Numerical simulation of a tunnel surrounded by sand under earthquake using a hypoplastic model

A numerical model of a centrifuge experiment on tunnel located in sand is being presented. The experiment was carried out under seismic loading using a dynamic actuator. The responses of the tunnel and of the sand were measured. The numerical model is based on a hypoplastic constitutive model with intergranular strains implemented in the FE-code TOCHNOG. The calculated accelerations in the sand match the measured results, while the surface settlement and the bending moments in the tunnel lining are only qualitatively reproduced by the numerical model.     

Measurement of wall friction in direct shear tests on soft soil

If the free vertical movement of the upper rigid part of the shear box is hindered during shearing, a frictional force is mobilized between the specimen and the vertical walls of the shear box. This causes either unloading (for contractant soils) or additional loading (for dilatant soils) of the specimen during shearing. If no correction of the applied vertical load with respect to the wall friction is taken into account, the resulting shear strength can be either underestimated (for contractant soils) or overestimated (for dilatant soils). For example, in a particular investigation of a normally consolidated soil, the measured friction angle from a direct shear test was almost 8° smaller than the angle from a triaxial test. This paper, therefore, presents a method for direct measurement of the frictional force at the contact between the vertical walls of the box and a fine-grained soil. If the wall friction is taken into account, the friction angle from the shear box coincides well with the angle from triaxial tests. If the wall friction cannot be measured during the test, a sufficiently large vertical gap should be adjusted in case of soft soils, in order to enable non-restrained settlement of the upper part of the box during specimen contraction.     

Analysis of the compression behavior of artificial lumpy composite materials

The landfills are dumped without any compaction and have a relatively open structure, which is similar to that of the granular materials. However, the original dumped material might be gradually transformed into a lumpy composite structure because of the influence of the climate. As a result, the lumps are randomly distributed in the reconstituted soil. In the presented study, the compression behavior of the lumpy composite soils was analyzed within the homogenization framework. Firstly, the volume of the composite soil was divided into four individual components. The inter‐lump porosity was introduced to account for the evolution of the volume fractions of the constituents, and it was formulated as a function of the overall porosity and those of its constituents. A homogenization law was then proposed based on the analysis of the lumpy structure together with a numerical method, which gives a relationship for tangent stiffnesses of the lumpy soil and its constituents. Finally, a simple compression model was proposed for the composite lumpy material, which incorporates both the influence of the soil structure and the volume fraction change of the reconstituted soil. The predictions of the model were validated against the test results, and the stress distribution within the lumpy composite was assessed. Copyright © 2016 John Wiley & Sons, Ltd.     

Strain-dependent slope stability

Acta Geotechnica - The state of equilibrium of a slope is usually interpreted and expressed by safety factors based on calculations with limit equilibrium methods. Different stress states, failure...     

A nonlinear Hvorslev surface for highly overconsolidated soils: elastoplastic and hypoplastic implementations

The clayfills are being produced in open-pit mining. The stress state in the stiff lumps of the clayfills is significantly lower than in situ level. As a result, their current states lie on the dry side of the critical state. The linear Hvorslev surface is widely used due to its simplicity and capability to model the limit stress condition of soils on the dry side. However, it may overestimate the strength at very low stress level. For this purpose, a series of drained triaxial tests were performed on a silty clay at very small stress levels. The failure points of the tested soil confirm a nonlinear relationship in \(p^{\prime }\)–q plane on the dry side of the critical state. The degree of nonlinearity increases after being normalized by the Hvorslev equivalent pressure, which can be well modeled by a nonlinear power law criterion proposed by Atkinson (Géotechnique 57(2):127–135, 2007). Based on the test data and the critical state concept, a new failure line is proposed with help of the equivalent Hvorslev pressure. The nonlinear Hvorslev surface is then incorporated into an elastoplastic model and a hypoplastic model. Comparisons between the experimental data and simulations reveal that the proposed models can well represent the behavior observed in the laboratory.     

Numerical investigation of undrained cavity expansion in fine-grained soils

Based on the spherical cavity expansion (SCE) problem, Cudmani and Osinov (Can Geotech J 38:622–638, 2001), Osinov and Cudmani (Int J Numer Anal Method Geomech 25:473–495, 2001) developed a semi-empirical method of interpretation of CPT for coarse-grained soils (sand, gravel) using a hypoplastic constitutive model. Using a material-independent shape factor, the cone penetration resistance was related to the limit pressure required to expand a spherical cavity. The shape factor was observed to be a function of the soil state only, in particular the pressure-dependent relative density. This paper presents an analogous interpretation technique for CPT in fine-grained soils using the shape factor concept, Cavity Expansion approach, and a hypoplastic constitutive model. Relations for the shape factor and the limit pressure have been proposed based on the parameters affecting these quantities. A validation of the proposed interpretation technique with experimental results has also been performed.