The effect of compacted prestress on compacted shale compressibility

The compressibility of the compacted New Providence Shale was studied as a function of the compaction variables, viz., water content, compactive foot pressure, compaction effort, and compacted density. All samples were compacted in the kneeding device to simulate low, standard and modified impact energy levels. All compression and/or heave was one-dimensional. Compactive prestress was found to increase with compaction pressure and to decrease with compaction water content. To simulate the effects of the service environment, compacted samples were confined and saturated with measurement of volume change. Such changes were either increases (heave) or decreases (settlement or collapse). Settlement was found to increase with increase in confining stress and with increase in initial void ratio. It also increased with decrease in initial water content. Heave increased with decrease in confining stress, with decrease in initial void ratio, and with increase in initial water content. The prediction equations developed in this report are of the type needed to predict settlements or heaves within compacted shale embankments in service. (FHWA)