The city of Jaipur, India is encircled by a wall six meters high and three meters thick, with seven gated openings. It is below these delicate and iconic structures that Jaipur’s first Metro, Line 1, travels. For this project, Contractor Continental Engineering Corporation (CEC) decided to refurbish their two 6.52 m (21.3 ft) diameter Robbins EPBs. The machines bored twin tunnels 2.3 km (1.4 mi) in length, directly beneath the historic Chandpole Gate.
In this particular project, the geology was not anticipated as a main concern. The bores consist mainly of silty sands, with a minor amount of clay and gravels. However, the geological conditions coupled with the extremely low overburden in the area of the launch shaft, especially the section passing beneath Chandpole gate, was cause for major concern.
The contractor opted to refurbish its two 6.52 m (21.3 ft) diameter Robbins EPBs originally used for the New Delhi Metro Project. The Robbins EPBs were refurbished in India, and customized for the Jaipur project. The original machines for New Delhi bored a straight tunnel and did not require active articulation, but a 430 m (1,410 ft) radius curve in Jaipur necessitated that the machines be articulated. The shields were essentially cut in half and another section put in to form articulation joints in the contractor’s casting yard. In addition, new a+b grouting systems were installed as well as sophisticated tunnel guidance systems to monitor each machine’s position.
Chandpole gate, one of the seven, is directly above the bore path for the Line 1 extension, and serves as a historical landmark. The construction materials of the walls and gate consist of irregularly-sized pieces of stone cemented together with lime mortar and faced with a sand and lime mortar render that provide little to no resistance to tunneling-induced settlement. Contractually the allowable limit for surface settlement was set at 4 mm (.16 in). Yet, there was also an Indian archeological law in place that made the stakes a bit higher. It states, “Whoever destroys, injures, mutilates, defaces, alters, removes, disperses, misuses, imperils or allows to fall into decay a protected monument, or removes from a protected monument any sculpture, carving image, bas-relief, inscription or other like object, shall be punishable with imprisonment for a term which may extend to six months with a fine which may extend to five thousand rupees or with both.”
The key to boring beneath the gate without any adverse effects relied on refining the TBM operating parameters before the TBM reached the zone of influence. As this had been achieved, these parameters were maintained after the restart and similar results were achieved up until ring No. 75, where surface heave increased slightly. Due to the increase in heave the EPB pressure was reduced to 1.2 bar and cutterhead speed to 1.1 RPM. These changes reduced the heave to within tolerance. Although vibrations levels were minimal, as the TBM approached the gate the cutterhead speed was further reduced to 1.0 RPM to reduce the risk of damage by vibration. The machine passed beneath the gate and through the zone of influence without incident using these parameters. The maximum recorded settlement in the vicinity of the gate was 2 mm (.08 in) and absolutely no adverse effects were sustained to the gate. The lessons learned on the first drive were applied to the second drive and TBM II also passed beneath the gate with minimal settlement and no damage to the gate.