Post-failure investigations are usually much more complex than design calculations. The latter can afford to be fairly simplistic, provided their inaccuracies remain on the conservative side. The interpretation of real accidents, however, must incorporate the precise mechanisms and behaviour of the materials; for that role, parameter values cannot be conservative, they are simply erronous.

Principia studied for the owner the failure of the Aznalcóllar tailings pond and established that the failure occurred because the design had not accounted for the slow consolidation of the underlying blue clays anf for the brittleness of their behaviour. To be able to interpret the accident, the numerical simulation must face many complex challenges: gradual construction over 20 years, the coupled character of structural and hydraulic phenomena, the marked brittleness of the clay behaviour, the strong accelerations undergone by the dam upon liquefaction of the tailings, etc.

The figures show an aerial photograph taken after the accident, together with an image generated for an animated explanation of the failure. In a fraction of a minute, the dam moved tens of metres along hundreds of metres of its length.

A lorry, travelling on the N-VI motorway with the tip body raised, impacted a large signal portal frame. The impact caused the frame to overturn, producing several fatalities and blocking all traffic in this major artery near Madrid for several hours. This accident is one of a number of them analysed by Principia, in this case collaborating with Esteyco.

Principia carried out the in-situ observations of the various elements, the effects caused by the accident and the marks left by it. This information allowed conducting realistic simulations of the events in the computer, as well as reproducing the failure process in a reliable manner.

The first photograph shows the underside of the lattice, with the marks left by the impact of the lorry. The second comes from an animated visualisation of the accident sequence, developped to explain its evolution and characteristics.

Having experienced a fire, as well as the collapse of an electricity pylon supporting two intermediate-voltage lines, the question posed was to determine the sequence of events. A preexisting fire could have caused the collapse of the pylon. Alternatively, arcs formed by the cables of the fallen pylon could have started the fire.

Principia analysed the existing information and investigated the various possibilities using numerical simulations of the relevant phenomena. As a result, it was possible to establish that the fire had preceded the collapse of the pylon and that the failure mechanism of the latter had been embrittlement by contact with liquid metal.

The photographs show the collapsed pylon and a virtual image from an animated sequence developped for explaining the events: the pylon emerges through a roof and its upper part is in the process of falling down.