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Water increases in slope layers induced failure of slopes. Water is the most important factor in most of the slope stability analysis. Although water does not directly lead to the slopes displacement, but is an important factor for the following reasons: (1) water increases due to rainfall and snow melt will lead to increases slope weight. (2) Water can change the angle of slope (angle of slope is an angle that slope is stable in this angle). (3) Water can be absorbed or excreted by minerals are available in the soil. After adding the water, the weight of the rock and soil increases. (4) Water can dissolve the cement between the seeds and cohesion between the seeds is lost. In this paper, the feasibility of using piles to stabilize layered earth slopes were studied. A set of physical modeling of foundations was performed adjacent to layered slopes. The deformation pattern and shear strains of soil near slope and below surcharge load were studied. For this purpose, a comprehensive set of tests and numerical analysis were undertaken on different slope models. In each step of loading, digital image of deformed soil was captured and image processing was applied with GeoPIV software for investigation of soil deformation on slope and below the footing. the effect of pile and saturated conditions effects on improvement ratio (safety factor of stabilized slope with pile / safety factor of the slope stability without piles), bearing capacity of foundations, slope stability and slip surface shape in layered slope were investigated. The results show that the slip surface of layered slopes differs depending strongly on the installed pile positions and layered saturation conditions. In consideration of the model tests and numerical analysis results, it is found that, when clayey layer was near ground surface, changes in clayey layers water content significantly affected on slip surface and layered slope stability. Consideration of slipe surface shape for different layers saturation canditions, it is found, saturation of below layers which is located below the slip surface, has not significant effects on slope stability and slip surface shape. But with increasing upper layers water content, large volume of soil were failed. Experimental and numerical results show, for stable slope before applied surcharge load or before water content increases, critical slipe surface occurred in front the installed pile. But for unstable slope, critical slip surface positions depend on layers saturation and soil properties and occurred in front or behind or in upper and lower part of pile. In general The critical slip surface location dependent on water table level conditions and location of pile. Also from the experimental and numerical results it is found, the optimum location of pile for increasing bearing capacity of foundation which is located on slope crest, is near slope crest and maximum magnitude of Bearing capacity ratio ((bearing capacity of reinforced slope/ bearing capacity of non- reinforced slope)(BCR)) was obtained when piles installed near slope crest. Also optimum location of pile for increasing slope stability are found near mid of slope. A close agreement between the experimental and numerical results in Failure mechanism and the critical values of the studied parameters is observed

Keywords: Layered slope, Pile, improvement ratio, PIV method, Physical Modeling.

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