Modelling the behaviour of soil-cooling tower-interaction
thesisposted on 17.02.2015, 08:57 by Emad H. Aly
Natural draught cooling towers belong to a category of exceptional civil engineering structures. These towers are an effective and economic choice among all technical solutions for the prevention of thermal pollution of natural water resources caused by heated cooling water in various industrial facilities. They are therefore widely used in most electric power generation units, chemical and petroleum industries and space conditioning processes. The cooling tower shell is the most important part of the cooling tower, both in technical and financial terms and also the most sensitive, since its collapse would put all or part of the cooling tower out of action for a considerable length of time. In this thesis, the 2D and 3D behaviour of soil-cooling tower-interaction, via the idealisation of the structure and soil on the resulting parameters, have been investigated, taking into consideration the effect of temperature changes in the cooling tower on the simultaneous interaction of the cooling tower and underlying soil. The temperature effect has been considered because it plays an important role in the design of the cooling towers. The capabilities of the two-dimensional Geotechnical Finite Element Analysis Program (GeoFEAP) have been updated in this project and the new version has been referred to as GeoFEAP2. New modelling capabilities and the ability to model 3D problems, with accompanying postprocessing features, were introduced, including 3D first order 8-noded hexahedrons. In addition, the Drucker-Prager yield criterion was programmed in GeoFEAP2 to model the elasto-plastic behaviour of the soil. A new 4-noded quadrilateral flat shell element, based on discrete the Kirchhoff's quadrilateral plate bending element, was also added to the software to model the elastic behaviour of the cooling tower shell. Furthermore, this element was modified to accommodate a temperature profile. The new software (GeoFEAP2) was then validated for soil behaviour and using several standard widely-used benchmark problems and the results compared well with the analytical and/or numerical results obtained by other researchers. A 3D finite element model was created, comprising the cooling tower, columns support, foundation, and elasto-plastic soil behaviour. The analyses of soil-cooling tower-interaction in this thesis have indicated the need to model the soil and structure as a combined problem, rather than by applying loads onto soil as geotechnical engineers' model, or by assuming the soil comprises springs and model the cooling tower, as structural engineers' model. The results have shown how unrealistic the latter two approaches are. In addition, the analysis necessitates the incorporation of thermal effects when modelling cooling tower problems. Moreover, from a design point of view, it has been recommended that circular footing with two cross-columns is better than pad footings and/or one column. Several other conclusions have been made that would improve the modelling of soil-cooling tower-interaction. Furthermore, the designer needs to ensure that enough modelling of soil conditions is done and an extensive site investigation is required to ensure that the variation in soil properties is represented correctly. Finally, the engineer needs also to ensure that the site tests performed to measure shear strength with depth via drilling and other methods needs to go deep enough into the ground to ensure that enough site information is available when designing the cooling tower.
- Architecture, Building and Civil Engineering