WHAT IS THE ROLE OF DRILLING MUD IN BOREHOLE STABILIZATION?

Drilling mud is a thin solution of clay in water. special soil is called bentoniteIts characteristicThixotropic (gelling) property‘, is used for this purpose. How such a humble material has revolutionized the practice of modern geotechnical engineering leaves little to professional geotechnical engineers, not to speak of the general public.

fig-1 digging in adjacent soil
fig-1 digging in adjacent soil

The edge of a bore hole (eg for building bumpy piles, and trenches, such as for building diaphragm walls), can ‘cave in’ or ‘slow in’ if left unsupported (Fig. ). This problem is particularly acute in the case of dry cohesive soils such as gravel, sand and silt, whose major shear strength parameter is Internal friction angle (Φ), with an occasional trace of reconciliation (c), If allowed, the caving would initiate a chain action resulting in the entire excavation being filled with soil from the sides. In the absence of physical support such as casing pipes used in bore holes, construction in such bore holes and trenches is impossible.


Mechanics of immobilization with thin bentonite solution

If water can stabilize the bore hole – which it can – it simply means that the pressure of the water (hydrostatic pressure) is greater than the pressure of the Earth. This is a fact that even civil engineers who have not been initiated into the comparatively modern discipline of geotechnical engineering will find it difficult to accept, because at first glance, water, which is a liquid, is comparable to soil. In is thin, in which there is a thick solid. step; The difference is that, while water does not have shear power, soil does.

A unit weight of water is 10 kN/m. It happens3, which is less than that of soil whose unit weight depends on its stage of compaction. Also there are zero values ​​for water I And C Due to which it does not have shearing power. On the other hand, the positive values ​​of soil are I And C which give it shearing power; the higher the value of I And C, the higher the shear strength. Ironically, this is why water is able to ‘retain’ and stabilize the soil.

Figure-2 Active Earth Pressure vs. Water Pressure
Figure-2 Active Earth Pressure vs. Water Pressure

Now let’s take a look at the pressure of the Earth. There are two limiting values ​​of earth pressure’Active‘ And ‘Inactive, Here we are concerned with the active value, which is the lowest value of earth pressure that a soil can exert on a yield support that tries to maintain it.


Let us now compare the pressure of the water to the pressure of the earth (for us ‘active’) in quantitative terms.


at any depth h (fig-2), water pressure Pwoo ,woo xh, where Iwoo The unit weight of water is. so we can tell Pwoo = 10 h, in the same depth h, (active) earth pressure, Pa = xhx ka, where I is the unit weight of the soil, and Ka, ‘Coefficient of active earth pressure’. the latter. decreases with increasing values ​​of I in uncontaminated soil. 15 kN/m . assuming a specific value like3 For I and 300 For I (for whom Ka = 1/3), we can write, Pa = 15 xhx 1/3 = 5 hours (Sketch No. 2).

This suggests that the water pressure is of the order of twice that of the active Earth pressure, and therefore has great potential to stabilize the edges of bore holes and trenches, we can assume that it is the active Earth pressure that causes the ‘resistance’. ‘ is not ‘ in an unsupported borehole whose sides therefore cave in.

If the hydrostatic pressure exerted by water is a potential stabilizing agent in the soil, the immediate question arises as to why add clay to it and make it the dilute solution we call drilling mud. This is because if we pour water into only one bore hole for the purpose of stabilizing the edges, the water will be constant seeping into adjacent soil, requires constant refilling, which makes it an impractical task as a means of stabilization on a practical scale.

role of soil in water

Fig-3 Filter skin formation
Fig-3 Filter skin formation

Soil present in water performs an important function which is described below.

In granular soils, water from the solution slowly drains into the voids, forming a thin deposit of clay on the wall of the hole. This thin layer of soil is called filter skin (Figure-3). It is highly impervious and prevents any further loss of slurry from seepage, which is essential in the interest of the economy. So the stabilizing action of drilling mud is purely “by exerting hydrostatic pressure on the sides of theseal“Wall – something that only water cannot achieve. In boring through soil, if the soil is already impermeable, the formation of a filter skin is not required, even though the stabilization action is purely hydrostatic pressure.” is one of.

Methods of ‘circulation’ of drilling mud

The drilling mud plays an additional role which keeps the cutting in suspension from the bore hole as the boring work progresses. here comes the ‘great harmony‘ The property of bentonite slurry is at play that holds the soil in suspension after being cut through a borehole, even when the solution is allowed to stand still (see Figure 3A).

Fig-4 Direct sludge circulation
Fig-4 Direct sludge circulation

There are two methods by which cut-off particles of clay are removed from the bore hole through drilling mud, and the drilling mud is recycled and sent back into the hole. these

  1. direct sludge circulation method and
  2. Reverse Mud Circulation Method

In straight way, the slurry is pumped into tubes to which a chisel shaped boring tool is attached through which it is discharged under pressure into the bottom of the bore hole. As a result, due to the repeated lifting and dropping action of the chisel, the earth is loosened, mixed with the slurry and discharged over the borehole where it is collected in the first compartment of the chisel.bentonite tank,’ where the soaked soil is allowed to settle and the bentonite solution is allowed to flow into the next compartment, from where it is pumped back into the tubes carrying the chisel (Fig. 4).

In reverse method, bentonite slurry is poured directly into the bore hole and mixed with the roughened soil is sucked through tubes using centrifuge pumps. One can note that when loose soil settles in the tank, the solution does not contain bentonite.

Also read: Quality Requirements Of Bentonite Powder Or Slurry In Piling Work

written by

Dr. Nain P. Kurien

Originally published on NPTEL site

Er. Mukesh Kumar

Photo of author
Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.