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A Primer in Coal Mine Surveying and Mapping by Raymond Shackleford.
When someone learns that I survey in underground mines, they very often ask,"What's
it like to survey underground?" This article will discuss the similarities and
differences between surveying in the sunlight and surveying deep underground.
In Alabama, where my coal mining experience has been, the seams are fairly
level and range in thickness from around 2' to as much as 15'. If the coal seam
or strata is thick enough to stand on, it is usually called high coal; if not,
it is called low coal. In the low coal mines, coal is moved by low profile
conveyor belts. In transportation areas, the roof is blasted higher than the
seam, and at conveyor belt headers and other key locations, rock is removed to
provide room to stand and work. The ideal height of a coal seam is around 7',
which permits walking upright while being able to reach the roof. Anything much less or
much more presents problems for the workers.
One of the first differences a new surveyor underground notices is the fact
that all the survey points are overhead. Because of the mud and the constant
traffic of mining equipment, it is not practical to set points in the bottom of
the seam. Surveys are generally run on the centerline of the mine areas. Points
are set by first drilling a small hole in the roof about 3/4" deep, into which a
round wooden plug is driven. Then the survey point is marked by driving a metal
spad into the plug. A spad looks like a small flat key. It has a round head with
a hole through it and a straight shaft with a rounded point. Once the spad has
been put on line, a plumb bob cord is threaded through the spad and tied with a
slip knot. Distance measurements are made to the cord with a surveyor's chain or
with an EDM. If electronic measurements are made, the prism must be mounted on a
special holder to hang it from the spad. In high coal, a 32-ounce bob is used, since the high volume
of air moving to the working faces makes it difficult to steady a lighter one. In low
coal it is sometimes necessary to use an extremely short bob, since there is so
little headroom.
A Survey Day in the Mines
Surveyors are used to carrying a lot of stuff. To do their work efficiently
they need plumb bobs, prisms and plumbing poles, measuring tapes, paint,
flagging, nails, tacks, stakes, hammers, field books, calculators and data
collectors, transit, EDM, tripod, radios... Underground mine surveyors need most
of the same stuff, but that is only the beginning. Every surveyor underground
wears a miner's hardhat and a wide leather belt that holds a self-rescuer to
supply oxygen in case of emergency, plus a heavy battery connected by a cable to
an electric lamp on the hardhat. Each crew carries a hand brace and drill bits
as well as a heavy-duty, battery-powered hand drill. Instead of stakes and hubs,
they carry wooden plugs and spads. They also carry a methane detector to avoid setting off an
explosion. Since parts of most coal mines are wet, everyone wears 16"-high
rubber safety boots. In thin coal seams the miners and surveyors must work on
their knees in the mud, so heavy duty k!
nee pads are standard equipment. In h igh seams the crew must carry a tall step
ladder to reach the roof.
Another difference in underground surveys is that there often must be very
short sights. In vertical shafts, it is often necessary to have two control
points less than 20' apart since the points are transferred down the vertical
shaft by wires or optical devices. And mine entries are often driven around 50'
apart. Instrument setups are done with utmost care. The transit is leveled under
the plumb bob (there is a mark on the top of the instrument), then the scope set
on a 90 degree zenith angle. Then the position under the bob is checked again to
ensure that there is no eccentricity. The string is hung from the hole through
the spad, making sure it is tied so as to hang from the lowest point in the hole.
To obtain longer sights, control surveys are run down the straightest of the
mine entries when possible. Check surveys are run regularly. When a new shaft is
constructed, it is possible to make a tie to surveys on the surface. Rough
checks can be made at bore holes that are put down for water or power, but since
drills can drift, these checks are only approximate. In some mines, azimuths are
determined from time to time using north-seeking gyroscopes. These instruments,
which can be mounted on a theodolite, can determine azimuths accurate to 10
seconds or so when used by a competent operator. Many mines have vertical shafts
for transporting miners and equipment to the work areas and slopes with conveyor
belts for bringing the coal out of the mine. Surveys must be made to keep the
slopes in alignment, both horizontal and vertical. These slopes are actually
rock tunnels. Survey points are often set both in the roof and in the walls of
these tunnels. If track is laid for rail haulage, points can sometimes be se t in the railroad ties. Surveys down the
slope provide a much more accurate transfer of azimuth to the coal seam.
Mine surveyors do not stay in the dark all the time. They also run surveys on
the surface, following the directions of the main courses of the mine, for the
purpose of locating power lines and staking out drill holes and shafts. Holes
are bored to provide electricity close to the working faces of the
mine--preferable to using extremely long cables underground. Other holes are
drilled for pumping water out of wet areas of the mine and for pumping rock dust
into the mine. Because fine coal dust can explode, the exposed coal must be
covered with a layer of powdered limestone. This is why all but the active
working faces of the mine are white instead of black. The dust is applied with
portable blowers and sometimes by hand. The surveys on the surface are tied to a
coordinate grid, which may be a state plane grid or a local one set up for the mine. All surveys are plotted on a master mine map, which is
updated daily.
Coal Mine Mapping
The map of a coal mine looks a great deal like a city map. The portions of the
mine where the coal is removed resemble streets and avenues, and the pillars of
coal left to support the roof look a little like city blocks. The mined areas
are called entries, courses or headings. Some parts of the mine are referred to
as tunnels, but these are usually where it's necessary to cut through rock
instead of coal. Mines are often five to seven courses across, each being 20 to
24' wide with crosscuts joining the courses at regular intervals. As mining
progresses, some of these crosscuts are sealed with walls called brattices. This
permits air flow to be directed. Until these walls can be built, the air flow is
routed by hanging heavy curtains across crosscuts. One or more air courses carry
air to the work faces (intake air courses), while others carry the air back to
the surface (return air courses).
There are several methods of extracting coal while maintaining roof support.
One is called longwall mining, which uses a giant coal cutter to remove long
strips of coal, after which the roof is permitted to cave in the mining area.
The percentage of coal recovered is very high, but this method does not lend
itself to all mine conditions. Another is room and pillar mining, in which the
coal is removed in fairly short parallel entries called rooms, leaving pillars
between the rooms. If conditions warrant, some or all of the pillars may be
removed as the room section is mined out. These rooms are lined up by setting a
spad at the mouth of each room and an additional spad, called a drive spad, in
line with the center of the room. Foremen can hang strings from these two spads
and use their cap lamps to signal the miners at the coal face and keep the room
on line. To avoid setting up the transit at every room spad, the surveyors use a
right angle prism suspended from the centerl!
ine spad. The transit man can look i nto the prism and line the drive spad a few feet away at right angles.
This requires good coordination between the transit person and the rod person,
and expert use of their miner's lamps.
One of the main entries contains a conveyor belt that moves coal to connecting
belts and eventually to the surface. The belt lines must be care- fully aligned
and leveled to ensure that the coal does not spill as it moves rapidly. The
survey crew sets a row of belt spads in the roof above one edge of the belt
framework for horizontal alignment. Levels are run on the spads, so grades can
be set on the belt framework. Most level measurements in the coal mines are also
in the roof, so level notes are reversed from the order used
out-side--backsights are minus, foresights are plus.
In addition to surveys for keeping the mine entries going in the right
directions, the crew must also make regular progress measurements--"as-built"
mine surveys. These measurements are used to plot the mine progress on the
master mine map. Copies of the map are furnished monthly to regulatory agencies as well
as company headquarters. The measurements are also used to compute volumes and
tonnages of coal and rock removed for comparison with weight measurements made
on the conveyor belts and outside coal stockpiles. The map shows not only the
underground workings but also the buildings and other facilities on the surface,
as well as boundary and lease lines. If mineral rights are leased, royalty
reports are prepared monthly. Features that must be avoided, such as wells, are
shown on the map, and the mining plan is laid out to allow adequate clearance.
Emergency escape routes are clearly marked, both underground and on the maps. In
the event of a mine emergency, management a!
nd rescue personnel can use the map to save valuable time--and perhaps lives.
Most construction projects, large or small, begin with surveyors. Before there
is a highway through the swamp, the survey crew must first cut lines to show
where it will go. The same is true underground. Miners do not wander around aimlessly. The mine plan is
carefully designed by the mine engineers. The survey crew then has the
responsibility of seeing that the mine continues to go where the plan indicates.
And when the map (the only evidence of mining that most people see) is made,
every detail is the result of the work of the underground surveyor.
Coal Mine Lingo
High coal - A seam thick enough to stand on.
Low Coal - A seam not thick enough to stand on.
Spad - Survey point marker that looks like a small flat key. Usually set in a
mine roof.
Entries - Openings in a coal seam to allow people or materials to enter. This
term is most often used to indicate a portion of the mine that is continuing,
much like streets or highways.
Courses - Entries especially used for movement of air. Although air must go to all parts of the mine, air courses
are designed to be kept free of obstructions to allow maximum movement of air.
Headings - Entries that define the major directions of the mine layout.
Crosscuts - Openings between entries. On the mine map the entries can be
thought of as avenues and the crosscuts as streets.
Brattices - Walls built across crosscuts after mining has proceeded past them
for the purpose of maintaining directed air flow.
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