The development in tunnelling has reached a point, where a wide range of difficult ground conditions is faced much more frequently than ever, due both to tunnel alignments through complex virgin neo-tectonic ground, or through urban environment with often unique boundaries. Both conditions host a similar problem, namely extremely varying stress conditions within a short interval of a tunnel. Varying stress conditions or stress gradients are likely to be invisible and they may not either be caught by deformation monitoring or they may be misunderstood. In all likelihood, they play a considerable role, when a tunnel collapses. This paper tries to identify the essence of rarely reported and often underestimated ground features, which can adversely impact budgets and timelines instantly with substantial effect.

1 Initial Stress State in the Ground

The stress state in the ground is usually identified in the exploration phase of a tunnel project through stress measurement cells (by using the over-coring or hydraulic fracking technique) which enable the definition of the stress state through σv (vertical stress from overburden σv = γ × h), σH as the major horizontal stress component and σh as the minor horizontal stress component in the ground as well as the orientation of the stresses acting in the ground. The values provided are derived with known constraints and deficits of the methods and subsequent with some uncertainties.

Before a tunnel is excavated, the "primary stresses" should in theory be more or less uniformly distributed. However in nature, it would be very rare that something is uniformly distributed. If good or bad, we have become used to such simplifications.

When a tunnel is then excavated, the stresses redistribute in the face of the tunnel excavation and act as σ1, σ2, and σ3 (induced stress) where σ1 is the largest compressive stress (major principal stress) and σ3 is the smallest compressive stress (minor principal stress) or the maximum tensile stress component acting onto a piece of rock.

The focuses of this paper are mainly high stress loads (anisotropic / horizontal stress) which appear to occur also relatively surface near (e.g. through faults with strike-slip modes) and not only deep situated. It concerns both brittle and ductile environment.

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