Wastewater sludge can be viewed as a geotechnical material from the moment it is placed on the ground, as landfill, lagoons or capping layers for landfill. Wastewater, or sewage sludges show unpredictable behaviour owing to the floccular nature of the ‘solid’ part of the material, and because of the high bonding, or adsorption, of the liquid phase within and around these flocs.

The authors have carried out a series of tests, including consolidation and shear strength tests, in order to attempt to characterise the geotechnical engineering and consolidation behaviour of these sludges. The great influence of the gaseous phase is discussed, both in the light of the test results, and with reference to published descriptions of sludge behaviour in an actual slip of lagooned wastewater sludge.

A model for the consolidation behaviour of lagooned wastewater sludge is postulated, based on published descriptions of pore fluid distribution within the floccular structure of sludges. This model has to take into account the different solid, liquid and gas phases in the sludges, the compressibility of both the ‘solid’ phase (the sludge flocs) and the liquid phase (the viscous pore fluid). Simplifications are necessary for the model to be useful in explaining test and field observations and in predicting future behaviour.