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 correlation to other parameters in some methods. For instance, Cone Penetration Test can be employed to estimate the relative density of sands as there is well established relation between cone tip resistance and relative density. On the other hand, shear wave velocity measurements using seismic CPT and Spectral Analysis of Surface Waves can be used to obtain relative density indirectly. 11. Why do compliance test be carried out some time after the completion of vibrocompaction?(R7) A process called “ageing” occurs after the operation of vibrocompaction. It is observed that sand fill shows an increase in strength and stiffness. Upon immediate completion of vibrocompaction, the apparent increase in strength of sand fill is due to dissipation of air and water pressure. Then the ageing process shall continue owing to creep and cementation. Hence, sufficient time shall be established for ageing of sand fill to develop prior to compliance testing (such as CPT test). Otherwise, it may result in non-compliant strength and requires re-compaction. 12. For reclamation by hydraulic filling with sand, it is commonly observed that the density of sand varies significant with depth. Why? In reclamation works, the density of filled sand varies with the method of placement. For example, bottom dumping is adopted for sand as the method of placement and this results in higher density of filled sand. For placement by pipeline discharge, the density of filled sand formed by this method is lower than that by bottom dumping. Hence a weak zone is formed for sands placed by pipeline discharge and hence these loose sand fill (relative density about 30%) may cause settlement when subjected to loading or vibrations. In general, the filled sand above water table is found to be higher in density as they are well compacted by bulldozers and traffic of constructional plant. 13. Why do prefabricated band drains become more popular than sand drains? There is a trend that sand drains are replaced by prefabricated band drains due to: 180
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU (i) The rate of installation of prefabricated band drains is high. As such, substantial cost and time could be saved by using this method of construction. (ii) The disturbance of soil during installation of drains is smaller when using prefabricated band drains. (iii) The ease of fabrication, quality control and storage associated with band drains. 14. What are the potential problems of smear around vertical drains? (R8) Smear zones are generated during the installation of vertical drains in which the zone of soils surrounding the band drains are disturbed. Soils in the smear zones are remoulded during the installation process and the effectiveness of band drains is reduced. For instance, the compressibility of surrounding soils is increased and this brings about the reduction of their permeability. In essence, with the reduced permeability of soils around band drains, it takes longer time to complete the consolidation process. To prevent the formation of smear zones, the raising and lowering of mandrel during drain installation should be minimized. Moreover, soil disturbance can be controlled by avoiding the use of vibratory hammers which serve to drive the drains into the ground [35]. Fig. Smear zones in band drain 15. For drained reclamation, what is the significance of smear zone induced by installation of band drains? (R8) During installation of band drains, smear zones are created in which a 181
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A Self-Learning Manual - Institution of Engineers Mauritius

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