The Direct Mud Circulation (DMC) method is commonly used for installing bored cast-in-situ piles. It is especially useful in areas where space is limited, such as industrial and power plant sites with numerous pipelines. Compared to other methods, DMC is a cost-effective way to install piles.
To carry out the DMC method, a tripod with a pulley, winch machine, wire rope, chisel, drill pipes, tremie pipes, and a pit for mixing bentonite powder with fresh water is required. The chisel is used to bore the earth and is connected to drill pipes using a wire rope that is operated by the winch machine. The drill pipes provide the depth required, and bentonite suspension is added to the bore using them. The number of drill pipes used is determined by the depth of the bore.
A vertical pump is installed in the bentonite pit to pump the bentonite solution through the drill pipes and chisel and into the borehole. The method is called Direct Mud Circulation because mud from the pile bore is circulated directly to the bentonite pit, where the pump reuses the bentonite slurry while the settled mud is left alone. Overall, the DMC method provides an efficient and cost-effective solution for installing piles in tight spaces.
Fig1. Direct Mud Circulation Method of Pile Installation
Procedure for Direct Mud Circulation Method of Pile Foundation Installation
Pile boring involves several steps. First, a tripod is set up around the target pile, with two legs positioned around the pile and one leg in the slot provided in the winch machine. A plumb bob is hung down from the pulley mounted on the tripod to aim at the center of the target pile and obtain the center for the chisel. The maximum tolerance allowed for piles with a diameter of 600mm or more is 75mm or D/2, whichever is more.
Once the position of the winch machine and tripod is fixed, a casing is installed into the earth while keeping the center point intact. A wall made of sacks filled with rubble or soil is created around the pile area 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.
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 the chisel. When the borehole is filled to the brink, it flows back to the bentonite pit along with the mud through the artificial conduit. This process continues until the mud from the bore keeps on moving up the borehole and is directed to the bentonite tank.
For the preparation of bentonite suspension, it is preferred to use fresh and clean water to avoid impurities that may alter the properties of the suspension. If 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, 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 can lead to the collapse of the vertical cut soil surface. To achieve the requisite specific gravity of the bentonite solution, the concentration of bentonite is calculated using the formula Ys=1+0.006 x Cs, where Cs is the concentration of bentonite by weight.
There are two stages of using bentonite suspension: first flushing when the earth is being bored, and second flushing when the bore has to be cleaned after completion. The 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 required depth is reached. Flushing is done all the while during the pile boring.
After the completion of the bore, the chisel is brought outside, and the second flushing of the borehole is done to remove all the mud and impurities. Cage insertion should be done immediately after the completion of the second flushing. It is recommended to provide temporary hooks to the cages to keep the verticality of the cage in view while inserting it down the borehole.
Finally, a tremie is inserted into the borehole attached to a hopper at the top hung by the wire rope. The joint between the hopper and the 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 the pile cap.
Fig: Direct Mud Circulation Method of Pile Foundation Installation