Creep Test On Soft Clay And The Identification Of Constitutive Model Study And Its Application

The forecast of subgrade settlement is one of the most importantproblems in highway construction, especially in areas of soft clay. Thesettlement analysis and forecast research for special subgrade whichcomposed of high liquid limit clay at the upper level and soft clay at thelower level are carried out by using test study, theoretical analysis andnumerical analysis methods. Based on the Zhucheng highway project, theforecast of subgrade settlement has been analyzed in detail. The mainresearch work is as following:1. Analysis on creep tests of soft clay(1) Based on a great deal of laboratory tests, the strength anddeformation characteristics of high liquid limit clay and soft clay havebeen studied. One-dimensional consolidation creep tests under differentconsolidation stresses and triaxial undrained creep tests under differentconfining pressures and different loading grades have been carried out onsoft clay, and the full-process creep test curves and pore pressure-timecurves, which reveals the special creep mechanism of soft clay, have beenobtained. The creep curves, equal-time stress-strain curves and stainrate-deviator stress curves processed by "Chen method" are analyzed indetailed.(2) Based on the triaxial creep tests data, the famous empiricalcreep models such as Singh-Mitchell model, Mesri model and thecorresponding revised models are applied respectively to predict the testresults. However, there are obvious differences between the actual testresults and the simulating results. Therefore, in this paper, a newempirical creep model is proposed, in which the power function is used for shear stress-strain behavior and hyperbolic function is used forstrain-time behavior. This model has few parameters and the simulationresults of creep tests on the soft clay in Zhucheng highway have beenremarkably improved. It should be noticed that empirical creep modelhas certain value in engineering application, but it lacks of rigoroustheoretical basis and is difficult for FEM numerical analysis. Advancedconstitutive model is needed to solve this problem.2. Investigation on identification theory of soft clay constitutivemodel based on genetic arithmetic and overlay model(1)The parameter identification theory based on genetic arithmeticand Elastic visco-plastic FEM program has been studied, and thecorresponding identification program GAPD has been programmed. Itshows that genetic arithmetic is applicable in parameter identification ofelastic-viscous-plastic models.(2) Based on the research on soft clay constitutive identificationtheory founded on genetic arithmetic and overlay model, thecorresponding numerical analysis software has been programmed.Combining with the triaxial creep tests, it can be concluded that7-parameter overlay model is well suitable for the creep characteristic ofthe soft clay subgrade settlement analysis in Zhucheng highway.3. Example on embankment settlementIn this paper, taking Zhucheng highway K8+270 for example, thesubgrade settlement laws are analyzed. The following methods areadopted:â‘ 7-parameter overlay model FEM;â‘¡the widely used FEManalysis progam-Plaxis considering creep or not;â‘¢the commonly usedtraditional revised empirical coefficient method in practical. Aftercomparing the methods mentioned above, conclusions helpful for design and construction in engineering are gained in this paper.In conclusion, based on a lot of creep tests, the creep deformation,the strength characteristics, and influence factors of soft clay subgradehave been studied. The suitable creep models including overlay model,empirical creep model, yield surface creep model and their parametersdetermination methods have also been discussed in this paper. Bycombining these creep constitutive models with FEM, differentcalculation methods of subgrade settlement which can reflect the creepcharacteristics of soft clay have been developed. These studies providetheoretical references for subgrade design of soft clay. The research workis of not only high academic value but also great significance forengineering application.