A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 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. 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). 10. What is the function of drilling fluid in rotary drilling in site investigation? (SI7) 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. Substitutes are available in market to replace water as drilling fluid (e.g. white foam). 11. What are the reasons of using compressed air as drilling fluid? (SI8) 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. 352
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 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. 12. Why is vibrocoring frequently used in marine ground investigation? (SI9) If only shallow marine ground geotechnical information is required for design purpose, vibrocoring is inevitably a good choice for sampling disturbed samples. In vibrocoring, a core barrel and an inner liner usually of 100mm diameter and 6m long are vibrated into the seabed. Since the installation of vibrocoring involves the vibration of barrels, there is considerable disturbance of recovered samples. Vibrocoring has the merit of the fast speed of sample recovery (e.g. up to 14 cores can be obtained in one day). Moreover, the cost of vibrocoring operation is low when compared with other viable marine geotechnical investigation options. 13. How can marble cavities and karstic features be detected in ground investigation? Marble is metamorphic rock derived from limestone and is dissolves in slightly acidic water to form cavities (partly filled with debris). It poses great problem for construction of tall buildings which requires the seating of firm foundation. One of the way to identify marble cavities and karstic features is to employ a combination of rotary drilling and micro-gravity method. Micro-gravity method involves the measurement of minute variations in gravitational pull of the Earth and interpretation of the presence of cavities from them. The principle of the technique is to locate areas of contrasting density in the sub-surface. As a cavity represents a lower density when compared with its surrounding soils, the subsequent small reduction in the pull of the Earth’s gravity is observed over the cavity. 353
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A Self-Learning Manual - Institution of Engineers Mauritius

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