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Study On The Ultimate Bearing Capacity Of The Footings Above Voids

Posted on:2008-06-15Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y Z YangFull Text:PDF
GTID:1100360215999004Subject:Geological Engineering
Abstract/Summary:PDF Full Text Request
Along with the rapid development of transportation and urbanconstruction, a growing number of underground voids have beenencountered in the design and construction of civil engineering. How todesign and construct the footings reasonably in these areas is an urgentproblem. However, the current theories for determination of ultimatebearing capacity of footings above voids can not meet the actual needs.Some of the relevant standards in this area are also in the skin-deep, andsystematical investigation of the determination of ultimate bearingcapacity of footings above voids is of high theoretical and practicalsignificance.The upper bound limit analysis, simplified structural mechanics, andfinite element analysis have been developed to determine the ultimatebearing capacity of footings above voids, and a great number ofcalculations and parameteric studies have been taken in this paper. Thefollowing results were obtained:1. The formula of ultimate bearing capacity of strip footing abovevoids are derived using upper bound limit analysis, and also thecorresponding Matlab program has been compiled. Firstly, byconstructing the failure modes and corresponding velocity field of stripfooting above voids, the gravity and other internal power as well asexternal energy dissipation have been calculated. And then the objectfunction of ultimate bearing capacity has been formed by power balanceequation. Finally, the upper bound result is obtained by the solution of anoptimize problem under the constraints which meet the physical andgeometric prerequisites of the failure mode. Based on the analysis in thispaper, it is shown that the upper bound limit analysis is a feasible andreliable way to solve the ultimate bearing capacity of strip footings abovevoids.2. The ultimate bearing capacity of foundation above voids has beensimplified as an beam and arch model, respectively. The formula ofstability of void roof have been derived usingthe structural mechanicsprinciple, and also the corresponding program has been compiled. Thecalculation results are evaluated by introduction of a capacity factor S. The results show that the rock properties has a great influence on theultimate bearing capacity. And the tensile stress is proved to be thecontrolled factor. With the increase of the thickness of the void roof, thelargest allowed cave span increases, and will reach a constant value.3. The influence factors of ultimate bearing capacity of circularfooting and strip footing above voids are investigated by commercialfinite element software ABAQUS. With constructed numerical modelsand analyzed results, the determination method of ultimate bearingcapacity has been put forward, and also a lot of numerical results havebeen obtained. The relationship between the ultimate bearing capacityand its various influence factors has been proposed by tables and pictures.The results provide a reference for actual design and construction offootings abve voids.4. Through the built up numerical model, lots of finite elementanalysis results of ultimate bearing capacity factors of strip footingsabove voids have been made. And the bearing capacity factors N_r and N_cwhich are proposed by tables and pictures are obtained by regressionanalysis for consideration of the width and thickness of voids roof,loading eccentricity, the footing width as well as other impact factors. Theresults can be of actual value for design and construction of footing abovevoids.
Keywords/Search Tags:void, footing, ultimate bearing capacity, limit analysis, finite element analysis, bearing capacity factor
PDF Full Text Request
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