Measurements on a tunnel lining in very dense till

Abstract

Measurements were made on a 4.3 m (14 ft) bored diameter, 3.0 m (10 ft) finished diameter tunnel for a sewer in Mississauga, Ontario. Measurements included deformation of the flexible temporary lining, pressures on the temporary and rigid concrete permanent linings, soil deformations and surface settlements as the tunnel was excavated, and the effect of the tunnel on groundwater levels.Because of the stiffness of the very dense till and the method of construction it was found that during the period of measurement the tunnel opening deformed and was largely self-supporting. Some increase of loads on the linings with time was observed, but the rate of increase was almost negligible after 6 months. The water table around the tunnel was lowered as the tunnel and the surrounding pea gravel fill acted as a drain.Indications are that the tunnel has nearly uniform all-around pressure on it, considerably less than the overburden pressure, and, therefore, that it could be considered in ring compression without bending moments in either lining. If bending moments are not present, the permanent lining could be reduced from the 460 mm (18 in.) used to half that thickness or less and still be quite conservative in supporting all-around loads equal to the overburden pressure and providing for wear.