ABSTRACT
SURFACE SUBSIDENCE The ground surface is deformed above a working coal face by a subsidence wave that 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 convex part of the wave), then a return to neutral, followed by compression (on concave part of the wave). This causes most of the structural damage in mining subsidence. Total strain is the sum of extension and compression, and is typically 1-10 mm/m or 0·001-0·01. Angular movement is tilt on the subsidence wave; mainly significant to tall chimneys and sensitive machines. Micro-earthquakes may occur due to movements in strong, massively jointed rocks under stress.
