Disclosed is a process for the slurry backfilling of subterranean in situ oil shale retorts, especially the improvement comprising introducing a slurry of sand and water into the subterranean retort so as to form drainage paths for the removal of water from the slurry of spent shale and water. Disclosed is a process for the subterranean in situ retorting of oil shale and disposal of spent shale from surface retorting comprising forming a subterranean in situ retort so that the subterranean retort contains a mass of rubblized oil shale; passing a retorting fluid through said rubblized mass so as to substantially retort said mass and recover hydrocarbons; cooling the subterranean retort; introducing a slurry comprising water and spent oil shale from surface retorting into said subterranean retort; and introducing a slurry comprising water and a material capable of forming a porous structure at one or more points in the subterranean retort so as to form one or more drainage paths for the removal of water from the slurry of water and spent shale from surface retorting; so that the spent shale from surface retorting is effectively disposed of and a mechanically and environmentally suitable underground formation is established.