Wet-Hole Method
A tremie pipe of 8 in (200 mm) or larger, with a temporary
closure applied to the bottom to make it watertight, is lowered to the bottom
of the shaft. When the concrete in the pipe has reached a level where the
concrete pressure exceeds the water pressure at the bottom, the pipe is raised
slightly to allow the seal to come off.
Construction Methods
In areas of high ground water level, sand and granular silt
cannot be removed to extend the hole past the bottom of the casing without
causing soil run-in. This run-in could result in loss of soil outside the
casing and a settlement of the ground around the excavation. However, this
problem can be handled by one of the three methods described below.
Water Head
The first method is to keep a positive head of water in the
drilled shaft so that the hydrostatic pressure is enough to prevent soil run-in
at the shaft bottom.
Casing
A second method is to advance the casing ahead of the
excavation with a vibratory pile driver attached to the casing. The agitation
of the cohesionless soil below water reduces the skin friction between the soil
and casing so that the casing can be sunk below the excavation level. Once the soil
is removed and the concrete is placed, the casing can be removed by the
vibratory driver without damaging the concrete.
Slurry
A third method is to stabilize the excavation by mud slurry.
The use of slurry allows a hole with straight walls to be drilled in dry,
moist, or saturated sand. This procedure requires an experienced driller who
understands slurry construction procedures.
The drilling equipment can be either rotary or auger type.
The mud slurry is premixed in a sump adjacent to the foundation areas. Water
and bentonite or attapulgite (clay) are mixed to the proper consistency, which
could have a unit weight of 65-75 lb/ft3 (10-12 kN/m3).
Drilling with an auger or mudding bit of the same diameter
as the required shaft is started with the mud slurry circulating from the sump
to the excavation. With a rotary drilling machine, the slurry is circulated
through the hollow drill stem into the bottom of the shaft.
The slurry is mixed with the soil cuttings as the bit
advances and is pushed back up and into the sump where the cuttings are
screened out before the mud is returned to the excavation. With an auger
machine, the slurry is pumped into the excavation and circulated by action of
the auger bit.
If the slurry mix is not stirred, it will turn into a gel
and hold the excavation wall in place and the cuttings in suspension. Some of
the larger diameter shafts are drilled by reverse circulation.
The mud slurry is placed in the top of the excavation as the
bit is advanced, and the slurry mixed with soil cuttings is pumped from the
bottom back into the sump. Before the reinforcing steel is placed, the slurry
should be stirred vigorously and circulated to remove the soil cuttings that
have settled to the bottom of the excavation.
After the hole has been excavated, prompt placement of steel
reinforcement and anchor bolts is necessary to prevent caving, water entry, and
difficulty in pulling the casing.
Concrete Placement by Tremie
Tremie methods are used to place concrete under the
following conditions:
1) When the water flow into the drilled shaft is too rapid
to be pumped down to allow the placement of the concrete in a dry excavation
2) In a static water table condition where the hydrostatic
head is required to prevent a blow-out at the base of the excavation,
3) Where the shaft has been drilled by the mud slurry
method.
The drilled shaft should be full of mud slurry, or the water
should have reached its stabilized elevation, before concrete is placed. The
concrete used in the tremie method should have a high slump of 7-10 in (180-250
mm) and be rich in cement with 7.5 bags/yd3. (One bag is 94 lb [418 N]).
The discharge end of the tremie should be raised slowly, but
it must be kept submerged in the concrete at a sufficient depth to maintain an
adequate seal during underwater placement. If the seal is broken, the tremie
must be withdrawn immediately from the shaft, resealed, and lowered below the
surface of the concrete. Then the pouring operation is restarted.
Once started, the tremie operation must proceed without
interruption until the concrete has reached the ground surface. It is then
continued until all foreign materials have been flushed from the top of the
pour. Vibration of the tremie concrete usually is not required, but it is
permissible to vibrate the tremie pipe under certain conditions when the flow
of concrete becomes sluggish.
Concrete Placement Using Pumps
In lieu of the tremie method, concrete pumps are gaining
wide use for placement of concrete under water. Pumping is especially useful
for large volume pours and sites where unsuitable terrain make it difficult to
locate delivery trucks and placement equipment efficiently.
Coordination is required between the contractor, concrete
supplier, and pumper to ensure that a proper mix is used, and that a machine
with adequate capacity is used. The concrete pump should be capable of pumping
a minimum of 60 yd3/hr (46 m3/hr) to a vertical height of 200 ft (60 m), and
the conductor pipe should not be less than 5 in (130 mm) inside diameter.
All the requirements and procedures for placement by the
tremie method discussed above apply to placement by pumping. If the contractor
chooses to pump concrete, a back-up pump should be available during concrete
operations, or a suitable hopper and tremie pipe should be on site.
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