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  Hard Rock TBM v . Soft Ground Fault - A case study in the Himalayas

  Twin 8.5 m diameter, 10.5 km long parallel headrace tunnels under overburdens of up to 1 870 m with high horizontal stresses were excavated for the Neelum Jhelum Hydroelectric Project located in the Azad Kashmir region of northeast Pakistan using two main beam gripper Tunnel Boring Machines (TBMs). The TBMs were introduced to improve tunnel excavation rates and were the only realistic methodology for accelerating progress on the portion of the tunnel alignment where additional adit or shaft access locations were difficult or uneconomical to implement due to high overburden and rugged, inhospitable terrain. The tunnel construction experienced two significant challenges; a major soft ground fault and over 1700 rock bursts. This paper deals with the soft ground fault, which was encountered after approximately 1 550 m of tunnel excavation and required extensive remedial work in the tunnel excavated by the lead TBM. Lessons learned from this incident allowed modifications to TBM operations for the trailing TBM, significantly reducing delays. This paper describes the fault zone, the techniques employed to cross it and introduces the concept of Computed Boring Pressure.

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