254 Excavation principles
Figure 15.16 A static pre-split formed using expanding grout (Isle of Lewis,
Scotland).An extreme example of the robustness of the technique is illustrated by
the rock face shown in Fig. 15.16, where pre-splitting has been successful
using an expanding grout instead of explosive-in this case it took several
weeks for the pre-split plane to be developed. This technique was used to
avoid vibration damage to an immediately adjacent building. Note that
there was a complete absence of the dynamic stress wave effect in this
situation, i.e. one of the fundamental mechanisms was not invoked, and
yet the pre-split plane formed.
75.3.2 Smooth-wall bl~~ting
We have seen the advantages and effectiveness of the pre-split method of
blasting as it applies to excavations at or near the ground surface. Can pre-
splitting be successful in an underground excavation? The answer is general-
ly no, because of the existence of the in situ stress field. The stress field is all
pervasive, and hence encourages any fracturing to form linear features, thus
not being conducive to assisting in the creation of a circular tunnel. There is,
however, an elegant method of utilizing the stress field to assist in producing
the geometry required-by post-splitting or smoothwall blasting.
As shown in Fig. 15.17, once an initial opening has been created, the
principal stress directions at the excavation boundary become radial and
tangential. The radial principal stress is reduced to zero, and the tangential