This paper presents a novel elasto-viscoplastic constitutive formulation based on the isotache concepts and the Nishihara model. Incorporating a novel viscoelastic body to include the delay elastic deformation of marine soft clays under the external load, the proposed model is used to evaluate the theories of consolidation-creep coupling, strain rate dependency and stress relaxation of saturated marine soft clays, and hence, the methodology used to determine the parameters of the model is discussed. Ningbo marine soft clay is selected as an example to interpret the determination of the model parameters on a field scale. A series of conventional oedometer tests are conducted as well. Eventually, we utilize the model to simulate several kinds of rheological tests, including one-dimensional (1-D) long-term compression tests on Ningbo marine soft clays, 1-D constant rate of strain (CRS) tests on Batiscan clays and 1-D stress relaxation tests on Hong Kong marine deposits. These findings indicate good agreement between the computational and experimental results, suggesting the given model can provide reliable forecasts for the rheological characteristics of marine soft clays.

The time-dependent compression characteristics of clay exhibit apparent differences when loaded from the over-consolidated and normally consolidated states. Based on the Nishihara model, this paper presents one dimensional isotache-based formulae for over-consolidated clays which consists of elastic, visco-elastic and visco-plastic deformation components. The newly constructed constitutive model is used to interpret the visco-elastoplastic compression characteristics of over-consolidated clays and incorporates effects including long-term compression, strain rate dependency and stress relaxation. Corresponding analytical solutions are obtained and the methodology to determine the model parameters is 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) are presented. These test data are used to verify the predictive capability of the proposed constitutive model, reasonable agreements between the computing and experimental results were found.

SummaryThe governing equations for the coupled processes of consolidation and creep of two‐layered soft soils are established. The Nishihara rheological model is adopted to simulate the elasto‐viscoplastic characteristics of soft soils, disregarding the effects of the soil self‐gravity. A semi‐analytical theory combined with numerical and analytical methods is introduced to solve the governing equations of the one‐dimensional rheological model. The computational procedure and the approximate solutions for two‐layered soft soils subjected to surface loading are obtained for two drainage conditions. The solutions and the computational procedure are used to study the effects of the two layers and constitutive parameters on rheological consolidation behavior of soft soils. It can be concluded that two layers affect the rate of excess pore water pressure dissipation and settlement development. The parametric studies show that when the parameters of the upper layer remain constant, increases in the permeability and elastic modulus in the lower layer accelerate the dissipation of the excess pore water pressure, and meanwhile increases in the viscosity coefficient and viscoplastic limit slows down the dissipation of the excess water pressure.

The threshold yield stress plays a key role in controlling the rheological consolidation behaviour of soft clay. A semi-analytical theory combining numerical and analytical methods is introduced that excludes the effects of clay self-weight. The governing equations assume the clay is an elasto-viscoplastic material. Approximate solutions as well as computational programme to surface loading for two drainage boundary conditions are obtained, while the clay profile is separated into material which is overconsolidated and normally consolidated. The effects of threshold yield stress and time-dependent loading on the rheological behaviour of soft clays are explored using the solutions developed.

In order to reveal the seepage characteristics of Ningbo marine soft clay under various hydraulic gradient,confining pressure and void ratio,eight series of constant head seepage tests were performed on both undisturbed and remolded soil samples utilizing the GDS apparatus.Fitting study of the widely used seepage models in academic field was conducted by the test results.It is shown that seepage coefficients of test samples decrease nonlinearly with decreasing void ratio,which behave more obviously on remolded soil samples.Similar seepage behaviors were obtained from both remolded and undisturbed soils as well,and the test results agreed well with fitting lines of the four existing seepage models.A new nonlinear empirical model was proposed,and test data of ten kinds of soils were collected to investigate its rationality and applicability.The comparison research on fitting results shows that the new model is better used than four existing seepage models,and the application of this new model turns to worth further research.