1 ABSTRACT
The exhaustion of shallow placers in Alaska, severe environmental restrictions on removal and disposal of the overburden, and the abundance of deep, gold-bearing, frozen placers in Alaska all indicate the need for development of an underground miming system. As a first step in this direction studies were undertaken to develop procedures for the design of pillars in frozen gravels. The studies were limited to laboratory testing of man-made samples. Gravel and data on grain size distribution and ice content from recent high pressure water Jet fragmentation of frozen gravels were used to simulate the natural material. Procedures for the freezing of samples without introducing residual stresses and for machining of samples were developed. It was assumed that the pillar strength is controlled by its size, shape, temperature, ice content, and straining rate. It was found that the ratio of the average compress lye strength of gravel and ice (at -3°C) is about 20, indicating that the frozen gravel strength is highly influenced by ice. Contrary to data reported on hard rock and coal, the strength of frozen samples showed no influence of shape and little influence of volume. Factors highly controlling the strength of frozen gravel are temperature and straining rate. Based on the studies, the influence on strength of the above variables was determined and incorporated in the final formula for the pillar strength.
2 INTRODUCTION
The objective of this research was to develop procedures for the design of pillars in frozen placers, be it for room-and-pillar, longwall, or any other mining system. Since data on the actual behavior of frozen pillars are not available, the main emphasis was placed on laboratory studies. The design and evaluation of the stability of any structure in rock or frozen ground has to include the collection of data on stress distribution as well as strength and deformational behavior of rocks in the vicinity of this structure. In the post-failure range and in rheologic materials the structure often cannot be designed, based exclusively on strength criterion. Additional assumptions have to be made concerning the adoptable deformation rate (closure rate of the opening) which in turn will depend on the function of the opening and its required stand-up time. The design of safe minimum-sized pillars in hard rook and coal mining has been a major task for at least the last seventy-five years. No research which addresses the design of frozen gravel pillars has been done in the western hemisphere. All of the practical approaches in pillar design commonly used today in the U.S. are based on a concept of the global safety faster (SF) which assumes that the compressive strength of a pillar san be directly sore pared with the predicted maximum stress in it, as follows: (mathematical equation) (available in full paper)
In more thorough, but rarely readily applicable concepts, a Failure criterion is introduced suitable for a particular material and a three dimensional state of stress in the structure is considered. Since the approach assumed in this paper is of an applied nature, the global safety fast or concept is adopted. Based on more resent extensive studies concerning pillar design, it is commonly assumed that the strength of a brittle pillar is a function of its size and shape.