Page 59 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
54 Geotechnical Engineering

Sand Boils
Sand boils are, as the name suggests, sand that has boiled up and spilled out at
the ground surface, usually creating a small cone shaped like a miniature volcano.
They have a considerable significance in engineering because the sand has become
fluidized by vibration, and buildings that are supported on sand that liquefies
as the result of an earthquake either sink into the ground or topple over. The
potential of a sand to liquefy is an important consideration in earthquake-prone
areas, and will be discussed in some detail later in this book. Radioactive dating
of sediments covering prehistoric sand boils can provide important clues to the
recurrence interval of major earthquakes—whether in tens, hundreds, or
thousands of years.

3.6 SUBSIDENCE OVER UNDERGROUND MINES

Mine Shafts
The most dangerous aspect of an abandoned mine is not ground settlement or
subsidence, but the vertical shafts that were used for ventilation and to go in and
out of the mine. Often the shafts are covered by a few loose boards and concealed
in weeds, ready to victimize anything or anybody who should happen to take a
false step. When an old mine shaft is discovered it can be sealed in the same
manner as a sink, with steel rails or beams laid across the opening to support
concrete rubble, which then is grouted to form a solid plug.

Mining Methods
Mines in igneous rock generally pose less of a problem for cave-ins than those in
sedimentary rocks that are more likely to have weak layers. Mines usually tunnel
into discrete rock layers or seams. In the case of coal or limestone mines entire
areas can be mined out in a waffle-like pattern of tunnels called room-and-pillar.
This method commonly takes out about 40 to 50 percent of a rock layer and
leaves the rest to support the mine roof. Additional support may be provided with
timbers, or the roof can be reinforced with rock bolts that are installed into holes
bored into the ceiling. The bolts themselves are steel rods that are grouted
or epoxied in, covered with a steel plate that acts as a washer, and tightened
to create an arch.

Longwall Mining
A relatively recent innovation in coal mining first developed in England is to
remove a long section of an entire coal seam with a cutter that runs back and forth
on a track. The roof behind the machine is temporarily supported by hydraulic
jacks that are individually released and moved forward to keep up with the
mining machine. The cutter is pushed into the seam with horizontal jacks, and
as mining proceeds the roof is let down, leaving little or no void. This procedure
is called longwall mining. While longwall mining is a near-ideal arrangement

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