ABSTRACT Abstract. A new method for the design of longwall chain pillars, called Analysis of Longwall Pillar Stability (ALPS), is described. The method provides estimates of the relative stability of different longwall pillar designs during all stages of their service life. The key issue addressed by ALPS is the magnitude and time-of-arrival of the longwall abutment loads applied to chain pillars. A field study was conducted and other available field data were analyzed in order to develop an approach for estimating the abutment loads. An example based on longwall experience in the Pittsburgh seam is used to illustrate how ALPS can be used in mine design.
INTRODUCTION
The gate entries are the lifelines of the longwall. Personnel, supplies, and ventilating air must pass through the gate entries, as must the coal being mined at the face. Given the large investment and high productivity associated with a longwall, it is essential that its opera- tion be uninterrupted by the instability in the gate entries. The function of the chain pillars in longwall mining is to maintain the stability of the gate entries. However, the stability requirements and the loading conditions of the chain pillars change during the course of mining. A typical chain pillar system (Figure 1) will be subjected to two sets of longwall loadings during the course of its service life, first as a headgate and then as tailgate. Other chain pillars, those located at the edges of multi-panel longwall blocks, may be designated as "single-use" because they will only be subjected to a single longwall loading. In either case the critical locations for stability move with the face and are located at and outby the face ends, where the headgate and tailgate entries intersect the longwall face. In the Eastern U.S., longwall chain pillars are almost always expected to support the over- burden and longwall loadings without failure. The implicit assumption is that unstable pillars will result in unstable gate entries. Experience has in fact provided many examples of mines where undersized pillars have been associated with intolerable entry conditions. In spite of the growing importance of longwall mining, however, relatively few quantitative guidelines for the sizing of longwall pillars have been developed (Choi and McCain, 1980, Carr and Wilson, 1982, Hsuing and Peng, 1985). Recent research at Penn State has attempted to fill this gap through the development of an empirical design approach for longwall pillars called ALPS (Analysis of Longwall Pillar Stability).
Figure 1. A Multi-Panel Longwall Block Showing Chain Pillar Systems (after Peng, 1976).(available in full paper)
estimation of the load applied to the pillars
estimation of pillar strength
calculation of "stability factors" (SF) based on a comparison of the load to the strength.
The ALPS approach includes the same three basic components of pillar design as developed for room and pillar mining. These are: In ALPS, these concepts have been adapted for longwall mining by considering the appropriate loading conditions and pillar geometry.
LOADS APPLIED TO LONGWALL PILLARS
The loading applied to longwall pillars can be divided into two parts.