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  Cross passage construction in soft soil by RCC Box pushing in MRTS

  After construction of two 1.67 Km long and 5.9 m Internal diameter twin bored tunnels, to connect Nizammudin to Ashram station in the phase-III expansion plan of Metro network, in New Delhi, five Cross Passages needs to be excavated to connect both tunnels as per NFPA guidelines. The ground consists of alluvium, associated with the Yamuna River, belonging to the Pleistocene, a very fine silty sandy soil with a water table almost at the surface level. After removing one segment from the main tunnel, to start the excavation of the first Cross Passage, very soft ground was found along with high seepage and local instabilities on the tunnel face, making the excavation very difficult with conventional methods. An alternative approach for the execution of the Cross Passages was designed, using Box Pushing method, jacking in the ground precast RCC boxes with a steel reaction frame taking thrust from the main tunnel. The paper describes the above mentioned method in detail, incorporating the challenges associated during the design and construction, and the instrumentation and monitoring scheme in the main tunnel; especially in the rings around the thrust points and reaction frame.

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