Vincent T. H. CHU

Packer test is used in unlined drillholes in rock to test the permeability. In single packer test, the hole is drilled to the bottom of first test section and the top of the test section is sealed off by a packer. Water is then delivered to the test section and it is kept at constant pressure and the flow is measured.

In highly fractured rock there is a high chance that water tends to leak around the packer which gives inaccurate result. As such, multiple packers are adopted instead in which three sections of the drillhole are sealed up and water is pumped to them at equal pressure. This eliminates the tendency for water to flow around the packers from the middle section.

Hence, a more accurate result could be obtained by measuring flow from the middle section alone.

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.

The maximum load acted on the soil specimen is limited because a highly deformed soil sample is not suitable for further testings. For instance, soft samples like clay display large failure strain and hence it may be not considered acceptable for multi-stage trial axial tests.

Moreover, multi-stage trial axial tests may not be suitable for residual coils whose cohesion is established based on the remaining rock strength mass. At the stage of shearing, part of cohesion may be destroyed and it is irrecoverable in other stages of triaxial tests.

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.

In multistage consolidated drained test, the soil sample is consolidated under all round pressure and then loaded by applying an axial stress. Prior to failure, loading is stopped and the specimen is consolidated under a higher confining pressure. The above steps are repeated for 3 stages to
obtain the failure envelope.

The main problem associated with multistage consolidated drained test lies in the practical difficulty in determining the failure state of the soil sample. Judgment has to be made regarding the condition of “immediately prior to failure” on stress strain curves. It is not uncommon that wrong estimation of the failure state occurs when interpreting the stress strain curves. When there is an underestimation of deviator stress at failure, it would result in overestimation of friction angle and underestimation of cohesion. In case actual failure of soil samples occurs before visual recognition, the sample undergoes overstressing so that the deviator stress at failure in later stages is reduced. As such, this leads to overestimation of cohesion and underestimation of friction angle.

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.

The test procedure is simpler for shear box test. However, it suffers from the demerit that drainage conditions are not easily controlled and pore water pressure cannot be measured. Moreover, the plane of failure is governed by the test itself rather than the properties of soil. It is likely that shear stress distribution across the soil sample is not uniform. The above limitations may affect the accuracy and reliability of test results.

One of the advantages of shear box is that the test could be continued to large strains so that residue shear strength could be determined. In fact, triaxial test has mostly replaced shear box test for normal application.

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.

The effective stress parameters of a soil sample can be obtained from both consolidated undrained test and consolidated drained test. However, consolidated undrained test is normally selected because of the following reasons:

(i) Time taken for consolidated undrained test is shorter than that of consolidated drained test. It is because consolidated drained test requires the full dissipation of excess pore water pressure of the soil during testing and it takes long time when soils of low permeability are tested.

(ii) Useful information can be obtained from the stress path of consolidated undrained test.

(iii) Failure occurs in lower stress level when compared with consolidated drained test.

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.

It appears that the selection of consolidation pressure is independent of in-situ soil stress theoretically. However, this may not be correct because the actual shear strength envelopes for soils are non-linear over a wide range of stresses. Therefore, consolidation pressure corresponding to the range of stresses relevant to site condition should be adopted.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.