ABSTRACT
This chapter discusses the technical aspects of the materials used in even the simplest pavements – soil, sand and stones. It discusses the way these have been classified, particularly by the “unified soil classification” system. It notes that fine-grained soils such as clay and silt can have a life of their own due to the water-related effects of surface tension and the electrochemical behaviour of free ions. Clay is formed by the natural weathering of some rocks, particularly basalts, and the resulting hydrated aluminosilicates are very complex materials, physically and chemically. As the water content of a clay changes, it can undergo disproportionately large volume changes. Soils are mixtures of fine stone particles, clays, silts and organic matter. As a result, they are influenced by their water content. Mixtures of sand and clay can make effective low-cost pavements. The term “gravel” is carefully defined, and the value of using broken stones is emphasised. Broken stones were often a by-product of masonry work. The relevant properties of stone are described, together with tests used to determine them. The properties of a pavement composed of a mix of these materials are described, and the importance of determining how much they have been compacted is emphasised. The behaviour of the mix under traffic is described, and the “fourth power law” explained.
