A climatic index of weathering and its application in road construction

Weathered rocks are the predominant natural road building materials in Southern Africa and their performance depends very much on the type of rock and the climatic environment in which such rocks weather. Climatic conditions which lead either to the predominance of disintegration, due to physical weathering, or to decomposition, due to chemical weathering, are defined in terms of an N value which is expressed as twelve times the computed free-water evaporation during the warmest month divided by the total annual precipitation. The particular feature of the formula is the use of climatic parameters for one particular month as well as for the whole year. The impact of the N value on weathering, on soil formation and on secondary transformations of soils (pedogenic materials), as well as on human activities in engineering and agriculture, are briefly described and the N value is compared with the moisture index I of Thornthwaite. Les roches alterees predominent parmi les materiaux naturels employes dans la c...