App C_Repair Maintenance Hangar B1033 Technical Memorandum 4.1.2020.pdf

Technical Memorandum

955 Malin Lane SW, Suite B • Tumwater, Washington 98501 • (360) 791-3178

TO: Devin Newton, PE, Project Manager, Tetra Tech

FROM: Daniel Simpson, PE

DATE: April 1, 2020

RE: Seismic and Foundation Recommendations Fairchild AFB Repair Maintenance Hangar B1033 Tetra Tech Project No. 200-38546-19018 Spokane County, Washington Landau Associates, Inc. Project No. 0043017.010.011

Introduction

Landau Associates, Inc. (LAI) prepared this technical memorandum in support of proposed repairs to

Hangar B1033 at the Fairchild Air Force Base (site; Figure 1).

Project Understanding

A seismic evaluation may be necessary to accommodate the proposed repairs. To determine whether a seismic evaluation is needed, a seismic design category (SDC, as defined in the American Society of

Civil Engineers’ [ASCE] Minimum Design Loads and Associated Criteria for Buildings and Other

Structures [ASCE/SEI 7]) must be assigned to Hangar B1033. Before an SDC can be assigned, the seismic site class of the structure must be determined (in accordance with Section 20 of ASCE 7). Tetra

Tech (prime engineer) has retained LAI’s services to help determine the seismic site class. If required, a seismic evaluation will be performed in accordance with Seismic Evaluation and Retrofit of Existing

Buildings, ASCE 41.

Proposed repairs to Hangar B1033 include construction of new interior concrete masonry unit (CMU) walls and a new slab-on-grade (mat) foundation. The walls will likely be constructed by cutting the existing slab and embedding new strip footings beneath the slab. The mat foundation will be constructed to smooth a grade change near the perimeter of B1033, and will be lightly loaded. LAI has provided the following information to support a seismic evaluation and design and construction of the walls:

Geotechnical conclusions regarding seismic hazards at the site, including the potential for soil liquefaction, surface fault rupture, and slope instability.

Recommended allowable soil bearing capacity for strip footings used to support the walls and a subgrade modulus for the mat foundation.

Recommendations for preparation of footing subgrades.

Subsurface Conditions

LAI used geotechnical data collected by others (CH2M Hill 1997, GeoEngineers 2002 and 2014, Shannon and Wilson 1976, see Attachment 1) to develop a conceptual model of site subsurface

Landau Associates

Seismic and Foundation Recommendations Fairchild AFB Repair Maintenance Hangar B1033 2 April 1, 2020 conditions. The model was used to evaluate seismic site class, to identify potential seismic hazards at the site, and to recommend geotechnical design parameters for wall footings.

Based on LAI’s review of available geotechnical data, site subsurface conditions consist of approximately 5 to 10 feet (ft) of loose to medium dense, sandy silt to silty sand alluvium. This unit may be seasonally saturated, beginning at approximately 3 to 5 ft below ground surface (bgs).

Weathered to competent basalt is likely present beneath the alluvium, at approximately 5 to 10 ft below ground surface. In areas surfaced with structural slab or pavement, several feet of alluvium has likely been overexcavated and replaced with fill.

Seismic Design Considerations

Seismic site class was calculated using Equation 20.4-2 in ASCE 7-16. Depth-to-bedrock was assumed to range from 5 to 10 ft bgs. Alluvium was assumed to have an average standard penetration test

(SPT) resistance of N = 10 blows per foot, and bedrock to have an SPT of N = 100 blows per foot. The resulting seismic site class is “C.” LAI recommends using the seismic design parameters in Table 1 to complete structural analysis.

Table 1. Seismic Design Parameters

Code Site Class PGA (g) FPGA Ss (g) Fa S1 (g) Fv

2015 IBC (ASCE 7-10) C 0.138 1.2 0.321 1.2 0.115 1.685

2018 IBC (ASCE 7-16) C 0.135 1.265 0.302 1.3 0.113 1.5

ASCE = American Society of Civil Engineers

FPGA, Fa, Fv = ground acceleration (PGA), acceleration (0.2-second period) and velocity (1.0-second period) site coefficients, respectively g = force of gravity

IBC = International Building Code

PGA = peak ground acceleration

Ss, S1 = 0.2-second and 1.0-second period spectral accelerations, respectively

Given the seasonal nature and limited thickness of potentially saturated soil relative to the overlying unsaturated crust, the relatively low design peak ground acceleration, and the relative density and fines content of site soils, the risk of liquefaction-induced structure damage appears negligible. The site is relatively flat, and is not located near active crustal faults. As such, the risk of seismic slope instability and fault rupture also appears to be negligible.

Foundation Recommendations for Concrete Masonry Unit Wall

Strip footings are proposed to support new CMU walls. Existing slabs-on-grade will likely be cut to accommodate installation of the footings. Native soil beneath the slab and slab fill likely consists of loose to medium dense, silty sand to sandy silt with a low bearing capacity. To limit settlement, soil

Fairchild AFB Repair Maintenance Hangar B1033 3 April 1, 2020 beneath the new footings should be overexcavated and replaced with structural fill. LAI recommends the following options for settlement mitigation:

Option 1. Overexcavate 1 ft of soil beneath the bottom of the footings, and replace with structural fill; the horizontal extents of the excavation can be neat with the edge of the footing. The bottom of the footings should be installed at least 12 inches below the top of slab or existing grade. Allowable net bearing capacities are summarized in Table 2.

Table 2. Allowable Net Bearing Capacity for Option 1 Overexcavation

Footing Width 1.5 ft 3 ft 4 ft

Allowable Bearing Capacity 2 ksf 1.5 ksf 1.3 ksf

Note: Increase values by one-third for transient (wind or seismic) loading.

ft = feet ksf = kips per square foot

Option 2. Overexcavate 2 ft of soil beneath the bottom of the footings and 1 ft beyond the edge of the footings, and replace with structural fill. The bottom of the footings should be installed at least 12 inches below the top of slab or existing grade. Allowable net bearing capacities are summarized in Table 3.

Table 3. Allowable Net Bearing Capacity for Option 2 Overexcavation

Footing Width 1.5 ft 3 ft 4 ft

Allowable Bearing Capacity 3.5 ksf 2.1 ksf 1.8 ksf

Note: Increase values by one-third for transient (wind or seismic) loading.

ft = feet ksf = kips per square foot

The subgrade should consist of existing native or fill soil, compacted with a vibratory plate compactor or similar compaction equipment. Before structural fill is placed, a qualified geotechnical engineer should evaluate the subgrade for soft or disturbed areas. Soft areas should be overexcavated to competent soil, moisture conditioned, and recompacted, or replaced with structural fill.

Structural fill should consist of Gravel Borrow that conforms to the requirements in Section 9-03.14(1) of the Washington State Department of Transportation’s 2020 Standard Specifications for Road, Bridge, and Municipal Construction (WSDOT 2020 Standard Specifications). Gravel Borrow should be placed in 6-inch, loose lifts and compacted to 95 percent of the maximum dry density, determined in accordance with the modified Proctor method (i.e., ASTM International standard test method D1557).

Controlled density fill or controlled low-strength material, per Section 2-09.3(1)E of the 2020 WSDOT

Standard Specifications, can be used in lieu of Gravel Borrow.

Foundation lateral resistance can be computed using a combination of base friction and lateral soil passive resistance. To limit foundation movement, passive resistance should be limited to two-thirds of the total (friction and passive) resistance. Frictional resistance should be calculated using a

Fairchild AFB Repair Maintenance Hangar B1033 4 April 1, 2020 coefficient of friction of 0.35 and dead loads only. Passive resistance can be calculated using an allowable equivalent soil fluid weight (includes a safety factor of 1.5) of 240 pounds per cubic foot, from the bottom of the existing slab to the bottom of the footing. If footings are situated outside of the building, passive resistance should be calculated from 2 ft bgs.

The minimum recommended footing depth for frost protection is 24 inches. Footings within a climate-controlled building enclosure need not be embedded for frost protection.

Mat Foundation Recommendations

LAI understands that a mat foundation may be constructed to facilitate a smoothed grade change associated with modifications or repairs to B1033. The approximate dimensions are 20 ft by 30 ft in plan and will be lightly loaded. The mat may be designed using the following moduli of subgrade reaction, depending on the subgrade condition. The provided moduli are for a 1ft by 1ft area and should be adjusted for the actual size of the mat. The moduli provided are applicable for mat loads up to 500 psf.

Option 1. Mat foundation directly on native soil. For this option, 12 inches of subgrade soils should be scarified and recompacted. Subgrade modulus, ks = 50 pci.

Option 2. Mat foundation on 2 ft of structural fill, as defined in the previous section. Subgrade modulus, ks = 120 pci.

Limitations

Landau Associates, Inc. (LAI) has prepared this technical memorandum for the exclusive use of Tetra

Tech for specific application to the Fairchild AFB Repair Maintenance Hangar B1033 in Spokane

County, Washington. No other party is entitled to rely on the information, conclusions, and recommendations included in this document without the express written consent of LAI. Reuse of the information, conclusions, and recommendations provided herein for extensions of the project or for any other project, without review and authorization by LAI, shall be at the user’s sole risk. LAI warrants that, within the limitations of scope, schedule, and budget, its services have been provided in a manner consistent with that level of skill and care ordinarily exercised by members of the profession currently practicing in the same locality under similar conditions as this project. LAI makes no other warranty, either express or implied.

Fairchild AFB Repair Maintenance Hangar B1033 5 April 1, 2020

Closing

We trust that the information provided herein is sufficient to proceed with the project. If you have questions or comments, please contact the undersigned at dsimpson@landauinc.com or at (360) 628-

5114.

LANDAU ASSOCIATES, INC.

Daniel Simpson, PE Associate

DCS/CAM/mcs [O:\0043\017.010\R\REPAIR MAINTENANCE HANGAR B1033 TECHNICAL MEMORANDUM 4.1.2020.DOCX]

Attachments: Figure 1. Vicinity Map Attachment 1. Supporting Data

Fairchild AFB Repair Maintenance Hangar B1033 6 April 1, 2020

References

ASCE. 2017. Minimum Design Loads and Associated Criteria for Buildings and Other Structures

(ASCE/SEI 7). American Society of Civil Engineers.

ASTM. 2003. D420-D5876: Annual Book of ASTM Standards. West Conshohocken, PA: ASTM

International.

CH2M Hill. 1997. Upgrade KC-135 Flightline Facilities, Washington Air National Guard, Fairchild AFB, Spokane, Washington.

GeoEngineers. 2014. Geotechnical Engineering Evaluation: Proposed Washington Air National Guard

DRBS Warehouse, Fairchild Air Force Base, Spokane County, Washington.

GeoEngineers. 2002. Geotechnical Engineering Report: Washington Air National Guard Clinic Addition

Project, Fairchild AFB, Washington. File No. 0781-009-00.

Shannon and Wilson. 1976. Steamline Explorations, WANG General Purpose Shops, Fairchild AFB.

WSDOT. 2019. M 41-10: Standard Specifications for Road, Bridge, and Municipal Construction 2020.

Washington State Department of Transportation. September 1.

Fairchild Air Force Base

FairchildFairchild

Fairchild AFBFairchild AFB

S Brooks Rd

ST902

£¤2

Data Source: Esri 2012.

Repair Maintenance Hangar B1033

Fairchild Air Force Base Spokane County, Washington Vicinity Map

Figure1

0 0.5 1

Miles

G:\ Pro jec ts\

\01

7\0 10\

\F0

1V icM ap .m xd 3/

24/

W a s h i n g t o nW a s h i n g t o nOlympia

Project Location

Tacoma Spokane

Everett Seattle

Project Location

Attachment 1

Supporting Data

Geotechnical Engineering Evaluation

Proposed Washington Air National Guard DRBS Warehouse Fairchild Air Force Base Spokane County, Washington for

ALSC Architects

August 4, 2014

Geotechnical Engineering Evaluation

Proposed Washington Air National Guard DRBS Warehouse Fairchild Air Force Base Spokane County, Washington for ALSC Architects

August 4, 2014

523 East Second Avenue Spokane, Washington 99202 509.363.3125

Table of Contents

INTRODUCTION

SCOPE OF SERVICES

SITE CONDITIONS

General Surface Conditions Published Information

Geologic Map Review Soil Survey Review

Subsurface Conditions General Fill Sand with Silt/Silty Sand Silt Silty Sand (Decomposed Basalt)

Groundwater Conditions

CONCLUSIONS AND RECOMMENDATIONS

General Site Preparation and Earthwork

Initial Site Preparation Slab and Pavement Subgrade and Footing Grade Preparation Excavation Support

Structural Fill General Use of On-Site Soil Imported Structural Fill Fill Placement and Compaction Criteria

Weather Considerations Foundation Support

General Footing Design Settlement Lateral Resistance

Floor Slab Support Seismic Considerations Pavements

General Material Specifications Flexible Pavement Section Design PCC Pad Design

Site Drainage Temporary Drainage Stormwater Considerations

August 4, 2014| Page i File No. 0781-019-00

LIMITATIONS

REFERENCES

LIST OF FIGURES

Figure 1. Vicinity Map Figure 2. Site Plan

APPENDICES

Appendix A. Field Explorations and Laboratory Testing Figure A-1 – Key to Exploration Logs Figures A-2 through A-7 – Logs of Borings Figure A-8 – Sieve Analysis Test Results Figures A-9 – Atterberg Limits Test Results

Appendix B. Report Limitations and Guidelines for Use

August 4, 2014| Page ii

INTRODUCTION

This report presents the results of our geotechnical engineering evaluation at the site of the proposed Washington Air National Guard (WANG) DRBS Warehouse at Fairchild Air Force Base (FAFB) in Spokane County, Washington. The site is located at the southwest corner of the intersection of Richmond Avenue and Grant Street, approximately as shown in the Vicinity Map, Figure 1.

We understand the approximate 8,000-square-foot, single-story warehouse will consist of a pre-engineered metal building with an approximate 4-inch-thick brick veneer. Project plans indicate the proposed DRBS Warehouse will be supported on conventional shallow spread foundations with a slab-on-grade floor at an approximate finished floor grade at Elevation 2,447.5 feet (North American Vertical Datum of 1988 [NAVD88]). Based on information provided by the structural engineer (Coffman Engineers), the proposed DRBS Warehouse will have maximum individual (column) foundation loads on the order of about 40 kips and continuous (wall) foundation loads on the order of about 1 kip per lineal foot (klf). The slab-on-grade will support heavy storage loads on the order of about 250 pounds per square foot (psf).

Additional site improvements will include landscaping, on-site stormwater management facilities, and a paved access road and parking lot around the proposed DRBS Warehouse. We understand heavy-duty pavements are required for support of forklifts and trucks configured to HS-20 loading. Based on our review of the “DRBS Grading and Storm Drainage Plan” developed by DCI Engineers, we anticipate that minor cutting and filling will be required to establish proposed final site elevations. The proposed improvements are shown relative to existing site features in the Site Plan, Figure 2.

GeoEngineers submitted a draft report containing our conclusions and recommendations on September 24, 2013. The information in this report confirms, revises and finalizes the information provided in our referenced 2013 draft report.

SCOPE OF SERVICES

Our services were completed in general accordance with our proposal dated April 2, 2013. Verbal authorization to begin our services was provided by you on August 12, 2013. Written authorization was provided in the contract between ALSC Architects and GeoEngineers dated September 10, 2013.

The purpose of our geotechnical engineering evaluation was to provide geotechnical recommendations for design and construction of proposed improvements based on site exploration, laboratory testing and engineering analyses. Our specific scope of services included:

1. Review of in-house files and information in the public domain for data that might be pertinent for use in developing our recommendations for the project.

2. Coordinating site access, staking exploration locations and checking for underground utility conflicts.

3. Exploration of soil and groundwater conditions by drilling a total six borings around the proposed building footprint and in pavement areas.

4. Laboratory testing to assess pertinent physical and engineering characteristics of soil encountered relative to the proposed construction.

August 4, 2014| Page 1

5. Recommendations for site preparation and fill placement, including: criteria for clearing, stripping and grubbing; an evaluation of the suitability of on-site soil for use as structural fill; criteria for identifying unsuitable soil at proposed foundation grade and floor slab subgrade within the proposed improvements and methods to mitigate presence of such material; gradation criteria for imported fill, if required; and criteria for structural fill placement and compaction.

6. Recommendations for design and construction of conventional shallow foundations for the proposed DRBS Warehouse, including: allowable soil bearing pressure; minimum width and depth criteria;

coefficient of friction and equivalent fluid densities for the passive earth pressure state of stress to estimate resistance to lateral loads; and estimates of total and differential foundation settlement. We also provide recommendations for treatment of unsuitable soil that might be encountered at proposed foundation grade during construction.

7. Recommendations for design and construction of concrete slab-on-grade floors, including: thickness, gradation and compaction criteria for an under-slab capillary moisture break layer; modulus of vertical subgrade reaction that may be used for thickness design of the slab; and guidance regarding the need for a vapor barrier, and criteria for design and construction of such a barrier, if warranted.

8. Recommendations for temporary cut and fill slope inclinations.

9. Geotechnical seismic design criteria based on the 2012 International Building Code (IBC).

10. Recommendations for design and construction of pavement parking lots and driveways at the site, including: subgrade preparation; thickness, gradation and compaction criteria for crushed rock base course; thickness of the portland cement concrete (PCC) pavement; and the thickness and compaction criteria for hot-mix asphalt (HMA).

11. An evaluation of the feasibility of on-site disposal of stormwater based on conditions encountered during site exploration, results of laboratory gradation analyses and information provided by the civil engineer.

SITE CONDITIONS

General

Soil and groundwater conditions at the site of the proposed DRBS Warehouse facility were explored on August 19, 2013 by drilling six exploratory borings (B-1 through B-6). The approximate locations of the borings are shown in Figure 2. The borings were drilled to depths ranging from about 7½ to 19 feet below existing site grade.

Soil samples obtained during exploration were transported to GeoEngineers’ laboratory for additional evaluation and testing. Detailed descriptions of our site exploration and laboratory testing programs, along with exploration logs and laboratory test results, are presented in Appendix A.

Surface Conditions

The proposed DRBS Warehouse facility is bounded to the north by Richmond Road, to the east by Grant Street, and to the south and west by currently undeveloped land. The site and surrounding areas are generally level, with occasional topographical variation from landscaping mounds. The site is vegetated by sparse weeds and grass and several deciduous trees. Partially to mostly buried cobbles, boulders, and

August 4, 2014| Page 2 concrete fragments and other debris are visible on the surface. Site topographic drawings and temporary markings at the site indicate the presence of underground utilities, including a gas line near the north property boundary extending in a generally east to west orientation, and a water line near the east property boundary extending in an approximate north-northeast to south-southwest orientation.

Published Information

Geologic Map Review

The Washington State Department of Natural Resources (DNR) “Geologic Map of the Spokane 1:100,000 Quadrangle, Washington – Idaho” compiled by N. Joseph (1990), maps the underlying geology in the vicinity of the site as Pleistocene outburst flood deposits of glacial Lake Missoula, predominantly gravel (Qfg).

These deposits consist of a poorly sorted, stratified mixture of boulders, cobbles, gravel and sand resulting from multiple episodes of catastrophic outbursts of glacier-dammed ancient Lake Missoula.

Soil Survey Review

The U.S. Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS) online Web Soil Survey map indicates surficial soil in the vicinity of the subject site consists of 3026—Phoebe, dry-Bong complex, 0 to 8 percent slopes, and 3044—Cheney ashy silt loam, 0 to 8 percent slope. The Phoebe soil is formed in sandy glaciofluvial deposits with minor amounts of volcanic ash and loess in the upper part and is located on outwash plains. The soil consists of sandy ashy loam, sandy loam and sand in the upper 60 inches and is considered somewhat excessively drained. The Cheney soil is formed in loess mixed with minor amounts of volcanic ash over sandy and gravelly glaciofluvial deposits, located on outwash plains.

The soil consists of ashy silt loam over extremely gravelly coarse sand in the upper 60 inches and is considered well drained.

Subsurface Conditions

General

Below the vegetation and surface debris, we encountered generally similar subsurface conditions in each of the explorations. For the purposes of this report, we characterize the soil encountered in our explorations into four general units, including: (1) fill; (2) sand with silt/silty sand; (3) silt; and (4) silty sand (decomposed basalt).

Fill

We encountered fill at the location of each boring. The fill encountered in the borings extended to a depth ranging between about 1½ and 3 feet below existing site grade. We describe the fill unit as loose to dense, silty fine to medium sand with variable debris (concrete). Although not specifically encountered in our boring explorations, we also observed cobbles and boulders at the surface of the fill unit. We characterize the fill as having low strength, low permeability, low to moderate compressibility, and moderate to high susceptibility to changes in moisture content.

Sand with Silt/Silty Sand

In each boring below the fill, we encountered loose to dense, fine to coarse sand with silt/silty sand. We encountered the sand with silt/silty sand to the depth explored in boring B-1 (7½ feet) and to depths ranging between about 7 to 10 feet below existing ground surface in the remaining borings. We

August 4, 2014| Page 3 characterize the sand with silt/silty sand as having moderate strength, low to moderate compressibility potential, low permeability, and moderate to high susceptibility to changes in moisture content.

Silt

At the locations of borings B-2 and B-3, and underlying the sand with silt/silty sand, we encountered an approximate 1- to 4-foot-thick layer of medium stiff to stiff silt with sand or silt with occasional gravel. We characterize this silt unit as having moderate strength, low to moderate compressibility potential, very low permeability, and very high susceptibility to changes in moisture content.

Silty Sand (Decomposed Basalt)

In borings B-2 through B-6, we encountered decomposed basalt underlying the silt with sand/silty sand and silt units described above. We describe the decomposed basalt as very dense, silty fine to coarse sand with gravel. The decomposed basalt extended to the depth explored in borings B-2 through B-6. We characterized the decomposed basalt as having high strength, low permeability and compressibility, and moderate to high susceptibility to changes in moisture content.

Groundwater Conditions

We encountered perched groundwater in boring B-3 at a depth of about 7½ feet below ground surface at the time of exploration. Based on previous experience at other sites in the nearby project vicinity, it is our experience that groundwater in this area often is perched above confining layers such as low permeability soil (silt or clay) or in-place basalt rock. The elevations and extent of perched groundwater could vary seasonally, generally being highest in the spring and early summer months. In addition, groundwater seepage and elevation could vary from year to year depending on annual precipitation, irrigation, or other sources of recharge.

CONCLUSIONS AND RECOMMENDATIONS

General

Based on the results of our site exploration, laboratory testing and engineering analyses, we believe the indigenous sand with silt/silty sand and underlying silt and decomposed basalt deposits encountered in our explorations are suitable for direct support of the proposed DRBS Warehouse. In our opinion, the on-site fill is suitable for support of a floor slab and improvements outside of the building footprint provided site soil is prepared as recommended in this report. In general, geotechnical-related conditions that could affect design and construction of the proposed improvements include:

■ The surface and near-surface fill at the site includes variable cobbles, boulders and debris. The contractor should be prepared to encounter and dispose of cobble- and boulder-sized material and other debris during general site excavation activities.

■ The indigenous sand with silt/silty sand generally contains a significant amount of fines (silt- and clay-sized particles). This soil will become more susceptible to disturbance as the moisture content increases. Above the optimum moisture content, this soil will be difficult, if not impossible, to place and compact to project specifications. For this reason, performing site grading activities during or immediately after wet weather should be avoided.

August 4, 2014| Page 4

■ Because of shallow perched groundwater and rock, standard drywells are not appropriate for this site.

However, in our opinion, surficial site soil is suitable for limited subsurface disposal of stormwater via swales.

■ The type, size and frequency of truck and forklift passes provided in this report were developed using best-available information at the time of this evaluation. Regardless, some general assumptions regarding wheel loads and tire pressure were required for our analysis. A summary of our assumptions are provided in the “Pavements” section of this report and should be reviewed by the project team to determine that pavement thicknesses are appropriate.

The following sections of this report present our specific conclusions and recommendations for site earthwork, foundation support and other geotechnical aspects of the project.

Site Preparation and Earthwork

Initial Site Preparation

Initial site preparation and earthwork operations will include clearing and grubbing, and limited site grading.

We recommend the contractor be prepared to strip surficial vegetation and remove such material from the site. Based on our experience in the vicinity of the site, we estimate that 2 to 3 inches of stripping could be required to remove surface vegetation. We understand that grading activities will be relatively minor across the site. Accordingly, it is our opinion the existing fill and indigenous soil units can be excavated using conventional excavating equipment and procedures capable of encountering and removing cobbles and boulders.

As noted previously, most of the site soil is moderately to highly moisture sensitive, and will be difficult to work or compact if moisture contents are about 2 to 4 percentage points greater or less than the optimum moisture content. Accordingly, earthwork during wet weather should be avoided, if possible. If earthwork activities cause excessive subgrade disturbance, recompaction or replacement with structural fill might be necessary. Disturbance to a greater depth should be expected if site preparation work is conducted during periods of wet weather when the moisture content of the site soil could exceed optimum.

Slab and Pavement Subgrade and Footing Grade Preparation

We recommend structural floor slabs and pavement areas be supported on a minimum of 12 inches of proof-compacted on-site soil. We further recommend individual and continuous spread footings for the proposed DRBS Warehouse be supported on proof-compacted indigenous sand with silt/silty sand, or on structural fill overlying the proof-compacted indigenous soil unit. Accordingly, we recommend the existing on-site fill, where present, be overexcavated and replaced with structural fill below and at least 2 feet or ½ the width of individual and continuous footings, whichever is greater.

After clearing and stripping are complete at proposed slab and pavement subgrade, and initial overexcavation to the indigenous sand with silt/silty sand unit at foundation grade, soil exposed at proposed foundation grade and at working subgrade within proposed building, structure and pavement limits, should be thoroughly proof-compacted before placing structural fill to establish final foundation grade, floor slab and pavement subgrade elevations or before placing forms and steel reinforcement for concrete. Proof-compaction consists of scarifying or disking exposed soil, moisture-conditioning to near the optimum moisture content and compacting to a depth of at least 12 inches, and to at least 95 percent of the maximum dry density (MDD) based on the ASTM International (ASTM) Standard Practices D 1557

August 4, 2014| Page 5 laboratory test procedure. Proof-compaction should extend at distances horizontally beyond the building slab and pavement area at least 5 feet. Proof-compaction beyond the footing perimeter should extend horizontally at least 2 feet, or ½ the width of the footing, whichever is greater.

Evaluation of compaction should be accomplished through in-place density testing of the prepared areas.

Alternatively, visual observations of proof-compaction and probing may be completed by the geotechnical engineer-of-record. The most appropriate method for evaluating soil compaction should be determined by the geotechnical engineer at the time the site work is performed. Soil, which cannot be properly compacted, should be excavated to firm bearing or a depth of 2 feet, whichever is less, and replaced with structural fill compacted as recommended in the following section. Additional fill required to establish proposed floor and pavement subgrade or foundation grade may be placed directly on proof-compacted native site soil or structural fill prepared as recommended above and in subsequent sections of this report.

Excavation Support

In our opinion, excavations are moderately susceptible to sloughing and caving. Excavations deeper than 4 feet should be shored or sloped at stable inclinations if workers are required to enter such excavations.

Shoring for utility excavations must conform with the U.S. Department of Labor, Occupational Safety and Health Administration (OSHA) Standard for Excavation (29 CFR 1926 Subpart P).

In our opinion, site soil may be classified as Soil Type C. More specifically, we believe that slopes inclined at 1½H:1V (horizontal to vertical) or flatter may be used for temporary cuts. This recommendation assumes that all surface loads are kept a minimum distance of at least ½ the depth of the cut away from the top of the slope. Flatter slopes will be necessary if surface loads are imposed above the cuts a distance equal to or less than one half the depth of the cut.

Regardless of the soil type encountered in the excavation, shoring, trench boxes or sloped sidewalls will be required under OSHA standards. While this report describes certain approaches to excavation, the contract documents should specify that the contractor is responsible for selecting excavation methods, monitoring the excavations for safety and providing shoring, as required, to protect personnel.

Structural Fill

General

Soil used to support foundations, slab-on-grade floors, and pavement is classified as structural fill for the purposes of this report. Structural fill material requirements vary depending upon its use as described below.

Use of On-Site Soil

The existing on-site fill should be generally suitable for use as structural fill during extended periods of dry weather provided it is free of debris, organic contaminants, frozen soil and particles larger than 4 inches in maximum dimension. This could require selective stockpiling and screening to remove cobbles, boulders and debris prior to reuse. Furthermore, the on-site fill, along with the indigenous sand with silt/silty sand observed in our explorations, generally contain a moderate to high percentage of fines with corresponding moderate to high sensitivity to moisture changes. Accordingly, these soil units could require considerable effort on the part of the contractor to properly adjust the moisture content to near the optimum and to achieve the recommended degree of compaction. Therefore, they will be suitable for reuse as structural

August 4, 2014| Page 6 fill only during periods of dry weather. For these reasons, we recommend that project planning include the provisions for importing select granular structural fill if construction takes place during the wetter periods of the year. We further recommend the geotechnical engineer of record determine the suitability of on-site fill material encountered during earthwork activities for reuse as structural fill.

In our opinion, the on-site topsoil and decomposed basalt (silt and silty sand) are not suitable for reuse as structural fill but may be used as fill in non-structural areas (such as landscaped areas) with the approval of the architect.

Imported Structural Fill

Imported fill also should be free of debris, organic contaminants, frozen soil and particles larger than 4 inches in maximum dimension. Depending on the intended use, imported structural fill should meet the specifications described below:

1. Structural fill placed as backfill in utility trenches outside of building and foundation areas, or within pavement limits should meet the criteria described in Section 9-03.15 of the 2014 edition of the Washington State Department of Transportation (WSDOT) Standard Specifications for Road, Bridge and Municipal Construction (Standard Specifications) for “Native Material for Trench Backfill.”

2. At a minimum, imported granular soil intended for use as structural fill placed within building limits, below structure foundations, floor slabs or pavements should consist of a well graded sand or sand and gravel mixture with less than about 10 percent fines. “Select Borrow” as described in Section 9-03.14(2) of the 2014 WSDOT Standard Specifications generally meets these requirements.

Select Borrow will be suitable for use as structural fill during dry weather conditions only. If imported soil is placed during wet weather, the structural fill should consist of “Gravel Borrow” as described in Section 9-03.14(1) of the 2014 WSDOT Standard Specifications or “Select Borrow” with the additional provision that the percent fines be less than 5 percent.

3. Structural fill placed as capillary break material below floor slabs should consist of 1½-inch-minus, free-draining crushed gravel with negligible sand or silt. Material in conformance with Section 9-03.1(4) C, “Grading No. 57” or Section 9-03.9(3) “Crushed Surfacing” of the 2014 WSDOT Standard Specifications generally meets these criteria.

4. Structural fill placed as crushed surfacing base course below pavements should meet the criteria for “Crushed Surfacing – Top Course and Keystone” as described in Section 9-03.9(3) of the 2014 WSDOT Standard Specifications.

5. Crushed rock placed as aggregate beneath PCC slabs should conform to WSDOT Specification 9-03.9(3), “Crushed Surfacing - Top Course and Keystone.”

Fill Placement and Compaction Criteria

Structural fill should be placed in loose lifts not exceeding 8 inches in thickness and mechanically compacted to a firm, non-yielding condition. Each lift should be conditioned to the proper moisture content and compacted to the specified density before placing subsequent lifts. We recommend structural fill be compacted to the following criteria:

August 4, 2014| Page 7

1. All structural fill within proposed building areas and below foundations, regardless of depth below floor subgrade or foundation grade, must be compacted to at least 95 percent of the MDD based on the ASTM D 1557 laboratory test procedures.

2. Structural fill in roadway areas, including utility trench backfill, should be compacted to 90 percent of the referenced MDD, except that the upper 2 feet below final pavement subgrade should be compacted to at least 95 percent of the MDD.

3. Structural fill placed as crushed surfacing base course below pavements should be compacted to at least 95 percent of the MDD.

4. Non-structural fill, such as fill placed in landscaped areas, should be compacted to at least 85 percent of the MDD. However, in areas intended for future development, a higher degree of compaction should be considered to reduce the settlement potential of the fill soil.

We recommend that a representative of GeoEngineers be on site during earthwork operations to observe site preparation and fill placement. Soil conditions should be evaluated by in-place density tests, visual evaluation, probing and proof-rolling of the structural fill and recompacted native soil as it is prepared to check for compliance with the contract documents and recommendations in this report.

Weather Considerations

Proof-compaction or structural fill placement will be difficult to accomplish if earthwork is performed during extended periods of wet or sub-freezing weather. We recommend that earthwork be scheduled for the normally warmer months unless delays in the construction schedule can be tolerated.

Unprotected site soil also can deteriorate under construction traffic if exposed to inclement weather.

Accordingly, to the degree possible, we recommend that construction equipment and personnel be prohibited from traversing prepared subgrade areas during wet weather conditions. Foundation excavations that are prepared before inclement weather should be re-inspected to identify areas requiring repair. Any such areas should be recompacted or overexcavated to firm bearing or a depth of 2 feet, whichever is less, and replaced with compacted structural fill as discussed in a previous section of this report.

Foundation Support

General

We recommend soil exposed at proposed foundation grade be prepared as recommended in the “Slab and Pavement Subgrade and Footing Grade Preparation” section of this report. Specifically, proof-compacted indigenous sand at proposed foundation grade or structural fill overlying the indigenous sand should provide suitable support for proposed DRBS Warehouse footing loads provided proof-compaction, and structural fill gradation and compaction conform to recommendations in this report.

Footing Design

We understand FAFB Design Standards require a minimum embedment of 4 feet for exterior footings for unheated structures. Based on our experience with similar projects in the nearby vicinity, a minimum embedment of 24 inches is generally sufficient to protect against frost heave for foundations supporting unheated structures in the Spokane region. Consequently, it is our opinion foundations for the proposed DRBS Warehouse may be founded on proof-compacted indigenous sand or structural fill placed over proof-

August 4, 2014| Page 8 compacted indigenous sand at a minimum depth of 24 inches below the nearest adjacent exterior finished grade.

We recommend individual and continuous footings be designed with minimum dimensions of 24 and 16 inches, respectively. Individual and continuous footings bearing on proof-compacted sand or structural fill prepared as previously described may be designed using an allowable net soil bearing pressure of 2,500 psf for dead plus real live loads. This value may be increased by one-third to account for short-term live loads such as those induced by wind and seismic conditions.

Settlement

Based on the foundation loads provided at the beginning of this report, we estimate that settlement of foundations should be less than about 1 inch. Differential settlement between adjacent foundations generally should be less than about ½ inch. Settlement should occur essentially as loads are applied.

Loose soil, on-site fill or topsoil not removed from footing excavations or disturbance of soil at foundation grade during construction could result in larger settlements than estimated. We should be contacted if the design assumptions stated at the beginning of this report do not reflect final design.

Lateral Resistance

The soil pressure available to resist lateral foundation loads is a function of the frictional resistance against the foundation base and the passive resistance which can develop on the face of below-grade elements of the structure as those elements move horizontally into the soil. For foundations bearing in soil prepared as recommended herein, the allowable frictional resistance may be computed using a coefficient of friction of 0.35 applied to vertical dead-load forces for the contact between concrete and proof-compacted on-site soil or structural fill.

The allowable passive resistance on the face of footings, grade beams or other embedded foundation elements may be computed using an equivalent fluid density of 260 pounds per cubic foot (pcf), triangular distribution, for on-site soil or structural fill. The above values include a factor of safety of about 1.5.

Floor Slab Support

Floor slabs and flat work may be supported on-grade provided the subgrade soil has been thoroughly proof-compacted or structural fill placed as recommended in previous sections of this report. We recommend that the slab be designed to resist point loads using a modulus of vertical subgrade reaction (k) of 250 pounds per cubic inch (pci) for thoroughly compacted soil.

To retard the upward wicking of moisture beneath the floor slab, we recommend that a capillary break be placed over the subgrade. To that end, we recommend floor slabs and flat work be underlain by 6 inches of free-draining crushed rock meeting the criteria outlined in the “Structural Fill” section.

A vapor retarder consisting of durable plastic sheeting also may be used in areas where the prevention of moisture migration through the slab could adversely influence performance of adhesives, which might be used to anchor carpet or tile to the slab. The architect should make the final determination regarding use of such a vapor retarder. Currently, the American Concrete Institute (ACI) does not recommend placing a moisture break layer of sand or crushed rock above plastic vapor retarders. If a moisture break layer is not employed, appropriate consideration should be given to the cement type used for the slab concrete, joint layout and curing operations to reduce the potential for curling of the slab.

August 4, 2014| Page 9

Seismic Considerations

Spectral response acceleration is estimated by classifying the site based on the average soil properties and/or rock shear wave velocity characteristics below the site to a depth of 100 feet. Based on subsurface conditions encountered in our explorations and our understanding of the geologic conditions in the site vicinity, it is our opinion the site may be characterized as Class B in accordance with the 2012 IBC.

Pavements

General

In our opinion, the on-site soil should provide adequate support for proposed pavements provided soil exposed at proposed subgrade is thoroughly proof-compacted to a depth of at least 12 inches before placing additional structural fill or base course, as recommended in the “Slab and Pavement Subgrade and Foundation Grade Preparation” section of this report. All soil placed as structural fill within proposed pavement areas should be compacted as outlined in the “Structural Fill” section of this report. Soil that cannot be compacted to the recommended density should be removed to a depth of 2 feet or firm bearing, whichever is less, and replaced with structural fill placed and compacted as recommended in a previous section of this report.

Material Specifications

We recommend pavement materials for improvements at the proposed DRBS Warehouse facility conform to the WSDOT Standard Specifications for Road, Bridge and Municipal Construction (2014 Edition).

Flexible Pavement Section Design

We completed HMA thickness design in general accordance with procedures in the American Association of State and Highway Transportation Officials (AASHTO) 1993 Guide for Design of Pavement Structures using the computer program WinPAS (Version 1.0.4), assumed traffic type and frequency, and estimated input parameters. The following assumptions and inputs were used in our analyses based on available information at the time of our evaluation:

■ Delivery trucks, generally defined as similar to an AASHTO HS-20 axle configuration and weight and estimated to make about 100 deliveries per year.

■ A resilient modulus (MR) value on the order of about 8,000 pounds per square inch (psi) for properly compacted subgrade soil.

■ Approximately 3,300 equivalent single-axle loads (ESALs) for delivery trucks.

■ A 20-year design life with no growth.

■ A reliability of 80 percent for local roadways.

■ A standard deviation of 0.45 based on new construction.

■ An initial serviceability index of 4.2 and a terminal serviceability index of 2.5.

■ Material factors of 0.40 for HMA and 0.14 for crushed surfacing base course.

■ A drainage factor of 1.0 for HMA and 0.95 for crushed surfacing base course.

August 4, 2014| Page 10

Based on the results of our analyses, and provided pavement subgrade soil is prepared as outlined herein, we recommend the pavement section for the proposed project consist of 3½ inches compacted thickness of HMA underlain by 6 inches of crushed rock base course compacted to at least 95 percent of the previously recommended MDD. We recommend that the crushed surfacing base course layer conform to WSDOT Specification 9-03.9(3), “Crushed Surfacing,” as outlined in the “Structural Fill” section of this report. HMA pavement should be compacted to a minimum of 92 percent of the theoretical Rice Density.

PCC Pad Design

We completed PCC pad thickness design in general accordance with procedures in the Portland Cement Association publication, Slab Thickness Design for Industrial Concrete Floors on Grade (Packard, 1996) using assumed traffic criteria to estimate input parameters. The following assumptions and inputs were used in our analyses based on information that was available at the time of our evaluation:

■ Forklifts consist of CAT 924 loader chassis weighing 28,380 pounds with two axels and single tires spaced about 74 inches on center.

■ Forklifts used to unload the trucks will consist of USAF 15k All Terrain Fork Lifts (ATFL) based on the CAT 924 loader design and will make about 1,000 trips per year.

■ Approximately 175,000 ESALs for loaded fork lifts.

■ Cargo loads will be less than 15 kips per trip and will load the front axels.

■ Tire inflation pressure will be about 60 psi for a 15.5 x 25 loader tire with a contact patch of about 240 in2 per tire.

■ A subgrade k-value estimated on the order of about 190 pci for at least 12-inches of proof-compacted native soil subgrade.

■ A safety factor of about 1.7 based on the periodic nature of the loading.

■ The slab will be constructed of PCC with a compressive strength of at least 4,000 psi.

Based on the results of our analyses, and provided pavement subgrade soil is prepared as outlined herein, we recommend the proposed PCC slab-on-grade aprons be a minimum of 6 inches in thickness for the design inputs described above. We recommend the slab-on-grade be underlain by at least 4 inches of crushed surfacing base course (CSBC), compacted to at least 95 percent of the previously recommended MDD. We also recommend the exterior edges of the slab be thickened to at least 9 inches to reduce the potential for cracking. The slab should thicken from 6 to 7½ inches in a taper starting at least 2 feet from the edge of the slab.

Site Drainage

Temporary Drainage

Some local ponding of water from precipitation could occur in excavations during construction. Site excavations should be provided with appropriate ditches and sumps to keep the exposed areas as dry as possible. Some pumping from sumps might be necessary during wet weather.

August 4, 2014| Page 11

Stormwater Considerations

General We recommend that all surfaces be sloped to drain away from the proposed building area. Pavement surfaces and open spaces should be sloped such that surface runoff is collected and routed to suitable discharge points. Roof drains should be tightlined to a suitable stormwater disposal system.

As noted previously, because of shallow perched groundwater, the presence of shallow basalt rock and the fines content of overlying on-site soil, standard drywells or infiltration trenches are not appropriate for this site. However, it is our opinion that site soil conditions are generally suitable for on-site disposal of stormwater through swales. The following section of this report presents our specific recommendations for stormwater disposal.

Swales The USDA, NRCS mapping of surficial soil in the project vicinity indicates the site is underlain by Phoebe, dry Bong complex with a small area of Cheney ashy silt loam near the northeast corner of the site.

Furthermore, the NRCS classifies Phoebe, dry Bong complex as Group A hydrologic soil, and…