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Borrow Pit Dewatering
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OUTLINE
• Administrative Issues
• Basics of gravitational settling
• Relationship between:
– Basin volume
– Detention time, and
– Pump rate.
• Borrow Pit as a Stilling Basin.
• Tier I Methods
– Stilling Basins
• Tier II Methods
CCPCUA Requirements
• Central Coastal Plain Capacity Use
Area
– Affects 15 Counties in Eastern NC.
• Locate number of wells within 1500 feet of pit.
boundary
– Use Latitude and Longitude Coordinates
• Determine whether adjoining dwellings are
served by county/municipal water supplies.
• Contractor to assume liability for impacts to wells
and surface waters.
• Average daily pump rate.
• Estimate number of pumping days for life of pit.
• Pit water elevation prior to pumping.
• Pit water elevation after pumping.
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Turbidity Limits
Surface Water
Classification
Streams
Lakes & Reservoirs
All Trout Waters
Turbidity
Not to Exceed Limit*
(NTU’s)
50
25
10
* Turbidity may not exceed upstream
background levels. If background levels are
less than the above limits, the above limits
must be met.
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Focus of This Discussion
• To find a low-cost, convenient
way to get the sediment
entrained in the borrow pit water
from the water.
• Let’s look at several possibilities.
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Settling Velocity and
Detention Time
• Using the Continuity Equation,
Q = AV (Clarifier Theory)
– Q = Pump Rate, L3/T
– Vs = Settling velocity, L/T
– A = Surface Area required to remove
sediment (by gravity), L2
• Therefore: A = Q/Vs
Settling Velocity
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Stilling Basin Requirements
10000.0
4 MGD
3 MGD
2 MGD
1 MGD
Area Required (acres)
1000.0
0.5 MGD
0.25 MGD
100.0
SILT
1.0
0.1
0.0001
0.001
0.01
• To use gravitational settling to
remove clays (even silts) is
unreasonable.
• Need either:
– Excessive detention times, or
– Excessive basin volumes.
10.0
CLAY
Conclusions
0.1
• May need additional technology
to yield the required NTU-Levels.
Particle Diameter (mm)
2
The Borrow Pit
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V
td =
Q
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Suggestion
• Use a step-wise, progressive
approach:
– First, try simpler/cheaper technology
(Stilling Basins).
– If Stilling Basins do not yield the desired
NTU levels,
• Basin Volume, V is a function of td and Q)
V = td Q
• ADD additional/better technologies.
Break for Worksheet; #21
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Borrow Pit an the Stilling Basin
• With few minor considerations you
maybe able to use the borrow pit and
its rim ditch as a stilling basin.
• The key is to eliminate (at least
minimize) resuspension of sediments
from within the borrow pit when the
pit is pumped.
Borrow Pit as the Stilling Basin
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Borrow Pit as a Stilling Basin
(Present)
Borrow Pit as a Stilling Basin
(Desired)
Water Flow
Water Flow
Plastic or Filter Fabric
Borrow Pit as a Stilling Basin
(Desired)
Water Flow
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• Tier I Methods
–
–
–
–
–
–
Outline
Stilling Basin for Pumped Effluent
Land Application (Irrigation)
Silt Bags
Alum
Gypsum
Polyacrylamide (PAM)
• Tier II Methods [Rare & unique resources]
–
–
–
–
–
Well Point Pumping
Impoundments
Cell Mining
Sand Media Filtration
Wet Mining
Break for Worksheet; #22
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The Next Step!
• If the water pumped
from the Borrow Pit
does not meet the
required NTU levels,
THEN
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– Land Application (Irrigation)
– Silt Bags
– Alum
– Gypsum
– Polyacrylamides (PAM)
– Pump the Borrow Pit water
into a Stilling Basin.
– Full Example in next
module.
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Irrigation
• Pump the borrow pit water into
and through a sprinkler irrigation
system.
• Irrigated water must be able to
infiltrate into the soil.
• Infiltrated water should be
sediment free.
The Next Step!
• If the water leaving the Stilling
Basin has an NTU level that is too
high, Look to other Tier I
Methods.
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Irrigation
5
Special Stilling Basin (Silt Bag)
• Geotextile bag that filters sediment from
pumped water.
• Used for small jobs near streams where basins
do not work well.
• Class A stone placed LEVEL under the silt bag.
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Flocculants
• Alum (Aluminum Sulfate)
• Gypsum
• Flocculants that are rarely used.
PAM
• Polyacrylamides (PAM) are
polymers (of proprietary
formulations).
– Available as liquids, powders, or in solid
form in “floc-logs”.
– Different formulations of PAM work
better in different soils.
PAM Mixing
• Should be applied at rate of about 1
mg/L.
– Flow 60 to 80 gpm over one Floc-Log in
turbulent flow.
• Inside a corrugated plastic pipe (no inner liner).
• Must have turbulent contact with the water flowing
past the logs.
– Powder
• Mix 1 pound of PAM per 100 gallons of water.
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PAM Injection
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PAM Injection Rate
4.0
PAM Injection Rate (gpm)
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3.5
3.0
2.0
1.5
1.0
0.5
0.0
500
1000
1500
2000
2500
3000
Pump Rate (gpm)
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PAM
• Follow PAM application with a
stilling basin.
– Stilling Basin; td = 2hrs
• Use skimming outlet
• Use 3 porous coir baffles
• Use a near bottom inlet
• Break for Worksheet; #23
• Following
appropriate mixing:
– Clay flocs form quickly.
– Flocs should settle from
water column in 2 to 6
hours.
– Some flocs may float.
– Porous baffles help to
capture flocs.
2.5
0
PAM
The Next Step!
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• If one of the Tier I approaches does
not yield water with an appropriate
NTU level,
• OR you have rare or unique resources
on the site, look to Tier II Methods.
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–
Well Point Pumping
Impoundments
Cell Mining
Sand Media Filtration
Wet Mining
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Well Point Pumping
• Place a row(s) of sand point wells
into the bottom of the borrow pit.
• Connect all wells to the pumping
system.
• Pump all wells simultaneously.
Well Point Pumping
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Water Removed via wells
& Filtered in the process
– Pumped water draws the water down
through the bottom of the borrow pit.
• Filtered water should be sediment free.
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Impoundments
• Pump polluted water into large
impoundments.
– Let the water stand until the water
quality improves sufficient to permit
stream discharge.
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Cell Mining
• Divide the borrow Pit into several
large cells separated by
embankments.
• Pump the polluted water from
the working cell into an unused
cell. Let the water stand until its
water quality improves and it can
be discharged.
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Sand Media Filtration
• Key component is chitin, a
coagulant/flocculant processed from
the shells of crab, shrimp, or lobster.
– Can be applied as a liquid (Liqui-FlocTM)
• Waters with 100 to 700 NTU
• Waters with pH of 6.5 to 8.5
– Can be applied as a gel (Gel-FlocTM)
• Waters with > 700 NTUs.
• Coagulated/flocculated clays must be
removed from the water by filtration.
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Wet Mining
• Excavate from the borrow pit
under the water level.
• Stack the excavated material
above the water.
– Gravitational water will drain from the
material.
– Drained material can then be taken to
construction site.
Wet Mining
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Questions?
Water Flow
9