In present days, intensity of traffic — both rail and road — is very heavy and the movement of vehicles cannot be disturbed for construction of under-bridges, canal crossings or drainage systems by the conventional or open cut system.
To address this issue, Noida Authority has started implementing the cut-and-cover method with the help of the box-pushing technique. Similarly, Delhi Metro Rail Corporation (DMRC), while constructing Metro structures, adopted the same technique whereby it avoided large-scale digging. DMRC used Reinforced Cement Concrete (RCC) boxes, pushed with the help of hydraulic jacks, to create a subway tunnel.
Under this technique, RCC boxes are cast outside in segments and pushed through heavy road embankments by jacking the required thrust bed. The line and level of precast boxes are also controlled.
The advantage of box pushing technique is no disruption in traffic movement, whether it be railways or roadways, and better quality control. The technique is also economical, time for completion is less, savings are realised in manpower & machinery and importantly, there is no involvement of cranes and heavy machinery.
The purpose of box pushing is to form a horizontal opening below the ground through the embankment by providing precast box units underground, without disturbing overhead amenities like traffic and various structures.
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 boxes are designed and constructed at the site itself. It takes about 21 days to cast one box.
Another method of box pushing is construction of precast RCC box segments over the thrust bed. The precast box is cast in segments of convenient pushing lengths. The box section is designed as per IRC codes of practice for loading.
The first segment of precast box is provided with a specially designed structure called a ‘cutting shield’ which forms the front working face with a cutting edge fabricated from MS plates and housed on the RCC box section with suitable anchor bolts. Necessary strengthening is provided by stiffeners and face plates of the box in front. On completion of jacking, the outer shell of the shield is cut and removed.
As the total length of a box is cast in segments, each segment is pushed turn by turn with necessary jacking force. For this, the necessary intermediate jacking station is provided, with jacking pockets in bed and walls.
Meanwhile, to reduce friction, a thin film of grease and a plastic sheet is provided between the thrust bed and the bottom of the box. A drag sheet system is also provided for reduced friction and distribution of the mass above the box.
With the progress of jacking, the front unit with the shield penetrates the embankment.
Thereafter, excavation within the shield is done manually or mechanically. The progress of pushing is kept continuous till the total length is pushed.