Tie Backs

In this method, no bracing in the form of struts or inclined rakes is provided. Therefore, there is no hindrance to the construction activity to be carried out inside the excavated area. The tie back is a rod or a cable connected to the sheeting or lagging on one side and anchored into the soil or rock out side of the excavation area (Fig-6.6.5). Inclined holes are drilled in to the soil Or Rock, and the tensile reinforcement (tendon) is then inserted and the hole is concreted. An enlargement or bell is usually formed at the end of the hole. Each tie back is generally prestressed before the depth of excavation is increased further to cope with the increased tension.

Fig-6.32 Tie Back

An alternative to use of sheeting and bracing system, which is being increasingly used these days, is the construction of slurry trenches around the area to be excavated. The trench is excavated and is kept filled with a heavy, viscous slurry bentonite of the clay water mixture. The slurry stabilizes the wall of the trenches, and thus the excavation can be done without sheeting and bracing. Concrete is then placed through a tremie. Concrete displaces the slurry. Reinforcement can also be placed before concreting, if required. Generally, the exterior walls are constructed in a slurry trench.