Drilling mud is thin slurry of clay in water. Special clay called bentonite, characterized by its ‘thixotropic (gelling) property’, is used for this purpose. How such a humble material has all but revolutionized the practice of modern geotechnical engineering is little known to professional geotechnical engineers themselves, not to speak of the lay public.
Stabilizing Action of Drilling Mud
Sides of bore holes (such as for constructing bored piles, and trenches, such as for construction of diaphragm walls), can ‘cave in’ or ‘slough in’ if left unsupported (Fig-1). This problem is particularly acute in the case of dry cohesionless soils such as gravel, sand and silt, whose major shear strength parameter is the angle of internal friction (Φ), with an occasional trace of cohesion (c). The caving in, if allowed, will initiate a chain action resulting in the entire excavation getting filled up with soil from the sides. In the absence of physical supports, like casing pipes used in bore holes, construction is impossible in such bore holes and trenches.
The Mechanics of Stabilization with Thin Bentonite Slurry
If water can stabilize bore holes – which it can – it simply means water pressure (hydrostatic pressure) is higher than earth pressure. This is a truth which even civil engineers, who are not initiated in the comparatively modern discipline of geotechnical engineering, will find it hard to accept, since, in the first look, water which is a liquid is thinner than soil which has a thick solid phase; the difference is, while water has no shear strength, soil has.
Water has a unit weight of 10 kN/m3, which is less than that of soil whose unit weight depends on its state of compaction. Also water has zero values for Φ and c because of which it has no shear strength. Soil, on the other hand, has positive values of Φ and c which impart it shear strength; the higher the values of Φ and c, the greater the shear strength. Ironically this is the very reason why water is able to ‘retain’ and stabilize soil.
Let us now take a look at earth pressure. Earth pressure has two limiting values ‘active’ and ‘passive’. Here we are concerned with the active value, which is the lowest value of earth pressure which a soil can exert on a yielding support which tries to retain it.
Let us now compare water pressure with earth pressure (‘active’ for us) in quantitative terms.
At any depth h (Fig-2), water pressure pw = γw xh, where γw is the unit weight of water. We can therefore state pw = 10 h. At the same depth h, (active) earth pressure, pa = γ x h x Ka, where γ is the unit weight of soil, and Ka, the ‘coefficient of active earth pressure’. The latter decreases with the increasing values of Φ in a cohesionless soil. Assuming typical values such as 15 kN/m3 for γ and 300 for Φ (for which Ka = 1/3), we can write, pa = 15 x h x 1/3 = 5 h (Fig-2).
This shows that water pressure is of the order of twice the active earth pressure, and hence it has great potential in stabilizing the sides of bore holes and trenches, to the extent we can assume that it is the active earth pressure which is not ‘resisted’ in an unsupported bore hole the sides of which therefore cave in.
If the hydrostatic pressure exerted by water is a potential stabilizing agent in soil, the immediate question that comes up is, why add clay to it and make it thin slurry which we call drilling mud. This is because if we simply pour water into a bore hole for the purpose of stabilizing the sides, water will continuously seep into the side soil, needing continuous refilling, which makes it an impractical task as a means of stabilization, on a practical scale.
The Role of Clay in Water
The clay present in water performs an important function which is described below.
In granular soil, water from the slurry slowly escapes into the voids, leaving a thin deposit of clay on the wall of the hole. This thin layer of clay is called filter skin (Fig-3). It is highly impervious and prevents any further loss of slurry by seepage, the avoidance of which is necessary in the interest of economy. So the stabilizing action of drilling mud takes place purely by exerting hydrostatic pressure on the sides of the “sealed” wall – something which water alone cannot achieve. In boring through clay, if the soil is already impermeable, the need for the formation of the filter skin does not arise, even though the stabilizing action is purely one of hydrostatic pressure.
Methods of ‘Circulation’ of Drilling Mud
The drilling mud plays an additional role which is keeping the cuttings from the bore hole in suspension as the boring work progresses. Here comes the ‘gelling’ property of the bentonite slurry into play which keeps the soil cut from the bore hole in suspension even when the slurry is allowed to remain stationary (see Fig-3).
There are two methods by which the cut soil particles are removed from the bore hole through the medium of the drilling mud, and the drilling mud recycled and sent back to the hole. These are
The direct mud circulation method &
The reverse mud circulation method
In the direct method, the slurry is pumped into tubes to which is attached the chisel-shaped boring tool through which it discharges under pressure into the bottom of the bore hole. As a result, the earth loosened due to the repeated lifting and dropping action of the chisel, mixes with the slurry and flushes out from the top of the bore hole where it is collected in the first compartment of the ‘bentonite tank,’ where the bored earth is allowed to settle and the bentonite slurry allowed to flow out into the next compartment, from where it is pumped back into the tubes carrying the chisel (Fig-4).
In the reverse method, the bentonite slurry is poured directly into the bore hole and the same mixed with the bored earth is sucked through the tubes using centrifugal pumps. One may note that while the loosened earth settles in the tank, the bentonite in the slurry does not.