The assessment of possible deformations in the fault zone

Abstract

One of the most relevant aspects for engineering geology regarding faults is the characterization of the changes induced by a fault and the assessment of possible fault movements. This zone cannot be clearly and well observed in field studies. However, it plays a negative role in the construction and operation of structures. In this zone, unfavorable phenomena, such as rock collapses, displacements, etc., may occur. The purpose of the present study is to estimate the size of fault changes, to reveal the peculiarities of stress distribution within the fault influence zone and to characterize free surface movements. The site of the Rogun HPP was chosen as the object of the study. Rogun HPP is located on Vakhsh River (Tajikistan) in a gorge of 400 - 500 m deep. The site of the dam, which will be about 350 m high, is located in a single tectonic block bounded by sub-parallel Ionakhsh and Gulizindan Regional Faults. The complex of Jurassic-Cretaceous sediments in this block lies monoclinically. According to the results of complex inclinometer-deformographic observations at the Rogun HPP site, the faults are tectonically active, with vertical displacements in 1-3 mm per year. In the scheme of numerical simulation, the width of the Ionakhsh Fault together with the crushing zone was taken as 20 m. For all rocks it was accepted: Young modulus – 3000 MPa; Poisson’s ratio - 0.3; density - 2.6 g/cm3. Horizontal compressive tectonic force was given as 12 MPa. The simulation showed that the dynamic influence zone is limited to 200 m on the side of the lying wing and 70 m on the side of the hanging wing, and the concentration zones are located at the mouth and in the middle part of the fault on the side of the lying wing and between the hanging side of the fault and the bottom of the valley. The analysis of the movements of eight points of the surface showed that the direction of movement corresponds to observed in nature. The total amplitude of vertical displacements of two points at the valley is 0.51 m. The values of the resulting displacements on the different sides of the fault are 0.22 and 0.49 m. According to geological data, the width of tectonic breccia zone of the Ionakhsh Fault is 120 m. However, according to numerical simulation the influence zone is about 270 m. Thus, the simulation showed that the size of real fault influence zone is more than twice as large as that measured in nature.