The role of suspension and dissolution on strength and deformation of soft carbonate rocks

Chalks are highly porous rocks formed of mainly carbonate grains bonded together by calcite bridges. From a microstructural point of view they are very similar to calcarenites. The above characteristics make them prone to water induced weathering, frequently featuring large caverns and inland natural underground cavities. This study is aimed to determine the main physical processes at the base of the short - and long-term weakening experienced by these rocks when interacting with water. We present the results of microscale experimental investigations performed on calcarenites from different sites in Southern Italy. SEM, thin sections, X-ray CT observations and related analyses are used for both the interpretation -definition of the structure changes and the identification -quantification of the degradation mechanisms. Two distinct types of bonding could be identified within the rock: depositional bonds and diagenetic bonds. The diverse mechanisms linked to these two types of bonding explain both the observed fast decrease in rock strength when water fills the pores (short-term effect of water), identified with a short-term debonding (STD), and a long-term weakening of the material, when the latter is persistently kept in water saturated conditions (long-term effect of water), identified with a long-term debonding (LTD). To highlight the micro-hydro-chemo mechanical processes of formation and annihilation of the depositional bonds and their role in the evolution of the mechanical strength of the material, mechanical tests on samples prepared by drying partially saturated calcarenite powder, or a mix of glass ballotini and calcarenite powder were conducted. Finally the use of transparent chemical consolidants as a more sustainable alternative to grouting is explored experimentally.