(TFT) What any good Dwarf should be able to tell you.

["Jay Carlisle" <Jay_Carlisle@charter.net>]

A tunnel is a long, narrow, essentially linear excavated underground 
opening, the length of which greatly exceeds its width or height.
A shaft can simply be thought of as a tunnel which is vertical rather than 
horizontal.
A cavern is an underground opening whose length and width are roughly 
similar.
Every underground excavation can be looked upon as a combination of tunnels 
and caverns.

Early excavation likely centered around clearing and enlarging naturally 
occurring caves and caverns for shelter and protection.
Cave systems are most easily detected by water flow in an area.
A prevalence of disappearing streams and emerging springs, especially in 
very rugged limestone terrain, are likely indicators of caves.
Faults, sinkholes, eroded surface rock, geothermal activity, and pronounced 
depressions are other signs of cave systems.

Stone like flint was mined well before the bronze age.
Flint often occurs in nodules found in chalk deposits, allowing mining with 
extremely primitive tools such as antlers, bone, wood, or flint.

Hard rock was often excavated by heating the rock-face with fire and cooling 
it with water, causing  expansion and spalling, then picking the fractured 
rock away with picks and wedges.
Egyptian and Roman mines were worked to depths of approximately 200 meters.
By the 6th century B.C., it has been estimated that the advance rate of a 
hand-worked tunnel in hard rock was perhaps 9 to 10 meters per year.
The use of explosives/magic increase the rate by an order of magnitude, with 
mechanization increasing the advance rate by another order of magnitude.

The process of digging a tunnel in rock, however, is not simply a case of 
deciding where the tunnel is to go and then pounding or blasting one's way 
through. Rock is a very treacherous medium through which to travel. Even 
"solid" rock often contains innumerable cracks, faults, folds, and 
discontinuities, the activation of any of which may become a trigger to a 
collapse of the tunnel.
The design and construction of any underground excavation must account for 
the mechanical properties of the surrounding rock, which includes not only 
the aforementioned cracks and discontinuities, but also the weathering and 
deterioration of the rock, the number and type of layers in the rock, strike 
and dip of these layers, underground water level, overburden, and the list 
goes on and on.

Sub-surface rock is commonly under stress causing strain that deforms it.
The three stresses are compression, tension, and shear.
Strain is either elastic, in which the effected mass returns to its 
pre-strained state, or plastic, where the mass retains the deformations 
after the strain is relieved.
Continuous plastic deformation occurs in ductile rock, causing folding, 
while brittle rock fractures and faults under strain.
Folding occurs as synclines (downward folds), anticlines (upward folds), and 
monoclines (gentler dips).
Breaks in brittle rock are fractures when there is no movement along either 
side of the break, while faults involve movement.
The mass of rock above the plain of the fault is the hanging wall while the 
rock below the fault plain is the foot wall.
The direction in which the fault runs is the strike, while the dip runs 
perpendicular to the strike.
Faults most commonly occur along tectonic plate junctions with diverging 
plates being more common than convergent plates (dip-slip faults).
Diverging plate faults usually create a situation in which the hanging wall 
is moving downwards while the footwall is rising, creating a normal fault 
(a.k.a. gravity fault) under tensional stress.
Converging plates create compression stresses commonly causing the hanging 
wall to rise while the foot wall sinks causing a reverse fault (thrust 
fault).
When the rock masses on either side of the fault are moving parallel to the 
fault plain a strike-slip fault results.

1. Hookian solid-body (elastic) behavior: In elastic behavior, the strain is 
completely proportional to the stress applied, so that a plot of stress vs. 
strain yields a straight line.

2. St. Venant's solid-body (plastic) behavior: In plastic behavior, an 
applied stress will not result in any strain until a certain stress is 
reached (yield point). At this point, only strain increases. Reduction of 
the stress to below the yield stress will result in the cessation of the 
strain.

3. Newtonian liquid (viscous) behavior: In viscous behavior, the rate of 
strain is proportional to the stress applied. That is, as the stress applied 
increases, the deformation does not increase, but the rate at which the body 
deforms does.

Rock Strength (resistance to deformation)
R0 = Extremely soft - 20-100psi
R1 = Very low strength - 100-1000psi
R2 = Low strength - 1000-4000psi
R3 = Moderate strength - 4000-8000psi
R4 = Medium high strength - 8000-16,000psi
R5 = High strength - 16,000-32,000psi
R6 = Very high strength - >32,000psi

Rock Weathering
W1
Fresh rock
No visible signs of weathering. Any breaks are across sound rock
W2
Slightly weathered
Slight discoloration and minor weakening of the rock material
W3
Moderately weathered
Fresh rock is still present, but up to 50% of the rock material has been 
decomposed into soil
W4
Highly weathered
Fresh rock is still present, but more than 50% of the rock material has been 
decomposed into soil
W5
Completely weathered
All of the rock material has been decomposed into soil, but the original 
rock mass structure has been preserved
W6
Residual soil
All of the rock material has been decomposed into soil, and there has been 
transport, and all original structure has been destroyed

Rock loads on excavation supports.
Intact rock: Intact rock contains neither joints nor hair cracks, and thus 
breaks across sound rock. Spalling conditions, which is when thin slabs of 
rock fall off the roof or walls of the tunnel, , and popping conditions, 
where rock slabs on the sides or roof of the tunnel spontaneously and 
violently detach, may occur for several hours or days after blasting.

Stratified rock: Stratified rock consists of individual strata with little 
or no resistance against separation along strata boundaries. Spalling 
conditions are quite common.

Moderately jointed rock: Moderately jointed rock contains joints and hair 
cracks, but blocks between the joints are locally grown together or so 
intimately interlocked that vertical walls do not require lateral support. 
Again, spalling and popping conditions may be encountered.

Blocky and seamy rock: This consists of chemically intact or nearly intact 
rock fragments which are entirely separated from each other and imperfectly 
interlocked. The vertical walls of the tunnel may require support.

Crushed rock: Crushed rock is chemically intact, but extensively fractured. 
If the crushed rock is small-grained and below the water table, it will 
exhibit the properties of a water-bearing sand.

Squeezing rock: Squeezing rock slowly advances into the tunnel without a 
perceptible volume increase. This condition requires a very high percentage 
of microscopic and submicroscopic micaceous minerals or clay minerals with a 
low swelling capacity.

Swelling rock: Swelling rock advances into the tunnel primarily by the 
expansion of the rock itself. This condition seems to be limited to rocks 
containing clays, such as montmorillonite, which have a high capacity to 
swell when hydrated.

Of all the hazards associated with mining, rockbursts are perhaps the most 
terrifying. A rockburst is the sudden, violent dislocation of slabs of rock 
in a tunnel, usually from the walls, but also potentially from the roof or 
even floor. Considered to be a "mining-induced seismic event," a rockburst 
can release enormous amounts of energy, and some have been measured at 4 on 
the Richter scale and one rockburst was recorded by a seismological station 
1200 miles distant. The danger is obvious and quantifiable: In a three-year 
period in the Kolar gold-field in India, rockbursts accounted for 50% of all 
fatalities.
In general, rocks have a high resistance to crushing. King (1996) states 
that the walls of a tunnel will not fail as a result of compression except 
at great depth - more than 2000 feet (600 m) for softer sandstones and more 
than 19,000 feet (5800 m) for the strongest rocks. However, the rocks are 
still under an immense amount of stress, and the rock left standing after 
tunnel or cavern excavation must bear a greater load than before.
The rocks affected are nearly always hard, strong, and brittle. These rocks 
may have an unconfined compressive stress of 15,000 to 60,000 psi. In the 
United States, the most common location for rockburst phenomena seems to be 
the Coeur d'Alene mining district of northern Idaho, where the galena mines 
run over a mile deep into the quartzites of the Revett Formation of the Belt 
Supergroup.
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