Forensic Excavation of Rock Masses: A Technique to Investigate Discontinuity Persistence

Abstract

True persistence of rock discontinuities (areas with insignificant tensile strength) is an important factor controlling the engineering behaviour of fractured rock masses, but is extremely difficult to quantify using current geological survey methodologies, even where there is good rock exposure. Trace length as measured in the field or using remote measurement devices is actually only broadly indicative of persistence for rock engineering practice and numerical modelling. Visible traces of discontinuities are treated as if they were open fractures within rock mass classifications, despite many such traces being non-persistent and actually retaining considerable strength. The common assumption of 100% persistence, based on trace length, is generally extremely conservative in terms of strength and stiffness, but not always so and may lead to a wrong prediction of failure mechanism or of excavatability. Assuming full persistence would give hopelessly incorrect predictions of hydraulic conductivity. A new technique termed forensic excavation of rock masses is introduced, as a procedure for directly investigating discontinuity persistence. This technique involves non-explosive excavation of rock masses by injecting an expansive chemical splitter along incipient discontinuities. On expansion, the splitter causes the incipient traces to open as true joints. Experiments are described in which near-planar rock discontinuities, through siltstone and sandstone, were opened up by injecting the splitter into holes drilled along the lines of visible traces of the discontinuities in the laboratory and in the field. Once exposed the surfaces were examined to investigate the pre-existing persistence characteristics of the incipient discontinuities. One conclusion from this study is that visible trace length of a discontinuity can be a poor indicator of true persistence (defined for a fracture area with negligible tensile strength). An observation from this series of experiments was that freshly failed surfaces through pre-existing rock bridges were relatively rough compared to sections of pre-existing weaker areas of geologically developed (though still incipient) discontinuities. Fractographic features such as hackle and rib marks were typical of the freshly broken rock bridges, whereas opened-up areas of incipient discontinuity were smoother. Schmidt hammer rebound values were generally higher for the rock bridge areas, probably reflecting their lower degree of chemical and physical weathering.

Tensile strength of large-scale incipient rock joints: a laboratory investigation

“…The light-brown areas (enclosed by red lines) were interpreted as areas of rock bridges; the black-mottled areas were assumed persistent. It can be seen that both incipient joints 1 and 2 were not fully persistent in contrast to the continuity shown by the exposed traces on the block surfaces, but the non-persistent nature can only be viewed after splitting [33]. The persistent areas (black-mottled areas) have been highly weathered.…”

“…If the areas enclosed by the red lines were assumed to be non-persistent (i.e. relatively fresh rock bridges), a persistence of 75% of the incipient joint 1 was calculated according to the definition of areal persistence [33]. An areal persistence of 63% was calculated for the incipient joint 2 (Fig.…”

Automatic Mapping of Discontinuity Persistence on Rock Masses Using 3D Point Clouds

“…Nevertheless, new available data can help develop new approaches to quantify the properties of discontinuities in a more realistic manner. Not surprisingly, true persistence is still considered difficult to be measured in practice (Shang et al 2017) and therefore, actual persistence seems to be impossible to be measured using data acquired from the surface. Only visible persistence can be measured when using field data (regardless of the use of geophysics).…”

A New Method for Determining the Critical Slip Surface of Fractured Rock Slope

“…The critical slip surface of a fractured rock slope is mostly controlled by the fractures developed in rock masses [1,2]. Due to its anisotropy and limitations of measurement, hidden within the rock mass, very little is known about true fractures [3]. Thus, it remains a difficult challenge to determine the critical slip surface of a fractured rock slope.…”