ABSTRACT

In tunnel excavation, contour hole blasting is generally accomplished by smooth blasting to minimize the blast-induced stress and fracturing of rock beyond the excavation line. In order to successfully Conduct smooth blasting, several factors have to be reasonably determined. They include types of explosives, spacing between contour holes and ratio of spacing to burden. Among these factors, choosing an appropriate spacing between contour holes according to the surrounding rock mass condition is essential in tunnel excavation. he objective of this work is to numerically investigate the optimal spacing between contour holes in five different rock mass class using Particle Flow Code. The extent of damage zone and propagation of fracture were monitored. Fracture information such as types, location, density, counts were tracked and analyzed. The optimal spacing between the contour holes was determined to be around 0.6 m in cases of rock mass class I & 11and 0.7 m in cases of class III, IV and V

INTRODUCTION

The rock beyond the excavation line in underground are Usually subject to excessive damage during conventional blasting. Therefore, controlled blasting techniques are adopted to reduce the damage to the rock itself and to improve the competence of the rock at the perimeter of the excavation by reducing the growth of uncontrolled cracks and over-breaks beyond the perimeter.

The technique most commonly used to control the damage in the final walls of excavations is smooth blasting. By carefully choosing the correct contour hole Spacing, a clean fracture can be created to run from blasthole to blasthole around the perimeter of excavation. Generally, it is suggested that the spacing between contour holes in smooth blasting to be 0.6~0.7 m with hole diameter of 43~48 mm (gustafsson, 1981). In most cases of underground excavation, however, the spacing was not determined by thorough consideration of surrounding rock mass condition.

The objective of this work is to numerically investigate the optimal spacing between contour holes under different rock mass class using PFC2D.

ROCK FRAGMENTATION PROCESS BY BLASTING

The process of rock fragmentation initiates with detonation of explosive, which results in a very rapid chemical reaction at the velocity normally between 4000 to 6000 m/s of a thermodynamically unstable substance producing gases at high temperature and high pressure in a very short time.

The first interaction between the rapidly expanding high-pressure gases and the surrounding rock occurs at the moment the explosion products impact the surface of the blasthole. This high-pressure on the surface instantaneously rises to its peak, and then decays roughly exponentially due to the cooling of the gases and their outward expansion.

The pressure of the explosion pulse greatly exceeds the dynamic compressive strength of the rock, and causes immediate crushing and fracturing of the rock around the blasthole. The crushed zone around a blasthole is formed with a limited extent less than two to four blasthole radii, while the extent of the fracture zone averages 20 borehole radii away and extends to 50 radii (Bhandari, 1997).

This content is only available via PDF.
You can access this article if you purchase or spend a download.