Theoretical And Experimental Study On Rheological Consolidation Behavior Of Marine Soft Clays

There is an ongoing discussion on the one dimensional consolidation theory in the geotechnical engineering community,while Rheology has been regarded as an independent discipline centuries after it was first proposed in the era of the ancient Greece.The combination between the one dimensional consolidation theory and Rheology has provided geotechnical researchers with new visions to analyze and solve problems in engineering practices.Loading history-dependent compression plays a key role in controlling the long-term settlement of clay and silt deposits,which are often abundant around estuaries and rivers in large populated centres around the world.Taking use of experimental,analytical,semi-analytical,and simulations methods,this research introduced the isochrone concepts and proposed certain constitutive formulae to explore the seepage,consolidation,long-term compression characteristics of marine soft clays under various loading conditions.The main works of this research are concluded as follows:(1)Eight series of constant head seepage tests were performed on both undisturbed and remolded soil samples utilizing the GDS apparatus.Similar seepage behaviors were obtained from both remolded and undisturbed soils as well,the test results agree well with fitting lines of the four existing seepage models.Thereafter,a new nonlinear empirical model was proposed,and test data of ten kinds of soils were collected to investigate its applicability.Through the comparison research on fitting results,the new model is thought to be better used than those four existing seepage models,and the application of this new model is still worth further research.(2)Referring to the robust Nishihara model,and assuming that the clay is an elasto-viscoplastic material,approximate solutions as well as computational programme under surface loading for two drainage boundary conditions were obtained,while the clay profile was separated into material which was overconsolidated and normally consolidated.The effects of visco-elastioplastic stress-strain relations,threshold yield stress and time-dependent loading on the rheological behaviors of soft clays were then explored using the deduced solutions.(3)The governing equations for the coupled processes of consolidation and creep of two-layered soft soils were formulated.The Matlab computational procedure and the approximate solutions for two-layered soft soils subjected to surface loading were obtained for two drainage conditions.The solutions and the computational procedure were used to investigate the effects of the two layers,Hooke body,Kelvin body and Bingham body on the developments of vertical deformation,excess pore pressure distribution and average consolidation degrees.(4)Adopting the GDSCTS and GDSTAS oedometer apparatus,Ningbo coastal soft clays were collected to conduct a certain indoor experiments,including:conventional oedometer tests,triaxial consolidation tests,rheological compression tests,constant strain rate loading tests,and stress relaxation tests.Regarding the tests results,we investigated the nonlinear deformation and loading history-dependency of testing specimens.(5)Incorporating a novel viscoelastic body to include the delay elastic deformation of marine soft clays under an external load,a new elasto-viscoplastic constitutive model based on the Suklje isotache concepts was proposed.Effects of consolidation-creep coupling,strain rate dependency and stress relaxations of saturated soft clays were evaluated,the parametric determination process was interpreted as well.The applicability of the given model was verified by the test results on Ningbo marine soft clay,Batiscan clays and Hong Kong marine deposits.(6)The Geocomp Load Trac Ⅲ and CRS oedometer apparatus were used to perform series of indoor tests,namely,conventional oedometer tests,rheological compression tests,constant strain rate loading tests,and stress relaxation tests on the marine clay samples from Australia’s North West Shelf(NWS).The rheological compression behaviors under complex loading paths were studied and stress-strain relations under various stress levels and strain rates were plotted.Regarding the tests results,we investigated the nonlinear deformation and loading history-dependent characteristics of testing specimens.(7)Novel one dimensional isochrone formulae for time-dependent compression characteristics of over-consolidated soft clays were established with concepts from the Nishihara model.The newly constructed constitutive model was used to interpret the rheological characteristics of over-consolidated clays and incorporated effects including consolidation-creep coupling,strain rate dependency and stress relaxation.Corresponding analytical solutions were obtained and the methodology to determine the model parameters was then introduced.New data from a series of oedometer,constant rate of strain(CRS)tests and stress relaxation tests on piston samples of marine,lightly over-consolidated,carbonate clay from the Australian North West Shelf(NWS)were presented.These test data have been used to verify the predictive capability of the proposed constitutive model.