Strip mining entails massive amounts of excavation, and machinery was developed on a scale commensurate for the work. After coal mining contractor Wright & Wallace used a modified land dredge—a machine designed to excavate drainage and irrigation canals—to strip overburden from a seam in the Mission coal field near Danville, IL in 1885, very large draglines and cable shovels came to dominate this work. Although shovels cycled faster than draglines and could dig harder material, draglines possessed two advantages that eventually contributed to the demise of purpose-built stripping shovels.
A shovel’s performance was limited by the depth of the overburden; it could dig only within a given arc, and it had to be able to dump to the top of the adjacent spoil bank. But a dragline could dig deeper from on top of the overburden than a shovel could from below, and only the dragline’s range, rather than the depth of the overburden, limited its digging depth. The giant stripping shovels were used at shallow-depth Midwestern mines that produced high-sulfur coal; as these mines played out or were rendered obsolete by environmental regulations requiring lower sulfur coal, these huge machines were retired, and draglines were chosen for the new shallow-depth coal mines in the western states because of their greater flexibility, reliability and technological improvements.
Also, while a dragline could move into more advantageous positions to gain digging and dumping reach, a stripping shovel was confined to working on the pit floor. Along with creating the expense and work of walking it into and out of the pit, the work had to be carefully planned in order to keep the shovel digging productively without working itself into a corner surrounded by spoil banks, excessively deep highwall and/or soft ground. Highwall conditions had to be constantly monitored, as slides could occur at any time in unstable material, putting men and machinery at risk, while draglines worked more safely on the surface.
A third machine, the bucket wheel excavator, is also used for removing very large quantities of overburden. These machines weigh up to 18,000 tons and are even more productive than the stripping shovels, but require material that is free of boulders that can damage or clog it; if boulders are present, draglines hold an advantage.
The Historical Construction Equipment Association (HCEA) is a 501(c)3 non-profit organization dedicated to preserving the history of the construction, dredging and surface mining equipment industries. With over 4,100 members in twenty-five countries, our activities include operation of National Construction Equipment Museum and archives in Bowling Green, Ohio; publication of a quarterly magazine, Equipment Echoes; and hosting an annual working exhibition of restored construction equipment.
Individual memberships are $30.00 within the USA and Canada, and $40.00 US elsewhere. We seek to develop relationships in the equipment manufacturing industry, and we offer a college scholarship for engineering students. Information is available at www.hcea.net, or by calling 419-352-5616 or e-mailing email@example.com.