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Cascade GeotechNW
4957 Lakemont Blvd SE, C-4, #325
Bellevue, WA 98006
cascadegeonw@gmail.com
206-491-0081
October 2, 2017
File No. 2017-15
Mr. Willy Cho
3037 - 67th Avenue SE
Mercer Island, WA 98040
Subject: Infiltration Assessment
Proposed Development
11840 SE 192nd Street, Renton, WA
King County Parcel # 6198400341
Dear Mr. Cho,
As requested, Cascade GeotechNW co nducted field exploration and field infiltration
testing to assist you and your project team with the proposed development at the above-
referenced site. This study was performed in general accordance with our mutually
agreed scope of work outlined in our proposal dated September 12, 2017, which was
subsequently approved by you on September 13, 2017. The following sections present
our understanding of the projects, the scope of services, document the subsurface
conditions encountered at the test locations, the field infiltration testing procedure, the
measured field infiltration rates, and the recommended long-term (design) infiltration
rate.
SITE DESCRIPTION AND PROJECT UNDERSTANDING
The subject site is an approximately 1.02-acre property located at 11840 SE 192nd Street
in Renton, Washington (see Vicinity Map, Figure 1). The site is bounded by SE 192nd
Street to the south, a vacant lot to the east, and existing single-family residences to the
west and north (see Figure 2). The site is currently occupied by a one-story house in the
southern portion of the site. The site is practically flat.
Based on information provided to us, we understand that you plan to remove the existing
house, and to subdivide the subject property into three single-family residential parcels
Infiltration Evaluation
Proposed Development – 11840 SE 192nd Street, Renton, WA
October 2, 2017
2017-15 11840 SE 192nd St Infiltration Rpt Page 2 PanGEO, Inc.
and to construct one single-family residence on each subdivided parcel. We further
understand that a geotechnical report is required by the City of Renton to evaluate the soil
and groundwater conditions at the site and the feasibility of infiltration characteristics of
the site soils, and to provide geotechnical recommendation related to the flow control and
BMP design.
The conclusions and recommendations outlined in this report are based on our
understanding of the proposed development, which is in turn based on the project
information provided. If the above project description is incorrect, or the project
information changes, we should be consulted to review the recommendations contained
in this study and make modifications, if needed.
PROJECT SCOPE
The purpose of our work is to characterize subsurface conditions at the project site, and to conduct
field infiltration testing and provide infiltration rate to support the stormwater drainage design.
The scope of our work for this project included the following tasks and work efforts:
1. Document Review – Review readily available geologic and geotechnical data for
the project area.
2. Site Reconnaissance – Conduct a site reconnaissance to observe the existing site
conditions, and to identify site conditions that may impact the proposed short plat
development from a geotechnical standpoint.
3. Infiltration Testing – Excavate four (4) test pits and conduct four small scale
pilot infiltration tests (PIT) at the site at about 5 feet deep. The infiltration
testing was performed in general accordance with the procedures outlined in
the 2017 Renton Surface Water Design Manual and 2014 WSDOE
Srormwater Management Manual For western Washington .
4. Report – Preparation of a geotechnical report summarizing our work on the project
and presenting our findings and opinions.
Please note that our current geotechnical evaluation is limited to evaluation of infiltration
feasibility of the site soils, and is not intended for providing geotechnical
Infiltration Evaluation
Proposed Development – 11840 SE 192nd Street, Renton, WA
October 2, 2017
2017-15 11840 SE 192nd St Infiltration Rpt Page 3 PanGEO, Inc.
recommendation for foundation design of the future houses. It should also be noted that
our scope of work does not include an evaluation of chemical properties of soil and
groundwater.
SITE GEOLOGY
According to the geology maps of the area (The Geologic Map of King County,
Washington: scale 1:100,000, Booth, D. B., Troost, K. A., and Wisher, A. P., 2007), the
project area is underlain by Vashon glacial till (Map Unit Qvt). Glacial till (Qvt) is a
very dense heterogeneous mixture of silt, sand, and gravel laid down at the base of an
advancing glacial ice sheet.
SUBSURFACE EXPLORATIONS AND CONDITIONS
Four test pits (TP-1 through TP-4) were excavated at the site on September 23 and 24,
2017, to explore the general subsurface conditions at the site and feasibility of infiltration
for stormwarer design. The approximate test pit locations were taped from existing site
features and are indicated on Figure 2. The test pits were excavated to depths of about 5
feet for infiltration testing and to about 7 feet to evaluate the conditions below the test
depth. The tests were excavated using a backhoe owned and operated by G & R
Excavating LLC of Hobart, Washington.
An engineer from Cascade GeotechNW was present during the field exploration to
observe the test pit excavations, obtain representative samples, and to describe and
document the soils encountered in the explorations. Summary test pit logs are included
in Appendix A of this report. The relative in-situ density of cohesionless soils, or the
relative consistency of fine-grained soils was estimated from the excavating action of the
excavator, and the stability of the test pit sidewalls. The stratigraphic contacts indicated
on the test pit logs represent the approximate depth to boundaries between soil units.
Where soil contacts were gradual or undulating, the average depth of the contact was
recorded in the logs. Actual transitions between soil units may be more gradual or occur
at different elevations. After PIT tests are completed and test pits were logged, the
excavation was backfilled with the excavated soils and the surface was tamped and re-
graded smooth.
Infiltration Evaluation
Proposed Development – 11840 SE 192nd Street, Renton, WA
October 2, 2017
2017-15 11840 SE 192nd St Infiltration Rpt Page 4 PanGEO, Inc.
In general, the test pits generally encountered 1 to 2 feet of topsoil consisting of brown to
dark brown, loose, damp, silty sand. Below the topsoil, the test pits generally
encountered light gray, loose to medium dense, moist, slightly silty to silty sand with
trace to some gravel that extended to the bottom of the test pits at about 7 feet below the
surface. No groundwater was observed in the test pits at the time of excavation. Detailed
descriptions of the subsurface conditions encountered at the test pit location are described
in the Test Pit Logs in Appendix A of this report.
According to Soil Survey map, King County Area, Washington (USDA, 1973), the site is
underlain by one soil type; AgB (Alderwood gravelly sandy loam). Our observations of
the soils encountered in the test pits appear to be consistent with this SCS soil type. This
soil type is described as to be moderately well drained for natural drainage class and
relatively low permeability.
IN-SITU INFILTRATION TESTING
We conducted four in-situ infiltration tests at the site (PIT-1 through PIT-4). The
approximate locations of the infiltration tests were specified by the project civil engineer
and are depicted on Figure 2. The infiltration tests were conducted in general accordance
with the procedure for small-scale pilot infiltration test (PIT) as outlined in the
Department of Ecology’s Stormwater Management Manual for Western Washington
(SMMWW) (DOE, 2012 as amended in 2014). The infiltration test procedure consisted
of the following:
• The test pits/small PIT locations were excavated using a 24- and 36-inch buckets
to the approximate design bottom of the proposed infiltration facility. The bottom
pit area was about 3 feet by 4 feet or 3.5 feet by 4 feet or base areas of 12 to 14
square feet.
• The pit was then pre-soaked by maintaining at least 12 inches of water in the
bottom of the small PIT for six hours. The water supply was sourced from a
garden hose bib at the existing residence.
• At the end of pre-soak period, a one hour steady state test was performed, using a
flow meter to measure the volume of water needed to maintain a constant head of
12 inches.
Infiltration Evaluation
Proposed Development – 11840 SE 192nd Street, Renton, WA
October 2, 2017
2017-15 11840 SE 192nd St Infiltration Rpt Page 5 PanGEO, Inc.
• After completing the steady state test, the water supply was turned off and a
falling head test was conducted by recording the drop the water level over regular
time intervals.
The infiltration rate was then calculated based on the results of the steady state testing
and the bottom area of the small PITs.
IN-SITU INFILTRATION TESTING RESULTS
After the pre-soak period, a constant head of 12 inches was maintained at each test
location. Based on the test surface area and the measured volume of water per hour
needed to maintain the constant head, we calculated a field infiltration rate for each test
location. The results are summarized in Table 1 below.
Table 1 – Summary of Field Infiltration Test Data
(No factor of safety included)
Test Location Constant Head Field Infiltration Rate (inches/hour)
PIT-1 0.50
PIT-2 0.45
PIT-3 0.45
PIT-4 0.50
Plate 1. Testing at PIT-1 Plate 1. Testing at PIT-2
Infiltration Evaluation
Proposed Development – 11840 SE 192nd Street, Renton, WA
October 2, 2017
2017-15 11840 SE 192nd St Infiltration Rpt Page 6 PanGEO, Inc.
After completing the infiltration test, the pit was excavated to a depth of 7 feet below the
test elevation to evaluate the presence of impermeable soils or groundwater mounding
that may impact the long-term performance of the infiltration system. We observed no
perched groundwater or seepage throughout the depth of excavation. For design
purposes, the field infiltration rates must be reduced through correction factors, as
discussed in the following section.
DESIGN INFILTRATION RATE AND RECOMMENDATIONS
The field test provides a short-term infiltration rate. To provide a long-term design
infiltration rate, field rate is adjusted by applying a series of correction factors to account
for site variability, the number of locations tested and the degree of influent control to
prevent siltation and bio-buildup.
Table 2 below, sourced from Table 3.3.1 of the DOE manual, outlines the correction
factors to be applied to the field infiltration rate in order to estimate the long-term design
infiltration rate for proposed infiltration facility.
Table 2 - Correction Factors to be used with In-Situ Saturated Hydraulic
Conductivity Measurements to Estimate Design Rates
(source: Table 3.3.1, Vol. 3, DOE Stormwater Manual)
Issue Partial Correction Factor
Site variability and number of locations tested CFv = 0.33 to 1.0
Test Method: Large-Scale PIT
Small-Scale PIT
Grain Size Method
CFt = 0.75
CFt = 0.50
CFt = 0.40
Degree of influent control to prevent siltation and bio-
buildup
CFm = 0.9
Total Correction Factor (CFT) = CFv * CFt * CFm
The partial correction factor for site variability (CFv) is selected based on the number of
locations tested and the consistency of the underlying soil conditions and ranges from
0.33 to 1.0. Based on our experience and engineering judgment, we recommend a
correction factor of 0.9 for site variability. From the Table 2 above, the partial correction
factor for CFm is 0.9 and the partial correction factor for CFt is 0.5 when using the small-
Infiltration Evaluation
Proposed Development – 11840 SE 192nd Street, Renton, WA
October 2, 2017
2017-15 11840 SE 192nd St Infiltration Rpt Page 7 PanGEO, Inc.
scale PIT method. Using the partial correction factors described above, in our opinion a
Total Correction Factor (CFT) of 0.405 is appropriate.
The total correction factor (CFT = CFv x CFt x CFm) is then applied to the field rate to
estimate to obtain a long-term design infiltration rate. The estimated long-term
infiltration rates for infiltration facility are provided in Table 3 below.
Table 3: Estimated Long-Term Infiltration Rates for Infiltration Facility
Test Location, Depth Correction Factor
(CFv x CFt x CFm)*
Long-Term Infiltration Rate (inches/hour)
PIT-1, 5’ 0.405 0.202
PIT-2, 5’ 0.405 0.182
PIT-3, 5’ 0.405 0.182
PIT-4, 5’ 0.405 0.202
*CFv = 0.9, CFt = 0.5, CFm = 0.9
Conclusions – In summary, based on the results of our field exploration, infiltration tests,
and observed soil texture, in our opinion, roof water disposal using the infiltration
method is likely infeasible. However, it is our opinion that limited infiltration may be
used for permeable pavement design and other on-site BMPs, as appropriate.
LIMITATIONS
This report has been prepared for the exclusive use of Mr. Willy Cho and the project
team for specific application to the proposed development. This report is intended to
provide infiltration feasibility and recommendations based on a site reconnaissance, field
infiltration testing, and our understanding of the project. The study was performed using
a mutually agreed-upon scope of work.
Variations in soil conditions may exist between the locations of the explorations and the
actual conditions underlying the site. The nature and extent of soil variations may not be
evident until construction occurs. If any soil conditions are encountered at the site that
are different from those described in this report, we should be notified immediately to
Infiltration Evaluation
Proposed Development – 11840 SE 192nd Street, Renton, WA
October 2, 2017
2017-15 11840 SE 192nd St Infiltration Rpt Page 8 PanGEO, Inc.
review the applicability of our recommendations. Additionally, we should also be
notified to review the applicability of our recommendations if there are any changes in
the project scope.
The scope of our work does not include services related to construction safety
precautions. Our recommendations are not intended to direct the contractors’ methods,
techniques, sequences or procedures, except as specifically described in our report for
consideration in design. Additionally, the scope of our work specifically excludes the
assessment of environmental characteristics, particularly those involving hazardous
substances. We are not mold consultants nor are our recommendations to be interpreted
as being preventative of mold development. A mold specialist should be consulted for all
mold-related issues.
This report has been prepared for planning and design purposes for specific application to
the proposed project in accordance with the generally accepted standards of local practice
at the time this report was written. No warranty, express or implied, is made.
This report may be used only by the client and for the purposes stated, within a
reasonable time from its issuance. Land use, site conditions (both off and on-site), or
other factors including advances in our understanding of applied science, may change
over time and could materially affect our findings. Therefore, this report should not be
relied upon after 24 months from its issuance. Cascade Group should be notified if the
project is delayed by more than 24 months from the date of this report so that we may
review the applicability of our conclusions considering the time lapse.
It is the client’s responsibility to see that all parties to this project, including the designer,
contractor, subcontractors, etc., are made aware of this report in its entirety. The use of
information contained in this report for bidding purposes should be done at the
contractor’s option and risk. Any party other than the client who wishes to use this report
shall notify Cascade GeotechNW of such intended use and for permission to copy this
report. Based on the intended use of the report, Cascade GeotechNW may require that
additional work be performed and that an updated report be reissued. Noncompliance
with any of these requirements will release Cascade Group from any liability resulting
from the use this report.
Infiltration Evaluation
Proposed Development – 11840 SE 192nd Street, Renton, WA
October 2, 2017
2017-15 11840 SE 192nd St Infiltration Rpt Page 9 PanGEO, Inc.
CLOSURE
We appreciate the opportunity to be of service and trust that the information outlined in
this letter meets your need at this time. Please call if you have any questions.
Sincerely,
10/2/2017
H. Michael Xue, P.E.
Principal Geotechnical Engineer
Enclosures:
Figure 1 Vicinity Map
Figure 2 Site and Exploration Plan
Appendix A Summary Test Pit Logs
A-1 thr. A-4 Logs of Test Pits TP-1 through TP-4
Infiltration Evaluation
Proposed Development – 11840 SE 192nd Street, Renton, WA
October 2, 2017
2017-15 11840 SE 192nd St Infiltration Rpt Page 10 PanGEO, Inc.
REFERENCES
Booth, D. B., Troost, K. A., and Wisher, A. P., 2007, The Geologic Map of King County,
Washington: scale 1:100,000.
City of Renton, 2017, Surface Water Design Manual.
King County Department of Natural Resources and Parks, 2016, Surface Water Design
Manual.
WSDOE, 2014, Stormwater Management Manual for Western Washington.
Figure No.Project No.2017-15
Infiltration Evaluation
11840 SE 192nd Street
Renton, Washington
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APPENDIX A
SUMMARY TEST PIT LOGS
2017-15 11840 SE 192nd St Infiltration Rpt Page A-1 Cascade GeotechNW
Test Pit TP-1/PIT-1
Approximate ground surface elevation: N/A
Ground Surface Conditions: Thick Blackberry Bushes
Depth (ft) Material Description
0 – 1.5 Loose, brown-dark brown, silty SAND (SM), some roots, damp
(Topsoil)
1.5 – 7 Loose to medium dense, light gray, silty SAND (SM), trace to some
gravel, moist
Test Pit terminated approximately 7 feet below ground surface.
No caving and groundwater/seepage observed in the test pit.
2017-15 11840 SE 192nd St Infiltration Rpt Page A-2 Cascade GeotechNW
Test Pit TP-2/PIT-2
Approximate ground surface elevation: N/A
Ground Surface Conditions: Thick Blackberry Bushes
Depth (ft) Material Description
0 – 2 Loose, brown to dark brown, silty SAND (SM), trace roots, damp to
moist (Topsoil)
2 – 7 Loose to medium dense, light gray-brown, silty SAND (SM), trace
gravel, moist
Test Pit terminated approximately 7 feet below ground surface.
No caving and groundwater/seepage observed in the test pit.
2017-15 11840 SE 192nd St Infiltration Rpt Page A-3 Cascade GeotechNW
Test Pit TP-3/PIT-3
Approximate ground surface elevation: N/A
Ground Surface Conditions: Short grass
Depth (ft) Material Description
0 – 1.5 Loose, brown-dark brown, silty SAND (SM), trace roots and gravel,
damp to moist (Topsoil)
1.5 – 7 Loose to medium dense, light gray-brown, silty SAND (SM), trace
gravel, moist
Test Pit terminated approximately 7 feet below ground surface.
No caving and groundwater/seepage observed in the test pit.
2017-15 11840 SE 192nd St Infiltration Rpt Page A-4 Cascade GeotechNW
Test Pit TP-4/PIT-4
Approximate ground surface elevation: N/A
Ground Surface Conditions: Gravel driveway
Depth (ft) Material Description
0 – 2 Loose, brown-dark brown, silty SAND (SM), trace roots and gravel,
damp to moist (Topsoil)
2 – 7 Loose to medium dense, light gray-brown, silty SAND (SM), trace to
some gravel, moist
Test Pit terminated approximately 7 feet below ground surface.
No caving and groundwater/seepage observed in the test pit.
Date Test Pits Excavated: September 23, 2017
Test Pits Logged by: HX
N ( blows/ft ) Approximate Consistency N (blows/ft ) Approximate
Relative Density (%) Undrained Shear
Strength (psf)
0 to 4 0 - 15 Very Soft 0 to 2 < 250
5 to 10 16 - 35 Soft 3 to 4 250 - 500
11 to 30 36 - 65 Medium Stiff 5 to 8 501 - 1000
31 to 50 66 - 85 Stiff 9 to 15 1001 - 2000
over 50 86 - 100 Very Stiff 16 to 30 2001 - 4000
Hard over 30 > 4000
CASCADE GEOTECHNW Project No. 2017-15 Figure A-5
KEY:
Indicates 3-inch OD Dames & Moore Sample.
Indicates 2-inch OD Split Spoon Sample (SPT).
Indicates Disturbed Sample.
Indicates No Recovery.
Indicates Bag Sample.
Indicates Shelby Tube Sample.
COMPONENT DEFINITIONS
COMPONENT SIZE RANGE
Boulders
Cobbles
Gravel
Coarse gravel
Fine gravel
Sand
Coarse sand
Medium sand
Fine sand
Silt and Clay
Larger than 12 in
3 in to 12 in
3 in to No 4 (4.5mm )
3 in to 3/4 in
3/4 in to No 4 ( 4.5mm )
No. 4 ( 4.5mm ) to No. 200 ( 0.074mm )
No. 4 ( 4.5 mm ) to No. 10 ( 2.0 mm )
No. 10 ( 2.0 mm ) to No. 40 ( 0.42 mm )
No. 40 ( 0.42 mm ) to No. 200 ( 0.074 mm )
Smaller than No. 200 ( 0.074 mm )
DESCRIPTIVE TERMS RANGE OF PROPORTION
Trace or little
Some
Clayey, silty, sandy,
gravelly
And
1 - 5%
6 - 12%
13 - 30%
31 - 50%
COMPONENT PROPORTIONS
MOISTURE CONTENT
DRY
DAMP
MOIST
WET
Absence of moisture, dusty,
dry to the touch.
Some perceptible
moisture; below optimum
No visible water; near optimum
moisture content
Visible free water, usually
soil is below water table.
RELATIVE DENSITY OR CONSISTENCY VERSUS SPT N -VALUE
COHESIONLESS SOILS COHESIVE SOILS
Density
Very Loose
Loose
Medium Dense
Dense
Very Dense
ATD : At Time of Drilling
BGS: Below Ground Surface
KEY:
Indicates 3-inch OD Dames & Moore Sample.
Indicates 2-inch OD Split Spoon Sample (SPT).
Indicates No Recovery.
Indicates Bag Sample.
COMPONENT DEFINITIONS
COMPONENT SIZE RANGE
Boulders
Cobbles
Gravel
Coarse gravel
Fine gravel
Sand
Coarse sand
Medium sand
Fine sand
Silt and Clay
Larger than 12 in
3 in to 12 in
3 in to No 4 (4.5mm )
3 in to 3/4 in
3/4 in to No 4 ( 4.5mm )
No. 4 ( 4.5mm ) to No. 200 ( 0.074mm )
No. 4 ( 4.5 mm ) to No. 10 ( 2.0 mm )
No. 10 ( 2.0 mm ) to No. 40 ( 0.42 mm )
No. 40 ( 0.42 mm ) to No. 200 ( 0.074 mm )
Smaller than No. 200 ( 0.074 mm )
DESCRIPTIVE TERMS RANGE OF PROPORTION
Trace
Few
Little
Some
And
1 - 5%
6 - 10%
11 - 20%
21 - 35%
36 - 50%
COMPONENT PROPORTIONS
MOISTURE CONTENT
DRY
DAMP
MOIST
WET
Absence of moisture, dusty,
dry to the touch.
Some perceptible
moisture; below optimum
No visible water; near optimum
moisture content
Visible free water, usually
soil is below water table.
RELATIVE DENSITY OR CONSISTENCY VERSUS SPT N -VALUE
COHESIONLESS SOILS COHESIVE SOILS
Density
Very Loose
Loose
Medium Dense
Dense
Very Dense
N ( blows/ft ) Approximate Consistency N (blows/ft ) Approximate
Relative Density (%) Undrained Shear
Strength (psf)
0 to 4 0 - 15 Very Soft 0 to 2 < 250
5 to 10 16 - 35 Soft 3 to 4 250 - 500
11 to 30 36 - 65 Medium Stiff 5 to 8 501 - 1000
31 to 50 66 - 85 Stiff 9 to 15 1001 - 2000
over 50 86 - 100 Very Stiff 16 to 30 2001 - 4000
Hard over 30 > 4000
Indicates Shelby Tube Sample.
CASCADE GEOTECHNW Project No. 2017-15 Figure A-6
ATD : At Time of Drilling
BGS: Below Ground Surface