Grain size distribution as a determinant of rock joints roughness

• Main Author: Kabeya, Kany Kanankoji
• Published: 2012
• Subjects: Surface roughness measurement; Granular materials; Joints (Engineering); Shear (Mechanics)
• Online Access: http://hdl.handle.net/10210/4346

D.Ing. === The quantification of the surface roughness in order to determine the shear behaviour of rock joints has been performed using many approaches, amongst which the most important are the joint roughness coefficient, statistical methods and the fractal approach. None of these approaches explicitly considers the contribution of grain size to the creation of surface roughness. An investigation has been undertaken to determine whether the grain size may be related to the surface roughness characteristics and also to assess its impact on the shear behaviour of rock joints. The influence of grain size on the shear behaviour of rock joints has been experimentally and numerically studied using a profile measurement approach followed by shear tests in the laboratory. Fresh rock joints in model material made of aggregates of no rite in resin and two natural rock types, namely norite and sandstone, were used for the investigation. A numerical analysis of the study has been performed using the Particle Flow Code PFC20 • Investigation of the model material has resulted in the conceptualisation of grain size parameters. Of the proposed grain size parameters, the average particle size index As has been successfully used to define the particle size distribution of a rock joint for both physical and numerical tests. From the profile measurements, four roughness parameters, n~mely the average inclination angle I, the root mean square of the first derivative Z2, the modified root mean square of the first derivative Z2' and the fractal dimension D, have been computed and a regression analysis conducted. Physical and numerical results have demonstrated that grain size distribution, expressed in terms of average particle size index As shows a good correlation respectively with the root mean square ofthe first derivative Z2, and its modified version Z2'. The root mean square of the first derivative Z2, and the modified root mean square of the first derivative Z2' increase with the average particle size index As. However, no correlation has been found between the grain size parameter and the fractal dimension D or the average inclination angle I. It has also been found that the joint roughness coefficient JRC and the peak friction angle ~P correlate very well with the average particle size index As. The greater the average particle grain size the higher the joint roughness coefficient JRC and the peak friction angle ~p· The type of correlation between JRC and ~P with As depends on the rock type. Exponential shear strength model has been derived where the average particle size index As is related to the peak friction angle via the residual friction angle ~r·