The Geomaterials Fracture by thermal Process

Abstract

Thermal spallation of the rock is promising alternative technique for rock drilling in civil engineering works and petrol industry as tunneling and wells drilling... Over the last century, many works were conducted to test and examine the functionality and the feasibility of thermal spallation to remove the rocky materials. Recently the radiation is the most examined fashion to deliver heat at the rock surface where we need high heat flux to spall the rock. However, the thermal spallation is firstly described by Preston et al. (1943). The Laboratory studies demonstrate that the required energy to produce fracture is huge due to high compression strength of the rocky materials. This energy varies between 0.5 and 14 MW/m² according to rock type. In addition, the energy loss in the fibers (to deliver the laser energy) is almost 60% for a kilometer away, which poses a problem of energy delivery to the rock surface in deep according to this high energy level. The present work offers an alternative method for generating thermal fracture of the rock. It is based on the introduction of the thermal contraction deformation. Accordingly tensile stresses potentially superior to tensile strength of the rock will be created. The tensile strength is much lower than that of compression as well known. So this is a hypothesis that supposedly reduces the required energy to fracture the rock. The proposed mechanism is a coupling of a local rapid heating followed by rapid local cooling of the treated surface. The rapid variation of the heat flow on the treated surface will suddenly reverse compressive stresses induced during the heating phase to tensile stresses during the cooling phase. Once induced tensile stresses exceed the tensile strength of the rock fracture should take place. A model of 2D axisymmetric finite element is used to demonstrate the procedure. The stone used is granite. The proposed mechanism is evaluated in several ways: (1) the thermal efficiency, (2) the possibility of fracturing the rock, (3) reducing the energy required to fracture the rock (4) and depth penetration.