B08__A16_Geotechnical_Data_Report_MC185_MC270_MC420.pdf

CRIP MAIN CANAL

CHECK STRUCTURES

November 2022

Prepared for:

Prepared by:

GEOTECHNICAL DATA REPORT

CRIP Main Canal Check Structures

Geotechnical Data Report

Prepared For:

Bureau of Indian Affairs

Office of Trust Services

Division of Water and Power

Task Order No. 140A1622F0021

Prepared By:

DOWL

222 N 32nd Street, Suite 700

Billings, MT 59101

November 3, 2022 \\dowl.com\j\Projects\26\12494-01\82Rpts\T2-3_GeotechnicalReport\CRIP_GeotechnicalReport.docx

Geotechnical Data Report: November 2022

Page i

TABLE OF CONTENTS

Executive Summary

1.0 Introduction

2.0 Project Understanding

3.0 Investigation

4.0 Subsurface Conditions

5.0 Engineering analysis & Recommendations

6.0 Geotechnical Design Continuity

7.0 LImitations

8.0 References

LIST OF TABLES

Table 1: Exploration Summary

Table 2: Laboratory Tests

Table 3: Dispersion Test Results

Table 4: Groundwater Depths During Drilling

Table 5: Documented Faults

Table 6: Seismic Design Parameters

Table 7: MC 420 Foundation Design Parameters

Table 8: Lateral Earth Pressures

Table 9: Engineering Properties

Table 10: Slope Stability Results

Table 11: Fill Specifications

Table 12: Compaction Specifications

Table 13: Soil Chemistry Test Results

LIST OF FIGURES

Figure 1: Vicinity and Location Map

Figure 2: Surficial Geologic Map

Figure 3: 185 Exploration Locations

Figure 4: 270 Exploration Locations

Figure 5: 420 Exploration Locations

LIST OF APPENDICES

Appendix A: Exploration Logs

Appendix B: Photograph Log

Appendix C: Laboratory Test Results

Appendix D: Calculations

EXECUTIVE SUMMARY

DOWL prepared this geotechnical report for the U.S. Bureau of Indian Affairs. This report addresses the recommendations for the Main Canal check structures MC 185, MC 270, and MC 420 of the Colorado River Irrigation Project. Based on the information obtained from our subsurface exploration and testing, the sites are suitable for rehabilitating two structures (i.e., MC 185 and MC 270) and reconstructing the third (MC 420) in conjunction with a slight canal realignment. Through the course of our evaluation, we identified the following geotechnical considerations:

• Generally, subsurface soils at MC 185 and MC 270 consist of one to two feet of gravel fill (i.e., the existing canal operations and maintenance road) over seven to seventeen feet of silty sand to sandy silt embankment fill overlying native sand. At the MC 420 check structure location (off of the existing canal embankment in the realignment reach), subsurface soils generally consist of one to two feet of gravel fill (gravel on the edge of the access road) overlying clay to a depth of 17 to 23 feet overlying sand.

• We encountered manmade fill in the borings drilled through the embankment. The fill generally consisted of sand at MC 185 and MC 270 and clay at MC 420.

• Based on our evaluation, the new structure at MC 420 can be constructed on a conventional mat foundation.

• On-site soils, including both native and fill materials, appear to be suitable for use as general fill (e.g., for embankment construction and structure backfill), but not as structural fill below foundations.

• Loose sands at the project sites are liquefiable under seismic events greater than the 250-year earthquake, with modeling indicating two to seven inches of settlement (depending on the severity of the event). Mitigation is possible to prevent liquification-induced settlement; however, this mitigation is costly and, even if select structures are mitigated, the remainder of the canal system would still likely be susceptible to settlement, damage, or failures possibly requiring the system to be taken out of service for repairs.

• Due to the presence of liquefiable soil, the site classifies and Site Class F. ASCE7 recommends a site-specific seismic response analysis.

This report does not address the subsurface conditions or provide geotechnical recommendations relevant to the relocation, embedment, or foundation design of electrical transmission and distribution poles within the project area. To the extent approved by the BIA, these features will be addressed in a separate, stand-alone geotechnical report by others.

1.0 INTRODUCTION

DOWL completed a geotechnical investigation for three check structures on the Colorado River Irrigation Project (CRIP) Main Canal near Parker, Arizona. The scope of geotechnical services involved reviewing existing geotechnical and geological information, field observations, subsurface exploration, laboratory testing, engineering analyses, and preparing this Geotechnical Report. The purpose of these services is to provide geotechnical-related recommendations for project planning and design. We conducted this referencing our work plan dated June 6, 2022.

Our geotechnical engineering scope of work for this project included drilling six borings to depths of 31.5 feet below existing site grades, lab testing for soil engineering properties, and engineering analyses to provide foundation recommendations.

2.0 PROJECT UNDERSTANDING

2.1 EXISTING SITE CONDITIONS

The project is located south-southwest of Parker, Arizona. The project area is east of the Colorado River.

The canal is part of CRIP, which is operated and maintained by the Bureau of Indian Affairs (BIA). We illustrate the project Location in Figure 1. The check structures are all located within the CRIP Main Canal.

The canal was constructed of two embankments with a concrete-lined channel. The canal may be accessed by a gravel-surfaced operations and maintenance roadway on top of the embankments on both sides. The canal is in a relatively flat topographic area with a gentle slope down to the south-southwest. The surrounding area is agricultural land and is predominately used for hay production. DOWL’s investigations focused on the areas surrounding the MC 185, MC 270, and MC 420 check structures. The structures are named for their approximate canal station with respect to the Main Canal heading and Headgate Rock Dam. For example, MC 185 is location at approximate canal station 185+00 (i.e., it is 18,500 feet downstream of the Main Canal heading). The check structures provide a control water surface allowing diversion of water into lateral canals off of the Main Canal. Lateral canals are named using a slightly different convention based on the check structure station at their point of heading off the Main Canal. For example, the Lateral 19L and Lateral 19R canals are the left and right laterals (respectively) with headings near the MC 185 structure. Similarly, laterals near the MC 270 canal are the Lateral 27L and Lateral 27R, while Lateral 42L is upstream of the MC 420 check.

The immediate area around structure MC 185 is covered with native shrubs. At this location, Lateral 19R departs from the Main Canal at a right angle, while Lateral 19L departs at an acute angle. There is a residence southwest of the structure, and an overhead electrical power line runs along the south side of Lateral 19L, east of the check structure (see Photograph 1 and Figure 3). This overhead power line is the power service for the electric gate actuators and control system of the MC 185 site.

Structure MC 270 is surrounded by hay fields on three sides, with native shrubs at the northwest corner.

Laterals 27L and 27R depart from the Main Canal at about 90 degrees. The overhead electrical power line approaches the structure from the east along the north side of Lateral 27L (see Photograph 2 and Figure 4). This overhead power line is the power service for the electric gate actuators and control system of the MC 270 site.

Structure MC 420 is located about 550 feet downstream of Lateral 42L, at an “S” curve in the Main Canal alignment. Lateral 42L flows east from the Main Canal at the intersection with Indian School Road. There are irrigated hay fields on the east side of the canal and native desert shrubs on the west side. There are overhead power lines on both sides of the Main Canal at the check structure, including electrical distribution lines and higher voltage (69 kV) transmission lines. The 69 kV electrical transmission line crosses over the Main Canal downstream of MC 420, within the project vicinity (see Photograph 3 and Figure 5). The electrical distribution line approaching the structure from the northwest provided service for the electric gate actuators and control system of the MC 420 site. Although electrical distribution and transmission lines will be relocated to accommodate Main Canal realignment near the MC 420 site, this Geotechnical Report does not include recommendations for the design of the distribution/transmission structures; rather, geotechnical recommendations to support these features will be provided in a separate report.

All elevations included within this report are in reference to the North American Vertical Datum of 1988

(NAVD88).

Photograph 1. 185 Check Structure (Looking Upstream)

Photograph 2. 270 Check Structure (Looking Upstream)

Photograph 3. 420 Check Structure (Looking Downstream)

Figure 1: Vicinity and Location Map

2.2 PROPOSED CONSTRUCTION

The BIA plans to rehabilitate check structures MC 185 and MC 270 and to replace check structure MC 420 in conjunction with a slight canal realignment. The existing structures were constructed in the early 1940s, and in recent years, BIA has implemented multiple gate repairs and coatings, and previous attempts at providing cathodic protection have been unsuccessful. The (preliminary) typical canal section in the canal realignment near MC 420 includes a 20-foot-wide base width, with a six-inch-thick concrete slab at the bottom overlying six inches of bedding gravel. The inside slope of the canal will be 1.5H:1V (horizontal to vertical) and will be covered with four inches of shotcrete to 13.5 feet above the flowline elevation. The embankments on each side of the canal will be 15.5 feet tall, measured from the flowline of the canal.

The tops of the embankments will be 16 feet wide, and the outer slopes will be 2.5H:1V.

Improvements at MC 185 will include new electrical equipment and instrumentation in a new control building, a new radial gate, and a new weir wall in the second (unused) gate bay.

Structure MC 270 will be partially reconstructed. The existing bypass channel on the right side of the Main Canal will be demolished and reconstructed. The new construction will include a new cast-in-place concrete retaining wall that will retain about eight feet of soil and a new bottom slab. A new radial gate will be installed in the Main Canal, and a control building for electrical equipment will be constructed on the left side of the check structure.

Structure MC 420 will be demolished and reconstructed on a new canal alignment. The new structure will be built off-alignment on the left (east) side of the Main Canal. Based on preliminary drawings, the new structure will be about 65 feet wide, 100 feet long and have 15-foot-tall walls. The water depth within the radial gate bay will be about 12 feet. The new structure will support a single 18.5-foot-wide radial gate.

Based on preliminary information from DOWL’s structural engineers, we anticipate a bearing pressure of about 1,600 pounds per square foot (psf) at the base of the slab including the water column.

Based on preliminary drawings, the floor elevation of the canal at structure MC 420 will be at approximate elevation 342 feet, with a 30-inch-thick slab bearing at elevation 339.5 ( about 14 feet below the existing grade). The top of the structure and canal embankments will be at approximate elevation 357.5 feet.

2.3 SITE GEOLOGY

A surficial geologic map of the project area (Wilson 1960) is presented in Figure 2. The project is located within the Parker Valley, part of the Lower Colorado River Valley region, in southwest Arizona. Based on the geologic map, the subsurface soil in the valley consists of sand, silt, and gravel deposited in the Holocene and late Pleistocene epochs. The soil was likely deposited from stream channels and flood plains of the Colorado River. East of the project area, older river terrace deposits are mapped. Bedrock in the project area generally appears to consist of both intrusive and extrusive volcanic rocks such as basalt, rhyolite, andesite, and granite.

CRIP MAIN CANAL CHECK STRUCTURES

GEOTECHNICAL INVESTIGATION

WWW.DOWL.COM SURFICIAL GEOLOGY MAP FIGURE 2

PROJECT LOCATION

Wilson, E.D., 1960, Geologic map of Yuma County, Arizona, Arizona Bureau of Mines, County Geologic Map Series M-3-11, 1:375,000.

3.0 INVESTIGATION

3.1 FIELD INVESTIGATION

DOWL performed fieldwork on August 22 and 23, 2022. Fieldwork consisted of site observations, drilling six (6) geotechnical borings, and constructing five (5) piezometers. We illustrate boring locations in Figure 3, Figure 4, and Figure 5. Enviro-Drill advanced the borings to depths of 31.5 feet below the ground surface. DOWL surveyed the boring locations during the topographic survey relative to the project datum.

Table 1: Exploration Summary

Boring Number

Drill Depth (feet)

Surface

Elevation (feet)

Northing (feet)

Easting (feet)

Location

185-1 31.5 364.4 1,140,323.0 529,049.9 SW of MC 185

270-1 31.5 360.5 1,132,415.8 525,986.2 SE of MC 270

420-1 31.5 355.8 1,119,512.2 525,960.1 NW of MC 420

420-2 31.5 350.6 1,117,064.0 525,617.5 SW of MC 420

420-3 31.5 356.0 1,116,775.1 525,276.8 S of MC 420

420-4 31.5 359.9 1,117,062.5 525,355.1 SE of MC 420

Enviro-Drill drilled the borings under the direction of a DOWL geologist using a CME-75 truck-mounted drill rig equipped with a 4.25-inch inside diameter (I.D.) hollow stem augers. We conducted field exploration referencing the following ASTM standards:

• ASTM D6151 Standard Practice for Using Hollow-Stem Augers for Geotechnical Exploration and Soil Sampling,

• ASTM D1586 Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils,

• ASTM D3550 Standard Practice for Thick Wall Ring-Lined, Split barrel, Drive Sampling of Soils

• ASTM D1587 Standard Practice for Thin-Walled Tube Geotechnical Sampling of Soils, Penetration testing was performed using a standard 2.0-inch outside diameter (O.D.) sampler and a 2.5-inch O.D., 18-inch-long California sampler. The samplers were driven with an automatic hammer weighing 140 pounds (lbs) and dropped 30 inches per blow. The Standard Penetration Test (SPT) is based on a sampler having a 2.0-inch O.D. and use of a 140-lb hammer with a 30-inch drop. Based on ASTM D3550, DOWL corrected the blow counts that were acquired with the California sampler to standard values. The method proposed by Lacroix and Horn (1973) uses a formula to convert a nonstandard penetration resistance, N1, to the Standard Penetration Resistance, NSPT, as follows:

𝑁𝑆𝑃𝑇 = 𝑁1 (

2𝑖𝑛

𝐷1

12 𝑖𝑛

𝐿1

𝑊1

140 𝑙𝑏𝑠

𝐻1

30 𝑖𝑛 where: NSPT = “field” Standard Penetration Test blow count (uncorrected for other factors such as overburden pressure, groundwater levels, etc.); D1 = nonstandard sampler diameter; W1 = nonstandard hammer weight; and H1 = nonstandard hammer drop height. Because the hammer weight and drop height were nominally standard, the correction for the nonstandard sampler diameter is simplified, as follows:

𝑁𝑆𝑃𝑇 = 𝑁1(2 𝑖𝑛 2.5 𝑖𝑛⁄ )2 = 0.64𝑁1

We have not corrected SPT values on the logs for hammer efficiency, sampler type, overburden stress, etc. The resistance, or N-value, can be used to estimate the relative density of granular soils and the relative consistency of cohesive soils. We provide the field N-value or resistance data on the exploration logs.

We provide exploration logs in Appendix A, which include soil and groundwater conditions, SPT information, and well schematics. Stratification boundaries on the boring logs represent the approximate location of changes in soil types; in situ, the transition between materials may be gradual and may vary.

In Appendix B, we present photographs of soil samples obtained during drilling.

We based the soil descriptions shown on the boring logs on field and laboratory testing referencing ASTM Standards D2487 or D2488. The soil and groundwater conditions depicted are only for the specific dates and locations reported and may not necessarily represent other locations and times.

LATERAL 19L

M

AI

N

CA

NA

LLATERAL 19R MC185 CHECK STRUCTURE

CRIP MAIN CANAL CHECK STRUCTURES

GEOTECHNICAL INVESTIGATION

WWW.DOWL.COM MC 185 EXPLORATION LOCATIONS FIGURE 3

B-1

LEGEND

GEOTECHNICAL BORING LOCATIONS

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MC270 CHECK STRUCTURE

CRIP MAIN CANAL CHECK STRUCTURES

GEOTECHNICAL INVESTIGATION

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B-1

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CHECK STRUCTURE

CRIP MAIN CANAL CHECK STRUCTURES

GEOTECHNICAL INVESTIGATION

WWW.DOWL.COM MC 420 EXPLORATION LOCATIONS FIGURE 5

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GEOTECHNICAL BORING LOCATIONS

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3.2 LABORATORY TESTING

DOWL transported samples to laboratories for testing after selecting representative field samples for testing based on visual examination of the soil and consideration of the design criteria. DOWL performed tests for index and engineering soil properties in Billings, Montana. Energy Labs of Billings, Montana, completed corrosion testing of select soil samples. Laboratory testing included:

Table 2: Laboratory Tests

Test Purpose

Natural Moisture Content

ASTM D 2216

Provides a measure of natural (in-situ) water content.

Unit Weight

ASTM D7263

Provides unit weight (density) of soil.

Atterberg Limits

ASTM D 4318

Provides an indicator of the consistency and swell potential of fine-grained soils.

Particle-Size Distribution

ASTM D 421

Provides a measure of grain sizes of the soils to classify and identify physical characteristics.

Hydrometer Analysis

ASTM D7928

Provides a measure of grain sizes of the fine-grained soil for identification of physical characteristics.

Hydraulic Conductivity

ASTM D5084

Provides a measure of the hydraulic conductivity (permeability) of soil.

Direct Shear

ASTM D3080

Provides an estimate of the shear strength of soil

Consolidated Undrained Triaxial Shear with Pore Pressure Measurements

ASTM D4767

Provides an estimate of both the shear strength of soil under consolidated-undrained and consolidated-drained conditions.

Consolidation

ASTM D2435

Used to estimate settlement.

Unconfined Compression

ASTM D2166

Provides an estimate of the undrained shear strength of the soil.

Double Hydrometer

ASTM D4221

Provides an estimate of the dispersion potential of soil.

Pin Hole Dispersion

ASTM D4647

Provides an estimate of the erodibility of soil.

Crumb Dispersion

ASTM D6572

Provides an estimate of the dispersion potential of soil.

Corrosion Tests

(pH, Resistivity, and Soluble Sulfates) MT-532

To determine the potential for corrosive interaction of soils with concrete and metal.

We present laboratory test results in the summary table and figures in Appendix C.

4.0 SUBSURFACE CONDITIONS

4.1 OBSERVED SOIL CONDITIONS

The generalized soil profile encountered at each check structure site consists of one to two feet of gravel fill over seven to seventeen feet of silty sand to sandy silt embankment fill overlying native sand.

Appendix A presents the exploration logs with lithology descriptions and other engineering properties. In the following paragraphs, we provide a general description of the soil strata.

4.1.1 Embankment Fill

We encountered embankment fill in all borings, except 420-2 (the only boring not located on the existing canal embankment). The fill typically consisted of one to two feet of gravel on the road surface with silty sand underneath. The fill is generally moist and has a relative density that ranges from very loose to medium dense. The moisture contents ranged from eight to 18 percent. Standard penetration test (SPT) results ranged from three to 24, with an average of eight.

4.1.2 Silty Sand and Sandy Silt

We encountered silty sand and sandy silt in all the borings, except 185-1, 270-1, and 420-3. The silty sand extended from the bottom of the embankment to depths ranging from 28 to 31.5 feet. The silty sand ranges from moist to wet, with a relative density ranging from very loose to medium dense. Moisture contents ranged from 12 to 32 percent, and SPT values ranged from two to 23 with an average of 10.

4.1.3 Poorly Graded Sand

We encountered poorly graded sand in borings 185-1, 270-1, and 420-3. The sand ranged in depth from nine to 28 feet and ranged in thickness from three feet to 21 feet. The sand is moist to wet, but it generally lies below the water table and has moisture contents ranging from two to 15 percent. The relative density of the sand ranges from very loose to medium dense, with SPT values ranging from three to 16 with an average of seven.

4.1.4 Clay

We encountered fat clay, sandy lean clay, and sandy, silty clay in borings 420-1, 420-2, and 420-3. The depth of the clay soil ranged from four to 28 feet in depth, and the thickness ranged from seven to 10 feet thick. The clay is moist to wet, with moisture contents ranging from 12 to 32 percent. The consistency of the clay ranged from very soft to stiff, with SPT values ranging from two to 13.

4.1.5 Dispersion

Physical dispersion tests were performed on samples from the canal embankment at each structure location. The test results show that the silty sand soil is dispersive, the silty clay has an intermediate dispersion potential, and the clay is nondispersive. Test results are summarized in Table 3.

Table 3: Dispersion Test Results

Sample Location Classification Pinhole

Dispersion1

Double Hydrometer2

Crumb Dispersion

185-1 @ 2.5-7.0 feet CL-ML D1 13 Intermediate

270-1 @ 5.0-6.5 feet SM D1 40 Intermediate

420-4 @ 5.0-6.5 feet CL ND3 8 Nondispersive 1D1 and D2 Dispersive, ND4 and ND3 Intermediate, ND2 and ND1 Nondispersive 2<30% Dispersive, 30-50% Intermediate; > 50% Dispersive

4.2 GROUNDWATER

Groundwater was encountered at depths of 14.5 to 20.5 feet below the ground surface in the borings at the time of field exploration. These observations represent groundwater conditions only at the time of the observations and may not indicate other times or locations. Groundwater conditions can change with varying seasonal and weather conditions and other factors. DOWL installed instruments in the piezometers to record groundwater over the irrigation seasons. These data were not available at the time this report was published. A separate report will be provided summarizing the results of the groundwater monitoring program.

Table 4: Groundwater Depths During Drilling

Boring Depth (ft) Elevation (ft)

185-1 18.5 349.5

270-1 14.5 348.9

420-1 18.5 340.6

420-2 14.5 338.9

420-3 19.5 336.5

420-4 20.5 339.1

4.3 SEISMICITY

4.3.1 Faulting

The proposed project is located in an area of Quaternary faulting. Based on the USGS Quaternary Fault Fold Database (USGS, 2018), two Quaternary faults exist in the project vicinity:

Table 5: Documented Faults

Fault Name Distance from

Project Site (mi) Recent

Earthquake Published Slip Rate (mm/yr)

Fault Length

(mi)

Average Strike

Blythe Graben 43 Late

Quaternary Unspecified 4 Unspecified

Chemehuevi Graben 47 Late

Quaternary Unspecified 3 Unspecified

Available publications do not include documentation that these faults have offset during the Holocene (last 15,000 years). Based on the fault locations, the risk of surface rupture at the site is low.

4.3.2 Seismic Loading

DOWL utilized site soil and geologic data, our knowledge of local geology, the project location, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 2016), and the National Earthquake Hazards Reduction Program (NEHRP) to estimate Seismic Site Classification of "F" at each of the project sites. The sites classify as Site Class F because the soil is liquefiable. ASCE7 recommends a site-specific site response for Site Class F. If the soil at the site were not liquefiable it would classify at Site Class E. We queried the ASCE 7 Hazard tool website (ASCE 2021) and, based on the soil conditions and project location, and provide seismic design parameters provided in Table 6.

Table 6: Seismic Design Parameters for Site Class E

Peak Acceleration, PGA

Short Period Acceleration, SDS

Long Period Acceleration, SD1

0.19 0.47 0.43

4.3.3 Liquefaction

Liquefaction is the partial or total loss of soil shear strength that can occur during strong earthquake motion of significant duration. Liquefaction is a process where high shear deformations result in a progressive build-up of pore water pressure. Because the seismic loading occurs rapidly, the excess pore pressures cannot dissipate and the effective stress is reduced resulting in a temporary loss of shear strength. Earthquake-induced liquefaction typically occurs only in loose, wet, granular soils during strong, long-duration ground motion. However, liquefaction can occur in low-plasticity silts and clays. Typically, liquefaction occurs within soil horizons less than 50 feet deep where groundwater is shallow (~10 feet deep).

Based on the laboratory test data, most of the native subgrade soils (except the clay layer at structure MC

420) are potentially liquefiable due to the gradation (sand) and non-plastic characteristics. DOWL followed the NCEER method (T. Y. Idriss 1997) and (R.B. Seed 2003) to evaluate the liquefaction potential for each site. The NCEER method calculates a cyclic stress ratio (CSR) and cyclic resistance ratio (CRR). The CSR represents the cyclic stresses induced by an earthquake and is based on peak ground acceleration adjusted for soil conditions (i.e., seismic site class), overburden pressure, earthquake magnitude, and depth. The CRR is based on the soil relative density (or consistency) as determined by Standard Penetration Testing (SPT). A factor of safety against liquefaction is calculated as CRR/CSR, and liquefaction is triggered when the CSR exceeds the CRR (or when the factor of safety is less than one).

DOWL used the Rocscience Settle3 software to evaluate the liquefaction potential for at least one boring at each structure site. For inputs, DOWL used the seismic loads from Table 7 and SPT blow counts from the field investigation. The SPT data were corrected for hammer energy, boring diameter, rod length, overburden and adjusted for the soil's fines content (i.e., the percent passing a No. 200 sieve). DOWL used the following assumptions to correct the SPT data:

• An automatic trip hammer is 75% efficient.

• Average fine contents from laboratory testing

○ Sandy Silt – 55%

○ Silty Sand – 25%

○ Poorly Graded Sand – 10%

Based on the results of DOWL’s analyses (see Table 7 and Appendix D), the low to non-plastic soils at each of the sites are liquefiable. The analyses indicate the soils do not liquify under seismic events through the 250-year event and no liquification-induced settlement is estimated for these events; however, liquification does occur under less frequent recurrence interval events. Modeling predicts the potential for two to six inches of liquefaction-induced settlement at each location. Further, the borings drilled during the field investigation attained approximate depths of 31 feet below the ground surface. It is likely that liquefiable layers extend beyond the depth of the borings, perhaps 20 or more feet below this depth.

If liquefiable layers extend below the depths explored, the results of the analyses may underestimate the total settlement potential.

Table 7: Liquefaction Induced Settlement

Seismic Data

Liquefies

Estimated Liquefaction Settlement

Return Period (years)

Annual Probability

PGA (g) M 185 270 420 Average

250 0.4% 0.091 7.0 no 0.0 0.0 0.0 0.0

500 0.2% 0.124 7.0 yes 1.5 1.8 1.8 1.7

1000 0.10% 0.180 6.9 yes 3.0 4.8 2.5 3.4

2500 0.04% 0.238 6.7 yes 4.0 5.3 2.8 4.0

5000 0.02% 0.276 6.5 yes 4.5 6.0 2.9 4.5

10000 0.01% 0.325 6.3 yes 5.0 6.2 3.0 4.7

The liquefiable soil layers begin between eight and 12 feet below the existing ground surface and extend to at least 30 feet in depth. To reduce the liquefaction potential, the loose granular soils at each site could be densified. For potential mitigation alternatives, DOWL recommends the consideration of either stone columns or compaction grouting. Stone columns are installed and compacted in large-diameter borings drilled into (or through) the liquefiable layer. Compaction grouting consists of pumping a low slump grout into the liquefiable soil layers. Each method acts to densify the soils within the areas treated.

Two other potential alternatives, dynamic compaction and vibro-compaction are likely not feasible due to the relatively high fines content of the soils. A final potential alternative involves retrofitting the structures with deep foundation elements, such as driven piles, that extend through the liquifiable zones.

However, based on other structures DOWL has been involved with in the project vicinity, pilings may extend to 100 feet or more below the ground surface.

In summary, the liquefaction potential of the soil at each site can be reduced, but the mitigation costs will likely be high. Further, if earthquake-induced liquefaction occurs at the locations of structures MC 185, MC 270, and MC 420, the event may also induce liquefaction failures throughout much of the canal system. While the improved structures (if mitigated for liquefaction-induced settlement) would be stable, the canal system would likely suffer damage in other areas through the system and may still need to be taken out of service to address failures at other locations. If the BIA wishes to pursue liquefaction mitigation alternatives further, additional exploration is warranted to determine the depth and nature of the liquefiable horizons.

5.0 ENGINEERING ANALYSIS &

RECOMMENDATIONS

5.1 FOUNDATIONS

Based on information from the subsurface exploration, laboratory testing results, and our analysis, it is our opinion that the proposed MC 420 structure can be supported on a conventional mat foundation system bearing on engineered fill over native soil. If encountered below the foundations, any undocumented fill should be removed. We provide specific recommendations in the following sections.

5.1.1 Conventional Mat Foundation

The hydraulic structure may be founded on a conventional foundation according to the parameters listed below.

• Prior to placement of structural fill, compact the exposed subgrade (see Section 5.4.1).

• A DOWL geotechnical engineer should observe the finished subgrade prior to placing fill or concrete to verify that foundation conditions are similar to those encountered in the borings.

• Remove and replace soft or loose zones or zones of unsuitable material, if encountered, with structural fill.

• Place a stabilization geotextile (see Section 5.4.9)

• Place a minimum of 18 inches of structural fill.

Note that there is potential for liquefaction settlement during/after an earthquake, as discussed in Section

4.3.3. Settlement calculations are presented in Appendix D.

Table 8: MC 420 Foundation Design Parameters

Footing Design Criteria Recommendations Notes

Mat Foundation Maximum Allowable Bearing Pressure

Static Loads (Dead & Sustained Live):

3,000 psf

Transient Loads (Wind & Seismic):

4,500 psf

Subgrade Modulus, k 150 psi/in

Maximum Estimated Settlement

Total (in) Differential (in) Based on 1,600 psf foundation pressure.

1 inch

0.5 inch over 25 feet

Subgrade Preparation and Structural Fill

Compact structural fill to 98% standard Proctor

Overexcavate 18 inches of soil below the foundation, lay stabilization geotextile, and place 18 inches of structural fill.

Design uplift of shallow foundations from wind and seismic events using the weight of the foundation and soil above the footing. You can include soil resistance in the shape of a truncated pyramid above the foundation. The pyramid edges are defined by straight lines extending from the top of the footing on either side at a 1H:2V (horizontal: vertical) slope.

DOWL used SIGMA/W software to evaluate the settlement of the structure and the proposed embankments. We estimated soil parameters from SPT values and consolidation testing. We assumed a load of 1,600 psf applied to the foundation. Because the foundation is about 10 feet below the existing grade, the structure will be within typical tolerances. We have used a bearing elevation of 339.5 feet.

Based on our analysis, we estimated the embankment will settle about two inches (see Appendix D).

Based on the model, the settlement occurs above the groundwater table, and it is our opinion that settlement will occur within one to two weeks. We recommend not placing concrete canal lining for at least two weeks after the embankments are constructed.

5.2 LATERAL EARTH PRESSURES

Design below-grade walls for retaining walls and any structure retaining soil to resist both lateral earth pressures from the retained soil adjacent to the structure and hydrostatic pressures from retaining water (if undrained, not recommended). Also, account for lateral surcharge loads from equipment, slopes, or vehicles adjacent to the walls in the structural wall design. We provide recommended lateral earth pressures for below-grade wall design in Table 9.

Table 9: Lateral Earth Pressures

Lateral Earth Pressure Case Equivalent Fluid

Pressure (pcf)

Above Water

At-rest (no wall movement) 61

Active (wall moves away from soil mass) 36

Passive (wall moves into soil mass) 390

Below Water

At-rest (no wall movement) 93

Active (wall moves away from soil mass) 80

Passive (wall moves into soil mass) 259 Based on a unit weight of 125 pcf, effective unit weight of 65 pcf below the water table, and friction angle of 31 degrees.

• Construct below-grade walls, retaining walls, or other retaining structures with adequate drainage as specified by the structural engineer to reduce the potential for instability or leakage.

• The retaining walls move away from or toward the soil to develop active and passive resistance, respectively. For walls that cannot tolerate movement, structurally design walls utilizing at-rest equivalent earth pressures.

• We based the above equivalent fluid pressures on the assumption that the surface of the backfill is level adjacent to the walls.

• Lateral surcharge pressures due to equipment (e.g., the control building, construction equipment and equipment necessary for installation of bulkheads or future gate repairs), slopes, storage loads, etc., are not included in the above lateral earth pressure recommendations. Include these loads in the design of the walls.

5.2.1 Seismic Earth Pressure

We recommend using the Mononobe-Okabe approach to determine the additional earth pressures due to earthquakes. Using the assumed unit weight of the retained soil at this project (125 pcf), and the design peak horizontal ground acceleration (0.19g) we estimate the equivalent additional fluid (active) earth pressure acting on the wall is: 8 pcf. We calculated this value using ½ the peak ground acceleration in the horizontal direction.

5.2.2 Coefficient of Friction

We recommend using a coefficient of friction of 0.45 between cast-in-place concrete and structural fill or granular native soil. The friction value may be combined with the passive pressure to resist horizontal loads.

5.3 EMBANKMENT SLOPE STABILITY

DOWL performed slope stability analyses of the proposed canal embankments at MC 420. DOWL evaluated the slope stability of the embankments using Geostudio's Slope/W and the soil properties shown in Table 10. The inside slope of the canal will be 1.5H:1V and will be covered with shotcrete to 13.5 feet above the flowline elevation. The tops of the embankments will be 16 feet wide, and the outer slopes will be 2.5H:1V. We summarize the slope stability results in Table 10 and the stability outputs in Appendix D.

Table 10: Engineering Properties

Material Moist Unit

Weight (pcf)

Effective Stress Total Stress

Friction Angle (degrees)

Cohesion (psf)

Friction Angle (degrees)

Cohesion (psf)

Embankment Fill 125 35 0 28.7 10

Clay 99 28 150 23 300

Sandy Silt 99 30 0 27 50

Sand 110 28 10 27 20

Concrete 145 0 6000 0 6000

Table 11: Slope Stability Results

Slope Location and Scenario Calculated Factor of Safety Minimum Factor of Safety*

Inside Slope - Static 1.82 1.5

Outside Slope – Static 1.84 1.5

Inside Slope - Pseudo-Static 1.53 1.0

Outside Slope – Pseudo-Static 1.49 1.0

* Based on (USBR 2011) and (Hynes-Griffin 1984)

5.4 EARTHWORK

5.4.1 Subgrade Preparation

• Soil containing vegetation and organics (topsoil) is generally not present. However, if topsoil or other organic soil is encountered, remove and replace it with structural fill.

• Remove uncontrolled fill below planned improvements or structures.

• Scarify, moisture condition, and compact upper subgrade soil as specified in Table 13.

• Grade the exposed subgrade surfaces to remove mounds and depressions, which could prevent uniform compaction. If unexpected fills or obstructions are encountered during site clearing or excavation, remove such features, and clean the excavation before placing backfill and construction.

• Soil disturbance negatively impacts the soil's performance. Disturbed soil is not allowed below any structure or pavement, especially at footing or slab subgrades.

• Moisture condition and compact disturbed soil or fill placed to achieve site grades to the requirements in Table 13: Compaction Specifications.

• Remove pumping or rutting subgrade areas to depths between 12 and 18 inches or as directed by

DOWL.

• Replace over excavations with granular structural fill. Contact DOWL's geotechnical engineer to review and approve the exposed subgrade.

• Once DOWL approves contractor’s prepared subgrades, it is the contractor's sole responsibility to protect subgrades from degradation.

5.4.2 Excavation

Based on the materials encountered in the soil borings, conventional earthmoving equipment should be capable of excavating the site soils.

5.4.3 Dewatering

During the investigation, we encountered groundwater within a few feet of the bottom of the proposed canal elevation. DOWL has installed groundwater monitoring devices; however, these data were not available as of the publish date of this report. Groundwater monitoring information will be published in a separate report. Dewatering may be necessary during construction. Depending upon the elevation of the groundwater surface at the time of construction, groundwater infiltration into the foundation excavation at MC 420 may occur. Significant fluctuations in groundwater elevation are common, particularly in sandy soils.

The groundwater should be drawn down at least two feet below the bottom of the foundation excavation during foundation construction. The groundwater should also be kept below the trench floor during the backfilling process until pipes are placed and backfilled above the groundwater elevation. This will aid in achieving and maintaining pipe alignment.

In-trench sump pumps may be applicable dependent on the groundwater elevation and inflows at the time of construction. A well-point system is a standard, but more intensive, method of dewatering during construction. This method, which dewaters from outside the excavation, reduces the risk of trench wall sloughing and potentially lowers the water table below the trench or pit floor.

Some dewatering from within the trench or pit may be possible depending upon the groundwater elevation at the time of construction and the soil type. This method should be used only when a minor amount of dewatering is necessary and where adequate water treatment for turbidity is possible before discharging the water into any waterway. Dewatering from within the excavation is usually ineffective if the groundwater table is more than 1.5 feet above the trench bottom. This method has a greater risk of trench-wall sloughing and trench-floor heave. Where excavations are dewatered from within the trench, the trench floors and walls will remain saturated.

Inadequate dewatering may result in unstable trench wall and bottom conditions requiring over-excavation and placement of gravel to stabilize the trench bottom. Proper compaction of bedding material will not be possible if adequate dewatering is not accomplished.

Plan excavations to allow for water collection points and utilize conventional sumps and pumps to remove nuisance water seeps, springs, or precipitation. If site soil excavations are not immediately backfilled, they may degrade when exposed to runoff and require over-excavation and replacement with structural fill.

We recommend construction activities, particularly earthwork, be performed as rapidly as possible or during drier conditions to reduce the potential for remedial earthwork.

Take care during dewatering to ensure adjacent structures are not damaged. Dewatering saturated clayey and sandy soils may initiate consolidation of load-bearing soils beneath foundations. The dewatering methods selected and utilized for construction should be designed and monitored by a qualified professional.

5.4.4 Temporary Slopes

Excavations must conform to OSHA Standards for Excavations, 29 CFR Part 1926.652 Appendix B to Subpart P. Based on field observations and laboratory tests, the soils at the site are classified as OSHA Type C. OSHA requires that Type C soil excavation slope angles do not exceed 1.5H:1V. The nature and extent of subsurface variations and groundwater conditions between the boring locations may not become evident until construction. Evaluate soil conditions at the time of construction by the contractor's responsible person to comply with OSHA requirements. Temporary excavation slopes may be required for soil improvement excavations and utility trenches. Conduct excavations and shoring following OSHA standards. Do not allow surcharges within a horizontal distance equal to half the excavation depth.

Construction vibrations can cause excavations to slough or cave. The contractor is solely responsible for site safety and excavation configurations.

Groundwater may be expected during excavations. If site soil excavations are not backfilled quickly, they may degrade when exposed to runoff and require over-excavation and replacement with structural fill.

We recommend construction activities, particularly earthwork, be performed as rapidly as possible or during drier conditions to reduce the potential for remedial earthwork.

5.4.5 Structural Fill

Consider fill placed within the planned structure footprint as structural fill. The sandy soil is suitable for canal embankment, retaining wall backfill, or below exterior concrete flatwork. However, the on-site silt and sand are not suitable for placement below the proposed structure, and we recommend structural fill as specified in Table 12.

Table 12: Fill Specifications

Soil/Fill Product Allowable Use Material Specifications

Non-Structural Fill

(Landscape Fill)

Any area that will not have structures or embankments (typically landscape areas)

• Soil classified as GM, GW, SM, SW, SC, CL, or ML according to the USCS.

• Soil may not contain particles larger than 8 inches in median diameter.

• The soil must contain less than 3% (by weight) of organics, vegetation, wood, metal, plastic, or other deleterious substances

General Fill

Site grading outside the structure footprints, utility backfill areas, canal embankment, non-structural fill, foundation backfill

• Soil classified as SM, SC, CL, or ML according to the

USCS.

• Site soil must have less than three percent vegetation, organics, and debris.

• Soil may not contain particles larger than 6 inches in diameter.

• The soil must contain less than 3% (by weight) of organics, vegetation, wood, metal, plastic, or other

Structural Fill

Over-excavations and soil improvements.

• Soil classified as GP, GM, GW, SP, SM, or SP, with at least 30 percent retained on a number 4 sieve and less than 15 percent passing a number 200 sieve.

• Soil may not contain particles larger than 2 inches in diameter.

• The soil must contain less than 3% (by weight) of organics, vegetation, wood, metal, plastic, or other deleterious substances

Unsatisfactory Soil

NONE

• Soil classified as MH, OH, CH, OL, or PT may not be used at the project site

• Any soil type not maintaining moisture content within 5% of optimum during compaction is unsatisfactory soil that must be disposed of and replaced.

• Any soil containing more than 3% (by weight) of organics, vegetation, wood, metal, plastic, or other

5.4.6 Compaction Requirements

Place fill material in lifts not exceeding 12 inches in uncompacted thickness. Moisture condition and compact fill according to Table 13.

Table 13: Compaction Specifications

Application Moisture Content (% of optimum)

Minimum Compaction

Subgrade ±4 95% ASTM D698

Below Foundations ±4 98% ASTM D698

Base and Subbase Courses ±4 97% ASTM D698

Utility Trenches ±4 95% ASTM D698

Site Grading Fill ±4 95% ASTM D698

Canal Embankment ±4 95% ASTM D698

Foundation Backfill ±4 95% ASTM D698

5.4.7 Testing and Observations

We recommend the following compaction testing frequencies:

• Structural Fill below Footing and Subgrade - One compaction test every 2,500 square feet (LF) of or 2 tests lift, whichever results in the greater number of tests, per each 1-foot lift of fill.

• Foundation/Retaining Wall Backfill - One compaction test every 100 LF of the wall or 2 tests per wall line, whichever results in the greater number of tests per each 1-foot lift of backfill.

• Trenches - One compaction test every 400 linear feet or 2 per trench, whichever results in the greater number of tests per each 1-foot lift of backfill

• To verify construction conforms to the intent of the specifications, we recommend DOWL be retained to observe and record the following:

○ Site preparation including grubbing, stripping, excavating, and proof-rolling;

○ Removal of topsoil and root zone beneath structures;

○ Excavations and subexcavations before placing backfill/fill materials or before construction of foundations; and

○ Approval of additional excavation, replacement, or stabilization if unsuitable soil is identified by the geotechnical engineer during excavation or proof-rolling operations.

5.4.8 Wet Weather/Soil Construction

• Ideally, perform earthwork construction when the soil moisture content is less than two percent above optimum.

• If possible, do not perform earthwork after rainfall when the soil is wet. Allow the soil to dry sufficiently to allow construction traffic without disturbing the subgrade.

• If the subgrade soil becomes wet, it may be necessary to perform earthwork with track-mounted equipment that reduces vehicular pressure applied to the soil if construction commences in wet areas or before the soil can dry enough to support wheeled vehicles.

• If the subgrade is soft, and it is necessary, the contractor may place an initial 16-inch lift of structural fill and a stabilization geotextile (see Section 5.4.9) to help reduce the compaction energy on the unstable subgrade. Thicker structural fill lifts may only be installed over sensitive subgrades and with DOWL's approval during construction. Initial thicker fill lifts and over excavations to remove soft, wet soil can only be placed after the contractor has unsuccessfully attempted to moisture condition and recompact the native soil.

5.4.9 Geosynthetics

Geosynthetic fabrics are applicable when constructing on soft or wet soil, for foundations soil improvement applications, as separation fabrics between drainage aggregate, below the construction access road, and at the base of over-excavations. Where required, apply geosynthetics directly on approved subgrades, taut, without wrinkles, over-lapped at least 12 inches, and in accordance with manufacturer’s recommendations. Consult DOWL to review geosynthetic applications or other subgrade improvement alternatives. Geotextile fabric placed at the bottom of the footing excavation must meet the requirements for stabilization fabric in Section 1014, Geosynthetics, of the Arizona Department of Transportation Standard Specifications for Road and Bridge Construction (ADOT 2021).

5.5 SOIL CHEMISTRY AND CORROSION

Based on the results shown in Table 14, concrete in contact with the on-site soil classifies as exposure class S2…