Hydraulic Conductivity of Saturated Sand-Bentonite mixtures

Doctoral thesis, 2004

The implementation of the EC directive concerning landfill liners and covers sets stricter technical and environmental requirements for landfill barriers. It is therefore essential to identify suitable materials that fulfil these requirements. There is a general ambition to promote a sustainable society, and it would be advantageous, both environmentally and economically, if industrial waste materials could be reused as landfill barriers. The overall objectives of the present study was to facilitate and increase the use of sand-bentonite mixtures, including industrial waste materials and mixtures, in landfill barriers, and to increase the knowledge and understanding of the hydraulic conductivity. For engineering purposes, it would be useful to establish a relationship between the hydraulic conductivity of sand-bentonite mixtures and some more easily determined parameters, as it is complicated and time-consuming to determine the hydraulic conductivity of a landfill barrier in situ. Therefore, a methodology to estimate semi-theoretical relationships between hydraulic conductivity, dry density and bentonite content for sand-bentonite mixtures with different types of bentonite was developed. A number of laboratory and field tests were performed in order to develop and verify the semi-theoretical relationships for sand-bentonite mixtures. The materials tested were bentonite, foundry green sand and quarry sand. In the laboratory, conventional test methods such as flexible wall permeameters (FWP), as well as alternative methods, such as constant rate of strain (CRS) and constant pore pressure gradient (CGT) tests, were investigated, improved and used for determination of hydraulic conductivity. Hydraulic conductivity was also determined in situ by means of sealed single ring infiltrometers (SSRI) and porous probes (BAT permeameters). Generally, there was a good agreement between the laboratory hydraulic conductivity test results and the corresponding semi-theoretical relationship. The field test results were on average somewhat higher, probably due to the macroscopic effects that were present in situ. Based on the semi-theoretical relationships and the test results, design charts were proposed in order to facilitate an environmentally, technically and economically suitable design process for landfill barriers.