Numerical modelling investigation of rock mass behaviour under gravity dams

The study of rock mass behaviour is a broad subject in the rock mechanics field which still needs more research and investigation, especially for geotechnical issues associated with dam construction. Since it is difficult to study rock mass behaviour at a large scale in the laboratory, the numerical modelling technique is an alternative method which can be used efficiently in this field. In this thesis two codes have been selected for this purpose. The first code was a continuum code FLAC (Fast Lagrangian Analysis of Continua), which was used to study the effect of a weak rock joint on the stability of a concrete gravity dam as well as to model the gravity dam with its foundation as a continuum. The second code was the Distinct Element Method (DEM) software package UDEC (Universal Distinct Element Code), which was used to study the fully jointed rock mass behaviour under the gravity dam. The equivalent strength and deformability parameters of jointed rock masses were also studied using UDEC. Furthermore, the cause of strain hardening behaviour in jointed rock samples was studied using UDEC and experimental methods. Based on UDEC models, it was concluded that only the low dip angle of the joints on downstream side, dipping upstream had a significant effect on the evaluated deformation of the dam. This was confirmed by using FLAC to model a single weak joint (using interfaces), which may exist in the upstream or downstream direction, on the stability of the dam. Furthermore, to obtain a better understanding of the rock mass behaviour, the equivalent strength parameters as well as the deformation modulus of a rock mass were determined using UDEC. A novel FISH (imbedded language in UDEC) function was developed for this purpose. A range of numerical simulations of uniaxial compressive strength (UCS) and triaxial tests were conducted on the numerical rock mass samples for these purposes. To validate this, the deformation modulus of the rock mass predicted by an analytical equation, quoted from literature, was compared with the numerical results that were given by UDEC. It was found that UDEC can efficiently be used to determine the strength and deformability parameters of rock masses and that at certain specific joint configurations, the rock mass behaviour was similar to granular material. In addition, it was concluded, for the first time, that the non-uniform distribution of axial loading on the jointed rock sample was the main cause of strain hardening especially for a joint that has a dip angle of 60 degree. To overcome this problem, a new loading configuration was developed for applying a uniform distribution of axial stress on the jointed rock samples in order to reduce the effect of platen-rock interaction on the axial stress-strain relationship, using UDEC and laboratory tests. Finally, the study of rock mass strength and deformability parameters of Surqawshan dam’s foundation in Iraq were determined using UDEC and the results were compared with rock classification systems: RMR and GSI. According to the UDEC results all methods give a similar evaluation of friction angle; however, GSI overestimates the cohesion. The UDEC and GSI gave reasonable estimations of deformation modulus, whereas RMR overestimated it. Generally, for rapid estimation of deformability and strength parameters, especially for weak sedimentary rock, the deformation modulus can be predicted from the GSI system, whereas the strength parameters can be estimated using RMR.