Structural safety assessment of concrete dams

Research focus

The Peer review has evaluated this group as Good

This reseach topic concerns structural mechanics problems of large concrete dams with focus on their diagnostic analysis and integrity assessment since most of the existing dams (in Italy as well as in other European countries) designed and built up several decades ago, are frequently affected by structural damages due to physical-chemical phenomena and/or to past extreme loadings, such as earthquakes. The evaluation of damages and of safety margins for such aging structures is a topic with obviously important social and economical implications, also with regard to the planning of possible rehabilitation works. The research activity at DIS on the above subject has been carried out in the framework of PRIN projects co-financed by the Italian Ministry for University and Research and of the European Network on 'Integrity Assessment of Large Dams' (IALAD) founded, among others, by the Milan research group and intended for the promotion and the dissemination of research results concerning dam safety assessment, diagnosis, maintenance, repair, rehabilitation and analysis tools for the numerical simulation of the structural behaviour of concrete dams. The various research activities on dams carried out in the present reporting period are briefly described in what follow. (A) Modelling of the alkali-aggregate reaction (AAR) in concrete. In several concrete dams designed decades ago, AAR causes the formation of a gel which swells in the concrete pores inducing micro-cracking with consequent strength and stiffness degradation. Research was aimed at developing a chemo-elastic model with stiffness reduction apt to achieve a good compromise between realistic- prediction capability and computational simplicity. (B) Overall diagnostic analysis of damages on the basis of the structural response to static or dynamic excitation. Reduction of elastic moduli can be assumed to be representative of damages of different source affecting dams. Fast or seasonal fluctuations of the reservoir water level can be exploited in inverse analysis procedures aimed at the identification of the above material parameters on the basis of experimental data obtained through traditional (pendula, collimators) and innovative (radar, laser) monitoring instrumentation. Seasonal variation of hydrostatic loading requires that thermal effects are accounted for: this was done by a novel method exploiting time periodicity. Traditional dynamic diagnostic analyses (via vibrodynes and accelerometers) have been improved by the evaluation of added masses and damping through decoupled boundary element analysis of the reservoir and by the use of Kalman filter for stochastic parameter identification. (C) Local diagnostic analysis by nondestructive in situ tests and inverse analysis. Traditional testing techniques, namely flat-jack tests superficially and in depth dilatometric tests, have been combined with computer simulations, inverse analysis and soft-computing (specifically neural networks) to economically assess in-plane stress components, anisotropic elastic moduli, compressive and tensile strength and fracture energy, crucial parameters for dam diagnosis in view of rehabilitation works. (D) Modelling of damage and fracture in concrete dams. The safety assessment of concrete dams requires the reliable description of the initial degradation and of the subsequent damage localization and evolution as fracture propagates. Research efforts have been hence devoted to the development of numerical strategies for nonlocal damage analyses and discrete crack propagation, implementing an effective extended finite element method with a proper continuum-discrete transition criterion. (E) The role of interfaces on dam failure/structural integrity. The role of the mechanical characteristics of the foundation interface in the failure mechanism of concrete gravity dams has been investigated by simplified direct analyses, considering possible water penetration into the joint. A so58 phisticated model for joints under cyclic loading has been developed for detailed analyses in critical situations for dam structural integrity. Inverse analysis techniques have been proposed to calibrate model parameters. (F) Failure phenomena in earth dams. Some research work has been carried out on the extention of some of the proposed models for concrete dams to description of the failure of earth dams, by considering strain localization in saturated soils or irreversible thermal effects in partially saturated soils. Research still in progress considers rockfill and blastfill dams by means of a multiscale approach based on the distinct element method. (G) Generalized limit analysis. Classical limit-analysis based on maximization of live-load factor up to plastic collapse has been generalized for safety margin assessment with respect to collapse, excessive deformation and local fracture, applicable also to full saturated poro-plasticity.

Dipartimento di afferenza

Dipartimento di Ingegneria Strutturale (DIS)

Docenti afferenti

Full Professors
Giorgio Novati (since Nov. 2005)
Claudia Comi
Umberto Perego
Associate Professors
Gabriella Bolzon
Giuseppe Cocchetti
Assistant Professors
Roberto Fedele (Post-Doc 2003-06)
Stefano Mariani