Evaluation of high local groundwater inflow to a rock tunnel by characterization of geological features

High local groundwater flow into rock tunnels is a technical and environmental problem for underground constructions. Geological features such as fault zones, open fractures and dykes can be the source of very high local inflows. However, prediction of possible groundwater inflow from these features using analytical and numerical tools often failed due to given hydrogeological assumptions and simplification of these heterogeneous media. In fact, the characteristics of geological features are not often detected by normal exploratory methods and missed from geological models. In order to identify factors that affect inflows from the geological features, a detailed investigation has been carried out at a tunnel excavated to supply potable water to the city of Semnan in Iran. The tunnel passes through formations of Cambrian age with thin and thickly laminated limestone, sandstone, shale and siltstone, which thrust on tuff Formation of Tertiary age by a main thrust fault. It was found out that:

(1)

High local inflows (from 20 up to 750 l/s) come from five geological features and much of tunnel is dry or has less inflow.

(2)

Maximum inflow occurs in an open fracture with amount of 750 l/s.

(3)

Faults have different hydraulic effect; they may act as conduits, barriers or combined conduit-barrier systems.

(4)

Dykes are generally barriers along which a large volume of water can be stored.

(5)

Evaluation of high local groundwater inflow to rock tunnels based on characterization of geological features is more reliable compared to available analytical and empirical estimation.