Strength and creep testing for artificial ground freezing
Artificial ground freezing (AGF) provides a means by which excavations can be given temporary or permanent structural support. It may also be used to control the movement of groundwater without the risk of pollution of potable aquifers. As AGF is called upon to strengthen ground at ever increasing depths, the design process needs to be adapted to account for the greater stresses encountered. In strong materials, the prime consideration is the short term strength of the materials and closed-form formulae can be used in design. In weaker materials, the time dependent creep behaviour of the frozen ground predominates and more complex analysis techniques have been devised (e. g. finite elements). Previous works in this field have been chiefly concerned with uniaxial states of stress. In this thesis, consideration is given to the problem of modelling creep under triaxial stress conditions. An introduction is followed by an outline of the general applications and design procedures currently used in ice wall design. Descriptions are then given of a selection of soils and weakly cemented rocks which have been incorporated into a programme of tests to investigate both short and long term strength behaviour. The apparatus available at the start of this project was suitable for uniaxial and low pressure triaxial tests only. Equipment subsequently developed to extend the confining pressure capability to 12 MPa, is described in detail. Short term strength tests show the increase in strength on freezing of ground materials is almost entirely due to the cohesion contributed by the ice matrix. Analysis of the creep test results leads to the development of a new empirically based triaxial creep equation for frozen soils. A sensitivity analysis of the parameters in this equation is followed by its application to a simplified design. Suggestions for further work in this field are included.