HomeMy WebLinkAboutRS_Geotechnical_Letter_08202020_v1Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
www.cobaltgeo.com (206) 331-1097
August 20, 2020
Sukhjinder Mangat
Sukhjinder.mangat@hotmail.com
RE: Limited Geotechnical Evaluation
Proposed Residence
1415 N. 24th Street
Renton, Washington
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to
discuss the results of our limited 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 1415 N. 24th Street in Renton, Washington. The site consists of one
rectangular parcel (No. 3344500234) with a total area of about 23,595 square feet.
The property is developed with a single-family residence and driveway. The remainder of the
property is undeveloped and vegetated with grasses, blackberry vines, ivy, ferns, and variable
diameter trees.
The site slopes gently downward from northeast to southwest at magnitudes of 4 to 8 percent and
relief of about 10 feet. There are locally steeper slopes further southwest and downgradient of the
property. These slopes have magnitudes of 15 to 30 percent.
The property is bordered to the east, west, and south by residential properties and to the north by
N. 24th Street.
The project includes construction of a new multi-story residence with garage and crawlspace
areas. Stormwater management may include dispersion, detention, or infiltration facilities
depending on feasibility.
Area Geology
The Geologic Map of King County indicates that the site is underlain by Vashon Recessional
Outwash and at depth by Vashon Glacial Till.
Vashon Recessional Outwash consists of sand, gravel and lacustrine deposited silt and clay.
These soils are variably permeable and highly erosive. Recessional outwash has not been
consolidated by glacial activity and can be permeable.
Vashon Glacial Till includes dense to very dense mixtures of silt, sand, clay, and gravel. These
deposits are typically impermeable.
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Limited Geotechnical Evaluation
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Soil & Groundwater Conditions
As part of our evaluation, we excavated two hand borings within the property to determine the
shallow soil and groundwater conditions, where accessible.
The hand borings encountered about 6 inches of topsoil and grass underlain by 3 to 4 feet of loose
to medium dense, silty-fine to fine grained sand with gravel (Weathered Glacial Till?). This layer
was underlain by medium dense to dense, silty-fine to fine grained sand with gravel (Glacial Till),
which continued to the termination depths of the hand borings.
Groundwater was not encountered in the explorations during the field work; however, mottled
soils were present at shallow depths. There is a chance that perched groundwater may develop
between the weathered and unweathered glacial till. The depth to groundwater would likely be 3
to 8 feet or more.
Stormwater Management Feasibility
The site is underlain by weathered and unweathered glacial till. These soils have a low
permeability which decreases with depth as soil density increases. The upper soils are mottled
which indicates that shallow groundwater could be present during the wet season.
Furthermore, the proposed development is adjacent to properties with basements and/or
crawlspaces that are downgradient of this property, as well as moderately steep slope areas to the
southwest. There is a high probability that if infiltration systems were utilized, the groundwater
would migrate laterally, eventually daylighting in a crawlspace or basement downslope of the
property, or as spring activity on downslope areas.
We performed an in-situ infiltration test (Small Scale PIT) in an excavation near HB-1 at a depth
of approximately 4 feet below grade. After testing and application of correction factors for site
variability (0.5), testing (0.5), and influent control (0.9), the infiltration rate was 0.22 inches per
hour. This is lower than what is considered to be feasible. We excavated below this area and
found the water perched on fine grained mottled soils.
We recommend direct connection of runoff devices to City infrastructure. 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.
Foundation Design
The proposed residence may be supported on a shallow spread footing foundation system bearing
on undisturbed 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 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.
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Limited Geotechnical Evaluation
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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 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 basement areas are proposed.
Wall Design Criteria
“At-rest” Conditions (Lateral Earth Pressure – EFD+) 55 pcf (Equivalent Fluid Density)
“Active” Conditions (Lateral Earth Pressure – EFD+) 35 pcf (Equivalent Fluid Density)
Seismic Increase for “At-rest” Conditions
(Lateral Earth Pressure)
11H* (Uniform Distribution)
Seismic Increase for “Active” Conditions
(Lateral Earth Pressure)
6H* (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 (10 percent probability of being exceeded in
50 years),
+EFD – Equivalent Fluid Density
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Limited Geotechnical Evaluation
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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
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.
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.
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.
August 20, 2020
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Limited Geotechnical Evaluation
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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
8/20/2020
Phil Haberman, PE, LG, LEG
Principal
PH/sc
Proposed Residence
1415 N. 24th 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
HB-2
Subject
Property
HB-1
N
PT
Well-graded gravels, gravels, gravel-sand mixtures, little or no fines
Poorly graded gravels, gravel-sand mixtures, little or no fines
Silty gravels, gravel-sand-silt mixtures
Clayey gravels, gravel-sand-clay mixtures
Well-graded sands, gravelly sands, little or no fines
COARSE
GRAINED
SOILS
(more than 50%
retained on
No. 200 sieve)
Primarily organic matter, dark in color,
and organic odor Peat, humus, swamp soils with high organic content (ASTM D4427)HIGHLY ORGANIC
SOILS
FINE GRAINED
SOILS
(50% or more
passes the
No. 200 sieve)
MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION
Gravels
(more than 50%
of coarse fraction
retained on No. 4
sieve)
Sands
(50% or more
of coarse fraction
passes the No. 4
sieve)
Silts and Clays
(liquid limit less
than 50)
Silts and Clays
(liquid limit 50 or
more)
Organic
Inorganic
Organic
Inorganic
Sands with
Fines
(more than 12%
fines)
Clean Sands
(less than 5%
fines)
Gravels with
Fines
(more than 12%
fines)
Clean Gravels
(less than 5%
fines)
Unified Soil Classification System (USCS)
Poorly graded sand, gravelly sands, little or no fines
Silty sands, sand-silt mixtures
Clayey sands, sand-clay mixtures
Inorganic silts of low to medium plasticity, sandy silts, gravelly silts,
or clayey silts with slight plasticity
Inorganic clays of low to medium plasticity, gravelly clays, sandy clays,
silty clays, lean clays
Organic silts and organic silty clays of low plasticity
Inorganic silts, micaceous or diatomaceous fine sands or silty soils,
elastic silt
Inorganic clays of medium to high plasticity, sandy fat clay,
or gravelly fat clay
Organic clays of medium to high plasticity, organic silts
Moisture Content Definitions
Grain Size Definitions
Dry Absence of moisture, dusty, dry to the touch
Moist Damp but no visible water
Wet Visible free water, from below water table
Grain Size Definitions
Description Sieve Number and/or Size
Fines <#200 (0.08 mm)
Sand
-Fine
-Medium
-Coarse
Gravel
-Fine
-Coarse
Cobbles
Boulders
#200 to #40 (0.08 to 0.4 mm)
#40 to #10 (0.4 to 2 mm)
#10 to #4 (2 to 5 mm)
#4 to 3/4 inch (5 to 19 mm)
3/4 to 3 inches (19 to 76 mm)
3 to 12 inches (75 to 305 mm)
>12 inches (305 mm)
Classification of Soil Constituents
MAJOR constituents compose more than 50 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 0 to 5 percent of the soil
(i.e., slightly silty SAND, trace gravel).
Relative Density Consistency
(Coarse Grained Soils) (Fine Grained Soils)
N, SPT, Relative
Blows/FT Density
0 - 4 Very loose
4 - 10 Loose
10 - 30 Medium dense
30 - 50 Dense
Over 50 Very dense
N, SPT, Relative
Blows/FT Consistency
Under 2 Very soft
2 - 4 Soft
4 - 8 Medium stiff
8 - 15 Stiff
15 - 30 Very stiff
Over 30 Hard
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Soil Classification Chart Figure C1
Log of Hand Boring HB-1
Date: August 12, 2020
Contractor:
Method: Hand Auger
Depth: 6’
Elevation: N/A
Logged By: PH Checked By: SC
Initial Groundwater: None
Sample Type: Grab
Final Groundwater: N/A
Material Description
SPT N-Value
Moisture Content (%)Plastic
Limit
Liquid
Limit
10 20 30 400 50
1
2
3
4
5
6
7
8
9
10
End of Hand Boring 6’
Vegetation/Topsoil
SM
Dense, silty-fine to medium grained sand trace gravel,
yellowish brown to grayish brown, moist. (Glacial Till)
SM
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Proposed Residence
1415 N. 24th Street
Renton, Washington
Hand
Boring
Log
Loose to medium dense, silty-fine to medium grained sand
with gravel and cobbles, mottled dark yellowish brown to
yellowish brown, moist. (Weathered Glacial Till)
Log of Hand Boring HB-2
Date: August 12, 2020
Contractor:
Method: Hand Auger
Depth: 6’
Elevation: N/A
Logged By: PH Checked By: SC
Initial Groundwater: None
Sample Type: Grab
Final Groundwater: N/A
Material Description
SPT N-Value
Moisture Content (%)Plastic
Limit
Liquid
Limit
10 20 30 400 50
1
2
3
4
5
6
7
8
9
10
End of Hand Boring 6’
Vegetation/Topsoil
SM
Dense, silty-fine to medium grained sand trace gravel,
yellowish brown to grayish brown, moist. (Glacial Till)
SM
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Hand
Boring
Log
Loose to medium dense, silty-fine to medium grained sand
with gravel and cobbles, mottled dark yellowish brown to
yellowish brown,moist. (Weathered Glacial Till)
Proposed Residence
1415 N. 24th Street
Renton, Washington