Improving the Performance of Clay as a Barrier to Water and Gas Flow through th e Landfill Cap Using Geofiber Mehrnaz Amini 2020 Department of Civil Engineering Isfahan University of Technology, Isfahan 84156-83111, Iran Degree: M.Sc Language: Farsi Dr.Sayyed Mehdi Abtahi Dr.Hajar Share Isfahani Although improvements have been made in the field of landfill worldwide, the scope of waste and the increasing rate of its production have led to more detailed studies and up-to-date knowledge of the new complexities discussed. Landfills are engineering structures that play the role of maintaining waste and controlling the effects of their pollution on the environment. These structures have faced various challenges throughout their life times. One of these challenges is the cracking of their final coating, which is usually made of clay. These cracks are generally caused by changes in climatic conditions and cause the infiltration of surface water such as rainwater into the landfill and the release of biogas produced by the decomposition of waste from the landfill to the surrounding environment. These changes, in addition to disrupting Landfill performance, cause environmental problems. In this study, according to previous studies and considering the positive results of using fibers in different soil characteristics, it was tried to use polyester, polypropylene and polyvinyl alcohol fibers for controlling cracks due to soil drying and reduce the permeability coefficient in wet condition. In this regard, two series of water and gas permeability tests were performed on samples made of clay, once without fibers and again with percentages of 0.2, 0.4 and 0.6 of the fibers. Water penetration tests were performed by constant water head method and the gas penetration tests were performed using a static three-axis test machine in both dry and wet conditions. In both series of water and gas penetration tests, due to the addition of a percentage of fibers, first the soil permeability coefficient decreased slightly and then with increasing the amount of fibers, this coefficient increased. The optimal percentage for achieving the minimum point of permeability coefficient in the diagrams depends on the fiber properties in addition to soil conditions. In water and gas penetration experiments, the process of permeability coefficient changes is similar and it was also observed that the addition of fibers is not only effective in reducing the rate of cracking but also reduces the ratio of permeability coefficient in dry to wet state. Among the fibers used in this study, polyvinyl alcohol fibers performed best by reducing the permeability coefficient by 65% ??in the water permeation test and 74% in the gas permeation test. Key Words Landfill, Geofiber, Permeability, Biogas, Clay Modification