REVIEWED RESUB2 BLD2022-1025+Geotechnical_Report+10.24.2022_11.05.16_AM+3180980PIT INFILTRATION TEST AND STANDARD SUBSURFACE INVESTIGATION
REPORT
October 11, 2022
INTRODUCTION
The subject property is located at 23511 70 Avenue W, Edmonds WA. The site is 25,976 sf in size
and is adjacent to Lake Ballinger. Once developed, the site will add a new home with a total of 3,100 sf of
new impervious area in the form of a roof for the home. Approximately 220 sf of existing gravel
driveway will be paved in front to the new garage. The existing rockeries and driveway will remain and
contains an existing home, garage, shed and driveway with a cover of lawn and landscaping. This report
is being prepared using Section 5 of the 2019 DOE Stormwater Management Manual as
adopted by the City of Edmonds
SUBSURFACE CONDITIONS
The soils were visually classified in general accordance with the Unified Soil Classification System,
presented in Figure 1. The logs of the test pits are attached to this report and are presented as Figure
3. The soils at the surface of Test Pits to down approximately 6-inches consisted of roots and top
soils. Underlying the surficial weathered soils from depth of 6-inches to just over 2.5 —feet,
the soils consisted of a clay silt/fill materials
HYDROGEOLOGIC CONDITIONS
The soils were moist with indications of mottling when the test pit was excavated down to 3 feet
after the PIT infiltration test was performed.
INFILTRATION TESTING:
A small-scale Pilot Infiltration Test (PIT) was performed per test procedures provided in "Reference
6-A" of the 2019 DOE Stormwater Management Manual as adopted by the City of Edmonds
(SL-1) to evaluate the infiltration of the soil. The infiltration test was conducted at a depth of 2.5 -
feet below the ground surface, which is close to the anticipated bottom of the infiltration facility (2.5-
feet below ground surface) if infiltration trenches are to be used.
Setup: The test pit dug was rectangular in shape, and measured roughly 4.0'x 4.5'x 2.6' (width x
length x depth). A vertical measuring rod was installed in the pit for the purpose of measuring the
depth of ponded water
Based on the geometry of the test pit dug for SL-1, the total plan area of the bottom of the test pit is
calculated as follows:
Test Pit Bottom Area =
(4.01)(4.51) Test Pit
Bottom Area = 18.0 ft2
Soaking Period: In preparation for the soaking period, the test pit was filled with water to a depth of
over 12-inches. A plastic tray was utilized as a splash block to help prevent water flowing into the
test pit from disturbing soils at the bottom of the pit, and the hose administering water to the pit was
held in place over the splash block using a clamp. The water level in the pit was maintained at a
depth of a minimum of 12-inches for over 6-hours, as required by the test procedure.
Measurement of Infiltration Rate: Upon completion of the soaking period, the water was allowed
to drain until achieving a water depth of between 8 and 14 inches, and the flow rate of the water into
Duffy Geotech Rpt Page 1
the test pit was adjusted to a rate that was expected to maintain the water level at the same point
between 8 to 14inches in depth over the course of an hour. The instantaneous flow -rate and water
depth were then measured and recorded every 15-minutes (approximately) over the course an hour. A
summary of this recorded data is provided in the table below
Table 1: Measured Infiltration Data During Flow Period
15-min
Interval
Time of
Measurement
Instantaneous Flow Rate
Water Depth
-
hour:minute
gal/min
inches
1
1:00
0.04
11 7/16
2
1:31
0.03
11 6/16
Stabilized Flow Period:
3
1:42
0.00
10 5/16
4
1:59
0.00
10 5/16
5
2:14
0.00
10 5/16
6
2:31
0.00
10 5/16
Average Stabilized Flow Rate = 0.00
The flow -rate was adjusted after the first two 15-minute intervals, so that a stabilized flow rate was
achieved that would maintain a depth of water at the same point over a one -hour time period.
Accordingly, readings shown in table 1 above represent data collected during this 1-hour period of
stabilized flow.
Measured Stabilized Flow Rate Calculation: From the data presented in table 1, the measured
stabilized flow infiltration rate was computed by dividing the average flow -rate by the horizontal bottom
area of the test pit and converting to inches per hour
Drawdown Time: Upon completion of the 1-hour stabilized flow period, the water flowing into the test
pit was shut off to allow the water -level in the test pit to drop. During this "drawdown" time,
measurements of the water -level were taken periodically for one -hour. The data measurements are
provided in table 2 below:
Table 2: Measured Infiltration Data for Drawdown Time
Time
A Time
Depth
A Depth
Infiltration
Rate
hour:minute
minutes
inches
inches
inches/hour
1:00
-
16
-
-
1:15
15
16
0.00
0.00
1:30
15
16
0.00
0.00
1:45
15
16
0.00
0.00
2:00
15
16
0.00
0.00
TEST
TERMINATED
Average Infiltration Rate = 0.00
Duffy Geotech Rpt Page 2
Measured Infiltration Rate: Based the measured infiltration data above it was determined that the
average infiltration rate is less than 0.30 inches per hour () Therefore, infiltration is not feasible.
I ....urea = 0.00 inches/hour
Ftesting = 0.50 For PIT test
F Geometry = 4 D/W + 0.05 = 4 (2/4) + 0.05 = 2.05
= 1.0 (Must be between 0.25 & 1)
F plugging = 0.7
I Design = TEST TERMINATED
Therefore, the use of Infiltration Or The Use Of Biorientation Is Not Feasible .
10/11/22
Duffy Geotech Rpt Page 3
SOILS EXPLORATION LOG
DEPTH (INCHES) USC SOILS DESCRIPTION
SL-1 TEST PIT
0-611 GRASS ROOTS
6" - 12" SC BLACK SILTY CLAY
1211- 3611 SC SILTY SANDY CLAY
EVIDENCE MOTTLING AT 24"
TP-1 INFILTRATION PIT
0-611 GRASS ROOTS
6"-24" SC FILL/SILTY CLAY
2411- 6011 SC SILTY SANDY CLAY
EVIDENCE MOTTLING AT 48"
�I
PETER CHO
t TM 27043200202900
N88° t6'26"E 206.00 — —
�j j a wood
11�-� �' Fwr:ox ans-IHF act\ to' I �� �//� �1 � Jl•SC"�: axe ;1.. ,ql / "`«-.�
aI L cr.• GAS -I" 10' BS
,°" METER - �' + *k f �'r,� )max• 'r I �
m k: 10YYL' :1
HOUSF1t P3511 .y.yb` .I
FOOTPRINT .'.e; .. O •,`-`, '
REST OF HOUSE DEMOS _ •j 1
- AT THE OF SUR1rEy AWROX, SIDE SEWER
60.0000
� t � \\\ I 1 I \\ WETLAND BUFFER -�
• iI. if 1 O
44.0' - g I/ 1 9.0000 I
7.
ti•
W ~> a I
I 27043200200100 +
;13 1 IIiS' jj 1 tee'- -- - x t \ • / I ±25,434 SO. FT. J .
l� BASE FL
4 NAVD88SEW'maoE j
�\ ASPHALT
0.8seL
AUREL'HEDGE
SOILS LOGS I f
11 j N88°1626"W 200.00';
n SL-1 NORTI
l$ -6" GRASS ROOTS IV
1 6-12" BLACK SILTY CLAY
12"- 36" SILTY SANDY CLAY
EVIDENCE MOTTLING AT 24"
P-1 INFILTRATION PIT
0-6" GRASS ROOTS
6-24" FILL/SILTY CLAY
24"- 60" SILTY SANDY CLAY
EVIDENCE MOTTLING AT 48"
FIGURE 2: SOILS LOG LOCATION
Duffy Geotech Rpt Page 4
UNIFIED SOIL CLASSIFICATION SYSTEM
GROUP
MAJOR DIVISIONS
GROUP NAME
SYMBOL
CLEAN
GW
WELL -GRADED, FINE TO COARSE GRAVEL
COARSE-
GRAVEL
GRAVEL
GP
POORLY -GRADED GRAVEL
GRAINED
MORE THAN 50 %
GRAVEL
GM
SILTY GRAVEL
OF COARSE FRACTION
RETAINED ON
SOILS
NO 4 SIEVE
WITH FINES
GC
CLAYEY GRAVEL
SAND
CLEAN
SW
WELL -GRADED SAND, FINE TO COARSE SAND
SAND
SP
POORLY GRADED SAND
MORE THAN 50
RETAINED ON
MORE THAN 50 %,
NO. 200 SIEVE
OF COARSE FRACTION
SAND
SM
SILTY SAND
PASSES NO 4 SIEVE
SC
WITH FINES
CLAYEY SAND
FINE -
SILT AND CLAY
ML
SILT
INORGANIC
GRAINED
LIQUID LIMIT
CL
CLAY
LESS THAN 50
SOILS
ORGANIC
OL
ORGANIC SILT, ORGANIC CLAY
SILT AND CLAY
MH
SILT OF HIGH PLASTICITY, ELASTIC SILT
INORGANIC
MORE THAN 50 %
PASSES
LIQUID LIMIT
CH
CLAY OF HIGH PLASTICITY, FAT CLAY
NO, 200 SIEVE
50 % OR MORE
ORGANIC
OH
ORGANIC CLAY, ORGANIC SILT
HIGHLY ORGANIC SOILS
PT
PEAT
NOTES
1) Field classification is based on visual SOIL MOISTURE MODIFIERS:
examination of soil in general
accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to
the touch
2) Soil classification using laboratory tests
is based on ASTM D 2488-93. Moist - Damp, but no visible water_
3) Descriptions of soil density or Wet - Visible free water or saturated,
consistency are based on usually soil is obtained from
interpretation of blowcount data, below water table
visual appearance of soils, and/or
test data.
Project Number
No.
Date
Revision
By
CI(
DECC10 Engineering
Figure 1(2M)
17217 7" AVENUE WEST
-374
20
(3495)W]4819-24
FIGURE l : UNIFIED SOIL CLASSIFICATION
Duffy Geotech Rpt Page 5