Thickness of Bituminous-Coal and Lignite Seams at All Mines and Thickness of Overburden at Strip Mines in the United States in 1950 Page: 2
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A survey of bituminous-coal mines for 1920 showed that 25 percent
of the total United States production came from seams less than L feet
thick, 45 percent from seams 4 to 6 feet thick, 25 percent from seams
6 to 8 feet thick, and 5 percent from seams thicker than 8 feet. A
similar survey for 1950 showed 34 percent of output from seams under
4 feet thick, 36 percent from seams 4 to 6 feet thick, 22 percent from
seams 6 to 8 feet thick, and 8 percent from seams over 8 feet thick.
Table 1 shows the number and production of bituminous-coal and lignite
mines classified by 1-foot intervals of seam thickness mined in 1920,
1945, and 1950.
Tables 2 and 3 contain 1950 data on productivity and average
thickness of seams mined. The correlation between thickness of seam
and productivity was not universal; instead, in a number of States
and districts great productivity was achieved in thin seams.
Conversely, several States and districts had low productivity in
thick seams. Tables t to 10 give data on mines and production,
classified by thickness of seam.
The coal seam of moderate thickness presents the least mining
difficulties. Any decided thinning limits production, decreases
recovery, and, therefore, increases cost. Thickening of the seam in
underground mining has the same effect; for, as the seam increases
in thickness, it becomes necessary to maintain larger pillars,
timbering becomes more difficult and finally impracticable, and
roof control is almost impossible. The limited information avail-
able indicates that, for maximum recovery in underground bituminous-
coal mines, the ideal seam thickness lies between 6and 8 feet.
Fig. 1 shows the percentage of bituminous coal produced, by
thickness of seam mined, in the 10 largest coal-producing States
and total United States in 1950. The States are arranged in order
of the percentage of output mined from seams less than 4 feet thick.
Fig. 2 shows the percentage of bituminous coal and lignite produced,
by thickness of seam mined, in each State at strip mines in 1950, and
fig. 3 shows similar data for underground mines.
Coal near the surface in relatively thin seams that are not
suited to underground mining frequently can be recovered by stripping.
In some areas, coal seams that could not be deep-mined profitably are
being stripped successfully. In 1950 strip mining was carried on in
11 States in coal seams having an average thickness less than that in
the deep mines in the same States. These 11 States produced 70 per-
cent of the total strip output in 1950.
As previously mentioned, the depth of cover or overburden to be
moved is equally as significant as the thickness of the coal in strip
mining. Table 11 gives details on overburden excavated at strip mines
in 1966 and 1950. The average thickness of overburden increased from
31.6 feet in 196 to 39.0 feet in 1950. The ratio of thickness of
overburden to thickness of coal seam is a very important factor in
stripping operations. In 1950 the average thickness of overburden
handled was eight times the average thickness of the coal mined.
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Young, W. H. & Anderson, R. L. Thickness of Bituminous-Coal and Lignite Seams at All Mines and Thickness of Overburden at Strip Mines in the United States in 1950, report, September 1952; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc67062/m1/8/: accessed July 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.