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  Construction of Road Tunnel Connecting AIIMS Hospital to JPN Apex Trauma Centre above Operational Metro Tunnels

  The work of constructing the AIIMS road tunnel over DMRC line-2 tunnels was very challenging, as this was the first time in India that any road tunnel was constructed above the operational metro tunnels with a minimum clearance of 1.6 m only. The two major challenges during the execution of work were:1. To ensure that the operational metro tunnels must not puncture during the construction of D-wall (for road tunnel) with D wall rig.2. To ensure non-ovalisation of DMRC line-2 tunnels due to release of over burden during construction phase within limits.Before the start of execution of work above operational line-2 metro tunnels, as built alignment was rechecked. After carrying the survey, it was found that the centre line of metro tunnels has a deviation of approx. 1.5 m from the available drawings. Hence, the D-wall panels of the road tunnel were shifted accordingly to adjust the central D-wall, which was to be constructed in between the two metro tunnels. Also, to ensure no damage to metro tunnels during construction of D-wall, sounding was carried at every 1.5 m and visual inspection of the segments of tunnels was carried during the whole process.To ensure non-ovalisation, construction was carried exactly as per the design. The design was made in such a way that overburden pressure above metro tunnels remains in limit during the excavation for base slab. At a time, excavation was limited to 3 m in length only. After casting of base slab of the excavated portion, further excavation was carried. This ensured no movement in operational metro tunnels. Also, 24 hour real time instrumentation monitoring was carried by DMRC in the operational metro lines. During the entire construction period, a vigilant watch was kept on the behaviour of the operational metro tunnels with the help of sophisticated instruments like Bi-reflex targets, Strain Gauges, Beam Sensors and liquid level settlement system placed inside the metro tunnels. The construction work above the metro tunnels was majorly carried in the night time when metro was not working.

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