07-0135 Stormwater Plan 1 031808Construction Stormwater Pollution
Prevention Plan
FAR
Taday's Dental
502 E Yelm Ave
Yelm, WA 98597
Marsh 14, 2008
Proponent: Judd and Sarah Sherman
[35U] 458-1975
Prepared hy: Lucas Johnson, E.I.T.
" Reviewed 6y: Justin Goroch, P.E.
k BCRA
21 fly Pacific Avenue, Suite 3UU
Tacoma, WA 9$4Q2
(253) 427-4367
RECEIVED
I~A~ 1$ 208
f hereby state that this report for the Today's Dental project has been prepared by
me or under my supervision and meets the standard of care and expertise which is
usual and customary in this community for professional engineers.
~~ ~- Go~,o ,~ j~~ joy
~~ _ of w Sir ~. A
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Table of Contents:
1. Construction Starmwater Pollution Prevention Elements
Element 1: Mark Clearing Limits .................................................................... ............ 1
Element 2: Establish Construction Access .................................................... ............ 1
Element 3: Control Flaw Rates ....................................................................... ............ 2
Element 4: Install Sediment Controls ............................................................ ............ 2
E[emen t 5: Stabilize Soils ................................................................................ ............ 2
Element 6: Protect 5lopes ............................................................................... ............ 3
Element 7: Protect drain Inlets ...................................................................... ............ 3
Element 8: Stabilize Channels and Out[ets .................................................. ............ 3
Element 9: Control Pollutants ........................:............................................... ............ 3
Element 1D: Control De-Watering .................................................................. ............ 3
Element ~ ~: Maintain ~MPs ............................................................................. ............ 4
Element 72: Manage the Project .................................................................... ............ 4
2. Project Description ................................................................................................. ............ 5
3. Existing Site Conditions ............................................................~--........................... ............ 5
4. Adjacent Areas ........................................................................................................ ............ 6
5. Critical Areas ........................................................................................................... ............ 6
6. Soil ............................................................................................................................. ............ 6
7. Potential Erosion Problem Areas ........................................................................... ........... 7
8. Construction Phasing ............................................................................................... ........... 7
9. Construction 5chedule ............................................................................................. ........... 7
1fl. Einanciall0wnership Responsibilities .....:............................................................. ........... 7
11. Engineering Calculatians ......................................................................................... ........... 8
Attachments:
Attachment A
Attachment 6
Attachment C
Vicinity Map
Site Plan
Geotechnical Report
ii
CONSTRUCTION STORMWATER
POLLUTION PREVENTION PLAN REPORT
1, Construction Stormwater Pollution Prevention Elements
The fallowing paragraphs describe how the twelve Construction Stvrmwater
Pollution Prevention minimum requirements well be satisfied for the Today's
^eetal project.
Element ~: Mark Clearing Limits
The clearing limits far the Today's Qental project are the property lines.
These clearing limits, along with any trees that are to be preserved, will be
clearly marked prior to any land disturbing activities with plastic, metal, ar
stake wire fence {BMP C103: High Visibility Plastic or Metal Fence or BMP C104:
Stake and Wire Fence). Alternatively, the clearing limits may be marked by Silt
fence. Items to be preserved will be clearly marked in order to prevent
damage to the site feature. There are no sensitive areas or buffers an ^r near
the site.
When possible, the duff layer, native topsoil, and natural vegetation will
remain undisturbed {BMP C101: Preserving Natural Vegetation}. However, if
they cannot be retained in place, they will be stockpiled an-site and covered to
prevent erosion. The site vegetation and topsoil will be replaced immediately
upon completion of the ground disturbing activities.
Element 2: Establish Construction Access
Construction vehicles will access the Today's Dental site via a single access
point in the east corner of the site {located on the southeast boundary line).
This entrance will be stabilized with a 12" thick pad of quarry spans ar crushed
rock (8MP 0105: Stabilized Construction Entrance}. Since the construction
process will be broken into three phases, it may be necessary tv relocate the
construction entrance to the middle of the southeast boundary line. If
relocation is necessary, it will take place following the completion of Phase 2
work. If the entrance becomes filled with dirt, it will be refurbished by
dislodging the riprap and reconstructing the pad. Alternatively, new material
will be added t^ the pad to provide storage for additional sediment.
Sediment transport is not foreseen to be a problem because the site is s^ small
{approximately 0.57 acres). Therefore a wheel wash ar fire bath will not be
required.
Should sediment be tracked on to the street, the roads will be cleaned
thoroughly at the end of each day, or more frequently during wet weather
(BMP C1D7: Construction RoadlParking Area Stabili~ation~. This will be done to
prevent sediment from entering waters ^f the state. All sediment removal will
be dune by means of shoveling or pickup sweeping. !t will then be transported
to a controlled sediment disposal area. Only after sediment is removed in this
manner can the streets be washed. All street wash wastewater will be pumped
back ont^ Site or discharged at an acceptable off-site location. Effort will be
taken to prevent dirty stormwater from discharging into systems tributary to
state surface waters.
Element 3: Control Flow Rates
The Today's dental project does not require a downstream analysis,
stormwater detentianlretention facility, permanent infiltration ponds, ^r any
other type of flaw control structure. This is due to the small site size and
phasing of construction.
Element 4: install Sediment Controls
All starmwater runoff from disturbed areas will pass through the silt fence
{BMP C233: Silt Fence} that is to be installed prior to any land disturbing
activities. It will be installed around the perimeter of each phase. The silt
fence will be inspected weekly at the end ^f the work week and subsequent to
each storm event. Any damage to the silt fence will be repaired immediately.
Sediment accumulation in excess of design limits will be removed from the silt
fence upon identification of the condition and prier to a forecasted storm
event. The construction superintendent responsible for these actions will be
responsible for maintenance of the erosion and sediment control facilities.
E[emerrt 5: Stabilize Soils
AU soils on the Today's Dental site, whether at final grade or not, will not
remain exposed and unworked for more than 2 days during the period from
~ctvber 1 to April 30. Soils will not remain exposed and unworked for more
than 7 days during the period from May 1 tv September 3~. Furthermore, sails
will be stabilized at the end of each shift prier to all holidays ar weekends if
needed. The need for stabilization will be based on the weather forecast.
Sail stabilization is nut foreseen as a problem since the slopes an the site are
mild and well vegetated. However, if stabilization is required, it may be done
with one or more of the fallowing practices: temporary and permanent
seeding, sodding, mulching, plastic covering, erasion control fabrics and
matting, soil application of palyacrylamide {PAM, the early application of
gravel base an areas to be paved, and dust control. If any stockpiles will be
located an site, they will be stabilized from erosion, protected with sediment
trapping measures, and located away from storm drain inlets, waterways, and
drainage channels.
2
Element 6: Protect Slopes
The slopes on the Today's Dental project are very mild and well vegetated.
They range from flat to approximately four percent. Because of these factors,
the site will not require any siape protection. Gff-site starmwater [run-on} is
not an erosion concern because the sail has such a high infiltration capacity.
The proposed site grading will cause runoff to flow from north to south, as it
currently does. However, the proposed site will be divided into two separate
drainage basins. Grading changes will not significantly change the direction of
flow or the potential for slope runoff. All excavated material shall be placed
on the uphill side ^f trenches, consistent with safety and space considerations.
The reasoning for this is to prevent runoff from entering the open trench and
causing erasion.
Element 7: Protect Qrain Inlets
As a means of preventing sediment from entering the conveyance system, all
storm drain inlets will be protected with catch basin inserts (BMP C220: Storm
Drain inlet Protection}. Inlet protection will be installed far existing inlets that
will collect runoff from the construction site and any newly constructed inlets.
Existing inlets in Yelm Avenue, Solberg Street, and Jefferson Street that are
adjacent to the site will be protected. The inlets will be inspected weekly at a
minimum and daily during storm events. At a minimum, sediment will be
removed from the inlet protection devices once sediment has filled one-third
^f the available storage. Approach roads will be kept clean in order to prevent
sediment from reaching inlets.
Element 8: Stabilize Channels and Outlets
The project site has no existing or proposed channels or outlets. Therefore nv
stabilization is required.
Element 9: Control Pollutants
Pollution of stormwater is not anticipated to be a problem an the Today's
Dental project. However, since concrete will be handled on site, BMP C751:
Car~crete Nand[irig will be implemented. This includes, but is not limited to,
washing out concrete only in farmed areas awaiting installation of concrete and
recycling unused concrete. In general, disposal of concrete will take place in
such a manner that it does not violate groundwater or surface water quality
standards.
F[emen# 10: Control ^e-Watering
The need to de-water is not anticipated. However, if it is encountered, water
from trench de-watering shall be conveyed tv a suitable infiltration location on
site yr transported off-site in a vehicle. Off-site transport will take place in a
vehicle such as a vacuum flush truck, for legal disposal in a manner that does
not pollute state waters. Highly turbid or otherwise contaminated dewatering
3
water, such as from construction equipment operation will be handled
separately from stormwater.
Element 11: Maintain 6MPs
All temporary and permanent erosion and sediment control BMPs shall be
maintained and repaired as needed to assure continued performance of their
intended function. All maintenance and repair will be conducted in
accordance with standard procedures far the BMPs.
Sediment control BMPs will be inspected weekly or after arunoff-producing
storm event during the dry season, and daily during the wet season.
Within 30 days after the site has been stabilized, all temporary erasion and
sediment control 6MP's shall be removed. If any 6MP's are required to be in
place longer than 30 days following site stabilization, they will be removed
ante they are no longer needed. All trapped sediment will be removed or
stabilized an site. Sail that has been disturbed due to the removal of BMP's ^r
vegetation shall be permanently stabilized.
F[ement 12: Manage the Project
Site clearing and grading, including the construction of the sediment trap, will
be performed ^nly after the erosion and sediment control measures have been
constructed. To further prevent soil erasion, the project will be divided into
three phases. Phasing the project construction will limit the amount of the
project area that is exposed to construction activities far any given time.
Clearing, grading, and other sail disturbing activities will not take place from
October 1 to April 30 unless it can lae shown to the satisfaction of the local
permitting authority that silt-laden runoff will be prevented from leaving the
site. The local permitting authority shall take enforcement action if sediment
leaves the site causing a violation of the surface water quality standard,
clearing and grading limits are violated, yr erosion and sediment control
measures are not maintained. Activities may be exempt from seasonal
restrictions. They include routine maintenance and repair ^f BMP's, routine
maintenance of public facilities ar existing utility structures that does not
expose soil or result in removal of vegetative cover, and activities where there
is 100% infi[trativn of surface water runoff within the site in approved and
installed erosion and sediment control facilities.
A Certified Professional in Erosion and Sediment Control, who can be on-site or
an-call at all times, will be identified by the contractor. All BMPs will be
inspected, maintained, and repaired as needed to assure continued
performance of their intended function.
4
The Construction SWPPP will he retainers an-site or at a nearby location. It will
be modified whenever there is a change that has, ar could have, a significant
effect on the discharge of pollutants to waters of the state.
2. Project Description
The Today's Qental project is located at the north corner of the Solberg Street
NW and Yelm Avenue W intersection. The site consists of parcels 21724142340
and 21724145220Q, both ^f which are zoned C-1 Commercial. The site lies in
Section 24, Township 17N, Range 1 E. See Appendix A for a vicinity map.
The project is comprised of a 0.57-acre site which will captain a newly
constructed dental office building and parking lat. Construction will be divided
into three phases as a means of maintaining building operations. The site will
have ingresslegress paints from Solberg Street. Site improvements include a
stormwater filtration system, starmwater infiltration system, domestic water
service, fire hydrapts, and STEP sewer system.
Nearly all of the 0.57 acre site will be disturbed during the construction
process. As previously mentioned, site construction will be completed in
phases, which will reduce the environmental impact of the construction
activities. See section 8. Construction Phasing for mare details.
Approximately 354 yd3 of material will cut from the site and less than 50 yd~ of
material will be filled.
3. Existing Site Conditions
The current site has a dental office building on the southwesterly parcel {No.
21724142300] and a single family residence on the northeasterly parcel {Na.
2172414224Q). The existing ground cover is a combination of lawn,
landscaping, and gravel with trees sparsely located around the site perimeter.
In general, the site slopes from north to south with slopes ranging from
relatively flat to 4~. In the current condition, site stormwater sheet flows
from north to south. Based on the soils information and the shallow site
grades, it appears that the site currently infiltrates all of the onsite storm
water.
The Today's Dental site has an upper sod and topsail layer that ranges from 6
inches to one foot in depth. This rests an top of a black ash layer with gravel
that ranges in depth Pram 2.5 to 3.0 feet. Underlying the ash layer is gravely
glacial autwash with cobbles and boulders. This autwash extends to the depth
explored, which was a maximum of 1 Q feet. N^ groundwater was encountered
in any of the soil explorations. The soils are capable ^f infiltrating runoff at a
5
high rate. The design infiltrate rate is suggested to be 1d incheslhour. See
Attachment C for the geotechnica[ report.
There have been no erosion problems associated with this site and we da not
foresee any difficult site conditions. Based on the survey, there appears to be
no fuel tanks or wells an the site.
There are currently no known critical ^r sensitive areas on or near the project
site. However, there is a septic tank and drain field that will require removal.
!t is located near the northernmost point of the southwesterly parcel. The
septic system will be decommissioned per Department of Ecology
requirements.
4. Adjacent Areas
The site is bordered to the northwest by multi-family residences and a
commercial building. Jefferson Avenue NW borders the site to the northeast,
Yelm Avenue W ASR 510 borders the site to the southwest, and Solberg Street
NW borders the site to the southeast.
Based on sail information provided in the geotechnical report, it appears that
the project area runoff infiltrates into the site soils. Since the site has such a
good infiltration ability, stormwater flaw management wilt be achieved by
utilizing an underground infiltration facility. A downstream analysis is not
provided in this report since infiltration will be utilized.
ft appears that the site does not accept any off-site runoff. Based on the
survey and generally mild slopes, current runoff from adjacent sites also
appears to infiltrate into the soil.
5. Critical Areas
There are no known areas on or immediately adjacent to the site that would be
classified as a critical area.
6. Soil
A geotechnical report was completed for the site by E3RA Inc, and has been
included as Attachment C. It indicates that the Today's Dental site has an
upper sod and topsoil layer that ranges from 6 inches to ^ne foot in depth.
This rests an top of a black ash layer with gravel that ranges in depth from ~.5
to 3.4 feet. Underlying the ash layer is gravely glacial outwash with cab6les
and boulders. This outwash extends to the depth explored, which was a
maximum of 10 feet. No groundwater was encountered in any of the sail
6
explorations. Further information regarding the an-site soils can be found in
the geotechnical report dated January 4, 2Qiab (See Appendix C).
E3RA, Inc. has classified the black ash as 5P-SM and the glacial outwash as SP
per the L15C5 Soil Classification system. The soils are capable of infiltrating
runoff at a high rate. The design infiltrate rate is suggested to be 10
ineheslhour.
7. Potential Erosion Problem Areas
All areas of the site that will have exposed underlying soils due to grading have
the potential far erasion problems. However, the erosion and sedimentation
control measures specified on the plans, and the generally flat terrain, will
minimize any potential for erosion and sedimentation problems.
8. Construction Phasing
The construction phasing will take place in the order listed below. Each phase
will include, but may not be limited ta, the indicated tasks. Some portions of
the steps may be performed out of sequence a5 conditions require.
Phase 1:
• lnsta[l STEP system and necessary pipe lines
^ Maintain current dental building operations
Phase 2:
^ Construct new dental building
^ Tie all utilities into new dental building
Phase 3:
^ fJemalish old dental building
• Install storm drainage structures and pipe lines within phase limits
• Construct new curb and gutter, Sidewalk, and parking lot
9. Construction 5ehedule
Construction is anticipated to begin in the spring of 2~0$. The civil site
development work is anticipated to be completed and stabilized during the fail
^f 2fl08. During the time period from October 1 to April 3~ [wet season) no
soils will remain exposed and unworked for more than two days. This will
prevent soils from becoming unstable and more susceptible to erosion.
10. Financiall~wnership Responsibilities
7
The property owner, Judd and Sarah Sherman, will be responsible far bonds
and other required securities.
1 ~ . Engineering Calculations
Due to the small size of the site and construction phasing, neither a sediment
trap nor a pond is required. Therefore engineering calculations are not
necessary.
8
Attachment A
Vicinity Map
J l -1 ~
Attachment B
Site Plan
Attachment C
Geotechnical Report
PG Box 44850
i ac~ma ~:'~'A 98444
253-X37-c4~r
253-537-9zJ1 rax
~~ RI®1
,fanuar;~~ 4, ?OOb
TD6347
Yelrra Dental Center, LLC
~U2 Yclm A~~enue West
Yelnr, WA 98579
Attention: Dr. Sarah Sherman
Subject: Geotechr~icxl Engineering Report
Planned Commercial Deve€opment
502 Yelm Avenue West. PIIJ 2 1 724 1 423DD
107 Solherg Street, PIN 2 1 724 1 422f]D
Yelm, Washington
I7ear Sarah:
F3RA is pleased to submit this report describing the results ofour geatechnicai engineering evaluationfor the
residential development planned at 502 Yelm Ave West and 107 Solherg Street in Yelm, WashinD o.n.
`T'his report has been prepared for the exclusive use ^f Yelm Dental Center, LLC and their consultants, for
specific application to this project, in accordance with generally accepted geotechnical engineering practice.
1.Q SITE AND PROJECT DESCRIPTIpN
T'he planned commercial development is located o.n the corner of SR 510 and SW Solberg St. in Yelrrr,
Washington, as shown ors the enclosed Location ~fap (Figure 1 }, it consists ^f two rectangular parcels that
measure about 80 feet fronting SR S 1 D and 3 DO feet fronting Solberg Street. The project site is curreutty
bordered by SR 5I0 to the southwest, Jefferson Ave to the northeast, and Solber St to the east. Surface
topography is relatively Ievel.
Plans call for the removal of the existing home and dental ^ffice and construction of a new dental office,
associated parking, and a storm water infiltration facility. The properties will have access from Solberg St.
Z.D EXPLORATORY METHODS
We explored surface and sul7surface conditions at the project site on November 18, 2D0G. ^ur exploration
program comprised t€re following elements:
• A surface reconnaissance of the two parcels;
• Ten test pits (designated TP-1 through TP-3}, advanced across the site;
• One Grair-r Size analyses of on-site soils;
Three ll~filtration Tests; and.
.Ianuary 5; 20fl7
T~~39? ! Yelm l~~ntal
A review oCpublished geolorric and seismologic reaps ar~d literature.
ERA. Inc.
'1'ahfe 1 sur~~n~arizes the appraxiznate functional locations «nd termination depths of our subsurface
exl~Ioratioris, and Figure 2 depicts their approxinate relative locations. The following text sections describe
the procedures used for excavation of test pits.
TABLE 1
Al'PRUXIMATE I~OCATIUNS AND DEl'TIIS OF EXPI.QRATIaI~'S
Termination
Depth
Exploration I+'unctionxl Location (feet}
*TP-1 Southwest site g
*TP-2 Central part east site I {}
*TP-~ Central site 1 p
* Includes infiltration test
Tl~e specific number and focatioirs of our explorations were selected in relation to the existing site features,
r.rnder the constraints of surface access, and underground utility conflicts.
It should be realized that the explorations performed and utilized far 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 signif cant 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.
~.1 Test Pit I'rQCedures
C?ur exploratory test pits were excavated with asteel-tracked excavator operated by an independent frrm
working under subcontract to E3IZA. An engineer from our firm continuously observed the test pit
excavations, logged the subsurface conditions, and collected samples. After we logged each test pit, the
excavator operator backfilied it with excavated soils and tamped tl~e surface.
~'lre enclosed Test Pit dogs indicate the vertical sequence of soils and materials encozmtered in each test pit,
based on our field classifications. Where a soil contact was observed to be gradational or undulating, our lags
indicate the average contact depth. We estimated the relative density and consistency of the in-situ soils by
means of the excavation characteristics and the stability of the test pit sidewalls. Our logs also indicate the
approximate depths of any sidewall caving or groundwater seepage observed in the test pits, as weal as all
sample numbers and sampling locations.
2~2 Iniiltratior~ Test Procedures
We performed fallinghead infiltration #ests at a depth afabout 7 feetwithin test pits'I~P-l, 2, and 3. All failiir~
head tests were performed in general accordance with the falling head type infiltration testing procedure
described in the EPA publication Can-site Wastewater Treatment andDisposal System 198, described below,
January 5, 2r747 f=31~A. Inc.
TOE39' 1 Yeftn rental
A 6-inch-diameter PVC pipe was ta.rrrpecl 3 to 5 inches into the soil of the upper part oft}ie infiltration layer,
their 2 inches of coarse, clean drain rocEc was placed at the bottorn of the pipe to prevent scauriiib. Soil was
placed and tamped outside the pipe far stabilization acid to prevent upwellirig oftcstwatcraround the pipe. T'lie
pipe was tlieri tilled twice with 1 foot of water tci pre-saturate iiie test soils. Because, in all cases, 1 foot of
tivater in:Fiitrated the test soils in Iess than 10 minutes, fu~-lher saturafiari was deeEned unitccessary and the
infiltration test proceeded.
The pipe was then tilled tivith fi inches of water, and, because site soils were found to be rapidly pernteable,
the time required for infiltration of the crrtire b inch column ^f water was recorded. We repeated this
procedure tlirce limes at each test location and used only the slowest oftlic 3 recorded irifiltratioti rates in
our analysis.
3.0 SITE CUNI]ITIONS
Tile following sections of text present our observations, measurements, findings, and interpretations regarding,
surface, sail, groundwater, seismic, liquefaction, and inCltrativn conditions.
3.I Surface Conditions
The project site is relatively level with no noticeable change in elevation.
q'he 542 Yelm Ave West parcel currently has an existing Yelm Dental Dffce and 147 Solberg Street currently
has a borne on the lot. The rerrtainder ^f the sites are yard or parking.
Vegetation onsite consists of grass and some small yard trees.
No signs of surface flow, such as stream channels or erosional scars, were noted during our reconnaissance.
No ponds are onsite. Nv seeps or springs were observed.
3.2 Soil Conditions
Qur on-site explorations revealed fairly nearly uniform near-surface sail conditions. Generally, we observed
ail upper sod and topsoil layer that ranged in thickness from'~z to a bit less than l foot ui thickness overlaying a
black ash layer with gravel ranging to a depth of about 2 112 to 3A feet. Underlying the ash layer, we
observed, tv the termi:ration of our explorations, which reached a maximum of about 14 feet, gravely glacial
outwash with cobbles and boulders. The soils appeared to become somewhat sandier arid sligh#ly less tv the
north and west. Caving was noted at deptl~s of about 4 112 to 5 feet in the test pits, suggesting that water will
readily infiltrate at these depths due to the mature of the soils and Iack of silts.
The enclosed exploration logs provide a detailed description of the soil strata encountered in our subsurface
explorations.
3.2.1 Laborafo Testin
Our Grain Size Analyses of the sandy gravel in test pits TP-1, found within the zone where inftltration will
likely occur {7 feet below current grades} indicate that the silt content is in the range of S percent.
l lie m~iSLUre Content of S3il~ i~Jitlriee the ii~iie of rnfltration is ab9i.it ~ perceni aS i~Jeil. ~Ve lnteiYret TiikvSt vft}'le
upper soils as being cIvse to optimum moisture.
The enclosed. labora#ary testing sheets graphically present our test results, and Table 2 summarizes these
results.
.Ianuary 5, 20fl7 ~3RA, Inc.
TD6397 1 Yalm dental
~ TAT3C,E 2
LA]3L?RAT() RY TEST RESULTS FDR NON-ORGANIC ON-SIT E SOILS
Sc}il Sample and Nloisture gravel Content Sand Content S~1t~Crylay ~
Depth Content
(percent}
(percent) Content
{percent} { ercent}
'I'1'-1, 5-1, 7 Feet
_~~ 5.1 __---
7tl
24.8 ~
5.2 --
3.3 Groundwater Conditions
fit the tirrze of our recor~naissance ~f~lovembcr 18, 2006), we did oat observe groundwater in any of our
explorations, wluch extended to depths of up to 1.0 feet. i~lo significant rnottiir~g was observed.
It is oat anticipated that ground water will be er~couritered during typical orisite construction activities.
3.5 Seismic Conditions
Based on our analysis of subsurface exploration lags ar~d our review of published geologic maps, we interpret
sail conditions on the site to correspond with a seismic site class S~, as defined by Table 1615.1.5 oftlze 2003
International $uilding Code (IBC). According to the IBC, tl~e site is Seismic Region 3.
3~5 Ligerefaction Potential
Liquefaction is a sudden increase in pore water pressure and a sudden lflss of soil shear strength caused by
shear strains, as could result from an earthquake. Research has shown that saturated, loose, fzne to medium.
sands with a fines (silt and clay} content less than about 20 percent are most susceptible to liquefaction. We
did not absezve easily liquefiable sails onsite.
3.7 Infiltration Conditions
A storm water infiltration facility is planned for the site. In our three test pits located in the vicinity of this
facility, test pits TF- l , 2, and 3, we observed loose silty sandy gravel with a fines content that averaged about 5
percent. According to the I;J.S.D.A. Textural Triangle, our laboratory analyses of this soil indicate that it is a
gravel, course sand type A with arr infiltration rate of 1 minutelinch.
The results of our infiltration tests are presented in Table 3. Because infiltration was moderately rapid, we
recorded ttre time necessary fora 6 inch high column ofwater to izzfiltrate completely as discussed in section
2.2 above. Based on our fseld testing, the Average lnfiltraiiozr Rate for soils at a depth of7 feet is 3.fl minutes
per inch. After incorporating a Factor of Safety of 2, we recommend a I]esigrr Inf [tration .Rate of 6 minutes per
inch ~ 14 inches per hour}.
TABLE 3
FIELD TNI'ILTATI~N TEST RESULTS
Test Depth below Field Infiltration Rate far 6
Number Location existing grades inch Column
feet
T-I TP-I, parl~ing area 7 20 min16 inches
T-2 TP-2, front yard of home 7 2fl miu16 inches
T-3 TP-3, back yard of home 7 8 minlb inches
January 5, 2007
T~639? ' Y?!m ^ental
E3RA. frc.
4.C3 C(7NCI.liSIflNS AND RECD1VIi1'IENUATIUNS
I'Ians call for the preparation of a new dental office, paved parking, and infiltration of storm water on site. We
~~ffcr the failowirig cnnclusio~~s and recommendations:
• 1_c~tsibility: Based on our freld explorations, research, and analyses, the proposed
development appears feasible From a gcotcchnical standpoint, provided that the
recommendatiar~s iri Section 4 arzd in this report arc followed.
• Foundation Qptiarts: We recommend conventional spread footings supported art firmly
compacted native soils. l~ecornmendations for spread footings arc provided in 5ectit~n 4_
• Floor Dpt10T]5: We recaz~~merid either a concrete slab-on-grade or joist-supported floors for
tlae proposed cornrnercial structure. Some over-excavation will be necessary for slah-ati-grade
floors. Recommendations for slab-on-grade floors are inclatded in Section 4.
• ^nsite Infiltration: }3ased an our onsitc infiltration tests and snits analyses, we recommend at
Design Infiltration Rate of 6 minutes per inch for soils in the vicinity of the planned
infiltration facility.
• Asphalt Pavement: Structrrral fill subbases appear do oat appear to be necessary provided that
sub-grades are compacted to 95 percent maximum dry derrsity. A pavement section,
consisting ^f 2 inches of asphalt pavemer~t aver a 4 inch crushed rock base, is recoxnrnended
for the planned parking area.
The following text sections ofthis report present our specific geotechnical conclusions and recommendations
concerning site preparation, spread footings, slab-on-grade floors, drainage, suhgrade walls, and structural fill.
The WSDDT Standard Specifications and Standard Plans cited herein refer to WS.DDTpuhlications M41-14,
Standard Specij~cations for ,Road, Bridge, and Municipal Construction, and M21-O1, S'tandard Plans for
Road, L'ridge, and llfunicipal Construction, respectively.
4_l Site Preparation
Preparation ofthe project site should involve erosion control, terrAporary drainage, clearing, stripping, cutting,
filling, excavations, and suhgrade compaction.
Erosion Control: Before new construction begins, an appropriate erosion control system should be installed.
This system should collect and f ]ter all surface run off through either silt fencing ar a series of properly placed
and secured straw bales. We anticipate a system of berms and drainage ditches around construction areas will
provide an adequate collection system. If silt fencing is selected as a filter, this fencing fabric should meet the
requirements of WSDDT Standard Specification 9-33.2 Table 3. In addition, silt fencing should embed a
minimum of b inches below existing grade. If straw baling is used as a filter, hales should be secured to the
ground so tlrat they will not shift under th.e weight of retained water. Regardless of the silt filter selected, an
erosion control system requires occasional observation and maintenance. Specifically, holes in the filter and
areas where the filter has shifted above ground srrrface should be replaced or repaired as soon as they are
identified.
Temporary Drainage: We recamrrtend intercepting and diverting any potential sources of surface ar
near-surface water witlxin the construction zones before stripping begins. Because the selection of an
appropriate drainage system will depend an the water quantity, season, weather conditions, construction
January 5. 2007 E3i~A, ]nc.
1-0539 ~ ,~ Yelm Dental
sequence, and contractor's methods, final decisions regarding drainage systems are best made in the Celd at the
time of construction. F3ased ^n our current understanding of the construction plans, surface anti subsurface
cor~ditinns, eve 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 wafer scnrrees have beery co~ztrolled, the construction
areas should be cleared and stripped vfall duff and topsc7ii. Qtrr explorations indicate that athickness of'/ to
i toot of topsoil will be encountered across the siic. Also, it should he reaIiced that if the sirippirrg operation
pr~nceeds during vvet weather, a generally greater stripping depth rniglzt be necessary to remove disturbed
moisture-sensitive soils; therefore, stripping is hest performed during a period of dry weather.
Site 1xcavations: ]3ased on our explorations, we expect that site excavations will encounter loose sails that
can be easily excavated by conventional earth working equipment.
Dervat__ erin~: We do not anticipate dewatering to be tieccssary on this project.
Terxtporary C,ut 5lnpes: All temporary soil slopes associated with site cut#ing or excavations should be
adequately inclined to prevent sloughing and collapse. Temporary cut slopes in site soils should be no steeper
than ]''/2 H:1 V, and should conform to W1SHA regulations.
Subgrade Compaction: exposed subgrades for Footings and floors should be compacted to a firm, unyielding
state before new concrete or fill sails are placed. Any local i7ed zones of looser granular soils observed within
a subgrade slrould be compacted to a density commensurate witty the surrounding soils. In contrast, an}r
organic, soft, ar pumping soils observed within a subgrade should be overexcavated and replaced with a
suitable structural fill material.
Site Filling: Qur conclusions regarding the reuse vfvn-site soils and our comments regarding wet-weather
filling are presented subsequently. Regardless ofsoil type, all fill should be placed and compacted according
to our recommendations presented in the Structural Fill section of this report. Specifcally, building pad fill
soil should be compacted to a uniform density ^f at ?east 95 percent {based on ASTM:17-1557}.
[]n-Site Soils: We offer the following evaluation ofthese vn-site soils in relation to potential use as structural
fill:
Su rcial Sod and 7'n soil: The sod and topsoil mantling the site is oat suitable for use as
structural fill under any circumstances, due to high organic content. L'onsequentty, these
materials can be used only for non-structural purposes, such as in landscaping areas.
Elack.4sh: T17e black silty sandy gravel that underlies the site is cut~ently near ^ptimum
moisture content and might possibly be reused as structural fill, depending on conditions at
time of construction. [t is more moisture sensitive then the outwash below and will be difficult
to reuse during wet weather conditions.
Glacial Qutwash: The sandy gravel with cobbles and boulders that underlies the site is
currently near optimum moisture content and can be reused as structural fill. This soil is less
moisture sensitive and can likely be reused in wet weather conditions.
4~2 Spread Footings
In our opinion, conventional spread footings wit[ provide adequate support #'or the proposed structure if the
subgrades are properly prepared. We offer the following comments and recommendations for purposes of
January 5. 2CJG7
rQ63G7 ,~ Yelm Dental
tooting desigtt and construction.
E3RA. Inc.
E~ooting 1-)epths and Widths: For frost and erosion proieclioi~, the base of alt exterior footings sltouLd bear at
least 24 inches bclo~v adjacent Outside grades. 'l'o limit past-constc~uction settlements, contirtuous (wall) and
is{plated [column} footings should be at (cast I 8 and 24 inches wide, respectively.
Bearing Sl<bgrade.s and Bearing Presstres: ~I'ho native ashy layer and glacial ot3twash underlying the prapttsed
building footprint at subgrade elevations will adequately support spread footings. In general, before footing
concrete is placed, any localized zac~es of loose soils exposed across the footicig subgrades should be
compacted to a firm, unyielding en~~dition, acid any localized zones of soft, organic, or debris-laden soils should
[3e t~ver-excavated and replaced with suitable structura[ till.
Sub rode Observation: Ail tooting subgrades should consist of either firm, unyielding, dative soils or strita.blc
structural fill materials. Footic~gs should never be cast atop loose, soft, or frocen soil, slough, debris, existing
uncontrolled fill, ar surfaces covered by standing water. We recommend thatthe condition of all subgrades be
observed by ati E3.IL4 representative before any concrete is placed.
Bearing Pressures: In our opinion, Far static loading, footings that hear on properly prepared subgrades can be
designed far the maximum allowable soil bearing pressures ^f 25Q0 psf: A one-third increase in allowable soil
bearing capacity may be used for short-term loads created by seiscnic or wind related activities.
Faotin Settlements: We estimate that total post-construction settlements of properly designed footings
bearing on properly prepared subgrades wilt not exceed 1 incl. 17ifferential settlements for comparably loaded
elements may approach one-half of this value over horizontal distances of approximately 50 feet.
Footing and.5temwall Backfill: To provide erosion protection and lateral Iaad resistance, we recommend that
all footing excavations be baGkfi.lled on bath sides ^f the footings, retaining walls, and stemwalls after the
concrete Itas cured. Either imparted stntctural fill ornon-organic on-site soils can be used for this purpose,
contingent an suitable moisture content at the time ofplacement. Regardless ofsoil type, all footing backfill
soil should be compacted to a density of at least 94 percent {based ^n ASTM:D-1557}.
Lateral Resistance: Footings that have been properly backfilled as recommended above will resist latera[
movements by means of passive earth pressure and base friction. We recommend using an allowable passive
earth pressure of 30Q psf for the granular hackfil[. We recommend an allowable base friction coefficient of
0.35 for granular sails.
4.3 Slab-Dn-Grade Floors
In our opinion, soil-supported slab-on-grade floors can be used in the proposed structure if the subgrades are
properly prepared. We offer the following comments and recommendations concerning slab-on-grade floors.
Floor Subbase; Structural fill suhhases do oat appear to be needed under soil-supported slab-orr-grade floors, if
the existing native subgrade can he thoroughly compacted. If subgrade compaction is oat feasible, we
recorrtmend that granular fll be placed to a depth of 12 inches below finish subgrade.
Capillary Break and Vapor Barrier: To retard the upward wicking of groundwater beneath the floox slab icy
areas where moisture sensitive floor coverings will be used, such as offices, we recommend that a capillary
bream be placed aver the subgrade. Ideally, this capillary break would consist of a 4-inch-flick layer of pea
gravel ar other clean, uniform, well-rounded gravel, but clean angular gravel can be used if it adequately
prevents capillary wicking. In additiatx, a layer of plastic sheeting (such as Crosstttff, Visqueen, or Moistop)
should be placed over the capillary break to serve as a vapor barrier. During subsequent casting ofthe concrete
slab, the contractor sltould exercise care to avoid punchrring this vapor barrier.
Januar„~ ~: 2Qfl i
X46397 i Yalm Qentai
4.4 Drair~aee Svstems
1r 3 Rti. inc.
~L'e offer the following recommendations artd comments for drainage design for construction purposes.
I'er~imeter Drains: We recommend that the buildings be encircled with a perimeter drain system to collect
seepage eater. This drain should consist of a 4-inch-diameter perforated pipe will-tin an envelope ofpea gravel
or washed ruck, extending at least fi iucltes on all sides of the pipe_ Tltc gravel envelope should he wrapped
with filter fabric to reduce the migration of rues from the surrounding sails. Ideally, the drain invert would be
installed no more than; 8 inches above the base of tl~e perimeter footings.
5ubfiaor Drains: 13ccause fl{3or subgra.des will on a granular material, we do not recommend the use of
subfloor d~~ains.
Discharge Considerations: If possible, all perimeter drains sllouid discharge to a suitable disccarge location.
Runoff VG'atcr: Roof-runoff and surface-runoff water should nU! discharge into the perimeter drain system.
Instead, these sources should discharge ir~ta separate tightline pipes and be routed away from the building to a
storm drain or other appropriate location,
[grading and Capping: Final site grades should slope downward awa~• from the building so that runoff water
will flaw by gravity to suitable collection poirzts, rather than ponding near the building. ideally, the area
surrounding the building would be capped with concrete, asphalt, or iow-permeability (silty] soils to minimize
or preclude surface-water infiltration.
4__,_3 Asphalf Pavement
Since asphaltic pavements will he used for the parking area and, possibly, driveways, we offer tl~e following
comments and recommendations far pavement design and construction.
Sub ode Pre oration: Ail soil subgrades should be thoroughly compacted, then proof-rolled with a Loaded
dump truck or heavy compactor. Any localized zones ofyielding 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.
i'avement Materials: For the base course, we recommend using imported crushed rock. Native materials shall
be adequate as a subbase.
Conventional Asphalt Sections: A conventional pavement section typically comprises an asphalt concrete
pavement over a crushed rock base course. Using the estimated design values stated above, we recommend
using the following conventional pavement sections:
Minimum Thickness
Pavement Course Parking Areas High Traffic arrd Drive~cvay
Areas
Asphalt Canorete Pavement 2 inches 3 inches
Cri1gh rf R nrk R
e_ aye Q inches ~ inrhec
Granular Fill Subbase (if needed} 12 inches I2
Compaction and Qbservation: All subbase and base course material should be compacted to at least 95 percent
January 5. 2ufl; E3RA, Enc.
i Ob39 i i" Yel?~ 17ental
ofthe Modified Proctor maximum dry density (A5"1~M ll-l X57), and all asphalt concrete should be compacted
to at least 92 percent of the Rice value (ASTM U-2[]41 }. We recommend that an E3R~1 representative he
retained to observe the compaction of each course before any overlying layer is placed. For the subbase and
pavement course, compactio~~ is best observed by means of fr'eguent density testing. For the base cot~rse,
me#hodology observatio~ls and hand-probing are snore appropriate than density testing.
Pavement i.ife and Maintenance: IVo asphaltic pavement is maintenance-free. The above described pavement
sections present our minimun; recommendatinnsfor an average level of performance during a ZO-year design
life; therefore, an a.vcrage level ofmaintei~ance will likely be required. Furtltermore, a2fl-year paverrtent life
typically assumes tl}at are overlay will he placed after about l U years. 'I']ricker asphalt andlar thicker base and
subbase courses would of'Icr better lot~g-term perfortnartce, but would cost mare initially; thinner courses
would be mare susceptible to "alligator" cracking and other failure triodes. As such, pavement design cart be
considered a compromise between a High initial cost and low maintenance costs versus a low initial cast and
higher maintenance costs.
4.6 Structural frill
The teen "structural fill" refers to any placed under foundations, retaining walls, slab-on-grade floors,
sidewalks, pavements, and other structures. fur comments, conclusions, and recommendations coz~cerning
structural ftll 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 barrow" or "pit-run"}, and miscellaneous
mixtures of silt, sand, and gravel. Recycled asphalt, concrete, and g3ass, which are derived from pulverizing
the parent materials, are also potentially useful as structural fill in certain applications. Soils used for structural
fill should oat cnrztain any organic matter or debris, nor any individual particles greater than about 6 inches itz
diatrreter.
Fill Placemetrt: Clean sand, granulithic gravel, crushed rock, soil mixtures, and recycled materials should be
placed in horizontal lifts not exceeding 8 inches in loose thickness, and each Tilt should be thoroughly
compacted with a mechanica3 cornpactor.
Compaction Criteria: Using the Modified Proctor test (ASTM:D-1557} as a standard, we recommend that
structural fill used for various an-site applications be compacted to the following minimum densities:
Fill Application Nlinimum
Campxctian
Footing subgrade and bearing pad 95 percent
Foundation backfil[ 9d percent
Slab-on-grade floor subgrade and subbase 95 percent
Pavement Subgrade (upper 2 feet) 95 percent
Pavetttent 5ubgrade (below ~ feet) 90 percent
Sub~'ade Observation and Compaction Testing: Regardless of material or location, al] structural fill should be
placed aver firm, unyielding subgrades prepared in accordance with the Site Preparation section ofthis 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
~iiaCcTiciit aid tiiai adcyiiaCy Of St7ii CV111pQct1V11 G~l'3rtj m&y uc BVaiuateu aS eaithPrDrk progreSSeS.
.lanuarv 5. 207 ~=3RA, [nc.
Tr~E39i 1 Ye[m Dental
Soil Iylc~isture Consideratior;s: The suitability or soils used for structural fill depends primarily on their
~7rain-sine distribution and moisture content when they are placed. As the "fines" content (that soil fraction
p~>"SSing the 11.5. Nc}. 2[70 Sieve) increases, soils bccart~e rnare sensitive to srnall changes in moisture content.
Soils containiFig more than about 5 percent tines (h}' ~~eight} cannt~t be consistently compacted to a firm,
rrr~yielding condition wl~e~~ tl~e roisture co~~lent is more than 2 percentage points above or below optimum.
For fill placement during ~~~et-weather site work, we recommend using "clean" rill, which refers to soils that
have a fuses content of 5 percent or less (by weight} based ore the soil fraction p~tssir~g tl~e U.S. No. 4 Sieve.
~.d RECf?IVIl17F1VDEl7 ADI7TTIC7I~IAL SERV(CF5
Because the future performance and integrity o}' the structural elements will depend largely on proper site
preparation, drainage, fill placement, and construction procedures, monitoring and testing by experienced
gevtecl~nica! persormcf should be considered a.n integral part ofthe construction process. Consequently, we
recommend that E~R~1 be retained to l~ravide the following post-report services:
• Review all construction plans and specifications to verity that our design criteria presented in
this report have been properly integrated into the design;
• Prepare a letter addressing relevant review cornrnertts cif required by the City of Yelm};
• C}.ieck all completed subgrades for footings and slab-an-grade floors before concrete is
poured, in ^rder to verify their bearing capacity; and
• Prepare apost-construction letter summarizing all field observations, inspections, and test
results (if required by the City of Sumner}.
GA CLQSLTRE
The conclusions and recommendations presented in th is report are based, in part, an the explorations that we
observed for this study; therefore, if variations in the subgrade conditions are observed at a later time, woe may
need to modify this report to re#lect 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 geotecl3n seal personnel should be considered an in#egral part of the
construction process. E3RA is available to provide geotechnical monitoring of soils throughout construction.
.~ar~uary 5, 2~DT
~d5397 J Y~km E]ental
f<3RA. fnc.
We appreciate the opportunity to be of service on this project. [fyou have any questiotas rcgardina this report
or at~y aspec#s of tl~e project, please feel free to contact onr office.
Sincerely,
1r3RA, Ir-rc.
iryS $' '. to ~ ~" ~ ~ `--~
6 - C,.-..sue ~ ~.} v' r~
f
r - ~ ~~ 'P ~ s~ t -
~.~~~ r~~A~Y j ~.. ~~~ r°~ G [ - Cis ~ d
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D3--l~-cam
~'asey R. Lnwe, .E.~.'r.
Staff Engineer
CRLIJEL
James E Brigham, P.L.
Principal Engineer
Enclosures: Figure l Location reap
Figure 2 - Site & Exploration Plan
~tttachrraent: Test Pii Logs TP-1 through TP-3, Sieve 14nalysi.s
January 5; 2007
i 06397 J Yelm ^entai
TEST PIT L~[:S -YeIm 1)elltEal []ffice
Dcnth (feet)
Test 1'if TP-Y
Material 13esc rin tin n
E3RA. fnc.
Location: Parking at existing dental off-ice
Approximate ground surface elevation: Littktto~~n
0.0 - U.7 Crushed rock
0.7 2.0 Medium dense, moist, black ash silty sandy gravel witf~ boulders and cobhies (S1'-SM)
2.0 - 3.5 Medium Dense, moist, tan sandy gravel wlsift, cabbies, and boulders (S1'}.
Test pit terminated at approximately 4.5 Feet
Moderate caving observed at 4.5 feet
No groundwater nr mottling noted
Death (feet}
Test Pit TP-2
Material Dcscriotion
Lncati.on: Existing home front yard
Approximate mound surface elevation: Unknown
0.0 - 0.5 Top Soil
0.7 - 2.5 Medium dense, moist, black ash silty sandy gravel with boulders and cobbles (SP-5M)
2.5 - 10A Medium Dense, moist, tan sandy gravel wlsilt, cobbles, and Moulders (SP).
Test pit terminated at apprnxirnately 1Q feet
Moderate caving observed at 5 feet
No groundwater or mottling noted
Death (feet)
Test Pit TP-3
Material ^escr?ptio^
.Location: Existing home backyard
Approximate ground surface clevativn: Unknown
O.Q - 0.6 Crushed ruck
Q.6 - 2.0 Medium dense, moist, black ash silty sandy gl-avel ~~~I boulders and cobbles (SP-SM}
2.0 - 5.5 Medium Dense, moist, tan sandy gravel with silt, cobbles, and boulders (SP}.
5.5 - i4.0 Medium Dense, tan sandy gravellgravelly sand (SP}.
Test pit terminated at approximately 10 feet
Slight ca~+ing observed at 5.5 feet
No groundwater or mottling noted
5am~le Nn.
S-1
5-2
Sawn.
S-1
Sample 1Va.
S-1
Date Excavated: 11118106 Logged by: CitL
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