Laboratory simulation and finite element analysis of irrigation-induced settlements of built environment overlying collapsible soil strata in United Arab Emirates

Vandanapu, Ramesh (2019) Laboratory simulation and finite element analysis of irrigation-induced settlements of built environment overlying collapsible soil strata in United Arab Emirates. (PhD thesis), Kingston University, .


Collapsible soils exist in some arid environments and have a unique deformation response upon saturation so they cannot be reliably assessed with conventional elastic and consolidation theories. Such soils may be stable in the dry state but suddenly collapse upon wetting due to loss of suction and breakage of inter-particle friction and cementing bonds. This necessitates the use of unsaturated soil mechanics theories or field wettingloading tests, both of which are too sophisticated and uneconomic for routine geotechnical design. This thesis emanated from two case studies in the United Arab Emirates (UAE) where a forensic geotechnical investigation had been undertaken, as a consequence of severe settlement and structural damage associated with infiltration of irrigation water deep into collapsible strata. Records from instrumentation and test boreholes drilled by a specialist consultant are analysed along with specific irrigation regimes actually applied in the case study areas. The primary aim of this thesis is to develop alternative cost-effective approaches to simulating the deformation characteristics of a collapsible soil and formulating mathematical solutions for collapsible soil settlement under specified irrigation patterns. A laboratory simulation test is developed to study the irrigationinduced response of a collapsible soil specimen under overburden and seepage conditions that represent the case study situations. The test incorporates a large steel tank in which a layer of collapsible soil, of variable thickness, sandwiched between two other layers and subjected to varying water levels and constant irrigation intensity and surcharge. The test procedure and variables are designed to model the case study site conditions as realistically as possible. Test results showed that the number of wetting cycles required for the soil to reach collapse state increases with increase in depth of water table. Also, upon the start of collapse, the rate at which it collapses remains identical regardless of its thickness. Using back-analysis and a 3D finite element approach, predictive methods are developed which can be used to estimate actual settlement in the field. The suggested numerical method is implemented in MidasTM software and tested against data from the case studies to demonstrate a remarkable agreement with the measured surface settlements and cracking patterns in the affected structures. The suggested method takes into account important factors including the depths and thicknesses of the collapsible strata, the in-situ stresses, transient water flow, irrigation cycles, water table depth and the soil-structure mechanical properties. The proposed method of assessing irrigation-induced settlement will assist future geotechnical designs as well as in selection of suitable methods of protecting existing structures built on collapsible strata.

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