Effects of hydraulic repeated loads on the hydromechanical response of an unsaturated silty soil

Abstract

Soils used in earth constructions are mostly unsaturated, and they undergo frequent wetting-drying cycles due to changes in the climatic conditions, particularly at shallow depths. Repeated hydraulic loads are also induced in levees and dykes by changes in the water height, which affect the extent of the unsaturated zone. Changes in water content significantly influence the hydromechanical behaviour of the construction material, which therefore has to be assessed for repeated hydraulic loads. This research work investigated the coupled hydromechanical behaviour of a silt, typically used in the construction of dykes, with the aim of providing a better understanding of the consequences of wetting-drying cycles in the field on the overall response of the material. The soil was studied in the laboratory by means of a comprehensive series of complementary experimental tests. The first series involved wetting-drying and loading-unloading stress paths applied in suction-controlled oedometer. Irreversible changes in the degree of saturation implied that hysteresis of water retention is induced by changes in the suction and volume of soil samples. A rate form of the water retention curve (WRC) model was proposed to simulate the hysteresis, in which the contact angle was used to express the dependence of the response on non-monotonic changes in suction and void ratio. A constitutive hypoplastic model was then proposed and coupled with the contact angle-WRC model to account for the coupling between the hydraulic and the mechanical behaviours. The model was then employed to simulate the experimental hydromechanical response observed in laboratory tests. The second series of experimental tests was performed to study the impact of wetting-drying cycles on microstructural features and hydraulic properties of ascompacted samples. When as-compacted samples were subjected to wettingdrying cycles, fabric changes took place due to interactions between different structural levels even though no significant total volumetric strain occurred. Accordingly, the water retention behaviour detected for such samples was different from the as-compacted one. The new WRC model was proposed accounting for different structure levels and an evolving pore size distribution of the tested materials. Eventually, the effects of repeated hydraulic loads on the mechanical behaviour of the silt were studied using constant water content triaxial tests. The hydraulic repeated loads changed the soil fabric by forming larger pores, resulting in fundamental changes in their hydromechanical response. The hydraulic repeated loads increased the compressibility of the soil at low-stress levels. Since samples subjected to repeated hydraulic loads experienced more volumetric contraction during compression, they became slightly denser than the as-compacted one being subjected to the same stress level. Moreover, when such samples subjected to the same suction, their hydraulic state changed with respect to their corresponding water retention curves, where samples subjected to repeated hydraulic loads expelled more water than the as-compacted one, and hence, such samples became stiffer and exhibited higher strength associated with dilative behaviour during subsequent shear loading.

Date of Award	Mar 2016
Original language	English
Awarding Institution	Polytechnic University of Milan
Supervisor	Cristina Jommi (Supervisor) & Guido Musso (Supervisor)