HomeMy WebLinkAboutCity Water Reservoir Site Geotechnical Report - 2010.08.11-t
EXHIBIT
ABPB GoNsttLTINc
G eorecn x t catl EaRrx ScreflcEs
Paut Boxlraa, P'E.G,
Axrc Burarq P'E.
12525 Willows Road, Suite B0
Kirkland, WA 98034
Phone: 425-820-2544
Fax: 425-820-2613
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August 11, 2010
Project No, 1295
CITY S PORT TOWNSINl)
DSD
Mr. Eric Sladky
PB Telecom Inc.
303 Battery Street
Seattle, Washington 98121
Subject: Geotechnical RePod
Proposed Jeffcom gllCommunication Tower Site
29tL - 20th Street
Pott Townsend, Washi ngton
Dear Mr. Sladlcy:
As requested, we have completed our geotechnical engineering study for the proposed
Jeffcom 911 communication tower to be constructed on the City of Port Townsend water
tank property in Port Townsend, Washington. The scope of our earlier work on this
project included conducting an Environment Site Assessment for the site. Following that
work, we have conducted a geotechnical study to develop design criteria for the tower.
This phase of work included drilling two test borings and conducting a Soil Resistivity
study at the tower location as well as the preparation of this repoft.
Project Description
The project will consist of the construction of an approximately 180 foot tall lattice
communication antenna tower and several small equipment enclosures on the site. The
project site is located just east of the existing large water tank on the propefi. The
approximate location of the site and the water tank propety is shown on the Vicinity Map,
Figure 1.
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Mr. Eric Sladky
August 11, 2010
Based on the plans for the tower site prepared by PB Telecom, the lattice tower will be
located in the eastern quadrant of the near rectangular multi-acre water tank propefi.
The area of the Jeffcom 911 tower facility is about 65 by 75 feet and will be within a new
irregular shaped planned fenced enclosure. The new Jeffcom tower, equipment shelters,
as well as the existing CingularfF-Mobile monopole and structures will all be located
within the new fenced enclosure.
The recommendations given in the following sections of this report are based on our
understanding of the above design features. If actual features vary or changes are made,
we should be informed and requested to review them in order to modifli our
recommendations as required.
Scope of Work
On August 3, 2010, we drilled two test borings at the site to a maximum depth of
approximately 35 feet below the existing sudace grade. Using the soil information
obtained, we performed analyses to develop geotechnical recommendations for project
design and construction. This report addresses site preparation and foundation design.
Site Conditions
Suface
The tower site is located several miles west of the central business district of Poft
Townsend, Washington. The City owned propefi is a rectangular shaped parcel located
on a gentle ridge-top at the end of 20th Street. Access to the City fenced property is from
the noftheast off 20th and Howard Streets. The planned new fenced enclosure at the
tower site is situated just east of the largest and older of the water tanks at the propefi.
An existing smaller water tan( telecommunication company monopole and pump building
are located immediately south of the planned lattice tower, The area of the tower and
surrounding propefties are forested with light to moderate sized deciduous trees and
scattered fir trees. The tower site lies at about Elevation 292 feet based on the
topographic map of the area. A gentle swale along the ridge-top site extends from the
planned lattice tower nofthward along the City property eastern line. The swale deepens
to about 10 feet in depth near the access road off 20th Street where the road crosses on a
fill embankment. The approximate proposed location of the lattice tower and facility is
shown on the Exploration Location Plan, Figure 2.
Subsuface
We drilled two test borings to a maximum depth of 35.5 feet below the ground suface.
The test borings encountered predominantly granular soils consisting of cemented silty
sands with occasional gravelly layers to the depths explored'
Project No. 1295
Page No. 2
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Mr. Eric Sladky
August 11, 2010
A few feet of mixed topsoil and suface fill was noted in Boring B-1 overlying a residual
soil layer consisting of loose to medium dense sifty Sand. The loose to medium dense
layer extended to a depth of 7 feet in Boring B-1 and 3.5 feet in B-2. No other significant
fills were noted around the boring locations. Below the loose to medium dense silty Sand,
we encountered a thick layer of cemented glacial till. This unit of glacial till soil
(Aldenruood Soil Type) extends down to the depths explored. The till is thought to
underlie much of the upland area around the water tank propefi. This soil is an
unsofted mixture of gravel, sand and silt deposited by glacial action about 12,000 to
15,000 years ago by an Ice Age glacier descending out of Canada. The overlying weight
of the ice consolidated the till to its relatively high strength and low permeability.
We do not believe any significant fills exist around the site location based on visual
obseruations and the proximity of the site to the ridge-top. The geologic map of this
area indicates the site vicinity is underlain by glacial till. These conditions are also shown
on the US Soil Conseruation Seruice maps as Aldenruood Soil group (Glacial Till). The site
soils encountered are generally consistent with the published descriptions.
The boring logs attached to this repoft as Figure 3 and 4 provide detailed descriptions of
the subsuface conditions encountered in the test borings. The approximate locations of
the borings are shown on Figure 2, Site Exploration Plan.
Groundwater
No significant seepage or groundwater was noted in the cemented glacial till soil down to
the base of the boring at 35.5 feet. We anticipate that any groundwater seepage within
or above the till will be at its highest during the wet season. Due to the limited
permeability of the glacial till, seepage may become perched on the cemented layer. We
do not anticipate that there will.be any significant seepage within the planned foundation
holes or excavations.
Seismic Design
This area of Western Washington falls within Seismic Zone 3, as classified by the 1997
Uniform Building Code (UBC). Based on the soil and anticipated dense till conditions, Site
Class C should be used in design, in accordance with the 2006 International Building
Code.
Liquefaction is a phenomenon where there is a reduction or complete loss of soil strength
due to an increase in water pressure induced by vibrations from a seismic event. It
mainly affects geologically recent deposits of fine-grained sands that are below the
groundwater table. Soils of this nature derive their strength from intergranular friction.
The generated water pressure or pore pressure essentially separates the soil grains and
eliminates this intergranular friction, thus reducing or eliminating the soil's strength.
Project No. 1295
Page No. 3
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Mr. Eric Sladky
August LL,20L0
In our opinion, based on the dense granular silty soils encountered in the test borings,
there is no apparent risk for liquefaction to occur at this site during an eafthquake.
Discussion and Recommendations
General
The soil conditions are suitable for suppofting the lattice tower on a drilled pier foundation
or large concrete mat footings. We recommend that the drilled piers extend no less than
10 feet below the existing ground suface. In our opinion, the tower and the equipment
cabinet foundation could also be supported on spread footings or a mat foundation
system. If spread footings or a mat foundation are used are used for the tower, they
should be based at least 5 feet deep or extended down to the cemented glacial till layer,
whichever is deeper.
Temporary excavations for foundation or utility construction may be made veftical to
depths of 4 feet. For depths greater than 4 feet, temporary excavations should be sloped
at a maximum inclination of 0.75:1 (horizontal: veftical). ,
To prepare the tower and equipment foundation subgrades, the existing topsoil and any
localized shallow loose near suface materials should be stripped from the structural area.
Due to the dense sandy and gravelly nature of the subsoils, the foundation subgrades
should be densely re-compacted with vibratory equipment. During the dry period of the
year, the site silty gravelly soils can be used as structural fill if they are protected from
damp weather. If general site fill is required, we recommend impofting a free-draining
granular material that meets the following grading requirements:
U.S. Sieve Size Percent Passing
3 inches 100
No.4 75 maximum
No. 200 5 maximumx
xBased on the 314-inch fraction
Structural fill should be placed in uniform loose layers not exceeding 12 inches and
compacted to a minimum of 95 percent of the soil's maximum dry density, as determined
by ASTM Test Designation D-698 (Standard Proctor). The moisture content of the soil at
the time of compaction should be within two percent of its optimum, as determined by
this ASTM standard.
Project No. 1295
Page No. 4
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Mr. Eric Sladky
August Lt,2010
Foundations
Two alternatives are available for supporting the tower. The first is to use drilled pier
foundations extending to a minimum depth of at least 10 feet below the existing ground
suface. Alternatively, the tower may also be supported on a mat foundation or spread
footings. If a mat foundation or spread footings are used, they should be based at a
minimum depth of 5 feet or be extended down to the cemented glacial till soils so as to
be below the upper looser native sand soils which may be susceptible to disturbance. The
equipment building may be suppofted on shallow spread footings bearing on the native
soils at a minimum depth of 18 inches.
Based on our study, we recommend designing foundations using the soil strength and
foundation design parameters included in the following section.
Soil Strenqth Parameters
Notes:
pcf = pounds per cubic foot
psf = pounds per square foot
Foundation Desiqn Parameters:
Alternative Spread Footinqs for Tower and Equipment Buildinq:
Allowable Soil Bearing:2,000 psf for equipment building
pad and 7500 psf for tower mat
foundation where based on Till soil.
(Tower Mat should be based at a
minimum depth of 5 feet or on the
cemented till, whichever is deeper)
Project No. 1295
Page No. 5
Depth
(feet)Soil Description
Effective Unit
Weight (pcf)
Cohesion
(psf)
Friction Angle
(degrees)
Unconfined
Compressive
Strength - psf
0-5 Medium Dense, Silty,
gravelly SAND
130 0 32 0
s-35 Very dense, sifty gravelly
SAND (Glacial Till)
13s 1000 42 4000
Base Friction Coefficient:0.4
Mr. Eric Sladky
August tL,20L0
Passive Earth Pressure:
Settlement
Drilled Piers for Lattice Tower:
Allowable End Bearing:
Allowable Shaft Friction:
Allowable Lateral Resistance
(Passive Pressure) :
350 pcf to 5 feet and 500 pcf below
5 feet (equivalent fluid weight).
Neglect upper 2 feet of depth.
1 inch or less
10 tsf (tons/foof)
2 to 5 feet: 500 psf
5 to 20 feet: 1000 psf
>20 feet: 2500 psf
2 to 5 feet: 350 pcf (equivalent fluid wt)
5 to 20 feet: 500 pcf
>20 feet: 500 pcf
2 to 5 feet: 10 tcf (tons/foof)
5 to 20 feet: 25 tcf
>20 feet: 40 tcf
Subgrade Modulus Coefficient (n6):
Notes:
The above friction values may be used for computing download and uplift capacity
Passive resistance may be taken as acting over two times the pier diameter.
The above values include a safety factor of 1.5
The Subgrade Modulus Varies Linearly with Depth: lq = nn(|e)
Where z = depth
WhereS=pierdiameter
Darnrnmanrlarl Minim m horrinn rlanth halnrnr arzicfinn rtr>rfa.
Spread Footings: The equipment cabinet pad footing should be based at a minimum
depth of 18 inches so as to be below the frost penetration depth. The Lattice Tower mat
foundation should be based at minimum depth of 5 feet or on the cemented till,
whichever is deeper. Drilled Piers should be at a minimum depth of 10 feet.
Project No. 1295
Page No. 6
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Mr. Eric Sladky
August 11,2010
The allowable soil bearing and end bearing capacities for spread footings and drilled piers
can be increased by one-third for short-term seismic and wind loading conditions. In
calculating passive resistance and shaft resistance, the upper two feet of soil should be
neglected because weather or future grading can affect them. Also, the passive eafth
pressure for spread footing design assumes the footing will be backfilled with compacted
structural fill. Foundation bearing sufaces must be relatively free of loose soil debris prior
to pouring concrete.
In our opinion, open hole drilling methods for a foundation pier system will be possible
with the soil conditions encountered down to the depths required. A local drilling
contractor with experience in the area should be consulted to evaluate the appropriate
equipment and methods to drill a clean shaft. Upon completion to the required bearing
depth, the hole should be cleaned to remove loose soils at the bottom.
Soil ResistiviW Testinq
We used a portable soil resistivity meter to conduct resistivity tests at the site. The tests
were conducted using the Wenner 4 Pole method. The approximate location of the
resistivity test line is shown on the Site Exploration Plan, Figure 2. The tests were
peformed with various probe spacings to evaluate the soil resistivity at varying depths.
The test results are given below:
Probe Spacing, feet
2
4
B
10
12
16
20
Soil Resistivity, ohm-cm
80,394
t07,240
79,664
78,480
75,800
58,190
43,260
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Additional Seruices
ABPB Consulting Inc. should review the final design and specifications to veriff that our
recommendations have been properly interpreted and incorporated into project design
and construction.
A qualified engineer should provide geotechnical inspection seruices during construction
to obserue compliance with the design concepts, construction specifications, and
recommendations. This will also allow expedient changes if the subsuface conditions
differ from those anticipated. These seruices should include obseruing footing subgrades
at the site and/or the installation of the drilled pier.
Project No. 1295
Page No. 7
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Mr. Eric Sladky
August It,20L0
Limitations
We prepared this report in accordance with generally accepted geotechnical engineering
practices. This report is the property of ABPB Consulting, Inc. and is intended for specific
application to the Jeffcom 911 Communication Tower project in lefferson County,
Washington. This repoft is for the exclusive use of PB Telecom, Inc. and their authorized
representatives. No other warranty, expressed or implied, is made.
The analyses and recommendations presented in this repoft are based upon data
obtained from the test borings drilled on-site. Variations in soil conditions can occur, the
nature and extent of which may not become evident until construction. If variations
appear evident, ABPB Consulting should reevaluate the recommendations in this repoft
before proceeding with construction.
If you have any question regarding our findings, please feel call.
Sincerely yours/
ABPB CONSULTING
Paul K. Bonifaci, P.G.
Engineering Geologist
Anil Butail, P.E.
Principal Geotechnical Engineer
Enclosures: Figure 1, VicinitY MaP
Figure 2, Exploration Location Plan
Figures 3 and 4, Boring Logs
Project No. 1295
Page No. B
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Ref: Wlndows Uve Maps
ABPB Consulting
Geotcchnical Consultantg
Kirkland, Waeh.
Vlcinlty Map
Pt. Townsend Communlcation Tower
Pt. Townsend, Washlnqton
ProJ. 1295 Date : 8-10 Flgure 1
T
Boring No. B-1
0
,-1
-2,-3
,4
-5
-6
-7'-8
-9
-10
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-12
-13
--14
-15
'-16
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-18
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-21
-22
-23
-24
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-26
'-27
-28'-29
-30
-31
'-32
-33
-34
,-35
8-3-10ProiectNo. 1295
292leetGlient : PB Telecom
Logged By: Paul BonifaciLocation: LatticeTower
SAMPLESUBSURFACE PROFILE Laboratory Results
Moisture Content
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U'oof,
A Standard Penetration
Resistance (SPT)
Blows/Foot10501
Soil DescriptionE
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Silty Sand: (24 inches of mixed old fill
and topsoil at surface) Mottled to red
tan, silty Sand with some roots and
gravel, loose to medium dense, dry
grading to moist
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Silty Sand: Tan grey grading to greY,
gravelly silty SAND, dense to very
dense, moist (GlacialTill)
No groundwater seePage at time of
drilling
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ABPB Consulting
Geotechnical Consu lta nts
12525 Wllows Road, Suite 80, Kirkland, Washington (425) 820'25+4
Figure 3ProjectName:Pt. Townsend Communication TowerDate :8 - 10
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Pt. T Gomm. Tower
Boring No. B-28-3-10DateProiect No. 1295 Elevation 290 feetPB TelecomClient:
Logged By: Paul BonifaciLocation: LatticeTower
SAMPLESUBSURFACE PROFILE Laboratory Results
Moisture Content
6
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(E3
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U)oa:)
A Standard Penetration
Resistance (SPT)
Blows/Foot10501
Soil Description
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Silty Sand: (12 inches of mixed old fill
and topsoil at surface) Mottled to red
tan, silty Sand with some roots and
gravel, loose to medium dense, dry
grading to moist
No groundwater seePage at time of
drilling
Silty Sand: Tan grey grading to grey,
gravelly silty SAND to silty SAND with
gravel, dense to very dense, moist
(GlacialTill)
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ABPB Consulting
Geotech n ica I Consulta nts
12525 Wllows Road, Sulte 80, Kirkland, Washington (425) 820-2544
Date :8 - 10 Project Name:Pt. Townsend Communication Tower Figure 4
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ABPB Consulting
Geotech n ical Consu ltants
Kirkland, Wash.
Exploration Location Plan
Proposed Communication Tower
Port Townsend, Washington
Proj. No. 1295 Date: B-10 Figure 2
r'lEflEtq
EllT IIr
s B-1
LEGEND
Approximate Boring Location and Number
Approximate Soil Resistivity Suruey LineLIJ
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trin-ur('*4I
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Approximate Scale
0 10' 20' 30' 40' 50'
Ref: Site Plan by Jeffcon 9-11 and PB Telecom, Inc. revised 6-29-10
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