Page 111 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

ROCK, SOIL, AND MUD 3.5

Rippability. This is a measure of the ease or difficulty with which a rock can be broken by heavy
rippers into pieces that can be economically moved by other equipment, usually scrapers.
At least three factors are involved: resistance to breakage of the rock material itself, the extent
to which it is weakened by bedding layers (lamination) or by joint cracks or fault movement, and
the degree to which the rock has been softened and weakened by weathering.
Many rocks are readily rippable at the surface, but become increasingly resistant with depth
because of less exposure to weathering. A generally rippable rock may contain seams or boulders
that are difficult or impossible to rip.
Some rocks are nonrippable in their natural condition, but can be ripped after shaking up by
light blasting.
In general the igneous rocks such as basalt, granite, diorite, felsite, and lava are not rippable
unless they are greatly weakened by weathering. They do not respond well to light blasting unless
they have a closely jointed structure.
Massive metamorphic rocks such as gneiss, marble, and quartzite are usually nonrippable, but slate
and thinly bedded schist can be ripped.
Some rocks have an absolute resistance to ripping. That is, they cannot be shattered by any amount
of pressure that can be exerted by a ripper tooth. More often, rippability is determined by how
much force can be applied. Limitations are the strength of teeth and shanks, the power available,
and the economics of applying the power.
There are also rocks that will break or tear rather readily, but remain in such large pieces that
they cannot be handled economically. Oversize is sometimes broken by walking the ripper-tractor
on chunks, by a crane or drop ball, or by blasting.
Ripping is usually not economical unless the product is fine enough for efficient scraper loading,
or can be made so by secondary work.
Seismic methods may be used. Rippable rocks seem to carry vibration differently from non-
rippable formations, as indicated in Fig. 3.4A.
Seismic testing is the recognized method of testing rippability. It depends on the relationship
between the cohesiveness of rock and the speed of vibration through it.
A diagram of the method used is shown in Fig. 3.4B.

Weight. Figure 3.5 gives the approximate weights per cubic yard of various materials.
Weight may limit the amount that can be dug or carried in a bucket or body, and the speed with
which the load can be hoisted or transported. It is a critical factor in selection of dragline and
clamshell bucket sizes, and in regard to the length and angle of the boom that carries them.

FIGURE 3.4A Rippability and seismic vibration.