Engineering performance of some coarse-grained arid soils in the Libyan Fezzan

Abstract

Arid engineering soils are those whose properties have been conditioned by an arid climate: they differ from soils in temperate or humid climates mainly in the role played by water. In temperate climates coarse-grained materials having less than 12 0.000000ines will generally behave as free draining soils. This case study shows that in a hot desert environment some sands with up to 30 0.000000ines, of low plasticity or no plasticity, also demonstrate behaviour which is consistent with that of free draining soils. The near-surface materials studied were the subject of a small investigation by the contractor during Phase II of the Great Man-made River Project, Libya. They were mainly rounded to subrounded granular particles resulting from mechanical action of wind and water. The particles are predominantly limestone and quartz fragments derived from the local Cretaceous and Palaeocene shallow marine, carbonate-rich bedrock sources: mid-Tertiary basic volcanics and younger Tertiary continental deposits also contribute. Windblown, mainly carbonate, silt-sized dust is characteristic of the local desert conditions. It is generally angular and often mixed with the sands. Occasional wetting and subsequent evaporation have resulted in a very weak cementation (calcite and occasionally gypsum) of the granular particles. This holds the grains, preventing them from being free-flowing, and the resulting soil has some special properties, including for example, large variations in grain size distribution between dry and wet sieving; low plasticity index of the fines in predominantly granular soils; high dry density upon compaction and good load bearing characteristics. These sands are considered typical of hot arid, coarse grained deposits in a carbonate rich geomorphological environment. Vast areas of sands in the Middle East and elsewhere with carbonate bedrocks are likely to be similar to these and their behaviour is also likely to be influenced by a carbonate fines content.