ABSTRACT
The assessment of tunnelling-induced ground settlements is used to verify the impact of tunnelling on existing infrastructure and establish the need of protective measures, being fundamental to limiting the impact to acceptable levels. The components of tunnel-induced settlements include those arising from the excavation deformation as well as those resulting from ground water draw-down, where applicable.
Conventionally, only positive changes in pore water pressure have been considered for the calculation of the effective stress changes due to groundwater drawdown. However, groundwater draw-down also leads to the development of negative pore water pressure (relative to the atmospheric air pressure) over time in the zone of soil located above the lowered groundwater table, a phenomenon referred to as soil drying. This negative pore water pressure causes an additional change in the effective stress, which is well documented in the literature (e.g. Bishop and Blight 1963). The changes in the effective stress due to suction in unsaturated soils should be considered for an accurate evaluation of the drawdown induced settlements.
This paper presents methods to quantify the draw-down settlements due to tunnelling based on the effective stress principle for unsaturated soils.
