Interface Strength of Rock Discontinuities

Abstract

The aim of this project is to focus on the effect of mechanical contrast and surface roughness as factors influencing the shear strength of the interface of different rock samples. The shear strength of an interface is significant in experimental projects that for instance address the containment and propagation analysis of hydraulic fractures.
A discontinuity can be defined as any type of interruption in mechanical and structural properties of a rock layer. The samples used in this project are mechanically discontinuous at the interface. These samples consist of 3 different sandstones, one granite and one monzodiorite samples. These samples were prepared to fit a certain casket that was specially designed for this project. The mechanical contrast between pairings of these samples were obtained through calculations for difference in unconfined compressive strength between the rock types. The samples were then placed in a direct shear box and sheared against each other to obtain shear force and vertical displacement profiles while under a specific normal load.
In three different sets of experiments, specific pairings of samples were analysed and processed. Each pairing showed a unique set of results which were then compared with one another. Each experiment set focused on a certain aspect. The first set of experiments focused on the effect of mechanical contrast. The second experiment set focused on the change in surface roughness caused by shearing at higher loads. The third set focused on the effect of predetermined changes on surface roughnesses and how this would affect the shear stress profiles.
The values obtained through these experiments were then mathematically processed and shear stresses and normal stresses were plotted against each other. From these plots the friction coefficients and angles of friction were calculated.
The results show that mechanical contrast has a direct effect on the shear stress profiles. A constant mechanical contrast of two different pairings of samples results in the same trend no matter what rock types were used. In other words, if two different pairs are sheared against each other with each pair having the same mechanical contrast, the shear stress profiles will have the same friction coefficient. The surface roughness also directly effects the shear stress profiles. The results showed that the higher the measure of surface roughness, the higher the shear stress will be. The predetermined surface roughnesses of the samples were 125 and 75 μm.