DISPERSIVE SOIL AMENDMENT USING LIME AND BENTONITE AND PLASTICITY IMPROVEMENT IN EARTH DAMS FOR REDUCING ARCHING EFFECTS

Problematic soils are sometimes needed to be used in dam construction. This usually occurs when appropriate materials are located far from the dam site and cannot be used for economic reasons. In this case, available problematic soils (e.g., dispersive soils) can be utilized after amendment. The amended soil is supposed to not show its problematic behavior before treatment. It is conventional to treat dispersive soils by the addition of the most used one being lime. However, the addition of lime may cause a considerable reduction in the plasticity index of the mixture that, in turn, can produce other problems. An important consequence caused by the low plasticity index of the amended soil used in earth dams is an arching phenomenon, which can finally lead to failure of the dam. In this research, lime and bentonite are used to amend dispersive soil samples taken from Mirzakhanloo-Dam site located in Zanjan Province. The aim was to treat the dispersivity of samples and, also, to avoid the arching problem. For this reason, a set of laboratory tests was conducted on the prepared samples. Lime was used in samples with amounts of 0.6, 1, 1.5, 3, 5 percents of dry weight of the soil. Moreover, bentonite was used with amounts of 3, 5, 10, 15, and 20 percents. Pinhole test, double hydrometer test, crumb test, and chemical test were performed on the pure soil and additive-soil samples to evaluate the dispersivity of soil samples. Moreover, Atterberg limits, particle size, and hydraulic conductivity of samples were examined. The result showed that the combination of 5% lime and 15% bentonite can solve the problem such that, on the one hand, the dispersivity of soil is treated and, on the other hand, the arching problem is controlled. Almost all previous research studies have recommended the lime and other additives for the amendment of dispersive soils regardless of the arching problem. This paper strongly recommends considering this problem. Since the creation of a small crack in dams can lead to failure with regard to the piping problem, it is important to carefully assess the outcome of soil improvement programs and control the plastic index of the amended soil.