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.
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.
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.
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]).
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.
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.