Geotechnical engineering of land disposal systems

Abstract

Land disposal of solid waste entails the placement of waste in above ground stockpiles or dike impoundments or conversely, burial/encapsulation in below ground vaults. Containment failure can result from either mass-stability failure of above ground structures or leakage from both types of disposal systems. Geotechnical considerations play an important role in preventing either type of failure.

Interactions between the waste (or its leachate) and the containment envelope significantly affect containment integrity. Leachates can increase the hydraulic conductivity of clay liners; however, many of the large (several orders-of-magnitude) increases reported in the technical literature were caused by experimental procedure e.g., the use of extremely high gradients and employment of fixed-wall permeameters. Pure organic liquids (solvents) can interact adversely with clays causing some shrinking and cracking with large attendant conductivity increases. On the other hand, dilute solutions of organics (in aqueous carriers) have little or no effect.

Leakage through landfill liners — either compacted clays or synthetic membranes — is dominated by flow through void volume defects. These defects include desiccations and/or syneresis cracks, fissures, root holes, pin holes, lift joints, tears, faulty seams, and porous inclusions (clods). The likelihood and the gravity of the consequences of void volume defects are greatest in very thin membranes.

Careful compaction/composition specifications are essential to insure both low permeability and good volume stability in clay liners. The latter objective will minimize the danger of desiccation/syneresis cracking. Generally both these objectives can be achieved by compacting at or near optimum water content to high relative densities, by using compaction plants that thoroughly mix and "homogenize" the clay, and by excluding the use of high plasticity clays.

Movement of leachate across clay liners occurs by both advection and diffusion. In cases of very low seepage velocity (characteristic of dense, natural or compacted clays) diffusional transport can be quite important. Solute breakthrough or transit times under diffusion vary as the square of the barrier thickness. In the absence of solute partitioning or adsorption these breakthrough times may be very low (less than 10 years) for thin barriers.