To ensure the stability of a man-made slope in a rock or soil mass throughout its envisaged engineering lifetime, the decrease of relevant strength properties of the slope material has to be taken into account. Following the initial stress release after excavation, weathering and erosion processes will start acting upon the newly exposed slope material and as field data shows, this may have a significant effect in less than one hundred years. In this study, a statistical approach to the trend analysis of actual weathering classification data is proposed, which makes use of the bootstrap percentile method and the Monte Carlo simulation technique. This methodology results in empirical relationships between strength properties and the age of the slope, with quantified probabilities. A fully quantified probabilistic slope stability assessment can then be made. In such an assessment, a slope design may be evaluated with regard to the slope stability over the full engineering lifetime of the slope and Consequently different designs can be compared.


Man-made slopes in rock and soil masses, such as road cuts, are generally designed to be stable throughout the so-called engineering lifetime, typically fifty to one hundred years. During that period the materials exposed in the slopes are subject to weathering and erosion. Although these are commonly regarded as processes acting on geological rather than human time scales, practice shows that the rock or soil masses in the slope may be significantly affected even in relatively short periods of time. Thus, these time-related processes may lead to a loss of structural integrity and deterioration of the geotechnical mass properties, causing a degradation of the materials in the slope and a decrease of the slope stability. To ensure safe construction of a slope, and to maintain a degree of stability that meets the requirements throughout the whole envisaged engineering lifetime, this degradation of the geotechnical mass with time should be incorporated in the initial design. However, the significance and extent of degradation of soil and rock mass properties within periods of up to one (or several) hundred years is only partially understood, and generally, relations are not quantified. In this study, a statistical evaluation of classification results provides us with an empirical relation between weathering degree and time with fully quantified uncertainties. From this, the decreasing slope stability is derived as a function of time as well.


The data presented in this paper was collected in the Tarragona province in North-eastern Spain. Here, the Southwest-Northeast orientated Catalan ides mountain range separates the coastal plain from the Ebro river basin. It is bounded in the south by the Ebro delta. The stratigraphy of the area consists of a sedimentary sequence of essentially Devonian through to Quaternary age. Igneous rocks occur, intruded into Carboni ferous formations as granodiorite bodies and aplitic dykes. The youngest formation in the Triassic (Keuper) is a sequence of shales and siltstones, in the lower part interbedded with limestones and dolomites.

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