Answer
There are several methods among which six methods are widely used-
1.Displacement borings
2.Wash boring
3.Auger boring
4.Rotary drilling
5.Percussion drilling and
6.Continuous sampling
1. Displacement borings
It is combined method of sampling & boring operation. Closed bottom sampler, slit cup, or piston type is
forced in to the ground up to the desired depth. Then the sampler is detached from soil below it, by rotating
the piston, & finally the piston is released or withdrawn. The sampler is then again forced further down &
sample is taken. After withdrawal of sampler & removal of sample from sampler, the sampler is kept in closed
condition & again used for another depth.
Features :
Simple and economic method if excessive caving does not occur. Therefore not suitable for loose sand.
Major changes of soil character can be detected by means of penetration resistance.
These are 25mm to 75mm holes.
It requires fairly continuous sampling in stiff and dense soil, either to protect the sampler from damage or to
avoid objectionably heavy construction pit.
2. Wash boring:
It is a popular method due to the use of limited equipments. The advantage of this is the use of inexpensive
and easily portable handling and drilling equipments. Here first an open hole is formed on the ground so that
the soil sampling or rock drilling operation can be done below the hole. The hole is advanced by chopping and
twisting action of the light bit. Cutting is done by forced water and water jet under pressure through the rods
operated inside the hole.
In India the “Dheki” operation is used, i.e., a pipe of 5cm diameter is held vertically and filled with water
using horizontal lever arrangement and by the process of suction and application of pressure, soil slurry
comes out of the tube and pipe goes down. This can be done upto a depth of 8m –10m (excluding the depth
of hole already formed beforehand)
Just by noting the change of colour of soil coming out with the change of soil character can be identified by
any experienced person. It gives completely disturbed sample and is not suitable for very soft soil, fine to
medium grained cohesionless soil and in cemented soil.
3. Auger boring
This method is fast and economical, using simple, light, flexible and inexpensive instruments for large to small
holes. It is very suitable for soft to stiff cohesive soils and also can be used to determine ground water table.
Soil removed by this is disturbed but it is better than wash boring, percussion or rotary drilling. It is not
suitable for very hard or cemented soils, very soft soils, as then the flow into the hole can occur and also for
fully saturated cohesionless soil.
4. Rotary drilling
Rotary drilling method of boring is
useful in case of highly resistant
strata. It is related to finding out
the rock strata and also to access
the quality of rocks from cracks,
fissures and joints. It can
conveniently be used in sands and
silts also. Here, the bore holes are
advanced in depth by rotary
percussion method which is similar
to wash boring technique. A heavy
string of the drill rod is used for
choking action. The broken rock or
soil fragments are removed by
circulating water or drilling mud
pumped through the drill rods and
bit up through the bore hole from
which it is collected in a settling
tank for recirculation. If the depth
is small and the soil stable, water
alone can be used. However,
drilling fluids are useful as they
serve to stabilize the bore hole.
Drilling mud is slurry of bentonite
in water. The drilling fluid causes
stabilizing effect to the bore hole
partly due to higher specific gravity
as compared with water and partly
due to formation of mud cake on
the sides of the hole. As the
stabilizing effect is imparted by
these drilling fluids no casing is
required if drilling fluid is used. This
method is suitable for boring holes
of diameter 10cm, or more
preferably 15 to20cm in most of
the rocks. It is uneconomical for
holes less than 10cm diameter.
The depth of various strata can be
detected by inspection of cuttings.