NUMERICAL ANALYSIS OF FAULT BEHAVIOR NEAR UNDERGROUND EXCAVATIONS

In this paper, the behavior of a fault is studied using a constant strain joint model. A
combination of slider and spring is used to simulate the shear behavior of faults in the plastic
region 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 the
represented shear plastic and dilation behavior of this joint element, and for this purpose a Matlab
based program called FEAFB (Finite element analysis of fault behavior) has been developed. The
corresponding normal and shear stresses, shear strengths and the factors of safety, for different
horizontal to vertical stress ratios and shear stiffness are analyzed and compared to the results of a
similar modeling in UDEC program. The analysis indicates that the normal and shear stresses, and
the shear strength are increased in the fault elements near the tunnel, and they are decreased in the
elements becoming far from the tunnel surface. However, the safety factor can either increase or
decrease as it becomes closer to the tunnel surface, depending on the horizontal to vertical stress
ratio. Moreover, it is also shown that safety factor depends upon the shear stiffness, i.e., as shear
stiffness increases, shear stress increases, and as a result, the safety factor decreases.