The problem of minor slope instabilities on the motorway and trunk road network has been described by Symons (1970) and Perry (1989). The problems are mainly confined to overconsolidated clays and to particularly high sections of embankments or cuttings, but the ongoing cost of repairs to the slope failures constitutes a significant amount of maintenance expenditure. The problem of balancing construction and maintenance expenditure on earthworks slopes was addressed by the development of a whole life cost model and computer programme. The resulting programme, WLCslope, enables comparisons to be made between different design options on a whole life cost basis rather than solely on the basis of capital cost. It also allows comparison of the residual maintenance costs for different options for DBFO contracts, where the responsibility for maintenance reverts to the government after 30 years. Application of the model to a range of soil types indicates that the maintenance component of the whole life cost may be up to 3.7 times the capital cost for soils with a high rate of slope failure. In some cases, the whole life cost for a 1:5 slope may be significantly lower than that for a 1:2 slope because of the slope repair costs associated with the steeper slope. Residual costs may be in the range 5% to 10% of the whole life cost at 30 years for steep slopes with high failure rates. Variations in land price and discount rate have a major impact on the results of the analysis.
Samenvatting