EFFECT OF DESICCATION CRACKING ON THE HYDRAULIC CONDUCTIVITY OF A COMPACTED CLAY LINER

Despite our best efforts to reduce the waste stream, there will always remain some residues which cannot be further treated and must be disp osed in landfills. One critical aspect of landfill construction is the integrity of the landfill liner. Current landfill liner technology in cludes a composite liner which consists of a FML component and a compa cted soil component. The primary characteristic for selecting a soil f or use in composite liner construction is that the soil have a saturat ed hydraulic conductivity of 1 x 10(-7) cm s(-1) or less. In the prese nt study the effects of desiccation cracks on the hydraulic conductivi ty of the compacted soil were measured. Two soils of diverse mineralog y and typical of soils used for clay liner construction were selected for use. Each was tested in its native state plus after the addition o f 30% sand. Laboratory measurements were made of the volumetric shrink age of each soil. In addition, the hydraulic conductivity was determin ed using 10 cm diameter fixed wall permeameters; Additional conductivi ty measurements were made using 60 cm diameter fixed wall double ring permeameters which had been exposed to 0, 1, and 2 periods of desiccat ion prior to hydraulic conductivity determinations. The data show that laboratory measurements using 10-cm diameter fixed wall permeameters underestimate the hydraulic conductivity of the same soils when packed in large diameter permeameters. It was also found that exposure to tw o cycles of desiccation resulted in large increases in hydraulic condu ctivity. The time required to reach a steady outflow volume decreased as the amount of desiccation increased. The hydraulic conductivities o f soils which had been allowed to dry were greater than those which we re not allowed to dry prior to measurement. The relationship between v olumetric shrinkage and the increase in hydraulic conductivity after d esiccation indicates that soils which exhibit less than 11% shrinkage in the laboratory, exhibit increases in K of less than a factor of 2 u pon desiccation. Clay soils with greater than 11% shrinkage can potent ially be amended with sand to decrease the volumetric shrinkage and th eir response to desiccation.