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 becomes effective. 20. Why does flexible wire rope nets effective in stopping rockfall? (SF8) A rockfall barrier has to serve basically the following two purposes: (i) It has to be sufficiently high so that it would not be jumped over by rock boulders. (ii) It has to resist the impact of rockfall without structural failure. For rockfall energy greater than 2,500kJ, it requires the installation of earth dams, underpinning (stabilized) or even complete removal. For rockfall energy less than 2,500kJ, it can be resisted by rigid structures made of concrete or steel. However, the recent trend is that most of these rigid structures are replaced by flexible structures like flexible wire rope nets. For total work on barrier system, it is defined as force times displacement. The merit of flexible system lies in the small forces generated with large displacement when compared with larges forces with little displacement in rigid structures. As a result, the flexible structures can be designed with low impact forces leading to better and economical design. 21. Will the posts of rockfall barrier be damaged when it is hit directly by rock boulders? (SF8) Rockfall barrier is intended to absorb the energy of rockfall safety without the need of future regular maintenance. The net of rockfall barrier is best to be arranged like Olympic Rings with interlacing (called ringnet) and it has proved to be very effective in its energy absorption capacity. The main function of post of rockfall barrier is to provide support to the net. Impacts to posts should not result in the collapse of the structure. In the event of directly impact of rocks with the posts, there is a break devise at the post’s base plate which is designed to allow the post from separating the base plate. In this way, it serves to protect the anchor of base plate from being damaged. 22. What are the potential problems of using clayey backfill in reinforced fill? (S7) Reinforced fill consists of reinforcement embedded in fill with facing. With backfilling material of clayey nature, it may pose problems to potential 298
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU corrosion of steel reinforcement. The low permeability of cohesive fill materials tends increase the duration of contact between reinforcement and water and may cause corrosion problems. Moreover, consideration should be given if cohesive materials could also achieve the required compaction. 23. How does geogrid function in reinforced fill? (S7) Geogrid allows the fill on one side of the grid can key with fill on the other side of the grid. Hence, it is different in its interlocking ability when compared with strip soil reinforcement. The keying-in of both sides of fill could be achieved by compaction and static load above the fill. To trigger shearing across the plane of geogrid, work has to be done to dilate the soils and overcome the frictional forces. 24. What are the effects of pile groups on slope stability? The ground conditions in hillsides of Hong Kong normally consist of colluvium overlying weathered and fresh rock. The groundwater flow in sloping ground may involve both perched and main water flow. As permeability of soil tends to decrease with depth, groundwater flow may take place in the upper aquifer which contains a perched water table. The presence of pile groups below groundwater level hinders the flow of groundwater, leading to an increase on groundwater level. For major development in hillsides with plenty of deep foundations, the effect of rise in water table is sever. The increase in water table owing to damming effects of pile groups decreases the stability of slopes by reduction of shear strength of soils and may eventually cause slope failures. The rise in groundwater levels tend to increase with slope angle and the depth of groundwater flow. 25. How can decomposed dolerite dykes affect slope stability? (S8) Decomposed dolerite dykes contain high clay content display high plasticity. Moreover, the material is generally of low permeability than decomposed granite which is found in many slopes. The presence of decomposed dolerite dykes in a direction parallel to slope surface may cause slope failure after heavy and prolonged rainfall. Owing 299
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A Self-Learning Manual - Institution of Engineers Mauritius

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