This paper presents the results of in situ stress measurements and the application of a global interpretation model for evaluation of the most likely in situ stress field in the rock mass of the underground powerhouse cavern of the Salamonde II re-powering scheme, located in a mountainous zone of the North of Portugal. The objective of the study was to obtain the stress field prior to the powerhouse cavern excavation, for design purposes. Overcoring tests were performed in boreholes, before construction started. During early construction stages, when direct access to the vicinity of the cavern was created, flat jack tests were performed. For estimating the most likely state of stress in the rock mass, a global inverse analysis was carried out, which combined the overcoring and the flat jack tests results, using an appropriate methodology and a three dimensional numerical model. In order to assess the influence of time dependent effects on the in situ stresses, a viscoelastic gravitational analysis was also performed using the same numerical model. The results obtained are compared and discussed.
In situstress tests in rock masses for the design of underground caverns or tunnels are necessarily performed in a limited number of locations and their results are scarce for evaluating the stress field in a representative rock mass volume. Several factors influence the in situ stresses, such as topography, nearby excavations, rock mass heterogeneity, tectonic forces or time dependent effects at the geological scale. Thus, for evaluation of the most likely natural stress field at a specific site, it is important to use global interpretation models, that consider the influence of those factors and integrate the results of all tests conducted at different locations, preferably using more than one testing method.
Several in situ stress measurementswere carried out for design of the Salamonde II hydroelectric scheme, which corresponds to the re-powering of a 50-years old scheme, located in a mountainous zone in the North of Portugal. In 2009, before construction started, six overcoring (STT) tests were carried out in two boreholes and a preliminary analysis of the stress field was done, but it was decided to complement them with additional tests, using flat jacks, as soon as access to the zone of the powerhouse cavern would exist. For this purpose, ten flat jack (SFJ) tests were performed in 2012, at three locations.
A large three dimensional numerical model was developed-which simulates the topography and the underground openings-and an appropriate inversion methodology was implemented in order to integrate all the test results and to estimate the most likely state of stress in the Salamonde II rock mass.