Optimized Inverse Analysis Applied To Risk Assessment Of Excavation

The continuous development of high density urban areas wasaccompanied by the increase of underground space exploitation, both interms of area and of depth demands. Ensuring safety during the excavationprocess became more and more challenging, which is why risk analysis ofexcavation takes more importance and is submitted to new requirements.Currently, risk evaluation on excavation engineering focused on thesemi-quantitative theory research, and controlled by the single-objectivethreshold according to standards during construction process. Combinedwith practical engineering cases, aiming at the uncertainty for excavationrisk, a risk criterion and the three assessment methods was proposedthrough a combination of theoretical analysis and numerical simulation,which was applied to excavation risk evaluation with differentrequirements, such as rapid assessment, accurate assessment andcomprehensive assessment. The main content of works and results isincluded as follow.(1) In order to eliminate the time uncertainty of risk, inverse analysismethod was put into use for predicting subsequent deformation of theexcavation. Based on elastic foundation beam method and existing finiteelement method, the model and solution was simplified according to theexcavation engineering. And a iteration process and convergence criteriawere proposed as well. A C++program was written by using this algorithmto computer proportionality coefficient m that makes subgrade coefficientincreased along the depth, and using this proportionality coefficient m to predict deformation of retaining structures in next stage.(2) Further study on inverse analysis method, and a method ofnumerical simulation&multi-objective inverse analysis was put forwardin solving soil parameter analysis learnt from Pareto optimization theory,which is one of the multi-objective optimization theories. An existingmulti-objective optimization algorithm was link to the numericalsimulation software. Selecting diaphragm deformation and groundsettlement as objects, weight coefficient ω was compared, and anappropriate value was determined. It realized the practical application ofthe method. Then both single-and multi-objective inverse analysismethods were put into use, and their results were compared.Single-objective one had a high accuracy in analyzing own object, but pooraccuracy in the other object. And multi-objective one had almost sameaccuracy with the own object of single-objective ones, which revealed theadvantage of multi-objective method. Further, they were used to predictionthe excavation deformation of following stage. It showed thatmulti-objective method was more stable than single-objective one. Andcompared with the single-step prediction, real-time prediction had betterpredicting results.(3) According to the mechanical relationship between internal forceand deformation when structures were subjected to bending, a kind ofsimplified method to calculate structural moment was raised up byemploying segmented numerical differentiation method. It was called UnitDeflection Method (UDM). In order to improve the calculation accuracy,reasonable value of unit length was determined by a case. The UnitDeflection Method is a fast way to get the instant moment andcomprehensive risk status. By using this method, and in view of thedeficiencies of the way to judging excavation’s risk based on controllingdisplacement in standards, a comprehensive risk criterion on retainingstructures by considering both displacement and internal force was broughtup, which is based on displacement test’s results. Both the traditional criterion and the proposed criterion were applied into a case. The resultshowed the comprehensive risk criterion was able to find the excitingdangerous position accurately.(4) This three kinds of evaluation methods are applied in practicalengineering to satisfy the demands of rapid evaluation, accurate evaluationand comprehensive evaluation to excavations. They accurately predictedthe diaphragm deformation and ground settlement, and successfully findthe dangerous points. The evaluation result is reliable.