A formalism for double structure hydromechanical coupled modelling of aggregated unsaturated soils has been developed. Independent coupled hydromechanical models are considered for each structural ...level, including independent measures of macromechanical and micromechanical effective stresses. The models are linked using a coupling function to obtain the global response. The individual components have been selected to represent the behaviour of compacted expansive clays. The macrostructural mechanical model is based on the existing hypoplastic model for unsaturated soils. Hydromechanical coupling at each structural level is efficiently achieved by linking the effective stress formulation with the water retention model. An essential component of the model is representation of microstructural swelling. It is demonstrated that its calibration on wetting induced expansion measured in oedometric (mechanical) tests leads to a correct global hydraulic response, providing a supporting argument for the adopted coupling approach. An interesting consequence of the model formulation is that it does not suffer from volumetric rachetting, which is often regarded as one of the main drawbacks of hypoplasticity. The proposed model has a small number of material parameters. Its predictive capabilities have been confirmed by simulation of comprehensive experimental data set on compacted Boom clay.
•New formalism for hydromechanical coupling in double-structure unsaturated soils is proposed.•Separate hydromechanical model for each structural level, including separate effective stress measures.•Microstructure swelling controls global expansion as well as global water retention curve.•Volumetric rachetting eliminated from hypoplasticity.
Large-diameter monopiles are the most commonly used foundation to support offshore wind turbines. Early designs usually adopted pile diameters (D) between 4 and 6 m, which is recently extended to 8 m ...and will target 10 m in the future. It is increasingly evident that the existing design method (i.e., API's p-y model) can significantly under-predict the lateral stiffness and capacity of large-diameter monopiles in soft clay, due to ignoring the soil resistances from base shear and base moment which become more pronounces as L/D reduces. In this study, a two-spring approach is proposed, aiming to predict the lateral behaviour of monopiles with varied L/D ratios in a unified manner. In light of the soil flow mechanisms around monopiles, the pure lateral soil resistance above the rotation point (RP) is quantified using a p-y model, while the resistances below the RP including the base shear and base moment are integrated into a moment-rotation spring (characterized by a MR-θR model) at the RP. It can naturally recover to a p-y model while analyzing flexible piles, where θR= 0 at RP. Formulations of the ‘p-y + MR-θR’ model (including diameter-related p-y and MR-θR models, and the depth of the RP) are proposed based on the results of a series of well-calibrated 3D numerical models. The proposed model has satisfactorily reproduced a number of field and centrifuge test results on laterally loaded monopiles with a wide range of L/D ratios (including flexible, semi-rigid and rigid piles), using a unified set of parameters. Compared to the standard p-y model, the adoption of the proposed ‘p-y + MR-θR’ model is shown to substantially reduce design conservatism.
•A unified model is proposed for monopiles in soft clay considering various length-to-diameter (L/D) ratios.•The model considers the soil resistance contributions from pile base shear and base moment.•Model formulations are developed based on a series of well-verified 3D finite element analyses considering a wide range of L/D.•The validity of the model has been well justified against a number of field and centrifuge model tests.
The central aim of this paper is to discuss the applicability of the effective stress principle as defined by Terzaghi (total stress minus pore-water pressure) to predict the behaviour of expansive ...clay aggregates. Phenomena occurring between individual clay minerals are reviewed first at the molecular level obtained in the colloid science research. In particular, it is noted that, for interparticle distances higher than approximately 1.5 nm, the pore-water pressure in the bulk equilibrium solution forms an additive component of the interparticle disjoining pressure. It is concluded that for these distances Terzaghi’s effective stress principle should be adequate to describe the clay behaviour. To support these developments, an extensive experimental database of nine different sodium and calcium bentonites available in the published literature was analysed. With the aid of double structure constitutive modelling, procedures were developed to extract information about the behaviour of clay aggregates from the experimental measurements. It was then shown that unconfined water retention curves, swelling pressure tests, swelling under constant load tests, and mechanical unloading tests are all uniquely related in terms of the dependency of dry density (or void ratio) of clay aggregate versus mean effective stress. By considering reversibility of aggregate behaviour and full saturation of aggregates, this implies that the effective stress principle is a valid way of predicting expansive clay aggregate volumetric deformation.
Bentonites are considered as a suitable buffer material for the high-level radioactive waste disposal. The long-term stability of the hydromechanical properties of the bentonite barrier at high ...temperature is one of the key conditions for the proper functioning of the bentonite barrier. This paper presents an experimental study of the swelling pressure evolution of compacted samples of BCV bentonite exposed to high temperatures. The samples at constant volume were fully saturated at room temperature from the as-compacted state and then rapidly heated to constant temperatures of 50–150 °C. Temperatures were then held constant and the swelling pressure evolution was measured for the next 30 days. In the next stage, temperatures were decreased back to laboratory conditions and the equilibration of the swelling pressures was monitored. Finally, hydraulic conductivity of the selected samples was determined at room temperature. An additional test was performed to investigate the effect of saturation after heating on swelling pressure development. The results showed a significant decrease in the swelling pressure with time at elevated temperatures in all the samples (denoted as thermal relaxation). This decrease was more significant at higher temperatures and at temperatures above 100 °C the swelling pressure did not reach a steady state value until the end of the high temperature stage. The rate of swelling pressure decrease was quantified by the coefficient of relaxation Crel. A permanent decrease in swelling pressure due to thermal relaxation was identified after cooling. The swelling pressures decreased to 90–41% of the original swelling pressures determined before heating. The magnitude of the swelling pressure reduction increased with the applied temperature. Similar behaviour was observed for samples of different dry densities. The same effect of thermal relaxation was observed for samples saturated before and after heating. Thermal relaxation did not affect the hydraulic conductivity of the bentonite after temperature cycle, which was similar to that of thermally untreated BCV samples.
•Significant thermal relaxation (swelling pressure decrease with time) of BCV bentonite was observed.•Decrease in swelling pressure accelerates with applied temperature.•Thermal relaxation resulted in permanent reduction of swelling pressure.•No effect of temperature cycle on hydraulic conductivity was observed.
Summary
This paper presents a new viscohypoplastic model for soft clays accounting for their typical features—strength anisotropy and rate dependency. The model is based on the hypoplastic model for ...clays enhanced by the anisotropic shape of the asymptotic state boundary surface. It has been shown that if the surface is skewed, the model predicts different ultimate strength in compression and in extension. Additional enhancement makes the tensor L bilinear in the strain rate, which more realistically predicts the stress paths of K0 consolidated samples. The new model has been evaluated by simulating laboratory experiments on soft marine clays (Singapore and Bangkok clays). The model can be easily calibrated using only undrained triaxial and odometer tests. The model is subsequently enhanced by the rate effects. The resulting viscohypoplastic model has been evaluated using experiments of remolded kaolin clay and St. Herblain clay. It is shown that the enhanced model can predict important features of soil viscous behavior, such as rate dependency of strength and preconsolidation pressure, relaxation, and creep.
A new rate-independent hypoplastic model for clays is developed. The model is based on the recently proposed approach enabling explicit incorporation of the predefined asymptotic state boundary ...surface and corresponding asymptotic strain rate direction into hypoplasticity. Several shortcomings of the existing hypoplastic model for clays are identified and corrected using the proposed approach. Thanks to the independent formulation of the individual model components, the new model is more suitable to form a basis for further developments and enhancements than the original one.
The paper presents evaluation of the newly proposed hypoplastic model for soft clays by modelling of the well-documented geotechnical failure of a deep excavation of Nicoll Highway, Singapore, in ...2004. The marine clays present at the site are typical normally consolidated soft clays, exhibiting large deformability and stiffness and strength anisotropy. These features - anisotropy of asymptotic state boundary surface and the effect of normal consolidation on stress paths and stiffness anisotropy - have been included into the newly proposed hypoplastic model. In the paper, it is first demonstrated that the model predictions compare well with experimental data from element tests on Singapore clay and improve predictions over original hypoplastic model. Secondly, numerical simulations of Nicoll highway excavation focusing on specific cross-section within the collapse area are presented, showing good fit between inclinometer data and simulations up to the final stages immediately preceding the excavation collapse. The simulation results are compared for previous version of the hypoplastic model, along with Mohr-Coulomb model with effective and total stress parameters. Subsequently, the influence of the model parameters on simulation results is studied and it is shown that the parameters affecting extension strength are more significant than parameters affecting pre-failure non-linearity.
Thermo-hydro-mechanical coupling is relevant in various natural processes and engineering applications involving clay soils. It can affect slope deformations and stability, as well as the functioning ...of clay barriers and energy piles. Temperature changes can alter the water retention capacity of expansive clays and, in turn, produce pressure, strength, and volume changes. In deep geological repositories, for instance, the design of bentonite buffers and the study of their interaction with the host formation must account for the heat released by radioactive decay. Here, to investigate how temperature controls the water retention capacity, vapor transfer experiments under adsorption/desorption (wetting/drying) paths were performed on the Czech B75 bentonite. The tests were conducted in a wide range of temperatures (20–80°C) and initial dry densities (0.6–1.9 g/cm3), at high total suction (4–400 MPa), without mechanical loads. The results showed a systematic loss of water retention capacity at high temperature, particularly at low suction, irrespective of the initial compaction. To predict the behavior at any temperature, a model was constructed from the Clausius–Clapeyron and the Guggenheim–Anderson–de Boer equations. It was calibrated and validated at various temperatures, also on a different bentonite (without further tuning), showing good performance. Dry density-specific calibrations did not affect the model predictions significantly, consistently with results that exclude an effect of initial compaction on water retention at high suction. The proposed model seems suitable for inclusion into thermo-hydraulic descriptions in comprehensive constitutive frameworks for expansive clays, potentially improving the understanding of some behaviors related to thermo-hydro-mechanical coupling.
•Vapor transfer experiments at 20–80°C were performed on the Czech B75 bentonite.•The water retention capacity decreased significantly at high temperature.•Initial compaction did not affect water retention at high suction.•A thermodynamic model was able to simulate the behavior at any temperature.•The model seems suitable for improving comprehensive constitutive formulations.
Tunnel driving inevitably induces changes in stress and deformation in the ground, which could cause ultimate and serviceability problems to an adjacent tunnel. The effects of induced stress on an ...existing tunnel and crossing-tunnel interaction are still not fully understood. In this study, a series of three-dimensional centrifuge tests were carried out to investigate the responses of an existing tunnel in sand to the excavation of a new tunnel perpendicularly below it. Three-dimensional tunnel advancement was simulated using a novel technique that considers the effects of both volume and weight losses. This novel technique involves using a “donut” to control volume loss and mimic soil removal in-flight. To improve fundamental understanding of the stress transfer mechanism during the new tunnel advancement, measured results were back-analyzed three-dimensionally using the finite element method. The maximum measured settlement of the existing tunnel induced by the new tunnel constructed underneath was about 0.3% of tunnel diameter, which may be large enough to cause serviceability problems. The observed large settlement of the existing tunnel was caused not only by a sharp reduction in vertical stress at the invert, but also by substantial stress transfer of overburden pressure at the crown. The section of the existing tunnel directly above the new tunnel was compressed vertically because the incremental normal stress on the existing tunnel was larger in the vertical direction than in the horizontal direction. The tensile strain and shear stress induced in the existing tunnel exceeded the cracking tensile strain and allowable shear stress limit given by the American Concrete Institute.
PDF
