HomeMy WebLinkAboutRS_Geotechnical_Report_210129_v1
Cobalt
Geosciences
P.O. Box 82243
Kenmore, Washington 98028
(206) 331-1097
August 22, 2016
Updated April 24, 2017
pmvrsingh@gmail.com
Mr. Satwant Singh
RE: Limited Stormwater & Geologic Hazard Evaluation
Proposed Six Lot Short Plat
13609 156th Avenue SE
Renton, Washington
Dear Mr. Singh,
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to
discuss the results of our limited stormwater and geologic hazard evaluation at the referenced
site.
The proposed development includes six single-family residences, access driveways, and
landscaped areas.
Site and Project Description
The site is located at 13609 – 156th Avenue SE in Renton, Washington (Figure 1). The site consists
of one rectangular parcel (No. 146340-0045) with a total area of approximately 1.3 acres.
The site is developed with a single-family residence and access driveway extending onto the
property from the north and near the east end of the property. There are several outbuildings
within the remainder of the property.
The site slopes gently downward toward the southwest with total topographic relief of about 25
feet. The site is vegetated with grasses, blackberry vines, and local deciduous and evergreen trees.
The site is bordered to the south and west by single-family residences, to the north by SE 2nd Place
and to the east by 156th Avenue SE.
The project consists of subdividing the existing parcel into six building lots. The existing
residence and outbuildings would be removed as part of site development.
Stormwater management will include a stormwater detention vault located in the west portion of
the site.
Area Geology
The Geologic Map of King County, indicates that the site is underlain by Vashon Glacial Till.
Vashon Glacial Till is typically characterized by an unsor ted, 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.
August 22, 2016
Updated April 12, 2017
Page 2 of 3
Limited Stormwater & Geologic Hazard Evaluation
Limited Soils Evaluation
We investigated the shallow soil conditions at three locations to confirm the mapped geologic unit
and to determine if infiltration is feasible. Figure 2 shows the test pit locations. The test pit logs
can be found in Figure 3.
All of the test pits encountered about 6 to 18 inches of vegetation and topsoil underlain by
approximately 1.5 to 2.5 feet of loose to medium dense, silty-fine to medium grained sand with
gravel (Weathered Glacial Till). This material was underlain by dense, silty-fine to medium
grained sand with gravel (Unweathered Glacial Till). The unweathered till was cemented in many
places.
We did not observe groundwater in the test pits during the excavation work in August, 2016. We
anticipate that perched groundwater may develop at the contact between the weathered and
unweathered glacial till during the wet season. We observed localized soil mottling from 3 to 4
feet in the test pits and the soils below these layers were partially cemented .
Limited Infiltration Evaluation
On April 21, 2017, we performed two small-scale Pilot Infiltration Tests (PIT) within the property
to evaluate the infiltration potential of the shallow soils. Two areas were excavated to a depth of
approximately 3 feet below grade and at least 40 square feet in area. Groundwater seepage was
observed entering the excavations at a depth of 2.5 feet below grade. These excavation locations
are indicated on the site plan in Figure 2.
The shallow soil and groundwater conditions indicate that infiltration of stormwater runoff is not
feasible. If groundwater was not present, the dense glacial till would likely have an infiltration
rate of less than 0.1 inches per hour, which is lower than the Washington State Department of
Ecology recommended minimum rate of 0.3 inches per hour.
Geologic Hazards
There are localized slope areas up to 25 percent in magnitude are located within and adjacent to
the western portion of the site. We conducted a reconnaissance of the area and did not observe
evidence of earth movements, erosion, or indicators of future instability.
Based on the site geology and field explorations, the site is underlain by relatively dense soil
deposits, which are not typically prone to landslide activity under the current conditions.
The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site
is underlain by Alderwood soils. We anticipate the soils to consist of very gravelly sandy loams
with a “Slight” 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 31 st to April 1st. Erosion
control measures should be in place before the onset of wet weather.
August 22, 2016
Updated April 12, 2017
Page 3 of 3
Limited Stormwater & Geologic Hazard Evaluation
Conclusions & Recommendations
The site is underlain by weathered and unweathered glacial till. The moderately steep slope in the
western portion of the site appears stable at this time. The proposed development will not
increase the likelihood of landslide activity or erosion on the property and adjacent areas
provided that the geotechnical firm reviews the final plans and periodically monitors grading
activities in the area during construction.
Recent subsurface explorations indicated dense glacial till with areas of perched groundwater at
shallow depths. Due to the presence of shallow groundwater along with very dense, fine -grained
soil conditions, infiltration of stormwater runoff is not feasible at this site.
We should review the final plans to determine if additional geotechnical provisions are warranted
as part of the proposed site development and stormwater system construction.
Closure
The information presented herein is based upon professional interpretation utilizing standard
practices and a degree of conservatism deemed proper for this project. We emphasize that this
report is valid for this project as outlined above and for the current site conditions, and should not
be used for any other site.
Sincerely,
Cobalt Geosciences, LLC
Phil Haberman, P.G., P.E.G.
Principal Engineering Geologist
PH/sc
N
VICINITY MAP
FIGURE 1
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
cobaltgeo@gmail.com
Cobalt
Geosciences
Proposed 6-Lot Short Plat
13609 - 156th Ave SE
Renton, Washington
Project
Location
Kent
WASHINGTON
SITE
SITE PLAN
FIGURE 2
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
cobaltgeo@gmail.com
Cobalt
Geosciences
N
TP-1
Proposed 6-Lot Short Plat
13609 - 156th Ave SE
Renton, Washington
Test Pit TP-1
0-0.5’ Grass/Topsoil
0.5-2.5’ Silty-SAND with Gravel (SM)
Loose to medium dense, silty-fine to
medium grained sand with gravel,
mottled yellowish brown to grayish brown,
dry to moist.
2.5-6.0’ Silty-SAND with Gravel (SM)
Dense to very dense, silty-fine to medium
grained sand with gravel, yellowish brown to
grayish brown, moist.
End of Test Pit 6’
No Groundwater
No Caving
SM
SM
Test Pit TP-3
Test Pit TP-2
TEST PIT LOGS
FIGURE 3
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
cobaltgeo@gmail.com
Cobalt
Geosciences
Paul Bunyan Training & Education Center
Puyallup, Washington
0-0.25’ Grass/Topsoil
0.25-3.0’ Silty Sand with Gravel (SM)
Loose, silty-sand with gravel,
yellowish brown, dry to moist.
3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel
with Sand (SM-GM)
Loose to medium dense, coarse gravel
with sand and silt, trace cobbles,
yellowish brown, moist.
5.0’-8.0’ Gravel with Sand to Silty-Gravel with
Sand (GP-GM)
Medium dense, gravel with sand and
silt, yellowish to grayish brown, moist.
End of Test Pit 8’
No Groundwater
SM
SM
0-0.5’ Grass/Topsoil
0.5-3.5’ Silty GRAVEL with sand (GM)
Loose to medium dense, fine to coarse
gravel with cobbles, sand, and silt,
yellowish brown, dry to moist (Drift).
3.5-4.0’ Dense to very dense, fine to coarse
gravel with cobbles, sand, and silt,
yellowish brown, moist (Drift/Till).
End of Test Pit 4’
No Groundwater
GM
GM
Test Pit TP-3
0-0.5’ Grass/Topsoil
0.5’-2.0’ Silty Sand with Gravel (SM)
Loose, silty-sand with gravel,
yellowish brown, dry to moist (Fill).
2.0’-7.0’ Silty Sand with Gravel (SM)
Loose to medium dense, silty-sand with
gravel, trace cobbles,
yellowish brown, moist.
End of Test Pit 7’
No Groundwater
SM
SM
Test Pit TP-2
0-0.25’ Grass/Topsoil
0.25-3.0’ Silty Sand with Gravel (SM)
Loose, silty-sand with gravel,
yellowish brown, dry to moist.
3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel
with Sand (SM-GM)
Loose to medium dense, coarse gravel
with sand and silt, trace cobbles,
yellowish brown, moist.
5.0’-8.0’ Gravel with Sand to Silty-Gravel with
Sand (GP-GM)
Medium dense, gravel with sand and
silt, yellowish to grayish brown, moist.
End of Test Pit 8’
No Groundwater
SM
SM-GM
GP-GM
Test Pit TP-2
0-0.25’ Grass/Topsoil
0.25-3.0’ Silty Sand with Gravel (SM)
Loose, silty-sand with gravel,
yellowish brown, dry to moist.
3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel
with Sand (SM-GM)
Loose to medium dense, coarse gravel
with sand and silt, trace cobbles,
yellowish brown, moist.
5.0’-8.0’ Gravel with Sand to Silty-Gravel with
Sand (GP-GM)
Medium dense, gravel with sand and
silt, yellowish to grayish brown, moist.
End of Test Pit 8’
No Groundwater
SM
SM-GM
GP-GM
Test Pit TP-2
0-0.25’ Grass/Topsoil
0.25-3.0’ Silty Sand with Gravel (SM)
Loose, silty-sand with gravel,
yellowish brown, dry to moist.
3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel
with Sand (SM-GM)
Loose to medium dense, coarse gravel
with sand and silt, trace cobbles,
yellowish brown, moist.
5.0’-8.0’ Gravel with Sand to Silty-Gravel with
Sand (GP-GM)
Medium dense, gravel with sand and
silt, yellowish to grayish brown, moist.
End of Test Pit 8’
No Groundwater
SM
SM-GM
GP-GM
Test Pit TP-2
0-0.25’ Grass/Topsoil
0.25-3.0’ Silty Sand with Gravel (SM)
Loose, silty-sand with gravel,
yellowish brown, dry to moist.
3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel
with Sand (SM-GM)
Loose to medium dense, coarse gravel
with sand and silt, trace cobbles,
yellowish brown, moist.
5.0’-8.0’ Gravel with Sand to Silty-Gravel with
Sand (GP-GM)
Medium dense, gravel with sand and
silt, yellowish to grayish brown, moist.
End of Test Pit 8’
No Groundwater
SM
SM-GM
GP-GM
Limited Stormwater Evaluation
Renton, Washington
0-1.5’ Grass/Topsoil
1.5-3.0’ Silty-SAND with Gravel (SM)
Loose to medium dense, silty-fine to
medium grained sand with gravel,
mottled yellowish brown to grayish brown,
dry to moist.
3.0-5.0’ Silty-SAND with Gravel (SM)
Dense to very dense, silty-fine to medium
grained sand with gravel, yellowish brown to
grayish brown, moist.
End of Test Pit 5’
No Groundwater
No Caving
Test Pit TP-3
0-1.5’ Grass/Topsoil
1.5-4.0’ Silty-SAND with Gravel (SM)
Loose to medium dense, silty-fine to
medium grained sand with gravel,
reddish brown to dark yellowish brown,
moist.
4.0-6.0’ Silty-SAND with Gravel (SM)
Medium dense to dense, silty-fine to medium
grained sand with gravel, yellowish brown to
grayish brown, moist.
End of Test Pit 6’
No Groundwater
No Caving
SM
SM