Circular 530 Channels
| Geologic Disturbances in Illinois Coal Seams | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Channels | Split Coal | Rolls | Limestone Bosses | Clay Dikes | White Top | Igneous Dikes | Joints | Coal Balls | Miscellaneous Disturbances | Acknowledgments |
Channels
A channel is the course of an ancient river that eroded part or all of a coal seam and/or the adjacent layers of rock. Because channels are widespread in the Illinois Basin Coal Field, many have serious effects on coal mining. (Contemporary river channels, which are easy to identify and avoid, cause no serious problems.) Miners call ancient river channels by various names: washouts and cutouts are commonly used when the same has been completely eroded.
In Illinois, coal mining is affected by channels formed during two geologic periods: (1) the Pleistocene Epoch, beginning about 2 million years ago, when this region was invaded repeatedly by glaciers; and (2) the Pennsylvanian Period, 315 to 280 million years ago, when layers of coal-forming peat accumulated in vast tropical swamps. Some Pennsylvanian channels already existed as the peat was, accumulating, while others developed later as sediment-covered peat beds became eroded.
Pleistocene Channels
For this report, Pleistocene channels are defined as valleys or watercourses filled with materials deposited during the Pleistocene Epoch. Although geologists dispute the ages of various channels, some of these channels were probably cut before the first advance of glacial ice over Illinois (Willman and Frye, 1970).
Four major stages of Pleistocene glaciation (ice ages) left their record in Illinois. At one time or another, glaciers covered nearly all the land now underlain by coal. They blocked and altered the courses of rivers, including the precursors of the Mississippi, Ohio, and Illinois Rivers. During the warm interglacial stages, they left thick deposits of clay, sand, gravel, and boulders to fill valleys and profound change the topography. Great torrents of meltwater scoured valleys and cut through older deposits into bedrock. When the ice returned, these valleys were buried again.
Consequently, many Pleistocene channels are completely buried, have little or no relationship to present drainage patterns (fig. I), and can be detected only by drilling or by other subsurface exploration. One of the largest and most important buried channels is the Mahomet Bedrock Valley. Created by the ancient Teays River, this channel originated in West Virginia, crossed Ohio and Indiana, and entered Illinois just north of Danville. The virtually level farmland north of Champaign and Danville gives no clue to the presence of this buried valley, which is several miles wide and more than 400 feet deep (Piskin and Bergstrom, 1975).
A variety of unconsolidated deposits, known collectively as glacial drift, fill Pleistocene channels. Many channels contain a type of drift called outwash, a well-sorted sand and gravel left by meltwater rivers. Outwash is highly permeable and may yield large quantities of groundwater. In fact, the thick sand and gravel deposits in the Mahomet Bedrock Valley are an important source of groundwater for central Illinois. Not all subsurface deposits commonly found in Pleistocene channels transmit or store water so readily. Fine-grained deposits from glacial lakes and sluggish streams are less permeable. The poorly sorted mixtures of clay, sand, gravel, and boulders, which are called till, also supply little groundwater. Figure 2 shows the generalized thickness of glacial drift in Illinois. Additional details on the thickness and nature of glacial drift are presented by Willman and Frye (1970) and Piskin and Bergstrom (1975).
Underground mining problems.
A Pleistocene channel is a hazard to underground mining. Not only do such channels hold large amounts of groundwater, but they also contain loose materials that are nearly impossible to support in the roof. Many fatalities in the early history of mining, including the drowning of 69 men in the Diamond Mine disaster of 1883, resulted from mining too closely under water-saturated deposits of sand and gravel. If such a channel is penetrated in mining, or undermined and exposed in a roof fall, the sand and gravel as well as the water could rush into the mine. Fortunately, such dangers can be avoided by subsurface exploration before mining. Careful logging of test holes is critical in areas of shallow underground mines so that the extent of Pleistocene channels can be assessed accurately.
Penetrating channels is not the only danger involved in shallow underground mining in areas of buried Pleistocene valleys. Roof failure, rib rashing, and squeezes occur more often under buried valleys than elsewhere. The shallower the mine and the greater the relief on the bedrock surface, the more severe the problems. In mountainous regions, such as West Virginia, the roofs of mines are frequently unstable under stream valleys. Here in Illinois, three documented cases of floor failure leading to squeezes and subsidence were related to mining under Pleistocene channels. In all three situations, the valleys were deeper than 100 feet, and the glacial drift was thicker than the bedrock above the coal (Hunt, Bauer, and DuMontelle, 1982).
Engineering studies based on computer modeling indicate that compressive stresses on pillars and roof corners (junctions of roof and rib) and tensile stresses on midspans of entries are higher under valleys than under hills (Wang, Ropchan, and Sun, 1974). Compression on pillars can produce rib rashing and squeezes, while tension on the midspan promotes roof failure. Ferguson (1967) states that rock under valleys is weakened by microfracturing invisible to the naked eye; such microfracturing is produced when the valleys are eroded and overburden pressures are relieved, allowing the rock to rebound into the valley. Mechanical testing of core samples a t the Illinois State Geological Survey reveals that shales beneath buried valleys are up to 25 percent weaker than the same shales away from valleys (R. A. Bauer, personal communication,1982).
Surface mining problems.
A few years ago, a surface mine was abandoned in Jackson County because of channels. The coal seam was not eroded, but the channels contained water-saturated sand that flowed like quicksand into the pits. In turn, movement of the sand triggered large-scale slumping of overlying till into the excavations. Water, sand, and till filled the pits faster than they could be removed. Channels at this site were as much as 50 feet deep, but only a few hundred feet wide. To accurately map channels with such dimensions, closely spaced drilling is necessary.
Pleistocene channels filled with sand and gravel may serve as sources of groundwater for homes, farms, and villages. Mining through such aquifers may deplete or pollute supplies. The risks must be considered in the planning of surface mines.
Pennsylvanian Channels
Many large river systems existed during the Pennsylvanian Period; some prevent peat from accumulating and some eroded existing peat beds. Three well known channel systems have profound effects on coal mining on Illinois:
- Anvil Rock Sandstone and younger sandstone fill channels that interrupt the Herrin (No. 6) Coal Member. These channels cut through the Herrin peat after it was covered with sediments.
- The Walshville channel was the course of a major river existing during and following accumulation of the Herrin peat.
- The Galatia channel was the course of a major stream coexisting with the peat that became the Springfield (No. 5) Coal Member.
Channels affecting coal seams other than the Herrin and Springfield are known in less detain and will be discussed last.
Channels of Anvil Rock Sandstone and younger sandstones
The Anvil Rock Sandstone Member is a rock unit that occurs above the Herrin (No. 6) Coal Member (figs. 3 and 4). In some parts of Illinois, however, the Anvil Rock Sandstone fills channels eroded into or completely through the Herrin Coal (fig. 5).
The largest channel meanders across southern Illinois from Gallatin to Randolph County (fig. 5). The Herrin Coal was eroded by this stream along a belt that ranged from slightly less than 1 mile to more than 4 miles wide. In some places, coal seams as deep as the Colchester (No. 2) Coal, 200 feet below the Herrin Coal, have also been cut out. Sandstone and siltstone mainly fill the channel, with some conglomerate near the base. The channel-fill deposits average 122 feet thick, and in some places, exceed 200 feet (Hopkins, 1958). Electric logs of oil-test holes have provided most of the data for this channel. Only one underground mine, the Clarkson Coal and Mining Company at Nashville (abandoned in 1940), mined up to its edge. No geologic details were available from this mine; nor was information obtained from a reported exposure of the channel at a surface mine in Randolph County. Only the closely spaced drill holes along the channel show the steep walls or banks where coal has been abruptly cut out and replaced by sandstone or siltstone.
Smaller channels filled with Anvil Rock Sandstone have been identified in several areas of Illinois. Active and abandoned underground mines in Christian, Macoupin, Montgomery, and Sangamon Counties provide the best documented examples (fig. 5). Most are a few hundred feet wide and at least several miles long. They show no preferred direction; some are nearly straight, but others branch and curve. In some channels, sandstone has completely replaced the Herrin (No. 6) Coal. In others, the sandstone or other channel-fill rocks form the immediate roof of a partly eroded coal seam. Still other channels eroded no coal at all, but scoured away the normal roof of black shale and limestone. Channels that incompletely eroded the coal or that only eroded and replaced roof strata are well documented (Simon, 1956; Potter and Mast,1963; DeMaris et al., 1979; Nelson and Nance, 1980).
A recent, detailed study shows that some channels, previously mapped as Anvil Rock, are actually filled with deposits younger than this sandstone. In central Illinois, for example, the Herrin (No. 6) Coal is completely eroded in two long, narrow, north-south trending channels. The first is in eastern Montgomery County where drill holes show a channel fill of sandstone younger than the Anvil Rock-probably the Gimlet Sandstone Member of the Modesto Formation (fig. 5); this channel is 200 to 300 feet wide and at least 6 miles long. To the west, a longer and wider cutout extends through Montgomery, Bond, Clinton, and Washington Counties; it is 1000 to 2000 feet wide and nearly straight to slightly curving. Either Anvil Rock or Gimlet Sandstone fills it. Both of these channels were encountered in now-abandoned underground mines; no geologic details on the channels are available.
No attempt was made to distinguish channel systems of different ages in figure 5. In some cases the evidence is incomplete. As to any effect on mining, the distinction is insignificant.
Mining problems.
Recently surface mines in Jackson, Williamson, and Gallatin Counties have exposed narrow channels of Anvil Rock Sandstone, locally replacing the Herrin (No. 6) Coal. Not enough information is available to map these cutouts away from the mines. In some cases, channel sandstone is porous and weakly cemented, so it conducts large amounts of water into the pits and slumps on the highwall. In other cases, the sandstone is hard and massive. To blast and remove it from the pit is difficult, especially for the small operator without a large dragline or stripping shovel. Furthermore, large blocks of sandstone may roll off the highwall without warning, crushing men and equipment below.
Underground mines present other problems (fig. 5); A southwest-trending channel about 500 feet wide completely cut out the coal at the abandoned Crown I Mine of Freeman Coal Mining Company in Montgomery County. It divided the mining property into two unequal parcels. To reach the coal northwest of the channel, Freeman had to drive sets of entries through solid rock. Mining in and near the channel was hazardous because of unstable, slickensided roof rock. Slickensides, which are common around the margins of many channels, apparently were formed by slippage as sand, peat, and mud compacted unevenly while turning into rock. At Crown 1 Mine, the coal was cut out sharply, but unevenly and at a low angle. The channel fill was mainly fine-grained sandstone, siltstone, and shale with some lenses of conglomerate (fig. 6).
