.5.2 Bearing Capacity

Based on the recommended remedial grading, the MSE wall may be designed for a bear-ing using an allowable bearing capacity of 3,000 psf. Additional overexcavation performed beyond this depth will yield an increased bearing capacity and a potential re-duction in differential and total settlements. The allowable bearing capacity may be increased by 1,000 psf for each additional foot of increase in the thickness of compacted fill materials up to a total bearing capacity of 5,000 psf.

The design and construction of the MSE wall should be performed in accordance with the recommendations presented above and the recommendations presented in the referenced reports. The MSE walls should be designed to tolerate differential settlement.

3.6 Pavement Structural Section Design

Pavement structural sections are based on Section 600 of the Caltrans Highway Design Manual. R-value testing should be performed on the roadway subgrade during construction.

Contractor is required to achieve the design R-value of 40 for subgrade material. If this is unachievable then the recommended pavement sections should be reevaluated for the lower R-values. Recommendations for R-value testing frequency during construction should be es-tablished in the construction quality control inspection procedures.

Pavement section recommendations were developed utilizing the results of R-value tests performed on materials collected from Lindavista and San Diego Formation during our sub-surface exploration. A traffic index (TI) of 6.0 was used in calculations utilizing the Caltrans Pavement Design Method. Based on our calculations, a pavement section consisting of 3 inches of asphalt concrete (AC) over 6 inches of Class II aggregate base (AB) is recom-mended as a preliminary design section.

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PROJECT NO.

106327001

DATE

4/08

FIGURE

BORING AND TEST PIT LOCATION MAP

SMUGGLER'S GULCH, BORDER FENCE PROJECT

SAN DIEGO, CALIFORNIA

NOTE: ALL DIRECTIONS, DIMENSIONS AND LOCATIONS ARE APPROXIMATE

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APPROXIMATE LOCATION OF SEISMIC LINE

APPROXIMATE LOCATION OF TEST PIT

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APPROXIMATE SCALE IN FEET

2.3. Groundwater