16-1 Introduction

Mining subsidence [1], like an earthquake [2], is a geologic hazard that can strike with little or no warning and can result in very costly damage [3]. However, unlike an earthquake, mining subsidence generally affects very few people. But, if a mine collapses [4] under an interstate highway, many lives and industries are affected.

Mining subsidence also can cause foundation damage to buildings, disrupt underground utilities, and be a potential risk [5] to human life.

16-2 Mechanics of Mine Subsidence [6]

Subsidence, in the context of underground mining, is the lowering of the Earth's surface due to collapse of bedrock and unconsolidated materials [7] (sand, gravel, silt, and clay) into underground mined areas.
����There are two types of subsidence: (1) pit, also called sinkhole or pothole, and (2) sag or trough. (The term "sinkhole" more properly refers to solution collapse features in limestone.) Pit subsidence [8] is characterized by an abrupt sinking of the surface, resulting in a circular steep-sided [9], craterlike feature [10] that has an inward drainage pattern. It is associated with roof collapse of mines that have total overburden (overlying unconsolidated material and rock) of less than 165 feet, weak roof rock of shale or mudstone, and a ratio of unconsolidated-material thickness to rock thickness of less than 1.2. Pit subsidence does not occur where the thickness of the unconsolidated overburden is more than 90 feet. Sag subsidence [11] is a gentle, gradual settling of the surface. It is associated with pillar crushing or pillar punching [12] of deeper mines (overburden of more than 75 feet). Sag-subsidence features may fill with water if the surface of the subsidence intersects the water table [13]. Pit-subsidence features generally do not hold water because the pit drains intoMining subsidence is controlled by many factors, including height of mined-out area, width of unsupported mine roof, thickness of overburden, competency (strength) [14] of bedrock, pillar dimensions, hydrology, fractures/joints, and time.The vertical component of subsidence is proportional to the height of the extraction area. Generally, the vertical component of subsidence does not exceed the height of the mine void. However, piping (subsurface erosion by water washing away fine-grained soil) of unconsolidated material can create a cavity deeper than the height of the mined area.
����The area of mining subsidence increases proportionally with increasing width of unsupported roof rock. The potential area of subsidence is equal to the extraction area plus an area surrounding the extraction area measured by an angle up to 35��, called the angle of draw [15], from the vertical at the edge of the extraction area. For example, roof collapse in a mine 160 feet deep could cause subsidence more than 75 feet beyond the edge of the mine. The deeper the mine, the larger the area potentially affected by mine subside

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