Void-induced liner deformation and stress redistribution

• Different tunnels with different void behind the liner are investigated numerically.
• The changes of liner stresses and deformation induced by the voids are studied.
• Liner stresses and deformation will change which may result in liner damage.
• Effect of void size, location, depth on liner stresses is studied.
• Tunnel shapes, lateral stress ratio and rock type are also examined.

Voids behind the liner may be created during or after construction of the tunnel liner because of insufficient backfilling, poor workmanship, water erosion or gravity. Survey on a number of tunnels that suffered collapse while in service showed that voids behind the liner were the main factors for the failure. The paper provides a better understanding of the stress redistribution that occurs in the rock mass around a void and quantifies the stresses induced in the liner due to the voids. The effects of cavities or voids due to different size, location and depth are explored numerically through an elasto-plastic finite element analyses. Other factors such as lining flexibility, in situ stresses and tunnel shape are also investigated. The analyses show that voids can induce a large change of the thrust and bending moment in the liner with respect to the case of a liner without a void. The bending moment can even reverse sign, which may result in cracks in the lining or even failure. The voids also change the distribution of the rock stresses around the tunnel, which may result in progressive failure of the rock and rock falls that can lead to additional damage to the liner. Because the void forms behind the liner, it may not be readily detected until significant damage to the liner has been produced, generally in the form of severe cracking, spalling or even failure.