NUMERICAL MODELLING OF JOINTED ROCK FOUNDATION SUPPORTING GRAVITY DAM

Abstract

Safety of dams has always been a major concern as it involves high potential hazard. Dam failures can cause huge loss of lives, property and environment and could be an irrecoverable loss. Thus it is important to analyze the dam in all aspects. A gravity dam is generally built on rocks as it requires strong foundation to resist the destabilizing forces through its weight. Thus stresses at heel and toe, strains and displacement of the dam body play an important role in the analysis of a gravity dam. These parameters are influenced by the orientation of joints in the rock foundation. This dissertation analyses a concrete gravity dam built on rock foundation consisting of multiple joints at a spacing of 5 m and dipping at the inclination of 0ο, 30ο, 60ο, 90ο, 120ο and 150ο and striking parallel to dam axis. Using PLAXIS 2D software, a finite element analysis of the dam is carried out for two load combinations: solely taking the dam's self-weight when the reservoir is empty (LC – A), and taking the dam's self-weight when the reservoir is fully filled (LC – B). The dam body and foundation is modelled using Mohr – Coulomb criterion. The joints are modelled using the discontinuity element present in PLAXIS, which accounts for the normal stiffness, shear stiffness, cohesion and angle of internal friction of the joints. The dam foundation is analyzed for stresses at heel and toe and crest displacement for each joint inclination. It is found that for LC – A higher stresses are generated at heel while for LC – B higher stresses are generated at toe. It is found that for LC – A 150ο is the most critical inclination as it generates maximum compressive stress at heel while for LC – B 120ο is the most critical inclination as it generates maximum compressive stress at toe of the dam. A specific case of the proposed Jamrani dam in Uttarakhand, India is analyzed using PLAXIS 2D. The dam foundation is modelled while considering the actual multiple joint sets present in the foundation. The joints are first plotted in Rocscience software while considering the effect of apparent dip. The coordinates thus obtained are used to plot the joints in PLAXIS. Heterogeneity within the foundation is also introduced at a particular location and is extended within the entire dam foundation. The dam is analyzed for crest displacement and stresses at heel and toe under two load combinations LC – A and LC – B. The maximum vertical displacement of the dam is obtained to be -63.77 mm under LC – B. The maximum compressive stress is obtained to be 10.48 MPa at toe of the dam under LC – B.