Strategy for Sill Pillar Mining in Underground Hard Rock Mines

CONSIDERATION OF POST-PEAK RESPONSE OF THE PILLAR-ROCK SYSTEM IN SILL PILLAR DESIGN

*Omkar N. S. Beruar1, 2 and Denis Thibodeau1, Shailendra K. Sharan2 and Ming Cai2

1VALE CANADA LTD. 18 Rink Street, Copper Cliff, Canada P0M 1N0 (*Corresponding author: [email protected])

2LAURENTIAN UNIVERSITY, 935 Ramsey Lake Road Sudbury, Canada P3E 2C6

ABSTRACT

Extraction of sill pillars has always been problematic because of high risk of seismicity and rockburst. Depending on a number of factors, a sill pillar can undergo different failure modes and cause problems in the mine. Of these, sudden and uncontrolled pillar failure leading to high-energy seismic events and rockbursts is a main concern because it is not possible to predict the location and time of a rockburst apart from its potential to cause extensive damage to the mine structures. To date, an appropriate technique for sill pillar design and recovery does not exist. Conventional methods of pillar design, such as the pillar stability graph method, are utilized to design stiff sill pillars. In these design approaches, the post- peak response of the pillar-rock system is not considered. This is in part due to a lack of clear and comprehensive understanding of the post peak behaviour and failure mechanism of sill pillars. In general, the strength of a rockmass is described using a failure criterion. When stresses reach the strength of the rockmass, the rockmass does not necessarily become unstable and knowledge of the loads alone is not enough to assess the instability conditions accurately. In fact, it is not possible to predict instability of rockmass based on the strength/stress ratio alone because instability depends not only on this ratio but also on the loading condition and loading system characteristics. The loading system dictates whether the rockmass will go through a stable or unstable failure process. This paper examines the influence of various parameters such as the pillar width to height ratio, the extraction rate, and placement of backfill on the Loading System Stiffness (LSS) and the Pillar Stiffness (PSS). The ultimate goal is to design yielding sill pillars with stable yielding so as to eliminate the risk of pillar rockburst. Plane strain finite element analysis is conducted using the generalized Hoek-Brown failure criterion with elastic-brittle-plastic parameters. Examples are given to demonstrate how to calculate these stiffness values and assess the stability condition of sill pillars. It is found that increasing the rate of extraction increases the likelihood of unstable yielding of the pillar. Placement of backfill in timely fashion in the stopes above and below the sill pillars increases both the rockmass and pillar stiffnesses but the increase in rockmass stiffness is much higher. This indicates that backfilling of stopes while creating the sill pillar increases the likelihood of stable yielding of the sill pillar.