Synthetic Slurry Used For Foundation Work at Baltimore /Washington International Airport
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By: John H. Berry, P.G.
Baltimore, Maryland – The Baltimore/Washington metropolitan area has
experienced a surge of population growth resulting in greatly increased
traffic at the Baltimore/Washington International Airport (BWI). As part
of a major expansion of BWI, Atlantic Caisson Corporation of Glen Rock,
Pennsylvania recently completed constructing the cast-in-place concrete
pilings for the curbside extension at the airport for the first two
stages. 95 of 142 caissons have been installed and the project is 67%
complete . Diameters of the shafts range from 48” to 60 “with average
depths of 38 lineal feet. Atlantic Caisson utilized a Watson 3100 CM HP
to drill all 95 shafts in 74 days.
Drilled Caissons Reduce Construction Footprint
Shallow footings were considered for support of the upper level
roadway structures; however, calculations showed that to support the
required loads would have resulted in large footings at least 15 feet
square. Due to the presence of numerous utilities and the need to
maintain traffic during construction, drilled caissons were selected as
the preferred foundation system. The project plans called for an
allowable shaft end bearing of 30 kips per square foot.
Slurry Installation of Caisson Foundations
Atlantic Caisson Corporation introduced the synthetic slurry method
of drilling foundation shafts into the Baltimore area in the fall of
1998, the state of the practice has gradually changed from the
telescoping dry method to the slurry displacement method. Shore Pac
slurry improves construction economics in many ways. The polymer slurry
requires less mixing/processing equipment, reducing capital investment,
jobsite congestion and fuel costs. It is highly concentrated so small
quantities of the dry polymer are required.
Consequently, Atlantic Caisson proposed this method to the General
Contractor, Kiewit Construction, and the slurry displacement method was
accepted due to past success using slurry in the Baltimore area.
First the slurry equipment is set up on-site. Atlantic Caisson owns
two 10,000 gallon slurry tanks, skid-mounted and fitted with platforms,
which reduces tank rental costs. Hoses and pumps have been fitted with
valves allowing slurry to be pumped from the tank holding fresh slurry
into the caisson under construction. As concrete is poured into the
drilled caisson, the slurry being displaced is recycled back into the
second slurry tank for reuse on the next hole. Air compressors run the
Sand Piper diaphragm pumps used to pump the slurry. Diaphragm pumps are
preferred over centrifugal pumps, which can shear the polymer, break its
molecular chains, and reduce its viscosity.
View of the slurry tank, diaphragm pumps, hoses, valves and air
compressor required to run a Foundation slurry drilling project.
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The crane lifts the rebar cage lowering it into the finished drilled shaft.
(click image to enlarge) |
View of spoils spinning from auger. The polymer can be
seen flying off the auger. Notice how dry the spoils
stay.
(click image to enlarge) |
Photo shows the entrance to BWI Airport and the expansion for additional gates. (click image to enlarge) |
The slurry Shore Pac is an easy mixing, water-soluble, polymer
supplied as a granular powder. When mixed into a stream of water
directed into the excavation the Shore Pac yields into a viscous clear
liquid that holds the hole open by hydraulic pressure and forming a
membrane along the sidewalls of the borehole. This polymer has been
designed specically for drilling large diameter foundation caissons and
it has been approved by Caltrans.
The polymer slurry is pre-mixed by adding it through a Venturi jet
mixer connected in-line to the discharge side of a centrifugal pump
which is plumbed to discharge the polymer/water mixture into the slurry
tank. Agitation is accomplished with compressed air delivered throughout
the bottom of the tank.
Prior to mixing the slurry, the pH of the make-up water is adjusted
by adding 5 lbs of soda ash per 1,000 gallons mix water. Chlorine from
municipal water is neutralized by adding ½ lb DE-CHLOR per 1,000 gallons
mix water.
Slurry Mixing Procedure
- Pre-treat water with soda ash 5 lb per 1,000 gallons mix water
for a pH of 8-10.
- Pre-treat water with DE-CHLOR ½ lb per 1,000 gallons mix water.
- Add Shore Pac GCV through Venturi type mixer @ 2.8 lbs per 1,000
gallons if pre-mixing in surface tanks, (if mixing directly within
excavation sprinkle into stream of water slowly as the excavation is
filled.)
- Monitor marsh funnel viscosity maintain at 54-60 sec/qt.
Depending on soil conditions.
Soil & Water Conditions at the Site
Water levels encountered at the site can were erratic and
unpredictable. This was due to irregular interbedding of impervious
silts and clayey silts with highly permeable sands. Shallow water levels
were encountered, representing perched water that when drilled into
would flood the shaft and dilute the slurry. Jim Hunt, Drilling
Superintendent for Atlantic, compensated for his problem by starting
with a viscosity of 100 sec/qt. “The water trapped in the sand between
the tight silt layers flowed into the hole when we drilled through it,
as though under pressure. The only way to prevent sidewall collapse was
to maintain a high viscosity,” Hunt said.
Slurry Recycling
Mark Suchy is a believer in maximizing the use of the Shore Pac
slurry by recycling. After the slurry testing has been completed and the
rebar has been placed, a suction line is dropped just inside the
excavation for recycling of the slurry. When concrete is tremie pumped
into the bottom of the hole, the displaced Shore Pac GCV slurry is
pumped from the top of the excavation into the holding tank. The last
three feet of slurry above the concrete interface is not recycled, this
slurry is contaminated from contact with the concrete. The impacted
slurry can be identified by its oatmeal-like appearance and is pumped to
a waste tank.
The recycled Shore Pac GCV slurry is tested for viscosity and pH.
While adding water to restore the original volume, additional soda ash
is added to adjust the pH. More Shore Pac GCV is added to restore the
slurry to full strength. Typically one-fourth the original amount of
Shore Pac added to the water filling up the holding tank is required to
restore the slurry to its initial strength.
Disposal of Slurry
Shore Pac slurry fluids are non-toxic and are readily degradable upon
completion of a slurry job to facilitate disposal. Upon completion of
the project, any remaining Shore Pac can be broken down with a chemical
oxidizer. The most common oxidizer for this purpose is 5% Sodium
Hypochlorite solution (household bleach).
Hypochlorite solution is added to the slurry at a rate of 1 gallon
per 100 barrels (4,200 gallons) of slurry to be treated. After the
hypochlorite breaker is added, it is necessary to ensure complete
oxidation of the polymer molecules. This can be accomplished by pumping
the slurry back into a holding tank or breaking it down in the hole and
agitating with the auger.
Atlantic Caisson can re-use the slurry, thereby minimizing disposal
costs, and maintaining a cleaner, more manageable construction site.
Shore Pac GCV is a dry synthetic granular polymer manufactured by CETCO
Drilling Products. The polymer offers greater frictional load bearing
capacity (skin friction) in bored piles and higher success rates on load
tests.
For information on drilling caissons using the slurry displacement
method with polymer slurry , and for information on equipment and
products mentioned in this article, contact John H. Berry, P.G.
Hydrogeologist/Product Manager with CETCO Drilling Products.
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