ABSTRACT

SURFACE SUBSIDENCE The ground surface is deformed above a working coal face by a subsidence wave which migrates at the same rate as the face advance - usually 10-20 m per week. This subsidence wave has a number of effects: Subsidence must be less than seam thickness, so usually about 1 m; may accumulate to > 15 m by multiple seam working over time; causes little structural damage but has impact on drainage and piped services. Ground strain develops first as extension (on the convex part of the wave), then a return to neutral, followed by compression (on the concave part of the wave). This causes most of the structural damage due to mining subsidence. Total strain is the sum of extension and compression values and is typically 1-10 mm/m or 0 0 0 1 - 0 0 1 . Angular movement occurs as tilt on the subsidence wave; usually minor and only signif icant to tall chimneys and sensitive machines. Micro-earthquakes may occur due to movements in strong, massively jointed rocks under stress. At any one site, subsidence movements are generally completed within no more than one year.