Question

Describe the characteristics of drilling methods that are commonly used for geotechnical assessments on land and in marine environments. In your answer you should describe: a) the drilling process, b) the materials that can be drilled, b) the nature of the sample that is retrieved, c) the typical depth of investigation, d) the relative merits of each technique that you recommend.

Answer 1

Drilling is a technique used for geotechnical assessment, oil extraction and many other fields related to Geotechnology. Below are listed some of the well equipped drilling methods :

Percussion or Cable Drilling

In this technique, a hammer bit is being attached to a long cable that is being losers into an open wide hole. This type of drilling is being preferred for the unconsolidated and consolidated formation such as sand, silt and gravel. This drilling can reach upto depth of 25 meters if done manually.

Rotatory Drilling

This drilling technique is one of the most used techniques in present time Specially for digging up different types of well and preferred near Marines also for crude oil extraction and many other production wells. The Equipment is mounted on a 40m high derrick. It also includes 27 metre long pipe sections which will be used in drilling. The pipe would have rotating capacity with the angular velocity of around 80-240 rpm. These methods are also preferred for the unconsolidated formations by the drag bit and would penetrate the consolidated sediments by the roller bit. The rotation speed is depended on the hardness of the rock or material that is being drilled.

Dual wall Drilling

This is a type of rotatory drilling in which controlled flow is being created by two concentric drill pipes. All the drilling is being done with the help of these two pipes and swiveling is also being done at surface. These have more depth of drilling around 50 meters. This method is suitable and being used for all types of geological formation and good sample recovery will also be provided which is an additional benefit of this drilling method.

The method which would be recommended most among the above ones is the rotatory drilling method due to the following benefits -

• Controls hole pressure
• Penetration rate is high
• Operate more efficiently than others
• Accuracy and stability in the holes drilled