NEW DELHI: Present day's intensity of traffic due to the fast development of industries and other infrastructure is very heavy and so it cannot be disturbed, for construction of underpass bridges by conventional cut and cover method.
So, to construct such structures, box pushing technique is developed where in RCC boxes in segments are cast outside and pushed through the heavy embankment of road by jacking the required thrust bed, as well as line and level of precast boxes are also controlled.
The method of box pushing broadly consists of construction of thrust bed, construction of precast RCC box segments over thrust Bed, front cutting shield, intermediate jacking station and pushing operation of precast box units.
The Thrust bed and keys are designed in RCC to resist the required thrust exorted by jacking force and transfer it to soil at bottom and sides. Provision for jacking supports is made by providing suitable pockets in the bed to accommodate pin supports.
Another method of Box Pushing is construction of precast RCC box segments over thrust bed.
The pre cast box is cast in segments of convenient lengths of total pushing length. The Box section is designed as IRC codes of practice for loading.
The First Segment of precast box is provided with a specially designed structure named as Cutting Shield which forms the front working face with a cutting edge fabricated from MS plates and housed on RCC box section with suitable anchor bolts. Necessary strengthening is provided by stiffeners and face plates of Box in front on completion of jacking the outer shell of the shield is cut and removed.
As total length of box is cast in segments, each segment is pushed turn by turn with necessary jacking force and so for this necessary intermediate jacking station are provided, with jacking pockets in bed and walls.
The purpose of pushing of box is to form a horizontal opening below ground through embankment by providing precast box units underground without disturbing the overhead amenities like traffic and structures.
To reduces friction a thin film of grease and plastic sheet is provided between the thrust bed and bottom of box.
A drag sheet system is also provided for least resistance of friction and distribution of mass above box.
With the progress of jacking, the front unit with shield penetrates into the embankment and there after excavation within the shield is done manually of mechanically. The progress of pushing is kept continuous till total length is pushed.