The process of weathering alters and usually weakens coal-bearing rocks. It is typically restricted to the surface layers. However, deep weathering can occur in permeable lithologies and along fractures as the result of oxidizing conditions, usually brought on by water percolation. The weathered strata most often are stained yellow to brown, the result of iron in the ferric state. The depth of weathering depends on many factors, the most important being rock type and climatic conditions. Weathered materials are weaker than fresh parent material.
In surface mines, weathered overburden is usually easier to strip (Pictures 1 and 2). However, because it is inherently weak, the weathered overburden is sometimes prone to highwall failure. Weak mechanical properties, decreased even further by meteoric water saturation, can promote slope failure (Picture 3). Weathered material usually does not provide a suitable base for a spoil pile. Because of its reduced mechanical strength, it could produce a weak plane for potential spoil pile failure.