The purpose of pushing of Box is to form a horizontal opening below ground or through embankments, can say a tunnel, by providing precast box units underground or an embankment, without disturbing the overhead amenities, like traffic & structures.
To reduce the frictional resistance, a thin film of Grease and thick gauge plastic sheet is provided between the top of the thrustbed & Bottom of the Box. This is done before casting of the Bottom slab of the box. A Glossy Epoxy layer is also provided to reduce friction on the top and prevent the disturbance of the mass over box to least during progress of pushing
Where mass of embankment is less or the soil is of poor quality, drag sheet system is also considered for least resistance of friction and disturbance of mass above box.
Auxiliary beds are also provided for casting of the other segments with thin film of grease and plastic and segments are then brought in alignment of pushing as and when required as pushing progresses.
With the progress of jacking, the front unit with shield penetrates into the embankment and there after excavation within the shield is done either manually or mechanically and the excavated stuff is transported outside the working area.
After the first segment advances to the end of the thrustbed, the second segment is brought in alignment of the front Box & intermediate jacking space is lift between the two. The progress of pushing is kept continuous and the system of side shifting of remaining box segments from Auxiliary bed and bring in pushing alignment is adopted till total length is pushed. |