46 / 2021-08-21 08:08:46

Backfill placement for the stability of crown and sill pillars in underground mines

Pillar stability, backfill placement, crown and sill pillars, underground mining

Stability of crown and sill pillars may be required to prevent ingress of water or mud into underground mining zones, constrain the potential caving boundary as part of the permit conditions, protect infrastructure in a local or regional scale or control dilution when the sublevel caving or open stoping methods are adopted for ore extraction in an underground mine. 

Delayed or no placement of fill may cause wall overbreak of mined out stopes or voids and subsequently lead to instability of crown or sill pillars. In particular, when the sublevel caving method is modified to introduce rock fill for supporting walls and as the waste blanket, backfill placement becomes a key factor in wall and pillar stability. In sublevel open stoping, if the filled stopes are interconnected and for various reasons the fill body is able to “flow” or move in a large scale, stability of the walls and pillars may become a concern for the mining operation.

Since crown or sill pillars at a shallow depth are generally under low stresses, their performance is similar to that of surface crown pillars. In cases where numerical modeling is not performed with a high confidence and as a preliminary assessment, the Carter’s scaled span approach is commonly used by engineers to evaluate the crown or sill pillar stability. However, application of the method implicitly assumes that the walls of mined out stopes are generally stable either with introduction of fill or under the allowable stable hydraulic radius.  

In this paper two cases where the modified sublevel caving method is used are presented to highlight the importance in prompt placement of fill in maintaining pillar stability and one case is discussed about the impact of movements of placed fill on pillar stability.