Freezing-thawing tests on natural pyroclastic samples

Artificial Ground Freezing is one of the construction techniques that were extensively adopted during construction of Line 1 of Napoli Underground, to ensure stability and waterproofing of the platform tunnels and inclined passageways during excavation below the ground water table through loose granular soils of pyroclastic origin (Pozzolana) and a fractured soft rock (Neapolitan Yellow Tuff). The aim of the paper is to study the behavior of the Pozzolana and Neapolitan Yellow Tuff subject to freezing and thawing cycle at constant vertical stress under oedometric condition. As the temperature decreases the ice content increases and a liquid ice-surface tension, σ-li, develops at the interface of the two phases as the temperature decreases. This tension must be balanced by the suction s defined as the difference in frozen and liquid water pressure Pi and Pl respectively. Two oedometers working under suction and temperature controlled conditions have been modified to work with temperature below zero degree. The stress path followed by samples reproduce the in situ vertical stress interested by freezing and thawing.