Optimal design of highway cuttings in residual soils

Abstract

It is argued that rainfall-induced slope failure is the dominant form of instability for highway cuttings in residual soils, and that the optimal slope design should be based on the total cost of the highway cutting over its design life. The maintenance cost in each year of operation can be evaluated by applying the estimated failure rate and the discount factor for that year to the volume of earthwork for repair. A preliminary methodology to predict the failure rate, or probability distribution, of a highway cutting throughout its life has been developed and is presented herein. The proposed design methodology is aimed primarily at the design of highway cuttings in residual soils, which do not contain pronounced structural features such as relict jointing or bedding, and adverse hydrogeological factors. A relatively simple but practical engineering model for the rainfall-induced instability of slopes in residual soils is proposed, based on the concepts of unsaturated soil mechanics. Using this model, failure probability for a highway cutting is estimated, which in turn gives the failure distribution with the age of the slope. The design approach is demonstrated by means of a numerical example. It can be used to choose between options for highway cut design in terms of whole-life cost rather than mainly on the basis of capital cost.