HidalgoProtLeveeIIGeotech.pdf

FINAL

GEOTECHNICAL INVESTIGATIONS AND ASSESSMENT

TECHNICAL REPORT

Improvements to the Hidalgo Protective Levee (Phase II)

From Approximately 0.5 Miles East of the Hidalgo Pumphouse To Just West of the Southbound Hidalgo –Reynosa International Bridge

Near the City of Hidalgo, Hidalgo County, Texas

Prepared for:

United States Section, International Boundary & Water Commission

Prepared by:

Project Manager: Daniel O. Rios, PE Deputy Project Manager: Sharlotte L. Teague, PE

In Cooperation With:

Contract No. IBM05D0001 Task Order No. IBM05T0025

July 31, 2008

Lower Rio Grande Flood Control Project Rehabilitation of Hidalgo Protective Levee (Phase II)

Contract No. IBM05D0001 Order No. IBM07T0025 i

ACKNOWLEDGEMENT / CERTIFICATION

The preparation of this document included the collaborative participation of the following parties:

US Section, International Boundary & Water Commission

• Colleen Jaye Elliott Contracting Officer El Paso

• Raymundo Aguirre, PhD, PE Technical Manager El Paso

Contractor Team

• Daniel O. Rios, PE Project Manager S&B, McAllen

• Sharlotte L. Teague, PE Deputy Project Manager S&B, McAllen

• Michael C. Riojas, PE Project Engineer S&B, San Antonio

• Ricardo Garcia, PE Project Engineer S&B, McAllen

• Manching Wei, PE Structural Engineer S&B, Houston

• Chris L. Schultz, PE Project Manager (Geotech) R-K, San Antonio

• John A. Focht III, PE Chief Geotechnical Engineer R-K, San Antonio

• Katrin Miovski, PE Geotechnical Engineer R-K, McAllen

All geotechnical sampling, testing, evaluations and engineering analysis for this document were prepared by Raba-Kistner Consultants, Inc.;

signed & sealed studies are included in the Appendices of this Technical Report.

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TABLE OF CONTENTS

Page

Purpose of Project I.1 Introduction / Background ……………………………...……… 1 I.2 Project Description ………………..…………………………. 2 I.3 Scope of Work ………………..…………………………... 2 I.4 References ………………………………………………… 4

Sampling and Testing II.1 Field Exploration and Sampling…………………………….….. 5

II.2 Testing……………………………………………….………….. 6

Site Conditions

III.1 Geology……………………………………………….…….….. 6 III.2 Stratigraphy……………….…….………………….………….. 7 III.3 Groundwater……………….…….………………….………….. 10

Engineering Analysis & Design Recommendations IV.1 Proposed Raise Levee and New Levee……….……….…….….. 10

IV.2 Floodwall…….…………….…….………………….………….. 12 IV.3 Intake Channel Crossing…….…….………………….………….. 13

“Geotechnical Engineering Study for Proposed Rehabilitation Improvements – Phase II (April 9, 2008)” by Raba-Kisnter Consultants, Inc.

Borehole Location Map

“Supplemental Recommendations to Geotechnical Engineering Study, Seepage Cutoff Wall and Drainage Blanket Recommendations, April 13 & 16, 2008” by Raba-Kistner Consultants, Inc.

Study, Intake Channel Crossing Recommendations, April 11 and 25, 2008” by Raba-Kistner Consultants, Inc.

Section I

Section II

Section III

Section IV

Appendix A

Appendix B

Appendix C

Appendix D

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Section I Purpose of Project

I.1 Introduction / Background The Lower Rio Grande Flood Control Project (LRGFCP) extends from the town of Peñitas, Texas at its upstream end, to twenty-eight (28) miles above the Gulf of Mexico, a distance of about one hundred fifty-eight (158) river miles. The LRGFCP has two diversion dams, Anzalduas Dam and Retamal Dam, approximately seventeen (17) and fifty-seven (57) river miles below Peñitas, which divert portions of the flood flows into the United States (US) Main Floodway and the Mexico Floodway, respectively. The LRGFCP was constructed to provide flood protection to urban, suburban, and irrigated lands in the Rio Grande delta in both the US and Mexico. Each country designed and constructed the portion of the project located in its territory.

The design flood for the LRGFCP is based on a flow in the Rio Grande of two hundred fifty thousand (250,000) cubic feet per second (cfs) at Rio Grande City, which attenuates to two hundred thirty-five thousand (235,000) cfs at Peñitas. Downstream of the diversion into the US Main Floodway, the design flood flow is one hundred thirty thousand (130,000) cfs from above Anzalduas Dam to just above the Hidalgo-Reynosa International Border Crossing, and one hundred twenty-five thousand (125,000) cfs from there on downstream to the point of diversion into the Mexico Floodway. During flood operations, Anzalduas Dam and Retamal Dam will each divert up to one hundred five thousand (105,000) cfs into the US and Mexico, respectively. Flow diversion during the flood operations will limit flood flows below Retamal Dam to twenty thousand (20,000) cfs.

The Hidalgo Protective Levee has been identified as one of the LRGFCP priority areas to improve flood containment. The existing Hidalgo Protective Levee was constructed in the 1970’s and extends from River Mile 155.7 to approximately River Mile 160.8, in the vicinity of the City of Hidalgo, in Hidalgo County, Texas. The Hidalgo Protective Levee System extends from approximately one-half (1/2) mile east of the Hidalgo Historic Pumphouse to the south levee of the Main Floodway (also know as the Common Levee) on the west side of the City of Hidalgo for a distance of four and a half (4.5) miles. Currently, the Hidalgo Protective Levee does not meet design criteria for the design flood event. Proposed improvements to the Hidalgo Protective Levee are planned to be constructed in two phases, each covering separate geographical reaches of the Hidalgo Protective Levee:

• Phase I encompasses the upstream three and three-tenths (3.3)-mile reach of the levee system, from the west boundary of the Hidalgo-Reynosa International Border Crossing northwest to the junction with the US Main Floodway.

• Phase II will include improvements from approximately one-half (1/2) mile east of the Hidalgo

Historic Pumphouse to just west of the Southbound Hidalgo –Reynosa International Bridge, for a total reach of one and two-tenths (1.2) miles.

This Geotechnical Investigations and Assessment Technical Report is for Phase II.

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The United States Section of the International Boundary and Water Commission (USIBWC) conducted a hydraulic model study which was reported in the 2003 document entitled, “Hydraulic Model of the Rio Grande and Floodways Within the Lower Rio Grande Flood Control Project (USIBWC, 2003)”. The study updated findings of a prior 1992 study by incorporating new structures and geometrical data, as well as changes associated with land use and agricultural practices and increased reliability of the hydraulic model with enhanced software capabilities. The hydraulic study indicated for the Hidalgo Protective Levee System, that an increase in levee height, ranging from four

(4) to nine (9) feet, would be required to meet design criteria for flood protection. These criteria included a levee freeboard of three (3) feet above anticipated water level during the design flood event.

In addition to the flood containment evaluation, an assessment of the levee system structural integrity was conducted for the Government in 2003 by the Engineering Research and Development Center of the US Army Corps of Engineers (ERDC - USACE). No structural deficiencies were reported for the existing Hidalgo Protective Levee (USACE 2003). Additionally, a finding of no significant impact was issued for the “Final Environmental Assessment, Hidalgo Protective Levee System: Alternatives for Improved Flood Control”.

I.2 Project Description: Rehabilitation Improvements to the Hidalgo Protective

Levee (Phase II) On July 11, 2007 a task order was issued by the USIBWC to S&B Infrastructure, Ltd. (S&B), as Contractor, to perform final design tasks for the rehabilitation improvements for Phase II of the Hidalgo Protective Levee. The majority of the proposed improvements under this task order involved improvements to approximately 1.2 miles of the Hidalgo Protective Levee, and included increasing the elevation of the existing earthen levee from an average elevation of 110 feet to 117.5 feet.

I.3 Scope of Work A general description of the work performed by S&B for the development of the Project included the following:

I.3(a) Geotechnical.

(1) Investigation. Conducted additional geotechnical investigations along the existing levee east of the Hidalgo Historic Pumphouse and the proposed location for the new levee along the south bank of the old intake channel to assess the need for construction of a barrier for control of seepage, as recommended in the report titled “Geotechnical Engineering Study for Proposed Rehabilitation Improvements – Phase II Hidalgo Protective Levee System” previously prepared by Raba-Kistner Consultants, Inc. (RKCI), as subcontractor to S&B.

(2) Analysis & Recommendations. Performed analysis and prepared a technical report containing the findings of the geotechnical investigations, an assessment of the potential for occurrence of seepage that may compromise the stability of the levees, and a recommendation on the need to provide a barrier or other works to control seepage.

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I.3(b) Engineering Design.

(1) Raise Existing Levee. Prepared engineered designs for raising the top of the existing levee as required at different points along the levee, from approximately one-half (1/2) mile east of the Hidalgo Historic Pumphouse to the east end of the Intake Channel (Sta. 0+00 to approximately Sta 25+00). The proposed top-of-levee elevations were provided by the USIBWC. The proposed levee was designed with side slopes of 3:1 (horizontal : vertical) or steeper, except that in no case are the side slopes steeper than 2.5:1. Footprint parameters included providing at least fifteen (15) feet between the right-of-way line and the limit-of-fill line on either side of the levee. The fill for raising the levee was located with its centerline coinciding with the existing levee centerline, when available right-of-way allowed. Otherwise, the centerline of the fill was placed with its centerline shifted towards or away from the river, as the existing right-of-way allowed. If, at any point along the levee, the existing right-of-way was insufficient even with fill side slopes of 2.5:1, additional right-of-way would be obtained by the USIBWC.

The crown of the existing levee incorporated a maintenance road and the reconstruction of the hike and bike trail with hot mix asphalt concrete pavement (HMACP) surfacing, totaling 18 feet in width, along the length of levee that overlaps an existing trail; a hike and bike trail transition was also designed from the top of the levee to the existing trail just east of the Hidalgo Historic Pumphouse. For the continuation of the levee west of the transition, the crown of the levee was designed as a maintenance road with a 6-inch caliche surfacing course 16 feet in width.

(2) New Levee. Prepared engineered designs for construction of a new levee from near the east end of the Intake Channel (approximately Sta. 25+00) for approximately one-half (1/2) mile (approximately Sta. 50+00) and then north across the intake channel to tie to the existing levee on the north side of the Intake Channel (Sta. 54+06). The design includes details for the tie-in with a proposed floodwall and for ramps from the top of the new levee east to the existing levee and west to the existing roadway with maximum slope of six percent (6%). The design of the Intake Channel Crossing, includes a gated culver structure to allow exchange of normal flows between the Rio Grande and the Intake Channel and to control flood flows in to the Intake Channel. The control gate was designed for a water surface elevation on the Rio Grande side at the LRGFCP design flood and on the Intake Channel side at normal Rio Grande flow. The proposed top-of-levee elevations were provided by the USIBWC. The proposed levee was designed with side slopes of 3:1 (horizontal : vertical) or steeper, except that in no case are the side slopes steeper than 2.5:1. Footprint parameters included providing at least fifteen (15) feet between the right-of-way line and the limit-of-fill line on either side of the levee. The crown of the levee was designed as a maintenance road with a 6-inch caliche surfacing course 16 feet in width.

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(3) Proposed Floodwall. Performed a preliminary analysis and prepared engineered design for a new proposed floodwall at the bridge crossings, from the tie-in at the Intake Channel (Sta. 54+06) to the tie-in to Phase I of the Hidalgo Protective Levee. The proposed new floodwall is located approximately 8 feet in front of the existing floodwall location (on the river side). It incorporates seepage control works in accordance with the recommendations from the “Geotechnical Engineering Study for Proposed Rehabilitation Improvements – Phase II (April 9, 2008)” by RKCI, and is included as Appendix A to this Technical Report. Additional discussions regarding the specifics on the seepage control works is included in Section IV.2 herein.

(4) Removable Flood Barriers. Prepared engineered designs for removable flood barriers to seal off spaces in the floodwall at the intersections of the floodwall line and the Northbound and Southbound Hidalgo-Reynosa International Bridges.

(5) Other Engineering / Design. Prepared engineered designs for existing drainage structure at Sta. 54+06; prepared construction drawings for all proposed improvements; prepared technical specifications; and prepared opinions of probable construction cost.

(6) Right of Way Mapping / Field Survey. Established right of way lines (existing and proposed); prepared strip map showing existing right of way, proposed right of way, approximate amount of surface area in each, and the names, addresses and telephone numbers of the owners; as well as, the existing levee centerline, the proposed raised levee centerline, the proposed levee-raising limit-of-construction lines, the proposed new levee centerline, the proposed new levee and crossing toe lines, and the floodwall limits of construction, as applicable; prepared metes and bounds; and obtained by field survey existing cross-sections every 500 feet .

I.4 References and Definition of Acronyms The following records and information were reviewed and provided for background:

(a) IBWC Minute No. 238, September 10, 1970, “Improvement of the International Flood Control Works of the Lower Rio Grande”

(b) USIBWC 2003 Hydraulic Study for LRGFCP

(c) USACE ERDC Levee Assessment Data:

(1) LIDAR Point Data

(2) LRGFCP Condition Assessment Plates

(3) “Assessment of U.S. International Boundary and Water Commission, LRGV Levees, South Texas”, dated October 2003

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(4) “Supplement to Condition Assessment of U.S. International Boundary and Water

Commission, LRGV Levees”, dated June 2004

Although some acronyms have been defined above, Table I-1 on the below provides a consolidated summary of terms used throughout this report.

Acronym Definition

ERDC Engineering Research Development Center (H:V) Slope, horizontal to vertical LRGFCP Lower Rio Grande Flood Control Project psf pounds per square foot PVR Potential Vertical Rise UU unconsolidated-undrained USACE US Army Corps of Engineers USFWS US Fish and Wildlife Service USIBWC United States Section, International Boundary and Water Commission

Table I-1: Definition of Terms

Section II Sampling and Testing

II.1 Field Exploration and Sampling As identified in the “Geotechnical Engineering Study for Proposed Rehabilitation Improvements – Phase II (April 9, 2008)” by RKCI, which is included as Appendix A to this Technical Report, geotechnical exploration within the Project limits included 19 soil borings to depths ranging from 22 to 100 feet below existing grades, as shown in Table II-1 on the next page. Boring locations were determined in the field with preliminary location maps provided by S&B and in coordination with the USIBWC. The borings were conducted utilizing straight flight augers in combination with mud rotary drilling techniques and were backfilled with the auger cuttings following completion of each day’s drilling operations. Sampling methods included Split-Spoon (with Standard Penetration Test, SPT) and Shelby Tubes.

A borehole location map is included in Appendix B.

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Area Number of Borings

Depth, feet Boring Identification

Floodwall

– International Bridge 5 100 B-101 thru B-105

New Levee Along South Bank 2 40 B-108 and B-111 of Old Intake Channel 6 22

B-106, B-107, B-109, B-110, B-112, and B-113

Existing Levee 2 40 B-115 and B-118

Existing Levee 4 22 B-114, B-116, B-117, and B-119

Table II-1: Borehole (Number, Depth, Location)

II.2 Testing Each sample was evaluated and visually classified in general accordance with the Unified Soil Classification System (USCS). Laboratory testing included natural moisture content, Atterberg Limits, Percent Passing No. 200 Sieve, Sieve Analysis, Unconfined Compressive Strength, Dry Unit Weigh, Unconsolidated-Undrained (UU) Triaxial, Consolidation, and Specific Gravity. With the exception of the Sieve Analysis, UU Triaxial, Consolidation, and Specific Gravity test results, the results of the field and laboratory tests are presented in the boring logs illustrated in Figures 2 thru 20 of the “Geotechnical Engineering Study for Proposed Rehabilitation Improvements – Phase II (April 9, 2008)” by RKCI, which is included as Appendix A to this Technical Report.

Section III Site Conditions

III.1 Geology From the “Geotechnical Engineering Study for Proposed Rehabilitation Improvements – Phase II (April 9, 2008)” by RKCI, which is included as Appendix A to this Technical Report, it states that the project site’s geology is as follows:

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

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(b) “Soil Survey of Hidalgo county, Texas”, published by the US Department of Agriculture –

Soil Conservation Service (USDA SCS), in cooperation with the Texas Agricultural Experiment Station, the project site appears to be located with the Hidalgo soil Association consisting of deep, moderately, permeable soils that typically have a dark grayish-brown, sandy clay loam layer. The corresponding soil symbols appear to be 6, Camargo silty clay loam; 55, Reynosa silty clay loam; 62, Rio Grande silt loam; and 68, Urban land.

III.2 Stratigraphy RKCI categorized the project site’s subsurface stratigraphy into four (4) generalized strata; Stratums I thru IV as follows:

(a) Stratum I – consists of brown to light brown; firm to hard; fat clay soils; lean clay soils; lean clay soils with sand; sandy lean clay soils; sandy, silty clay soils; silt soils with sand; and sandy silt soils; with calcareous nodules and black and orange ferrous stains. This layer was noted for the borings identified in Table III-1(A); and soil properties for this stratum are identified in Table III-1(B).

STRATUM I

Bore

Depth Range

B-101 thru B-104 From 0 extending to 10 ~ 35 feet B-105 From 0 extending to 2 feet, from 5 extending to 13 feet

B-106, B-107 From 0 extending to termination depth B-108 thru B-112 From 0 extending to 10 ~ 35 feet

B-113, B-114 From 0 extending to termination depth B-115 From 0 extending to 10 ~ 35 feet B-118 From 0 extending to 10 ~ 35 feet

Table III-1(A): Borings Containing Stratum I

STRATUM I

Property

Value Moisture Content 1% to 31%

Plasticity Non-plastic to plastic; PI = 6% to 34% SPT N-Values 4 blows to 33 blows per foot of penetration

USCS Soil Classifications CH, and/or CL, and/or CL-ML, and/or ML

Table III-1(B): Stratum I Properties

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(b) Stratum II – consists of brown to light brown, loose to very dense, poorly-graded sand soils;

poorly-graded sand soils with silt; silty sand soils; clayey sand soils; and clayey, silty sand soils with orange ferrous stains and clay lenses. This layer was noted for the borings identified in Table III-1(C); soil properties for this stratum are identified in Table III-1(D).

STRATUM II

B-101 thru B-104 Beneath Stratum I, extending to 35 ~ 53 feet from surface

B-105 Intermixed with Stratum I, from 2 extending to 5 feet, from 13 extending to 52 feet B-108 thru B-110 Beneath Stratum I, extending to termination depth

B-111 Beneath Stratum I, extending to 35 ~ 53 feet from surface B-112 Beneath Stratum I, extending to termination depth B-115 Beneath Stratum I, extending to termination depth B-118 Beneath Stratum I, extending to termination depth

Table III-1(C): Borings Containing Stratum II

STRATUM II

Moisture Content 1% to 49% Plasticity Non-plastic to moderately plastic; PI = 7% to 15%

Percent Passing No. 200 Sieve Percent Fines, 4% to 45% SPT N-Values 5 blows to 50 blows per foot of penetration

USCS Soil Classifications SP, and/or SM, and/or SC, and/or SC-SM

Table III-1(D): Stratum II Properties

(c) Stratum III – consists of brown to light brown, firm to hard, fat clay soils; fat clay soils with sand; lean clay soils; and sandy lean clay soils. This layer was noted for the borings identified in Table III-1(E); soil properties for this stratum are identified in Table III-1(F).

STRATUM III

B-101 thru B-102 Beneath Stratum II, extending to 77 ~ 90 feet from surface

B-103 Beneath Stratum II, extending to 60 feet from surface; from 90 feet to termination depth B-104 Beneath Stratum II, extending to 63.5 feet from surface; from 75 feet to 95 feet B-105 Beneath Stratum II, extending to 77 ~ 90 feet from surface B-111 Beneath Stratum II, extending to termination depth

Table III-1(E): Borings Containing Stratum III

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STRATUM III

Moisture Content 23% to 38%

Plasticity Moderately plastic to plastic; PI = 11% to 39% Percent Passing No. 200 Sieve Percent Fines, 65% to 99%

SPT N-Values 7 blows to 50 blows per foot of penetration USCS Soil Classifications CH and/or CL

Table III-1(F): Stratum III Properties

(d) Stratum IV – consists of brown, medium dense to very dense, poorly-graded sand soils;

poorly-graded sand soils with silt; and clayey sand soils. This layer was noted for the borings identified in Table III-1(G); soil properties for this stratum are identified in Table

III-1(H).

STRATUM IV

B-101 thru B-102 Beneath Stratum III, extending to termination depth B-103 Intermixed with Stratum III, from 60 extending to 90 feet B-104 Intermixed with Stratum III, from 50 extending to 63.5 feet; from 95 feet to term depth B-105 Beneath Stratum III, extending to termination depth

Table III-1(G): Borings Containing Stratum IV

STRATUM IV

Moisture Content 21% to 35%

Plasticity Non-plastic; two PI’s could not be determined Percent Passing No. 200 Sieve Percent Fines, 11% to 42%

SPT N-Values 25 blows to 50 blows per foot of penetration USCS Soil Classifications SP, and/or SC

Table III-1(H): Stratum IV Properties

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III.3 Groundwater Groundwater was encountered in the exploratory borings during drilling operations, as tabulated in Table III-2 below.

Boring Identification

Approximate Groundwater Depth Encountered During Drilling Operations feet

Approximate Caved-In Depth Upon Completion of the

Drilling Operations feet

B-101 25.0 36.8 B-102 30.0 79.0 B-103 45.0 33.6 B-104 23.4 40.3 B-105 28.0 38.3 B-106 Dry 20.9 B-107 Dry 21.0 B-108 27.5 28.0 B-109 Dry 20.0 B-110 Dry 21.0 B-111 20.0 36.0 B-112 19.0 21.0 B-113 Dry 21.5 B-114 Dry 18.4 B-115 22.0 33.4 B-116 Dry 19.5 B-117 Dry 20.0 B-118 22.0 38.0 B-119 20.0 19.0

Table III-2: Groundwater Depths

Section IV Engineering Design Analysis

IV.1 Proposed Raised Levee and New Levee RKCI’s evaluation of the stability for the Proposed Raised Levee and the New Levee utilized the evaluation parameters outlined in Table IV-1 on the next page.

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Parameter

Selected Value Levee Height 20.5 feet

Riverside Slope 2.3 : 1 Landside Slope 2.6 : 1

Groundwater Depth 38.5 feet Below Top of Levee Cohesion, c 400 psf

Angle of Internal Friction 13 degrees

Table IV-1: Slope Stability Evaluation Parameters

• Slope Stability. RKCI’s evaluation of the slope stability for the proposed raise levee and the new levee indicates that the probable minimum factor of safety for the proposed design, utilizing clay soils similar to those present in the existing levee, will exceed the minimum factor of safety recommended by the US Army Corps of Engineers for all three cases (Case I – End of Construction, Case II – Long-Term (Steady Seepage), and Case III – Rapid Drawdown).

• Settlement. Incorporating results from the consolidation analysis, and including compression settlement within the new levee fill material, the existing embankment fill, and the underlying native soil, RKCI identifies probable settlements on the order of about 4 inches at the edge and about 11.5 inches on center should be considered for the proposed levee improvements. Post-construction related settlement can be limited with a high quality fill and using good construction techniques.

• Seepage Controls Under Levees. Based on the subsurface stratigraphy encountered, RKCI recommended that under long-term flood conditions (several weeks or more) consideration should be given to the possible implementation of the following seepage control measures:

(1) Slurry Trench Cut-off Wall with Instrumentation to Monitor Seepage Pressures; and,

(2) Relief Wells with Instrumentation to Monitor Seepage Pressures.

RKCI recommends the Slurry Trench Cut-off Wall due to (1) it’s performance advantage penetrating to the sands through the underlying clays as opposed to the Relief Wells disadvantage of hot-spots in between wells where the seepage underneath the levee would be higher than the wells resulting in reduced effectiveness when compared to the slurry trench;

and (2) it’s minimal maintenance cost as opposed to the relief wells life-long maintenance, including periodic testing, servicing of the pumps and screens annually, as well as the back-up power source.

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• Slope Surfaces. RKCI recommends that vegetative cover of the levee should follow USACE

Engineering Manual 1110-2-301 “Guidelines for Landscape Planting and Vegetation Management at Floodwalls, Levees, and Embankment Dams,” dated January 1, 2000.

IV.2 Floodwall A floodwall is proposed at the bridge crossings, from the tie-in at the Intake Channel (Sta. 54+06) to the tie-in to Phase I of the Hidalgo Protective Levee. The proposed new floodwall is located approximately 8 feet in front of the existing floodwall location (on the river side). It is proposed to be an “inverted T” stem wall, with a design top of wall elevation of 115.40 feet above mean sea level and expected foot bearing elevations ranging from 100 feet to 150 feet above mean sea level.

Results of the Phase II field exploration and laboratory testing for final design generally confirmed RKCI’s findings in the previous preliminary analysis of 2006 for a stem wall:

• Poor soil support for a shallow foundation system;

• Underseepage beneath the stem wall will lead to stability problems; and,

• Unknown condition and dimensions of the existing sheetpiling.

IV.2(a) Existing Sheetpiling.

Based on the uncertainties and unknowns, RKCI recommended that the existing sheetpiling be removed or abandoned in place.

IV.2(b) Foundation System.

Regarding the proposed foundation system of the floodwall, RKCI recommended the following:

(1) The number of transitions in the wall footings should be kept few as possible; and the footings should generally be placed at a consistent elevation along the length of the floodwall.

(2) The footings be initially sized using a design maximum allowable bearing pressure of

1,000 pounds per square foot for the maximum flood condition; footing should be checked for stability assuming full hydrostatic head directly beneath the footing; assume maximum angle of internal friction of 20 degrees with no cohesion; and footing design should be checked for all modes of failure (rotation, sliding, bearing).

IV.2(c) Seepage Control Under Floodwall.

RKCI’s report entitled “Geotechnical Engineering Study for Proposed Rehabilitation Improvements – Phase II (April 9, 2008)” by RKCI, which is included as Appendix A to this Technical Report, identifies underseepage as a major concern for the proposed floodwall. RKCI’s subsequent report entitlted “Supplemental Recommendations to Geotechnical Engineering Study, Seepage Cutoff Wall and Drainage Blanket Recommendations, April 16, 2008” (included in Appendix C) outlines some approaches to seepage control under the floodwall, including:

• soil-bentonite “slurry wall”;

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• dewatering wells; or,

• steel sheetpiling cutoff wall.

Soil-Bentonite Slurry Wall. According to RKCI, the use of a slurry wall would not be practical at this site. In general, a slurry wall will require a relatively large lay-down area for use by the specialty construction contractor to blend the excavated soils with both dry and slurried bentonite. Additionally, based on RKCI’s prior experience with similar sands, RKCI does not believe that the appropriate permeability could be achieved at this site by adding bentonite slurry alone. This process would be very messy, slow and prone to unexpected delays and potential damage to nearby structures should the temporary excavations become unstable and experience collapse. Estimated construction cost for approximately 54,235 square feet at $40.00 per square foot is $2,169,400.

Dewatering Wells. For this project site, RKCI recommends that a minimum of two rows of wells would be required, with center-to-center well spacings in the range of 20 feet to 70 feet. Additionally, while technically feasible, the use of dewatering wells would represent a significant ongoing maintenance expense for the USIBWC. And, dewatering well systems also require electricity to operate, a back-up power system, and full-time monitoring during an actual event. Estimated construction cost for approximately 42 wells at $20,000 per well is $840,000; and for yearly maintenance and test operation costs approximately 42 wells at $2,500 per well is $105,000 per year.

Steel Sheetpiling Cutoff Wall. RKCI reports that the sheetpile offers the more significant advantage, in that it can be installed while using a relatively small overall footprint; it should require very little annual maintenance, and will not require power to operate. Estimated construction cost for approximately 47,595 square feet at $40.00 per square foot is $1,903, 800.

RKCI’s ultimate recommendation is the construction of a steel sheetpile cutoff wall with a drainage blanket. The steel sheetpile cutoff wall should be designed to completely penetrate the Stratum II silty sands and extend at least 10 feet into the underlying Stratum III clay soils (about 44 feet mean sea level). The drainage blanket would be installed in a zone between the existing and proposed floodwall structures. Additional construction guidelines for this seepage control for the floodwall is outlined in RKCI’s report entitled “Supplemental Recommendations to Geotechnical Engineering Study, Seepage Cutoff Wall and Drainage Blanket Recommendations, April 16, 2008” (included in Appendix C).

IV.3 Intake Channel Crossing Because of existing conditions, the Intake Channel Crossing also requires special construction considerations, including a temporary retaining wall structure to protect and isolate the construction work area from the existing channel water; dewatering; site preparation; excavation / backfill; and slope and erosion protection. These construction guidelines are outlined in RKCI’s reports entitled “Supplemental Recommendations to Geotechnical Engineering Study, Intake Channel Crossing, April 11, 2008” and “Supplemental Recommendations to Geotechnical Engineering Study, Intake Channel Crossing, April 25, 2008” (both included in Appendix D).

Contract No. IBM05D0001 Order No. IBM05T0025

APPENDIX A

APPENDIX A

“Geotechnical Engineering Study for Proposed Rehabilitation Improvements

– Phase II (April 9, 2008)” by Raba-Kistner Consultants, Inc

APPENDIX B

APPENDIX B

Borehole Location Map

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B 118

B 117

B 116

B 115 B 114B 113

B 112 B 111

B 110 B 109B 108

B 107 B 106

B 105 B 104

B 103

B 101

B 102

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4t h

Esperanza

9t h

Bridge

A 10 th

Judy

Pirate Patsy

Camelia th

1s t

Gardenia

Levee

Monterrey

Azalia

Ki tty

Joe Pate

El Mercado

6t h

D r J os e

Bugambilia

Lu pi ta

Dalia

1s t

Camelia

8t h

4t h

R :\P ro je ct \G

IS

_D

A

TA

\M A

P S\

S ha rlo tte

\m ap s-

\g eo bo re p ha se

II

.m xd

£ INTERNATIONAL BOUNDARY

AND WATER COMMISSION

HIDALGO PROTECTIVE LEVEE SYSTEM

REHABILITATION PROJECT

GEOTECH BORE HOLE LOCATION

1 inch equals 500 feet

Legend S 100 ft

? 22 ft © 40 ft

APPENDIX C

APPENDIX C

“Supplemental Recommendations to Geotechnical Engineering Study, Seepage Cutoff Wall and Drainage Blanket Recommendations, April 13 & 16, 2008” by Raba-Kistner Consultants, Inc.

APPENDIX D

APPENDIX D

Study, Intake Channel Crossing Recommendations, April 11 and 25, 2008” by Raba-Kistner Consultants, Inc.