Experimental study on the effect of granular backfill with various gradations on the mechanical behavior of rock

Backfilling is a common practice in the mining industry and the backfilling performance plays a significant role in supporting the surrounding rock mass. To evaluate the backfilling performance, an experimental apparatus has been developed to understand how backfill affects the compressive strength of sandstone specimens in the laboratory. Pebbles were selected to model the backfill and divided into six groups with different particle sizes using a set of standard sieves. The backfilling pebbles with three types of particle size compositions were then produced, i.e. single gradation, two adjacent gradations, and increasingly widening gradations. A series of compressive tests were carried out to study the mechanical behavior of the sandstone specimens confined by these pebbles. The effects of the gradations of the filled pebbles on the peak and residual compressive strengths were analyzed. It is found that the increasing amount of the compressive strength is over 10% in most cases, even up to 20%. Based on the experiment data, the increasing amount was also estimated theoretically under some assumptions and it further confirmed the experimental results. The effects are closely related to the gradations of the filled pebbles except for their dense degree.