A numerical framework for infinite slope stability analysis under transient unsaturated seepage conditions

The soil-water characteristic curve (SWCC), which describes the relationship between water content in soil and its matric suction, is important for studying the mechanical properties of unsaturated soil. Because measuring SWCC is time-consuming and labor-intensive, many empirical methods have been proposed for assessing SWCC. However, the uncertainty associated with empirical methods is rarely quantified. The purpose of this paper is to propose a probabilistic method for assessing the SWCC of fine-grained soils based on the particle size distribution and index properties. First, the van Genuchten model, which is a widely used SWCC model, is briefly presented. A comprehensive soil database was then compiled and the parameters of the van Genuchten model were calibrated for each soil sample. After that, a regression analysis was performed to estimate the mean value of the parameters of the van Genuchten model, and a correlation analysis was performed to estimate the covariance matrix of the parameters of the van Genuchten model. Finally, a joint probability density function of the parameters of the van Genuchten model is constructed. In this paper, Johnson distributions are used to construct the joint probability distribution of the parameters in the van Genuchten model that do not obey the multivariate normal distribution. The proposed method has been rigorously tested to assess its accuracy and applicability. The method proposed in this paper can provide a practical tool for estimating SWCC with limited data and accounting for the associated uncertainty.

This study aims at developing an efficient and accurate methodology to estimate the resilient modulus of subgrade soils. First, a new resilient modulus model incorporating stress dependence and moisture dependence was proposed. Second, prediction models were developed to conveniently and accurately determine model parameters of SWCC and resilient modulus model. In order to characterise the moisture dependence of subgrade soils, the matric suction was added into the proposed model. The matric suction was measured by the pressure plate test and the soil-water characteristic curve (SWCC) was used to determine the matric suction value at any given moisture contents. In order to develop prediction models for model parameters of SWCC and resilient modulus model, the laboratory experiments and multiple regression analysis were conducted on 22 soil samples. A series of performance-related soil properties were measured and used to develop the coefficients prediction models. The developed coefficients prediction models using the performance-related soil properties have high R-squared values and were validated by comparing the measured and predicted values of resilient modulus. Therefore, when the basic physical properties of soils were obtained, the resilient modulus can be predicted for the subgrade soils at any given matric suctions and stress states.

Determination and Compare Analysis for Soil Water Characteristic Curve Model in Planting Soil in Saline Region of Tianjin

Several researchers have reported that Bishop’s mean effective stress decreases in unsaturated soils under fully undrained cyclic loading conditions, and unsaturated soils are finally liquefied in a similar manner as saturated soils. This paper presents a series of simulations of such fully undrained cyclic loading on unsaturated soils using an elastoplastic model of the unsaturated soil. This model is formulated using the Bishop’s effective stress tensor incorporating the following concepts: the volumetric movement of the state boundary surface containing the normal consolidation line and the critical state line due to the variation in the degree of saturation, a soil water characteristic curve model considering the effect of specific volume and hysteresis, the subloading surface model, and Boyle’s law. Comparisons between the simulation results and the experimental ones show that the model agreed well with the unsaturated soil behavior under cyclic loading. Finally, the typical cyclic behavior of unsaturated soils under fully undrained conditions, such as the mechanism of liquefaction of unsaturated soils, the compression behavior, and an increase in the degree of saturation, are described through the proposed simulation results.

Road structures are more often found in unsaturated conditions. Despite this, at present there is a limited volume of literature on the hydraulic properties of soils used for road construction evaluated under unsaturated conditions. In this context, this paper presents the characterization and measurement of the hydraulic properties of different soils used for road construction in the city of Santander, Northern Spain. The soils were extracted from a road structure. The hydraulic properties of soils have been measured under saturated and unsaturated conditions, in order to determine the saturated hydraulic conductivity (Ks) and the Soil Water Characteristic Curve (SWCC), according to the van Genuchten model of four-parameter. Finally, the procedures followed in this particular study case can give guidelines for the determination of the hydraulic parameters of soils used for road construction, especially when unsaturated conditions needs to be take into account. Besides, the hydraulic parameters obtained with the described methods can be effectively used for modeling the water flow and contaminants through road structures using FEM-based numerical simulation.

Rainfall-induced landslides are a common geological hazard. Analyzing this problem necessitates the use of the unsaturated soil theory, with the soil-water characteristic curve (SWCC) being a crucial component. Most existing unsaturated soil seepage models are established based on the static soil-water characteristic curve model. However, the dynamic capillary pressure has been observed in experiments conducted in previous studies. In this study, experiments were performed to explore the dynamic impact of the SWCC. The influence of different infiltration rates was studied, and a rate-dependent SWCC model under dynamic conditions was established. The model was validated through experiments. The behavior of an unsaturated soil slope under different rainfall intensities was analyzed based on the equivalent model. The equation for the slope safety factor was then derived using the Bishop method. The slope safety factors based on both the rate-independent and rate-dependent SWCC models were compared. The results indicated that the safety factor continuously decreased with increasing rainfall intensity. The dynamic effect reduced the safety factor, making the slope more susceptible to instability.

The experimental research, theory models and numerical simulation methods of unsaturated loess are conducted in this paper. Firstly, a series of indoor unsaturated loess experimental studies including physical properties test and triaxial test were done based on the landslide disaster happened on the Loess Tableland in Xi’an, Western China on September 17, 2011. The soil-water characteristic curves of different unsaturated loess samples were obtained by using Pressure Plate Extractor. Influences of dry density, initial moisture content and sand content on soil-water characteristic curves were analyzed. Secondly, three soil-water characteristic curve models based on experimental research were analyzed comparatively. It was found out that VG Model is more suitable for the loess samples than Fredlund and Xing model or Gardner model and calculation parameters of VG Model were obtained. Finally, based on test results, combined with field loess landslide scale, calculation model of high steep saturated-unsaturated soil slope was established. Finally, Slope stability with the unsaturated seepage flow under different rainfall intensity was analyzed.

Improving understanding of Arctic ecosystem functioning and parameterization of models that simulate freeze-thaw dynamics require advances in quantifying soil and snow properties. Due to the significant spatiotemporal variability of soil properties and the limited information provided by point-scale measurements (e.g., cores), geophysical methods hold potential for improving soil and permafrost characterization. In this study, we evaluate the use of Ground Penetrating Radar (GPR) to estimate thaw layer thickness, snow depth and ice-wedge characteristics in polygonal-shaped tundra in Barrow, AK. To this end, we analyze GPR and point-scale measurements collected along several parallel transects at the end of the growing season and the end of winter. A synthetic study is also performed to understand the GPR signal response to ice-wedge characteristics. Results show that GPR data collected during the growing season provide reliable estimates of thaw depth although strong spatial heterogeneity in soil properties can impair the estimates. While previous studies have documented the value of GPR for providing thaw depth estimates during the growing season, results from this study suggest that GPR methods are also useful for estimating active layer thickness during the frozen season, snow thickness and geometry of permafrost features, such as ice-wedges.

Construction and demolition of buildings together with the reconstruction of roads produce a lot of solid wastes every year. These wastes create problems related to cost and space for disposal, especially for countries with limited land area like Singapore. Therefore, it is important to choose suitable landfill management system (i.e. cover system) for optimization of landfill area in Singapore. Landfill cover offers many geo-environmental benefits, including reducing water infiltration, isolating waste and controlling landfill gases. In Singapore the cover system is located within the unsaturated zone above the groundwater table. Therefore, it is necessary to incorporate unsaturated soil mechanics principles in designing the cover system for the landfill. Soil properties in the unsaturated soil zone affect the rate of wetting front movement from the ground surface. As a result, the rates of changes in pore-water pressures during and after rainfall will vary in accordance with the characteristics of unsaturated soil properties. In this study, the performance of dual capillary barrier (DCB) in minimizing rainwater infiltration into a sanitary landfill in Singapore was investigated. The DCB consists of fine recycled asphalt pavement (RAP) and coarse RAP as the materials for the fine- and coarse-grained layers, respectively. The recycled materials were used in the DCB to reduce the cost associated with the construction of a cover system in the field and to maintain environmental sustainability. Laboratory tests were conducted to characterize the index properties and hydraulic properties (i.e. soil–water characteristic curve (SWCC) and permeability function) of the RAP for the cover system under saturated and unsaturated conditions. The SWCC and permeability function were used in the finite element seepage analyses to study the effect of climate change on the soil cover system. The results from the seepage analyses show that the DCB was effective in minimizing rainwater infiltration into the sanitary landfill.

The soil-water characteristic curve (SWCC) is often used to estimate unsaturated soil properties (e.g. strength, permeability, volume change, solute and thermal diffusivity). The SWCC of soil samples is significantly affected by cyclic wetting-drying. To examine how water content and cyclic wetting-drying affect the SWCC of disintegrated carbonaceous mudstone (DCM), SWCC tests were implemented using a pressure-plate apparatus. In addition, SWCC models for DCM considering the initial gravimetric water content and cyclic wetting-drying were developed. The test results showed that the volumetric water content (θ) of the DCM first decreased rapidly and then became stable as matric suction (s) increased. The initial water content affected the SWCC by altering the pore structure of the DCM. For a given number of wetting-drying cycles, the higher the initial water content, the higher the stabilized θ. At a given s value, θ decreased as the number of wetting-drying cycles increased, which suggests that cyclic wetting-drying reduces the water-holding capacity of DCM. The Gardner model for DCM was constructed considering initial water content and cyclic wetting-drying, and was effective at describing and predicting the SWCC model for DCM.

The engineering behavior of natural untreated and compacted lime-treated expansive soils, which typically exist in a state of unsaturated condition, can be better-explained using concepts from unsaturated soil mechanics. The soil water characteristics curve (SWCC) is the key unsaturated soil property for obtaining unsaturated soil property functions (USPFs). However, there are limited studies on the effect of lime on the SWCC of lime-treated expansive soils. This study investigated the effect of lime on the SWCC of lime-treated expansive soils. The drying portion of SWCCs for untreated natural soil and lime - treated soil samples with three different lime contents (3 %, 6 % and 9 %) with 7 days of curing were studied. The SWCCs were determined by using pressure plate apparatus in the suction range of 0 – 1400 kPa. The shrinkage curve (SC) was also determined to evaluate the change in volume of the different soil samples. The experimental results indicate that SWCC is affected by lime treatment and there is a change in the SWCC parameters and in the shape and position of SWCC as the percentage of lime is changed. The SWCC of the lime-treated soil samples show a higher rate of desaturation as the lime content increases. The Air Entry Value (AEV) and residual water content of lime-treated soil decreases with increase in percentage of lime and the SWCC shifts towards the left side as the AEV decreases. The differences in AEV obtained from gravimetric- water-content-based SWCC (w-SWCC) and degree-of-saturation-based SWCC (S-SWCC) for the lime-treated soil samples were small when compared to the untreated natural soil sample.Keywords: Air-entry value, Expansive soil, Lime treatment, Shrinkage curve, Soil–water characteristic curve, Unsaturated soil.

5th generation district heating and cooling networks operating at near ground temperature offer a low-cost, zero-carbon energy solution. Detailed understanding and accurate estimation of ground behaviour for its heat storage and recharge potential are of paramount importance for the success of such networks. In this paper, an advanced modelling tool, based on a coupled Thermal-Hydraulic (TH) modelling framework, is presented to calculate and predict temperature and soil-moisture behaviour of a shallow ground under complex atmospheric, temperature and hydraulic boundary conditions. Atmospheric data e.g., solar radiation, rainfall, humidity, air temperature, wind velocity is considered together with subsurface soil data to investigate thermal and hydraulic responses of the ground, and its individual soil layers. Furthermore, a transient method for estimating shallow ground source heat (SGSH) resources is proposed based on the simulated temperature and saturation distributions of the ground. The model is applied to predict the long-term ground temperature and saturation level of a test site located in Warwickshire County, UK. The total heat content per unit area and the annual/seasonal/monthly net heat content per unit area of the site are predicted for a five-year period. The total heat content of the sandy clay layer varied between 2.32 and 11.6 MJ/m2, silty clay from 34.0 to 50.5 MJ/m2, and mudstone from 50.7 to 55.0 MJ/m2. A parametric sensitivity study is also conducted to investigate the effects of soil types and hydraulic drainage conditions on the ground heat supply potential, and it revealed that the spatial and temporal distributions of ground heat is significantly affected by the underlying soils. This study highlights the influences of atmospheric conditions and coupled ground processes, and the parameters that should be considered for designing a 5th generation low-temperature heat network.

Predicting coastal hazards for sandy coasts with a Bayesian Network

Spatial and temporal occurrence of rainfall-induced shallow landslides of flow type: A case of Sarno-Quindici, Italy

Carex sempervirens tussocks induce spatial heterogeneity in litter decomposition, but not in soil properties, in a subalpine grassland in the Central Alps