The explosive compaction of spent shale provides a potential means by which the void volume in previously burned in situ retorts can be recovered and transferred to adjacent newly created retort modules. The technical and economic feasibility of an oil shale processing operation based on the compaction of spent shale will depend directly on the explosive energies required to compact the spent shale, fragment and introduce a uniformly distributed void volume into the fresh shale and the coupling characteristics between these two processes if carried out simultaneously. Static and dynamic laboratory experiments on spent shale compaction have been combined with one- and two-dimensional finite-difference calculations to evaluate the coupling between the compaction and fragmentation processes and the effects of retort geometry and shothole charge timing. The numerical analyses show that the additional explosive energy required to effect a desired degree of spent shale compaction is less than that required for the fragmentation of shale when blasted into a mined out void volume and that the processes of compaction and fragmentation can be coupled such that the overall explosive requirements are not excessively large. The development of a 9.8 m by 9.8 m square (in cross-section) retort in direct juxtaposition to a burned retort (of 108 ml/kg or 26 gpt oil shale and with 22% initial fracture void volume) required explosive energies of 12.91 Joules per gm of fresh shale processed (approximately 6.9 lbs of explosive per ton). These explosive energies would be acceptable to a commercial operation and field tests are now required to further evaluate the spent shale compaction concept. 15 refs., 19 figs., 1 tab.