Studies have estimated that 156 billion tons of coal – or two thirds of the US’ mineable reserves – are subject to multiple seam mining influences. In some coalfields, particularly in Central Appalachia and the West, most mines are operating above and-or beneath previously mined seams.

The effects of multiple seam interactions can include roof falls, rib spalling, floor heave, and bumps that can seriously disrupt mining operations and threaten the safety of miners. In early 2006, a West Virginia coal miner was killed by rib roll that occurred in a high-stress zone beneath a remnant structure in an overlying mine.

Fortunately not every multiple seam situation results in hazardous conditions. Indeed, most do not. Accurate prediction of which interactions are likely to be higher risk allows mine planners to prepare for them or avoid them.

Multiple seam mining is most significant in the central Appalachian region of southern West Virginia, eastern Kentucky, and southwestern Virginia, where mining has been ongoing for nearly 150 years. Recent studies have indicated that perhaps 70% of the ultimate reserve base in the region has already been mined out.

One consequence of the maturity of the central Appalachian coalfields is that nearly every remaining underground reserve has been impacted by past mining activity. The mountains there are honeycombed with worked-out mines, located above, below, and adjacent to today’s and tomorrow’s operations. Full-extraction is also widely practiced in the central Appalachian coalfields.

While only eight mines use the longwall method, a recent survey indicated that about 315 mines, accounting for 58% of the room and pillar production in the region, engage in pillar recovery – which adds greatly to the potential for multiple seam interactions.

The Western US is the next most significant area for multiple seam mining. In Utah, Colorado, Wyoming, and New Mexico, nearly half of the 13 longwall operations are operating in multiple seam configurations. In contrast to Central Appalachia, in the West the same mining company is usually responsible for all the mining on a property. As a result, a greater degree of multiple seam planning is normally possible.

On the other hand, when combined with deep cover and strong roof and floor rock, multiple seam interactions can contribute to deadly bump hazards.

Hazards of multiple seam mining

Multiple seam interactions can cause roof falls, floor heave, rib rolls, and pillar failure. The type of ground instability, and the severity of the interaction, depends on the mining method, the mining sequence, and the thickness of the interburden. Other potential hazards are associated with inflows of water, gas, or oxygen-deficient air.

There are four main types of multiple seam interactions:

Undermining occurs when the upper seam has been mined first and the lower seam is the active seam (see Figure 1). In an undermining situation, damage is caused by load transfer from highly stressed remnant structures associated with full-extraction mining in the overlying seam.

Two types of remnant structures can cause undermining interactions (see Figure 2). A gob-solid boundary carries a single, distributed abutment load, while an isolated remnant pillar is subjected to two, overlapping abutments. As a result, the stress concentration on an isolated remnant pillar is usually significantly larger than that on a gob-solid boundary, and its impact on underlying seams proportionally greater.

Overmining occurs when the upper seam is extracted after mining is complete in the lower seam (see Figure 3). Load transfer occurs in this situation just as it does in undermining (in other words, gob-solid boundaries and isolated remnant pillars cause stress concentrations both above and below). In addition, however, full extraction of the lower seam normally results in subsidence of the overlying beds.

Research has indicated that when the interburden thickness exceeds approximately 6-10 times the lower seam thickness, the upper seam should be largely intact, though the roof and floor strata may be fractured or otherwise damaged.

Dynamic interactions are much more severe than other types of multiple seam interactions. They occur whenever active mining occurs above or beneath open entries that are in use.

The most destructive dynamic interactions occur when the lower seam is longwalled or pillared, resulting in active subsidence of the open overlying workings.

Less predictable are instances in which delayed subsidence of underlying works has the same destructive effect on overlying entries. In one instance, a set of mains were developed 180 feet above pillared works, and conditions were excellent for two years. Then the roof began to deteriorate dramatically, and heavy supplemental support was required to prevent major roof collapses.

Ultra-close mining is the fourth type of interaction, and normally the only one in which development mining alone is significant. The primary concern is failure of the interburden between the two seams. Ultra-close scenarios are most likely to occur near where a thick seam splits, or where a rider coalbed is of mineable thickness.

Columnization of the pillars is considered the standard design practice when ultra-close interactions are a concern. Ultra-close interactions are unlikely when the two seams are more than 20-30ft apart.

Case histories

Undermining. At a West Virginia longwall operation, mining was completed in the overlying seam first. The interburden above the lower seam was only 60-90ft, and consisted largely of sandstone. In the highly mountainous terrain the cover varied from 300-1100ft. The immediate roof above the lower seam was initially a strong siltstone.

The lower-seam layout was planned to minimize the potential for interactions. Particular care was taken to ensure that no development crossed beneath the heavily loaded upper-seam chain pillars. The lower-seam mains were developed beneath the overlying ones, but the longwalls were laid out so that the gates were offset by about one-quarter of a panel width.

Because the lower-seam panels were longer than the overlying ones, the gates crossed both the upper-seam start and stop line barrier pillars. At both crossings, the entry widths were reduced to 18ft and the crosscut spacing was increased. Extra support, including truss bolts and steel props, were installed in the belt entry.

When the first panels were retreated in the lower seam, no serious gate entry stability problems were encountered. The high stress zones beneath the stop and start lines were clearly visible, however, particularly when the depth of cover exceeded 800ft. Although the upper-seam chain pillars were located above the longwall face, they did not seriously impact ground conditions either.

As one miner put it: “Beneath the chain pillars, the coal was so broken that the shearer Click here to read Part 2