Module 4: Soil Assessment for Bioretention

Module 4: Soil Assessment for Bioretention
Dennis Shallenberger, PE, GE
Earth Systems Pacific
Comparison of Infiltration Test Methods
Fairfax County, Virginia – July 2012
Wisconsin Dept. of Natural Resources – February 2004
Michigan LID Manual – 2008
Western Washington Storm Water Manual – August 2012
City of Portland, Oregon – February 1, 2010
City of Corvallis, Oregon – 2011 (?)
City of San Diego – 2011
City of Los Angeles – January 1, 2011
City of Salinas – October 2008
City of Santa Barbara BMP Guidance Manual – 2008
Pre-Investigation Research
Recommended / Required
• Least:
None recommended
• Most:
Complete geologic evaluation including
geologic units, structure, current and
historic groundwater elevation, expansive
soils, strikes and dips of units, slope
stability, groundwater mounding, faulting,
etc. required
• Typical:
Cursory office evaluation and assessment
of topography, USDA soil type,
groundwater elevations
USDA Soil Chart
Subsurface Profile Investigation
Recommended / Required
• Least:
None required if project is less than 20
acres; If required, depth and method not
• Most:
Test pits to 11’ below the proposed facility
• Typical:
Borings or test pits to proposed facility
invert (or about 5‘ below)
Borings allowed but discouraged in favor
of pits
Number of Percolation / Infiltration
Tests Recommended / Required
• Least:
None required
• Most:
One test per 50 linear ft or 2,000 sq ft
of facility
• Typical: One test per 100 linear ft or 1 test per
5,000 sq ft; 2 tests minimum
Depth of Infiltration Test Recommended
/ Required
• Least:
None specified
• Most:
4 feet below invert of proposed facility
• Typical: About at depth of proposed facility
Test Diameter or Square Footage
Recommended / Required
• Least:
6” or none specified
• Most:
100 sq ft
• Typical: 6” to 12” diameter
Pilot Infiltration Test (PIT)
Pilot Infiltration Test (PIT)
Test Method Recommended /
• Least:
none specified
• Most:
Infiltration Testing in Double Ring
Infiltrometers; laboratory sieve and
plasticity index testing of all soil layers
• Typical:
Testing in borings or Double Ring
Infiltrometer at engineer’s discretion;
Constant head for infiltrometers, falling
head generally recommended for boring
Double Ring Apparatus
Test Duration
(presoak, if any, plus actual test)
• Least:
2 hours
• Most:
Difficult to determine, probably 30
hours to presoak and read until stabile
• Typical: 4 hours to 1 day
Standards from Nearby Jurisdictions
Only applies to sites > 20 acres
1 profile boring or test pit to 15’
Evaluate seasonally high ground water
Conduct Double Ring Infiltrometer test at depth
of proposed facility
• Alternate methods may be approved by City
Standards from Nearby Jurisdictions
City of Santa Barbara
• Preliminary site evaluation including soil types,
geology, topography, drainage patterns, etc.,
• Excavate and log “many” test pits to 11’ below
infiltration facilities
• Excess disturbance of the soil is not recommended
• Perform Standard Penetration Tests in test pit at 2’
to 3’ intervals
• Infiltration tests in 2 locations in each test pit, at
least one at bottom-of-facility elevation
Standards from Nearby Jurisdictions
City of Santa Barbara, cont’d
• Implies creating test excavations in bottom of each
test pit, size and depth not specified
• Remove smeared sides of test excavations
• Case excavations; size, height, material,
perforations, not specified
• Presoak by filling casing to unspecified depth for 24
• Refill casing (to unspecified depth) and read for 1
Standards from Nearby Jurisdictions
City of Santa Barbara, cont’d
• Repeat 4 times
• Pull casings and backfill pits
• If infiltrating into fill, limit compaction in the fill
area & test fill for soil for hydraulic conductivity
A Few Specific Excerpts
• Farifax County, Virginia: Use “snug fitting”
casing; “septic system percolation tests are not
an acceptable alternative”
• City of Portland: Use tightly sealed casing
• City of San Diego: Identify surrounding Brown
Field sites
• City of Los Angeles: Size of facility is based
upon infiltration, void ratio of soil and a factor of
safety; no method specified for infiltration testing
or determination of void ratio
Paper by Eric Phillips & William Kitch
Compared several infiltration methods on 3 Southern
California sites
• Double Ring Infiltrometer (ASTM D3385-3) [sic]
• Borehole Test (City of San Bernardino, 1992)
• Borehole Test (California Test 750, 1986)
• Hazen empirical formula (1892, 1911)
• Kozney-Carmen empirical formula (1927, 1938, 1936,
• Massman empirical formula (2003)
• Pilot Infiltration Test (Washington State Dept. of Ecology,
• Cone Penetration Test
Phillips & Kitch Findings
Site A - 3.5’ of silty sand over silty clay
• Cal. 750 borehole test rate showed 4 times
faster percolation than Double Ring Infiltrometer
• Cal 750 rate was 2,000 times faster than
predicted by CPT
Phillips & Kitch Findings
Site B - well and poorly graded sand to 10’
• Two San Bernardino County borehole tests, one
29% faster and one 29% slower than Double
Ring Infiltrometer
• CPT predicted about 10% to 50% of the Double
Ring Infiltrometer rate
• Empirical rates about equal to, to 4 times faster
than borehole test
• PIT tests 1/3 to 1/7 the rate of the borehole test
Phillips & Kitch Findings
Site C - poorly graded and silty sand to 10’; silt and
silty sand below 10’
• San Bernardino County borehole test and CPT
just about equal
• 35-fold scatter in empirical methods
Proposed Method
(Shallow Infiltration Facilities; Sizing by Infiltration Rate
• Drill 6” to 8” diameter profile boring to 15’, log
cuttings and samples to min. 5’ below invert
• Drill 6” to 8” diameter test borings ranging from
depth of infiltration facility invert, to 2’ below invert
• Generally, 1 boring per 100 linear ft or 1 boring per
2,000 sq ft of infiltrator; 2 test borings, minimum
Proposed Method – cont’d
Perform test by:
• Add water to elevation of top of facility, maintain
head for 30 minutes
• Shut off water, record volume
• Read falling head at 1 to 30 minute intervals for
2 hours
Proposed Method – cont’d
• If test runs dry within 2 hr. period, refill and read
falling head test to end of original 2-hr. period; 2
refills maximum
• If fall < 6” in 2 hrs., continue reading for an
additional 2 hrs.
Closing Thoughts
• For the last century, civil and geotechnical
engineers have tried to get runoff off of a site as
quickly as possible
• Now we are trying to keep it on-site and into the
ground; the whole subject of LID is new to most
of us
• Where do we go from here?
Closing Thoughts
• Need consistent test methods
• Need economical test methods
• Need appropriate scale factors, factors of safety,
other factors
Closing Thoughts
• Need geotechnical input early in design
• Need open discussion among owner, designer
and geotechnical engineer
Final Though
• We are at the bottom of a new learning curve
• If we work together, we can steepen & shorten it

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