Assessment of stress–strain behavior of shaft lining in bottomhole area during sinking by real-time monitoring and computer modeling data

Abstract

With an increase in the depth of shafting and subsequent risk growth, it becomes necessary to make a maximum accurate estimate of the actual stress–strain behavior of the shaft lining. Real-time monitoring and numerical simulation of the bottomhole zone of a shaft in the process of sinking is proposed as an effective approach to the solution of this problem. The results of an experimental check in three sites of shafts in similar engineering–geological conditions are presented. The real-time monitoring provided control of tensile forces in rock bolts before installation of the main support. Rock bolts 1.8 m long were used with full- and partiallength grouting in the hole. In each test area, 18 strain gauged rock bolts were installed with tensometric sensors glued along the rock bolts. As a result of the research, an array of data on 486 values of tensile forces is obtained. The graphs of the force variation along the rock bolts are plotted at different sizes of its grouting in the borehole. In all experimental sites, the minimum tensional forces in the rock bolts were observed when the bolts were fully grouted in the borehole. When the bolts were partially grouted, the forces were 1.4–1.7 times greater. At the second stage of the research, the 3D finite element models were built for the bottomhole area of vertical shafts in three experimental areas. The calculation took into account the deformation nonlinearity by using stepwise iterative procedures. The comparison of the experimental and numerical simulation results shows the similar qualitative distribution of forces along the rock bolts. According to the quantitative analysis, the maximum deviation of the force values never exceeds 19%. The higher calculated values can be explained by the fact that numerical modeling neglects structural yielding of the rock bolts. The obtained comparative data confirm the correctness of the numerical models. Based on that, the numerical models were then used to find the maximal stresses in the shaft lining, and the structure strength was estimated. As the main method for increasing the bearing capacity of the lining, it is recommended to use concrete of higher strength at depths greater than 700 m.