Morrow Insurance and Professional Services

MINE SUBSIDENCE

How it can affect your structure

Pennsylvania Mining History

Room-and-pillar mines have been active in Pennsylvania's bituminous coalfields since the late-1700s. Bituminous coal was first mined in Pennsylvania at "Coal Hill" (Mount Washington), just across the Monongahela River from the city of Pittsburgh. The coal was extracted from drift mines in the Pittsburgh coal seam, which outcrops along the hillside, and transported by canoe to the nearby military garrison. By 1830, the city of Pittsburgh consumed more than 400 tons per day of bituminous coal for domestic and light industrial use. Development of the anthracite coalfields in eastern Pennsylvania had progressed to the point where "hard coal" had captured the eastern markets. Consequently, bituminous coal production in western Pennsylvania grew principally with western population growth, expansion and development of rail and river transportation facilities to the west, and the emergence of the steel industry. Towards the last half of the nineteenth century, the demand for steel generated by the explosive growth of the railroad industry and ship building concerns, began to further impact bituminous coal production in western Pennsylvania (Puglio, 1983). Until the maturation of modern longwall mining in the 1960s, Pennsylvania's underground bituminous coal production came almost exclusively from room-and-pillar mines.

Early room-and-pillar mines did not include retreat mining; they relied on manual labor to cut the coal at the working face and the coal was hauled from the mine by horse and wagon. Today, many room-and-pillar mines use mechanized continuous mining machines to cut the coal and a network of conveyors that transports the coal from the working face to the surface (continuous haulage). The room-and-pillar mining method is used in all of Pennsylvania's underground bituminous coal mines including longwall mining operations, where it is used to develop the haulage and ventilation systems, and to delineate and support the longwall panels. Until the relatively recent advent of modern longwall mining, room-and-pillar mining had been the prime method for underground bituminous coal extraction in Pennsylvania. While room-and-pillar mining is still an important player in Pennsylvania, longwall mining continues to capture a growing portion of the Commonwealth's total underground production. Recent trends include a decline in the large high-extraction room-and-pillar mining operations, and some increase in small room-and-pillar operations that utilize continuous haulage.

Underground Coal Mining in the Bituminous Coalfields

Recent data

During 1997 over 73 million tons of bituminous coal was mined in Pennsylvania. More than 75% of the total production came from underground mines. The ratio of underground production versus total production has steadily increased over the past decade and is currently at levels not seen since the mid-1950s. The annual surface mining production has been generally declining since the late 1970s. In contrast, annual underground production was relatively constant during the 1980s and has experienced a marked upswing since 1993.

The major types of underground mining conducted in Pennsylvania's bituminous coalfields are room-and-pillar mining, room-and-pillar with retreat mining, and longwall mining. Room-and-pillar mining involves driving tunnel-like openings to divide the coal seam into rectangular or square blocks. These blocks of coal, or pillars, are sized to provide support for the overlying strata. The openings are referred to as rooms or entries. In older mines, entries normally ranged from 8 to 30 feet (2.4 to 9.1 meters) wide, while pillar sizes varied considerably. In modern-day room-and-pillar mines, the dimensions of the mining equipment (cut width and reach of the continuous miner) and the type of haulage system employed largely determine the pillar dimensions.

Coal recovery is relatively low using the room-and-pillar method, normally ranging between 35 and 70 percent. The highest coal recovery is normally achieved when retreat mining is combined with room-and-pillar mining. This method is often referred to as room-and-pillar with retreat mining. Retreat mining is a systematic removal of coal support pillars once a mining section has been developed using standard room-and-pillar mining. The retreat phase (also known as "second mining") typically results in immediate or quick collapse of overburden into the unsupported opening. Room-and-pillar with retreat mining is a high-extraction mining method, generally recovering greater than 70% of the target coal seam.

What Is Mine Subsidence

Mine Subsidence - An Overview

In order to consider potential impacts of underground mining on overlying structures, water resources, and surface land, it is first necessary to have some understanding of the mechanics of mine subsidence.

Mine subsidence can be defined as movement of the ground surface as a result of readjustments of the overburden due to collapse or failure of underground mine workings. Surface subsidence features usually take the form of either sinkholes or troughs.

Sinkhole subsidence is common in areas overlying shallow room-and-pillar mines. Sinkholes occur from the collapse of the mine roof into a mine opening, resulting in caving of the overlying strata and an abrupt depression in the ground surface. The majority of sinkholes usually develop where the amount of cover (vertical distance between the coal seam and the surface) is less than 50 feet. This type of subsidence is generally localized in extent, affecting a relatively small area on the overlying surface. However, structures and surface features affected by sinkhole subsidence tend to experience extensive and costly damages, sometimes in a dramatic fashion. Sinkhole subsidence has been responsible for extensive damage to numerous homes and property throughout the years.

Sinkholes are typically associated with abandoned mine workings, since most active underground mines operate at depths sufficient to preclude the development of sinkhole subsidence. In accordance with the current regulations, the Department will not authorize underground mining beneath structures where the depth of overburden is less than 100 feet (30.5 m), unless the subsidence control plan demonstrates that proposed mine workings will be stable and that overlying structures will not suffer irreparable damage.

Subsidence troughs induced by room-and-pillar mining can occur over active or abandoned mines. The resultant surface impacts and damages can be similar, however the mechanisms that trigger the subsidence are dramatically different. In abandoned mines, troughs usually occur when the overburden sags downward due to the failure of remnant mine pillars, or by punching of the pillars into a soft mine floor or roof. It is difficult, if not impossible, to predict if or when failure in an abandoned mine might occur, since abandoned mines may collapse many decades after the mining is completed, if the mine workings were not designed to provide long-term support.

For more information contact the Morrow Insurance Agency, or visit: www.dep.state.pa.us/MSIHomeowners