Depending upon the type of soil and the purposes of boring, the following methods are used for drilling the holes.
- Auger Boring
- Wash Boring
- Rotary Drilling
- Percussion Drilling
An auger is a drilling device, or drill bit, that usually includes a rotating helical screw blade called a “flighting” to act as a screw conveyor to remove the drilled out material. The rotation of the blade causes the material to move out of the hole being drilled.
Auger Boring consists of a shank with a cross wise handle for turning and having central tapered feed screw. It can be operated manually or mechanically.
Diameter of hand auger is about 15 to 20 cm and are suitable for advancing holes up to a depth of 3 to 6 m in soft soil.Hand auger is attached to the drill rod of 18 mm diameter. The hole is advanced by rotating the cross arm and at the same time applying force in the downward direction. Then same is withdrawn with the help of post hole auger.
These are driven by power and are used for making holes in hard soil up to a depth of 12 m.For depths greater than 12 m, mechanical auger becomes inconvenient then other methods of boring are used.
Auger boring is used in soils which can stay open without casing like as clayey soil.The main disadvantages of auger boring is that the soil samples are highly disturbed and can not be used for lab testing
In this method, rotary drilling consists of a rotary mechanism and an arrangements for applying downward pressure. Bore hole is advanced by rotating a hollow drill rod with a cutting bit attached to its lower end. As the drill rod is rotated, the cutting bit cut the soil particles in the forms of chips. The water with higher pressure is introduced through the drilling rod to the bottom of the hole. This water mixed with soil particles and comes out from annular space between the drilling rod and the walls of the hole.This drilling fluid also cools the drilling bit. During sampling process, the drilling rod is taken out and the drilling bit is replaced by a sampler.
Rotary drilling can be used in clay, sand and rocks but can not used for containing large sized gravels.
This method is used for making hole in hard strata of soil i.e boulders, rocks etc. A heavy chisel is attached with drilling rod is lifted and dropped in a vertical hole. Due to this repetition, the material cut off. If the point where the chisel strikes is above the water table, then water is added to the hole, added water forms slurry and removed by a sand pump at regular intervals of time.
The main advantages of this method is that it can be used in any kind of soil strata ( all types of materials ) and one major disadvantage is that the material at bottom of the hole is get highly disturbed by blows of heavy chisel and by this there is no chance to get high quality of undisturbed soil samples.