Rock mass behavior under hydropower embankment dams with focus on fracture erosion and rock mass stability

Abstract: In Sweden there are about 190 large hydropower dams of varying age. According to the definition by the International Commission of Large Dams (ICOLD), if the height of the dam exceeds 15 m it is referred as large. Majority of these dams are of embankment type. The peak of the dam construction in Sweden was between 1950 and 1980, hence, the major part of the dams are between 30 and 60 years old. It is well known that the strength of dam body and foundation rock deteriorates with time. For this reason there are ongoing concerns for the hydropower industry regarding the production and safety of the dams. Currently, the majority of research efforts are concentrated on degradation of hydropower dams whereas less attention is paid to the bedrock under the dam, which is a critical factor for construction integrity and functionality. This work is focused on: 1. Better understanding of the rock mass response to the loads caused by construction and exploitation of a hydropower dam, i.e. the loads from the weight of the dam and the water in the reservoir; 2. Validation of the developed conceptual model on the case study; 3. Investigation of how static and cyclic loads of the hydropower dam effect the fracture erosion and rock mass stability and grout curtain based on case study (in long-term perspective). An extensive literature review of this field is performed. Based on it a series of factors are selected which are important in terms of opening and shearing along geological structures under dams and reservoirs. Developed conceptual model together with numerical code UDEC (2D) helped to identify the most significant (stress conditions, structural geology and properties of discontinuities) conditions of the rock. The construction of a dam on rock foundation (with its water reservoir) causes redistribution of the stress field, and affects the state of mechanical and hydro-geological properties of the rock mass beneath the dam. The combination of sub-horizontal discontinuities (bankning discontinuities) with sub-vertical discontinuities, which are perpendicular to the river valley, give rise to a water leakage under the dam. This behavior depends on the direction of the dip angle of the sub-horizontal discontinuities, either downstream or upstream. The adverse effect is caused by downstream direction. Magnitude and direction of the in-situ stress field, density of discontinuities, and friction angle of joints are also major factors which affect the behavior of the rock mass considerably. All these findings are in good agreement with the results of other authors. As the developed numerical model showed good agreement with previous research it was adopted for a case study. Håckren dam (Sweden) had been selected for this purpose. A series of field (mapping, RMR, Q, GSI) and laboratory tests (Bulk density, Porosity, Water content, Point Load Strength index) were applied. Results of numerical models showed a good agreement with monitored leakage into the inspection gallery. The deformations within the foundation rock mass has not been monitored therefore numerical results has been evaluated based on the literature review. Results from the numerical models in terms of shear and normal deformations (openings) in the bedrock did not show significant deformations (around 1 mm for shear deformation and less than 0.1 mm in case of opening). Cyclic loading of the water in the reservoir results in accumulation of the deformations within the rock mass. Such processes may create favorable conditions for development of erosion within the bedrock. Numerical results showed an increase of accumulated shear deformations within the rock mass within the first years but then it stabilized. These deformations are occurring along subhorizontal discontinuities in downstream side of the dam and along intermediate joints on the upstream side. The magnitude of opening of the discontinuities is staying low, less than 0.3mm. The inflow into the inspection tunnel increased only during the first years, and then stabilized. However the flow velocity around the inspection tunnel showed a small increase along several simulated years. Numerical simulations showed the stability of the foundation rock of the Håckren dam after construction and within 10 idealized years of the exploitation of the dam. The used approached showed its validity and applicability for preliminary evaluation of the foundation rock of the embankment dams, based on the Håckren dam case.