BankerFldwy-SouthLeveeGeotech.pdf

Executive Summary

The existing subsurface conditions of the South Banker Floodway Levee (Common Levee Extension) Improvements Project (from Common Levee to Spur 115 - Sta. 238+68.33 to 246+22.82) were explored by 2 exploratory borings (BH-1 – BH-2) drilled on top of the existing levee to a minimum depth of 100 ft. below the ground surface elevation existing at the top of the levee at the time of our study.

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 soil profile within the limits of the South Banker Floodway (Common Levee Extension) can be generally characterized as a top layer of moderately plastic sandy clay “CL/CH” (identified as the existing levee embankment) followed by alternating strata of primarily lean to fat hard clays “CL to CH”. All areas of the existing levee within these segments consisted of suitable material which can be left in place. A summary of the generalized soil profiles utilized in our analysis can be found in Appendix A.

We performed a detailed engineering analysis to assess the condition of the existing levee and underlying soils within this project. Our main focus was to quantify the engineering properties of the in-situ soils and their ability to withstand the additional loads imposed by the proposed embankment in the specified areas of deficiency. The scope of this analysis is explained in detail in the body of this report.

The calculated global stability factors of safety are higher than the minimum factors of safety required by the USACE Manual EM 1110-2-1902 ‘Slope Stability’. Levee slopes were analyzed utilizing 3.0:1.0 (horizontal:vertical) slopes and a 16 ft access road top width (minimum traffic loading was incorporated into analysis). Analysis and summary of results can be found in Appendix B.

A settlement analysis was performed, based on consolidation tests performed on samples retrieved from the field operations of the underlying native soils. Using methods and guidelines in the USACE Manual EM 1110-1-1904 ‘Settlement Analysis’, we estimated a total anticipated settlement 3.0 inches in the areas of anticipated levee raise. These values incorporate a 1% consolidation of the newly placed fill material (assuming density control measures are followed during construction). The high strength parameters of the existing foundation soils and relatively small increases in levee deficiency can attribute to the low values of settlement. Analysis is described in further detail in the body of the report and consolidation reports can be found in Appendix G.

The ultimate bearing capacity of the underlying soils were established and provided in this report to assist in the design of the proposed improvements. The factors of safety for these segments proved to be in compliance to the USACE guidelines required factor of safety of 2.0.

Analysis and computed factors of safety can be found in Appendix D.

We conducted a detailed through/underseepage analysis that adheres to both the USIBWC and USACE requirements along this stretch. The soils from the borings were classified and “k” values for permeability were assigned to each of the varying strata. This allowed us to conduct a very detailed seepage analysis to ensure that Seepage Remediation Techniques were only iii specified in areas that truly warrant their implementation. The seepage analysis concluded that no seepage remediation techniques were needed for this stretch of levee. The analysis and findings can be found in detail in Appendix C.

It is our understanding that the subject project considers raising the existing levee a maximum of approximately 3.0 feet above the existing top of levee (including caliche for access road).

For recommendations with regard to construction means and methods and/or new embankment specifications refer to the section of this report titled, “General Considerations During Construction”. This area provides the designer recommendations as it relates to Site Drainage, Site Preparation, Embankment Fill Placement Recommendations, and Embankment Construction Benching Recommendations.

The geotechnical engineer certifies that existing levee reaches, where no alterations are proposed, meet the requirements of the Federal Emergency Management Administration (FEMA) found in 44 CFR Section 65.10 (Code of Federal Regulations, Title 44 – Emergency Management Administration, Section 65.10 – Mapping of Areas Protected by Levee Systems). In alteration areas of the existing levees, the construction contractor must adhere to the construction methodology presented in this report for levee certification.

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.

iv v

INTRODUCTION

GENERAL PROJECT OVERVIEW

Project Description Proposed Improvements Proposed Improvements (At Spur 115 Bridge) Scope and Limitations of Investigation

EXISTING LEVEE SURFACE & SUB-SURFACE CONDITIONS

Site Location / Description Previous Geotechnical Studies Subsurface Stratigraphy Rainfall Waterstrike

LEVEE GEOTECHNICAL INVESTIGATION

Soil Borings and Laboratory Tests

LABORATORY INDEX TESTING

Moisture Content Plasticity Index Minus 200 / Sieve Analysis / Hydrometer Summary of Index Testing

LABORATORY STRENGTH & COMPRESSIBILITY TESTING

Unconfined Compression Test Consolidation Test

SLOPE STABILITY ANALYSIS

Minimum Factors of Safety for Overall Stability Analysis Soil Design Parameters Levee Reaches Analyzed

SETTLEMENT ANALYSIS

SEEPAGE ANALYSIS

BEARING CAPACITY ANALYSIS

GENERAL CONSIDERATIONS DURING CONSTRUCTION

Site Drainage Recommendations Site Preparation Recommendations Special Site Recommendations (At Spur 115 Bridge) Embankment Fill Placement Recommendations Embankment Construction Benching Recommendations

REFERENCES

APPENDIX A – BORING LOGS/LOCATION MAPS/PROFILES.................A

APPENDIX B – SLOPE STABILITY ANALYSIS.............................................B APPENDIX C – SEEPAGE ANALYSIS..............................................................D

APPENDIX D – BEARING CAPACITY ANALYSIS........................................D

APPENDIX E – GRADATIONS/HYDROMETERS..........................................E

APPENDIX F – UNCONFINED COMPRESSION TESTS............................... F

APPENDIX G – CONSOLIDATION REPORTS .............................................. G APPENDIX H – ADDITIONAL REFERENCES............................................... H

INTRODUCTION

L&G Engineering Laboratory, L.L.C. was contracted by S&B Infrastructure, Ltd. to perform a subsurface geotechnical investigation and engineering analysis on a portion of the Rio Grande Flood Control System. This report covers the South Banker Floodway Levee (Common Levee Extension) of the rehabilitation and improvements project that extends from Common Levee to Spur 115 (Refer to Appendix A for a detailed location map). Contained in this report is a general project overview which includes; the techniques and testing procedures used to perform the subsurface exploration and testing, engineering analysis consisting of settlement, through/underseepage, slope stability, bearing capacity, and soil strength parameters derived from extensive laboratory testing.

GENERAL PROJECT OVERVIEW

Project Description

The proposed 2009 USIBWC Rio Grande Flood Control System Projects 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 interior levee system of the Lower Rio Grande Flood Control Project. The improvements proposed for the existing levee include a raising of the top of levee elevation to appropriate heights designed by the USIBWC, detailed engineering analysis to evaluate the adequacy of existing levee material and possible seepage issues, evaluation of the existing levee and proposed levee configurations for compliance with USACE requirements for levee certification, and utilization of fill material to increase the top crown width of the levee embankment to accommodate a 16 ft. access road.

Proposed Improvements

The South Banker Floodway (Common Levee Extension) project site begins at approximately 100 feet NE of the Hidalgo Loop Levee/Common Levee intersection (Station 238+68.33), and concludes at Spur 115 (Station 246+22.82). At the time this report was written, the scope of the proposed improvements included raising the existing top of levee elevations in areas not meeting the height requirements designed by USIBWC, widening the top of levee crown to 16 feet and placing an all-weather surface (caliche/gravel roadbed) to allow for an access road atop the levee. In addition, proposed levee slopes both landside and riverside (floodway side) were to be graded to maximum 3.0:1.0 slopes (only tie-ins to the existing slope required in areas flatter than 3.0:1.0). The areas of this segment which require levee raising were provided to us by S&B Infrastructure via cross-sections identifying existing and proposed levee heights. A detailed breakdown of the critical cross-sections in this segment used in our geotechnical analysis can be found on the “Typical Section Summary” sheet in Appendix A. All station numbers are references from a centerline file provided by S&B Infrastructure, Ltd.

Proposed Improvements (At Spur 115 Bridge)

The South Banker Floodway (Common Levee Extension) project crosses a major transportation corridor at the Spur 115 Bridge. At the time this report was written, the current structure consisting of twin bridges was being widened to the center through a Texas Department of

Transportation (TxDOT) project (CSJ: 1804-01-057). The proposed structure widening project would leave an open area between the bridges of approximately 30 feet and would incorporate concrete riprap on the slopes (see Appendix H for TxDOT Spur 115 Plan Sheets as provided by S&B Infrastructure, Ltd.). As per S&B Infrastructure, Ltd. this area would require a minimal top of levee elevation raise of approximately 1 foot. Through coordination between TxDOT and S&B Infrastructure, Ltd., it is the understanding of L&G Engineering Laboratory that TxDOT will issue a supplemental to their current contract to include the necessary work. A detailed breakdown of the critical cross-sections in this segment used in our geotechnical analysis can be found on the “Typical Section Summary” sheet in Appendix A.

Scope and Limitations of Investigation

This report has been prepared in general accordance with requirements from both the USACE 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 S&B Infrastructure, Ltd. 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) 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 current levee and underlying strata, and analyze proposed levee reconstruction and rehabilitation 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 S&B Infrastructure, Ltd. A Location Map of these borings can be found in Appendix A of this report. S&B Infrastructure, Ltd. also provided survey and tie down of 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 minimum factors of safety for overall stability and bearing capacity

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 settlement for this stretch of levee

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.

EXISTING LEVEE SURFACE & SUB-SURFACE CONDITIONS

Site Location / Description

The South Banker Floodway Levee (Common Levee Extension) is located in the Pharr area and runs along the Common Levee portion (parallel to the North Banker Floodway Levee) of the Lower Rio Grande Floodway System. Figure 1 illustrates the general location of the geotechnical investigation beginning from 100 feet NE of the Hidalgo Loop Levee/Common Levee intersection (Station 238+68.33), and ending at Spur 115 (Station 246+22.82). Proposed construction for this area consists of raising the top of levee elevation in deficient areas and providing an all-weather levee access road. Typical 3.0:1.0 embankment slopes are being utilized throughout. Refer to Appendix A for a detailed break-down of the typical sections being used in our analysis.

Figure 1: General Location Map

Previous Geotechnical Studies

L&G has previously performed a detailed geotechnical engineering analysis at the western adjoining Common Levee (from Hidalgo Loop Levee to Banker Weir) in February/March 2008.

The project was performed for the Hidalgo County Drainage District as a portion of the Hidalgo County Levee Rehabilitation and Improvements Project. The project was constructed and recently completed in June 2009.

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 soil profile within the limits of the South Banker Floodway (Common Levee Extension) can be generally characterized as a top layer of moderately plastic sandy clay “CL/CH” (identified as the existing levee embankment) followed by alternating strata of primarily lean to fat hard clays “CL to CH”. 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 in April/May of 2009 was conducted during a relatively moderate to dry climatic period. According to the National Weather Service no uncommonly extreme amounts of rainfall occurred prior to or during our sub-surface exploration.

Waterstrike

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

During the drilling operations, a waterstrike was encountered in both 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 existing levee elevation at the time of exploration. Table 1 shows the waterstrike elevation with respect to top of hole elevation at each boring in tabular form.

Boring No.

Water Strike Depth from Top of Hole

(ft.)

1 22.00

2 24.00

Table 1: Water Strike Elevation Summary

LEVEE GEOTECHNICAL INVESTIGATION

Soil Borings and Laboratory Tests

Subsurface conditions at the site were evaluated by two (2) exploratory borings advanced to 100 ft below the top of levee. All borings were drilled at the approximate locations shown on the Boring Location Maps located in Appendix A of this report. S&B Infrastructure, Ltd.

provided survey and tie downs of the exact locations of the completed borings. All borings were drilled and logged by representatives of L&G Engineering Laboratory. All laboratory testing was performed by L&G Engineering Laboratory. The soil borings were drilled in general accordance with American Society of Testing Materials Procedure D 420 using a truck mounted 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 determination of soil strength parameters. SPT tests 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 and on the test summary tables. 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

37 split barrel samples and 7 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 Client.

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 9 percent to 41 percent. The variance in these percentages can be attributed to range in depth and distance between samples. 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 may inherently absorb more water than others. 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 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 7 (low plasticity sandy silts) to 45 (high plasticity clays).

Minus 200 / Sieve Analysis / Hydrometer

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 11% (sands with little to no fines) to 100% (high plasticity clays with little to no coarse grained material). 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 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 E.

Summary of Index Testing

A summary of the index testing performed on the collected samples can be found on the boring logs in relation to their corresponding depths in Appendix A. Table 2 is a table representing the total number of index tests performed for the South Banker Floodway Levee (Common Levee Extension).

Test Name & ASTM Designation Number of Tests

Performed

Moisture Content (ASTM D 2216-98) 44

Plasticity Index (ASTM D 4318-00) 24

% Finer than No 200 (ASTM D 1140-00) 24 Hydrometer & Sieve Analysis (ASTM D 422 – 63) 11

Table 2: Index Testing Summary

LABORATORY STRENGTH & COMPRESSIBILITY TESTING

Unconfined Compression 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. 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.

Unconfined compression tests were performed due to the high volume of stiff cohesive clay samples obtained from the field subsurface exploration (maximum 1 performed per boring). The Shelby tube samples were extracted and cut to a length / diameter ratio of approximately two to one. Refer to Appendix F to see the sample depth and location, along with the corresponding unconfined compressive 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 (end of construction case).

Consolidation Test

The one dimensional consolidation tests were performed to determine the magnitude and rate of volume decrease that a laterally confined soil would undergo when subjected to different vertical pressures. The main purpose of this test is to obtain data that will be used to predict the rate and amount of approximate settlement of the levee structure. 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. This data is useful in determining the compression index, the recompression index and the pre-consolidation pressure (or maximum past pressure) of the soil. In addition, the data obtained can also be used to determine the coefficient of consolidation and the coefficient of secondary compression of the soil.

The consolidation properties determined from the consolidation test were used to estimate the magnitude of total anticipated settlement. Estimates of this type are vital in the design of engineered structures and the evaluation of their performance. Consolidation input values and strain readings can be found in tabular form in Appendix G of this report.

SLOPE STABILITY ANALYSIS

For purposes of analyzing the stability of the levee in its final configuration, the method as outlined in USACE EM 1110-2-1902, “Slope Stability”, 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 man-made slopes. The fundamental principles 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 through a comparison between forces, moments and stresses tending to cause instability of the mass versus those that resist instability and the conditions for static equilibrium are applied to analyze the problem.

Two-dimensional (2-D) sections are analyzed and plane strain conditions (no deformation of plane surfaces and no strain until 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 (the capacity) to the shear stress required to maintain equilibrium of the failure mass under the applied loads (the demand). This method is the most appropriate for this application and has traditionally been used in the analysis of man-made earth structures such as levees.

For this project, the levee reach analyzed consisted of 2 sections. The first section generally covered Sta. 238+68.33 to 245+00. This section consisted of a raising of the existing top of levee a maximum of 3.0 (ft.) and a reconstruction of the landside and riverside (floodway) slopes to proposed 3.0:1.0 grades (maximum grades allowable as per S&B Infrastructure Ltd.). The second section generally covered Sta. 245+00 to 246+22.82 and consisted of a raising of the existing top of levee a maximum of 1.0 (ft.) and a reconstruction of the riverside (floodway) slopes to 3.0:1.0 grades incorporating concrete riprap.

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 as follows:

Case I: End of Construction (Figure 2) – This case generally reflects the condition of the levee directly following completion of the proposed construction. The riverside (floodway) of the raised levee is not subject to flooding under this condition.

Figure 2: End of Construction

Case II: Long-Term Steady-State Seepage (Figure 3) – This case generally reflects a condition where a pool level reaching the maximum allowable height (3.0 feet below proposed top of levee elevation) has persisted for a long period of time on the riverside (floodway) of the levee, such that a steady-state seepage condition has developed through the existing and newly reconstructed levee.

Figure 3: Steady State Seepage

Case III: Rapid Drawdown (Figure 4) – This case generally reflects a condition where a pool level reaching the maximum allowable height (3.0 feet below proposed top of levee elevation) has persisted for a long period of time on the riverside (floodway) of the levee, resulting in a steady-state seepage condition through the existing and newly reconstructed levee prior to the occurrence of a rapid drawdown.

Figure 4: Rapid Drawdown

The stress-strain properties of the materials in both the existing fill and foundation govern the potential location 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. The minimum factor of safety for the steady state seepage case is 1.4, and 1.0 to 1.2 for the sudden drawdown case. Stability under earthquake (FEMA Case 5) loadings was not analyzed because the existing/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 a global slope stability analysis are stratification, unit weight, and shear strength. The stratification was developed from the boring logs and laboratory tests obtained for the levee reaches. 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 slope stability section analyzed and is shown in Appendix A.

Levee Reaches Analyzed

For purposes of stability analysis for this reach of the levee, 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 were derived from data collected from a combination of SPT and laboratory testing. The cross-sections provided to us by S&B Infrastructure Ltd. varied slightly throughout this stretch and therefore were summarized into 2 critical cross-sections.

The minimum factors of safety for global stability of the levee were calculated for both riverside (floodway) and landside of the existing/proposed levee. 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.

South Banker Floodway Levee (Common Levee Extension)

Typical Section #

Stations Soil

Profile Slope Analyzed Calculated Minimum Calculated Minimum Calculated Minimum

1 238+68.33 - 245+00 A Riverside

(Floodway)

3.08 1.30 3.31 1.40 1.80 1.00

2 245+00 - 246+22.82 B Riverside

(Floodway)

6.14 1.30 2.70 1.40 1.64 1.00

Typical Section #

Stations Soil

Profile Slope Analyzed Calculated Minimum Calculated Minimum Calculated Minimum

1 238+68.33 - 245+00 A Landside 3.18 1.30 2.11 1.40 2.11 1.00

*2 245+00 - 246+22.82 B Landside ~ 1.30 ~ 1.40 ~ 1.00

* - No Landside Slope Stability Analysis performed at Spur 115 due to gradual slopes not impacting stability analysis

Factor of Safety

Short-Term Analysis Steady State SeepageEnd of Construction Rapid Drawdown

Long-Term Analysis

Factor of Safety

Long-Term Analysis

Factor of Safety

Rapid Drawdown Long-Term Analysis

Factor of Safety

Short-Term Analysis Long-Term Analysis

Factor of Safety Factor of Safety

Steady State SeepageEnd of Construction

Table 3: Summary of Slope Stability Results

Analysis of Results

Referencing Table 3, all cases are passing the minimum requirements as per USACE EM 1110- 2-1913, and no countermeasures (i.e. retaining walls, sheet pilling, stone columns, soil densification measures, etc.) are required. It is important to note some of the lower end values in the rapid drawdown and steady state seepage cases. These values are conservative in nature and will not result in catastrophic type failures; however, sloughing or sliding of the outer face of slopes on the floodway side may occur in major flood events thus possibly requiring some future maintenance. In addition, it is important to note that in this particular case since the floodway is an interior levee system, the hydraulic durations are minimal compared to a more critical river protective levee. To achieve true steady state seepage and rapid drawdown the hydraulic duration of a flood event would need to persist for an excessive period of time. Although we feel that this will never be the case, we have provided a complete and detailed analysis including all cases. Detailed stability analysis can be found in Appendix B.

SETTLEMENT ANALYSIS

Settlement of in-situ soils within and beneath the levees is characterized as a consolidation or a decrease in void fraction of soil particles associated with changes in stress (proposed embankment increase), water content, soil mass, and climatic effects (seasonal variation in moisture and humidity). It is imperative to accurately estimate the total settlement of the existing soil and proposed embankment to adhere to the minimum height specifications (100 year storm water level plus freeboard) set forth by the USIBWC and FEMA.

Coarse-grained soils and fine-grained soils exhibit very different behavior with respect to settlement. Coarse-grained soils (sands, clayey sands, silty sands and gravel) generally exhibit rapid consolidations usually occurring immediately after the new load is applied and similar to elastic deformation. This immediate settling or elastic settling is due to the fast drainage of pore water between the coarse particles. Fine-grained, cohesive soils (clays, silts, and other organic soils) usually settle considerably from compression and may take long periods of time to reach full consolidation. This time delayed consolidation is exhibited in clays because the expulsion of pore water from the soil voids takes a significant amount of time to dissipate between the smaller fine-grained particles.

Soils within our field exploration of these levee reaches can be generally characterized as a top layer of moderately plastic sandy clay “CL/CH” (identified as the existing levee embankment) followed by alternating strata of primarily lean to fat hard clays “CL to CH”. These soils were considered mostly normally consolidated clays; which means that the present effective overburden pressure (pressure from the existing environment and embankments) is the maximum pressure that the soil has been subjected to in the past. For a more detailed representation of the in-situ soil profiles and strata used for analysis, refer to the soil profile shown in Appendix A of this report.

The total settlement for the underlying soils beneath the existing levee and the proposed embankment was estimated from the results of a series of one dimensional consolidation tests (ASTM D2435) performed on the samples retrieved from the field exploration. These samples were extracted from Shelby tube samplers and returned to the laboratory for subsequent testing.

The calculated settlement was estimated using the following equation for normally consolidated clays from the USACE Manual EM1110-1-1904, titled “Settlement Analysis”:

ρc = * log * H σ'f___ σ'o 1 + eo

Cc___ where: ρc = ultimate one-dimensional consolidation settlement Cc = compression index (slope of virgin compression 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

Estimated settlement is shown as a function of the increase in embankment height (or effective overburden pressure). The maximum total settlement in areas requiring proposed embankment fill (areas of deficiency with USIBWC 100 year flood elevations) were approximately 3.0 inches. These calculated values include an estimated additional settlement on the order of approximately 1.0% of the total height of newly placed fill embankment. Density and moisture control measures should be followed during the construction and rehabilitation of this levee reach to minimize any additional settlement.

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 ground water wells are located. The extraction of this natural moisture has the potential to reduce the volume of the in-situ soil, causing some marginal settlement to occur. The estimated settlement does not consider subsidence due to these wells or reduction of soil mass due to erosion, piping, or shallow seepage. Construction techniques, atmospheric and climatic changes, as well as seasonal moisture variance can also cause settlements exceeding our estimated maximum recommendations.

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.

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. As previously discussed, it is important to note that in this particular case since the floodway is an interior levee system, the hydraulic durations are minimal compared to a more critical river protective levee. In essence the minimalism of this hydraulic duration directly corresponds to the reliability of a seepage analysis since seepage is time dependent necessitating long durations in order for the water to be allowed to seep through the soil. The hydraulic durations for this stretch of floodway were 106 to 203 hours (2 to 3.5 days) at full capacity provided by S&B Infrastructure, Ltd. (originally from IBWC) based on gauge data from Hurricane Beulah (1967).

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 E), we were able to accurately model the soil profiles and effects of seepage flow and substratum pressures within the various reaches studied. 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 involved in the underseepage analysis see Appendix C.

Due to the extremely low permeability makeup and impervious nature of the existing levee soils encountered (mostly moderate plasticity clays), only a minimal detailed study of through seepage was analyzed for the area in accordance with USACE EM 1110-2-1913. The seepage flows through the levee were very minimal and ranged from approximately 1.4 x 10-6 to 2.1 x 10-6 ft3/sec/ft. These flows in conjunction with the short hydraulic durations should have minimal effect on the structural integrity of the levees. For further illustrations, calculations and results involved in the through seepage analysis see Appendix C.

BEARING CAPACITY ANALYSIS

The bearing capacity of the existing natural ground is defined as the ability of a foundation to safely support the imposed loadings (surcharge) from the existing and proposed levee embankments, without experiencing any form of shear failure. The ultimate bearing capacity is a measure of the soil’s maximum resistance immediately prior to a bearing capacity failure. The ultimate bearing capacity was estimated using the methods and equations, as recommended by the USACE in Manual EM 1110-1-1905 titled “Bearing Capacity of Soils”:

qu = c Ncζc + ½ B γh Nγζγ + σ Nqζq 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, Nq = Terzaghi’s bearing capacity factors ζc ζγ ζq = dimensionless correction factors for cohesion, soil unit weight, and surcharge

Nc , Nγ , and Nq are the dimensionless bearing capacity factors developed by Karl Terzaghi for general shear failure listed in Table 4-1 of EM 1110-1-1905. Our analysis was based on modeling the existing and proposed embankments resting on the natural ground as a strip foundation model (Terzaghi). Utilizing this strip foundation model allowed for the correction factors ζc , ζγ and ζq to be taken as 1.0 throughout or analysis (Bowles 1988). Due to variations in the soil profile, the bearing capacity analysis was divided into 2 general soil profiles. These profiles and their limits can be found in the “Soil Profile Summary” in Appendix A. Cohesion values for clays were estimated by using values of undrained shear strength derived from a combination of the laboratory unconfined compression tests performed on the recovered samples (ASTM D2166), pocket penetrometer readings, and standard penetration test results performed in the field. All clay strata were assigned an angle of internal friction equal to 0º to maintain a conservative nature in analysis. The reports of the unconfined compression tests and plots can be found in Appendix F of this report. In sand strata, the angles of friction were derived through correlations with the standard penetration tests performed in the filed. All correlations used were in accordance with the applicable USACE manuals.

The factor of safety for our analysis was calculated by dividing the Ultimate Bearing Capacity by the maximum proposed surcharge from the proposed levee embankments. Generally, for all structures and embankments a factor of safety of 2.0 or greater is recommended as standard engineering practice when addressing bearing capacity, as noted in chapter 1 of EM 1110-1- 1905. Table 4 is a summary of the bearing capacity analysis performed for this section of levee improvements. All soil profiles met or exceeded the minimum required factors of safety.

BEARING CAPACITY ANALYSIS RESULTS

South Banker Floodway Levee (Common Levee Extension)

Soil Profile Designation

Ultimate Bearing

Capacity (psf)

Maximum Surcharge (psf)

Calculated F.S.

**Minimum Required F.S.

A 5,472 2,160 2.5 2.0

B 10,260 1,800 5.7 2.0 ** from USACE Manual EM 1110-1-1905

Table 4: Summary of Bearing Capacity Analysis

GENERAL CONSIDERATIONS DURING CONSTRUCTION

Site Drainage Recommendations

Drainage is one of the most important aspects to be addressed to ensure the successful performance of any foundation. Positive surface drainage should be implemented prior to, during and maintained after construction to prevent water ponding at or adjacent to the embankments under construction. It is recommended that the levee rehabilitation plans include some form of channel runoff to designated detention areas or to existing storm drains.

Site Preparation Recommendations

Levee rehabilitation areas should be stripped of all vegetation and organic topsoil where embankment fill will be placed. Exposed levee banks should be thoroughly compacted in order to locate and re-compact any weak, compressible soft spots. Embankment compaction shall be performed in general conformance with TxDOT 2004 Specification Item 132 “Embankment”.

The ‘density control’ procedure method shall be used for all embankment slope construction (Item 132.3.D.2). Proof rolling should be performed on the level top surface of the levee where local traffic is expected. Proof rolling shall be performed in general accordance with TxDOT 2004 Specification Item 216. Proof Rolling operations should be observed by the Geotechnical Engineer or his representative to document existing levee condition and preparation. Weak or soft areas identified during proof rolling should be treated with a minimum of 1% lime admixture by weight. Prior to Embankment Fill placement, the exposed levee surfaces should be moisture-conditioned and re-compacted to adequate compaction to support the proposed loadings. Compaction should be a minimum 95 percent of the maximum dry density as determined in accordance with ASTM D698, moisture-density relationship. If a higher compactive effort than is attained by ASTM D698 is desired ASTM D1557 may be utilized and references to ASTM D698, relative to compactive effort, should be changed to ASTM D1557.

The moisture content of the Subgrade should be maintained within the range of three (3) percentage points below optimum to three (3) percentage points above the optimum moisture content until permanently covered. Any excavation/backfill activities should be observed by L&G representatives to document existing levee conditions and proposed embankment preparation.

Special Site Recommendations (At Spur 115 Bridge)

As previously noted in this report, the area around the Spur 115 Bridge will have a minimum 1 foot raise approaching from the west of the structure and between the twin structures with concrete riprap on maximum 3:1 bridge header slopes. In addition to the “Site Preparation Recommendations” section, this portion of levee should be constructed in accordance with the current TxDOT “Concrete Riprap and Shoulder Drains” (CRR) Standard (should incorporate at a minimum ¾ in. Expansion Joint Material between the faces of the abutment wingwalls of the twin structures and the proposed concrete riprap). See Appendix H for TxDOT CRR standard.

Embankment Fill Placement Recommendations

Select fill shall be placed in loose lifts not to exceed 8 inches (6 inches compacted) and compacted to a minimum 95 percent of the maximum dry density as determined in accordance with ASTM D 698. If a higher compactive effort than is attained by ASTM D698 is desired ASTM D1557 may be utilized and references to ASTM D698, relative to compactive effort, should be changed to ASTM D1557. The water content of the fill shall be maintained within the range of three (3) percentage points below optimum to three (3) percentage points above the optimum moisture content until the fill is permanently covered. The site fill should be properly compacted in accordance with these recommendations and tested for compaction as specified.

Embankment fill material shall consist of soils with a maximum liquid limit of 50 and a plasticity index equal to or greater than 10, but not to exceed 30.

Embankment Construction Benching Recommendations

This project consists of generally thin slope fills (small sliver fills) which historically have proven problematic from a geotechnical standpoint. Proper compaction of these fills is especially difficult and poor compaction can lead to erosion problems and potential sliding and shear failures along the sloping interface between the existing surface and proposed fill.

Benching or stepping the slopes improves the stability of the final product by inhibiting the development of the shear plane along the interface. We recommend that benching be utilized for all embankments placed on or against a slope steeper than 8:1 to assure the new embankment is knitted together with the existing levee soils. Figure 6 shows a recommended typical benching scenario which we developed for this project.