Depth of Soft Cohesive Soil Underlying Highway Embankment for Limiting Residual Settlement

Residual settlement of highway embankment is studied for different compressibility of cohesive subsoil. Parametric study is carried out for different height of road embankment and different depth of natural soft soil underlying the embankment. The Residual Settlement is considered as the sum of 70% of consolidation settlement and elastic settlement due to axle load only which are to be occurred after construction of pavement and before the first maintenance of road pavement. The values of residual settlement (Sr) for different depths of soft subsoil (Hs) are obtained and presented graphically for different SPT Value (N60) and different Compression Ratio (CR). The tolerable limit of residual settlement is 0.100m for rigid pavement and flexible pavement in approach to bridge or culvert. For flexible pavement in general road sections this tolerable limit is taken as 0.200m. A design guideline is developed for construction of highway embankment in Bangladesh underlain by soft cohesive clayey subsoil to limit the residual settlement with in mentioned tolerable limit considering ESAL factor of 10 and for the ranges of Field SPT value, Liquid limit and Natural void ratio of 1-4, 30%-90% and 0.6-1.8 successively. Design tables, design charts and empirical equations are incorporated in this guideline. Simplified values of the ratio of embankment height (He) to soft subsoil depth (Hs )are obtained corresponding to satisfying tolerable or limiting level of the residual settlement. The developed guideline may be used in assessment of necessity of ground improvement to satisfy tolerable settlement limit. The ground improvement is only necessary when the residual settlement is not within tolerable limit corresponding to the soft subsoil depth.