Theoretical estimation of loosening pressure on tunnels in soft rocks

This paper presents a theoretical method predicting the development and progress of plastic zones around tunnels in soft rocks and determining the loosening pressure which has to be taken by rock bolts or linings. Influence effects of the mechanical properties of soft rocks on the loosening pressure are examined and discussed. The judgment method used to assess the extent of rock failure is presented according to the ground characteristic curve. The ground characteristic curve shows a lowest point that refers to the minimum support pressure and the controlled tunnel wall strain when the competency factor (i.e. ratio of compressive strength to the initial in-situ pressure) is smaller than a critical value, which can be estimated from the proposed theoretical equations. (C) 2001 Elsevier Science Ltd. All rights reserved.