Development of a new hybrid ANN for solving a geotechnical problem related to tunnel boring machine performance

Engineering with Computers - Prediction of tunnel boring machine (TBM) performance parameters can be caused to reduce the risks associated with tunneling projects. This study is aimed to introduce...     

A theoretical solution for analysis of tunnels below groundwater considering the hydraulic–mechanical coupling

An analytical solution for the analysis of tunnels below groundwater table in plane strain axisymmetric condition is presented. Seepage body force and secondary permeability of the rock mass due to the mechanical–hydraulic coupling are taken into account. The strain-softening behavior model and Hoek–Brown empirical strength criterion for the rock mass are used in the analysis. As the derived analytical equations do not have closed form solutions, a computer program has been prepared for solving the corresponding equations numerically and examining the analysis. It is shown that the tunnel stability depends on the seepage and the pore water pressure particularly in the case of high pore pressure gradient.     

Experimental and numerical analysis of indirect and direct tensile strength using fracture mechanics concepts

Brazilian tests were conducted on an isotropic limestone with different length-to-diameter ratios and various bearing strip widths. The direct tensile strength of the rock specimens was also obtained using direct tension test apparatus and a servo-control testing machine. The Brazilian test was modeled using the computer program (Abaqus 6.7-1) and both smeared rotating crack and cohesive crack models were selected for the analysis of crack propagation. Comparison of the experimental and numerical analyses showed that the results from the smeared rotating crack model were closer to the experimental results than those from the cohesive crack model. Appropriate testing conditions for the Brazilian test are proposed, in order to achieve the results closest to the direct tension test.     

A theoretical approach for analysis of the interaction between grouted rockbolts and rock masses

Rock-support interaction concepts, which are applied to support systems such as concrete linings and ungrouted rockbolts, cannot be applied to grouted rockbolts, because of the nature of the structural interaction of the rock mass and grouted rockbolts. In this paper, with consideration of a non-linear strength criterion for rock mass and on the basis of two material behavior models and rock-support interaction concepts, corresponding relationships are derived. In this method, the equation of the ground response curve which has been reinforced with grouted rockbolts is also derived for a circular underground excavation under hydrostatic stress field. Because of algebraic complexity, it is not possible to obtain closed-form solutions; therefore a computer program was prepared for solving the problem on the basis of numerical methods.     

Effect of time on the stress–strain behaviour of a single rock joint

The time-dependent behaviour of discontinuities is important in many geotechnical applications. To investigate the stress–strain behaviour of jointed specimens tested at various rates of strain and at constant deformation in various stress conditions, three cases were examined: (a) increasing compressive load at different constant strain rates, (b) increasing compressive load at changing strain rates, (c) stress variations with a constant amount of displacement along the complete stress–strain curves for 5 min (stress relaxation). Triaxial tests were carried out on samples of the Penrith and Stainton Sandstones using a 5 MN servo-controlled stiff testing machine and confining pressures in the range of 0–30 MPa. Both intact and jointed specimens with saw cut and split joints were tested. The joints were produced with orientations of 30, 45 and 60° to the direction of the principal stress. Three different strain rates were applied.      

Analytical approach for the design of active grouted rockbolts in tunnel stability based on convergence-confinement method

Assuming that grouted rockbolts increase internal pressure within a broken rock mass, a new procedure for computation of ground response curves for a tunnel reinforced with active grouted rockbolts is presented, while the effect of distance of bolted section to tunnel face has been also considered. This analytical solution for a circular underground excavation under hydrostatic stress field, and with consideration of a non-linear strength criterion for rock mass and on the basis of two material behavior models has been developed. In this work, the equation of the ground response curve for a tunnel which has been reinforced with passive grouted rockbolts is also derived. The proposed model allows one to take, the effect of the distance of the bolted section to the tunnel face, the effect of increasing rockbolts spacing, the influence of increasing pretension load in calculating of the ground response curve, and the effect of increasing the cross-section area of rockbolts, into account. The results show that decreasing rockbolts spacing increase the support system stiffness rather than preloading of them.     

Analytical solution for the excavation of circular tunnels in a visco-elastic Burger’s material under hydrostatic stress field

By excavating an underground space, the state of stress and displacement are changed in comparison to the initial state. This variation depends on the advance of the tunnel and on the rheological behavior of the hosting rock mass. In this paper, the effect of creep, in the response of the tunnel is discussed. The objective of the paper is to predict time–dependent displacement of the tunnel wall, after stopping the excavation or after installing the support system. The rock mass is assumed to be isotropic and homogenous and incompressible. The tunnel is assumed to be circular and driven in a hydrostatic stress field. The rate of excavation is considered to be infinitely large. The Burger’s body which is able to model the primary and secondary creep regions of the rock mass is applied. In such a condition, an analytical solution for predicting time-dependent deformation of tunnel wall is derived. Thereafter, the application of the proposed solution is illustrated through three examples. The examples are analyzed numerically using the finite difference code FLAC. The results obtained from numerical analyses show proper agreement to the results of proposed solution.     

Intelligent modelling of sandstone deformation behaviour using fuzzy logic and neural network systems

Bulletin of Engineering Geology and the Environment - A realistic analysis of rock deformation in response to any change in stresses is heavily dependent on the reliable determination of the rock...     

Uniaxial compressive strength prediction through a new technique based on gene expression programming

Proper estimation of rock strength is a critical task for evaluation and design of some geotechnical applications such as tunneling and excavation. Uniaxial compressive strength (UCS) test can be measured directly in the laboratory; nevertheless, the direct UCS determination is time-consuming and expensive. In this study, feasibility of gene expression programming (GEP) model in indirect determination of UCS values of sandstone rock samples is examined. In this regard, several laboratory tests including Brazilian test, density test, slake durability test and UCS test were conducted on 47 samples of sandstone which were collected from the Dengkil, Malaysia. Considering multiple inputs, several GEP models were constructed to estimate UCS of the rock and finally, the best GEP model was selected. In order to indicate capability of the proposed GEP model, linear multiple regression (LMR) was also performed. It was found that the GEP model is superior to LMR one in terms of applied performance indices. Based on coefficient of determination (R ²) of testing datasets, by proposing GEP model, it can be improved from 0.930 (which was obtained by LMR model) to 0.965. As a result, it is concluded that the proposed models in this study, could be utilized to estimate UCS of similar rock type in practice.