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 Level Two (Advanced FAQs) Part I: Testing 1. In conducting triaxial test to determine shear strengths for soil samples, what consolidation pressures should be specified? (TT1) 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. 2. In determining the effective stress parameters of a soil sample, which test is preferable, consolidated undrained test or consolidated drained test? (TT2) 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. 3. Why is shear box test not a better alternative to triaxial test in determining shear strengths of soils? (TT2) 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. 354
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 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. 4. Multistage triaxial test may not be preferable for consolidated drained test. Why? (TT3) 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. 5. What are the limitations in multi-stage test of triaxial tests? (TT3) 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. 6. Why is multiple packer test instead of single packer test sometimes adopted in testing permeability of rock? (PT1) Packer test is used in unlined drillholes in rock to test the permeability. In 355
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A Self-Learning Manual - Institution of Engineers Mauritius

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