Gonyea_Playfield_Geotechnical_Report.pdf

Geotechnical Engineering Services Gonyea Playfield

13422 10th Avenue South Parkland, Washington for Site Workshop, LLC

October 7, 2025

1101 Fawcett Avenue, Suite 200 Tacoma, Washington 98402

253.383.4940

Geotechnical Engineering Services

Gonyea Playfield 13422 10th Avenue South

Parkland, Washington

File No. 04406-018-01 October 7, 2025

Prepared for:

Site Workshop, LLC 3800 Woodland Park Avenue North, Suite 200 Seattle, Washington 98103

Attention: Luke Vannice

Prepared by:

GeoEngineers, Inc.

1101 Fawcett Avenue, Suite 200 Tacoma, Washington 98402 253.383.4940

Archie A. Escobido Stuart S. Thielmann, PE Geotechnical Engineer Senior Geotechnical Engineer

Dennis J. “DJ” Thompson, PE Associate Geotechnical Engineer

AAE:SST:DJT:atk

Disclaimer: Any electronic form, facsimile, or hard copy of the original document (email, text, table, and/or figure), if provided, and any attachments are only a copy of the original document. The original document is stored by GeoEngineers, Inc. and will serve as the official document of record.

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File No. 04406-018-01

Table of Contents

1.0 Introduction and Project Understanding

2.0 Purpose, Scope and Previous Study

3.0 Site Description

4.0 Existing Information Review

4.1 Geologic Setting

4.2 Soil Survey

4.3 Nearby Subsurface Explorations and TEsting

4.4 Seismic Site Assessments

5.0 Exploration Program

6.0 Subsurface Conditions

6.1 Soil Conditions

6.1.1 Fill

6.1.2 Steilacoom Gravel

6.2 Groundwater

7.0 Geotechnical Conclusions and Design Recommendations

7.1 Geotechnical Summary

7.2 Geologic Hazards

7.2.1 Landslide Hazard

7.2.2 Erosion Hazard

7.2.3 Seismic Hazards

7.3 Seismic Design

7.3.1 Seismic Design Parameters

7.3.2 Liquefaction Potential

7.3.3 Lateral Spreading and Slope Instability

7.3.4 Faulting and Surface Rupture Potential

7.4 Shallow Foundation Support

7.4.1 Overexcavation and Bearing Surface Preparation

7.4.2 Wet Weather Considerations

7.4.3 Foundation Design Parameters

7.4.4 Foundation Settlement Estimates

7.4.5 Lateral Resistance

7.4.6 Footing and Building Drainage

7.5 Slab-On-Grade Floors

7.5.1 Slab Subgrade Preparation and Capillary Break

7.5.2 Slab-on-Grade Design

7.6 Below Grade Structures

7.6.1 Lateral Earth Pressures

7.6.2 Bearing and Resistance

7.6.3 Wall Drainage

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7.7 Pavements

7.7.1 General Pavement Design Criteria

7.7.2 Pavement Drainage

7.7.3 Pavement Subgrade Preparation

7.7.4 Recommended Pavement Sections

7.7.5 Additional Considerations

7.8 Stormwater Infiltration

7.8.1 Stormwater Infiltration Methodology

7.8.2 Soil Grain-Size Analysis Method

7.8.3 Recommended Design Infiltration Rate

7.8.4 Infiltration Design Discussion

7.8.5 Stormwater Treatment

7.8.6 Construction Considerations

8.0 Site Development and Earthwork

8.1 Stripping, Clearing and Subgrade Preparation

8.2 Debris and Obstacles

8.3 Subgrade Preparation and Evaluation

8.4 Subgrade Protection and Wet Weather Considerations

8.4.1 Soil Types

8.4.2 Surface Drainage

8.4.3 Wet Weather Considerations

8.5 Temporary Groundwater Handling

8.6 Temporary Excavations, Cut Slopes and Shoring

8.7 Permanent Cut and Fill Slopes

8.8 Fill Materials

8.8.1 Fill Requirements

8.8.2 Reuse of On-site Soils as Structural Fill

8.8.3 Topsoil Strippings

8.8.4 Imported Structural Fill

8.9 Fill Placement and Compaction

8.9.1 Lifts

8.9.2 Compaction Criteria

8.9.3 Retaining Wall and Below Grade Structure Backfill

8.9.4 Trench Backfill

8.9.5 Slopes

9.0 Additional Geotechnical Services

10.0 Limitations

11.0 References

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List of Figures Figure 1. Vicinity Map Figure 2. Site Plan (Existing Conditions) Figure 3. Exploration Plan and Conceptual Layout

Appendices Appendix A. Subsurface Explorations and Laboratory Testing Figure A-1 – Key to Exploration Logs Figures A-2 through A-8 – Logs of Hand Augers Figures A-9 through A-14 – Logs of Test Pits Figure A-15 – Laboratory Testing Results Summary Figures A-16 through A-18 – Sieve Analysis Results Figure A-19 – Chemical Analysis Results Appendix B. Report Limitations and Guidelines for Use

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1.0 Introduction and Project Understanding

This report presents updated geotechnical engineering considerations and recommendations for the proposed Gonyea Playfield project located at 13422 10th Avenue South in Parkland, Washington. Our understanding of the project is based on our previous work at the site, discussions with Site Workshop (project architect), attending project meetings and reviewing provided schematic site layouts. The Vicinity Map, Figure 1 and Site Plan, Figure 2, show the approximate property and project extents.

The overall project consists of renovations and new construction at the existing park facility. GeoEngineers, Inc. (GeoEngineers) has been involved with this project since initial planning and conceptual design, including advancing shallow hand explorations and providing a geotechnical study titled “Preliminary Geotechnical Site Assessment” dated June 17, 2024. Since issuing our preliminary assessment, a preferred concept has been selected and advanced to 30 percent design. Specific project elements include new at-grade structures (restrooms and picnic shelters), walking trails, basketball courts, play areas and grass lawns. Shallow swales and infiltration areas are anticipated to manage stormwater runoff. New parking and frontage improvements are planned along 10th Avenue South (eastern site boundary).

Proposed site grading will be minor with cuts on the order of about 2 feet and isolated “fill mounds” anticipated to be up to about 5 feet deep.

2.0 Purpose, Scope and Previous Study

The purpose of our geotechnical services is to complete subsurface explorations at the site to evaluate soil and groundwater conditions and provide geotechnical recommendations and reporting to support project planning, design and construction. Our services have been provided in accordance with our agreement with Site Workshop dated April 24, 2024 and contract amendment dated May 22, 2025. Details of our scope of services are provided in these documents and can be made available upon request. Where necessary, we have incorporated the conclusions and recommendations presented in our June 17, 2024 study “Preliminary Geotechnical Site Assessment” study. The observations, conclusions and recommendations provided in this report supersede those presented in our June 17, 2024 study.

3.0 Site Description

Gonyea Playfield is an approximate 12.4-acre rectangular parcel, approximately 1,340 feet (north-south) and 430 feet (east-west) in dimension. The site is generally bound by 132nd Street Court South (north site boundary), 10th Avenue South (east), 136th Street Court South (south), and single-family residential properties (west). Surrounding properties consist of single-family residential developments. Gonyea Playfield was developed in the 1950s; only minor improvements have taken place since initial development. Park amenities include asphalt surfaced basketball courts, playground structures and a restroom facility in the central portion of the site. North and southern portions of the site consist of grass playfields and dirt surfaced baseball/softball infields. Paved parking stalls and driveway are located along the east property boundary adjacent to 10th Avenue South.

The site is relatively level with gradual slopes between about Elevation 305 to 308 feet in the northern portion of the site, and between Elevation 303 to 305 feet in the southern portion of the site. Surrounding properties in the vicinity are at similar grades and elevations. Site elevations and grades referenced within this report are based on survey data provided by Site Workshop. Elevations presented in this report refer to the North American Vertical Datum of 1988 (NAVD88) and should be considered approximate.

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4.0 Existing Information Review

4.1 GEOLOGIC SETTING

We reviewed published geologic mapping of the area, including 1:24,000-scale mapping by Troost (2004) and 1:100,000-scale mapping by Schuster et. al. (2015). The reviewed maps indicate the site and surrounding areas are underlain by Steilacoom gravel deposits, a subunit of glacial outwash. Soil types are described as granular material ranging in size from fine gravel to boulders with variable silt, sand and cobble content. Lenses of sand and silty sand are also present. Steilacoom gravel soils are normally consolidated (not glacially overridden) and can range from loose to dense. Average thickness of this soil type is reported on the order of about 6 meters (20 feet).

4.2 SOIL SURVEY

The Natural Resources Conservation Service (NRCS) Web Soil Survey (WSS) provides soil survey data and information for general planning and engineering applications; the data is approximate and not site-specific. The WSS can be used to assess generalized information about local soils, including soil properties and suitability for various land uses (e.g., construction and site development). The site and immediate surroundings are mapped as underlain by Spanaway gravelly sandy loam. These soils are described as forming within outwash plains and consist mainly of deep, well drained to excessively drained gravelly sands. Soils are rated as moderately permeable, slight erosion hazard and Hydrologic Soil Group A (high infiltration rate and low runoff potential when thoroughly wet).

4.3 NEARBY SUBSURFACE EXPLORATIONS AND TESTING

During preparation of this report we reviewed our in-house files and online portals from the Washington State Department of Ecology (Ecology), Washington State Department of Natural Resources (DNR), Washington State Department of Transportation (WSDOT) and National Water Information System (NWIS) for previous subsurface explorations completed in the project vicinity. Our review included nine (9) drilled borings/wells and seven (7) test pit excavations within about a 1-mile radius of the site. Depths generally extended between about 6.5 and 25 feet below ground surface (bgs); however, one water well was drilled to 390 feet.

Typical generalized soil descriptions reported in the logs consist of gravel with variable silt, sand and cobble, which we interpret to be consistent with Steilacoom gravel outwash. Some of the borings were advanced below these gravels and encountered very dense silty sand or “hardpan” at depths ranging from about 20 to 80 feet bgs; which we interpret to be glacial till. Intermittent groundwater readings are reported in some of the wells. Groundwater depths within about a ½-mile radius of the site were reported between approximately 12 to 58 feet bgs at the time of measurement (seven total measurements). Some of the borings reported “dry” conditions and no groundwater to depths of about 25 feet (bottom of boring). Long-term monitoring data was not provided.

Our in-house files include review of a study that included four (4) Pilot Infiltration Tests (PITs) about 1-mile southeast of the project site. The PITs were completed at depths of about 3.5 to 4.75 feet bgs (corresponding to approximate Elevation 338 to 347 feet, NAVD88). Relatively rapid infiltration of water was observed within “clean” Steilacoom Gravel deposits (GP and GP-GM), with field measured (uncorrected) infiltration rates between approximately 26 and 180 inches per hour. Reduced infiltration on the order of 1.4 inches per hour (uncorrected) were measured within upper weather layers (GM).

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4.4 SEISMIC SITE ASSESSMENTS

We reviewed geophysical testing data and seismic site class assessments for school campuses across the state available from the Washington Geological Survey (2019). The nearest site assessments within similar geologic mapping (Steilacoom gravel outwash) are Fern Hill Elementary (approximately 3 miles north) and Camas Prairie Elementary (approximately 3 miles southeast). Shear wave measurements at each of these sites indicate seismic site class C (stiff soil) based on criteria outlined in the 2021 International Building Code (IBC). The sites are also rated severe ground shaking potential and very low to low liquefaction hazard.

5.0 Exploration Program

We investigated subsurface conditions at the site by advancing hand augers and test pit excavations.

Exploration depths ranged between about 1 to 9 feet below ground surface (bgs). Approximate locations are shown in Figure 3, Exploration Plan and Conceptual Layout, which includes an overlay of the proposed schematic layout. Details of the exploration program, summary logs and laboratory test results are presented in Appendix A.

We also completed a geologic site reconnaissance while completing field work for the exploration program.

Reconnaissance consisted of visual observation of site conditions such as vegetation, exposed soil types, pavement cracking, groundwater seepage and wet soils. During our time on site, we noted cracked asphalt within the basketball courts, parking areas and sidewalk hardscaping. Some of the pavement cracking appeared to be within the root zone of adjacent trees.

6.0 Subsurface Conditions

A summary of subsurface conditions expected at the site is provided in the sections below and considers the explorations completed for this study, as well as reviewed subsurface information in the vicinity.

6.1 SOIL CONDITIONS

Completed explorations were typically advanced with grass playfield areas and encountered about 2 to 6 inches of sod at ground surface. One hand auger (HA-4) was advanced within a baseball infield area and encountered approximately 6 inches granular fill (sand with trace silt) at ground surface. Below the exposed surfaces noted above, we interpret two general soil units to be present at the site based on the completed explorations and reviewed information: (1) Fill and (2) Steilacoom gravel outwash. A brief description of each unit is provided below.

6.1.1 Fill

Soils we interpret to be fill were encountered in all but two explorations (TP-5 and TP-6 in the east-central portion of the site). Fill material generally consisted of loose to medium dense sands with silt and gravel (SP-SM) and silty gravels with sand (GM). Occasional cobble and trace organics (fine roots) were also encountered throughout. Based on the dark coloration, observed densities and location within grass playfield areas, we interpret fill materials encountered to be landscaping fill associated with previous site grading and development. Landscaping fill appeared to consist of a mixture of topsoil amendment and reworked/weathered Steilacoom Gravel outwash soils (described below). Fill depths varied up to approximately 3.5 feet bgs at exploration locations.

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6.1.2 Steilacoom Gravel

Soils we interpret to be native Steilacoom gravel outwash were encountered below the fill to the full depths explored. Soil types generally consisted of gravel with sand, occasional cobble and trace silt (GP). At test pit TP-5 the upper approximate 6 inches consisted of silty gravel (GM), indicating a weathered condition.

Outwash densities ranged from loose to dense and excavations encountered caving conditions at depth.

6.2 GROUNDWATER

We did not observe groundwater seepage or wet soils at the time of our explorations, which were advanced as deep as about 9 feet bgs. We note that our explorations were completed in May (hand augers) and July (test pits); which are outside of the typical wet season for Western Washington. Based on reviewed well logs and groundwater readings, static groundwater levels in the immediate vicinity appear to range between about 12 to 40 feet bgs. We anticipate static groundwater levels in the project area are below the depths of our explorations (as deep as 9 feet bgs) and are likely much deeper.

Perched groundwater and wet soils could be encountered at shallower depths throughout the site depending on the time of year. The presence, quantity and location of perched groundwater (if encountered) is expected to be discontinuous, intermittent and dependent on infiltration of surface water that slows or terminates atop underlying less permeable layers.

7.0 Geotechnical Conclusions and Design Recommendations

7.1 GEOTECHNICAL SUMMARY

It is our opinion that the proposed development is feasible from a geotechnical engineering standpoint. We did not identify geotechnical related conditions that would prohibit or significantly constrain overall project design and construction. Geotechnical recommendations are provided below to support civil and structural design, cost estimating and site planning.

7.2 GEOLOGIC HAZARDS

We reviewed portions of the Pierce County Code (PCC) related to geologic hazards, specifically chapters for landslide, erosion and seismic hazards. These geologic hazards are not expected to limit the planned development, and we anticipate hazards identified at this site can be mitigated through engineering controls (e.g., site layout, grading, horizontal offsets, etc.).

7.2.1 Landslide Hazard

Landslide hazard areas are susceptible to mass ground movement down a slope, and are based on a combination of geologic, topographic, hydrologic and other factors. The PCC defines landslide hazard areas to include areas with historic slope movement, slopes greater than 20 percent inclinations with vertical relief greater than 20 feet or slopes with indicators such as bedding planes or groundwater seepage. Pierce County Landslide Hazard Areas mapping indicates potential shallow landslide susceptibility in areas of the southwest corner of the site. Historical landslide maps available online from the Department of Natural Resources (DNR) and United States Geologic Survey (USGS) indicate no mapped landslides in the area.

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Site grades overall, and in the southwest corner, consist of gentle slopes and undulating ground surface, with total elevation change on the order of about 5 feet which is less than the slope height as defined by PCC for a landslide hazard. During our time on-site we did not observe evidence of groundwater seepage, ground settlement, surface depressions, land sliding or soil movement. Based on our observations and encountered soil/groundwater conditions, we interpret the risk of landslide hazard at the site to be low.

7.2.2 Erosion Hazard

Erosion is the process of particle migration, displacement and/or removal of surface soils, typically through wind, rain, stream or wave action. Concentrated stormwater runoff is a major cause of erosion and soil loss.

The site and surrounding areas are not mapped within erosion hazard areas by Pierce County. Reviewed soil survey mapping indicates soil types in the area have a slight erosion hazard. It is our opinion that the overall risk of erosion at the site is low. However, site soils will be susceptible to erosion if left exposed and erosion control measures will need to be considered for construction and development. We anticipate erosion can be mitigated through typical erosion control measures (e.g., limited ground disturbance, site grading, drainage, plantings, etc.) in accordance with applicable city/county standards.

7.2.3 Seismic Hazards

The site is located within western Washington, a high seismic hazard region with the potential for large ground shaking intensities and long durations. Detailed discussion of seismic hazards and recommended design parameters are provided below. Seismic code will need to be considered as a part of structural design. Overall, we do not see mitigation of seismic hazards required for this site aside from code prescribed requirements and as such, it is our opinion that seismic hazards are not a limiting factor for the proposed development.

7.3 SEISMIC DESIGN

We understand structures will be designed in accordance with the International Building Code (IBC). The 2021 IBC states structures shall be designed to resist earthquake ground motions in accordance with American Society of Civil Engineers (ASCE) 7-16. We evaluated seismic design parameters in accordance with ASCE 7-16 using map-based methods as recommended by the USGS.

7.3.1 Seismic Design Parameters

Medium dense to dense Steilacoom gravel outwash deposits were encountered in explorations completed at the site (as deep as about 9 feet bgs). We anticipate soils extending to depths of 100 feet consist of similar outwash soils and dense to very dense glacially consolidated deposits. Shear wave testing in the vicinity and similar soil conditions correspond to values in the Site Class C range. Based on our studies, we recommend using IBC Site Class C for seismic design. Recommended design parameters based on this site class and the site location are provided in the table below.

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TABLE 1. RECOMMENDED SEISMIC DESIGN PARAMETERS

SEISMIC DESIGN PARAMETER (2021 IBC & ASCE 7-16) RECOMMENDED VALUE

Site Class C

Mapped Spectral Response Acceleration at Short Period, Ss (g) 1.315

Mapped Spectral Response Acceleration at 1-second Period, S1 (g) 0.458

Mapped Peak Ground Acceleration, PGA (g) 0.5

Design Spectral Acceleration at Short Period, SDS (g) 1.052

Design Spectral Acceleration at 1-second Period, SD1 (g) 0.458

Site Modified Peak Ground Acceleration, PGAM (g) 0.6

7.3.2 Liquefaction Potential

Liquefaction refers to a condition where vibration or shaking of the ground, usually from earthquake forces, disturbs the soil structure (i.e., the arrangement of individual soil particles) within saturated and unconsolidated soils. Soil particles may lose contact with each other during shaking, resulting in the affected soil deposit losing much of its stiffness and strength. Soils most susceptible to liquefaction include loose to medium dense “clean” sands below the water table; however, other loose granular soils such as silty sands, low-plasticity silts and gravels may also be susceptible to liquefaction. Ground settlement, loss of bearing capacity, foundation settlement and/or lateral movement may result from soil liquefaction.

Liquefaction susceptibility maps of the area by Palmer et al. (2004) and Pierce County (2005) indicate the site has “very low” potential for liquefaction. Based on soil and groundwater conditions observed in our explorations and our interpretation of the regional geology and groundwater table, it is also our opinion that the potential for liquefaction at the site is low.

7.3.3 Lateral Spreading and Slope Instability

Lateral spreading related to seismic activity typically involves lateral displacement of large, surficial blocks of non-liquefied soil when an underlying soil layer loses strength during seismic shaking. Lateral spreading usually develops in areas where sloping ground or large grade changes are present. In addition to lateral spreading, seismic shaking can also trigger instability of slopes in either non-liquefied or liquefied conditions. Based on our understanding of the subsurface conditions, liquefaction risk, site topography and proposed site grading, it is our opinion that the risk of lateral spreading and slope instability is low.

7.3.4 Faulting and Surface Rupture Potential

We reviewed geologic seismic feature maps of the project vicinity, including the “Faults and Earthquakes in Washington State” (Czajkowski and Bowman 2014). The nearest mapped fault is the Tacoma fault zone, approximately 10 miles north of the project site. Location of this fault zone has been inferred from geophysical studies and there are no known surface expressions of the fault. Based on our review and understanding of local geology it is our opinion that the risk for seismic surface rupture at the site is low.

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7.4 SHALLOW FOUNDATION SUPPORT

Proposed development includes new at-grade structures consisting of restrooms and picnic shelters. We envision these lightly loaded structures can be adequately supported on shallow foundations and slab-on-grade floors bearing on medium dense or better Steilacoom gravel outwash deposits.

7.4.1 Overexcavation and Bearing Surface Preparation

Occasional layers of loose and/or organic-rich soils are present within upper soil layers (fill and weathered materials) at the site. Thickness of these soil types varies throughout the site and were noted at depths up to about 3.5 feet. Depending on final foundation grades these soils could be present at planned bearing surface elevations. While generally suitable for the foundation support, as described further in this report, these shallow layers could contain excessive organics and/or become easily disturbed and require removal.

The necessity for overexcavation will ultimately have to be determined in the field at the time of construction.

Bearing surfaces must be confirmed and/or compacted (as necessary) to a uniformly firm and unyielding condition. Bearing surfaces should be evaluated by GeoEngineers prior to formwork, steel and foundation construction. Foundation bearing surfaces and subgrades should be prepared as discussed in Section 8.3 below.

7.4.2 Wet Weather Considerations

Foundation bearing surfaces must not be exposed to standing water, as wet surfaces can become easily disturbed. If water pools in the base of the excavation, it should be removed before placing structural fill, formwork or reinforcing steel. Structural fill and concrete should be placed as soon as practical after preparation of the foundation excavations. If foundation excavations are exposed to extended wet weather conditions, a 2- to 3-inch-thick layer of lean mix concrete or controlled density fill (CDF) can be considered for subgrade protection. Alternatively, 6 to 8 inches of compacted crushed rock or select granular fill can also be considered for subgrade protection.

7.4.3 Foundation Design Parameters

Exterior footings should be established at least 18 inches below the lowest adjacent grade for bearing and for frost protection. Interior footings may be founded a minimum of 12 inches below the top of the floor slabs, where insulated. Continuous footings should have a minimum width of 18 inches. Isolated column footings should have a minimum width of 24 inches.

For footings prepared as described we recommend an allowable soil bearing pressure of 3,000 pounds per square foot (psf) be used for design. This allowable bearing pressure is appropriate for footings bearing on level ground and medium dense or better Steilacoom gravel outwash soils. This recommended bearing pressure applies to the total of dead and long-term live loads and may be increased by one-third when considering total loads, including earthquake or wind loads. This is a net bearing pressure; the weight of the footing and overlying backfill can be ignored in calculating footing sizes.

Higher allowable bearing pressures can be considered with exposure of dense gravel at depth and additional design review. If higher pressures would be advantageous, we can provide alternative recommendations as building layout and elevations are finalized.

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7.4.4 Foundation Settlement Estimates

We estimate settlement of footings designed and constructed as recommended will be on the order of 1 inch or less. Differential settlements could be on the order of about ¼ to ½ inch between comparably loaded isolated column footings or over a distance of about 50 feet along continuous wall footing.

Settlement is expected to occur rapidly, essentially as loads are applied. Settlements could be greater than estimated if loose or disturbed soil is present beneath footings.

These estimates are based on footings under design loads of about 4,000 pounds per lineal foot or 30,000 pounds per column and proportioned using the recommended allowable bearing pressures above.

We should be notified if foundation loads exceed those presented above so we can review and revise our settlement estimates or footing design recommendations, if necessary.

7.4.5 Lateral Resistance

The ability of the soil to resist lateral loads is a function of frictional resistance, which can develop on the base of footings and slabs, and passive resistance, which can develop on the face of below-grade elements as these elements move into the soil. For footings prepared in accordance with the recommendations presented above, the allowable frictional resistance on the base of the footing may be computed using a coefficient of friction of 0.40 applied to the vertical dead-load forces. The allowable passive resistance on the face of the footing or other embedded foundation elements may be computed using an equivalent fluid density of 300 pounds per cubic foot (pcf) for proof-compacted non-organic site soils or structural fill extending out from the face of the foundation element a horizontal distance at least equal to 2.5 times the depth of the element.

The above values include a factor of safety of about 1.5. The passive earth pressure and friction components may be combined provided the passive earth pressure component does not exceed two-thirds of the total. The passive earth pressure value is based on the assumptions that the adjacent grade is level and groundwater levels remain below the base of the footing throughout the year. The top foot of soil should be neglected when calculating passive lateral earth pressure unless the area adjacent to the foundation is covered with pavement or slab-on-grade.

7.4.6 Footing and Building Drainage

It is our opinion that footing drains are not necessary to maintain bearing support conditions as provided in this report. However, we suggest footing drains be included around the base of structure footings to maintain dry conditions, for general building perimeter maintenance and to aid in capture of incidental seepage from below grade or outside sources (e.g., irrigation). We expect the majority of standard civil design sections for foundation drains (i.e., perforated pipe surrounded by clean gravel wrapped in a geotextile fabric, installed at base of footing) will be adequate for these purposes, and some flexibility in this design may be considered. The perforated pipe material should consist of solid wall rigid polyvinyl chloride (PVC), or equivalent, and be at least 4 inches in diameter. We can review and/or provide specific recommendations for design of foundation drains, if requested, once final structure locations and elevations have been determined.

We recommend the ground surface be sloped away from structures. Downspouts should be tightlined away from the building foundation areas and should also be discharged into appropriate areas such as a stormwater disposal system. Downspouts should not be connected to footing drains.

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7.5 SLAB-ON-GRADE FLOORS

We anticipate floor loads will be on the order of about 150 psf for planned structures. In our opinion satisfactory subgrade support for the considered loading can be obtained provided subgrades are prepared as recommended in this report. For floor slabs with larger design loads, we recommend we be contacted to review the proposed design and determine if additional/revised recommendations are required.

7.5.1 Slab Subgrade Preparation and Capillary Break

We anticipate subgrade soils could be variable and range from relatively loose to dense depending on final layout and elevations. In all cases, slab subgrade must be confirmed or compacted (as necessary) to a uniformly firm and unyielding condition. Slab subgrades preparation is discussed further in Section 8.3 below.

We recommend slab-on-grade floors be underlain by a minimum 6-inch-thick capillary break layer to provide uniform support and reduce the potential for moisture migration into the slab. This minimum section thickness is recommended, in part, to assist in bridging areas where the slab spans may span between soils of varying densities. Capillary break material, placement and compaction should conform to recommendations provided in Section 8.0 below.

In our opinion an underslab drainage system is not necessary for slab subgrades within a few feet of ground surface. If dry slabs are required (e.g., where adhesives are used to anchor carpet or tile to slab), a waterproof liner may be placed as a vapor barrier below the slab. The vapor barrier should be selected by the architect and/or structural engineer as a part of the overall floor section design.

7.5.2 Slab-on-Grade Design

For subgrades constructed as described above we recommend a modulus of subgrade reaction of 250 pounds per cubic inch (pci) for slab design. This recommended value is based on a 1-foot by 1-foot square plate loading condition. Ultimately, the coefficient of subgrade reaction for a foundation will vary based on its minimum width according to the following equation:

ks = ks1[(B+1)/2B]2

Where ks is the coefficient of subgrade reaction, ks1 is the coefficient of subgrade reaction for a 1-foot by 1-foot plate (250 pci) and B is the minimum width or lateral dimension of the mat/footing. For the design of smaller mat foundations and typical structural footings and for use with some common structural programs, we recommend the use of ks. If foundation size is accounted for and for larger slabs and mat foundations, the above value of ks1 equal to 250 pci may be used.

Settlement for floor slabs designed and constructed as recommended is estimated to be less than 1-inch for long-term static loads of 150 psf. We estimate differential settlement of floor slabs will be ½ inch or less over a span of about 50 feet.

7.6 BELOW GRADE STRUCTURES

Conventional retaining walls and below-grade structures may be considered as a part of site development.

Recommendations below are based on a maximum structure/wall height of about 8 feet and are based on drained conditions. Areas that cannot be adequately drained will need to consider hydrostatic loading conditions. If these assumptions are not appropriate, we should be contacted to review conditions and provide revised design recommendations.

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7.6.1 Lateral Earth Pressures

We anticipate retaining walls and below grade structures at the site will retain existing fill and/or glacial soils as encountered in our explorations or be backfilled with structural fill. We recommend walls and subsurface structures, as described above, be designed using the lateral earth pressures provided below assuming drained conditions.

■ Active soil pressure may be estimated using an equivalent fluid density of 35 pcf for the level backfill condition (triangular distribution). For walls with backfill sloping upward behind the wall at 2H:1V , an equivalent fluid density of 52 pcf should be used. If the slope is shallower than 2H:1V, the active lateral earth pressures can be linearly interpolated between the two values above using the slope angle (from horizontal) in degrees. The active soil pressure condition assumes the top of the wall is not structurally restrained and is free to rotate and deflect a distance of at least 0.001*H (where H is the wall height).

■ At-rest soil pressure may be estimated using an equivalent fluid density of 55 pcf for the level backfill condition. For walls with backfill sloping upward behind the wall at 2H:1V, an equivalent fluid density of 80 pcf should be used. The at-rest condition is applicable where walls are restrained against deflection (not allowed to rotate).

■ For seismic considerations, a uniform lateral surcharge pressure of 13*H psf (where H is the height of the retaining structure or the depth of a structure below ground surface) should be added to the lateral earth pressure.

■ Surcharge effects should be considered if loads are applied closer than one-half of the structure height from the wall face. A typical traffic surcharge of 250 psf may be represented by assuming walls will contain an additional 2 feet of retained fill.

■ Other surcharge loads, such as construction equipment, soil stockpiles, structures, etc., should be considered on a case-by-case basis and will be based on load, load geometry and location;

GeoEngineers can provide additional recommendations upon request.

7.6.2 Bearing and Resistance

Below grade elements may be supported on shallow foundations designed and prepared in accordance with recommendations presented in Section 7.4 above, including allowable bearing and resistance values provided. We estimate settlement of retaining structures will be similar to the values previously presented for structure foundations.

7.6.3 Wall Drainage

A positive drainage system behind walls and below grade structures must be constructed to collect water and prevent the buildup of hydrostatic pressure against the wall or structure. We recommend a zone of free-draining material behind the retaining structure with perforated pipes to collect seepage water. The drainage material should consist of coarse sand and gravel containing less than 5 percent fines (material passing the U.S. No. 200 sieve) by weight. The drainage zone should extend horizontally at least 18 inches from the back of the retaining structure. Other systems and materials may also be considered, where appropriate, and as approved by the project engineer. Other systems, such as corrugated “waffle” drainage systems/strips and weep holes may also be options for management of water behind retaining structures and these systems could be considered on a case-by-case basis.

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A perforated, rigid, smooth-walled drainpipe with a minimum diameter of 4 inches should be placed along the base of the structure within the free-draining backfill and extend for the entire wall length. The drainpipe should be rigid polyvinyl chloride (PVC) pipe and be sloped to drain by gravity. Discharge should be routed to appropriate discharge areas and to reduce erosion potential. Cleanouts should be provided to allow routine maintenance. We recommend roof downspouts or other types of drainage systems not be connected to retaining wall drain systems.

7.7 PAVEMENTS

Proposed improvements will include new at-grade parking, driveway and sidewalk areas. We envision vehicular pavements will support passenger automobiles as well as traffic for occasional delivery trucks/vans, maintenance vehicles and garbage trucks.

Construction traffic has not been considered in our pavement design. We assume pavement subgrade preparation and pavement construction will be completed at the end of the project after heavy construction vehicles (such as concrete trucks and construction material delivery trucks) will no longer access the site.

If this is not the case and areas are to be paved as part of site construction with heavy traffic, additional pavement thickness may be necessary to prevent pavement damage during construction, or repairs could be necessary.

7.7.1 General Pavement Design Criteria

Specific vehicle loading information (vehicle type, frequency, etc.) was not provided. We have assumed vehicular pavement areas will include light-duty and heavy-duty traffic areas.

■ Light-Duty: Light-duty pavements are considered those accessed only by passenger automobile traffic and delivery vans (i.e., general automobile parking and driveway areas only).

■ Heavy-Duty: Heavy-duty pavements are considered those within the drive path of operations, maintenance, garbage trucks and other large vehicles. We considered limited heavy-duty traffic to include vehicles up to about 30,000-pound vehicle weight and 18,000-pound axle loads.

■ Both heavy and light duty pavement sections are capable of supporting fire engines and ladder type vehicles during infrequent emergency access situations.

7.7.2 Pavement Drainage

Long-term pavement performance is influenced significantly by drainage conditions beneath the pavement section. Final improvements surrounding the pavements should be designed and constructed such that stormwater or irrigation water from landscape areas does not accumulate below the pavement section or pond on pavement surfaces. Positive drainage can be accomplished by establishing grades to promote drainage, including crowning of subgrades. If pavements in parking areas slope inward (toward the center of the parking area), full depth curbs or other measures should be used to prevent water from entering and ponding on the subgrade and within the base section.

We expect pavement subgrades will consist of Steilacoom gravel outwash soils and are anticipated to have adequate infiltration capacity to dissipate water that may accumulate around pavement areas. In our opinion the recommended subbase layer may be omitted provided that clean granular materials are exposed at the subgrade. However, fill and weathered materials are also present, and omitting the subbase layer will ultimately have to be determined during construction.

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7.7.3 Pavement Subgrade Preparation

Because of the variable near surface conditions encountered in the explorations, it is possible that areas of loose, soft or organic soils could be present below planned pavement subgrade elevations. In all cases pavement subgrade must be confirmed or compacted (as necessary) to a uniformly firm and unyielding condition as discussed in Section 8.3 below and evaluated by GeoEngineers prior to paving.

7.7.4 Recommended Pavement Sections

Recommended pavement design sections are presented below and based on our experience with similar projects. These sections can be refined for specific vehicle loading or traffic conditions, as necessary.

TABLE 2. RECOMMENDED PAVEMENT SECTIONS

SECTION

MINIMUM THICKNESS (INCHES)

PCC AC SUBBASE

Sidewalk (Pedestrian Areas Only, No Vehicle Loading)

- 2 -

4 - -

Light Duty (Automobile Traffic Only)

- 2.5 6

6 - 6

Heavy Duty (Access Lanes for Bus/Transit Vehicle Maintenance)

- 4 6

9 - 6

7.7.5 Additional Considerations

Crushed surfacing materials (base course and/or top course) should consist of material as recommended and be moisture-conditioned to near optimum moisture content (OMC) and compacted as recommended in Section 8.0 below. Hot-mix asphalt (HMA) should conform to applicable sections of 5-04, 9-02 and 9-03 of the WSDOT Standard Specifications. We recommend HMA consist of Class ½-inch (maximum aggregate size), with binder performance grade (PG) of either 58-22 or 64-22 (depending on anticipated climate temperatures the asphalt will be exposed).

Our design PCC pavement sections assume concrete pavements in vehicular areas will be jointed and that dowel bars will be included at expansion joints to assist in load transfer. Dowels should not be included between exterior pavement slabs and interior pavement slabs to reduce the risk of cracking due to differential settlements. We also assume concrete shoulders or curbs will be installed. We recommend PCC with a minimum 14-day flexural strength of 650 pci.

Some areas of pavement may exhibit settlement and subsequent cracking over time; this can be a common part of the pavement lifecycle. Cracks in the pavement will allow water to infiltrate to the underlying base course, which could increase the amount of pavement damage caused by traffic loads. To prolong the effective life of the pavement, cracks should be sealed as soon as possible.

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7.8 STORMWATER INFILTRATION

Stormwater management will include shallow swales and infiltration areas. Current conceptual design envisions the bottom of infiltration facilities within about 1 to 2 feet of existing grade. The site is underlain by Steilacoom gravel outwash deposits, which typically have moderate to high infiltration potential.

Reduced infiltration performance is expected within upper fill and weathered layers, which include a higher percentage of fine-grained soil particles. The sections below provide our interpretation of the suitability of site soils for infiltration, including recommended rates for design.

7.8.1 Stormwater Infiltration Methodology

We understand stormwater facilities will be designed in accordance with the 2021 Pierce County Stormwater Management and Site Development Manual (SMSDM). In accordance with the SMSDM, design infiltration rates can be determined using the Soil Grain-size Analysis Method for sites underlain by Hydrologic Soil Group A soils. Based on observed soil types and reviewed soil survey mapping, Hydrologic Soil Group A soils are present at the site (Steilacoom gravel outwash) and therefore the grain-size method is appropriate. Our recommendations also consider our experience with in-situ infiltration testing completed within similar soil types and within about 1-mile of the project site.

In accordance with the 2021 Pierce County SMSDM, groundwater monitoring and mounding analysis are typically required when there is less than a 3-foot separation distance between the bottom of proposed infiltration facilities and the seasonal highwater mark, “hardpan” (e.g., glacial till), or other low permeability layer. Based on subsurface conditions encountered, we anticipate infiltration facilities within a few feet of existing grade can be designed without consideration of groundwater or low permeability layers.

7.8.2 Soil Grain-Size Analysis Method

We calculated infiltration rates of selected soil samples from our explorations using the Soil Grain-size Analysis Method and procedures outlined in the SMSDM. Long-term design infiltration rates were calculated through correction factors to account for uncertainties in the testing method, facility geometry and long-term plugging. Per the SMSDM, the maximum design infiltration rate (I-design) is calculated with the following equation:

𝐼𝐼𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 = 𝐼𝐼𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 ∗ 𝐹𝐹𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 ∗ 𝐹𝐹𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 ∗ 𝐹𝐹𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝

We considered the following correction factors in our analysis:

■ Ftesting = 0.4 for the soil grain-size analysis method.

■ Fgeometry = 0.25. This is the lowest recommended value recommended by the SMSDM and in our opinion is appropriate for facilities with a large footprint.

■ Fplugging = 1.0 for coarse sands or cobbles as observed in our explorations.

Table 2 below summarizes the calculated initial (short-term) and design (long-term) infiltration rates.

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TABLE 2. CALCULATED INFILTRATION RATE SUMMARY

EXPLORATION

DEPTH

(FT)

ELEVATION

(FT)

GEOLOGIC UNIT

USCS

SOIL TYPE

PERCENT

FINES (%)

KSAT, INITIAL

(IN/HR)1

KSAT, DESIGN

(IN/HR)2,3

TP-2 2.5 302.5 Outwash GW 1 1,445 145

TP-3 1.5 305.5 Fill GM 17 12 1.2

TP-3 6 301 Outwash GP 4 180 18

TP-4 1.5 305.5 Fill GM 18 9.3 0.9

TP-5 0.5 306.5 Outwash (Weathered) GM 13 12 1.2

TP-6 1 304 Outwash GW 1 3,355 336

TP-6 4 301 Outwash GW 1 658 66

HA-5 1 307 Outwash GP 3 104 10

HA-7 0.75 304 Fill GP-GM 10 186 19

HA-7 1.5 303.5 Outwash GP 4 234 23

Notes:

1 Initial saturated hydraulic conductivity as calculated by the grain-size analysis method without correction factors.

2 Design (long-term) infiltration rate as defined by the Pierce County SMSDM using the calculated by the grain-size analysis method including correction factors, per Pierce County SMSDM.

3 In accordance with the Pierce County SMSDM, maximum allowable design infiltration rate is 30 inches per hour.

7.8.3 Recommended Design Infiltration Rate

Ultimately, final design infiltration rates will depend on the geometry of proposed facilities and actual soil types present at the bottom of the facility. We recommend infiltration facilities target “cleaner” outwash deposits, generally encountered in the explorations at depths ranging between about 1 to 3.5 feet bgs.

Soils above this depth include landscaping fill and weathered zones, which include higher percentage of fine-grained soil particles that will limit infiltration potential.

We recommend a design infiltration rate of 1 inch per hour for facilities near existing grade; this recommended rate assumes infiltration through upper silty fill/weathered zones. We recommend a design infiltration rate of 10 inches per hour for facilities within “clean” Steilacoom gravel outwash soils (deeper than about 2 to 3.5 feet below existing grade).

7.8.4 Infiltration Design Discussion

Correction factors were selected based on recommended values in the SMSDM, our project understanding, observed soil conditions and our experience. We should be contacted once facilities have been finalized to confirm the correction factors used in our analysis are still appropriate.

Required vertical and horizontal separations (e.g., wells, structures, drainfields, slopes, etc.) will need to be considered during layout and design as presented in the SMSDM and as required by local regulations, building code requirements or other state regulations. Additional limitations than those referenced above might be required and additional review and/or verification could be necessary.

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7.8.5 Stormwater Treatment

Stormwater treatment could be required to reduce the potential for increased run-off and pollutants. Cation exchange capacity (CEC) and organic content (OC) testing results of selected soil samples are summarized in the table below for stormwater treatment design, if applicable and necessary.

TABLE 3. CHEMICAL SOIL ANALYSIS RESULTS

EXPLORATION DEPTH

(FT) SOIL TYPE GEOLOGIC UNIT ORGANIC CONTENT

CATION EXCHANGE CAPACITY

(MEQ/100G)

TP-2 1 GP-GM Fill 9.7 20.7

TP-2 2 GW Outwash 3.2 8.7

TP-4 1.5 GM Fill 13.8 28.9

TP-6 1 GW Outwash 1.9 5.2

Notes:

meq = milliequivalents; g = grams

7.8.6 Construction Considerations

Construction equipment should not be permitted in infiltration areas after they are excavated because of the potential for compaction of the subgrade that could reduce the infiltration rate. To help reduce clogging of infiltration facilities, we recommend they be protected during construction with siltation control facilities such as temporary settling basins, silt fences and hay bales. Suspended solids can clog the soil and reduce the infiltration rate. Periodic sweeping of paved areas, during and following construction, will help extend the life of the infiltration facilities.