23-101282-TIR-11.09.23www.tebaldiengineering.com P.O. Box 121, Sumner, WA 98390 (206) 450-5096
Drainage Memorandum
To: City of Federal Way
From: Chris Tebaldi, P.E., Tebaldi Engineering, LLC
Date: July 21, 2023
Re: Barnes Short Plat
7/21/23
This project is located in the City of Federal way on tax parcel number 5153200260. The project proposed a
2 lot short plat on the subject property. The property currently has an existing single-family residence that
will remain as a part of the project. The remaining lot will be utilized for the construction of a new single-
family residence along with the associated driveway and utilities. The existing topography ranges from
elevation 326 to elevation 270. The site gently slopes from east to west and the slope ranges from 0 to 40
percent. The property currently has an existing single-family residence that will remain. The property has a
geological hazard area on-site as in the form of slopes over 40%. Those slopes will remain undisturbed as a
part of the development.
The proposed development consists of the new impervious surface as indicated below.
The proposed project is subject to the City of Federal way Requirements which follow the 2021 King County
Stormwater Manual (KCSWDM). Per the drainage review requirements flow chart, the project qualifies for
Simplified Drainage Review since it is inside of the Urban Growth Area on till soils with less than 7,947 square
Area Summary
Developed
Proposed house 2,500 SF
Proposed permeable driveway 993 SF
Proposed on-site walk/patio 500 SF
Proposed offsite sidewalk 946 SF
Total 2,124 SF
feet of target impervious. As the project is on a lot larger than 22,000 square feet the Large Lot BMP
Requirements apply. The Large Lot BMP Requirements have been summarized below for feasibility:
Full Dispersion: This BMP is infeasible due to lack of available flow path and native vegetation.
Full Infiltration: This BMP is infeasible as the existing soils are not outwash soils.
Limited Infiltration: This BMP is feasible and will be utilized as a part of the development. The sandy loam
soils will be used to size an infiltration trench for the proposed residence using 36 lineal feet of 2-foot wide
trench (72 square feet) per 1,000 square feet of tributary impervious.
Bioretention: Not applicable as infiltration trenches and permeable pavement are proposed.
Permeable Pavement: Permeable pavement is proposed for new driveway and on-site sidewalk surfaces.
Standard cement concrete sidewalk is proposed for the frontage sidewalk as the area has been subjected to
compaction by vehicular and pedestrian traffic.
Basic Dispersion: Not applicable as infiltration trenches and permeable pavement are proposed.
Reduced Impervious Surface Credit: Not applicable as infiltration trenches and permeable pavement are
proposed.
Native Growth Retention Credit: Not applicable as infiltration trenches and permeable pavement are
proposed.
Soil Amendment: All disturbed soils will be amended in accordance with the King County manual. Topsoil
retention details have been added to the plans.
Perforated Pipe Connection: Not applicable as infiltration trenches and permeable pavement are proposed.
Based on the above analysis, the BMPs and storm drainage design for the proposed development have been
designed in accordance with the City of Federal Way and King County requirements. The rooftop will be
infiltrated via infiltration trench. The driveways and on-site walkways will utilize permeable pavement. The
proposed improvements will not have an adverse impact on the downstream storm drainage system.
& A S S O C I A T E S, I N C.
GEOTECHNICAL ENGINEERING ENVIRONMENTAL ENGINEERING
CONSTRUCTION TESTING &INSPECTION
Offices Serving the Western United States
825 Center Street, Suite A Tacoma, Washington 98409 (253) 939-2500 Fax: (253) 939-2556
May 24, 2023 Krazan Project No.062-22023
Page 1 of 4
Mr. Ron Barnes Tel:(206)688-6698
Innovative Panel, LP Email:ron.barnes@fastwalls.com
17837 1st Ave. S, #531
Normandy Park, WA 98148
Reference:Geotechnical Engineering Investigation Addendum Letter
Proposed Barnes Short Plat
1298 301st Street
Federal Way, WA
Dear Mr.Barnes,
This addendum letter presents the results of our infiltration evaluation for the proposed Barnes Short Plat
project located at 1298 301st Street in Federal Way, WA.
On November 30, 2022, Krazan and Associates prepared a Geotechnical Engineering Investigation report
for this project. Based on our communication with you and the project civil engineer, we understand that
the stormwater runoff from the proposed development will need to be managed on-site,and infiltration
trenches are being considered.
Our original report did not include an evaluation of infiltration feasibility of the on-site soils. Therefore,
we have been requested to provide an evaluation of infiltration feasibility of the on-site soils and provide
design infiltration rates, if feasible.Our services were performed in general accordance with our change
order for this project,dated May 4, 2023 (Krazan proposal number G23016WAT), and included the
following:
Exploration of the subsurface soil and groundwater conditions at the potential infiltration location
by observing and documenting two (2) test pits excavated by the client. The test pits will likely be
excavated to about 6 to 7 feet below existing ground surface (bgs).
Obtain soil samples and perform laboratory testing. Laboratory testing will include grain size
analysis.
Provide comprehensive test pit logs including soil stratification and classification, and groundwater
levels where applicable.
KA Project No.062-22023
Proposed Barnes Short Plat
Federal Way, WA
May 24, 2023
Page No.2
Offices Serving the Western United States
825 Center Street, Suite A Tacoma, Washington 98409 (253) 939-2500 Fax: (253) 939-2556
Perform two (2) infiltration tests utilizing EPA Falling Head Percolation Test Procedure in
accordance with Department of Ecology (DOE) 2014 Stormwater Management Manual for
Western Washington (SWMMWW) as referenced in the 2021 King County Surface Water Design
Manual (KCSWDM), Appendix C.
Perform engineering analysis utilizing the data acquired from the test pits,infiltration tests and
laboratory testing, and prepare an engineering addendum letter.
Field Investigation and Findings
On May 9, 2023, two (2) exploratory test pits were completed to evaluate the subsurface soil and
groundwater conditions at the potential infiltration location. Two test pits, designated TP-1 and TP-2, were
excavated by an excavator operated by the client.The test pits were initially excavated to about 3.5 feet
bgs in order to perform the infiltration tests. The infiltration tests were performed in accordance with the
DOE 2014 SWMMWW as referenced in the 2021 KCSWDM, Appendix C.
Test Procedure:After the excavation to the planned testing elevation, a six-inch diameter PVC pipe was
placed at the bottom of excavation. The excavated soils were backfilled around the pipe. Approximately
two-inches of clean gravel was placed in the bottom of the PVC pipe to protect the soil from scouring as
water was poured into the apparatus. Water was added to the test holes in order to saturate the soils prior
to taking the timed water level measurements. After the soaking period, the elapsed times for changes in
water levels were measured to determine the approximate uncorrected infiltration rates for the soils.
After the infiltration tests were completed, the test pits were over-excavated to evaluate whether water from
the test had mounded on a shallow restrictive layer or if the water had continued to flow deeper into the
subsurface.TP-1 and TP-2 were over-excavated to depths of about 6.5 and 6.0 feet bgs respectively.We
did not observe signs of seasonal high groundwater (mottling) or restrictive layer or mounded water from
the infiltration tests up to the maximum explored depths of each test pit.
The test pits exposed approximately 2 feet of forest duff consisting of an accumulation of organic forest
debris and roots in a dark brown silty sand matrix. The duff was underlain by medium dense, moist, tan
sand (SP) containing occasional cobbles and boulders in TP-1, with tan poorly graded sand with silt (SP-
SM)and occasional cobbles in TP-2, to the maximum explored depths of each test pit.Detailed test pit logs
are provided as an attachment to this letter.The approximate location of each test pit is shown on the Site
Plan (Figure A).
A geologist from Krazan &Associates performed the infiltration tests, examined the soils and geologic
conditions exposed, obtained samples of the subsurface soils, and maintained logs of the exposed soils and
documented the infiltration test results.
The soils exposed in the excavation were examined and visually classified in accordance with the Unified
Soil Classification System (USCS).Representative samples of the exposed soils in the test pits were
KA Project No.062-22023
Proposed Barnes Short Plat
Federal Way, WA
May 24, 2023
Page No.3
Offices Serving the Western United States
825 Center Street, Suite A Tacoma, Washington 98409 (253) 939-2500 Fax: (253) 939-2556
collected and sealed in plastic bags.These samples were transported to our laboratory for further
examination and testing.
Laboratory Testing
The laboratory testing program was developed primarily to determine the index properties of the soils. Test
results were used for soil classification and as criteria for determining the engineering suitability of the
subsurface materials encountered. The laboratory test results are provided as an attachment with this letter.
Conclusions and Recommendations
The design infiltration rate was developed utilizing the EPA Falling Head Percolation Test Procedure in
accordance with the DOE 2014 SMMWW as referenced in the 2021 KCSWDM,Appendix C.This method
provides a field test to measure the percolation rate for the subsurface soils encountered below the proposed
infiltration facilities.Correction factors are then applied to this measured value to account for testing
methods, facility geometry and plugging of soils. The correction factors utilized are listed below:
Fgeometry =1
Ftesting = 0.3
Fplugging = 0.7 (for medium sands)
Based on the infiltration tests results, it is our opinion that the infiltration trench maybe designed using a
long-term infiltration rate of 2 inches per hour utilizing the native sands exposed in the northwest portion
of the site. In our opinion, the design infiltration rate should be appropriate provided that the planned
pretreatment measures for control of total suspended solids are adequately maintained.The on-site
stormwater infiltration system should be designed by a Washington State Licensed Civil Engineer.
Changes in soil conditions and the corresponding infiltration rates are possible at different locations and
depths.Accordingly, we recommend that the subsurface soils be evaluated during construction by a
representative of the geotechnical engineer.
We understand that a single-family residence may be designed and constructed at the site. A setback
between the foundation and infiltration trench may be needed. We should evaluate the setback once the
location of the structure and infiltration have been finalized.
All the recommendations presented in our original geotechnical report, dated November 30, 2022 should
be followed.
KA Project No.062-22023
Proposed Barnes Short Plat
Federal Way, WA
May 24, 2023
Page No.4
Offices Serving the Western United States
825 Center Street, Suite A Tacoma, Washington 98409 (253) 939-2500 Fax: (253) 939-2556
Closure
We appreciate the opportunity to provide service to you on this project. If you have any questions, or if we
may be of further assistance, please do not hesitate to contact our office at (253) 939-2500.
Respectfully submitted,
KRAZAN & ASSOCIATES, INC.
Vijay Chaudhary, P.E.
Project Engineer
KC:VC
Attachments:Site Plan (Figure A)
Test Pit Logs
Laboratory Results
& A S S O C I A T E S, I N C.
Site Plan
Reference: Conceptual site Plan prepared by
Tebaldi Engineering LLC and dated August 1,
2022.
Project: Proposed Barnes Short Plat CO#1, 1298 SW 301st Street, Federal Way, WA
.
Date: May 2023 Project Number: 062-22023 CO#1
Drawn By: KC Not To Scale Figure A
Number and Approximate Location
of Test Pit
Project Number:
0
1 of 1
DUFF - Accumulation of organic debris and roots, in a
dark brown organic silty sand matrix, soft, moist
Tan Poorly Graded SAND (SP), medium dense,
moist, occasional cobbles up to 10-inches
Grab S-1
% G = 5
% Sa = 91
% Si/Cl = 4
5.9.2023
Contractor:
1290 SW 301st Street, Federal Way, WA Innovative Panel, LP
DateEquipment:
Vijay Chaudhary 5.9.2023 Bobcat E32 Excavator
Ground Elevation:Total Depth:
Not Encountered 279 +/- feet 6.5 feet
Elev. (feet)Depth (feet)Sample IDGraphic LogSample TypeClient:Test Pit No.:TP-1Proposed Barnes Short Plat 062-22023 Innovative Panel, LP
Kelby Carambot 5.9.2023
Project Manager:
Field Engineer:
Project:
Groundwater Depth:
Backfilled:
3
4
5
6
Page
Test pit terminated at 6.5 feet
Started:
Completed:
Location:
Soil
Classification Notes
1
2
Infiltration testing
performed at
about 3.5 feet.
Project Number:
0
1 of 1Page
Test pit terminated at 6.0 feet
5
6
3
Grab
2
Tan Poorly Graded SAND (SP-SM) with silt, medium
dense, moist, occasional cobbles, boulders
S-1 % G = 4
% Sa = 91
% Si/Cl = 54
Notes
DUFF - Accumulation of organic debris and roots, in a
dark brown organic silty sand matrix, soft, moist1Elev. (feet)Depth (feet)Sample TypeSample IDGraphic LogSoil
Classification
Equipment:
Vijay Chaudhary
Groundwater Depth:Ground Elevation:Total Depth:
Not Encountered 279 +/- feet 6.0 feet
Kelby Carambot 5.9.2023
Backfilled:
5.9.2023
Project: Client:Test Pit No.:TP-2Proposed Barnes Short Plat 062-22023 Innovative Panel, LP
S-2Grab
Becomes Dense
5.9.2023 Bobcat E32 Excavator
Field Engineer: Completed:
Location: Contractor:
1290 SW 301st Street, Federal Way, WA Innovative Panel, LP
Project Manager:DateStarted:
Infiltration testing
performed at
about 3.5 feet.
Particle Size Distribution Report
PERCENT FINER0
10
20
30
40
50
60
70
80
90
100
GRAIN SIZE - mm.
0.0010.010.1110100
% +3"Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
0 0 5 24 21 46 46 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Test Results (ASTM C117 & ASTM C136)
Opening Percent Spec.*Pass?
Size Finer (Percent)(X=Fail)
Material Description
Atterberg Limits (ASTM D 4318)
Classification
Coefficients
Date Received:Date Tested:
Tested By:
Checked By:
Title:
Date Sampled:Source of Sample: TP-1 Depth: 3.5'
Sample Number: 23L242
Client:
Project:
Project No:Figure
Brown Sand
Sampled onsite
Sampled by K.Carambot3/4"
5/8"
1/2"
3/8"
#4
#8
#10
#16
#20
#40
#60
#80
#100
#200
100
98
98
98
95
78
71
58
56
50
16
7
5
3.7
NP NV NP
SP A-1-b
3.4747 2.8767 1.3654
0.4257 0.3109 0.2436
0.2121 6.44 0.33
sample ID:23L242
Sample date: 5/9/23
5/10/23 5/11/23
I.Teriong
V.Chaudhary
Project Engineer
5/9/23
Innovative Panel, LP
Proposed Short Plat
062-22023
PL=LL=PI=
USCS (D 2487)=AASHTO (M 145)=
D90=D85=D60=
D50=D30=D15=
D10=Cu=Cc=
Remarks
*(no specification provided)
Particle Size Distribution Report
PERCENT FINER0
10
20
30
40
50
60
70
80
90
100
GRAIN SIZE - mm.
0.0010.010.1110100
% +3"Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
0 0 4 30 26 35 56 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Test Results (ASTM C117 & ASTM C136)
Opening Percent Spec.*Pass?
Size Finer (Percent)(X=Fail)
Material Description
Atterberg Limits (ASTM D 4318)
Classification
Coefficients
Date Received:Date Tested:
Tested By:
Checked By:
Title:
Source of Sample: TP-2 Depth: 3.5' Date Sampled: Sample Number: 23L243
Client:
Project:
Project No:Figure
Brown Sand with Silt
Sampled onsite
Sampled by K.Carambot1/2"
3/8"
#4
#8
#10
#16
#20
#40
#60
#80
#100
#200
100
99
96
74
66
47
46
40
14
7
6
5.0
NP NV NP
SP-SM A-1-b
3.5416 3.0205 1.7823
1.3486 0.3427 0.2560
0.2166 8.23 0.30
Sample ID:23L243
Sample date: 5/9/23
5/10/23 5/11/23
I.Teriong
V,Chaudhary
Project Engineer
5/9/23
Innovative Panel, LP
Proposed Short Plat
062-22023
PL=LL=PI=
USCS (D 2487)=AASHTO (M 145)=
D90=D85=D60=
D50=D30=D15=
D10=Cu=Cc=
Remarks
*(no specification provided)