Soils that exhibit nonassociated flow may, according to stability postulates by Drucker and by Hill, become unstable when exposed to certain stress paths inside the failure surface. Series of conventional triaxial tests on fully saturated and on partly saturated specimens were performed under drained and undrained conditions to study the regions of stable and unstable behavior. For specimens that compress and have degrees of saturation higher than critical, undrained conditions lead to effective stress paths directed within the region of potential instability, and instability was observed provided the yield surface opens up in the outward direction of the hydrostatic axis. Thus, instability occurs inside the failure surface. Instability is not synonymous with failure, although both may lead to catastrophic events. The location of the instability line is discussed. Examples of a shallow submarine slope and a nearly fully saturated steeper slope representing a tailings dam, which both should remain stable a...

Closure of "Static Instability and Liquefaction of Loose Fine Sandy Slopes"

It is known that the distribution of active earth pressure against a translating rigid wall is not triangular but nonlinear, owing to arching effects in the backfill. In the present paper, a new formulation is proposed for calculating the active earth pressure on a rigid retaining wall undergoing horizontal translation. It takes into account the arching effects that occur in the backfill (or retained soil mass). In order to check the accuracy of the proposed formulation, the predictions from the equation are compared with both existing full-scale test results and values from existing equations. The comparisons between calculated and measured values show that the proposed equations satisfactorily predict both the earth pressure distribution and the lateral active earth force on the translating wall. In order to facilitate calculation of active earth pressures using the proposed equations, a modified active pressure coefficient and an effective height of application of the lateral active force are also prov...

Estimation of active earth pressure against rigid retaining walls considering arching effects

The paper focuses on investigating the destructuration process during time-dependent stress-strain evolution. For this purpose, various oedometer tests and triaxial tests on intact and reconstituted samples of soft sensitive Vanttila clay were carried out. Based on experimental observations, a new elastic viscoplastic model, extended from the overstress theory of Perzyna, is developed. The proposed model accounts for inherent and induced anisotropy, interparticle bonds and bond degradation, and viscosity. The determination of model parameters is discussed, demonstrating how all model parameters can be determined in a straightforward way and no additional test is needed for the proposed model compared to the modified Cam clay model. The model is implemented into a finite-element code, which enables coupled consolidation analyses. The model is used to simulate various strain-rate and creep tests under one-dimensional and triaxial conditions on the intact samples of Vanttila clay. The comparisons between experimental results and simulations show that the model has good predictive ability on the time-dependent behavior of a soft sensitive clay.

Modeling time-dependent behavior of soft sensitive clay

Modeling shear behavior and strain localization in cemented sands by two-dimensional distinct element method analyses

Soft structured clays usually exhibit complex behaviors, which can lead to difficulties in the determination of parameters and high testing costs. This paper aims to propose an efficient optimization method for identifying the parameters of advanced constitutive model for soft structured clays from only limited conventional triaxial tests. First, a new real-coded genetic algorithm (RCGA) is proposed by combining two new crossover and mutation operators for improving the performance of optimization. A newly developed elastic–viscoplastic model accounting for anisotropy, destructuration and creep features is enhanced with the cross-anisotropy of elasticity and is adopted for test simulations during optimization. Laboratory tests on soft Wenzhou marine clay are selected, with three of them being used as objectives for optimization and others for validation. The optimization process, using the new RCGA with a uniform sampling initialization method, is carried out to obtain the soil parameters. A classic genetic algorithm (NSGA-II)-based optimization is also conducted and compared to the RCGA for estimating the performance of the new RCGA. Finally, the optimal parameters are validated by comparing with other measurements and test simulations on the same clay. All comparisons demonstrate that a reliable solution can be obtained by the new RCGA optimization combined with the appropriate soil model, which is practically useful with a reduction in testing costs.

An efficient optimization method for identifying parameters of soft structured clay by an enhanced genetic algorithm and elastic–viscoplastic model

A viscoplastic constitutive model is proposed that can reproduce both strain rate dependence of natural soft clay behaviour and destruction processes (DPs) induced by strain or stress increments. The model tracks changes in the yield locus from the initial position (INYL), which reflects the presence of metastable structures, to the intrinsic position (IRYL), which represents structurally stable conditions, where DPs are almost complete. The model is able to simulate in oedometric conditions Bjerrum's 1967 framework, and makes a clear distinction between the vertical effective yield stress, σ′vy, above which significant irrecoverable time-dependent strains are irreversibly activated, and the preconsolidation pressure, σ′vp, from Casagrande's construction, helping to avoid possible sources of error in consolidation settlement predictions. Notwithstanding some simplifications, the model fits well the experimental results of oedometric tests on a natural soft clay with distinct σ′vy and σ′vp for which viscop...

Modelling of natural soft clay destruction processes using viscoplasticity theory

Understanding sampling disturbance and behaviour of structured clays through constitutive modelling

This paper presents a proposal, developed as part of the EC-sponsored SafeLand project, to supplement existing landslide classifications with a more detailed description of the geomechanical characteristics of the materials to include the effects of “structure” and stress history, as determined by depositional and post-depositional processes. This provides a valuable insight into and a rational basis for the modelling of the physical mechanisms that govern the triggering and subsequent development of landslides. The classification, behaviour and minimum requirements for the constitutive modelling of saturated clays and sands are summarized here. The use of advanced constitutive models which include, where necessary, progressive damage to the microstructure of the material helps to predict the conditions and the style of triggering and of subsequent movement of landslides, depending on the type of material, even in first time landslides. Numerical examples are presented, illustrating the different response to rapid perturbation of sensitive clays versus mechanically overconsolidated clays and of “loose” sands versus “dense” sands.

Maurizio Fontana

Papers

Modelling of natural soft clay destruction processes using viscoplasticity theory

Understanding sampling disturbance and behaviour of structured clays through constitutive modelling

Description of soils based on geomechanical criteria for improved landslide classification