Abstract

Manar Abd-El-Hameed
Cost Optimization Of Replacement Layer Beneath Shallow Footings Resting On Medium Clay
Construction of structures on a medium clay soil causes consolidation settlement over time. Settlement is considered to be a major problem and may lead to soil and foundation failure. Several techniques can be used to improve the soil behavior, increase bearing capacity and decrease consolidation settlement of medium clay. Soil replacement is one of the most familiar techniques it is implemented by removing the weak soil and replacing it with a better compacted soil. It can be more economical and may require less delay to construction time in comparison with other techniques. But till this moment, the determination of replacement layer thickness is questionable because it is based on experience. The main aim of this thesis is to determine the optimum replacement layer thickness and the most suitable type of replacement soil which could be used to achieve minimum total direct cost of foundation works and reduce consolidation settlement under foundation in certain circumstances. The literature review was started with considering two main topics such as ‘soil improvement techniques especially soil replacement' and ‘nonlinear programming technique’. Mathematical model formulation was developed for the thesis problem. Then data base through Excel spread sheet was carried out to designate the factors affecting the foundation construction direct cost in preliminary phases of the work to be the basis of the computer model. Verification and validation activities were executed to ensure that model is correct and reliable for specific limitations. Model verification was performed on three different cases by comparing the model solutions to the analytical solutions. While model validation was carried out by comparing the model solutions for four real case studies with the solutions recommended by consultants based on experience. A parametric study was carried out to study the effect of five parameters on determining the optimum type and depth of replacement soil to be used in different circumstances of original soil profile. The studied parameters were relevant to the soil profile and included the thickness of clay layer (Hc), the clay compression index and initial voids ratio in terms of ( Cc/(1+e0) ),the saturated unit weight of clay layer (γsat), the clayey soil cohesion (C) and the depth of ground water table (G.W.T). The results of this study showed the relationship between the thickness of replacement layer (Hr) and the total direct cost of foundation works, the optimum thickness corresponding to the minimum cost through the different studied soil profiles, the suitable type of replacement layer and finally the relationships between the optimum thickness of replacement layer and the studied parameters. The developed computer model may help geotechnical engineers to predict the optimum thickness and type of replacement layer used to improve the clayey soil and achieve minimum total direct cost of foundation works at the same time.