02_Edinburg PS Geotech Report 20151118.pdf

GEOTECHNICAL EVALUATION REPORT

(FINAL SUBMITTAL)

REHABILITATION FOR THE LEVEES AT

EDINBURG PUMP STATION –

LOWER RIO GRANDE FLOOD CONTROL

PROJECT

Penitas, Texas

Contract No. IBM09D0008 Task Order No. IBM14T0012

Prepared for:

The United States Section of The International Boundary and Water Commission 4171 N. Mesa, Bldg. C 100 El Paso, TX 79902

November 2015

Prepared by:

URS Group, Inc.

9400 Amberglen Blvd.

Austin, TX 78729 TBPE Firm No. 3162

TABLE OF CONTENTS

ii Levees at Edinburg Pump Station Rehab

Geotechnical Evaluation Report (Final)

1.0 INTRODUCTION ................................................................................................ 1-1

1.1 Terms of Reference ..................................................................................... 1-1

1.2 Project Overview ......................................................................................... 1-1

1.3 Objective ...................................................................................................... 1-2

1.4 Scope .......................................................................................................... 1-2

1.5 Organization of This Report ......................................................................... 1-3

2.0 PROJECT INFORMATION ................................................................................ 2-1

2.1 Site Location ................................................................................................ 2-1

2.2 Existing Conditions ...................................................................................... 2-1

2.2.1 Land Use and Site Features ......................................................... 2-1

2.2.2 Topography and Surface Conditions ............................................ 2-2

2.3 Region and Site History ............................................................................... 2-9

2.3.1 LRGFCP Project........................................................................... 2-9

2.3.2 Original Edinburg PS River Levee Construction

(1950’s) ........................................................................................ 2-9

2.3.3 Levee Rehabilitation Improvements (2007-2012) ......................... 2-9

2.3.4 Intake Channel Improvements (2013-2014) ............................... 2-10

2.4 Recent Flood Performance and Emergency Repairs ................................ 2-11

2.5 Proposed Improvements............................................................................ 2-13

3.0 GEOTECHNICAL INVESTIGATION .................................................................. 3-1

3.1 General ........................................................................................................ 3-1

3.2 Field Investigation ........................................................................................ 3-1

3.2.1 Soil Borings .................................................................................. 3-1

3.2.2 Soil Sampling ............................................................................... 3-2

3.2.3 Borehole Permeability Tests ........................................................ 3-2

3.3 Laboratory Testing Program ........................................................................ 3-3

3.4 Supplemental Geotechnical Data (by Others) ............................................. 3-3

3.4.1 Geotechnical Engineering Report for Design (L&G, 2008) ............................................................................................ 3-3

3.4.2 Geotechnical Engineering Report (L&G, 2009) ............................ 3-3

3.4.3 Geotechnical Engineering Services Report (PSI, 2012) ............... 3-4

4.0 SUBSURFACE CONDITIONS ........................................................................... 4-1

4.1 Geology ....................................................................................................... 4-1

4.1.1 Regional Geology ......................................................................... 4-1

4.1.2 Site Geology ................................................................................. 4-1

4.1.3 Near Surface Soil Mapping .......................................................... 4-2

4.1.4 Faulting ........................................................................................ 4-3

4.1.5 River Geomorphology .................................................................. 4-3

4.2 Site Stratigraphy .......................................................................................... 4-4

4.2.1 General ........................................................................................ 4-4 iii Levees at Edinburg Pump Station Rehab

4.2.2 Proposed Alignment Stratigraphy ................................................. 4-4

4.2.3 Landside Stratigraphy .................................................................. 4-6

4.2.4 Riverside Stratigraphy .................................................................. 4-6

4.3 Summary of Advanced Field and Laboratory Test Results .......................... 4-7

4.3.1 General ........................................................................................ 4-7

4.3.2 Shear Strength ............................................................................. 4-7

4.3.3 Stress History ............................................................................. 4-10

4.3.4 Hydraulic Conductivity ................................................................ 4-12

4.3.1 Hydrometer ................................................................................ 4-13

4.3.2 Dispersion Potential ................................................................... 4-15

4.4 Groundwater .............................................................................................. 4-16

4.5 Seismic Hazards ........................................................................................ 4-19

5.0 PROJECT DESIGN CRITERIA .......................................................................... 5-1

5.1 Overview ...................................................................................................... 5-1

5.2 Design Water Surface Elevations ................................................................ 5-1

5.3 Horizontal and Vertical Control .................................................................... 5-1

5.4 Geotechnical Analysis Criteria ..................................................................... 5-1

5.4.1 Underseepage .............................................................................. 5-1

5.4.2 Through-Seepage ........................................................................ 5-2

5.4.3 Steady-State Slope Stability ......................................................... 5-2

5.4.4 End-of-Construction Case ............................................................ 5-2

5.4.5 Sudden-Drawdown Case ............................................................. 5-3

6.0 CHARACTERIZATION OF ANALYSIS SECTIONS .......................................... 6-1

6.1 General ........................................................................................................ 6-1

6.2 Analysis Section Characteristics .................................................................. 6-1

6.2.1 Analysis Section #1 ...................................................................... 6-1

6.2.2 Analysis Section #2 ...................................................................... 6-2

6.2.3 Analysis Section #3 ...................................................................... 6-3

6.2.4 Analysis Section #4 ...................................................................... 6-3

6.3 Levee Rehabilitation Improvements and Remedial Measures ..................... 6-4

6.3.1 Overview ...................................................................................... 6-4

6.3.2 Levee Raise ................................................................................. 6-4

6.3.3 Removal of Dispersive Soils ......................................................... 6-5

6.3.4 Chimney Drain & Blanket Drain .................................................... 6-5

6.3.5 Floodwalls .................................................................................... 6-5

6.3.6 Cement-Bentonite Slurry Cutoff Wall ............................................ 6-7

6.3.7 Levee Access Road ..................................................................... 6-7

6.3.8 Future Floating Bollard Fence ...................................................... 6-7

6.3.9 Gates ............................................................................................ 6-8 iv Levees at Edinburg Pump Station Rehab

7.0 GEOTECHNICAL ANALYSIS AND EVALUATION .......................................... 7-1

7.1 Overview ...................................................................................................... 7-1

7.2 Seepage Analysis ........................................................................................ 7-1

7.2.1 Methodology ................................................................................. 7-1

7.2.2 Material Parameter Characterization ............................................ 7-1

7.2.3 Boundary Conditions of Model ..................................................... 7-2

7.3 Slope Stability Analysis ............................................................................... 7-2

7.3.1 Methodology ................................................................................. 7-2

7.3.2 Analysis Conditions ...................................................................... 7-2

7.3.3 External Loads ............................................................................. 7-3

7.3.4 Material Parameter Characterization ............................................ 7-3

7.4 Floodwall Analysis ....................................................................................... 7-7

7.4.1 General ........................................................................................ 7-7

7.4.2 Lateral Earth Pressures ................................................................ 7-7

7.4.3 External Loads ............................................................................. 7-7

7.4.4 Shallow Foundation Alternative .................................................... 7-7

7.4.5 Deep Foundation Alternative ........................................................ 7-8

7.5 Settlement Analysis ................................................................................... 7-12

7.5.1 Earth Levee ................................................................................ 7-12

7.5.2 Floodwall Settlement .................................................................. 7-12

8.0 GEOTECHNICAL ANALYSIS RESULTS .......................................................... 8-1

8.1 Levee Analysis Results ............................................................................... 8-1

9.0 RECOMMENDATIONS FOR DESIGN & CONSTRUCTION ............................. 9-1

9.1 Borrow Source for Levee Fill ....................................................................... 9-1

9.2 Design Criteria for Proposed Improvements ................................................ 9-1

9.2.1 Levee Embankment Raise ........................................................... 9-1

9.2.2 Vegetative Cover .......................................................................... 9-2

9.2.3 Cement-Bentonite Slurry Cutoff Walls .......................................... 9-2

9.2.4 Chimney Drain & Blanket Drain .................................................... 9-2

9.2.5 Levee Access Road ..................................................................... 9-3

9.3 Construction Considerations ........................................................................ 9-4

9.3.1 Clearing and Grubbing ................................................................. 9-4

9.3.2 Dewatering ................................................................................... 9-4

9.3.3 Subgrade Preparation .................................................................. 9-4

9.3.4 Levee Raise ................................................................................. 9-5

9.3.5 Cutoff Walls .................................................................................. 9-5

9.3.6 Floodwalls .................................................................................... 9-6

9.4 Considerations for Future Tie-In to Existing Pump Station .......................... 9-7 v Levees at Edinburg Pump Station Rehab

10.0 LIMITATIONS .................................................................................................. 10-1

11.0 REFERENCES ................................................................................................. 11-1 vi Levees at Edinburg Pump Station Rehab

Tables

Table 3-1 Table 3 1. Summary of Soil Borings (from PSI GDR)

Table 3-2 Summary of Soil Borings from Previous Studies (by others)

Table 4-1 Consolidated Drained Direct Shear Test Results (from PSI GDR)

Table 4-2 Consolidated-Undrained Triaxial Shear with Pore-Pressure Measurements (from PSI GDR)

Table 4-3 Unconfined Compression Test Results (from PSI GDR)

Table 4-4 Unconfined Compression Test Results from Previous Studies (by others)

Table 4-5 Unconsolidated-Undrained Triaxial Compression Test Results from Previous Studies (by others)

Table 4-6 Consolidation Test Results (from PSI GDR)

Table 1-7 Consolidation Test Results from previous studies (by others)

Table 4-8 Falling-Head Hydraulic Conductivity Test Results (from PSI GDR)

Table 4-9 Borehole Infiltration Test Results and Estimated Hydraulic Conductivity Values (from PSI GDR)

Table 1-10 Hydrometer Test Results (from PSI GDR)

Table 4-11 Hydrometer Test Results from Previous Studies (By Others)

Table 4-12 Dispersion Potential Test Results from Previous Studies (by Others)

Table 4-13 Dispersion Potential Test Results (PSI GDR)

Table 4-14 Short-Term Groundwater Levels (from PSI GDR)

Table 4-15 Short-Term Groundwater Levels from Previous Studies (by Others)

Table 6-1 Summary of Analysis Sections

Table 6-2 Rehabilitation Segment Summary

Table 7-1 Seepage Design Parameters

Table 7-2 Slope Stability Design Parameters

Table 7-3 Unfactored Loads for Floodwall Deep Foundations

Table 7-4 LPILE Design Parameters for Floodwall Drilled Shaft Foundations – Section #2 (Station 3+82 to 4+62)

Table 7-5 LPILE Design Parameters for Floodwall Drilled Shaft Foundations – Section #3 (Station 6+40 to 7+00)

Table 7-6 Recommended Design Parameter For Axial Capacity of Drilled Piers

Table 8-1 Summary of Seepage and Slope Stability Analysis Results

Table 8-2 Summary of Floodwall Drilled Shaft Foundation Analysis Results vii Levees at Edinburg Pump Station Rehab

Embedded Figures

Figure 2-1 As-built plan and section view of existing Edinburg Pump Station.

Figure 5-1 Hydrograph of Rio Grande Discharge and Stage for the 1967 Hurricane Beulah at Rio Grande City (S&B, 2008)

Figure 7-1 Proposed drilled pier-supported floodwall cross-section

Figure 7-2. Equivalent footing method for settlement of pier groups.

Figures

Figure 1 Site Vicinity and Location Map

Figure 2 USIBWC Proposed Levee Improvements

Figure 3 Historic Levee Construction Plan (1953)

Figure 4 Plan of Borings (2008 Aerial Imagery)

Figure 5 Plan of Borings (2014 Aerial Imagery)

Figure 6 Geologic Map

Figure 7 NRCS Soils Map

Figure 8 Historic Topographic Map (1916)

Figure 9 River Geomorphology viii Levees at Edinburg Pump Station Rehab

Appendices

Appendix A Geotechnical Data Report (PSI 2015)

Appendix B Historic Geotechnical Data from Previous Consultants

Appendix C Subsurface Profiles

Appendix D Seepage and Slope stability Analysis

SECTION 1.0 INTRODUCTION

vi Levees at Edinburg Pump Station Rehab

Geotechnical Report (100% Submittal)

Acronyms and Abbreviations

Term Description ASTM ASTM International cm/sec centimeters per second

EM Engineering Manual

EOC end-of-construction

ERDC Engineering Research and Development Center

ETL Engineering Technical Letter

FEMA Federal Emergency Management Agency

GDR Geotechnical Data Report

L&G L&G Engineering Laboratory, LLC

LRGFCP Lower Rio Grande Flood Control Project mm millimeters

NAD 83 North American Datum of 1983

NAVD 88 North American Vertical Datum of 1988 pcf pounds per cubic foot

PGA peak ground acceleration psf pounds per square foot

PSI Professional Service Industries, Inc.

TxDOT Texas Department of Transportation

URS URS Group, Inc.

USACE U.S. Army Corps of Engineers

USCS Unified Soil Classification System

USIBWC United States Section of the International Boundary and Water Commission

1-1 Levees at Edinburg Pump Station Rehab

1.0 INTRODUCTION

1.1 Terms of Reference

This report presents the findings of a geotechnical investigation and analysis for the proposed Rehabilitation Improvements for the Levees at Edinburg Pump Station (“Edinburg PS Levees”) located in Penitas, Hidalgo County, Texas, as part of the design improvements for the Lower Rio Grande Flood Control Project.

The work reported herein was completed by URS Group, Inc. (“URS”) for the United States Section of the International Boundary and Water Commission (“USIBWC”) in accordance with the scope of work identified under Task Order IBM14T0012. The work was executed under terms and conditions of Contract IBM09D0008.

The report was prepared by Mr. Lance Finnefrock, P.E. of URS. The report was reviewed by Messrs. Longjie Hong, P.E. and Francke Walberg, P.E., also of URS, in accordance with URS’ requirements for internal review.

1.2 Project Overview

The USIBWC has been designing and constructing the rehabilitation of the levees within the Lower Rio Grande Flood Control Project (LRGFCP) to meet FEMA requirements for the safe containment of 100-year flood flows since 2009. The LRGFCP extends from the town of Penitas, Texas, at its upstream end, to 82 miles above the Gulf of Mexico, a distance of about 158 river miles. The LRGFCP has two (2) diversion dams, Anzalduas Dam and Retamal Dam, approximately 17 and 57 river miles below Penitas, which divert portions of the flood flows into the Main Floodway in the United States and the floodway in Mexico, respectively. The LRGFCP was constructed to provide flood protection to urban, suburban, and irrigated lands in the Rio Grande delta.

The majority of the levees within the LRGFCP have been designed and/or constructed, but there are several existing gaps in the levee system that still need to be addressed. A “gap” is defined as a portion of the levee system that does not meet the required protection for the 100-year flood elevation. Presently, the existing Edinburg Pump Station represents one such gap in the levee system.

The existing Edinburgh Pump Station and adjoining Levees are located on the north side of the Rio Grande River. The location of Edinburg Pump Station is shown in Figure 1. The gap in the levee system extends beyond both the west and east sides of the pump station, with total lengths of about 345 linear feet and 365 linear feet, respectively. Existing floodwall/border fence infrastructure and earthen levees are present at the edges of the gap both east and west of the pump station. The purpose of this project is to close the gap in levee system within the USIBWC’s right-of-way (ROW) by constructing new levee and/or floodwalls between the existing floodwall/border fence and the sides of the Edinburg Pump Station. The USIBWC’s proposed alignment for levee rehabilitation improvements is provided in Figure 2. The scope of this project does not include evaluation of the pump station as a flood retention structure.

1-2 Levees at Edinburg Pump Station Rehab

1.3 Objective

The purpose of the project is to improve the existing flood control system to meet the Federal Emergency Management Agency (FEMA) design criteria for a 100-year flood event and obtain the corresponding FEMA certification. This levee improvements project includes the following requirements:

Design for raising the top-of-levee by as much as 10 feet to Elevation 136.8 feet to provide containment of design flood flows with a minimum freeboard of three (3) feet for water surface elevations calculated in the USIBWC 2003 hydraulic model for the Rio Grande, as needed;

Topographic survey for design and for USIWBC internal use and records;

Conducting additional geotechnical investigations and testing to determine the type and extent of required remediation improvements due to slope stability, seepage, levee settlement, erosion, or other geotechnical issues that may cause levee failure during a 100-year flood event;

Assessing existing structures within the proposed alignment and rehabilitating as needed (e.g. floodwalls, gatewells, etc.). The design must allow for closure of structures during flood events, and must encompass any necessary structural repair or replacement. The pump station is excluded from this scope of work;

Design of seepage and slope stability remediation, as needed along the length of the project limits.

This Geotechnical Evaluation Report has been prepared to support the URS design documentation for the levee improvements. Additionally, this Geotechnical Evaluation Report expands on the information needed to demonstrate the adequacy of the proposed levee improvements.

1.4 Scope

The scope of geotechnical services for this project contained herein includes the tasks described below.

Task 1: Exploratory Borings and Field Testing

a) Drilling borings to depths varying between 35 and 90 feet below existing grade

b) Perform Standard Penetration Testing (SPT)

c) Collecting soil samples using disturbed split-spoon sampling and relatively undisturbed thin-wall tube sampling methods

d) Perform in-situ permeability testing using borehole infiltration test methods outlined by ASTM and the US Bureau of Reclamation (USBOR)

Task 2: Laboratory Testing of Selected Soil Samples

a) Classification Tests (moisture content, unit weight, Atterberg limits, and gradation)

1-3 Levees at Edinburg Pump Station Rehab

b) Advanced Testing o Consolidation o Permeability o Unconfined Compression (UC) o Unconsolidated-Undrained (UU) Triaxial Shear o Consolidated-Undrained (CU) Triaxial Shear w/ pore pressure measurements o Consolidated-Drained Direct Shear (CDDS)

Task 3: Geotechnical analyses

a) Identifying and evaluating the existing soil composition and strength parameters of the levee and underlying strata;

b) Characterizing the engineering properties of the top stratum and the substratum and evaluating the groundwater conditions;

c) Performing steady-state seepage analyses to evaluate under-seepage and through-seepage considering the 100-year design water surface elevation;

d) Performing slope stability analyses for end of construction, steady-state, and sudden-drawdown conditions to evaluate the levee’s overall stability.

e) Performing calculations to estimate settlement, bearing capacity, and lateral earth pressures.

Task 4: Preparation of Geotechnical Evaluation Report

a) The geotechnical exploration findings (soil boring logs, laboratory test results, and in-situ permeability test results);

b) Evaluate and provide summary information about regional groundwater, geology, geomorphology, and past studies

c) Summarize the project background information, including water surface elevations, geometry criteria for levees, and criteria for geotechnical analyses

d) Discussion of the analysis methodologies and results for seepage, slope stability, and settlement and the respective results of these analyses;

e) The design and construction recommendations to assist in preparation of plans and specifications.

1.5 Organization of This Report

The remainder of this report is organized as follows:

• Section 2 reviews the presently available project information;

• Section 3 describes the field investigation;

• Section 4 describes the surface and subsurface conditions at the site;

• Section 5 provides project design criteria;

1-4 Levees at Edinburg Pump Station Rehab

• Section 6 describes the characterization of analysis sections;

• Section 7 describes characterization of geotechnical material parameters;

• Section 8 presents geotechnical analysis approach and summary of results; and

• Section 9 provides recommendations for design and construction.

The report is supported by Appendices A through D. Appendix A presents the relevant geotechnical data referenced herein, including the Geotechnical Data Report prepared by PSI, Inc. for URS. Appendix B presents historic site geotechnical data by previous consultants. Appendix C provides geotechnical subsurface profiles. Appendix D E provides geotechnical calculations supporting the recommendations contained herein.

SECTION 2.0 PROJECT INFORMATION

2-1 Levees at Edinburg Pump Station Rehab

2.0 PROJECT INFORMATION

2.1 Site Location

The project site is the Edinburgh Pump Station and surrounding area located in Penitas, Hidalgo County, Texas. The site is located approximately 1,500 feet north of the Rio Grande River and just southwest of the intersection of Penitas Street (FM 1427) and Military Highway. The project site is bounded by the existing levees and floodwall/border fence infrastructure to the west and east of the pump station, private property to the north, and the pump station intake channel to the south. The project vicinity and site location is shown on Figure 1.

2.2 Existing Conditions

2.2.1 Land Use and Site Features

The site consists primarily of developed agricultural infrastructure. Surrounding areas to the north primarily consist of private parcels of agricultural farmland. Areas to the south of the site towards the Rio Grande and Mexico border are largely undeveloped.

The Edinburg Pump Station is the major infrastructure feature at this site. The Edinburg Pump Station was constructed in the mid-1920's to provide irrigation water to the surrounding agricultural areas. The pump station is currently owned and operated Hidalgo County Irrigation District No. 1 (HCID No. 1) and still provides irrigation water for agricultural purposes. The Edinburg Pump Station is currently eligible to become registered as a historic structure by the Texas Historical Commission.

Water is supplied to the pump station via an earthen intake channel (also referred to as a “canal”) leading from the Rio Grande River nearly 1,500 feet south. Water enters the pump station on the adjacent south wall, and is then distributed to irrigation customers by HCID No.

1. The pumps and control units are enclosed by a housing unit consisting of reinforced concrete walls on all sides and a steel truss roof.

As-built drawings of the pump station dated September 21, 1925 indicate the pump house structure is founded on reinforced concrete slab foundation bearing at Elev. 101.7 feet. The walls forming the pump house consist of counterfort walls with spread-footing foundations that bear at about Elev. 104 to 105 feet. On the south side of the building, the middle one-third of the wall extends below the building interior floor level and into the intake channel, where it is founded on a reinforced concrete slab bearing at Elev. 94.2 feet. Three oversized counterfort stems span between the slab and the pump house walls. In plan, the building exterior is 103.2 feet long by 49.7 feet wide. Counterfort stems and footings extend to a maximum of 10 feet beyond the exterior walls on the north, east, and west sides, and a maximum of between 10 and 21 feet on the south side. The plan and section view of the pump station from the as-built drawings are depicted in Figure 2-1.

A maintenance yard on the north side of the pump station includes several small sheds and wells/risers connected to various piping. A “surge wall” consisting of a large, hollow reinforced-concrete box approximately 18 feet tall is oriented parallel to the pump station in

2-2 Levees at Edinburg Pump Station Rehab the southeast-northwest direction. The surge wall serves as a surge chamber to prevent damage from changes in pressure, typical of pump stations that pump water directly into an underground pipeline (TxDOT, 2009).

The pump station incorporates a complicated system of ditches and piping to convey water to various locations throughout the site. A water storage pond nearly-circular in plan is located to the northeast of the pump station. Existing piping appears to carry water both into and out of the pond. A ditch located to the north of the pump station, oriented north-south, appears to carry off-site stormwater from Penitas Street into the pump station. It is unclear whether the pump station can discharge stormwater into the canal during flood events. A buried pipe near the site entrance (oriented east-west) conveys flow through eroded soils and into the aforementioned ditch; the source of flow is not evident, but two concrete standpipes are located within about 15 feet to the north. Standpipes in the Lower Rio Grande Valley are typically used to prevent excessive pressure from building in pipelines that can result from trapped air and water hammer, and may also contain gates and valves to control and distribute water (TxDOT, 2009).

Evidence of numerous utilities were observed on-site, including but not limited to underground gas and petroleum pipelines, water and sewer lines, buried cables, and overhead power and telephone lines. Based on as-built plans and utility locater field markings, utilities do not appear to impact the proposed project alignment. A system of dirt roads provides access around the perimeter of the pump station and to the riverside of the existing levee system. A view of the general project site is depicted in Photos 2-1 and 2-2.

2.2.2 Topography and Surface Conditions

The topography of the area surrounding the project is relatively flat, generally ranging from about Elev. 120 to 130 feet. The ground surface gently slopes toward the Rio Grande, where the riverbank slopes steepen locally with crest elevation ranging from approximately Elev. 110 to 120 feet. Ground surface contours generated by USIBWC 2005 LIDAR data indicate the water level in the Rio Grande and intake channel was approximately Elev. 106 feet at that time.

Steep slopes in the immediate vicinity of the pump station are due to man-made cut and/or fill associated with original facility construction, and subsequent modifications and flood protection improvements. Ground surface elevations in the immediate vicinity of the pump station vary from Elev. 131.3 feet on top of adjacent existing levee to approximately Elev.

100 feet at the intake channel invert. The top of existing levee fill within the proposed alignment varies from about Elev. 124 (access ramp crossing the alignment about 200 feet west of the pump station) to about 136 feet (western and eastern extents of the project alignment). At the western and eastern extents of the project, existing floodwall/border fence with retained levee embankment on the landside of the wall is present; these were constructed as part of the previously-proposed levee alignment.

Ground cover was relatively sparse at the time of the exploration, and generally consisted of patches of grass with bare exposed soil within the proposed levee improvements alignment.

More robust grass cover was present to the immediate north and northeast of the pump station, and in some areas along the intake channel. Spotty vegetative cover was present on

2-3 Levees at Edinburg Pump Station Rehab existing riverside levee slopes. Examination of historical aerial photos dating back as far as 1995 indicates similar trends in vegetative cover.

The existing levee fills present to the immediate west and east sides of the pump station feature over-steepened riverside slopes on the order of 1.5H:1V to 2H:1V, and reach a maximum height of approximately 21 feet as measured from the riverside toe at Elev. 110 feet. The landside slope and height is more modest at these locations, generally on the order of 2H:1V to 3H:1V and with heights from 4 to 10 feet as measured from the landside toe, respectively. Riverside slope surfaces on either side of the pump station featured large chunks of concrete rubble and rocks, evidence of previous attempts to stabilize surficial soils from erosion (discussed in next section). The slope just east of the pump station also featured exposed geogrid partially buried under soil and vegetation. Conditions of the riverside slopes of existing levee adjacent to the pump station are presented in Photos 2-3 and 2-4.

Minor erosion rills, typically on the order of 4 inches or less in width and/or depth, were present over most of the existing slopes comprising levee fills and the intake channel. More substantial erosion gullies measuring up to 3 feet in width and/or depth were present along the eastern bank of the intake channel. A moderately-sized erosion gully approximately 12 inches wide is present on the landside slope of the existing levee, located approximately 10 to 15 feet east of the pump station. A general representation of the varying degrees of erosion observed on-site is provided in Photos 2-5 through 2-7.

Ongoing erosion alongside the pump house was evident based on some soil loss observed where existing levee fill meets the immediate west side of the pump station. This includes some undermining around concrete at fence post foundations at the crest and toe of the existing levee. Washout erosion of soil along the building envelope was apparent as well.

The steepness of existing slopes here due to limited width of the pump house and constrained riverside toe, as well as lack of good vegetative cover, are likely factors contributing to erosion. See photos 2-8 through 2-11.

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Figure 2-1. As-built plan and section view of existing Edinburg Pump Station.

2-5 Levees at Edinburg Pump Station Rehab

Photo 2-1. General project site, viewing north/northwest from east bank of intake channel

Photo 2-2. General project site, viewing south from top of existing levees

2-6 Levees at Edinburg Pump Station Rehab

Photo 2-3. Riverside Slope of Existing Levee, East Edge of Pump Station (viewing northeast)

Photo 2-4. Riverside Slope of Existing Levee, West Edge of Pump Station (viewing east)

2-7 Levees at Edinburg Pump Station Rehab

Photo 2-5. Erosion Rills on Existing Levee Slopes west of pump station

Photos 2-6 and 2-7. Erosion Gullies on East Bank of Intake Channel, viewing northwest (left) and east (right)

2-8 Levees at Edinburg Pump Station Rehab

Photos 2-8 and 2-9. Soil loss along west side of intake channel, near pump station recirculation line (left) and alongside the pump station wall counterforts (right)

Photos 2-10 and 2-11. Soil loss along west side of pump station, looking south from top of embankment (left) and looking northeast from mid-height of riverside slope (right)

2-9 Levees at Edinburg Pump Station Rehab

2.3 Region and Site History

2.3.1 LRGFCP Project

The LRGFCP Project covers 180 miles of river from Penitas, Texas to the mouth of the Rio Grande in the Gulf of Mexico. The LRGFCP extends through Hidalgo, Cameron, and Willacy counties. The project was the result of a 1932 agreement between the United States and Mexico to provide flood protection for residents, businesses, and farms in both countries.

Originally built as a system of floodways and levees in the 1930s, improvements were made in the 1950s and 1960s. After Hurricane Beulah struck the Lower Rio Grande Valley in 1967, causing serious damage to Harlingen, the McAllen Airport, and other urban and agricultural lands in Texas and Mexico, the IBWC decided to build Retamal Diversion Dam to enable diversions to a new Mexican floodway. Additionally, levees were raised and floodways modified. From 1968 to 1977, the USIBWC has made significant economic investment in project improvements. The U.S. portion currently has 270 miles of levee, 30,000 acres of interior floodway, 420 drain structures, 180 irrigation structures, and 6 bridges (USIBWC, 2015).

2.3.2 Original Edinburg PS River Levee Construction (1950’s)

The Edinburg Pump Station and intake channel pre-date original construction of the levee system at the project site. An archive drawing provided by USIBWC entitled “River Levee, Borrow Area, and Revetment – Flood Control Project, Lower Rio Grande, Texas, Hidalgo County Water Conservation and Improvement District” dated January 16, 1953 depicts the original construction plans for the existing levee system near the pump station. The first levee at the site was likely constructed shortly after the drawing date (approximately 60 years ago). The figure indicates the borrow source used to construct the levee was present to the south of the site on both the west and east banks of the intake channel.

The contents of this drawing have been digitized and overlain onto a map of existing topographic contours and site features, and are presented as Figure 2.

2.3.3 Levee Rehabilitation Improvements (2007-2012)

Design plans to address the gap in the levee system at the Edinburg pump station were previously prepared by Dannebaum Engineering and date March 17, 2010. The project alignment differed from that currently proposed, and included crossing the intake channel approximately 300 feet south of the pump station. Passage of water through the intake channel was to be accomplished using large box culverts built to span across the intake channel. An earthen levee berm with concrete-armored slopes was to be built over the box culverts to span across the intake channel and provide flood protection for the pump station.

The subsurface investigation did not explore conditions at the bottom of the channel, and were unknown to the design/construction team. Soft and wet silty soil conditions encountered during construction posed problems in dewatering the channel area. The thickness of the sedimentation at the bottom of the channel was found to extend to much

2-10 Levees at Edinburg Pump Station Rehab deeper depths than expected. It was reported that attempts to dewater the channel area had failed several times. The soils at the bottom of the channel appeared to be wet, soft silty soils and unsuitable for the support of the box culverts, flood gate well structures and the levee

(PSI, 2012).

In 2011, PSI was contracted by Dannenbaum to perform additional field exploration services at the subject site in order to evaluate the construction site conditions within the limits of the existing channel and to develop recommendations for the proposed construction. At the time, an earthen cofferdam had already been constructed across the intake channel to create working platform in the channel construction area. Additionally, a bypass channel had been constructed along the west side of the intake channel to the existing pump station. The bypass channel was constructed using a sheet pile wall system that was reportedly designed by Lloyd Engineering, Inc. PSI’s geotechnical recommendations for crossing the channel included several options to accommodate construction in soft/saturated conditions, including soil overexcavation/replacement, ground improvement with GeoPiers, or support by driven pile foundations (PSI, 2012). The option ultimately selected by the designers included overexcavation/replacement of soft sediments.

Construction resumed, but the overexcavation of soft sediments was not completed.

Consequently, this section of the alignment was not built, which has created the current gap in the levee.

Remnants of the initial construction include a nearly-finished gatewell riser structure located at the western extent of the proposed levee alignment (approximately STA 2+25). A buried storm drain enters the gatewell riser from the landside, and a buried culvert extendss from the gatewell and under or through the floodwall and outlets into a ditch partially lined with riprap on the riverside of the existing floodwall/border fence. The gatewell will be incorporated into the proposed levee improvements as part of this project, and will be furnished with a sluice gate to prevent riverside floodwaters from entering the stormdrain.

2.3.4 Intake Channel Improvements (2013-2014)

Intake channel improvements consisting of sheetpile walls with concrete caps and internal bracing (“struts”) were constructed sometime between January/May 2013 and May 2014, based on review of aerial photos. Aerial imagery indicating conditions before and after construction of these improvements is shown in Figure 3 and 4, respectively. Design plans provided by HCID No. 1 for the Phase 2 channel improvements indicate the intake channel flowline is Elev. 95 feet within the structure (Lloyd Engineering, 2014). The Phase 2 plans address the last 105 feet of the intake channel structure, furthest from the pump house.

Plans specify proposed sheetpile tip elevations no lower than Elev. 75.2 feet and Elev. 78.5 feet on the west and east sides, respectively, due to the presence of an existing gas line than crosses under the channel. The gas line is also identified in the URS design plans. The top of the gas line is shown to be approximately 3 feet lower than specified sheepile tip elevations. It is not known whether sheetpiles were fully driven to the limiting elevations, or if driving refusal was encountered at shallower depths. Phase 1 channel improvements, consisting of the portion of braced sheetpile walls adjoining the pump station, were not available for review.

2-11 Levees at Edinburg Pump Station Rehab

2.4 Recent Flood Performance and Emergency Repairs

In July 2010, heavy rainfall associated with Hurricane Alex caused flooding in the Rio Grande and through the Lower Rio Grande Valley. According to the high water mark observed at the site, flood levels as high as Elev. 128.6 feet reached the existing levee.

These levels correspond to a level approximately 2.5 feet below existing maximum crest elevation.

URS discussed the events with USIBWC staff, and reviewed “Situation Reports” prepared by Hidalgo County Office of Emergency Management (HCEM) during Hurricane Alex. The reports were numbered 3a thru 22 and dated June 10 thru July 22, respectively (http://www.co.hidalgo.tx.us/index.aspx?NID=970).

Elevated flood levels reportedly caused significant erosion of the existing levee located at the immediate east and west sides of the pump station on July 12, 2010. According to USIBWC staff present during flooding, development of “eddy pools” along the pump house walls and wave action against the riverside slopes appeared to have been causing the progressive degradation of the steeper levee slopes near the pump station. The USIBWC staff speculated that observed slope erosion may have been a combination of “washout” from noted hydraulic action and shallow sloughing. No evidence of through-seepage or under-seepage (e.g. sand boils, seeps, etc.) on the landside slopes was reported.

Emergency repairs were initiated the next day and completed on July 14, 2010. Repairs consisted of:

Widening of the levee crest from 8 feet to 20 - 25 feet in the vicinity of the pump house using approximately 14,000 cubic yards of soil fill.

Placement of erosion control materials consisting of 5,000 square-feet of “geo-mat slope protection” (presumably the geogrid observed on-site and discussed previously in Section 2-2) and concrete debris cover onto the riverside slope to reinforce and stabilize the slope surface.

Following repairs, floodwaters were still believed to be a threat to the existing Edinburg Pump House due to the age of the structure. There was concern that the pump house could fail under flood conditions, and the surrounding levee improvement may place increased pressure on the pump house during flooding. Consequently, additional repairs were initiated on July 16, 2010 and included:

Construction of an 8- to 10-feet high temporary ring levee (referred to as “earthen protection berm” by HCEM). The horse-shoe shaped ring levee was built around the back of pump house and surge wall (landside) to serve as a secondary flood control structure against the risk of floodwaters breaching the pump house. The berm was completed on July 17, 2010.

Some details pertaining to the 2010 flooding are not clear based on review of the situation reports and other documents (USIBWC, 2010a and 2010b), and are described below:

2-12 Levees at Edinburg Pump Station Rehab

It was reported that a concrete floodwall attached to the pumping plant had provided flood protection in the past (HCEM, 2010). According to discussions with USIBWC staff, this is a reference to the counterforts extending from the pump station walls on either side. Photo 2-11 shows that a portion of this “wall” had been broken off at some point. There is no indication that a formal floodwall structure previously existed at this location.

A photo in USIBWC (2010a) showed water being discharged into the intake channel through a hose or pipe. The source of the water discharge, and whether or not it contributed to slope erosion, is not known at this time based on available data.

It was reported that cracking developed on the river levee located downstream of the pump house (USIBWC, 2010a). However, the location of this distress was not readily apparent from available information, and was not observed during the URS’ site visit.

A structural firm hired by HCID No. 1 recommended steel plate reinforcements for the east wall of the pump station (HCEM, 2010). It is not clear why the specific repairs were needed, or if they were implemented.

At the time of the 2015 URS field investigation, remnants of the 2010 emergency repairs were readily apparent, including most of the ring levee, concrete debris rubble fill on the riverside slope, and pieces of geogrid protruding from the embankment east of the pump station. However, the distress that occurred prior to emergency response could not be observed since it is obscured by the repair. From the descriptions of the distress, the underlying causes are not definitively known. The photo from the HCEM, 2010 report suggests water was being discharged into the intake channel below the slope on the west side of the pump station, but it is not clear whether the discharge point is on the levee slope or at the waters’ edge. Similar distress occurring on both sides of the pump station would suggest a common cause of erosion, which would support the anecdotal evidence that eddy currents or wave action may have contributed to erosion of the steeper slopes adjacent to the pump house.

The pump station walls viewed from the intake channel were observed to have slightly off-vertical cracking midway between the counterfort walls across the front of the pump station, suggesting the possibility of differential settlement of the structure. Since the pump station is an old structure, it is possible that material was being lost through open cracks during the operation of the facility. If undetected over time, soil loss can lead to development of voids beneath and adjacent to structures; however, until further information can be developed, this is only speculation as to the potential cause of the distress. Photos 2-8 through 2-11 do suggest further loss of ground apparently since the flood. The cause of soil loss here is not well understood; it could represent loss of material into the pump station, surface erosion due to steep slopes consisting of potentially dispersive soils, and/or loss of material through the contact between the pump station and the newly-constructed sheet pile structure in the intake channel.

2-13 Levees at Edinburg Pump Station Rehab

2.5 Proposed Improvements

Due to difficulties associated with the original project alignment that crossed the intake channel, a new alignment was selected to eliminate the channel crossing. The alignment for proposed levee improvements would close the flood protection gap between the pump station and existing flood protection located to the east and west. The existing flood protection away from the pump station to east and west includes a riverside floodwall/border fence with retained earthen levees on the landside. Proposed improvements will tie-in to these earthen levees to the east and west. However, the proposed improvements under this scope of work do not provide levee system tie-in to the existing Edinburg Pump Station because the pump station is located within the HCID No. 1 ROW. The future tie-in will be performed by others contracted directly by HCID No. 1.

The proposed improvements for the project site include raising the top of levee to provide containment of the 100-year flood Water Surface Elevation (WSE) with at least 3-feet of freeboard plus overbuild to compensate for settlement. The 100-year WSE used for design is 133.80 feet. The typical levee section will include a 21-foot wide crest to accommodate the future installation of a bollard floating fence by the Department of Homeland Security (DHS). The levee will include ramps providing access to both the landside and riverside for DHS, USIBWC staff, and HCID #1maintenance crews. The top of levee will include a flexible base access roadway.

Proposed levee-raising earthen fill will be built with side slopes of 3H:1V (horizontal:vertical) on the riverside and 3H:1V on the landside. USIBWC requires at least 15 feet of horizontal distance from the existing easement line of the HCID No. 1 property to the limit-of-fill line resulting from the proposed raising of the levee. At sections where the present design results in this distance being less than 15 feet, steepened slopes and/or floodwalls will be required.

Based on the age of the existing pump station, its potential to become a national historic structure, photos showing the extensive cracking and distress of the structure and past structural engineer recommendations for strengthening of the structure, only proposed improvements that would not add additional loading to the existing structure were considered for this project. Accordingly, to accommodate the future tie-in design and construction by others, the proposed levee improvements will include two (2) T-wall monoliths (floodwalls) to transition from proposed earthen levee to an area adjacent to the pumping station. The T-walls will terminate at an offset distance to be determined by the geotechnical analysis, but no closer than 10’.