ABSTRACT:

Axial and lateral ground deformation due to axial freezing are experimentally studied with sand and clay samples from two tunnel projects. For the project a special cryogen 3D-triaxial deformation apparatus was developed to perform frost heave tests under stress and drainage conditions that are comparable with the tunnel locations. Results and characteristics of deformation behaviour between sand and clay are discussed.

INTRODUCTION

In the Netherlands two major tunnels projects will be constructed with tunnel bore machines. The Westerschelde Tunnel has a maximum depth of 60m below the Westerschelde estuary and shall be bored partly through stiff Oligocene overconsolidated Boom Clay. The Green Hart Railway Tunnel is constructed onshore at a maximum depth of 30m below surface and will run mainly through unconsolidated Pleistocene sands. In the Westerschelde Tunnel (motorway) numerous transverse service galleries shall be constructed between the bored main tunnels for safety purposes. The connecting transverse galleries will be built between the tunnels with ground freezing techniques. With these studies frost heave – that is expected between the two main tunnel linings – is investigated experimentally. Theoretical and experimental studies on stress development during ground freezing operations for tunnelling projects are financed by the Project Organisation HSL-Zuid.

ARTIFICIAL GROUND FREEZING AND STRESS DEVELOPMENT

Artificial ground freezing enhances the strength of soil temporarily and is an effective technique used in civil engineering. In construction practice cooled salt water or liquid nitrogen is pumped through a net of tubes that have been installed in the soil to be frozen. Normally a temperature of –20°C is used. Ground freezing increases the strength of the soil necessary during the construction phase of the connecting transverse tunnels. Deformation and stress development is expected because of cryosuction and ice lens formation.

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