In modified in-situ (MIS) oil shale retorting, large chambers of rubblized oil shale are formed by mining out about 20% to 40% of the in-place shale and blasting the balance into the created void. The mined-out material is brought to the surface and oil is recovered from it by surface retorting. The in-place material is pyrolyzed to recover oil, leaving large numbers of abandoned retort chambers underground. This type of oil shale processing may result in a number of environmental problems, including in-situ leaching of the abandoned retorts, low resource recovery (large pillars are required to support the overburden), and subsidence. These problems may be mitigated by filling abandoned retorts with a grout that would fill the void space, thus improving retort structural strength and stiffness, and reducing retort permeability to groundwater flow. If sufficient strength and stiffness can be developed, it may be possible to design retorts so that the pillars can be retorted and resource recovery improved. This document describes a computer program that was developed and used to evaluate the strength and stiffness required in a grouted retort to support the overburden and to improve resource recovery. The program SUBSID was developed to predict subsidence over a field of MIS retorts under various conditions of retort backfill. Where subsidence must be prevented or limited, backfilling retorts can permit retort design with a higher extraction ratio than would otherwise be possible. SUBSID can be used to relate the desired increase in extraction ratio to the required structural properties of the retorts after backfilling, thus allowing costs and benfits to be balanced against each other.