MINIMUM HEIGHT OF ROAD EMBANKMENT TO SATISFY TOLERABLE RESIDUAL SETTLEMENT OF GRANULAR SUBSOIL OVERLAIN BY STRONGER SOIL STRATA

The Residual settlement of granular subsoil of different strength underlying Road Embankment overlain by stronger soil layer is studied. A parametric study is conducted considering variable heights of embankment and variable ESAL factors (vehicle equivalence factor). The residual settlement (Sr) is stated as sum of elastic settlements of granular subsoil due to axle induced stress and self-weight of pavement layers. Values Sr for various heights of road embankment (He) are obtained and delivered as a set of design charts for different values of SPT and ESAL factor. In case of rigid pavement tolerable limit of the residual settlement is 0.1m. In case of flexible pavement in bridge or culvert approach and general sections except bridge or culvert approach, tolerable limit of the residual settlement considered as 0.2m and 0.3m successfully. A comprehensive guideline is developed for design of Road Embankment underlain by Granular subsoil underlying a stronger soil strata to satisfy the tolerable limit of the residual settlement. In this research study the range of both of ESAL factor and SPT value (N60) is 1-10. Through parametric study, this is observed that, no ground improvement is necessary when the thickness of overlying stronger soil strata (d) is minimum 0.45m thick in case of flexible pavement in general road section if ESAL factor≤8 and He≥1.5m. For flexible pavement in bridge or culvert approach d≥1.0m required to avoid ground improvement if ESAL factor≤8 and He≥1.5m. For rigid pavement d≥2.0m required to avoid ground improvement. A guideline is prepared including tables and charts to find out minimum allowable height of road embankment to keep the residual settlement within mentioned tolerable limit. Allowable minimum height of the embankment including pavement layers (He,0.1 , He,0.2 or He,0.3) are obtained corresponding to tolerable safe level of the residual settlement of granular subsoil for different values of N60, d and ESAL factor. The developed guideline is intended to be used in identification of the requirement of ground improvement in case of Granular subsoil underlying Road Embankment overlain by stronger soil strata to ensure tolerable level of the residual settlement. The ground improvement is only to be necessary when the residual settlement of subsoil is more than tolerable level or in other word if He is less than He,0.1, He,0.2 or He,0.3.