The effect of scale and defect planes on the failure process and strength of rock with applications in open pit mining

The ability of engineers to design deep excavation into Earth’s surface has advanced significantly over the last century. The increased power and accessibility of computer systems capable of conducting various permutations of slope stability analysis has meant that solutions to complex geotechnical problems can now be found relatively rapidly. Although, some fundamental problems still exist which challenge modern computing power and the ability of geotechnical engineers to accurately model the geo materials in which they seek to excavate. These problems include the area of discontinuous rock masses and the effect of scale on strength. In this research, the effect of measurable joint and fracture properties on rock mass strength has been examined through the use of experimental modelling techniques and statistical analysis of new and previously published datasets. Furthermore, the effect of sample size on rock strength has also been examined in order to bridge the gap and quantify the relationship between the strength of laboratory scale rock samples and the scale of the designed slopes. This research has sought to improve the understanding of these areas through the use of; laboratory studies, data analysis and empirical model development using statistical assessment methods. The laboratory testing conducted in this research has included the development and use of high pressure triaxial testing to simulate the in situ stress state of rock at depth. Additionally, uniaxial testing has provided widely comparable measurements of the strength characteristics of samples while emulating near face stress conditions. The specifications of, and procedures followed to build and conduct the tests has been extensively outlined in this work. The high pressure multi-phase triaxial testing apparatus was built to evaluate the strength of intact and jointed rock and mortar specimens. A key requirement of this equipment was its capacity to measure specimen volume change in order to quantify Poisson’s ratio. A comprehensive review of published models and, more importantly, the experimental data relating to the effect of sample size on UCS underpins the research conducted on the correlation between sample size and rock strength. Using the data found in this review, together with a substantial amount of new data, comprehensive statistical analysis was undertaken to characterise the strength-size effect more accurately for different rock types. Original experimental data pertaining to specimens ranging from 63 mm to 300 mm in diameter was also produced and included in this analysis. Additionally, the effect of specimen shape (cylindrical versus rectangular prismatic specimens) on unconfined strength, elastic modulus and Poisson’s ratio were examined. Based on the current analysis no relationship was found between Young’s modulus and specimen size, nor Poisson’s ratio and specimen size, nor specimen shape (right-cylinders versus rectangular prisms of the same length to diameter ratio) and strength. However, the nexus between specimen size and strength was confirmed although some models currently used to represent this relationship provide inaccurate predictions. Recommendations have been given to improve the accuracy of the strength change with specimen size. In Chapter 5 the progressive damage stages of quartzite are defined by both stress-strain and AE methodologies. Additionally, recommendations have been made as to the practical implications of these thresholds. The recommendations include the application of the knowledge of failure stage boundaries to more consistently estimate the elastic modulus and Poisson’s ratio. This methodology allows a modulus and Poisson’s ratio to be selected based on the desired design application. It may also remove some parameter variation caused by subjective interpretations of the linear regions over which to calculate parameters. These findings can be built upon and applied to other similar rock types. The strength of discontinuous rock mass containing defects, specifically the effect of a single joint, double parallel joints and a discontinuous plane of weakness or intact rock bridge were studied. The relationship between joint orientation and persistency has been quantified in a set of empirical relationships. Acoustic emission readings were collected from tests in order to build on the conclusions of Chapter 5 and study crack growth within the specimen. The assessment of this methodology was of particular importance to AE assessment of failure stages and has not traditionally been applied to specimens containing defined weakness planes. Chapter 6 developed empirical relationships for non-persistent joints of a single roughness, while Chapter 7 quantifies the effect of roughness and orientation on the strength of a rock mass. Open joints of defined roughness and orientation were subjected to triaxial stress to define an anisotropic strength criterion. Accuracy and repeatability of joint properties was ensured through the use of a water jet cutting system to cut CAD modelled JRC joint profiles into intact rock. Testing occurred under multiple in situ stress conditions to define the criterion over a stress range applicable to open pit slope designs. This data was used to create novel empirical strength versus joint orientation relationships, additionally it was analysed using the published relationships of other researchers. It was found that the empirical relationship of McLamore and Gray (1967) yielded an exceptionally good fit to both the experimental data presented in this research but also a range of experimental data contained in the literature. This finding was consolidated and recommendations have been presented as to the values of appropriate anisotropic fit variables applicable to different rock types and joint roughness. Based on these results a practical methodology for applying this anisotropic strength relationship to the modelling of anisotropic strength in slope design has been proposed.