Experimental evaluation of hydraulic conductivity of compacted soil liner

The construction of a compacted mineral barrier for covering a wide MSW landfill involves many relevant problems. As the wastes underlying the barrier are often very compressible, a specific degree of compaction may be difficult to achieve. Furthermore, climate conditions have a great influence on the actual compaction energy and water content of clayey liner and then on its hydraulic conductivity. This study shows the results of an experimental investigation, carried out on a full-scale landfill liner. The soil liner was first compacted by a heavy dumper, the effective degree of compaction was checked by in situ density measurements, and permeability tests were then carried out with Guelph and Boutwell permeameters, in order to validate data from previous laboratory tests and to verify whether regulations for compaction degree and barrier hydraulic conductivity had been respected. The test site is located at Grumolo delle Abbadesse (province of Vicenza, NE Italy). One of the functions of a permanent cover system for MSW landfill is to control the percolate production, to limit the infiltration of meteoric water. In these conditions, the key parameter controlling the efficiency of the mineral barrier is the coefficient of hydraulic conductivity. This coefficient can be determined by laboratory or/and on site permeability testing. The on site tests provide a value of permeability more realistic than one evaluated by laboratory tests, since greater volumes of soil are involved and therefore the important effects of soil macrostructure are also taken into account. The rollers used to compact cohesive soils are generally heavy sheep foot rollers with feet capable of penetrating the layer properly, to produce a kneading type of compaction. The choice of type of roller depends on the nature of the soil, on the availability and requirements of each site. The site operators have a powerful dumper, used on the site for other purposes, which was tested as a compaction vehicle, as its rear compartment has been loaded with different weights. Crucial aspects turned out to be not only the type of soil, but also how it had been prepared before compaction (choice of water content and clod size) which influenced the value of the coefficient of hydraulic conductivity. Using both types of in situ permeameter turns out to be essential: the Guelph instrument can be used to check the quality of compaction in a relatively short period of time during compaction and to intervene during preparation of the clay cover; testing with the Boutwell permeameter takes longer, but the final result is a coefficient which is reliable and can be used as a test parameter of the barrier. The initial results of compaction operations showed that the dumper could provide adequate compaction energy, changing the weight of the load on the rear compartment. Overall, the test results also highlighted the important aspect of driver training. The workers were familiar with the dumper, and quickly acquired the expertise to use it in this new way, as shown by the good spatial homogeneity of the characteristics of the final barrier.