HomeMy WebLinkAboutRS_Drainage_Report_Gorban_Shortplat_180905_v1.pdfRECEIVED
09/19/2018
amorganroth
PLANNING DIVISION
Table of Contents
· Section 1: Project Overview
· Section 2: Conditions and Requirements Summary
· Section 3: Offsite Analysis
· Section 4: Flow Control and Water Quality Analysis and Design
· Section 5: Conveyance System Analysis and Design
· Section 6: Special Reports and Studies
· Section 7: Other Permits
· Section 8: CSWPPP Analysis and Design
· Section 9: Bond Quantities, Facility Summaries, and Declaration of Covenant
· Section 10: Operations and Maintenance Manual
TIR Section 1: Project Overview
This proposed project is to create 2 single family lots from two existing King County tax parcels
(#334390-1360) located in the northerly portion of the City of Renton. The property address is
2213 NE 28th Street. The project consists pf a single parcel of 18,326 square feet. There is an
existing house, paved driveway (west side) and a shed. These features will be removed for the
new development. The current property is a single-family residence located in the middle of the
parcel.
Access to the property is from NE 28th Street, a developed public road along the north side of the
property. The proposed access to the two new lots will be from NE 28th Street. Public widening is
required by the City for NE 28th Street. The owner is requesting a fee be paid in-lieu-of the
widening. The developed project will be reviewed for drainage under the 2017 Renton Storm
Water Manual. The soils on the property and adjacent are Vashon Recessional Outwash medium
sands and gravels. These soils are feasible for full infiltration of developed hard surfaces.
NO
YES
YES
YES
GORBAN SHORT PLAT
NO
TIR Section 2: Conditions and Requirement Summary
Section 1.2 Core Requirements
Core Requirement #1: Discharge at Natural Location
The subject property has a natural discharge into the subsoil by means of infiltration.
There does not appear that the property has a natural overland flow discharge. The
proposed developed project will be to infiltrate into the underlying sands.
Core Requirement #2: Offsite Analysis
The property was walked up-gradient and down gradient from the subject property. The
properties to the east (up-gradient) infiltrate into the surrounding yard. The down gradient
properties to the south and west discharge onto the lawns and infiltrate into the
surrounding landscape areas. There are no inflows onto or discharge from the subject
property.
Core Requirement #3: Flow Control
The subject property is located within the City of Renton, Flow Control Duration Standard,
Forested Site Conditions. A flow control analysis has been performed and included within
the TIR Section 4. The subject development is exempt from flow control since the
developed 100-year runoff is less than 0.15 cfs from the existing 100-year forested
condition.
Core Requirement #4: Conveyance System
There is no conveyance system proposed for this project.
Core Requirement #5: Erosion and Sediment Control
A Temporary Erosion and Sediment Control Plan will be provided as part of the utility
permit application.
A Washington State Department of Ecology construction discharge NPDES permit is NOT
required for this project since the property is less than 1 acre in size.
Core Requirement #6: Maintenance and Operations
The maintenance and operation of the onsite single-family BMP’s will be provided as part of
the building permit application and recorded on the subject property as Covenants.
Core Requirement # 7: Financial Guarantees and Liability
No infrastructure is proposed; therefore, Financial Guarantees and Liabilities are not
required.
Core Requirement No. 8 – Water Quality
The proposed development is for 1,616 square feet of PGHS (Pollution Generating Hard
Surface) which is less than the threshold of 5,000 square feet, therefore, water quality is
Not required.
Section 1.3 Special Requirements
Special Requirement #1: Other Adopted Area-Specific Requirements
This project is not in a designated area included in an adopted master drainage plan, basin
plan, salmon conservation plan, stormwater compliance plan, flood hazard reduction plan,
or shared facilities drainage plan as identified under the Renton Surface Water Manual.
Special Requirement #2: Flood Hazard Area Delineation
This project does not contain or adjacent to a flood hazard area for a river, stream, lake,
wetland, closed depression, marine shoreline within the City of Renton.
Special Requirement #3: Flood Protection Facilities
The subject property does not rely on any flood protection facilities or proposes to modify
or construct a new flood protection faciality.
Special Requirement #4: Source Control
The proposed project does not require a commercial building or commercial site
development permit.
Special Requirement #5: Oil Control
The proposed project is a single-family residential subdivision. This project does not meet
the requirements of a high-use site.
Special Requirement #6: Aquifer Protection Area
The proposed site development is not within an Aquifer Protection Area (APA) zone 1. The
subject property is located within an Aquifer Zone 2 area.
TIR Section 3: Offsite Analysis
The property and surrounding properties were inspected and evaluated for existing runoff during a
minor storm event in September 2018. The existing house and driveway infiltrate into the
surrounding on-site soils. There was no runoff leaving the site during the storm event. The
property to the west was developed into two single family houses with infiltration systems,
designed by Offe Engineers. There were no indications of discharge leaving the site, all storm
water runoff was infiltrating onsite. The surrounding properties were infiltrating as will.
TIR Section 4: Flow Control and Water Quality Analysis and Design
Existing Site Hydrology (Part A):
The project site is 18,326 square feet in size. The existing characteristics and land cover will not
be utilized for this project. The project is located within the “Forested site conditions” area;
therefore, the project will utilize a fully forested condition as the pre-developed modeling. The
existing site soil conditions are Vashon Recessional Outwash, consisting of medium sands and
gravels.
The existing site hydrology is infiltration of existing runoff.
Developed Site Hydrology (Part B):
Drainage Review Requirements
Project Characteristics:
Total Site Area = 18,326 square feet
Offsite Improvements: None proposed
Total Improvement: $0k
Developed Impervious = 8,416 square feet
Drainage Review:
Based upon Table 1.1.2.A., the project drainage review meets “Directed Drainage Review”.
Developed Site data:
2 single-family lots ranging from 7971 to 8712 square feet in size.
Proposed Deeded R/W 1643 square feet
Developed Site Conditions:
Lot 1:
House 3,400 sq. feet
Driveway (PGHS) 600 square feet
Landscape Area 3,971 square feet
Lot 2:
House 3,400 square feet
Driveway (PGHS) 600 square feet
Landscape Area 4,712 square feet
Deeded R/W:
Driveways (PGHS) 416 square feet
Landscape Area 1,227 square feet
Performance Standards (Part C):
Area-specific flow control facility standards: From the “Flow Control Application Map”,
reference 11-A, the project discharge is required to meet Flow Control Duration Standard
(Forested Conditions). Flow control BMP’s are required for this project and will be further
evaluated within Part D of this Section.
Flow Control System (Part D):
Based upon the Renton Storm Water Manual, Section 1.2.9.2.1 Small Lot BMP requirements:
1. Full Dispersion – INFEASIBLE – The property does not have adequate flow path
lengths to allow for full dispersion;
2. Full Infiltration of Roof Area – FEASIBLE – The soils are medium sands and
gravels. Infiltration dry wells will be sized for the proposed house (roof area) runoff.
Drywell for full infiltration (Section C.2.2.4)
Medium Sands 90 cu. feet of gravel / 1,000 square feet of roof area
Proposed houses 3,400 sq. feet roof area
Proposed Drywells – 90 * 3400/1000 = 306 cubic feet gravel
Use 8’ x 8’ x 4.75’ deep = 306 cu. feet per house
Modeling – full infiltration subtracts out roof area from total site area <6,800 sq. ft.>
3. Permeable Pavement – FEASIBLE (Section C.2.7.2) for driveway areas
Proposed driveways = 808 sq. feet each to edge of existing roadway (NE 28th Street)
Modeling – Permeable Pavement no underdrain – 50% imperious surface/50% pervious
surface
Impervious surface = 808 sq. feet
Pervious surface = 808 sq. feet
—————————————————————————————————
MGS FLOOD
PROJECT REPORT
Program Version: MGSFlood 4.46
Program License Number: 201110001
Project Simulation Performed on: 08/01/2018 12:06 PM
Report Generation Date: 08/01/2018 12:06 PM
—————————————————————————————————
Input File Name: Offe_Renton_08_1_18.fld
Project Name: Offe Project_Renton
Analysis Title: 08_1_18
Comments: SMW
———————————————— PRECIPITATION INPUT ————————————————
Computational Time Step (Minutes): 15
Extended Precipitation Time Series Selected
Climatic Region Number: 14
Full Period of Record Available used for Routing
Precipitation Station : 96003605 Puget East 36 in_5min 10/01/1939-10/01/2097
Evaporation Station : 961036 Puget East 36 in MAP
Evaporation Scale Factor : 0.750
HSPF Parameter Region Number: 1
HSPF Parameter Region Name : USGS Default
********** Default HSPF Parameters Used (Not Modified by User) ***************
********************** WATERSHED DEFINITION ***********************
Predevelopment/Post Development Tributary Area Summary
Predeveloped Post Developed
Total Subbasin Area (acres) 0.416 0.256
Area of Links that Include Precip/Evap (acres) 0.000 0.000
Total (acres) 0.416 0.256
----------------------SCENARIO: PREDEVELOPED
Number of Subbasins: 1
---------- Subbasin : Subbasin 1 ----------
-------Area (Acres) --------
Outwash Forest 0.416
----------------------------------------------
Subbasin Total 0.416
----------------------SCENARIO: POSTDEVELOPED
Number of Subbasins: 1
---------- Subbasin : Subbasin 1 ----------
-------Area (Acres) --------
Outwash Grass 0.240
Impervious 0.016
----------------------------------------------
Subbasin Total 0.256
************************* LINK DATA *******************************
----------------------SCENARIO: PREDEVELOPED
Number of Links: 0
************************* LINK DATA *******************************
----------------------SCENARIO: POSTDEVELOPED
Number of Links: 0
**********************FLOOD FREQUENCY AND DURATION STATISTICS*******************
----------------------SCENARIO: PREDEVELOPED
Number of Subbasins: 1
Number of Links: 0
----------------------SCENARIO: POSTDEVELOPED
Number of Subbasins: 1
Number of Links: 0
***********Groundwater Recharge Summary *************
Recharge is computed as input to Perlnd Groundwater Plus Infiltration in Structures
Total Predeveloped Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: Subbasin 1 91.169
_____________________________________
Total: 91.169
Total Post Developed Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: Subbasin 1 68.000
_____________________________________
Total: 68.000
Total Predevelopment Recharge is Greater than Post Developed
Average Recharge Per Year, (Number of Years= 158)
Predeveloped: 0.577 ac-ft/year, Post Developed: 0.430 ac-ft/year
***********Water Quality Facility Data *************
----------------------SCENARIO: PREDEVELOPED
Number of Links: 0
----------------------SCENARIO: POSTDEVELOPED
Number of Links: 0
***********Compliance Point Results *************
Scenario Predeveloped Compliance Subbasin: Subbasin 1
Scenario Postdeveloped Compliance Subbasin: Subbasin 1
*** Point of Compliance Flow Frequency Data ***
Recurrence Interval Computed Using Gringorten Plotting Position
Predevelopment Runoff Postdevelopment Runoff
Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs)
----------------------------------------------------------------------------------------------------------------------
2-Year 3.069E-04 2-Year 5.916E-03
5-Year 3.285E-04 5-Year 8.554E-03
10-Year 3.322E-04 10-Year 1.087E-02
25-Year 3.351E-04 25-Year 1.707E-02
50-Year 3.356E-04 50-Year 2.190E-02
100-Year 3.356E-04 100-Year 3.906E-02
200-Year 3.732E-04 200-Year 4.199E-02
** Record too Short to Compute Peak Discharge for These Recurrence Intervals
Water Quality System (Part E):
The proposed project has 1,616 sq. feet of PGHS which is less than the threshold of 5,000 square
feet therefore, Water Quality is NOT required.
TIR Section 5: Conveyance System Analysis and Design
There is no conveyance system proposed for this project.
TIR Section 6: Special Reports and Studies
Geotechnical Investigation
Proposed Two-Lot Subdivision
2213 NE 28th Street
Renton, Washington
June 3, 2018
GEOTECHNICAL INVESTIGATION
RENTON, WASHINGTON
i
Table of Contents
1.0 INTRODUCTION ............................................................................................................. 1
2.0 PROJECT DESCRIPTION .............................................................................................. 1
3.0 SITE DESCRIPTION ....................................................................................................... 1
4.0 FIELD INVESTIGATION ............................................................................................... 1
4.1.1 Site Investigation Program ................................................................................... 1
5.0 SOIL AND GROUNDWATER CONDITIONS .............................................................. 2
5.1.1 Area Geology ........................................................................................................ 2
5.1.2 Groundwater ........................................................................................................ 2
6.0 GEOLOGIC HAZARDS ................................................................................................... 3
6.1 Erosion Hazard .................................................................................................... 3
6.2 Seismic Hazard .................................................................................................... 3
7.0 DISCUSSION ................................................................................................................... 4
7.1.1 General................................................................................................................. 4
8.0 RECOMMENDATIONS .................................................................................................. 4
8.1.1 Site Preparation ................................................................................................... 4
8.1.2 Temporary Excavations ........................................................................................ 4
8.1.3 Erosion and Sediment Control.............................................................................. 5
8.1.4 Foundation Design ............................................................................................... 5
8.1.5 Stormwater Management ..................................................................................... 6
8.1.6 Slab-on-Grade ...................................................................................................... 6
8.1.7 Groundwater Influence on Construction .............................................................. 7
8.1.8 Utilities ................................................................................................................ 8
8.1.9 Pavements ............................................................................................................ 8
9.0 CONSTRUCTION FIELD REVIEWS ...........................................................................10
10.0 CLOSURE ...................................................................................................................10
LIST OF APPENDICES
Appendix A — Statement of General Conditions
Appendix B — Figures
Appendix C — Test Pit & Hand Boring Logs
GEOTECHNICAL INVESTIGATION
RENTON, WASHINGTON
June 3, 2018
1
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
206-331-1097
1.0 Introduction
In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt) has completed a geotechnical
investigation for the proposed two-lot subdivision located at 2213 NE 28th Street in Renton, Washington
(Figure 1).
The purpose of the geotechnical investigation was to identify subsurface conditions and to provide
geotechnical recommendations for foundation design, stormwater management, earthwork, soil
compaction, pavements, and suitability of the on-site soils for use as fill.
The scope of work for the geotechnical evaluation consisted of a site investigation followed by engineering
analyses to prepare this report. Recommendations presented herein pertain to various geotechnical
aspects of the proposed development, including foundation support of the new buildings and stormwater
management.
2.0 Project Description
The project includes subdivision of the parcel into two building lots followed by construction of two new
residences, driveways, and stormwater management systems.
Anticipated building loads are expected to be light and site grading will include cuts and fills on the order
of 4 feet or less. We should be provided with the final plans to verify that our recommendations have been
incorporated into the design.
3.0 Site Description
The site is located at 2213 NE 28th Street in Renton, Washington (Figure 1). The site consists of one
rectangular parcel (No. 3343901360) with a total area of 17,913 square feet.
The property is developed with a single-family residence located in the central portion of the property. A
gravel driveway extends on to the property near the northwest corner. The site is nearly level and
vegetated with grasses, bushes, and sparse evergreen/deciduous trees.
The property is bordered to the east, west, and south by single-family residences and to the north by NE
28th Street.
4.0 Field Investigation
4.1.1 Site Investigation Program
The geotechnical field investigation program was completed on May 30, 2018 and included excavating
and sampling one test pit and two hand borings within the property for subsurface analysis.
The soils encountered were logged in the field and are described in accordance with the Unified Soil
Classification System (USCS).
GEOTECHNICAL INVESTIGATION
RENTON, WASHINGTON
June 3, 2018
2
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
206-331-1097
A Cobalt Geosciences field representative conducted the explorations, collected disturbed soil samples,
classified the encountered soils, kept a detailed log of the explorations, and observed and recorded
pertinent site features.
The results of the test pit and hand boring explorations are presented on the exploration logs enclosed in
Appendix C.
5.0 Soil and Groundwater Conditions
5.1.1 Area Geology
The site lies within the Puget Lowland. The lowland is part of a regional north-south trending trough that
extends from southwestern British Columbia to near Eugene, Oregon. North of Olympia, Washington,
this lowland is glacially carved, with a depositional and erosional history including at least four separate
glacial advances/retreats. The Puget Lowland is bounded to the west by the Olympic Mountains and to
the east by the Cascade Range. The lowland is filled with glacial and non -glacial sediments consisting of
interbedded gravel, sand, silt, till, and peat lenses.
The Geologic Map of King County, indicates that the site is located near the contacts between Vashon
Glacial Till and Vashon Recessional Outwash.
Vashon Glacial Till is typically characterized by an unsorted, non -stratified mixture of clay, silt, sand,
gravel, cobbles and boulders in variable quantities. These materials are typically dense and relatively
impermeable. The poor sorting reflects the mixing of the materials as these sediments were overridden
and incorporated by the glacial ice.
Vashon Recessional Outwash includes normally consolidated sands with areas of gravel and interbeds of
silt and clay. These materials are typically less than 20 feet thick and overlie Vashon Glacial Till.
Explorations
Test Pit TP-1 encountered approximately 12 inches of topsoil and vegetation underlain by approximately 2
feet of loose, silty-fine to medium grained sand (Weathered Recessional Outwash). These materials were
underlain by loose to medium dense, fine to medium grained sand trace gravel (Recessional Ou twash)
which continued to the termination depth of TP-1.
Both hand borings encountered approximately 12 inches of grass and topsoil underlain by about 2 to 2.5
feet of loose, silty-fine to medium grained sand trace gravel (Weathered Recessional Outwash). These
materials were underlain by loose to medium dense, fine to medium grained sand (Recessional Outwash),
which continued to the termination depths of the hand borings.
5.1.2 Groundwater
Groundwater was not encountered in any of the explorations at the date and time of our investigation.
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.
GEOTECHNICAL INVESTIGATION
RENTON, WASHINGTON
June 3, 2018
3
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
206-331-1097
Water levels at the time of the field investigation may be different from those encountered during the
construction phase of the project.
6.0 Geologic Hazards
6.1 Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site is
underlain by Indianola loamy sand (5 to 15 percent slopes). These soils have a slight to moderate erosion
potential in a disturbed state.
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 1st. Erosion control measures should be in place before
the onset of wet weather.
6.2 Seismic Hazard
The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the 2015
International Building Code (2015 IBC). A Site Class D applies to an overall profile consisting of dense to
very dense soils within the upper 100 feet.
We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values
for SS, S1, Fa, and Fv. The USGS website includes the most updated published data on seismic conditions.
The site specific seismic design parameters and adjusted maximum spectral response acceleration
parameters are as follows:
PGA (Peak Ground Acceleration, in percent of g)
SS 143.40% of g
S1 54.20% of g
FA 1.00
FV 1.50
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.
We anticipate that the relatively dense soils below the site will have a low potential for liquefaction.
GEOTECHNICAL INVESTIGATION
RENTON, WASHINGTON
June 3, 2018
4
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
206-331-1097
7.0 DISCUSSION
7.1.1 General
The site is underlain by normally consolidated recessional outwash sands. Some re-compaction or
overexcavation and replacement of locally loose soils may be necessary below foundation elements. The
depth of overexcavation or re-compaction will likely be less than 2 feet below new footings.
Infiltration of stormwater runoff is feasible at the site. Shallow infiltration trenches are suitable to
manage runoff from new impervious surfaces.
8.0 Recommendations
8.1.1 Site Preparation
Trees, shrubs and other vegetation should be removed prior to strippi ng of surficial organic-rich soil and
fill. Based on observations from the site investigation program, the stripping depth will range from 8 to
12 inches. Deeper excavations may be required below large trees and in any areas where fill may be
present.
The native soils consist of silty-sand and poorly graded sands (Recessional Outwash). These soils may be
used as structural fill provided they achieve compaction requirements and are within 3 percent of the
optimum moisture. . 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.
8.1.2 Temporary Excavations
Based on our understanding of the project, we anticipate that the grading could include local cuts on the
order of approximately 4 feet or less for foundation and utility placement. Any deeper excavations should
be sloped no steeper than 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 1.5H:1V,
where room permits.
Temporary cuts should be in accordance with the Washington Administ rative 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
GEOTECHNICAL INVESTIGATION
RENTON, WASHINGTON
June 3, 2018
5
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
206-331-1097
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 procee d 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.
8.1.3 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 thr ough 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.
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.
8.1.4 Foundation Design
The proposed residences may be supported on shallow spread footing foundation systems bearing on
undisturbed medium dense or firmer native soils or on properly compacted structural fill placed on the
suitable native soils. If structural fill is used to support foundations, then the zone of structural fill should
extend beyond the faces of the footing a lateral distance at least equal to the thickness of the structural fill.
We anticipate that bearing soils will be encountered between 3 and 4 feet below existing site elevations.
GEOTECHNICAL INVESTIGATION
RENTON, WASHINGTON
June 3, 2018
6
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
206-331-1097
For shallow foundation support, we recommend widths of at least 18 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 2,000 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 25-foot exterior wall footing, or between adjoining column footings,
should be less than ½ 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 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 0.40
acting between the base of foundations and the supporting subgrades. Lateral resist ance for footings can
also be developed using an allowable equivalent fluid passive pressure of 275 pounds per cubic foot (pcf)
acting against the appropriate vertical footing faces (neglect the upper 12 inches below grade in exterior
areas).
The allowable friction factor and allowable equivalent fluid passive pressure values include a factor of
safety of 1.5. The frictional and passive resistance of the soil may be combined without reduction in
determining the total lateral resistance. A 1/3 increase i n the above values may be used for short duration
transient loads.
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 surfac e by the geotechnical engineer or his representative.
8.1.5 Stormwater Management
Because the Vashon Recessional Outwash has not been overridden by glacial ice, this soil unit is
considered normally-consolidated. The Washington State Department of Ecology 2015 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. Based on
this equation and our sieve analyses, infiltration rates of 6 to 8 inches per hour are suitable. Alternatively,
the systems may be designed using the Medium Sand criteria from the 2016 Surface Water Design
Manual (King County).
GEOTECHNICAL INVESTIGATION
RENTON, WASHINGTON
June 3, 2018
7
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
206-331-1097
The soil classifications and water quality laboratory results at potential infiltration system depths are
presented in the following table:
We would classify the upper weathered soils (0-3 feet below grade) as Loamy Sand (USDA) and silty-sand
to poorly graded sand (SM-SP per USCS). Soils below this level consist of Sand (USDA) and poorly
graded sand (SP per USCS). The shallow soils that underlie the s ite (below 3 to 3.5 feet) consist of
medium sand as described in the King County Surface Water Design Manual (2016 SWDM).
From the SWDM, infiltration trenches in medium sand may accept runoff from 1,000 square feet of
impervious surface. Each trench (per 1,000 square feet of runoff) should be at least 2 feet wide and 30
feet long. As noted above, we recommend a bottom of trench elevation of 3 to 5 feet below existing site
elevations. More information can be found in the SWDM on pages C-50 and C-51 (and others).
Infiltration trenches/galleries should be set back at least 5 feet from property lines and 10 feet from
adjacent building footing foundations or utility trenches. The bottom of infiltration trenches or galleries
should be at least one foot lower than adjacent building footing foundations or utility trenches.
Infiltration trenches/galleries should be installed at several locations within the project site to disperse
disposed water over a wide area under the site to minimize potential problems from concentration of
disposed stormwater.
The trenches/galleries should be at least 24 inches wide. The side walls (but not the bottom) of the
trenches/galleries should be lined with a layer of non-woven filter fabric (MIRAFI 140N). The
trenches/galleries are then to be filled with clean washed 3/4 to 1-1/2 inch gravel to within about 12 inches
of finish grade. The gravel particles should be clean and not coated with mud.
8.1.6 Slab-on-Grade
We recommend that the upper 12 inches of the native soils within slab areas be re-compacted to at least
95 percent of the modified proctor (ASTM D1557 Test Method).
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 o f
the vapor barrier below concrete slabs. Exterior slabs typically do not utilize vapor barriers.
The American Concrete Institutes ACI 360R-06 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.
Exploration
Number
Sample/
Test
Depth
Organic
Content
Cation
Exchange
Capacity
Soil Classification
(USDA/USCS)
TP-1 3’ 4.4% 6.5 meq Medium Sand/SP
HB-1 3.5’ 5.3% 7.1 meq Medium Sand/SP
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Slabs on grade may be designed using a coefficient of subgrade reaction of 180 pounds per cubic inch (pci)
assuming the slab-on-grade base course is underlain by structural fill placed and compacted as outlined in
Section 8.1.
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 s tormwater 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.
8.1.7 Groundwater Influence on Construction
Groundwater was not encountered in any of our explorations. We do not anticipate that groundwater will
be encountered in shallow excavations at the site.
8.1.8 Utilities
Utility trenches should be excavated according to accepted engineering practices following OSH A
(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 dry ing 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 soils were encountered at shallow depths in the exploratio ns at this site. These soils
have very 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.
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8.1.9 Pavement Recommendations
The near surface subgrade soils generally consist of silty sand and poorly graded sand. These soils are
rated as fair to good for pavement subgrade material (depending on silt content and moisture conditions).
We estimate that the subgrade will have a California Bearing R atio (CBR) value of 8 and a modulus of
subgrade reaction value of k = 180 pci, provided the subgrade is prepared in general accordance with our
recommendations.
We recommend that, at a minimum, 18 inches of the existing subgrade material be moisture conditioned
(as necessary) and re-compacted to prepare for the construction of pavement sections. Deeper levels of
recompaction or overexcavation and replacement may be necessary in areas where loose soils are present.
The subgrade should be compacted to at least 95 percent of the maximum dry density as determined by
ASTM Test Method D1557. In place density tests should be performed to verify proper moisture content
and adequate compaction. However, if the subgrade soil consists of firm and unyielding nativ e glacial
soils a proof roll of the pavement subgrade soil may be performed in lieu of compaction tests.
The recommended flexible and rigid pavement sections are based on design CBR and modulus of
subgrade reaction (k) values that are achieved, only foll owing proper subgrade preparation. It should be
noted that subgrade soils that have relatively high silt contents will likely be highly sensitive to moisture
conditions. The subgrade strength and performance characteristics of a silty subgrade material m ay be
dramatically reduced if this material becomes wet.
Based on our knowledge of the proposed project, we expect the traffic to range from light duty (passenger
automobiles) to heavy duty (large trucks). The following tables show the recommended pavement
sections for light duty and heavy duty use.
ASPHALTIC CONCRETE (FLEXIBLE) PAVEMENT
LIGHT DUTY
Asphaltic Concrete Aggregate Base* Compacted Subgrade* **
2.0 in. 6.0 in. 18.0 in.
* 95% compaction based on ASTM Test Method D1557
** A proof roll may be performed in lieu of in place density tests
HEAVY DUTY
Asphaltic Concrete Aggregate Base* Compacted Subgrade* **
3.5 in. 8.0 in. 18.0 in.
* 95% compaction based on ASTM Test Method D1557
** A proof roll may be performed in lieu of in place density tests
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PORTLAND CEMENT CONCRETE (RIGID) PAVEMENT
Min. PCC Depth Aggregate Base* Compacted Subgrade*
**
6.0 in. 8.0 in. 18.0 in.
* 95% compaction based on ASTM Test Method D1557
** A proof roll may be performed in lieu of in place density tests
The asphaltic concrete depth in the flexible pavement tables should be a surface course type asphalt, such
as Washington Department of Transportation (WSDOT) ½ inch HMA. Aggregate base includes crushed
surfacing top course or base course (5/8” or 1-1/4” minus crushed rock).
The rigid pavement design is based on a Portland Cement Concrete (PCC) mix that has a 28 day
compressive strength of 4,000 pounds per square inch (psi). The design is also based on a concrete
flexural strength or modulus of rupture of 550 psi.
9.0 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
Monitor infiltration system excavations
Observe slab-on-grade preparation
Monitor proofrolls of pavement areas
Observe excavation stability
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.
10.0 Closure
This report was prepared for the exclusive use of Svetlana Gorban and her 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.
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Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is the
responsibility of Svetlana Gorban 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.
Respectfully submitted,
Cobalt Geosciences, LLC
Original signed by:
Exp. 6/26/18
Phil Haberman, PE, LG, LEG
Principal
PH/sc
APPENDIX A
Statement of General Conditions
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 party 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 loca tions.
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 i n the presence of a qualified
geotechnical engineer; Cobalt Geosciences cannot be responsible for site work carried out without being
present.
10.2
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APPENDIX B
Figures: Vicinity Map, Site Plan
SITE
N
Project
Location
Renton
WASHINGTON
Proposed Two-Lot Subdivision
2213 NE 28th Street
Renton, Washington
SITE PLAN
FIGURE 1
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
N
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
SITE PLAN
FIGURE 2
TP-1
Proposed Two-Lot Subdivision
2213 NE 28th Street
Renton, Washington
TP-1
APPENDIX C
Exploration Logs
TEST PIT
& HAND
BORING
LOGS
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-1
0-1’ Vegetation/Topsoil
1-3’ Silty Sand to Poorly Graded Sand (SM-SP)
Loose to medium dense, silty-fine to medium grained sand trace
gravel, yellowish brown to reddish brown, moist.
(Weathered Recessional Outwash)
3-10’ Poorly Graded Sand (SP)
Medium dense, fine to medium grained sand trace gravel, grayish
brown, moist. (Recessional Outwash).
End of Test Pit 10’
No Groundwater
Caving in Upper 6’
SM-SP
Topsoil/Vegetation
Weathered Recessional Outwash
1’
USCS Graphic
3’
Recessional OutwashSP
Proposed Two-Lot Subdivision
2213 NE 28th Street
Renton, Washington
Hand Boring HB-1
0-1’ Vegetation/Topsoil
1-3.5’ Silty Sand to Poorly Graded Sand (SM-SP)
Loose to medium dense, silty-fine to medium grained sand trace
gravel, yellowish brown to reddish brown, moist.
(Weathered Recessional Outwash)
3.5-7’ Poorly Graded Sand (SP)
Medium dense, fine to medium grained sand trace gravel, grayish
brown, moist. (Recessional Outwash).
End of Hand Boring 7’
No Groundwater
No Caving
SM-SP
Topsoil/Vegetation
Weathered Recessional Outwash
1’
USCS Graphic
3.5’
Recessional OutwashSP
Hand Boring HB-2
SM-SP
Topsoil/Vegetation
Weathered Recessional Outwash
1’
USCS Graphic
3’
Recessional OutwashSP
Not to Scale
0-1’ Vegetation/Topsoil
1-3’ Silty Sand to Poorly Graded Sand (SM-SP)
Loose to medium dense, silty-fine to medium grained sand trace
gravel, yellowish brown to reddish brown, moist.
(Weathered Recessional Outwash)
3-7’ Poorly Graded Sand (SP)
Medium dense, fine to medium grained sand trace gravel, grayish
brown, moist. (Recessional Outwash).
End of Hand Boring 7’
No Groundwater
No Caving
TIR Section 7: Other Permits
City of Renton Building Permits
City of Renton Demolition Permit
City of Renton Side Sewer Permits
City of Renton Water Meter Permits
TIR Section 8: CSWPPP Analysis and Design
A Temporary Erosion and Sediment Control Plan (TESCP) implementing Best Management
Practices will be provided for the demolition permit and individual building permits.
TIR Section 9: Bond Quantities, Facility Summaries, and Declaration of
Covenant
TIR Section 10: Operations and Maintenance Manual
The individual building permits will have the required operation and maintenance for each BMP
attached to the building permits and Covenants.