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 that 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 borehole through drill pipes and a 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 Installation1) 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 borehole 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 lignosulphonate 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 CsWhere “ys” is the density of the bentonite suspension and “Cs” is the concentration of bentonite by weight. “Concentration of bentonite” means the 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 the 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.
Fig: Direct Mud Circulation Method of Pile Foundation Installation
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