On the assessment of the effect of the anisotropy in in-situ stress on support pressure in tunnels

This paper presents a newly empirical approach to estimate support pressure in rock tunnels. The new approach takes the rock mass quality (Geological Strength Index: GSI), the disturbance degree, the post-failure behavior, the size of tunnel, the state of in-situ stresses, and the squeezing-prone condition into consideration. A parametric study was, too, carried out using numerical analysis to investigate the influence of anisotropy in field stresses and the effect of the various shapes and sizes of tunnels on failure-height and support pressure in a variety quality of rock masses. Therefore, a correction factor for stress ratio (horizontal to vertical stress ratio: k) is recommended to adjust the support pressure. For this purpose, the rock masses with various ranges of quality were considered to stimulate the very poor, fair, and good quality rock masses whereas arch-shaped and rectangular tunnels were examined in an anisotropic state of field-stress

An empirical method for design of grouted bolts in rock tunnels based on the Geological Strength Index (GSI)

The procedure presented in this paper has been developed for the design of grouted rock bolts in rock tunnels during preliminary design stage. The proposed approach provides a step-by-step procedure to set up a series of practical guidelines for optimum pattern of rock bolting in a variety of rock mass qualities. For this purpose, a new formula for the estimation of the rock load (support pressure) is recommended. Due to its wide-spread acceptance in the field of rock engineering, the Geological Strength Index (GSI) is adopted in support pressure equation. For poor and very poor rock mass where the GSI < 27, the use of Modified-GSI is, instead, recommended. The supporting action is assumed to be provided by rock bolts carrying a total load defined by the rock load height. The mechanism of bolting is assumed to rely on roof arch forming and suspension principle. Integrated with support pressure function, the bolt density parameter is modified in order to provide an optimized bolt pattern for any shape of tunnel. The modified bolt density can also be used in analysis of a reinforced tunnel in terms of Ground Reaction Curve (GRC) in such a way as to evaluate the reinforced rock mass and the tunnel convergence. By doing so, the effectiveness of the bolting pattern is well evaluated. The proposed approach based on GSI is believed to overcome constrains and limitations of existing empirical bolt design methods based on RMR or Q-system, which are doubtful in poor rock mass usage. The applicability of the proposed method is illustrated by the stability analysis and bolt design of a rail-road tunnel in Turkey

A study on truss bolt mechanism in controlling stability of underground excavation and cutter roof failure

The truss bolt reinforcement system has been used in controlling the stability of underground excavations in severe ground conditions and cutter roof failure in layered rocks especially in coal mines. In spite of good application reports, working mechanism of this system is largely unknown and truss bolts are predominantly designed based on past experience and engineering judgement. In this study, the reinforcing effect of the truss bolt system on an underground excavation in layered rock is studied using non-linear finite element analysis. Different indicators are defined to evaluate the reinforcing effects of the truss bolt system. Using these indicators one can evaluate the effects of a reinforcing system on the deformation, loosened area, failure prevention, horizontal movement of the immediate layer, shear crack propagation and cutter roof failure of underground excavations. Effects of truss bolt on these indicators reveal the working mechanism of the truss bolt system. To illustrate the application of these indicators, a comparative study is conducted between three different truss bolt designs. It is shown that the design parameters of truss bolt systems, including tie-rod span, length, and angle of the bolts can have significant effects on the reinforcing capability of the system

Improvement in soft ground tunnelling using an innovative technique

ABSTRACT: The primary stabilization of tunnel face in soft ground tunnelling by means of soil nailing has been found to be an effective and economical method. This paper focuses on some aspects of the performance of two different techniques for the stabilization of the tunnel face and surrounding ground, with reference to a real tunnel project in Southern Italy. The difficult conditions met during tunnelling, due to poor quality of the rock mass and presence of high pore water pressures, required a design solution featuring a preliminary ground improvement and a heavy tunnel support. An innovative technique for ground improvement was applied using special soil nail consisting of a fibreglass bar element and an external sheath devised to contain the injected grout, which can also be integrated with a coaxial drain. The high strength of the nails, evaluated by field pull out tests, and the ability to reduce the pore water pressures ahead of the tunnel face resulted in an effective increase of the stability during excavation