The International Information Center for Geotechnical Engineers

Ground Freezing

 

1.0 INTRODUCTION

 

This report provides a detailed review of Artificial Ground Freezing (AGF) as a technique to improve site conditions for civil engineering projects.

                    

AGF is a ground improvement technique in which a soil mass of a certain geometry is frozen using a refrigeration process involving a coolant, either chilled brine or liquid nitrogen, which is circulated through freeze pipes embedded in the ground.  AGF is typically used for ground stabilization and groundwater control for a wide variety of applications involving all soil types.  

                                                                                                                   

This report is based on a review of the available literature on ground freezing and provides a brief history of ground freezing and its effects on typical geotechnical engineering properties.  It goes on to discuss considerations for implementing ground freezing in the field, as well as the advantages and disadvantages of the process.  Finally, two case studies of AGF implementation in the field are reviewed.  

 

2.0 HISTORY

 

Artificial ground freezing (AGF) is a soil stabilization technique involving the removal of heat from the ground to freeze a soil’s pore water.  The concept of ground freezing was first introduced in France, and industrial applications date back to 1862 where it was used as a mine shaft construction method in South Wales (Schmidt 1895).  The method was eventually patented by German mining engineer F.H. Poetsch in 1883 (sometimes termed Poetsch Process).  The method involves a system of  pipes consisting of an outer pipe and concentric inner feed-pipes where a chilled coolant (calcium chloride brine, typically) is circulated.  The coolant is pumped down the inner pipe and back up the outer pipe.  It is then cooled again through a refrigeration process and returned through the pipe system.  A further development on the AGF technique occurred in France in 1962, when liquid nitrogen (LN2) was pumped into the freeze pipes instead of chilled calcium chloride brine.  This allows for much faster ground freezing if necessary.  The liquid nitrogen runs through the freeze pipes and is allowed to evaporate into the atmosphere (Sanger and Sayles 1979).  

 

Currently, AGF has been applied to a wide variety of engineering projects where stability, groundwater conditions, and containment are an issue.  Example situations include: vertical shaft construction for mining or tunneling, stabilization of non-engineered earth fills (large obstructions), sites that require horizontal access (e.g. a TBM canopy for cross passage construction), lateral and vertical contaminant containment, contaminant redirection, groundwater cutoff (can be tied into bedrock), and emergency support/stabilization using LN2 (Schmall and Braun 2006).  

During the process, heat is removed from the soil in a cylindrical pattern around freeze pipes.  This produces columns of frozen soil.  The columns continue to expand until they intersect.  From here, the frozen mass will expand outwards creating a wall or solid ring of frozen soil (Sanger and Sayles 1979).

 

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