24-100334-Preliminary TIR-01.31.24TECHNICAL INFORMATION REPORT
FOR
SAAD 2951h SHORT PLAT
CLIENT:
Emerald Real Estate Investments LLC
6208 S 300th St
Phone: (253) 394-6346
PREPARED BY:
Daley-Morrow-Poblete, Inc.
726 Auburn Way North
Auburn, WA 98002
Phone: (253) 333-2200
PROJECT No. 23-440
DATE: Jan. 8, 2024
4�
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2 52
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STAMP NOT VALID UNLESS SIGNED & DATED
TABLE OF CONTENTS
DESCRIPTION PAGE
SECTION I — PROJECT OVERVIEW
Project Overview
1
Figure 1 - TIR Worksheet
2-6
Figure 2 - Vicinity Map
7
Figure 3 — Basin Map
8
Figure 4 - Soils Map
9 - 10
SECTION II — CONDITIONS AND REQUIREMENTS SUMMARY
Core Requirements 1-2
SECTION III — OFF -SITE ANALYSIS
Off -Site Analysis 1 —3
SECTION IV — FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND
DESIGN
Existing Site Hydrology
1
Developed Site Hydrology
1
Performance Standards
1
Flow Control System
1-2
Water Quality System
2
Peale Flow Analysis
3 - 4
SECTION V — CONVEYANCE SYSTEM ANALYSIS AND DESIGN N/A
SECTION VI — SPECIAL REPORTS AND STUDIES
Geotechnical Evaluation 1 - 16
SECTION VII — OTHER PERMITS N/A
SECTION VIII — C.S.W.P.P. PLAN ANALYSIS AND DESIGN
T.E.S.C. 1
SECTION IX — BOND QUANTITIES, FACILITY SUMMARIES AND DECLARATION OF
COVENANT
Bond Quantities Worksheet To be added if required
Storm Facility Summary Sheet N/A
Declaration of Covenant To be added if required
SECTION X — OPERATIONS AND MAINTENANCE MANUAL
Operation and Maintenance 1-3
SECTION I
PROJECT OVERVIEW
PROJECT OVERVIEW
The project site, with Tax parcel 515180-0035 is within the northeast quarter of Section 5,
Township 21 North, Range 4 East, Willamette Meridian, King County, Washington. The
property is on the south side of S 295th Place and is approximately 450 feet east of the
intersection of S 295th Place SE and 9th Place S. The site address is 1013 S 295th Place, Federal
Way, WA 98003.
The 0.55-acre property is zoned RS9.6 and is surrounded by single-family homes. The
irregularly -shaped project site is presently developed with a single-family home with
landscaping and a wooded area in the south side of the lot. The site slopes to the northwest and
southwest. Site soils are classified as Arents, Alderwood material (AmC) by the USDA Natural
Resources Conservation Services. However, the Geotechnical Evaluation in Section VI indicates
that the site is underlain by Vashon Recessional Outwash and is suitable for infiltration.
It is proposed to subdivide the property into 2 lots, with the existing home remaining in Lot 1.
Utility services will be provided to the proposed Lot 2. Flow Control BMPs will be provided to
the proposed impervious surfaces.
We believe that with implementation of the Flow Control BMPs, this project should not have any
adverse effects on the downstream properties.
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
Part 1 PROJECT OWNER AND
PROJECT ENGINEER
Project Owner 04CX41.0 AC4,e
I11,9-Y "CIV 7S I. L C
Phone K?-513),2 Y!g— 43 �14
Address 2,o 5 3 0o lW
AIddemd, WA 9 001
Project Engineer XZ I-K 190
Company 1/11/f ,
Phone 2. 3) 3.3.3- Z Z00
Part 3 TYPE OF PERMIT APPLICATION
U Land use (e.g.,Subdivision Short Sub / UPD)
❑ Building (e.g.,M/F / Commercial / SFR)
❑ Clearing and Grading
❑ Right -of -Way Use
❑ Other
Part 2 PROJECT LOCATION AND
DESCRIPTION
Project Name .S AA 0 2 ?5 7-11
DLS-Permitting
Permit #
Location Township 2/ /t/
NE
Range
Section
Site Address /0/3 , jo 2!?(rrl'l
FG,9F-/�A e- WAY
Part 4 OTHER REVIEWS AND PERMITS' I
❑
DFW HPA
❑
COE CWA 404
❑
ECY Dam Safety
❑
FEMA Floodplain
❑
COE Wetlands
❑
Other
❑ Shoreline
Management
❑ Structural
Rockery/Vault/
❑ ESA Section 7
Part 5 PLAN AND REPORT INFORMATION
Technical Information Report
Site Improvement
Plan
(Engr. Plans)
Full
Type of Drainage Review
❑ Targeted
Plan Type (check
❑
❑
Full
(check one):
❑ Simplified
one):
Modified
❑ Large Project
❑
Simplified
Date (include revision
dates):
❑ Directed
JAAh ria 20 0 y
Date (include revision
dates):
Date of Final:
Date of Final:
Part 6 SWDM ADJUSTMENT APPROVALS
Type (circle one): Standard / Experimental / Blanket
Description: (include conditions in TIR Section 2)
Approved Adjustment No.
Date of Approval:
DFW: WA State Dept. of Fish and Wildlife. HPA: hydraulic project approval. COE: (Army) Corps of Engineers. CWA: Clean
Water Act. ECY: WA State Dept. of Ecology. FEMA: Federal Emergency Management Agency. ESA: Endangered Species Act.
2021 Surface Water Design Manual 1 Last revised 7/23/2021
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
Part 7 MONITORING REQUIREMENTS
Monitoring Required: Yes No
Start Date:
Completion Date:
Describe:
Re: KCSWDM Adjustment No.
Part 8 SITE COMMUNITY AND DRAINAGE BASIN
Community Plan DC,�,4 G41,4
Special District Overlays: ItIoA/
Drainage Basin: WA
Stormwater Requirements: eoAjsc,Q t/47,-/o/L/ 5g-o ' e-- 41isu-, wArez a bA61
Part 9 ONSITE AND ADJACENT SENSITIVE AREAS
❑ River/Stream
❑ Steep Slope
❑ Lake
❑ Erosion Hazard
❑ Wetlands
❑ Landslide Hazard
❑ Closed Depression
❑ Coal Mine Hazard
❑ Floodplain
❑ Seismic Hazard
❑ Other
❑ Habitat Protection
Lj
Part 10 SOILS
Soil Type Slopes
❑ High Groundwater Table (within 5 feet)
❑ Other
❑ Additional Sheets Attached
❑ Sole Source Aquifer
❑ Seeps/Springs
Erosion Potential
2021 Surface Water Design Manual 2 Last revised 7/23/2021
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
Part 11 DRAINAGE DESIGN LIMITATIONS
REFERENCE
❑ Core 2 — Offsite Analysis
❑ Sensitive/Critical Areas
❑ SEPA
❑ LID Infeasibility
❑ Other
n
I ❑ Additional Sheets Attached
LIMITATION / SITE CONSTRAINT
Part 12 TIR SUMMARY SHEET
(provide one TIR Summary Sheet per Threshold Discharge Area)
Threshold Discharge Area:
(name or description)
Core Requirements (all 8 apply):
Discharge at Natural Location
Number of Natural Discharge Locations:
Offsite Analysis
Level: 1 / 2 / 3 dated: /fZ-Z 9123
Flow Control (include facility
Level: 1 / 2 / 3 or Exemption Number
summary sheet)
Flow Control BMPs
Conveyance System
Spill containment located at:
Erosion and Sediment Control /
CSWPP/CESCL/ESC Site Supervisor: ]-o Sc p/Lv t,1/1)
Construction Stormwater
Contact Phone: '4/opa T9 t aLPzVc�n`
`�
Pollution Prevention
After Hours Phone:
Maintenance and Operation
Responsibility (circle one): Private / Public
If Private, Maintenance Log Required: Yes / No
Financial Guarantees and
Provided: Yes / No % e-" 1 VI A
Liability
/le6Qul /'c
Water Quality (include facility
Type (circle one): Basic / Sens. Lake / Enhanced Basic / Bog
summary sheet)
or Exemption No.
Landscape Management Plan: Yes / o
For Entire Project:
Total Replaced Impervious surfaces on the site O
% of Target Impervious that had a
Total New Pervious Surfaces on the site
feasible FCBMP a
implemented 00
/
ReDI. Imp. on site mitigated w/flow control facility D
Repl. Imp. on site mitigated w/water quality facility D
Repl. Imp. on site mitigated with FCBMP D
2021 Surface Water Design Manual 3 Last revised 7/23/2021
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
Part 12 TIR SUMMARY SHEET
(provide one TIR Summary Sheet per Threshold Discharge Area)
Special Requirements (as applicable):
Area Specific Drainage
Type: CDA / SDO / MDP / BP / LMP / Shared Fac. Non
Requirements
Name:
Floodplain/Floodway Delineation
Type (circle one): Major / Minor / Exemption / None
100-_near Base Flood Elevation (or range):
Datum:
Flood Protection Facilities
Describe: IV11
Source Control
Describe land use:
(commercial / industrial land use)
Describe any structural controls:
Oil Control
High -use Site: Yes / o
Treatment BMP:`
Maintenance Agreement: Yes /f No
with whom?
Other Drainage Structures
Describe:
Part 13 EROSION AND SEDIMENT CONTROL REQUIREMENTS
MINIMUM ESC REQUIREMENTS
MINIMUM ESC REQUIREMENTS
DURING CONSTRUCTION
AFTER CONSTRUCTION
Clearing Limits
Stabilize exposed surfaces
Cover Measures
emove and restore Temporary ESC Facilities
Ld Perimeter Protection
Yclean and remove all silt and debris, ensure
Ef Traffic Area Stabilization
operation of Permanent Facilities, restore
operation of Flow Control BMP Facilities as
Sediment Retention
necessary
❑ Surface Water Collection
❑ Flag limits of SAO and open space preservation
Dewatering Control
areas
,-❑,(
❑ Other
LI Dust Control
Flow Control
,❑
iJ Protection of Flow Control BMP Facilities
(existing and proposed)
Maintain BMPs / Manage Project
2021 Surface Water Design Manual 4 Last revised 7/23/2021
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
Part 14 STORMWATER FACILITY DESCRIPTIONS (Note: Include Facility Summary and Sketch)
Flow Control
Type/Description
Water Quality
Type/Description
❑ Detention
❑ Infiltration
❑ Regional Facility
❑ Shared Facility
❑ Flow Control BMPs
❑ Other
❑ Vegetated Flowpath
❑ Wetpool
❑ Filtration
❑ Oil Control
❑ Spill Control
❑ Flow Control BMPs
❑ Other
I?A5—IL hzii4
- G
Part 15 EASEMENTS/TRACTS
Part 16 STRUCTURAL ANALYSIS
❑ Drainage Easement
❑ Cast in Place Vault
❑ Covenant
Retaining Wall
❑ Native Growth Protection Covenant
❑ Rockery > 4' High
❑ Tract
❑ Structural on Steep Slope
❑ Other
❑ Other
I Part 17 SIGNATURE OF PROFESSIONAL ENGINEER I
I, or a civil engineer under my supervision, have visited the site. Actual site conditions as observed were
incorporated into this worksheet and the attached Technical Inform ion Report. To the best of my
knowledge the information provided here is accurate.
�,i' 6- a/1 8/24 Z,
2021 Surface Water Design Manual Last revised 7/23/2021
. _ -
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_
The information included on this map has been compiled by King County staff from a variety of sources and is
subject to change wihout notice. King County makes no representations orwarranties, express or implied,
as to accuracy,complateness, burst has a, or rights to the use of such nfonnation.This document is not!ntended
r� ,m�
'r I I f /
014. E 2-
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A
lor use as a survey pmduct. King County shal not be liable for any general, special, indirect incidental or
'
wnsequential damagesindudng, but not fmlted to, lostrewnues or Istprofitsmsulting from the use or misuse
of the information contained on this map. Any sale of this map or nfonnationon this map is prohibited except by
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VICINITY N A
written permissionof King County.
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King County
Date: 11/29/2023 Notes:
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20.3, BLDG � nR �` i 101J SOUTH 295ACTH PL0.3' 7.3' ROOF
moe t / Q
15.a' STAIRS �.
ROOF LINE „\ \
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3 Soil Map —King County Area, Washington N
(SOILS MAP)
470 20' 18" N
470 2Y 14° N
551250 5512.0 551270 551280 551290 551300 551310 551320
Map Scale: 1:679 ff printed on A portra t (8.5' x I V) sheet.
Meters
N 0 10 20 40 60
�et
o so so 120 ,ao
Map projection: Web Mercator Comer coordinates: WGS84 Edge tics: UTM Zone 10N WGS84
USDA Natural Resources Web Soil Survey
Conservation Service National Cooperative Soil Survey
47o 2918' N
0
N
N�
N
47o 2U 14' N
55133f) 55134U 55135U
3
11/27/2023
Page 1 of 3
Soil Map —King County Area, Washington
Map Unit Legend
SOILS MAP
Map Unit Symbol
Map Unit Name
Acres in AOI
Percent of AOI
AmC
Arents, Alderwood material, 6
to 15 percent slopes
1.4
100.0%
Totals for Area of Interest
1.4
100.0%
USDA Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
11 /27/2023
Page 3 of 3
01
SECTION II
CONDITIONS AND REQUIREMENTS
SUMMARY
CORE REQUIREMENT #1: DISCHARGE AT THE NATURAL LOCATION
Discharge from the proposed development will be infiltrated. Any release from the site will
discharge to the natural location.
CORE REQUIREMENT #2: OFFSITE ANALYSIS
See Section III.
CORE REQUIREMENT #3: FLOW CONTROL FACILITIES
This project proposes subdividing the property into 2 lots. The existing home will remain in Lot
1. Lot 2 will contain a new home. It was assumed that the house will have a roof area of
approximately 3,419 sf, while approximately 1,644 sf of new driveway will be added. New
target surfaces will be the additional driveway, new house and landscaped areas within Lot 2. A
comparison of the pre and post -development 100-yr peak flow rate in Section IV shows that the
increase will be less than 0.15 cfs; therefore, this project is has met Exception 2 and does not
need to have a flow control facility.
CORE REQUIREMENT #4: CONVEYANCE SYSTEM
There are no conveyance facilities being proposed.
CORE REQUIREMENT #5: CONSTRUCTION STORMWATER POLLUTION
PREVENTION
Silt fencing, stabilized construction entrance and site stabilization will be provided consistent
with developments this size. Less than 1-acre of disturbance will be created; therefore, this
project is exempt from the NPDES permit requirement. A Residential TESC plan will be
included with the building permit for Lot 2.
CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS
On -site facilities will be privately maintained. See Section X for the maintenance guidelines.
CORE REQUIREMENT #7: FINANCIAL GUARANTEES AND LIABILITY
Will be provided if required by the City.
CORE REQUIREMENT #8: WATER QUALITY
It is projected that no more than 5,000 sf of new and replaced pollution generating impervious
surfaces (PGIS) may be created on this lot. This project is therefore exempt from this
requirement. Furthermore, either basic dispersion or permeable pavement will be used for the
PGIS, which provides water quality treatment.
6
CORE REQUIREMENT #9: FLOW CONTROL BMPs
The property falls under the Small Lot BMP requirements. The following BMPs were considered for
this project:
Full Dispersion - Full dispersion is not feasible due to lack of available native vegetated
flowpath.
Full Infiltration —Medium sands may be used for this project per paragraph 5 on page 8 of the
Geotechnical Evaluation in Section VI; therefore, this BMP is feasible. A drywell will be
used for the roof runoff.
Limited Infiltration — Full infiltration is already being used for the roof runoff.
Bioretention — There is no safe overflow pathway to a municipal or private storm system.
Therefore, this BMP is infeasible.
Permeable Pavement — Since most of the proposed pavement will be in fill and if is on the
upstream side of the proposed house, water that infiltrates would threaten the below grade
floors, this BMP is not feasible.
Basic Dispersion — Most of the proposed pavement will be dispersed, except for the new
pavement at the entrance since there is no sufficient area to disperse.
Lot 2 will be bigger than 11,000 sf. Per Section C.1.3.1 of the 2021 KCSWDM, if is required to
provide BMPs to 20% of the lot size. With a lot size of 14,162 sf, it is required to provide BMPs to
2,833s£ It is proposed to provide BMPS to 5,063 (3,419 sf roof+ 1,644 sf driveway). Therefore,
this project meets this requirement.
SPECIAL REQUIREMENT #1: OTHER ADOPTED AREA -SPECIFIC REQUIREMENTS.
N/A
SPECIAL REQUIREMENT #2: FLOOD HAZARD AREA DELINEATION
N/A
SPECIAL REQUIREMENT #3: FLOOD PROTECTION FACILITIES
N/A
SPECIAL REQUIREMENT #4: SOURCE CONTROL
N/A
SPECIAL REQUIREMENT #5: OIL CONTROL
This project is not a "High Use Site".
Eel
SECTION III
OFF -SITE ANALYSIS
OFF -SITE ANALYSIS
As previously stated, there are two drainage points for the site. Approximately half of the
existing house and the area north of it drains to the northwest, while the other half of the house
and the area south of it drains to the southwest. Any runoff leaving the proposed improvements
will drain to the southwest. Runoff from the southwest will sheet flow across private property,
while runoff to the northwest drains to S 295'h Place. Both flows combine at the intersection of S
295'h Place SE and 9'h Place S, which is approximately 450 feet west of the site. Since the flows
combine within a quarter mile, there is only one threshold discharge area.
There is approximately 1.5 acre upstream of the site that potentially contributes runoff through
the site. However, since said area is partially wooded and the soils in the area have infiltrative
capacity, we believe that very little runoff will actually enter the site. Any runoff that does enter
the site, will do so near the southeast corner of the site and exit near the southwest corner. No
sensitive areas were identified.
As previously mentioned, flows to the southwest initially enter private property; therefore, we
are not able to inspect the flow path. Since both flows recombined within a quarter mile, this
analysis will therefore begin at the northwest corner of the site.
Runoff from the northwest corner of the site drains to S 295'h Place and flows along the south
side of S 295'h Place for a distance of approximately 210'feet to a catch basin. A 12" pipe
conveys water across the street for a distance of approximately 28 feet to another catch basin.
From said catch basin, a 12" pipe carries water for approximately 220 feet to a catch basin on the
northwest corner of the intersection of S 295'h Place and 9'h Place S. Water then travels for a
distance of approximately 30 feet through a 12" pipe across 9'h Place S to another catch basin.
From said catch basin, water flows north for approximately 360 feet to a catch basin at the
intersection of 9'h Place S and S 294'h Place. From said catch basin, water is conveyed north by a
12" pipe for approximately 155 feet to another catch basin. Water then travels north for a
distance of approximately 230 feet to a catch basin at the southwest corner of the intersection of
9'h Place S and S 293rd Street. Water then travels west along the south side of S 293rd Street
within a 15" pipe for a distance of approximately 350 feet to a catch basin at the southwest
corner of the intersection of S 293rd Street and 8'h Avenue S. Said catch basin being 1,583 feet or
more than a quarter mile downstream from the site. Water then continues north and eventually
drains into the Puget Sound. The downstream path is shown on the attached map.
INS
King County Map
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King County '4�
>
The information included on this map has been compiled by King County staff from a variety of sources and is subject to
change without notice. King County makes no representations orwarranfies, express or implied, as to accuracy, Date: 11/29/2023 N
completeness, timeliness, or rights to the use of such information. This documentis not intended for use asasurvey
product. King County shall not be liable for any general, special, indirect, incidental, or consequential damages including, LIZ
but not limited to, lost revenues or lost profits resulting from the use or misuse of the information contained on this map. Notes: A
Any sale of this map or information on this map is prohibited except by written permission of King County. IV7--G King County
I I
NAIIVA-irne- NAF
OFF -SITE ANALYSIS DRAINAGE SYSTEM TABLE
KING COUNTY SURFACE WATER DESIGN MANUAL, CORE REQUIREMENT #2
P4��
sOiT�
Subbasin
Subbasin
Date
`%oZ
Name:
Number:
Symbol
Drainage
Component Type,
Name, and Size
Drainage
Component
Description
Slope
Distance
from site
discharge
Existing
Problems
Potential
Problems
Observations of field
inspector, resource
reviewer, or resident
see map
Type: sheet flow, swale,
stream, channel, pipe,
pond, flow control/wq
BMP; Size: diameter,
surface area
drainage basin, vegetation,
cover, depth, type of sensitive
area, volume
%
'/4 ml = 1,320 ft.
constrictions, under capacity, ponding,
overtopping, flooding, habitat or organism
destruction, scouring, bank sloughing,
sedimentation, incision, other erosion
tributary area, likelihood of problem,
overflow pathways, potential impacts
G UT7C-12
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ti ,�
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ILJO N 5
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�733-
2021 Draft Surface Water Design Manual
Last revised 4/24/2016
H
SECTION IV
QUALITY FACILITY ANALYSIS AND
DESIGN
EXISTING SITE HYDROLOGY (PART A)
As previously stated, there are two drainage points for the site. Approximately half of the
existing house and the area north of it drains to the northwest, while the other half of the house
and the area south of it drains to the southwest. Any runoff leaving the proposed improvements
will drain to the southwest. Runoff from the southwest will sheet flow across private property,
while runoff to the northwest drains to S 295"' Place. Both flows combine at the intersection of S
295"' Place SE and 91h Place S, which is approximately 450 feet west of the site. Since the flows
combine within a quarter mile, there is only one threshold discharge area.
DEVELOPED SITE HYDROLOGY (PART B)
Runoff from the site will be dispersed or fully infiltrated. Any release from the site will continue
to discharge to its natural drainage path after construction of the house and associated driveway.
PERFORMANCE STANDARDS (PART C)
The project site is within the Conservation Flow Control/Level,2 Flow Control and the
Basic Water Quality Treatment Area.
FLOW CONTROL SYSTEM (PART D)
This project proposes the construction of a single-family home with a roof area of approximately
3,419 sf and additional driveway of approximately 1,644 sf. A comparison of the 100-yr pre and
post -development peak flow rates in this Section shows that the increase is less than 0.15 cfs;
therefore, this project is exempt from this requirement. The following BMPs are sized as follows
(See Basin Map in Section I for tributary Areas):
Drywell:
Tributary roof area = 3,419 sf
Required gravel volume for medium sands for drywells = 90 cy per 1,000sf
Required gravel volume (V) = 3,419 x 90 / 1,000 = 307.7
Drywell height (H) = 4 ft
Required drywell radius (R) = (V / (71 x H))A o.s
= (307.7 / (n x 4))A0.5
= 4.95 ft, say 5 ft
Required drywell diameter (D) = R x 2
=5x2
= 10,
EO
Dispersion trench
Tributary Area = 1,644 sf (266 sf at the entrance will not be treated due to
location)
Required dispersion trench per Section C.2.4.4 = 10 if per 700
Length Required = 1,644 x 10 / 700 = 23.5 ft, use 24 ft
WATER QUALITY SYSTEM (PART E)
A total of 1,644 sf of pollution generating impervious surface will be created (PGIS). Since the
PGIS is less than 5,000 sf, this project is exempt from this requirement. Furthermore, basic
dispersion will be used for most of the new PGIS, which provide water quality treatment.
■
l'otAl Ld / IVAL Y5-15
WWHM2012
PROJECT REPORT
Project Name: BASIN
Site Name: SAAD 295th
Site Address: 1013 S 295th Pl
City : Federal Way
Report Date: 1/24/2024
Gage : Seatac
Data Start : 1948/10/01
Data End : 2009/09/30
Precip Scale: 1.00
Version Date: 2023/01/27
Version : 4.2.19
PREDEVELOPED LAND USE
Name : Basin 1
Bypass: No
GroundWater: No
Pervious Land Use
acre
C, Forest, Steep
.29
Pervious Total
0.29
Impervious Land Use
acre
DRIVEWAYS STEEP
0. 03
'&yt- WA y ToAC"AIAI
Impervious Total
0.03
AV11/11- -11R n n a
Basin Total
0.32
MITIGATED LAND USE
Name : Basin 1
Bypass: No
GroundWater: No
Pervious Land Use acre
C, Lawn, Steep .18
Pervious Total 0.18
Impervious Land Use acre
ROOF TOPS FLAT 0.08
DRIVEWAYS STEEP 0.06 1--- Aj`t) o cam'`° "f ` ,
Impervious Total 0.14
Basin Total 0.32
H
Predeveloped Landuse Totals for POC #1
Total Pervious Area:0.29
Total Impervious Area:0.03
Mitigated Landuse Totals for POC #1
Total Pervious Area:0.18
Total Impervious Area:0.14
Flow Frequency Return Periods for Predeveloped. POC #1
Return Period
Flow(cfs)
2 year
0.023558
5 year
0.033126
10 year
0.040035
25 year
0.049426
50 year
0.056908
100 year
0.064812
Flow Frequency Return Periods for Mitigated. POC #1
Return Period
Flow(cfs)
2 year
0.076652
5 year
0.103958
10 year
0.123553
25 year
0.150086
50 year
0.171174
100 year
0.193423
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SECTION V
CONVEYANCE SYSTEM ANALYSIS AND
DESIGN
SECTION VI
SPECIAL REPORTS AND STUDIES
E � COBALT
GEOSCIENCES
November 9, 2023
Muhammad Saad Mohyuddin
saadmohi(&googlemail.com
RE: Geotechnical Evaluation
Proposed Residence
1013 S. 295th Place
Federal Way, Washington
Cobalt Geosciences, LLC
P.O. Box 1792
North Bend, WA 98045
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to
discuss the results of our geotechnical evaluation at the referenced site.
The purpose of our evaluation was to determine the feasibility of utilizing infiltration devices for
stormwater runoff management along with providing recommendations for foundation and
retaining wall design.
Site and Project Description
The site is located at 1013 S. 295th Place in Federal Way, Washington. The site consists of one
nearly rectangular shaped parcel (No. 5151800O35) with a total area of about 23,753 square feet.
The northern portion of the property is developed with a single family residence with partial
basement and driveway. The remainder of the property is undeveloped and vegetated with
grasses, bushes, ferns, ivy, and spares trees.
The developed portion of the site is nearly level to slightly sloping downward to the northwest and
southwest with low relief. The southern half of the site slopes downward to the north and south
into a shallow ravine -like feature. Slope areas have been previously graded and have magnitudes
of about 15 to 50 percent and local relief of io feet within the property. There is a taller slope that
is locally faced with walls south of the subject parcel sloping downward to the north into the
ravine. The ravine is very shallow and does not have evidence of seasonal streams or surface
waters.
The property is bordered to the east, west, and south by residential properties and to the north by
S. 295th Place.
The project includes subdivision of the parcel and construction of a residence in the southern half
of the site. A driveway will extend to the area along the west property line. Figure 1 shows the
current conditions and planned layout.
Grading could include cuts of 8 feet or less if a daylight basement is utilized, and foundation loads
will generally be light. Stormwater management may include dispersion, detention, or infiltration
facilities depending on feasibility. We should be provided with the plans when they become
available so that we may determine if additional recommendations are warranted.
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Page 2 of 11
Geotechnical Evaluation
Area Geology
The Geologic Map of the Poverty Bay Quadrangle, indicates that the site is underlain by Vashon
Glacial Till and near the contacts with Vashon Advance Outwash.
Vashon Glacial Till consists of dense mixtures of silt, sand, clay, and gravel. These deposits are
typically impermeable below a weathered zone. Vashon Advance Outwash includes fine to
medium grained sand trace gravel with local interbeds of silt and clay. These deposits become
denser below a weathered zone.
The till and outwash can locally be overlain by Vashon Recessional Outwash. These deposits
include normally consolidated sand and gravel with lesser silt and clay. These deposits can be
highly permeable.
Soil & Groundwater Conditions
As part of our evaluation, we excavated two test pits within the property to determine the shallow
soil and groundwater conditions, where accessible.
The test pits encountered about 6 inches of topsoil and grass underlain by approximately 2.5 to 3
feet of loose to medium dense, silty -fine to fine grained sand trace to with gravel (Weathered
Recessional Outwash). This layer was underlain by medium to locally dense, fine to medium
grained sand with gravel trace cobbles (Recessional Outwash?), which continued to the
termination depths of the explorations.
Groundwater was not observed in the test pits during our evaluation. Based on the soil
conditions, it appears that perched groundwater would likely be 8 or more feet below grade
during later winter and spring months.
A review of available well logs (from the DOE well log site) within 2,000 feet of the site indicates
groundwater at more than 77 feet below grade.
Water table elevations often fluctuate over time. The groundwater level will depend on a variety
of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and
soil permeability. Water levels at the time of the field investigation may be different from those
encountered during the construction phase of the project.
Steep Slope Hazard
Most critical area ordinances designate slopes with magnitudes greater than about 40 percent and
vertical relief of at least 10 feet as potentially geologically hazardous (steep slope/landslide
hazards).
The developed portion of the site is nearly level to slightly sloping downward to the northwest and
southwest with low relief. The southern half of the site slopes downward to the north and south
into a shallow ravine -like feature. Slope areas have been previously graded and have magnitudes
of about 15 to 50 percent and local relief of 10 feet within the property.
There is a taller slope that is locally faced with walls south of the subject parcel sloping downward
to the north into the ravine. The ravine is very shallow and does not have evidence of seasonal
streams or surface waters. Slope areas to the south are well vegetated.
It is our opinion that the modified and natural slopes with.magnitudes over 40 percent within and
near the properly are stable at this time. We did not observe evidence of erosion, landslide
activity or soil creep. These areas are not consistent with landslide hazard areas.
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Geotechnical Evaluation
Provided all stormwater runoff during construction is fully managed, and stormwater is
infiltrated or dispersed in areas with magnitudes of about 20 percent or more, the proposed
development should not increase the risk of soil movements on the subject property and adjacent
areas.
Any new foundation systems should be embedded an adequate depth in order to create a
minimum 7 foot effective setback from adjacent slope systems (downslope areas) where slope
magnitudes are 30 percent or more. This is the horizontal distance from the lower outside face of
the footing to the face of the adjacent slope.
Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site
is underlain by Arents, Alderwood material (6 to 15 percent slopes). These soils would have a
slight to moderate erosion potential in a disturbed state depending on the slope magnitude.
It is our opinion that soil erosion potential at this project site can be reduced through landscaping
and surface water runoff control. Typically, erosion of exposed soils will be most noticeable
during periods of rainfall and may be controlled by the use of normal temporary erosion control
measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The
typical wet weather season, with regard to site grading, is from October 31st to April ist. Erosion
control measures should be in place before the onset of wet weather.
Seismic Parameters
The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the
International Building Code (IBC). A Site Class D applies to an overall profile consisting of
medium dense to very dense soils within the upper loo feet.
We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to
obtain values for Ss, S1, Fa, and F,,. The USGS website includes the most updated published data
on seismic conditions. The following tables provide seismic parameters from the USGS web site
with referenced parameters from ASCE 7-10 and 7-16.
Seismic Design Parameters (ASCE 7-16)
Site
Spectral
Spectral
Site
Design Spectral
Design
Class
Acceleration
Acceleration
Coefficients
Response Parameters
PGA
at 0.2 sec. (g)
at 1.o sec. (g)
Fa
K
SDs
SM
D
1.343
0.461
1.0
Null
o.895
Null
0.568
Additional seismic considerations include liquefaction potential and amplification of ground
motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a
high groundwater table. The site has a low likelihood of liquefaction. "Null' indicates see Section
11.4.8 of the ASCE.
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Geotechnical Evaluation
Conclusions and Recommendations
General
The site is underlain by sand and gravel generally consistent with Vashon Recessional Outwash.
The proposed residence may be supported on a shallow foundation system bearing on medium
dense or firmer native soils or on structural fill placed on the native soils. Local overexcavation
or recompaction of loose weathered native soils may be necessary depending on the proposed
elevations and location of the new residence.
Any new foundation systems should be embedded an adequate depth in order to create a
minimum 7 foot effective setback from adjacent slope systems (downslope areas) where slope
magnitudes are 30 percent or more. This is the horizontal distance from the lower outside face of
the footing to the face of the adjacent slope.
Infiltration is generally feasible in the soils consistent with the coarser outwash soils at depth.
Trenches or drywells should be located in areas with slope magnitudes of 20 percent or less.
Dispersion devices and permeable pavements are also generally feasible depending on their
elevations and locations. We can provide additional recommendations once a site plan has been
prepared.
Site Preparation
Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich
soil and fill. Based on observations from the site investigation program, it is anticipated that the
stripping depth will be 6 to 12 inches. Deeper excavations will be necessary below large trees
where root systems can extend to greater depths, in areas of existing foundation systems, and in
any areas underlain by undocumented fill.
The native soils consist of silty -sand with gravel and poorly graded sand/gravel with silt and sand.
Most of the native soils may be used as structural fill provided they achieve compaction
requirements and are within 3 percent of the optimum moisture. Some of these soils may only be
suitable for use as fill during the summer months, as they will be above the optimum moisture
levels in their current state. These soils are variably moisture sensitive and may degrade during
periods of wet weather and under equipment traffic.
Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of
3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve).
Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted
to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the
ASTM D 1557 test method.
Temporary Excavations
Based on our understanding of the project, we anticipate that the grading could include local cuts
on the order of approximately 8 feet or less for foundation and utility placement. Any deeper
temporary excavations should be sloped no steeper than 1.5H:iV (Horizontal:Vertical) in loose
native soils and fill and 1H:1V in medium dense, native soils. If an excavation is subject to heavy
vibration or surcharge loads, we recommend that the excavations be sloped no steeper than
2H:1V, where room permits.
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Geotechnical Evaluation
Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part
N, Excavation, Trenching, and Shoring. Temporary slopes should be visually inspected daily by a
qualified person during construction activities and the inspections should be documented in daily
reports. The contractor is responsible for maintaining the stability of the temporary cut slopes
and reducing slope erosion during construction.
Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather,
and the slopes should be closely monitored until the permanent retaining systems or slope
configurations are complete. Materials should not be stored or equipment operated within 10 feet
of the top of any temporary cut slope.
Soil conditions may not be completely known from the geotechnical investigation. In the case of
temporary cuts, the existing soil conditions may not be completely revealed until the excavation
work exposes the soil. Typically, as excavation work progresses the maximum inclination of
temporary slopes will need to be re-evaluated by the geotechnical engineer so that supplemental
recommendations can be made. Soil and groundwater conditions can be highly variable.
Scheduling for soil work will need to be adjustable, to deal with unanticipated conditions, so that
the project can proceed and required deadlines can be met.
If any variations or undesirable conditions are encountered during construction, we should be
notified so that supplemental recommendations can be made. If room constraints or
groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed
by the WAC, temporary shoring systems may be required. The contractor should be responsible
for developing temporary shoring systems, if needed. We recommend that Cobalt Geosciences
and the project structural engineer review temporary shoring designs prior to installation, to
verify the suitability of the proposed systems.
Foundation Design
The proposed residence may be supported on a shallow spread footing foundation system bearing
on undisturbed medium dense or firmer native soils or on properly compacted structural fill
placed on the suitable native soils. Any undocumented fill should be removed and replaced with
structural fill below foundation elements. Structural fill below footings should consist of clean
angular rock 5/8 to 2 inches in size.
For shallow foundation support, we recommend widths of at least 16 and 24 inches, respectively,
for continuous wall and isolated column footings supporting the proposed structure. Provided
that the footings are supported as recommended above, a net allowable bearing pressure of 1,500
pounds per square foot (psf) may be used for design.
A 1/3 increase in the above value may be used for short duration loads, such as those imposed by
wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted
to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing
excavations should be inspected to verify that the foundations will bear on suitable material.
Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or
adjacent exterior grade, whichever is lower. Interior footings should have a minimum depth of 12
inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower.
If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch.
Differential settlement, along a 25400t exterior wall footing, or between adjoining column
footings, should be less than 1/2 inch. This translates to an angular distortion of 0.002. Most
settlement is expected to occur during construction, as the loads are applied. However, additional
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Geotechnical Evaluation
post -construction settlement may occur if the foundation soils are flooded or saturated. All
footing excavations should be observed by a qualified geotechnical consultant.
Resistance to lateral footing displacement can be determined using an allowable friction factor of
0.4o acting between the base of foundations and the supporting subgrades. Lateral resistance for
footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds
per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12
inches below grade in exterior areas). The frictional and passive resistance of the soil may be
combined without reduction in determining the total lateral resistance.
Care should be taken to prevent wetting or drying of the bearing materials during construction.
Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the
footing excavations, should be removed prior to placing concrete. The potential for wetting or
drying of the bearing materials can be reduced by pouring concrete as soon as possible after
completing the footing excavation and evaluating the bearing surface by the geotechnical engineer
or his representative.
Concrete Retaining Walls
The following table, titled Wall Design Criteria, presents the recommended soil related design
parameters for retaining walls with a level backslope. Contact Cobalt if an alternate retaining wall
system is used. This has been included if any concrete walls are proposed in yard areas.
Wall Design Criteria
"At -rest" Conditions (Lateral Earth Pressure — EFD+)
55 pef (Equivalent Fluid Density)
"Active" Conditions (Lateral Earth Pressure — EFD+)
35 pcf (Equivalent Fluid Density)
Seismic Increase for "At -rest" Conditions
(Lateral Earth Pressure)
14H* (Uniform Distribution) 1 in 500 year event
Seismic Increase for "Active" Conditions
(Lateral Earth Pressure)
7H* (Uniform Distribution)
Passive Earth Pressure on Low Side of Wall
(Allowable, includes F.S. =1.5)
Neglect upper 2 feet, then 250 pcf EFD+
Soil -Footing Coefficient of Sliding Friction (Allowable;
includes F.S. =1.5)
0.40
*H is the height of the wall; Increase based on one in 500 year seismic event (io percent probability of being exceeded in
50 years),
+EFD — Equivalent Fluid Density
The stated lateral earth pressures do not include the effects of hydrostatic pressure generated by
water accumulation behind the retaining walls. Uniform horizontal lateral active and at -rest
pressures on the retaining walls from vertical surcharges behind the wall may be calculated using
active and at -rest lateral earth pressure coefficients of 0.3 and 0.5, respectively. A soil unit weight
Of 125 pcf may be used to calculate vertical earth surcharges.
To reduce the potential for the buildup of water pressure against the walls, continuous footing
drains (with cleanouts) should be provided at the bases of the walls. The footing drains should
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Geotechnical Evaluation
consist of a minimum 4-inch diameter perforated pipe, sloped to drain, with perforations placed
down and enveloped by a minimum 6 inches of pea gravel in all directions.
The backfill adjacent to and extending a lateral distance behind the walls at least 2 feet should
consist of free -draining granular material. All free draining backfill should contain less than 3
percent fines (passing the U.S. Standard No. 200 Sieve) based upon the fraction passing the U.S.
Standard No. 4 Sieve with at least 30 percent of the material being retained on the U.S. Standard
No. 4 Sieve. The primary purpose of the free -draining material is the reduction of hydrostatic
pressure. Some potential for the moisture to contact the back face of the wall may exist, even with
treatment, which may require that more extensive waterproofing be specified for walls, which
require interior moisture sensitive finishes.
We recommend that the backfill be compacted to at least 90 percent of the maximum dry density
based on ASTM Test Method D1557• In place density tests should be performed to verify
adequate compaction. Soil compactors place transient surcharges on the backfill. Consequently,
only light hand operated equipment is recommended within 3 feet of walls so that excessive stress
is not imposed on the walls.
Slab -on -Grade
We recommend that the upper 18 inches of the existing native soils within slab areas be re -
compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). Any fill
should be fully removed and replaced with imported fill.
Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor
barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture
typically requires the usage of a vapor barrier. A materials or structural engineer should be
consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs
typically do not utilize vapor barriers.
The American Concrete Institutes ACI 36oR-o6 Design of Slabs on Grade and ACI 302.1R-04
Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier
selection and floor slab detailing.
Slabs on grade may be designed using a coefficient of subgrade reaction of 150 pounds per cubic
inch (pci) assuming the slab -on -grade base course is underlain by structural fill placed and
compacted as outlined above. A minimum 4 inch thick capillary break should be placed over the
prepared subgrade. These materials should consist of 5/8 inch clean angular rock or pea gravel.
A perimeter drainage system is recommended unless interior slab areas are elevated a minimum
Of 12 inches above adjacent exterior grades. If installed, a perimeter drainage system should
consist of a 4 inch diameter perforated drain pipe surrounded by a minimum 6 inches of drain
rock wrapped in a non -woven geosynthetic filter fabric to reduce migration of soil particles into
the drainage system. The perimeter drainage system should discharge by gravity flow to a
suitable stormwater system.
Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate
surface water flow away from the building and preferably with a relatively impermeable surface
cover immediately adjacent to the building.
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Geotechnical Evaluation
Stormwater Management Feasibility
The site is underlain by Vashon Recessional Outwash. Infiltration is suitable in these soil
deposits. Groundwater was not encountered in the explorations. Groundwater is likely more than
8 and possibly more than 50 feet below site elevations based on a review of nearby boring logs.
Because the recessional deposits have not been overridden by glacial ice, this soil unit is
considered normally -consolidated. The Washington State Department of Ecology Stormwater
Management Manual for Western Washington allows determination of infiltration rates of this
soil unit by Soil Particle Size Distribution testing. This method involves using a logarithmic
equation and grain size values along with correction factors for testing type, soil homogeneity,
and influent control.
The equation in conjunction with sieve analysis results yields design infiltration rates of between
2.5 to 3.5 inches per hour at depths of 3 to 6 feet below grade. These rates reflect application of
correction factors for variability (0.6 used), influent control (0.9), and testing analysis type (0.4).
We also performed an infiltration test in TP-1 at 4 feet below grade. Following saturation, testing,
and application of the correction factors noted above, the design rate was determined to be 2.5
inches per hour. We recommend using this rate for system design.
We note that the subsurface soils vary from Medium Sand to Coarse Sand/Gravel if sizing is
determined using the Textural Triangle information in the King County Surface. Water Design
Manual (SWDM). We recommend using Medium Sand for any systems.
Infiltration systems should have a depth of at least 3 feet below existing grades and located at
least 10 feet apart. Any fine grained soils or interbeds of fine grained soils must be removed prior
to rock placement.
We should be provided with final plans for review to determine if the intent of our
recommendations has been incorporated or if additional modifications are needed. We should
observe the soil conditions in infiltration systems during construction to confirm soils are
consistent with our investigation.
Erosion and Sediment Control
Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to
wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment
control measures should be implemented, and these measures should be in general accordance
with local regulations. At a minimum, the following basic recommendations should be
incorporated into the design of the erosion and sediment control features for the site:
• Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance
of the site soils, to take place during the dry season (generally May through September).
However, provided precautions are taken using Best Management Practices (BMP's), grading
activities can be completed during the wet season (generally October through April).
• All site work should be completed and stabilized as quickly as possible.
• Additional perimeter erosion and sediment control features may be required to reduce the
possibility of sediment entering the surface water. This may include additional silt fences, silt
fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration
systems.
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Page 9 of ii
Geotechnical Evaluation
• Any runoff generated by dewatering discharge should be treated through construction of a
sediment trap if there is sufficient space. If space is limited other filtration methods will need
to be incorporated.
Utilities
Utility trenches should be excavated according to accepted engineering practices following OSHA
(Occupational Safety and Health Administration) standards, by a contractor experienced in such
work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent
to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be
avoided. Depending upon the location and depth of some utility trenches, groundwater flow into
open excavations could be experienced, especially during or shortly following periods of
precipitation.
In general, sandy and gravelly soils were encountered at shallow depths in the explorations at this
site. These soils have low cohesion and density and will have a tendency to cave or slough in
excavations. Shoring or sloping back trench sidewalls is required within these soils in excavations
greater than 4 feet deep.
All utility trench backfill should consist of imported structural fill or suitable on site soils. Utility
trench backfill placed in or adjacent to buildings and exterior slabs should be compacted to at
least 95 percent of the maximum dry density based on ASTM Test Method D1557. The upper 5
feet of utility trench backfill placed in pavement areas should be compacted to at least 95 percent
of the maximum dry density based on ASTM Test Method D1557. Below 5 feet, utility trench
backfill in pavement areas should be compacted to at least 90 percent of the maximum dry
density based on ASTM Test Method D1557. Pipe bedding should be in accordance with the pipe
manufacturer's recommendations.
The contractor is responsible for removing all water -sensitive soils from the trenches regardless of
the backfill location and compaction requirements. Depending on the depth and location of the
proposed utilities, we anticipate the need to re -compact existing fill soils below the utility
structures and pipes. The contractor should use appropriate equipment and methods to avoid
damage to the utilities and/or structures during fill placement and compaction procedures.
CONSTRUCTION FIELD REVIEWS
Cobalt Geosciences should be retained to provide part time field review during construction in
order to verify that the soil conditions encountered are consistent with our design assumptions
and that the intent of our recommendations is being met. This will require field and engineering
review to:
■ Monitor and test structural fill placement and soil compaction
■ Observe bearing capacity at foundation locations
■ Observe slab -on -grade preparation
■ Monitor subgrade preparation of roadways
■ Observe excavation stability
■ Verify soil conditions at any stormwater management systems
Geotechnical design services should also be anticipated during the subsequent final design phase
to support the structural design and address specific issues arising during this phase. Field and
engineering review services will also be required during the construction phase in order to
provide a Final Letter for the project.
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Geotechnical Evaluation
CLOSURE
This report was prepared for the exclusive use of Muhammad Saad Mohyuddin and his appointed
consultants. Any use of this report or the material contained herein by third parties, or for other
than the intended purpose, should first be approved in writing by Cobalt Geosciences, LLC.
The recommendations contained in this report are based on assumed continuity of soils with
those of our test holes, and assumed structural loads. Cobalt Geosciences should be provided with
final architectural and civil drawings when they become available in order that we may review our
design recommendations and advise of any revisions, if necessary.
Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is
the responsibility of Muhammad Saad Mohyuddin who is identified as "the Client" within the
Statement of General Conditions, and its agents to review the conditions and to notify Cobalt
Geosciences should any of these not be satisfied.
Sincerely,
Cobalt Geosciences, LLC
NY
4
5d896
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NA6
11/9/2023
Phil Haberman, PE, LG, LEG
Principal
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Page ii of ii
Geotechnical Evaluation
Statement of General Conditions
USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its
agent and may not be used by any third party without the express written consent of Cobalt
Geosciences and the Client. Any use which a third parry makes of this report is the responsibility
of such third party.
BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this
report are in accordance with Cobalt Geosciences present understanding of the site specific
project as described by the Client. The applicability of these is restricted to the site conditions
encountered at the time of the investigation or study. If the proposed site specific project differs
or is modified from what is described in this report or if the site conditions are altered, this report
is no longer valid unless Cobalt Geosciences is requested by the Client to review and revise the
report to reflect the differing or modified project specifics and/or the altered site conditions.
STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in
accordance with the normally accepted standard of care in the state of execution for the specific
professional service provided to the Client. No other warranty is made.
INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and
statements regarding their condition, made in this report are based on site conditions
encountered by Cobalt Geosciences at the time of the work and at the specific testing and/or
sampling locations. Classifications and statements of condition have been made in accordance
with normally accepted practices which are judgmental in nature; no specific description should
be considered exact, but rather reflective of the anticipated material behavior. Extrapolation of in
situ conditions can only be made to some limited extent beyond the sampling or test points. The
extent depends on variability of the soil, rock and groundwater conditions as influenced by
geological processes, construction activity, and site use.
VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be
encountered that are different from those described in this report or encountered at the test
locations, Cobalt Geosciences must be notified immediately to assess if the varying or unexpected
conditions are substantial and if reassessments of the report conclusions or recommendations are
required. Cobalt Geosciences will not be responsible to any party for damages incurred as a result
of failing to notify Cobalt Geosciences that differing site or sub -surface conditions are present
upon becoming aware of such conditions.
PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and
specifications should be reviewed by Cobalt Geosciences, sufficiently ahead of initiating the next
project stage (property acquisition, tender, construction, etc), to confirm that this report
completely addresses the elaborated project specifics and that the contents of this report have
been properly interpreted. Specialty quality assurance services (field observations and testing)
during construction are a necessary part of the evaluation of sub -subsurface conditions and site
preparation works. Site work relating to the recommendations included in this report should only
be carried out in the presence of a qualified geotechnical engineer; Cobalt Geosciences cannot be
responsible for site work carried out without being present.
www.cobaltgeo.com
(2o6) 331-1097
Y
1
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t
wes
t
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acs
3
1
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Provided figures
TP-1
Approximate
Test Pit
Location
Cobalt Geosciences, LLC
Proposed Residence SITE PI AN P.O. Box 1792
North Bend, WA 98045
COBALT 1013 S. 295th Place (206) 331-1097
GEOSCIENCES Federal Way, Washington FIGURE 1 wcvw•cobaltgeo.com
phil(&cobaltgeo.com
Unified Soil Classification System (USCS)
MAJOR DIVISIONS
TYPICAL DESCRIPTION
Clean Gravels
JBOL
0w
Well -graded gravels, gravels, gravel -sand mixtures, little or no fines
Gravels
(less than 5%GP
(more than 50%
fines)
Poorly graded gravels, gravel -sand mixtures, little or no fines
of coarse fraction
;t-
GM
retained on No. 4
Gravels with
Silty gravels, gravel -sand -silt mixtures
COARSE
sieve)
Fines.
GC
Clayey gravels, gravel -sand -clay mixtures
GRAINED
(more than 12%
fines)
SOILS
(more than 50%
retained on
Clean Sands
:•, sw
Well -graded sands, gravelly sands, little or no fines
No. 200 sieve)
Sands
(less than 5%
SP
(5o% or more
fines)
Poorly graded sand, gravelly sands, little or no fines
of coarse fraction
Sands with
sit
Silty sands, sand -silt mixtures
passes the No. 4
sieve)
Fines
sc
(more than 12%
fines)
Clayey sands, sand -clay mixtures
ML
Inorganic silts of low to medium plasticity, sandy silts, gravelly silts,
or clayey silts with slight plasticity
Silts and Clays
Inorganic
cL
Inorganic clays of low to medium plasticity, gravelly clays, sandy clays
(liquid limit less
silty clays, lean clays
FINE GRAINED
than 50)
OL
SOILS
Organic
Organic silts and organic silty clays of low plasticity
(50% or more
passes the
MH
Inorganic silts, micaceous or diatomaceous fine sands or silty soils,
No. 200 sieve)
elastic silt
Silts and Clays
(liquid limit 50 or
Inorganic
C4'1
Inorganic clays of medium to high plasticity, sandy fat clay,
more)
or gravelly fat clay
Organic
oxOrganic
clays of medium to high plasticity, organic silts
HIGHLY ORGANIC
Primarily organic matter, dark in color,
PT
Peat, humus, swamp soils with high organic content (ASTM D4427)
SOILS
and organic odor
Classification of Soil Constituents
MAJOR constituents compose more than 5o percent,
by weight, of the soil. Major constituents are capitalized
(i.e., SAND).
Minor constituents compose 12 to 50 percent of the soil
and precede the major constituents (i.e., silty SAND).
Minor constituents preceded by "slightly" compose
5 to 12 percent of the soil (i.e., slightly silty SAND).
Trace constituents compose o to 5 percent of the soil
(i.e., slightly silty SAND, trace gravel).
Relative Density
(Coarse Grained Soils)
Consistency
(Fine Grained Soils)
N, SPT,
Relative
N, SPT,
Relative
Blows/FT
Density
Blows/Fr
Consistency
0-4
Very loose
Under 2
Very soft
4 - 10
Loose
2-4
Soft
10 - 30
Medium dense
4-8
Medium stiff
30 - 50
Dense
8 -15
Stiff
Over 50
Very dense
15 - 30
Very stiff
Over 3o
Hard
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98o28
(2o6) 331-1097
www.cobaltzeo.com
cobaltgeo(@gmail.com
Grain Size Definitions
Description
Sieve Number and/or Size
Fines
<#200 (o.o8 nun)
Sand
-Fine
#200 to #40 (0.08 to 0.4 mm)
-Medium
#40 to #10 (0.4 to 2 mm)
-Coarse
#10 to #4 (2 to 5 mm)
Gravel
-Fine
#4 to 3/4 inch (5 to 19 mm)
-Coarse
3/4 to 3 inches (19 to 76 mm)
Cobbles
3 to 12 inches (75 to 305 mm)
Boulders
>12 inches (305 mm)
Moisture Content Definitions
Dry Absence of moisture, dusty, dry to the touch
Moist Damp but no visible water
Wet Visible free water, from below water table
Soil Classification Chart
Figure C1
Test Pit TP-1
Date: October 2023
Depth: 8'
Groundwater: None
Contractor: Jim
Elevation:
Logged By: PH Checked By: SC
o>
O
o
Moisture Content (%)
Plastic 1 I Liquid
�
U
N
Limit Limit
Q
a
Material Description
a)?
o
DCP Equivalent N-Value
O
0 10 20 30 40 50
------
----
--
Topsoil/Grass
---------------------------------------------
1
SM
Loose to medium dense, silty -fine to medium grained sand with gravel
SP
yellowish brown, moist. (Weathered Recessional Outwash)
2
T .'
---------------------------------------------------
SP
Medium dense to locally dense, fine to medium grained sand trace
4
gravel to gravel with sand, grayish brown, moist.
'%
(RecessionalOutwash?)
;..;
Local caving limits test pit depth
b
t
7
ti
End of Test Pit 8'
9
10
Cobalt Geosciences, LLC
Proposed Residence P.O. Box 82243
Test Pit Kenmore, WA 98028
COBALT1013 S. 295th Place (2o6) 331-1097
• Federal Way, Washington Logs 6v%%�v.cobaItgeo.com ��
cobaltgeo(@gmail.com
Test Pit TP-2
Date: October 2023
Depth: 8'
Groundwater: None
Contractor: Jim
Elevation:
Logged By: PH Checked By: SC
0)
O
o
Moisture Content (%)
Plastic I Liquid
U
T
3
Limit
Q
o
�
Material Description
o
O
?
'o
DCP Equivalent N Value
O
0 10 20 30 40 50
----
--
Topsoil/Grass
---------------------------------------------
------
1
SM
Loose to medium dense, silty -fine to medium grained sand with gravel
SP
yellowish brown, moist. (Weathered Recessional Outwash)
2
+ ~ i
Local yard waste at ground surface
3
; ,•
------
4
----
-}
; :,':::
SP
---------------------------------------------
Medium dense to locally dense, fine to medium grained sand trace
gravel to gravel with sand, grayish brown, moist.
5::i::
(RecessionalOutwash?)
L•i 11
6
; �`
i�• ti•
Local caving limits test pit depth
7
1 y
End of Test Pit 8'
9
10
Proposed Residence
1013 S. 295th Place
Federal Way, Washington
Cobalt Geosciences, LLC
P.O. Box 82243
Test Pit Kenmore, WA 98028
(2o6) 331-1097
Logs lb
-,%,i%-%v.cobaltgeo.com
cobalteeo(@gmail.com
SECTION VII
SECTION VIII
T.E.S.C.
Silt fencing, stabilized construction entrance and site stabilization will be provided consistent
with developments this size. Less than 1-acre of disturbance will be created, therefore, this
project is exempt from the NPDES permit requirement. A Residential TESC plan is included
with the submittal.
SECTION IX
BOND QUANTITIES, FACILITY SUMMARIES
AND DECLARATION OF COVENANT
a wa
SECTION X
OPEIaATIONS AND MAINTENANCE MANUAL
OPERATIONS AND MAINTENANCE
Maintenance of the different BMPs proposed for this project is the responsibility of the property
owner. The following pages include the maintenance guidelines for the gravel fill trench and
drywell.
EO
NO.25 - DRYWELL BMP
Maintenance
Defect or Problem
Conditions When Maintenance is Needed
Results Expected When
Component
Maintenance is Performed
Preventative
Plugging, obstructions
Any cause limiting flow into drywell.
Drywell able to receive full flow prior
to and during wet season.
Site
Trash and debris
Trash or debris that could end up in the drywell is
No trash or debris that could get into
evident.
the drywell can be found.
Pipes
Inlet is plugged
The entrance to the pipe is restricted due to
The entrance to the pipe is not
sediment, trash, or debris.
restricted.
Vegetation/roots
Vegetation/roots that reduce free movement of
Water flows freely through pipes.
water through pipes.
Plugged
Sediment or other material prevents free flow of
Water flows freely through pipes.
water through the pipe.
Broken or joint leaks.
Damage to the pipe or pipe joints allowing water
Pipe does not allow water to exit
to seep out.
other than at the outlet.
Structure
Basin leaks
Holes or breaks in the basin allow water to leave
Basin is sealed and allows water to
the basin at locations other than per design.
exit only where designed.
Filter Media
Filter media plugged
Filter media plugged.
Flow through filter media is normal.
Inspection
Frequency
Annually and prior to and following significant
Inspect drywell system for any
storms.
defects of deficiencies.
■
NO. 27 - GRAVEL FILLED DISPERSION TRENCHBMP
Maintenance
Defect or Problem
Conditions When Maintenance is Needed
Results Expected When
Component
Maintenance is Performed
Preventative
Blocking, obstructions
Debris or trash limiting flow to dispersion trench
Dispersion trench able to receive full
or preventing spreader function.
flow prior to and during wet season.
Site
Trash and debris
Trash or debris that could end up in the
No trash or debris that could get into
dispersion trench is evident.
the dispersion trench can be found.
Pipes
Inlet is plugged
The entrance to the pipe is restricted due to
The entrance to the pipe is not
sediment, trash, or debris.
restricted.
Vegetation/roots
Vegetation/roots that reduce free movement of
Water flows freely through pipes.
water through pipes.
Plugged
Sediment or other material prevents free flow of
Water flows freely through pipes.
water through the pipe.
Broken joint or joint
Damage to the pipe or pipe joints allowing water
Pipe does not allow water to exit
leaks.
to seep out.
other than at the outlet to the trench.
Cleanout caps
Cleanout caps are broken, missing, or buried.
Cleanout caps are accessible and
intact.
Structure
Flow not reaching
Flows are not getting into the trench as designed.
Water enters and exits trench as
trench
designed.
Perforated pipe
Flow not able to enter or properly exit from
Water freely enters and exits
plugged
perforated pipe.
perforated pipe.
Flow not spreading
Outlet flows channelizing or not spreading evenly
Sheet flow occurs at the outlet of the
evenly at outlet of
from trench.
trench.
trench
Cleanout/inspection
The cleanout/inspection access is not available.
Cleanout/inspection access is
access does not allow
available.
cleaning or inspection
of perforated pipe
Filter Media
Filter media plugged
Filter media plugged.
Flow through filter media is normal.
Inspection
Inspection -?
Frequency
Annually and prior to and following significant
Inspect dispersion trench system for
storms.
any defects of deficiencies.
U