NUMERICAL ANALYSIS OF FAULT BEHAVIOR NEAR UNDERGROUND EXCAVATIONS

Abstract

In this paper, the behavior of a fault is studied using a constant strain joint model. Acombination of slider and spring is used to simulate the shear behavior of faults in the plasticregion in contrast to the previous models that have only used an elastic shear spring. Furthermore,the proposed joint element was used to study the behavior of a fault crossing a tunnel regarding therepresented shear plastic and dilation behavior of this joint element, and for this purpose a Matlabbased program called FEAFB (Finite element analysis of fault behavior) has been developed. Thecorresponding normal and shear stresses, shear strengths and the factors of safety, for differenthorizontal to vertical stress ratios and shear stiffness are analyzed and compared to the results of asimilar modeling in UDEC program. The analysis indicates that the normal and shear stresses, andthe shear strength are increased in the fault elements near the tunnel, and they are decreased in theelements becoming far from the tunnel surface. However, the safety factor can either increase ordecrease as it becomes closer to the tunnel surface, depending on the horizontal to vertical stressratio. Moreover, it is also shown that safety factor depends upon the shear stiffness, i.e., as shearstiffness increases, shear stress increases, and as a result, the safety factor decreases.