Page 324 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 21

Method of Excavation

Blasting for underground construction purposes is a cutting tool, not a bombing operation.
Svanholm, Persson, and Larsson (1977)

EXCAVATION TECHNIQUES

Excavation of rock or soil is an important aspect of a civil engineering project. The
excavation techniques or the methods of excavation of rocks differ from those in soil.
Similarly, these also change with the type of project.

Broadly, methods of excavation can be classified according to their purpose, that is,
whether the excavation is for foundations, slopes, or underground openings. Methods of
excavation in a broader sense can be divided into three types:

1. Digging
2. Ripping
3. Blasting

A system was proposed by Franklin, Broch, and Walton (1972) to classify methods
of excavation based on rock material strength (Figure 21.1). Figure 21.1a shows
a plot between the point load strength index of rocks and fracture spacing, whereas
Figure 21.1b is drawn between point load strength index and rock quality. Using these
figures, we can select a method of excavation for a particular rock; for instance, a
rock of medium strength and medium fracture spacing is classified as medium rock
(Figure 21.1a) and should be excavated by ripping (Figure 21.1b). There is too much
confusion on the soil–rock boundary line. ISO 14689 (2003) defined a geological (rock)
material having a uniaxial compressive strength (UCS) less than 1 MPa as soil.

This classification is useful when estimating the cost of excavation, which should be
paid to a contractor who may not prefer to change the method of excavation according to
rock condition.

ASSESSING THE RIPPABILITY

Assessing the rippability is also an important aspect of excavation. Even stronger rocks
such as limestones and sandstones, when closely jointed or bedded, are removed by
heavy rippers to at least the limit of weathering and surfacial stress relief.

Sedimentary rocks are usually easily ripped. Rippability of metamorphic rocks, such
as gneisses, quartzites, schists, and slates, depends on their degree of lamination and mica
content. Igneous rocks are often not possible to rip, unless they are very thinly laminated
as in some volcanic lava flows.

Engineering Rock Mass Classification 281
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