ABSTRACT:

A new borehole breakdown model with external in-situ stresses as well as internal grouting pressure is proposed in this study. For this, a bifurcation analysis together with a tensile fracturing model have been employed to simulate the crack propagation and eventual breakdown of the borehole. Numerical examples are also introduced to verify the proposed model on crack propagation and comparison with experimental data on shear and tensile cracking mode are made wherever possible.

INTRODUCTION

A grouting technique which has been widely used to increase the bearing capacity of in-situ soil/rock can be classified into several methods according to the characteristics and properties of the grout. Among others, a permeation or penetration grouting can be used where rock joints are scattered along the civil structures such as tunnels and dams and, in this case, grouting material is penetrated into the rock joints so that the material properties of in-situ rock will be enhanced. In this regards, a new design approach on the permeation grouting has been proposed by authors [1].

Meanwhile, a fracturing grouting can be considered where in-situ soil/rock properties are weathered and cracks due to high injection pressure are expected. In this case, artificially generated cracks are filled with grouting material which is composed of cement and additional admixture. However, fracturing process and design approach on the fracturing grouting have not yet been properly established. In this study, the fracturing mechanism of the borehole (grouting hole) due to high injection pressure is investigated to estimate the fracturing pressure and, using this, reinforcement design of the grout can be performed. Theory of the hydraulic fracturing or borehole breakout can be directly used to the analysis and design of the fracturing grouting. As is well known, the hydraulic fracturing was introduced to estimate the in-situ stresses and several theories on the hydraulic fracturing have been suggested and new field of applications have also been proposed

[2-4].

In this study, two modes of cracking process, i.e. a shear and a tensile crack, are investigated using a bifurcation theory and a tensile strength model, respectively. Also considered here is the combined cracking mode in which an initial shear crack is subsequently changed into the tensile crack mode by external stresses. However, a compresslye crack model which has been reported in [5] is not considered here. Currently, the fracturing mode is limited to the newly generated cracks and no interaction with the preexisting joints or cracks is accounted for. Future work will have to include the interaction mechanism between joints or cracks and, from this, the grouting effect will be considered accordingly.

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