Constructional problems at interventions into slaty sedimentary rocks in Slovenia in terms of fabric and composition

In Slovenia, several open excavations and tunnels have been constructed in tectonically weakly metamorphosed shaly, silty to sandy sedimentary rock formations of Carboniferous age. During a typical process of excavation rocks are subjected to changes in stress direction, which can result in different types of failure. To a large extent, these failures depend on the direction of loading in relation to the most pronounced types of discontinuities. During design stage of road structures in Slovenia, however, incomparably greater attention is paid to the relevant laboratory geomechanical factors than to the effect of the textural-structural and mineralogical properties of the rocks. It is only in recent years that some attention has been paid to the research into the connection between the textural-structural and mineralogical properties of slates and clay-containing rocks and their weak geomechanical behaviour. Nevertheless, these researches are not taken into serious consideration yet. In order to determine more accurately the type of relationship between the petrographic characteristics of these rocks and their resistance to the point load, a series of petrographical analyses and corresponding point load strength index tests were performed. Samples were taken from trial boreholes and at excavation sites of the rock mass for the tunnels of the Ljubljana area as well as from the wider area of earthworks for the Ljubljana - Celje motorway in central Slovenia. The investigated rocks have mud to sand grain size. From the structural point of view, they exhibit clear dynamo-metamorphic changes. They are manifested with the occurrence of folds, crenulations, strong secondary foliation s1, differential cleavage and fracture porosity. In contrast to primary foliation s0, where the grains are intergrown, secondary foliation s1 is smooth, and only insignificant intergrowing of the grains can be observed. At the micro level these deformations are expressed as recrystallization (mainly degradational), new growth, pressure shadows, kinking of the phyllosilicates, and mechanical reorientation of the mineral components along s1. Preferred orientation of the phyllosilicates and slaty cleavage (s1) have developed, and represent one of the most pronounced structures along which the rock prefers to split. It was shown that in all cases a good correlation exists between the average point load strength index of the rock samples, the direction of loading, and the petrographic/fabric type in the samples without pre-failed surfaces. Failures in the slate and metasiltstone follow concentrations of preferred oriented phyllosilicates of the s1 slaty cleavage in the first place. Other parameters (e.g. average grain size, granoblastic texture, quantity of quartz etc.) are of secondary importance and find principal expression in homogeneous rocks and in cases when they were loaded transversely to the main discontinuity. In the pre-failed rocks, the direction of loading played the major role. The relevance of the above stated facts is clearly manifested in the field of geotechnical works where their disregard led to landslides on the surface and to several mass collapses in tunnel works. The predominant direction of failure in-situ additionally depends on the geological macrostructure, and on the direction of intervention into the rock. Determination of the fabric and compositional properties of such rocks is a key factor for the identification of the weakest directions and for the appropriate and timely adaption of method and direction of excavation.