ABSTRACT

The paper discusses structural considerations of rock strength under high pressures. Medium grained sandstones from Mucharz were used as test material, in which streaks emphasizing lamination can be observed and a distinct direction of longer grain axes. Rock samples were cut out in three perpendicular directions depending on the structural surfaces. Two types of triaxial tests were carried out: triaxial single failure tests and triaxial multiple failure tests. The analysed rocks show greater strength in the plane parallel to lamination than in the plane perpendicular to that structural surface.

INTRODUCTION

The structural considerations of rock strength under high pressure are a highly important issue in Geornechanics. In the world literature, anisotropy of strength features of bedded or laminated rock is usually described on the basis of cylindrical sample testing under conventional triaxial compression conditions, i.e. under axisymmetric compressive stress conditions (σ12 = σ3) (McCabe & Koerner 1975; Niandou et al. 1997; Kwasniewski 2002). Much less frequently the effect is described under true triaxial stress state conditions(σ1 > σ 2 = σ3) (Reik & Zacaas 1978; Kwasniewski 2002). In general, it is believed that under conventional triaxial compression test conditions, rock strength determined in the direction perpendicular to structural surfaces (layering or lam- Ination) is usually greater than the strength measured parallel to them.

However, in this paper, that statement will not be confirmed for flysch sandstones from Mucharz. The results of triaxial compressive testing(σ1 > σ 2 = σ3) Will be presented for rock specimens cut out in three perpendicular directions depending on the structural features of rock.

GEOLOGICAL CHARACTERISTICS OF THE TESTED ROCKS

The test material was heavy-bedded flysch sandstone excavated from the Górka-Mucharz quarry situated at the eastern end of Beskid Maly Mountains, about 12km south of the town of Wadowice. The quarry is located within the Silesian nappe, north of the Magura trust front (Fig. 1)

(Figure in full paper)

The excavated rocks are medium-grained sandstones of the lower Krosno strata, which date back to Upper Eocene - Oligocene. In macroscopic terms, they are blue-grey rocks with carbonate or carbonate and clay cement with disorderly large grains of muscovite embedded in fine-grained quartz mass with mica. They are also characterized by a slightly directional texture and size-based arrangement of grains. In petrographic terms, the tested material is moderately granular lithic or arkose wacke with a diverse quantity of feldspars and lithoclasts with a slightly directional texture marked with numerous light and dark micas.

A macroscopic analysis, confirmed by a microscopic analysis of thin plates, shows that the rocks in question exhibit marked lamination emphasised by streaks of dark iron compounds and certain directionality in the arrangement of mineral grains. In order to examine the effect of those structural surfaces on

(Figure in full paper)

the strength of Krosno sandstones from Mucharz, several rectangular roof and floor-oriented rock blocks were prepared at the quarry.

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