The observations and measurements carried out at Kalhovd Dam (Statkraft) and Nesjen Dam (Sira-Kvina) during the summer of 2017 have shown that in many pillars a series of cracks are emerging in the concrete and progress differently compared to the theoretical estimation. It is likely that other dams suffer from similar symptoms. We believe that one reason behind this behavior is the presence of large differences in topographical gradients at the concrete rock interface. Consequently, it was hypothesized that the topographical profile affects the strength and the stability of the concrete dams.
During spring of 2017, PhD candidate Dipen Bista and Dr Gabriel Sas conducted a series of laboratory tests at LTU laboratory facilities. The objective of the testing program was to study the effect of large asperities on the stability of dams. Twenty-two tests on unbonded concrete to rock joints have been carried out under different loading conditions. The joint was composed of a concrete part, at the top, and a rock part, at the bottom. Each rock sample had a triangularly shaped asperity carved in the rock part. Three different locations were chosen for the asperity along the profile, start, middle and end, with respect to the loading directions. Two different loading schemes have been used, pure shear and eccentric shear loading with respect to the position of the contact zone.
The tests have been monitored using a novel non-contact method, digital image correlation. The measurements consist of photographing a predefined pattern during the loading procedure. With this method virtually any displacement point on the monitored area can be measured with high precision, therefore the failure progress and can be continuously recorded and quantified. The samples have been scanned before and after testing with a high precision 3D scanner based on the blue light technology. The method could provide important measurement data into identifying the influence of the material variability and geometrical imperfections on the failure plane. These are an unprecedented measurement technique within our field, and they have been tested and will be evaluated as a secondary objective in order to understand if it can be used in real field applications.
The results have shown that the position of the asperity has a great influence on the strength of the interface under the influence of eccentric load which is typical for dams. The further the asperity from the loading direction the greater the strength is.
This experimental program is the first step in attempting to define a realistic potential failure path in small concrete plate dams, the aim of Dipens’ research work.