A Self-Learning Manual - Institution of Engineers Mauritius

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.
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