This paper proposes a new water retention model for unsaturated soils, which takes into account capillary condensation of adsorbed water. In this model, the degree of saturation of a soil is ...separated into that based on capillary water and that based on adsorbed water. Through analysis of a partially saturated two-cylinder system, a new shear strength criterion for unsaturated soils is proposed, in which only the degree of saturation based on capillary water contributes to the variation of shear strength with suction. The proposed shear strength criterion is justified against thermodynamic principles and is compared against existing criteria in the literature, which shows that it provides a much improved prediction of the experimental data for a wide range of suction values.
Conductive rubber fibres filled carbon black (CB)/cementitious composites were developed to achieve the cement-based sensors with excellent piezoresistivity in this study. Ameliorations on the ...conductivity and piezoresistive sensitivity of CB filled composites were mainly explored with conductive rubber fibres embedded. Their compressive strengths were investigated to evaluate the practical application possibility. The results indicated that the composites with CB content <4.0 wt% possessed acceptable compressive strengths. In terms of conductivity and piezoresistivity, both conductivity and piezoresistivity of composites filled with 0.5 wt% CB increased with the rubber content, and their gauge factor raised to 91 when embedded with 80 rubber fibres (1.27 vol%). Moreover, phenomenon of “piezoresistive percolation” was observed by sharp fractional changes of resistivity for the composites filled with 1.0 wt% CB, where existed highest gauge factor reaching 482 when embedded with same rubber fibres. However, because of the excellent conductivity of 2.0 wt% CB filled composites, the gauge factor firstly increased but then slightly decreased around 100 with increase of rubber fibre content. Overall, conductive rubber fibres can significantly improve the piezoresistivity of CB/cementitious composites by the increased gauge factor.
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•CB/cementitious composites with w/b ratios from 0.31 to 0.5 exhibit acceptable workability.•The compressive strength of cementitious composites is sensitive to CB content.•The electrical conductivity of CB/cementitious composites can be improved by conductive rubbers.•Conductive rubber filled CB/cementitious composite is promising for structural health monitoring.
This paper presents machine learning aided stochastic reliability analysis of spatially variable slopes, which significantly reduces the computational efforts and gives a complete statistical ...description of the factor of safety with promising accuracy compared with traditional methods. Within this framework, a small number of traditional random finite-element simulations are conducted. The samples of the random fields and the calculated factor of safety are, respectively, treated as training input and output data, and are fed into machine learning algorithms to find mathematical models to replace finite-element simulations. Two powerful machine learning algorithms used are the neural networks and the support-vector regression with their associated learningstrategies. Several slopes are examined including stratified slopes with 3 or 4 layers described by 4 or 6 random fields. It is found that with 200 to 300 finite-element simulations (finished in about 5 ~ 8 h), the machine-learning generated model can predict the factor of safety accurately, and a stochastic analysis of 105 samples takes several minutes. However, the same traditional analysis would require hundreds of days of computation.
In this paper, an actual landslide process that occurred in Southern China is simulated by a continuum approach, the particle finite element method (PFEM). The PFEM attempts to solve the ...boundary-value problems in the framework of solid mechanics, satisfying the governing equations including momentum conservation, displacement-strain relation, constitutive relation as well as the frictional contact between the sliding mass and the slip surface. To warrant the convergence behaviour of solutions, the problem is formulated as a mathematical programming problem, while the particle finite element procedure is employed to tackle the issues of mesh distortion and free-surface evolution. The whole procedure of the landslide, from initiation, sliding to deposition, is successfully reproduced by the continuum approach. It is shown that the density of the mass has little influence on the sliding process in the current landslide, whereas both the geometry and the roughness of the slip surface play important roles. Comparative studies are also conducted where a satisfactory agreement is obtained.
The role of vapor flow in unsaturated freezing soils is unclear, and usually ignored when analyzing water migration. Indeed, vapor flow is seldom considered in geotechnical design. On the other hand, ...frost heave damages were recently observed in a cold and arid region in China, where the groundwater table is about 20m deep, and cannot be explained using the classical frost heave theory which focuses on the coupled flow of heat and liquid water. This paper presents a new approach for modeling moisture and heat movement in unsaturated freezing soils, in which the phase change of evaporation, condensation of vapor flow are taken into account. The method enables numerically stable solutions with energy and mass conservation. The performance of the proposed model was evaluated by a series of laboratory freezing experiments, which involve a 60cm long soil column subjected to different initial conditions and boundary temperatures. It is shown that the proposed model is capable of effectively simulating the freezing process of an unsaturated soil. The vapor flux is shown to be significant, especially at the end stage where the freezing process is stabilized. Parametric analysis was carried out to clarify the role of vapor in moisture migration. The results show that vapor flow apparently contributes to ice formation and generally accounts for more than 10% of total water flux. The percentage of vapor flux in total flux (vapor flux ratio) positively relates to temperature gradient and freezing depth, but negatively relates to initial water content. Relations between vapor flux ratio and hydraulic parameters are less clear.
•A new approach for modelling moisture and heat movement in unsaturated freezing soils is presented with considering the phase changes of evaporation, condensation and desublimation.•Vapor flow apparently contributes to ice formation and generally accounts for more than 10% of total water flux.•The percentage of vapor flux in total flux positively relates to temperature gradient and freezing depth, but negatively relates to initial water content.
The relationship between the unfrozen water content in frozen soil and negative temperature is defined as the soil freezing characteristic curve (SFCC). The SFCC plays a crucial role in understanding ...the hydrothermal migration, frost heaving and constitutive relations of frozen soil. The mechanism of the SFCC is less understood, especially in the low-temperature range. Considering the adsorption and capillary action between soil particles and the ice interface, a new SFCC model for saturated frozen soil is proposed from the pore scale. The predicted results of the model show that the unfrozen water content gradually decreases with the increasing particle radius at the same temperature. The SFCC of monodisperse silica microspheres and quartz sand samples were tested based on nuclear magnetic resonance technology. The experimental results were compared with the mathematical model, which showed that the new SFCC model is in good agreement with the experimental results. The new model has a clear physical meaning and can provide a theoretical basis for understanding the phase change process in frozen soils, which is of great significance to the application of the proposed model.
•A new SFCC model is proposed that can consider the adsorption and capillary action.•A reasonable phase transition trajectory is described for the soil melting process.•The SFCC of silica microspheres and quartz sand are measured by an NMR device and the test results validate the new model.
This paper presents an alternative method to couple the hydraulic component with the mechanical component in a constitutive model for unsaturated soils. Some pioneering work on hydromechanical ...coupling is reviewed. Generalized constitutive relations on coupled hydromechanical behaviour are introduced. These generalized constitutive relations are then incorporated into existing mechanical and hydraulic models for unsaturated soils. A new coupling mechanism is proposed based on the fact that soil-water characteristic equations are usually obtained for constant stress, not constant volume. The proposed coupling mechanism also satisfies the intrinsic relationship between the degree of saturation and the volumetric strain for undrained compression. Numerical examples are presented to show the performance of the proposed model in predicting soil behaviour along drying and loading paths. Finally, the model is validated against experimental data for different soils.
The significance of rainfall pattern in the assessment of rainfall-induced landslide is widely recognized. However, much work so far is limited to several simplified typical rainfall patterns. In ...this study, the random rainfall pattern (RRP) is introduced and generated using random cascade model based on the rainfall event characterized by average rainfall intensity and duration. The stability of unsaturated slope considering RRPs is studied from three perspectives: deterministic analysis by means of safety factor under different generated RRPs, probabilistic analysis through conditional failure probability considering the diversity of generated RRPs based on Monte Carlo method and risk assessment analysis by introducing annual failure probability (AFP) considering also the occurrence frequencies of rainfall events. Three typical rainfall patterns are introduced for comparison analysis. The results show that slope stability is sensitive to the RRP and is strongly depend on the temporal distribution of rainfall intensity in RRP. High likelihood of slope failure may occur considering the variety of RRPs even though the slope is in a stable state in terms of deterministic analysis. The AFP considering RRPs increases rapidly with increasing rainfall duration and is significantly different from those under typical rainfall patterns. The findings lead to the conclusion that RRPs should be considered in the estimation of unsaturated slope stability.
•The stability of unsaturated soil slopes is studied considering the random rainfall patterns (RRPs).•The assessment of slope stability could be significantly different under different RRP.•High slope failure probability may occur considering the variety (or uncertainty) of RRPs.•The annual failure probability of slope under RRP is significantly different from those under typical rainfall patterns.
The Harbin–Dalian high-speed railway in northeastern China has a significant portion of track foundation built on seasonally frozen ground. Wide-spread frost heave was observed during the first ...winter of its operation and the heave occurred mainly in coarse fills that were considered not susceptible to frost heave. This paper first presents the field data of frost heave and frost depth observed along the railway. It then analyses alternative mechanisms that have been considered to have caused the observed frost heave. The three most likely mechanisms are poor quality control of fine content in the coarse fill, the top-down water supply mechanism, and the bottom-up water supply mechanism. The likelihoods of these mechanisms are analysed against observed field data, using a one-dimensional frost heave model. The results indicate that the most likely explanation for the unexpected frost heave is a combined action of different mechanisms.
Coarse-grained soils were considered not susceptible to frost heave. However, substantial frost heave has been observed in unsaturated coarse fills in high-speed railway embankments. Recent ...experimental results in the literature show that vapour transfer has a considerable influence on the frost heaving of coarse-grained soil. However, vapour transfer has rarely been considered in modelling frost heave. This study presents a new frost heave model that considers vapour transfer and its contribution to ice formation. An updated computer program (PCHeave) is developed to account for the vapour transfer in unsaturated coarse-grained soils, where the rigid ice theory is applied to initiate ice lens formation in the frozen fringe. The results of the proposed model are compared with laboratory test results, which show reasonable agreement. The frost heave data monitored in 2013–2014 along the embankment of the Harbin–Dalian Passenger Dedicated Railway are also used to validate the proposed model. The prediction of the model agrees well with the measured results of frost heave and frost depth. This indicates that the proposed model can reasonably reflect the process of frost heave caused by vapour transfer in unsaturated coarse-grained soils.
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