The development of the Cracked-Chevron-Notched Brazilian Disc method for rock fracture toughness measurement and tunnelling machine performance prediction

Abstract

This thesis consists of two parts: Part I The Development of Cracked-Chevron-Notched Brazilian Disc Method for Rock Fracture Toughness Measurement. Part II The Prediction of Tunnelling Machine Performance. In part I, a new novel method-Cracked-Chevron-Notched Brazilian disc (CCNBD) specimen is developed for rock fracture toughness measurement based on three-dimensional numerical analysis and experimental validation. A programme was undertaken to investigate the dependence of rock fracture toughness using CCNBD method on the dimensions of the specimen. A comparison with chevron-notched bending specimens and chevron-notched shod rod specimens were performed. Mixed-mode rock fracture investigations and mode II rock fracture toughness measurement has been studied. It is found that CCNBD method gives very comparable results with that of chevron-notched bending specimen and chevron-notched short-rod specimen recommended by the testing commission of the ISRM. The only requirements are that the diameter of specimen should be larger than 50 mm and a/R should be less than 0.85. It is found that mode II rock fracture toughness testing results using the CSTBD method depends on its dimensionless crack length. The short crack length CSTBD specimen is recommended for further testing. Mode I rock fracture toughness testing using the CSTBD method also shows its dependence on dimensionless crack length a/ Il, short crack length could generate comparable results with that by the CCNBD, SR and CB methods. The CCNBD method has the following advantages: (1) The loading and displacement measurement apparatus is very simple; (2) The magnitude of failure load is generally larger than 1 kN which is of considerable benefit to those loading machines with a preload of 1 kN; (3) can be used for mixed-mode rock fracture investigations and mode II rock fracture toughness measurements; (5) It needs only a small sample for testing; (6) It is convenient for measuring rock fracture toughness in different orientations; (7) Specimen preparation is very simple. Author proposed that this method could be used for the third chevron notched specimen for the measurement of mode I rock fracture toughness. On the part II, the prediction of tunnelling machine performance was analysed based on comprehensive analysis of existing methods for the prediction of tunnelling machine performance and curvilinear regression ,analysis of intact rock cuttability using a large database including rock physical, mechanical, energy, textural and fracture properties. A full review of existing models for the drag tool rock cutting mechanisms is presented. A drag tool rock cutting mechanism based on rock fracture properties is presented. A new model based on mixed mode rock fracture mechanism for drag tool rock cutting was recommended for future research. The following conclusions were drawn: (1) the toughness index, moduli ratio are very important rock properties for the prediction of rock cuttability. (2)new prediction equations analysed by new statistical analysis give more accurate prediction than results analysed by Me Feat Smith; (3) rock cuttability prediction has strong dependence on rock lithology determination; (4) the most important variables for the prediction of rock specific energy are: cone indenter, toughness index, moduli ratio. For the sandstone group, the most important prediction variables are: cone indenter, moduli ratio and toughness index; (5) the most important variables for the prediction of rock cutter wear are: grain roundness and lathe abrasivity index. For the sandstone group, the most important prediction variables are: cone indenter and lathe abrasivity index; (6) the most important prediction variables for coarseness index are: impact strength index, lathe abrasivity index and toughness index. For the sandstone group, the most important prediction variables are: toughness index and grain density; (7) rock fracture toughness has strong correlation with rock specific energy.