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. |
