FIO_W912PL-18-B-0002_GeotechnicalReport.pdf

Geotechnical Engineering Report

OCEAN VIEW CHANNEL C06 IMPROVEMENT

Orange County, California

Prepared for:

United States Army Corps of Engineers Los Angeles District

915 Wilshire Boulevard Los Angeles, California, 90017

Prepared by:

Tetra Tech 1360 Valley Vista Drive

Diamond Bar, California 91765

March 3, 2017

1360 Valley Vista Drive * Diamond Bar, CA 91765 * Tel: 909-860-7777 * Fax: 909-860-8017

TET 16-97E

March 3, 2017

Ms. Anne Hutton United States Army Corps of Engineers Los Angeles District 915 Wilshire Boulevard Los Angeles, California

Subject: GEOTECHNICAL ENGINEERING REPORT

OCEAN VIEW CHANNEL C06 IMPROVEMENT

Orange County, California

Dear Ms. Hutton:

Tetra Tech is pleased to submit the results of our geotechnical investigation for the proposed improvements of Ocean View Channel C06 between Beach Boulevard and the confluence with East Garden Grove-Wintersburg Channel C05 in Orange County, California. The improvements will include repair of existing erosion cutting and slope instability within the channel slopes and construction of a concrete lining along the channel slopes and invert. This report includes a brief description of the proposed project, discussions regarding field and laboratory investigative efforts, subsurface conditions, local geology, engineering seismology, and geotechnical conclusions and recommendations for design and construction of the proposed channel improvements. The appendices to the report include logs of borings, results of laboratory tests, seismic demand analyses, liquefaction and sensitivity analyses, slope stability analyses, and photographs of the site.

We appreciate the opportunity to provide our professional services on this project. If you have any questions regarding this report or if we can be of further service, please do not hesitate to contact the undersigned.

Respectfully submitted, Tetra Tech, Inc.

Andrew McLarty, C.E.G.

Project Engineering Geologist

Yonglang Li, Ph.D., P.E.

Project Engineer

Douglas Bell, G.E.

Principal Engineer

Distribution: Addressee (4 hardcopies)

Filename: Ocean View Channel Improvements GEO RPT 100% 2017-03-03.docx

Ocean View Channel C06 Improvement March 3, 2017 i

TABLE OF CONTENTS

Page

INTRODUCTION

SCOPE OF SERVICES

PROPOSED PROJECT AND SITE DESCRIPTION

FIELD EXPLORATION AND LABORATORY TESTING

4.1. FIELD EXPLORATION

4.2. LABORATORY TESTING

GEOLOGIC AND SUBSURFACE CONDITIONS

5.1. REGIONAL GEOLOGY

5.2. SITE GEOLOGY

5.3. SUBSURFACE CONDITIONS

ENGINEERING SEISMOLOGY

6.1. GENERAL SEISMIC SETTING

6.2. SEISMIC HAZARDS AND POTENTIAL FOR SURFACE FAULT RUPTURE

6.3. LIQUEFACTION POTENTIAL AND DYNAMIC SETTLEMENT

DESIGN RECOMMENDATIONS

7.1. GENERAL

7.2. SLOPE STABILITY ANALYSES

7.3. CONSTRUCTION CONSIDERATIONS

7.4. CONCRETE LINING

7.5. SUBDRAINAGE SYSTEM

7.6. RETAINING STRUCTURES

7.7. COMPRESSIBLE / COLLAPSIBLE SOILS

7.8. EXPANSIVE SOILS

7.9. SOIL CORROSION

8 GENERAL SITE GRADING RECOMMENDATIONS

9. DESIGN REVIEW AND CONSTRUCTION MONITORING

9.1 PLANS AND SPECIFICATIONS

9.2 CONSTRUCTION MONITORING

10. LIMITATIONS

11. SELECTED REFERENCES

ii

TABLE OF CONTENTS

(continued)

List of Figures

Figure 1 – Site Location Map Figure 2 – Geologic Mapping and Boring Location Map Figure 3 – Geologic Map Figure 4 – Generalized Soil Profile Figure 5 – Historic High Groundwater Map Figure 6 – Regional Fault and Seismicity Map Figure 7 – Seismic Hazard Zone Map Figure 8A – Benching Details (Eroded Areas) Figure 8B – Benching Details (Surficial Slide Areas) Figure 9 – Design Pressure for Concrete Lining Figure 10 – Subdrain Details

Appendices

Appendix A – Logs of Exploratory Borings Appendix B – Laboratory Testing Appendix C – Seismic Demand Analyses Appendix D – Liquefaction and Sensitivity Analyses Appendix E – Slope Stability Analyses Appendix F – Site Photos

INTRODUCTION

This report presents the results of the geotechnical and geologic engineering evaluation performed by Tetra Tech for the proposed improvements to Ocean View Channel C06 Improvement between Beach Boulevard and the confluence with East Garden Grove-Wintersburg Channel C05 in Orange County, California. This study was performed for the United States Army Corps of Engineers – Los Angeles District (LAD) as part of Contract W912PL-15-D-0007, Task Order 0003. The approximate location of the project site is shown in Figure 1.

The purpose of this study was to evaluate the subsurface conditions along the subject channel alignment and to provide geotechnical recommendations for the design and construction of the proposed improvements. This report summarizes the data collected and presents the findings, conclusions, and recommendations.

SCOPE OF SERVICES

The scope of services performed by Tetra Tech for this project consisted of the following tasks:

Review of available background data, including several previous phases of field investigation performed by the LAD, geotechnical literature, geologic maps, and seismic hazard maps relevant to the subject site.

A site reconnaissance to observe the site surface conditions and to mark boring locations.

Field geologic mapping of the existing channel.

A subsurface evaluation, including the excavating, logging, and sampling of 5 exploratory borings to a depth of approximately 31.5 feet below the existing grade. Soil samples obtained from the borings were transported to a geotechnical laboratory for visual classification and testing.

Laboratory testing of selected samples recovered from the borings to evaluate geotechnical engineering properties of the on-site soils.

Engineering evaluation of the data collected to develop geotechnical recommendations for the design and construction of the proposed channel improvements, including the following items:

An evaluation of general subsurface conditions and description of types, distribution, and engineering characteristics of subsurface materials.

An evaluation of the liquefaction and seismic settlement potential of on-site materials.

An evaluation of slope stability of the improved channel.

Recommendations for site grading, including subgrade preparation, fill specifications and placement, temporary dewatering, and temporary shoring.

Recommendations for design of foundation systems including allowable bearing capacity, lateral resistance, and settlement estimates.

An assessment of the expansion, consolidation and hydrocollapse potential of the on-site soils.

An evaluation of the corrosion potential of the on-site soils to buried concrete and steel.

Preparation of this report, including the provision of reference maps and illustrations, a summary of the collected data, conclusions, and geotechnical recommendations for the design and construction of the proposed improvements.

PROPOSED PROJECT AND SITE DESCRIPTION

The proposed project includes improvements to a portion of the Ocean View Channel C06 located between Beach Boulevard and the confluence with East Garden Grove - Wintersburg Channel C05 in Orange County, California. The channel is located in a relatively close proximity to both residential and commercial developments on both banks. Improvements immediately adjacent to the right-of-way boundary are typically roadways, parking areas and lightly loaded carports.

However, along the left (southern) bank of the channel, a three-story building is located very close to the right-of-way boundary between approximate Station 8+00 and 9+50. The impact of loading from this building, as well as the traffic loading, has been included in the slope stability analyses performed for this study.

Within the right-of-way limits an unpaved access road is located along the left bank of the channel.

The current earthen channel was constructed as an open trapezoidal channel with sideslopes at 1.5(H):1(V) and about 15 to 16.5 feet high. The channel sideslopes have undergone significant erosion and slumping (Tetra Tech, 2012). Much of this damage occurred during the storm events between December 16 and 24, 2010. This has led to fairly pervasive instability and slumping of the channel sideslopes. The backscarps of these slumps are steep, generally ranging from about 1(H):1(V) to 0.5(H):1(V) with heights up to about 12 feet, with localized areas in a near vertical condition for heights up to about 8 feet. Tetra Tech (2012) reported that the channel invert has been scoured up to about 1.5 feet.

It is our understanding that the proposed improvements to the subject channel will consist of restoration of the channel to its original design configuration by placement of earthen fill and concrete lining along the channel banks and invert. The design section of the improved channel consists of sideslopes at 1.5(H):1(V) with an invert width of 8 feet. The length of proposed improved channel is approximately 2,115 feet. Re-grading and lining of the channel slopes will also require the reconstruction of several existing storm drain outlets into the channel.

FIELD EXPLORATION AND LABORATORY TESTING

4.1. Field Exploration

Prior to initiation of the field exploration program, a geotechnical exploration and laboratory testing plan was prepared to define the quantity and depth of borings and laboratory testing requirements deemed necessary to develop a sufficient level of geotechnical data for the project design. The plan also identified all proposed equipment, site access roads, materials, and testing procedures. The plan was submitted to the LAD for review and comments, and was approved for execution.

The subsurface soil and groundwater conditions beneath the site were explored on September 12 and 13, 2016 and included the drilling, logging, and sampling of 5 hollow stem auger borings (B-1 through B-5). Approximate locations of the borings are shown in Figures 2A through 2C. Prior to starting the field exploration program, a field reconnaissance was conducted to observe surface conditions and to mark the locations of the planned boreholes. Underground Service Alert was notified of the exploratory borings locations at least 48 hours prior to drilling.

The borings were excavated using a truck-mounted drill rig equipped with an 8-inch diameter hollow stem auger, and were advanced to a depth of about 31.5 feet. Both driven ring-type and bulk samples were retrieved at selected depths during drilling. The driven samples were collected utilizing a California-type sampler driven by a 140-pound hammer with a drop of 30 inches.

Standard Penetration Testing (SPT) was also performed using the same hammer and drop as for the ring-type samples in general accordance with ASTM D1586.

All borings were surface-logged by a California Certified Engineering Geologist (CEG), who also prepared the recovered samples for subsequent reference and laboratory testing. Borings were backfilled with cement bentonite grout at the conclusion of drilling. Soil boring logs are presented in Appendix A.

4.2. Laboratory Testing

Laboratory tests were performed on selected samples obtained from the borings in order to aid in the soil classification and to evaluate pertinent engineering properties of the foundation soils. The following tests were performed:

In-situ Moisture Content and Dry Density;

Grain Size Distribution;

Atterberg Limits;

Maximum Density and Optimum Moisture Content;

Direct Shear;

Unconfined Compressive Strength;

Consolidation;

Expansion Index; and Soil Corrosion Tests.

Testing was performed in general accordance with applicable ASTM Standards and California Test Methods. Results of all laboratory tests are presented in Appendix B. For ease of referral to the soil profile, selected laboratory results are presented on the boring logs in Appendix A.

GEOLOGIC AND SUBSURFACE CONDITIONS

5.1. Regional Geology

The subject site is situated within the central block of the Los Angeles Basin, a broad alluviated coastal lowland plain located near the northerly end of the Peninsular Ranges Geomorphic Province (Norris, 1990). The physiographic basin is bounded on the east and southeast by the Santa Ana Mountains and San Joaquin Hills, on the northwest and north by the Santa Monica Mountains, and on north and northeast by the Elysian, Reppetto, Puente Hills and the San Gabriel of the Transverse Ranges Geomorphic Province. The central block of the basin is bordered on the south by the active Newport Inglewood fault zone and in the northeast by the Whittier-Elsinore fault zone. The inactive Bolsa-Fairview fault is located approximately 1.6 mile southwest of the site.

5.2. Site Geology

The subject site is situated along the Ocean View Channel which drains westward into Bolsa Chica Wetlands. Based on the USGS Geologic Map of the San Bernardino and Santa Ana 30’x 60’ Quadrangle map, the subject site is characterized primarily by Holocene- age fluvial deposits consisting of younger alluvial (Qya) typically found in canyon floors and drainages. These deposits are present along the exposed channel sideslopes. Both the fill and the native sediments overlie older deposits of the early- to late-Pleistocene age Repetto, Pico, San Pedro, and Lakewood Formations and Miocene-age Monterey and Puente Formations, which in turn rest on Mesozoic schist basement complex (see Figure 3 - Geologic Map).

A geologic reconnaissance of the existing channel was performed by a Tetra Tech CEG on September 7, 2016. Surficial geologic units and details on the existing geomorphology features including erosion, scour, and slope instability were noted and recorded on the field plan.

Significant geomorphic features observed during field geologic mapping included:

Erosion rills and gullies – incised rills up to several inches deep and extending from the top to about the lower third of the embankment slope were observed in the western 250 feet of the channel. Two gullies, upwards of 3 feet in width and extending from near the top to the lower third of the embankment were also observed within the same channel reach of the alignment on the north bank.

Animal burrows – isolated areas of animal burrows were noted along the alignment on both the north and south banks. Burrows were observed at the top of the channel as well as on the channel sideslopes, typically in clusters.

Surficial slumps – local areas of ground loss were observed as triangular to arcuate shaped slump scars along both the north and south banks. Features were typically 0.5 to 1.5 feet in depth as measured perpendicular to the slope face. Slump scars were noted in the lower to upper third of the bank slopes and, in some cases, associated with animal burrows.

Vertical or near vertical erosion slope face surfaces on the order of 2 to 4 feet in height at the lower third of both the north and south banks.

Vertical or near vertical slope failure scarps on the order of 3 to 14 feet in height along the north bank in the eastern several hundred feet of the alignment and also opposite outlet structures.

The predominant geomorphology features were mapped and are presented in Figure 2.

5.3. Subsurface Conditions

Based on the findings from our subsurface investigation, the top of the existing channel sideslopes along the left (south) bank are generally mantled by fill soils approximately 2 to 3 feet thick.

Within the downstream right (north) bank considerable deeper fill was encountered. This material was likely placed during the original construction of the confluence of Ocean View Channel C06 and East Garden Grove - Wintersburg Channel C05. Beneath the fill, alternative layers of silt, silty clay, fat clay, and silty fine sand were encountered to the maximum depths explored.

Generalized descriptions of the encountered units are provided below. Detailed descriptions of the encountered soil conditions are presented on the boring logs in Appendix A. A generalized soil profile along the channel bank is presented in Figure 4.

5.3.1. Fill

Fill was encountered in Borings B-1 through B-4 advanced for this study to depths ranging from

1.5 to 2.5 feet. Fill was encountered in Boring B-5 at the right bank near the downstream end of the project to a depth of about 16 feet. The fill soils generally consisted of dark gray to brown firm to stiff lean clay, fat clay and silt.

5.3.2. Native Alluvium

Native alluvial soils were encountered below the fill soils and typically consisted of yellowish brown to pale yellow medium dense to dense poorly graded sand, well graded sand, silty sand and clayey sand; and dark gray to yellowish brown firm to very stiff lean clay, fat clay and silt. These soils extended to the total exploration depth of all the borings.

5.3.3. Groundwater

At the time of our exploration, groundwater was measured in all borings B-1 through B-5 ranging in depths from approximately 15 to 21 feet below the existing top of channel grade shortly after the completion of drilling.

Mapping by the State of California (California Department of Conservation, Division of Mines and Geology, 1997) for the Anaheim and Newport Beach Quadrangle indicates that the historic high groundwater level at the site is estimated to be between 10 to 15 feet below the ground surface as shown in Figure 5 - Historic High Groundwater Map.

It should be noted that groundwater levels may fluctuate due to seasonal variations, rainfall, irrigation, or other factors. Evaluation of such factors is beyond the scope of the current work.

ENGINEERING SEISMOLOGY

6.1. General Seismic Setting

The Southern California region is known to be seismically active. Earthquakes occurring within approximately 60 miles of the site are generally capable of generating ground shaking of engineering significance to the proposed construction. The project area is located in the general proximity of several active and potentially active faults, as shown on Figure 6. Active faults are defined as those that have experienced surface displacement within Holocene period (approximately the last 11,000 years). The closest active fault to the site is the Newport Inglewood fault zone (South Los Angeles Basin section) located approximately 3.7 km southwest of the site.

The San Andreas Fault is located about 80 km northeast of the site. Regional fault and seismicity map is presented in Figure 6.

Historical earthquakes in Southern California of potential significance to the project include:

2008 magnitude M5.4 Chino Hills earthquake near the Whittier and Chino faults [Epicenter location: 33.96°N, 117.76°W; approximately 34 km northeast of the site];

1994 magnitude M6.7 Northridge earthquake on blind thrust fault (low angle faults that are not expressed at the ground surface) [Epicenter location: 34.21°N, 118.54°W; approximately 137 km northwest of the site];

1992 magnitude MW7.3 Landers earthquake on the Johnson Valley fault (as well as several others) [Epicenter location 34.16oN, 116.4oW; approximately 155 km northeast of the site];

1991 magnitude ML5.6 Sierra Madre earthquake on the Clamshell – Sawpit fault [Epicenter location 34.259oN, 118.001oW, approximately 60 km north of the site];

1987 magnitude M5.9 Whittier Narrows earthquake on Puente Hills Blind Thrust Fault [Epicenter location: 34.06°N, 118.08°W; approximately 39 km north-northwest of the site];

1971 magnitude M6.4 San Fernando earthquake which occurred on the San Fernando Fault (the easternmost fault of the Sierra Madre system) [Epicenter location: 34.42°N, 118.37°W;

approximately 85 km northwest of the site];

1933 magnitude M6.3 Long Beach earthquake on the Newport Inglewood Fault [Epicenter location: 33.63°N, 118.00°W; approximately 9.5 km south of the site].

6.2. Seismic Hazards and Potential for Surface Fault Rupture

The engineering seismology study for the subject site included reviewing local and regional fault maps and the review of historical earthquake data. Specifically, the following engineering seismology issues were addressed:

6.2.1. Seismic Hazard Zones

Maps of seismic hazard zones are issued by the California Geological Survey (CGS, formerly California Department of Conservation, Division of Mines and Geology (CDMG)) in accordance with the Seismic Hazards Mapping Act enacted in April 1997. The intent of the Seismic Hazards Mapping Act is to provide for a statewide seismic hazard mapping and technical advisory program to assist cities and counties in developing compliance requirements to protect the public health and safety from the effects of strong ground shaking, liquefaction, landslides, or other ground failure and other seismic hazards caused by earthquakes.

Based on the review of the Newport Beach Quadrangle Official Map of Seismic Hazard Zones issued April 15, 1998 (Figure 7), the proposed project is located within an area identified by the State of California as potentially subject to the hazard of liquefaction, but is not located within an area identified to be subject to the hazard of seismically induced landsliding. However, site-specific liquefaction analyses indicated that the on-site soils are not susceptible to liquefaction as discussed later in Section 6.3.

6.2.2. Surface Fault Rupture

Official Maps of Earthquake Fault Zones were reviewed to evaluate the location of the project site relative to active fault zones. Earthquake Fault Zones (known as Special Studies Zones prior to 1994) have been established in accordance with the Alquist-Priolo Special Studies Zones Act enacted in 1972. The Act directs the State Geologist to delineate the regulatory zones that encompass surface traces of active faults that have a potential for future surface fault rupture. The purpose of the Alquist-Priolo Act is to regulate development near active faults in order to mitigate the hazard of surface fault rupture.

The site is not located within a designated Earthquake Fault Zone for fault surface rupture hazard.

Surface traces of any active or potentially active faults are not known to pass directly through or project towards the site. Neither our field exploration nor literature review disclosed an active fault trace projecting to the ground surface in the project area. Therefore, the potential for surface rupture due to faulting occurring beneath the site during the design life of the proposed development is considered low.

6.3. Liquefaction Potential and Dynamic Settlement

Liquefaction of soils can be caused by ground shaking during earthquakes. Research and historical data indicate that loose, relatively clean granular soils and low plasticity silts are susceptible to liquefaction and dynamic settlement, whereas the stability of the majority of clayey silts, silty clays and clays are not typically adversely affected by ground shaking. Liquefaction is generally known to occur in saturated or near-saturated cohesionless soils at depths shallower than about 50 feet.

6.3.1. Soil Description

Evaluation of liquefaction potential for the on-site materials was performed based on soil stratigraphy encountered in our field explorations. The on-site soil materials encountered during our field exploration generally consisted of stiff to very stiff lean clay and fat clay and silt, and medium dense to dense poorly graded sand, well graded sand and silty sand. Review of boring logs indicate that layers of sand in Boring B-3 between a depth of about 10 and 20 feet and in Boring B-4 between a depth of 10 and 25 feet have a grain size distribution that warrants liquefaction evaluation. For all 5 borings, layers of silty sand, clayey sand, or poorly or well graded sand were encountered at depths of approximately 28 to 31.5 feet, with SPT blowcounts varying from about 20 to 35. These layers were included in the liquefaction analyses. The bottom elevation of this sandy layer was not explored since the total exploration depth for this field exploration effort was about 31.5 feet for each of the 5 borings.

6.3.2. Groundwater Level

Groundwater was encountered in all the borings B-1 through B-5 at depths of about 15 to 21 feet during our field explorations. The historical high groundwater at the site was mapped by CDMG (Anaheim and Newport Beach Quadrangle, 1997) at a depth of about 10 to 15 feet. Therefore, a groundwater depth of 10 feet was conservatively assumed for liquefaction analyses.

6.3.3. Seismic Demand

The seismic demand at the site was evaluated by a probabilistic seismic hazard analysis approach.

The evaluation utilized the USGS Probabilistic Seismic Hazard Deaggregation website https://geohazards.usgs.gov/deaggint/2008/ as a tool to calculate probabilistic peak ground acceleration. The attenuation relationships used for ground motion prediction include the Next Generation Attenuation (NGA) relationships of Boore and Atkinson (2008), Campbell and Bozorgnia (2008), and Chiou and Youngs (2008). An assumed average shear wave velocity in the top 30 meters (Vs30) of 270 meters per second was used in the model based on correlations with SPT blowcounts. The peak ground accelerations for various return periods were estimated from the USGS website. USACE criteria for design of structures require various return period values for Operating Basis Earthquake (OBE) and Maximum Design Earthquake (MDE). As the project is considered as a non-critical structure, the MDE has been assigned as an event with a 949 year return period. A summary of the estimated peak ground acceleration values for OBE and MDE event, as well as an intermediate 475 year return period event, are presented in Table 1. A printout of the seismic demand analysis is included in this report in Appendix C.

Table 1 Estimated Peak Ground and Spectral Accelerations

Return Period Peak Ground Acceleration

Spectral Acceleration

0.2 second 0.3 second 1 second

144 years (OBE) 0.22g 0.50g 0.49g 0.26g

475 years 0.35g 0.77g 0.76g 0.42g

949 years (MDE) 0.44g 0.97g 0.96g 0.55g

6.3.4. Liquefaction Potential and Sensitivity Evaluation

The liquefaction potential of cohesionless (sandy) soils was evaluated based on the SPT blowcounts and laboratory test results utilizing procedure published in Youd and Idriss (2001) consensus publication on liquefaction evaluation, and as recommended in the CDMG Special Publication 117A (CDMG, 2008).

The analyses based on SPT blowcounts considered the energy ratio correction factor CE of 1.3.

This ratio is based on Table 5.2 of the Recommended Procedures for Implementation of DMG Special Publication 117, Guidelines for Analyzing and Mitigating Liquefaction in California (SCEC, 1999). For an automatic trip hammer the table suggests the energy ratio correction factor range from 0.9 to 1.6 (modified from Youd and Idriss, 1997). Consequently, the selected design energy ratio correction factor of 1.3 is an average and reflects a hammer efficiency of approximately 78 percent, which is consistent with our experience with similar equipment. The blowcounts recorded for soils driven with the 3-inch O.D. California Sampler with brass rings were converted to an equivalent SPT blowcounts using a reduction factor of 0.7. Borehole diameter correction factor CB of 1 based on the internal diameter of the hollow stem auger system used for the drilling was utilized in our liquefaction evaluation.

Results of liquefaction analyses of granular soils are presented in Appendix D and indicate that the on-site saturated sandy soils are not susceptible to liquefaction.

As discussed previously, exploration did not extend below a depth of 31.5 feet. It is acknowledged that loose, potentially liquefiable sandy soils could theoretically exist below a depth of 31.5 feet at the site. In order to perform a cursory evaluation of liquefaction potential below 31.5 feet, analyses were performed assuming sandy soils with SPT blowcounts equal to the lowest values measured in the sandy soils encountered during the field exploration. This conservative evaluation indicated that a limited amount of liquefaction (deformation of less than 0.25 inch) could occur at depth. However, the potential for limited liquefaction at this depth causing either vertical seismic settlement or lateral spreading at the ground surface is considered minimal.

Liquefaction potential and seismic sensitivity of fine-grained soils was further evaluated per County of Los Angeles GME-3 Memorandum, “Review of Geotechnical Reports Addressing Liquefaction”, dated February 24, 2009. The fine-grained soils are classified in the following 3 categories:

1. Soils with Plasticity Index < 12 and moisture content greater than 85 percent of the Liquid Limit are classified as fine-grained soils susceptible to liquefaction (typically silts);

2. Soils with Plasticity Index > 18 are classified as fine-grained soils potentially susceptible to significant loss of strength during seismic shaking and require additional evaluation.

The sensitivity of the on-site fine-grained soils was evaluated based on the water content, Atterberg limits, and effective vertical stresses using the procedures suggested by Terzaghi, Peck and Mesri (1996).

3. Fine-grained soils falling outside the above two categories are considered to behave like clays and not susceptible to liquefaction.

The alluvial lean clay and fat clay soils has a moisture content varying from about 18 to 38 percent, a Liquid Limit of 44 to 56 percent, and a Plasticity Index of 21 to 30. Analyses of the seismic sensitivity of the saturated alluvial lean clay and fat clay indicated low sensitivity based on the estimated sensitivity ratios of less than 2. Consequently, the potential for significant loss of strength of the alluvial lean clay and fat clay during design seismic shaking is considered low.

Results of sensitivity analyses are presented in Appendix D.

DESIGN RECOMMENDATIONS

7.1. General

Based on the results of the field exploration and engineering analyses, it is Tetra Tech’s opinion that the proposed construction is feasible from a geotechnical standpoint, provided that the recommendations contained in this report are incorporated into the design plans and implemented during construction. The primary considerations for this project from a geotechnical standpoint include:

Slope stability of trapezoidal open channel sideslopes;

Requirements for minimum concrete lining strength to ensure adequate static slope stability of channel sideslopes;

Installation of a permanent subdrainage system beneath the channel sideslopes and below the channel invert for groundwater control;

Potential needs for temporary surface water diversion system and dewatering during construction, depending on the actual construction schedule;

Potential need for temporary shoring system, particularly along the north bank, for protection of adjacent structures during channel sideslope grading, depending on the final grading requirements.

Presence of high plastic clay will require specific moisture-conditioning during construction.

Laboratory tests indicate that the on-site soils have negligible levels of water-soluble sulfates.

Additionally, the on-site soils are expected to exhibit moderate to severe corrosion potential to ferrous metals based on the results of laboratory corrosivity tests. The design recommendations presented below reflect these conditions.

7.2. Slope Stability Analyses

Improvement of the existing channel will involve construction of fill slopes against the existing banks coupled with placement of concrete lining along both banks and the invert. The design channel sideslope inclination is 1.5(H):1(V). The bottom width and height of the open channel are generally 8 feet and 15 to 16 feet, respectively.

Slope stability analyses of the proposed channel sideslopes were performed using the computer program SLOPE/W. Three critical cross-sections A-A’, B-B’, and C-C’ were analyzed for slope stability. These sections were located in areas where significant slope instability and movement were observed during the field reconnaissance. Locations of the critical cross-sections are shown in Figures 2A through 2C.

7.2.1. Shear Strength Parameters

The shear strength values of the on-site material were evaluated by laboratory testing of saturated relatively undisturbed samples obtained during the field investigation. For most applications, the ultimate (strength at 10 percent strain) shear strength parameters were utilized in slope stability, earth pressure and bearing capacity analyses. For earthquake and short-term construction conditions, the peak shear strength parameters were utilized. The applicability of the estimated shear strength parameters, especially for clayey soils, was evaluated by performing back-calculations on the critical cross-sections. For these back-calculations, the original 1.5(H):1(V) slope angle was used and a parametric study was conducted that considered various heights of near vertical erosion cuts at the toe of slope. Erosion cuts with heights of roughly 3 to 4 feet and a partially drained drawdown condition from historic high groundwater level yielded Factor of Safety values of about 0.94 to 0.97. These failure conditions are considered to fairly well represent conditions encountered during the December 2010 flood events and corroborate the assumed shear strength parameters. Results of the back-calculations are presented in Figures E-1 through E-3 in Appendix E. For the proposed concrete liner shear strength parameters of c = 2,000 psf and phi = 40 degrees were conservatively assumed. These values are considered conservative for a 2,500 psi compressive strength concrete. A summary of shear strength parameters adopted for engineering analyses is presented in Table 2.

Table 2 Summary of Soil Shear Strength Parameters

Material Total Unit

Weight (pcf)

Effective Shear Strength Undrained Shear Strength

(psf) Cohesion (psf) Friction Angle

(degrees)

Clay 125 50 (ultimate) 100 (peak)

Ф1 = 27 (σ’v < 1,000 psf)1 Ф2 = 22 (σ’v > 1,000 psf)1 1,800

Silt 125 100 24 2,000

Sand 125 0 32 (ultimate)

37 (peak) N/A

Concrete Liner 140 2,000 40 N/A 1 Bilinear shear strength model based on laboratory testing and slope stability back-calculations.

7.2.2. Stability Analyses Methodology

Stability analyses were performed in general accordance with the methodology outlined in the Engineering Manual EM 1110-2-1902 (USACE 2003). Selected graphical outputs of the pertinent SLOPE/W models are presented in Appendix E. The load cases considered for the stability analyses are discussed below. A uniform surcharge of 250 psf was included in each stability model in order to account for traffic loading at the top of slope. In addition, a loading equivalent to 10 kips per lineal foot was included to account for the 3 story building located near the right-of-way boundary on the left bank of the channel.

Case I: End of Construction. The proposed improvements will require placement of fill in some areas to re-establish the channel sideslopes to design grade. Review of boring data indicated that the existing groundwater levels are generally at or below the invert elevation of the channel.

Therefore, the end of construction condition was conservatively evaluated by assuming the groundwater table to be at the invert elevation of the proposed channel. Undrained shear strength was assumed in the saturated clay and silt materials below the groundwater, while effective strength parameters were used for unsaturated clay and silt. Effective strength parameters were used for sandy soil layers. This procedure yielded acceptable Factor of Safety values.

An additional end of construction analysis was performed in areas where significant depths and horizontal extent of fill will be required to achieve the design grade (Section C-C’). In this procedure an initial excess pore pressure equal to the pressure from the estimated weight of fill required to obtain design grade was conservatively assigned to the saturated soils below the invert.

This model yielded Factors of Safety less than the required value of 1.3. A second model assumed excess pore pressure of 50 percent of the fill load and yielded Factor of safety greater than 1.3.

Both models are conservative in that the assumed excess pore pressure is uniform through the depth of the foundation layer. However, the analysis does indicate that providing a waiting period after placing fill and before constructing the channel lining in areas where fill depth are significant would be a prudent procedure. Utilizing a typical value for the coefficient of consolidation (cv) of 1 x 10-3 cm2/sec for medium to high plastic clayey soils (Lambe & Whitman 1969) and assuming a 10 feet thick layer of clay underlying the invert, a consolidation period of about 2 months was estimated to achieve 50 percent completion of consolidation. This waiting period should be included in project specifications where vertical fill depths (measured from existing grade to the bottom of the concrete liner) exceed 4 feet over significant distances. For fill depths less than 4 feet the excess pore pressure is not anticipated to lower the Factor of Safety below 1.3. Based on a review of preliminary construction drawings the settlement waiting period should be performed between approximately Stations 21+30 to 25+90.

Case II: Sudden Drawdown. For the sudden drawdown analysis, it was assumed that the groundwater within the channel banks is at the historic high level and that water in the channel has rapidly dropped to a dry channel condition. This assumption is conservative in that it does not account for any drainage of groundwater in the channel banks into the proposed subdrain.

Case III: Steady State Seepage from Full Flood stage. The stability analyses for the case of steady seepage were performed assuming that the design flood event level remains for a period sufficient to saturate the bank soils.

Case IV: Earthquake. In order to evaluate the overall gross stability of the channel sideslopes under earthquake loading, a pseudo-static analysis was performed utilizing a horizontal coefficient kh of 0.22g, which is equal to the OBE peak ground acceleration and 50 percent of the MDE peak ground acceleration. In the analyses the groundwater level was assumed to be at the invert elevation of the proposed channel.

In addition to the above cases, slope stability analysis was also performed for a high groundwater and no channel flow condition. For this case, a long term groundwater condition and no stabilizing effect of water in the channel was assumed. The effect of the recommended subdrain system was included in the analysis. The graphical output for the stability analyses for Section A-A’, B-B’ and C-C’ is presented in Appendix E (sheets E-4 through E-20).

Minimum Required Factors of Safety. The required minimum Factors of Safety used for each of the load cases were developed using the criteria in EM 1110-2-1902, Slope Stability (USACE, 2003). Table 3-1 in the manual presents the required minimum Factors of Safety for new embankment dam slopes. However, in Section 3-4 of the EM, there is discussion of the minimum required Factors of Safety for use in the stability analyses of other slopes. Within paragraph 3-4, the EM states:

…Typical minimum acceptable values of factor of safety are about 1.3 for end of construction and multistage loading, 1.5 for normal long-term loading conditions, and 1.1 to 1.3 for rapid drawdown in cases where rapid drawdown represents an infrequent loading condition. In cases where rapid drawdown represents a frequent loading condition, as in pumped storage projects, the factor of safety should be higher.

Based on this guidance, required minimum safety factors of 1.3, 1.5, and 1.1 were selected for the end of construction case, the long-term design flood level steady state seepage, and the sudden drawdown case, respectively. It is our opinion that the sudden drawdown case represents an infrequent loading condition in the channel so a lower required minimum Factor of Safety is appropriate for this case. For the unusual condition of high groundwater and no water in the channel, a Factor of Safety of 1.4 was considered appropriate.

7.2.3. Stability Analyses Results

The results of the stability analyses are summarized in Table 3 and presented in Appendix E, Figures E-4 through E-20.

Table 3 Results of Slope Stability Analyses

1 USACE EM 1110-2-1902 recommends 1.3 for end of construction.

2 USACE EM 1110-2-1902 recommends a factor of safety of 1.1 to 1.3. A value of 1.1 was used.

3 USACE EM 1110-2-1902 does not currently specify an acceptable Factor of Safety. A value of 1.1 is a typical standard of practice in southern California.

4 Assumes a settlement waiting period prior to liner construction

It is acknowledged that the above Factors of Safety assume some component of resistance from the channel lining. In order to assist the Structural Engineer in evaluating the internal stability of

Stability Case

USACE EM1110-2-1913

Minimum Required Value

Factor of Safety

A-A’ B-B’ C-C’

Case I End of Construction

1.31 1.53 1.52 1.484

Case II Rapid Drawdown

1.12 1.34 1.23 1.26

Case III Steady State Seepage from Design Flood

1.5 1.64 1.64 1.72

Case IV Earthquake (kh = 0.22g)

1.13 1.23 1.17 1.21

High Groundwater / No Channel Flow 1.4 1.53 1.48 1.48 the liner system additional slope stability analyses were performed to estimate the nominal soil loading on the liner system for various load conditions. This loading is deemed to be equivalent to the resistance required to be provided by the concrete liner to achieve a Factor of Safety of 1.

Analyses were performed on Cross-Section A-A’ for the rapid drawdown, earthquake, and static with historic high groundwater condition. Varying triangular pressure distributions were placed against the slope face, without the concrete liner layer, for each condition until a Factor of Safety of 1.0 was achieved. For the earthquake condition, the horizontal seismic coefficient (kh) value was taken as the kyield value back-calculated from deformation analysis per the procedures presented by Bray and Travasarou (2007). The kyield value (0.13) was estimated based on a slope deformation of 15 cm under the MDE loading condition. The maximum value of the calculated pressure distributions to achieve a Factor of Safety of 1 were 22 psf, 32 psf, and 47 psf for static, rapid drawdown and earthquake, respectively. These pressure distributions are presented on Figure 9 and the graphics of the stability analyses are presented in Appendix E, Figures E-21 through E-23. The Structural Engineer should design the concrete liner to resist these pressures with appropriate Factors of Safety.

Temporary stability of construction slopes was also evaluated as presented in Appendix E, Figures E-24 and E-26. The analysis indicates that, provided groundwater is lowered to the bottom of the excavation, benched construction slopes at a gradient of 1.5(H):1(V) should have an adequate Factor of Safety. Where excavation to install new storm drain outlets or the amount of benching required to remove all loose or disturbed soil on the existing channel slope requires slopes steeper than 1.5(H):1(V) it is recommended that shoring or controlled slot excavation be implemented.

7.3. Construction Considerations

7.3.1. Clearing and Grubbing

The surface should be cleared of any vegetation, trash and debris, prior to commencement of the earthwork. Any subterranean installations not to be preserved, such as pipes, utility collectors, tanks, etc., should be abandoned in accordance with applicable regulations.

7.3.2. Temporary Excavation

The on-site soils are not expected to pose unusual excavation difficulties, and therefore, conventional earth-moving equipment may be used. Localized sloughing/raveling of exposed sandy soil intervals should be anticipated. All excavations should be performed in compliance with local regulatory requirements and currently applicable OSHA guidelines. Excavated materials should not be stockpiled on the top of the channel banks.

7.3.3. Invert Subgrade Preparation

All existing failure debris or disturbed material should be removed from the channel invert and slope. It is anticipated that recent channel deposits will be encountered below the channel invert during the grading of the existing channel. The density and strength of the recent channel deposits are anticipated to be relatively low and somewhat variable, therefore, it is recommended that the upper 2 feet of the material below the existing channel invert be overexcavated and replaced with compacted engineered fill.

The exposed subgrade or overexcavation grade should be probed and accepted by the Geotechnical Engineer. The subgrade soils should be scarified to a depth of 6 inches, moisture-conditioned to at least 120 percent of the optimum moisture content, and compacted to at least 90 percent of the maximum dry density, as evaluated by the latest version of ASTM D1557.

It is anticipated that localized zones of loose and/or unstable soils may be encountered during excavation. Such zones should be overexcavated to competent alluvium, and the overexcavated subgrade should be prepared and backfilled with compacted engineered backfill as outlined in this report. If loose/soft/wet areas are encountered that are not practical to be excavated, Table 4 below provides options for stabilizing the subgrade. The specific type of remediation and associated area limits will need to be evaluated, determined, and documented in the field by the representative of the Geotechnical Engineer.

Table 4 Conceptual Options for Handling Unstable Materials at the Excavated Subgrade

Limited areas where the soils are soft and/or unstable

Overexcavate at least 2 feet Stabilize the soft subgrade by working open-graded aggregate material (typically 3/4” or 1.5” crushed rock, coarser for softer subgrade) at least 4 to 6 inches into the soil.

Place non-woven geotextile, Mirafi 180N or approved equivalent, over the stabilized subgrade.

Place and compact well-graded fill (e.g., AB, CMB) or general approved backfill material to specified compaction over the geotextile.

Larger areas where the soils are excessively soft and/or unstable

Overexcavate at least 2 feet Improve the soft subgrade by working in open-graded aggregate material as much as possible/practical into the subgrade.

Place non-woven geotextile, Mirafi 180N or approved equivalent, over the exposed soil.

Place at least 8 inches (12-18 inches preferred) of well graded aggregate material (e.g., AB, CMB); only reasonably achievable compaction is required.

Place non-woven geotextile, Mirafi 180N or approved equivalent, over the aggregate layer.

Place and compact fill to specified compaction over the geotextile.

The Geotechnical Engineer should observe the excavation progress so that appropriate modifications to the excavation design may be recommended, if necessary, due to encountered conditions differing from the design assumptions. If pervasive unsatisfactory conditions are observed at the overexcavation subgrade as judged by the Geotechnical Engineer, additional subsurface investigation and subgrade improvements consisting of further overexcavation, construction of a stiffening blanket from select fill materials and/or use of geogrids may be required.

7.3.4. Slope Preparation and Fill Placement

The design fill slope inclination is 1.5(H):1(V) for the proposed improvements. Prior to placement of fill materials, benches should be cut within the existing channel sideslopes in order to remove any weathered material or soil that has been disturbed or displaced by previous slumps or slope movement. The benching should expose undisturbed competent alluvium. To the extent practicable, the benches should be at least 2 feet wide and no more than 5 feet high, as shown in Figures 8A and 8B.

Excavated on-site soils may be re-used as compacted fill provided they are free of organics, deleterious materials, debris and particles over 3 inches in largest dimension. It should be anticipated that excavated material below the proposed invert will be saturated and will need significant drying prior to use as compacted fill.

Any soil materials (including backfill or base course materials) imported to the site should be sampled, tested, and approved by the Geotechnical Engineer prior to arrival on-site. In general, any soils imported to the site for use as fill should have fines content between 15 and 40 percent, and Liquid Limit less than 30. Additional recommendations for site grading are provided in the “General Site Grading Recommendations” section of this report.

Fill should be placed in horizontal lifts not more than 8 inches in loose uncompacted thickness. In order to adequately compact the face of reconstructed fill slopes, it is strongly recommended to overbuild the slopes by 1 to 2 feet and to cut the slope back to the final design configuration. If this method is not practical, the contractor must be prepared to skillfully compact the outer slope edge and face to meet the compaction requirements. The edge of the constructed slope should be placed slightly elevated and not be allowed to roll off. Sandy fills (fines content less than 40 percent) should be moisture-conditioned to at least 110 percent of the optimum moisture content. Clayey fills should be moisture-conditioned to at least 125 percent of the optimum moisture content. All fill should be compacted to at least 90 percent of the maximum dry density

(ASTM D1557).

As discussed previously, due to the depth of required fill, a settlement waiting period of 2 months is recommended after placement of fill and before construction of the concrete liner between approximate Stations 21+30 to 25+90.

7.3.5. Temporary Dewatering

Groundwater was encountered during site exploration at depths ranging from about 15 to 21 feet below the top of the channel banks, i.e., at or roughly one foot above to 5 feet below the proposed channel invert. Surface water on the order of about one foot depth was observed in the existing channel during the field exploration. Depending on local groundwater conditions and flow levels in the channel at the time of construction, significant temporary dewatering measures may be required to maintain a dry and safe working environment during subgrade preparation and other construction activities.

If present, the surface water within the channel should be temporarily diverted from the construction areas. Depending on the water flow conditions at the time of subgrade preparation, appropriate water diversion measures may be required during the construction.