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 such, the cost of frequent replacement of bentonite slurry is substantially increased. (ii) Clayey materials to clog and cause blockages in slurry system leading to sudden pressure surges. 3. Should closed-end or open-end pipes be used in pipe ramming It is more common to adopt open-end pipes in pipe ramming because it is not readily to undergo surface heaving or pipe deflection when compared with closed-end pipes. Moreover, the use of open-end pipes requires lower ramming force. Closed-end pipes are only used in the following conditions: (i) Ground with poor self-support so that inflow of soils inside the pipes would render ground settlement and loss of support to utility services. (ii) Small size of pipes. 4. What is the difference between “cut-and-cover” method and “cover-and-cut” method in tunnel construction “Cut-and-cover” method involves the construction of open cut at the first place, followed by the construction of tunnels under open excavation. Upon completion of the structure, backfilling and reinstatement would be subsequently carried out. “Cover-and-cut” method involves firstly the construction of cover followed by the second stage in which construction activities are carried out under the cover. As such, the disturbance to the public owing to constructional activities could be reduced to a minimum. 5. Would pipe ramming increase the vertical soil loads on installed pipes Consider a certain cross section along the line of pipe ramming. When the pipe is rammed close to the cross section, the horizontal and vertical soil pressure would increase owing to the effect of soil compaction brought about by dynamic ramming operation. Upon reaching the cross section, soil pressure is redistributed around the pipe such that the vertical pressure above the pipes is reduced while the vertical pressure in pipe abutment 60
A Closer Look at Prevailing Civil Engineering Practice, What, Why and How Vincent T. H. CHU locations is increased. When the pipe is advanced further, the load on pipes tend to increase owing to reorientation of soils around pipe wall. Finally, when the pipe is rammed some distance away from the cross section, a stable state is achieved in which there is smaller earth pressure on the pipe’s top and higher vertical soil pressure on soils at both sides of the pipe. 6. What are the situations which warrant the use of pipe ramming instead of other trenchless methods There are two main distinct advantages of pipe ramming: (i) Settlement and heaving of existing ground. Where a pipe have to be installed under an existing railway or heavily-trafficked highway, it is almost impossible to install the pipes by open excavation. In particular, if the pipes to be installed are of shallow depth, the use of some trenchless methods (e.g. pipe jacking and heading) may cause considerable ground settlement because the soil loss within shallow zone would induce larger settlement. As such, the use of pipe ramming could resolve this concern. Pipe ramming is a displacement method which generally would not result in ground loss. For open-ended steel casing, the soils inside the pipe are not removed until the entire casing is installed in place. (ii) Muck Disposal For microtunneling, spoils are removed from the excavation face in a slurry so that the spoil are wet. Hence, sufficient space has to be provided to allow for drying of spoils or the wet spoils have to be removed off site immediately. For pipe ramming slurry is not used so that the spoil retains only it natural moisture content. Therefore, it is easier to handle in-situ soils than wet spoils. On the other hand, the amount of spoil produced by pipe ramming is smaller when compared with pipe jacking and microtunneling. 7. What is the difference between close mode, open mode and mixed mode of trenchless methods (i) Close mode It refers to mechanically operated TBM using bulkhead or slurry to balance earth pressure and groundwater. There is no manual access to the face of excavation. 61
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