SYNOPSIS

A major part of the stability problems causing larger slides when tunnelling in crystalline rock are caused by joints or zones filled with swelling clay. the occurrence of swelling clay minerals in faults and joints is discussed as well as the importance of continuous mapping of the encountered rock and its discontinuties as the tunnelling proceeds. Field and laboratory methods used to classify clay minerals and to determine their activity are described. When the highly consolidated clay in the joints is exposed to water through the tunnelling it will expand and a swelling pressure will be built up if the expansion is prevented by rigid supports. A radical drop in the Swelling pressure occurs if the clay is allowed to expand. Thus an expansion of 5 per Cent lowers the swelling pressure with about 50 per cent. In order to lower the swelling pressure, concrete and shotcrete supports have been designed with a compressible material (insulating mat) in contact with the clay zone. The design of supports in connection to clay filled joints and zones considering their geometry as well as the cross-section of the tunnel or cavern is discussed.

INTRODUCTION

Improved tunnelling technique has success-sively made it more and more feasible to utilize underground openings for storage, traffic systems and for water and sewage transport. In the crystalline rocks encountered within the major part of Sweden the Strength of the rock material normally allows for spans up to 20m. Stability problems generally only are met in connection to tectonically disturbed zones. The most costly strengthening efforts and also a number of major slides have occured in Portions where clay filled joint systems are encountered. Especially dangerous conditions are to be expected when the clay Can be classified to be swelling type This paper describes some investigation and tunnel support methods used in rock with clay filled zones.

NATURE OF SWELLING CLAY

Swelling clay, normally montmorillonite, has been encountered in all types of crystalline rock. The occurence of clay rock is mainly depending on one or a combination of the following processes, weathering at low pressures and normal temperature conditions or hydrothermal alteration at high pressures and temperatures. Near the rock surface clay can also be deposited by sedimentation in open joints.

The danger for slides is governed by a number of factors such as:

  • width of the clay zone

  • strike and dip of the zone in relation to the tunnel

  • the mineralogical composition of the clay

  • the degree of consolidation in the clay

  • availability of water

  • the character of the rock in connection to the joint, its strength, joint spacing and tightness

The clay encountered in joints is generally highly overconsolidated and its water content low. If the clay is of swelling type it will start to expand as soon as it is exposed to the humidity of the air in the tunnel. An even faster development of the swelling process will occur if the clay is exposed to free flowing water.

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