ABSTRACT

Twin 8.5 m diameter, 10 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 in north-eastern Pakistan using two main beam gripper Tunnel Boring Machines (TBMs). Rockbursts were encountered, and presented a significant danger to personnel and equipment. Rockburst characteristics were compiled on both TBMs to create a database for future reference. Correlation of rockburst characteristics improved the ability to predict the likelihood, location, severity and number of rockburst events. This paper presents an analysis of the rockburst events relative to category, tunnel separation, TBM activity, location of TBM cutterhead within quartile sections of each sandstone bed, radial location and most susceptible sandstone bed thickness. These results permitted the more effective application of counter measures including the drilling of horizontal and vertical relief holes, and the placement of full ring steel supports.