A Practical Method For Evaluating Deformation Of Deep Excavation In Soft Soil Considering Uncertainty Of Soil Properties And Construction Parameters

Deformation caused by deep excavation may lead to the destruction of adjacent buildings and facilities.Scholars have developed various empirical models to characterize the deformation of deep excavation.Amongst these empirical models,KJHH model has been used widely in practice.The KJHH model is able to consider the influence of soil properties and the effect of deep excavation dimensions,whereas is not able to account for the the influence of construction factors?such as over-excavation,reduction of strut stiffness,and deviation in strut position?on the deformation behavior.As a matter of fact,the problems such as over-excavation and deviation in strut position are very common in deep excavation practice.Moreover,the performance of strut tends to degrade?i.e.,strut stiffness tends to reduce gradully?owing to the fact that the steel tube struts are usually reused and some damage or deformation may take place.Under this situation,the results predicted from the KJHH model may deviate significantly from the actual values.Therefore,this thesis aims to identify the influence of both soil properties and construction factors on the deformation behavior of deep excavation in soft soil,and develop a practically reasonable method for evaluating deep excavation deformation considering uncertainty of both soil properties and construction factors.Specific work includes:?1?By using FLAC^3D software and selecting the plastic-hardening?PH?constitutive model,the numerical modelling of a typical deep excavation case is carried out,and the ground surface settlement and diaphragm wall deflection are obtained.The numerical simulation results are in good agreement with the measured data,demonstrating the rationality of the numerical simulation model.Parametric studies are performed by using the verified numerical simulation model to analyze the sensitivity of deep excavation deformation to the three main construction factors,including over-excavation depth,deviation in strut position,and reduction rate of strut stiffness.Results indicate that all the three construction factors will produce additional deep excavation deformation,and the additional impact is substantial and not negligiable.?2?FLAC^3D numerical simulations and KJHH model-based analytical analyses are performed for 432 different cases which cover various soil properties,deep excavation dimensions,over-excavation depths,reduction rates of strut stiffness,and deviation magnitudes of strut position.Comparison between the numerical modelling results and analytical analyses demonstrates that the KJHH model will significantly underestimate the lateral displacement of enclosure structure of deep excavation when over-excavation,reduction of strut stiffness and/or deviation of strut position exist in the actual construction.Therefore,multivariate regression analysis is conducted to process the numerical simulation results for the 432 cases to identify the quantitative relationship between the deep excavation deformation and over-excavation depth?reduction rate of strut stiffness,or deviation magnitude of strut position?.A modified KJHH model,which is able to account for all the factors mentioned above,is developed for practical evaluation of deep excavation deformation.By comparing the results from the modified KJHH model and numerical simulation,it can be found that the prediction accuracy of the modified KJHH model is substantially improved in comparison with the existing KJHH model.?3?By introducing the deviation coefficient?,the model uncertainty of the modified KJHH model is analyzed.It is shown that the COV?coefficient of variation?of?is merely at a low level,suggesting the insignificant model uncertainty.Further,on the basis of Monte-Carlo simulation,the influence of uncertainty of soil properties and construction parameters on the uncertainty of deep excavation deformation is studied by using the modified KJHH model.It is concluded that:the uncertainty of soil parameters?E_i/?'_v and s_u/?'_v?has the most significant effect on the uncertainty of the deep excavation deformation;while the uncertainty of construction factors such as over-excavation also shows substantial impact,and is necessary to be taken into consideration in evaluating the deformation risk of deep excavation in soft soil.