1. Designing Around Seasonal High Groundwater
Jennifer Deal, Graeme Bowles, Don Hullings
January 31, 2017

2. Introduction
Seasonal high groundwater elevation affects many landfill design aspects
Design elevation
Grading and fill requirements
Construction dewatering
Stormwater management
Airspace
Monitoring well construction
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3. Seasonal High Groundwater Determination
Site specific
Regional
Soil survey
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4. Seasonal High Groundwater Determination
Site Specific Seasonal High
Goal
Data sources
Considerations
Data outliers
Climatic patterns
Topography
Goal – provide a number with relative certainty considering the data set available and local experience.
Data sources
local, state or fed databases - series of ROMP wells in several FL locations, long data record, multiple units monitored
client data collection - adjacent sites and current development site, prior permitting efforts and hydraulic assessments, current wells and consideration for planning in advance
adjacent sites – reporting requirements for multiple reasons, FDEP Occulus, Water Management Districts
Considerations
Outliers, anomalies, bad data, artificial short term reason, reasonable test with assessment against the trend for the well and the well set, elimination.
Climatic patterns – designing in wet vs. dry season, swing of as much as 25 inches annually, “normal” is 53 in Sarasota, long term trend influencing conservative design, NOAA record (go to pic) seasonal cycle – 5 to 7 years, with outliers, magnitude of the bars is relative to amount of precip.
Topography – affects how water behaves in the subsurface, shallows at the toe of slopes, mounds at water divides, responds to changes in topography to a degree, reliant on upslope hydraulic pressure, wetland offsets closing out ponds, canals, swales or ditches, level topography will not show the same as historic water table, even pressure not lower outlet (subsurface connections remain).
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5. Seasonal High Groundwater Determination
Climatic patterns – designing in wet vs. dry season, swing of as much as 25 inches annually, “normal” is 53 in Sarasota, long term trend influencing conservative design, NOAA record (go to pic) seasonal cycle – 5 to 7 years, with outliers, magnitude of the bars is relative to amount of precip.
Palmer Drought Severity Index - NOAA
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6. Seasonal High Groundwater Determination
Regional
Relative comparison
Setting
History
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7. Seasonal High Groundwater Determination
Soil survey
Original intent – soil conservation
Focus on flora and fauna
Vadose zone
Hydric vs. non-hydric
Predictions for uses of soils other than farming, grazing, wildlife habitat, and forestry have tended to concentrate on limitations of soils for the intended uses.
Not intended to provide water table information.
Design purposes
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8. Example – St. Marks C&D Facility
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9. Example – St. Marks C&D Facility
Shallow wells on-site but no record of data
Several years of data for adjacent site
Compared lithology from Nine Mile and St. Marks to confirm consistency between the sites
Determined seasonal high at Nine Mile relative to ground surface and applied to St. Marks
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10. Example – St. Marks C&D Facility
Seasonal high one-foot below ground surface
Construction below seasonal high
Earthfill
Permittable FAC (11)
“equivalent degree of protection for the environment”
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11. Example – St. Marks C&D Facility
Additional 60-mil geomembrane and GCL
Preliminary approval from FDEP, application currently under review
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12. Expense of Raising Grades
One foot of earthfill per acre is 1613 CY/ac At $3/CY = $4840/ac At $10/CY = $16,130/ac Value of lost airspace At $5/CY = $8067/ac
I think we are know about federal subtitle regs and how they are adopted by individual states. Post-closure care is required and while most people plan for 30 years,, it can be shorter although unlikely, but quite possibly longer. How much longer can be determined by the state on an individual basis
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13. Costs for Going Below SHGWT
One foot: Assume no dewatering as “actual” groundwater is more than a foot above SHGWT Cost of placing additional layer of geomembrane below GCL to address effects of groundwater per FAC sf (40x40) x $0.60/sf = $1,000 Cost to convince FDEP = Priceless
Subgrade
SHGWT
Groundwater Level at time of construction
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14. Costs for Going Below SHGWT
Two feet: Two well points = $1000 Header pipe = $1000 Pump rental for several months = $1000 Pump operations = $4000 ($40/day for 100 days) Added Geomembrane = $1000 Dewatering Permit (simple recharge) = $2000 Say $10,000
Subgrade
SHGWT
Groundwater Level at time of construction
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15. Costs for Going Below SWGWT
Subgrade at SWGWT
8 ft
Ditch grade
sump
Isolates area below groundwater to ditch and sump
Dewater effectively to 8+ at sump
Achieve an average of 4 feet or earthfill/capacity
Plan view
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16. Cost For Going Below SHGWT
Four feet:
Extensive Dewatering = $75,000 (check out YouTube)
Geomembrane for sump and ditch = $25,000
Extra Gravel = $100,000
Dewatering Permit = $20,000
Say $220,000
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17. Cost and Benefits of Dewatering
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18. Questions?
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