36C25919R0007-0002001.pdf

Geotechnical Engineering Report 436-113 VA Fort Harrison Construct Primary Care Mental Health

Integration Project

Fort Harrison, Montana

November 3, 2017

Terracon Project No. C4175024

Prepared for:

Guidon Design

905 N. Capitol Avenue, Suite 100

Indianapolis, Indiana

Prepared by:

Terracon Consultants, Inc.

1392 13th Ave SW

Great Falls, Montana

Terracon Consultants, Inc. 1392 13 th Ave SW Great Falls, MT 59404

P [406] 453 5400 F [406] 761 6655 terracon.com

GUIDON Design

905 N. Capitol Avenue, Suite 100

Indianapolis, Indiana 46204

Attn: Ms. Kari Allen, AIA, EDAC, LEED AP

P: 317-800-6388 ext. 112

E: kallen@guidondesign.com

Re: Geotechnical Engineering Report

436-113 VA Fort Harrison Construct Primary Care Mental Health Integration Project

Fort Harrison, Montana

Terracon Project Number: C4175024

Dear Ms. Allen:

Terracon Consultants, Inc. (Terracon) has completed the geotechnical engineering services for the above referenced project. This study was performed in general accordance with our proposal number PC4175024 dated May 18, 2017 and subsequent signed agreement dated

June 28, 2017. This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerning earthwork and the design and construction of foundations and floor slabs 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.

Matthew D. Hoffmann, P.E. for Brian J. Williams, P.E., P.G.

Office Manager Senior Project Geotechnical Engineer

Geotechnical Services APR Reviewer

NAM Reviewer: R.L. (Levi) Denton II, P.E.

Enclosures cc: 1 – Client (PDF)

1 – File mailto:kallen@guidondesign.com

TABLE OF CONTENTS

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Page

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

1.0 INTRODUCTION

2.0 PROJECT INFORMATION

2.1 Project Description

2.2 Site Location and Description

3.0 SUBSURFACE CONDITIONS

3.1 Surficial Geology

3.2 Generalized Subsurface Profile

3.3 Groundwater

4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION

4.1 Geotechnical Considerations

4.2 Earthwork

4.2.1 Site Preparation

4.2.2 Material Requirements

4.2.3 Compaction Requirements

4.2.4 Fill Construction and Observation Testing

4.2.5 Utility Trench Backfill

4.2.6 Grading and Drainage

4.2.7 Earthwork Construction Considerations

4.3 Foundations

4.3.1 Foundation Design Recommendations

4.3.2 Foundation Construction Considerations

4.4 Floor Slabs

4.4.1 Floor Slab Design Recommendations

4.4.2 Floor Slab Construction Considerations

4.5 Seismic Considerations

4.6 Lateral Earth Pressures

4.7 Soil Chemistry and Corrosivity

5.0 GENERAL COMMENTS

APPENDIX A – FIELD EXPLORATION

Exhibit A-1 Field Exploration Description

Exhibit A-2 Site Vicinity Map

Exhibit A-3 Boring Location Site Plan

Exhibit A-4 to A-8 Boring Logs

APPENDIX B – SUPPORTING INFORMATION

Exhibit B-1 Laboratory Test Descriptions

Exhibits B-2 to B-5 Laboratory Data

APPENDIX C – SUPPORTING DOCUMENTS

Exhibit C-1 General Notes

Exhibit C-2 Unified Soil Classification System

Primary Care Mental Health Integration Project ■ Fort Harrison, Montana November 3, 2017 ■ Terracon Project No. C4175024

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EXECUTIVE SUMMARY

A geotechnical investigation has been performed for the proposed Primary Care Mental Health

Integration Project at the VA Montana Health Care System located in Fort Harrison, Montana. A total of six borings were planned for the site, but due to conflict with buried utilities at one boring location only five borings were drilled within the building footprint. The borings were drilled to depths between 19.3 and 31.5 feet below existing grade. This report provides geotechnical recommendations for the design and construction of foundations, floor slabs, and other earthwork related aspects of the proposed project. These recommendations are based on the preliminary project information currently available and as described herein.

The surficial geology of the site is controlled surficial existing fill overlying native alluvial deposits of clayey/silty sand and gravel overlying dense gravel/cobble deposits at relatively shallow depths.

Based on these geologic conditions, the following geotechnical design considerations have been applied in the development of this report:

The preliminary design information provided indicates that the project will include the construction of a two-story addition to the existing Primary Care Facility with basement and first floor levels indicated on the preliminary plans provided to Terracon. Based on discussions with Ms. Kari Allen, AIA with Guidon, we understand that the basement level is actually walk-in level from the south end of the existing facility. The proposed layout would include a crawlspace below this level, with 8 foot walls in the crawlspace area. The basement level and first level will be above grade. Based on review of available as-built drawing for the existing facility, we understand that the existing foundations are embedded approximately 2.5 feet below existing exterior grade. With the crawlspace construction of the addition and the understanding that the existing facility foundation is embedded approximately 2.5 feet below grade, the additions crawlspace slab will be approximately 5.5 feet below existing bearing elevation. Collaboration has been conducted with design team members to limit the potential distress that the existing building foundations could experience without a proper approach to limit potential for loss of bearing support of existing foundations during construction of the addition, i.e. reduce risk for undermining foundations by means of underpinning or supporting existing foundations during and post construction. Based on this collaboration, it is our understanding that the addition foundations will be constructed in a stepped manner from west to east such that the foundations adjacent the existing facility will be founded near the 2.5-foot embedment elevation and the addition foundations will gradually be stepped to embedment on the order of 8 feet below grade. As project design continues, the loading conditions should be reviewed by Terracon to ascertain that the recommendations provided within this report are consistent with the final plan to reduce risk of differential movement between addition and existing facility as well as provide reliable bearing support within native alluvial deposits without compromising support of adjacent facility.

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Based on preliminary plans provided, conversations with Guidon, and collaboration on addition support concepts, the following concept has been developed for crawlspace construction. The excavation for the foundation system at the addition will be conducted in a stepped fashion that will extend approximately 8 feet below existing grade as excavation progresses east. The majority of excavation is likely to result in complete removal of existing uncontrolled fill, special observation will be necessary to ascertain that all fill is removed adjacent the structure, while limiting potential for undermining concerns. The removal of all existing fill will provide structure support for the new foundations within a bearing stratum consisting of native alluvial clayey sand or clayey gravel. Based on Standard Penetration

Testing (SPT) N-values, the bearing capacity will increase substantially from 10 feet to 13 feet, at which elevation the majority of the borings encountered a gravel layer. The SPT N-values at 10 feet are variable from 5 to 22 blows per foot within the predominant clayey sand, while the N-values at 15 feet increase to 30+ blows per foot. The use of shallow foundations founded on a nominal Structural Fill extending to native alluvial sands/gravels at depths near

13 feet (for crawlspace areas to the east) are recommended to limit differential performance associated with variable loose pockets within the sand as well as provide sufficient load carrying capacity for foundation bearing. The new foundations to be established closest to the existing facility, will be founded near the existing foundation elevations to limit potential for undermining of the existing structures. Therefore, the higher profile foundations will include a minimum 4-foot thickness of Structural Fill, to remove all uncontrolled fill, overlying native sand deposits that are generally loose to medium dense, and proper preparation of subgrade in accordance with recommendations of this report will be important to limiting differential settlement. The use of a Structural Fill working pad will increase bearing uniformity, limit potential differential performance, and reduce the risk of potential point loading on large gravels/cobbles potentially encountered within the alluvial deposit.

Foundation excavations adjacent the existing facility are to be carefully conducted to limit the potential for undermining of existing foundation elements. Separation of new foundation elements from existing elements will need to be carefully reviewed by the project structural engineer in coordination with additional geotechnical input from Terracon. The concept of stepping the new foundation element from existing foundation elevation to 8-foot depth for full crawl space will aid in reducing the risk of undermining of existing structures. Providing new foundation elements at the same bearing elevation along with appropriate separation will reduce impacts of stress overlap of structural loads within the bearing materials. The lateral interaction (seismic) of structures will need to be reviewed as well to limit potential concerns of differing frequency structures colliding during a seismic event, i.e. reduce lateral earth pressure conditions if possible. For adequate assessment of this interaction, existing loads will need to be made available for review along with design loads for addition.

Close monitoring of the construction operations discussed herein will be critical in achieving the design subgrade support. We therefore recommend that the Terracon be retained to monitor this portion of the work.

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This summary should be used in conjunction with the entire report for design purposes. It should be recognized that 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.

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GEOTECHNICAL ENGINEERING REPORT

PRIMARY CARE IMPROVEMENTS PROJECT

FORT HARRISON, MONTANA

Terracon Project No. C4175024

1.0 INTRODUCTION

A geotechnical investigation has been performed for the proposed Primary Care Mental Health

Integration Project at the VA Montana Health Care System located in Fort Harrison, Montana. The focus of the geotechnical investigation was to characterize subsurface conditions at the site and to provide recommendations for foundation design and construction, floor slabs, and other earthwork related considerations. A total of six borings were planned for the site, but due to conflict with buried utilities at one boring location only five borings were drilled within the building footprint. The borings were drilled on August 23 and 24, 2017 utilizing Terracon’s CME 55 truck-mounted drill rig. Borings were advanced to depths ranging between 19.3 and 31.5 feet below existing grade. Boring locations are shown on the Exploration Plan attached in Appendix A.

The purpose of these services is to provide information and geotechnical engineering recommendations relative to:

subsurface soil conditions foundation design and construction groundwater conditions slab design and construction earthwork seismic considerations

2.0 PROJECT INFORMATION

2.1 Project Description

Item Description

Structure

The project consists of the addition of a new 18,400 square foot facility housing Primary Care in support of the Patient Aligned Care

Team (PACT) model of care. The addition will be connected to the existing primary care facility, adjacent the existing south entrance, and will require renovation of approximately 2,000 square feet of the existing space

Building Construction

We expect the multi-story structure to consist of a combination of structural steel and brick/CMU construction with crawlspace foundations.

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Finished floor elevation Planned finished floor is set to match existing building floor

Maximum loads, provided by

Guidon Design

Column: 140 kips

Walls: 9 kips per linear foot

Floor slab: 250 psf

Grading Minimal grading to be conducted

Below grade areas Crawlspace construction is anticipated along with elevator pits

2.2 Site Location and Description

Location

The building is located at 3687 Veterans Drive in Fort Harrison, Montana. Approximate GPS coordinates of the addition are

46.61857° N / 112.1011° W

Existing improvements The project is to adjoin the existing primary care facility

Current ground cover Varied cover from asphalt pavement, concrete sidewalk, and landscaping.

Existing topography Relative level site

3.0 SUBSURFACE CONDITIONS

3.1 Surficial Geology

The site is located on the western edge of the Helena Valley, to the northwest of Tenmile Creek.

Topography of the site is indicative of alluvial and colluvial influences from the Cherry Creek and

Sevenmile Creek drainages emanating from the low hills to the west of the site and the Tenmile

Creek flood plain to the south of the project site. Soils are primarily alluvial sand and gravel materials, with some interbedding of silt and clay soils that likely overbank flood deposits.

3.2 Generalized Subsurface Profile

Based on the results of the borings, subsurface conditions on the project site can be generalized as follows:

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Stratum

Approximate Depth to

Bottom of Stratum

(feet)

Material Description Consistency/

Density

Topsoil

Layer/Asphalt

0.2 to 0.3

TOPSOIL, organic matter

(Borings B1, B2 and B4) / Asphalt

Concrete 3.5 inches thick (Boring

B5)

N/A

Stratum 1

(existing fill)

2.0 to 7.0

Existing FILL, predominantly reworked native alluvial material, clayey sand with scattered gravels

N/A

Stratum 2

Alluvial Sands and

Gravels

28.0 to 31.5+

Variation between Clayey

SAND/Clayey SAND with

Gravel/Clayey GRAVEL with

Sand/Silty SAND, fine to coarse sands throughout, some clay beds, scattered larger gravels and potential cobbles

Loose to Very

Dense

Stratum 3

Alluvial Clay

30.0 to 31.5+ (Borings

B1, B2, & B4)

Lean CLAY/Lean CLAY with

Sand, moderate plasticity, scattered gravels

Stiff to Very Stiff

Subsurface conditions encountered are described in further detail on the Logs of Boring in Appendix

A of this report and in the paragraphs below. Stratification boundaries on the Logs represent the approximate location of changes in soil types as could be estimated from field observation and laboratory test data; the in-situ transition between materials may be gradual.

The upper, existing fill layer (Stratum 1) appears to be the product of backfilling of the existing building foundations and site grade raising for development of the adjacent parking and landscaping. The existing fill is predominately clayey sand with scattered gravels; existing fill is assumed to be uncontrolled for support characterization. Uncontrolled fill is not considered a reliable material for foundation support and any zones of fill below bearing surfaces should be removed to native alluvial deposits.

The alluvium layer (Stratum 2) is a by-product of downcutting, progressive meandering, and periodic flooding associated with the surrounding Cherry Creek, Sevenmile Creek and Tenmile Creek. The alluvial deposits are primarily coarse-grained materials with scattered fine-grained deposits in-filled throughout the depth of the borings. The alluvial material is generally loose to very dense with

Standard Penetration Test (SPT) N-values in the range of 5 to 50+ blows per foot (bpf), with lower blow counts occurring in the finer grained sand deposits and blow counts typically greater than 30 bpf in the gravels. The wide range indicates that some areas of the cobble and gravel field were likely filled with subsequent sand and finer-grained deposits, causing the blow counts to be somewhat erratic in those zones. The natural moisture contents ranging from 4 to 19 percent varied

Responsive ■ Resourceful ■ Reliable 4 randomly. Index testing (Atterberg Limits and Particle-Size Distribution) results are presented in

Appendix B, Exhibits B-2 through B-4. Laboratory resistivity testing (discussed further in the paragraphs below) indicated a saturated resistivity of 2,000 ohm-cm, with pH of 8.0 and an extractable sulfate content of 0.014 percent (Exhibit B-5).

The clay layer (Stratum 3) is a by-product of periodic flooding associated with the surrounding Cherry

Creek and Tenmile Creek drainages. The alluvial clay is predominately lean clay with sand, with moderate plasticity and scattered gravels. The clay is generally stiff to very stiff at the depth encountered with Standard Penetration Test (SPT) N-values in the range of 11 to 23 blows per foot.

The natural moisture content of the clay was on the order of 11 to 23 percent.

3.3 Groundwater

The borings were observed while drilling and immediately after completion for the presence and level of groundwater. Groundwater was not observed in any of the borings. The field investigation does not fully reflect seasonal or long-term groundwater conditions which will be influenced by precipitation, hydrologic impacts originating off-site, and other factors beyond the scope of this investigation. Therefore, groundwater levels during construction or at other times in the life of the structure may vary from the conditions indicated on the Log.

4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION

4.1 Geotechnical Considerations

The primary geotechnical consideration for the planned addition is to provide reliable support of the addition while minimizing the potential for differential movement, both between the main building and addition, as well as along the new foundation lines. It will be important that new foundation elements immediately adjacent the existing building are established at the same elevation, or very near the same elevation, as the existing Primary Care Facility foundation elements to limit the potential for undermining of the existing footings or stress induction of new loads into below grade walls. It is our understanding that the use of stepped continuous foundations, in which the new footings are stepped from existing foundation level to a deeper level, on the order of 8 feet to accommodate crawlspace area, will be utilized to reduce risk of undermining. This will also limit the potential for stress induction from existing foundations to new foundation walls bearing below existing foundation elevation. At a minimum, lateral separation of new continuous footing should be on the order of 1 to 2 times the footing width from existing footings to limit stress overlap. At the current stage of project development, preliminary structural loading has been provided for our review and incorporated in our analysis described further below.

It is recommended that when final structural loading is development, we be allowed to review the loading along with foundation plans to assess impacts of new loads adjacent the existing structure

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Based on the results of the subsurface exploration, laboratory testing, and our analyses, it is our opinion that the proposed building addition can be supported on shallow foundations that extend through the existing uncontrolled fill, which should be fully accomplished by excavation to crawlspace elevation of approximately 8 feet below grade as preliminarily planned, and placed on a minimum 12 inches of newly compacted Structural Fill. For footings directly adjacent the existing facility, the minimal thickness on the order of 4 feet of Structural Fill will be required below footings bearing at or near existing footing elevation to limit potential for undermining of existing foundations. Geotechnical considerations for this project are addressed in the subsequent sections.

4.2 Earthwork

4.2.1 Site Preparation

Prior to conducting site earthwork or foundation construction, all vegetation, topsoil, and any fill or otherwise unsuitable material should be removed from the construction areas. After stripping, the foundation subgrade should be excavated to at least one foot below base of footing elevation by use of a backhoe/excavator. Disturbed material should be recompacted to at least 98 percent of maximum laboratory dry density per ASTM D698 prior to placement of a nominal 12 inches of

Structural Fill which will provide near surface uniformity and reduce potential for point loading of oversized gravels and cobbles where likely to be encountered.

4.2.2 Material Requirements

Compacted Structural Fill should meet the following material property requirements:

Fill Type 1 USCS Classification Acceptable Location for Placement

Structural Fill1,2 GP, GW, GM (and dual symbols)

Below foundation elevation and as replacement backfill.

On-site soils3, 4 SC, GC, CL

Exterior backfill above foundation elevation, site grade raising material, subgrade for parking and landscaped areas, as well as utility trench backfill

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Fill Type 1 USCS Classification Acceptable Location for Placement

1. Structural Fill should consist of approved materials that are free of organic matter and debris, and do not include soft, degradable, or deleterious particles. Frozen material should not be used, and fill should not be placed on frozen subgrade. Each proposed fill material type should be sampled and evaluated by the geotechnical engineer prior to its delivery and/or use.

2. Structural Fill, defined as imported aggregate, should meet the following criteria outlined below:

Percent finer by weight

Gradation (ASTM C136)

3”………………………………………………………………………………………………100

1 ½”………………………………………………………………………………………..85-100

No. 4 Sieve…………………………………………………………………………………30-60

No. 200 Sieve…………………………………………………………………………..12(max)

Liquid Limit………………………………………………………………………………25(max)

Plastic Index…………………………………………………………………………….10(max)

3. Significant moisture conditioning of the native clay soil, where encountered and considered for reuse, may be necessary to obtain compaction requirements; this will require mechanical reduction in clay clod size (disking, etc.) to a maximum 1-inch dimension to facilitate moisture conditioning; the necessary moisture adjustment will be difficult during wet/cold seasons.

4. Existing clayey sand/clayey sand with gravel fill may be considered for reuse if properly sorted on site to remove topsoil and other organics.

4.2.3 Compaction Requirements

Fill Lift Thickness

9 inches or less in loose thickness when heavy, self-propelled compaction equipment is used

4 to 6 inches in loose thickness when hand-guided equipment (i.e. jumping jack or plate compactor) is used

Minimum Compaction Requirements

Structural Fill and Native Alluvial Materials (subgrade) below foundation elevation: 98% of the maximum Standard

Proctor dry density (ASTM D 698)

On-site gravels above foundation elevation and backfill:

95% of the maximum Standard Proctor dry density (ASTM

D 698)

Pavement area subgrade and base course: 95% of the maximum Standard Proctor dry density (ASTM D 698

Landscape or Green Areas: 90% of the maximum Standard

Proctor dry density (ASTM D 698

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Compaction Water Content 2

Workable moisture levels – generally within the range of +/-

3% of the optimum moisture content value as determined by the Standard Proctor test at the time of placement and compaction

1. We recommend that engineered fill be tested for moisture content and compaction during placement. Should the results of the in-place density tests indicate the specified moisture or compaction limits have not been met, the area represented by the test should be reworked and retested as required until the specified moisture and compaction requirements are achieved.

2. Moisture levels should be maintained to allow for satisfactory compaction to be achieved without the fill material yielding/pumping.

4.2.4 Fill Construction and Observation Testing

The exposed subgrade and each subsequent lift of compacted fill should be tested, evaluated, and reworked, as necessary, until approved by the geotechnical engineer’s representative prior to placement of additional lifts. We recommend that prepared subgrade and each lift of fill be tested for density and moisture content at a frequency of one test for every 1,500 square feet of compacted fill in the building areas (below slabs and bulk fill), one test for every 75 lineal feet of footing lines, one test per isolated interior column location, and one test for every 10,000 square feet in pavement areas. We recommend one density and moisture content test for every 100 lineal feet of compacted utility trench.

4.2.5 Utility Trench Backfill

All trench excavations should be made with sufficient working space to adequately permit construction operations including backfill placement and compaction. Utility trenches are a common source of water infiltration and migration. All trenches that penetrate beneath the building foundations should be effectively sealed to restrict water intrusion and flow through the trenches that could migrate beneath the structure. We recommend constructing an effective clay

“trench plug” that extends at least 5 feet out from the foundation. The plug material should consist of clay compacted at a water content at or above the optimum water content. The clay fill should be placed to completely surround the utility above the bedding zone and be compacted in accordance with recommendations of this report. The bedding zone for utility installation immediately adjacent the structure should consist of flowable fill formed to cradle the pipe in a manner that will inhibit underflow toward the foundation area.

4.2.6 Grading and Drainage

Site grading should provide properly compacted backfill, and positive surface drainage should be established around the foundation locations to divert surface runoff and prevent ponding and resulting moisture infiltration. A 10 percent grade away from the foundation lines is recommended in the 10 feet adjacent to the structures. Where ADA access prohibits such grading, positively sloped pavement/flatwork with joints sealed on a routine basis may also serve as an acceptable deterrent to infiltration. Roof downspouts should be discharged to a positively graded surface beyond the

Responsive ■ Resourceful ■ Reliable 8 structure backfill zone. In addition, irrigation in the backfill zones should be avoided in favor or dry landscaping. Use of a clayey material placed as controlled fill for landscaping purposes would be generally advantageous for diverting and shedding surface runoff; clayey materials should be readily available on-site. Placement of controlled fill necessary to accomplish positive surface drainage and building/surficial backfilling should be conducted in accordance with the recommendations in

Sections 4.2.2 and 4.2.3. Calculations based on the assumed loading conditions discussed below are valid for proper surface drainage conditions to limit potential moisture infiltration to foundation locations. If poor drainage exists, and drainage is allowed to seep into foundation areas, the values of total and differential settlement discussed below are no longer to be held valid.

4.2.7 Earthwork Construction Considerations

The contractor is responsible for designing and constructing stable, temporary excavations and should shore, slope or bench the sides of the excavations as required, to maintain stability of both the excavation sides and bottom. All excavations should be sloped or braced to comply with applicable local, state and federal safety regulations, including the current OSHA Excavation and

Trench Safety Standards.

Upon completion of site preparation earthwork, care should be taken to maintain the subgrade moisture content prior to construction of slabs and foundations. Heavy construction traffic over the completed subgrade should be avoided to the extent practical. The site should also be graded to prevent ponding of surface water on the prepared subgrades or in excavations. If the subgrade should become frozen, desiccated, saturated, or disturbed, the affected material should be removed or these materials should be scarified, moisture conditioned, and recompacted prior to construction. Unstable subgrade conditions could develop during general construction operations, particularly if the soils are wet and/or subjected to repetitive construction traffic. Should unstable subgrade conditions develop, stabilization measures will need to be employed such as scarification/recompaction, or reinforcement measures as recommended by our engineer.

Our geotechnical engineer or representative should be retained during the construction phase of the project to perform necessary tests and observations during foundation and subgrade preparation, placement and compaction of Structural Fill, and backfilling of excavations.

4.3 Foundations

Based on the subsurface conditions, and the preliminary design information indicating crawlspace

(on the order of 8 feet below finished floor) level foundation system, it is our opinion that the proposed addition can be supported by shallow foundations bearing on a minimum 12 inches of

Structural Fill material extending to properly prepared native alluvial sand with gravel or native gravel. For the foundations adjacent existing facility that will be stepped from higher elevation bearing to 8-foot depth, the use of a minimum 4-foot thickness of Structural Fill will be required to remove all existing uncontrolled fill and provided bearing on native alluvial sands. The upper zone sands are generally loose to medium dense, but with the inclusion of on the order of 2 to 4 feet

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(as the footings step down in elevation) of Structural Fill the stress dissipation will be sufficient to limit settlement to acceptable levels. The use of Structural Fill working pad is recommended to increase bearing surface uniformity in the variable native alluvial deposit and reduce potential for differential settlement.

If a crawlspace depth is lessened, the use of frost depth shallow foundations established on

Structural Fill extending through existing uncontrolled fill (approximately 7 feet below existing grade) to properly prepared native alluvial sands will also provide reliable bearing support while limiting differential movements. It should be noted that if foundations are supported higher in the profile, the native sands above 13 feet have higher variability in relative density and additional

Structural Fill may be required to reduce anticipated settlement based on actual loading conditions. See analysis comments below.

4.3.1 Foundation Design Recommendations

Description Column Wall

Net allowable bearing pressure 1

Compacted Structural Fill extending to properly prepared native alluvial sand/gravel

4,500 psf 4,000 psf

Minimum dimensions 42 inches 18 inches

Minimum embedment below finished grade 2 12 inches 48 inches

Approximate total settlement from foundation loads 3 <1 inch < 1 inch

Estimated differential settlement from foundation loads 3

< ¾ inch between columns < ¾ inch over 40 feet

Ultimate passive pressure 4

Compacted Structural Fill extending to properly prepared native alluvial sand/gravel

250 pcf, equivalent fluid density

Ultimate coefficient of sliding friction 4 0.32

1. The recommended net allowable bearing pressure is the pressure in excess of the minimum surrounding overburden pressure at the footing base elevation. Assumes any unsuitable existing fill or soft soils, if encountered, will be undercut and replaced with compacted Structural Fill. Based upon a minimum Factor of Safety of 3.

2. For frost protection and to reduce the effects of seasonal moisture variations in the subgrade soils.

For perimeter footing and footings beneath unheated areas. Interior isolated footings are assumed to be embedded a minimum depth below slab elevation, where perimeter footings are frost protected.

3. The above settlement estimates from assumed foundation loads shown in Section 2.1 have assumed that the maximum footing width is 6.5 feet for column footings and 1.5 feet for continuous footings.

4. The spread footing foundation excavation sides must be nearly vertical and the concrete should be placed neat against these vertical faces for the passive earth pressure values to be valid. If the

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Description Column Wall loaded side is sloped or benched, and then backfilled, the allowable passive pressure will be significantly reduced. Passive resistance in the upper 3 feet of the soil profile should be neglected.

If passive resistance is used to resist lateral loads, the base friction should be neglected.

4.3.2 Foundation Construction Considerations

Given the preliminarily proposed building configuration and the anticipated use of crawlspace level foundation embedded approximately 8 feet below finished grade, the bearing surface anticipated will consist of native alluvial sand and/or gravel. The native clayey/silty sands or clayey gravel with sand should be properly prepared to recover construction disturbance caused by excavation activities. It is therefore recommended that following excavation to foundation elevation, the subgrade be moisture conditioned and compacted to obtain a minimum value of 98 percent of the maximum laboratory dry density value as determined by ASTM D698. This will provide for near surface uniformity to improve bearing as well as limit potential settlement associated with disturbed subgrade conditions.

The base of all foundation excavations should be free of water and loose soil and rock prior to placing concrete. Concrete should be placed soon after excavating to reduce bearing soil disturbance. Should the soils at bearing level become excessively dry, disturbed or saturated, or frozen, the affected soil should be removed prior to placing concrete. Place a lean concrete mud-mat over the bearing soils if the excavations must remain open over night or for an extended period of time. We recommend that the geotechnical engineer be retained to observe and test the soil foundation bearing materials.

If unsuitable bearing soils are encountered in footing excavations, the excavation could be extended deeper to suitable soils and the footing could bear directly on these soils at the lower level or on lean concrete backfill placed in the excavations. As an alternative, the footings could also bear on properly compacted structural backfill extending down to the suitable soils.

Overexcavation for compacted Structural Fill placement below footings should extend laterally beyond all edges of the footings at least 8 inches per foot of overexcavation depth below footing base elevation. The overexcavation should then be backfilled up to the footing base elevation with well graded granular material placed in lifts of 9 inches or less in loose thickness (6 inches or less if using hand-guided compaction equipment) and compacted to at least 98 percent of the material's standard effort maximum dry density (ASTM D 698). The overexcavation and backfill procedure is described in the following figure.

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4.4 Floor Slabs

The floor slab performance will be a function of subgrade preparation, and it is recommended that the floor slab should be supported on newly placed engineered fill, underlain by properly prepared suitable native soils. A nominal thickness of 12 inches of engineered fill is recommended throughout the floor slab to provide near surface uniformity of the slab support material. Preparation/compaction of the subgrade should be in accordance with Section 4.2.2 and Section 4.2.3.

4.4.1 Floor Slab Design Recommendations

Floor slab support Nominal 12-inch thickness of newly placed low plasticity engineered fill underlain by suitable soils, Modulus of subgrade reaction 125 pounds per square inch per in (psi/in) for point loading conditions

Aggregate base course/capillary break 2 4 inches of free draining granular material

1. Floor slabs should be structurally independent of any building footings or walls to reduce the possibility of floor slab cracking caused by differential movements between the slab and foundation.

2. Free-draining granular material should have less than 5 percent fines (material passing the #200 sieve). Other design considerations such as cold temperatures and condensation development could warrant more extensive design provisions.

Where appropriate, saw-cut control joints should be placed in the slab to help control the location and extent of cracking. For additional recommendations refer to the ACI Design Manual. Joints or any cracks in slab areas that develop should be sealed with a water-proof, non-extruding compressible compound specifically recommended for heavy duty concrete sections and wet environments.

The use of a vapor retarder should be considered beneath concrete slabs-on-grade that will be covered with wood, tile, carpet or other moisture sensitive or impervious coverings, or when the slab will support equipment sensitive to moisture. When conditions warrant the use of a vapor

Responsive ■ Resourceful ■ Reliable 12 retarder, the slab designer should refer to ACI 302 and/or ACI 360 for procedures and cautions regarding the use and placement of a vapor retarder.

Where floor slabs are tied to perimeter walls or turn-down slabs to meet structural or other construction objectives, our experience indicates that any differential movement between the walls and slabs will likely be observed in adjacent slab expansion joints or floor slab cracks that occur beyond the length of the structural dowels. The structural engineer should account for this potential differential settlement through use of sufficient control joints, appropriate reinforcing or other means.

4.4.2 Floor Slab Construction Considerations

On most project sites, the site grading is generally accomplished early in the construction phase.

However, as construction proceeds, the subgrade may be disturbed due to utility excavations, construction traffic, desiccation, rainfall, etc. As a result, the floor slab subgrade may not be suitable for placement of aggregate base and concrete and corrective action will be required. Additional protection, stabilization measures may be necessary and requires specific field evaluation. We recommend floor subgrades be maintained in a relatively moist condition until floor slabs are constructed. If the subgrade should become desiccated prior to construction of floor slabs, the affected material should be removed or the materials scarified, moistened, and recompacted. Upon completion of grading operations in the building areas, care should be taken to maintain the recommended subgrade moisture content and density prior to construction of the building floor slabs.

We recommend the area underlying the floor slab be rough graded and then have the Terracon geotechnical engineer perform evaluation prior to final grading and placement of aggregate base.

Particular attention should be paid to high traffic areas that were rutted and disturbed earlier and to areas where backfilled trenches are located. Areas where unsuitable conditions are located should be repaired by removing and replacing the affected material with properly compacted fill. All floor slab subgrade areas should be moisture conditioned and properly compacted to the recommendations in this report immediately prior to placement of the aggregate base and concrete.

4.5 Seismic Considerations

A site specific Probabilistic Seismic Hazard Analysis (PSHA) has been performed for the project, and will be provided under separate cover as an amendment to this report. The PSHA consist of two phases, with the first phase including measurement of shear-wave velocity in the upper 100 feet of the subsurface profile to determine site classification. The second phase includes determining the source location of the earthquake risks at the site, considering the distance to the site from the source, and the magnitude of potential earthquakes. Based on the field geophysical surveys, the average shear wave velocity in the upper 100 feet was on the order of 1,700 ft/s.

Based on 2015 International Building Code (IBC) Table 1613.5.2 and applicable ASCE 7, Chapter

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20 the Site Classification is C. This will be confirmed or modified, if necessary, in the forthcoming

PSHA report addendum.

4.6 Lateral Earth Pressures

Reinforced concrete walls with unbalanced backfill levels on opposite sides should be designed for earth pressures at least equal to those indicated in the following table. Earth pressures will be influenced by structural design of the walls, conditions of wall restraint, methods of construction and/or compaction and the strength of the materials being restrained. Two wall restraint conditions are shown. Active earth pressure is commonly used for design of free-standing cantilever retaining walls and assumes wall movement. The "at-rest" condition assumes no wall movement. The recommended design lateral earth pressures do not include a factor of safety and do not provide for possible hydrostatic pressure on the walls.

Earth Pressure Coefficients

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Earth

Pressure

Conditions

Coefficient for Backfill

Type

Equivalent

Fluid

Density

(pcf)

Surcharge

Pressure, p1 (psf)

Earth Pressure, p2 (psf)

Active (Ka) Structural Fill – 0.27

Native Sand/Gravel - 0.33

Native Lean Clay - 0.49

(0.27)S

(0.33)S

(0.49)S

(35)H

(40)H

(50)H

At-Rest (Ko) Structural Fill – 0.43

Native Sand/Gravel - 0.50

Native Lean Clay - 0.66

(0.43)S

(0.50)S

(0.66)S

(55)H

(60)H

(70)H

Passive (Kp) Structural Fill – 3.7

Native Sand/Gravel - 3.0

Native Lean Clay - 2.0

Applicable conditions to the above include:

For active earth pressure, wall must rotate about base, with top lateral movements of about

0.002 H to 0.004 H, where H is wall height. Typically, basement or perimeter walls do not deflect enough to mobilize active earth pressure. Therefore, at-rest earth pressures are typically employed for such design.

For passive earth pressure to develop, wall must move horizontally to mobilize resistance

Uniform surcharge, where S is surcharge pressure

In-situ soil backfill weight a maximum of 135 pcf (Structural Fill), 125 pcf (native sand/gravel), 110 (native lean clay)

Horizontal backfill, compacted between 95 and 98 percent of standard Proctor maximum dry density

Loading from heavy compaction equipment not included

No hydrostatic pressures acting on wall

No dynamic loading

No safety factor included

Ignore passive pressure in frost zone

Backfill placed against structures should consist of granular soils or low plasticity cohesive soils.

For the granular values to be valid, the granular backfill must extend out and up from the base of the wall at an angle of at least 45 and 60 degrees from vertical for the active and passive cases, respectively. To calculate the resistance to sliding, a value of 0.32 should be used as the ultimate coefficient of friction between the footing and the underlying soil.

A perforated rigid plastic drain line installed behind the base of walls that extend below adjacent grade is recommended to prevent hydrostatic loading on the walls. The invert of a drain line around a below-grade building area or exterior retaining wall should be placed near foundation bearing level. The drain line should be sloped to provide positive gravity drainage or to a sump

Responsive ■ Resourceful ■ Reliable 15 pit and pump. The drain line should be surrounded by clean, free-draining granular material having less than 5 percent passing the No. 200 sieve. The free-draining aggregate should be encapsulated in a filter fabric. The granular fill should extend to within 2 feet of final grade, where it should be capped with compacted cohesive fill to reduce infiltration of surface water into the drain system.

As an alternative to free-draining granular fill, a pre-fabricated drainage structure may be used.

A pre-fabricated drainage structure is a plastic drainage core or mesh which is covered with filter fabric to prevent soil intrusion, and is fastened to the wall prior to placing backfill.

If controlling hydrostatic pressure behind the wall as described above is not possible, then combined hydrostatic and lateral earth pressures should be calculated for lean clay backfill using an equivalent fluid weighing 90 and 100 pcf for active and at-rest conditions, respectively. For granular backfill, an equivalent fluid weighing 85 and 90 pcf should be used for active and at-rest, respectively. These pressures do not include the influence of surcharge, equipment or pavement loading, which should be added. Heavy equipment should not operate within a distance closer than the exposed height of retaining walls to prevent lateral pressures more than those provided.

Other earth pressure conditions may occur where new foundations are constructed in close proximity to existing foundations, but at different elevations. In that case, the footing higher in elevation will import lateral earth pressure on the foundation wall of the lower footing. The degree of such earth pressure is dependent upon the geometry of the two footing areas and the structural loads imported by the upper footing, and the later earth pressures developed are unique to the foundation loading and footing proximity. Analysis of such conditions is beyond our current scope of work, but we are available to analyze such conditions and provide design earth pressures. As

Responsive ■ Resourceful ■ Reliable 16 the basis for such analysis, we will require detailed cross-sections of the footing arrangement and the loads imposed by both foundations.

4.7 Soil Chemistry and Corrosivity.

Laboratory saturated paste resistivity testing was conducted on a discrete sample recovered from

Boring B1 at a depth of 7.5 feet below existing grade. Laboratory resistivity testing was conducted in general accordance with AASHTO T288 using the Miller Soil Box Method to evaluate a soil paste to determine minimum resistivity. Based on this testing the clayey sand encountered at 7.5 feet had a minimum resistivity of 2,000 ohm-cm.

SOIL RESISTIVITY (ohm-cm) CORROSION POTENTIAL

0 to 1,000 Very High

1,000 to 2,000 High

2,000 to 5,000 Moderate

> 5,000 Mild

Data suggests that the soil pH should not be a dominant soil variable affecting soil corrosion if the soil pH in the range of 5 to 8. Based on analytical laboratory pH testing, the soil tested has a pH of 8.0. The pH of the sample falls at the upper end of the recommended range and indicates that the soil pH may provide a minor contribution to corrosion.

The sulfate test results indicate the water soluble sulfate concentration at Boring B1 at 7.5 feet was 140 mg/kg.

This data indicates that the controlling subsurface profile is a moderately corrosive environment for buried metals. Any planned buried metals should be designed in accordance to the recommendation of a corrosion engineer were necessary.

Based on our local experience and results of the laboratory testing, the sulfate contents of the clay materials are generally to be considered as having the negligible potential for severe sulfate attack on concrete. Based on our experience, concrete in contact with native soils, in the presence of groundwater transporting the sulfate concentrations indicated, or subject to freeze-thaw cycles should be designed according to PCA Design and Control of Concrete Mixtures, utilizing Type I-II cement with minimum 28-day compressive strength of 4,500 psi and minimum water-cement ratio of 0.45.

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5.0 GENERAL COMMENTS

Terracon should be retained to review the final design plans and specifications so comments can be made regarding interpretation and implementation of our geotechnical recommendations in the design and specifications. Terracon also should be retained to provide observation and testing services during grading, excavation, foundation construction and other earth-related construction phases of the project.

The analysis and recommendations presented in this report are based upon the data obtained from the borings performed at the indicated locations and from other information discussed in this report. This report does not reflect variations that may occur between borings (it should be noted that the site has been heavily developed over time and variations are highly likely to exist between the discreet soil column profiles investigated and the excavation to be conducted for the addition footprint), across the site, or due to the modifying effects of construction or weather. The nature and extent of such variations may not become evident until during or after construction. If variations appear, we should be immediately notified so that further evaluation and supplemental recommendations can be provided.

The scope of services for this project does not include either specifically or by implication any environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or identification or prevention of pollutants, hazardous materials or conditions. If the owner is concerned about the potential for such contamination or pollution, other studies should be undertaken.

This report has been prepared for the exclusive use of our client for specific application to the project discussed and has been prepared in accordance with generally accepted geotechnical…