The many advantages afforded by building underground are of little worth if the geological conditions are not favourable. Fortunately, the rock underlying much of Sweden is mostly suitable for building tunnels and rock caverns.
Much of the country lies on a very hard basement of Precambrian Shield rocks, which either form the surface or are covered by a thin layer of soil. The basement mainly comprises granitoid and metamorphosed rocks, transformed crystalline rock types that contain varying degrees of fractures and crushed zones. The solid rock is generally very strong, but permeability in the fractures and crushed zones can vary considerably, affecting the conditions for rock construction.
In some parts of Sweden, the crystalline basement is overlain by sedimentary rocks, mainly sandstones, limestones and shales. The sedimentary rocks differ from the crystalline basement in two main ways. One is that the strength of the individual rock types varies, from very strong to very weak. The other is the layered nature of the rocks. Often, the solid sedimentary rock types are more porous, which can have a negative effect on deformation and permeability properties.