This paper presents the results of an investigation conducted to determine the causes for failure of Saveh irrigation canal lining. Study of the technical reports of the Saveh Irrigation Project indicates that concrete lining of the main canal has failed in the first year of operation and repair works has not been successful since then. Most of the previous reports on the failure causes have assumed that gypsum in the foundation soil is responsible for the failure. In the current research, a detailed field survey was performed as the first step and all previous technical reports in this relation were studied. In the second stage, several test pits were dug at 500 meter intervals along the canal embankment as well as its center line and soil samples were taken from different depths at 50 cm intervals. All samples were tested for their chemical, physical and mechanical properties. The over all results of investigations indicated that the foundation material is classified as difficult soil and is responsible for all damages. Considering the feature of damages, soil dispersivity and solubility were assumed as the main causes of failures. The results of grain size analysis and Atterberg Limits tests show that most samples (60-70%) are sandy and classified as SC and SM. The results of chemical tests indicate that the white grains in the soil are all calcareous and gypsum content of the soil is very low. Using the Sherard Method and Double Hydrometer test, dispersivity potential of samples was evaluated and the samples were classified as non-dispersive. However, the results of Pin Hole tests showed that some samples are medium to highly dispersive. Thus, considering the sandy texture of the samples and low quantity of sodium
ions implied that the foundation soil has potential of physical dispersivity. Based on the over all results of the field and laboratory investigations, the main causes of the lining damages were related to three groups, i.e., design parameters, construction quality, and geotechnical properties of the foundation soil. Among these causes, the latter, which is related to sandy texture of the soil layer, is mainly responsible for the failures. Sandy layers under the lining are easily eroded by water flow from seepage and surface runoff and cavities are formed extensively which in turn causes lack of support for the concrete slabs, resulting a general failure.
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