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Clay soils and their related abnormal behavior such as excessive shrinkage, swelling, consolidation settlement and cracking on drying has been the subject of many investigations. Previous studies mainly evaluated the effects of additives such as lime, cement and sand on these characteristics. Initial results indicated that the soil characteristics were improved. However, reportedly in many cases, these additives resulted in a decrease in plasticity and increase in hydraulic conductivity. As a result, there has been a growing interest in soil/fiber reinforcement. The present investigation has focused on the impact of short random fiber inclusion on consolidation settlement, swelling, hydraulic conductivity, shrinkage limit and the development of desiccation cracks in compacted clays. To examine the possible improvements in the soil characteristics, samples consisting of 75% kaolinite and 25% montmorillonite were reinforced with 1, 2, 4 and 8 percent fibers as dry weight of soil with 5, 10 and 15mm lengths. Results indicated that consolidation settlements and swelling of fiber reinforced samples reduced substantially whereas hydraulic conductivities increased slightly by increasing fiber content and length. Shrinkage limits also showed an increase with increasing fiber content and length. This meant that samples experienced much less volumetric changes due to desiccation, and the extent of crack formation was significantly reduced.

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Hydrologic Evaluation of Landfill Performance (HELP) model is one of the most accepted tools to simulate

the hydrological attributes of landfills. Although some major deviations from real values has been reported about the

calculated results for leachate generation by HELP model but other researchers and/or engineers in practice have

used it in some places to estimate amount of leachate produced in the landfills. On the Other hand this model is

elaborated and mainly used in developed countries with the waste having low moisture content and also in climatic

conditions with high precipitation. This research investigated the applicability of the model in arid areas, by

construction of two 30m× 50m (effective horizontal length) test cells in Kahrizak landfill (longitude=51°, 20',

latitude= 35° 27' degrees), and monitoring the real leachate generation from each one. A set of field capacity and

saturated water conductivity tests were also performed to determine basic hydrologic properties of municipal waste

landfilled. A comparison was made between values calculated by HELP model and recorded values, shows that a

prediction of leachate on annual basis can be done by HELP model with acceptable accuracy but when the infiltration

of water to waste body increases due to leachate production, the model intents to underestimate water storage capacity

of the landfill, which lead to deviation of calculated values from real ones.

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The main task in the design and construction of impermeable liners in landfills is to block the migration of pollutants to the groundwater

systems or to reduce its rate to a reasonable amount. That is why environmental regulations force governments to construct engineered

waste dumps for waste management purposes. These liners are exposed to various types of chemical, biological, and physical processes

and are affected by the leachate which is produced from decomposition of waste materials accompanying methane gas. The leachate

includes a lot of components such as water and different types of salts. For this reason, the geotechnical characteristics of clay liners

which are evaluated in laboratories using distilled water or tap water might be far different from the representative sample of the in-situ

conditions. There are some evidences regarding the effect of these salts on the physical and mechanical properties of clay barriers which

could affect the long-term performance of these liners. Since the main criterion for impermeable bottom liners in landfills is their

hydraulics conductivity, the increase of this parameter could have a considerable environmental impact. This paper embraces the results

of a recent study on the effect of three inorganic salts, NaCl, CaCl2 and MgCl2 on some geotechnical properties of a common used clay

soil in impermeable bottom barrier in Kahrizak landfill, the main waste disposal center of the Tehran Metropolitan. Also the effect of

bentonite content by adding different percentage of this special clay mineral, 10 and 20 percent, on these properties was investigated.

Laboratory tests like liquid limit, compaction, 1D consolidation and free swell tests were performed for this purpose. Results indicated

that all of these salts could have a considerable effect on the geotechnical properties of the mixtures. The main reason of such effects is

the changes which occur in diffuse double layer of clay particles.