A Closer Look at Prevailing Civil Engineering Practice - European ...

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A Closer Look at Prevailing Civil Engineering Practice, What, Why and How Vincent T. H. CHU carried out some time latter after initial tightening. 6. What are the limitations of CCTV regarding the inspection of sewers CCTV is a well-established and prevalent method for pipe inspection. However, there are several limitations of CCTV technology: (i) It could only provide the view of pipe internal surface above waterline. (ii) It does not provide the view of soil conditions surrounding the pipe. (iii) Most CCTV system is incapable of measuring pipe gradient. (iv) It does not provide any structural data on the integrity of pipe wall. 7. What is the difference between ductile iron pipes and steel pipes in resisting external loads Ductile iron pipes normally possess thicker pipe walls and are generally stiffer than steel pipes. As such, it replies less on side fill soils to support external loads. Hence, it is not necessary to achieve highly-compacted soils for ductile iron pipes for sustaining external loads. For steel pipes, owing to less stiffness associated with thinner pipe walls, it relies heavily on the stiffness of backfill soils in resisting external loads. Hence, to enhance the external load-carrying capacity of steel pipes, the most convenient methods are to improve the quality of backfill materials and to increase the level of soil compaction. 8. What does the pipe thickness of ductile iron pipes generally larger than that of steel pipes Both steel pipes and ductile iron pipes use hoop stress equation to model internal pressure design. The difference in pipe thickness arises as a result of more conservative approach in DI pipes. For ductile iron pipes, surge pressure is considered as part of design pressure and they are added together before applying a safety factor of 2 as follows: F( P + S) D t = 2Y 72
A Closer Look at Prevailing Civil Engineering Practice, What, Why and How Vincent T. H. CHU where t = Pipe thickness F = Factor of Safety of 2 P = Working pressure S = Surge pressure Y = Yield strength of ductile iron For steel pipes the design of working pressure is based on 50% of steel yield strength (i.e. a factor of safety of 2). The presence of surges could be allowed to increase the stress in pipe to 75% of yield strength. The design is based on the following steps: (A) If surge pressure is less than or equal to one-half of working pressure, the pipe shall be designed using working pressure only with 50% yield strength as allowable stress. PD t = where Y = 50% of yield strength 2Y (B) If surge pressure is more than or equal to one-half of working pressure, the pipe shall be designed using working pressure and surge pressure only with 75% yield strength as allowable stress. ( P + S) D t = where Y = 75% of yield strength 2Y For case A, the use of 50% yield strength is essentially the same of adopting a safety factor of 2 in DI pipe design. However, as surge pressure is not considered, the thickness calculated is smaller than that in DI pipe design. For case B, the use of 75% yield strength is essentially the same of adopting a safety factor of 1.33 in DI pipe design. As such, the thickness calculated is smaller than that in DI pipe design. 9. What are the controversial health issues of using asbestos cement pipes for watermain There are suspected health hazard for using asbestos cement pipes for watermain. In drinking water the gastrointestinal tract cancer risk depends on the amount of asbestos swallowed. When asbestos cement pipes are in good condition, there should be little safety problem. However, when the 73
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