Evaluation on the Stress Induced Brittle Failure along the Headrace Tunnels of Neelum Jhelum HPP, Pakistan

Abstract

The Neelum Jhelum Hydropower Project is currently under construction in northeastern Pakistan. When completed, the power plant will have a maximum capacity of 969 MW, producing renewable energy to the growing population of the region. Two 11 km headrace tunnels are excavated by TBM machines through alternating beds of sandstone, siltstone, mudstone and shale. Due to high in-situ stress and the presence of strong and brittle rock, high magnitude rockbursts have caused considerable damage during tunnel excavation. The focus of this thesis is on the brittle failure and related geological conditions in the twin headrace tunnels of Neelum Jhelum HPP.

A statistical analysis is carried out to investigate the relationship between geological conditions and brittle failure intensity. No significant correlation is found between rockburst intensity and local overburden. The results indicate that other geological factors such as tectonic activity and rock anisotropy influence stress distribution and rockburst occurrence around the tunnels. Approximately 68% of the rockbursts have occurred in sandstone, the strongest rock unit in the formation. Additionally, the analysis points to a possible connection between stress anisotropy, rockburst intensity and faults and sheared zones crossing the tunnel alignment.

A brittle failure analysis is carried out with semi-analytical and numerical approaches. The brittle failure potential is assessed with approaches from Diederichs (2007) and the NGI Q-system. The depth of brittle failure is predicted with the Martin and Christiansson (2009) semi-empirical relationship. The obtained results are compared with in-field observations to assess the applicability of the approaches to the existing geological conditions. The results from Diederichs (2007) and the Q-system approaches are in reasonable agreement with observations. The Martin and Christiansson (2009) approach was found to overpredict the depth of brittle failure in the tunnels.

The numerical analysis methods include an elastic model with the Hoek-Brown brittle parameters (HBBP), and a non-linear model with the Damage Initiation and Spalling Limit (DISL) approach. The elastic model was found to produce unrealistic results for high stress to strength ratios. The non-linear DISL model produce results that correspond well to rockburst damage observed in the tunnels.

The varying geological conditions in the Murree formation makes it difficult to assess the rockburst potential for the area as a whole. Hence, the validated methods should be used in combination with practical judgement and local geological knowledge to be of value in the remaining stages of tunnel excavation.