Infrastructure development is important for developing country like India. Road construction plays a major role in it. Hence, good quality roads are necessary for any type of infrastructure development. Flexible pavements are divided in to layers, which consists of subgrade layer (soil as a foundation), sub-base layer, base course layer and surface course layer (bituminous layer). If any layer becomes unstable or weak, it may lead to failure of the pavement. It is observed that, sometimes pavements fail due to low strength of subgrade soil .The subgrade soil is internal part of the pavement. Hence, sound understanding of the properties of soil subgrade is essential for designing a pavement. The properties of soil can be modified by mixing the fiber rein for cements in the subgrade soil. Natural fibers like coir, jute, bamboo have been used to change the index properties of soil. The expansive soil which is also known as problematic soil was selected for the study. Pavement construction on such type of soil is a major problem. In this paper, coir fibres are used as reinforcement and its effects on the various index properties, i.e. Optimum Moisture Content (OMC), Maximum Dry Density (MDD), Unconfined Compressive Strength (UCS), Swell Pressure and California Bearing Ratio (CBR) are studied. The study also includes effect of percentage reinforcement on the subgrade soil for the pavement design. The pavement design for soil and coir fiber mixture results in to decrease the thickness of subgrade layer.