Simulation study on seismic response of ground surface above an underground longwall goaf

The dynamic varied elastic modulus and damping coefficient of underground goaf are most important factors which describes the goaf absorption ability on seismic wave energy. This numerical simulation study have focused on the effect of goaf area on earthquake-wave propagation and seismic response on the ground surface in a coal mine area. The laboratory measurements were carried out by 135 porous samples cases consisting rocks and coal particles 0.12–0.25 mm in size to estimate the dynamic elastic modulus and damping coefficient of the fragmented rock masses. In this simulation, a new model on damping coefficient of longitudinal and transverse waves has been presented in considering rock size (D), effective stress (re), porosity (u) and compaction time (t). The empirical correlation between the damping coefficient and effective stress (depth) as well as the compaction time of the goaf area has been derived and used for each goaf and strata grid blocks in the simulation. The results showed that the peak ground acceleration (PGA) above the goaf area exhibits 9–20% reduction compared to that of unexcavated condition (original coal seam), and around 10–35% seismic energy is trapped in the goaf area as goaf varied from 100 to 700 m. In the meantime, the peak ground displacement (PGD) is amplified up to seven times for the goaf depth ranging from 100 to 700 m compared to that of the unexcavated condition. Both PGA and PGD above goaf area are smaller than those above the undisturbed coal seam.