The shape effect, widely studied by researchers as Hustrulid (1973), Protodyakonov (1964), Obert and Duvall (1967) and Bieniawski (1984), has had its importance recognized in the scientific and technical community since a long time ago (Bunting apud Hustrulid, 1973; Bunting, 1911). In all its technical applications however, the rock mass structure has not been taken into account. But as Bell (2000) also stresses, it is of fundamental importance for the determination of compressive strength in rock masses. This study aimed to investigate the variability of the results obtained in uniaxial compressive stress tests, using prismatic and cylindrical specimens, considering their slenderness (respectively L/H and D/H), which Bieniawski (1984a,b) defines as the relationship between the smaller size of the section and its height. In order to quantify the influence of the rock mass structure on the result of the shape effect expression, one intended to correlate its parameters with the value of Q-system of Barton (2007).

1 Introduction

The literature assigns the first attempts at rational design of pillars in underground mines to Bauschinger who, as mentioned by Bunting, verified in 1876 the compressive strength variation of 23 prismatic specimens of sandstone (Switzerland) according to the relation between their width and height, obtaining the first equation which related these two variables (Bunting apud Hustrulid, 1973). In 1911 Bunting, studying the design of pillars in an anthracite mine, carried out tests on 13 prisms between 2" and 6". Based on his experience he observed the behavior of 8 pillars (from which only one failed) and obtained the same equation of Bauschinger (Bunting, 1911).

It is worth mentioning that Bunting obtained the following the equation:

CP= 1750 + 750 L / H (psi)

where compressive strength in psi, L=the smallest pillar width in inch; and H =pillar height in inch.

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