Measurement of in situ stresses in the concrete of the Cahora Bassa dam

Abstract

The 166 m high Cahora Bassa arch dam, built in the Zambezi River in Mozambique more than 40 years ago, is undergoing a process of swelling reactions of the concrete, which has been accompanied since it was detected in its early stages. Several measures were imple-mented, namely the upgrade of the monitoring system and the use of numerical models for analysis of the dam’s behaviour. From the results of these analyses, it was concluded that the compressive stresses in the concrete dam’s body were slowly increasing and it was decided to confirm on site the values obtained by means of in situ stress measurements. A program of stress measurements was established, focusing on critical locations where the calculated stresses reached higher values. The overcoring method was used, with equip-ment originally developed by the U.S. Bureau of Mines (USBM cell). This method was developed for stress measurements in rock masses, where it is used on a routine basis, but its application in dam’s concrete is not usual. This is an innovative aspect of the work done. The paper presents the field work program, details on the USBM stress measure-ments, the values obtained and some considerations on the results.