Engineering Geology

Chapter 9
The ease of drilling in rocks in which there are many discontinuities is influenced by their
orientation in relation to the drillhole. Drilling over an open discontinuity means that part of the
energy controlling drill penetration is lost. Where a drillhole crosses discontinuities at a low
angle, this may cause the bit to stick. It also may lead to excessive wear and to the hole going
off line. Drilling across the dip is generally less difficult than drilling with it. If the ground is
badly broken, then the drillhole may require casing. Where discontinuities are filled with clay,
this may penetrate the flush holes in the bit, causing it to bind or deviate from alignment.
Spacing of the blastholes is determined on the one hand in relation to the strength, density
and fracture pattern within the rock, and on the other in relation to the size of the charge.
Careful trials are the only certain method of determining the correct burden and blasting
pattern in any rock. As a rule, spacing varies between 0.75 and 1.25 times the burden.
Generally, 1 kg of high explosive will bring down about 8–12 tonnes of rock. Good fragmentation
reduces or eliminates the amount of secondary blasting while minimizing wear and tear
on loading machinery.
Rocks characterized by high specific gravity and high intergranular cohesion with no preferred
orientation of mineral grains cause difficulties in blasting. They have high tensile
strength and very low brittleness values, the high tensile strength resisting crack initiation and
propagation upon blasting. Examples are provided by gabbros, breccias and greenstones.
A second group that provides difficulties includes those rocks, such as certain granites,
gneisses and marbles, which are relatively brittle with a low resistance to dynamic stresses.
Blasting in such rocks gives rise to extensive pulverization immediately about the charged
holes, leaving the area between almost unfractured. These rocks do not give an effective
energy transfer from the detonated charge to the rock mass. The third category of rocks
giving rise to difficult blasting is those possessing marked preferred orientation, mica schist
being a typical example. The difficulty arises from the influence of the mechanical anistropy
due to the preferred orientation of the flaky minerals. These rocks split easily along the
lineation but crack propagation across it is limited.
In many excavations, it is important to keep overbreak to a minimum. Apart from the cost of
its replacement with concrete, damage to the rock forming the walls or floor may lower its
strength and necessitate further excavation. What is more, smooth faces allow excavation
closer to the payline and are more stable. There are two basic methods that can be used for
this purpose, namely, line drilling and presplitting.
Line drilling is the method most commonly used to improve the peripheral shaping of excavations.
It consists of drilling alternate holes between the main blastholes forming the edge of the
excavation. The quantity of explosive placed in each line hole is significantly smaller and
indeed if these holes are closely spaced, from 150 to 250 mm, then explosive may be placed
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