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 Sometimes, raking struts are observed in long flying shores across excavation and these struts serve to decrease the span length of struts. Fig. Diagonal strutting and long flying shores in braced excavation 6. What is the purpose of installation of “grout curtain” around excavation? (FE2) When excavation work is carried out in grounds with highly permeable soils, other than the installation of well points to lower down the groundwater table, consideration may be given to the injection of grout to the soils [60]. The purpose of the injection of grout is to fill the pore spaces and cavities of soils with grout and to reduce the permeability of soils. The method of grouting is effective in coarse soils but not for sands. In essence, “grout curtain” is constructed around the excavation by installation of several rows of injection holes for grouting. 7. What is the significance of free length and fixed length in tiebacks in anchored excavation? (FE3) The use of tiebacks in deep excavation allows uninterrupted earth moving within the excavation zone owing to the absence of interior obstructions. The spacing of tieback should not be placed too close as this may impair the capacity of tieback because of the interference between adjacent grouted zones. A tieback is made first by drilling a hole by a drill rig, followed by placing a bar in the drilled hole. Concrete is then poured in the hole and the 304
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU connection of tieback with wall is made lastly. A tieback anchor consists of an anchorage located in a bearing layer and the anchor is tensioned at the front face of the wall. The portion of the anchor which transmits the force to the surrounding soil is called the "fixed length". One the other hand, the "free length" of tieback transfers the force from the fixed length through the anchor head to the wall. Fig. Free length and fixed length in tieback 8. What are the functions of diaphragm walls? (DW1) The functions of diaphragm walls are as follows: (i) It is designed to retain soils during the construction of underground structures. (ii) It helps to control the movement of ground during construction. (iii) It is intended to take up high vertical loads from aboveground structures during construction (e.g. top-down approach). In addition, during the servicing of the completed structures, the diaphragm walls, internal piles and basement raft act together as a single unit to perform as piled raft. 9. Why should excess slurry head be maintained in diaphragm wall? (DW2) For the construction of diaphragm walls adjacent to buildings, previous experience showed that excess slurry head above groundwater level had to be maintained to limit the ground settlements during the construction of diaphragm walls. In fact, the excess slurry head can be achieved by the following methods. The first one is to construct a ring of well points to lower the piezometric level to achieve a higher excess slurry head in diaphragm walls. Alternatively, guide walls may be raised above ground level to accommodate the slurry column. 305
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A Self-Learning Manual - Institution of Engineers Mauritius

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