Determination of the load bearing capacity of pre-stressed expandable props for ground support in underground mines

Abstract

This paper aims to determine the load bearing capacity of pre-stressed expandable props with different geometries and load eccentricities for flexible support in underground mining or excavation. It is deduced that the expandable device could have much higher strength (>89 MPa) by laboratory tests, and the load bearing capacity of the expandable prop may depend on the stability of the supporting steel pipe structure. A good agreement was found between the laboratory test and numerical results in terms of the load bearing capacity and the final macro-bending failure pattern for expandable props with heights of 1.5 and 2.7 m, and the theoretical calculation for the strength of traditional steel structures is not directly suitable for the expandable props. Moreover, additional numerical simulations were performed for the expandable props with different normalized slenderness ratios λn and loading eccentric distances e. The variation of stability coefficient of the expandable prop is in line with the Perry-Robertson equation and its correlation coefficients are fitted as a of 0.979 and b of 0.314. For estimating the load bearing capacity of the expandable props, the strength equation for traditional steel structures is improved by introducing a bending magnification factor and by modifying the normalized slenderness ratio to a converted slenderness ratio. Based on the underground field monitoring for the strength of expandable props with different heights, the empirical eccentric distances were back calculated, and a safety factor is introduced to obtain the designed strength of the expandable prop. In addition, a four-step design procedure is proposed for the expandable prop.