Decima Conferenza Annuale A. Croce “Crystal growth and geotechnics”

Crystal growth is a mechanism leading to extreme expansive phenomena and very high swelling pressures against engineering  works.

The lecture presents three cases which resulted in damage and expensive corrective measures in a tunnel excavated in anhydritic claystone, in a viaduct founded on piles and in the compacted access embankments to another viaduct. In the first two cases gypsum precipitation in rock discontinuities led to severe tunnel floor heaving, to very high pressures against tunnel lining and tothe unexpected heave of the central pillars of the viaduct. The third case describes the massive growth of ettringite and thaumasite in embankments reinforced by Portland cement.

As a result the bridge was axially compressed and damaged. In addition the heave rates of embankment surface required a full excavation of upper embankment layers and the construction of additional supporting structures. The paper also describes the fundamentals of gypsum crystal growth and presents a coupled Hydro Mechanical and Chemical  model formulated in a porous media. The model was applied to interpret and simulate the heave experienced by the viaduct. Modelling capabilities were checked against the long term history of viaduct heave and also, against the foundation response when the vertical load from a surface embankment was added to counteract swelling