Sub Surface drainage is a key element in the design of pavement system, because it is often lead directly to the pavement failure if laid with lack of preliminary studies need to be conducted for getting good performance of pavement structure throughout its design life . The sub surface drainage problems encountered on each section of highways may be different and will require specific consideration and treatment depending upon site condition. Proper construction and QC/QA testing operations can help to ensure good performance of the sub-base layer. Excessive compaction can alter the gradation and create additional fines that may result in lower permeability than determined in laboratory tests and used in the pavement system design. However, the optimization of structural contributions from high stability, versus the need to provide adequate drainage for pavement materials is still a point of debate. Hence it is very essential to study and compare Drainage characteristics of Granular Subbase in terms of Coefficient of Permeability so that along with stability it’s possible to have a clear idea on effect of its material characterization on their effective drainage property as wells before actual execution on site. From this study an attempt has been made to determine and compare drainage characteristics of GSB layer on field using semi model pavement. Such that Permeability characteristics are examined for all six GSB gradations specified in the MoRT&H 5th revision and AASHTO-57 GSB Grade as shown in Table 1 and 2 by varying hydraulic gradient with constant head permeability concept in metal chamber which simulates semi model pavement, so that optimum hydraulic gradient (camber to GSB Layer) is determined for each grades at which maximum hydraulic conductivity is expected. Amongst all six GSB grades specified in MoRT&H (5th revision) Grade III gives maximum permeability of 3675.66 m/day at optimum gradient 2.5% & which is most suitable for all locations where drainage is an at most important consideration. Test conducted on Grade II & Grade I at all gradients, showed no flow through them, because thick particle interlocking. Therefore it is suggested to use in less rainfall areas or areas where there is need of structural support to pavement rather than drainage requirement.
