ABSTRACT:

On the basis of linear fracture mechanics, a tunnel excavation method by combination of static demolisher with slit cut drilling was designed by giving due consideration to the effect of in-situ stresses. Several types of slit patterns were studied for their effectiveness on the strength of numerical analysis, laboratory experiments and a few experimental excavation in in-situ rock mass in order to propose the appropriate design plan for tunneling in underground formations.

RESUME:

Une methode d'excavation de tunnel utilisant un demolisseur statique et une haveuse/carotteuse à fente a ete conçue sur la base de la mecanique des fractures lineaires, en tenant compte de l'effet des contraintes existant sur le site. Plusieurs types de fentes ayant des formes differentes furent etudies pour analyser leur efficacite sur la force d'une analyse numerique, d'experiences en laboratoire et de quelques excavations experimentales dans la masse rocheuse du site afin de proposer un plan de conception appropriee pour percer des tunnels dans des formations souterraines.

ZUSAMMENFASSUNG:

Auf der Basis der linearen bruchmechanik wurde eine Methode zur Stollenausschachtung Konzipiert, die eins Kombination von Statikzerstör und Schlitzbohrung verwendet und am Ort auftretende Druckauswirkungen in Betracht zieht. Verschiedene Arten von Schlitzmustern wurden durch numerische Analysen, Laborversuche und einige experimentelle Ausschachtungen in Gesteinsmassen vor Ort auf ihre Wirksamkeit untersucht, um einen angemessenen Konstruktionsplan zum Stollenbau in Untergrundformationen zu entwickeln.

1 INTRODUCTION

When adopting the conventional drill and blast method for tunnel excavation, it would be practically impossible to completely protect the surrounding rock from damage induced by the shock generated during the detonation of explosives even though the smooth blasting technique may be adopted. The excavation method described herein uses the so-called static demolisher which is a kind of expansive cement instead of explosives. After the hole drilled in the rock mass is charged with the static demolisher in slurry state, the hole wall will be gradually pressurized while the slurry commences to harden and expand. In consequence, fractures initiate from the hole wall and they grow in a rather stable manner. Therefore this method incorporates a special characteristic of being able to induce fractures in the rock mass without vibration and to complete a tunnel having section of a prescribed dimension, free from any damage to the tunnel wall. The attainable maximum pressure due to the expansion of this material is as high as 50 to 70 Mpa under favorable conditions. However, this value may be insufficient to effectively induce fractures into the rock mass which is loaded by the in-situ stresses and is located ahead of the tunnel with little available free surfaces area. Therefore, some suxiliary aids are required. In this paper a method of tunnel excavation by using static demolisher in combination with slit cut drilling is introduced with some theoretical considerations and both laboratory and field experimental results.

2 FRACTURING MECHANISM OF ROCK MASS BY USING STATIC DEMOLISHER

Pressure is gradually generating the static demolisher into the expansion of the material recrystallization. It should be after injection of hole accompanying due to its noticed that this pressure is completely retained within the borehole in the rock since the material of solid state can not intrude into the emanated cracks. This is disadvantageous from view-point of effective fracturing since the intense fracture propagation could only be realized under the condition that the pressure of substantial magnitude acts along the cracks. On the other hand, this brings easiness of precise prediction on the crack behaviors since the boundary condition of simplified nature is prescribed everywhere along the boundary. A computing scheme which can numerically predict, based on the linear fracture mechanics, the crack behaviors developing from the charge holes of prescribed number and spatial distribution have been developed by using the body force method (Nishitani 1978). In this scheme, the effect of rock pressure can be also taken into consideration. Some exemplified results obtained by using this scheme will be mentioned.

(1) Fracture behavior emanating from a single hole in an infinite plate At the initial stage of fracturing, several small cracks initiate in the radial direction from the hole wall. Consequently, two symmetrical cracks grow from these during the course of pressurization. When these cracks extend to some length, then additional two cracks perpendicular to the first pair ones emerge and finally four predominant symmetrical cracks are formed.

(2) The fracture behavior emanating from holes of equal spacing arranged along a line pararrell to the free surface in a semi-infinite plate. In this case, growth of two equal length fractures toward the direction connection each of the borehole is most prominent, if the distance between the hole line and free surface is not so large.

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