While numerical methods are increasingly becoming important tools for predicting the outcomes of difficult and costly experiments, lack of experimental data for calibrating these models, especially in the dynamic field, results in unreliable predictions. A series of laboratory-scale blasting experiments has been done in Barre and Laurentian granites to measure the changes in the pressure with distances from the blasting holes. Drucker-Prager strength model available in Autodyn, an advanced explicit code for transient nonlinear dynamics simulations, is used to find the pressures on the borehole walls for different combinations of charge loads, coupling media, borehole diameters and rock types. Attenuation factors of these rocks are estimated from the results of numerical and experimental studies.
The behavior of rocks subjected to intense transient blasting loads is very complicated and depends on complex interaction among the properties of explosive, rock, and coupling media. After detonation, a strong stress wave travels into the rock and causes shear and tensile failures. At the same time gaseous reaction products expand and penetrate into the cracks that result from the stress wave. This pressure measurement study is a part of a wider project in which we intend to find the effect of only the shock wave on the fracture patterns in two granitic rocks. We prevent explosion gas penetration into the cracks by lining the borehole wall with copper tube. Experimental pressure values were measured at different distances from the borehole wall. Carbon resistor gauges were used to accomplish these measurements. To estimate the pressures on the borehole walls, a series of 2D planar symmetry simulations were done in Autodyn environment. Pressures obtained from experiments and numerical studies are used to predict the attenuation factors of the two granitic rocks under investigation.
In a series of explosive loading tests on rocks, shock pressures have been measured at different distances from the end of core samples of Lithonia granite. These samples were shock loaded by explosive charges placed at one end of the sample [1]. Shock and particle velocities were measured by using a streak camera from which shock pressures were calculated.
In a series of preliminary experiments with no copper tube installed in the boreholes, we tried to find the expected range of pressures and the applicability of the method used for pressure measurements. For the rest of the experiments, copper tubes of appropriate diameters were inserted into the boreholes for intimate contact with the rock so as to prevent gas penetration into the cracks. In addition, this minimizes the load of numerical model calibration process. Measuring dynamic pressures around the boreholes in blasting experiments is a difficult task to accomplish. Gauge response time, rock anisotropy, impedance mismatch between rock and gauge embedding environment and gauge calibration must be taken into account in these experiments.
