AESI AESI 6/17/2015 HOW AND WHERE DOES INFILTRATION WORK? o Context: Summary of Geologic History. o Constraints/benefits for different geologic units

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1 PRESENTED June 17, 2014 BY Jim Brisbine, PE HOW AND WHERE DOES INFILTRATION WORK? o Context: Summary of Geologic History o Constraints/benefits for different geologic units o Key geologic and groundwater flow parameters critical to site planning/engineering o Subgrade preparation Reference: D. Molenaar,

2 TYPICAL PUGET SOUND STRATIGRAPHY o Recessional outwash: Bedded and sorted sand, gravel. River deposits flowing from wasting and retreating ice o Till: Unsorted mixture of clay, silt, sand, gravel, cobbles nature s concrete 5 to 30 feet thick on average. Compacted beneath ice sheet o Advance outwash: Bedded and sorted gravel and sand at top: River deposits flowing from advancing ice; wellbedded clay and silt at base: deposits of lakes (or salt water) farther in front of the ice RECESSIONAL OUTWASH Constraints: o Thin Benefits: o o Removed during grading Shallow ground water Downslope impacts (slope stability, springs, wetland hydrology High permeability Dispersed infiltration options LODGEMENT TILL Constraints: o Thin weathered horizon removed during grading o Very low permeability parent material Good for earthen dams/berms In-situ amendments not feasible o Ground water mounding o 1 to 1-1/2 inches/month of recharge through till o to inches/hour 2

3 ADVANCE OUTWASH o Constraints Depth Variable receptor soil characteristics Downslope impacts (slope stability) o Benefits Adequate receptor soil Recharges aquifer system Only viable solution at many sites SITE ASSESSMENT OVERALL PROJECT LEVEL CONSTRAINTS o Geology/soil characteristics o Ground water conditions o Infiltration potential o Water balance issues Wetlands Springs Water Supply o Final Site Use Commercial Residential Industrial SITE ANALYSIS o Exploration Exploration pits Deep exploration borings o Testing o Modeling 3

4 READILY AVAILABLE RESOURCES o USGS and DNR Geologic Maps o USDA Maps o In House Previous Work RECEPTOR SOILS o Organic Content o Infiltration rate of native soils o Cation Exchange Capacity o Grain Size Distribution INFILTRATION RATE TESTING SPECIAL CASE ONLY: o Grain Size Distribution o Published Soil Infiltration Rates OUTDATED: o Percolation Test (Single Standpipe) o Double Ring Infiltrometer PREFERRED: o Large-Diameter Single Ring o Pilot-Scale PIT 4

5 Percent Finer 6/17/2015 INFILTRATION RATE DETERMINATION Ecology 2012, Sieve Analysis (USDA/ASTM) Recessional Outwash or Holocene only US STANDARD SIEVE NOS. #200 sieve - Silt/Clay Grain Size, mm USDA METHOD, ECOLOGY 2005 REMOVED FROM ECOLOGY 2012 Table 3.7 Recommended Infiltration Rates based on USDA Soil Textural Classification Clean sandy gravels and gravelly sands *Short-Term Infiltration Rate (in/hr) Correction Factor Estimated Long- Term (Design) Infiltration Rate (in/hr) Sand Loamy Sand Sandy Loam Loam OLD SCHOOL INFILTRATION Falling Head Test (EPA) Double Ring Test 5

6 SMALL SCALE INFILTRATION Pilot Infiltration Test (PIT) Pilot Infiltration Test (PIT) LARGE DIAMETER RING INFILTRATION TEST () Modified PIT Test TESTING FREQUENCY 2012 ECOLOGY MANUAL o Commercial Sites 1 test per 5,000 sq. ft. Groundwater thru wet season o Residential Sites 1 Test per 200 feet of road and every lot Groundwater thru wet season 6

7 MODELING ANALYSIS o Depth to water table o Infiltration rate of native soils o Hydrographs o MODRET STORMWATER INFILTRATION -SUMMARY o Characterization of Receptor Soils Hydraulic Parameters water quality characteristics o Infiltration Rate Laboratory/Field Measurements o Depth to Water Table Thickness of Unsaturated Zone o Groundwater Flow Direction Impacts to Environment/Wells o Depth of Aquitard Aquifer Capacity o Design Storm Event Peak Flow Rate/Total Volume SUBGRADE PREPARATION o Stripping depth- know where receptor soils are o Uniformity of subgrade support Look for soft spots o Proof Roll use a fully loaded dump truck o Firm and Unyielding o Geotextile only for separation not always advisable 7

8 CONSTRUCTION o Truck traffic to a minimum, establish haul routes o Erosion control o Finished product protection o Siltation from other areas o Truck Traffic o Concrete washouts SECTION PLACEMENT o Ballast placement push forward without tracking on subgrade o Static Roll or minor vibratory rolling, minimize compaction to the receptor soils 8