Estimation of the Rock Load in Non-squeezing Ground Condition Using the Post Failure Properties of Rock Mass

Estimation of rock load is very important parameter to design the support system because it is a function of many parameters such as stress magnitude, rock mass behavior, excavation method and etc. Several methods are used to estimate this parameter such as experimental, empirical and numerical methods. In this study based on the actual collected data from five tunnels in non squeezing ground condition, a new empirical method is proposed to estimate the rock load with considering the post failure behavior of rock mass and it is estimated using the drop to deformation modulus ratio (was named η). Finally the relation between the rock load and the drop to deformation modulus ratio, η, in non squeezing ground condition is estimated. Based on the statistical analysis, the maximum correlation between both parameters is achieved using of Eqs. 9–11 to estimate the drop modulus. It is cleared that the amplitude of η, is high and to increase the correlation between mentioned parameters, the classification of data is performed in two methods, in the first method, all data is classified in two classes such as \(\eta \le 0.1\) and \(\eta > 0.1\) and in the second method, all data is classified in five classes [according to the proposed classification by Hoek and Brown (1997) and Osgoui and Ünal (2009)] as very weak (GSI < 30, ICR < 25, without filling and η < 0.01) to very good classes (10 ≤ η < 10,000 and 65 ≤ GSI < 90). Also a statistical analysis is performed to estimate the rock load using the mentioned parameter (η) in any class. The result shows that there is an inverse relation between both parameters and the best correlation is achieved using of logarithmic equations to estimate the rock load. Also the correlation of first equations obtained from the first method (including two classes such as \(\eta \le 0.1\) and η > 0.1) is higher than other equations (including five classes) so it is proposed that the mentioned equations are used to estimate the rock load.