Page 61 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
56 Geotechnical Engineering

Answer: At the end of collapse the depth X that equals the initial height of the tunnel will
contain 10 percent voids. Therefore
0:10 X ¼ 2m
X ¼ 20 m ð60 ftÞ
As 20 m550 m depth, the answer is no.

Boring into a collapsed mine tunnel therefore may only mean boring into
some loose rock, which may be difficult to detect. The best procedure is to
use a recirculating fluid such as a drilling mud to carry cuttings up out of the hole,
because if loose rock is encountered there will be a drop in pressure of the drilling
fluid.

Mine Maps
The most useful tool for discovery of an abandoned mine should be a map made
at the time of mining, but records often are inaccurate and may no longer exist.
As many small mines were marginal operations using mules or horses to take out
the coal or other rock, they may not have included the luxury of accurate
mapping. The main sources of maps are state and federal geological surveys
and bureaus of mines, and mining companies if they still exist.

Drilling in Rock
The most common drill bit used for production drilling in rock is the Hughes
tricone bit that has three toothed rollers that impact the rock. In softer rocks such
as shale a hardened steel ‘‘fishtail’’ bit may be used. Where rock cores are required
for identification or testing, a hollow-core bit laced at the end with industrial
diamonds is used.

Regardless of the type of bit, some type of circulating fluid is required to cool
the bit and bring up cuttings. Most common is a mud made by mixing
bentonite, which is expansive clay mineral, with water. This is down the hollow
‘‘drill stem’’ to come out at the drill bit. The mud is pumped down under pressure
and carries drill cuttings upward in the annulus between the drill pipe and the
wall of the boring.

A bentonite mud is used because in the event of an interruption in drilling, it is
‘‘thixotropic,’’ or touch-sensitive, so it sets into a weak gel that keeps the rock
particles suspended instead of sinking down on top of the drill bit. The same muds
are used in construction of retaining walls by excavating vertical slots in the soil,
called slurry trenching.

The next chapter deals with sedimentary soils that are not cemented into
rocks and comprise the bulk of engineering soils, and Chapter 5 deals with the
weathering changes that result in a soil profile that can include expansive clays.

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