All unsupported excavation surfaces are principal stress planes 39shear stresses acting on them and are therefore principal stress planes. This
results from Newton's Third Law ('to every action there is an equal and
opposite reaction'). Furthermore, and also from Newton's Third Law, the
normal stress component acting on such surfaces is zero. Thus, we
know at the outset that the stress state at all unsupported excavation
surfaces will be
or in principal stress notation
expressed, respectively, relative to an x-, y-, z-axes system with x
perpendicular to the face, and the principal stresses acting as shown in
Fig. 3.8.
In Fig. 3.8(a), the pre-existing stress state is shown in terms of the prin-
cipal stresses. In Fig. 3.8(b) the stress state has been affected by excavation:
both the magnitudes and directions of the principal stresses have
changed. Neglecting atmospheric pressure, all stress components acting on
the air-rock interface must be zero.
It should also be noted that the air-rock interface could be the surface
of an open fracture in the rock mass itself. Thus, as we will discuss further
in Chapters 4,7 and 14, the rock mass structure can have a significant effect
on the local stress distribution.
on TXy = Txl = 0
excavation surface(a) (b)
Figure 3.8 (a) Before excavation. (b) After excavation.