IBM09TO020-GeotechReport.pdf

Engineers, Geologists, Hygienists and Environmental Scientists

San Antonio, TX

Austin, TX Brownsville, TX El Paso, TX

Mexico McAllen, TX Houston, TX

GEOTECHNICAL EXPLORATION AND ENGINEERING EVALUATION OF

LEVEE SYSTEM

FOR

LATERAL A TO DONNA PUMP TO RETAMAL DAM

LOWER RIO GRANDE FLOOD CONTROL PROJECT

FINAL TECHNICAL MEMORANDUM

Engineering • Testing • Environmental • Facilities • Infrastructure

Raba-Kistner Consultants, Inc.

800 E. Hackberry McAllen, TX 78501

(956) 682-5332 FAX (956) 682-5487

www.rkci.com

Austin Brownsville El Paso Houston McAllen Mexico San Antonio

Project No. AMA09-035-00 September 22, 2009

Ms. Sharlotte L. Teague, P.E., Deputy Project Manager S&B Infrastructure, Ltd.

5408 North 10th Street McAllen, Texas 78504

RE: Geotechnical Exploration and Engineering Evaluation of Levee System

Final Technical Memorandum Lateral A to Donna Pump to Retamal Dam

Lower Rio Grande Flood Control Project Hidalgo County, Texas USIBWC IDIQ Contract No. IBM05D0001 S&B Project No. U0450.502 S&B Purchase Order No. U0450-Z009 Work Authorization No. 9

Dear Ms. Teague:

Raba-Kistner Consultants, Inc. (R-K) is pleased to submit our Final Technical Memorandum for the geotechnical explorations and engineering evaluation of the levee system for The Rio Grande Flood Control project from Lateral A to Donna Pump to Retamal Dam reach (except for a

0.9 mile length of existing levee/floodwall combination in the Lateral A to Donna Pump reach, just northwest of Donna Pump) within Hidalgo County, Texas. These services were performed in accordance with the scope of work pursuant to the executed Subcontract for Consultant Services between S&B Infrastructure, Ltd. (CLIENT) and R-K for the above-referenced indefinite delivery - indefinite quantity contract (IDIQ) with the United States International Boundary and Water Commission (USIBWC). Written notice to proceed was received by our office via facsimile.

The purpose of this study was to drill borings along the existing alignment of the levee, to perform laboratory testing to classify and characterize subsurface conditions, and to prepare a Technical Memorandum presenting our geotechnical analyses of the levee in general accordance with the U.S. Army Corps of Engineers (USACE) Engineering Manual (EM) 1110-2- 1913 Design and Construction of Levees and the Federal Emergency Management Agency (FEMA) Requirements for Mapping Levees (44 CFR 65.10), as they pertain to geotechnical evaluations.

Raba-Kistner

We appreciate the opportunity to be of service on this project, and look forward to working with you throughout the remaining phases. If you have any questions, please call.

Very truly yours, RABA-KISTNER CONSULTANTS, INC.

Russell L. Thomas, Jr., P.E. John A. Focht, III, P.E.

Senior Project Manager Chief Geotechnical Engineer

Chris L. Schultz, P.E. Katrin Miovski, P.E.

Senior Vice President Manager, Engineering Services

RLT/JAF/CLS/KM/ns

Attachments

Copies Submitted: Above (1)

GEOTECHNICAL EXPLORATION AND ENGINEERING EVALUATION OF LEVEE SYSTEM

LATERAL A TO DONNA PUMP TO RETAMAL DAM

LOWER RIO GRANDE FLOOD CONTROL PROJECT

HIDALGO COUNTY, TEXAS

USIBWC IDIQ CONTRACT NO. IBM05D0001

FINAL TECHNICAL MEMORANDUM

for

U.S. SECTION OF THE INTERNATIONAL BOUNDARY AND WATER COMMISSION

(USIBWC)

Prepared for

S&B INFRASTRUCTURE, LTD.

McAllen, Texas

Prepared by

RABA-KISTNER CONSULTANTS, INC.

McAllen, Texas

PROJECT NO. AMA09-035-00

EXECUTIVE SUMMARY

The purpose of this study was to drill borings along the existing alignment of the Lateral A to Donna Pump to the Retamal Dam Reach of The Lower Rio Grande Flood Control Project levee system, to perform laboratory testing to classify and characterize subsurface conditions, to prepare a Technical Memorandum presenting our geotechnical analyses of the levee in general accordance with the U.S. Army Corps of Engineers’ (USACE) Engineering Manual (EM) 1110-2- 1913 Design and Construction of Levees and the Federal Emergency Management Agency (FEMA) Requirements for Mapping Levees (44 CFR 65.10) as they pertain to geotechnical evaluations, and to provide geotechnical engineering design recommendations with engineering solutions for the levee against the provided 100 year flood water surface elevations.

Subsurface conditions along The Lower Rio Grande Flood Control Project levee system were evaluated by drilling 70 borings to a maximum depth of about 50 feet each and four borings to a maximum depth of about 100 feet each below the existing top-of-levee elevation using truck-mounted drilling rigs. The borings were advanced using straight flight augers in combination with mud rotary drilling techniques and were backfilled with cement-grout following completion of each day’s drilling operations.

The subsurface conditions observed in the borings performed for this study were relatively consistent along the entire length of levee. In general, fill materials associated with the existing levee were observed overlying naturally deposited soils common to the area. The thickness of these fill soils were observed to range from about 8 to 22 feet. The vast majority of the fill materials observed in the borings performed for this study consisted of lean clays (CL) and fat clays (CH). The remaining fill materials were primarily comprised of clayey sand and silty sands (SC and SM), and silts (ML).

Areas where sand and/or silt layers were observed within the existing fill soils are of particular concern. Non-cohesive soils have a much higher hydraulic conductivity than cohesive soils, which will allow water to flow through the earthen levee at higher rates within these non-cohesive layers than through cohesive soil layers. When observed, these sand and silt layers were about 1/2 to 6 feet in thickness.

Beneath the fill soils, naturally deposited cohesive soils with interbedded layers of sands and silts were observed. The naturally deposited soils vary over the length of the levee project in both depth and thickness. The native soils encountered in the borings conducted for this study were predominately comprised of cohesive soils, consisting of lean clays (CL) and fat clays (CH). Encountered to a lesser extent were cohesive sandy fat clays, sandy lean clays, silty sandy clays, and silty clays. The amount of fat clays observed in the naturally deposited soils is significantly higher than that observed in the fill soils, where lean clays predominated. The native non-cohesive soils observed in our borings were predominately silty sands (SM), with some areas of clayey sands (SC); silts (ML), and sands (SP-SM or SP).

A detailed description of the soil conditions observed at each boring location has been presented in the Boring Logs, Figures A-2 through A-75, of Appendix A. The lines designating the interfaces between strata on the boring logs represent approximate boundaries. Transitions between strata may be gradual.

The existing and modified levees should meet three important criteria: they should be resistant to the forces of erosion, should exhibit a suitable allowable factor of safety with respect to slope stability, and should have an acceptable factor of safety against instability due to through- or under-seepage conditions. The levee structure must meet these criteria so that the calculated risk of failure is in accordance with criteria established by the USACE guidelines.

The slope stability analyses indicate that embankment slopes of 3:1 (horizontal:vertical) or flatter should exhibit factors of safety consistent with USACE EM 1110-2-1913 when the levees are modified as recommended in this Final Technical Memorandum. Where the embankments are composed entirely of clays and the underlying soils consist of at least 10 feet of clay, no treatment of the embankment is required. Where the embankments contain layers of silts or sands, the river side levee face must be plated with at least 2 feet of clay. Land side drainage blankets are required in areas where sands and silts of limited depth were observed directly below the embankment; land side drainage blankets are also recommended where a relatively thin clay layer lies directly beneath the embankment that is underlain by sands and silts of limited thickness.

Deep layers of sands and silts were observed directly below the embankment fills at six boring locations. Following the written authorization received by our office on Tuesday, July 21, 2009, four additional, 100-feet deep borings were conducted at the areas where deeper seepage control measures needed to be evaluated. Upon review of the field and laboratory data obtained from these four deeper borings, a deep slurry trench or steel sheetpiling will have to be installed in order to control deeper seepage. Alternatively, a combination of toe drains and gravel blankets may be installed in these areas. The latter alternative will not prevent underseepage, but will assist in controlling its location and reduce damage caused to the levee system resulting from underseepage. Hence, this alternative will provide flood protection during a base flood for a one percentage annual-chance level flood, as required for FEMA levee certification; however, post-damage flood repairs may be needed in localized areas.

This Executive Summary is intended for information purposes only, and should not be utilized separately from the entire Final Technical Memorandum. The following Final Technical Memorandum contains a more definitive explanation of the study performed and the results of our field and laboratory tests. In addition, the Final Technical Memorandum contains a greater discussion on failure mechanisms, the methods and results of our analyses, and our design considerations.

TABLE OF CONTENTS

i

INTRODUCTION

PROJECT BACKGROUND

PROJECT DESCRIPTION

RISK

LIMITATIONS

FIELD EXPLORATION

LABORATORY TESTING

STRENGTH TESTS

CONSOLIDATION TESTS

HYDRAULIC CONDUCTIVITY (PERMEABILITY) TESTS

GENERAL SITE CONDITIONS

SITE DESCRIPTION

SITE GEOLOGY

SEISMIC COEFFICIENTS

STRATIGRAPHY

IMPACT OF SAMPLING AND TESTING PROCEDURES ON

MEASURED AND DESIGN SOIL PROPERTIES

PERMEABILITY

INFERRED PERMEABILITY

DEPTH-TO-WATER

EARTHEN LEVEE SYSTEM

DESIGN CONSIDERATIONS

Probable Failure Modes

SLOPE STABILITY

Minimum Factor of Safety Slope Configurations Method of Analysis Loading Conditions

SOIL PARAMETERS

Results of Analyses

SEEPAGE ANALYSIS

ii

Drainage layer Seepage barrier Relief wells Other approaches

SETTLEMENT

SOIL PARAMETERS

Results of Analyses

TRAFFICABILITY

CONCLUSIONS

RECOMMENDATIONS

EARTHEN LEVEE SYSTEM

CONSTRUCTION RECOMMENDATIONS

GENERAL CONSTRUCTION CONSIDERATIONS

VEGETATIVE COVER FOR CLAY-PLATED LEVEES

SITE PREPARATION

CRUSHED LIMESTONE BASE

CLAY FILL MATERIAL

EXCAVATION SLOPING AND BENCHING

EXCAVATION EQUIPMENT

iii

ATTACHMENTS

Appendix A (Field Testing Summary) Boring Location Map ................................................................................................................. A-1 Logs of Borings ....................................................................................................A-2 through A-75 Key to Terms and Symbols..................................................................................................... A-76 Results of Soil Sample Analyses ...................................................................A-77a through A-77z

Appendix B (Laboratory Testing Summary) Grain Size Curves................................................................................................B-1 through B-31 Unconfined Compression Test Reports .............................................................B-32 through B-43 Consolidated Undrained (CU) Triaxial Compression Test Reports ...................B-44 through B-59 Consolidated Drained (CD) Triaxial Compression Test Reports .......................B-60 through B-62 Direct Shear Test Reports .................................................................................B-63 through B-67 Hydraulic Conductivity (Permeability) Test Reports...........................................B-68 through B-97 One-Dimensional Consolidation Test Reports.................................................B-98 through B-111 Moisture-Density Relationships (Proctor) ......................................................B-112 through B-118 Crumb Tests ..................................................................................................B-119 through B-124 Additional Grain Size Curves .........................................................................B-125 through B-129

Appendix C (Results of Geotechnical Analyses) Slope Stability and Seepage Analysis .................................................................. C-1 through C-5 Total Hydraulic Gradient ....................................................................................... C-6 through C-9 Standard Detail – Drainage Blanket........................................................................................ C-10 Standard Detail – Toe Drain ................................................................................................... C-11 Standard Detail – Drainage Blanket and Toe Drain................................................................ C-12

INTRODUCTION

Raba-Kistner Consultants, Inc. (R-K) has completed the authorized subsurface exploration and engineering analyses for the proposed rehabilitation improvements to the Lateral A to Donna Pump to Retamal Dam Reach of the Lower Rio Grande Flood Control Project (LRGFCP) of the U.S. Section of the International Boundary and Water Commission (USIBWC) in Hidalgo County, Texas. This Final Technical Memorandum briefly describes the procedures utilized during this study and presents our findings along with our geotechnical analyses of the levee, which was performed in general accordance with the USACE document EM 1110-2-1913 Design and Construction of Levees and FEMA Requirements for Mapping Levees (44 CFR 65.10) as it pertains to geotechnical evaluations.

PROJECT BACKGROUND

We understand that the USIBWC has recently conducted studies to assess the ability of the United States levees of the LRGFCP to safely contain flood flows of the Rio Grande River. The LRGFCP extends from the Town of Peñitas, Texas, at its upstream end, to 28 miles above the Gulf of Mexico, a distance of about 158 river miles. The LRGFCP has two diversion dams, Anzalduas Dam and Retamal Dam, approximately 17 and 57 river miles below Peñitas, respectively, which divert portions of the flood flows into the Main Floodway and the Mexican Floodway, respectively. The LRGFCP was constructed to provide flood protection to urban, suburban, and irrigated lands in the Rio Grande delta in both the United States and Mexico. Each country designed and constructed the portion of the project located in its respective territory.

PROJECT DESCRIPTION

We understand several areas along the LRGFCP levee system have been identified as needing rehabilitation and/or improvements to meet current USACE and/or FEMA guidelines. One of these areas has been indentified as the Lateral A to Retamal Dam segment. The Lateral A to Retamal Dam segment is approximately 14 miles in length, extending from the Carlson Settling Basin to the Retamal Diversion Dam located within Hidalgo County, Texas. This study is concerned with this 14 mile segment as well as an approximately 0.9 mile length of levee extending from Lateral A to Donna Pump.

The USIBWC studies indicate that the existing levee along this reach has height deficiencies ranging from about 2 to 7 feet. The proposed rehabilitation improvements consist of raising the top of levee elevation by the addition of earthen fill. The top elevation of these improvements will be as required to provide containment of design flood flows with a freeboard of 3 feet.

The scope of our study for the proposed rehabilitation improvements consists of performing a geotechnical investigation for the design of the proposed improvements along the existing levee alignment, including slope stability, levee settlement, slope stability evaluations, and seepage analyses.

RISK

The geotechnical engineering design recommendations contained in this Final Technical Memorandum are intended to provide S&B Infrastructure, Ltd. and the U.S. Boundary and

Water Commission with design information. We have attempted to provide guidance regarding the impact of various design choices on calculated factors of safety with respect to calculated slope stability. The design factors of safety used for this study were taken from USACE EM 1110-2-1913 dated April 30, 2000. The selection of a given design factor of safety implicitly includes a decision regarding the level of acceptable risk for a project. This study did not include an independent evaluation of risk with respect to levee performance, and adopted the published USACE design factors of safety as the current performance standards for these types of structures. Levees are only designed to provide protection to a certain flood level; R-K did not determine the chosen design flood level, and the selection of this parameter is outside our area of expertise. We understand that USIBWC used the computer program HEC-2 to evaluate the water surface profile for the 100-year design flood in accordance with the current FEMA Levee Certification criteria. Furthermore, for the purposes of this Final Technical Memorandum, it has been assumed that the maximum time that the levee would be subjected to the design flood level is 14 days.

It is important to note that the levee systems are being designed to perform a required function (retain flood waters) under stated conditions (during a 100-year flood event) for a specified period of time (presently unknown.) The design may not be adequate for a different storm event or longer performance period, and unexpected damage to the levees may result in the need for repairs to the levee and/or levee improvements over time. Our conceptual design recommendations inherently assume that the levee will be properly maintained and regularly inspected by trained and qualified personnel. Our conceptual design recommendations also assume that any identified repairs will be made promptly, and the adequacy of the levee system will be regularly assessed, fully taking into account changes in weather, land use and evolving design and performance standards, with changes made as appropriate.

The geotechnical properties of the soils encountered in this study involve significant variability.

This variability includes some spatial variability; however, the spatial variability appears to occur over relatively short distances. It is important to note that levees differ from other types of structures, such as drilled piers or driven piles, in that the performance of the levee involves local, not average, soil conditions.1 The selection of design parameters for this project was based on a review of the available geotechnical data, our knowledge of the project area, design calculations using idealized geometries. The results of our analyses were then reviewed with respect to important trends and general design concepts, keeping these conditions and limitations in mind. Our conceptual recommendations are based on a conservative approach to design as is warranted for all levee structures. We believe that the combination of observed conditions and probable failure modes justifies this approach.

LIMITATIONS

This Final Technical Memorandum has been prepared in accordance with accepted Geotechnical Engineering practices in the region of South Texas for the use of S&B Infrastructure, Ltd. (CLIENT) and its representatives for design purposes. This Final Technical Memorandum may not contain sufficient information for purposes of other parties or other uses.

1 Focht, J.A. Jr. and Focht, J.A. III, “Factor of Safety and Reliability in Geotechnical Engineering, Discussion and Closure”, ASCE JGGE Vol.

127 No. 8, pp.700-721, August 2001.

The recommendations submitted in this Final Technical Memorandum are based on the data obtained from 74 borings drilled along the alignment, our understanding of the project information provided to us, and the assumptions with regard to water elevations and durations that are discussed in greater detail in the following Final Technical Memorandum. If the project information described in this Final Technical Memorandum is incorrect, is altered, or if new information is available, we should be retained to review and modify our recommendations.

This Final Technical Memorandum may not reflect the actual variations of the subsurface conditions across the site. This is particularly true with regard to existing groundwater levels that are likely to be highly variable based on precipitation events and the percent of sands versus fine grained materials. The nature and extent of variations along the levee may not become evident until construction commences. The construction process itself may also alter subsurface conditions. If variations appear evident at the time of construction, it may be necessary to reevaluate our recommendations after performing on-site observations and tests to establish the engineering impact of the variations.

The scope of our current study does not include an environmental assessment of the air, soil, rock, or water conditions either on or adjacent to the site. No environmental opinions are presented in this Final Technical Memorandum.

If final top-of-levee elevations are significantly different from those provided by S&B Infrastructure (more than plus or minus 12-inches), our office should be informed about these changes. If needed and/or if desired, we will reexamine our analyses and make supplemental recommendations.

FIELD EXPLORATION

Subsurface conditions along the subject site were evaluated by 74 borings (designated as “LA1-”) drilled from April 14, 2009 through May 6, 2009, and from July 23, 2009 through July 27, 2009 at the locations shown on the Boring Location Map, Figure A-1.

The boring locations shown on the Boring Location Map are based on our review of an undated site plan titled “LA1 – Lateral A to Retamal Dam (includes Retamal Dike)”, provided to us by the CLIENT via electronic-mail attachment on April 10, 2009 and on July 23, 2009. The ground surface elevation of each boring location was provided to us by the CLIENT via electronic-mail attachment on July 28 and September 21, 2009. Seventy of the borings were drilled to a depth of about 50 feet each, while the other four were drilled to a depth of about 100 feet each below the existing top-of-levee elevation using truck-mounted drill rigs, with the borings located near the centerline of the levee cross-section to a depth equal to twice the expected final levee height. This depth of exploration is consistent with the USACE EM 1110-2-1913 Design and Construction of Levees. The borings were advanced using straight flight augers in combination with mud rotary drilling techniques and were backfilled with cement-grout following completion of each day’s drilling operations. During drilling operations, the samples tabulated in the table shown on the following page were collected:

Type of Sample Number Collected Split-Spoon (with Standard Penetration Test, SPT) 368 Shelby Tubes 638 Bulk Samples for Proctor 7

The SPT and Shelby tube samples were performed in accordance with accepted standard practices. Selected portions of the samples were sealed in containers to reduce moisture loss, labeled, packaged, and transported to our laboratory for subsequent testing and classification.

LABORATORY TESTING

In the laboratory, each sample was evaluated and visually classified by a member of our geotechnical engineering staff in general accordance with the Unified Soil Classification System (USCS). The geotechnical engineering properties of the strata were evaluated by the laboratory tests summarized in the following table:

Type of Test Number Complete

Natural Moisture Content 1006

Atterberg Limits 331

Percent Passing a No. 200 Sieve 508

Grain Size Curves (Particle Size, Part I) 164

Hydrometer (Particle Size, Part II) 23

Specific Gravity 36

Unconfined Compressive Strength 60

Dry Unit Weight 137

Consolidated-Undrained (CU) Triaxial 16

Consolidated-Drained (CD) Triaxial 3

Direct Shear (DS) 5

One-Dimensional Consolidation 14

Hydraulic Conductivity (Permeability) 30

Moisture-Density Relationship (Proctor) 7

Crumb Tests 130

With the exception of the laboratory grain size curves (particle size, Part I), hydrometer tests (particle size, Part II), specific gravity tests, hydraulic conductivity (permeability) tests, direct shear tests, one-dimensional consolidation tests, CU triaxial tests, CD triaxial tests, moisture-density relationship (Proctor) tests, and crumb tests, the results of the field and laboratory tests are presented in graphical or numerical form on the boring logs illustrated on Figures A-2 through A-75. A key to the classification of terms and symbols used on the logs is presented on

Figure A-76. The results of the laboratory and field testing are also tabulated on Figures A-77a through A-77z for ease of reference.

SPT results are noted as “blows per foot” on the boring logs and on Figures A-77a through A-77z, where “blows per foot” refers to the number of blows required for a falling 140-pound hammer to penetrate 12 inches into the subsurface materials. Where hard and/or very dense materials were encountered, the tests were terminated at 50 blows even if one foot of penetration had not been achieved.

Samples will be retained in our laboratory for 30 days after submittal of this Final Technical Memorandum. Other arrangements may be provided at the written request of the CLIENT.

STRENGTH TESTS

Shear strength parameters of selected samples were evaluated by several methods. Field tests for measuring these parameters included pocket penetrometer, hand-held Torvane, and standard penetration testing. Laboratory tests for measuring these parameters included unconfined compression, multi-stage consolidated undrained (CU) triaxial compression with pore pressure measurements, multi-stage consolidated drained (CD) triaxial compression, and single-stage direct shear (DS).

Results of the field tests, as well as the unconfined compression tests, are presented in both graphical and numerical form in both Appendix A (on the boring logs and the Results of Soil Sample Analyses) and Appendix B (where the stress-strain behavior of each unconfined compression test is presented). Results of the laboratory strength tests are presented in the following tables. Detailed summary sheets are presented both graphically and numerically in Appendix B for each test.

Multi-stage CU tests were used to determine both total and effective soil strength parameters.

During CU testing, each stage was subjected to an effective consolidation pressure of about 1, 2, and 3 times the in-situ confining stress.

The following table presents the results of our multi-stage CU tests.

Multi-Stage CU Test Results Effective Total

Boring No.

Depth* feet

Principle Stress

Difference ksf

Axial Strain

Friction Angle, φ' deg

Cohesion, c' ksf

Friction Angle, φ deg

Cohesion, c ksf

Effective Consolidation

Pressure ksf

LA1-5 4 5.0394 7.0 26.0 0.62 16.0 1.14 2.63

LA1-5 4 7.7362 7.6 26.0 0.62 16.0 1.14 5.48

LA1-5 4 11.7728 3.0 26.0 0.62 16.0 1.14 7.97

LA1-9 6 5.0679 7.0 25.0 0.75 18.0 1.19 1.98

LA1-9 6 7.0076 7.6 25.0 0.75 18.0 1.19 4.04

LA1-9 6 10.8083 11.7 25.0 0.75 18.0 1.19 5.78

LA1-13 8 3.1557 7.0 26.0 0.44 17.0 0.56 1.98

LA1-13 8 4.4100 7.5 26.0 0.44 17.0 0.56 3.95

Effective Total

Boring No.

Depth* feet

Principle Stress

Difference ksf

Axial Strain

Friction Angle, φ' deg

Cohesion, c' ksf

Friction Angle, φ deg

Cohesion, c ksf

Effective Consolidation

Pressure ksf

LA1-13 8 8.6014 8.8 26.0 0.44 17.0 0.56 5.94

LA1-17 4 3.2557 7.0 25.0 0.45 20.0 0.40 2.00

LA1-17 4 5.4996 7.6 25.0 0.45 20.0 0.40 4.11

LA1-17 4 9.7430 11.4 25.0 0.45 20.0 0.40 5.76

LA1-22 6 5.5583 7.0 19.0 1.20 13.6 1.55 2.61

LA1-22 6 7.6352 7.6 19.0 1.20 13.6 1.55 5.31

LA1-22 6 12.0546 2.9 19.0 1.20 13.6 1.55 7.78

LA1-26 8 2.6331 7.0 23.0 0.42 15.9 0.32 2.36

LA1-26 8 4.3950 7.7 23.0 0.42 15.9 0.32 5.03

LA1-26 8 9.5590 11.9 23.0 0.42 15.9 0.32 7.50

LA1-30 10 3.0713 7.0 27.8 0.34 17.1 1.02 2.33

LA1-30 10 6.8110 7.5 27.8 0.34 17.1 1.02 5.35

LA1-30 10 12.7903 11.7 27.8 0.34 17.1 1.02 7.75

LA1-30 30 9.1883 7.0 35.8 0.32 31.0 0.53 3.40

LA1-30 30 15.7663 7.6 35.8 0.32 31.0 0.53 6.42

LA1-30 30 26.0650 5.3 35.8 0.32 31.0 0.53 9.31

LA1-34 4 3.9844 7.0 27.0 0.38 24.5 0.08 2.61

LA1-34 4 7.4250 7.6 27.0 0.38 24.5 0.08 5.50

LA1-34 4 16.5651 11.4 27.0 0.38 24.5 0.08 8.24

LA1-40 15 6.5684 7.0 20.0 1.40 15.0 1.80 2.62

LA1-40 15 8.3077 7.5 20.0 1.40 15.0 1.80 5.33

LA1-40 15 13.6014 9.3 20.0 1.40 15.0 1.80 7.93

LA1-44 2 4.1692 7.0 0.0 1.85 0.0 1.85 1.44

LA1-44 2 3.7101 7.5 0.0 1.85 0.0 1.85 2.72

LA1-44 2 4.5248 10.2 0.0 1.85 0.0 1.85 4.05

LA1-48 20 4.5500 7.0 35.8 0.00 37.8 0.00 2.71

LA1-48 20 13.5173 7.5 35.8 0.00 37.8 0.00 5.27

LA1-48 20 31.6765 5.7 35.8 0.00 37.8 0.00 7.77

LA1-50 10 3.8980 7.0 17.4 0.92 8.0 1.22 2.63

LA1-50 10 4.8022 7.5 17.4 0.92 8.0 1.22 5.27

LA1-50 10 6.7695 10.5 17.4 0.92 8.0 1.22 7.52

LA1-60 2 4.6880 7.0 5.1 1.90 2.1 2.20 2.06

LA1-60 2 4.4793 7.6 5.1 1.90 2.1 2.20 4.05

LA1-60 2 6.5606 8.5 5.1 1.90 2.1 2.20 6.15

LA1-64 4 4.6129 7.0 17.0 0.85 11.0 1.40 2.66

LA1-64 4 5.8494 7.6 17.0 0.85 11.0 1.40 5.51

LA1-70 6 5.8690 7.0 21.0 1.00 16.6 1.40 2.64

LA1-70 6 8.1137 7.5 21.0 1.00 16.6 1.40 5.38

Effective Total

Boring No.

Depth* feet

Principle Stress

Difference ksf

Axial Strain

Friction Angle, φ' deg

Cohesion, c' ksf

Friction Angle, φ deg

Cohesion, c ksf

Effective Consolidation

Pressure ksf

LA1-70 6 13.9442 11.7 21.0 1.00 16.6 1.40 8.06

*Depth below the top of levee surface elevation existing at the time of our field study.

Single-stage CD tests were used to determine total soil strength parameters. During CD testing samples were subjected to an effective consolidation pressure of about 1 to 2 times the in-situ confining stress. The results of the single-stage CD tests presented graphically and numerically in Appendix B, are also summarized in the table below.

Single-Stage CD Test Results Total

Boring No.

Depth* feet

Principle Stress

Difference ksf

Axial Strain

Friction Angle, φ deg

Cohesion, c ksf

Effective Consolidation

Pressure ksf

LA1-4 6 6.19 10.0 37.3 0 2.00

LA1-36 40 11.67 10.0 34.3 0 4.49

LA1-44 6 8.64 10.0 37.5 0 2.77

*Depth below the top-of -levee elevation existing at the time of our field study.

Direct shear tests have been completed on selected samples. These samples were subjected to various normal loads and sheared at a rate of 0.007 inches per minute. The results of the direct shear tests presented graphically and numerically in Appendix B, are also summarized in the table below.

Direct Shear Test Results

Boring No.

Depth* feet

Normal Stress ksf

Max Shear Stress ksf

Cohesion Ksf

Friction Angle, φ deg Visual Classification

LA1-30 25 2.90 2.01 0 34.7 Silt LA1-36 25 2.80 2.18 0 37.9 Clayey Sand LA1-40 10 1.20 0.67 0 29.2 Silt LA1-42 30 3.30 2.20 0 33.6 Silty Sand LA1-54 8 1.20 0.81 0 34.2 Silt

*Depth below the top-of -levee elevation existing at the time of our field study.

CONSOLIDATION TESTS

One-dimensional incremental consolidation tests were performed on selected samples to define the compressibility characteristics of both the existing fill soils and the bearing soils along the levee alignment. Results of the consolidation tests are presented graphically and numerically in Appendix B and are also summarized in the following table.

Consolidation Test Results

Boring No.

Depth* feet

Initial γdry pcf

Initial γtotal pcf

Po ksf

Pc ksf OCR Ccε Crε

Liquid Limit

LA1-2 2 102.7 121.0 0.2 9.0 44 0.109 0.021 54

LA1-4 4 98.6 121.0 0.4 8.0 20 0.132 0.013 45

LA1-8 8 98.7 122.0 0.8 5.0 6 0.140 0.030 48

LA1-17 8 102.4 125.2 0.8 9.0 11 0.110 0.023 61

LA1-22 10 112.6 131.6 1.1 12.0 11 0.089 0.015 39

LA1-26 15 97.0 119.5 1.5 9.0 6 0.118 0.029 71

LA1-34 8 95.8 119.1 0.8 4.5 6 0.130 0.025 71

LA1-40 6 107.9 119.9 0.6 9.0 14 0.098 0.011 33

LA1-46 10 94.7 121.5 0.9 5.0 5 0.134 0.026 60

LA1-50 6 95.3 122.1 0.6 9.0 16 0.155 0.033 66

LA1-58 10 108.5 128.6 1.1 10.0 9 0.118 0.014 41

LA1-62 4 99.1 123.5 0.4 10.0 25 0.113 0.031 65

LA1-66 10 94.0 121.4 0.9 12 13 0.142 0.037 66

LA1-70 10 108.9 131.2 1.1 8 7 0.097 0.012 39

Note: The compression and recompression indices shown are “modified” indices as described by Holtz and Kovacs, 1981. These indices were taken from a strain-log P curve, rather than an e-log P curve.

*Depth below the top-of-levee surface elevation existing at the time of our field study.

HYDRAULIC CONDUCTIVITY (PERMEABILITY) TESTS

Hydraulic conductivity (permeability) tests were performed on relatively undisturbed Shelby tube samples obtained from 30 selected samples of cohesive-type soils. These cohesive soil samples were tested for vertical hydraulic conductivity to assess the seepage velocities of the embankment and cohesive subsurface soils. A calculation summary sheet for each hydraulic conductivity (permeability) test is presented in Appendix B. The results are also summarized in the table below.

Permeability Test Results

No.

Depth feet

Hydraulic Conductivity cm/sec

Boring No.

Depth feet

Hydraulic Conductivity cm/sec

Boring No.

Depth feet

Hydraulic Conductivity cm/sec

LA1-1 4 2.30E-08 LA1-22 8 5.60E-08 LA1-48 10 1.30E-08

LA1-3 15 4.70E-07 LA1-24 6 8.30E-08 LA1-50 8 6.50E-08

LA1-5 8 3.30E-08 LA1-26 10 9.00E-09 LA1-52 6 3.00E-08

LA1-7 10 4.70E-08 LA1-28 4 7.30E-07 LA1-55 8 1.20E-06

LA1-9 8 9.00E-08 LA1-30 15 5.50E-08 LA1-58 8 8.60E-08

LA1-11 6 5.00E-08 LA1-32 6 3.20E-08 LA1-60 8 2.70E-08

LA1-15 4 4.20E-08 LA1-36 6 1.90E-08 LA1-62 10 8.50E-07

LA1-17 6 3.10E-08 LA1-40 8 1.30E-05 LA1-66 4 9.70E-09

LA1-18 10 4.80E-08 LA1-43 2 6.90E-08 LA1-68 15 2.70E-08

LA1-20 10 8.40E-08 LA1-46 4 2.20E-08 LA1-70 8 1.30E-08

Based on the USACE EM 1110-2-1901 Seepage Analysis and Control for Dams indirect methods for determining hydraulic conductivity may also be used. For uniform, loose clean sands classified as SP (poorly graded sand), the Hazen2 equation is suggested for use. The following graph presents the results calculated using the Hazen equation compared to the traditional testing for cohesive soils discussed previously.

0 5 10 15 20 25

Samples Tested

1E-009

1E-008

1E-007

1E-006

1E-005

0.0001

0.001

0.01

0.1

H yd ra ul ic C on du ct iv ity , c m /s ec

K values calculated using Hazen formula k values from laboratory testing on reconstituted cohesive samples

GENERAL SITE CONDITIONS

SITE DESCRIPTION

This study pertains to the existing levee reach between Lateral A to Retamal Dam, with the exception of an approximately 0.9-mile length of existing levee/floodwall combination in the Lateral A to Donna Pump reach, just northwest of Donna Pump. The entire project lies within Hidalgo County, Texas. At the time of our field activities, the project site can generally be described as an earthen levee with grass-covered slopes and a caliche-surfaced roadway spanning the levee crest. During our site reconnaissance, heavily vegetated areas were observed along several portions of the existing levee alignment. These vegetated areas were cleared by USIBWC representatives prior to commencement of our drilling operations. In addition, areas of undeveloped, vegetated land were also observed along both sides of the existing levee alignment. Irrigation ditches were observed along some portions of the alignment. These ditches were observed to be both dry and wet (full of water) at the time of our field activities.

2 Hazen, Allen, “Discussion of ‘Dams on Sand Foundations’ by A.C. Koenig,” Trans. American Society of Civil Engineers, Vol. 73, p. 199, 1911.

SITE GEOLOGY

The levee project is located within the confines of the Gulf of Mexico Basin, which formed from down-faulting Paleozoic basement rocks during the break-up of Pangaea in the Late Triassic Period3,4. Continental highlands provided a continued source of sediments into the basin causing rapid accumulation of thick sediment deposits in a fluvial-deltaic or shallow-marine environment3,4.

Repeated sea-level changes with continued basin subsidence generated cyclic assemblages of river, eolian, and lake sediments. Eighteen major depositional episodes have been mapped in the Gulf of Mexico Basin using lithofacies, thickness, stratigraphic architecture, and facies association3.

A cursory review of the Geologic Atlas of Texas, (McAllen-Brownsville Sheet, dated 1976), published by the Bureau of Economic Geology at the University of Texas at Austin, indicates that the existing levee alignment appears subject site appears to be located within Alluvium (floodplain) deposits consisting of clays, silts, sands, and gravel deposits of the Quaternary epoch (Holocene period).

According to the Soil Survey of Hidalgo County, Texas, published by the United States Department of Agriculture - Soil Conservation Service, in cooperation with the Texas Agricultural Experiment Station, the existing levee alignment appears to be located within Harlingen-Runn Reynosa and Rio Grande-Matamoros soil associations. The Harlingen-Runn Reynosa soil association consists of deep, very slowly, slowly, and moderately permeable soils, that typically have a grayish-brown clay, silty clay, or silty clay loam surface layer. The Rio Grande-Matamoros soil association consists of deep, moderately and slowly permeable soils, that typically have a light brownish-gray or grayish-brown silt loam or silty clay surface layer.

The corresponding soil symbols within the existing levee alignment appear to be 5, Camargo silt loam; 6, Camargo silty clay loam; 7, Cameron silty clay; 15, Grulla clay; 19, Harlingen clay; 34, Matamoros silty clay; 55, Reynosa silty clay loam; 62, Rio Grande silt loam; 63, Rio Grande silty clay loam; 64, Runn silty clay; and 65, Runn silty clay, saline.

SEISMIC COEFFICIENTS

Based upon a review of Section 1613 Earthquake Loads of the 2006 International Building Code (IBC), the following information has been summarized for seismic considerations associated with the planned levee alignment. Note that the project covers a substantial geographic range. As such, the values taken from the IBC charts also vary across the project. The following parameters are presented as a range and represent the western most boring (LA1-1) to the eastern most boring (LA1-70); these values presented are for Boring LA1-1 and Boring LA1-70, respectively.

• Mapped Maximum Considered Earthquake Ground Motion for a 0.2 sec., Spectral Response Acceleration (Figure 1613.5(1)): Ss = 0.048 to 0.049g.

3 Chowdhury, A., Turco, M., Geology of the Gulf Coast Aquifer, Texas; Proceedings of the Gulf Coast Aquifer Conference, Corpus Christi, Texas Water Development Board Report 365 4 Morrison, R.B., 1991, Quaternary nonglacial geology:conterminous U.S.; The Geology of North America; v. K-2. Geological Society of America, Chp. 19, Quaternary geology of the Gulf of Mexico Coastal Plain, & Plate 8.

• Mapped Maximum Considered Earthquake Ground Motion for a 1 sec., Spectral Response Acceleration (Figure 1613.5(2)): S1 = 0.013g.

STRATIGRAPHY

The subsurface conditions observed in the borings performed for this study varied along the entire length of levee. In general, fill materials associated with the existing levee were observed overlying naturally deposited soils common to the area.

Embankment fill soils within the existing levee: Levee fill soils consisting of locally available soils were observed at the levee crest extending to the natural ground elevations. These fill soils were observed at thicknesses ranging from about 7-1/2 to 23 feet. The thickness of these fill soils were observed to range from about 8 to 22 feet.

The majority of the fill materials observed in the borings performed for this study consisted of lean clays (CL) and fat clays (CH). These cohesive soils are considered marginally plastic to very highly plastic, with measured plasticity indices ranging from 7 to 53 percent, indicating a wide range of potential shrink-swell type movements associated with changes in moisture content. The remaining fill materials were primarily comprised of clayey and silty sands (SC and SM), and silts (ML). These non-cohesive soils are considered non-plastic to marginally plastic, with measured plasticity indices ranging from 7 to 10 percent, and one Atterberg limit laboratory test that could not be determined.

Areas where sand and/or silt layers were observed within the existing fill soils are of particular concern. Non-cohesive soils have a much higher hydraulic conductivity than cohesive soils, which will allow water to flow through the earthen levee at higher rates within these non-cohesive layers than through cohesive soil layers. When observed, these non-cohesive layers typically ranged from about 2 to 4 feet in thickness, with the exception of Boring LA1- 44, where the non-cohesive layer was noted to be about 6 feet thick. The borings in which non-cohesive soils were observed in the existing fill soils are tabulated below:

LA1-4 LA1-36

LA1-8 LA1-42

LA1-12 LA1-43

LA1-16 LA1-44

LA1-24

The above table does not include the approximately 6 to 9 inches thick layer of clayey sands with gravel (caliche-based access road) that was observed at the top of the levee alignment in a number of borings. This caliche-based access road was not considered as part of the levee height to achieve the 3-foot of freeboard in our analysis.

Native soils beneath existing levee: Beneath the fill soils, naturally deposited cohesive soils with interbedded layers of non-cohesive soils were observed. The naturally deposited soils vary over the length of the levee project in both depth and thickness. The native soils encountered in the borings conducted for this study were predominately comprised of cohesive soils, consisting of lean clays (CL) and fat clays

(CH). Encountered to a lesser extent were cohesive, sandy fat clays, sandy lean clays, silty sandy clays, and silty clays.

The native non-cohesive soils observed in our borings were predominately silty sands (SM), with some areas of clayey sands (SC); silts (ML), and sands (SP-SM or SP).

A detailed description of the soil conditions observed at each boring location has been presented in the Boring Logs, Figures A-2 through A-75, of Appendix A. The lines designating the interfaces between strata on the boring logs represent approximate boundaries. Transitions between strata may be gradual.

IMPACT OF SAMPLING AND TESTING PROCEDURES ON

MEASURED AND DESIGN SOIL PROPERTIES

PERMEABILITY

Soils were deposited along the Rio Grande River floodplain through a series of natural processes, primarily due to transport by water and wind. The soils beneath the levee embankments are primarily clays with a significant amount of sands and silts, and were generally deposited by the Rio Grande River system, with some relatively thin, fine layers of sand and silt deposited by wind during dust storms. The soil deposits exhibit spatial variability, with resulting anisotropy inherent in the gradation and permeability of the natural deposits. In general, the horizontal permeability of each soil strata is expected to be at least 2 to 5 times greater than their respective vertical permeability.

The potential impacts of natural soil and compacted soil fill layering on permeability are significant. Under ideal conditions, the field exploration and laboratory testing programs would be aimed at harvesting and testing undisturbed soil samples so that the in-place soil permeability could be measured. Unfortunately, the soil types of greatest interest (sands) encountered in this study are very difficult to sample and test in an undisturbed condition. As a result, conventional practice is to obtain disturbed soil samples and perform the various laboratory tests on reconstituted, or remolded, samples, and to make adjustments to the measured values when evaluating their probable behavior in the field. The sampling process destroys the structure and orientation of layers of differing characteristics, and distributes the fine-grained silt and clay fraction throughout the bulk sample. The process of harvesting large volumes of soil and remolding samples has the effect of homogenizing the samples, generally resulting in lower tested permeability. Laboratory measured permeability can be several orders of magnitude lower than the actual maximum field permeability.

The laboratory permeability tests were conducted using ordinary tap water as the permeant in accordance with industry practice. This choice can affect the tested permeability where the soils contain significant amounts of soluble salts, or the tap water contains unusually high amounts of total dissolved solids (TDS). We do not believe that the tested samples contained significant amounts of TDS, and the tap water used in the tests meets current EPA drinking water standards. Given the soil types tested for this study, we do not believe that changes in the tested samples’ TDS, if any, had a significant impact on the measured results.

The laboratory permeability tests were performed using hydraulic gradients as high as 11. This test condition is much higher than the typical expected field gradient of less than 0.5. Excessive laboratory hydraulic gradients can cause migration of silt and clay particles within the sample, significantly reducing the measured permeability. However, we do not believe that particle migration significantly affected the test results. Particle migration often occurs when the gradient is applied instantaneously. The test gradient was gradually applied over several minutes in an effort to avoid this problem.

INFERRED PERMEABILITY

The laboratory grain size analyses were used in conjunction with the Hazen formula to provide an estimate of permeability. The Hazen2 formula was developed using filter sands with effective sizes between 0.1 and 3 mm and uniformity coefficients of less than 5. For this limited range of clean sands, permeability estimates computed using the formula generally was within 20 percent of the measured values.

The soils tested for this study generally do not meet the effective size or fines fraction requirements of the soils Allen Hazen used to develop his correlation. However, in many cases, the effective sizes of the tested sands are close enough to the sands tested by Hazen that the equation can be used to provide some indication of the variability of permeability of the soils. In general, we would expect that the laboratory permeability will be somewhat lower than the value calculated using Hazen’s correlation. This approach is one of the two methods preferred by USACE; the other preferred method involves performing field pumping tests to evaluate site permeability.5

DEPTH-TO-WATER

Groundwater was observed in all of borings conducted for this project as tabulated on the following table.

No.

First Observed Depth of

Groundwater, feet*

Groundwater Level 15 Minutes After First Observed Depth of Groundwater, feet*

Boring No.

First Observed Depth of

Groundwater, feet*

Groundwater Level 15 Minutes After First Observed Depth of Groundwater, feet*

LA1-1 20.0 11.5 LA1-41 23.0 21.0

LA1-2 20.0 13.5 LA1-42 29.0 17.0

LA1-3 20.0 13.0 LA1-43 23.0 20.0

LA1-4 18.0 16.0 LA1-44 25.0 17.0

LA1-5 17.0 14.5 LA1-45 20.0 17.5

LA1-6 19.0 17.5 LA1-46 20.0 17.0

LA1-7 17.0 14.0 LA1-47 10.0 8.0

LA1-8 17.0 14.5 LA1-48 20.0 20.0

LA1-9 19.0 18.0 LA1-49 21.0 19.0

LA1-10 10.0 7.5 LA1-50 25.0 25.0

LA1-11 19.0 17.0 LA1-51 23.0 20.0

LA1-12 25.0 ** LA1-52 30.0 22.0

LA1-13 19.0 17.0 LA1-53 33.0 30.0

5 EM 1110-2-1913 "Design and Construction of Levees", US Army Corps of Engineers, 30 April 2000, para. 3-9, p. 3-8.

No.

First Observed Depth of

Groundwater, feet*

Groundwater Level 15 Minutes After First Observed Depth of Groundwater, feet*

Boring No.

First Observed Depth of

Groundwater, feet*

Groundwater Level 15 Minutes After First Observed Depth of Groundwater, feet*

LA1-14 19.0 17.0 LA1-54 15.0 11.0

LA1-15 19.0 17.0 LA1-55 29.0 26.0

LA1-16 20.0 14.0 LA1-56 35.0 35.0

LA1-17 19.0 16.0 LA1-57 29.0 24.5

LA1-18 25.0 17.0 LA1-58 30.0 30.0

LA1-19 25.0 20.0 LA1-59 22.0 20.0

LA1-20 25.0 19.0 LA1-60 21.0 21.0

LA1-21 29.0 20.0 LA1-61 25.0 23.0

LA1-22 25.0 20.0 LA1-62 25.0 20.0

LA1-23 25.0 21.0 LA1-63 23.0 20.5

LA1-24 30.0 26.0 LA1-64 25.0 20.0

LA1-25 30.0 21.0 LA1-65 23.0 19.0

LA1-26 25.0 23.0 LA1-66 25.0 25.0

LA1-27 25.0 20.0 LA1-67 24.0 21.0

LA1-28 40.0 35.0 LA1-68 28.0 **

LA1-29 25.0 18.0 LA1-69 24.0 20.5

LA1-30 23.0 16.5 LA1-70 27.0 20.0

LA1-31 26.0 24.0 LA1-71 20.0 22.0

LA1-32 20.0 16.5 LA1-72 31.5 31.0

LA1-33 25.0 23.0 LA1-73 30.0 29.0

LA1-34 27.0 18.0 LA1-74 29.0 25.0

LA1-35 19.0 17.0

LA1-36 25.0 18.5

LA1-37 20.0 18.0

LA1-38 30.0 29.0

LA1-39 21.0 19.0

LA1-40 25.0 21.0

*Depth below the top of levee surface elevation existing at the time of our field study.

** Groundwater could not be measured due to caving of borehole.

The depth-to-water levels recorded during drilling are considered short-term water level readings.

Due to the presence of non-cohesive soils, bentonite drilling mud was often introduced after observing free water to keep the saturated sands from “blowing up” the bottom (or unraveling) of the open borehole. Obtaining long-term depth-to-water level readings was not part of our scope of work. At the completion of each day’s drilling, each boring was backfilled with cement-bentonite grout from the bottom-up using a tremie pipe.

As presented on the precedent table, the depth-to-water level readings ranged from 10 to 35 feet below the top of the existing levee. A waiting a period of about 15 minutes was used to observe any immediate changes in the depth-to-water levels. In general, the depth-to-water levels rose from about 1 to 9 feet from their initial readings.

It should be noted that all depth-to-water level readings were observed in the subsurface soils beneath the levee structure. Free water was only observed within the levee embankment fill soils in Borings LA1-1, LA1-4, and LA1-22. It is possible for groundwater to exist beneath this site at shallower depths on a transient basis following periods of precipitation and/or flooding.

Fluctuations in groundwater levels occur due to variations in rainfall and surface water run-off.

The construction process itself may also cause variations in the groundwater level.

EARTHEN LEVEE SYSTEM

DESIGN CONSIDERATIONS

The existing and modified levees should…