Laboratory alteration of gypsiferous materials below a landfill

Some urban solid waste landfill sites in Spain are located on geological substrates of gypsiferous lithology. Although this type of substrate is assumed to be of low permeability, it can develop secondary pores by dissolution and, under favourable environmental conditions, form a karstic system that may pose serious geotechnical problems in the medium and long term. The purpose of this work was to study alterations caused by selective tests in various sections of a gypsum lithological column obtained from the Colmenar de Oreja landfill site (Spain). The tests were used to assess the influence of individual environmental factors and involved the addition of milli-RO water, solutions containing a 1%, 5% or 10% concentration of landfill leachate, and 2, 5, 10 and 15 mg l^?1 solutions of NaCl in successive immersion–drying cycles at ?15, 20 or 60 °C. Differences in alterability between the six studied segments of the lithological column were found to be due to differences in lutite content, specific crystal habit and type of cement. Segments with specular gypsum were the strongest in the tests, while the segment with the highest amount of detrital materials was the most responsive to temperature and moisture changes. The treatments that produce greater alterations are those that contain only milli-RO water.