P907 TPS Geotech Report_0006.pdf

Geotechnical Engineering Services

P-907 Fuel Facility and Vessel Maintenance Facility Naval Base Kitsap Bangor, Washington for The United States Navy and Notkin Mechanical Engineers

May 17, 2019

Geotechnical Engineering Services

P-907 Fuel Facility and Vessel Maintenance Facility Naval Base Kitsap Bangor, Washington for The United States Navy and Notkin Mechanical Engineers

May 17, 2019

17425 NE Union Hill Road, Suite 250 Redmond, Washington, 98052 425.861.6000

May 17, 2019 | Page i File No. 0144-488-00

Table of Contents

1.0 INTRODUCTION

2.0 PROJECT DESCRIPTION

3.0 FIELD EXPLORATIONS AND LABORATORY TESTING

3.1. Field Explorations

3.2. Laboratory Testing

4.0 SITE CONDITIONS

4.1. Geology

4.2. Seismicity

4.2.1. Earthquake Source Zone

4.2.2. Benioff Source Zone

4.2.3. Cascadia Subduction Zone Interplate Source Zone

4.2.4. Shallow Crustal Source Zone

4.3. Surface Conditions

4.3.1. Fuel Facility and Vessel Maintenance Facility

4.4. Subsurface Soil Conditions

4.4.1. Fuel Facility Site

4.4.2. Maintenance Facility Site

4.5. Groundwater Conditions

4.5.1. Fuel Facility Site

4.5.2. Vessel Maintenance Facility Site

5.0 CONCLUSIONS AND RECOMMENDATIONS

5.1. Summary of Geotechnical Considerations

5.2. Earthquake Engineering

5.2.1. Liquefaction

5.2.2. Other Seismic Hazards

5.2.3. Seismic Design Information – Fuel Facility and Vessel Maintenance Facility

5.3. Shallow Foundations for VMF

5.3.1. Allowable Bearing Pressures

5.3.2. Static Settlement

5.3.3. Lateral Resistance

5.4. Slab-on-Grade Floor for VMF

5.4.1. Subgrade Preparation

5.4.2. Design Parameters

5.5. Drainage Considerations

5.5.1. Perimeter Footing Drains for Buildings

5.5.2. Other Drainage Considerations

5.6. Stormwater Infiltration Considerations

5.7. Earthwork

5.7.1. Excavation Considerations

5.7.2. Stripping, Clearing and Grubbing

5.7.3. Subgrade Preparation

5.7.4. Erosion and Sedimentation Control

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5.7.5. Structural Fill

5.7.6. Temporary Slopes

5.7.7. Permanent Slopes

5.8. Site Retaining Walls

5.8.1. Drainage

5.8.2. Construction Considerations

5.9. Below-Grade Vaults

5.9.1. Foundation Support

5.9.2. Lateral Earth Pressures

5.9.3. Construction Considerations

5.10. UST Excavation and Tank Buoyancy

5.11. Pavement Recommendations

5.11.1. Subgrade Preparation

5.11.2. Asphalt Pavement

5.11.3. Portland Cement Concrete Pavement

5.12. Geotechnical Special Inspection

6.0 LIMITATIONS

7.0 REFERENCES

LIST OF FIGURES

Figure 1. Vicinity Map Figures 2A and 2B. Site and Exploration Plan

APPENDICES

Appendix A. Field Explorations Figure A-1 – Key to Exploration Logs Figures A-2 through A-10 – Log of Borings

Appendix B. Laboratory Testing Figures B-1 and B-2 – Sieve Analysis Results Figure B-3 – Atterberg Limits Analysis Results

Appendix C. Results of Analytical Testing Appendix D. Report Limitations and Guidelines for Use

May 17, 2019 | Page 1

1.0 INTRODUCTION

This report presents the results of GeoEngineers’ subsurface explorations and geotechnical evaluation for the upland portions of the P-907 Pier and Vessel Maintenance Facility (VMF) project at Naval Base Kitsap, Bangor, Washington. The upland portion of the work consists of the VMF and a separate fuel facility. The project site is shown relative to surrounding physical features on the Vicinity Map (Figure 1).

The purposes of our services are to evaluate subsurface conditions at the proposed upland portions of the project site and to provide recommendations for the proposed project components which include a new underground fuel storage tank facility and new VMF. The existing site conditions at the proposed underground fuel storage facility are shown in Figure 2A; the existing site conditions at the proposed VMF are shown in Figure 2B (Site and Exploration Plans).

GeoEngineers’ services were completed in general accordance with the Professional Services Subconsultant Agreement, Indefinite Quantity Contract, Project N44255-16-D-2006, between Notkin Mechanical Engineers and GeoEngineers, Inc. (GeoEngineers) dated September 29, 2017. Our specific scope of services for this portion of the project includes:

■ reviewing previous geotechnical explorations prepared for projects near the site;

■ completing upland borings and test pits to characterize subsurface conditions for the site;

■ completing laboratory testing on selected soil samples obtained from the explorations;

■ developing geotechnical engineering design recommendations for the project; and

■ preparing this report.

The contract was later modified to include additional offshore borings and finite element modeling (Consultant Agreement-02 dated July 6, 2018). The results of the offshore borings and finite element modeling will be included in a separate report.

2.0 PROJECT DESCRIPTION

GeoEngineers’ understanding of this project is based on the Facility Design Criteria (FDC) provided by the United States Navy (Navy) and discussions with the project team.

We understand that the fuel facility project includes the installation of two 20,000-gallon underground storage tanks (USTs) with an excavation on the order of 15 feet. We expect that some grading will occur at the site to create a level pad area. We understand the USTs will be located near GEI-8 shown in Figure 2A.

The VMF will be a single story, steel framed, metal panel building with a slab-on-grade for maintenance of boats. The building may include some gantry or fixed cranes. We understand that the building loads will be relatively light. The proposed outline of the building footprint is shown in Figure 2B.

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3.0 FIELD EXPLORATIONS AND LABORATORY TESTING

3.1. Field Explorations

The subsurface soil and groundwater conditions at the site were evaluated by reviewing available geologic maps and drilling three borings (GEI-8 for the proposed fuel facility, GEI-9 and GEI-10 for the proposed VMF). Six test pits were also completed, TP-1 at the proposed fuel facility and TP-2 through TP-6 at the proposed VMF. Borings were completed to depths of about 30½ to 31½ feet below ground surface (bgs) and the test pits were completed to 12 feet bgs. Borings GEI-8 through GEI-10 were completed on February 7 and 8 and the test pits TP-1 through TP-6 were completed on January 29 and 30, 2018. The borings were advanced continuous-flight, hollow-stem auger drilling equipment.

The approximate locations of the explorations completed for this project are presented on the Figures 2A and 2B. Details of the field exploration program and logs of the explorations are presented in Appendix A.

3.2. Laboratory Testing

Soil samples were obtained during the drilling program and taken to GeoEngineers’ laboratory for further evaluation. Selected samples were tested for the determination of moisture content, fines content (material passing the U.S. No. 200 sieve), grain-size distribution (sieve analysis) tests and Atterberg limits.

A description of the laboratory testing and the test results are presented in Appendix B.

4.0 SITE CONDITIONS

4.1. Geology

Published geologic information for the project vicinity includes the Washington State Department of Natural Resources (DNR) map titled “Geologic Map of the Seabeck and Poulsbo 7.5-minute quadrangles, Kitsap and Jefferson Counties, Washington” (Polenz et al. 2013). The geologic map indicates that the surficial deposits in the vicinity of the proposed fuel and maintenance facility sites consist of ice contact deposits.

Glacial till is also mapped in the area. The units are described below in order of deposition, starting with the most recent.

■ Ice contact deposits consist of medium dense to dense cobble, gravel, sand, lacustrine mud and isolated boulders with discontinuous variations of glacial till; ablation, flow and lodgment till.

■ Glacial till consists of a dense to very dense, non-sorted mixture of clay, silt, sand, gravel, cobbles and boulders.

4.2. Seismicity

4.2.1. Earthquake Source Zone

The Puget Sound area is located near the convergent continental boundary known as the Cascadia Subduction Zone (CSZ). The CSZ is the zone where the westward advancing North American Plate is overriding the subducting Juan de Fuca Plate. The interaction of these two plates results in two potential seismic source zones: (1) the Benioff source zone and (2) the CSZ interplate source zone. A third seismic source zone, referred to as the shallow crustal source zone, is associated with the north-south compression resulting from northerly movement of the Sierra Nevada block of the North American Plate.

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4.2.2. Benioff Source Zone

Benioff source zone earthquakes are also referred to as intraplate, intraslab or deep subcrustal earthquakes. Benioff zone earthquakes occur within the subducting Juan de Fuca Plate between depths of 20 and 40 miles and typically have no large aftershocks. Extensive faulting results as the Juan de Fuca Plate is forced below the North American Plate and into the upper mantle.

The Olympia 1949 (magnitude 7.1), the Seattle 1965 (magnitude 6.5) and the Nisqually 2001 (magnitude 6.8) earthquakes are considered to be Benioff source zone earthquakes. The Benioff source zone is characterized as being capable of generating earthquakes up to magnitude 7.5. The recurrence interval for large earthquakes originating from the Benioff source zone is believed to be shorter than for the shallow crustal and CSZ source zones; damaging Benioff source zone earthquakes in Western Washington occur every 30 years or so. The deep focal depth of these earthquakes tends to dampen the shaking intensity when compared to shallow crustal earthquakes of similar magnitudes.

4.2.3. Cascadia Subduction Zone Interplate Source Zone

The CSZ is an approximately 650-mile-long thrust fault that extends along the Pacific Coast from mid-Vancouver Island to Northern California. CSZ interplate earthquakes result from rupture of all or a portion of the convergent boundary between the subducting Juan de Fuca Plate and the overriding North American Plate. The fault surfaces approximately 50 to 75 miles off the Washington coast. The width of the seismogenic portion of the CSZ interplate fault varies along its length. As the fault becomes deeper, materials being faulted become ductile, and the fault is unable to store mechanical stresses.

The CSZ is considered to be capable of generating earthquakes of magnitude 8 to 9. No earthquakes on the CSZ have been instrumentally recorded; however, through the geologic record and historical records of tsunamis in Japan, it is believed that the most recent CSZ event occurred in the year 1700 (Atwater 1996 and Satake et al. 1996). Recurrence intervals for CSZ interplate earthquakes are thought to be on the order of 400 to 600 years. Paleogeologic evidence suggests that five to seven interplate earthquakes may have been generated along the CSZ over the last 3,500 years at irregular intervals.

4.2.4. Shallow Crustal Source Zone

The shallow crustal source zone is used to characterize shallow crustal earthquake activity within the North American Plate. Shallow crustal earthquakes typically occur at depths ranging up to 12 miles. The shallow crustal source zone is characterized as being capable of generating earthquakes up to about magnitude 7.5. Large shallow crustal earthquakes are typically followed by a sequence of aftershocks. The largest known earthquakes associated with the shallow crustal source zone in Western Washington include an event on the Seattle Fault about A.D. 900 and the 1872 North Cascades earthquake. The Seattle Fault event was believed to have been magnitude 7 or greater (Johnson et al. 1999), and the 1872 North Cascades earthquake is estimated to have been between magnitudes 6.8 and 7.4. The location of the 1872 North Cascades earthquake is uncertain; however, recent research suggests that the earthquake’s intensity center was near the south end of Lake Chelan (Bakun et al. 2002).

The project site is located approximately 9 miles from the Seattle Fault Zone (United States Geological Survey [USGS] 2010). The Seattle Fault Zone is a 2- to 4-mile-wide, west-trending zone of three or more south-dipping reverse faults (Johnson et al. 1999). The Seattle Fault ruptured about 1,100 years ago and

May 17, 2019 | Page 4 caused broad uplift and subsidence on either side of the fault. The rate of recurrence on the Seattle Fault is thought to be on the order of thousands of years.

4.3. Surface Conditions

4.3.1. Fuel Facility and Vessel Maintenance Facility

The fuel facility and VMF sites are currently undeveloped forested areas. Vegetation consists of large trees with small to medium trees, brush and understory vegetation. The fuel facility site has an elevated north-south trending “lobe” type feature, with slopes downward in all directions from the high spot of about Elevation 115 feet; the slopes are steepest downward toward W. Shore Boulevard Road as shown in Figure 2A. The site grades at the VMF slope to the west from approximately Elevation 120 to 95 feet.

Existing utilities are present along the adjacent roadways including power, water and storm drains.

4.4. Subsurface Soil Conditions

Based on the geologic map, the fuel facility and VMF sites are anticipated to consist of ice contact deposits overlying glacial till. The subsurface soil conditions observed in the explorations were consistent with the anticipated geology as summarized below.

4.4.1. Fuel Facility Site

Boring GEI-8 and test pit TP-1 were completed at the proposed fuel facility area at the locations shown on Figure 2A. Boring GEI-8 encountered a surficial weathered till layer consisting of medium dense silty sand with gravel overlying unweathered glacial till consisting of very dense silty sand with gravel. The weathered horizon extended to approximately 5 feet bgs, and the glacial till extended to the bottom of the exploration at 31 feet bgs. Test pit TP-1 encountered a surficial forest duff layer, ice contact deposits consisting of dense silty gravel and sand overlying glacial till consisting of very dense silty sand with gravel. The ice contact deposits extended to approximately 3 feet below ground surface (bgs).

4.4.2. Maintenance Facility Site

Borings GEI-9 and GEI-10 and test pits TP-2 through TP-6 were completed at the proposed VMF area at the locations shown on Figure 2B. The explorations encountered soils consisting of surficial forest duff and ice contact deposits overlying glacial till soils. The ice contact deposits were extremely variable and graded from soft to medium stiff silt to dense gravel with sand, and typically extend to approximately 3 to 5 feet bgs; however, in explorations GEI-9, TP-2 and TP-3 the ice contact deposits extended to 7½ to 12 feet bgs.

TP-2 terminated in the ice contact unit. Glacial till consisting of very dense silty sand was observed to the depth explored of 30½ to 31½ feet bgs in GEI-9 and GEI-10.

4.5. Groundwater Conditions

4.5.1. Fuel Facility Site

Groundwater was not observed during drilling of boring GEI-8 or excavation of TP-1. We anticipate that perched groundwater may be present within the ice contact deposits because of the low permeability of glacial till soils. Perched groundwater levels will typically fluctuate as a function of season, precipitation and other factors.

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4.5.2. Vessel Maintenance Facility Site

Perched groundwater was typically observed near the transition between the ice contact deposits and glacial till at 1½ to 3½ feet bgs in test pits TP-2 through TP-6. Borings GEI-9 and GEI-10 encountered perched groundwater at 7½ to 10 feet bgs. The perched groundwater condition occurs within the ice contact deposits because of the underlying relatively low permeability of glacial till soils, or occasionally within sandy zones of the till. Perched groundwater levels will typically fluctuate as a function of season, precipitation and other factors.

5.0 CONCLUSIONS AND RECOMMENDATIONS

5.1. Summary of Geotechnical Considerations

A summary of the primary geotechnical considerations is provided below. The summary is presented for introductory purposes only and should be used in conjunction with the complete recommendations presented in this report.

■ We recommend a seismic Soil Profile Type C per ASCE 7-10 and the 2013 Unified Facilities Criteria

(UFC).

■ The regional groundwater table was not encountered in the borings completed for this upland portion of the project (which extended to approximately Elevation 73 feet). However, perched groundwater was observed in some of the explorations completed for this project. Temporary dewatering by means of local sumps and pumps within excavations is anticipated to be sufficient to remove groundwater seepage.

■ The UST excavation will encounter very dense glacial till. The excavation can be completed with conventional earthmoving equipment; however, large horsepower excavators will be much more efficient. The glacial till can create a “bathtub effect,” therefore, we recommend the USTs be anchored to a hold down slab.

■ The proposed VMF structure may be supported on shallow foundations bearing directly on dense native soils or structural fill placed over approved subgrade. Properly supported shallow foundations can be designed using an allowable soil bearing pressure of 4 kips per square foot (ksf).

■ We estimate that the VMF structure supported on shallow foundations could experience up to ¾ inch of settlement, with up to ½ inch of differential settlement.

■ The existing fill is suitable for support of on-grade floor slabs. We recommend that the upper 12 inches of exposed subgrade within the building footprint be recompacted to 95 percent of the maximum dry density (MDD) estimated in general accordance with ASTM D 1557.

■ We recommend that the pavement sections in areas subjected to trailer and truck traffic/parking areas consist of 4 inches of hot-mix asphalt (HMA) concrete overlying 6 inches of crushed surfacing base course (CSBC) and approved subgrade.

These geotechnical issues and other considerations are discussed in greater detail, and conclusions and recommendations for the geotechnical aspects of the project are presented in the following sections.

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5.2. Earthquake Engineering

5.2.1. Liquefaction

Liquefaction refers to the condition by which vibration or shaking of the ground, usually from earthquake forces, results in the development of excess pore pressures in saturated soils with subsequent loss of strength. In general, soils that are susceptible to liquefaction include very loose to medium dense, clean to silty sands that are below the water table.

Only perched groundwater was encountered at the site. Our analysis indicates that the soils that underlie the proposed USTs and building area have a low risk of liquefying because of the density and gradation of the soils underlying the building.

5.2.2. Other Seismic Hazards

Due to the location of the site and the site’s topography, the risk of adverse impacts resulting from seismically induced slope instability, differential settlement or surface displacement due to faulting is considered to be very low.

5.2.3. Seismic Design Information – Fuel Facility and Vessel Maintenance Facility

We understand that the proposed structures will be designed in accordance with the 2013 UFC (UFC 3-301-01). UFC 3-301-01 provides seismic design parameters near the center of the specific installation/city and states that ASCE 7-10 seismic data specific to the project site may be used with approval of the Authority Having Jurisdiction.

We recommend that the site is classified as Site Class C in accordance with the 2013 UFC 3-301-01 and ASCE 7-10. The UFC 3-301-01 provides seismic parameters at Department of Defense installations within the United States in Table E-3. The seismic parameters presented in Table E-3 in the UFC 3-301-01 vary somewhat from those determined by the USGS Seismic map web application for ASCE 7-10. Table 1 is based on UFC 3-301-01 Table E-3 while Table 2 is based on ASCE 7-10. For Table 1, note that the SS and S1 values in the tables are for Site Class B. The FA and FV values modify the parameters to the recommended Site Class D. The ASCE 7-10 site specific design spectra parameters for site class, adjusted maximum considered earthquake spectral response acceleration for short periods, SMS, and at 1-second period, SM1, design spectral response acceleration parameters at short periods, SDS, and at 1-second period, SD1, are presented in Table 2.

TABLE 1. UFC 3-301-01 PARAMETERS – FUEL FACILITY AND VESSEL MAINTENANCE FACILITY

2013 UFC 3-301-01 Parameters Recommended Value

Site Class C

Peak Ground Acceleration (PGA) (percent g) 66

Short Period Spectral Response Acceleration, SS (percent g) 157

1-second Period Spectral Response Acceleration, S1 (percent g) 61

Seismic Coefficient, FA 1.0

Seismic Coefficient, FV 1.3

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TABLE 2. ASCE 7-10 PARAMETERS – FUEL FACILITY AND VESSEL MAINTENANCE FACILITY

ASCE 7-10 Parameters Recommended Value

Site Class C

Adjusted maximum considered earthquake spectral response acceleration for short periods, SMS (percent g) 133

Adjusted maximum considered earthquake spectral response acceleration at 1-second period, SM1 (percent g) 70

Design spectral response acceleration at short periods, SDS (percent g) 89

Design spectral response acceleration at 1-second period, SD1 (percent g) 46

5.3. Shallow Foundations for VMF

Shallow foundations are planned for the VMF. The site is undeveloped and will require stripping the existing forest duff and vegetation. The exposed native soils will consist of variable ice contact deposits or glacial till.

Most of the ice contact deposits encountered in the explorations were medium dense to dense, with the exception of soft to medium stiff silt in TP-3 and TP-4. We recommend that the shallow foundations be founded on dense native soils or a maximum of 2 feet of densely compacted structural fill over the dense soils to minimize differential settlement across the building. Additionally, we recommend that the upper 12 inches of the exposed ice contact deposits be compacted to a minimum of 95 percent of the MDD estimated in general accordance with ASTM D 1557.

5.3.1. Allowable Bearing Pressures

For shallow foundations founded directly on dense native soils or on a maximum of 2 feet of structural fill over approved subgrade, an allowable bearing pressure of 4 ksf may be used for design. The allowable soil bearing pressure applies to the total of dead and long-term live loads and may be increased by up to one-third for wind or seismic loads.

Depending on the final grades, some silt/silty sand ice contact soil may require overexcavation and replacement with structural fill.

We recommend that the exterior footings be founded a minimum of 18 inches below the lowest adjacent grade. Interior footings should be founded a minimum of 12 inches below the top of the slab. Continuous wall footings should have a minimum width of 18 inches, and column footings should have a minimum width of 24 inches.

5.3.2. Static Settlement

The postconstruction static settlement of shallow footings supported as recommended above is estimated to be less than ¾ inch. Postconstruction differential settlement is estimated to be about ¼ to ½ inch between similarly loaded column footings or along 25 feet of wall foundations provided that unsuitable soils are removed and the maximum structural fill thickness is 2 feet. These static settlements should be expected to occur rapidly, essentially as the loads are applied.

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5.3.3. Lateral Resistance

Lateral foundation loads may be resisted by passive resistance on the sides of the footings and by friction on the base of the footings. For footings supported on dense native soils or structural fill, the allowable frictional resistance may be computed using a coefficient of friction of 0.4 applied to vertical dead-load forces.

The allowable passive resistance may be computed using an equivalent fluid density of 400 pounds per cubic foot (pcf). The allowable passive resistance is for horizontal soil conditions in front of the footing and is applicable provided that the footings are surrounded by structural fill. The structural fill should extend out from the face of the foundation element for a distance at least equal to three times the height of the element and be compacted to at least 95 percent of the MDD estimated in general accordance with ASTM D 1557. Passive pressure resistance should be calculated from the bottom of adjacent floor slabs or below a depth of 1 foot where the adjacent area is unprotected, as appropriate.

These allowable frictional resistance and passive resistance values include a factor of safety of about 1.5.

5.4. Slab-on-Grade Floor for VMF

The VMF building floor slab will be supported on-grade and subject to relatively high loads from transporting marine vessels. The native ice contact subgrade soils are generally suitable for support of floor slabs;

however, some soft silt/silty soils with organics were encountered which may require overexcavation.

5.4.1. Subgrade Preparation

The exposed subgrade should be evaluated after site grading is complete. Proof-rolling with heavy, rubber-tired construction equipment should be used for this purpose during dry weather and if access for this equipment is practical. Probing should be used to evaluate the subgrade during periods of wet weather or if access is not feasible for construction equipment. Ideally, we recommend that the upper 12 inches of the exposed subgrade be compacted to a minimum of 95 percent of the MDD estimated in accordance with ASTM D 1557. Disturbed areas should be recompacted if possible or removed and replaced with compacted structural fill. We recommend that any structural fill within the building footprint be compacted to the same density.

5.4.2. Design Parameters

Conventional slabs may be supported on-grade, provided the subgrade soils are prepared as recommended above. For slabs designed as a beam on an elastic foundation, a modulus of subgrade reaction of 150 pounds per cubic inch (pci) may be used for subgrade soils prepared as recommended.

We recommend that the slab-on-grade floors be underlain by a 6-inch-thick capillary break consisting of ¼- to 1½-inch, clean, crushed gravel with less than 10 percent sand and 3 percent silt. Where moisture-sensitive floor coverings or moisture-sensitive equipment will be present above the slab-on-grade, we recommend installation of a vapor barrier consisting of 10-mil plastic sheeting below the slab to reduce the potential for migration of moisture. It may also be prudent to apply a sealer to the slab to further retard the migration of moisture through the slab. The contractor should exercise care to avoid damaging the vapor barrier during slab construction.

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Provided that loose soil is removed and the subgrade is prepared as recommended, we estimate that postconstruction static settlement of the slabs-on-grade will be minimal provided that soft/loose material is overexcavated and replaced with structural fill.

5.5. Drainage Considerations

5.5.1. Perimeter Footing Drains for Buildings

We recommend that the VMF perimeter building footings be constructed with conventional footing drains because of the perched groundwater condition. The drains should consist of 4-inch-diameter perforated collector pipe enveloped within a minimum thickness of 6 inches of gravel backfill for drains conforming to Section 9-03.12(4) of the 2018 Washington State Department of Transportation (WSDOT) Standard Specifications. The gravel backfill for drains should be wrapped with a non-woven geotextile filter fabric meeting the requirements of construction geotextile for underground drainage in Section 9-33 of the 2018 WSDOT Standard Specifications. The pipe should not be wrapped in a geotextile fabric.

We recommend using either heavy-wall solid pipe (SDR-35 polyvinyl chloride [PVC]) or rigid corrugated polyethylene pipe (ADS N-12, or equivalent) for the collector pipe. We recommend against using flexible tubing for footing drainpipe.

The pipes should be laid with a minimum slope of ½ percent and discharge into the stormwater collection system to convey the water to a suitable disposal location. The pipe installations should include a cleanout riser with cover located at the upper end of each pipe run. Permanent drainage systems should intercept surface water runoff at the top and/or bottom of cut and fill slopes to prevent the water from flowing in an uncontrolled manner across the site.

5.5.2. Other Drainage Considerations

We recommend that finished ground surface adjacent to new buildings be sloped so that surface water runoff flows away from the structure. Pavement surfaces should also be sloped such that surface water is collected and routed to suitable discharge points.

The building should be designed with roof drains. Roof drains should be tightlined to an appropriate discharge point. The roof drains should not be connected to the perimeter footing or retaining wall drains.

5.6. Stormwater Infiltration Considerations

The Washington State Department of Ecology 2014 Stormwater Management Manual for Western Washington (2014 SMMWW) provides guidelines for determining infiltration rates for stormwater systems.

The SMMWW allows for use of grain size analyses to determine infiltration rates on sites with soils unconsolidated by glacial advance. The upland sites are underlain by ice contact deposits and glacial till and therefore the grain size method is not appropriate. The fines content of the glacial till ranged from 19 to 35. Due to the density, high fines content and low permeability of the underlying glacial till layer, the site soils are considered unsuitable for infiltration. If necessary, to evaluate feasibility, we recommend a preliminary infiltration rate of not more than 0.1 inches per hour be used for evaluation of infiltration facilities in the native glacial till soils. We anticipate that bioswales and detention are the most likely stormwater management strategies.

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5.7. Earthwork

5.7.1. Excavation Considerations

Dense ice contact deposits, and dense glacial till were observed in the explorations within the anticipated excavation depths with the exception of soft to medium stiff silt in TP-3 and TP-4. We anticipate that these soils can be excavated with conventional excavation equipment, such as trackhoes or dozers. Although not encountered in the explorations, boulders are frequently encountered in glacial soils. The earthwork contractor should be prepared to encounter boulders at this site. Large horsepower excavators will be more efficient excavating at depth in the very dense glacial till.

5.7.2. Stripping, Clearing and Grubbing

Vegetation including the root mass and organic-rich topsoil should be stripped and removed from the pavement and building areas. Our test pits typically encountered 6 inches of forest duff. For preliminary planning we recommend a stripping depth of 8 inches to account for site variability.

5.7.3. Subgrade Preparation

The exposed subgrade should be evaluated after site grading is complete. The majority of the site should expose dense granular materials. Exposed granular materials should be compacted and proof rolled.

Proof-rolling with heavy, rubber-tired construction equipment should be used for this purpose during dry weather. Probing should be used to evaluate the subgrade during periods of wet weather. Soft areas noted during proof-rolling or probing should be excavated and replaced with compacted structural fill. Soft to medium stiff silt is anticipated to be exposed in some areas. Equipment should be kept off silty subgrades to the extent possible. Additionally, we recommend that the exposed subgrade in the footing and building areas be compacted to a minimum of 95 percent of the MDD estimated in general accordance with

ASTM D 1557.

5.7.4. Erosion and Sedimentation Control

Potential sources or causes of erosion and sedimentation depend upon construction methods, slope length and gradient, amount of soil exposed and/or disturbed, soil type, construction sequencing and weather.

Implementing an erosion and sedimentation control plan will reduce the project impact on erosion-prone areas. The plan should be designed in accordance with applicable city, county and/or Navy standards. The plan should incorporate basic planning principles that include:

■ scheduling grading and construction to reduce soil exposure;

■ retaining existing vegetation whenever feasible;

■ revegetating or mulching denuded areas;

■ directing runoff away from denuded areas and away from the shoreline;

■ reducing the length and steepness of slopes with exposed soils;

■ decreasing runoff velocities;

■ preparing drainage ways and outlets to handle concentrated or increased runoff;

■ confining sediment to the project site; and

■ inspecting and maintaining control measures frequently.

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In addition, we recommend that slope surfaces in exposed or disturbed soil be restored so that surface runoff does not become channeled. We recommend that graded and disturbed slopes be tracked in place with the equipment running perpendicular to the slope contours so that the track grouser marks provide a texture to help resist erosion. Some sloughing and raveling of slopes with exposed or disturbed soil should be expected.

Temporary erosion protection should be used and maintained in areas with exposed or disturbed soils to help reduce erosion and reduce transport of sediment to adjacent areas and receiving waters. Permanent erosion protection should be provided by reestablishing vegetation using hydroseeding or landscape planting.

Until the permanent erosion protection is established and the site is stabilized, site monitoring should be performed by qualified personnel to evaluate the effectiveness of the erosion control measures and to repair and/or modify them as appropriate. Provisions for modifications to the erosion control system based on monitoring observations should be included in the erosion and sedimentation control plan.

5.7.5. Structural Fill

5.7.5.1. Materials

Materials used to construct building pads, driveways and parking areas are classified as structural fill for the purpose of this report. Structural fill material quality varies depending upon its use, as described below:

■ Structural fill placed as crushed surfacing base course below sidewalks and pavements should meet the requirements of crushed rock base course in conformance with Section 9-03.9(3) of the WSDOT Standard Specifications.

■ Structural fill placed within 18 inches behind retaining walls, for drainage purposes, should meet the requirements for gravel backfill for walls in Section 9-03.12(2) of the WSDOT Standard Specifications.

■ Structural fill placed as utility trench backfill, to support structures or placed in sidewalk or parking areas should meet the criteria for common borrow as described in Section 9-03.14(3) of the WSDOT Standard Specifications. Common borrow will be suitable for use as structural fill during dry weather conditions only. If structural fill is placed during wet weather, the structural fill should consist of gravel borrow as described in Section 9-03.14(1) of the WSDOT Standard Specifications, with the additional restriction that the fines content be limited to no more than 5 percent.

■ Structural fill placed within 6 inches of perimeter foundation or wall drains (drainage zone aggregate) should meet the requirements for gravel backfill for drains in conformance with Section 9-03.12(4) of the WSDOT Standard Specifications.

■ Structural fill used for capillary break material below slabs should consist of 1½-minus clean crushed gravel with negligible sand or silt in conformance with Section 9-03.1(4)C, grading No. 57 of the WSDOT Standard Specifications.

5.7.5.2. On-site Soils

Some of the silt/silty soils on the sites are moisture-sensitive and generally have natural moisture contents higher than the anticipated optimum moisture content for compaction. These soils might require moisture conditioning in order to meet the required compaction criteria during dry weather conditions and will not be suitable for reuse during wet weather. The gravelly ice contact soils may be suitable for reuse as

May 17, 2019 | Page 12 structural fill and could be approved by the Engineer during construction. Most of the fill soils required for the project have specific gradation requirements that must be met

5.7.5.3. Fill Placement and Compaction Criteria

Structural fill should be mechanically compacted to a firm, non-yielding condition. In general, structural fill should be placed in loose lifts not exceeding 12 inches in thickness. The actual lift thickness will depend on the structural fill material used and the type and size of compaction equipment. Each lift should be conditioned to near the optimum moisture content and compacted to the specified density before placing subsequent lifts.

Structural fill should be compacted to the following criteria:

■ Structural fill placed below foundations should be compacted to 95 percent of the MDD estimated in general accordance with ASTM D 1557.

■ Structural fill placed behind below-grade or retaining walls, within a distance equal to the height of the wall, should be compacted to between 90 and 92 percent of the MDD estimated in general accordance with ASTM D 1557. Care should be taken when placing fill near the face of walls to avoid over-compaction and, hence overstressing the walls.

■ Structural fill placed in new pavement or hardscape areas, including utility trench backfill, should be compacted to 95 percent of the MDD estimated in general accordance with ASTM D 1557.

■ Structural fill placed for permanent slopes should be compacted to 90 percent of the MDD estimated in general accordance with ASTM D 1557.

5.7.5.4. Weather Considerations

The on-site soils contain a sufficient percentage of fines (silt and clay) and are moisture-sensitive. When the moisture content of these soils is more than a few percent above the optimum moisture content, these soils become muddy and unstable, and operation of equipment on these soils is difficult. Additionally, disturbance of near-surface soils should be expected if earthwork is completed during periods of wet weather. During wet weather, we recommend the following:

■ The ground surface in and around the work area should be sloped so that surface water is directed away from the work area.

■ The ground surface should be graded such that areas of ponded water do not develop.

■ The contractor should take measures to prevent surface water from collecting in excavations and trenches.

■ Measures should be implemented to remove surface water from the work area.

■ Earthwork activities should not take place during periods of heavy precipitation.

■ Slopes with exposed soils should be covered with plastic sheeting.

■ The contractor should take necessary measures to prevent soils to be used as fill from becoming wet or unstable. These measures may include covering stockpiles with plastic sheeting, sumps with pumps, and grading. The site soils should not be left uncompacted and exposed to moisture. Sealing the surficial soils by rolling with a smooth-drum roller prior to periods of precipitation will help reduce the extent that these soils become wet or unstable.

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■ Construction traffic should be restricted to specific areas of the site, preferably areas that are surfaced with materials not susceptible to wet weather disturbance.

■ Construction activities should be scheduled so that the length of time that soils are left exposed to moisture is reduced to the extent practicable.

5.7.6. Temporary Slopes

Temporary slopes may be used around the site during construction. We recommend that temporary slopes be inclined no steeper than 1½H:1V (horizontal to vertical) for the ice contact deposits, for deeper excavations into the glacial till we recommend no steeper than 1H:1V. Flatter slopes may be necessary if seepage is present on the face of the cut slopes or if localized sloughing occurs.

The above cut slope recommendations apply to fully dewatered conditions. Flatter slopes may be necessary if seepage is present on the cut face or if localized sloughing occurs. For open cuts at the site we recommend that:

■ no traffic, construction equipment, stockpiles or building supplies be allowed at the top of the cut slopes within a distance of at least 5 feet from the top of the cut;

■ construction activities be scheduled so that the length of time the temporary cut is left open is reduced to the extent practical; and

■ the general condition of the slopes should be observed periodically by a geotechnical engineer to confirm adequate stability.

Because the contractor has control of the construction operations, the contractor should be made responsible for the stability of cut slopes, as well as the safety of the excavations. Shoring and temporary slopes must conform to applicable local, state and federal safety regulations.

5.7.7. Permanent Slopes

We recommend that permanent cut and fill slopes be constructed no steeper than 2H:1V. To achieve uniform compaction, we recommend that fill slopes be overbuilt slightly (1 to 2 feet) and subsequently cut back to expose properly compacted fill. We recommend that the finished slope faces be compacted by track walking with the equipment running perpendicular to the slope contours so that the track grouser marks provide a texture to help resist erosion.

To reduce erosion, newly constructed slopes should be planted or hydroseeded shortly after completion of grading. Until the vegetation is established, some sloughing and raveling of the slopes should be expected.

This may require localized repairs and reseeding. Temporary covering, such as clear heavy plastic sheeting, jute fabric, loose straw or excelsior matting should be used to protect the slopes during periods of rainfall.

5.8. Site Retaining Walls

The south side and southwest corner of the site will require retaining walls on the order of 8 to 10 feet tall.

The walls will be constructed as conventional cast-in-place (CIP) concrete walls.

For walls that are free to yield at the top at least 0.1 percent of the height of the wall, soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing. Assuming that the walls are

May 17, 2019 | Page 14 backfilled, and drainage is provided as outlined in the following sections, we recommend that yielding walls supporting horizontal backfill be designed using an equivalent fluid density of 35 pcf (triangular distribution), while non-yielding walls supporting horizontal backfill be designed using an equivalent fluid density of 55 pcf (triangular distribution). For seismic loading conditions, a rectangular earth pressure equal to 7H psf (where H is the height of the wall in feet) should be added to the active/at-rest pressures. A traffic surcharge pressure of 75 psf should also be included in the design, where traffic will be present above the wall. Other surcharge loading should be applied as appropriate.

Lateral resistance for conventional CIP walls can be provided by frictional resistance along the base of the wall and passive resistance in front of the wall. The walls must be founded on dense native soils (transitional glacial deposits or glacial outwash) or compacted structural fill placed over these soils. The allowable frictional resistance may be computed using a coefficient of friction of 0.4 applied to vertical dead-load forces. The allowable passive resistance may be computed using an equivalent fluid density of 300 pcf.

The above coefficient of friction and passive equivalent fluid density values incorporate a factor of safety of about 1.5.

Potions of the wall will have a 3H:1V toe slope. Where the 3H:1V slope is present, we recommend that the walls be embedded an additional 1.5 feet to provide adequate passive resistance.

The above soil pressures assume that wall drains will be installed to prevent the buildup of hydrostatic pressure behind the walls, as discussed below.

5.8.1. Drainage

Positive drainage should be provided behind CIP retaining walls by placing a minimum 18-inch-wide zone of free draining backfill directly behind the wall. The drainage material should meet the requirements for gravel backfill for walls in Section 9-03.12(2) of the WSDOT Standard Specifications. A perforated drainpipe should be placed near the base of the wall to provide the drainage. The drainpipe should consist of 4-inch-diameter perforated collector pipe enveloped within a minimum thickness of 6 inches of gravel backfill for drains in conformance with Section 9-03.12(4) of the WSDOT Standard Specifications. The gravel backfill should be wrapped with a geotextile filter fabric meeting the requirements of construction geotextile for underground drainage (Section 9-33 of the WSDOT Standard Specifications).

We recommend using either heavy-wall solid pipe (SDR-35 PVC) or rigid corrugated polyethylene pipe (ADS N-12 or equivalent) for the collector pipe. We recommend against using flexible tubing for footing drainpipe.

The pipes should be laid with a minimum slope of ½ percent and discharge into an appropriate outfall.

The pipe installations should include a cleanout riser with cover located at the upper end of each pipe run.

We recommend that the cleanouts be covered and be placed in flush-mounted utility boxes or monuments.

5.8.2. Construction Considerations

In order to prevent overstressing the concrete retaining walls and causing bulging or rotation, we recommend that the structural fill placed against the back of the wall be compacted within the range of 90 to 92 percent of the MDD estimated in accordance with ASTM D 1557. Backfill should be placed after the concrete has had sufficient time to cure and develop the necessary strength.

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5.9. Below-Grade Vaults

The following recommendations should be used for the design of below-grade vaults for this project.

We anticipate that small electrical or other utility vaults might be required as a part of this project. Assuming that the vaults will be 3 to 5 feet below existing grades, we anticipate that dense native soils will be exposed at the bottom of vault excavations.

5.9.1. Foundation Support

The vaults may be supported on shallow foundations in accordance with recommendations provided in Section 5.3. If desired, the vaults be supported on a crushed rock leveling pad. The crushed rock should be tamped or rolled to the extent possible.

5.9.2. Lateral Earth Pressures

We recommend that vaults be designed for an equivalent fluid density of 55 pcf for a level ground surface behind the vault. Surcharge and seismic pressures should be added as described below.

Consideration should also be given to designing the below-grade walls for seismic earth pressures. Seismic earth pressures should be determined using a rectangular distribution of 7H in psf, where H is the wall height.

If vehicles can approach the tops of vault walls to within one-half the height of the wall, or if materials will be stockpiled adjacent to the vault, a surcharge should be added to the wall pressure. For parking areas, the traffic surcharge can be approximated using a lateral surcharge of 55 psf on the vault wall. For truck parking areas and access driveway areas, a lateral surcharge of 110 psf on the vault wall should be used.

Other surcharge loads should be considered on a case-by-case basis.

The soil pressure available to resist lateral loads is a function of the frictional resistance against the vault base and the passive resistance that can develop on the face of below-grade elements of the structure as those elements move horizontally into the soil. For a vault foundation bearing on compacted crushed rock or quarry spalls prepared as recommended in this report, an allowable coefficient of sliding friction of 0.4 between concrete and the compacted crushed rock or quarry spalls. The allowable passive resistance on the face of embedded foundation elements may be computed using an equivalent fluid density of 300 pcf.

5.9.3. Construction Considerations

In order to prevent overstressing the concrete retaining walls and causing bulging or rotation, we recommend that the structural fill placed against the back of the wall be compacted within the range of 90 to 92 percent of the MDD estimated in accordance with ASTM D 1557. Backfill should be placed after the concrete has had sufficient time to cure and develop the necessary strength.

If critical utilities tie into the vault we suggest that flexible utility connections be considered for use to accommodate possible settlement of the vault.

5.10. UST Excavation and Tank Buoyancy

Provided that the base of the UST excavation is lower than Elevation 85 feet, base of the excavation will be completed in very dense, low permeability glacial till. The regional water table was not encountered in our explorations. While no groundwater seepage was encountered in the explorations at the time of the site

May 17, 2019 | Page 16 evaluation, a seasonal perched groundwater can occur (and was observed at the VMF site). Deep excavations into glacial till that are backfilled with granular soils can develop a “bathtub condition” and become saturated. We assume that providing permanent drainage is not desirable because of monitoring systems. Therefore, we recommend that the USTs be anchored to a hold down slab constructed over the subgrade. After the excavation is completed, we recommend placing a minimum 4- to 6-inch layer of crushed rock similar in gradation to capillary break described in Section 5.7.5.1 above.

5.11. Pavement Recommendations

5.11.1. Subgrade Preparation

We recommend that the subgrade soils in new pavement areas be prepared and evaluated as described above in the “Earthwork” section of this report. In areas where existing granular soils are encountered at subgrade elevation and are not disturbed by construction activities during the wet season, we recommend that the upper 12 inches of the existing site soils be compacted to at least 95 percent of the MDD estimated…