INITIAL EXPERIMENTAL TEST ON THE RATE OF MECHANICAL EROSION OF LIMESTONE

The mechanical erosion of limestone has been very neglected in the study of karst geology. Even it might not be a significant factor in the stage of epikarst, its contribution might be overlooked once the karst system is opened to phreatic flow. This study investigated the significance of mechanical erosion on limestone, evaluated the parameters that played in the process, and determined the best way to estimate the rate of mechanical erosion in karst conduit development. Twelve Devonian limestone slabs are eroded in a circulated water tank with neutral water. Water without solid particles, or with fine silt size quartz and feldspar, or with #600 silicon carbide is used to test the rate of erosion. The flow speed is set to 0.5 m/sec, and the angle of impact is set to vertical. The rate of erosion is measured as the weight loss of the sample, and is calculated according to the area of erosion, the time of erosion and the concentration of erodent. The calculated erosion ranged from 0.0001 cm/day in clear water to 0.002 cm/day in a 2 gm/liter turbid flow. When compared to a normal chemical dissolution rate in the range of 1 mm/yr, the measured rate of mechanical erosion is very significant. When the original and the eroded limestone samples are examined under SEM, it is revealed that the porosity and other rock textures played critical roles in the mechanical erosion. Fine silt to clay size micritic crystals composed numerous micropores and are vulnerable to mechanical disintegration. The overall strength of the rock samples in terms of the resistance to mechanical erosion is evaluated by ultrasonic cavitation test. The result indicated that the weight loss caused by ultrasonic cavitation is linearly proportional to the weight loss caused by water erosion. Since the textural variation of limestones may cause more than 10 times difference on the rate of mechanical erosion, the petrology of involved rock formations in a karst area should be a critical part in the evaluation of karst conduits enlargement.