Las rocas blandas o de baja resistencia: la tiza como caso de análisis

Abstract

The following paper presents general comments about chalk as a soft rock, emphasizing on its mechanical classification and laboratory analysis. Considerinq their caracterization, the mechanical tests analized in the laboratory were: pulse ultrasonic velocity test, point load test, uniaxial and triaxial tests at different misture contents. The pulse ultrasonic velocity test helped establish some correlation bettween the velocity and the moisture content. It was observed that when the moisture content of the chalk decreased the propagation of velocity increased (and viceversa) to an specific value of moisture content when the rock's behaviour was totally opposed. In the point load test samples on the same size but with some moisture content showed a 43% reduction in their resistance value when compared with the dry samples. In the uniaxial compressive strength test, dry and moist core samples of 50 mm diameter and 125mm length were analized. The observations showed that the rock's water content increased, the uniaxial compressive strength of 4.8 MN/m2 and those with some water content showed an average value of 2.6 MN/m2. Finally, during the triaxial test some samples with 38 mm of diameter of 76 mm of length under dry conditions and with 26 % of moisture content were compared with other samples with a diameter Of 76 mm and 152 mm of length also under dry conditions. According to the obtained data, it is possible to conclude that these samples with different diameter (38 and 76 mm) under dry conditions showed aproximately the same value of cohesion (15-17 Kgff/cm2 and a basic friction angle (34-35°). The principal difference was found in those with a moisture content of 26% which gave a lower value of cohesion (7 Kgrf/cm2)than those under dry conditions, for die presence of water reduces the strength characteristics and the cohesion of the rocks.