This document summarizes the Direct Mud Circulation (DMC) method for bored cast-in-situ concrete pile foundations. Key points include:
- DMC involves boring a hole and circulating bentonite mud from the pile bore directly to a settling pit, where the mud is reused after settling out solids.
- A tripod rig supports a winch, DMC rods attached to a chisel are used to bore the hole and add rods as depth increases, removing soil which is settled from bentonite water.
- Once at design depth, rods and chisel are removed and a reinforced concrete cage is lowered then tremie pipes are used to pour concrete from bottom-up, displacing
1. PRESENTATION ON DMC PILE FOUNDATION
B Y: S A M I R S A R K A R
A S E / C I V I L
2. INTRODUCTION
Piles are the columnar structure, the most common type of deep foundation, that safely
transfer loads from the super-structure to more competent sub-surface strata by means
of :
Friction,
End Bearing or
A combination of both.
Based on the method of construction, Piles can be broadly classified into the
followings:
Driven (Displacement) Cast-in-situ Piles.
Bored (Replacement) Cast-in-situ Piles.
Driven (Displacement) Precast Piles.
Bored (Replacement) Precast Piles
3. Based on the materials, Piles can further be classified into the followings:
Concrete
Timber
Steel
Composite
Sand
Among these, the most common type of piles used for majority of construction works
in India are bored cast-in-situ concrete piles.
4. BORED CAST-IN-SITU PILE BY DMC METHOD
Direct Mud Circulation method of pile foundation is a type of bored cast-in-situ piles.
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.
Direct Mud Circulation method is best suited:
When working-space is a huge constraint.
In Industries and power plants where there is a network of pipelines snaking all
around.
While the other methods are comparatively expensive.
Where the sub-soil be cohesive in nature.
Diameter of Pile varies from 500 mm to 1000 mm and can dug a depth upto 30 m.
6. Main Shear Leg- Longer than other two leg,
attached with winch. Length- 7.50 m and can be
adjusted with a 20 mm dia pin.
Side Shear Leg- There are two side shear legs,
6.30 m long and they mainly support the tripod
system. The side legs must be placed at least 1.5m
away from the center of the piles.
Typical arrangement of bentonite tank at site.
7. DMC ROD
Length (m) 2.70 3.0 3.25
Diameter (mm) 95 75 65
Thread Length (mm) 120
Thread Dia (mm) Tapered
WINCH MACHINE
Capacity 3 Ton (Other capacities can be of 1 Ton
2.5 Ton 5 Ton 7.5 Ton etc.)
Driven wheel dia (mm) 900
TREMIE PIPE
ID (mm) 125
OD(mm) 150
Thickness (mm) 12
8. PROCEDURE
DMC piling Rig set-up and initiation of boring using the Bailer.
Survey is carried out from main benchmark, and pile points (by verifying co-
ordinates) are marked with iron pegs. Minimum two reference points are kept (at
least 2.00 m away from center of pile) at site to cross-check the center of the Pile.
After the point for boring operation is set out by the surveyor the tripod is paced
such that each side legs are at least 1.50 m away from the center of borehole.
The Bailer is then attached with the rope and the boring operation is initiated.
9. Insertion of the guide casing.
After bailer reaches its full length into the soil, the guide casing (different dia &
different length) is placed on the top of the bore hole and hammered by the bailer to
insert it into the soil. This will be withdrawn after concreting is over.
Boring with Chisel.
Once the guide casing is placed, the boring is started with chisel. The chisel has 4
nozzles from where bentonite slurry jet is forced on the hard soil layer as the boring
progresses.
10.
11. Progression of boring and addition of DMC Rods.
DMC rods are added one after another as the depth increases.
DMC head are attached on top of the DMC rods which has the plunge which
receives the slurry from delivery hose.
The whole system is pulled up by the rope and the D-Shackle with the help of the
winch system and then left to fall free, so that the chisel at the bottom cuts the soil
and the boring progresses.
The loose soil produced by boring, mixes with water and comes up and is stored in
the wash vat, where the soil gets precipitated and the water is again usable.
12. Taking out of the DMC rods and Chisel and Insertion of the reinforcement
cage.
After the boring is complete up to the desired depth, the DMC rods are removed one
by one using the DMC fork and the chisel is also removed.
Prefabricated reinforcement cage is cleaned out of loose material and maintaining
the proper cover throughout the length of the cage. Cage may be made in two or
three parts. Cage lowering to be done from bottom to top (one after another) by
welding the main vertical reinforcement. The cage should be welded in between the
main vertical rod and stiffeners to maintain rigidity
13. Concreting and finalization of the process
The tremie pipes are inserted one by one using the Tremie fork and then the
bore hole is washed with the Bentonite water and the Muck is completely
removed and the hole is filled with bentonite slurry.
The Transit mixer arrives and the Hopper is attached
on top of the tremie pipe.
The TM puts concrete mix into the hopper and the
valve opens. The bore hole is gradually filled with
concrete which is compacted by gradual vibration
of tremie pipes. This filling of concrete from the
bottom is done to avoid segregation of the concrete
mix.
14. Concreting and finalization of the process
As the concrete fills the hole from bottom, Bentonite slurry escapes the hole.
The tremie pipes are detached one by one and thus the hole gets filled
completely.
After the hole is fully filled, the Guide casing shall be lifted immediately and
no pouring shall be allowed thereafter. Concrete is left 28 days to gain full
strength.
15. Drilling Mud (Bentonite) confirming IS: 2911-2010.
The liquid limit of bentonite when tested in accordance with IS 2720 (Part 5) shall be
400 percent or more.
The bentonite suspension shall be made by mixing it with fresh water using a pump for
circulation.
To achieve the requisite specific gravity of the bentonite solution, the concentration of
bentonite per 1 cum of water is calculated as under:
Ys=1+0.006 x Cs
Ys= Desired specific gravity of bentonite., Cs= Concentration of bentonite (by weight
per cum of water).
So, Cs=1/6 x (Ys -1) x1000 =% by weight of 1000 L water.
First flushing: When the earth is being bored, bentonite suspension of specific gravity
between 1.03 to 1.10 is to be maintained.
16. PROCEDURE
The density of bentonite after contamination with deleterious material in the bore
hole may rise and should be brought down to at least 1.12 g/ml by flushing before
concreting.
The marsh viscosity of bentonite suspension when tested by a marsh cone shall be
between 30 to 60 stokes; in special cases it may be allowed upto 90 s.
The pH value of the bentonite suspension shall be between 9 and 11.5.
18. CONCLUSION
DMC piling is the most common piling methods used in India. For moderate sized
piles, that is 450mm to 1000mm diameter and 25m to 30m depth piles; it is also most
convenient and economic way of piling. Larger piles that are of diameter up to
1500mm can also be founded by this method, but those are not that convenient. Other
process like Reduced Mud Circulation (RMC) is also in use.
