The technique of wash boring involves a simple process of developing a considerable deep borehole into the ground as clearly illustrated in the figure 1.
In wash boring process, a piece of casing with a diameter about 50 to 100 mm and to a depth of 1.5 m to 3.0 m is directly driven into the ground. The casing is provided with a chopping bit fastened on the lower end of a wash pipe that is inserted inside the casing in order to remove the soil inside the casing. At first water is forced through the wash pipe which emerges along the chopping bit through a small opening with a high velocity. Due to the high velocity of water, the soil is cut into fragments and soil water slurry comes up through the annular space between the wash pipes and casing. As shown in figure 1, the T-connection fixed at the top aids the collection of wash water in a sump made nearby. During boring a hole, the wash water thus collected is again pumped through a hose into the wash pipe. As the process continues, a borehole is developed and additional casing pipe and drill rods are added. A swivel head provided between the water hose and water pipe so that the wash pipe and chopping bit can be twisted and pressed down whereby the borehole is advanced further into the soil. But, while we drive the casing into the soil, the swivel head is removed. Following the same process repeatedly, a borehole of required depth is obtained.
This type of method of boring is suitable for all types of soil except gravels and boulders. And, to add more this process is very fast. For clayey soil, casing is not required. Observing the slurry flowing out of the hole, we can predict the type of soil. Similarly, the change of color in the wash water indicates the change in the soil strata.
Samples are taken from the materials settled in the sump. But, the samples thus taken would be highly disturbed and provides only a rough prediction of the soil type. For adequate accuracy of the samples taken, we follow a suitable sampling procedure at any desired depth by replacing the chopping bit with a sampling tube. Generally, this method is used for the preliminary investigation phase of the exploration.