IBM10B0006_Ft.Hancock_geotech.pdf

Geotechnical Engineering Report

International Boundary and Water Commission Upper Rio Grande Levee Evaluation

Fort Hancock, Texas December 21, 2009

Terracon Project No. 88095074

Prepared for:

S & B Infrastructure, Ltd.

McAllen, Texas

Prepared by:

Terracon Consultants, Inc.

Pharr, Texas

Project No. 88095074

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 – Fort Hancock 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 – Fort Hancock Reach in Hudspeth 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 Hudspeth County between International Dam and Riverside Weir in 2007 to meet the 100-

December 21, 2009 year, 3-foot 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 Fort Hancock 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 Fort Hancock Acala to McNary ~8.5 559502 X

10486226 Y 583629 X 10457167 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. 88095074, dated November 3, 2009, and Draft Reports on November 18, 2009 and December 8, 2009.

Project Description The Fort Hancock Reach, here-on referred to as FHE1, is located within the vicinity of Acala to McNary in Hudspeth County, Texas. The levee reach extends from about 1,000 feet north of Mile Marker 55 of URGFCS, west of Acuna, Texas, to about 1,500 feet east of Mile Marker 36 of URGFCS, south of McNary, Texas. The total distance of this reach is about 8.5 miles. This reach of the URGFCS has the Rio Grande Protective Fence at its land side toe from boring FHE1-19 (567563.969, 10476144.549) (X,Y) to the end of the levee, excluding boring FHE1-28 (573283.283, 10469207.257). 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 FHE1 will need to be raised from 0.23 to 3.81 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 46 test borings along the levee reach from 30 to 60 feet below existing surface at the top of the levee about every 1,000 feet 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 FHE1-7 and FHE1-32 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

December 21, 2009

• 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 46 borings at the project site.

The 46 borings (FHE1-1 through FHE1-46) were drilled on the Fort Hancock levee at about every 1,000 feet, as identified in the Statement of Work (SOW), from coordinates (559496.622, 10486224.351) (X,Y) to coordinates (583889.277, 10457088.484). The borings were drilled from about 30 to 60 feet deep from the top of the levee. The 60 foot

December 21, 2009 borings were drilled about every 5,000 feet beginning at FHE1-2 (558605.204, 10485758.076).

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, 16 Seismic CPTU soundings (conducted adjacent to borings FHE1-1, FHE1-4, FHE1-7, FHE1-10, FHE1-13, FHE1-16, FHE1-19, FHE1-22, FHE1-25, FHE1-28, FHE1-31, FHE1-34, FHE1-37, FHE1-40, FHE1-43 and FHE1-46) were performed on the Fort Hancock 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

December 21, 2009 classification and laboratory tests were performed in general accordance with the appropriate ASTM standards.

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 FHE1 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 FHE1-6, FHE1-10 to FHE1-11, from FHE1-13 to FHE1-19, FHE1-21 to FHE1-22, FHE1-24, FHE1-26, FHE1-29, FHE1-33, FHE1-37, FHE1-39 and FHE1-41.

• Some superficial tire ruts are evident on the river side slopes in the vicinity of borings FHE1-6, FHE1-18, between FHE1-23 to FHE1-24, FHE1-25 to FHE1-26, FHE1-28, FHE1-29, between FHE1-33 and FHE1-34 and FHE1-36 to FHE1-37.

• Several ramps use to access the levee from a river side patrol road are located throughout the levee reach in the vicinity of borings FHE1-8, between FHE1-11 and FHE1-12, FHE1-15, FHE1-16, FHE1-22, FHE1-28, FHE1-31 and FE1-44. 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.

December 21, 2009

• 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.

• Some areas of concern on the FHE1 levee include a floodgate structure at the Fort Hancock Port of Entry and an irrigation canal that abuts the land side of the levee just west of boring FHE1-28. In addition, an irrigation canal runs along the length of the levee, about 15 feet from the land side toe, starting between borings FHE1-2 and FHE1-3 to the end of the levee, with the exception of from borings FHE1-28 to FHE1-31. The canal ranges from 15½ to 36 feet deep (on the levee side) and about 31 to 64 feet wide (from crest to crest). Some slope failure at the crest of the irrigation canal (on the levee side) was evident at various spots from the vicinity of boring FHE1-31 to FHE1-46, the end of the levee.

• 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).

However, more flexbase material is present in the areas of the levee adjacent to the Pedestrian Fence (from borings FHE1-19 to FHE1-46). 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 FHE1-19, FHE1-22, FHE1-24, FHE1-25, FHE1-26, FHE1-27, FHE1-29, FHE1-30, FHE1-42, FHE1-44 and FHE1-45.

• 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 FHE1-2 (land side), FHE1-4 (land side), FHE1-9 (river side), FHE1- 10 (river side), FHE1-12 (river side), and FHE1-17 (river side), FHE1-19 (river side), FHE1-20 (river side), FHE1-25 (land side), FHE1-28 (land side), FHE1- 35 (river side), FHE1-40 (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 Fort Hancock levee section is located on the Alluvium of the Rio Grande

December 21, 2009

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:

• Clay (CH and CL); medium stiff to hard;

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

• Sand (SP, SC, SM, SP-SM and SP-SC); loose 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 to their termination depths or 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.

December 21, 2009

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.

Geotechnical Considerations Based on information provided by S&B, which was based on IBWC Top of Levee Elevations (TOLE), we understand that the existing FHE1 levee reach TOLE ranges from elevation (El.)

3519.77 feet (ft.) to El. 3558.28 ft. In order to maintain a 3-foot freeboard for a 100-year flood design, the proposed TOLE ranges from El. 3519.44 ft. to 3541.86 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 FHE1-1 through FHE1-7, FHE1-11 through FHE1-15, FHE1-18 and FHE1-43 through FHE1-45.

Grade changes in the remaining boring vicinities will range from 0.23 ft. to 3.81 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 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, existing condition, are in the vicinity of borings FHE1-2 (land side), FHE1-4 (land side), FHE1-9 (river side), FHE1-10 (river side), FHE1-12 (river side), and FHE1-17 (river side), FHE1-19 (river side), FHE1-20 (river side), FHE1-25 (land side), FHE1-28 (land side), FHE1-35 (river side), FHE1-40 (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 for some levee segments may not allow for extension of the land side right-of-way.

The proposed grade changes and slope finishes will result in result in insufficient slope finishes for the following segments: FHE1-2 (steepest land side), FHE1-22 (land side), FHE1-25 (land side), FHE1-27 (river side), FHE1-28 (land side) and FHE1-40 (land side).

Extending the levee base out towards the riverside will result in the required corresponding slope for segments FHE1-27 and FHE1-28. Segments FHE1-2, FHE1-22, FHE1-25 and FHE1-40 will be checked for slope stability. Cross section drawings for these segments of the FHE1 levee are provided in Appendix H.

December 21, 2009

The proposed grade changes and slope at the other locations referenced above 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 FHE1-19, FHE1-22, FHE1-24, FHE1-25, FHE1-26, FHE1-27, FHE1-29, FHE1-30, FHE1-42, FHE1-44 and FHE1-45 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.

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

December 21, 2009 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 0.23 ft. to 3.81 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.

We anticipate maximum imposed loads from the added fill material for this section to be less than or equal to about 450 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 (FHE1- 28, 28½ - 30 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 1½ inches 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 FHE1 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 FHE1-2, FHE1-3 (canal), FHE1-22, FHE1-25, FHE1-28 and FHE1-45 (canal), and the results are presented in the table below. Slope stability analysis were

December 21, 2009 performed on the adjacent irrigation canal (levee side) for segments FHE1-3 and FHE1-45 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 FHE1-2 consisting of medium dense Sand followed by stiff to hard Clay and dense to very dense Sand. This section had an east land side slope of about 5(H):1(V) and a west land side slope of about 1(H):1(V). The west land side slope is considered too steep as per EM 1110-2-1913.

Therefore, a slope stability analysis was performed on the cross section to verify the factors of safety and the applicability of the resulting side slopes. The east land side and west land side heights of the levee at this segment are about 7 feet and 11 feet, respectively. Grade at this section of the levee will not be raised.

A slope stability analysis was performed on the irrigation canal cross section of boring FHE1-3. The subsurface soils encountered at FHE1-3 and that were used to model the cross section of the adjacent canal consisted of medium dense Clayey Sand followed by loose to dense Sand with Silt. This levee side slope of the canal is about 2.2(H):1(V). The canal side height of the canal at this segment is about 19 feet. Grade at this section of the levee will not be raised. This section was chosen as a typical section of the canal for slope stability analysis.

The proposed grade changes in the vicinity of boring FHE1-22, will result in an insufficient side slope on the land side. As shown in Appendix H, a river side slope of about 3(H):1(V) and a land side slope of about 2(H):1(V) will result. Both side 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. The cross section of boring FHE1-22 consisted of medium dense to very dense Clayey Sand and Silty Sand followed by very dense Sand. The land side and river land side heights of the levee at this segment are about 5½ feet and 4 feet, respectively. This section of the levee will be rehabilitated with about 2.51 feet of fill material.

The proposed grade changes in the vicinity of boring FHE1-25, will result in an insufficient side slope on the land side. As shown in Appendix H, a river side slope of about 3(H):1(V) and a land side slope of about 2(H):1(V) will result. Both side 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. The cross section of boring FHE1-22 consisted of stiff to hard Sandy Clay followed by medium dense to dense Silty Sand. The land side and river land side heights of the levee at this segment are about 5 feet and 6½ feet, respectively. This section of the levee will be rehabilitated with about 3.07 feet of fill material.

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A slope stability analysis was performed on the cross section of boring FHE1-28 consisting of very stiff Sandy Clay followed by medium dense to dense Sand with Silt. This section had a land side slope of about 1.8(H):1(V) and a river side slope of about 6(H):1(V). The land side slope is considered too steep as per EM 1110-2-1913. In addition, an irrigation canal with about 4 feet of water abuts the land side slope. Therefore, a slope stability analysis was performed on the cross section for both the existing and rehabilitated cases (see figure in Appendix H for proposed, rehabilitated side slopes) to verify the factors of safety and the applicability of the resulting side slopes. The land side height of the levee is about 3½ feet to the surface of the adjacent canal water and the river side height of the levee at this section is about 4½ feet, respectively. This section of the levee will be rehabilitated with about 3.66 feet of fill material.

A slope stability analysis was performed on the irrigation canal cross section of boring FHE1-45. The subsurface soils encountered at FHE1-45 and that were used to model the cross section of the adjacent canal consisted of dense to very dense Silty Sand followed by medium dense to dense Silt. This levee side slope of the canal is about 1.4(H):1(V). The canal side height of the canal at this segment is about 18 feet. Grade at this section of the levee will not be raised. This section was chosen for slope stability analysis on the canal because of the steep levee side slope.

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 FS for the three locations for both conditions including the minimum FS for existing levee slope stability presented in EM 1110-2-1913.

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

FHE1-2 1.6 1.8

FHE1-3 (canal) 1.5 1.61

FHE1-22 2.1 2.3

FHE1-25 (modified)2 4.4 2.8 FHE1-28 (existing)2 8.23 --- FHE1-28 (existing)2 2.74 --- FHE1-28 (modified)2 4.43 3.13

FHE1-28 (modified)2 2.54 --- FHE1-45 (canal) 1.2 1.31

EM 1110-2-1913 Min. 1.4 1.0 to 1.2

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1. For a dry canal condition

2. Existing conditions represent the existing levee and modified conditions represent the levee after the proposed grade changes.

3. For a full river side condition

4. For a full canal condition

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.

• 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.

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Six segments of FHE1 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 FHE1-2, FHE1-7, FHE1-18, FHE1-20, FHE1-22 and FHE1- 28, 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 FHE1 Levee Section &

Boring River / Land Side Slope River / Land Side

Height Soils Present

FHE1-2 5.0(H):1(V)/1.0(H):1(V)1 7 / 111 SM, CL

FHE1-7 3.0(H):1(V)/4.0(H):1(V) 7½ / 1½ CL, SP-SM, SM

FHE1-18 3.0(H):1(V)/3.3(H):1(V) 9 / 5½ SM, SP-SM

FHE1-20 2.8(H):1(V)/3.3(H):1(V) 7 / 4½ SM, CL, SP-SM

FHE1-22 4.0(H):1(V)/2.5(H):1(V) 4 / 5½ SC, SM, SP

FHE1-28 6.0(H):1(V)/1.8(H):1(V) 4½ / 3½2 CL, SM, SP

1. The slopes and heights are for the land side east and land side west, respectively.

2. The land side height is from the top of the levee to the surface of the water in the adjacent canal.

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

FHE1-2 0.10

FHE1-7 0.29

FHE1-18 0.39

FHE1-20 0.22

FHE1-22 0.52

FHE1-28 0.23

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 light to no seepage condition may be assumed for the above referenced FHE1 sections.

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As noted earlier in this text, an irrigation canal traverses adjacent and along the length of 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 six 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 at three boring locations deemed to be the most critical in regards to liquefaction potential based on SPT blow counts and fines content (percent of silt and clay). Our analysis determined that the clean loose sands at cross section FHE1-14 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 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

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863 ft./sec. and 771 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 Cone Penetration Testing (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 SCPTU probe consists of a seismic adapter threaded to a standard CPTU probe. The SCPTU probe records tip resistance, sleeve friction, pore pressure, and tilt angle, and the seismic adapter is used to measure the shear wave velocities. The SCPTU seismic logs 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 levee FHE1, our analysis was performed for cross-sections FHE1-12, FHE1-14, FHE1-32 and FHE1-42. Our analysis indicated that the likelihood of liquefaction is very low (FSmin = 1.4) for Scenario 1, and that only cross-section FHE1-14 in Fort Hancock contained a zone of sand with a FS = 1.1 for Scenario 1. Hence, liquefaction is not a concern for Scenarios 2 and 3.

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.

Conclusions and Recommendations

In order to evaluate seismic performance of the levees, we computed the liquefaction potential of soil encountered in our…