Question

You are an engineer evaluating a major road construction project in an area with variable cover of unconsolidated sediment and diverse bedrock. Describe drilling techniques that might be suitable for geotechnical investigations of both the cover thickness and bedrock materials in this environment. In your answer you should describe the drilling process, the nature of the samples recovered, and the relative merits of each technique that you recommend. Illustrate your answer with diagrams as appropriate.

Answer 1

There are two methods of carrying out drilling in rocks-

(i) Conventional core drilling . In this method of drilling technique , the entire drill string needs to be pulled out of the bore hole for each drilling cycle.

This means that all the drill rods must be removed from the hole and the threads must be taken apart and rejoined for each core sample. It is a tedious method and less preferred nowadays.

(ii) Wireline core drilling- In this method a part of equipment known as overshot is sent down the hole to release and retrieve the inner tube.

The overshot and inner tube are then brought up to the ground using a wireline hoist, permitting the rods and core bit to remain in the hole.

A new inner tube is then sent down and locked into place.

The wireline method is more common and widely used these days as it is more efficient and safe.

I recommend the wire line core drilling method because it is advanced and efficient.

Procedure-

Three major operations involved are

(i) Drilling into the ground ,

(ii) Retrieving the core sample from the core barrel using either conventional core driliing or wireline core drilling.

(iii) Replacing the equipment with a clean, lubricated inner tube assembly which will allow you to resume the drilling cycle.

These three steps make up a drilling cycle. After each drilling cycle, the bore hole is made deeper by the length of the core barrel.

While the drilling cycle is in process , drilling fluids are pushed down into the drill string. These fluids will be under pressure and will flush past the face of the drill bit as it drills, cooling it and bringing the drill cuttings back up to the surface. When the drill bit starts to wear out, it will no longer cut well. It will need to be replaced immediately and the entire drill string will need to be removed from the bore hole in both conventional and wireline methods.

The core sample

Getting the core is the ultimate goal of this process and there are products that can be added to drilling fluids to make core recovery easier and extend the life of drilling equipment.

For an instance , these products can help solidify the sample, stabilize boreholes, lubricate equipment, obstruct rust and minimize torque.

After performing each drilling cycle, the core sample is stored in a core box and logged with the date and time and specific location of the drilling.

Obtained samples are sent to the laboratory for performing various tests which will help in knowing the property of rock.

We have also three methods of rock drilling as-

(i) In single tube sampler, the core barrel of the sampler rotates and this poses the possibility of disturbing the sample by shearing the sample along particular weak planes. However , the cored samples are subjected to erosion and disturbance by the drilling fluid.

(ii) The double tube samplers, the tube samplers do not rotate with the core barrels and the samplers are not protected against the drilling fluid. The logging operation of samples presents difficulty for highly fractured rock.

(iii) The triple core barrel basically consists of a double core barrel sampler including an addition of a stationary liner which is intended to protect the cored samples during extraction.

Therefore the quality sample obtained from triple core barrel is the best among the three types of barrels .