J.08_20110414_Geotech_Paisano_Feasibility_Raba_Kistner.pdf

FEASIBILITY STUDY AND CONCEPTUAL DESIGN

For

PAISANO REACH (PHASE II)

UPPER RIO GRANDE FLOOD CONTROL SYSTEM

RECTIFICATION PROJECT

EL PASO, EL PASO COUNTY, TEXAS

GEOTECHNICAL MEMORANDUM

Raba-Kistner

Project No. AEA10-050-00 April 14, 2011

Mr. Juan C. Vargas, PE URS Corporation 1950 North Stemmons Freeway, Suite 6000 Dallas, TX 75207

RE: Feasibility Study and Conceptual Design for Paisano Reach (Phase II)

Upper Rio Grande Flood Control System Rectification Project El Paso, El Paso County, Texas Geotechnical Memorandum USIBWC IDIQ Contract No. IBM09D0008 USIBWC IDIQ Task Order No. IBM10TO41 URS Project No. 25338511; Vendor No. 1147327

Dear Mr. Vargas:

Raba-Kistner Consultants, Inc. (R-K) is pleased to submit our Geotechnical Memorandum for the feasibility study and conceptual design for the approximately 2-mile long Paisano Phase II Reach levee system in El Paso, El Paso County, Texas. These services were performed in accordance with the scope of work pursuant to the executed Subcontract for Consultant Services between URS Corporation (CLIENT) and R-K for the above-referenced indefinite delivery - indefinite quantity contract (IDIQ) with the United States International Boundary and Water Commission (USIBWC).

The purpose of this study was to drill borings along the existing and/or proposed alignment of the levee, to perform laboratory testing to classify and characterize subsurface conditions, and to prepare a Technical Memorandum presenting our geotechnical analyses for the conceptual design of the subject levee reach in general accordance with the U.S. Army Corps of Engineers (USACE) documents EM 1110-2-1913 Design and Construction of Levees and EM 1110-2-2502 Retaining and Flood Walls, as applicable, and the Federal Emergency Management Agency (FEMA) Requirements for Mapping Levees (44 CFR 65.10) as they pertain to geotechnical evaluations of levees.

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

Very truly yours, RABA-KISTNER CONSULTANTS, INC.

Russell L. Thomas, Jr., P.E. John A. Focht, III, P.E.

Senior Project Manager Chief Geotechnical Engineer

4/14/2011

Chris L. Schultz, P.E.

Senior Vice President

RLT/JAF/CLS/mem Attachments Copies Submitted: Above (4)

Engineering • Testing • Environmental • Facilities • Infrastructure

Raba-Kistner Consultants, Inc.

12821 W. Golden Lane San Antonio, Texas 78249

(210) 699-9090 FAX (210) 699-6426

www.rkci.com TBPE Firm F-3257

Austin Brownsville Dallas El Paso Houston McAllen Mexico San Antonio

FEASIBILITY STUDY AND CONCEPTUAL DESIGN FOR

PAISANO REACH (PHASE II)

UPPER RIO GRANDE FLOOD CONTROL SYSTEM RECTIFICATION PROJECT

EL PASO, EL PASO COUNTY, TEXAS

GEOTECHNICAL MEMORANDUM

for

U.S. SECTION OF THE INTERNATIONAL BOUNDARY AND WATER COMMISSION

(USIBWC)

Prepared for

URS CORPORATION

Dallas, Texas

Prepared by

RABA-KISTNER CONSULTANTS, INC.

San Antonio, Texas

PROJECT NO. AEA10-050-00

a

EXECUTIVE SUMMARY

The purpose of this study was to drill borings along the existing and/or proposed alignment of the levee, to perform laboratory testing to classify and characterize subsurface conditions, and to prepare a Technical Memorandum presenting our geotechnical analyses for the conceptual design of the subject levee reach in general accordance with the U.S. Army Corps of Engineers (USACE) documents EM 1110-2-1913 Design and Construction of Levees and EM 1110-2-2502 Retaining and Flood Walls, as applicable, and the Federal Emergency Management Agency (FEMA) Requirements for Mapping Levees (44 CFR 65.10) as they pertain to geotechnical evaluations of levees.

As requested, subsurface conditions along the approximately 2-mile segment of the Paisano Phase II reach were evaluated by drilling and sampling 16 borings as shown on the following table.

Number of Borings Depth, feet Boring Identification

9 50 PB-1 through PB-4, PB-12, PB-14 through PB-16

7 40 PB-5 through PB-11, PB-13

Borings PB-1 through PB-5, PB-9 and PB-10 were drilled in the existing floodplain. Borings B-6, B-7, B-8, B-11 and B-12 were drilled on top of the existing access road. Borings B-13 through B-16 were drilled on the riverside slope and at the top of the existing levee. The borings were spaced at approximately 700 ft. intervals along the levee alignment. The borings were drilled at the locations shown on the Boring Location Map, Figure A-1, from November 10 through November 18, 2011 and on December 16, 2010. The boring locations were estimated by R-K using satellite maps and approved by USIBWC. Final locations were surveyed in the field by the project surveying consultant, Bohannan Houston, Inc.

Four of the borings (Borings PB-1 through PB-4) were drilled using environmental drilling techniques. All borings were advanced using a modified hollow-stem auger procedure. Hollow-stem augers were used from the ground surface until caving soils below the groundwater table were encountered. Mud rotary methods were then employed to the boring termination depths.

For two of the borings (Borings PB-5 and PB-12), Nx rock coring methods were introduced once bedrock was encountered at depths of about 2 feet and 34 feet, respectively. Upon completion of drilling, bentonite-cement grout was used to backfill the holes using tremie methods in accordance with EM-1110-1-1906 guidelines.

Levee embankment fill soils observed within the levee in those borings drilled on top of the levee consisted of locally available non-cohesive and cohesive soils, predominately silty sands (SM) with interbedded clayey and/or gravel layers, which overlay highly variable subsurface profiles consisting of interbedded clayey sands (SC); silty clayey sands (SC-SM); well-graded gravels with silt and sand (GW-GM); gravelly silty sands (SM); gravelly clayey sands (SC); silty gravel with sand (GM); clayey silts (ML); silty clays with sand (CL-ML); lean clays with sand (CL); and sandy lean clays (CL). Subsurface conditions beneath the existing levees were highly variable but consisted of predominately granular soils. Due to the variability of subsurface conditions, the b boring logs should be consulted for detailed stratigraphic information at the individual boring locations.

Three identifiable anomaly subsurface profiles were encountered in Borings PB-2, PB-5 and PB- 14 which likely require additional field exploration as similar conditions were not encountered in the adjacent borings. In Boring PB-2, an approximately 9 ft. layer of soft, saturated lean clays were observed near the surface. In Boring PB-5, shallow Andesite rock was encountered at a depth of 2 ft. below the existing ground surface, which extend to at least the 40 ft. termination depth of drilling. In Boring PB-14, construction debris and/or waste was encountered within the existing levee fill materials, and potentially in a fill layer beneath the levee. Each of these conditions will require further analyses, as discussed in a subsequent section of this report.

To evaluate the levees, for the purposes of this study, the Paisano Phase II reach was divided into four separate subreaches based on the proposed remediation options and the subsurface conditions encountered. A general description of the subreaches is discussed below.

Subreach No. 1

Subreach No. 1 consists of the downstream end of the project, and extends to the area where the proposed floodwall begins, near Station No. 81+00. In this area, improvements are proposed to the existing levee, and the American Canal runs immediately adjacent and parallel to the land side toe of the existing and proposed levee alignment. The American Canal in this subreach as well as the others in which it is present is reportedly lined with approximately 3 in. concrete pavements which cover the canal bottom and side slopes. Our analyses and recommendations assume the condition of the concrete lining to be in reasonably good condition, which must be verified during the final design phase. In this subreach, the levee is at or near the proposed design flood elevation and it will not be required to add additional height to the levee.

Subreach No. 2

Subreach No. 2 consists of the floodwall portion of the proposed flood control system. In this area, due to site restrictions, it will be required to construct the floodwall. This subreach extends from approximately Station 25+00 to Station 81+00. A culvert section associated with the American Canal is also located near Station 25+00.

Subreach No. 3

Subreach No. 3 consists of an existing levee which extends from Station 14+00 to Station 25+00 (beginning of the floodwall). Conditions in this subreach are similar to Subreach No. 1 in that the American Canal runs parallel to the land side to of both the existing and proposed levee alignment. However, in this subreach, it will be required to raise the levee to reach the required flood design elevation.

Subreach No. 4

Subreach No. 4 extends from Station 0+00 at the American Dam to Station No. 14+00. In this subreach, the existing levee will be enhanced, and will also be raised to account for current flood design elevations. However, over a majority of this subreach, the American Canal diverges from c the toe of the levee, and is located as far as 160 ft. from the toe. Additionally, subsurface conditions in this subreach included a unique subsurface soil profile at Boring PB-2 which consist of a relatively shallow, approximately 7 to 9 ft. layer of soft, lean clays, as discussed in greater detail in a subsequent section of this report.

On the basis of our analyses, the existing levee embankments were constructed using soils that are silts and sands that are highly susceptible to erosion and seepage. Seepage through and/or beneath the existing embankments represents a serious risk to the levee system during a design flood event. Additionally, the American Canal is located immediately at the land side toe of the levee, and runs parallel to the levee over a majority of Subreaches 1 and 3. The American Canal in this reach is concrete lined, and has relatively steep side slopes of 1.5:1 (horizontal:vertical).

The results of our seepage analyses for steady state flow indicate that a significant risk of damage and failure of the levee and side slopes of the adjacent American Canal. The slope stability analyses indicated similar results.

As a result of the analyses, various seepage control and slope stability control measures were evaluated to develop mitigation options for the levee and canal system. The existing levee may be subject to serious seepage-related distress if new operating procedures for the American Canal are not implemented during flood events. The new operating procedure must involve filling the canal within 6 hours of the beginning of flood events. All of the mitigation options considered requires that the American Canal be full of water during flood events. Additionally, our recommendations include the installation of pressure relief wells at the bottom of the canal to prevent bottom instability and heave.

Mitigation measures to control seepage and slope stability failures will be required, which are summarized below for the various subreaches of the levee.

Subreach No. 1

• Build the levee to final grade with a compacted clay with a flood side levee slope of 3:1 (horizontal:vertical) or flatter, and land side levee slope of 2.5:1 (horizontal:vertical) or flatter.

• Relocate the levee to a distance where the toe of the land side slope is a minimum of 10 ft. from the top of the canal lining.

• Install two passive pressure relief wells, 10 ft. deep, in the bottom of the canal lining with center to center distances to be determined by additional analyses following field permeability testing.

• A 3 ft. gravel drainage blanket will be placed on the ground extending from the top of canal concrete lining to the landside face of the levee.

• Overbuild the levee height by 2 in. to account for potential settlements.

• The American Canal must be full during the design storm event.

Subreach No. 2

In Subreach No. 2, a floodwall will be required due to current infringements on the adjacent right of way. On the basis of the wall configuration and the subsurface soil conditions encountered, a T-wall founded on shallow foundations over improved subgrades was selected. A summary of the floodwall foundation requirements is presented below.

d

• Overexcavate to a minimum of 4 ft. beneath the bottom of the footing and backfill the resulting excavation with cement treated base materials.

• The top of footing must be a minimum of 4 ft. below final ground on the river side and land side of the levee.

• On the land side, in addition to the 4 ft. embedment, the footing should be covered with a minimum of 4 ft. of fill extending out until it daylights into the natural ground elevation. If this does not occur, general fill materials may be utilized until daylighting occurs, or to a maximum horizontal distance of 35 ft.

from the back of the wall.

• A gravel drainage trench with a minimum width of 3 ft will be required at the back of the cement treated base that extends from the bottom of the base to the ground surface.

• Provide scour protection on the riverside of the wall to protect the shallow foundation. Various methods of protection may be used, including articulated mats, stone riprap and concrete slope paving.

Subreach No. 3

(horizontal:vertical) or flatter.

• Relocate the levee to a distance where the toe of the land side slope is a minimum of 12 ft. from the top of the canal lining.

• Install three passive pressure relief wells, 10 ft. deep, in the bottom of the canal lining with center to center distances to be determined by additional analyses following field permeability testing.

• A 3 ft. gravel drainage blanket will be placed on the ground extending from the top of canal concrete lining to the landside face of the levee.

Subreach No. 4

• Build the levee to final grade with a compacted clay with a flood side levee slope of 3:1 (horizontal:vertical) or flatter, and land side levee slope of 2.5:1 (horizontal:vertical) or flatter.

• In areas where the canal is located within 17 ft. of the landside toe, relocate the levee to a distance where the toe of the land side slope is a minimum of 17 ft.

from the top of the canal lining.

• A toe drain should be installed on the land side toe of the levee. The toe drain should extend 3 ft. up (measured from vertical) the landside face. The toe drain width extends horizontally a minimum of 10 ft. from the landside toe into the levee.

• If subsequent field operations confirm that soft soil conditions are encountered beneath the levee, the levee must be rebuilt with a biaxial geogrid located in a 12

in. cement sand layer at the bottom of the levee. The geogrid should extend e from a point inside the embankment located 12 ft. from the riverside toe (to maintain 4 ft. of cover) through the remaining embankment and gravel blanket, terminating outside of the levee at a distance of 10 ft. from the landside toe.

• Install two passive pressure relief wells, 10 ft. deep, in the bottom of the canal lining with center to center distances to be determined by additional analyses following field permeability testing.

In addition to the above recommendations, further subsurface exploration is recommended before final design, as discussed in detail in the ADDITIONAL SUBSURFACE EXPLORATION section of this report.

The following report contains a more detailed description of our field and laboratory testing services, engineering analysis, and recommendations. This Executive Summary should not be utilized independently of the following report.

TABLE OF CONTENTS

i

INTRODUCTION

PROJECT BACKGROUND

PROJECT DESCRIPTION

SUBREACH 1

SUBREACH 2

SUBREACH 3

SUBREACH 4

RISK

LIMITATIONS

FIELD EXPLORATION

LABORATORY TESTING

GENERAL SITE CONDITIONS

SITE DESCRIPTION

General Levee Geometry

SITE GEOLOGY

SEISMIC COEFFICIENTS

STRATIGRAPHY

DEPTH-TO-WATER

DESIGN PERMEABILITY

DESIGN SHEAR STRENGTH

EARTHEN LEVEE SYSTEM

DESIGN CONSIDERATIONS

PROBABLE FAILURE MODES

Embankment Seepage Under Seepage Slope Erosion Slope Stability

LEVEE SEEPAGE ANALYSIS

ACTIVE RELIEF WELLS

PARTIAL PENETRATING SEEPAGE BARRIER

TABLE OF CONTENTS

ii

PASSIVE RELIEF WELLS IN CANAL BOTTOM

LEVEE SLOPE STABILITY ANALYSIS

MINIMUM FACTOR OF SAFETY

RESULTS OF ANALYSES

LEVEE EROSION PROTECTION

SETTLEMENT ASSOCIATED WITH THE PROPOSED RAISED LEVEE

LEVEE TRAFFICABILITY

FLOODWALL SYSTEM

DESIGN CONSIDERATIONS

DESIGN RECOMMENDATIONS

STABILITY ANALYSIS

SEEPAGE ANALYSES

CONCLUSIONS

RECOMMENDATIONS

CONSTRUCTION RECOMMENDATIONS

VEGETATIVE COVER FOR CLAY LEVEES

ATTACHMENTS

Appendix A (Field Testing Summary) Boring Location Map ............................................................................................................... A-1 Logs of Borings .................................................................................................. A-2 through A-17 Key to Terms and Symbols ................................................................................................... A-18 Results of Soil Sample Analyses ........................................................................................... A-19

Appendix B (Laboratory Testing Summary) Grain Size Curves ................................................................................................ B-1 through B-7 Liquid Limit, Plastic Limit, and Plasticity Index of Soils ....................................... B-8 through B-29 Moisture-Density Relationships (Proctor) .............................................................................. B-30 Analytical Soil Tests ....................................................................................... B-31 through B-198

Appendix C (Results of Geotechnical Analyses)

Project No. AEA10-050-00 1

INTRODUCTION

Raba-Kistner Consultants, Inc. (R-K) has completed the authorized subsurface exploration and engineering analyses of the approximately 2-mile segment of the Paisano Phase II Levee Reach in El Paso, El Paso County, Texas. This Geotechnical Memorandum briefly describes the procedures utilized during this study and presents our findings along with our geotechnical analyses for the conceptual design of the subject levee reach in general accordance with the U.S. Army Corps of Engineers (USACE) documents EM 1110-2-1913 Design and Construction of Levees and EM 1110-2-2502 Retaining and Flood Walls, as applicable, and the Federal Emergency Management Agency (FEMA) Requirements for Mapping Levees (44 CFR 65.10) as they pertain to geotechnical evaluations of levees.

PROJECT BACKGROUND

The Rio Grande Rectification Project (RGRP) of the Upper Rio Grande Flood Control System is a flood control and water delivery project completed in 1938 along the Rio Grande in El Paso and Hudspeth Counties, Texas that extends approximately 91 miles from El Paso to Fort Quitman.

The reach for this study begins at the American Dam and ends at the International Dam and is called the Paisano Reach (Phase II).

The United States Section of the International Boundary and Water Commission (USIBWC) has conducted hydraulic, aerial, geophysical, and exploratory geotechnical studies of the Rio Grande Rectification Project segment of the Upper Rio Grande Flood Control System to assess the ability of the U.S. levee to safely contain flood flows in the Rio Grande. In 2006, FEMA decertification of the USIBWC levees in El Paso and Hudspeth Counties, Texas, resulted in public and local government desire to upgrade the levees to FEMA criteria. The purpose of this investigation is to support the upgrade of the Paisano Reach (Phase II) in order to meet FEMA criteria.

PROJECT DESCRIPTION

The USIBWC, jointly with the USACE, conducted a hydraulic study to assess the ability of the levees of the RGRP to safely contain the 100-year flood flows in the Rio Grande. Results from this study and other hydraulic modeling studies have been used by the USIBWC to establish preliminary required top-of-levee elevations for a conceptual levee improvement project designed to contain the 100-year flood flows with 3 feet of freeboard. From these studies, the USIBWC has determined that certain reaches require improvements for safe containment of the design flood flows.

The approximate coordinates of the Paisano Reach (Phase II) improvements are listed below.

The approximate length of our study was about 2 miles long.

The Paisano Reach (Phase II) Latitude Longitude

Beginning Station (end of the American Dam Parapet Wall Addition)

31° 47’ 02.77” N 106° 31’ 39.70” W

Ending Station (Beginning of International Dam) 31° 45’ 41.54” N 106° 30’ 34.78” W

Project No. AEA10-050-00 2

We understand that floodwall design is necessary at one segment due to the USIBWC not having Right-of-Way (ROW) between the river bank and an existing water line. Therefore, the containment of the 100-year flood flows will need to be met by the construction of a floodwall in this Subreach.

We also understand that the upstream portion of the Paisano Reach (Phase II) is in the vicinity of the ASARCO Smelter, and USIBWC’s property has been contaminated by metals such as arsenic, cadmium, lead, and selenium, which have migrated from the Smelter. The dangers of lead and arsenic contamination in soils and groundwater are well known, and the contamination in the El Paso area is well documented. ASARCO executed a State enforcement order with the Texas Commission on Environmental Quality (TCEQ) in 1996 relating to the ASARCO Smelter, but has still not implemented corrective actions to eliminate the off-site migration of metals from the ASARCO property. Continuing contamination from the ASARCO property has forced USIBWC to address the dangers of lead and arsenic in carrying out its responsibilities at the American Dam and adjacent areas. Due to the uncertainty of soil contamination in the USIBWC levee near ASARCO, precautions were taken to minimize potential health hazards. Our field personnel were advised to utilize protective equipment and to implement dust suppression techniques, such as soil wetting.

For the purposes of this study, the Phase II levee reach was divided into four separate subreaches based on the proposed remediation options and the subsurface conditions encountered. A general description of the subreaches is discussed below.

SUBREACH 1

Subreach No. 1 consists of the downstream end of the project, and extends to the area where the proposed floodwall begins, near Station No. 81+00. In this subreach, improvements are proposed to the existing levee, and the American Canal runs immediately adjacent and parallel to the land side toe of the existing and proposed levee alignment. The American Canal in this subreach as well as the others in which it is present is reportedly lined with approximately 3 in. concrete pavements which cover the canal bottom and side slopes. Our analyses and recommendations assume the condition of the concrete lining to be in reasonably good condition, which must be verified during the final design phase. In this reach, the levee is at or above USIBWC required top of levee elevation and it will not be required to add additional height to the levee.

SUBREACH 2

Subreach No. 2 consists of the floodwall portion of the proposed flood control system. In this area, due to site restrictions, it will be required to construct the floodwall. This subreach extends from approximately Station 25+00 to Station 81+00. A culvert section associated with the American Canal is also located near Station 25+00.

SUBREACH 3

Subreach No. 3 consists of an existing levee which extends from Station 14+00 to Station 25+00 (the end of the floodwall). Conditions in this subreach are similar to Subreach No. 1 in that the American Canal runs parallel to the land side to of both the existing and proposed levee alignment. However, in this subreach, it will be required to raise the levee to reach the required flood design elevation.

Project No. AEA10-050-00 3

SUBREACH 4

Subreach No. 4 extends from Station 0+00 at the American Dam to Station No. 14+00. In this section, the existing levee will be enhanced, and will also be raised to account for current flood design elevations. However, over a majority of this reach, the American Canal diverges from the toe of the levee, and is located as far as 160 ft. from the toe. Additionally, subsurface conditions in this subreach included a unique subsurface soil profile at Boring PB-2 which consist of a relatively shallow, approximately 7 to 9 ft. layer of soft, lean clays, as discussed in greater detail in a subsequent section of this report.

RISK

Levee systems are designed to perform a required function (retain flood waters) under stated conditions for a specified period of time (presently unknown). The levee’s design may not be adequate for a different flood event or longer performance period, and unexpected damage to the levees may result in the need for repairs to the levee and/or levee improvements over time.

Our conceptual design recommendations inherently assume that the levee will be properly maintained and regularly inspected by trained and qualified personnel. Our conceptual design recommendations also assume that any identified repairs will be made promptly, and the adequacy of the levee system will be regularly assessed, fully taking into account changes in weather, land use and evolving design and performance standards, with changes made as appropriate. This Technical Memorandum includes an assessment of various design options that may be used to modify the levees to meet the 100-year flood criteria, as provided by USIBWC, FEMA and others.

The subsurface profiles along this section of the levee are relatively heterogeneous, and the geotechnical properties of the soils encountered in this study involve significant variability. This variability includes significant spatial variability; and the spatial variability appears to occur over relatively short distances. It is important to note that levees differ from other types of structures, such as drilled piers or driven piles, in that the performance of the levee involves local, not average, soil conditions.1

LIMITATIONS

The selection of design parameters for this project was based on a review of the available geotechnical data, our knowledge of the project area, and design calculations using idealized geometries. The results of our analyses were then reviewed with respect to important trends and general design concepts, keeping these conditions and limitations in mind. Our conceptual recommendations are based on a conservative approach to design as is warranted for all levee structures. We believe that the combination of recent incipient failures, observed conditions and probable failure modes justifies this approach.

This Geotechnical Memorandum has been prepared in accordance with accepted Geotechnical Engineering practices in the region of West Texas for the use of URS Corporation (CLIENT) and its representatives for design purposes. This Geotechnical Memorandum may not contain sufficient information for purposes of other parties or other uses.

1 Focht, J.A. Jr. and Focht, J.A. III, “Factor of Safety and Reliability in Geotechnical Engineering, Discussion and Closure”, ASCE JGGE Vol. 127 No. 8, pp.700-721, August 2001.

Project No. AEA10-050-00 4

The recommendations submitted in this Geotechnical Memorandum are based on the data obtained from the borings drilled along the alignment, our understanding of the project information provided to us, and the assumptions with regard to water elevations and durations that are discussed in greater detail in the following Geotechnical Memorandum. If the project information described in this Geotechnical Memorandum is incorrect, is altered, or if new information is available, we should be retained to review and modify our recommendations.

This Geotechnical Memorandum may not reflect the actual variations of the subsurface conditions across the site. This is particularly true with regard to the identified anomalies encountered in three of the borings, as discussed in detail in this report, and with regard to the existing groundwater levels that are likely to be highly variable based on precipitation events and the percent of sands versus fine grained materials. The nature and extent of variations along the levee may not become evident until construction commences. The construction process itself may also alter subsurface conditions. If variations appear evident at the time of construction, it may be necessary to reevaluate our recommendations after performing on-site observations and tests to establish the engineering impact of the variations.

The scope of our current study does not include an environmental assessment of the air, soil, rock, or water conditions either on or adjacent to the site. No environmental opinions are presented in this Geotechnical Memorandum.

If final heights along the levee are significantly different from those provided from URS (more than plus or minus 12-inches), our office should be informed about these differences. If needed and/or if desired, we will reexamine our analyses and make supplemental recommendations.

FIELD EXPLORATION

Representatives of the USIBWC, URS and R-K developed the field exploration program during several phone conferences at the time of contract negotiation. The number of borings, locations, and depths reflect the results of studies by others, our experience with similar soil conditions, and the immediate needs of the USIBWC.

As requested, subsurface conditions along the approximately 2-mile Paisano reach (Phase II) were evaluated by drilling and sampling 16 borings as shown on the following table.

Number of Borings Depth, feet Boring Identification

9 50 PB-1 through PB-4, PB-12, PB-14 through PB-16

7 40 PB-5 through PB-11, PB-13

Borings PB-1 through PB-5, PB-9 and PB-10 were drilled in the existing floodplain. Borings B-6, B-7, B-8, B-11 and B-12 were drilled on top of the existing access road. Borings B-13 through B-16 were drilled on the riverside slope and at the top of the existing levee. The borings were spaced at approximately 700 ft. intervals along the levee alignment. The borings were drilled at the locations shown on the Boring Location Map, Figure A-1, from November 10 through November 18, 2011 and on December 16, 2010. The boring locations were estimated by R-K

Project No. AEA10-050-00 5 using satellite maps and approved by USIBWC. Final locations were surveyed in the field by the project surveying consultant, Bohannan Houston, Inc.

Four of the borings (Borings PB-1 through PB-4) were drilled using environmental drilling techniques. All borings were advanced using a modified hollow-stem auger procedure. Hollow-stem augers were used from the ground surface until caving soils below the groundwater table were encountered. Mud rotary methods were then employed to the boring termination depths.

For two of the borings (Borings PB-5 and PB-12), Nx rock coring methods were introduced once bedrock was encountered at depths of about 2 feet and 34 feet, respectively. Upon completion of drilling, bentonite-cement grout was used to backfill the holes using tremie methods in accordance with EM-1110-1-1906 guidelines. During drilling operations, the following samples were collected in general accordance with the American Society for Testing and Materials (ASTM) and EM 1110-2-1913 and EM 1110-1-1906 guidelines:

Type of Sample Number Collected Split-Spoon (with Standard Penetration Test, SPT) 178

NX Core (core runs up to 5 ft.) 11

Grab Samples 1

The samples were obtained in general 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.

It should be noted that multiple attempts were made to collect Shelby tube samples in the isolated clay layers that were encountered in the borings, particularly in Boring PB-2. However, due to the soft and saturated condition of the clays, testable samples could not be retrieved.

LABORATORY TESTING

In the laboratory, which is USACE validated for soils testing, 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

Natural Moisture Content 162

Atterberg Limits 72

Sieve Analysis 31

Minus 200 Sieve 60

Moisture-Density Relationship (Proctor) 1

With the exception of the laboratory moisture-density relationship (proctor) and analytical soils 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-17, of Appendix A. A key to the classification of terms and symbols used on the logs is presented on Figure A-18, of Appendix A. The results of the laboratory and field testing are also tabulated on Figure A-19, of

Project No. AEA10-050-00 6

Appendix A for ease of reference. Please note where the results of the Atterberg Limits testing indicated the soil to be “non-plastic” the results of the testing are not included in Figure A-19.

However, the test results indicating “non-plastic” are included on the individual boring logs. The results of the remaining test are presented in Appendix B of the report.

SPT results are noted as “blows per ft” on the boring logs and on Figure A-19, of Appendix A, where “blows per foot” refers to the number of blows required for a falling automatic trip 140 -pound hammer to penetrate 12 inches into the subsurface materials, in general accordance with ASTM D 1586 procedures. 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. As referenced above, it should be noted that multiple attempts were made to collect Shelby tube samples in the isolated clay layers that were encountered in the borings, particularly in Boring PB-2, for the purpose of performing other strength and consolidation testing. However, due to the soft and saturated condition of the clays, testable samples could not be retrieved.

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

GENERAL SITE CONDITIONS

SITE DESCRIPTION

General

The Paisano Reach (Phase II) is situated along the U. S. bank of the Rio Grande River from the American Dam to approximately 80 feet upstream of the International Dam, on the central-west side of El Paso, El Paso County, Texas. The country of Mexico is located west of the site.

A few residential and commercial structures were observed along the far south east portion of the levee and the former ASARCO Smelter Company was observed at the east side of the north half of the project site in the U.S. territory, and mostly residential and commercial structures along the Mexican side. The American Canal and West Paisano Drive run parallel along the eastern side of the project levee. Multiple utilities run parallel to the Rio Grande and along the project site.

Observed structures along the site consist of a metal fence running parallel and east of the Rio Grande, an abandoned pump house in the vicinity location of Boring PB-5, and several monitoring wells.

Where present, the existing levee system along the project alignment was observed to have heights ranging from about 5 to 12 feet, the crest widths ranged from about 12 to 30 feet, and were relatively level and free of vegetation. Where levees were not present, the site appears to be relatively level and sparsely covered with grass and bushes.

Levee Geometry

URS Corporation provided surveyed cross-sections at 200-foot intervals along the existing alignment, as well as proposed top of levee and/or top of wall elevations. Additionally, it is our understanding that all of the existing and/or enhance levees will be reconstructed to maintain a slope of 3:1 (horizontal:vertical) or flatter on the river side and 2.5:1 or flatter on the land side of the levee, with a minimum levee crest of 16 ft. For Subreaches 1, 3 and 4, the final levee

Project No. AEA10-050-00 7 design height was approximately 12 ft. for our analyses, which is near the current levee height in Subreach No. 1, but requires up to 4 to 5 ft. of additional height in segments 3 and 4. The free wall height on the riverside for Subreach No. 2 utilized for analyses was estimated to be approximately 14 ft.

SITE GEOLOGY

Based on the Soil Survey for El Paso County, Texas, prepared by the U.S. Department of Agriculture Soil Conservation Service, dated 1971 and the USGS map, Geological Map of West Hueco Bolson, El Paso Region prepared by the Bureau of Economic Geology, The University of Texas at Austin (2000), the predominate soil type encountered along the project site consists of Made Land, Gila Soil Material (MG) with Igneous rock land (IG), and Delnorte-Canutillo, hilly (DCD) association located adjacent east, within a few hundred feet. The MG association follows the Rio Grande and grades into the DCD association at the southern and northern portions of the project alignment but in the middle section, the MG abruptly changes into the IG association.

The MG association lies on the recent floodplain of the Rio Grande and is comprised of silty clays to fine sands deposited by the Rio Grande during episodic flooding. It has been reworked by human activity during the construction of the levees and alignment of the Rio Grande channel and subsurface stratigraphy is minimal to non-existent, except at a depth greater than what was reworked by humans.

The Igneous rock land (IG) consists of very steep outcrops of andesite or syenite rock that weathers relatively easy, forming a weak, easily breakable surface. A hummocky surface results where low areas are filled with gravelly soil, capable of supporting sporadic desert plant colonies (i.e. creosote bush).

The DCB association is a very gravelly soil that occurs on foot slopes and over outwash material from igneous and limestone outcrops. A strongly cemented to indurated 24 inch thick caliche layer underlies the 6 to 12 inch gravelly soil surface.

The project site is located south of the Franklin Mountains within and near the limits of a shallow trachyandesitic intrusion that intruded through Paleozoic and Cretaceous age marine deposits.

During the mid Cretaceous Period, the El Paso area was located along the western coastline of the Cretaceous Inland Sea. Several periods of transgression and regression (sea level rise and fall) occurred during the Cretaceous and intertidal lagoonal and mud flats, tidal estuaries and channel deposits, to intermediate water reef structures, and numerous marine fauna are present in the marine sediments. These deposits are deposited as shales, sandstones, siltstones, and limestones.

Approximately 100-million years ago (mya), uplift in the region began occurring, and 70-mya the North American Plate began to collide with the oceanic Farallon Plate southwest of El Paso. This collision resulted in compressional stresses throughout western North America (i.e., Laramide Orogeny).

During the early Tertiary, continued subduction of the Farallon Plate resulted in a shallow, 6 mile wide belt of andesitic intrusions in the region. Andesite represents the end product of melting oceanic crust and lower continental crust along active subduction margins. However, during the early Tertiary Period, subduction of the Farallon Oceanic Plate under the North American Plate

Project No. AEA10-050-00 8 was occurring hundreds of miles southwest of the El Paso region. Geophysical models suggest that a segment of the Farallon Plate was subducted at a shallow angle, allowing the oceanic slab to reach the El Paso region before subsequent melting. The resulting intrusion, referred to as the Campus Andesite near the University of Texas as El Paso and Sierra de Cristo Rey intrusion to the north, intruded through a thick Precambrian section overlain by mostly Paleozoic and Cretaceous carbonates. The intrusion deformed the sediments, causing them to become highly folded and faulted. Subsequent erosion of the carbonates exposed the andesite sills and the marine deposits are only present near Sierra de Cristo Rey and at isolated locations near Executive Boulevard.

During the Plio-Pleistocene Epoch, the Rio Grande River flowed into the region north of the Franklin Mountains, filling the Hueco Bolson with local and exotic clasts. These sediments are up to several thousand feet thick. During the mid-Pleistocene, continued uplift of the Franklin Mountains forced the Rio Grande River to flow along the western and southern side of the Franklin Mountains, forming the Narrows. The Narrows is a narrow, possible fault-controlled pass between the Franklin Mountains and the Sierra de Juarez Mountain range in Mexico and dissects through the Campus Andesite near the former ASARCO Smelter.

The project site is situated within the Narrows, a possibly fault controlled erosion channel of the Rio Grande that has dissected through the Campus Andesite. The Rio Grande dissected through this area, north and west of the andesite intrusion and through the Narrows, depositing several tens to hundreds of feet of fluvial sediments (Camp Rice Formation). These sediments are comprised of local and exotic clasts, deposited in discontinuous sand, silt, and gravel deposits that are difficult to correlate. Several periods of aggregation and degradation have in-filled and eroded the Rio Grande Channel prior to human occupation in the region. The Rio Grande has been channelized by humans and the upper most surface of the fluvial deposited sediments have been reworked by human activity and show little to no subsurface stratigraphy.

The Geological Map of West Hueco Bolson; El Paso Region, Texas 2000, depicts the site as encompassing Alluvium of the Rio Grande floodplain (Qarg) in the southern portion of the site to Undivided alluvium of drainageways, young fans, and young arroyo terraces located along the Rio Grande valley border (Qavb) in the middle and northern portion of the site (i.e., near the ASARCO Smelter).

Arroyos originating in the Franklin Mountains and the Campus Andesite drain into the Rio Grande along the length of the site, but much of them have been buried with slag under the former ASARCO Smelter, while others have been channelized due to the numerous roadways in the vicinity.

Depth to bedrock along the site varies from several 10’s of feet below the overlying fluvial deposits to being exposed at the surface near the former ASARCO Smelter. Bedrock in the southern portion of the site consists of Andesite while in the northern portions of the site bedrock consists of Cretaceous marine deposits.

Due to the closeness of the Rio Grande River, the depth to groundwater is associated with the irrigation season. The average depth to groundwater would be expected at approximately 2 – 15 feet below ground surface and be closely associated with water levels in the Rio Grande.

Project No. AEA10-050-00 9

SEISMIC COEFFICIENTS

The following Spectral Response Accelerations with a 5% probability of exceedance in 50 years (950 years return period) are recommended for design at the El Paso area, based on the 2008 U.S. Seismic Hazard Maps published by USGS at http://earthquake.usgs.gov/hazards:

• Spectral Response Acceleration for a 0.2 sec. period: Ss = 0.19g.

• Spectral Response Acceleration for a 1.0 sec. period: S1 = 0.06g.

STRATIGRAPHY

Levee embankment soils observed within the levee in those borings drilled on top of the levee consisted of locally available non-cohesive and cohesive soils, predominately silty sands (SM) with interbedded clayey and/or gravel layers, which overlay highly variable subsurface profiles consisting of interbedded clayey sands (SC); silty clayey sands (SC-SM); well-graded gravels with silt and sand (GW-GM); gravelly silty sands (SM); gravelly clayey sands (SC); silty gravel with sand (GM); clayey silts (ML); silty clays with sand (CL-ML); lean clays with sand (CL); and sandy lean clays (CL). Subsurface conditions beneath the existing levees were highly variable but consisted of predominately granular soils. Due to the variability of subsurface conditions, the boring logs should be consulted for detailed stratigraphic information at the individual boring locations.

Three identifiable anomaly subsurface profiles were encountered in Borings PB-2, PB-5 and PB- 14 which likely require additional field exploration as similar conditions were not encountered in the adjacent borings. In Boring PB-2, an approximately 9 ft. layer of soft, saturated lean clays were observed near the surface. In Boring PB-5, shallow Andesite rock was encountered at a depth of 2 ft. below the existing ground surface, which extend to at least the 40 ft. termination depth of drilling. In Boring PB-14, construction debris and/or waste was encountered within the existing levee fill materials, and potentially in a fill layer beneath the levee. Each of these conditions will require further analyses, as discussed in a subsequent section of this report.

DEPTH-TO-WATER

Borings were drilled using dry-auger techniques in an attempt to observe groundwater during drilling operations. The depth-to-water levels recorded during drilling are considered short-term water level readings. Obtaining long-term depth-to-water level readings was not part of our scope of work as each boring was backfilled with cement-bentonite grout upon completion of drilling.

The depth-to-water level readings measured at the time of drilling ranged from about 7 to 19 feet below the top of the levee and/or ground elevation existing at the time of our study. In several cases, this equates to water depths of less than 3 ft. below the toe of the levee. It should be noted however, that all depth-to-water level readings were observed beneath the original ground elevations as evidenced by the existing ground elevations on both the riverside and landside toes of the levee. Free water was not observed within the levee embankment fill soils.

It is likely for groundwater to exist beneath this site at shallower depths on a transient basis following periods of precipitation and/or flooding. Fluctuations in groundwater levels occur due to variations in rainfall and surface water run-off. The construction process itself may also cause variations in the groundwater level.

http://earthquake.usgs.gov/hazards�

Project No. AEA10-050-00 10

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 Hazen2 equation is suggested for use. We have also used the Hazen equation to estimate poorly graded sands with some silt (SP-SM) as well as the limited number of gravel samples (GP, GP-GM, GW and GW-GM.) The following graph presents the results calculated using the Hazen equation plotted against the sample fraction passing the No. 200 sieve.

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 1·10-6 cm/sec for the lean clays. The following table summarizes the assumed horizontal permeability values used in our analyses.

2 Hazen, Allen, “Discussion of ‘Dams on Sand Foundations’ by A.C. Koenig”, Trans. American Society of Civil Engineers, Vol. 73, p. 199, 1911.

Permeability Values Calculated Using Hazen's Equation

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Material

Horizontal Saturated Design Permeability*, cm/sec

Gravels (GP, GP-GM, GW, GW-GM) 1.00E-01

Poorly Graded Sands (SP) 1.00E-02

Poorly Graded Silty Sands (SM) 1.00E-04

Lean Clays (CL) 1.00E-06

Gravel (Drainage Blanket) 1.00E+00

Sand (Drainage Blanket) 1.00E-02

Concrete 1.00E-08

Cement Treated Base 1.00E-02

Lean Clay FILL (CL) 1.00E-06

Relief Well (Gravel Filter) 1.00E-01 *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 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

Gravels (GP, GP-GM, GW, GW-GM) 14 - 50+ 35-43

Poorly Graded Sands (SP), Silty Clayey Sands and with Gravel (SC-SM), Silty Clayey Gravel and Sand (GC-GM), Poorly Graded Sand with Silt (SP-SM)

2 – 50+

30 - 43

Poorly Graded Silty Sands (SM), Sandy Silt (ML), Silty Gravel (GM)

2 - 20 30 - 35

Clayey Sand with Gravel (SC) 2 – 37 30 - 37

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

Project No. AEA10-050-00 12

Undrained Shear Strength, psf

Material SPT Blow Count Range Inferred Range

Lean Clays (CL) 2 - 9 250 – 1,125

Fat Clays (CH) 37 4,625

The boring logs were reviewed and the classification of the various strata into one of the above materials…