Provision of Reinforced Concrete Lining in Poor Rock Mass - A Case Study of Sawra-Kuddu HEP (111 MW) of HPPCL

The construction of a medium and large Hydro Electric Project in Himalayan region generally involves underground project components and head race tunnel is one of the most important part of water conductor system of an HEP and its length is also significant in most of the cases. Traversing through varying lithological features, the construction of head race tunnel becomes one of the most challenging tasks in execution of Hydroelectric Projects specially in lower & middle Himalayas due to the occurrence of various geological surprises which not only pose risk to men, material and machinery but also need special treatment at critical reaches which require extra time and lead to long gestation period of execution and resultant cost-over-runs as well.

Comprehensive planning in execution of HRT & provision of adequate and safe temporary and permanent support is essential to ensure safety and tackling geological uncertainties and also for long term stability of tunnel. Uncertainties due to varied ground conditions lead to increase in construction period, and cost. Any delay in execution of tunnel with concurrent rock support system directly leads to overall time-over run of the project execution as driving of a Head Race Tunnel of significant length (Sawra Kuddu HEP, HRT- 11365m) with designed rock support system, is most of time on the critical path of execution of a Hydroelectric Project.

After excavation and rock supports provided, another one such activity which requires utmost application of engineering is concrete lining and provision of reinforcement in concrete lining of tunnels because reinforcement in lining ensures safety but its laying is time consuming and increases the unit cycle of concrete lining.

Based on rock mass characteristics designated Q-values the tunnel reaches are categorised good, fair, poor, very poor and extremely poor. Following practice is in general being adopted by various engineers:

P.C.C. is provided in the reaches which involve good and fair rock masses.

R.C.C. is provided in the reaches where poor, very poor and extremely poor rock mass is encountered.

R.C.C. is also provided in special reaches like junction transitions, inlet, outlet and low cover reaches irrespective of rock class.

Provision of P.C.C. in good and fair rock class and R.C.C. in very poor, extremely poor and special reaches as defined above had never been an issue of debate among the engineers. However, the provision of reinforcement in the poor rock class which generally covers a considerable portion of tunnel becomes the issue of debate whether to provide reinforcement or not in poor class as laying of reinforcement requires lot of time.

This paper presents a case study of Sawra-Kuddu HEP (111 MW) of HPPCL wherein the need of provision of reinforced concrete lining in poor rock mass has been analysed.