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What are the reasons of using compressed air as drilling fluid?

For rotary drilling in ground investigation works, drilling fluid is normally used to clear and clean the cuttings from the drilling bits and transport them to the ground surface. Moreover, it also serves to produce a cooling effect to the drilling bit. In addition, the stability of boreholes can be enhanced and the drilling fluid also produces lubricating effect to the bits.

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Compressed air when used as a drilling fluid possesses several advantages. Firstly, the use of compressed air can reduce the loss of fluid during circulation which is commonly encountered for water being used as drilling fluid. Secondly, the efficiency of air to clean drilling bits is higher than other types of drilling fluids. Thirdly, the moisture condition of in-situ soils would not be affected by air when compared with water as drilling fluid.

In addition, in cold countries the occurrence of freezing of drilling water/mud can be avoided by using air. However, special attention should be taken to avoid breathing the generated dust when compressed air is employed as drilling fluid and dust suppression measures have to be properly implemented.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the function of drilling fluid in rotary drilling in site investigation?

Drilling fluid in rotary serves two main purposes:

(i) Facilitate the rotation of drilling tube during rotary drilling;

(ii) Act as a cooling agent to cool down heat generated during drilling operation.

Traditionally, water is normally employed as drilling fluid. However, it suffers from the following drawbacks:

(i) It affects the stability of nearby ground with the introduction of water into the borehole (borehole for soil; drillhole for rock);

(ii) It affects the quality of sample by changing the water content of soil samples collected from the borehole/drillhole.

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Substitutes are available in market to replace water as drilling fluid (e.g. white foam).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the difference between standpipe, standpipe piezometer and piezometer?

A standpipe normally contains plastic pipes with perforated holes at the base. The annular space between the perforated tube and casing is filled with gravel or sand backfill. Under such an arrangement, standpipe is used to measure water level of a certain region.

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A standpipe piezometer is a type of piezometer which measures pore water pressure at a certain level. It consists of plastic pipes without holes. The tip of the standpipe piezometer is perforated and the annular space between the tip of the piezometer and soil is filled with sand while the annular space between other parts of plastic tube and soil is filled with cement/bentonite grout to seal off water from entering the region of piezometer tip. This enables the pore water pressure in the region of piezometer tip to be measured. In essence, standpipe piezometers are installed to study the pore water pressure of a specified depth below ground. However, it suffers from the disadvantage that the response time is relatively slow in clayey soils. Reference is made to Marius Tremblay (1989).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

How do Piston samplers function?

In sampling clays or silts, Piston sampler is lowered into boreholes and the piston is locked at the bottom of the sampler. This prevents debris from entering the tube prior to sampling. After reaching the sampling depth, the piston is unlocked so that the piston stays on top of the sample going into the tube. Prior to the withdrawal of the sampler, the piston is locked to prevent the downward movement and the vacuum generated during the movement of the piston from the sampler’s end aids in retaining the samples recovered. As such, sample recovery is increased by using Piston samplers.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What are the differences among single tube sampler, double tube sampler and triple tube sampler?

Core barrel samplers are originally designed to sample rock. 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 certain weak planes. Moreover, the cored samples are subjected to erosion and disturbance by the drilling fluid.

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For 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 of samples presents difficulty for highly fractured rock. 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 mentioned above.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.