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Why is shear box test not a better alternative to triaxial test in determining shear strengths of soils?

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.

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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.

In determining the effective stress parameters of a soil sample, which test is preferable, consolidated undrained test or consolidated drained test?

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.

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(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.

In conducting triaxial test to determine shear strengths for soil samples, what consolidation pressures should be specified?

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.

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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.

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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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.

Why is vibrocoring frequently used in marine ground investigation?

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.

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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.