Mechanical Properties and Failure Behavior of Mortar-Encased Coal Bodies under Impact Loads: Insights from Experimental Investigation

Failure of residual coal pillars under dynamic load disturbances can induce goaf collapse, ground subsidence, or coalbursts. Encasing the residual coal pillar in mortar is an effective method for reinforcing the residual coal pillar. However, the mechanical behaviors of mortar-encased coal bodies under impact loads remain poorly investigated. In this study, impact tests were conducted on coal, mortar, and mortar-encased coal specimens using a split Hopkinson pressure bar (SHPB) system. The mechanical properties and failure behavior of the mortar-encased coal specimens under impact loading were systematically investigated in terms of several metrics including dynamic stress-strain curves, failure patterns, strength change characteristics, and energy consumption laws. Results show that, owing to the different mechanical properties of the coal and mortar elements in the composite specimens, the mortar-encased specimen has a nonlinear deformation characteristic. The mortar has a higher energy absorption rate compared to the coal. Additionally, increasing the thickness of the external mortar body is helpful for absorbing more stress wave energy and increasing the dynamic strength of the mortar-encased coal specimens. Furthermore, under low strain rate loading, the external mortar body of the composite specimen initially experienced axial splitting failure. With increasing strain rate, axial splitting failure occurred in both the external mortar body and inner coal body. This study provides useful guidelines for reinforcing residual coal pillars in underground engineering.