Unsaturated Coarse-grained Soils Research Articles

Effect evaluation of grass roots on mechanical properties of unsaturated coarse-grained soil

The utilization of vegetation to prevent shallow slope failure and erosion has received significant attention in recent years, since it is recognized that vegetation has the mechanical and hydrological strengthening effect on soils. To accurately evaluate the mechanical properties of grassed soil and effect of grass age, grassed soils with grass ages of 2, 4, 6 and 8 months as well as bare soil are cultivated and prepared. Then, considering that grass layer in nature is usually situated above ground water table in the non-saturation state, a sequence of unsaturated triaxial tests are performed for soil specimens with different grass ages and matric suctions. Experimental results reveal that both grass root and matric suction enhance soil shear strength. The peak shear strength and cohesion of grassed soils tend to increase with increasing grass ages or matric suctions. Moreover, it is recognized that compared with grass root, the matric suction causes a more significant reinforcement effect for increasing total cohesion of soil sample. On the other hand, grassed soil has a higher water retention capacity, and under the same matric suction conditions, the soil specimen with older grass age have higher volumetric water content. Furthermore, the measured total cohesion for unsaturated soil is validated by the proposed calculation equation based on Vanapalli model, and the calculated results fit the measured results well. Calculated and experimental results have good agreement showing the proposed equation can properly reflect the increase in the unsaturated mechanical properties of grassed soil.

Effect of Water Content on Apparent Cohesion of Soils from Landslide Sites

There are many empirical equations published for unsaturated fine-grained soils. However, there is only one empirical equation established for silty sand using the shear-box test and filter-paper-based suction test, but with the suction range of 0 to 200 kPa. It is reported that there is a significant discrepancy between the predicted values and test results of apparent cohesion within the range of 0 to 100 kPa for unsaturated coarse-grained soils. The purpose of this research is to study the effect of water content on apparent cohesion and predict apparent cohesion for coarse-grained soils within the range of 0 to 100 kPa using shear-box test and suction test results without much inconsistency. In this research, soil samples from the rainfall-induced landslide sites were obtained; laboratory tests such as soil-classification tests, shear-box tests and consolidated undrained triaxial tests were carried out. Test results were analyzed, and the findings are presented. When the water content is increased from 0% to 30%, there is a reduction of 89% in apparent cohesion on average. A newly developed prediction model for apparent cohesion based on the low range of matric suction from 0 to 100 kPa for unsaturated coarse-grained soils is introduced in this paper and compared with published models.

Overview on failures of urban underground infrastructures in complex geological conditions due to heavy rainfall in China during 1994–2018

• A database consisting of 378 failure cases was established for analyses. • Trends and distributions of heavy rainfalls in China were characterized. • Spatio-temporal distribution patterns of the failures were identified. • Urbanization, climate and geological conditions were key factors. Currently, global warming is increasing rainfall intensities worldwide, particularly in China. This climate change has caused numerous underground infrastructure failures, which threatens the sustainable development of urban environments. Most failures brought about secondary hazards in densely populated areas, which often caused massive damages and casualties and/or injuries. Thus, a summary of such failures and their contributing factors on a national scale is essential for understanding the adverse influences of climate change on infrastructures, which can help urban planners, designers and engineers to improve city resilience. In this paper, an overview of 378 underground infrastructure failures in China associated with heavy rainfall was conducted. First, the latest rainfall characteristics in China were analyzed. Then, a review was conducted in four aspects, i.e., infrastructure types, involved strata, and temporal and geographic distributions. Finally, contributing factors of the failures were discussed. Several major findings were obtained: (1) the failure numbers had increased significantly since 2010, mainly attributed to the intensification of rainfall in rapidly urbanizing regions; (2) the resilience of underground infrastructures against heavy rainfall is governed by the performance of existing drainage facilities nearby; and (3) more failures clustered in the urban environments featuring highly concentrated rainfalls and thick-layered unsaturated coarse-grained soils.

Liquid water–vapour migration tracing and characteristics of unsaturated coarse-grained soil in high-speed railway subjected to freezing and different load types

Unsaturated coarse-grained soil (CGS) is typically used as fillings in high-speed railway subgrade. Herein, the frozen depth, external water intake, water content distribution, liquid water migration height, and frost-heaving process were investigated in vapour migration and liquid water–vapour migration modes. Liquid water–vapour migration characteristics under gravity, static load, and dynamic load were compared. Vapour migration is dominant in the CGS external water intake; vapour affects the frozen zone of CGS specimens. The frost-heaving duration under dynamic load is longer than under gravity and static load. More attention should be paid to the frost heaving induced by vapour migration.

Shear Properties and Mechanism of Freeze-Thaw Interface in Unsaturated Coarse-Grained Soil from Qinghai-Tibet Plateau

Freeze-thaw interface in unsaturated coarse-grained soil (CGS) is a weak plane which can cause slope failures in cold regions. This study presents a series of large-scale direct shear tests on freeze-thaw interface in CGS through a temperature control system. The tested soil was taken from a high slope in the Qinghai-Tibet Plateau. It was remolded with three dry densities (1.9, 2.0, and 2.15 g/cm3) and three moisture contents (9.0%, 11.5%, and 14.0%). With testing results, direct shear curves mainly performed as hardening deformation, and they were affected considerably by specimen conditions. The shear strength increased with both the increasing dry density and normal stress, but it was opposite with moisture content changed. The cohesion and internal friction angle increased with the increase in dry density but decreased with the moisture content. The particle movement and water migration of freeze-thaw interface in CGS during the test were significant, and they had close relations with the shear properties of specimens. And, an empirical model was produced to express the effect of pore ice on the shear strength of interface during the shear test. The tests and analysis in this study may provide useful references for CGS slope stability analysis in high cold regions.

A Numerical Model of Vapour Transfer and Phase Change in Unsaturated Freezing Soils

In recent studies, vapour transfer is reported to lead to remarkable frost heave in unsaturated soils, but how to better model this process has not been answered. In order to avoid the great uncertainty caused by the phase change term of vapour-water-ice in the numerical iteration process, a new numerical model is developed based on the coupled thermal and hydrological processes. The new model avoids using the local equilibrium assumption and the hydraulic relations that accounts for liquid water flow, which provides a new way for the water-heat coupling movement problem. The model is established by using COMSOL Multiphysics, which is a multiphysics simulation software through finite element analysis. The model is evaluated by comparing simulated results with data from column freezing experiments for unsaturated coarse-grained soils. Simulated values of the total water content compare well with experimental values. The model is proved to be applicable and numerically stable for a high-speed railway subgrade involving simultaneous heat and moisture transport. An agreement can be found between the predicted and measured frost/thawed depth and soil moisture profiles, demonstrating that the model is able to simulate rapidly changing boundary conditions and nonlinear water content profiles in the soil.

Modelling frost heave in unsaturated coarse-grained soils

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.

A frost heave model of unsaturated coarse-grained soil considering vapour transfer

Substantial frost heave has been observed in coarse fills in high-speed railway embankments. These coarse fills have low fine contents and very low water content. The groundwater table is located below the coarse fills. The coarse fills were considered not susceptible to frost heave. Recent experimental results in the literature showed that vapour transfer has a considerable influence on the frost heaving of unsaturated coarse-grained soil. But vapour transfer has been rarely considered in the modelling of frost heave. This study presents a new frost heave model with considering vapour transfer and its contribution to ice formation. The rigid ice theory is applied to initiate an ice lens formation in the frozen fringe. An updated computer programme PCHeave is developed by considering the vapour transfer. The results of the proposed model are compared with laboratory test results, which show reasonable agreement. The prediction of the model agrees well with the measured frost heave and frost depth, which indicates that the proposed model can reasonably reflects the process of frost heave in unsaturated coarse soil.

Experimental study of ice accumulation in unsaturated clean sand

A series of laboratory experiments is carried out to replicate moisture accumulation in an unsaturated coarse-grained soil underneath an impervious cover. The results show that significant moisture accumulation occurs in relatively dry specimens when the temperature at the cover drops below the freezing point. The tested soil is a coarse-grained sand and is not expected to generate much moisture accumulation according to existing frost susceptibility criteria in the literature. The primary mechanism of moisture migration in the soil is observed to be vapour diffusion, and the primary mechanism of moisture accumulation is ice formation by way of vapour–ice desublimation. It is also observed that two peak values exist along the total water content profile of the 13·5 cm long specimen. For a non-freezing condition, water content gradually decreases from the warmer end to the colder end without any peak value, and the amount of moisture accumulation is less than occurs under freezing conditions. The test results are considered to be useful for understanding vapour diffusion in unsaturated freezing soils, and for validating theoretical and numerical models.

Water-vapor-heat behavior in a freezing unsaturated coarse-grained soil with a closed top

A series of one-side freezing experiments with closed tops were performed to illustrate moisture migration and frost heave mechanisms of the coarse-grained soil for high speed railway subgrades in cold regions, as well as silty clay and standard sand as controls. The experimental results show that the pot effect can be formed in the soils with different unsaturated porosities when their tops are closed. The vapor migration and frost heave of unsaturated soils become more significant under larger temperature gradient. Water supply can not determine the formation of the pot effect, and only influence its intensity. Vapor migration is prominent when the initial water contents of soils are low (e.g. saturation of 30%), however, the water migration is dominant when the initial moistures of soils are high (e.g. saturation of 70%). It is also found that the initial water contents of the soils only influence the in-situ frost heave amounts; however, the total frost heave amounts are related to the combined actions of soil frost susceptibility and water migration, etc.

Investigating the role of stratigraphy in large-area physically-based analysis of December 1999 Cervinara shallow landslides

The paper describes a large-area analysis of the triggering zones of shallow landslides on a case of unsaturated layered volcanic air-fall (pyroclastic) soil deposits in Cervinara site (18 km2), Southern Italy. The physically-based model TRIGRS (Transient Rainfall Infiltration-Based Grid Regional Slope-Stability) is used, which is used with either saturated or unsaturated conditions and implemented in a GIS platform. In addition to using the TRIGRS model to simulate some recent landslides, a new simplified approach is also tested to take into account the actual layered soil stratigraphy. The consistency check of the model and of the input data is performed with reference to slope stable conditions before rainfall. The performances of the models are evaluated through the ROC curves and two other quantitative indexes taken from the literature referring to the slope failures caused by December 1999 rainstorm. Notwithstanding the simplifications and limitations of the present work, both unsaturated conditions and layered stratigraphy are outlined as key factors for the slope stability of shallow deposits of unsaturated coarse-grained soils subjected to short heavy rainfall.

A New Simple Shear Apparatus and Testing Method for Unsaturated Sands

This paper deals with a new simple shear apparatus for the investigation of mechanical soil behavior of unsaturated coarse-grained soils under monotonous and cyclic loading conditions. Non-cohesive soils, such as sands, typically encounter low capillary effects, and little research has been dedicated to them so far. This holds true especially for effects of repeated loading. The presented test setup allows control of small matric suction up to 85 kPa within the soil specimens with the help of a vacuum control method. Although the pore water pressures are negative as occurring in nature, the pore air pressure is kept at an atmospheric level. Depending on the pore water drainage condition of the specimen boundaries, either drained constant suction tests (CS-tests) or undrained constant water content tests (CW-tests) can be performed. The focus of the research is placed on the interaction of loading and volume change with matric suction and degree of saturation, i.e., the hydraulic-mechanical coupling, during monotonous and cyclic shear. Central questions are the impact of matric suction and degree of saturation on the densification behavior as well as, vice versa, the effect of cyclic loading on a change of matric suction and degree of saturation. The new simple shear apparatus and testing method will be explained, their benefits and shortcomings will be discussed, and selected results of a series of tests will be presented.

Bimodal Shear-Strength Behavior of Unsaturated Coarse-Grained Soils

AbstractThe microstructure, compressibility, and shear strength of an unsaturated widely graded coarse soil were investigated through a series of laboratory tests. The results show that coarse soil has a dual-porosity structure that has both intra- and interaggregate pores. The soil-water characteristic curve (SWCC) shows bimodal features. Accordingly, the shear strength and compressibility over the entire suction range also show two distinct modes. The pore water drains from the interaggregate pores in the first mode while the intraaggregate pores remain saturated. The shear-strength behavior during this drainage period resembles that of uniform coarse sand. The apparent cohesion reaches a peak and then decreases with increasing suction, whereas the compressibility decreases first and then increases. The clay aggregates become unsaturated in the second mode as the pore water drains from the intraaggregate pores. The compressibility decreases and the apparent cohesion increases with suction as in a typica...

Finite Element Simulation of the Bearing Capacity of an Unsaturated Coarse-Grained Soil

The mechanical behaviour of partially saturated soils can be very different from that of fully saturated soils. It has long been established that for such soils, changes in suction do not have the same effect as changes in the applied stresses, and consequently the effective stress principle is not applicable. A procedure was proposed to define the soil water characteristic curve. Then this relation is converted to relation correlating the void ratio and matric suction. The slope of the latter relation can be used to define the H-modulus function. This procedure is utilized in the finite element analysis of a footing on unsaturated coarse grained soil to investigate its bearing capacity. The finite element results demonstrated that there is a significant increase in the bearing capacity of the footing due to the contribution of matric suction in the range 0 to 6 kPa for the tested compacted, coarse-grained soil. The ultimate pressure increases from about 120 kPa when the soil is fully saturated to about 570 kPa when the degree of saturation becomes 90%. This means that an increase in the bearing capacity of about 375% may be obtained when the soil is changed from fully saturated to partially saturated at a degree of saturation of 90%. This development in the bearing capacity may exceed 600% when the degree of saturation decreases to 58%.

A model for predicting the modulus of elasticity of unsaturated soils using the soil-water characteristic curve

A simple model was proposed by Oh et al. (2009) to predict the variation of modulus of elasticity with respect to matric suction for unsaturated coarse-grained soils (i.e., sandy soils with plasticity index, <i>I_p</i> = 0). This model requires the modulus of elasticity under saturated condition (<i>E_sat</i>) and the Soil-Water Characteristic Curve (<i>SWCC</i>) along with two fitting parameters (α and β). In the present study, this model has been extended and a general model has been proposed such that the modulus of elasticity can be estimated for all soils (i.e., both coarse- and fine-grained soils). The proposed model is developed using model footing and in-situ plate load test results of six different unsaturated soils. The study shows that there is a good comparison between the measured and the predicted modulus of elasticity with respect to matric suction for all the six soils whose plasticity index, <i>I_p</i> values are in the range of 0 to 16%.

Constant Flow Method for Concurrently Measuring Soil-Water Characteristic Curve and Hydraulic Conductivity Function

Abstract A constant-flow laboratory testing method (CFM) is presented for concurrently measuring the soil-water characteristic curve (SWCC) and hydraulic conductivity function (HCF) of unsaturated coarse-grained soils. Two computer-automated syringe pumps are employed to control the volumetric water content of a specimen and to periodically impose constant volumetric flow rates through the specimen, respectively. Hydraulic conductivity (k) corresponding to each water content increment is determined from Darcy's law by measuring the steady-state gradient induced by the applied constant flow. Matric suction is maintained by axis translation using elevated pore air pressure and high-air-entry ceramic disks. Diffused air bubbles are removed using unique passive bubble traps. SWCCs and HCFs are obtained for three sandy soil specimens, requiring about 25–35 days to obtain both functions for each soil. The range of the system is demonstrated for k from about 10−4 cm/s to 10−9 cm/s at corresponding matric suction between about 0 kPa (saturated) and 40 kPa. Results are validated by comparison with independent SWCC measurements obtained using Tempe cells and HCFs estimated using a statistical model.