HomeMy WebLinkAboutRS_Geotechnical_Addendum_170817_v1
13705 Bel-Red Road · Bellevue, Washington 98005
Phone 425/649-8757 · Fax 425/649-8758
Geotechnical Engineers, Geologists,
& Environmental Scientists GEO Group Northwest, Inc.
August 8th, 2017 G-4362
Mr. Bill Moorman
bill@billsmpteam.com
Subject: Geotechnical Addendum with Infiltration Testing
2004 Shattuck Ave S
Renton, WA
Dear Mr. Moorman,
We are pleased to provide geotechnical consulting services for the proposed short plat residential
development in Renton. A visit to the project site was made on August 1st and 2nd, 2017 to
perform infiltration testing and provide soil conditions evaluation at infiltration testing locations.
This report summarizes the findings and test results.
Subsurface Investigation:
While on-site, three 2-foot deep and approximately 6” in diameter holes were dug using a postal
digger on the property at 2004 Shattuck Ave S in Renton. These holes are referred to as test
holes (TH) no. 1 thru 3 in this report. The approximate locations for each of the test holes are
indicated on the aerial map of the property shown in fig. 1 below.
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Fig. 1: Aerial map depicting approximate locations of test hole no.’s 1 thru 3
Soil Logs: The following soil logs, labeled test pit (TH) no.’s 1 through 3, summarize the soil
conditions observed for each of the three excavations shown in fig. 1. Both the Unified Soil
Classification System (USCS) and United States Department of Agriculture’s (USDA) Textural
Triangle classification systems were utilized to classify the soil.
TH-1:
Depth Classification Soil Description Comments
USCS USDA
0’-0” to 2’-0” MH
Silt Brown silt with
occasional gravel
Dense, slightly damp
1’-8” to 2’-0” MH Silt Brown silt with
occasional gravel
Medium dense, damp
TH-2:
Depth Classification Soil Description Comments
USCS USDA
0’-0” to 2’-0” MH Silt Brown silt with
occasional gravel
Dense, slightly damp
TH-3:
Depth Classification Soil Description Comments
USCS USDA
0’-0” to 1’-8” MH Silt Brown silt with
gravel
Dense, slightly damp
1’-8” to 2’-0” GM Silt Brown silty gravel Dense, slightly damp
No evidence of water seepage was observed at any of the test pit locations logged above.
Additional soils logs for this site from subsurface investigation performed 5/4/17 are documented
in GEO Group Northwest’s geotechnical engineering report dated 5/23/17.
Infiltration Testing:
The Falling Head Percolation Test Procedure, based on the test method described in
Environmental Protection Agency’s (EPA) Design Manual for On-site Wastewater Treatment
and Disposal Systems (1980), was performed at each of the three test pit locations to evaluate the
infiltration of the soil.
Setup: As mentioned previously, approximately three test holes were prepared measuring a little
over 2 feet deep and approximately 6-inches in diameter. Next, the bottom 1-2” of each hole was
filled with clean gravel, and a 6” diameter PVC pipe was placed into the holes in the vertical
position to prevent sloughing off of soil into this hole.
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Soaking Period: Each test hole was filled with water with the water level maintained above 12”
while I was on-site, and then allowed to soak overnight.
Measurement of Percolation Rate: The water in each hole was adjusted to 6” of water, and
allowed to drain for 30 minutes. After 30 minutes, the water level was measured and recorded,
and this step was repeated two more times for each test pit hole.
For test hole no. 1, the water completely seeped away in less than 30 minutes, but not less than
10 minutes. For this reason, the measurements for the water level were taken after 10-minute
time intervals for infiltration testing, rather than 30 minutes called for by the test procedure.
Tables 1 thru 3 shown below summarize the recorded data for drainage times and calculated
percolation rates.
Table 1: Infiltration Data and Calculated Percolation Rates for TH-1
Time Starting Depth Measured Depth Falling Head Percolation Rate
(min) (inches) (inches) (inches) (min/inch)
10 6 3.25 2.75 3.64
10 6 3.25 2.75 3.64
10 6 3.25 2.75 3.64
Average Percolation Rate = 3.64
Table 2: Infiltration Data and Calculated Percolation Rates for TH-2
Time Starting Depth Measured Depth Falling Head Percolation Rate
(min) (inches) (inches) (inches) (min/inch)
10 6 4.5 1.5 6.67
10 6 4.625 1.375 7.27
10 6 4.75 1.25 8.00
Average Percolation Rate = 7.31
Table 3: Infiltration Data and Calculated Percolation Rates for TH-3
Time Starting Depth Measured Depth Falling Head Percolation Rate
(min) (inches) (inches) (inches) (min/inch)
10 6 3.125 2.875 3.48
10 6 3.3125 2.6875 3.72
10 6 3.375 2.625 3.81
Average Percolation Rate = 3.67
Table 4 shown below summarizes the measured infiltration rate data converted to units of inches
per hour for each of the test holes:
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GEO Group Northwest, Inc.
Table 4: Summary of Measured Percolation Rates for TH-1 thru TH-3 in Inches Per Hour
TH-1 TH-2 TH-3
(inches/hour) (inches/hour) (inch/hour)
16.50 2.73 5.45
Design Infiltration Rate:
The “Simplified Method” described in the King County Surface Water Design Manual, 2009 was
followed in adjusting the measured percolation rate to determine the design infiltration rate of the
soil. A sample calculation showing how the measured infiltration rate for TH-1 is converted to
the design infiltration rate is delineated as follows:
1. Facility Geometry Factor, Fgeometry
𝐹𝑔𝑑𝑙𝑙𝑑𝑟𝑟𝑦=4 (𝐷
𝑊)+.05
For the equation above, the width of the facility, W was assumed to be 2’, which is a
typical width of an infiltration trench. The depth to the water table or impervious layer, D
was taken to be 16” (1.3’), since this was the lowest depth that groundwater seepage was
encountered during subsurface investigation from May 4th, 2017 (see soil logs from
geotechnical report dated May 23rd, 2017).
𝐹𝑔𝑑𝑙𝑙𝑑𝑟𝑟𝑦=4 (1.3′
2′)+.05
𝐹𝑔𝑑𝑙𝑙𝑑𝑟𝑟𝑦=2.65
Since the value for Fgeometry must fall between .25 and 1.0, the value of 2.65 calculated
above becomes 1.0.
𝐹𝑔𝑑𝑙𝑙𝑑𝑟𝑟𝑦=1.0
2. Plugging Factor, Fplugging
𝐹𝑙𝑙𝑟𝑔𝑔𝑖𝑙𝑔=.8 (for fine sands and loamy sands)
3. Testing Factor, Ftesting
𝐹𝑟𝑑𝑟𝑟𝑖𝑙𝑔=.30 (for EPA testing method)
4. Measured Infiltration Rate, Fmeasured
𝐹𝑙𝑑𝑎𝑟𝑟𝑟𝑑𝑑=16.5 �ℎ𝑛𝑐�𝑒𝑟/�𝑛𝑢𝑟 (calculated previously for TH-1)
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5. Design Infiltration Rate, Idesign
𝐼𝑑𝑑𝑟𝑖𝑔𝑙=𝐼𝑙𝑑𝑎𝑟𝑟𝑟𝑑𝑑×𝐹𝑟𝑑𝑟𝑟𝑖𝑙𝑔×𝐹𝑔𝑑𝑙𝑙𝑑𝑟𝑟𝑦×𝐹𝑙𝑙𝑟𝑔𝑔𝑖𝑙𝑔
𝐼𝑑𝑑𝑟𝑖𝑔𝑙=(16.5 �ℎ𝑛𝑐�/�𝑟)×(.30)×(1.0)×(.8)
𝐼𝑑𝑑𝑟𝑖𝑔𝑙=3.96 �ℎ𝑛𝑐�/�𝑟
The maximum design infiltration rates calculated for each test hole are summarized in the table
below:
Table 5: Summary of Max. Design Percolation Rates for TH-1 thru TH-3 in Inches Per Hour
TH-1 TH-2 TH-3
(inches/hour) (inches/hour) (inch/hour)
3.96 0.66 1.31
Discussion and Conclusion:
The measured infiltration rates presented in this report exceed the minimum infiltration rate of
0.3 inches per hour recommended by the City of Renton. Because of the variation in infiltration
rates for each test hole, GEO Group Northwest recommends using the lowest and most
conservative design infiltration rate of 0.66 inches per hour for design purposes.
Please feel welcome to contact GEO Group Northwest for questions.
Sincerely,
GEO GROUP NORTHWEST, INC.
David Crawford, E.I.T.
Staff Engineer
William Chang, P.E.
Principal