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 (iii) For pipes with spigot and socket joints, it enables pipes to be supported along pipe lengths instead of pipe sockets. Otherwise, uneven stress may be induced and it may damage the pipes; (iv) To provide a platform for achieving correct alignment and level during and after construction. 6. What is the purpose of granular bedding for concrete pipes? (ED2) In designing the bedding for concrete drainage pipes, granular materials are normally specified instead of soils containing a wide range of different particle sizes. The main reason of adopting granular material free of fine particles is the ease of compaction as it requires very little tamping effort to achieve a substantial amount of compaction and the crushed aggregates readily move to suitable place around the pipes [67]. However, the use of granular materials has the drawback that a stable support can hardly be provided for the drainage pipes. In particular, it cannot maintain an accurate slope and level for the bedding of concrete pipes. Most pipes are gravity pipes and the accuracy in level is essential to maintain the flow capacity. Fig. Bedding of concrete pipes 7. Should engineers consider embankment condition and trench condition when considering flexible pipes? (DD3) The structural capacity of flexible pipes (e.g. plastics and metals) is derived from ring bending stiffness. Owing to creep or relaxation the ring bending stiffness decreases with time. Flexible pipes are liable to failure by excessive vertical deflections, ring bending strain and buckling. For rigid pipes, the shape of embedment determines the amount of loads on pipes. For trench condition, the side walls of the trench provide frictional 122
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU support to resist the filling material on top of pipes. For embankment condition, the fill materials on either sides of the pipe settle more than the soils on top of the pipe leading to increased loads on the pipe. However, for flexible pipes, it distort in the vertical direction at least as much as the embedment. The friction effects can hardly be developed to increase the loads on the pipes more than the loads on soil on its top. Hence, the shape of embedment is generally not considered when determining the loads on flexible pipes. 8. Proper installation is essential for flexible pipes. Why? (DT1) Flexible pipes are pipes that can deflect at least 2% of the pipe diameter without any damage. When compared with rigid pipes such as precast concrete pipes, flexible pipes are comparatively weak and they count predominantly on the composite action between pipe deflection and backfilled soils to achieve structural stability. On the other hand, rigid pipes rely mainly on their inherent structural strength to carry imposed earth and traffic loads. Hence, improper installation of flexible pipes would compromise structural performance and results in risk failure. The design of flexible pipes is complex as the soil boundary conditions include not only the backfilling trench but also the envelope of original soils outside the trench. It requires detailed assessment of theoretical pipe deflection, long-term bending strain and buckling pressure. 9. What are the functions of different layers in the trench of flexible pipes? (DT1) A typical pipe trench for flexible pipes is divided into the following layers: (i) Final backfill This region has little influence on the performance of pipes. However, as it is close to existing road surface, it highly affects functioning of roads and structures. (ii) Initial backfill This zone provides some assistance in supporting pipe loads. It mainly serves to prevent the flexible pipe from damage upon placement of final backfill. It is beneficial to increase the depth of this region. (iii) Haunching 123
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A Self-Learning Manual - Institution of Engineers Mauritius

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