Rapidly growing population and proportionate increase in vehicular traffic in metropolitan cities have left little or no space for further expansion of infrastructure on the ground surface. Flyovers and underpasses are inevitable, and these need to be constructed without major disruptions to the live traffic. Sometimes in thickly populated areas, road diversion, safety consideration and other site constraints arise during the construction of flyovers. In such a situation, the construction of underpasses below the existing highways or railways become the best and viable option.
The construction of an underpass is generally taken up by the cut and cover method, which usually require extra soil cutting. Sometimes, the removal of soil may trigger caving in, and the surface collapses. The removal of social can also be a sudden collapse and can be harmful to nearby structures. The cut and cover method of the construction process also takes many months.
In the earlier days, the underpass constructions were generally not done due to soil instability and other site constraints. But, in recent times, the box jacking technique with ground improvements is a non-disruptive technique for underpass construction without disturbing the live traffic. The technique is also inherently safe, simple, economical, requiring less equipment and enabling speedy construction.
Box jacking involves the fabrication of a series of large size of precast RCC box segments. The box segments are pushed with the help of jacking force applied through hydraulic jacks installed at the rear end of the segment. The jacking reaction is obtained from the pocket provided in the thrust bed and rear wall.
In the last two decades, the construction of an underpass by jack pushing without disturbing the existing structures (road/rail track etc.,) has become popular. To date, the technique has been used to create underground space primarily for rails and roads, car parking, water pipelines, drainages, and pedestrian accesses, etc. This is usually done after the implementation of some site-specific ground improvement techniques/measures.
Whenever a precast box tunnel is pushed to create an underpass beneath any existing structure, the overburdened soil (OB) experiences a considerable increase in the pressure on either side of walls and above the box face. The excessive increase in pressure may lead to differential settlements (upheave or settlement) or collapses. Similarly, the redistribution of stresses at the box tunnel face also occurs due to soil particles' lateral movement. Sometimes, the magnitude of the stresses and strains at the box tunnel face crosses its allowable limits of the soil results in instability on its box face or the differential settlement experiences at the ground surface.
The challenging site conditions demand a novel ground improvement technique, which could have stabilised the overburden and box/tunnel face in controlled conditions during box jacking operation and regular running rail traffic. In many box jacking projects, ground improvement techniques like; ground freezing, grouting, and chemical stabilisation were used to stabilise soil to reduce the settlements or displacement of soil during box jacking. Sometimes, these techniques may not be feasible due to environmental or site conditions besides being time-consuming.
It is essential to stabilise the soil at the box face during the box pushing operation. Many a time, it has been observed that the soil slope in front of the box suddenly collapses, and workmen get buried during box pushing operation. The project becomes more challenging and risky when such underpasses have to be constructed through cohesionless soil such as sandy soil.
CSIR-CRRI invented a technique for reinforcing or strengthening the soil at the box face during box jacking operations. Soil nailing is one of the ground improvement methods, which has been widely used for controlling the displacements and protection of natural and man-made slopes. CSIR-CRRI' Soil Nailing with Box jacking' method has been patented in India and abroad, including the US, Great Britain, Singapore, Sri Lanka, and India (https://patents.google.com/patent/WO2014013508A2/en).
This technique was successfully used for the first time in the world to construct three adjacent large-size underpasses below double rail tracks on the Old Delhi-Shahdara section near Salimgarh Fort, Yamuna Bazaar Delhi, India. After completion of Yamuna Bazaar rail underpass, many more projects have been completed or are underway, including Apsara Border (Delhi), Sahibabad (Ghaziabad), Pragati Maidan (Delhi), Gurgaon, Jhalawar, Rajasthan (ongoing), Kota, Rajasthan (ongoing) and Road underpass at Mahipal Pur, New Delhi.
The team of CRRI scientists, namely Dr Kanwar Singh, Sudhir Mathur and Dr P.S Prasad, was awarded the Supreme Engineer's Award, 2012 by Economic Research of India, Mumbai, for the Yamuna bazaar project.