Studies on the development and control of desiccation cracks in compacted clay liner soils

Abstract

In the present day scenario of rapid urbanization and industrialization, large quantities of wastes in different physical forms are generated. Despite all efforts to minimize waste and to neutralize it, the requirement for storage or disposal still exists for which landfills form one of the solutions. The modern landfill is a very complex structure engineered to protect the environment from the liquid that seeps from the waste called leachate. This leachate tends to percolate downward into the ground as well as from the products of decomposition that takes place in the waste which may last for decades. An important component of a land fill is a layer of compacted, low permeability soil that is intended to act as a hydraulic barrier and minimize infiltration of water into the mass, when it is part of a cover system, or prevent the leachate from contaminating the ground water, when the soil is part of a liner system. According to regulations laid down by Environmental Protection Agency, the soil liners shall ensure that the hydraulic conductivity is equal to or less than 10-9 m/sec, which obviously is the prime criterion in the selection of liner material. During certain stages in the life of a landfill, it could be subjected to seasonal changes, resulting in significant variation of water content leading to the desiccation of clay liner material and thus posing a major threat to the integrity of the system as a hydraulic barrier. The application of bentonite is currently the most accepted practice for lining purposes. The ideal bentonite sand combination, which satisfies the liner requirements is 20% bentonite and 80% sand, was selected as one of the liner materials for the investigation of development of desiccation cracks. Locally available sundried marine clay and its combination with bentonite were also included in the study. The desiccation tests on liner materials were conducted for wet/dry cycles to simulate the seasonal variations. Digital image processing techniques were used to measure the crack intensity factor (CIF), a useful and effective parameter for quantification of desiccation cracking. The repeatability of the tests could be well established, as the variation in CIF values of identical samples had a very narrow range of 0 to 2%. The studies on the development of desiccation cracks showed that the CIF of bentonite enhanced sand mixture (BES) was 18.09%, 39.75% and 21.22% for the first, second and third cycles respectively, while it was only 9.83%, 7.52% and 4.58% respectively for sun dried marine clay (SMC).