Artificial ground freezing: from applications and case studies to fundamental research

The use of Artificial Ground Freezing (AGF) to form earth support systems has had applications worldwide. These cover a variety of construction problems, including the formation of frozen earth walls to support deep excavations, structural underpinning for foundation improvement, and temporary control of ground water in construction processes. Excavation of some of the stations of Line 1 of Napoli Underground through loose granular soils and a fractured soft rock provides a recent example of an extensive and successful application of AGF. Construction was accompanied by an intense programme of monitoring designed to measure and control the effects on adjacent structures, which, for its extension and completeness, represented a unique opportunity to collect field data on the performance of AGF. Significant thaw settlements were systematically recorded at all sites, in many cases larger than the heave recorded during the freezing stages. The paper describes the main phenomena that were observed during construction and some of the early analyses that were carried out to interpret different aspects of the process, and then illustrates some recent work conducted in the framework of an international co-operative research project involving academics and technical personnel and engineers of Napoli underground. This includes a preliminary experimental investigation of the behaviour of pyroclastic soils on freezing/thawing cycles, the calibration of a fully coupled termohydro-mechanical constitutive model against available experimental data, and its application to the back analyses of Toledo Station. The final goal of the research is to be able to model the AGF process accurately, thus gaining confidence in the design of other works from the engineering point of view.