Module 4: Soil Assessment for Bioretention

1. Module 4: Soil Assessment for Bioretention Dennis Shallenberger, PE, GE Earth Systems Pacific

2. 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

3. 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

4. USDA Soil Chart

5. Subsurface Profile Investigation Recommended / Required Least: None required if project is less than 20 acres; If required, depth and method not specified Most: Test pits to 11’ below the proposed facility invert Typical: Borings or test pits to proposed facility invert (or about 5‘ below) Borings allowed but discouraged in favor of pits

6. 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

7. Depth of Infiltration Test Recommended / Required Least: None specified Most: 4 feet below invert of proposed facility Typical: About at depth of proposed facility

8. Test Diameter or Square Footage Recommended / Required Least: 6” or none specified Most: 100 sq ft Typical: 6” to 12” diameter

9. Pilot Infiltration Test (PIT)

10. Pilot Infiltration Test (PIT)

11. Test Method Recommended / Required 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 tests

12. Double Ring Apparatus

13. Test Duration(presoak, if any, plus actual test) Least: 2 hours Most: Difficult to determine, probably 30 hours to presoak and read until stabile readings Typical: 4 hours to 1 day

14. Standards from Nearby Jurisdictions Salinas 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 Engineer

15. Standards from Nearby Jurisdictions City of Santa Barbara Preliminary site evaluation including soil types, geology, topography, drainage patterns, etc., recommended 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

16. SPT WITH TRIPOD

17. 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 hours Refill casing (to unspecified depth) and read for 1 hour

18. 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

19. 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

20. 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, 2003) Massman empirical formula (2003) Pilot Infiltration Test (Washington State Dept. of Ecology, 2005) Cone Penetration Test

21. 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

22. 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

23. 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

24. Proposed Method (Shallow Infiltration Facilities; Sizing by Infiltration Rate Required) 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

25. 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

26. 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.

27. 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?

28. Closing Thoughts Need consistent test methods Need economical test methods Need appropriate scale factors, factors of safety, other factors

29. Closing Thoughts Need geotechnical input early in design Need open discussion among owner, designer and geotechnical engineer

30. Final Though We are at the bottom of a new learning curve If we work together, we can steepen & shorten it