05_Geotech Report DHS Segment O-4B 20091109.pdf

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Executive Summary

The DHS Segment O-4B project includes a re-alignment of the existing levee to cross the Peñitas Pump House Intake Channel just south of the historic Peñitas Pump House and an increase in top of levee elevation to meet the minimum design heights established by the USIBWC with an additional three feet of freeboard. The existing subsurface conditions of DHS Segment O-4B were evaluated by four (4) one hundred foot (100 ft) exploratory borings (B1 to B4). Laboratory tests were performed on selected specimens obtained from the exploratory borings to evaluate the engineering characteristics of the various soil strata encountered during the field exploration activities. The general soil profile encountered in the exploratory borings consisted of a top stratum comprised of light brown to dark brown silty clay with small amounts of granular sand and calcium formations (CL to CH) followed by a thick stratum of silt and sand with traces to fair amounts of gravel. Below these strata were alternating layers of clayey gravel, gravely clay, and gravely sand. It should be noted that very few samples from the B-2 boring contained any gravel, however, at depths grater than fifty feet, there were samples containing very stiff to hard cemented clays. Through due diligence and for verification through our internal QA/QC program, a fifth hole (B-5) was drilled and tested to a depth of 50 feet to provide additional verification of soil strata configuration and strengths. In addition we re-sampled borings B1, B2, and B3 (B-1A, B-2A, and B-3A) in the top ten feet to recover undisturbed Shelby tube samples for shear strength verification and consolidation testing.

We performed a detailed engineering analysis to assess the condition of the underlying soils within Segment O-4B. Our focus was on quantifying the engineering properties of the soils and their ability to withstand the additional loads imposed by the proposed improvements (ie, concrete flood protection structure, earthen levee, access ramps). The scope of this analysis is explained in detail in the body of this report. A specialty consultant with experience in saturated and collapsible soils was hired to analyze the section crossing the Peñitas Pump House Intake Channel. Their analysis and results are included in this report as Appendix G.

The ultimate bearing capacities of the underlying soils were established and provided in this report to assist in the design of the flood protection structure. For typical sections A-A and B-B, it is the responsibility of the wall designer to adhere to the minimum factor of safety of 3.0 for bearing capacity set forth by the USACE. Typical sections C-C, E-E, and F-F, met the minimum required factor of safety (2.0) set forth by the USACE. Typical section D-D was analyzed for bearing capacity by a specialty consultant (PSI) and the results of their analysis can be found in appendix G.

The calculated minimum global stability factors of safety are higher than the minimum factors of safety required by the USACE for the stability analysis of concrete structures and earthen levees (Typical sections A-A, B-B, C-C, E-E, and F-F). Typical section D-D was analyzed for global stability by a specialty consultant (PSI) and the results of their analysis can be found in appendix G. Consolidation tests performed on samples of the underlying native soils were used to estimate a total anticipated settlement of 0.6 inches for Typical Section A-A, 2.75 inches for Typical Section B-B, 3.0 inches for Typical Section C-C, 6.0 inches for Typical Section E-E, and

3.75 inches for Typical Section F-F. Typical section D-D was analyzed for settlement by a specialty consultant (PSI) and the results of their analysis can be found in appendix G.

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A detailed through and underseepage analysis that adheres to both the USIBWC and USACE requirements was also performed for this project. The seepage analysis concluded that no seepage remediation techniques were needed for typical sections A-A, B-B, C-C, E-E, and F-F.

Calculations for this analysis can be found in Appendix F of this report. Typical section D-D was analyzed for seepage by a specialty consultant (PSI) and the results of their analysis can be found in appendix G.

This summary is only intended to provide a brief overview of the findings and recommendations.

The following report discusses the results of this study in greater detail.

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INTRODUCTION............................................................................................... - 1 - GENERAL PROJECT OVERVIEW ............................................................... - 1 -

Project Description ...............................................................................................................- 1 - Proposed Improvements.......................................................................................................- 1 - Scope and Limitations of Investigation...............................................................................- 2 - Previous Geotechnical Investigations..................................................................................- 3 -

EXISTING LEVEE SURFACE & SUB-SURFACE CONDITIONS............ - 3 -

Site Location / Description ...................................................................................................- 3 - Subsurface Stratigraphy ......................................................................................................- 4 - Rainfall...................................................................................................................................- 4 - Water Strike ..........................................................................................................................- 4 -

LEVEE GEOTECHNICAL INVESTIGATION............................................. - 5 - Soil Borings and Laboratory Tests......................................................................................- 5 -

LABORATORY INDEX TESTING ................................................................. - 6 - Moisture Content ..................................................................................................................- 6 - Plasticity Index ......................................................................................................................- 6 - Minus 200 / Sieve (Hydrometer) Analysis ..........................................................................- 6 - Summary of Index Testing...................................................................................................- 7 -

LABORATORY STRENGTH & COMPRESSIBILITY TESTING ............ - 7 -

Unconfined Compression and UU Triaxial Test ................................................................- 7 - Consolidation Test ................................................................................................................- 8 -

GEOTECHNICAL ANALYSIS ........................................................................ - 8 -

TYPICAL SECTION A-A & B-B (CONCRETE FLOOD PROTECTION WALL)...- 11 -

Overturning Stability Analysis.............................................................................................- 11 - Sliding Failure Analysis ......................................................................................................- 12 - Bearing Capacity Analysis...................................................................................................- 12 - Settlement Analysis ..............................................................................................................- 13 - Overall (Rotational) Stability Analysis................................................................................- 14 - Seepage Analysis ..................................................................................................................- 16 - TYPICAL SECTION C-C ANALYSIS ............................................................................- 17 - Slope Stability Analysis ........................................................................................................- 17 - Bearing Capacity Analysis...................................................................................................- 20 - Settlement Analysis ..............................................................................................................- 21 - Seepage Analysis ..................................................................................................................- 22 - TYPICAL SECTION D-D ANALYSIS ............................................................................- 22 - TYPICAL SECTION E-E & F-F ANALYSIS .................................................................- 22 - Slope Stability Analysis ........................................................................................................- 22 - Bearing Capacity Analysis...................................................................................................- 26 - Settlement Analysis ..............................................................................................................- 27 - Seepage Analysis ..................................................................................................................- 28 -

GENERAL CONSIDERATIONS DURING CONSTRUCTION ................ - 28 -

Site Drainage Recommendations.......................................................................................- 28 - Site Preparation Recommendations..................................................................................- 28 - Embankment Fill Placement Recommendations .............................................................- 29 -

REFERENCES.................................................................................................. - 31 -vi

APPENDIX A – LOCATION MAP, TYP SECTIONS & BORING LOGS.....A

APPENDIX B – SOIL COMPRESSION TESTS ................................................B APPENDIX C – CONSOLIDATION TESTS......................................................C

APPENDIX D – GRADATIONS/HYDROMETERS..........................................D

APPENDIX E – STABILITY ANALYSIS ...........................................................E APPENDIX F – SEEPAGE ANALYSIS .............................................................. F

APPENDIX G – PSI ANALYSIS OF TYPICAL SECTION D-D .................... G

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INTRODUCTION

L&G Engineering Laboratory, L.L.C. was contracted by the Hidalgo County Drainage District #1 to perform a subsurface geotechnical investigation and engineering analysis on a portion of the Rio Grande Flood Control System. This report covers DHS Segment O-4B of the USIBWC Mission Levee Improvements project that extends from west of the Peñitas Pump House, across the Peñitas Pumphouse Intake Channel, and ends east of the Peñitas Pump House (refer to Appendix A for a detailed location map). Contained in this report are a general project overview which includes the techniques and procedures used to perform the subsurface exploration and subsequent testing, and an engineering analysis consisting of a settlement estimate, through/underseepage, slope stability, bearing capacity, and soil strength parameters derived from extensive laboratory testing. This analysis will evaluate the adequacy of existing subgrade material, identify possible seepage issues, and evaluate the proposed levee configurations for compliance with USACE and FEMA requirements for levee certification

GENERAL PROJECT OVERVIEW

Project Description

The proposed “DHS Segment O-4B Project” has commenced to provide adequate safety and protection against a 100 year flood event and the loadings that an event of that magnitude would impose on the existing river protective levee system of the Lower Rio Grande Flood Control Project. The project includes a re-alignment of the existing levee to cross the Peñitas Pumphouse Intake Channel just south of the historic Penitas Pump House and an increase in top of levee elevation to meet the minimum design heights established by the USIBWC with an additional three feet of freeboard.

Proposed Improvements

At the time of our study, the majority of the DHS Segment O-4A and O-4C projects had been constructed with a vertical concrete face on the riverside of the levee. (See Appendix A – Location Map for O-4A and O-4C alignments). As stated previously, the Segment O-4B project will re-align the existing levee to cross the Peñitas Pumphouse Intake Channel south of the historic Penitas Pump House, instead of through the pump house, as is the existing configuration. The project will begin at station 50+00 with a vertical concrete flood protection structure (cantilever type retaining wall) that ties into the Segment O-4C project and runs south up to station 55+56.87. This retaining wall will be bound by a drainage ditch on the west side and the Peñitas Pumphouse Intake Channel on the east side (See typical sections in Appendix A for details). The project with then turn east, and cross the Intake Channel, incorporating an embankment levee with rip rap protected slopes up to station 60+73+32. This section will include a series of six (6) seven by seven foot box culverts with gate wells to control water flow though the channel (See typical sections in Appendix A for details). Once across the channel, the typical section will then transition back to the retaining wall type structure and tie in to the constructed Segment O-4A flood protection structure. In addition to the re-alignment, the top of levee elevation will be built to the appropriate heights designed by the USIBWC, and bollards will be installed along the top of the levee. All cross-sections and stationing used throughout this report were provided by Dannenbaum Engineering Corporation.

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Scope and Limitations of Investigation

This report has been prepared in general accordance with requirements from the USACE, USIBWC, and FEMA. No specific warranty program or other standards, except acceptable industry standards, were followed during the course of this investigation. This report is intended for use by the Hidalgo County Drainage District No. 1 and their representatives. This report may not contain sufficient information for purposes of other parties or agencies for uses in determining construction means and methods.

The stratigraphies shown on the boring logs (included in Appendix A of this report) represent the conditions at the boring locations. These stratifications represent approximate boundaries between subsurface materials; their actual transition may be gradual. The purpose of this study is to identify and evaluate the existing soil composition and strength parameters of the underlying strata, and analyze the proposed earthen levee and concrete flood protection cross-sections for stability, settlement, seepage, and their ability to resist applied loads as a composite section. It should be noted that the borings were drilled within the limits of the proposed levee improvements as approved by Hidalgo County Drainage District No. 1. A Location Map of these borings can be found in Appendix A of this report. Valley Data Inc. was sub-contracted to survey and tie down the exact location of each boring. The location of each boring, along with the top of hole elevation and station/offset with respect to the centerline provided to us, is located on every boring in Appendix A.

The benchmarks for this study are to:

1. explore the existing subsurface conditions at the site and create a generalized soil profile that will be used for engineering analysis

2. evaluate the relevant engineering properties of the samples collected and create boring logs for representation of in-situ soils

3. calculate strength parameters for the existing levee soil and underlying strata to be used in structural evaluation

4. provide ultimate bearing capacity and minimum factors of safety for overall stability

5. provide a detailed seepage analysis

6. highlight potential constructability issues and state general considerations for the construction process

7. provide a settlement analysis, with maximum anticipated settlement for each section.

The scope of this geotechnical engineering study does not include an environmental assessment of the air, soil, rock, or water conditions on or adjacent to the existing levee system. No environmental options are presented in this report. No study was performed on any hazardous materials, or the effect that construction may have on the existing ecosystems at or nearby the proposed construction. If environmental assessments are needed to begin construction, please contact our office for further assistance.

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Previous Geotechnical Investigations

L&G has performed two geotechnical investigations in the vicinity of the proposed project. The first was DHS Segment O-4 Special (L&G Report number GL08006-04Sp, dated 6/6/08) and the second was DHS Segment O-4 (L&G Report number GL08006-04, dated 6/30/08). Boring logs, soil compression test results, and consolidation tests results from these previous studies were used to compliment and analyze the data collected though the current investigation. Borings BH-590+11 and BH-588+90 were performed during the Segment O-4 Special Project, and borings BH-7 and BH-8 were performed during the Segment O-4 project. These boring logs and test results can be found in the respective appendices of this report.

EXISTING LEVEE SURFACE & SUB-SURFACE CONDITIONS

Site Location / Description

Segment O-4B of the flood protection and improvements project is located in Peñitas, Texas and runs adjacent to the Rio Grande River. Figure 1 shows the general location of the geotechnical investigation. Again, proposed construction for this segment consists of re-aligning the existing levee and raising the of top levee elevation to the appropriate heights designed by the USIBWC, as well as providing a flood protection structure through a portion of the project.

Figure 1: General Location Map

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Subsurface Stratigraphy

On the basis of our soil borings, there are several generalized strata that possess similar physical and engineering characteristics at the boring locations. The general soil profile encountered in the exploratory borings consisted of a top stratum comprised of light brown to dark brown silty clay with small amounts of granular sand and calcium formations (CL to CH) followed by a thick stratum of silt and sand with traces to fair amounts of gravel. Below these strata were alternating layers of clayey gravel, gravely clay, and gravely sand. It should be noted that very few samples from the B-2 boring contained any gravel, however, at depths grater than fifty feet, there were samples containing very stiff to hard cemented clays. The lines designating the interfaces between strata on the boring logs represent approximate boundaries. Transitions between strata may be gradual. Refer to Appendix A for a graphical representation of the subsurface profile.

Rainfall

The annual average precipitation for Hidalgo County is approximately 23.0 inches. Of this rainfall, 14 inches, or approximately 60 percent of annual rainfall occurs during the months of April thru September. According to the United States Department of Agriculture, Soil Survey of Hidalgo County, our geotechnical field investigation performed during the dates 9/9/09 to 9/16/09 was conducted during a relatively wet climatic period with heavy rains prior to, during, and after our exploration. According to the National Weather Service, approximately 2.0 inches of precipitation was observed during this period. This amount of precipitation did affect our encountered water strike elevations and 24-hour water level readings. It also influenced the moisture in the top five feet of our exploration.

Water Strike

Both dry auger and mud rotary drilling techniques were used to complete the soil borings.

During the drilling operations, a waterstrike was encountered in all of the borings. It should be noted that fluctuations in water levels are influenced by variations in rainfall and surface water run-off from season to season. The construction process itself may also cause variations in the water level. If the subsurface water elevation is critical to the construction process the contractor should check the subsurface water conditions just prior to construction excavation activities or contract L&G Engineering Laboratory to install and monitor piezometer wells for water level evaluation. Subsurface water and cave-in depths are based on measurements referenced from the top of boring elevation at the time of exploration. Table 1 below shows the waterstrike depths at each boring in tabular form.

Boring No.

Water Strike Depth from Top of Hole

(at time of drilling) Water Strike Depth from Top of Hole (after 24 hours)

B-1 18.0’ no reading taken

(hole was backfilled by others)

B-2 18.0’ 3.75’

B-3 15.0’ 3.50’

B-4 28.0’ 37.75’

B-5 15.0’ n/a

Table 1: Water Strike Elevation Summary

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LEVEE GEOTECHNICAL INVESTIGATION

Soil Borings and Laboratory Tests

Subsurface conditions at the site were evaluated by four (4) exploratory borings. The borings were advanced to 100 feet below the existing natural ground. Through due diligence and for verification through our internal QA/QC program, a fifth hole (B-5) was drilled and tested to a depth of 50 feet to provide additional verification of soil strata configuration and strengths. In addition we re-sampled borings B1, B2, and B3 (B-1A, B-2A, and B-3A) in the top ten feet to recover undisturbed Shelby tube samples for shear strength verification and compressibility testing of cohesive samples. The borings were drilled at the locations shown on the Boring Location Maps located in Appendix A at the end of this report. Valley Data Inc. was sub-contracted to survey and tie down the exact locations and top of hole elevations of the completed borings. These borings were drilled and logged by representatives of L&G Engineering Laboratory and all laboratory testing was performed by L&G Engineering Laboratory as well. The soil borings were drilled in general accordance with American Society of Testing Materials Procedure D 420 using a truck mounted (Model 52800) drilling rig with dry auger and wet rotary techniques.

As part of the drilling procedures, Standard Penetration Tests (SPT) were performed at five foot intervals for use in the determination of soil strength parameters and engineering properties.

SPT test were executed in compliance with American Society of Testing Materials procedures (ASTM D1586) and results for SPT are reported as blows per foot on the boring logs. A 140 pound hammer was used to drive the split barrel sampler through three six inch increments. The first six inch increment is referred to as the seating drive. The number of blows required to drive the sampler through the second and third increments is recorded as the SPT result. Where dense or hard material was encountered the SPT was terminated after one of the following occurrences:

∗ a total of 50 blows was applied during any one of the 6 inch increments ∗ a total of 100 blows was applied ∗ no advancement of the sampler was observed during the execution of 10 consecutive blows

95 split barrel samples and 24 thin walled Shelby tube samples were collected during the drilling operations in general compliance with American Society of Testing Materials procedures (ASTM D3550). The Shelby tube samples were used for laboratory testing of both soil consolidation and unconfined compressive strength testing. Shelby tube samples were collected when cohesive clays were encountered, or when samples could successfully be extracted for testing.

In the laboratory, each sample was evaluated and visually classified by a member of our geotechnical engineering staff. The moisture content, Atterberg limits, and particle size analysis were evaluated, and a summary of the results and their corresponding depths are presented on the boring logs in Appendix A. Samples will be retained in our laboratory for 30 days after submittal of this report. Other arrangements may be provided at the request of the Drainage District.

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LABORATORY INDEX TESTING

Moisture Content

The moisture content of a soil is defined as the ratio of the weight of the water in the sample to the dry weight of the soil sample. The moisture contents for the samples obtained as part of our geotechnical exploration varied from 8.0 percent to 51.0 percent. The variance in these percentages can be attributed to range in depth and distance between samples. Typically the deeper a sample is located the higher the associated moisture content is. Another major factor could be the location of the sample relative to the encountered water table at that particular location. Lastly, the variation could be caused by the difference in soil classification, as some soils (fat high plasticity clays) may inherently absorb and retain more water than others (sands and gravels). A list of all the moisture contents can be found on the boring logs in Appendix A.

Plasticity Index

The plasticity index is known as the difference between the liquid limit and the plastic limit of a soil. These limits are commonly referred to as the Atterberg limits, which describe the consistency of soils with respect to their varying moisture contents. The liquid limit is defined as the moisture content at which soil begins to transition from a plastic to a liquid state and begins to behave as a liquid material. The plastic limit refers to the water content of a soil at the point of transition from a semisolid to a plastic state where soil starts to exhibit plastic behavior. A soils behavior can be divided into four basic states: liquid, plastic, semisolid and solid. The plasticity index shows the range in which a soil acts in a plastic state. Experience has shown that the more plastic a soil is the more expansive and compressive it will act. PI values for the borings performed range from 0 (non-plastic poorly graded sands and gravels classified as SP- SC, SC, GM and GC) to 44 (fat high plasticity clays classified as CH).

Minus 200 / Sieve (Hydrometer) Analysis

The standard grain size analysis is used to determine the relative proportions of different grain sizes as they are distributed along a range of different sized sieves. The minus 200 sieve analysis is used commonly as a tool for soil classification and identification using the Unified Soils Classification System. Results for this test are reported as a percentage of soil passing the particular opening or sieve number. This test is also used to determine the suitability of soil for construction purposes. Generally a % -200 greater than 50% indicates a non-granular cohesive soil with large amounts of fines in the soil composition. The % -200 values for the samples collected range from 1.0 % (poorly graded sands and gravels classified as SP-SC and GP-GC, and GM) to 100.0 % (fat high plasticity clays classified as CH). Materials with extremely high or low percentages may present constructability problems due to runoff and difficulty maintaining proper moisture content.

Standard gradation analysis involves two parts, the sieve analysis and the hydrometer analysis.

The sieve analysis consists of stacking progressively finer sieves and passing a soil mass through. The sieve sizes correspond to different particle sizes within a soil and allow for the construction of a gradation curve. Hydrometer analyses are used primarily in fine grained soils

- 7 -but are also very useful in establishing the ‘tail-end’ of a gradation curve for soils having a mixture of coarse grained and fine grained soil constituents. Gradation curves based on sieve analyses and hydrometers analyses can be found in Appendix D.

Summary of Index Testing

A summary of the index testing performed on the collected samples with respect to depth below natural ground is located on the boring logs in Appendix A. Table 2 is a tabular representation of the total number of index tests performed for Segment O-4B.

Test Name & ASTM Designation Number of Tests

Performed

Moisture Content (ASTM D 2216-98) 115

Plasticity Index (ASTM D 4318-00) 66

% Finer than No 200 (ASTM D 1140-00) 106

Sieve Analysis (ASTM D 422 – 63) 23

Hydrometer (ASTM D 422 – 63) 20

Table 2: Index Testing Summary

LABORATORY STRENGTH & COMPRESSIBILITY TESTING

Unconfined Compression and UU Triaxial Test

The unconfined compression test is a special type of unconsolidated undrained triaxial test that is commonly used for clay samples. In this test procedure, drainage of pore water from the soil sample is not permitted and the test sample is sheared to failure by the application of deviator (axial) stress. The axial load is applied rapidly to the sample and the resulting shear failure force is referred to as the unconfined compressive strength of the sample. This result is then divided by two to derive the undrained shear strength of saturated clays. The unconsolidated undrained triaxial test is similare to the unconfined compression, except that a confining pressure us used to simulate the in-situ overburden and lateral pressures of the existing soil. Theoretically, the unconfined compression test and the unconsolidated undrained triaxial test should result in similar undrained shear strength for saturated clays, however, it has been proven that the unconfined compression test provides a more conservative result than the triaxial test.

Several unconfined compression tests and an unconsolidated undriained triaxial test were performed on the recovered samples. The Shelby tube samples were extracted and cut to a length / diameter ratio of approximately two. Refer to Appendix B to see the sample depth and location, along with the corresponding unconfined compressive and shear strength recorded from that test. The undrained cohesion values were used in the soil stratification and the calculation of the ultimate bearing capacity and overall slope stability.

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Consolidation Test

The one dimensional consolidation tests were performed to determine the magnitude of volume decrease that a laterally confined soil would undergo when subjected to different and increasing vertical effective pressures. The main purpose of this test is to obtain data that will be used to predict the amount of approximate settlement of the proposed concrete structure and earthen embankments. The test specimens were placed inside a metal ring with porous stones on both top and bottom. The loadings were then applied and settlement was recorded. From the measured data, the consolidation curve (pressure-void ratio relationship) was plotted in log format. Consolidation input values and strain readings can be found in tabular form in Appendix C (Consolidation Tests) of this report with the corresponding log plots.

GEOTECHNICAL ANALYSIS

The geotechnical analysis for the Segment O-4B project was divided into 6 general typical sections (as shown in the plan set provided by Dannenbaum). The limits of Typical Section A-A are from station 50+00 to 51+20. This section includes a vertical concrete flood protection structure (cantilever type retaining wall) that is bound by a drainage ditch on the southwest side (riverside) and an earthen slope (3:1) on the northeast side (landside). This section was analyzed using soil profile A (see Appendix E for Soil Profile Summary). The limits of Typical Section B-B are from station 51+20 to 55+60. This section includes a vertical concrete flood protection structure (cantilever type retaining wall) that is bound by a drainage ditch on the west side (riverside) and the Peñitas Pumphouse Intake Channel on the east side (land/channel side). This section was analyzed using soil profile B (see Appendix E for Soil Profile Summary). Typical Section C-C includes the removal of a portion of the DHS Segment O-4A concrete floodwall and construction of a maintenance bench with 3:1 earthen slopes on both sides. This section was analyzed using soil profile C-C. Typical Section D-D is composed of an earthen levee crossing the Peñitas Pumphouse Intake Channel from station 55+56.87 to 60+73.32. This section has rip rap protected slopes with a series of six (6) seven by seven foot box culverts with gate wells to control water flow though the canal. This section was analyzed by PSI. Typical Sections E-E and F-F both are comprised of earthen levee extensions that incorporate a rip rap protected slope (2:1) on the canal side and 3:1 earth slopes on the opposite side with access ramps. All six sections are shown on the following pages. Soil profiles E&F were used to analyze these sections respectively.

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Figure 2 - Typical Section A-A

Figure 3 - Typical Section B-B

Figure 4 - Typical Section C-C

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Figure 5 - Typical Section D-D

Figure 6 - Typical Section E-E

Figure 7 - Typical Section #F-F

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TYPICAL SECTION A-A & B-B (CONCRETE FLOOD PROTECTION WALL)

Cantilever spread footing walls are generally composed of a footing, key, and a wall stem which are typically made of reinforced cast in place concrete. The footing and key are poured first, allowed to dry, and then the form work for the wall stem is tied together. The footing and wall stem are relatively thin and heavily reinforced to resist the bending forces present. The weight of the soil above the heel of the footing helps keep the wall stable. To properly analyze such walls some basic parameters of the soil retained behind the wall, as well as the underlying soils below the base of the footing, need to be established. These parameters include unit weight, angle of friction, and cohesion. These soil parameters are in turn used to establish both the lateral earth pressure coefficients, and the allowable resisting vertical plane earth pressure distribution. As previously stated, all exploratory borings and lab testing of the recovered materials were preformed by L&G Engineering Laboratory L.L.C. and the necessary soil parameters were established from these borings and tests.

The stability of a retaining wall can be broken down into five categories of analysis:

• Overturning Stability Analysis

• Sliding Failure Analysis

• Bearing Capacity Analysis

• Settlement Analysis

• Overall (Rotational) Stability Analysis

It should be noted that Overturning and Sliding Failure analysis will be discussed in this report, but actual thorough analysis of these failure conditions is the responsibility of the wall designer.

Overturning Stability Analysis

The basic definition of overturning in retaining wall stability analysis is the relationship between the moments of forces acting to overturn the wall and the moments of forces acting to resist the overturning about a chosen point. Forces acting to overturn the wall include the horizontal component of the lateral active earth pressure acting on the back of the wall-soil unit and any hydrostatic forces acting behind the wall-soil unit. Forces acting to resist the overturning include the vertical component of the lateral earth pressures acting on the back of the wall-soil unit, the weight of the wall-soil unit and surcharge loads acting above the wall-soil unit. In the case of cantilever walls, overturning is taken about the toe of the wall and the relationship of the moments previously stated yields a factor of safety:

FS(overturning) = Σ MR / Σ MO where: Σ MO = sum of the moments of forces acting to overturn about the toe Σ MR = sum of the moments of forces acting to resist overturning about the toe.

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Sliding Failure Analysis

The basic definition of sliding in retaining wall stability analysis is the relationship between the driving forces acting to slide the wall and the resisting forces acting to resist the sliding. Forces acting to slide the wall include the horizontal component of the lateral active earth pressure acting on the back of the wall-soil unit and any hydrostatic forces acting behind the wall-soil unit. Forces acting to resist the sliding include the lateral passive earth pressures acting on the front of the wall-soil unit, the sliding friction along the bottom of the footing and any hydrostatic forces acting in front of the wall-soil unit. In the case of cantilever walls, sliding is taken distributed along the base of the wall and the relationship of the forces previously stated yields a factor of safety:

FS(overturning) = Σ FR / Σ FD where: Σ FR = sum of the horizontal resisting forces Σ FD = sum of the horizontal driving forces

Bearing Capacity Analysis

The bearing capacity of a foundation is the ability of a soil and footing to support the imposed loadings from the existing embankment and proposed flood protection system. It is also the theoretical maximum pressure that can be resisted before general shear failure of the soil occurs.

The bearing capacity was estimated using Karl Terzaghi’s bearing capacity equation, as recommended by the USACE manual EM 1110-1-1905 titled “Bearing Capacity of Soils”:

where: qu = ultimate bearing capacity c = soil cohesion B’ = effective width of foundation γh = effective unit weight of soil within failure zone σ = effective soil surcharge pressure at depth

Nc, Nγ, and Nq are dimensionless bearing capacity factors developed by Karl Terzaghi for general shear failure. The bottom of footing elevation (115’) was taken from the cross-sections provided by Dannenbaum Engineering along with the requirement of a 2.0 foot minimum cover were used to calculate ultimate bearing capacity of the soil. No foundation shape correction factors were used during our calculations for ultimate bearing capacity (as recommended in the EM 1110-1-1905) because the wall footing was assumed to be a continuous strip footing.

Cohesion values were estimated by using conservative values of undrained shear strength derived from the laboratory unconfined compression tests performed on the recovered samples (ASTM D2166) and standard penetration and pocket penetrometer test results performed in the field. All correlations used were in accordance with the applicable USACE manuals. The results of the unconfined compression tests results and plots can be found in Appendix B of this report.

Typically, retaining wall type structures are required to have a minimum factor of safety of 3.0 as recommended in chapter 1 of EM 1110-1-1905. The ultimate bearing capacity for the soil qu = c Nc + ½ B’ γh Nγ + σ Nq

- 13 -beneath the footing was 6,630 pounds per square foot for Typical Section A-A and 15,146 pounds per square foot for Typical Section B-B. The factor of safety can be calculated by dividing the ultimate bearing capacity by the total applied load. It is the responsibility of the wall designer to adhere to the minimum factor of safety of 3.0 for bearing capacity.

The ultimate bearing capacity is valid to the approximate elevation and embedment depth as previously noted. Bottom of footing elevations shall not be above the approximate elevation of

115.0 (with an allowable range of 2 feet). If this elevation restriction is not withheld, L&G should be advised to re-evaluate our capacity recommendations.

Settlement Analysis

Settlement is caused both by soil compression and lateral movements of adjacent soils. Cohesive soils usually settle from compression while cohesionless soils such as sands and some silts exhibit settlement through lateral failure. Our field exploration showed that the existing soil profile consisted generally of a top stratum comprised of light brown to dark brown silty clay with small amounts of granular sand and calcium formations (CL to CH) followed by a thick stratum of silt and sand with traces to fair amounts of gravel. Below these strata were alternating layers of clayey gravel, gravely clay, and gravely sand. For a more detailed representation of the in-situ soil classifications, refer to the soil profile shown in Appendix A of this report.

The total settlement analysis for underlying soils below the footing foundation was estimated from the results of a series of one dimensional consolidation tests (ASTM D2435) performed on the samples retrieved from the field exploration. Individual test reports are provided in Appendix C. The total anticipated settlement for the existing levee and underlying soils was estimated using the following equation for normally consolidated clays from EM1110-1-1904 “Settlement Analysis”:

where: ρc = ultimate one-dimensional consolidation settlement Cc = compression index (slope of virgin consolidation curve) eo = initial void ratio σ'f = final effective vertical pressure in stratum σ'o = initial effective vertical pressure in stratum H = thickness of stratum

The calculations resulted in a total settlement of approximately 0.60 inches for Typical Section A-A and 2.75 inches for Typical Section B-B throughout this section of improvements.

Additional settlement will also occur in the newly placed fill behind the flood protection structure. This settlement is on the order of approximately 1.0% of the total height of newly placed embankment. This estimate was made assuming that the new embankment fill will be density controlled and placed in compliance to the “Embankment Fill Placement” section of this report.

ρc = * log * H Cc___

1 + eo σ'f___ σ'o

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It should be noted that other factors may affect settlement of the existing levee embankment and underlying soils. The existing levee embankment is in the general vicinity where natural gas and ground water wells are located. The extraction of these natural resources has the potential to reduce the volume of the in-situ soil, causing some settlement to occur. Construction techniques, atmospheric and climatic changes, as well as seasonal moisture variance can also cause settlements exceeding our estimated maximum recommendations.

Overall (Rotational) Stability Analysis

For purposes of analyzing the stability of the concrete flood protection structure in its final configuration, the method as outlined in USACE EM 1110-2-1902, “Slope Stability”, USACE EM 1110-2-2100 “Stability Analysis of Concrete Structures”, and USACE EM 1110-2-1913, “Design and Construction of Levees”, was used. The limit equilibrium method of analysis is the most commonly used method of analyzing the stability of both natural and manmade slopes as well as retaining wall structures. The fundamental principals behind this method are that the soil mass above a potential failure surface acts as a rigid body, and the shear strength of the material is fully engaged at all points along the surface at the moment of initial movement. A failure criterion is adopted and the conditions for static equilibrium are applied to analyze the problem.

This method of analysis assumes that no strain takes place until the failure condition is reached.

The results of the analyses are expressed in terms of a safety factor in the form of a ratio of the available shear strength along the potential failure surface to the shear stress required to maintain equilibrium of the failure mass under the applied loads. This method has traditionally been used in the analysis of man-made earth structures such as levees and retaining wall structures for analysis of global stability.

The 3 cases that require analysis as per the USACE EM 1110-2-1902, “Slope Stability”, and USACE EM 1110-2-1913, “Design and Construction of Levees” are the Case I – End of Construction, Case II – Long-Term Steady State Seepage and the Case III – Rapid Drawdown.

The global rotational stability analysis was adjusted and modified for this reach and the latter two cases were removed based on the fact that the concrete flood protection system would eliminate the chance of water saturating the levee material. Therefore the only case analyzed in this section is the following:

Case I (MOD): End of Construction – This case generally reflects the condition of the levee directly following completion of the proposed construction. It is important to note that this case was modified to analyze both the short term and long term conditions of the levee. The riverside of the raised levee is not subject to flooding under this condition, although to remain conservative for the long term case a water table was placed at the riverside surface and directly below the flood protection structure.

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Figure 8 - End of Construction

The stress-strain properties of the materials in both the existing fill and foundation govern the potential location of failure surfaces. The addition of the flood protection system also effects the analysis of failure surfaces Actual failure surfaces are very complex and can never be known in advance; however, research into the behavior of soils have yielded methods of analysis that can be applied to some general surfaces for the purpose of predicting actual field behavior of the materials. Using the Modified Bishop Method, circular failure surfaces were investigated using the slope stability program GSTABL7 with STEDwin analysis software version. 2.005.

Minimum Factors of Safety for Overall Stability Analysis

The current criteria listed in USACE EM 1110-2-1913 states that for the end of construction case, the minimum factor of safety for slope stability is 1.3. As previously stated, the steady state seepage and rapid drawdown cases did not apply to the proposed model and therefore no analysis of these cases was conducted. Stability under earthquake (FEMA Case 5) loadings was also not analyzed because the proposed levee is located in Seismic Zone 0 (1997 Uniform Building Code) where seismicity is not considered a design issue.

Soil Design Parameters

The soil design parameters required for an overall/global rotational stability analysis are stratification, unit weight, and shear strength. The stratification was developed from the boring logs and laboratory tests obtained for the reach of levee. The unit weight assigned to the various soils was based on experience and geotechnical investigations. The shear strength model used for analysis was based on experience in the area, boring logs, and laboratory testing. A shear strength model was developed for each global stability section analyzed and is shown in Appendix E.

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Levee Reaches Analyzed

For purpose of stability analysis for Typical Sections A-A and B-B, the soils were classified into 2 different profiles. These profiles can be found on the “Soil Profile Summary” page in Appendix A of this report. Soil profiles derived from data collected from a combination of SPT and laboratory testing. The cross-sections provided to us by Dannenbaum Engineering Corporation were fairly consistent throughout this stretch and therefore summarized into 2 Typical Sections.

The minimum factors of safety for global stability of the levee were calculated for both riverside and landside of the flood protection system. This analysis was done for all combinations of the most critical cross-sections, and their corresponding soil profiles. The results of this analysis are summarized in Table 3 located at the top of the next page.

Table 3: Summary of Slope Stability Results

The stability analysis is based on wall height “H” (ft.) shown on the “Retaining Walls Summary Sheet” as provided by Dannenbaum Engineering Corporation. Dimensions used in analysis are based on the dimensions provided in the TxDOT Retaining Walls standard RW1(L) C for the given wall height. Any deviation in design or dimensions of the wall by the wall designer will null and void the results shown and will thus put the responsibility of the rotational stability model on the wall designer. All cases shown above are passing the minimum requirements as per USACE EM 1110-2-1913, and no countermeasures are required.

Seepage Analysis

By definition, seepage is the movement or flow of a fluid through a porous medium. In the construction of earth dams and levees this is critical where pervious or highly permeable materials are existent within or in the foundation layers beneath the structure. These conditions can lead to excessive hydrostatic pressures and soil erosion problems with the potential to affect stability in the long term.

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The seepage analysis was conducted as outlined in USACE EM 1110-2-1913, “Design and Construction of Levees,” 30 April 2000 and USACE ETL 1110-2-569 “Design Guidance for Levee Underseepage,” 1 May 2005. Basic calculations for seepage entail evaluating levee toe exit gradients against factor of safety adjusted exit gradients to determine need for seepage control measures. The analysis was modified slightly to incorporate the addition of the proposed concrete floodwall on the riverside.

The analysis method used for underseepage analysis is based on a top blanket theory that assumes the site soils consist of two layers; an impervious or semipervious top stratum (mostly fine grained soils including clays, silts) and a pervious substratum (mostly coarse grained soils including clean sands and gravels). The assumptions and criteria set forth in this type of analysis were provided in USACE EM 1110-2-1913 and are as follows:

a. Seepage may enter the pervious substratum at any point in the foreshore (usually at riverside borrow pits) and/or through the riverside top stratum.

b. Flow through the top stratum is vertical.

c. Flow through the pervious substratum is horizontal.

d. The levee (including impervious or thick berms) and the portion of the top stratum beneath it is impervious.

e. All seepage is laminar.

Utilizing the gradation curves and -200 sieve analyses from the geotechnical exploration (see Appendix D), we were able to accurately model the soil profiles and effects of seepage flow and substratum pressures within the various reaches studied. Noting the proposed addition of gate valves to control water in the Peñitas Pumphouse Intake Channel during flood events, we conservatively modeled the full head condition on the riverside with a zero head condition on the landside. From these models we calculated exit gradients which proved to be in compliance with the USACE requirements (as per ETL 1110-2-569) of a minimum exit gradient of i = 0.5.

For further illustrations, calculations and results…