TNC Phase 3 Geotechnical Engineering Report-7-26-2023.pdf

REPORT COVER PAGE

Geotechnical Engineering Report

Tahoma National Cemetery - Phase 3 Development Kent, King County, WA

October 17, 2022 Terracon Project No. 81215088

Prepared for:

Anderson Engineering Plymouth, Minnesota

Prepared by:

Terracon Consultants, Inc.

Mountlake Terrace, Washington

Terracon Consultants, Inc. 21905 64 th Ave. W, Suite 100 Mount lake Terrace, WA 98043

(425) 771 3304 F (425) 771 3549 www.terracon.com

REPORT COVER LET TER T O SIGN

October 17, 2022

Anderson Engineering 13605 1st Avenue N, Suite 100 Plymouth, Minnesota 55441

Attn: Michael Brandvold P.E. – Senior Project Manager P: (763) 412-4017 E: MBrandvold@ae-mn.com

Re: Geotechnical Engineering Report Tahoma National Cemetery - Phase 3 Development 18600 SE 240th Street Kent, King County, WA Terracon Project No. 81215088

Dear Mr. Brandvold:

We have completed the Geotechnical Engineering services for the above referenced project. This study was performed in general accordance with Terracon Proposal No. P81215088 dated June 8, 2021. This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerning earthwork, the design and construction of building and structure foundations, floor slabs and pavements 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.

Nithybhan Chandaresan Dennis R. Stettler, P.E.

Staff Geotechnical Engineer Senior Engineering Consultant

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REPORT TOPICS

INTRODUCTION

SITE CONDITIONS

PROJECT DESCRIPTION

GEOTECHNICAL CHARACTERIZATION

GEOTECHNICAL OVERVIEW

SEISMIC CONSIDERATIONS

LIQUEFACTION

EARTHWORK

SHALLOW FOUNDATIONS

FLOOR SLABS

LATERAL EARTH PRESSURES

STORMWATER MANAGEMENT

PAVEMENTS

CORROSIVITY

GENERAL COMMENTS

FIGURES

ATTACHMENTS

Note: This report was originally delivered in a web-based format. For more interactive features, please view your project online at client.terracon.com.

ATTACHMENTS

EXPLORATION AND TESTING PROCEDURES

SITE LOCATION AND EXPLORATION PLANS

EXPLORATION RESULTS

PREVIOUS EXPLORATION RESULTS

SUPPORTING INFORMATION

Note: Refer to each individual Attachment for a listing of contents.

http://client.terracon.com/

Tahoma National Cemetery - Phase 3 Development ■ Kent, King County, WA October 17, 2022 ■ Terracon Project No. 81215088

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REPORT SUMMARY

Topic 1 Overview Statement 2

Project Description

Project is identified by the Veterans Administration (VA) as ‘Phase 3 Development’ of the Tahoma National Cemetery (TNC). In accordance with the master plan for the entire 160-acre site, this project will develop approximately 15 acres of undeveloped land to provide approximately 27,000 to 30,000 gravesites, including both casket and cremation sites in new burial sections, columbarium niches and pre-placed crypts. This expansion will provide for an additional 15-year inventory of interments and includes:

■ New paved roadways will be constructed to serve and access the new cemetery areas at northern portion of cemetery - preliminary depiction of the roadway alignment is shown on Exploration Plan

■ Three columbaria courts totaling 21,300 additional niches are expected to be constructed.

■ Additional gravesite crypts and cremains.

Modification, renovation, or additional investigation of existing development included as part of Phase 3 work includes:

■ Expansion of Administration Building at the north end

■ Construction of Honor Guard building (1,400 GSF) located north of existing

Administration Building

■ Construction of remote public restroom (720 GSF) near Loop Road

■ Construction of material storage building and maintenance yard including

Quonset and covered materials area.

Geotechnical Characterization

Subsurface conditions generally consist of the following:

■ Topsoil is underlain by approximately 1 foot of loose silty sand.

■ Beneath the silty sand layer exists loose to medium dense sand with variable silt content and gravel/cobble to about 4½ feet.

■ Medium dense to very dense silty sand and sandy silt with gravel/cobble is present below about 1 to 4½ feet and extending to the bottom of the explorations.

■ Perched groundwater as shallow as 1½ feet below the ground surface.

■ Existing fill was observed at B-06 through B-09 and B-11.

Earthwork

■ Remove the topsoil and organic-rich soils.

■ Existing granular soils can be reused for engineered fill, but are moisture sensitive due to an appreciable fines content (percent passing the #200 sieve).

■ Near-surface soils are moisture sensitive and could become unstable when exposed to excessive moisture and/or disturbance.

■ Utility trenching may require minor dewatering efforts due to the perched groundwater.

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Shallow Foundations

Summary of foundation recommendations (Refer to Shallow Foundations) Allowable bearing pressures for shallow foundations:

■ Structural fill or native soil: 3,000 psf

Expected static settlement: < 1 inch total, < ½ inch differential Detect and remove topsoil and organics as noted in Earthwork

Pavements

With subgrade prepared as noted in Earthwork Concrete:

■ Minimum 5 inches Portland Cement Concrete (PCC) over minimum 4 inches granular base in Light Duty and Heavy Duty areas

Asphalt:

■ Minimum of 3 inches Asphaltic Concrete (AC) over minimum 6 inches granular base in Light Duty areas Minimum of 3 inches Asphaltic Concrete (AC) over minimum 10 inches granular base in Heavy Duty areas Pervious pavement could be placed if decided by the owner. Assumptions of ESALS for Light Duty and Heavy areas are provided in Pavement section.

General Comments

This section contains important information about the limitations of this geotechnical engineering report.

1. If the reader is reviewing this report as a pdf, the topics above can be used to access the appropriate section of the report by simply clicking on the topic itself.

2. This summary is for convenience only. It should be used in conjunction with the entire report for design purposes.

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INTRODUC TION

Geotechnical Engineering Report Tahoma National Cemetery - Phase 3 Development

18600 SE 240th Street Kent, King County, WA

Terracon Project No. 81215088 October 17, 2022

INTRODUCTION

This report presents the results of our subsurface exploration and geotechnical engineering services performed for the proposed Veterans Administration (VA) Tahoma National Cemetery (TNC) – Phase 3 Development located at 18600 SE 240th Street in Kent, King County, WA. The purpose of these services is to provide information and geotechnical engineering recommendations relative to:

■ Subsurface soil conditions ■ Seismic considerations and liquefaction

■ Groundwater conditions ■ Foundation design and construction

■ Site preparation and earthwork ■ Lateral earth pressures

■ Floor slab design and construction ■ Pavement design and construction

The geotechnical engineering Scope of Services for this project included the advancement of fourteen (14) soil borings and twenty-one (21) test pits to depths ranging from approximately 15 to 25 feet and 5 to 10 feet below existing site grades, respectively.

Maps showing the site and exploration locations are shown in the Site Location and Exploration Plan sections, respectively. The results of the laboratory testing performed on soil samples obtained from the site during the field exploration are included on the boring logs and test pit logs and/or as separate graphs in the Exploration Results section.

SITE CONDITIONS

The following description of site conditions is derived from our site visit in association with the field exploration and our review of publicly available geologic and topographic maps.

Item Description

Site Information The project is located at 18600 SE 240th Street in Kent, King County, WA.

Latitude: 47.3953 Longitude: -122.0988 See Site Location

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Item Description

Existing Improvements

The Phase 3 developments will be situated primarily in the north end of the existing Tahoma National Cemetery; however, elements of the Phase 3 development also include modifications, upgrades, and rehabilitation of existing structures within the footprint of previous Phase 1 and Phase 2 development – some of which includes a geotechnical component. The north end of the cemetery includes current equipment and material storage facilities in otherwise vacant, undeveloped land. Phase 3 elements occurring within the footprint of previous Phases 1/2 development include, expansion of the Administration Building, construction of Honor Guard Building to the north of the Administration Building and a public restroom. A settlement investigation of Columbaria I/J/K (near the southwest corner of the cemetery) may be conducted once the results of settlement monitoring are available.

Current Ground Cover

Based upon recent aerial photography and previous (2011 and 1995) topographic mapping, current ground cover at the north end of the site consists primarily of mature and dense mixed conifer-and-deciduous forest.

Within the area of the maintenance yard, gravel or dirt-surfaced roads are present, and vegetation in this area appears to consist of shrubs or grasses.

Existing Topography

■ Previous topographic mapping of the undeveloped Phase 3 area indicates irregular terrain north and west of the existing Assembly Area, and the area slopes downward generally to the southeast. However, this overall trend is interrupted by several shallow “hollows” which tend to coincide with a number of small, delineated wetlands. East of the maintenance yard, an unnamed creek crosses through the northeastern corner of the parcel approximately 15 to 20 feet lower than the bench on which the maintenance facilities are located.

■ Earthmoving activities completed previously in the developed portions of the Phase 1 and Phase 2 project areas have resulted in a muted, flat topography which slopes gently to the south and southeast.

■ Within the Phase 3 area of new development, elevations range from about 500 to 550 feet above mean sea level, with the creek bottom at approximate el. 500 feet, and a topographic high (el. +/-550 feet) near the northwest corner. Areas to the south – in the Phase 1 and 2 areas – are situated between an elevation of between about 510 and 480 feet.

■ No existing landslides, suspected landslides, or erosion hazard areas are indicated on ‘Environmental Critical Areas’ (ECAs) map layers in King County’s online mapping tool or observed in our reconnaissance of the site.

■ Small, potential steep slope hazard areas are mapped on the west side of the creek on neighboring property to the north of the project area, and also mapped to the south of the project area (on current VA property), but no such slopes are mapped within the current project extent.

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Item Description

Geology

Geologic mapping, the current geotechnical explorations, and test pit investigations in 1995 and exploration borings in 2011 indicate subsurface materials at most locations are likely to consist of a thin forest duff and topsoil layer atop several feet of weathered, medium dense to dense glacial till. Below the weathered zone, the glacial till is generally very dense. These previous and recent explorations are consistent with published geologic mapping of the area. Although explorations avoided delineated wetlands, soils within such areas should be expected to include accumulations of soft and/or compressible soils and shallow groundwater; these materials are likely situated atop the underlying and generally low-permeability glacial till.

PROJECT DESCRIPTION

Our initial understanding of the project was provided in our proposal and was discussed during project planning. A period of collaboration has transpired since the project was initiated, and our final understanding of the project conditions is as follows:

Item Description

Information Provided

Master Plan North Expansion of the Phase 3 area has been provided by Anderson Engineering, along with preliminary and design development drawings. The purpose of the geotechnical study is primarily to characterize the site subsurface conditions, identify areas of problematic soils and favorable soils for particular development purposes, and to assist the design team with developing site layout alternatives that integrate geotechnical considerations with those of other design disciplines. Following development of the site layout, the geotechnical characterization has been used to develop geotechnical recommendations for design and construction of the project.

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Item Description

Project Description

Project is identified by the VA as ‘Phase 3 Development’ of the TNC. In accordance with the master plan for the entire 160-acre site, this project will develop approximately 15 acres of undeveloped land to provide approximately 27,000 to 30,000 gravesites, including both casket and cremation sites in new burial sections, columbarium niches and pre-placed crypts. This expansion will provide for an additional 15-year inventory of interments and includes.

■ New paved roadways will be constructed to serve and access the new cemetery areas at northern portion of cemetery - depiction of the roadway alignment is shown on the Exploration Plan

■ Three columbaria courts totaling 21,300 additional niches are expected to be constructed.

■ Additional gravesite crypts and cremains will be constructed.

Modification, renovation, or additional investigation of existing development included as part of Phase 3 work includes:

■ Expansion of Administration Building at the north end

■ Construction of Honor Guard building (1,400 GSF) located at the north of existing Admin. Building

■ Construction of public restroom (720 GSF) near Loop Road A

■ Construction of material storage building and maintenance yard including Quonset and covered materials area.

Grading/Slopes

Project Phase 3 development consists of cutting and filling soils in the expansion area. Some of these areas have been previously modified by placement of cemetery earthen spoils in material dumping grounds. This spoil zone is located at B-06 through B-09 and is about 9 to 10 ft thick fill.

Site development and cost estimates will need to account for this unsuitable fill to be exported off-site. The volume of poor soils will be estimated by Anderson engineering.

Project grading work is being designed with the aim of balancing the site earthwork to the extent possible. As such, the roads and cemetery element areas are comprised of a mixture of cut and fill sections, also keeping in mind haul distances.

Retaining Walls No major retaining walls will be constructed, although low landscaping walls will be present in some areas.

Pavements Paved vehicle drives and pedestrian walkways will be elements of site design, and one or more parking pullouts are likely be constructed. We assume that both rigid and flexible pavements may be incorporated into design.

Applicable Building Code(s)

International Building Code – Version 2018 (2018 IBC) American Society of Civil Engineers Standard 7 – Version 2016 (ASCE 7-16)

Estimated Start of Construction 2023

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GEOTECHNICAL CHARACTERIZATION

We have developed a general characterization of the subsurface conditions based upon our review of the subsurface exploration, laboratory data, geologic setting and our understanding of the project. This characterization, termed GeoModel, forms the basis of our geotechnical calculations and evaluation of site preparation and foundation options. Conditions encountered at each exploration point are indicated on the individual logs. The individual logs can be found in the Exploration Results section and the GeoModel can be found in the Figures section of this report.

As noted in General Comments, the characterization is based upon widely spaced exploration points across the site, and variations are likely.

As part of our analyses, we identified the following model layers within the subsurface profile. For a more detailed view of the model layer depths at each boring location, refer to the GeoModel.

Model Layer Layer Name General Description

1 Topsoil Silty Sand (SM) or Sandy Silt (ML) with abundant organics mixed with variable amounts of forest duff, with variable gravel content, dark brown.

2 Existing Fill Silty Sand (SM) or Sandy Silt (ML) or Silty Gravel (GM) with sand, with variable gravel content, typically grayish brown, with variable organic content, loose to medium dense or medium stiff.

3 Palustrine Deposits

This unit generally underlays Model Layer 2 and is composed of Organic Sandy Silt (OL) with abundant organics and variable gravel content, dark brown, moist, non-plastic, medium stiff to stiff.

4 Weathered Glacial Till

Silty Sand (SM) with gravel, variable cobble and organic content, tannish brown, loose to medium dense.

5 Glacial Till Silty Sand (SM) or Sandy Silt (ML) or Gravel (GW/GP) with sand, variable gravel and cobble content, olive brown, olive gray, or light gray, fine to medium grained, dense to very dense.

Stratification boundaries on the boring and test pit logs represent the approximate location of changes in native soil types; in situ, the transition between materials may be gradual.

Groundwater Conditions

The boreholes and test pits were observed while drilling and after completion for the presence and level of groundwater. The water levels observed in the boreholes can be found on the boring and test pit logs in Exploration Results and are summarized below.

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Exploration Number Approximate Depth to Groundwater while

Excavating or Drilling (feet) 1, 2

TP-01 4

TP-02, TP-12, TP-17, TP-20 2½

TP-03 3½

TP-04 6½

TP-05 9

TP-06 1

TP-07 2

TP-08 7

TP-09 5½

TP-11 3

TP-16 1½

TP-18, B-08 9½

TP-19 5

B-03, B-11 4½ B-05 13½

B-06, B-07, B-13 7½

TP-10, TP-13, TP-14, TP-15, TP-21, B-01, B-02,

B-04, B-09, B-10, B-12, B-14 Groundwater not observed

1. Below ground surface

2. In test pits, the presence of groundwater was based on observation of seepage from the test pit sidewalls; in borings, it was inferred from change in sample moisture or from groundwater on drill sampling/equipment

Perched groundwater was observed during test pit excavation in the form of seepage coming from the test pit sidewalls. The seepage typically originated near the top of very dense, glacially consolidated soil. The geologic units at this site are often associated with perched groundwater in which the vertical infiltration of water from rainfall events or other sources is slowed by low permeability layers. The infiltrating water tends to collect or “perch” above the very dense, low permeability soil. Although groundwater was observed seeping from the sidewalls of the test pit explorations, this perched groundwater condition likely does not represent the actual groundwater elevation and the very dense soil beneath this perched groundwater typically does not produce free water into an excavation. Although not directly observed in the soil borings, zones of perched groundwater may be present where soil above a dense layer appears to be wet or free water is observed on the drilling sampling equipment within the perched groundwater zone. Such conditions were noted on the boring logs when encountered.

Groundwater level fluctuations occur due to seasonal variations in the amount of rainfall, runoff and other factors not evident at the time the borings were performed. Therefore, groundwater

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GEOTECHNICAL OVERVIEW

The site is underlain primarily by loose to medium dense weathered glacial till overlaying dense to very dense glacial till down to exploration termination depths. Following appropriate site clearing and grading, proposed building structures could be founded on the natural soil present on the site or on compacted structural fill placed over undisturbed natural soil.

Undocumented existing fill (Model Layer 2) was encountered at borings B-06 through B-09 to depths ranging from about 9½ to 10½ feet and boring B-11 to depths of about 3½ feet. The fill was underlain by sandy silt (Model Layer 3) with appreciable organic content, which indicated that poor or no site clearing and grubbing were performed prior to fill placement.

We assume there will be large scale clearing to develop the project site. The grading be limited to only what is necessary to level the site. A minimum upper 1 foot of the topsoil layer is largely organic material and should be removed prior to subgrade preparation for placement of fill, structures, or paving. The soils present at the subgrade elevation are moisture sensitive due to the significant fines content and may become unstable when exposed to excessive moisture and/or disturbance such as construction traffic. Therefore, we recommend earthwork be performed during warmer and drier months to facilitate more workable site conditions.

Additionally, effective surface water control and drainage should be completed early in the construction sequence and maintained after construction to avoid potential issues. If grading is performed during the winter months, an increased risk for possible undercutting and replacement of unstable subgrade will persist. Additional site preparation recommendations, including subgrade improvement and fill placement, are provided in the Earthwork section.

The Shallow Foundations section addresses support of the building and other structures bearing on native soil or compacted structural fill. The slab-on-grade support of the buildings is discussed in the Floor Slabs section of this report.

The potential for localized perched groundwater should be considered in the civil engineering design for site grading, utility construction, and pavements. A flexible pavement system and a rigid pavement system are recommended for this site. The Pavements section addresses the design of pavement systems.

Specific conclusions and recommendations regarding these geotechnical considerations, as well as other geotechnical aspects of design and construction of foundation systems and other earthwork related phases of the project are outlined in the following sections. The

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SEISMIC CONSIDERATIONS

Description Value 2018 International Building Code (IBC) Site Classification D

Site Latitude 47.3953

Site Longitude -122.0988

SS – Short Period Spectral Acceleration 2 1.258 g

S1 – 1-Second Period Spectral Acceleration 2 0.431 g

PGA - ASCE 7, Peak Ground Acceleration 0.535 g

1. The IBC requires a site profile extending to a depth of 100 feet for seismic site classification. Borings were extended to a maximum depth of 25½ feet. The site properties below the boring depth to 100 feet were estimated based on our experience and knowledge of geologic conditions of the general area.

2. These values were obtained using online seismic design maps and tools provided by OSHPD (https://seismicmaps.org/).

Surface-Fault Rupture

The hazard of damage from onsite fault rupture appears to be low based on review of the USGS Earthquake Hazards Program Quaternary Faults and Folds Database available online (https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf 88412fcf) accessed on April 8, 2022. The closest mapped fault is the Seattle fault zone, which lies approximately 6.3 miles to the northeast.

LIQUEFACTION

Liquefaction is the phenomenon where saturated soils develop high porewater pressures during seismic shaking and lose their strength characteristics. This phenomenon generally occurs in areas of high seismicity, where groundwater is shallow and loose granular soils or relatively non-plastic fine-grained soils are present. Based on the site geology and subsurface groundwater conditions, the risk of liquefaction of the site soils is low during a design level earthquake.

https://seismicmaps.org/ https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf

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EARTHWORK

Earthwork is anticipated to include clearing and grubbing, removal of the topsoil layer and any organic-rich soils encountered above Model Layer 4, fill placement for raising site grades, and excavations for foundation elements and utility trenches. The following sections provide recommendations for use in the preparation of specifications for the work. Recommendations include critical quality criteria, as necessary, to render the site in the state considered in our geotechnical engineering evaluation for foundations, floor slabs, and pavements.

Site Preparation

Prior to placing fill, existing vegetation, root mats and organic-rich soils should be removed.

Complete stripping of the topsoil should be performed in all non-landscape areas. The depth of necessary stripping to remove topsoil and heavy root accumulations will typically be about 1 foot or more. However, the required depth can be highly variable and locally the required stripping thickness could be more or less than the typical depth.

The subgrade should be proofrolled with an adequately loaded vehicle such as a fully-loaded tandem-axle dump truck prior to placing fill. The proofrolling should be performed under the observation of the Geotechnical Engineer. Areas excessively deflecting under the proofroll should be delineated and subsequently addressed by the Geotechnical Engineer. Excessively wet or dry material should either be removed or moisture conditioned and recompacted.

Existing Fill

As noted in Geotechnical Characterization, borings B-06 through B-09 encountered existing fill to depths ranging from about 9½ to 10½ feet and B-11 encountered existing fill to depths of about 3½ feet. The fill appears to have been placed in an uncontrolled manner and we have no records to indicate the degree of fill compaction control. The existing fill also contains variable amounts of organic material and organic debris. Support of footings, floor slabs, and pavements, on or above existing, undocumented fill soils presents a risk of poor performance. However, even with the recommended construction procedures, there is inherent risk for the owner that compressible fill or unsuitable material, within or buried by the fill, will not be discovered. This risk of unforeseen conditions cannot be eliminated without completely removing the existing fill, but can be reduced by following the recommendations contained in this report.

We recommend that any existing, undocumented fill that is present within a planned building areas, paved areas, or areas with improvements that could be damaged by differential settlement be overexcavated, removed, and replaced with compacted structural fill. Once the existing fill materials have been removed, the entire area should be proofrolled with heavy, rubber tire construction equipment, to aid in delineating remaining areas of soft or otherwise unsuitable soil.

Once unsuitable materials have been remediated, and the subgrade has been observed and

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If the owner elects to construct the concrete sidewalk and pavements on the existing fill to reduce initial construction costs in exchange for increased potential longer-term distress, the following protocol should be followed. After the planned grading has been completed, the area should be undercut 2 feet within and 2 feet beyond the lateral limits of the concrete sidewalk and pavement.

Following this overexcavation, the entire area should be compacted and proofrolled with heavy, rubber tire construction equipment, to aid in delineating areas of soft or otherwise unsuitable soil.

Once unsuitable materials have been remediated, and the subgrade has passed the proofroll test, backfill to finished subgrade elevation can begin. The existing undocumented fill that was removed can be evaluated for reuse as structural fill. Areas of soft or otherwise unsuitable material should be undercut and replaced with either new structural fill or suitable, existing on-site materials.

Fill Material Types

Fill required to achieve design grade should be classified as structural fill and common fill.

Structural fill is material used below, or within 10 feet of structures and apertures, pavements, and constructed slopes. Common fill is material used to achieve grade outside of these areas. Earthen materials used for structural and common fill should meet the following material property requirements:

Fill Type Recommended Materials Acceptable Location for Placement

Structural Fill

9-03.9(1) Ballast1

9-03.9(3) Crushed Surfacing Base Course 1

9-03.12(1)A Gravel Backfill for Foundations Class A1

9-03.14(1) Gravel Borrow1

On-site Soils (i.e. Model Layers 4 or 5)2,3

Beneath and adjacent to structural slabs, foundations, building appurtenances, and pavement subgrades

Common Fill Section 9-03.14(3) Common Borrow1

On-site Soils (i.e. Model Layers 4 or 5)2,3

Grade filling, utility trench backfill outside the building foundations, paved areas, and appurtenances

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Fill Type Recommended Materials Acceptable Location for Placement

Free-Draining Granular Fill

Structural Fill 4

9-03.9(2) Permeable Ballast1

9-03.12(2) Gravel Backfill for Walls1

9-03.12(4) Gravel Backfill for Drains1

Backfilling in wet weather, drainage layers for walls, sump drains, footing drains5

1. WSDOT Standard Specifications

2. Structural and common fill should consist of approved materials free of organic matter and debris. Frozen material should not be used, and fill should not be placed on a frozen subgrade. A sample of each material type should be submitted to the Geotechnical Engineer for evaluation prior to use on this site.

3. May contain local areas of higher fines content that could make this material moisture sensitive. Particles with a nominal diameter greater than about 3 in. should be removed.

4. Material provided must be specified to be less than 5-percent passing the #200 sieve for the portion of material passing the #4 sieve.

5. Minimum particle size must be greater than drain pipe perforations.

Other earthen materials may be suitable for use in addition to the options presented in the table above. All materials should be approved by the Geotechnical Engineer prior to use.

Fill Compaction Requirements

Structural and common fill should meet the following compaction requirements.

Item Structural and Free-Draining Fill Common Fill

Maximum Lift Thickness

8 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

Same as Structural fill

Minimum Compaction Requirements 1

95% of max. below foundations and floor slabs and within 1 foot of finished pavement subgrade 92% of max. above foundations and more than 1 feet below finished pavement subgrade

92% of maximum dry density

Water Content

Range 1 Typically within 2% of optimum As required to achieve min.

compaction requirements

1. Maximum density and optimum water content as determined by the modified Proctor test (ASTM D 1557).

Utility Trench Backfill

All trenches should be wide enough to allow for compaction around the haunches of the pipe, or material such as pea gravel (provided this is allowed by the pipe manufacturer) should be used below the spring line of the pipes to eliminate the need for mechanical compaction in this portion of the trenches. If water is encountered in the excavations, it should be removed prior to fill

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Placement and compaction of recommended materials for utility trench backfill should be in accordance with the recommendations presented herein for Earthwork. In our opinion, the initial lift thickness should not exceed one foot unless recommended by the manufacturer to protect utilities from damage by compacting equipment. Light, hand-operated compaction equipment in conjunction with thinner fill lift thicknesses may be utilized on backfill placed above utilities if damage resulting from heavier compaction equipment is of concern. Flexible connections for utilities that pass through building foundations are recommended to reduce potential stress associated with differential settlement that may occur between the building foundation and the improvements located outside of the building footprint.

Grading and Drainage

All grades must provide effective drainage away from the building during and after construction and should be maintained throughout the life of the structure. Water retained next to the building can result in soil movements greater than those discussed in this report. Greater movements can result in unacceptable differential floor slab and/or foundation movements, cracked slabs and walls, and roof leaks. Gutters and downspouts should be routed into tightline pipes that discharge directly into an appropriate drainage facility. Splash-blocks should also be considered below hose bibs and water spigots.

Site grades should be established such that surface water is directed away from foundation and pavement subgrades to prevent an increase in the water content of the soils. Adequate positive drainage diverting water from structures, open cuts, and slopes should be established to prevent erosion, ground loss, and instability. Locally, flatter grades may be necessary to transition ADA access requirements for flatwork. After building construction and landscaping, final grades should be verified to document effective drainage has been achieved. Where paving or flatwork abuts the structure a maintenance program should be established to effectively seal and maintain joints and prevent surface water infiltration.

Earthwork Construction Considerations

Shallow excavations for the proposed structure are anticipated to be accomplished with conventional construction equipment. Upon completion of filling and grading, care should be taken to maintain the subgrade water content prior to construction of floor slabs. Construction traffic over the completed subgrades should be avoided. The site should also be graded to prevent

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As a minimum, excavations should be performed in accordance with OSHA 29 CFR, Part 1926, Subpart P, “Excavations” and its appendices, and in accordance with any applicable local, and/or state regulations. Construction site safety is the sole responsibility of the contractor who controls the means, methods, and sequencing of construction operations. Under no circumstances shall the information provided herein be interpreted to mean Terracon is assuming responsibility for construction site safety, or the contractor's activities; such responsibility shall neither be implied nor inferred.

Construction Observation and Testing

The earthwork efforts should be monitored under the observation of the Geotechnical Engineer.

Monitoring should include documentation of adequate removal of vegetation and topsoil, proof-rolling and mitigation of areas delineated by the proof-roll to require mitigation.

Each lift of compacted fill should be tested, evaluated, and reworked as necessary until approved by the Geotechnical Engineer prior to placement of additional lifts. Each lift of fill should be tested for density and water content.

In areas of foundation excavations, the bearing subgrade should be evaluated by the Geotechnical Engineer. In the event that unanticipated conditions are encountered, the Geotechnical Engineer should recommend mitigation options.

In addition to the documentation of the essential parameters necessary for construction, the continuation of the Geotechnical Engineer into the construction phase of the project provides the continuity to maintain the Geotechnical Engineer’s evaluation of subsurface conditions, including assessing variations and associated design changes.

Wet Weather Earthwork

The near-surface soils have variable fines content based on our visual observations and lab testing and are considered moisture sensitive. The soils will exhibit moderate erosion potential and may be transported by running water. Silt fences and other best-management practices will be necessary to control erosion and sediment transport during construction.

The suitability of soils used for structural fill depends primarily on their grain-size distribution and moisture content when they are placed. As the fines content (the soil fraction passing the U.S.

No. 200 Sieve) increases, soils become more sensitive to small changes in moisture content.

Soils containing more than about 5 percent fines (by weight) cannot be consistently compacted

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If inclement weather or in situ soil moisture content prevents the use of on-site material as structural fill, we recommend use of materials specified in Fill Material Types for free-draining granular fill. Stockpiled soils should be protected with polyethylene sheeting anchored to withstand local wind conditions and preservation of the soil’s moisture content.

SHALLOW FOUNDATIONS

If the site has been prepared in accordance with the requirements noted in Earthwork, the following design parameters are applicable for shallow foundations.

Design Parameters – Compressive Loads

Description Spread Footing Wall Footing

Net Allowable Bearing Pressure 1 n Native Soil or Compacted Structural Fill Over Native Soil

3,000 psf 3,000 psf

Minimum Dimensions 24 inches 18 inches

Minimum Embedment Below Finished Grade 2 18 inches 18 inches

Approximate Static Total Settlement <1 inch <1 inch

Estimated Static Differential 3 About 2/3 of total settlement

Ultimate Passive Pressure 4,5

■ Compacted Structural Fill 400 pcf (equivalent fluid unit weight)

Ultimate Coefficient of Sliding Friction6 0.40

1. The maximum net allowable bearing pressure is the pressure in excess of the minimum surrounding overburden pressure at the footing base elevation. An appropriate factor of safety has been applied. These bearing pressures can be increased by 1/3 for transient loads unless those loads have been factored to account for transient conditions. Assumes that exterior grades are relatively level adjacent to the structure.

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. For sloping ground, maintain depth below the lowest adjacent exterior grade within 5 horizontal feet of the structure.

3. Differential settlements are as measured over a span of 50 feet. We should review the settlement estimates after the foundation plan has been prepared by the structural engineer.

4. Use of passive earth pressures require the sides of the excavation for the spread footing foundation to be nearly vertical and the concrete placed neat against these vertical faces or that the footing forms be removed and compacted structural fill be placed against the vertical footing face.

5. Passive resistance in the upper 2 feet of the soil profile should be neglected.

6. Can be used to compute sliding resistance where foundations are placed on suitable soil/materials. Should be neglected for foundations subject to net uplift conditions.

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Construction Adjacent to Existing Building

Differential settlement between the additions and the existing building is expected to approach the magnitude of the total settlement of the addition. Expansion joints should be provided between the existing building and the proposed addition to accommodate differential movements between the two structures. Underground piping between the two structures should be designed with flexible couplings and utility knockouts in foundation walls should be oversized, so minor deflections in alignment do not result in breakage or distress. Care should be taken during excavation adjacent to existing foundations, to avoid disturbing existing foundation bearing soils.

New footings should bear at or near the bearing elevation of immediately adjacent existing foundations. Depending upon their locations and current loads on the existing footings, footings for the new addition could cause settlement of adjacent walls. To reduce this concern and risk, clear distances at least equal to the new footing widths should be maintained between the addition’s footings and footings supporting the existing building.

Foundation Construction Considerations

As noted in Earthwork, the footing excavations should be evaluated by the Geotechnical Engineer at the time of construction. The base of all foundation excavations should be free of water and loose soil, prior to placing concrete. Concrete should be placed soon after excavating to reduce bearing soil disturbance. Care should be taken to prevent wetting or drying of the bearing materials during construction. Excessively wet or dry material or any loose/disturbed material in the bottom of the footing excavations should be removed/reconditioned before foundation concrete is placed.

If unsuitable bearing soils are encountered at the base of the planned footing excavation, the excavation should be extended deeper to suitable soils, and the footings could bear directly on these soils at the lower level or on lean concrete backfill placed in the excavations. This is illustrated on the sketch below.

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Over-excavation for structural fill placement below footings should be conducted as shown below.

The over-excavation should be backfilled up to the footing base elevation as recommended in the Earthwork section.

Foundation Drains

We recommend the building be encircled with a perimeter foundation drain to collect exterior seepage water. This drain should consist of a 4-inch diameter perforated pipe within an envelope of washed rock, extending at least 6 inches on all sides of the pipe. The washed rock should conform to WSDOT Section 9-03.12(4), Gravel Backfill for Drains or 9-03.12(5), Gravel Backfill for Drywells. The washed rock envelope should be wrapped with filter fabric (such as Mirafi 140N, or equal) to reduce the migration of fines from the surrounding soil. Ideally, the drain invert would be installed no more than 8 inches above or below the base of the perimeter footings. The perimeter foundation drain should not be connected to roof downspout drains and should be constructed to discharge into the site storm water system or other appropriate outlet. These recommendations are summarized in the figure below.

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FLOOR SLABS

Design parameters for floor slabs assume the requirements for Earthwork have been followed.

Specific attention should be given to positive drainage away from the structure and positive drainage of the aggregate base beneath the floor slab.

Floor Slab Design Parameters

Item Description

Floor Slab Support 1

Minimum 6 inches of free-draining material of either of the following:

■ washed drain rock

■ 9-03.12(1)A Gravel Backfill for Foundations Class A (compacted to at least 95% of ASTM D 1557) 3,4

Estimated Modulus of Subgrade Reaction 2

200 pounds per square inch per inch (psi/in) for point loads 45 psi/in for distributed loads

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

2. Values of modulus of subgrade reaction are estimated for subgrade conditions where non-yielding, native soils are present, or compacted structural fill placed above competent native soil is present.

3. Free-draining granular material should have less than 5% fines (material passing the No. 200 sieve).

4. WSDOT Standard Specifications

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The use of a vapor retarder should be considered beneath concrete slabs on grade 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 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.

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 cracks should be sealed with a water-proof, non-extruding compressible compound specifically recommended for heavy duty concrete pavement and wet environments.

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

Floor Slab Construction Considerations

Finished subgrade, within and for at least 10 feet beyond the floor slab, should be protected from traffic, rutting, or other disturbance and maintained in a relatively moist condition until floor slabs are constructed. If the subgrade should become damaged or desiccated prior to construction of floor slabs, the affected material should be removed and structural fill should be added to replace the resulting excavation. Final conditioning of the finished subgrade should be performed immediately prior to placement of the floor slab support course.

The Geotechnical Engineer should approve the condition of the floor slab subgrades immediately prior to placement of the floor slab support course, reinforcing steel, and concrete. Attention should be paid to high traffic areas that were rutted and disturbed earlier, and to areas where backfilled trenches are located.

LATERAL EARTH PRESSURES

Design Parameters

Structures with unbalanced backfill levels on opposite sides should be designed for earth pressures at least equal to values 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 retained. Two wall restraint conditions are shown in the diagram below. Active earth pressure is commonly used for design of retaining walls that allow wall movement such as cantilever walls, gravity walls, and MSE walls. The “at-

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Responsive ■ Resourceful ■ Reliable 19 rest” condition assumes no wall movement and is commonly used for basement walls, loading dock walls, or other walls restrained at the top. The recommended design lateral earth pressures are for horizontal backfill and do not include a factor of safety and do not provide for possible hydrostatic pressure on the walls (unless stated).

Lateral Earth Pressure Design Parameters

Earth Pressure Condition 1

Coefficient for Backfill Type2

Uniform Pressure 3, 4, 5 p1 (psf) Effective Fluid

Pressures (psf) 2, 4, 5

Active (Ka) 0.28 (0.28)S (35)H At-Rest (Ko) 0.44 (0.44)S (55)H Passive (Kp) 3.54 --- (440)H

Seismic 6 --- (7)H –…