Collapsing loess-like soil in western Saudi Arabia

Earth fissures have developed at Wadi Al-Yutamah, western Saudi Arabia. The fissues are associated with land subsidence which is considered to be due to both rapid draw down of the ground-water level and hydrocompaction of th e wad soil after flooding. This phenomenon is relatively recent in the area . The wadi soil was investigated and classified in the field, and disturbed a nd undisturbed samples were collected for laboratory testing and analysis. The engineering properties of the wadi soils were determined, including in situ field density, specific gravity, liquid limit, plastic limit, plastici ty index, shrinkage limit, and consolidation characteristics such as total settlement and coefficient of subsidence (collapse). Four types of silty soil of different colours were identified, in addition to yellowish-brown sand (representing the dunes) which accumulated on the s urface of the study area. The silty soils include yellowish-brown silty san d, yellowish-gray silt, pale brown silt and yellowish-brown clayey silt. Th e results of X-ray diffraction analysis indicated that the clay is mainly k aolinite and illite with minor smectite. The dominant soil type in the stud y area is silt of low plasticity, high void ratio and low density which dec reased with depth. This soil was classified as loess-like materials. The studied soils are of a collapsing type, and settlement is greatly incre ased by excessive wetting under constant pressure. The calculated coefficie nts of subsidence (collapse) of the soils at different depths generally inc reased with decreasing soil density and ranged between 3.1% and 10.8%. The wadi soils are considered to pose a moderate problem when wetted. (C) 1999 Academic Press.