A back-analysis of Beliche Dam

Abstract

A back-analysis of the Beliche Dam in southern Portugal is described. This dam was the subject of a 'Class A' prediction which grossly underpredicted the actual settlement at the end of construction. The analysis described here, which incorporated an elasto-plastic critical state model applied in terms of effective stress, simulated the effect of collapse settlement and by so doing reproduced a amximum settlement which was close to that measured. The earlier underprediction is attributed mainly to the fact that that analysis did not include the collapse settlement due to the partial impounding of the reservoir which occurred before construction was complete. The back-analysis described here covers the period from the start of construction in 1984 to approximately four years after the first complete impounding in January 1988. Comparisons are made between the calculated and measured movements and stresses. Apart from the good agreement with the maximum recorded settlement, the measured settlements generally exceeded the calculated values. Part of the underprediction of settlements is attributed to creep which the modelling did not attempt to simulate and part to the in situ relative density being less than that used in the tests from which the parameters were determined. The computed stress distributions indicated a favourable load transfer from shell to core, thus confirming the safety of the dam against internal erosion due to hydraulic fracture. It was, however, possible to estimate from the settlement records how much of the settlement is due to creep, and this has been done. The conclusions underline the usefulness of the collapse settlement methodology.