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Coal seams, in many cases, contain a foreign rock layer (commonly known as partings) and are thus in homogeneous in structural sense as opposed to the common notion of being a homogeneous body. It has also been observed in the field that for the pillars in these coal seams, which have been standing long after mining, the stability depends very much on the weathering action of the parting in the presence of mine moisture. It was observed in the mine that most of the pillar deterioration, in abandoned mining areas, initiated as the parting got weathered and changed into a clay like material. The weathering action continues to degrade the pillars as the weathered portion of the parting peeled off and mine moisture penetrated deeper into the pillar. The rate of moisture penetration differs from parting to coal and the depth of the weathered zone depends chiefly on time. Since the weathering tends to degrade the coal and parting in terms of their load bearing capacity, it influences the long term stability of the pillars. This research investigates the effect of weathering action in coal and parting on the long term stability of the pillar as a whole. In the laboratory, the slaking indices have been compared for the coal and the parting and the effect of moisture on the uniaxial compressive strength of coal and parting have been studied. An extensive non-linear finite element modeling has been done to study the effect of presence of a parting on the stress distribution at various level in the pillar and the behavior of the coal-parting interfaces by using gap elements. The field observations, the laboratory studies of coal and parting and the finite element modeling provide the insight of the structural degradation of the pillar and lead to conceptualize the time dependent strength deterioration of coal and parting individually and then collectively for the whole pillar. In the field, the strength profile in the coal and the parting and the effect of time (in terms of the pillar age) on them have been studied by using borehole penetrometer. In this study, an attempt has also been made to establish the formulae for time dependent strength of coal and parting and a hypothetical case study has been analyzed by using the developed time dependent strength equations.