Many a times, building sites extend to the edges of the property lines or are adjacent to other sites, over which some structures may already be existing. Under these conditions, it is mandatory that the sides of the excavation must be made vertical and should be usually supported. Several methods are available in such occasions. Two common and simple methods are explained below.
If the depth of excavation is less than 4 m, it is common practice to drive vertical planks knowing as sheeting around the boundary of the proposed excavation. The depth of sheeting is kept near to the bottom of the excavation in progress. The sheeting is held in position by means of horizontal beams called wales. These wales are in turn are commonly supported by horizontal struts extending from side to side of the excavation. The struts are usually are of timber for the excavation not more than 1.5 m wide. For wider excavation metal pipes called trench braces are commonly used (Fig. 15.14).
If the excavation is too wide, the wales may be supported by inclined struts known as rakes. Rakes can be used to provide the supporting soil is firm enough to withstand the forces (Fig. 15.15).
Figure 15.14 Sheeting of shallow excavation
Figure 15.15 Shallow bracing
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