The observation of the behaviour of concrete hydraulic structures, using foundation piezometry and measurement rates of water flow through the drainage system, has made it evident that the uplift pressure figures can vary throughout the year, and that the structure/foundation system can be strongly affected by the environmental thermal variations of a seasonal nature. The authors have queried the historical hehaviour of the piezometric variations of eight hydroelectric schemes in Brazil and have arrived at the conclusion that this phenomenon can be present in any kind of concrete dam structure. Throughout the work, the authors attempt to prove the validity of this claim and to query the physical nature of the phenomenon, going a little beyond its simple evidence.
The hydraulic concrete structure behaviour observed through foundation piezometry and low rate measurement in the drainage system has shown that the uplift pressure figures may not be constant throughout the year and that the structure foundation system may be strongly affected by ambient thermal variations of a seasonal nature.
This result was first seen in physical environments subject to severe climatic Conditions, where the annual thermal oscillations reach, on an average, some tens of centigrade degrees per year. Even so, the phenomenon was observed at first when analysing the behaviour of hollow concrete structures such as buttress dams, Or example. In fact, it is in structures of this type that the thermal oscillations can induce more significant volumetric variations.
The authors of this work, have queried historical behaviour of the piezometric variations in hydroelectric schemes in Brazil, and have concluded that this phenomenon can be present not only in hollow concrete but also in gravity dams. They also concluded that seasonal oscillation up lift pressures can appear in enviro nments subject to considerably reduced thermal variations.
The authors attempt to prove the validity of this statement, as well as query the physical nature of the phenomenon. They are also convinced that the concept of uplift pressure figures susceptible to periodic variations may reflect in the design hypotheses and adopted uplift pressure criteria.
The authors had access to the data file of CESP - Companhia Energetica de São Paulo, Central Civil Engineering Laboratory,that supervises the operation of all its plants.
The chronological records of eight power plants (Barra Bonita, Ibitinga, Promissao Jupia,Nova Avanhandava, Água Vermelha, Jurumirim and SaIto Grande)were investigated, totalling 752 piezometers and water level gauges. A seasonal cyclic presence was observed in the uplift pressure levels of six out of the eight power plants.
The seasonal cyclic nature of the piezometric oscillations was noted in the six power plants and recorded in 25 of the 641 instruments analysed, that is, about 4% of the apparatus. In most cases, they were instruments installed near the rock/concrete contact, in the upstream side of the structures, at a short distance from the reservoir. In this way, the percentage increased around 10% if only contact piezometers were considered.