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Geotechnical Engineering Report
Grand Stand Improvements
North Mason High School
300 East Campus Drive
Belfair, Washington RECEIVED
MAY 0 8 2019
April 19, 2019 615 W.Alder Street
prepared for:
North Mason School District
c/o Erickson McGovern Architects
Attention: Ray Mow, Principal Architect
101 E 26th Street, Suite 300
Tacoma, Washington 98421
prepared by:
Migizi Group, Inc.
PO Box 44840
Tacoma,Washington 98448
(253) 537-9400
MGI Project P1584-T19
TABLE OF CONTENTS
Page No.
1.0 SITE AND PROJECT DESCRIPTION.............................................................................................. 1
2.0 EXPLORATORY METHODS............................................................................................................2
2.1 Auger Boring Procedures......................................................................................................3
3.0 SITE CONDITIONS............................................................................................................................3
3.1 Surface Conditions.................................................................................................................3
3.2 Soil Conditions.......................................................................................................................4
3.3 Groundwater Conditions......................................................................................................5
3.4 Seismic Conditions.................................................................................................................5
3.5 Liquefaction Potential...........................................................................................................6
3.6 Infiltration Conditions...........................................................................................................6
4.0 CONCLUSIONS AND RECOMMENDATIONS............................................................................6
4.1 Site Preparation......................................................................................................................7
4.2 Spread Footings......................................................................................................................9
4.2.1 Concrete Caissons................................................................................................10
4.3 Slab-On-Grade Floors..........................................................................................................11
4.4 Asphalt Pavement................................................................................................................12
4.5 Retaining Walls....................................................................................................................13
4.6 Structural Fill........................................................................................................................14
5.0 RECOMMENDED ADDITIONAL SERVICES.............................................................................15
6.0 CLOSURE...........................................................................................................................................16
List of Tables
Table 1. Approximate Locations and Depths of Explorations.............................................................................2
List of Figures
Figure 1. Topographic and Location Map
Figure 2. Site and Exploration Plan
APPENDIX A
Soil Classification Chart and Key to Test Data..................................................................................................A-1
Logsof Auger Borings B-1 through B-3....................................................................................................A-2...A-4
I
MIGIZI GROUP, INC.
PO Box 44840 PHONE (253)537-9400
Tacoma, Washington 98448 FAX (253)537-9401
April 19, 2019
North Mason School District
c/o Erickson McGovern Architects
101 E 261h Street, Suite 300
Tacoma,Washington 98421
Attention: Ray Mow,Principal Architect
Subject: Geotechnical Engineering Report
Grand Stand Improvements
North Mason High School
300 East Campus Drive
Belfair,Washington
MGI Project P1584-T19
Dear Mr. Mow:
Migizi Group,Inc.(MGI)is pleased to submit this report describing the results of our geotechnical
engineering evaluation of the new improvements planned at the North Mason High School Main
Campus in Belfair, Washington. The undersigned, while working under the umbrella of E3RA,
Inc.,previously prepared a Geotechnical Engineering Report,dated October 25,2013,addressing the
new high school development. Furthermore,we prepared numerous supplementary soil reports
as the scope of the project expanded.
This report has been prepared for the exclusive use of North Mason School District, and their
consultants, for specific application to this project, in accordance with generally accepted
geotechnical engineering practice.
1.0 SITE AND PROJECT DESCRIPTION
The proposed work area is located towards the south end of the large North Mason School District
property, immediately northeast of the intersection between state routes WA-3 and WA-302 in
Belfair,Washington, as shown on the enclosed Topographic and Location Map (Figure 1).
Improvement plans call for the construction of a new covered grand stand on the south side of
the existing high school football field. The new grand stand will replace the existing home grand
stand and will be large enough to accommodate the home team while the existing grand stand
on the north side of the field will accommodate visiting teams.
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Construction of the new grand stand will also require a new access road along the southeast side
of the existing field, south of the existing fence to accommodate walking and delivery access.
Given the sloped conditions of this region, it is our understanding that the majority of the
roadway will be constructed of fill material,supported by an approximately 8 foot high modular
block retaining wall. Storm water collected from the new improvements will be infiltrated below
this road using pervious pavement, if possible, and through diversion to an existing
retention/detention pond to the south.
2.0 EXPLORATORY METHODS
We explored surface and subsurface conditions at the project site on March 4, 2019. Our
exploration and evaluation program comprised the following elements:
• Surface reconnaissance of the site;
• Three auger boring explorations (designated B-1 through B-3), advanced adjacent
to proposed improvements on March 4,2019;and
• A review of published geologic and seismologic maps and literature.
Table 1 summarizes the approximate functional locations and termination depths of our
subsurface explorations,and Figure 2 depicts their approximate relative locations. The following
sections describe the procedures used for excavation of auger borings.
TABLE 1
APPROXIMATE LOCATIONS AND DEPTHS OF EXPLORATIONS
Termination
Exploration Functional Location Depth
(feet)
B-1 West side of existing bleachers,grass clearing above pond slope 311/2
B-2 East side of bleachers,inside pedestrian walkway outside of track 311/2
B-3 East end of track,inside of pedestrian walkway outside of track 111/2
The specific numbers and locations of our explorations were selected in relation to the existing
site features, under the constraints of surface access, underground utility conflicts, and budget
considerations.
It should be realized that the explorations performed and utilized for this evaluation reveal
subsurface conditions only at discrete locations across the project site and that actual conditions
in other areas could vary. Furthermore, the nature and extent of any such variations would not
become evident until additional explorations are performed or until construction activities have
begun. If significant variations are observed at that time,we may need to modify our conclusions
and recommendations contained in this report to reflect the actual site conditions.
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2_1 Auger Boring Procedures
Our exploratory borings were advanced through the soil with a hollow-stem auger,using a truck-
mounted drill rig operated by an independent drilling firm working under subcontract to MGI.
An engineering geologist from our firm continuously observed the borings,logged the subsurface
conditions, and collected representative soil samples. All samples were stored in watertight
containers and later transported to a laboratory for further visual examination. After the borings
were completed, the borehole was backfilled with bentonite chips.
Throughout the drilling operation, soil samples were obtained at intervals of 21/2 to 5 feet by
means of the Standard Penetration Test (SPT) per American Society for Testing and Materials
(ASTM:D-1586). This testing and sampling procedure consist of driving a standard 2-inch-
diameter steel split-spoon sampler 18 inches into the soil with a 140-pound hammer free-falling
30 inches. The number of blows required to drive the sampler through each 6-inch interval is
counted, and the total number of blows struck during the final 12 inches is recorded as the
Standard Penetration Resistance, or "SPT blow count". If a total of 50 blows are struck within
any 6-inch interval, the driving is stopped, and the blow count is recorded as 50 blows for the
actual penetration distance. The resulting Standard Penetration Resistance values indicate the
relative density of granular soils and the relative consistency of cohesive soils.
The enclosed boring logs (Appendix A) describe the vertical sequence of soils and materials
encountered in the borings, based primarily on our field classifications and supported by our
subsequent laboratory examination and testing. Where a soil contact was observed to be
gradational, our logs indicate the average contact depth. Where a soil type changed between
sample intervals, we inferred the contact depth. Our logs also graphically indicate the blow
count, sample type, sample number, and approximate depth of each soil sample obtained from
the boring, as well as any laboratory tests performed on these soil samples. If any groundwater
was encountered in the borehole,the approximate groundwater depth is depicted on the boring
log. Groundwater depth estimates are typically based on the moisture content of soil samples,
the wetted height on the drilling rods, and the water level measured in the borehole after the
auger has been extracted. The soils were classified visually in general accordance with the system
described in Figure A-1, which includes a key to the exploration logs. Summary logs of our
exploration is included as Figures A-2 through A-4.
3.0 SITE CONDITIONS
The following sections present our observations, measurements, findings, and interpretations
regarding, surface, soil, groundwater, and infiltration conditions.
3.1 Surface Conditions
As previously indicated, the proposed work area is located towards the south end of the large
North Mason School District property, immediately northeast of the intersection between state
routes WA-3 and WA-302 in Belfair, Washington. The aforementioned North Mason School
District property is comprised of three tax parcels, encompasses ± 73.5 acres, and contains the
new high school campus, Hawkins Middle School, a NMSD administration building, various
tennis courts,practice fields,outbuildings,and towards its southwest corner,the primary athletic
field, which ultimately hosts high school football games.
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Existing bleachers are located along the south side of the track and field in the proposed footprint
of the new grand stand separated from the stadium running track by a chain-link fence.
Immediately outside of this fence to the south is an existing infiltration/detention pond, which
has been utilized by the North Mason School District property since its inception,with the outfall
to the pond traveling directly beneath the aforementioned bleachers. The pond bottom is located
12 to 15 feet below ground levels adjacent to the bleachers,with the moderate to steep slope which
marks this transition, starting just outside of the fence line.
Vegetation within the school campus, adjacent to the proposed work area, generally consists of
lawn grasses and ornamental trees and/or shrubs. Adjacent to the pond area,and further regions
outside of the fence line and west of the track and field, are occupied by a mature woodland of
fir, cedar, alder and maples, with an understory of ferns, salal, blackberry bushes, and other
brush.
3.2 Soil Conditions
Subsurface conditions were observed in the vicinity of the proposed improvements through the
advancements of three auger boring explorations;two adjacent to the proposed grand stand,and
one further east along the alignment of the supplementary access road.
Auger boring exploration B-1 was performed along the west side of the existing bleachers, in a
small clearing directly above the slope break down to the pond. In this region, underlying a
surface mantle of sod and topsoil, we encountered upwards of 14 feet of fill soils comprised of
fine to medium sand with silt in a very loose to medium dense condition. This material
transitioned to native outwash deposits consisting of medium dense to dense sands and gravels,
which extended through a depth of 30 feet. From a depth of 30 to 311/2 feet,the termination depth
for this exploration,we encountered glacial till soils comprised of very dense gravelly silty sand.
Auger boring explorations B-2 and B-3 were advanced along the east side of the bleachers and
further inland than the previous boring in order to miss existing utilities. No fill was encountered
in either boring. These explorations observed a similar soil sequence to that of B-1,with outwash
deposits overlying glacial till soils. The depth to this soil contact in the vicinity of B-2 was
approximately 25 feet. Auger boring B-3 encountered solely outwash deposits, extending only
111/2 feet below existing grade.
Belfair,and the larger Puget Sound area in general,has been glaciated a number of times over the
last 2.4 million years. The most recent of these glacial events, the Vashon Stade of the Fraser
Glaciation,receded from this region approximately 13,500 years ago. The majority of near surface
soils encountered within the Belfair area are either directly associated with or have been
physically altered by the Vashon glacial event. Outwash soils generally consist of variably
consolidated sands and gravels deposited along meltwater streams/rivers during the latter end
of a glacial event, during an extended period of ablation, and regression of glacial ice. Glacial till
is typically described as being a compact, coherent mixture of gravel, silt, clay and sand-sized
clasts deposited along the base of glacial ice during a period of localized advancement. This
material is generally encountered in a compact relative consistency given the fact that it was
overridden by the ice mass shortly after deposition.
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In the Geologic Map of the Belfair 7.5-minute Quadrangle, Mason, Kitsap, and Pierce Counties,
Washington, as prepared by the Washington State Department of Natural Resources (WSDNR)
(2009),the project site is mapped as containing Qgic, or Vashon-aged glacial ice-contact deposits.
As the name would indicate, these deposits formed in the presence of meltwater alongside ice,
generally towards the end of the glaciation, and is thus commonly accompanied by stagnant ice
features, such as kettles and less orderly hummocky topography, eskers and subglacial or
subaerial outwash channels. Deposits generally consist of sands and gravels,minor silt and clay
beds, with intermittent till showing regions of localized advanced during an extended period of
glacial ice regression.
The National Cooperative Soil Survey(NCSS)for the Mason County area classifies soils onsite as
consisting of lb, or Indianola loamy sand, 5 to 15 percent slopes. This soil series is generally
associated with recessional outwash deposits. Our field observations generally correspond with
site classification prepared by the WSDNR and NCSS.
The enclosed exploration logs (Appendix A) provide a detailed description of the soil strata
encountered in our subsurface explorations.
3.3 Groundwater Conditions
At the time of our reconnaissance and subsurface explorations (March 4, 2019), we were unable
to clearly identify localized groundwater levels, given the fact that we were utilizing the mud-
rotary drilling method, which obscures the moisture content of split-spoon samples. However,
we anticipate that the depth to groundwater across the proposed work area likely mirrors the
elevation of water levels within the infiltration/detention pond along the south side of the track
and field. Additionally,groundwater levels fluctuate with localized geology and precipitation.
3.4 Seismic Conditions
Based on our analysis of subsurface exploration logs and our review of published geologic maps,
we interpret the onsite soil conditions to generally correspond with site class D, as defined by
Table 20.3-1 in ASCE 7, per the 2015 International Building Code(IBC).
Using 2015 IBC information on the USGS Design Summary Report website,Risk Category I/II/III
seismic parameters for the site are as follows:
Ss= 1.438 g SMs =1.438 g SDs=0.959
Si=0.576 g SMi=0.864 g SD1=0.576
Using the 2015 IBC information,MCER Response Spectrum Graph on the USGS Design Summary
Report website, Risk Category I/II/III, S.at a period of 0.2 seconds is 1.44g and S.at a period of
1.0 seconds is 0.58g.
The Design Response Spectrum Graph from the same website, using the same IBC information
and Risk Category,S.at a period of 0.2 seconds is 0.96g and S.at a period of 1.0 seconds is 0.58g.
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3_5 Liquefaction Potential
Liquefaction is a sudden increase in pore water pressure and a sudden loss of soil shear strength
caused by shear strains, as could result from an earthquake. Research has shown that saturated,
loose,fine to medium sands with a fines(silt and clay)content less than about 20 percent are most
susceptible to liquefaction. Subsurface explorations performed within the confines of the project
area did not encounter any saturated, loosely consolidated sandy soils, and we interpret the
potential for seismically induced liquefaction to be low within the subject property.
3.6 Infiltration Conditions
As indicated in the onset of this report, preliminary design concepts have the new access road
servicing the proposed grand stands being constructed utilizing pervious pavement materials, if
feasible, and infiltrating runoff water into the subgrade. The proposed alignment for this
roadway is along the sloped region directly above the pond, southeast of the existing bleachers.
Our explorations adjacent to the slope break indicate that the southern margin of the existing
athletic field was constructed utilizing fill soils during initial development of the site.
Furthermore, substantial fill soils will be utilized in developing the subgrade for the roadway. It
is our understanding that governing design manuals do not allow infiltration within fill soils.
Given such,we do not see infiltration as being feasible for this project and recommend collecting
and diverting stormwater produced by the proposed development and dispersing into the
existing pond system towards the south end of the project area.
4.0 CONCLUSIONS AND RECOMMENDATIONS
Improvement plans call for the construction of a new covered grand stand on the south side of
the existing high school football field. The new grand stand will replace the existing home grand
stand and will be large enough to accommodate the home team while the existing grand stand
on the north side of the field will accommodate visiting teams.
Construction of the new grand stand will also require a new access road along the southeast side
of the existing field, south of the existing fence to accommodate walking and delivery access.
Given the sloped conditions of this region, it is our understanding that the majority of the
roadway will be constructed of fill material, supported by an approximately 8 foot high modular
block retaining wall. Storm water collected from the new improvements will be infiltrated below
this road using pervious pavement, if possible, and through diversion to an existing
retention/detention pond to the south. We offer the following recommendations:
• Feasibility: Based on our field explorations, research and evaluations, the
proposed structures and pavements appear feasible from a geotechnical
standpoint.
• Foundation Options: Foundation elements should be constructed on medium
dense or denser undisturbed native soils, or on structural fill bearing pads that
extend down to medium dense or denser native soils. We anticipate that adequate
bearing soils will be encountered at relatively shallow depths (less than 3 feet)
across much of the proposed building area. Recommendations for Spread
Footings are provided in Section 4.2,and recommendations for Concrete Caissons
are provided in Section 4.2.1.
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• Pavement Sections: Given the topography of the proposed access road alignment,
the majority of the road subgrade will be constructed utilizing fill material. We
recommend a conventional pavement section comprised of an asphalt concrete
pavement over a crushed rock base course over a properly prepared (compacted)
subgrade or a granular subbase, depending on subgrade conditions during
pavement subgrade preparation. All structural fill should be compacted
according to our recommendations given in the Structural Fill section.
Specifically, the upper 2 feet of soils underlying pavement section should be
compacted to at least 95 percent(based on ASTM D-1557),and all soils below 2 feet
should be compacted to at least 90 percent.
All soil subgrades should be thoroughly compacted, then proof-rolled with a
loaded dump truck or heavy compactor. Any localized zones of yielding subgrade
disclosed during this proof-rolling operation should be over-excavated to a depth
of 12 inches and replaced with a suitable structural fill material.
• Infiltration Conditions: Existing soil conditions and proposed grading activities
dictate that the proposed access road servicing the new grand stands will be
constructed utilizing, or directly underlain, by fill material. It is our
understanding that governing design manuals do not allow infiltration within fill
soils. Given such, we do not see infiltration as being feasible for this project and
recommend collecting and diverting stormwater produced by the proposed
development and dispersing into the existing pond system towards the south end
of the project area.
• Geologic Hazards: During our site reconnaissance, advancement of subsurface
explorations, and general evaluation of the proposed development, we did not
observe any erosional, landslide, seismic, settlement, or other forms of geologic
hazards within the subject property. Given this fact, we recommend that no
buffers,setbacks,or other forms of site restraints be implemented to address these
potential hazards.
The following sections of this report present our specific geotechnical conclusions and
recommendations concerning site preparation, spread footings, slab-on-grade floors, pavement,
and structural fill. The Washington State Department of Transportation (WSDOT) Standard
Specifications and Standard Plans cited herein refer to WSDOT publications M41-10, Standard
Specifications for Road, Bridge, and Municipal Construction, and M21-01, Standard Plans for
Road, Bridge, and Municipal Construction, respectively.
4_1 Site Preparation
Preparation of the project site should involve erosion control, temporary drainage, clearing,
stripping,excavations, cutting,subgrade compaction, and filling.
Erosion Control: Before new construction begins, an appropriate erosion control system should
be installed. This system should collect and filter all surface water runoff through silt fencing.
We anticipate a system of berms and drainage ditches around construction areas will provide an
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adequate collection system. Silt fencing fabric should meet the requirements of WSDOT Standard
Specification 9-33.2 Table 3. In addition,silt fencing should embed a minimum of 6 inches below
existing grade. An erosion control system requires occasional observation and maintenance.
Specifically,holes in the filter and areas where the filter has shifted above ground surface should
be replaced or repaired as soon as they are identified.
Temporary Drainage: We recommend intercepting and diverting any potential sources of surface
or near-surface water within the construction zones before stripping begins. Because the selection
of an appropriate drainage system will depend on the water quantity,season,weather conditions,
construction sequence, and contractor's methods, final decisions regarding drainage systems are
best made in the field at the time of construction. Based on our current understanding of the
construction plans,surface and subsurface conditions,we anticipate that curbs,berms,or ditches
placed around the work areas will adequately intercept surface water runoff.
Clearing and Stripping: After surface and near-surface water sources have been controlled, sod,
topsoil, and root-rich soil should be stripped from the site. Our explorations and field
observations indicate that the topsoil horizon is typically 6 inches thick across the project area.
Site Excavations: Based on our explorations, we expect that excavations will encounter medium
dense sandy soils which can be easily excavated using standard excavation equipment.
Dewatering: Given our drilling method, we did not observe groundwater elevations during our
subsurface explorations. We anticipate that localized groundwater levels will mirror water
elevations within the pond system towards the south end of the project area. If groundwater is
encountered, we anticipate that an internal system of ditches, sumpholes, and pumps will be
adequate to temporarily dewater shallow excavations.
Temporary Cut Slopes: All temporary soil slopes associated with site cutting or excavations
should be adequately inclined to prevent sloughing and collapse. Temporary cut slopes in site
soils should be no steeper than 11/2H:1V and should conform to Washington Industrial Safety and
Health Act(WISHA) regulations.
Subgrade Compaction: Exposed subgrades for the foundations of the planned structures should
be compacted to a firm,unyielding state before new concrete or fill soils are placed. Any localized
zones of looser granular soils observed within a subgrade should be compacted to a density
commensurate with the surrounding soils. In contrast, any organic, soft, or pumping soils
observed within a subgrade should be over-excavated and replaced with a suitable structural fill
material.
Site Filling: Our conclusions regarding the reuse of onsite soils and our comments regarding wet-
weather filling are presented subsequently. Regardless of soil type, all fill should be placed and
compacted according to our recommendations presented in the Structural Fill section of this
report. Specifically, building pad fill soil should be compacted to a uniform density of at least
95 percent(based on ASTM:D-1557).
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Onsite Soils: We offer the following evaluation of these onsite soils in relation to potential use as
structural fill:
• Surficial Organic Soil and Organic-Rich Topsoil: Where encountered, surficial
organic soils, like duff, topsoil, root-rich soil, and organic-rich fill soils, are not
suitable for use as structural fill under any circumstances, due to high organic
content. Consequently,this material can be used only for non-structural purposes,
such as in landscaping areas.
• Sandy Outwash: Underlying the entirety of the proposed work area, extending
through depths of 25 to 30 feet below existing grade,we encountered either native
sandy outwash soils, or repurposed fill materials consisting of a similar material
type. This soil group is relatively impervious to the moisture content variations,
and can be utilized as a fill source year-round, under most weather conditions.
Permanent Slopes: All permanent cut slopes and fill slopes should be adequately inclined to
reduce long-term raveling,sloughing,and erosion. We generally recommend that no permanent
slopes be steeper than 2H:1V. For all soil types,the use of flatter slopes(such as 21/2HJV) would
further reduce long-term erosion and facilitate revegetation.
Slope Protection: We recommend that a permanent berm, swale, or curb be constructed along
the top edge of all permanent slopes to intercept surface flow. Also, a hardy vegetative
groundcover should be established as soon as feasible, to further protect the slopes from runoff
water erosion. Alternatively, permanent slopes could be armored with quarry spalls or a
geosynthetic erosion mat.
4_2 Spread Footings
In our opinion, conventional spread footings will provide adequate support for the proposed
grand stands if the subgrades are properly prepared. We offer the following comments and
recommendations for spread footing design.
Footing Depths and Widths: For frost and erosion protection, the bases of all footings should
bear at least 18 inches below lowest adjacent outside grade. To reduce post-construction
settlements, continuous (wall)and isolated (column) footings should be at least 18 and 24 inches
wide,respectively.
Bearing Subgrades: Footings should bear on medium dense or denser, undisturbed native soils
which have been stripped of surficial organic soils and vigorously surface compacted, or on
properly compacted structural fill which bears on the soils just described. Based on our field
observations, we anticipate that adequate bearing subgrades will be encountered within
relatively shallow depths across the proposed footprint of the new grand stands(less than 3 feet),
in the medium dense outwash soils.
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In general,before footing concrete is placed,any localized zones of loose soils exposed across the
footing subgrades should be compacted to a firm,unyielding condition, and any localized zones
of soft, organic, or debris-laden soils should be over-excavated and replaced with suitable
structural fill.
Lateral Overexcavations: Because foundation stresses are transferred outward as well as
downward into the bearing soils, all structural fill placed under footings should extend
horizontally outward from the edge of each footing. This horizontal distance should be equal to
the depth of placed fill. Therefore,placed fill that extends 24 inches below the footing base should
also extend 24 inches outward from the footing edges.
Subgrade Observation: All footing subgrades should consist of firm,unyielding,native soils,the
existing medium dense or denser gravelly fill, or structural fill materials that have been
compacted to a density of at least 95 percent(based on ASTM:D-1557). Footings should never be
cast atop loose, soft, or frozen soil, slough, debris, existing uncontrolled fill, or surfaces covered
by standing water.
Bearing Pressures: In our opinion, for static loading, footings that bear on properly prepared
subgrades of native outwash soils can be designed for a maximum allowable soil bearing pressure
of 3,000 psf. A one-third increase in allowable soil bearing capacity may be used for short-term
loads created by seismic or wind related activities.
Footing Settlements: We estimate that total post-construction settlements of properly designed
footings bearing on properly prepared subgrades will not exceed 1 inch. Differential settlements
for comparably loaded elements may approach one-half of the actual total settlement over
horizontal distances of approximately 50 feet.
Footing Backfill: To provide erosion protection and lateral load resistance, we recommend that
all footing excavations be backfilled on both sides of the footings and stem walls after the concrete
has cured. Either imported structural fill or non-organic onsite soils can be used for this purpose,
contingent on suitable moisture content at the time of placement. Regardless of soil type, all
footing backfill soil should be compacted to a density of at least 90 percent (based on ASTM:D-
1557).
Lateral Resistance: Footings that have been properly backfilled as recommended above will resist
lateral movements by means of passive earth pressure and base friction. We recommend using
an allowable passive earth pressure of 250 psf for both the glacial outwash and fill on site and an
allowable base friction coefficient of 0.35 for both soil types.
4.2.1 Concrete Caissons
Based on the existing topography present across the proposed footprint for the new grand stands,
it is our understanding that portions of this structure may be supported by drilled shafts or
caissons. We offer the following comments and recommendations for caisson design.
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Bearing Subgrades: Caissons should bear on medium dense or denser, undisturbed native soils.
Based on our field observations, we anticipate that adequate bearing subgrades in the medium
dense outwash soils will be encountered within relatively shallow depths across the proposed
footprint of the new grand stands (less than 3 feet),. In our opinion, for static loading, footings
that bear on properly prepared subgrades of native outwash soils can be designed for a maximum
allowable soil bearing pressure of 3,000 psf. This value may be increased for greater embedment
depths if we are consulted. A one-third increase in allowable soil bearing capacity may be used
for short-term loads created by seismic or wind related activities.
Lateral Resistance: Properly constructed column supports will resist lateral movements by means
of passive earth pressure and base friction. We recommend using an allowable passive earth
pressure of 275 pcf and an allowable base friction coefficient of 0.35. Any backfill foundation
should be placed and compacted to 95 percent of the maximum dry density according to the
recommendations presented below.
Caisson Embedment: All concrete caissons should have sufficient embedment below the lowest
adjacent grade to provide adequate "kick-out" resistance to horizontal loads. We recommend a
minimum embedment of 3 feet, or equal to the deepest planned excavations within about
10 horizontal feet of the caisson, whichever is greater. However, deeper embedment might be
needed to develop adequate vertical capacity or passive resistance at specific locations.
Drilling Conditions: Based on our explorations, we predict that medium dense sandy outwash
will be encountered in most or all caisson holes. This soil can be worked using conventional
augers or small excavator. We do not anticipate the presence of any obstructions to be present in
potential caisson locations.
4.3 Slab-On-Grade Floors
In our opinion, soil-supported slab-on-grade floors can be used in the construction of the grand
stands if the subgrades are properly prepared. We offer the following comments and
recommendations concerning slab-on-grade floors.
Floor Subbase and Modulus: Generally,structural fill subbases do not appear to be needed under
soil-supported slab-on-grade floors. Surface compaction of slab subgrades is recommended. A
subgrade modulus of 225 pci should be used for design.
If a subbase is required, it should be compacted to a density of at least 95 percent (based on
ASTM:D-1557).
Capillary Break and Vapor Barrier: To retard the upward wicking of moisture beneath the floor
slab, we recommend that a capillary break be placed over the subgrade. Ideally, this capillary
break would consist of a 4-inch-thick layer of pea gravel or other clean, uniform, well-rounded
gravel, such as "Gravel Backfill for Drains" per WSDOT Standard Specification 9-03.12(4).
Alternatively, angular gravel or crushed rock can be used if it is sufficiently clean and uniform to
prevent capillary wicking.
Migizi Group, Inc. Page 11 of 16
NMSD—NMHS Grand Stand Improvements,300 E Campus Dr,Belfair,WA April 19,2019
Geotechnical Engineering Report P1584-T19
Vapor Barrier: We recommend that a layer of durable plastic sheeting (such as Crosstuff,
Moistop,or Visqueen)be placed directly between the capillary break and the floor slab to prevent
ground moisture vapors from migrating upward through the slab. During subsequent casting of
the concrete slab,the contractor should exercise care to avoid puncturing this vapor barrier.
4_4 Asphalt Pavement
Asphalt pavements will be utilized in the construction of the supplementary access road servicing
the new grand stands, with the proposed alignment being towards the southeast corner of the
project area, along the sloped region between the existing pond and track and field. Given the
topography of this region, the majority of the road subgrade will be constructed utilizing fill
material. We offer the following comments and recommendations for pavement design and
construction.
Subgrade Preparation: All soil subgrades should be thoroughly compacted, then proof-rolled
with a loaded dump truck or heavy compactor. Any localized zones of yielding subgrade
disclosed during this proof-rolling operation should be over excavated to a maximum depth of
12 inches and replaced with a suitable structural fill material. All structural fill should be
compacted according to our recommendations given in the Structural Fill section. Specifically,
the upper 2 feet of soils underlying pavement section should be compacted to at least 95 percent
(based on ASTM D-1557), and all soils below 2 feet should be compacted to at least 90 percent.
Pavement Materials: For the base course, we recommend using imported washed crushed rock,
such as "Crushed Surfacing Base Course" per WSDOT Standard Specification 9-03.9(3)but with
a fines content of less than 5 percent passing the No.200 Sieve. Although our explorations do not
indicate a need for a pavement subbase, if a subbase course is needed, we recommend using
imported, clean, well-graded sand and gravel such as 'Ballast" or "Gravel Borrow" per WSDOT
Standard Specifications 9-03.9(1) and 9-03.14, respectively.
Conventional Asphalt Sections: A conventional pavement section typically comprises an asphalt
concrete pavement over a crushed rock base course. We recommend using the following
conventional pavement section for the proposed access road:
Pavement Course
Asphalt Concrete Pavement 3 inches
Crushed Rock Base 6inches
Granular Fill Subbase(if needed) 12 inches
Compaction and Observation: All subbase and base course material should be compacted to at
least 95 percent of the Modified Proctor maximum dry density (ASTM D-1557), and all asphalt
concrete should be compacted to at least 92 percent of the Rice value (ASTM D-2041). We
recommend that an MGI representative be retained to observe the compaction of each course
before any overlying layer is placed. For the subbase and pavement course, compaction is best
observed by means of frequent density testing. For the base course, methodology observations
and hand-probing are more appropriate than density testing.
Migizi Group, Inc. Page 12 of 16
NMSD—NMHS Grand Stand Improvements,300 E Campus Dr,Belfair,WA April 19,2019
Geotechnical Engineering Report P1584-T19
Pavement Life and Maintenance: No asphalt pavement is maintenance-free. The above described
pavement sections present our minimum recommendations for an average level of performance
during a 20-year design life; therefore, an average level of maintenance will likely be required.
Furthermore,a 20-year pavement life typically assumes that an overlay will be placed after about
10 years. Thicker asphalt and/or thicker base and subbase courses would offer better long-term
performance but would cost more initially; thinner courses would be more susceptible to
"alligator" cracking and other failure modes. As such, pavement design can be considered a
compromise between a high initial cost and low maintenance costs versus a low initial cost and
higher maintenance costs.
4_5 Retaining Walls
Based on conversations with members of the design team, it us our understanding that the
proposed access road servicing the new grand stand will be constructed utilizing fill soils. The
base of the road subgrade will be supported by a modular block retaining wall. The retaining
wall will likely be constructed in conjunction with the roadway. The following general
recommendations should be applied to the design of retaining walls.
Retaining walls should be designed to resist earth pressures and applicable surcharge loads. A
minimum wall embedment depth of 2 feet or greater below lowest adjacent grade is
recommended. We recommend the following parameters be used for retaining wall design:
• Active Earth Pressure (Yielding Wall) 35 pcf (equivalent fluid) (for walls with
horizontal backslopes)
55 pcf (equivalent fluid) (for walls with
backslopes up to 1H:1V)
• Passive Resistance 225 pcf(equivalent fluid)
• Coefficient of Friction 0.35
• Allowable Bearing Pressure 2,000 psf
Wall Foundations: In our opinion, conventional spread footings will provide adequate support
for the proposed walls if the subgrade is properly prepared. A 12-inch crushed rock base, such
as "Crushed Surfacing" per WSDOT Standard Specification 9-03.9(3) should be placed along the
keyway of the wall alignment. The rock base should extend a minimum of 12 inches in front and
behind the block members. The bottom of the wall should be embedded a minimum of 18 inches
below adjacent grades.
Wall Drainage: Drainage should be provided behind subgrade and retaining walls by placing a
zone of drain rock containing less than 3 percent fines (material passing No. 200 sieve) against
the wall. This drainage zone should be at least 12 inches wide (measured horizontally) and
extend from the base of the wall to within 1 foot of the finished grade behind the wall. Smooth-
walled perforated PVC drainpipe having a minimum diameter of 4 inches should be embedded
within the sand and gravel at the base of the wall along its entire length. This drainpipe should
discharge into a tightline leading to an appropriate collection and disposal system.
Migizi Group, Inc. Page 13 of 16
NMSD—NMHS Grand Stand Improvements,300 E Campus Dr,Belfair,WA April 19,2019
Geotechnical Engineering Report P1584-T19
Backfill Soil: Ideally, all subgrade wall backfill would consist of clean, free-draining, granular
material, such as "Gravel Backfill for Walls" per WSDOT Standard Specification 9-03.12(2). A
non-woven geotextile should be placed between the drainage zone and the backfill soil to prevent
drain clogging.
Backfill Compaction: Because soil compactors place significant lateral pressures on subgrade
walls, we recommend that only small, hand-operated compaction equipment be used within
2 feet of a backfilled wall. Also, all backfill should be compacted to a density as close as possible
to 90 percent of the maximum dry density (based on ASTM:D-1557); a greater degree of
compaction closely behind the wall would increase the lateral earth pressure, whereas a lesser
degree of compaction might lead to excessive post-construction settlements.
Grading and Capping: To retard the infiltration of surface water into the backfill soils, we
recommend that the backfill surface of exterior walls be adequately sloped to drain away from
the wall. Ideally, the backfill surface directly behind the wall would be capped with asphalt,
concrete, or 12 inches of low-permeability (silty) soils to minimize or preclude surface water
infiltration.
Vertical Slope of Wall Face: The wall should be designed using a vertical inclination between 1:6
and 1:8 (H:V).
Wall Settlements: We estimate that the settlement of the wall foundation, constructed as
recommended, will be on the order of 1 inch or less. Most of this settlement is expected to occur
as soon as the loads are applied. Differential settlement along the walls is expected to be 1 inch
or less over a 50-foot span.
4.6 Structural Fill
The term "structural fill' refers to any material placed under foundations, retaining walls, slab-
on-grade floors, sidewalks, pavements, and other structures. Our comments, conclusions, and
recommendations concerning structural fill are presented in the following paragraphs.
Materials: Typical structural fill materials include clean sand, gravel, pea gravel, washed rock,
crushed rock,well-graded mixtures of sand and gravel(commonly called"gravel borrow"or"pit-
run"),and miscellaneous mixtures of silt,sand,and gravel. Recycled asphalt,concrete,and glass,
which are derived from pulverizing the parent materials,are also potentially useful as structural
fill in certain applications. Utilizing recycled content may require approval from the Tacoma
Pierce County Health Department for placement in an aquifer recharge area. Soils used for
structural fill should not contain any organic matter or debris,nor any individual particles greater
than about 6 inches in diameter.
Fill Placement: Clean sand,gravel,crushed rock,soil mixtures,and recycled materials should be
placed in horizontal lifts not exceeding 8 inches in loose thickness, and each lift should be
thoroughly compacted with a mechanical compactor.
Migizi Group, Inc. Page 14 of 16
NMSD—NMHS Grand Stand Improvements,300 E Campus Dr,Belfair,WA April 19,2019
Geotechnical Engineering Report P1584-T19
Compaction Criteria: Using the Modified Proctor test (ASTM:D-1557) as a standard, we
recommend that structural fill used for various onsite applications be compacted to the following
minimum densities:
Fill Application Minimum Compaction
Footing subgrade and bearing pad 95 percent
Foundation backfill 90 percent
Slab-on-grade floor subgrade and subbase 95 percent
Asphalt pavement base and subbase 95 percent
Asphalt pavement subgrade(upper 2 feet) 95 percent
Asphalt pavement subgrade(below 2 feet) 90 percent
Subgrade Observation and Compaction Testing: Regardless of material or location, all structural
fill should be placed over firm, unyielding subgrades prepared in accordance with the Site
Preparation section of this report. The condition of all subgrades should be observed by
geotechnical personnel before filling or construction begins. Also, fill soil compaction should be
verified by means of in-place density tests performed during fill placement so that adequacy of
soil compaction efforts may be evaluated as earthwork progresses.
Soil Moisture Considerations: The suitability of soils used for structural fill depends primarily
on their grain-size distribution and moisture content when they are placed. As the"fines"content
(that soil fraction passing the U.S.No. 200 Sieve) increases, soils become more sensitive to small
changes in moisture content. Soils containing more than about 5 percent fines(by weight)cannot
be consistently compacted to a firm,unyielding condition when the moisture content is more than
2 percentage points above or below optimum. For fill placement during wet-weather site work,
we recommend using"clean"fill,which refers to soils that have a fines content of 5 percent or less
(by weight)based on the soil fraction passing the U.S. No.4 Sieve.
5.0 RECOMMENDED ADDITIONAL SERVICES
Because the future performance and integrity of the structural elements will depend largely on
proper site preparation, drainage, fill placement, and construction procedures, monitoring and
testing by experienced geotechnical personnel should be considered an integral part of the
construction process. Subsequently, we recommend that MGI be retained to provide the
following post-report services:
0 Review all construction plans and specifications to verify that our design criteria
presented in this report have been properly integrated into the design;
• Prepare a letter summarizing all review comments (if required);
0 Check all completed subgrades for footings and slab-on-grade floors before
concrete is poured,in order to verify their bearing capacity;and
• Prepare a post-construction letter summarizing all field observations,inspections,
and test results (if required).
Migizi Group, Inc. Page 15 of 16
NMSD—NMHS Grand Stand Improvements,300 E Campus Dr, Belfair, WA April 19,2019
Geotechnical Engineering Report P1584-T19
6.0 CLOSURE
The conclusions and recommendations presented in this report are based, in part, on the
explorations that we observed for this study; therefore, if variations in the subgrade conditions
are observed at a later time, we may need to modify this report to reflect those changes. Also,
because the future performance and integrity of the project elements depend largely on proper
initial site preparation, drainage, and construction procedures, monitoring and testing by
experienced geotechnical personnel should be considered an integral part of the construction
process. MGI is available to provide geotechnical monitoring of soils throughout construction.
We appreciate the opportunity to be of service on this project. If you have any questions
regarding this report or any aspects of the project, please feel free to contact our office.
Respectfully submitted,
MIGIZI GROUP, INC. of
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mecca "
Zach L. Logan U James E. Brigham, P.E.
Staff ologist Senior Principal Engineer
Migizi Group, Inc. Page 16 of 16
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NOTE: 253-537-9400 DESIGNER: JRB JOB NO.P1584-T19
BOUNDARY AND TOIvIAPPING DRAWN BY: JRB SCALE: NTS
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APPENDIX A
SOIL CLASSIFICATION CHART AND
KEY TO TEST DATA
LOGS OF AUGER BORINGS
MAJOR DIVISIONS TYPICAL NAMES
CLEAN GRAVELS GW WELL GRADED GRAVELS,GRAVEL-SAND MIXTURES
GRAVELS WITH LITTLE OR
MORE THAN HALF NO FINES G p a: a POORLY GRADED GRAVELS,GRAVEL-SAND MIXTURES
o'•D:
COARSE FRACTION
J > o SILTY GRAVELS,POORLY GRADED GRAVEL-SAND-SILT
.T IS LARGER THAN GM o MIXTURES
o NO.4 SIEVE GRAVELS WITH
uj OVER 15%FINES CLAYEY GRAVELS,POORLY GRADED GRAVEL-SAND-CLAY
zn GC MIXTURES
O
w CLEAN SANDS SW WELL GRADED SANDS,GRAVELLY SANDS
SANDS WITH LITTLE
O o OR NO FINES SP POORLY GRADED SANDS,GRAVELLY SANDS
MORE THAN HALF
COARSE FRACTION
IS SMALLER THAN SM SILTY SANDS,POORLY GRADED SAND-SILT MIXTURES
SANDS WITH
NO.4 SIEVE OVER 15%FINES
SC fflffl�
CLAYEY SANDS,POORLY GRADED SAND-CLAY MIXTURES
INORGANIC SILTS AND VERY FINE SANDS,ROCK FLOUR,
ML SILTY OR CLAYEY FINE SANDS,OR CLAYEY SILTS WITH
SILTS AND CLAYS SLIGHT PLASTICITY
a� INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY,
0 .N LIQUID LIMIT LESS THAN 50 CL GRAVELLY CLAYS,SANDY CLAYS,SILTY CLAYS,
o LEAN CLAYS
O o —_
o OL — ORGANIC CLAYS AND ORGANIC SILTY CLAYS OF LOW
w v = PLASTICITY
z - —
MH INORGANIC SILTS,MICACEOUS OR DIATOMACIOUS FINE
OSANDY OR SILTY SOILS,ELASTIC SILTS
z , SILTS AND CLAYS
w CH INORGANIC CLAYS OF HIGH PLASTICITY,FAT CLAYS
LIQUID LIMIT GREATER THAN 50
OH ORGANIC CLAYS OF MEDIUM TO HIGH PLASTICITY,
ORGANIC SILTS
HIGHLY ORGANIC SOILS Pt PEAT AND OTHER HIGHLY ORGANIC SOILS
8 Modified California RV R-Value
® Split Spoon SA Sieve Analysis
Pushed Shelby Tube SW Swell Test
[j] Auger Cuttings TC Cyclic Triaxial
® Grab Sample TX Unconsolidated Undrained Triaxial
Sample Attempt with No Recovery TV Torvane Shear
CA Chemical Analysis UC Unconfined Compression
CN Consolidation (1.2) (Shear Strength,ksf)
CP Compaction WA Wash Analysis
DS Direct Shear (20) (with%Passing No.200 Sieve)
o PM Permeability Water Level at Time of Drilling
PP Pocket Penetrometer T Water Level after Drilling(with date measured)
a
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SOIL CLASSIFICATION CHART AND KEY TO TEST DATA
Z
Z Figure A-10=40wft
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Migizi Group,Inc. BORING NUMBER B-1
PO Box 44840 PAGE 1 OF 1
>®bumim Tacoma,WA 98448 Figure A-2
Telephone: 253-537-9400
Fax: 253-537-9401
CLIENT North Mason School District PROJECT NAME NMHS Grand Stand Improvements
PROJECT NUMBER P1584-T19 PROJECT LOCATION 300 E Campus Dr,Belfair,Washington
DATE STARTED 3/4/19 COMPLETED 3/4/19 GROUND ELEVATION HOLE SIZE
DRILLING CONTRACTOR Holt Services,Inc. GROUND WATER LEVELS:
DRILLING METHOD Mud Rotary/Track Mounted Drill Rig AT TIME OF DRILLING ---
LOGGED BY ZLL CHECKED BY JEB AT END OF DRILLING -
NOTES AFTER DRILLING ---
W c
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w m w p Z U a MATERIAL DESCRIPTION
�� �Q O
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EL (SIP-SM)Brown fine to medium sand with silt(loose,moist)(Fill)
EL
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0
SS 3-2-2
z S-1 6 (4)
it
°m 5
m
�7 SS 9 1-2-1
w- -X S-2 (3) Grades to very loose
a SP-
~ SS 9 4-5-6 SM
S-3 (11) Grades to medium dense
IL
z 10
SS 4-5-6
S-4 10 (11)
z
m
z
z
fn 14.0
a 15 (SP)Gray fine to medium sand with some gravel(medium dense,moist)
LU SS 12 7-10-9
o S-5 (19)
Y
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Uj
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22.0
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o D
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W S-7 (68) GP Q
o D
co Q
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S-8 181 (79) 1 R31.5
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Migizi Group,Inc. BORING NUMBER B-2
PO Box44840 PAGE 1 OF 1
®t Tacoma,WA 98448 Figure A-3
Telephone: 253-537-9400
Fax: 253-537-9401
CLIENT North Mason School District PROJECT NAME NMHS Grand Stand Improvements
PROJECT NUMBER P1584-T19 PROJECT LOCATION 300 E Campus Dr,Belfair,Washington
DATE STARTED 3/4/19 COMPLETED 3/4/19 GROUND ELEVATION HOLE SIZE
DRILLING CONTRACTOR Holt Services, Inc. GROUND WATER LEVELS:
DRILLING METHOD Mud Rotary/Track Mounted Drill Rig AT TIME OF DRILLING ---
LOGGED BY ZLL CHECKED BY JEB AT END OF DRILLING ---
NOTES AFTER DRILLING
u1 c
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w v W g >C! Q U a 0 MATERIAL DESCRIPTION
o a7) O- mp> ��
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'a 6 inches crushed rock
° (SP)Brown fine to medium sand with gravel(medium dense, moist)
c�
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z S-1 0 (26)
m 5
SS 0 13-15-16
S-2 (31)
a
rn
a SS 6 10-13-11 SP
S-3 (24)
a
z 10
SS 10-9-9
z S 4 0 (18)
0
0
m
z
z
15 15.0
WSS 11 12-20-22 (SP)Gray fine to coarse sand with some gravel(dense,moist)
o -X S-5 (42)
Y
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w 20 20.0 _
w
SS 12 32 45 ° (GP)Gray gravel with medium to coarse sand(very dense,moist)
S-6 (50/5") GP °
22.0
(SM)Gray silty sand with gravel(very dense,moist)(Till)
M
25
SS 4 8-44-55
w S-7 (99) 131.5
°
LL
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30 30.0
coSS 6 39-45Till
S-8 (50/5")
LU Bottom of borehole at 31.5 feet.
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Migizi Group, Inc. BORING NUMBER B-3
PO Box 44840 PAGE 1 OF 1
�� Owt Tacoma,WA 98448 Figure A-4
Telephone: 253-537-9400
Fax: 253-537-9401
CLIENT North Mason School District PROJECT NAME NMHS Grand Stand Improvements
PROJECT NUMBER P1584-T19 PROJECT LOCATION 300 E Campus Dr Belfair,Washington
DATE STARTED 3/4/19 COMPLETED 3/4/19 GROUND ELEVATION HOLE SIZE
DRILLING CONTRACTOR Holt Services,Inc. GROUND WATER LEVELS:
DRILLING METHOD Mud Rotary/Track Mounted Drill Rig AT TIME OF DRILLING ---
LOGGED BY ZLL CHECKED BY JEB AT END OF DRILLING - -
NOTES AFTER DRILLING ---
w c
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w -j g >rr �D Q O MATERIAL DESCRIPTION
Q' � J
QZ L) COUOZ U
ui
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3 inches asphalt concrete
i 6 inches crushed rock
(SP)Brown fine to medium sand with silt and gravel(medium dense,moist)
° SS 5-6-6
z S-1 6 (12)
m° 5
SS 9-10-9 SP
S-2 9 (19)
a
rn
~ M SS 9 6-10-12
S-3 (22)
a
z_ 10 10.0
SS 7-4-3 (SM)Brown silty sand with some organics and gravel(loose,moist)(Pond Bottom)
z S-4 8 (7) SM 111.5
o Bottom of borehole at 11.5 feet.
m
z
a
m
V-
a
m
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m
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