Field and simulation study for rock bolt loading characteristics under high stress conditions

Rhomboid shaped coal pillars (35 m x 30 m to 26 m x16 m) were formed by a modified Room and Pillar method below 850 m depth from surface at the CSM mine in the Czech Republic. The pillars were developed in a shaft protective pillar by driving roadways of 3.5-4.5 m in height and 5.2 m in width within Panel V of Seam No. 30. Development of pillars at such great depth is prone to spalling/fracturing (pillar rib dilation) due to redistribution of the high stress regime. The induced stress driven dilation was measured during partial extraction of the coal seam within the shaft protective pillar using rib extensometers. In order to stabilize the pillar ribs, four rows of rock bolts with 2.4 m length were installed into the pillar from all sides at different heights. The immediate roof was also supported by rock bolts at a 1 m grid pattern. Three-way intersections were made to control the deformation of developed pillars and other underground structures. Further, an attempt was made to understand the rock bolt loading characteristics at different stages of rib dilation using numerical modelling with the available properties of rock mass and reinforcement for the studied site. Elastic and Mohr Coulomb strain-softening constitutive models are considered in FLAC3D to evaluate the performance of the rock bolts. Results obtained on numerical models were found to be in good tune with the rock bolt loading characteristics monitored during the field study. This paper presents a discussion about the impact of rib bolting on pillar safety factor and induced load on rock bolt with respect to the dilation/spalling of pillar ribs at the studied site.