03-_Geotechnical_Addendum _Subreach_I.pdf

GEOTECHNICAL ADDENDUM – SUBREACH 1

FOR

THE LOWER RIO GRANDE FLOOD CONTROL PROJECT

LEVEE SYSTEM – FROM DONNA PUMP TO BROWNSVILLE LEVEE REACH

HIDALGO COUNTY AND CAMERON COUNTY, TEXAS

Raba Kistner Consultants, Inc.

12821 W. Golden Lane San Antonio, TX 78249

P.O. Box 690287 San Antonio, TX 78269‐0287 www.rkci.com

P 210 :: 699 :: 9090 F 210 :: 699 :: 6426 TBPE Firm F‐3257

San Antonio • Austin • Brownsville • Corpus Christi • Dallas • El Paso • Houston • McAllen • New Braunfels • Mexico • Salt Lake City

12821 W. Golden Lane San Antonio, TX 78249

P.O. Box 690287

San Antonio • Austin • Brownsville • Corpus Christi • Dallas • El Paso • Houston • McAllen • New Braunfels • Mexico • Salt Lake City

THIS PAGE LEFT INTENTIONALLY BLANK

Project No. AMA08-115-06 June 4, 2013

Mr. Andy Gong, P.E.

Tetra Tech, Inc.

17885 Von Karman Avenue, Suite 500 Irvine, California 92614

RE: Geotechnical Addendum – Subreach 1

The Lower Rio Grande Flood Control Project Levee System – From Donna Pump to Brownsville Levee Reach

Hidalgo County and Cameron County, Texas Tetra Tech, Inc., Job No. 100-SWW-T27062

Dear Mr. Gong:

Raba-Kistner Consultants, Inc. (RKCI) is pleased to submit our geotechnical addendum to our original Technical Memorandum (RKCI Report No. AMA08-115-00, dated July 24, 2009) for the above referenced project. Specifically, this addendum presents the results of a supplemental geotechnical exploration and engineering evaluation of the levee system for the final design of the levee system identified as Subreach 1 of the Donna Pump to Brownsville Levee, from Station 199+70.94 to 307+66.36 and from Station 637+40.37 to 0+43.03 (Cameron County). The purpose of the supplemental geotechnical exploration was to drill borings along the existing alignment of the levee in locations where subsurface information was not obtained during the original geotechnical exploration. Laboratory testing was performed on samples from these new borings to classify and characterize subsurface conditions to assist in the performance of additional stability evaluations.

The analysis prepared for our original Technical Memorandum was reviewed based on the updated survey and levee geometry data provided by Tetra Tech and upon the additional boring information just described.

On the basis of our review of the new data, it was determined that the levee geometry and landside conditions warranted additional analysis to evaluate seepage and slope stability conditions in this subreach.

Additionally, RKCI was asked to provide input on the existing structures along this subreach, and to provide additional construction recommendations related to the placement of the embankment fill materials and the need for the toe drain recommended for portions of this subreach in our original report. This report should not be utilized separately from the referenced, original Technical Memorandum. These services were performed in accordance with the scope of work pursuant to the executed Subconsultant Professional Services Agreement between Tetra Tech, Inc. (CLIENT) and RKCI dated February 3, 2011 for the above-referenced Tetra Tech, Inc. Job with the United States, Section of the International Boundary and Water Commission (USIBWC).

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

Very truly yours, RABA-KISTNER CONSULTANTS, INC.

John A. Focht, III, P.E. June 4, 2013 Chris L. Schultz, P.E., PMP Chief Geotechnical Engineer President and CEO

JAF/CLS/dlc Attachments Copies Submitted: Above (4)

12821 W. Golden Lane San Antonio, TX 78249

P.O. Box 690287

San Antonio • Austin • Brownsville • Corpus Christi • Dallas • El Paso • Houston • McAllen • New Braunfels • Mexico • Salt Lake City jfocht seal jfocht signature

GEOTECHNICAL ADDENDUM – SUBREACH 1

THE LOWER RIO GRANDE FLOOD CONTROL PROJECT LEVEE SYSTEM

FROM DONNA PUMP TO BROWNSVILLE LEVEE REACH

HIDALGO COUNTY AND CAMERON COUNTY, TEXAS

for

U.S. SECTION OF THE INTERNATIONAL BOUNDARY AND WATER COMMISSION (USIBWC)

Prepared for

TETRA TECH, INC.

Irvine, California

Prepared by

RABA KISTNER CONSULTANTS, INC.

San Antonio, Texas

PROJECT NO. AMA08-115-06

TABLE OF CONTENTS

i

INTRODUCTION

LIMITATIONS

FIELD EXPLORATION

LABORATORY TESTING

STRENGTH TESTS

CONSOLIDATION TESTS

HYDRAULIC CONDUCTIVITY (PERMEABILITY) TESTS

SELECTION OF CROSS-SECTIONS FOR ANALYSIS

FIRST SORTING - SEEPAGE PROPERTIES

SECOND SORTING – GEOMETRIC PROPERTIES

DESIGN PERMEABILITY

DESIGN BLANKET PERMEABILITY

DESIGN SHEAR STRENGTH

End of Construction Condition Steady State Condition Rapid Drawdown Condition Summary

LEVEE SEEPAGE ANALYSIS

LEVEE SLOPE STABILITY ANALYSIS

MINIMUM FACTOR OF SAFETY

RESULTS OF ANALYSES

POWER POLES BETWEEN STATIONS 714+88.17 AND 737+55.0

SETTLEMENT

STRUCTURE REVIEW

DRAINAGE STRUCTURES

ROADWAY CONDITIONS

CONCLUSIONS

CONSTRUCTION RECOMMENDATIONS

GENERAL CONSTRUCTION CONSIDERATIONS

TABLE OF CONTENTS

ii

CLAY FILL LEVEE MATERIALS

VEGETATIVE COVER FOR CLAY LEVEES

FLEX BASE

TOE DRAIN MATERIAL

ATTACHMENTS

Field Data ...................................................................................................................................... Appendix A Laboratory Test Results ................................................................................................................ Appendix B Slope Stability and Seepage Analyses ........................................................................................... Appendix C

INTRODUCTION

Raba-Kistner Consultants, Inc. (RKCI) has completed its review of: 1) our original Technical Memorandum (RKCI Report No. AMA08-115-00, dated July 24, 2009) for Subreach 1 of the Donna Pump to Brownsville Levee, from Station 199+70.94 to Station 307+66.36 and from Station 637+40.37 to Station 0+43.03 (Hidalgo County and Cameron County); 2) the Tetra Tech Design Memo “Reach 1 - Canal Alternatives Study - Levee Improvements Adjacent to Canals”, dated November 23, 2012; 3) our Summary Report (RKCI Report No. AMA 08-115-01, dated April 14, 2011) on Evaluation of Placed Fill; 4) the results of the supplemental geotechnical subsurface investigation; and, 5) the subsurface conditions at the specific locations of existing power poles between Stations 714+88.17 and 737+55.02. It is important to note that all recommendations included in this report are based upon the assumption that all recommendations from our Evaluation of Placed Fill report will be fully implemented. These prior recommendations must be implemented prior to the placement of any additional fill or other installed features. In addition, the levee segments evaluated as a part of this study are not contiguous with each other.

The analysis prepared for our original Technical Memorandum was reviewed based on the updated survey and levee geometry data provided by Tetra Tech and upon new soil boring information collected for this study. On the basis of our review of the new data, it was determined that the levee geometry and landside conditions warranted additional analysis to evaluate seepage and slope stability conditions in this reach. The Mercedes Settling Basin, the Mercedes Basin Canal and the Mercedes Concrete Canal run adjacent to the levee through a portion of Reach 1. Both the Mercedes Settling Basin and the Mercedes Basin Canal have water levels that are above the 100-yr design flood for this reach. As such, there are no seepage issues associated with this portion of Reach 1 with respect to the 100-yr flood.

At the location of the power poles between Stations 714+8.17 and 737+55.02, the actual levee crest and associated berm road for the Mercedes Basin Canal are more than 40 ft wide, combined. In addition, the potential seepage layer identified in the new borings is significantly below the bottom of the telephone poles such that if these were to be blown over during heavy winds, there a low probability for levee instability.

RKCI was also asked to provide input on the existing structures along this reach, many of which have been abandoned, and to provide additional construction recommendations related to the placement of the embankment fill materials and the need for the toe drain recommended for portions of this reach in our original report. This report should not be utilized separately from the referenced, original Technical Memorandum. These services were performed in accordance with the scope of work pursuant to the executed Subconsultant Professional Services Agreement between Tetra Tech, Inc. (CLIENT) and RKCI dated February 3, 2011 for the above-referenced Tetra Tech, Inc. Job with the United States International Boundary and Water Commission (USIBWC).

LIMITATIONS

This geotechnical addendum has been prepared in accordance with accepted Geotechnical Engineering practices in the region of South Texas for the use of Tetra Tech, Inc. (CLIENT) and its representatives for design purposes. This addendum may not contain sufficient information for purposes of other parties or other uses.

The recommendations submitted in this summary report are based on the data obtained in completion of the original Technical Memorandum, and should not be utilized separately from the Technical Memorandum. All other recommendations and limitations remain as stated in that report.

FIELD EXPLORATION

Subsurface conditions along the subject site were evaluated by 24 supplemental borings (designated as DP-301 through DP 324) drilled from August 8, 2012 through August 24, 2012 at the locations shown on the Boring Location Map, Figure A-1.

The boring locations shown on the Boring Location Map are approximate and are based on information provided to us by the CLIENT via electronic-mail attachments. The borings were drilled to a depth of about 50 feet each below the existing top of levee using truck-mounted drill rigs, with the borings located near the centerline of the levee cross-section to a depth equal to twice the expected final levee height. This depth of exploration is consistent with the USACE EM 1110-2-1913 Design and Construction of Levees. The borings were advanced using straight flight augers in combination with mud rotary drilling techniques and were backfilled with cement-grout following completion of each day’s drilling operations. During drilling operations, the samples shown on the following table were collected:

Type of Sample Number Collected

Split-Spoon (with Standard Penetration Test, SPT) 147

Shelby Tubes 191

The SPT and Shelby tube samples were performed in accordance with accepted standard practices.

Selected portions of the samples were sealed in containers to reduce moisture loss, labeled, packaged, and transported to our laboratory for subsequent testing and classification.

LABORATORY TESTING

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

Type of Test Number Complete

Pocket Penetrometer 159

Natural Moisture Content 347

Atterberg Limits 95

Percent Passing a No. 200 Sieve 149

Grain Size Curves (Particle Size, Part I) 55

Unconfined Compressive Strength 32

Dry Unit Weight 46

One-Dimensional Consolidation 5

Hydraulic Conductivity (Permeability) 4

With the exception of the laboratory grain size curves (particle size, Part I), hydraulic conductivity (permeability) tests, and one-dimensional consolidation tests, the results of the field and laboratory tests are presented in graphical or numerical form on the boring logs illustrated on Figures A-2 through A-25. A key to the classification of terms and symbols used on the logs is presented on Figure A-26. The results of the laboratory and field testing are also tabulated on Figures A-27 through A-36 for ease of reference.

SPT results are noted as “blows per feet” on the boring logs and on Figures A-2 through A-25, where “blows per foot” refers to the number of blows required for a falling 140-pound hammer to penetrate 12 inches into the subsurface materials. Where hard and/or very dense materials were encountered, the tests were terminated at 50 blows even if one foot of penetration had not been achieved.

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

STRENGTH TESTS

Shear strength parameters of selected samples were evaluated by several methods. Field tests for measuring these parameters included pocket penetrometer and standard penetration testing. Laboratory tests for measuring these parameters included unconfined compression testing.

Results of the field tests as well as the unconfined compression tests are presented in both graphical and numerical form in both Appendix A (on the boring logs and the Results of Soil Sample Analyses) and Appendix B (where the stress-strain behavior of each unconfined compression test is presented).

CONSOLIDATION TESTS

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

Consolidation Test Results

Boring No.

Depth* feet

Initial γdry pcf

Initial γtotal pcf

Po ksf

Pc ksf

OCR Ccε Crε Liquid Limit

DP-304 31 106.9 129.3 4.0 8.5 8.7 0.16 0.05 59

DP-307 13 97.2 122.2 1.6 14.7 9.2 0.28 0.04 45

DP-311 11 101.8 123.7 1.4 19.7 29.0 0.21 0.05 60

DP-316 11 95.6 120.6 1.3 6.4 4.9 0.20 0.04 40

DP-321 7 98.0 121.0 0.8 11.9 14.2 0.20 0.03 44

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

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

HYDRAULIC CONDUCTIVITY (PERMEABILITY) TESTS

Hydraulic conductivity (permeability) tests were performed on relatively undisturbed Shelby tube samples obtained from 4 selected samples of cohesive-type soils. These cohesive soil samples were tested for vertical hydraulic conductivity to assess the seepage velocities of the embankment and cohesive subsurface soils. A calculation summary sheet for each hydraulic conductivity (permeability) test is presented in Appendix B. The results are also summarized in the table below.

Permeability Test Results

Boring No.

Depth feet

Hydraulic Conductivity cm/sec

DP-306 25 - 26.5 3.09E-04

DP-312 10 - 12 2.70E-05

DP-315 18.5 - 20 7.49E-05

DP-324 10 - 12 4.76E-06

SELECTION OF CROSS-SECTIONS FOR ANALYSIS

In selecting the levee sections for analysis, we considered both seepage properties and geometric properties and performed a two-stage sorting based upon both criteria. A brief description of this sorting process is presented below. This approach is intended to identify possible circumstances where a poor seepage properties and moderate geometric properties, or poor geometric properties and moderate seepage properties represent the critical case with respect to underseepage.

FIRST SORTING - SEEPAGE PROPERTIES

We first sorted the boring logs presented in RKCI Project No. AMA08-115-00 dated July 24, 2009 and in the recent boring logs developed for this project (DP-301 through DP-324) into three categories: 1) sands shallower than 20 ft; 2) sands between 20 ft and 30 ft; and 3) sands below 30 ft. Within these categories, the boring logs were then resorted from perceived highest permeability of the topmost seepage layer to the lowest.

SECOND SORTING – GEOMETRIC PROPERTIES

Using the most critical locations based upon seepage properties from the first sorting, we then identified the corresponding levee/structure geometries and sorted the seepage categories from thinnest blanket to thickest blanket in order to develop the critical analysis section. This initial sorting process identified the cross-section at Station 837+70 as the most critical to evaluate.

Subsequent to the initial two-stage sorting, two additional sections were identified in order to provide adequate coverage in design reach. One of these is at Station 302+10 and the other is adjacent to the Mercedes Concrete Canal at Station 748+80.

These three sections (Station 302+10, 748+80 and 837+70) were then evaluated for seepage and stability and the final results are presented herein.

The levee reach between Station 639+00 and 680+00 runs immediately adjacent to the Mercedes Basin and the levee reach from Station 680+00 and 738+00 runs immediately adjacent to the Mercedes Basin Canal. The Mercedes Basin and Mercedes Basin Canal are continuously full of water unless the Mercedes Pump Station is not operating. The actual water level in the Mercedes Basin and the Mercedes Basin Canal can vary depending upon demand for water. The normal water level is above the 100-yr flood elevation. Hence, levee cross sections along these two features will not experience adverse seepage conditions during 100-yr flood event because the flow will be from the Basin and Basin Canal towards the river.

The top of the existing levees along the Mercedes Basin and the Mercedes Basin Canal are approximately 40 to 50 ft wide. In addition, an access road runs along the Mercedes Basin Canal between the levee and the canal. This road is several feet higher than the top of the levee. Given the overall width of the levee sections along the Mercedes Basin and Mercedes Basin Canal, the only construction activities planned in these areas involve the removal of an existing flex base and the placement of a new 16 ft wide flex base on the top of the levee with only minor rising to meet the minimum top of levee provided by the USIBWC. Accordingly, no final seepage and stability calculations were perform for the levee adjacent to the Mercedes Basis and the Mercedes Basin Canal.

Landside bank erosion is occurring along Mercedes Basin due to surface wave action in the Basin. This erosion will need to be monitored by the Irrigation District as a part of normal operation and maintenance procedures. Given the width of the levee section in this region, the current amount of bank erosion would need to progress more than 20 ft beyond its present location before it begins to interfere with the design levee prism.

DESIGN PERMEABILITY

Based on the USACE EM 1110-2-1901 Seepage Analysis and Control for Dams, indirect methods for determining hydraulic conductivity were employed for this study. For uniform, loose clean sands classified as SP (poorly graded sand) using the Unified Soil Classification System, the Hazen1 equation is suggested for use. We have also used the Hazen equation to estimate poorly graded sands with some poorly graded sand with silt (SP-SM).

Since the silty sands (SM) at this site contained significant quantities of silt, we chose a design horizontal permeability of 10-4 centimeters per second for these soils based on the silt fraction as well as prior area experience with similar soils. Similarly, we chose a design horizontal permeability of 10-6 cm/sec for the lean clays and fat clays. The following table summarizes the assumed horizontal permeability values used in our analyses.

Material Horizontal Saturated Design Permeability*, cm/sec

Poorly Graded Sands (SP-SM) 10-2

Poorly Graded Silty Sands (SM) 10-4

Lean Clays (CL) 10-6

Gravel (Drainage Blanket) 1

Clay FILL (CL and CH) 10-6 *Vertical permeability was estimated to be 20% of horizontal permeability

The selection of the design permeabilities was based on the discussions presented above, as well as the relative density of the materials based on SPT values and the plasticity of the materials. The boring logs were reviewed and the classification of the various strata into one of the above materials was based predominately on the gradation and percentage of fine grained materials encountered in our borings.

DESIGN BLANKET PERMEABILITY

The selection of the design permeability values for the surficial “blanket” soils were developed using a combination of guidance from USACE DIVR 1110-1-400, prior area experience, and engineering judgment.

These blanket permeabilities are a function of thickness of the blanket. The following table contains the design values used for both the riverside and landside of the levees.

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

199, 1911.

DESIGN SHEAR STRENGTH

The selection of the design shear strength values was based on several parameters including but not limited to the soil classification testing (PI and gradation), the moisture contents, the results of the SPT testing, and our experience with similar soils.

For granular soils, the following table summarizes the range in angle of internal friction values based on the results of our field and laboratory testing.

Angle of Internal Friction, degrees

Material SPT Blow Count Range Inferred Range

Poorly Graded Sands (SP), Poorly Graded Sand with Silt (SP-SM) 11 – 50+ 30 - 43

Poorly Graded Silty Sands (SM), Sandy Silt (ML), Silty Clayey Sand (SC-SM), Clayey

Sand (SC) 2 - 50+ 30 - 35

For cohesive soils, the following table summarizes the range of undrained shear strength values based on the results of our field and laboratory testing.

Undrained Shear Strength, psf

Material SPT Blow Count Range Inferred Range

Lean Clays (CL) 3 - 27 250 – 3,750

Fat Clays (CH) 5 - 31 375 - 4,500

The boring logs were reviewed and the classification of the various strata into one of the above materials was based predominately on the soil classification and the SPT test results. The design shear strength parameters for this study were selected based on the results of laboratory strength tests, resistance to driving while performing the SPT test, prior area experience, and professional judgment. The following tables summarize the design values used in our analyses.

Material Design Permeability cm/sec Riverside Landside

Silt with Sand (ML) and Poorly graded Silty Sands (SM) 1.3x10-4 2x10-4

Clays (CL and CH) (0.03 to 1.5)x10-5 (0.08 to 3.0)x10-5

End of Construction Condition

Total Stress

Material Total Unit Weight, pcf

Cohesion, psf

Angle of Internal Friction degrees

Poorly Graded Sands with Silt (SP-

SM)

117 0 33

Poorly Graded Silty Sands (SM) 117 0 32

Clays (CL and CH) 120 400-425 0

Gravel (Drainage Blanket / Toe Drain)

125 0 35

Clay FILL (CL and CH) 120 400-425 0

Steady State Condition

Effective Stress

Material Total Unit Weight

Cohesion, psf

Angle of Internal Friction degrees

Poorly Graded Sands with Silt

(SP-SM)

117 0 33

Poorly Graded Silty Sands (SM) 117 0 32

Clays (CL and CH) 120 200 24

Gravel (Drainage Blanket / Toe Drain)

125 0 35

Clay FILL (CL and CH) 120 200 24

Rapid Drawdown Condition

The rapid drawdown analyses utilized the design soil parameters from both the end of construction (total stress) and steady state (effective stress) conditions. The computer program SLOPE/W evaluates the sliding resistance under both conditions, and uses a value close to the lower of the two calculated values for that portion of the assumed sliding surface. This comparison is performed for every soil “slice” within a given trial surface, and it is possible for both total stress and effective stress soil parameters to be utilized within the calculated critical failure surface. This type of analysis is often referred to as a multi-stage rapid drawdown analysis.

Summary

A review of the above indicates that for our analyses, the lower range of strength values for a given soil type was typically selected based on the above parameters in lieu of average values based on statistical analyses. The use of the lower range of strength values for design purposes was considered appropriate as levees must be designed for local, not average, soil conditions, as discussed in detail in the Risk section of the original Technical Memorandum. The exception was the lower range of blow counts recorded for some of the granular strata.

Some of the SPT blow counts at depth for this material were less than 5, suggesting these materials are weaker than we have assumed for design. However, published correlations2 between SPT and relative density would imply that for an angle of internal friction of 32 degrees, the soil’s relative density would be less than 30 percent. Based on the observed condition of the overlying soils during field exploration and sampling, we do not believe that in situ relative densities lower than 30 percent are likely, and consider the minimum likely angle of internal friction in granular soils to be 32 degrees at this site.

LEVEE SEEPAGE ANALYSIS

We performed a steady-state seepage analysis of proposed slope geometries using SEEP/W, a commercially available engineering program produced by Geo-Slope International which utilizes finite element analyses to evaluate ground water seepage. Our analyses included a geometry of 3:1 slopes on the riverside and 2.5:1 slopes on the landside where these are proposed by Tetra Tech. Otherwise the existing conditions were analyzed based upon final cross sections provided by Tetra Tech. These levees analyzed had a maximum height of 14.4 ft on the riverside and 10.9 ft on the landside.

The lateral boundaries of each model on the landside were located at a distance from the levee profile following the guidelines established in USACE DIVR 1110-1-4003. The full hydrostatic pressure associated with the design 100 year storm was assumed to act on the boundary on the riverside of each model boundary, while a total head equivalent to the ground surface elevation in the far field (as determined by the previously mentioned USACE DIVR guidelines) was assumed to act on the landside of each model boundary. Potential seepage face conditions were assumed at the landside ground surface and along the landside slope of the levee. The bottom boundary of each model was assumed to be 50 ft below the crest (maximum depth of soil exploration in borings associated with this report). No flow conditions were assumed at the bottom boundary. The analyses were performed assuming that the hydraulic properties of the soils, other than blanket soils, exhibited a horizontal permeability that was five times greater than the vertical permeability. The blanket soils were assumed to have a vertical permeability that was five times greater than the horizontal permeability.

A vertical exit gradient of 0.1 was predicted at the bottom of the Mercedes Concrete Canal at Station 748+80 under a dry canal condition during the 100 yr flood. However, the phreatic surface was predicted to daylight above the toe of the slope. Although the exit gradient is acceptable and this canal section is lined with concrete, we are recommending that a minimum of 1 ft of water be placed in this canal

2 “Penetration Tests and Bearing Capacity of Cohesionless Soils”, Meyerhof, G.G., JSMFD, ASCE, Vol. 82, SM 1, pp. 1-19.

3 USACE DIVR 1110-1-400, “Landside Seepage Berms for Mississippi River”, December 12, 1998.

between Stations 738+00 and 750+00 in order to prevent the phreatic surface from daylighting on the landside slope of the levee in this location.

As previously mentioned, the Mercedes Basin and the Mercedes Basin Canal are maintained full of water as long as the Mercedes Pump Station is pumping. Their normal water levels are above the 100-yr flood elevation and hence there are no seepage concerns for the adjacent levees.

In addition to the ponding requirements in the Mercedes Concrete Canal, there are locations in Reach 1 where the phreatic surface under steady state conditions is expected to daylight above the landside toe of the levee. In these locations it will be necessary to place an embedded toe drain in the levee section to lower the phreatic surface and prevent such daylighting. The results of these seepage analyses are provided in Appendix C and are summarized in the following table.

Reach 1 Seepage Results

Station Canal Condition Analyzed iexit at Toe Phreatic Surface

302+10 - Dry 0.05 Daylights at Levee Toe

748+80 Mercedes Concrete

Canal Dry 0.13 Daylights on Levee Slope 1 ft Above Canal Bottom

1 ft of Water in Canal 0.01 Daylights at Canal Water Surface

837+70 -

Dry 0.09 Daylights 2 ft Above Toe

Dry; Embedded Toe Drain Phreatic Surface Below Ground Surface in Landside Slope

A review of the geometry of the levee also indicates that there are various roadways that pass over it. In these locations, a toe drain will not be necessary. There is also a locations where an adjacent pond is at the landside toe of the levee section making the installation of a toe drain unnecessary. The following table summarizes the locations where toe drains are not required under these circumstances. An embedded toe drain will be required for Reach 1 from Station 800+00 to Station 0+43.30 (Cameron County), except for segments as indicated in the Table.

Stations Where No Toe Drain Is Required Station

Gap Distance (ft.) Description Begin End

800+00 801+55.49 155.49 Landside toe is outside of the

USIBWC ROW

LEVEE SLOPE STABILITY ANALYSIS

We performed end-of-construction, steady-state, and multi-stage rapid drawdown slope stability analyses of proposed slope geometries using SLOPE/W, a commercially available engineering program produced by Geo-Slope International which evaluates slope stability. Our analyses was based on the Spencer method, and included a slope declination of 3:1 (horizontal:vertical) for the riverside of the levee and 2.5:1 for the landside of the levee where new construction is planned, or was based on existing levee slopes where only minor construction activities are planned. For all slope stability evaluations, a uniform traffic surcharge load of 300 psf applied to the crest of the levee. The levee crest elevation for these analyses was taken from the levee profiles provided by Tetra Tech.

MINIMUM FACTOR OF SAFETY

For a given slope configuration, the forces that “drive” slope failure (including gravity and external loading conditions) are compared to the slope’s resistance to failure, which is a function of dewatering controls and internal shear strength (cohesion and internal angle of friction) of both the foundation soils and the fill soils utilized for construction of the levees.

For levee projects, the USACE has specified minimum safety factors against slope failure with respect to loading conditions. The minimum acceptable factors of safety for levees, provided in Table 6-1b on Page 6-5 of EM 1110-2-1913, are listed in the following table.

Condition Required Factor of Safety

End of Construction 1.3

Sudden (“Rapid”) Drawdown 1.0 to 1.2*

Long Term (Steady Seepage) 1.4

* Sudden drawdown analysis. FS = 1.0 applies to pool levels prior to drawdown for conditions where these water levels are unlikely to persist for long periods preceding drawdown. FS = 1.2 applies to pool level, likely to persist for long periods prior to drawdown. For this project, 1.0 is considered applicable.

We consider a significant slope failure to involve a volume of slope material that is large enough to substantially impair the serviceability or operation of the levee or that could imperil human life.

Shallow, sloughing slope failures that involve a relatively small volume of material or that can be repaired locally without substantially impacting the levee operations are considered to be minor slope failures and do not control the conclusions of our stability analyses.

RESULTS OF ANALYSES

The results of these analyses indicate that the slopes of the levee are not of primary concern. The following table contains a summary of the results from our slope stability analyses for loading conditions and slope configuration discussed above. A graphical presentation of the most critical failure surfaces from our SLOPE/W analyses can be found at the end of this Geotechnical Memorandum in Appendix C.

Computed Factors of Safety For Each Section Analyzed

End of Construction-

Riverside

End of Construction-

Landside

Steady State at Flood Stage –

Riverside

Steady State at Flood Stage –

Landside

Sudden Drawdown –

Riverside

Station 302+10

1.8 2.1 2.2 2.1 1.8

Station 748+801

1.9 1.6 3.3 2.3 1.9

Station 837+702

1.3 1.5 2.8 2.4 1.4 1 Requires 1 ft of water in the Mercedes Concrete Canal at this location 2 Requires embedded toe drain

Based on the results of our analyses, the factors of safety computed for riverside and landside slopes of 3:1 (horizontal: vertical) and 2.5:1, respectively, or flatter meet the USACE requirements for minimum safety factors for the assumed flood event based off of the improved levee configurations. However, as discussed in the seepage analyses section of the report, ponded water to 1 ft of depth will be required in the Mercedes Concrete Canal between Stations 738+00 and 750+00 and an embedded toe drain will be required in a portion of Reach 1 between Station 800+00 and 0+43.30 (Cameron County) except at specific locations as presented the Levee Seepage Analysis section. These additional control measures will be necessary to prevent isolated slope and/or seepage failures.

POWER POLES BETWEEN STATIONS 714+88.17 AND 737+55.02

Existing power transmission poles are located along the levee section adjacent to the Mercedes Basin Canal at or near the crest between Stations 714+88.17 and 737+55.02. If power transmission poles are toppled during a storm event, these can pose a threat to the stability of the levee cross section if they create holes deep enough to compromise the crest of the levee, or if they are located along the landside slope or in the landside toe area and their tips are deep enough to penetrate into a seepage layer.

In this particular location, all of these poles are located along the crest or on the riverside slope. Based upon information provided by Tetra Tech, the tips of these poles are embedded approximately six (6) feet below ground surface. A comparison of the ground surface elevation at each pole location and the corresponding soil boring information indicates that the closest that the tip of one of these poles gets to and underlying seepage layer is approximately 16 ft.

Given that the levee crest width (including the access road along the Mercedes Basin Canal) ranges from approximately 40 ft to 60 ft wide, and that the tips of these power poles are at least 16 ft above the underlying seepage layer, there is very little concern for the stability of the levee during a flood event if one or more of these is toppled under high winds.

SETTLEMENT

In general, the on-site soils are overconsolidated as evidenced by the geologic history of the area and the pre-consolidation pressures observed as part of the laboratory one-dimensional consolidation tests performed for the original geotechnical study. That is, the native soils have experienced a stress history greater than the overburden pressures that will be imposed by the planned improvements.

For our original study, settlement estimates were prepared for four case conditions based on the underlying subsurface soil conditions. The method used for estimating settlement was based on NAVFAC DM-7.1, which references Osterberg’s stress distribution theory for earthen embankments. We also used a commercially available computer program, Settle3D.

On the basis of our review of the conditions encountered along Subreach 1, where fill is to be added to the riverside slope, maximum estimated settlements of 3 to 3 1/2 in. were calculated. It is our understanding that the new levee construction will include a 4 in. overbuild to account for settlement.

Additionally, the design includes a 3 ft. freeboard. Consequently, settlement is considered to be a low risk for this levee segment. The original report should be reviewed for more detail on the settlement calculations.

STRUCTURE REVIEW

DRAINAGE STRUCTURES

Multiple drainage structures that penetrate the levee are located in Subreach 1. These structures are quite old and some of them have been abandoned and/or removed. Although a detailed seepage and slope stability analysis of these structures was not included in the study scope, RKCI was asked to review the as-built drawings to determine if obvious concerns existed with regard to the as built conditions.

In general, it appears that most if not all of the structures were constructed over 50 years ago. Although some structures were “buried”, immediately identifiable seepage or stability concerns were not observed other than the common use of seepage collars, which is discussed further below. On the basis of the age of the structures and our understanding that significant seepage or stability problems have not been observed to date, RKCI recommends that all of the structures be observed during and immediately following flood events to determine if remediation measures and/or additional analyses are required in the future.

As referenced above and in conversations with the USIBWC, our review of the as-built drawings indicate that seepage collars were utilized as a seepage control measure for a significant majority, if not all of the structures within Reach 1. It is our understanding that the US Army Corps of Engineers does not accept seepage collars as an acceptable seepage control measure. However, the USIBWC has decided that existing seepage structures that are still in use and that have seepage collars will remain in place and require no further evaluation. RKCI recommends that if there are drainage structures that are no longer in service, these should be properly sealed and abandoned in place.

ROADWAY CONDITIONS

It is our understanding that several roads utilized by the Border Patrol run perpendicular to and over the levee along this reach and that some of these were constructed by DHS without consulting the USIBWC staff. The preferred recommendation would be that these roads be closed. However, if they remain operational, it is our understanding that rutting conditions were observed at several of these crossings.

RKCI recommends that where unauthorized ramps exist, these be removed and that the levee faces be repaired. Provided the rutting is less than 24 in. in depth, the repairs can be made by excavating the rutted areas and recompacting soils on the existing slope. RKCI further recommends that authorized ramps be graded and that flex base be placed on them to reduce the risk of future rutting.

CONCLUSIONS

The results of our original and current analysis indicate seepage, settlements and slope stability are considered to be low risk for the proposed reconstructed levee. However, the phreatic surface is expected to daylight above the bottom of the canal on the landside slope of the levee adjacent to the Mercedes Concrete Canal and between Stations 800+00 and 0+43.30 (Cameron County). A toe drain must be installed between Stations 800+00 and 0+43.30 and the Mercedes Concrete Canal between Stations 738+00 and 750+00 must be maintained with at least 1 ft of water to prevent isolated slope and/or seepage failures during flooding events. Provided the USIBWC develops new operational programs to ensure the filling of this canal, and the remaining geotechnical recommendations presented herein are incorporated into the design and construction, the reconstructed levees will satisfy 44CFR65.10 criteria from a geotechnical perspective.

The following table summarizes all of the geotechnical recommendations presented in this report.

Summary of Final Recommendations

Reach 1 Station Canal or Basin Recommendation

199+70.94 to 307+66.36 - No specific geotechnical recommendations

639+00 to 680+00 Mercedes Basin Watch wave erosion from Mercedes Basin and repair if it extends more than 20 ft beyond present condition.

680+00 to 738+00 Mercedes Basin Canal No specific geotechnical recommendations

738+00 to 800+00 Mercedes Concrete Canal

Maintain at least 1 ft of water in canal during a flood event between Stations 738+00 and 750+00.

801+55.49 to 0+43.30 (Cameron County) - Toe drain required on landside toe of levee between Stations 801+55.49 and 0+43.30 (Cameron County)

CONSTRUCTION RECOMMENDATIONS

GENERAL CONSTRUCTION CONSIDERATIONS

As clay will be utilized as an engineered fill, it is important for the clay fill and existing clays be permitted to develop some mechanical bond. Typically, the existing embankment face is excavated so that a series of keys and benches are cut into the slope face. The keys and benches reduce the risk of formation of a preferential failure surface at the old/new fill interface. Bench heights should be limited to 3.5 ft. or less.

In all areas of the levee, including the levee faces, the upper 6 in. of material is anticipated to be topsoil and should be removed prior to new embankment construction. The remaining levee embankment materials can be stockpiled and reutilized as fill if they meet the requirements of the Clay Fill Levee Materials presented in a subsequent section of this report.

Additionally, it is our understanding that the caliche base and an additional 6 in. of levee embankment materials will be removed prior to new levee construction

CLAY FILL LEVEE MATERIALS

Low to moderate plasticity clay soils may be used to protect the existing embankment soils from the effects of erosion as well as limit seepage through the embankment. The clay soils should be clean and free of trash, organics, hazardous compounds, asbestos or other deleterious materials. The clay soils should be rated as Grade 1 (non-dispersive) when tested in accordance with ASTM D 6572 (“Crumb Test”) and either ND1 or ND2 as evaluated using ASTM D 4647 (“Pin Hole Test”). The materials should be classified as CL, CL-ML and/or SC in accordance with the Unified Soil Classification System (USCS). Soils classified as PT, CH, OL, OH, ML, MH, sands (other than SC) or gravels should not be permitted. These materials should have a Plasticity Index in the range of 20 to 30, with a maximum Liquid Limit of 45.

The surface of the previous lift should be scarified just prior to fill placement. The clays should be compacted in relatively thin, horizontal lifts with a maximum loose lift thickness of 6 inches. The project site is in a region where dry, hot conditions can exist for extended periods. The soils must be maintained in a moist condition at all times and properly compacted in a timely, workmanlike manner. The soil should be compacted to at least 95 percent of optimum density as determined by ASTM D1557 at a moisture content that is within two percent of optimum moisture. Once placed, the compacted materials should be maintained in a moist condition until the next lift is placed. The compacted mixture should be protected from freezing, although the probability of prolonged, significant freezing conditions in Hidalgo and Cameron Counties is considered unlikely. Any soil materials that are damaged, disturbed, or desiccated after placement should be removed and replaced with like materials. Some degree of desiccation cracking should be anticipated for clay fill materials.

Samples of the fill soils should be taken at a minimum rate of one set per 10,000 square feet of compacted fill with a minimum of one test per lift, and checked for plasticity and crumb test grade. Samples should be taken every 50,000 square feet and checked for non-dispersivity by the pin hole test. Any materials that do not meet the recommended crumb test and pin hole test criteria outlined above should be removed and replaced with suitable soils.

VEGETATIVE COVER FOR CLAY LEVEES

The recommendations presented in Sections 3-1 and 3-2 of the USACE ETL 1110-2-571 titled “Guidelines for Landscape Planting and Vegetation Management at Floodwalls, Levees, and Embankment Dams,” should be incorporated into the design. Additionally, RKCI recommends that sod and/or vegetative cover not be constructed over the exposed faces of the proposed toe drain.

FLEX BASE

Flex base should meet the gradation and plasticity requirements of TxDOT Item 247 Type A or C, Grade 3.

Blending of 2 or more sources is allowed to achieve the following physical characteristics.

Type A or C, Grade 3:

Grading Requirements

U.S. Standard Sieve Size Percent Retained

2 ½ inch 0

1 ¾ inch 0-10

No. 4 45-75

No. 40 50-85

Max Liquid Limit 40

Max Plasticity Index 12

Base course should be placed in lifts with a maximum thickness of 8 inches and compacted to a minimum of 95 percent of the maximum density at a moisture content within the range of 2 percentage points below to 2 percentage points above the optimum moisture content as determined by ASTM D 698.

TOE DRAIN MATERIAL

Toe Drain Material shall be concrete aggregate in accordance with requirements of aggregate gradation in ASTM C33-08 to meet ASTM 33 Table 2, Size Numbers 57, 67 or 357. The gravel must be separated from the underlying subgrade utilizing a geotextile filter fabric, such as Mirafi 160N or approved equal.

The following Appendices are attached and complete this revised supplemental report:

Field Data ...................................................................................................................................... Appendix A Laboratory Test Results ................................................................................................................ Appendix B Slope Stability and Seepage Analyses ........................................................................................... Appendix C

APPENDIX A

FIELD DATA

!?DP

-30

DP

-31

DP

-31

DP

-31

DP

-31

DP

-30

DP

-30

DP

-30

DP

-30

DP

-30

DP

-30

DP

-30

DP

-30 by R ab a-K ist ne r C on su lta nts , In c.

c

BORING LOCATION MAP GEOTECHNICAL MEMORANDUM - SUBREACH 1

THE LOWER RIO GRANDE FLOOD CONTROL PROJECT

LEVEE SYSTEM - DONNA PUMP TO BROWNSVILLE LEVEE REACH

HIDALGO COUNTY, TEXAS

RE

VIS

IO

NS

No

DA

TE

DE

SC

RI

PT

IO

N

PR

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EC

T N o.:

AM

A0 8-1

-07

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BY

ISS

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AT

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KE

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BY

MJ

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7/2 7/2

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S

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: T his

D raw ing is

Pr ov ide d f or Illu str ati on O nly , M ay N ot be to S ca le an d i s N ot

Su ita ble fo r D es ign or

C on str uc tio n P urp os es

FIG

UR

E 1a

SOURCE: AERIAL PROVIDED BY BING (2011)

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DP

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-31

DP

-31

DP

-31 by R ab a-K ist ne r C on su lta nts , In c.

c

BORING LOCATION MAP GEOTECHNICAL MEMORANDUM - SUBREACH 1

THE LOWER RIO GRANDE FLOOD CONTROL PROJECT

LEVEE SYSTEM - DONNA PUMP TO BROWNSVILLE LEVEE REACH

HIDALGO COUNTY, TEXAS

RE

VIS

IO

NS

No

DA

TE

DE

SC

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PR

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T N o.:

AM

A0 8-1

-07

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Pr ov ide d f or Illu str ati on O nly , M ay N ot be to S ca le an d i s N ot

Su ita ble fo r D es ign or

C on str uc tio n P urp os es

FIG

UR

E 1b

SOURCE: AERIAL PROVIDED BY BING (2011)

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Flexible Base Material (FBM) - 8 inches

FILL: FAT CLAY (CH)

hard to very stiff, brown, with calcareous nodules

- with gravel below a depth of about 2-1/2 ft

- with orange ferrous stains below a depth of about 7 ft

FAT CLAY (CH)

very stiff to hard, brown

- becomes light brown in color below a depth of about 16 ft

- with black ferrous stains, and gypsum nodules and crystal below a depth of about 18 ft

- becomes brown in color below a depth of about 20 ft

LEAN CLAY (CL)

stiff, light brown, with black ferrous stains

LOG OF BORING NO. DP-301

PL

AS

TI

CI

TY

IN

D

EX

SURFACE ELEVATION: 87.9 ft

Straight Flight Auger & Mud Rotary

-2

DRILLING

METHOD: LOCATION:

PLASTIC

LIMIT

LIQUID

LIMIT

WATER

CONTENT

BL

O

W S

PE

R

FT

10 20 30 40 50 60 70 80

DESCRIPTION OF MATERIAL

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

SHEAR STRENGTH, TONS/FT2

U N

IT

D

RY

W

EI

G

H T, p cf

See Figure 1

N O

TE

: T

H

ES

E

LO

G S

SH

O

U

LD

N O

T

BE

U

SE

D S

EP

AR

AT

EL

Y

FR

O M

T H

E

PR

O

JE

CT

R

EP

O

RT

DEPTH DRILLED:

DATE DRILLED:

DEPTH TO WATER:

DATE MEASURED:

SY

M

BO

L

SA

M

PL

ES

Lower Rio Grande Flood Control Project Levee System Donna Pump to Brownsville - Subreach 1

Hidalgo County, Texas

29.5 ft 8/8/2012

D

EP

TH

, F

T

50.0 ft 8/8/2012

AMA08-115-07

A-2a

PROJ. No.:

FIGURE:

TBPE Firm Registration No. F-3257

LEAN CLAY (CL)

stiff, light brown, with black ferrous stains

(continued)

- with sand below a depth of about 40 ft

- with gravel below a depth of about 45 ft

Boring terminated at a depth of about 50 ft.

NOTES:

1. Free water was first observed at about

30 feet; after 20 minutes a depth to water level of about 29.5 feet was observed.

2. Mud rotary methods used after a depth of about 30 feet.

3. Backfilled with cement bentonite grout.

4. The boring elevations and coordinates were provided to us on Thursday, October 18, 2012, by Tetra Tech, Inc.

LOG OF BORING NO. DP-301

PL

AS

TI

CI

TY

IN

D

EX

SURFACE ELEVATION: 87.9 ft

Straight Flight Auger & Mud Rotary

-2

DRILLING

METHOD: LOCATION:

PLASTIC

LIMIT

LIQUID

LIMIT

WATER

CONTENT

BL

O

W S

PE

R

FT

10 20 30 40 50 60 70 80

DESCRIPTION OF MATERIAL

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

SHEAR STRENGTH, TONS/FT2

U N

IT

D

RY

W

EI

G

H T, p cf

See Figure 1

N O

TE

: T

H

ES

E

LO

G S

SH

O

U

LD

N O

T

BE

U

SE

D S

EP

AR

AT

EL

Y

FR

O M

T H

E

PR

O

JE

CT

R

EP

O

RT

DEPTH DRILLED:

DATE DRILLED:

DEPTH TO WATER:

DATE MEASURED:

SY

M

BO

L

SA

M

PL

ES

Lower Rio Grande Flood Control Project Levee System Donna Pump to Brownsville - Subreach 1

Hidalgo County, Texas

29.5 ft 8/8/2012

D

EP

TH

, F

T

50.0 ft 8/8/2012

AMA08-115-07

A-2b

PROJ. No.:

FIGURE:

TBPE Firm Registration No. F-3257

Flexible Base Material (FBM) - 4 inches

FILL: FAT CLAY (CH)

hard to very…