Persistence and Tensile Strength of Incipient Rock Discontinuities Available to Purchase

Rock discontinuities are fundamentally important to most rock engineering projects but predicting or measuring their properties such as roughness, aperture, shape and extent (persistence) are fraught with difficulty. So far, the solution of how to measure or predict persistence is poorly researched partly because the concept of how to investigate the extent of rock discontinuities within a rock mass seems intractable, by any economical methods. In many engineering applications, it is a fairly widespread practice to follow a conventional approach, assuming a 100% persistence value. However, that is certainly incorrect even if usually a conservative assumption. This project is a small step towards resolving this issue. A series of laboratory and field research activities were carried out to investigate incipient nature of rock discontinuities and the extent of rock bridges. Uniaxial tensile strength of incipient discontinuities was quantified in the laboratory using cylindrical rock samples. The tested samples included incipient joints, mineral veins and bedding. It has been confirmed that such visible yet incipient features can have high tensile strength, approaching that of the parent rock. Factors contributing to the tensile strength of incipient rock discontinuities have been investigated. It is concluded that the degree of incipiency of rock discontinuities is an important factor that should be differentiated as part of the process of rock mass classification to inform more realistic engineering design and that this might best be done with reference to the tensile strength relative to that of the parent rock. I will then report an original methodology that has been developed in the laboratory using expansive chemical splitters in drillholes, to quantify the tensile strength of large-scale incipient rock joints. In these tests, smaller tensile strengths were obtained, which probably was the result of localised stress concentration, low pressurization rate and unavoidable variations of expansive tensile force arising from the chemical splitter. In the final part of this project, a technique ‘Forensic Excavation of Rock Masses (FERM)’ was established and employed to investigate areal extent and incipient nature of discontinuities in the field. Large rock blocks, containing incipient features, were split using similar expansive grout techniques as developed in the laboratory. Test results were interpreted and discussed with respect to fracture mechanics, fractographic features, as well as geological conditions affecting the incipiency of the tested discontinuities including degree and extent of weathering and mineralisation. One common observation from the tests conducted is that breakage of nonpersistent sections of incipient rock joints (rock bridges) leads to the development of rough surfaces over those freshly broken areas, and this may have implications for rock fracture development more generally. Despite rock bridge failure, the freshly formed surfaces might be expected to have relatively high strength compared to the pre-existing persistent sections. An important conclusion from this research is that areal extent of open rock discontinuities (persistence) can be investigated realistically using the FERM technique. It has been found that estimates of persistence from trace mapping on rock exposures can be wildly inaccurate and it is concluded that field studies using FERM techniques can be used to understand and quantify better the true nature of rock mass fracture network connectivity and extent that are important parameters for many rock engineering endeavours.