IBM10B0006_Fabens_geotech_Final.pdf

Geotechnical Engineering Report

International Boundary and Water Commission Upper Rio Grande Levee Evaluation

Fabens, Texas December 21, 2009

Terracon Project No. 88095073

Prepared for:

S & B Infrastructure, Ltd.

McAllen, Texas

Prepared by:

Terracon Consultants, Inc.

Pharr, Texas

Project No. 88095073

TABLE OF CONTENTS

PURPOSE OF GEOTECHNICAL STUDY

Introduction Project Description Scope of Services

SITE EXPLORATION AND TESTING PROCEDURES

Field Exploration and Sampling Laboratory Testing

SITE CONDITIONS

Levee Characteristics Area Geology Subsurface Soil Conditions Groundwater Conditions

ENGINEERING EVALUATION

Geotechnical Considerations

ENGINEERING RECOMMENDATIONS

Site Preparation Settlement Analysis Slope Stability Analysis Seepage Analysis Seismic Considerations Evaluation of Liquefaction Potential

Design Earthquakes and Ground Motions Liquefaction and Its Effects Liquefaction Potential and Consequences Conclusions and Recommendations

Lateral Earth Pressures for Retaining Walls and Flood Walls Retaining Wall Drainage Selection and Placement of Fill Maintenance of Existing Levee Construction Services

GENERAL COMMENTS

APPENDIX A - Boring Location Plans APPENDIX B - Logs of Borings APPENDIX C – NAD 83 Boring Location Coordinates APPENDIX D – Cone Penetration Tests Logs APPENDIX E – Laboratory Test Results APPENDIX F – Photograph Log APPENDIX G – Existing and Final Top of Levee Elevations APPENDIX H – Typical Cross Sections APPENDIX I – Slope Stability Analysis Results APPENDIX J – Seepage Analysis Results APPENDIX K – Seismic Considerations APPENDIX L – Moisture Density Relationship of Borrow Material

APPENDIX M – ASFE

IBWC Upper Rio Grande Levee Evaluation – Fabens Reach

December 21, 2009

PURPOSE OF GEOTECHNICAL STUDY

The purpose of our services will be to serve as a Geotechnical Engineer subconsultant in support of S & B Infrastructure (S&B) Design and Rehabilitation Improvements for the Upper Rio Grande Levee – Fabens Reach in El Paso County and Fort Hancock Reach in Hudspeth County, Texas. Terracon, the Geotechnical Engineer, conducted geotechnical testing and analysis required for compliance determination with regards to our Scope of Work as defined in Section E.1 of Contract No. IBM05D0001 and presented in our Scope of Services section below.

Terracon provides data, documentation, and the engineers’ declaration that the levee system, where no alterations are proposed, provides reasonable assurance of protection for the base flood (100-year) event and meet the requirements of 44 CFR 65.10 (Code of Federal Regulations, Title 44 – Emergency Management Administration 65.10 - Mapping of Areas Protected by Levee Systems), assuming that the standard of care is based upon the guidance documents of the USACE EM (Engineering Manual) 110-2-1913 with regards to our Scope of Work. In alteration areas of the existing levees, the construction contractor must adhere to the construction methodology presented in this report for levee compliance.

Upon timely receipt of the LCR, its review, and its acceptance by FEMA, the levee system will be accredited on National Flood Insurance Program (NFIP) maps.

Please note that as per 44 CFR 65.2 b, a certification by a registered professional engineer or other party’s services does not constitute a warranty or guarantee of performance, expressed or implied. Certification of data is a statement that the data is accurate to the best of the certifier’s knowledge.

Introduction This Geotechnical Engineering Report presents the results of our geotechnical investigation, evaluation, and engineering analysis regarding the proposed raising of the International Boundary and Water Commission (IBWC) Upper Rio Grande Levee – Fabens Reach in El Paso County, Texas.

The United States Section of the IBWC has conducted hydraulic, aerial, geophysical, and exploratory geotechnical studies of the Rio Grande Rectification Project segment of the Upper Rio Grande Flood Control System (URGFCS) to assess the ability of the associated levees to safety contain flood flows in the Rio Grande in June 2004. The design is for a 100-year flood with a 3-foot freeboard. From these studies, the USIBWC has determined that certain reaches of the levees require rehabilitation improvements for safe containment of the URGFCS design flood flows.

The 2006 Federal Emergency Management Agency (FEMA) decertification of the USIBWC levees in El Paso and Hudspeth Counties prompted raising portions of the levee in el Paso County between International Dam and Riverside Weir in 2007 to meet the 100-year, 3-foot

December 21, 2009 freeboard criteria. The Fabens and Fort Hancock Reaches of the URGFCS levees were found to be in need of rehabilitation improvements. Terracon Consultants, Inc. (Terracon) has been retained as the Geotechnical Engineer subconsultant in support of S&B Infrastructure (S&B) in Design and Rehabilitation Improvements for the URGFCS Fabens Reach and Fort Hancock Reach. This Geotechnical Engineering Report presents the evaluation and preparation of the Fabens Reach which is defined as follows:

Table 1: Upper Rio Grande Flood Control System – Fabens Reach Coordinates* Reach Name Location/Description Length (mi) Begin End

Fabens Reach South of Fabens to Tornillo

~6.9 466700 X 10545377 Y

497048 X 10527281 Y

* Provided in NAD83 State Plane Coordinate System

This project was approved by Ms. Sharlotte Teague, P.E., with S & B Infrastructure (S&B) on September 24, 2009 by issuing Work Authorization No. 2 under S&B Purchase Order No.

U0450-Z0018 and S&B Project No. U0450.535. Our scope of services for this project was outlined in Terracon Proposal No. P88090143, dated April 10, 2009. Terracon submitted a Preliminary Report Letter No. 88095073 , dated November 3, 2009, and Draft Reports on November 18, 2009 and December 8, 2009.

Project Description The Fabens Reach, here-on referred to as FE1, is located within the vicinity of Fabens and Tornillo in El Paso County, Texas. The levee reach extends from about 2,500 feet west of Mile Marker 32 of URGFCS, south of Fabens, Texas, to about 2,500 feet east of Mile Marker 38 of URGFCS, in Tornillo, Texas. The total distance of this reach is about 6.9 miles. This reach of the URGFCS has the Rio Grande Protective Fence at its land side toe for the entire length of the levee. At some areas, the land side of the levee does not have a downslope because of the proximity of the Protective Fence. These areas are located at and near the fence gates. In addition, a series of on and off ramps to access the levee from a patrol road located about 30 feet south of the levee’s riverside toe.

The proposed levee rehabilitation improvements consist of raising the top-of-levee elevation, conducting geotechnical investigations and testing to determine the type and extent of any required remediation improvements due to slope stability, seepage, levee settlement, erosion, and any other geotechnical issues that may cause levee failure during a 100-year flood event. We understand that the levee structures will be assessed by others. Based on information and plan and profile drawings provided by S&B, we understand that some portions of FE1 will need to be raised from 0.02 to 1.87 feet.

December 21, 2009

Scope of Services This geotechnical investigation is for the purpose of determining foundation stability, for the added loading as well as to evaluate potential for seepage through or below the levee in critical and questionable areas, in preparation for design of the levee rehabilitation projects.

To achieve the objectives of this study, our scope of services consisted of:

• Visual inspection of the levee sections;

• Review of the Condition Assessment of U.S. International Boundary and Water

Commission, Texas and New Mexico Levees, June 2004;

• Drilling a total of 38 test borings along the levee reach from 30 to 60 feet below existing surface at the top of the levee to help determine subsurface stratigraphy and subsurface water conditions;

• Performing Seismic Cone Penetration Tests with pore pressures (CPTU) at about every 3,000 feet at depths between 30 to 60 feet (until refusal) to help determine subsurface stratigraphy and soil parameters;

• General field procedures included:

• Subsurface water level readings during and upon completion of the drilling activities and backfilling the boreholes with soil cuttings.

Installation of piezometers was not part of this scope;

• Standard geotechnical drilling techniques were utilized and no environmentally sensitive boring sites were included;

• All drilling activities were logged by Terracon;

• Performing a Refraction Microtremor (ReMi) analysis in the vicinity of borings FE1-7 and FE1-32 to determine the average shear wave velocity of the soils to help determine the seismic site classification;

• Performing appropriate laboratory tests on recovered soil samples to evaluate the physical and engineering properties of the subsoils;

• Performing engineering analyses to include:

• Prepare boring logs to represent in-situ soils

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• Identify soil parameters, based on collected field and laboratory data, of the current levee and underlying strata to be used in structural evaluation;

• Determine foundation stability with regards to increased load including providing factors of safety against failure at critical points along the levee reach;

• Calculate factors of safety on representative cross sections of slope stability using steady state and rapid drawdown analysis and a seepage analysis of critical and typical sections to evaluate flow quantities and critical exit gradients both through and below the embankments. Shear strength was determined by analyzing average strength values for the soils encountered in the soil test borings and CPTUs and reduced strengths to represent seismic conditions;

• Perform a liquefaction analysis to determine the presence of liquefiable soils based on recovered field data;

• Provide sketches of typical section alternatives for areas to be rehabilitated,

• Construction methods, sequences, and potential difficulties that may arise during construction;

• Provide recommended material specification requirements for a typical levee section; and

• Preparing a geotechnical engineering memorandum that will be used by S&B to prepare an Engineering Technical Memorandum.

SITE EXPLORATION AND TESTING PROCEDURES

Field Exploration and Sampling The subsurface conditions were evaluated by drilling a total of 38 borings at the project site.

The 38 borings (FE1-1 through FE1-38) were drilled on the Fabens levee at about every 1,000 feet, as identified in the Statement of Work (SOW), from coordinates (466701.434, 10545376.463) (X,Y) to coordinates (497526.776, 10526693.932). The borings were drilled from about 30 to 60 feet deep from the top of the levee. The 60 foot borings were drilled about every 5,000 feet beginning at FE1-2 (467478.464, 10544728.696).

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The four-inch nominal diameter borings were drilled using conventional truck-mounted drilling equipment at the approximate locations shown on the Boring Location Plans provided in Appendix A. This Boring Location Plan was provided for preliminary purposes by S&B and was thereafter adopted by Terracon as the final boring locations. The borings were located in the field with the assistance of S&B. Boring depths were measured from the existing ground surface at the time of our field activities. The boring elevations were provided by S&B and are presented on the Logs of Borings. The Logs of Borings, presenting the subsurface soil descriptions, type of sampling used, and additional field data, are provided in Appendix B. Description of our field activities are also included in this Appendix.

See Appendix C for NAD83 State Plane Coordinates of the referenced boring locations.

Soil samples were recovered by means of Shelby Tubes and Standard Penetration Test (SPT) in general accordance with the appropriate ASTM standards. The SPT consists of measuring the number of blows (N-value) required for a 140-pound hammer free falling 30 inches to drive a standard split-spoon sampler 12 inches into the subsurface material after being seated six inches. The sampling depths, penetration distances, and the N-values are reported on the boring logs. Samples were removed from samplers in the field, visually classified, and appropriately sealed in sample containers to preserve their in-situ moisture contents.

In addition, 12 Seismic CPTU soundings (conducted adjacent to borings FE1-2, FE1-5, FE1- 8, FE1-11, FE1-14, FE1-17, FE1-20, FE1-23, FE1-26, FE1-29, FE1-32 and FE1-35) were performed on the Fabens reach about every 3,000 feet. The CPTU procedure involves the hydraulic advancement of a steel cone shaped device attached to steel rods with flush-joint couplings. The sounding unit has electronic strain gauges that measure the point resistance, sleeve friction and pore-water pressure and the seismic adapter is used to measure the shear wave velocities. A depth encoder device monitors penetration as the rods are hydraulically pushed into the ground. The system is computer interfaced and records the referenced parameters every two to four centimeters. These parameters can be correlated to a variety of soil properties, including strength and density. The in-situ data and the approximate soil types empirically estimated from the data are reported on the CPTU sounding logs. The CPTU sounding logs, presenting the in-situ field data and approximate soil types, are provided in Appendix C.

Laboratory Testing Samples obtained during the field program were visually classified in the laboratory by a Terracon geotechnical engineer in general compliance with ASTM D-2488. A testing program was conducted on selected samples, as directed by the geotechnical engineer, to aid in classification and evaluation of engineering properties required for analyses. Soil classification and laboratory tests were performed in general accordance with the appropriate ASTM standards.

December 21, 2009

Laboratory tests were performed on selected samples and include Atterberg limits, sieve analysis (material finer that No. 200 sieve and gradation), unconfined compression strength, one-dimensional consolidation, unconsolidated-undrained and consolidated-undrained with pore pressure strength in triaxial compression, hydrometer, and moisture content. The results of the laboratory tests are presented in Appendix E along with a summarization of the testing procedures.

All samples were returned to our laboratory in Pharr, Texas. Samples not tested in the laboratory will be stored for a period of 30 days subsequent to submittal of this report and will be discarded after this period, unless we are notified otherwise.

SITE CONDITIONS

Levee Characteristics Terracon conducted a visual inspection of the FE1 levee reach of the URGFCS levees for the purpose of identifying areas of concern. During the field exploration, the following general observations were reported:

• Section FE1 of the levee is in generally good condition. The levee is stable and generally not critically-eroded. Some superficial run-off water erosion and sloughing of the side slopes has occurred in the vicinity of borings FE1-1, FE1-9, FE1-25, FE1-26, FE1-29, FE1-30, FE1-35 and FE1-36.

• Some superficial tire ruts are evident on the river side slopes in the vicinity of borings FE1-2, FE1-3, FE1-5, FE1-6, FE1-23 and FE1-34.

• Several ramps use to access the levee from a river side patrol road are located throughout the levee reach in between borings FE1-1 and FE1-2, at FE1-5, FE1-10, between FE1-15 and FE1-16, FE1-18, FE1-25, FE1-27, FE1-

35. Information regarding construction considerations for the ramps is provided in the Maintenance of Existing Levee section of this report.

• In some areas near Protective Fence gates, no land side slope is present and the Protective Fence is adjacent to the land side end of the levee road.

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• The ground cover on the levee slopes consisted of desert grass and small brush. Ground cover on the river side of the levee toe consisted of desert grass, brush and trees. Ground cover on the land side of the levee toe consisted of mainly cultivated crops and brush, tress and tall grass in an adjacent canal. A thick tree root was observed on the river side slope in the vicinity of boring FE1-9.

• Some areas of concern on the FE1 levee include a floodgate structure between borings FE1-31 and FE1-32. In addition, an irrigation canal runs along the length of the levee (on the north side of the Protective Fence) about 15 feet from the land side toe with the exception of in the vicinity of between borings FE1-30 and FE1-31 to between borings FE1-31 and FE1-32. The canal ranges from 10 to 29½ feet deep (on the levee side) and about 26 to 59 feet wide (from crest to crest).

• The top of the levee shows some signs of rutting and less than the required 6 inches of flexbase material (Engineering Manual (EM) 1110-2-1913). In general, the levee road appears to achieve the 16-foot width requirement as per Exhibit II Section II.A of Work Authorization No. 2 throughout the length of the levee with the exception of near borings FE1-5, FE1-6, and FE1-19.

• As noted above, the existing levee slopes appear stable showing some superficial erosion, sloughing and rutting, and in general the 3(H):1(V) river side slope and 2.5(H):1(V) land side slope, as required by Exhibit II Section II.A of Work Authorization No. 2, is maintained for most of the levee reach.

Areas that do not appear to meet the slope requirement are in the vicinity of borings FE1-6 (river side), FE1-27 (river and land side), FE1-28 (river side), FE1-29 (river side) and FE1-33 (river side).

Photographs of the levee system and some areas of concern along the levee system are presented in Appendix G of this report.

Area Geology The “Geologic Atlas of Texas” Van Horn – El Paso Sheet was used to map this levee system. The Fabens levee sections are located on the Alluvium of the Rio Grande Formation (Qalr) of the Holocene (Recent) Period of the Quaternary Age. The soils are mostly composed of clay, silt, sand and gravel.

Subsurface Soil Conditions The generalized subsurface stratigraphy, as evaluated from our recent field and laboratory programs, is described as:

December 21, 2009

• Clay (CH and CL); soft to hard;

• Silt (ML); loose to medium dense; non-plastic

• Sand (SP, SC, SM, SP-SM and SP-SC); very loose to very dense; and

• Gravel (GP); medium dense to very dense.

The above referenced soils were not encountered in every boring and, when encountered, they were at various depths along the length of the levee. The strata thicknesses and depths to their interfaces are approximate. Our measurements are referenced from the ground surface at the time of our drilling activities. Subsurface conditions may vary between boring locations.

The Logs of Borings presenting the stratum descriptions, types of sampling used, limited laboratory test data, and additional field data, are presented in Appendix B. Additional laboratory test data is presented in Appendix E please see General Notes sheet, which defines the terms and descriptive symbols used on the boring logs, is also presented in this Appendix.

Groundwater Conditions The borings were advanced using dry drilling techniques until subsurface water was encountered, and completed using wet rotary drilling techniques, in an effort to evaluate groundwater levels at the time of our investigation. Subsurface water was encountered in the borings during drilling activities. The use of drilling fluid does not allow for subsurface water readings once wet rotary drilling techniques have commenced. The borings were backfilled with soil cuttings and sealed with bentonite grout upon completion of the drilling operations and subsurface water observations.

Short-term field observations do not permit an accurate evaluation of the subsurface water levels. Subsurface water levels are influenced by seasonal and weather conditions, which generally result in fluctuations in the elevation of the subsurface water level over time.

Specific information concerning subsurface water levels at the boring locations is also noted on the boring logs presented in Appendix B of this report.

ENGINEERING EVALUATION

The following evaluation and recommendations are based upon the data obtained in our field and laboratory programs, project information provided to us, and on our experience with similar subsurface and site conditions.

December 21, 2009

Geotechnical Considerations Based on information provided by S&B, which was based on IBWC Top of Levee Elevations (TOLE), we understand that the existing FE1 levee reach TOLE ranges from elevation (El.)

3587.28 feet (ft.) to El. 3615.38 ft. In order to maintain a 3-foot freeboard for a 100-year flood design, the proposed TOLE ranges from El. 3588.93 ft. to 3615.57 ft. Some areas of the existing levee meet or exceed the proposed TOLE required to maintain a 3-foot freeboard for a 100-year flood design. Those areas are in the vicinity of borings FE1-17, FE1-19, FE1-20, FE1-22, FE1-23, FE1-26, FE1-27, FE1-29 and FE1-36. Grade changes in the remaining boring vicinities will range from 0.02 ft. to 1.87 ft. Information concerning existing and final elevations and necessary grade changes are presented in Appendix G of this report.

Only localized areas of the levee core have soft Clays or very loose to loose Silts and Sands. In general, the medium stiff to hard Clay (CH and CL) soils at the core and medium dense to very dense Sands (SP, SC, SM, SP-SM and SP-SC ) at the core and at the base of the levee system are competent to support the proposed grade changes to the TOLE and anticipated temporary construction loads.

Based on cross sections of the existing levee provided by S&B, in general the 3(H):1(V) river side slope and 2.5(H):1(V) land side slope, as required by Exhibit II Section II.A of Work Authorization No. 2, is maintained for most of the levee. Areas appearing not to meet the slope requirement are in the vicinity of borings FE1-6 (river side), FE1-27 (river and land side), FE1-28 (river side), FE1-29 (river side) and FE1-33 (river side). We understand that the IBWC prefers not be extend the right-of-way limits on either side of the existing levee. In addition, the proximity of the Protective Fence on the land side of the levee may not allow for extension of the land side right-of-way. The proposed grade changes and slope finishes in the vicinity of boring FE1-33, result in insufficient slope finishes on both the land and river sides. Extending the river side levee base out 3 feet will result in a 3(H):1(V) river side slope in the vicinity of boring FE1-27. The proposed grade changes and slope finishes at the other three locations will result in sufficient slopes after construction. Typical cross-section drawings for the levee areas requiring enlargement and rehabilitation are provided in Appendix H.

Areas in the vicinity of borings FE1-5, FE1-6 and FE1-19 do not meet the 16-foot wide levee road as required by the project specifications. Typical cross-section drawings for the levee areas requiring enlargement and rehabilitation are provided in Appendix H.

Expansive soils are present at the site. Atterberg Limit tests indicate that the clayey soils present at the site generally exhibit moderate to high expansion potential. However, levee structures are not very susceptible to shrink/swell movements as a result of moisture fluctuation. Therefore a Potential Vertical Rise (PVR) analysis was not conducted.

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Recommendations regarding levee design, evaluation, construction and other issues related to the geotechnical aspects of the project are presented in the following sections.

ENGINEERING RECOMMENDATIONS

Terracon analyzed the levee in accordance with USACOE EM 1110-2-1913, entitled “Design and Construction of Levees” and the calculations showed the findings to meet the safety factors and hydraulic gradient requirements established by this manual for slope stability and seepage.

Considerations should be given when considering the liquefiable potential at the site. See the section “Evaluation of Liquefaction Potential” for more information.

The top of levee shall be designed as a minimum 16-foot wide maintenance road consisting of a 6-inch layer of flexbase material directly placed on the finished embankment material. This flexbase material should not count towards the required elevation change. The USIBWC and S&B provided the required TOLE to attain a 3-foot freeboard.

Site Preparation All care should be taken to ensure good bond between the enlargement and the surface of the existing levee. Site preparation for the proposed levee rehabilitation sections should consist of stripping all existing vegetation, granular flexbase, loose and organic topsoil and other unsuitable materials encountered on-site.

The exposed levee subgrade soils should be scarified to a depth of about 6 inches and moisture conditioned at the optimum moisture content and then compacted to at least 95 percent of the Standard Effort (ASTM D 698) maximum dry density of the in-situ soils. The exposed soil subgrade area should be proofrolled with a 20-ton roller or loaded dump truck to detect weak areas. Weak areas detected during proofrolling, as well as zones of fill containing organics or debris, should be removed and replaced with select fill or soils exhibiting similar classification, moisture content, and density as the adjacent in-situ soils.

Proofrolling and removal of organics should be performed under the direct supervision of the geotechnical engineer or his representative.

Settlement Analysis We understand that TOLE will be raised on FE1 from 0.02 ft. to 1.87 ft. at various areas throughout the levee. Evaluation of post-construction settlement as a result of consolidation of clays or settlement of sands at the embankment or foundation will be a factor if a loss of freeboard is achieved. Our settlement analysis was performed on areas with weaker soils under load conditions representing the maximum grade change.

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We anticipate maximum imposed loads from the added fill material for this section to be less than or equal to about 250 pounds per square foot (psf). In addition, we anticipate maintenance equipment live loads to be about 250 psf. These applied loads may lead to an immediate settlement of granular soils likely occurring mostly during construction. In addition, consolidation settlement may occur in clayey soils over a much longer period of time.

A consolidation test was conducted on a relatively undisturbed, recovered sample (FE1, 13- 15 feet) to determine the degree of settlement the levee may undergo with the above referenced surcharge. The results of the consolidation test, including the estimated Compression Index (Cc),, Recompression Index (Cr) and the Preconsolidation Pressure (σp) are provided in Appendix E. In addition, empirical correlations with the CPTU data presented in Appendix D were also used to estimate the magnitude of settlement to expect at these sections of the levee. It was determined that long term and short term settlement expected at this section of the levee is about 1 inch or less.

Slope Stability Analysis Slope failures occur when the balance of slopes are disturbed either by nature or by humans. Goals of a slope stability analysis should be to assess the stability under specified conditions, assess risk of slope failure, and determine the effect of proposed changes on a slope. Engineers determine the maximum angle at which a slope is stable and study the slope stability in terms of the factor of safety (FS). The FS is equal to the shear strength available divided by the mobilized strength. Terracon conducted the slope stability analysis using SLIDE 5.0 software.

Six sections of FE1 were chosen for a slope stability analysis based on soil conditions, such as stratigraphy and general strength, and cross-sectional geometry of the segment. Those segments are FE1-8, FE1-16, FE1-22 (canal), FE1-26, FE1-33 and FE1-36 (canal), and the results are presented in the table below. Slope stability analysis were performed on the adjacent irrigation canal (levee side) for segments FE1-22 and FE1-36 to verify stability of the canal levee side slope and it’s effect on the levee stability. The remaining sections of the levee were not evaluated for slope stability due to the limited height of the levee at these locations, favorable soils conditions and/or favorable levee side slopes after the proposed rehabilitation.

A slope stability analysis was performed on the cross section of boring FE1-8 consisting of very stiff to medium stiff Silty Clay followed by loose to dense Silt and Sand. This section had a river side slope of about 3(H):1(V) and a land side slope of about 2.5(H):1(V). The river side and land side heights of the levee at this section are about 16 feet each. This section of the levee will be rehabilitated with about 1.35 feet of fill material.

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A slope stability analysis was performed on the cross section of boring FE1-16 consisting of medium stiff Clay followed by loose to medium dense Clayey Sand and Sand. This section had a river side slope of about 5(H):1(V) and a land side slope of about 3(H):1(V). The river side and land side heights of the levee at this section are about 16 feet and 9 feet, respectively. This section of the levee will be rehabilitated with less than 1 foot of fill material.

A slope stability analysis was performed on the irrigation canal cross section of boring FE1-

22. The subsurface soils encountered at FE1-22 and that were used to model the cross section of the adjacent canal consisted of very stiff to stiff Clay followed by medium dense to very dense Sand. This levee side slope of the canal is about 1.2(H):1(V). The canal side height of the canal at this section is about 11 feet.

A slope stability analysis was conducted on the cross section of boring FE1-26 because the soil profile consists of loose to dense Silty Sand from the surface to 30 feet deep. A river side slope of about 3(H):1(V) and a land side slope of about 2.5(H):1(V), the maximum permitted by EM 1110-2-1913. The river side height of the levee is about 6½ feet and the land side height is about 4 feet. This section of the levee will not need to be raised based on the 3-foot freeboard, 100-year flood design.

The proposed grade changes in the vicinity of boring FE1-33, will result in insufficient side slopes on both the land and river sides. As shown in Appendix H, a river side slope of about 2(H):1(V) and a land side slope of about 2(H):1(V) will result. These slopes are acceptable as per EM 1110-2-1913. However, a slope stability analysis was performed on the cross section to verify the factors of safety and the applicability of the resulting side slopes.

A slope stability analysis was performed on the irrigation canal cross section of boring FE1-

36. The subsurface soils encountered at FE1-36 and that were used to model the cross section of the adjacent canal consisted of medium stiff to very stiff Clay followed by medium dense to very dense Sand. This levee side slope of the canal is about 1.2(H):1(V). The canal side height of the canal at this section is about 13½ feet.

Two conditions were considered in the analysis: steady state seepage (using effective stress parameters) and rapid drawdown (using effective and total stress parameters).

Earthquake loadings were also considered due to the seismic classification of the site (see the section of this report titled Seismic Conditions below for more information regarding seismic site classification). The following table presents the minimum Factor of Safety for the six segments for both conditions including the minimum Factor of Safety for existing levee slope stability presented in EM 1110-2-1913.

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Table 2: Factors of Safety for Slope Stability for URGFCS – Reach FE1 Factor of Safety Levee Section & Boring Steady State Condition Rapid Drawdown Condition

FE1-8 6.3 5.4

FE1-16 2.1 1.6

FE1-22(canal) 3.1 2.6

FE1-26 2.2 1.4

FE1-33 3.7 3.7

FE1-36(canal) 1.7 1.7 EM 1110-2-1913 Min. 1.4 1.0 to 1.2

As shown in Appendix H, the river side and a land side slopes of the segments provided in the table above are acceptable as per EM 1110-2-1913. The side slopes proposed in Appendix H are acceptable for both steady state and rapid drawdown conditions. The canal levee side slope at section FHE1-45 has a factor of safety less than that proposed by the design manual. The location of the canal critical surface is far enough away from the levee that the initial failure should not affect the levee. However, the adjacent irrigation canals should be maintained if failures occur to minimize future affects of the instability in the canals on the levees. Slope stability cross sections along with the assigned soil parameters are presented in Appendix I.

Seepage Analysis Through seepage in an embankment that emerges on the landside slope can soften fine-grained soils in the vicinity of the landside toe, causing sloughing of the slope, or even lead to piping of fine sand or silt materials. Double hydrometers were performed to determine the likelihood of piping through the soils. The laboratory results are provided in Appendix E.

Groundwater seepage goes from areas of high hydraulic pressure to areas of low hydraulic pressure.

Terracon performed a groundwater seepage analysis for the condition of the riverside floodway full, to a 3-foot freeboard, and the landside of the levee empty. The seepage analysis was performed using the flow net method with the aid of SLIDE 5.0 software. The flow net method evaluates seepage using a graphical technique that shows the decrease in hydraulic head along the flow path.

The following assumptions were made in order to conduct the seepage analysis. If any of the following assumptions are determined to not be accurate, Terracon should be informed so that the appropriate changes to our seepage analysis may be made. Any changes to the seepage analysis may result in different results.

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• The clayey soils used to construct the existing levee do not extend horizontally past the levee toe in both the river and land side directions; and

• The water level of the adjacent irrigation canal was taken at the bottom of the canal as per EM 1110-2-1913, Sect. 8-17.

Seven segments of FE1 were chosen for a slope stability analysis based on soil conditions, such as stratigraphy and general soil properties, and cross-sectional geometry of the segment. Those segments are FE1-2, FE1-8, FE1-14, FE1-22, FE1-26, FE1-31 and FE1-33 and the results are presented in the table below. Based on the soil profiles presented in the Logs of Borings and the CPTU sounding logs, other areas of the levee consist of mainly sand soils at relatively short levee sections. These sections were not evaluated for seepage because the heights of the levee at these locations are relatively short and a full riverside condition would produce a minimal hydraulic head. The following table identifies and describes the cross sections chosen for seepage analysis.

Table 3: Factors of Safety for Slope Stability for URGFCS – Reach FE1

Levee Section & Boring

River / Land Side Slope

River / Land Side Height

Soils Present

FE1-2 4.5(H):1(V)/3.5(H):1(V) 13½ / 3 CH, SM, CL

FE1-8 3.0(H):1(V)/2.5(H):1(V) 16 / 16 CL, SP

FE1-14 3.0(H):1(V)/3.3(H):1(V) 19 / 12½ SM, SP

FE1-22 3.8(H):1(V)/3.2(H):1(V) 7 / 2 CH, CL, SP, SM

FE1-26 3.3(H):1(V)/2.5(H):1(V) 13 / 8 SM

FE1-31 3.3(H):1(V)/3.3(H):1(V) 12 / -- CH, SP

FE1-33 2.5(H):1(V)/2.5(H):1(V) 5 / 4 CH, CL, SP, SM

The following table presents the maximum hydraulic gradient from the landside toe of the levee for all locations.

Table 4: Exit Hydraulic Gradient for URGFCS – Reach FE1 Levee Section & Boring Hydraulic Exit Gradient

FE1-2 0.17

FE1-8 0.25

FE1-14 0.25

FE1-22 0.02

FE1-26 0.25

FE1-31 0.16

FE1-33 0.28

* See commentary regarding exit gradient below.

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As per USACOE Engineering Technical Letter (ETL) No. 1110-2-569, a hydraulic exit gradient of less than 0.5 results in light to no seepage and minimizes the downstream effects of sand boils. As indicated in the table above, a no seepage condition may be assumed for the above referenced FE1 sections.

As noted earlier in this text, an irrigation canal traverses adjacent to the existing levee on its landside. As indicated in section 8-16. Construction of Ditches Landside of Levee of USACOE Engineering Manual 1110-2-1913, drainage ditches should be located such that the exit gradient in the bottom of the ditch does not exceed 0.5 at the landside levee toe and does not exceed 0.8 at a distance 150 feet landward of the landside levee toe and beyond. The evaluated sections of the existing levee generally meet the requirements for seepage.

Flow nets used to perform the seepage analysis, the resulting downstream discharges, and exit hydraulic gradients for all seven cross sections are presented in Appendix J.

Seismic Considerations Earthquake loadings are not normally considered in analyzing the stability of levees because of the low probability of earthquake coinciding with periods of high water. Levees constructed of loose cohesionless materials or founded on loose cohesionless materials are particularly susceptible to failure due to liquefaction during earthquakes. A preliminary liquefaction analysis was conducted for FE1, and it was determined that the clean loose sands at some sections of the levee are potentially liquefiable.

Table 6-1b of EM 1110-2-1913 of the USACOE refers to ER 1110-2-1806 Earthquake Design and Evaluation for Civil Works Projects in regards to levee slope stability under the earthquake stability condition. Section 5.h.2 of ER 1110-2-1806 states that "Site-specific studies should be conducted for all zone 3 and 4 projects, and for zone 2A and 2B projects for which earthquake loadings control the design." According to the Appendix C (Uniform Building Code Seismic Zone Map) of ER 1110-2-1806, the project site maps in zone 1.

Therefore, a site-specific analysis was not performed for this site and standard methods of analysis were used.

Terracon utilized the SeisOpt® ReMi™ method to develop the full depth shear wave velocity profile at the site for use in determining the seismic site class. This method employs non-linear optimization technology to derive one-dimensional S-wave velocities from refraction microtremor (ambient noise) recordings using a typical seismograph and standard, low frequency, refraction geophones. We utilized 24 receivers (geophones) set along a straight-line array with 15±-foot receiver spacing for a total line length of about 350 feet. Unfiltered, 30-second records were recorded using the background ‘noise’ created by moving vehicles and other ambient vibrations. The testing was conducted in the vicinity of borings FE1-7 and FE1-32. The collected data, the response spectrum in the 5- to 40-Hz range, was processed

December 21, 2009 using the computer software SeisOpt® ReMi™ by Optim, LLC with the results plotted as a conventional shear wave velocity vs. depth profile. Based on the testing results, the composite average shear-wave velocities in the vicinity of borings FE1-7 and FE1-32 are 850 ft./sec. and 784 ft./sec., respectively. This information was used to classify the site. The site is located in an area designated as Seismic Design Category “D” as defined by IBC 2006 Sections 1616.4.1 through 1616.4.4. The corresponding shear wave velocity profiles are included in Appendix K.

Seismic CPTU was also performed during advancement of the CPT soundings conducted at the locations referenced in the Field Exploration and Sampling section of this report. This type of testing provides a rapid method for directly measuring shear wave velocity of soils in situ. The Seismic CPTU probe consists of a seismic adapter threaded to a standard CPTU probe. The Seismic CPTU probe records tip resistance, sleeve friction, pore pressure, and tilt angle, and the seismic adapter is used to measure the shear wave velocities. The Seismic CPTU seismic logs for the areas deemed critical are presented in Appendix K.

Evaluation of Liquefaction Potential

Design Earthquakes and Ground Motions

To be consistent with ER 1110-2-1806 in our evaluation of seismic performance of the levees, we considered more than one design earthquake. Ground motion values for the earthquakes were obtained from the U.S. Geological Survey (USGS) internet website and correspond to exceedance probabilities of 2, 5, and 10 percent in 50 years. We estimated the ground surface acceleration, amax, values by obtaining the peak ground acceleration values from the USGS National Seismic Hazard Maps for rock sites (Site Class B) and adjusting these values for Site Class D to account for soil amplification effects.

Deaggragation of the USGS peak ground acceleration computations allowed us to identify the three mean earthquake magnitude, M, values.

The following table summarizes the seismic design parameters for the three scenarios.

Table 5: Summary of Earthquake Magnitude and Ground Surface Acceleration Values

Scenario Probability of Exceedance Earthquake Magnitude Ground Surface

Acceleration (g)

1 2% in 50 years 6.3 0.22

2 5% in 50 years 6.2 0.13

3 10% in 50 years 6.1 0.08

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Liquefaction and Its Effects

Liquefaction is a phenomenon that usually takes place in saturated, relatively clean loose fine sand deposits when subjected to ground shaking. Liquefaction causes a sudden drop in the shear strength of saturated sands under undrained conditions. This is usually triggered by the repeated application of small shear-stress increments and decrements caused by ground shaking associated with an earthquake or explosion. The loss of shear strength is so significant that the sand temporarily assumes the consistency of a dense liquid.

Cohesionless soil layers are potentially liquefiable if they are loose enough to be contractive and are of sufficiently low permeability to experience no or little drainage during the duration of ground shaking. Clean fine sands containing less than 5 percent fines (silt and clay content) are the most susceptible to liquefaction. This is due to the likeliness of these soils being loosely deposited and them having enough low permeability to allow little drainage during ground shaking. As indicated by our borings, soundings, and laboratory test results, these conditions are currently present at the subject sites.

On the other hand, coarse sands and gravel are less susceptible to liquefaction. This is due to their high void ratios and high permeability which enables them to quickly drain unless they are significantly thick or bounded by less pervious layers. Very loose to medium dense clayey sands (SC) are presumed not liquefiable, but the earthquake motion may disturb these soils causing a temporary reduction in their shear strength and their ability to support foundation loads.

The triggering of liquefaction may cause movements/failure at the ground surface or within the near surface soils which may be broadly classified as:

• Slope instability and displacement when the static shear stresses in the slope exceed the shear strength of the liquefied soil.

• Lateral movements and spread failures which may occur along gentle slopes and may cover large areal extents, especially at sites adjacent to a free face like a stream or river channel.

• Vertical settlements caused either by reconsolidation of cohesionless soils following cessation of earthquake ground shaking and/or resulting from lateral movements within the underlying or adjacent soil layers.

• Localized or widespread disruption of the ground surface caused by the movement of water from underlying saturated cohesionless soil deposits to the ground surface, which may manifest in the formation of sand boils.

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• Shaking or oscillation of nonliquefied soil layers at the ground surface with reduced bearing support in underlying liquefied soil masses.

The liquefaction potential of a particular soil layer is judged by comparing the dynamic shear stresses caused by an earthquake to the soil’s undrained shear strength at yield. Seismic forces driving the triggering of liquefaction are quantified using Cyclic Stress Ratios (CSR), which is the cyclic shear stress the soil is subjected to, expressed as a fraction of the initial effective confining stress. The capacity of the soil to resist seismically induced liquefaction is defined using Cyclic Resistance Ratios (CRR) which is the Cyclic Stress Ratio required to trigger liquefaction. CRR values were estimated using measured SPT blowcount data corrected for hammer energy ratio, rod length, effective overburden stress, fines content and earthquake moment magnitude, as appropriate, and compared to the design CSR (function of design earthquake scenario). CRR values were also estimated based on cone tip resistance values measured during Cone Penetration Testing. Factors of Safety (FS) against triggering of liquefaction were computed as the ratio of the CRR to the CSR values.

Liquefaction Potential and Consequences

Our analysis was performed based on the results of the Standard Penetration Testing (SPT) and Cone Penetration Testing (CPT) for selected cross-sections of the levee where relatively loose clean sands were encountered. For Fabens levee (FE1), we analyzed soil profiles for cross-sections FE1-7, FE1-8, FE1-17, FE1-18, FE1-20, FE1-21, FE1-26, FE1-31 and FE1-32. Our analysis indicated that the likelihood of liquefaction is very low (FSmin = 1.4) for Scenario 3, and that only cross-section FE1-18 contained a zone of sand with a FS < 1.1 for Scenario 2. For Scenario 1, cross-sections FE1-8, FE1-18, FE1-20, FE1-31 and FE1-32 contained soils with FS < 1.1.

Liquefaction in the foundation soils may result in deformation of the overlying levees. To quantify the vertical settlement, we applied relationships for volumetric reconsolidation developed from case histories during past earthquakes. Stability and displacement of the levee slopes were evaluated using limit equilibrium methods and Newmark-type displacement analyses. The post-liquefaction slope stability analyses required assigning reduced soil strength values to the liquefied layers. We modeled a 5-foot thick layer of liquefied sand with an undrained residual shear strength value of 115 psf at an approximate depth of 25 feet below the crest of the levee. For saturated soil layers above the liquefied layer, we reduced the static shear strength by 10 percent to account for pore pressure increases. Figure 1 shows the post-liquefaction limit equilibrium model we developed for cross-section FE1-18.

The results of our slope stability analyses for Scenario 1 are shown in Figure 2 for the case of no flood and Figure 3 with the added hazard of a base flood condition. The combination of Scenario 1 ground motion and base flood resulted in FS = 1.0 for cross-section FE1-18 and indicates marginal stability. Displacement estimates for the combined hazard case shown in

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Figure 3 require numerical methods and were not included in our scope of work, but would be on the order of several feet. We computed Newmark-type displacements for Scenario 1 with no flood (Figure 2) to be a maximum of 3 inches.

The following table summarizes the stability and deformation analyses.

Table 6: Summary of Stability and Deformation Analyses for Cross-Section FE1-18 (Fabens)

Scenario Reconsolidation Settlement (in.) Slope Stability and Displacement

Post-Liquefaction Factor of Safety Displacement (in.)

w/ base flood < 2 1.0 > 12 w/ no flood

< 2 1.21 < 3

2 < ½ > 1.3 0

3 0 > 1.3 0

Conclusions and Recommendations

In order to evaluate seismic performance of the levees, we computed the liquefaction potential of soil encountered in our explorations for three design earthquakes. Cross- section FE1-18, located in a section of the Fabens levee, was found to have the greatest likelihood for liquefaction. Subsequent deformation analyses for cross-section FE1-18 led to the conclusion that the levees should experience no settlement or displacement for Scenario 3, negligible deformation for Scenario 2, and no more than a few inches of deformation for Scenario 1 unless the base flood condition exists at the time of the earthquake.

ER 1110-2-1806 describes two design earthquakes: the Maximum Design Earthquake (MDE) and the Operating Basis Earthquake (OBE). The MDE is defined as “the maximum level of ground motion for which a structure is designed or evaluated” and the OBE “is an earthquake that can reasonably be expected to occur within the service life of the project.” Equating the three scenarios of our analysis to the design earthquakes of ER 1110-2-1806 is a risk-based decision for the owner, but if the MDE corresponds to Scenarios 2 or 3, we anticipate acceptable performance of the levees. If the MDE equates to Scenario 1, and the owner is concerned about a simultaneous flood condition, we recommend more detailed analyses.

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The model used for the stability analysis and the results are shown in Figures 1 through 3 in Appendix K.

Lateral Earth Pressures for Retaining Walls and Flood Walls We understand that retaining walls are planned along the land side of the levee from Station (Sta.) 1634+00.00 to Sta. 1651+00.00 and Sta. 1667+00.00 to Sta. 1686+00.00 and a flood wall is planned along the river side of the levee over the same segments. Continuous footings foundation system may be used to support the planned retaining or flood walls. The footing should bear no less than 2 feet below final outside grade. The footings should be designed for a total load net bearing pressure of 1,500 psf. This includes a factor of safety of 2.

The footings can provide some uplift resistance due to wind or other induced structural loading.

The uplift resistance of a footing may be computed using the effective weight of the soil above the footing along with the weight of the footing and structure.