Upper bound limit analysis for finding interference effect of two nearby strip footings on sand

The ultimate bearing capacity of two closely spaced strip footings, placed on a cohesionless medium and loaded simultaneously to failure at the same magnitude of failure load, was determined by using an upper bound limit analysis. A logarithmic spiral radial shear zone, comprising of a number of triangular rigid blocks, was assumed to exist around each footing edge. The equations of the logarithmic spiral arcs were based on angles φ_L and φ_R rather than soil friction angle φ; the values of φ_L and φ_R were gradually varied in between 0 and φ. The ultimate bearing capacity was found to become maximum corresponding to a certain critical spacing between the footings. For spacing greater than the critical, the bearing capacity was found to decrease continuously with increase in the spacing. The extent of the spacing corresponding to which the ultimate bearing capacity becomes either maximum or equal to that of a single isolated footing increases with increase in φ. The results compare reasonably well with the available theoretical and experimental data.     

Relationship between small and large strain solutions for general cavity expansion problems in elasto-plastic soils

This paper presents a closed-form relationship between small and finite strain cavity expansion solutions. Its derivation is based on the non-linearly elastic–perfectly plastic cylindrical (or spherical) problem considering a general Mohr’s criterion and constant plastic dilatancy. It is shown, however, that it is sufficiently accurate for general expansion problems not obeying plane-strain rotationally (or spherically) symmetric conditions and involving strain-hardening/softening constitutive behaviour. Therefore, this relationship quantifies the error stemming from the computational assumption of small deformations and provides a simple and efficient way of accounting for geometric non-linearity based entirely on conventional computational methods: ‘self-correction’ of small strain analyses results.     

A discrete approach for anisotropic plasticity and damage in semi-brittle rocks

In this paper, we propose an anisotropic plastic damage model for semi-brittle geomaterials based on a discrete thermodynamic approach. The macroscopic plastic deformation is generated by frictional sliding of weakness planes. The evolution of damage is related to growth of such weakness planes. The local frictional sliding in each family of weakness planes is described by a non-associated plastic model taking into account material softening and volumetric dilatancy. The damage evolution is coupled with plastic deformation and modelled by an isotropic damage criterion. The proposed model is applied to modelling mechanical responses of typical sandstone under different loading paths. There is good agreement between numerical predictions and experimental data. Further, the anisotropic distributions of plastic deformation and induced damage are analysed and discussed.     

Critical-state-based Mohr-Coulomb plasticity model for sands

This paper presents a refined Mohr-Coulomb model for sands based on the critical state theory. The refined model adjusts a dilatancy angle based on the state parameter with respect to the critical state line. Furthermore, a friction angle is decomposed into the critical state friction angle and a portion of the dilatancy angle to capture the peak phenomenon of dilative sands. The elemental simulations of the drained and undrained triaxial compression tests on Toyoura sand using the refined model showed much better performance than the conventional Mohr-Coulomb model.     

岩石力学性质及可钻性分级研究

岩石力学性质和可钻性是分析和评价岩石抵抗破碎和钻进的基本性质，也是合理选择碎岩工具和工艺的基础。因此，对岩石力学性质和可钻性的深入研究，采用能够反映某种碎岩方式实质的综合力学性质指标评定岩石可钻性，应用数理统计学原理，依据岩石力学性质和可钻性指标，建立表示岩石可钻性的数学模型，并以此模型进行岩石可钻性分级，可及满足岩土工程施工生产的急需。     

Implicit and explicit procedures for the yield vertex non-coaxial theory

The yield vertex non-coaxial model is different from classical elastoplastic models, in that there is an additional plastic strain rate tangential to yield surfaces, as well as the plastic strain rate normal to yield surfaces, when orientations of principal stress change. This feature raises concerns on its finite element implementations. In nonlinear finite element numerical iterations, a large tangential plastic strain rate is likely to make the trial total strain rate direct inside a yield surface, which entails convergence difficulty. Some modifications are introduced on the non-coaxial model itself to make numerical convergence easier in the work published in Yang and Yu (2010) [20]. This paper is an extension of the previous work. Instead of modifying the non-coaxial model itself, this paper concerns the use of finite element explicit procedure, which is suitable for highly discontinuous problems. The simulations of shallow foundation load-settlement responses indicate that the finite element explicit procedure, assisted with a robust and explicit automatic substepping integration scheme of the non-coaxial model, does not encounter numerical difficulty. In addition, the overall trends of implicit and explicit simulations are similar.     

Improving force resultant model for anchors in clays from large deformation finite element analysis

Abstract

This paper presents an improved plasticity force-resultant model for anchors deeply embedded in clays, developed from large deformation finite element analyses. The current available force-resultant models for anchors are mainly developed from small strain finite element analysis while experimental approach has not been used due to technical challenges. The advantage of large deformation finite element analysis is that it provides much more data points to fit the yield surface than small strain finite element analysis, in addition to avoiding excess mesh distortion problems. Furthermore, the flow rule or normality can be effectively checked in the large deformation finite element analysis and further used to improve the fitting quality. After validated against retrospective simulations, the better performance of the developed plasticity force-resultant model is demonstrated by comparing with available experimental observations from centrifuge test.     

Bearing capacity of strip and circular footings in sand using finite elements

Design of shallow foundations relies on bearing capacity values calculated using procedures that are based in part on solutions obtained using the method of characteristics, which assumes a soil following an associated flow rule. In this paper, we use the finite element method to determine the vertical bearing capacity of strip and circular footings resting on a sand layer. Analyses were performed using an elastic–perfectly plastic Mohr–Coulomb constitutive model. To investigate the effect of dilatancy angle on the footing bearing capacity, two series of analyses were performed, one using an associated flow rule and one using a non-associated flow rule. The study focuses on the values of the bearing capacity factors N_q and N_γ and of the shape factors s_q and s_γ for circular footings. Relationships for these factors that are valid for realistic pairs of friction angle and dilatancy angle values are also proposed.     

A geological study of clayey laterite and clayey hydromorphic material of the region of Yaoundé (Cameroon): a prerequisite for local material promotion

A geological survey carried out in the Yaoundé (Cameroon) region has revealed the presence of homogeneous clayey laterite in the upper part of a laterite cover on interfluves, thickest on hills (780–800 m altitude) where ferricrete is absent, and clayey heterogeneous hydromorphic material in valleys. We present in this paper the physical, mineralogical and geochemical properties of these occurrences and discuss their potential as raw material for pottery, manufacture of bricks and tiles. These clayey raw materials are mostly made up of fine particles (ranging from 55 to 60% clay + silt in the clayey laterite, more than 70% clay + silt in the clayey hydromorphic material). Their chemical composition is characterized by silica (<60% SiO₂), alumina (<35% Al₂O₃) and iron (ranging from 3 to 14% Fe₂O₃). Their main clay minerals are disorganized and poorly crystallized kaolinites. The average limits of liquidity (44.56% versus 91.58%) and limits of plasticity (22.4 versus 45.93) revealed that clayey hydromorphic material has the greatest plasticity. The studied raw materials are suitable for making pottery as well as the manufacture of bricks and tiles. However, the high iron content in the clayey laterite (between 11 and 12% Fe₂O₃) prevents their efficient use in the manufacture of ceramics.