Atch_H_-_Geotech_Engineering_Report_-_Gila_Substation.pdf

APPENDIX A

Geotechnical Engineering Report

Terracon Consul tants, Inc. 4685 South Ash Avenue, Suite H-4, Tempe, Ar izona 85282 P [480] 897-8200 F [480]-897-1133 terracon.com

November 2, 2015

Civil Design & Engineering, Inc.

340 East Palm Lane, Suite A-138 Phoenix, Arizona 85004

Attn: Christine Laguna, P.E.

Re: Addendum No. 1 to the Geotechnical Engineering Report Western Area Power Administration Gila Substation Near Yuma, Arizona Terracon Project No. 65145275

Dear Ms. Laguna:

At your request, Terracon Consultants, Inc. (Terracon) has prepared this addendum report regarding the planned temporary access road and laydown area for the Gila Substation project.

Terracon previously completed the original geotechnical engineering report for the project, with our geotechnical engineering recommendations provided in the Terracon Report No. 65145275 dated September 2, 2014. The recommendations provided in the original report remain applicable with the following supplemental recommendations.

Introduction

We understand that Western will provide a laydown yard immediately southeast of the planned substation and will construct an access road between the laydown yard and new substation location. Although specific traffic loading conditions and frequencies have not been provided, we understand the access road must be capable of supporting heavy truck loads hauling transformers and other substation equipment.

Recommendations

Based on the geologic/geotechnical exploration of the site completed for the original geotechnical engineering report, the soil conditions at the locations of the proposed access road and laydown yard will likely consist of loose, fine to medium grained sand with little fines.

Considering the soil types and the planned construction traffic, compacted subgrade alone will not support the planned traffic loading without the need for some form of continual maintenance.

Therefore, we recommend the surface along the access road and within the laydown yard be improved with one of the following options:

n Cement Treated Subgrade; or, n Mechanically Stabilized Subgrade

Addendum No. 1 to the Geotechnical Engineering Report Gila Substation ■ Near Yuma, Arizona November 2, 2015 ■ Terracon Project No. 65145275

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Cement Treated Subgrade – The soil type at the site is well-suited for improvement with cement. The addition of cement will provide a firm wearing surface capable of supporting the planned temporary construction traffic. If utilized, we recommend that the cement treated layer have a minimum thickness of six (6) inches. For planning purposes, we recommend a 4% to 6% cement content (by dry weight of soil) be utilized, as necessary to result in a minimum compressive strength of the soil-cement of 500 at 28 days of age. A soil-cement mix design should be completed by the contractor to determine the appropriate cement content to meet the minimum compressive strength requirements.

Mechanically Stabilized Subgrade – Another alternative that is considered acceptable is to construct a mechanically stabilized subgrade layer consisting of geogrid reinforcement with an aggregate base course wearing surface. We recommend the geogrid layer consist of Tensar TX-140 or equivalent with a minimum aggregate base course thickness of six (6) inches. The aggregate base course should be compacted to a minimum of 100% of the maximum dry density as determined by ASTM D 698.

Subgrade Preparation

Prior to construction of either option as outlined above, the underlying subgrade soils should scarified to a minimum depth of eight (8) inches, moisture conditioned to within +3% of optimum water content, and should be compacted to a minimum of 98% of the maximum dry density as determined by ASTM D 698. In-lieu of subgrade preparation below the cement treated subgrade option, the cement treated thickness may be increased to 10 inches.

Grading and Drainage

If the subgrade soils become elevated in moisture content, the overall performance of the aggregate surfaced pavement areas will be reduced, and could result in excessive rutting. To help direct surface water over the aggregate surface, we suggest surface slopes of 2% to 3% be constructed and maintained. For both options, surface drainage should be directed away from the roadway and laydown yard, and no ponding of water should be allowed adjacent to the edges of either type of improved surface.

Construction Specifications

Materials and construction of either option outlined in this report should be constructed in accordance with the following listed Maricopa Association of Governments (1MAG, 2015). A copy of each of these recommended specifications is attached to this addendum report.

n Cement Treated Subgrade – MAG Section 311 n Mechanically Stabilized Subgrade – MAG Section 306

1 Maricopa Association of Governments, 2015, Uniform Standard Specifications and Details for Public Works Construction, AZ.

Addendum No. 1 to the Geotechnical Engineering Report Gila Substation ■ Near Yuma, Arizona November 2, 2015 ■ Terracon Project No. 65145275

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We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report, or if we may be of further service, please contact us.

Sincerely, Terracon Consultants, Inc.

Jesse R. Huston, P.E. Donald R. Clark, P.E.

Senior Project Manager Senior Principal

Copies to: Addressee (1 via email)

Attachments: MAG Section 311: Placement and Construction of Cement Treated Subgrade MAG Section 306: Mechanically Stabilized Subgrade – Geogrid Reinforcement

SECTION 311

PLACEMENT AND CONSTRUCTION OF CEMENT TREATED SUBGRADE

311.1 DESCRIPTION:

This item shall consist of a cement treated subgrade composed of a mixture of local soil, Portland cement, and water compacted at optimum moisture content.

311.2 MATERIALS:

Portland cement and water shall comply with Sections 725. The soil for the mixture shall consist of the material in the area to be paved. The material shall not contain more than 5 percent gravel or stone retained on a 3 inches sieve. It shall be demonstrated by laboratory tests that the plasticity and strength characteristics as defined in Section 311.4.5 of the soil will be adequately modified by the specified cement content.

311.3 EQUIPMENT:

An ample number of machines, combination of machines and equipment shall be provided and used to produce the complete soil cement treated layer meeting the requirements for soil pulverization, cement distribution, water application, incorporation of materials, compaction, finishing, and for application of the curing material as provided in these specifications.

Mixing shall be accomplished by means of multiple-pass soil-cement mixer, single-pass soil-cement mixer or central plant mixer.

Water may be applied through the mixer or with the water trucks equipped with pressure sprays. Water trucks providing fine fog-type sprays shall be furnished for finishing and curing. Properly adjusted garden type nozzles on a pressure bar may be used to produce fog spray if approved by the Engineer.

Cement spreader shall be a specially constructed device to distribute bulk cement at the specified rate. The spreader shall have the ability to maintain a consistent spread rate over variable travel speeds.

311.4 CONSTRUCTION METHODS:

Prior to construction, the contractor shall remove all deleterious material, organic material, and particles retained on the 3 inch sieve from the area to be treated. The soil shall be brought to a compacted condition, true to line and grade as directed by the Engineer or as shown on the plans. The compacted soil and surface shall be approved by the Engineer prior to proceeding with mixing.

The material shall be scarified, pulverized, mixed with water and cement, compacted, finished and cured in lengths permitting the full roadway width to be complete in not more than 4 hours from the time that cement is exposed to water. Such lengths will generally be not less than 600 feet or the length of one City block and preferably more. Where a gutter section exists the material shall be pulled back from the gutter face for the full depth of the course before processing.

311.4.1 Pulverizing: Prior to application of cement, soil to be processed shall be scarified to depth of base. The material shall be damp at time of scarifying to reduce the dust generation and to aid in pulverization. Soil shall be pulverized until not less than 80 percent, exclusive of gravel or stone, will pass a No. 4 sieve.

311.4.2 Application of Cement: The quantity of cement shall be by weight as a percentage of the dry weight of the soil as determined by the laboratory and/or as directed by the Engineer and shall be applied uniformly on the soil in a manner satisfactory to the Engineer. The allowable deviation in uniformity shall not exceed 10 percent. The entire operation of spreading and mixing shall be conducted in such a manner as will result in a uniform soil cement and water mixture for the full design width and depth.

The percentage of moisture in the soil, at the time of cement application, shall not exceed the quantity that will permit a uniform and intimate mixture of the soil and cement during mixing operations, and it shall not exceed the specified optimum moisture content for the soil cement mixture.

Revised 2014 311-1

311.4.3 Mixing: Mixing with addition of water as required shall be continued until the product is uniform in color and at optimum moisture content to +4% of optimum moisture content as determined in accordance with ASTM D558. Any mixture of soil and cement which has not been compacted and finished shall not remain undisturbed for more than 30 minutes but shall be agitated by remixing.

311.4.4 Optimum Moisture: Optimum moisture requirements and field tests of moisture density shall be determined in accordance with ASTM D558, and D6938, with moisture content periodically corrected in accordance with AASHTO T-217 on representative samples of soil cement mixture obtained from the area being processed. At the time of compaction, the moisture content shall not be below optimum moisture, and shall be less than that quantity which will cause the base course to become unstable during the compaction and finishing process. Any area which becomes so unstable shall be removed and replaced with new cement stabilized material.

311.4.5 Compressive Strength: Laboratory compressive strength testing of the cement treated subgrade is required to evaluate the proposed amount of cement and/or verify the compressive strength achieved during construction. Laboratory compressive strength testing shall be done in accordance with ARIZ-241.

311.4.6 Compaction: After mixing is complete, the mixture shall be carefully placed in a uniform loose depth which will provide a surface true to grade and section when compacted. Unless otherwise directed by the Engineer, initial compaction shall be by means of a tamping, grid, or pneumatic roller. After the tamping roller has partially walked out, pneumatic rollers shall be used. Density of final product shall be not less than 95 percent as determined by ASTM D6938 as specified above.

311.4.7 Finishing: As compaction nears completion, the surface of the base course shall be shaped to required lines, grades and cross-section. When required, the surface shall be lightly scarified with spike tooth harrows or other approved equipment to remove imprints left by equipment or to prevent slippage planes. During the finishing process the surface shall be kept moist by means of fog-type sprays. Surface finish and final compaction shall be completed in not more than 2 hours from the time the cement is exposed to water. The completed base course shall be true to line, grade, cross-section and shall not vary more than ½ inch in thickness and not more than 1 inch in surface tolerance when tested with a 10 foot straight edge. It shall be free of surface cleavage planes, cracks, or loose material. As a final operation, the surface shall be very lightly scalped with a motor grader, wet with a fog spray and rolled with a pneumatic roller as directed by the Engineer.

311.4.8 Thickness Deficiency: The Engineer may choose to have cores obtained to evaluate the thickness of the treated cement stabilized subgrade layer. Should the thickness of the treated layer not meet the project specifications, the Engineer may require the contractor to submit an Engineering Analysis (EA) to address the pavement section. The EA will provide an opinion as to the anticipated performance of the pavement section as a result of the reduced cement treated layer thickness and make recommendations on possible corrective actions. The Engineer shall determine what corrective actions, if any, are required.

311.4.9 Curing: Each layer of cement treated subgrade shall be maintained in a moist condition until the next layer of pavement structure is placed. If required, a fog seal for curing in compliance with MAG Section 333, shall be furnished and applied to the surface of the final layer of the cement stabilized material as soon as possible after completion of final rolling and before the ambient temperature falls below 40º F. Curing seal shall be applied at a rate between 0.10 and 0.20 gallons per square yard of surface. The exact rate shall be determined by the Engineer.

After curing begins, all traffic, except necessary construction equipment shall be kept off the cement stabilized subgrade for a minimum of 7 days or until the final pavement structure layer(s) are placed. As an alternative, the contractor may place a loose lift of aggregate base course over the curing subgrade. The aggregate base course shall be kept moist during the curing process.

311.4.10 Construction Joints: At the end of each day's work, a construction joint shall be made transverse to the centerline of the road by cutting back into the work to provide a full depth vertical joint. Except where specifically authorized by the Engineer, no other construction joints will be permitted. Where authorized, such joints shall be full depth vertical joints.

311.4.11 Maintenance: The Contractor shall maintain the surface until it has been covered with the designated bituminous wearing course. In case it is necessary to replace any soil cement, it shall be for the full depth. No skin patches or soil cement will be permitted. Minor surface pits may be filled with compacted bituminous surfacing, if authorized by the Engineer.

Immediately prior to the placing of the bituminous wearing course, the surface shall be broomed to remove all loosened material from the surface.

Revised 2012

311-2

311.5 MEASUREMENT:

Measurement of soil cement will be the number of square yards constructed to the required depth, completed and accepted.

Measurement of Portland cement will be the number of tons of cement mixed with local soil.

311.6 PAYMENT:

Payment will be made for the applicable items at the contract unit prices bid in the proposal, and shall constitute full payment for furnishing all material, equipment, tools, labor and incidentals necessary to complete the work and for carrying out the maintenance provisions.

No measurement or payment will be made for any imported earth materials.

- End of Section -

311-3

SECTION 306

MECHANICALLY STABILIZED SUBGRADE - GEOGRID REINFORCEMENT

306.1 DESCRIPTION:

Mechanically stabilized subgrade shall consist of furnishing and placing a geogrid material within or below untreated base to provide a stabilized platform on which paving materials are placed. Geogrid type, fill thickness, pavement cross-section and associated details, shall be as shown on the contract drawings.

306.2 MATERIALS:

The geogrid material shall be supplied in accordance with and conform to the material requirements of Section 796 and Table 796-4.

306.3 SUBGRADE PREPARATION:

The geogrid shall not be placed when weather or surface conditions do not meet the manufacturer’s recommendations for installation.

306.3.1 Placing Geogrid on Soft Subgrade: Prior to placement of geogrid material, soft subgrade shall be lightly proof rolled to provide a firm surface, brought to grade and shaped to conform to the typical sections, lines and grades as shown on the plans. The surface on which the geogrid will be placed shall be free of rock and other material that could damage the geogrid.

The placement of the geogrid shall be approved by the Engineer before placement of overlaying materials.

Subgrade tolerances shall be in accordance with MAG Section 301.4.

306.3.2 Placing Geogrid Within Untreated Base: Subgrade shall be prepared in accordance with MAG Section 301.

306.4 EQUIPMENT:

Mechanical or manual laydown equipment shall be capable of laying the geogrid properly and smoothly, in compliance with the manufacturer's recommendations.

306.5 GEOGRID PLACEMENT:

The geogrid shall be installed in accordance with the installation guidelines provided by the manufacturer or as directed by the Engineer.

The geogrid may be temporarily secured in place with ties, staples, pins, sand bags or acceptable fill material as required by fill placement procedures, weather conditions, or as directed by the Engineer. A 12-inch minimum secured overlap is required at all joints (both transverse and longitudinal). At transverse joints, the preceding roll shall overlap the following roll in the direction that the aggregate base will be placed. The geogrid shall be rolled out along the alignment in the direction of advancing construction. All wrinkles and folds shall be removed.

The geogrid shall be tensioned by hand and anchored to the ground at the edges, including overlaps, and in the center of the roll at 30-foot intervals along the roll length, at the corners if applicable, or as directed by the Engineer. Securing locations may be reduced or eliminated if it can be shown to the satisfaction of the Engineer that an alternative installation process will provide satisfactory results.

Geogrid shall be placed to obtain full coverage of the indicated area. Placement of geogrid on irregular shaped areas and radii may require cutting of the geogrid material and the use of diagonal overlapping joints. Buckling of geogrid material will not be allowed.

306-1

SECTION 306

306.6 PLACING AND COMPACTING AGGREGATE FILL:

The aggregate shall be back dumped and spread in a uniform lift maintaining the design aggregate thickness at all times. The aggregate material shall be bladed on the geogrid in such a manner that the aggregate rolls forward onto the grid ahead.

When underlying substrate is trafficable with minimal rutting, rubber-tired equipment may pass over geogrid reinforcement at slow speeds (less than 10 mph). Sudden stops and turning by trucks shall be avoided on the geogrid. Traffic shall not be allowed onto coated geogrid materials. A minimum loose fill thickness of 6 inches is required prior to operation of tracked vehicles over the geogrid. Turning of tracked vehicles shall be kept to a minimum to prevent tracks from displacing the fill and damaging the geogrid.

Any ruts which develop during spreading or compacting aggregate fill shall have additional aggregate added rather than bladed from surrounding areas. Placing additional aggregate into the rutted areas limits disturbance to the underlying geogrid keeping it intact.

Untreated base shall be compacted as specified in Section 310. Untreated base material shall not be mixed or processed on the geogrid. Base materials will be uniformly blended and sampled for acceptance prior to placement on the geogrid material.

Contamination and segregation of base materials during placement shall be minimized.

306.7 REPAIR:

Any geogrid material damaged before, during or after installation shall be replaced by the contractor at no additional cost to the Agency.

Replacement of geogrid reinforcement shall consist of removal and replacement of the geogrid and aggregate fill from the defective area. The aggregate fill shall be removed at least 3 feet beyond the limits of the defective area. The replacement geogrid shall be installed with proper overlaps. Aggregate fill replacement shall not commence until placement of the geogrid material has been inspection and approved.

306.8 PAYMENT:

The surface area of accepted in-place geogrid reinforcement will be measured to the nearest square yard.

Payment for geogrid reinforcement at the contract unit price shall be full compensation for furnishing all labor, material, equipment, and installing complete in place the geogrid as shown on the project plans.

- End of Section -

306-2

Western Area Power Administration

Gila Substation Near Yuma, Arizona

September 2, 2014 Terracon Project No. 65145275

Prepared for:

Civil Design & Engineering, Inc.

Phoenix, Arizona

Prepared by:

Terracon Consultants, Inc.

Tempe, Arizona

Terracon Consul tants, Inc. 4685 South Ash Avenue, Suite H-4, Tempe, Ar izona 85282 P [480] 897-8200 F [480]-897-1133 terracon.com

Civil Design & Engineering, Inc.

340 East Palm Lane, Suite A-138 Phoenix, Arizona 85004

Attn: Christine Laguna, P.E.

Re: Geotechnical Engineering Report Western Area Power Administration

Gila Substation Near Yuma, Arizona

Dear Ms. Laguna:

Terracon Consultants, Inc. (Terracon) has completed the geotechnical engineering services for the proposed Western Area Power Administration (Western) Gila Substation in Yuma, Arizona.

These services were performed in general accordance with our Proposal No. P65140474 dated November 8, 2013 and subsequent Task Order No. 154. This geotechnical engineering report presents the results of the subsurface exploration and provides geotechnical engineering recommendations concerning earthwork and the design and construction of foundations and other earth connected phases for the proposed project.

We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report, or if we may be of further service, please contact us.

Sincerely, Terracon Consultants, Inc.

Jesse R. Huston, P.E. Donald R. Clark P.E.

Senior Project Manager Senior Principal

Copies to: Addressee (1 via email)

Gila Substation Near Yuma, Arizona September 2, 2014 Terracon Project No. 65145275

Resourceful Responsive Reliable

TABLE OF CONTENTS

Page

EXECUTIVE SUMMARY ............................................................................................................. i

1.0 INTRODUCTION

2.0 PROJECT INFORMATION

2.1 Project Description

2.2 Site Description

3.0 SUBSURFACE CONDITIONS

3.1 Site Geology

3.2 Liquefaction Potential

3.3 Subsurface Soil Condtions

3.3.1 Existing Fill Soils

3.3.2 Native Soils

3.3.3 Laboratory Testing Summary

3.4 Groundwater Conditions

3.5 Seismic Considerations

3.6 Field Resistivity Test Results

4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION

4.1 Geotechnical Considerations

4.2 Mat Foundations

4.3 Spread Footings

4.4 Drilled Shaft Foundations

4.4.1 Drilled Shaft Design Criteria

4.4.2 Lateral Loading Design Criteria

4.4.3 Drilled Shaft Construction Considerations

4.5 Earthwork

4.5.1 Site Preparation

4.5.2 Excavations

4.5.3 Subgrade Preparation

4.5.4 Fill Materials and Placement

4.5.5 Compaction Requirements

4.5.6 Grading and Drainage

4.6 Corrosion Considerations

5.0 GENERAL COMMENTS

Exhibit No.

Appendix A – Field Exploration Site Plan and Test Locations ....................................................................................... A-1 Field Exploration Description ....................................................................................... A-2 General Notes ............................................................................................................. A-3 Unified Soil Classification System ................................................................................ A-4 Boring Logs .................................................................................................. A-5 thru A-15

TABLE OF CONTENTS con’t.

Exhibit No.

Appendix B – Laboratory Testing Laboratory Test Description ......................................................................................... B-1 Atterberg Limits Results............................................................................................... B-2 Grain Size Distribution .................................................................................... B-3 and B-5 Moisture-Density Relationship ........................................................................ B-6 thru B-8 Consolidation Test Results .......................................................................................... B-9 Direct Shear Test Results ........................................................................... B-10 thru B-16 Summary of Laboratory Results ................................................................. B-17 thru B-19

Appendix C – Strength Limit State Design Load Graphs Drilled Shafts Compression Loading ............................................................................ C-1 Drilled Shafts Uplift Loading ........................................................................................ C-2

EXECUTIVE SUMMARY

A geotechnical engineering exploration has been performed for the proposed Western Area Power Administration’s Gila Substation located in Yuma, Arizona. Terracon’s geotechnical scope of work included the advancement of 11 test borings to approximate depths of 30 to 50 feet below existing site grades, laboratory testing on representative samples of the subsurface materials, engineering analyses and development of engineering recommendations for design and construction of foundations to support the proposed structures.

Based on the information obtained from our engineering analyses of the field and laboratory data, the site appears suitable for the proposed construction based upon geotechnical conditions encountered in the test borings, and provided our recommendations contained in this report are properly implemented in the design and construction. The following geotechnical considerations were identified:

A large stockpile of existing fill soil is present in the western quarter of the site. The approximate limits of the soil stockpile are presented on Exhibit A-1 in Appendix A. The stockpiled fill is likely associated with excavation for the adjacent canal and/or dredging operations within the canal. Borings B-5 and B-5A were advanced on top of the stockpile and encountered fill soils to depths of 34 and 29.5 feet, respectively. Topographic mapping also indicates that this stockpile is on the order of 30 to 40 feet in height. Based on borings performed on top of the stockpile, the upper eight (8) to 14 feet of fill soils consisted of silty clayey sand and clayey sand, underlain by fine to medium grained sand fill soils with variable amounts of silt. Debris or deleterious materials were not encountered within the existing fill soils at these boring locations.

The existing stockpile is comprised of uncontrolled fill and is not considered suitable for support of the proposed improvements. Therefore, we recommend that the soil stockpile be removed from the planned construction areas. The existing fill soils can be re-used as engineered fill on the project, provided the materials are moisture conditioned and compacted during placement. Recommendations for the construction of engineered fill are provided in the report.

Below the soil stockpile and from the surface at the remaining borehole locations, the native soils generally consisted of fine to medium grained sand with variable amounts of silt. The relative density of the native soils was generally loose to medium dense.

Groundwater was not encountered to the maximum depth explored in any of the borings.

The Yuma area is located in a seismically active zone, and valley areas in and around Yuma are known to be susceptible to seismically induced liquefaction. As part of our engineering evaluation, we have reviewed a liquefaction hazard potential report for Yuma, Arizona. Based on this review, the project site is not located within an area that has been mapped as being susceptible to liquefaction. Additionally, considering the medium relative density of the soils in the upper 30 feet and the depth to groundwater, the potential for liquefaction of soils on this site is considered low.

We recommend that transformers and other slab mounted equipment be supported on mat foundations constructed on a minimum of 12 inches of engineered fill.

We recommend that the planned service building be supported on a shallow spread footing foundation system bearing on a minimum of 12 inches of engineered fill.

We recommend that pole and truss-mounted equipment be supported on drilled shaft foundation systems. If utilized, fully embedded steel poles should be embedded in accordance with the pole manufacturer’s recommendations. Drilling of foundations to design depths should be possible with conventional drilling equipment using single flight power augers. However, the sandy and dry nature of the subsurface soils may necessitate the use of temporary casing during construction of drilled shaft foundations.

The on-site soils, including the existing fill soils, are considered suitable for use as engineered fill for the project.

The 2012 International Building Code seismic site classification for this site is Site Class D.

This executive summary should be used in conjunction with the entire report for design and/or construction purposes. It should be recognized that specific details were not included or fully developed in this section, and the report must be read in its entirety for a comprehensive understanding of the items contained herein. The section titled General Comments should be read for an understanding of the report limitations.

GEOTECHNICAL ENGINEERING REPORT

WESTERN AREA POWER ADMINISTRATION

GILA SUBSTATION

YUMA, ARIZONA

1.0 INTRODUCTION

This report presents the results of our geotechnical engineering services performed for the proposed Western Area Power Administration’s (Western) Gila Substation located in Yuma, Arizona. The purpose of these services is to provide information and geotechnical engineering recommendations relative to:

subsurface soil conditions groundwater conditions earthwork foundation design and construction seismic considerations lateral earth pressures

Our geotechnical engineering scope of work for this project included drilling 11 borings for subsurface exploration, field resistivity testing, laboratory testing, geotechnical engineering analysis, and preparation of this report. Logs of the borings along with a Site Plan and Test Locations diagram (Exhibit A-2) are included in Appendix A of this report. The results of the laboratory testing performed on soil samples obtained from the site during the field exploration are included in Appendix B of this report. Descriptions of the field exploration and laboratory testing are included in their respective appendices. Strength limit design load charts for drilled shaft foundations are presented in Appendix C.

2.0 PROJECT INFORMATION

2.1 Project Description

ITEM DESCRIPTION

Site Layout See Exhibit A-1 in Appendix A.

Description 230-kV Electrical Substation.

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Structures

The new substation will include slab and pole mounted equipment supported on reinforced concrete foundations, fully embedded steel poles and/or drilled shaft foundations. A single-story service building will be constructed in the southeast corner of the planned construction area.

Maximum loads Not provided. Maximum vertical loads are anticipated to be less than 500 kips.

Grading

A large stockpile of soil exists in the western portion of the site. If utilized as engineered on-site, existing grades could be raised on the order of 2 to 4 feet, although the grading plan for the site is unknown at this time.

2.2 Site Description

Location The project site is located northeast of the intersection of Araby Blaisdell Road and S. Avenue 9E near Yuma in Yuma County, Arizona.

Description The site consists of a rectangular shaped 17.9 acre parcel that is situated just east of the Gila Gravity Canal. The proposed construction will be located in the western half of the site.

Existing improvements

Outside of the existing soil stockpile, the site appears to be vacant and undeveloped. However, the planned construction area is located directly north of an existing 69kV substation, and existing north-south oriented overhead power lines are present on the east side of the planned construction area. The Gila Gravity Canal is located just west of the site, and the associated Gila Mesa pumping plant is located just southwest of the site.

Existing topography A large stockpile of soil with a height of 30 to 40 feet is present in the western quarter of the site. Outside of this stockpile area, the surface of the site is relatively flat.

Current ground cover Bare soil with sparse desert shrubs.

3.0 SUBSURFACE CONDITIONS

3.1 Site Geology

The project area is located in the Basin and Range physiographic province (1Cooley, 1967) of the North American Cordillera (2Stern, et al, 1979) of the southwestern United States. The

1Cooley, M.E., 1967, Arizona Highway Geologic Map, Arizona Geological Society.

2Stern, C.W., et al, 1979, Geological Evolution of North America, John Wiley & Sons, Santa Barbara, California.

Resourceful Responsive Reliable 3 southern portion of the Basin and Range province is situated along the southwestern flank of the Colorado Plateau and is bounded by the Sierra Nevada Mountains to the west. Formed during middle and late Tertiary time (100 to 15 m.y. ago), the Basin and Range province is dominated by fault controlled topography. The topography consists of mountain ranges and relatively flat alluviated valleys. These mountain ranges and valleys have evolved from generally complex movements and associated erosional and depositional processes.

Typically, the ranges in this area are of small aerial extent, but protrude significantly above adjacent wide alluviated plains and valleys. The basin rims are formed by the mountain ranges, which consist of sedimentary, igneous, and metamorphic materials that have been subjected to recurrent faulting and tilting, and in some places volcanic and intrusive events. As a result of erosion, the valleys have experienced partial infilling with sedimentary material deposited as alluvial fans. Occasionally, the valleys may become interlocking as a result of coalescing alluvial fans, which are referred to as bajadas.

Surficial geologic conditions mapped in the project vicinity (3Richard, et al, 2000) consist of Quaternary surficial deposits. These materials have been described as unconsolidated to strongly consolidated alluvial and eolian deposits. This unit includes: coarse, poorly sorted alluvial fan and terrace deposits on middle and upper piedmonts and along large drainages;

sand, silt and clay on alluvial plains and playas; and wind-blown sand deposits. The soils encountered in the test borings are consistent with the mapped geologic conditions.

3.2 Liquefaction Potential

The Yuma area is located in a seismically active zone, and valley areas in and around Yuma are known to be susceptible to seismically induced liquefaction. As part of our engineering evaluation, we have reviewed a liquefaction hazard potential report for Yuma, Arizona prepared for the City of Yuma by Southland Geotechnical (4Stringer, 1997). Based on this review, the project site is not located within an area that has been mapped as being susceptible to liquefaction. Additionally, liquefaction is typically a hazard at sites where loose sandy soils exist below groundwater, and considering the medium relative density of the soils in the upper 30 feet of the borings and the depth to groundwater, the potential for liquefaction of soils on this site is considered low.

3.3 Subsurface Soil Conditions

Specific conditions encountered at each boring location are indicated on the individual boring logs included in Appendix A of this report. Stratification boundaries on the boring logs represent the

3 Richard, S. M., Reynolds, S.J., Spencer, J. E., and Pearthree, P. A., 2000, Geologic Map of Arizona: Arizona Geological Survey

Map 35, 1 sheet, scale 1:1,000,000.

4 Stringer, Shelton L., November 5, 1997; Liquefaction Hazard Evaluation for Yuma, Arizona; Southland Geotechnical, Inc., Report No. Y97036.

Resourceful Responsive Reliable 4 approximate location of changes in soil types; in-situ, the transition between materials may be gradual. Details for each of the borings can be found on the boring logs included in Appendix A of this report.

3.3.1 Existing Fill Soils

A large stockpile of soil is present in the western quarter of the site. The approximate limits of the soil stockpile are presented on Exhibit A-1 in Appendix A. Based on a review of historic aerial photographs and topographic mapping5, it appears the soil stockpile was placed prior to 1965. The fill pile is likely associated with excavation for the adjacent canal and/or dredging operations within the canal.

Borings B-5 and B-5A were advanced on top of the stockpile and encountered fill soils to depths of 34 and 29.5 feet, respectively. Based on borings performed on top of the stockpile, the upper eight

(8) to 14 feet of soil consisted of silty clayey sand and clayey sand fill soils, underlain by fine to medium grained sand fill soils with variable amounts of silt. The relative density of the fill soils was generally loose. Debris or deleterious materials were not encountered within the existing fill soils encountered at these boring locations.

3.3.2 Native Soils

Beneath the existing fill soils encountered in Borings B-5 and B-5A, and from the surface at the remaining boring locations, the native soils generally consisted of fine to medium grained sand with variable amounts of silt. The relative density of the native soils was generally loose to medium dense. Thin interbedded layers of lean clay were encountered at isolated locations throughout the boring depths. Isolated zones containing gravel were also encountered in several of the borings.

3.3.3 Laboratory Testing Summary

Laboratory tests were conducted on selected soil samples and the test results are presented in Appendix B. The upper silty clayey sand and clayey sand soils encountered within the existing fill stockpile exhibited low plasticity characteristics. The poorly graded sand and silty sand soils encountered in the borings are nonplastic.

Excluding the thin isolated layers of lean clay encountered at isolated locations, the fine fraction (i.e. silt and clay content) within the soil matrix is generally less than 20 percent, and the moisture contents are less than five (5) percent in samples obtained to the maximum depth explored of 30 feet below the native site grade. In response to wetting, the sand soils exhibit low potential for moisture induced compression (i.e, collapse) at in-situ moisture and density.

Direct shear testing performed at in-situ moisture content and density (i.e., unsaturated) on the upper sand soils had friction angles ranging from 32 to 42 degrees, and apparent cohesion ranging from 0 to 84 psf.

5 U.S. Geologic Survey. Fortuna Quadrangle, Arizona-Yuma County [map]. 1965. 1:24,000. 7.5 Minute Series.

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3.4 Groundwater Conditions

Groundwater was not observed in any test boring at the time of field exploration, nor when checked immediately upon completion of drilling. These observations represent groundwater conditions at the time of the field exploration and may not be indicative of other times, or at other locations. Groundwater conditions can change with varying seasonal and weather conditions, and other factors.

Based on information obtained from the Arizona Department of Water Resources – Groundwater Data website (https://gisweb.azwater.gov/waterresourcedata/GWSI.aspx), the depth to regional groundwater was measured in February 2007 to be approximately 140 feet below the ground surface (approximate elevation of 140 feet above mean sea level) at an Arizona Department of Water Resources (ADWR) monitored well site (Local ID C-29-21 07AAD) located approximately two miles southeast of the site.

3.5 Seismic Considerations

The following table provides the seismic design criteria in accordance with the 2012 International Building Code at the approximate center of the site, obtained from the USGS Earthquake Hazards website (http://earthquake.usgs.gov/hazards/designmaps/javacalc.php):

Location Site Class1

Site Latitude (ºNorth)

Site Longitude

(ºEast)

Ss - Spectral Acceleration for a Short

Period

S1 - Spectral Acceleration for a 1-Second Period

Fa - Site Coefficient for a Short

Period

Fv - Site Coefficient for a 1-Second Period

B-3 D 32.68568 -114.47058 0.625g 0.239g 1.3 1.9 1 Note: In general accordance with the 2012 International Building Code, Section 1613.3.2, which refers to Chapter 20 of ASCE 7-10.

The procedure outlined in Chapter 20 of ASCE 7-10 relies on a site soil profile determination extending to a depth of 100 feet for seismic site classification. The current scope does not include the required 100 foot soil profile determination. Borings extended to a maximum depth of 30 feet. Additional exploration to deeper depths would be required to change the current seismic site classification.

3.6 Field Resistivity Test Results

Field measurements of soil resistivity were performed on August 23, 2014, in general accordance with ASTM Test Method G 57, and IEEE Std. 81, using the Wenner Four-Electrode Method. The soil resistivity testing was performed along two perpendicular lines oriented in northeast-southwest and northwest-southeast directions at the location of the project. The locations of the field resistivity lines, identified as FR-1 and FR-2, are shown on Exhibit A-1 in Appendix A.

The soil resistivity measurements were performed using a MiniSting, Memory Earth Resistivity and IP Meter, manufactured by Advanced Geosciences, Inc. The Wenner arrangement (equal electrode spacing) was used with the “a” spacing incrementally increasing in 5 foot increments https://gisweb.azwater.gov/waterresourcedata/GWSI.aspx http://earthquake.usgs.gov/hazards/designmaps/javacalc.php

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Location “a” spacing (feet)

Measured Resistance (ohms)

Average Resistivity (ohm-ft)

Average Resistivity (ohm-cm)

FR-1

Soil Resistivity Test Line 1, North-South

Orientation

5 45.50 1,429 43,547

10 29.20 1,834 55,893

15 20.00 1,884 57,424

20 13.23 1,662 50,648

25 9.29 1,459 44,456

30 7.50 1,413 43,068

35 7.25 1,594 48,571

40 6.60 1,658 50,533

45 5.78 1,633 49,787

50 5.44 1,708 52,065

55 5.14 1,775 54,113

60 4.89 1,843 56,161

65 4.46 1,821 55,491

FR-2

Soil Resistivity Test Line 2, East-West

Orientation

5 60.30 1,893 57,711

10 36.00 2,261 68,909

15 22.10 2,082 63,454

20 15.54 1,952 59,492

25 13.15 2,065 62,927

30 11.71 2,206 67,244

35 9.82 2,158 65,789

40 8.62 2,165 66,000

45 7.03 1,987 60,554

50 5.36 1,683 51,299

55 4.23 1,461 44,533

60 3.56 1,341 40,886

65 3.10 1,265 38,570

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4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION

4.1 Geotechnical Considerations

Based on the geotechnical engineering analyses, subsurface exploration and laboratory test results, we recommend that transformers and other slab mounted equipment be supported on mat foundations, and that the planned service building be supported on a shallow spread footing foundation system, both bearing on engineered fill. We recommend that any pole and truss-mounted equipment be supported on drilled shaft foundation systems or fully embedded steel poles. Drilling of foundations to design depths should be possible with conventional drilling equipment using single flight power augers. Specialized drilling equipment is not considered necessary for this project based on the subsurface conditions encountered in the test borings.

However, the sandy and dry nature of the subsurface soils may necessitate the use of temporary casing during construction of drilled shaft foundations.

A large stockpile of existing fill soil is present in the western quarter of the site. The existing soil stockpile is comprised of uncontrolled fill and is not considered suitable for support of the proposed improvements. Therefore, we recommend that the soil stockpile be removed from the planned construction areas. The existing fill soils can be re-used as engineered fill on the project, provided the materials are moisture conditioned and compacted during placement.

We understand that Load and Resistance Factor Design (LRFD) will be used for the design of foundations on this project. All foundation design recommendations are based on nominal (ultimate) resistances. Appropriate LRFD resistance factors should be applied in the design.

Geotechnical engineering recommendations for foundation systems and other earth connected phases of the project are outlined below. The recommendations contained in this report are based upon the results of field and laboratory testing (which are presented in Appendices A and B), engineering analyses, and our current understanding of the proposed project.

4.2 Mat Foundations

Reinforced mat foundations are recommended for support of transformers and other slab mounted equipment on the project. The mat foundations should be designed based on the criteria outlined in the following table:

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DESIGN ITEM DESCRIPTION/RECOMMENDATION

Foundation Type Mat Foundations for Equipment Slabs

Bearing Material A minimum 12 inches of engineered fill placed within the geometric configurations below mat foundations as outlined in

Section 4.5.3 of this report Design Modulus of Subgrade Reaction, k 250 pci Modulus Correction Factor* kc=k((b+1)/2b)2

Nominal (Unfactored) Bearing Resistance Any practical value up to 5,550 psf

Minimum Strength Resistance Factor 0.45

Total Estimated Settlement 1 inch or less

Minimum Embedment for Frost Penetration N/A *It is common to reduce the k-value to account for dimensional effects of large loaded areas. Where kc is the corrected or design modulus value and b is the mat width in feet.

The reinforced mat should be designed for sufficient rigidity to resist differential movement of ¾-inch acting over about 4 feet. Subgrade supporting reinforced mats should be prepared as recommended in the Earthwork section of this report.

4.3 Spread Footings

Spread footings can be considered for support of any proposed buildings on the project. The spread footings should be designed based on the criteria outlined below:

DESIGN ITEM DESCRIPTION/RECOMMENDATION

Foundation Type Conventional Shallow Spread Footings

Bearing Material A minimum 12 inches of engineered fill placed within the geometric configurations and depths below footings as outlined in

Section 4.5.3 of this report Nominal (Unfactored) Bearing Resistance 5,550 psf Minimum Strength Resistance Factor 0.45 Minimum Embedment Depth Below Finished Grade 12 inches Minimum Embedment for Frost Penetration N/A

Minimum Footing Dimensions Isolated Column Footings: 24 inches Continuous Wall Footings: 16 inches

Total Estimated Settlement (at the strength limit design) 1 inch or less

Estimated Differential Settlement (at the strength limit design) ½ of the total settlement

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Finished grade is defined as the lowest adjacent grade within five feet of the foundation for perimeter (or exterior) footings and finished floor level for interior footings. The factored foundation bearing pressure applies to factored dead loads plus design live load conditions.

The factored design bearing pressure may be increased by one-third when considering factored total loads that include wind or seismic conditions. The weight of the foundation concrete below grade may be neglected in dead load computations.

Total and differential settlements should not exceed predicted values, provided that:

the strength limit factor is applied to the ultimate bearing pressure, foundations are constructed as recommended, and essentially no changes occur in water contents of foundation soils.

Additional foundation movements could occur if water from any source infiltrates the foundation soils; therefore, proper drainage should be provided in the final design and during construction.

Footings, foundations, and masonry walls should be reinforced as necessary to reduce the potential for distress caused by differential foundation movement. The use of joints at openings or other discontinuities in masonry walls is recommended.

Foundation excavations should be observed by the geotechnical engineer. If the soil conditions encountered differ significantly from those presented in this report, supplemental recommendations will be required.

4.4 Drilled Shaft Foundations

4.4.1 Drilled Shaft Design Criteria

Drilled shaft foundations drilled a minimum of 10 feet or 4B (where B is the shaft diameter), whichever is greater, into natural soils are recommended for support of proposed pole mounted equipment or transmission lines on the project. If utilized, fully embedded steel poles should be embedded in accordance with the pole manufacturer’s recommendations.

As requested in the Western RFP, the geotechnical design parameters have been determined for the subsurface profile, and are summarized in the following table. The values in the following table have been based on foundations being constructed at the native grade of the site. If any substantial cut or fill placement is expected, the design values presented in the following table should be modified accordingly.

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Geotechnical Engineering Design Parameters

Depth to Bottom of Layer (ft)

Unit Weight

(pcf)

SPT

‘N’

Value

Friction Angle ( ) (degrees)

Cohesion (psf)

Nominal End

Bearing Capacity1

(ksf)

Nominal Skin

Friction Compression2

(ksf)

Nominal Skin

Friction Uplift3 (ksf)

Depth To

Water (ft)

5 105 10 30 --- --- 0.25 0.25

N/A 10 105 10 30 --- --- 0.80 0.80 15 110 18 32 --- 21 1.25 1.25 20 110 18 32 --- 21 1.70 1.70 25 120 25 34 --- 30 2.00 2.00

1 A strength resistance factor of 0.40 should be applied…