20210695 Ver 3_Appendix D - Bentonite Remediation Plan_20240523

Bentonite Mitigation Plan
What is bentonite used for:
Bentonite in drilling mud is used to both assist the drilling and boring process, as
well as condition the ground depending on the soil type. A predominant property
of bentonite is its ability to absorb water and increase its own volume by several
times, forming a gelatinous and viscous fluid. In a drilling or boring operation
bentonite is used for:
1. Extracting the drill cuttings to increase hole cleaning capability.
2. Forming a thin layer of low permeability and sealing the drill hole against
seepage to prevents wall cavities and leakages to the outside of the bore.
3. Promoting hole stability in poorly cemented formations (sand, shales)
4. Cooling and lubricating the drill bit.
Boring 101:
The boring process starts with the entry and exit pits being established,
equipment set-up and the bore path marked out on the surface. A pilot hole is
then bored from beginning to end that will act as a guide for the reamer. The
reamer will then follow that pilot hole and bore a hole typically 1.5 times the size
of the utility line to be installed. For example a 30" outside diameter pipe would
require a 45" bore hole. The hole is both reamed out and spray filled with a
water/bentonite slurry to prevent the hole from collapsing and to lubricate the
reamer. Bentonite slurry is pumped through drill stem and out through drill bit in
spray nozzles. The hole will then be swabbed (resprayed/lubricated) and the
utility will be pulled back to the drill rig. Bear in mind that once the contractor
begins to pullback the utility line, the process cannot be shut down for more than
a few hours without the risk the bore hole collapsing. Bore hole failure causes loss
of the underground utility (buried) and requires a new location to be selected and
re -bored. This will cause cost over runs and can pose further issues with frac-outs
due to a less desirable drill path being selected.
Trenchless Drilling and Frac-Outs:
A frac-out occurs when drilling fluid penetrates fractured bedrock, or seeps/flows
into the underlying soils and travels toward the Earth's surface. A frac out
frequently is the result of excessive down -hole pressure and or weak overlying
soil cohesion. Typically sandy and other types of unconsolidated soils pose the
greatest risk. In addition areas that have been previously cleared are more likely
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Bentonite Mitigation Plan
to have frac-outs. This is due to weakened path to the surface being created by
the decomposition of tree roots and the void created. When the soil fractures, the
fluid will travel along the path of least resistance to alleviate the pressure within
the bore. Almost always, that path leads to the surface above the trenchless work
area. However a frac-out can occur 10 to 100 feet or more away from the drill
path based on overlying soil cohesion. This means the contractor will have to
provide additional labor for monitoring and cleanup. A contingency and
remediation plan addressing the possibility of frac-outs during a drilling operation
is one measure to reduce environmental damage and extra costs to the
contractor associated with bore hole failure, large scale spills and clean-up. The
focus of this plan is to reduce the size of the impacted area once a frac-out
begins.
The boring company can take several steps to mitigate the spill. The following list
outlines various steps and actions:
1. During Drilling Operations:
• Workers shall constantly walk the bore path looking for signs of frac-outs.
• The drilling rig operator is also closely monitoring the system for pressure
drops, which if significant, will indicate a frac-out. The amount of drilling
fluid returning to the entry and exit pits is also an indication of a frac-out.
• When frac-outs occur, the boring process can be temporally halted to allow
the drilling crew to begin remediation techniques. Most frac-outs occur
during the pullback process, where the utility line to be installed is pulled
back through the bore hole, displacing the bentonite fluid. The larger the
diameter of the bore and the longer the bore length will both increase the
likelihood of frac-outs.
2. Frac-outs within the bounds of the construction project:
• Inform DOT staff so they can assist with monitoring and advise mitigation.
• Frac-outs shall be immediately addressed because they are typically in the
proximity of permitted sites (stream and wetlands). Failure to address
these types of frac-outs could allow run-off into permitted sites.
• Vacuum trucks in conjunction with grading/trenching areas can contain the
frac-out and allow a sufficient clean-up to occur without further
environmental issues.
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• Silt fencing, if not already installed, can also aid in reducing runoff of the
slurry.
• As long as the frac-out can be maintained, without spilling out into
permitted areas, the drilling operation shall continue. This is a maintenance
operation and not the clean-up.
3. Frac-outs in wetlands:
• Immediately inform DOT staff so they can inform the regulatory agencies.
• DOT staff will assist with monitoring and advise mitigation.
• Wattles should be immediately available to isolate and minimize the area of
the frac-out. Wattles shall be properly installed around the perimeter of the
frac-out completely encircling the area.
• If necessary, soil mounds can be created, using natural/native wetland soil,
to berm the area and prevent the spread of the slurry.
• Small trenches can be hand dug or with a mini excavator, to divert flows
away from the wetland for collection. Flow should be diverted back toward
the project boundaries or to an area where removal is more efficient.
Equipment should only be used when absolutely necessary to contain a
large frac-out or one that will pose a risk to the environment or humans.
• Once the frac-out is contained the area can be maintained by pumping out
or vacuuming out the slurry. If the area is inaccessible with equipment,
then shovels and buckets will be utilized.
• Trenches installed on the project side of a frac-out are useful to collect
slurry for removal. Typically these trenches will be along the silt fencing.
• After the utility pulled back is complete, the frac-out area(s) can be
shoveled to remove large deposits and then low pressure washed and
pumped/vacuumed to remove the remaining material. The washing
technique works better in vegetated areas. Rakes and squeegees work to
move slurry out of permitted areas and into an area where it can be
collected. For larger areas a mini excavator can be used to remove the
slurry material
• Care should be taken not to compact or remove excess insitu soils.
• Once the slurry has been removed consult with regulatory agencies to
determine if native wetland soil needs to be added to replace soil removed
during remediation. Once this is decided, the area can then be seeded with
a wetland seed mix and straw mulched.
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4. Frac-outs in water courses:
• Immediately inform DOT staff so they can inform the regulatory agencies.
• DOT staff will assist with monitoring and advise mitigation.
• Isolate the watercourse with an impervious dike of sandbags wrapped in
plastic liner, first downstream then up stream of the frac-out location.
• Install a pump to divert the watercourse around the release location.
• The pump outfall should be onto geotextile, so as to not cause any turbidity
and erosion as the water returns to the downstream area.
• Dewater the frac-out area by means of pumps and silt bags on pallets. If the
water is too turbid to pump through the silt bag it shall be removed with a
vacuum truck or other such method that does not impact the stream bed.
• Clean-up in the area will be by vacuum truck removal and/or with shovels
and buckets. Low pressure washing of the bentonite slurry and vacuum
extraction has been a proven low impact method to remove the slurry.
• Care should be taken not to remove excess insitu soils.
• A resource specialists will continually monitor the area for clean-up and
observation any impacts to fish and wildlife.
This document is not designed to limit remediation efforts but to provide
guidance and techniques that can be quickly and efficiently implemented in the
field. This document is also not all inclusive and other mitigation and clean-up
methods should all be explored to reduce the size of the frac-out and to provide
the best clean-up methods. Methodology outside of this document shall be
discussed with DOT staff and regulating agencies prior to implementation. This is
to provide protection to both the contractor and the environment.
Material Needed for Mitigation: (all standard erosion and sediment control
devices)
• Coir fiber wattles: coconut fiber with a 10" length and a minimum 12"
diameter. (See standard dwg. 16)
• Wooded Stakes: 2 feet with a 2" x 2" cross section, sharpened or beveled
on one end.
• Staples: 0.125" diameter steel wire, "U" shaped with a minimum 12"
length.
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• Special Stilling Basin (Silt Bag): used with wooden pallets. (See standard
dwg. 1630.06)
• Silt fencing with metal post/stakes. (See standard dwg. 1605.1)
• Vac -trucks in conjunction with low pressure hosing down of areas. This
method works best in vegetated areas.
• A mini excavator.
• If areas are inaccessible, shovels, rakes, squeegees, and buckets.
• Geotextile, Type 2 or greater will suffice.
• Plastic liner 5mm or greater will be needed.
Reference Sources:
BMPs' for Construction and Maintenance Activities NCDOT: Aug 2003
NCDOT Roadway Standard Drawings: January 2018
NCDOT Standard Specifications for Roads and Structures: January 2018
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