Spalling can be a serious threat to underground excavation works in hard, good quality rock masses at great depth. Spalling endangers drill & blast advance or TBM drives with severe damage of tunnel walls or the face. In mining the concern is about spalling at pillars and at the face. Research on spalling is mainly associated with the definition of Crack Initiation (CI) and Crack Damage (CD) stresses and the definition of the S-shaped spalling strength envelope. Particularly CI stress at low confining stress (σC/σ3 < 0.05 and σ1/σ3 < 10) is important for defining spalling around an underground excavation in hard rock. We summarize 59 UCS tests and 197 triaxial tests (σ3 < 2.5 MPa) with sedimentary, magmatic, and metamorphic rocks, respectively, and defined robust CI and CD stresses. In this paper, we focus on the distortional strain energy at CI and CD stresses. From a rock mechanical point of view, crack initiation and crack damage are associated with stress (or strain) deviation. Analyses of the very well instrumented tests shows that for any rock type (saturated and dry) crack initiation (CI) occurs at 20% and crack damage (CD) at 75% of the distortion strain energy at failure. This approach was tested at the well-known case of spalling at the test tunnel in the AECL Underground Research Laboratory. Mechanical properties for the Lac du Bonnet granite were used to estimate distortional energy at CI and to model breakout depth and shape.
Spalling is becoming a recognized problem when excavating underground openings in a competent rock mass under high in-situ stresses. It is agreed that rock masses with RMR > 75, GSI > 70 or Q > 40 qualify for competent rock mass prone to stress driven failure [1, 2]. Spalling may endanger pillars [3] or tunnel faces [4]. Recently, [5] reported about TBM problems in Mixed-Face-Conditions due to spalling.
Spalling is known to occur in hard rock under minor principal stress in the range of σ3 < 0.05 ÷ 0.1 UCS and below σ1/σ3 » 20. The latter comes from research about stable tensile fracture propagation in glass and may not necessarily be applicable to brittle rocks. This area around an underground opening is referred to as the inner shell where stress rupture dominates. In the outer shell shear failure dominates [1]. This leads to the well-known S-shaped strength criterion for brittle rocks (figure 1a). Figure 1b shows the minor principal stresses around a 8 m dia. circular tunnel at approximate 750 m of overburden for different k (= σH/σV).