08_-_Vado_Geotech_Raba_Kistner_20110525.pdf

DESIGN OF REHABILITATION IMPROVEMENTS

For

RIO GRANDE CANALIZATION FLOOD CONTROL PROJECT

VADO SEGMENT – EAST LEVEE

VADO, DOÑA ANA COUNTY, NEW MEXICO

GEOTECHNICAL MEMORANDUM

Engineers, Geologists, Hygienists and Environmental Scientists

San Antonio, TX

Austin, TX Brownsville, TX El Paso, TX

Mexico McAllen, TX Houston, TX

Project No. AEA10-051-00 May 25, 2011

Mr. Yen-Hsu Chen, Principal Project Manager Tetra Tech, Inc.

17885 Von Karman Avenue, Suite 500 Irvine, California 92164

RE: Design of Rehabilitation Improvements for the

Rio Grande Canalization Flood Control Project (RGCFCP) Vado Segment – East Levee

Vado, Doña Ana County, New Mexico Geotechnical Memorandum USIBWC IDIQ Contract No. IBM10T0044 Tetra Tech Project No. 100-SWW-T26735

Dear Mr. Chen:

Raba-Kistner Consultants, Inc. (R-K) is pleased to submit our Geotechnical Memorandum for the feasibility study and final design for the approximately 1.08-mile long section of East levee system (referred to as Vado Segment) within the vicinity of Vado, Doña Ana County, New Mexico. These services were performed in accordance with the scope of work pursuant to the executed Subcontract Professional Services Agreement between Tetra Tech, Inc. (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 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 final 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

Chris L. Schultz, P.E.

Senior Vice President

RLT/JAF/CLS/mem Attachments Copies Submitted: Above (6)

Engineering • Testing • Environmental • Facilities • Infrastructure

Raba-Kistner Consultants Inc.

7002 Commerce El Paso, Texas 79915

(915) 778-5233 FAX (915) 779-8301

www.rkci.com TBPE Firm F-3257

Austin Brownsville Dallas El Paso Houston McAllen Mexico San Antonio

DESIGN OF REHABILITATION IMPROVEMENTS

RIO GRANDE CANALIZATION FLOOD CONTROL PROJECT

VADO SEGMENT – EAST LEVEE

VADO, DOÑA ANA COUNTY, NEW MEXICO

GEOTECHNICAL MEMORANDUM

for

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

(USIBWC)

Prepared for

TETRA TECH, INC.

Irving, California

Prepared by

RABA-KISTNER CONSULTANTS, INC.

San Antonio, Texas

PROJECT NO. AEA10-051-00

Raba-Kistner

EXECUTIVE SUMMARY

The purpose of this study was to drill borings along the existing 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 final 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 1-mile segment of the East levee Vado Segment) were evaluated by drilling and sampling five borings as shown on the following table.

Number of Borings Depth, feet Boring Identification

5 100 * B-1, and B-3 through B-6

* below the toe (riverside) of levee elevation existing at the time of our study.

Boring B-2 could not be performed, due to its proposed location within an existing railroad easement. The borings were spaced at approximately 700 feet to 1,000 feet intervals along the levee alignment. The borings were drilled at the locations shown on the Boring Location Map, Figure A-1, from November 8 through November 15, 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, Zia Engineering and Environmental Consultants, LLC.

The completed 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.

Upon completion of drilling, bentonite-cement grout was used to backfill the holes using tremie methods in accordance with EM-1110-1-1906 guidelines.

Fill soils observed within the levee consisted of locally available non-cohesive and cohesive soils, predominately silty sands (SM) and sands (SP) with interbedded near surface silt (ML) layers and deeper gravel (GW, GP and GW-GM) layers. 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.

The levee should be constructed using lean clays placed with 3:1 (horizontal:vertical) or flatter slopes. We understand that due to right-of-way constraints, some landside slopes may be constructed 2.5:1 slopes with a maximum slope height of 4 ft. The proposed levees should not experience global problems with either slope stability or underseepage, although we do suggest that the Del Rio Drain be filled during flood events as a precaution against the formation of sand boils. Additionally, a toe drain is recommended from Stations 642+00 to 658+00. The recommendation to include toe drains in this area is based on the results of steady state seepage analyses of the levee during a design flood event. Given the relatively short duration of flood events, we do not anticipate that steady state seepage will actually develop, and the quantity of seepage collected in the toe drains is expected to be relatively small. The calculated steady state seepage per foot of alignment into the toe drains may be used to provide an upperbound estimate of the total quantity of seepage that could seep through the drains by multiplying the seepage rate and the time period when the river level will be at or above the drain elevation. The actual quantity of seepage is expected to be a fairly small fraction of this calculated quantity.

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

RISK

LIMITATIONS

PREVIOUS STUDY

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 Del Rio Drain

ITERATIVE DESIGN PROCESS/RECOMMENDATIONS

LEVEE SEEPAGE ANALYSIS

TOE DRAIN

LEVEE SLOPE STABILITY ANALYSIS

MINIMUM FACTOR OF SAFETY

ii

RESULTS OF ANALYSES

LEVEE EROSION PROTECTION

SETTLEMENT ASSOCIATED WITH THE PROPOSED RAISED LEVEE

FOUNDATION DESIGN RECOMMENDATIONS

BEARING CAPACITY

ESTIMATED SETTLEMENT

LATERAL AND SURCHARGE EARTH PRESSURE

MODULUS OF SUBGRADE REACTION (“K-VALUE”)

SEISMIC DESIGN CONSIDERATIONS

LIQUEFACTION

DRAIN CONSIDERATIONS

DRAIN INSPECTION REQUIREMENTS

DRAIN FILLING REQUIREMENTS

IMPACT OF CHANNEL DREDGING AND CHANNEL IMPROVEMENTS

LEVEE TRAFFICABILITY

CONCLUSIONS

RECOMMENDATIONS

CONSTRUCTION RECOMMENDATIONS

GENERAL CONSTRUCTION CONSIDERATIONS

CLAY FILL LEVEE MATERIALS

VEGETATIVE COVER FOR CLAY LEVEES

CRUSHED ROCK BASE

TOE DRAIN MATERIAL

ATTACHMENTS

Appendix A (Field Testing Summary) Boring Location Map ................................................................................................................. A-1 Logs of Borings ...................................................................................................... A-2 through A-6 Key to Terms and Symbols ....................................................................................................... A-7 Results of Soil Sample Analyses ......................................................................... A-8 through A-10 iii

Appendix B (Laboratory Testing Summary) Grain Size Curves .................................................................................................. B-1 through B-4 Liquid Limit, Plastic Limit, and Plasticity Index of Soils ............................................................. B-5 Moisture-Density Relationship (Proctor) ................................................................................... B-6

Appendix C (Results of Geotechnical Analyses)

Project No. AEA10-051-00 1

INTRODUCTION

Raba-Kistner Consultants, Inc. (R-K) has completed the authorized subsurface exploration and engineering analyses of the approximately 1.08-mile segment of the East Levee (referred to as Vado Segment) within the vicinity of Vado, Doña Ana County, New Mexico. This Geotechnical Memorandum briefly describes the procedures utilized during this study and presents our findings along with our geotechnical analyses for the final 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 United States Section of the International Boundary and Water Commission (USIBWC) has conducted hydraulic and aerial geophysical studies of the Canalization Project to assess the ability of the associated levees to safely contain flood flows in from the Rio Grande River. From these studies, the USIBWC has determined that certain reaches of the levees require rehabilitation improvements in order to meet FEMA requirements for the safe containment of 100-year flood flows. The purpose of this investigation is to support the upgrade of the Vado Segment in order to meet FEMA criteria.

The Rio Grande Canalization Project was completed in 1943 pursuant to Public Resolution No.

4-74th Congress, Approved February 13, 1935, Control and Canalization of the Rio Grande- Caballo Dam, New Mexico to El Paso, Texas. The Project extends for 105.6 miles along the Rio Grande from Percha Dam to the American Dam at El Paso. The Canalization Project consists of a normal flow channel and a leveed floodway. The normal channel carries the irrigation releases from Elephant Butte and Caballo Dams to the headworks of irrigation projects developed by the U.S. Bureau of Reclamation in New Mexico and Texas and also carries waters released for delivery to Mexico under the Treaty of 1906. The normal flow channel has a depth of 3 to 5 ft, a width ranging from 110 to 500 feet, and a capacity of 2,500 cubic feet per second (cfs) above Leasburg Dam to 1,200 cfs at El Paso. The floodway varies from about 50 to 2,100 feet in width.

Its bordering levees are 3 to 15 feet in height and have a total length of 130 miles. The crowns of the levees are gravel surfaced to facilitate access during flood emergencies and maintenance.

Some of the floodway lands are used for recreational purposes and the remainder largely for grazing and agriculture. The reach for this study begins at the upstream of Del Rio Drain Outlet to Three Saints Lateral and is called the Vado Segment.

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.

Project No. AEA10-051-00 2

The approximate coordinates of the Vado Segment for improvements are listed below. The approximate length of our study was about 1.08 miles long.

The East Levee Vado Segment Latitude Longitude

Beginning Station 32° 6’ 14.44” N 106° 39’ 36.05” W

Ending Station 32° 5’ 24.53” N 106° 39’ 28.74” W

The proposed levee rehabilitation improvements consists of: 1) raising the top of levee elevation;

2) providing seepage control structures, as required, to control through-seepage and under-seepage; and 3) rehabilitating existing pipeline/culvert structures and appurtenances located within the levees by modifying them to be in general accordance with the raised levee cross-section, repairing gates and gatewells, as required, and adding gates to ungated structures.

We understand that portions of the subject levee alignment’s right-of-way (ROW) are located within an existing railroad easement, which will dictate the side slope inclination of the raising of the top of levee elevation.

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 final design recommendations inherently assume that the levee will be properly maintained and regularly inspected by trained and qualified personnel. Our final 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 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 final 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.

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. AEA10-051-00 3

LIMITATIONS

This Geotechnical Memorandum has been prepared in accordance with accepted Geotechnical Engineering practices in the region of south New Mexico for the use of Tetra Tech (CLIENT) and its representatives for design purposes. This Geotechnical Memorandum may not contain sufficient information for purposes of other parties or other uses.

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 Tetra Tech, Inc. (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.

PREVIOUS STUDY

R-K has previously performed a Geotechnical Engineering Study along the East Levee (Canutillo Segment), which included borings located along the subject Vado Segment (R-K Project No.

AEA08-020-03). The results of this geotechnical engineering study are on file in our office. Our previous data was utilized as supplementary information in the preparation of this report.

FIELD EXPLORATION

Representatives of the USIBWC, Tetra Tech 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.

Project No. AEA10-051-00 4

As requested, subsurface conditions along the approximately 1-mile segment of the East levee Vado Segment) were evaluated by drilling and sampling five borings as shown on the following table.

Number of Borings Depth, feet Boring Identification

5 100 * B-1, and B-3 through B-6

* below the toe (riverside) of levee elevation existing at the time of our study.

The borings were generally spaced at approximately 700 feet to 1,000 feet intervals along the levee alignment. The borings were drilled at the locations shown on the Boring Location Map, Figure A-1, from November 8 through November 15, 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, Zia Engineering and Environmental Consultants, LLC.

Boring B-2 could not be performed, due to its proposed location within an existing railroad easement. During the irrigation season, the proposed levee alignment is within the Rio Grande channel, and is inaccessible to conventional drilling. River flows dropped significantly in late November 2010. At that time, access would have required significant field work to prepare a working pad and access road for the drill rig, as well as permitting from both the railroad and

USACE.

The completed 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.

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) 115

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.

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:

Project No. AEA10-051-00 5

Type of Test Number Complete

Natural Moisture Content 91

Atterberg Limits 49

Sieve Analysis 17

Minus 200 Sieve 32

Moisture-Density Relationship (Proctor) 1

With the exception of the laboratory moisture-density relationship (proctor) test, 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-6, of Appendix A. A key to the classification of terms and symbols used on the logs is presented on Figure A-7, of Appendix A. The results of the laboratory and field testing are also tabulated on Figures A-8 through A-10, of Appendix A for ease of reference. 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 Figures A-8 through A-10, 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 following seating of the spoon. 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.

Undisturbed sampling was not performed for this project, primarily because very few clay soils were observed in thin strata in only two borings. A 1 ft thick sandy clay layer was observed near the ground surface in Boring B-4, while a 5 ft clay layer was observed at the surface in Boring B-5. These clays are very low in plasticity, and are borderline CL (very low plasticity clay) materials with Plasticity Indices of 10 to 13. The 5 ft lean clay layer observed at the ground surface in boring B-5 did not behave as a cohesive material; the upper portion was relatively dry with a moisture content about 8 percent below the soils’ Plastic Limit, while the deeper portion of this layer was relatively wet with a moisture content a few percent above the soils’ Liquid Limit.

Since this soil exhibited silt-like behavior, neither sample would have been sampled successfully with a Shelby tube sampler, and the use of SPT sampling techniques was determined appropriate for the conditions.

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 project site is situated within the limits of the relic floodplain along the Rio Grande Levee near Vado, New Mexico, between El Paso, Texas and Las Cruces, New Mexico. Farm fields and a few residential structures were observed along the project alignment. Railroad tracks were observed to run parallel along the project east area. The site appears to be relatively level and sparsely covered with grass and small bushes.

Project No. AEA10-051-00 6

Please note that approximately 360 feet portion of the subject levee alignment (from approximately Sta. 660+11.77 to Sta. 656+51.86) falls within the existing ROW of an existing railroad easement. Hence, due to easement restriction, Boring B-2 could not be conducted.

Levee Geometry

Tetra Tech, Inc. provided surveyed cross-sections at 250-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 both the river side and land sides of the levee, with a minimum levee crest of 12 ft in new levee areas, while the crest will be maintained at 16 ft for the existing levee. We understand that due to right-of-way constraints, some landside slopes may be constructed 2.5:1 slopes with a maximum slope height of 4 ft.

SITE GEOLOGY

The site is located within the limits of the relic Rio Grande River floodplain, northwest of the Franklin Mountains, within the limits of the Mesilla Bolson. The Mesilla Bolson is located on the west side of the Franklin Mountains and extends northward into New Mexico, and is filled with lacustrine and fluvial deposits.

The Franklin Mountains are a horst block, linear 23-mile long and less than 5 mile wide north-south trending mountain range, situated near the eastern margin of the Basin and Range Physiographical Province and within the Rio Grande Rift, a compilation of en echelon extensional basins extending from central Colorado, through New Mexico, and potentially into northern Mexico.

During the Pleistocene, the Rio Grande was a stream, originating in Colorado that flowed into the basin lakes and progressively overtopping each basin, flowing into the southern adjacent basin.

By the late Pleistocene, the Rio Grande became a through flowing stream to the Gulf of Mexico.

Periods of episodic entrenching and flooding of the Rio Grande, correlating to seasonal and climate fluctuations, began forming stream terraces and fluvial deposits throughout the region.

Sediments within these basins consist of mixed-rounded gravels, sand, silts, clays and conglomeratic sediment of the Santa Fe Group. The Santa Fe Group sediments have been informally divided into the upper, middle and lower and collectively vary in thickness from several hundred to several thousands of feet thick throughout the course of the Rio Grande. In the El Paso/Las Cruces region, the upper Santa Fe Group sediments have been divided into the Fort Hancock Formation and Camp Rice Formation.

The Fort Hancock Formation is loose, unconsolidated Plio-Pleistocene age sediments containing very fine-grained clay with minor sand that formed in a fluvial and lacustrine environment associated with Lake Cabeza de Vaca. Lake Cabeza de Vaca was located within the Hueco Bolson (east of the Franklin Mountains), the Mesilla Bolson, and the Bolson De Los Muertos, located southwest of Ciudad Juarez, Mexico.

The Pleistocene Camp Rice Formation overlies the Fort Hancock Formation and is predominantly fluvial sediments associated with the ancestral Rio Grande River, and was deposited between

Project No. AEA10-051-00 7

~2.5 mya and 0.78 mya (Late Pliocene to middle Pleistocene). The Camp Rice Formation contains predominantly local clasts, but clasts from northern New Mexico are not uncommon.

Well cemented caliche horizons are associated with the Camp Rice Formation and are derived from the dissolution and reprecipitation of the limestone clasts.

Below the Fort Hancock Formation, relatively unconsolidated basin fill materials, ranging from upper clean sand and rounded gravel through sand to lower clays, silts, and sands with varying amounts of angular to sub-rounded gravel are present. Lenticular bodies of ash occur in the upper Santa Fe sediments. The ash is derived from Tertiary volcanics in the area and ash zones as much as 5 feet thick consisting of white, uncemented, massive to laminated glass shards of coarse silt and fine-sand are present.

Generally the underlying strata (middle Santa Fe Group) are evenly bedded with minimal exotic clasts, indicating the sediments were primarily deposited in a closed basin lacustrine system.

Around the margins of the lakes, lacustrine sediments inter-finger into ephemeral streams originating in the surrounding highlands. The overlying sediments typically consist of fluvial deposited gray coarse-grained, cross-bedded gravel, sand, silt, and volcanic ash. The Upper Santa Fe Group is unevenly bedded, has a wide range of particle size, and was deposited mainly in stream channels and flood plains. Gravel clasts within the deposits and channel gravels comprise a wide variety of distant and local resistant rock type sources, indicating a through flowing depositional system.

The site is located along the levee of the Rio Grande which was channelized in the 1930’s as part of a valley-wide flood control and irrigation project. The Rio Grande Floodplain deposits represent the most recent incision of the Rio Grande into the Upper Santa Fe Group Sediments and were deposited during the late Pleistocene through the Holocene Epochs. The thickness of the Rio Grande Floodplain deposits are up to 200 feet thick and comprised of distant and local source sediments ranging from gravel to silt size.

Crevasse splays are the typical deposition pattern encountered on the Rio Grande floodplains and are comprised of thin fans of sand and silt. These sediments are spread across the floodplain during a break in a natural or artificial levee caused by a flood event. The splay grows uniformly progressively coarser and sandier until new channels are cut into the top of the splay. These channels are then filled with coarse sediment that becomes progressively finer-grained upward, opposite the majority of the splay deposition. These channels are discontinuous, often become commingled, and are difficult to correlate across short distances.

The New Mexico Geologic Map, 1996, depicts the site as encompassing Quaternary age fluvial sediments and depth to bedrock along the site is anticipated at several 100’s to 1,000’s of feet below the overlying fluvial deposits.

Do 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 5 – 10 feet below ground surface.

SEISMIC COEFFICIENTS

Project No. AEA10-051-00 8

The project site was evaluated for peak ground acceleration (PGA) for three design earthquakes:

the Operating Basis Earthquake (OBE), the Maximum Design Earthquake (MDE) and the Maximum Credible Earthquake (MCE.) The criteria for the OBE and MDE events were chosen based on USACE and Tetra Tech, Inc. criteria. Since the project is relatively limited in size, the values were calculated using the latitude and longitude of boring B-1, which is very close to the Del Rio Drain box culvert structure. The following parameters were calculated using the U.S.

Geologic Survey’s “Seismic Hazard Curves and Uniform Hazard Response Spectra” Java applet (named ‘NSHMP_HazardApp.jar’) version 5.1.0 dated February 10, 2011.

Condition Peak Ground Acceleration (g)

Operating Basis Earthquake (OBE) 0.0256

Maximum Design Earthquake (MDE) 0.0811

Maximum Credible Earthquake (MCE) 0.13

The PGA for the OBE was calculated assuming the design event has a 50 percent probability of exceedance in a 100 year observation period, while the PGA for the MDE was calculated assuming a 10 percent probability of exceedance in a 100 year period. The PGA for the MCE was calculated using the USGS “DesignMaps” Detailed report, which is based on the 2009 NEHRP (https://geohazards.usgs.gov/secure/designmaps/us/application.php).

STRATIGRAPHY

Subsurface conditions within the borings conducted for this study mainly consisted of locally available non-cohesive and cohesive soils, predominately silty sands (SM), poorly-graded sands (SP), and poorly-graded sands with silt (SP-SM) with interbedded clay, silt, and/or gravel layers.

Due to the variability of subsurface conditions, the boring logs should be consulted for detailed stratigraphic information at the individual boring locations.

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 7-1/2 to 11- 1/2 feet below the toe (riverside) of the levee and/or ground elevation existing at the time of our study. 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.

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

Project No. AEA10-051-00 9 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

Project No. AEA10-051-00 10

Material Horizontal Design Permeability*, cm/sec

Gravels (GP, GW) 1.00E-01

Poorly Graded Sands (SP) 1.00E-02

Poorly Graded Silty Sands (SM) 1.00E-04

Gravel (Toe Drain) 1.00E-01

Sand (Toe Drain) 1.00E-02

Lean Clay FILL (CL) 1.00E-06 *Vertical permeability was estimated to be 20% of horizontal permeability

DESIGN SHEAR STRENGTH

The design shear strength parameters for this study were selected based on the resistance to driving while performing the SPT test, prior area experience, and professional judgment. The following tables summarize the design values used in our analyses.

End of Construction Condition

Total Stress

Material Total Unit

Weight

Cohesion, psf Angle of Internal Friction degrees

Gravels (GP, GW) 135 0 35

Poorly Graded Sands

(SP, SP-SM)

117 0 33

Poorly Graded Silty Sands (SM)

117 0 32

Lean Clay FILL (CL) 120 1,000 0

Gravel (Toe Drain) 135 0 35

Sand (Toe Drain) 125 0 32

Steady State Condition

Effective Stress

Material Total Unit

Weight

Cohesion, psf Angle of Internal Friction degrees

Gravels (GP, GW) 135 0 35

Poorly Graded Sands

(SP, SP-SM)

117 0 33

Poorly Graded Silty Sands (SM)

117 0 32

Lean Clay FILL (CL) 120 200 24

Gravel (Toe Drain) 135 0 35

Sand (Toe Drain) 125 0 32

Project No. AEA10-051-00 11

Rapid Drawdown Condition

The design soil properties used for the rapid drawdown analyses are identical to the effective stress parameters used in the steady state analyses and the total stress parameters from the end of construction condition. The analyses compare the resistance to sliding using both total and effective stresses, and implement the lower of the two calculated values for a given portion of the assumed slide surface.

EARTHEN LEVEE SYSTEM

DESIGN CONSIDERATIONS

The project will consist of an earthen levee system. The existing and modified levees should meet three important criteria: they should be resistant to the forces of erosion, should exhibit a slope stability design allowable factor of safety of at least 1.3 for end of construction, 1.4 for steady state and 1.0 for rapid drawdown, and the calculated exit gradients should not be greater than 0.5 and have a factor of safety of at least 1.4 against heave at the toe where a “blanket” condition exists. The levee structure must meet these criteria so that the calculated risk of failure is consistent with criteria established by the U.S. Army Corps of Engineers and USIBWC design guidelines.

PROBABLE FAILURE MODES

Our review of the site and expected conditions indicates that the following major modes of failure could affect the levee system during a flood event:

Embankment (“Through”) Seepage Under seepage Slope erosion Slope stability Instability of the Del Rio Drain

The following sections address each of these failure modes. Please note that while these failure modes have been separated for ease of discussion, it is possible that two or more of the failure modes may occur simultaneously.

Embankment Seepage

Embankment seepage consists of water flowing through the embankment as a result of the rise in river level during the design flood event. The principal failure mechanism related to embankment seepage occurs when the seepage gradients in the soils are sufficient to transport fine soil particles through the soil mass and create a “pipe.” However, as discussed in detail in a subsequent section, the new levees will be constructed utilizing clay fill materials. A small toe drain should be included to protect the new levee from the potential for damage from through seepage. Provided this recommendation is implemented, we generally consider embankment seepage to be a relatively low risk for the modified levee embankments. The recommendation to include a toe drain is based on the results of steady state seepage analyses of the levee during a

Project No. AEA10-051-00 12 design flood event. Given the relatively short duration of flood events, we do not anticipate that steady state seepage will actually develop, and the quantity of seepage collected in the toe drain is expected to be relatively small. The calculated steady state seepage into the toe drain per foot of alignment may be used to provide an upperbound estimate of the total quantity of seepage that could seep through the drains by multiplying the seepage rate and the time period when the river level will be at or above the drain elevation. The actual quantity of seepage is expected to be a fairly small fraction of this calculated quantity.

Under Seepage

We generally consider under seepage to be the major risk for the proposed levee embankments.

Under seepage consists of water flowing beneath the embankment as a result of a rise in river level during the design flood event. The principal failure mechanisms related to under seepage occur when: 1) the seepage force equals or exceeds the buoyant weight of the soil in the toe area;

and/or 2) when the seepage gradients in the soils are sufficient to transport fine particles through the foundation soils and create a pipe. The first process, often referred to as a “quick condition”, represents a potentially serious condition for overall levee stability. The second process is referred to as piping and is also potentially dangerous to the overall stability of the levee system.

When the upward force of water equals or exceeds the buoyant weight of the soil, the soil particles can become dislodged and carried away by the water. The soil particles are lost where the flow exits the ground surface; this “loss of ground” can migrate toward the river as more soil is lost, forming a “pipe” or open channel. If a pipe becomes large enough, and collapses during the flood event, the levee can be breached in a sudden, catastrophic event.

Slope Erosion

Significant quantities of the existing levee embankment materials, where present, are granular and are susceptible to slope erosion failures. However, similar to embankment seepage, provided the new levees are constructed of clay fill material, the risk of slope erosion is considered low. Properly compacted clays are generally resistant to erosion, although all soils will erode if subjected to a sufficiently swift current, overtopping or unusual turbulence.

Slope Stability

For the proposed levee configuration, slope stability is not considered to be a concern for the levees evaluated for this project. For the proposed enhanced conditions which include slopes of 3:1 (horizontal:vertical) or flatter on the land side and 3:1 on the flood side, the levees constructed as recommended herein are not considered to have a significant risk of slope instability. We understand that due to right-of-way constraints, some landside slopes may be constructed 2.5:1 slopes with a maximum slope height of 4 ft. These relatively short slopes are not considered to have a significant risk of slope instability.

Del Rio Drain

As referenced in a previous section of this report, the Del Rio Drain is located at least 30 ft from the toe of the levee alignment, when the canal is closest to the levee. For this condition, the results of our analyses indicate that under seepage will not result in pressures that will heave the bottom of the canal since the canal is unlined and the subgrade soils consist of silty sands. The

Project No. AEA10-051-00 13 calculated seepage exit gradients were below 0.5, indicating a low probability that piping would occur in this area.

ITERATIVE DESIGN PROCESS/RECOMMENDATIONS

It should be noted that the analyses of the levee system, as well as potential remedial activities, is an iterative design process. Furthermore, remedial activities performed to correct one deficiency will impact the results of the remaining analyses for the various failure modes. As referenced above, several failure modes were identified for the levee portions within this reach.

During the design process for the proposed levees, a final recommended design solution was determined, which consists of the following recommendations:

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 3:1 or flatter, or 2.5:1 on the land side levee slope where the slope height will not exceed 4 ft.

A toe drain should be installed in the landside levee toe to control seepage through the levee during steady state seepage conditions.

Overbuild the levee height by 6 in. to account for potential settlement.

The Del Rio Drain should be full during the design storm event as a precautionary measure, and must be full if the canal is lined.

The analyses based on these improvements, as well as discussions on other options considered, is provided in the following sections of this report.

LEVEE SEEPAGE ANALYSIS

We performed a steady-state seepage analysis of proposed slope geometries using SEEP/W.

Our analyses included the slope geometry discussed in a previous section of this report. The lateral boundaries of the model on the land side were located at a distance from the levee profile following the guidelines established in USACE DIVR 1110-1-4003. The full hydrostatic pressure associated with the design 100 year storm was assumed to act on the boundary on the flood side model boundary at the Rio Grande, while a hydrostatic pressure associated with the groundwater at the ground surface was assumed to act on the land side model boundary. The analyses were performed assuming that the hydraulic properties of the soils exhibited a horizontal permeability that was five times greater than the vertical permeability.

Conditions exceeding the maximum allowable seepage exit gradient of 0.5 were not observed for the levee profiles evaluated for this study. These analyses do not indicate the need to lengthen the seepage flow paths, provide pressure relief, and/or intercept the seepage with one or more drainage layers.

Our analyses indicate the low areas directly behind the proposed levee would experience significant upward seepage during steady state conditions, with the rate of seepage influx on the order of 700 to 800 gallons per day per foot of levee alignment. The potential for flooding in these areas can be eliminated by filling these areas with soils that are similar to the silty sand subgrade

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

Project No. AEA10-051-00 14 soils. In general, we do not recommend filling these areas with clays or clayey soils, since the use of these materials in the low-lying areas behind the levee would create a blanket condition that could result in problems during steady state conditions. It appears that the silty sands needed to fill these low-lying areas may be available from the soils to be excavated and/or dredged from the river bottom as a part of the planned improvements to this section of the river.

TOE DRAIN

The under seepage was analyzed assuming a toe drain, as described below, was installed at the land side toe to control the exit point of the water. The results of the analyses with the toe drain indicated it sufficiently controlled the seepage through the levee at the landside levee face.

The toe drains should extend 1.5 ft. up (measured from vertical) the landside face. Additionally, the drain should penetrate the subgrade at the landside toe for a minimum of 1.5 ft. The toe drain width extends horizontally a minimum of 3 ft. The temporary cut slopes for the sides of the toe drain should extend no steeper that 1:1 (horizontal:vertical). The toe drain should be constructed with gravel meeting the gradation requirements of ASTM D 448 coarse concrete aggregate Size No. 57, 67 or 357. The gravel must be entirely wrapped with a geotextile fabric (Mirafi 160N or equivalent). The geotextile fabric should be buried 6 in. beneath the exposed surfaces of the gravel to prevent damage.

Toe drains should be installed from Sta.642+00 to Sta.658+00. Based on the results of the SEEP/W analyses performed for this study, the calculated flow rate into the toe drain is 700 to 800 gallons per day per foot of levee alignment.

LEVEE SLOPE STABILITY ANALYSIS

We performed a steady-state slope stability analysis of proposed slope geometries using SLOPE/W. Our analyses included a slope declination of 3:1 (horizontal:vertical) for the flood side of the levee and 3:1 for the land side of the levee.

MINIMUM FACTOR OF SAFETY

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

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

Condition Required Factor of Safety

End of Construction 1.3

Sudden (“Rapid”) Drawdown 1.0 to 1.2*

Long Term (Steady Seepage) 1.4

Project No. AEA10-051-00 15

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

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

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

RESULTS OF ANALYSES

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

Computed Factors of Safety

Station 646+00

End of Construction-

Flood Side

End of Construction-

Land Side

Steady State at Flood Stage –

Flood Side

Steady State at Flood Stage –

Land Side

Sudden Drawdown – Flood Side

2.4 2.6 2.9 1.9 1.8

Station 650+00

End of Construction-

Flood Side

End of Construction-

Land Side

Steady State at Flood Stage –

Flood Side

Steady State at Flood Stage –

Land Side

Sudden Drawdown – Flood Side

2.4 2.5 2.9 1.8 1.8

Based on the results of our analyses, the factors of safety computed for flood side and land side slopes of 3:1 (horizontal: vertical) and 3:1, respectively, or flatter meet the USACE requirements for minimum safety factors for the assumed flood event based off of the…