Depending on the degree of ground stabilisastion and on the tunnelling technique the amount of ground lost during tunnelling varied from 0.3 to 7.1 percent of the excavated tunnel volume; settlements at the ground surface varied from 1.3 to 42.2mm. It is concluded that the bentonite is likely to provide more effective ground support than compressed air in cohesionless soil and while chemical treatment was more effective than either, the possibility of some additional settlement during the injection process had to be considered