Analysis of the cone penetration test in layered clay

This paper presents an analysis of the cone penetration test in multi-layered clays using the commercial finite-element code Abaqus/Explicit. The von Mises yield criterion and its associated flow rule are assumed to model the plastic behaviour of elastoplastic undrained clays. An arbitrary Lagrangian–Eulerian scheme and an enhanced hourglass algorithm are adopted to preserve the quality of mesh throughout the numerical simulation. Initially, the behaviour of the penetration resistance is examined in a soil with only two layers. The bottom layer is the weaker of the two and the behaviour of the penetration resistance when the cone approaches the lower layer is studied. The investigation is then extended to study the cone penetration test in a multi-layered clay by sandwiching a weaker clay layer between two stronger clay layers. The thickness of the weaker clay layer is varied and the behaviour of the penetration resistance is studied in relation to the thickness and relative strength of the soil layers. The results are discussed with respect to the soil mechanisms that are present when the cone moves past the relevant layer boundaries so that the position of these boundaries can be determined more accurately

Discrete element modelling of material non-coaxiality in simple shear flows

We investigate the quasi-static simple shear flow of a two-dimensional assembly of cohesionless particles using discrete element method (DEM) simulations. We focus on the unsteady flow regime where the solid would experience significant evolution of stresses, mobilised shear strength and dilation. We construct the DEM model using a discretised-wall confined granular cell where the apparent boundary is allowed to dilate or contract synchronously with the confined solid. A rather uniform simple shear field is achieved across the whole assembly, which benefits rheological studies in generalising constitutive laws for continuum methods. We examine two aspects of the simple shear behaviour: macroscopic stress and strain rate evolution, particularly the non-coaxiality between the principal directions of the two; and micromechanics such as evolution of fabric. For an initially anisotropic specimen sheared under constant normal pressure, the direction of principal stress rotates towards that of the principal strain rate, gradually reducing the degree of non-coaxiality from about 45° to fluctuating around 0°. The rate in approaching coaxiality is slower in samples with larger initial porosity, stress ratio and mean stress. Generally, a faster rate in approaching coaxiality in simple shear is observed in a more dilatant sample, which often shows a larger degree of mobilised fabric anisotropy, suggesting the possible important role of instantaneous internal friction angle. The evolution of principal fabric direction resembles that of the principal stress direction

Correlations between the stress paths of a monotonic test and a cyclic test under the same initial conditions

In most experimental studies on liquefaction, cyclic loadings are applied on specimens with various initial conditions. However, few studies compared cyclic test results with monotonic results under the same initial conditions. The relation between monotonic tests and cyclic tests is crucial for understanding liquefaction mechanics and liquefaction resistance. This work compares the stress paths of a monotonic test with those of a cyclic test under the same initial conditions, and concluded that the stress path of monotonic tests envelops the stress path of cyclic tests under the same initial conditions. In addition, a new parameter, Level of Liquefaction Index (LI) is proposed to evaluate the liquefaction resistance of specimens under various initial conditions, and a linear relationship between LI and number of cycles at failure is found

Micro mechanics of isotropic normal compression

Discrete element modelling has been used to investigate the micro mechanics of isotropic normal compression. One-dimensional (1D) normal compression has previously been modelled in three dimensions using an oedometer and a large number of particles and without the use of agglomerates, and it was shown that the compression index was solely related to the strengths of the particles as a function of size. The same procedure is used here to model isotropic normal compression. The fracture of a particle is governed by the octahedral shear stress within the particle (due to the multiple contacts) and a Weibull distribution of strengths. The octahedral shear stresses, due to local anisotropic stresses within a sample with isotropic boundary stresses, are shown to give rise to a normal compression line (NCL) and the evolution of a distribution of particle sizes. The compression line is parallel to the 1D NCL in log e–log p space, in agreement with traditional critical state soil mechanics and confirming that the compression index is solely a function of the size effect on average particle strength, which determines the hardening law for the material. The paper shows, for the first time, how local octahedral shear stresses induced in the particles within the sample generate an isotropic normal (clastic) compression line

Undrained Cavity-Contraction Analysis for Prediction of Soil Behavior around Tunnels

The cavity-contraction method has been used for decades for the design of tunneling and prediction of ground settlement by modeling the cavity-unloading process from an in situ stress state. Analytical solutions of undrained cavity contraction in a unified state-parameter model for clay and sand (CASM) are developed in this paper to predict soil behavior around tunnels. The overall behavior of clay and sand under both drained and undrained loading conditions could be properly captured by CASM, and the large-strain and effective-stress analyses of cavity contraction provide the distributions of stress/strain within the elastic, plastic, and critical-state regions around a tunnel. The effects of ground condition and soil model parameters are investigated from the results of stress paths and cavity-contraction curves. Comparisons of the ground-reaction curve and the excess pore pressure are also provided between the predicted and measured behavior of tunneling by using data of centrifuge tunnel tests in clay

Monotonic direct simple shear tests on sand under multidirectional loading

Stress–strain responses of Leighton Buzzard sand are investigated under bidirectional shear. The tests are conducted by using the variable direction dynamic cyclic simple shear (VDDCSS), which is manufactured by Global Digital Systems (GDS) Instruments Ltd., U.K. Soil samples are anisotropically consolidated under a vertical normal stress and horizontal shear stress and then sheared in undrained conditions by applying a horizontal shear stress acting along a different direction from the consolidation shear stress. The influence of the orientation and magnitude of the consolidation shear stress is investigated in this study. There are only a few previous studies on soil responses under bidirectional shear, of which most studies do not consider the impact of the magnitude of the consolidation shear stress. They are compared with current studies, indicating both similarities and differences. Generally, all test results indicate that a smaller angle between the first and second horizontal shear stress leads to more brittle responses with higher peak strengths, and a larger angle leads to more ductile responses. In addition, the consolidation shear tends to make soil samples denser, and both the magnitude of consolidation shear stress and its direction influence the following stress–strain responses of soil samples

Interpretation of cone penetration test data in layered soils using cavity expansion analysis

Cavity expansion theory plays an important role in many geotechnical engineering problems, including the cone penetration test (CPT). One of the challenges of interpreting CPT data is the delineation of interfaces between soil layers and the identification of distinct thin layers, a process which relies on an in-depth understanding of the relationship between penetrometer readings and soil properties. In this paper, analytical cavity expansion solutions in two concentric regions of soil are applied to the interpretation of CPT data, with a specific focus on the layered effects during penetration. The solutions provide a large-strain analysis of cavity expansion in two concentric regions for dilatant elastic-perfectly plastic material. The analysis of CPT data in two-layered soils highlights the effect of respective soil properties (strength, stiffness) on CPT measurements within the influence zones around the two-soil interface. Results show good comparisons with numerical results and elastic solutions. A simple superposition method of the two-layered analytical approach is applied to the analysis of penetration in multilayered soils. A good comparison with field data and numerical results is obtained. It is illustrated that the proposed parameters effectively capture the influence of respective soil properties in the thin-layer analysis. It is also shown that results based on this analysis have better agreement with numerical results compared with elastic solutions

Stress-force-fabric relationship for unsaturated granular materials in pendular states

This paper explores the particle-scale origin of the additional shear strength of unsaturated granular materials in pendular states induced by the capillary effect by applying the stress–force–fabric (SFF) relationship theory to unsaturated granular material stress analysis. The work is based on discrete element simulations with the particle interaction model modified to incorporate the capillary effect. By decomposing the total stress tensor into a contact stress tensor originating from contact forces and a capillary stress tensor due to the capillary effect, the directional statistics of particle-scale information are examined. The observations are used to support the choice of the appropriate analytical approximations for the directional distributions associated with the solid skeleton and water bridges. The SFF relationship for unsaturated granular materials is formulated, and is shown to match the material stress state with good accuracy and is used to interpret the material strength in terms of the relevant microparameters. Macro and micro observations are carried out on both relatively dense and loose samples in triaxial shearing path to the critical state. The capillary force remains nearly isotropic during triaxial shearing. Anisotropy in the water bridge probability density, however, develops alongside the anisotropy in contact normal density, which decreases when the suction level decreases and the water content increases. The anisotropy effect in the water phase is much smaller than the solid skeleton, and a coupling effect with the solid phase makes the fabric anisotropy in wet materials smaller than that in dry materials. Combined with the SFF function, the increased solid coordination numbers and mean contact forces by the water bridge effect are more important factors for the suction-induced shear strength

Advanced monitoring and numerical techniques for assessing the stability of tunnels

This research aims to develop an advanced monitoring technique for assessing the stability of tunnels, which may substitute or supplement the conventionally manual procedures. In this paper, ‘laser scanning technique’ is primarily studied to determine the utilisation of laser scanners for condition monitoring of lined tunnels. A series of model tests have been carried out byusing laser scanning technology, in which models were set up simulating tunnel conditions. After scanning and processing in thecorresponding software, 3D coordinates of the models were obtained as well as 3D ‘point clouds’ models. Thus various defect targets like cracks and deformation on the tunnel wall could be identified and measured efficiently in these digital models. Precision and limitations relatedto laserscanning were also highlighted. In addition, numerical simulation would also be conducted using FLAC2D and FLAC3D software to link the simulatedtunnelbehaviourstooverallstructuralstability. This study indicates that laser scanning technique has potential for executing condition monitoring, such as depth and width of cracks, deformation of tunnels, with high accuracy in a static mode of scanning. By these observed information combined with numerical analysis, the stability of the tunnels couldbeassessedforsafety

A Binary-Medium Constitutive Model for Artificially Structured Soils Based on the Disturbed State Concept and Homogenization Theory

Triaxial compression tests were carried out on artificially structured soil samples at confining pressures of 25, 37.5, 50, 100, 200, and 400 kPa. A binary-medium constitutive model for artificially structured soils is proposed based on the experimental results, the disturbed state concept (DSC), and homogenization theory. A new constitutive model for artificially structured soils was formulated by regarding the structured soils as a binary medium consisting of bonded blocks and weakened bands. The bonded blocks are idealized as bonded elements whose deformation properties are described by elastic materials, and the weakened bands are idealized as frictional elements whose deformation properties are described by the Lade-Duncan model. By introducing the structural parameters of breakage ratio and local strain coefficient, the nonuniform distribution of stress and strain within a representative volume element can be given based on the homogenization theory of heterogeneous materials. The methods for determination of the model parameters are given on the basis of experimental results. Comparisons of predictions with experimental data demonstrate that the new model provides satisfactory qualitative and quantitative modeling of many important features of artificially structured soils