Direct Mud Circulation method of pile foundation installation is used for bored cast in-situ piles. Direct Mud Circulation method is best suited when working-space is a huge constraint.
In Industries and power plants where there is a labyrinth of pipelines snaking all around, DMC comes to the rescue. While the other methods are comparatively expensive, DMC is a fairly cheaper method to install piles.
The apparatus basically comprises a Tripod with pulley, winch machine, wire-rope, Chisel, drill pipes, tremie pipes, a vertical pump and a pit where bentonite powder is mixed with fresh water.
A tripod as the name suggests is a three-legged structure which is mounted with a pulley at the apex to maneuver the chisel that bores the earth, which is further appended to drill pipes in tandem with a wire rope that is ultimately knotted to the winch machine. To make it sound simple, the chisel is the primary boring tool and the drill pipes are mere appendages that provide the depth, and also, suffuse the bore with bentonite suspension.
The number of drill pipes required varies depending on the depth of the bore required. Besides this, a vertical pump is installed in the bentonite pit so as to pump the bentonite solution into the bore hole through drill pipes and chisel.
The method is termed as Direct Mud Circulation because it involves the circulation of mud from the pile bore directly to the bentonite pit where the pump reuses the bentonite slurry leaving alone the settled mud.
Fig1. Direct Mud Circulation Method of Pile Installation
Procedure for Direct Mud Circulation Method of Pile Foundation Installation
1) The Tripod is made to stand around the target pile (two legs around the pile and one into the slot provided in winch machine) and a plumb bob is then hung down the pulley (mounted on the tripod) aiming the center of the target pile so as to obtain the center for the chisel. The maximum tolerance permissible for piles with diameter 600mm or more is 75mm or D/2, whichever is more.
2) Once the position of winch machine and the tripod is fixed, casing is installed into the earth keeping the center point intact. A wall made up of sacks filled with rubble or soil is created around the pile-area so as to channel an artificial conduit directed towards the bentonite pit. This channel directs the mixture of mud and bentonite slurry coming out from the bore to the bentonite pit.
3) The topmost drill pipe is connected with a hose emerging from the vertical pump (installed in the bentonite) pit sending fresh bentonite slurry to the borehole through a fine opening in chisel and when the bore hole is filled to the brink, it flows back to the bentonite pit along with the mud through the artificial conduit. In this way the mud from the bore keeps on moving up the borehole and is directed to the bentonite tank.
4) It is preferred to use fresh and clean water for the preparation of bentonite suspension as the minutest impurities may alter the properties of a suspension. In case, clean water is not available at the site, special additives such as Ferrosome lignosulphate in combination with soda ash, phosphates and flocculants are added.
The specific gravity of the bentonite suspension should be between 1.05 to 1.12 as it is responsible for the retention of the vertical cut soil surface of pile borehole. Cohesiveness and surface tension develop among the soil particles if the specific gravity of the suspension is overlooked which eventually leads to the collapse of vertical cut soil surface.
To achieve the requisite specific gravity of the bentonite solution, the concentration of bentonite is calculated as under:
Ys=1+0.006 x Cs
Where “ys” is the density of the bentonite suspension and “Cs” is the concentration of bentonite by weight.
“Concentration of bentonite” means percentage of bentonite (by weight), to be mixed with water.
So, Cs=1/6 x (1.05-1) x1000 =8.33% by weight.
i.e In 1m3 of water 83.3kg bentonite shall be mixed to get 1.05 specific gravity of the bentonite solution.
There are two stages of use of bentonite suspension:
- First flushing: When earth is being bored, bentonite suspension of specific gravity between 1.05 to 1.12 is to be maintained.
- Second flushing: When the bore has to be cleaned after the completion.
5) Chisel is then driven inside the casing and the earth is bored until the chisel has fully gone down inside the earth. Drill pipes are attached to the chisel in tandem and the boring progresses by maneuvering the winch machine until the requisite depth is bored. Flushing is done all the while during the pile boring. After the completion of bore, chisel is brought outside and the second flushing of the bore hole is done to remove all the mud and impurities.
6) Cage Insertion should be done immediately after the completion of second flushing. It is recommended to provide temporary hooks to the cages so as to keep the verticality of the cage in view while inserting it down the bore hole.
7) Tremie are finally inserted into the bore hole attached to a hopper at the top hung by the wire rope. The joint between the hopper and tremie pipe is closed with a steel plug before the first charge of concrete. The hopper is then filled with concrete to its full capacity. Once filled, the steel plug is removed to allow the concrete to flow down the tremie pipes thereby replacing the bentonite solution.
Concreting is done up to at least 1m above the cut-off levels to ensure good concrete for proper embedment into pile cap.
Fig: Direct Mud Circulation Method of Pile Foundation Installation
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