For a better understanding of the directional strength behaviour of siltstone and shale rocks point load tests were carried out applying different load angles. For this purpose, the specimen had to undergo a special preparation procedure. Although, standard application recommendations stood against performing point load index tests on soft rocks the results look comprehensible. Formerly performed tests on similar rock types in the same region could be confirmed. The compressive strength for load conditions perpendicular to the foliation/stratification (load angle α = 0°) decreased to about 20% for load angles of about α = 60 – 70°. By these test results also the strong variance of results of compressive strength tests performed before the point load test campaign could be explained.
The point load index test is a simple test for estimation of rock parameters. Usually, a correlation between the uniaxial compressive strength and the point load index is developed. For strong rocks the correlation factor c reaches values c > 20, for soft rocks it might be c = 10 or less.
In most cases the orientation of discontinuities is not of interest as long as strong rock conditions are prevailing and the shear failure is not dominated by joints, fissures, foliation or similar discontinuities. For soft sedimentary rocks the failure mechanism is depending on the direction of loading. A simple assumption might result in the idea that loads parallel to discontinuities may result in low strength values, whereas the rock shows stronger resistance for perpendicular loads.
Few authors and studies paid attention to this effect since usually soft sedimentary rocks are classified as very poor to poor rocks and, therefore, strength is weak and strengthening by engineering measures might be required or the design and/or location of the construction is changed in favour of meeting better conditions. For the design of tunnels and caverns in large soft rock formations where the feasibility of a project may be depending on the shear strength behaviour of the rock the situation is different. A precise knowledge of the directional strength behaviour might be required in order to accomplish design and approve the technical and economic feasibility.
In order to investigate the directional shear strength behaviour of the present soft sedimentary rock point load index tests were carried out by a number of 106 of which 94 were successful. The results confirmed the uniaxial compressive test results showing very weak to weak shear strength varying with the load direction. The results of the case study could confirm the rarely existing benchmark values for sedimentary rocks. Additionally, the variation of the strength depending on the load direction angle could be determined, which is helpful for design works when designing and modelling underground structures.
