Soil_Infiltration.pdfA
Edmonds, WA 98026
Prepared for Gus and Morgan Kays
Project Number 2016024
FEB 25 295
DEVELOPMENT SERVICES
COUNTER
February 10, 2016
Element Solutions
1812 Cornwall Avenue
Bellingham WA, 98225
360.671.9172
info@elementsolutions.org
Fable of Contents
Tableof Contents...............................................................,..............................................1
1 Introduction................................................................................................................,....1
1.1 Purpose.........................................................................................................................1
1.2 Definitions.....................................................................................................................1
2 Desktop Analysis............................................................................................................ 2
2.1 Location and Physiography............................................................................................. 2
2.2 NRCS Soil Classification.................................................................................................. 3
2.3 Geologic Mapping.......................................................................................................... 3
3 Field Investigation........................................................................................................... 3
3.1 Subsurface Explorations...................................................................................................... 3
3.2 Infiltration Measurement Methods.................................................................................... 4
3.3 Small Scale Infiltration Pit Test Results........................................................................... 6
3.4 Summary....................................................................................................................... 7
3.S Conclusions and Recommendations............................................................................... 7
4 Closure.......................................................................................................................... 7
Appendix I - Pit Locations
Appendix II - Small Scale Infiltration Test Pits Measurement Data
Appendix III — January 27, 2016 Field Photos
Appendix IV - Soil Lab Data
Introduction
1.1 Purpose
The purpose of this analysis was to investigate the soil conditions and measure infiltration rates
within the study area, located at the project site at 8705 Maplewood Lane, Edmonds, WA 98026
In Snohomish County, Washington (Figure 1 below), to inform stormwater infiltration suitability
and design related to a proposed development project. The proposed project will involve
development of up to two single family residences.
Figure 1: Site vicinity (modified from Google Earth, 2015)
The following definitions from the USDA Natural Resources Conservation Service (NRCS) are used
in the discussion of infiltration:
Infiltration capacity - The maximum rate at which water can infiltrate into a soil under a given set
of conditions.
Infiltration rate - The rate at which water penetrates the surface of the soil at any given instant,
usually expressed in inches per hour. The rate can be limited by the infiltration capacity of the soil
or the rate at which water is applied at the surface.
i obi.,nrww 10, 20:+5
Popp, I
ELEMENT
1812 Cornwall Avenue Belling1—o AIA 95225
(36)J 671 4172 Imfoi�elercenr;duhons nrg
Intake rate - The average rate of water entering the soil under irrigation. Most soils have a fast
initial rate; the rate decreases with application time. Therefore, intake rate for design purposes is
not a constant but is a variable depending on the net application. The rate of water intake in
inches per hour is expressed as follows:
Less than 0.2 ...............
0.2 to 0.4 .....................
0.4 to 0.75 ..................
0.75 to 1.25 .................
1,25 to 1.75 ................
1.75 to 2.5 ...................
More than 2.5 ............
...................... very low
....................... low
.............. I....... moderately low
...................... moderate
.......................moderately high
...I...................high
........................very high
2.3. Location and Physiography
The study area is located in at 8705 Maplewood Lane, roughly 1 mile northeast of downtown
Edmonds, Washington. Elevations across the site are approximately 400 feet (NAVD 88). In
general the regional topography slopes from the northwest towards the west (Figure 2). The site
has gentle topography that is forested with a mature tree canopy (predominantly Western Red
Cedar and Douglas Fir) with native top soils. The infiltration test pit had intact, native soils.
Figure 2: Topography in the vicinity of the site (USGS — USA Topo)
ELEMENT
2.2 NRCS Soil Classification
The Natural Resource Conservation Service (NRCS) identifies the local soil type in the study area as
Alderwood-Urban land complex, 2 to 8 percent slopes. The soil is interpreted to be derived from
glacial till/drift. It is typically moderately well drained and has a restrictive layer 20 to 40 inches
and water table at 18 to 36 inches. The soil is characterized by the USDA as a Hydrologic Soil
Group (HSG) B soil; these soils have moderately low runoff potential when thoroughly wet. Water
transmission though HSG B soils is limited by the limiting layer (Ksat), which typically has between
0 and 0.06 inches per hour. Mean annual precipitation is interpreted as between 25 to 60 inches.
2.3 Geologic Mapping
Washington State Department of Natural Resources 1:100,000 scale geologic mapping indicates
that the subject area is underlain by Pleistocene Vashon Stade continental glacial till. The most
recent glaciation in the Puget Lowland, known as the Fraser Glaciation, occurred during the
Pleistocene epoch and includes several alternating periods of glacial advance and retreat, referred
to respectively as stades and interstades. The Vashon Stade till deposits are described as compact
(dense) diamicton (silt or sand rich with gravel with local boulders).
3.1 Subsurface Explorations
The subsurface soil investigation and infiltration pit test was performed on January 27, 2016.
Locations of the Test Pits are shown in Appendix I. Test Pit No. 1 (TPI) was excavated to a depth
of 5 feet and Test Pit No. 2 (TP2) was excavated to a depth of 2 feet below ground surface (bgs).
The base dimension of the infiltration pit was 4.0 feet by 4.0 feet (16 square feet). Test Pit No. 1
was excavated to ground water and the underlying restrictive layer.
Observed Conditions
Seepage was visible at approximately 5.0 feet in TP 1. Dark brown to brown, loose, moist silty sand
with gravel subsoil was encountered from the surface to 5.0 feet. Browhish grey dense soil was
encountered at 5.0 feet and was interpreted as the restrictive layer. Groundwater seepage was
observed at the contact at 5.0 feet. Similar soils were encountered in TP 2. TP 2 was used as the
infiltration test and is located in the approximate area of the proposed stormwater facility. Table
1 below summarizes the results of the soils test pit analysis. Appendix III presents photographs of
the soils. A sieve analysis performed and lab analysis is presented in Appendix IV.
Table 1: Summary of Soil Pit Analysis
Soil
Total
Thickness
Pit
Pit
of topsoil fi
USDA Soil Type
Depth
ID
(bgs) 1
(TD)
Loamy sand to
TP1
5 feet
0 - 5'
loamy sand
TPI
>5 feet
NA
TP2
3 feet
>3'
Loamy sand to
loamv sand
Maplewood Lane Soil and Infiltration Assessment
February 10, 2016
Page 3
Interpreted Strata Type
Weathered till and drift
Glacial till
Weathered till and drift
„, u,uuuuu
LT
302 Infiltration Measurement Methods and Characteristics
There are several methods for estimating infiltration rates from soil characteristics, including soil
texture. The NRCS soil map described the soil at the subject area as moderately well drained and
conditions supporting moderate to high infiltration rates could be anticipated based upon field
observations. Anecdotal field information supporting this infiltration characterization include no
evidence of ponding, surface water runoff, or saturated soil conditions at a time during the year
when soil saturation and groundwater conditions approach their maximum. At depth, less
permeable geologic conditions occur, and we anticipate that groundwater may locally migrate
laterally along the highly permeable layers when it encounters less permeable strata below.
To characterize the soils and estimate potential infiltration at the site, the USDA soil texture
classification system that is based on grain size (chart below) is often used. Using this method and
the recommended infiltration values developed by Washington State Department of Ecology
(Ecology), the site would be able to infiltration approximately 0.25 to 0.75 -inches of rain per hour.
It is generally accepted however that the Ecology method of estimating infiltration rates is very
conservative (Figure 4).
Figure 3: USDA Soil Texture Classification
Texture of soils
encountered at
subject site
i0O"
� ro
(dl.w
elk
day
m %Y" N'a'
a GK 1w� �n�� uIs
Clay ks;
%alstG —
21M
a�
In
✓G/J 7J O � J Y/J �G �J �G
.. Sund Separate, %
COMPARISON OF PARTICLE SIZE NCALES
IN
Maplewood Lane Soil and Infiltration Assessment „r.°iz014050,
February 10, 2015 , LM Ml'ONT'
Page 4
Referenced infiltration and conductivity rates for soils characterized by USDA soil texture are
available in the WADOE manual and in other literature. The Ecology infiltration rates apply a
correction factor to a set of field measured infiltration rates to obtain a long-term infiltration rate.
The correction factor ranged from 4 to 9 times measured rates. A study conducted for the
Washington State Transportation Commission and US Department of Transportation (Massman,
et. al., 2001) shows a comparison of other methods and field data and finds that the Ecology
estimates are conservative. Figure 6 (following page) shows the comparisons of one study
(Meyers, et. al, 1997) and the Ecology rates. The results from Meyers showed that observed
infiltration rates were at least 4 times the Ecology rates, and they recommended using a factor of
4 as a conservative correction factor to account for uncertainty in saturated hydraulic conductivity
resulting from "plugging" caused by micro -stratification, siltation or bio -buildup; this correction is
consistent with the King County manual.
Figure 4: Meyers et. al. 1997 versus representative rates from Ecology
N
1
d
0.1 .
Field tested mean Intl
(showing 5 and 95 percentiles)
. t
Sand I oalyny Sandy
Sand goat
The methods for this site that were used to quantify actual site saturated hydraulic conductivity
(Ksat) rates followed the Ecology Stormwater Manual - Volume III (2012) Small -Scale Pilot
Infiltration Test methods. The assessment methods included:
• Excavation of two test pits (TP1 and TP2); one to the estimated surface elevation of the
proposed infiltration facility, the other to investigate the depth of the soils and determine
site uniformity.
o The horizontal surface area of the bottom of the test pits was 16 feet
• Installation of vertical measuring rods adequate to measure the ponded water depth,
marked in half-inch increments and placed in the center of the pit bottom.
• Use of a rigid pipe with a splash plate on the bottom to broadcast water application to the
pits to reduce side-wall erosion or excessive disturbance of the pond bottom.
y'-r"be"Mdw"yO .10, 2'01.6 HLEMENT
y;,,,���,,„J i3] iJ�r1l'tlr��fll%P/IUP L�FI�IIi(]1)(11R V/A 93225
Y 3GU) v?J-
Q1,12 I mJn(��alan:enl:n;�r,;o�a orq
• A pre-soak period of 6 hours, taking measurements of application volume, rate, and water
depth every 30 minutes
• After the pre-soak period, the water was turned off and the rate of infiltration (the drop rate
of the standing water) in inches per hour was recorded until the pit was empty.
• Calculation and recording of the saturated hydraulic conductivity rate in inches per hour.
We deviated from the method guidelines in that we did not measure the maximum ponding depth
following the pre-soak period, and we did not over -excavate the pit to test for groundwater
mounding. These two methods were omitted from the study based on the observed infiltration
conditions. Data from the pit test was collected and tabulated and is presented in Appendix II.
3.3 Small Scale Infiltration Pit Test result's
A summary of our findings is presented below; actual test data is presented at the end of this
report in Appendix II and Appendix IV.
Test Pit No. 1 (TP1)
1728 gallons of water were applied over 6 hours to the test pit during the pre-soak period for TP1,
which had 16 square feet of infiltration surface area. The average application rate was
approximately 4.8 gallons per minute, or 288 gallons per hour. The maximum depth of water
reached in the infiltration pit during the pre-soak period was 13 inches, measured from the
bottom of the pit floor.
A correction factor for the pit assessment was applied to the measured infiltration rate. Utilizing
the following values from Table 3.3.1 in the Ecology Stormwater Manual - Volume III (2012), our
correction factor was calculated as follows:
CFT = CFv x CFt x CFm
CFT = Total Correction Factor
CFv = Site Variability (0.33 –1.0; we use 0.901)
CFt = Small-scale PIT method (0.50)
CFm = Bio -buildup (0.9)
CFT = (0.90)*(0.50)*(0.9) —o CFT = 0.40
When applied to the saturated infiltration rate observed at conclusion of the infiltration test (7.5
inches per hour), the recommended correction factor (CFT) of 0.40 yields a corrected infiltration
rate of 2.4 inches per hour. This value fits within the range of infiltration values determined by
Meyers et al and is at or below the mean infiltration values calculated in their study.
1 The partial correction factor (CFv) used for this study was based upon our professional consideration of the
potential for site variability; the observed similarities in the substrate composition among the four excavated pits
justified an upper -range partial correction factor representing low site variability. All corrections were determined
in accordance with the criteria described in section 3.3.6 of the 2012 WA DOE Stormwater Manuel.
Maplewood Lane Soil and Infiltration Assessment ���,,,nm„oll
February 10, 2016 SLSMENT
POge 6 Bellu?gAum V/A 98225
!3ii'J)G%I n1.2 fin/�(.�'el �rcartru'u[inut org
3.4 Summary
Using the observed and corrected infiltration rates to provide an additional factor of safety for
informing stormwater management design in the subject area, a corrected infiltration rate of 2.4
inches per hour is indicated for the subject site. This value is associated with high rates of water
uptake, and indicates good transmissivity in the substrate. The depth of the water table is seen as
the only potential limitation in designing an infiltration facility at this site.
®S Conclusions and eco menda ions
The native in situ substrate encountered during the subsurface exploration appears well suited for
stormwater infiltration due to the highly permeable alluvial gravel and cobble substrate observed
in all excavated test pits. It is our opinion that an infiltration rate of at least 2.4 inches per hour is
supported by the observed and measured site conditions for the native soils after with applying
the recommended factor of safety corrections.
This report was prepared and submitted by:
Exp 05/21/2016
Paul D. Pittman, L.E.G.
Earth and Environmental Sciences Manager, Principal
Statement of Limitations
This document has been prepared by Element, a wholly owned subsidiary of Pacific Surveying and Engineering, for the exclusive use and benefit
of the client. No other party is entitled to rely on any of the conclusions, data, opinions, or any other information contained in this document.
This document represents Element Solutions best professional judgment based on the information available at the time of its completion and
as appropriate for the project scope of work. Services performed in developing the content of this document have been conducted in a manner
consistent with that level and skill ordinarily exercised by members of the geologic engineering profession currently practicing under Similar
conditions. No warranty, expressed or implied, is made.
l'apl uvuvo d j ane SO oaa+:i lnfiffi'utoon Assc,^ a;mae'af"
a r soma 6 ), BLIEMENT
15, P Av-nue 1,42I
�rnzr1!11m„1 ulq
Appendlix I - Pit Locations
'rld 11198
CI
M.W L*.00S D
Appendix 11 Sism ali,Scale Infiltration Test IlPits Measurement Data
SMALL SCALE PIT TEST DATA SHEET
PIT test per 2012 DOE Manual
MAX PONDING DEPTH PROPOSED BY ENGINEER = 2 ft
PIT BOTTOM SURFACE AREA (12-32 SF REQUIRED) = 16 sf
METER READING PRIOR TO PRE-SOAK PERIOD = 11,502
METER READING AT COMPLETION OF PRE-SOAK = 13,230
1. Pre-soak period: Add water to the pit so there is standing water for at least 6 hours. Maintain the presoak water level at least 12
inches above the bottom of the pit.
2. At the end of the pre-soak period, add water to the pit at a rate that will maintain a 6-12 inch water level above the bottom of the
pit over a full hour. The depth should not exceed the proposed maximum depth of water expected in the completed facility.
3. Every 15 -minutes, record the cumulative volume and instantaneous flow rate in gallons per minute necessary to maintain the water
level at the same point (between 6 and 12 inches) on the measuring rod. Depth should be the same as the proposed maximum
ponding depth.
4, After one hour, turn off the water and record the rate of infiltration (the drop rate of the standing water) in inches per hour from
the measuring rod data, until the pit is empty,
5. At the conclusion of testing, over excavate the pit to see if the test water is mounded on shallow restrictive layers or if it has
continued to flow deep into the subsurface. The depth of the excavation varies depending on soil type and depth to hydraulic
restricting layer, and is determined by the engineer or certified soils professional. The soils professional should determine if a
mounding analysis is necessary.
STEP 1 - PRE-SOAK DATA RECORDING
Page 1 of 2
STEP 3 - CONSTANT WATER LEVEL DATA RECORDING
Time -Win)
Depth (in)
Meter Reading (Rai)
0
8
13230
15
8
13255
30
8
13283
45
7.75
13311
60
7.5
13338.
STEP 4 - DRAWDOWN DATA RECORDING
Time (min),
Depth (in)
Meter Reading (gal)
Time _mire
Depth in
MeterReadingigaij
0
7.5
0
135
15
6,5
0
150
30
45
3.51
2.5
0
0
165
180
60
1
O195
75
0
01210
90
105
225
240
120
255
Page 2 of 2
f
Photo 1: Undisturbed topsoil conditions
Photo 2: Pit test pre-soak period
Photo 3: Soil collected for lab analysis
100
90
80
70
W 60
Z
LL
Z 50
W
U
W 40
CL
30
20
10
0
Sieve Analysis Test Report - T27/T11
%+3" E
�, „%a Gravel
,,,.,. ,,,.. .
Infiltration - Rain Garden
Coarse 1 Fine
0
22 17
SIEVE
PERCENT SPEC." PASS?
SIZE_
FINER PERCENT (X=NO)
3"
100
1-1/2"
90
3/4"
78
3/8"
72
#4
61 1
#10
52
#20
45
#40
39
#60
33
#100
28
#200
22
l7 rCF111V JILC - II IIII.
Sand % Fines
Coarse Medium Fine Silt
...,.....
9 13 17 22 .
Materiaii Dgscrlptierl
Infiltration - Rain Garden
SP2 at 2.0 feet
silty sand with gravel
Atterberg Limits
PL= NP LL= NV
PI= NP
Coefficients
D90= 37.7593 D85= 28.7041
D60= 4.3650
D50= 1.5617 D30= 0.1830
D15=
D10= Cu=
Cc=
Classification
USCS= SM AASHTO=
A -1-b
Remarks
Specification was not provided by client.
100
90
80
70
W
W 60
Z
LL
Z 50
W
U
G:
W 40
CL
30
20
10
0
Sieve Analysis Test Report - T27/T11
oda +3.1 , % Gravel .... ..
"'
Coarse.Fine
0 9 . 20
SIEVE PERCENT SPEC.'
PASS?
SIZE FINER PERCENT
I (X=NO)
3" 100
1-1/211 100
3/4" 91
silty sand with gravel
3/8" 82
#4 71
#10 62
l -L= N
#20 53
#40 45
#60 38
D5= 12.1811
#100 31
D50= 0.6272
#200 23
i
I
UR/"1.1, I, V, JILC - 111111.,
Sand € % Fines
Coarse
e Medium Fine Silt
9 17 22 23
kno specincaaon provIoeu)
Location: Kays Infiltration - Native - SPI at 4.0 feet
Sample Number: 7346
Project: Element eral Solutions
Services
x,hy Cs� 5l' i
w.
��� h Project No: 16-0077
Tested By. SEI -11 _ ._ Checked By: DL
Date: 1-27-16
Floure SA002
NMaterlal Description
Infiltration - Rain
Garden
SP 1 at 4.0 feet
silty sand with gravel
Attarberg Limits
PL= NP
l -L= N
PI=
Coefficients
D90= 17.5766
D5= 12.1811
D60= 1.6965
D50= 0.6272
D30= 0.1353
D15=
D10=
Cu=
Cc=
Classification
USCS= SM
AASHTO=
A -1-b
Remarks
Specification was
not provided by client.
kno specincaaon provIoeu)
Location: Kays Infiltration - Native - SPI at 4.0 feet
Sample Number: 7346
Project: Element eral Solutions
Services
x,hy Cs� 5l' i
w.
��� h Project No: 16-0077
Tested By. SEI -11 _ ._ Checked By: DL
Date: 1-27-16
Floure SA002