Two-dimensional numerical simulation and design of fully grouted bolts for underground coal mines.

The fully grouted bolt is a passive support. It occupied the greatest portion of the annual roof bolt consumption in the U.S. There are three components of the fully grouted bolt; steel rebar (bolt), resin annulus, and rock/grout interaction. Fully grouted bolts are loaded in three ways: axial load, shear load and bending moment. Moreover, vertical shear stress is developed along the grout/rock interface. Using ABAQUS 5.8, finite element models were developed to simulate the components of the fully grouted bolt and its interaction with the surrounding roof strata. Several models were conducted in order to investigate the effect of the fully grouted bolt on roof stability, and the load transfer from the rock to the bolt and vice versa under different geological, mining conditions and bolt parameters. The behavior of the simulated fully grouted bolt was verified with a case study during the development and mining stages. Different failure modes of the fully grouted bolts, such as bolt axial failure, bolt shear failure, and grout/rock interface shear failure, were defined. Bolt and roof stability measures were presented in order to evaluate the bolt and roof stability. The effects of bedding planes location with respect to roof line and their properties and the strata sequence on roof or bolt stability measures were studied. Moreover in this study, different factors such as in-situ horizontal stress, overburden depth, bolt length, bolt diameter, and hole roughness, were considered. According to the plastic strain distribution for different geological conditions, four modes of failure were discussed. Therefore, the immediate roof has been classified into four roof types, I, II, III, and IV, based on different strata sequences and the different modes of failure. Based on the results finite element models, four regression equations were developed for each type of roof to estimate the roof and bolt stability. The regression equations relate the roof stability measures with overburden depth and entry width and assumed values of bolt length and bolt diameter. The estimated roof and bolt stability measures will be compared with the corresponding critical values. If the comparison shows a safe roof and bolt conditions, the assumed values for bolt length and bolt diameter will be accepted as appropriate bolt design. Otherwise the bolt length and/or the bolt diameter will be increased. The roof and bolt stability measures will be revaluated with the new values of bolt length and diameter. These procedures will repeat until a safe roof and bolt design is obtained