Direct exposure of soil to certain atmospheric agents, such as water, can influence adversely or favourably the engineering behaviour of the soil. For instance, saturated and unsaturated/partially saturated soils behave differently, so do soils under seepage and hydrostatic pressures. Many theories in soil mechanics idealise soils as either cohesive or non-cohesive, and this has allowed much research to be done on saturated cohesive soils. However, non-cohesive soils have not received as much attention, apart from recent strength and dilatancy theories, yet in some parts of the world, certain non-cohesive soils pose significant risk to structures built on them. The most problematic examples of such soils are collapsible soils that may not be detected and properly considered in routine ground investigation activities. In this paper some case studies of collapsible soils in the United Arab Emirates are examined to analyse the effect of their collapse on infrastructure and the possible techniques to ameliorate the situation. The case studies include various sites that were found to suffer structural damage traceable to collapsible soils. It is found that in most cases the soil collapse was due to infiltration of rainwater or water from sustained irrigation activities at the surface.
The heterogeneous nature of soil as a load bearing material, coupled with varying environmental conditions, pose challenges to geotechnical engineers in their quest to characterize and understand ground behavior for safe design of structures. Standard procedures for checking bearing capacity and settlement alone may sometimes be insufficient to achieve an acceptable degree of durability and in-service performance of a structure, particularly under varying environmental conditions, whether natural or manmade. There exists a wide variety of problematic soils that exhibit swelling, shrinkage, dispersion and collapse characteristics occasioned by changes in moisture content. Specific examples are collapsible soils, which occur mainly in arid and semi-arid regions, are generally capable of resisting fairly large loads in the dry condition but suffer instability and significant strength loss when in contact with water. A number of case studies in the United Arab Emirates were examined, where lightly loaded structures such as boundary walls, pavements and footpaths had been built on ground overlying collapsible soil strata. Sustained irrigation of the dry landscapes was found to have caused uneven settlement of the collapsible soils leading to continuous distress to the structures as evident from cracking and deformation. To help address the problem, an opportunity has been taken to develop a laboratory method of simulating the loaded behavior of collapsible soils in varying situations and to measure its deformation at constant surcharge and ground water infiltration rates. Finally, relationships were developed to estimate the time and magnitude of settlement, if thickness of collapsible soil is known.
The collapsibility of soils is one of the various phenomena by which soils pose threats to structures built upon them. This behaviour is more pronounced in clays and some silty sands in arid and semi-arid zones. In this study, an attempt was made to decrease the collapsibility of three different clay soils (soil I, soil II and soil III with different clay content and plasticity indices) by treatment with cement–polymeric fibres as an admixture at various percentages (1, 2, 3, 4 and 5%). The collapsibility potential of all soils was found to be ‘moderate to high’ in the remoulded initial state. Standard double oedometer tests were conducted to measure the collapsibility of the soils before and after stabilisation. It was found that an increase in the cement:fibre ratio and/or the admixture content percentage significantly decreased the collapse strain at inundation. Moreover, the collapsibility index decreased on increasing the admixture content beyond 3% for soil I and 2% for soils II and III. For an admixture content of <2–3%, the reduction in the collapsibility index was found to be negligible. For all soils used in this study, with a 5% admixture content and a cement:fibre ratio of 6:1 by weight, the soils were found to change from the very high collapsibility range to the almost no-collapsibility range.
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