Numerical modeling of coal wall spall in a longwall face

•Longwall face spall mechanism was revealed numerically and verified by field observations.•Mechanical behaviours of coal wall were presented versus longwall face advancing.•Tensile stress concentration near the coal wall dominates the tensile splitting failure as laminas after coal wall exposed.•Shear strain bands initiating from the outer ends of tensile areas results in shear slipping failure as blocks subsequently.