Portal: Underground

Historically, in Western Canada, the stabilization of rock slopes and construction of excavations have been achieved using methods such as soldier piles and lagging or construction of cast-in-place concrete retaining walls. In the case of reinforced cast-in-place concrete, there is a requirement for erection of formwork, fixing of reinforcement, pouring the concrete mixture, and vibration to ensure good concrete consolidation and steel encapsulation. These methods have proven to be relatively ineffi-cient and costly in many cases. In recent decades, however, the use of shotcrete for ground support has seen increased use, as shotcrete has allowed

Tere has long been a need for a reliable, simple-to-use means of determining the early-age rate of strength gain in shotcrete.During approximately the first 24 hours after shotcrete has been placed, its compressive strength is typically too low to measure using standard core-extraction and testing procedures. Monitoring the rate of early-strength development in shotcrete is important in tunneling, mining, and other applications such as the underpinning of structures. Recent studies by the authors have demonstrated that there is a simple, direct method for determining the early-age compressive strength development of shotcrete. It involves the shooting of a set of beams in a standard steel mold and testing the beams after stripping, using an adaptation of ASTM C 116, œStandard Test Method for Compressive Strength of Concrete using Portions of Beams Broken in Flexure. This œTechnical Tip describes a procedure for deter-mining the compressive strength of shotcrete beams and presents results of tests conducted with plain and accelerated shotcretes produced by both the wet- and dry-mix shotcrete processes.

Developments in Shotcrete in Hobart, Tasmania, Australia in April 2001. One of the outstanding papers presented at the Conference was a paper by Grant, Ratcliffe and Papworth on œDesign Guidelines for the use of SFRS in Ground Support. Frank Papworth was asked to submit an updated paper on the subject for publication in the ASA Shotcrete Magazine and so here it is. It is more technical than most of the papers published in the ASA Shotcrete Magazine, but was selected because it was considered that it would be of considerable value to designers of fiber-reinforced shotcrete linings for ground support in civil and mining applications.
Abstract: There are presently no design guide-lines based on toughness for the use of fiber-reinforced shotcrete (FRS) in ground support for underground mine development. Typically, in the Australian mining environment, the approach to the use of FRS has been one of borrowing experiences from other mines and a œtrial-and-error method of design, installation, and assessment. There is a need for a ground support design guide that can be simply applied by œfront-line personnel.
This paper provides an overview of the performance characteristics of FRS and how the various shotcrete guides specify its use. Practical experiences with the use of FRS in Australia and Canada in various applications and ground conditions are combined with existing empirically based ground support-design methods to develop a ground support guideline that incorporates the concept of toughness. An assessment of structural synthetic fibers shows that their low modulus makes their performance characteristics different from those of steel fibers, and that they are not likely to be economical in linings where crack widths are limited, but that they are preferable where large deflections are permissible.
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Fiber-reinforced shotcrete (FRS) has been used successfully for ground support for more than