EFFECT OF INFILL MATERIAL THICKNESS ON STRENGTH BEHAVIOR OF JOINTED ROCK MASS UNDER DIFFERENT ORIENTATION

Abstract

Rock masses used in engineering structures such as tunnels, slopes, foundations, underground openings, and mining excavations generally contain joints, fractures, and discontinuities that greatly influence their mechanical behavior and stability. In many practical situations, these joints are filled with weak materials such as clay, gypsum, weathered deposits, or gouge materials, which reduce the strength and stiffness of the rock mass. The presence of infill material changes the stress transfer mechanism between rock blocks and increases the possibility of deformation and failure under loading conditions. Therefore, understanding the combined influence of joint orientation and infill thickness on the behavior of jointed rock masses is essential for safe and economical rock engineering design. The present experimental investigation focuses on the compressive behavior of artificial jointed rock specimens prepared using gypsum plaster under laboratory-controlled conditions. Artificial rock blocks were casted using gypsum material due to its brittle nature, ease of preparation, and suitability for simulating weak rock behavior. Three individual blocks of dimensions 150 mm × 150 mm × 50 mm were assembled together to form cube specimens of overall dimensions 150 mm × 150 mm × 150 mm. Artificial discontinuities were introduced at joint orientations of 0°, 30°, and 90° to study the effect of crack inclination on strength and failure characteristics. In addition to clean joint specimens, infill layers of 2 mm and 4 mm thickness were provided between the joints to simulate realistic infilled rock conditions. Uniaxial compression tests were conducted on all specimens using a Compression Testing Machine (CTM). During testing, observations related to crack initiation, crack propagation, deformation behavior, and failure mechanisms were carefully recorded. Experimental results revealed that joint orientation has a major influence on compressive strength. Specimens with 0° joint orientation exhibited the highest compressive strength due to direct axial load transfer through the specimen, whereas specimens with 30° orientation showed minimum strength because of dominant shear sliding along the inclined joint plane. The 90° joint orientation specimens displayed moderate strength and mainly failed through tensile splitting. The study also showed that the presence of infill material significantly reduces the compressive strength of jointed specimens. Increasing the infill thickness from 2 mm to 4 mm further decreased the load-carrying capacity because the thicker infill layer reduced frictional resistance and interlocking between adjacent blocks. The experimental observations confirmed that thicker infill layers promote deformation and sliding failure under compression loading. The findings of the present investigation highlight the importance of considering both discontinuity orientation and infill thickness while analyzing the stability of rock engineering structures. The study provides valuable insight into the failure behavior of infilled jointed rock masses and offers a simple and economical experimental approach for simulating field conditions in laboratory studies.