During percussion drilling A. Ground water observations are hindered due to entry of the slurry in the soil below the bottom of the hole B. Caving or mixing of strata are caused in soft soils or cohesion-less soils C. The soil to a considerable depth below the bottom of the hole gets disturbed D. All the above

The correct answer is D. All of the above.

During percussion drilling, a rotating bit is used to break up the soil and rock. The broken material is then removed from the hole using a bailer or airlift. This process can cause a number of problems, including:

• Groundwater observations are hindered due to the entry of the slurry in the soil below the bottom of the hole. The slurry is a mixture of water, soil, and rock particles that is used to lubricate the bit and remove the broken material from the hole. The slurry can enter the soil below the bottom of the hole and make it difficult to determine the groundwater level.
• Caving or mixing of strata are caused in soft soils or cohesion-less soils. When the bit breaks up the soil, it can cause the soil to collapse or mix with the underlying strata. This can make it difficult to maintain the stability of the hole and can also create problems for subsequent construction activities.
• The soil to a considerable depth below the bottom of the hole gets disturbed. The percussion drilling process can disturb the soil to a considerable depth below the bottom of the hole. This can cause problems for subsequent construction activities, as the disturbed soil may not be able to support the weight of the structure.

In order to minimize these problems, it is important to use proper drilling techniques and to take appropriate precautions. For example, it is important to use a slurry that is compatible with the soil type and to monitor the groundwater level during drilling. It is also important to take steps to stabilize the hole, such as using casing or grout.