Unsaturated Samples Research Articles

Field and laboratory investigation of rainfall-triggered slope failure in unsaturated loess soils, New Zealand

Shallow rainfall triggered slope failures occur frequently in loess and loess-derived deposits across the South Island, New Zealand. These failures, which occur in both natural slopes and engineered cuttings, impact road infrastructure, residential housing and rural land use. When dry, the loess can form near vertical cuttings. However, with increase in moisture content loess slopes become susceptible to shallow slope failures. To date, the influence of negative pore-water pressure (suction) on the stability of loess slopes in New Zealand has not been well understood. In this paper, data from long term in situ field monitoring of rainfall, suction and volumetric water content from a loess slope in Banks Peninsula, Canterbury are presented with laboratory triaxial test results undertaken on undisturbed unsaturated loess samples. Field and laboratory soil responses to wetting and drying are compared, and the characteristics of rainfall events which reduce suction in situ and therefore slope stability are discussed.

The effect of microbial calcite precipitation on the retention properties of unsaturated fine-grained soils: discussion of the governing factors

In recent years, biogeotechnology has been introduced as a novel and environmentally friendly technique for soil improvement. The need to address global warming and the adverse environmental effects of the chemical additives have led to the emergence and development of the techniques which use calcite producing microorganisms in order to improve soil mechanical properties. While the effects of microbial induced calcite precipitation (MICP) on the hydraulics and mechanics of saturated coarse-grained soils have been well examined and studied, there is not yet much information on the effects these microorganisms would have on the unsaturated soil mechanical behaviour. The first step, in this regard, is to understand the effect of the processes involved in the MICP on the soil retention properties. Soil water suction is a key factor controlling soil hydraulic and mechanical behaviour. In this study, the influence of MICP on the soil total suction in an unsaturated fine-grained soil sample has been explored using filter paper experiment. The results of this study revealed that by increasing the amount of bacterial solution, the soil saturation-total suction curves are significantly affected. The soil water retention changes are attributed to the change in double layer thickness as well as the precipitation of calcite crystals.

Effects of calcium carbonate on hydro-mechanical properties of bentonite

To simulate characteristic changes of the bentonite mixing with CaCO3 crystals that precipitate from groundwater around the high-level radioactive waste repositories, different amounts of CaCO3 powders were mixed in the bentonite. In this study, four series of bentonite/CaCO3 mixtures were prepared with different contents of CaCO3 powders. A series of swelling, compression and direct shear tests were conducted on the bentonite/CaCO3 samples using deionized water. Terzaghi consolidation theory was adopted to compute the hydraulic conductivity of bentonite/CaCO3 mixtures. The results demonstrated that the final swelling strain decreased with increasing vertical stress and the content of CaCO3 powders. The compression index decreased slightly with increasing CaCO3 content. It was found that as the CaCO3 content increased, the hydraulic conductivity of bentonite/CaCO3 mixtures decreased. There was a peak point in the shear strength of unsaturated samples with the differences of the CaCO3 content. The cohesion of unsaturated samples increased first and then decreased, but saturated samples presented the opposite character. It could be seen from the scanning electron micrograph that the existence of CaCO3 powders filled pores of bentonite/CaCO3 mixtures and made the structure denser.

Hydromechanical behaviour of overconsolidated unsaturated soil in undrained conditions

Hydromechanical behaviour of an unsaturated silt with various suctions and different overconsolidated ratios (OCRs) was investigated through a series of undrained triaxial tests (constant water contents, CW). All the samples were prepared from the slurry state. Different OCRs (= 1, 2, 4, and 8 in net stress) were achieved by unloading the samples to 400, 200, 100, and 50 kPa from an initial confining net pressure of 400 kPa. Then the samples were dried to various suctions (0, 100, 200, 300, and 400 kPa). Unsaturated samples with different OCRs were then sheared at CW conditions following the conventional triaxial compression (CTC) paths. Full hydromechanical responses including the changes in deviator stress, stress ratio, volumetric strain, suction, and degree of saturation with axial strain were monitored and are presented in this paper. Some key findings include (i) the critical state for unsaturated soils with different OCRs can be well defined by Bishop’s effective stress; (ii) the peak strength in Bishop’s effective stress increases with increase of OCR, but decreases with increase of suction in the undrained condition; and (iii) the volume change of unsaturated soils in undrained conditions is related to OCRs and the volume of pore air.

Correlation between Electrical Resistivity and Suction for Unsaturated Coastal Saline Soil

An experimental research on joint application of engineering-geological and geophysical measurements is presented in this paper. Experimental results obtained from the filter paper method and four electrode resistivity measurements were presented to characterize unsaturated saline soil samples collected in a test area in Lian Yungang City (in Jiangsu Province), which aimed at defining both soil suction and electrical resistivity curves versus water content at different salt contents. From the matching of the experimental data, the relationships between electrical resistivity and suctions were retrieved for the investigated soil. It was concluded that suctions increase with the increase of electrical resistivity and the increase of soil salt content. The outcomes of the study would provide proper understanding about the influence of physical characteristics of unsaturated soils on their electrical responses.

Determining Osmotic Suction Using a Chilled Mirror Device

Abstract To properly define stress states controlling the mechanical and hydraulic behavior of unsaturated soil, osmotic and matric suction should be determined, since they affect soil behavior differently. A novel and simple method is presented for determining osmotic suction in soil using a device that measures pore air relative humidity. The method is developed using a chilled mirror device (CMD) and validated by conducting a series of experiments on three different soils mixed with aqueous solutions with various concentrations of different salts. The proposed method involves determining the water content of an unsaturated sample, mixing it with pure water to achieve saturation, and then determining the total suction and water content at the saturated state. At saturation, the measured total suction is equal to the osmotic suction in the sample; thus, by using the water content and osmotic suction for the saturated sample, the osmotic suction corresponding to the water content in the unsaturated condition can be determined. Also, by determining the total suction of the unsaturated sample before mixing with water, matric suction can be computed. While this method extends the usefulness of the CMD, there are limitations associated with the measurement accuracy of the device and solubility limit of salt in the pore water that are discussed in this article.

Experimental study of shear wave velocity in unsaturated tailings soil with variant grain size distribution

This study presents experimental tests on the influence of grain size distribution on the shear wave velocity (Vs) of unsaturated tailings. Combined with other scholars' experiments and research methods, more than 40 unsaturated samples from the Xiaodae copper tailing dam with four different grain size distributions were tested on a tri-axial testing instrument with bending elements at different effective stresses and void ratios. The test results showed that the soil water characteristic curve of the unsaturated tailing soils is similar to that of other sandy soils, and Vs is strongly related to effective stress, grain size, and void ratio. The Vs empirical formula is obtained with the characteristic parameters of the grain size distribution by fitting the experimental data. The fitting results showed that parameter Cu has a better correlation than D50 and Cc under the samples. The predicted Vs empirical formula can also be applied to the Xiaodae field test (drilling Vs test). Finally, the similarities and differences of the Vs (Gmax) fitting formulas with those from other scholars are compared in terms of the choice of the parameters of the grain size characteristics. The coefficients of variation of D50 and Cu can be used as the basis for selecting two parameters to characterize the particle size in fitting Vs (Gmax) to obtain a better predicted effect and application efficiency.

Unsaturated behavior of rammed earth: Experimentation towards numerical modelling

This study concerns the coupled hydro-mechanical behavior of rammed earth, which is a real difficulty for the development of this construction technique. Unconfined compressive strength tests on samples conditioned at different relative humidities were performed to determine the variation of mechanical capacity with suction. Both compressive strength and Young’s modulus increase with suction. Further, the effect of shear characteristics with the hydric conditions was studied by direct shear test. As suction induced cohesion contributes a significant part of strength, the apparent cohesion reduced with the reduction of suction. In addition, a considerable variation was observed in the friction angle. Unconsolidated undrained triaxial tests on unsaturated samples were performed to plot the failure envelope for a greater value of normal stress and to complete the failure envelope. These three test makes it possible to put in evidence a non-linearity in the failure envelope over all the suction range studied. Consolidated undrained triaxial tests on saturated samples were performed to determine the intrinsic cohesion and intrinsic friction angle. It was observed that Mohr-Coulomb criterion is not realistic for rammed earth and the failure envelope is non-linear for unsaturated conditions. Thus, a failure criterion was proposed in which both cohesion and friction angle are dependent on normal stress applied. The expression for Bishop’s effective stress for unsaturated soil was evaluated from the results of triaxial and unconfined compressive strength test together. The non-linear failure criterion obtained for a particular suction can be extended to other suction states through Bishop’s effective stress formulation. It can further be used for coupled hydro-mechanical modeling of rammed earth structures.

Characteristic Research on Dynamic Strength of Remolded Unsaturated Loess Under Different Moisture Content

Luoyang remolded unsaturated loess samples under different natural moisture content were prepared, and then their dynamic strength was tested. The dynamic strength parameters of original loess were obtained when corresponding failure vibration frequency Nf was 10 and 20 times. According to dynamic strength parameters, the present research analyzed changing rules of dynamic strength parameters of remolded unsaturated loess with moisture content and failure vibration frequency. The paper further explores impact of natural moisture content and failure vibration frequency on unsaturated loess dynamic strength index in the region, and fits relationship curve between dynamic strength parameter of unsaturated remodeled loess and natural moisture content, thus propose mathematical expressions. It is shown from the results that there is an obvious linear relationship between dynamic internal friction angle and moisture content. When both dynamic cohesion and moisture content are logarithmized, they will present a good symbol linear relationship.

Mechanisms of failure in saturated and unsaturated clayey geomaterials subjected to (total) tensile stress

The paper presents an experimental investigation into the mechanisms of tensile failure in clayey geomaterials under saturated and unsaturated conditions. An experimental apparatus was developed to test specimens in uniaxial tension with the facility to monitor suction (pore-water tension) using high-capacity tensiometers. This allowed interpretation of failure data in terms of effective stress and average skeleton stress for saturated and unsaturated specimens, respectively. Experimental data from normally consolidated samples showed that failure under uniaxial tension occurs in shear and tensile cracks form as a combination of mode I and mode II fracture. In the saturated range, when samples were prepared with de-aired water, tensile failure occurred at deviatoric stresses corresponding to the critical state line derived from triaxial and uniaxial compression tests. When using non-de-aired water and at suction levels approaching the air-entry value, failure occurred at deviatoric stresses lower than the ones corresponding to the critical state line derived from compression tests. It has been suggested that water cavitation may be one of the mechanisms that control premature rupture of saturated clay when subjected to a (total) tensile stress state. Finally, tensile failure data from unsaturated samples showed that there is continuity between saturated and unsaturated states.

Effects of water content on the shear behavior and critical state of glacial till in Tianmo Gully of Tibet, China

Glacial tills are widely distributed in Tibet, China, and are highly susceptible to landslides under intense rainfalls. Failures of the slope during rainfall are closely related to the shear behavior of glacial tills at different moisture conditions. This study investigates the shear behavior and critical state of saturated and unsaturated glacial tills through a series of drained direct shear tests. The tests were conducted on two types of compacted glacial tills with different water contents and total normal stresses. A strain softening mode of failure is observed for all water content conditions accompanied by noticeable dilation. Dilatancy is found to decrease with increasing water content. Unsaturated samples showed increased rates of dilation as water content is decreased for all applied normal stresses a behavior which cannot be predicted well by classical stress-dilatancy models. Furthermore, it was found that the Critical State Line (CSL), plotted on the (e-lnσn) plane, can be used to define the shear behavior of unsaturated glacial tills at different water contents. The CSL of saturated glacial tills run parallel to this line. The experimental results in this study are aimed to provide a basic understanding to the underlying failure mechanisms of glacial tills.

Calibration of DEM material parameters to simulate stress-strain behaviour of unsaturated soils during uniaxial compression

Discrete element method (DEM) is an appealing technique to simulate soil deformation. However, the calibration of the material parameters in the model remains challenging due to the high computational cost associated with it. In this study, the stress-strain relation of 125 unsaturated soil samples was simulated using DEM and material parameters were calibrated with Kriging. The Young's modulus and friction angle were found to be the most sensitive DEM material parameters, and they had a significant correlation with the bulk density, clay content and water content of the undisturbed soil samples. The DEM simulation showed that the deformation process was a combination of elastic and plastic processes, also at low stresses. Further improvement such as the coupling with (and calibration of) a fluid dynamics model might allow to more accurately simulate the dynamic behaviour of unsaturated soil compression.

Water retention and diffusion in unsaturated clays: Connecting atomistic and pore scale simulations

Molecular diffusion is the dominant solute transport process in clays and claystones that are considered as sealing materials in the deep geological disposal of radioactive waste. These materials are typically water saturated, but during construction and later, at elevated temperatures or when gas may be produced, unsaturated conditions prevail. Investigating the clay's water retention properties as well as solute transport under unsaturated conditions is therefore mandatory. Here, functional dependencies of these properties were derived from atomistic and pore-scale simulations. In the absence of tomographic maps that resolve all pores in clays, model clay structure maps with different pore size distributions were generated using a previously developed algorithm. Upscaled water retention functions and upscaled diffusion coefficients of unsaturated samples were derived from these maps based on the shifted Young-Laplace equation that considers film adsorption and capillary condensation. Pore-scale parameters (water film thickness, diffusion coefficients) used for the upscaling were taken from Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, thus connecting molecular and pore-scale simulations. We focused on effects of the pore size distribution and of the adsorbed water film on upscaled parameters. Sample-scale diffusion coefficients were clearly reduced in unsaturated samples compared to the saturated state, with less reduction when including adsorbed water films. The reduction was stronger in samples with a narrow size distribution of the interparticle pores as compared to those with a wide distribution (but equal mean size). The results follow the trends of the experimental data, even though the scale of the simulations is still clearly smaller than that of typical experiments.

Influences of degree of saturation and stress cycle on gas permeability of unsaturated compacted Gaomiaozi bentonite

Compacted bentonite is usually used as the backfill material in underground storage of radioactive waste due to its swelling properties and low permeability. To understand the effects of the degree of saturation and stress cycle on the barrier properties of compacted bentonite, gas permeability tests were performed on highly compacted bentonite with different dry densities, water contents and stress cycles. The testing results show that the degree of saturation and stress cycle both have significant influences on the gas permeability of unsaturated compacted bentonite samples. As the saturation or confining stress increases, gas permeability of the samples is decreased by no more than 3 orders of magnitude. Moreover, under the condition of the same dry density, the greater the degree of saturation, the faster the decline of gas permeability of samples during loading; under the condition of the same degree of saturation, it shows that larger initial dry density can cause gentler decrease of the gas permeability of samples. A slight recovery in gas permeability was observed during unloading process of confining stress, and the decline of gas permeability caused by increasing confinements is mainly irreversible. Different from saturated/nearly-saturated samples, gas migration in the low/medially saturated compacted bentonite sample is considered to be dominated by single-phase advection in the connected pores of the unsaturated zone. The present work implicates that the effect of the degree of saturation and the stress path on the gas permeability of the backfill material should be taken into account during the construction period and early stage of operation of the radioactive waste.

The effective thermal conductivity of methane hydrate-bearing seasand

The thermal properties of hydrate-bearing sediments can help to evaluate the stability of hydrate reservoir and improve the safety of natural gas hydrate exploitation. In this work, the thermal conductivities of methane hydrate and methane hydrate-bearing seasand were investigated by the transient plane source (TPS) technique. The high-quality samples of hydrate were prepared by synergistic uses of a variety of methods, i.e., the “seeding” ice, simultaneous slowly heating and slowly supplying gas, temperature oscillation and aging method. The measured thermal conductivity of methane hydrate is 0.4877–0.5467 W·m−1·K−1 with the temperature 263.2–283.1 K and the pressure 4.0–9.5 MPa. The effective thermal conductivities of the saturated seasand sample, supersaturated seasand sample and unsaturated seasand sample (Here, the “saturated”, “unsaturated” and “supersaturated” refers to the three relationships between the pore volume of seasand and the volume of ice powder, respectively) are 1.1310–1.2703 W·m−1·K−1, 1.0070–1.1490 W·m−1·K−1, 0.9221–1.1980 W·m−1·K−1 with the pressure 8.0 MPa and the temperature 253.8–281.2 K, respectively. The effective thermal conductivities of the saturated sample, supersaturated sample and unsaturated sample are 1.1280–1.1402 W·m−1·K−1 (1.1331–1.5974 W·m−1·K−1), 1.0172–1.0252 W·m−1·K−1 (1.0181–1.2032 W·m−1·K−1), 0.9154–0.9388 W·m−1·K−1 (0.9401–1.1543 W·m−1·K−1) with the pressure 4.0–10.0 MPa and the temperature 263.2 K (275.2 K), respectively. The relationship between the effective thermal conductivity and methane hydrate saturation in the sample are λe,saturated > λe,supersaturated > λe,unsaturated. The results suggest that the thermal conductivities of methane hydrate and methane hydrate-bearing seasand are positively correlated with the temperature and the influence of the gas phase pressure is negligible. Besides, the effective thermal conductivity shows a sudden increase near to the phase equilibrium temperature. Due to the complex of actual samples, the same classic theory model cannot accurately predict three samples simultaneously.

Investigating the use of Boyle's Law to relate pore air pressures and volume changes in unsaturated triaxial samples

Measuring the volume changes of unsaturated triaxial soil samples is one of the main challenges when performing unsaturated triaxial tests. Under fully undrained conditions, Boyle's Law can be used to calculate the sample's volume changes caused by the compression of air, as Boyle's Law relates changes in volume to changes in pore air pressures within the sample. This method has been used to calculate the volumetric strains and pore air pressures in unsaturated samples tested under cyclic loading conditions. However, the volume changes calculated from using Boyle's Law and the increases in pore air pressures, have not been compared with the actual measured volume changes of unsaturated samples. This study presents the pore pressures and compares the measured and predicted volumetric strains calculated from using Boyle's Law, in unsaturated triaxial samples that were tested cyclically. In some cases, using Boyle's Law was found to be similar to the volume changes estimated from direct measurement, but sometimes the method did not appear to work. Reasons for the discrepancies will also be discussed.

Mechanical behaviour of unsaturated undisturbed black volcanic ash soil under static and cyclic loading

The mechanical behaviour of unsaturated undisturbed black volcanic ash soils was investigated using a constant volume direct shear apparatus. A series of static and cyclic tests were conducted under unsaturated and saturated samples. The cyclic test under two patterns were adopted. First type of pattern, cyclic one-sided shearing was applied. For the second type, two-sided shearing was conducted. For further understanding of the chemical content and microstructure, X-Ray Fluorescence analysis (XRF) and Scanning Electron Microscope (SEM) were evaluated. It was found that, under static shearing, the unsaturated sample exhibits a higher apparent cohesion and friction angle in comparison to the saturated sample. The normalized vertical stress under one-sided cyclic shearing in both overconsolidated and normally consolidated samples rapidly reduced at the beginning of shearing. This might be attributed to increase of the pore water pressure during shearing. Furthermore, there is a significant difference between the normalized shear stress under cyclic one-sided and two-sided shearing. For the two-sided cyclic shearing test, the normalized shear stress value is higher than the one-sided cyclic shearing test. In addition, the normalized shear stress under unsaturated condition is significantly larger. This can be related to the suction forces to the total strength of soils. The main chemical content of the black volcanic ash soil is allophane. Further, the microstructure of the black volcanic ash changes due to shearing. For the overconsolidated sample, particles of soil more severe damage than normally consolidated sample.

The effect of suction on ground surface settlement for a tunnel

This paper addresses the physical modelling of a tunnel for saturated and unsaturated sand as well as tests in dry condition. A series of tests were conducted for a 50 mm diameter tunnel buried 25 mm into a poorly graded fine sand under different normal stresses: 0.78, 1.56, 2.34 and 3.31 kPa. Suction was controlled by means of the hanging column technique (HCT). Particle image velocimetry (PIV) was also utilised to obtain the vector displacement and compared with the results of the physical modelling. The results of the fully saturated sample showed that the sample experienced a sudden spatial collapse above and around the tunnel's crown. In addition, large displacement on top of the tunnel's crown was also observed for the fully dry case. However, the unsaturated sample did not experience any sudden collapse. This was attributed to the effect of the suction contribution on strength.

Experimental study on the volume and strength change of an unsaturated silty clay upon freezing

In order to investigate the frost deformation and shear strength increment characteristics of unsaturated soil, a comprehensive experimental study was conducted to unsaturated slity clay samples with considering the influence of soil initial dry density and water content. Micro-structure observation tests, three-dimensional frost deformation tests, direct shear tests and soil compression tests are carried out. The frost deformation test results demonstrate that frost shrinkage happens when soil initial degree of saturation (Sr) is low, whereas frost heave and frost cracking happen when soil initial Sr is high. There is a critical degree of saturation at which frost shrinkage will turn into frost expansion. For the silty clay tested, this critical value is about 0.75. The frost shrinkage of soil is considered to be owing to the increase of cohesion upon freezing. The frost expansion of soil is induced by the fabric rearrangement which happens when pore-water changes into ice upon freezing. The shear strength test results demonstrate that soil friction angle does not vary much upon freezing, whereas soil cohesion increases a lot because of the generation of ice cementation and the increase of capillary cohesion. With assuming the increment of capillary cohesion upon freezing is equal to that upon drying for soil samples with same reduction of pore-water content, the ice cementation of frozen soil is found to be linear to its volumetric ice content. If the volume shrinkage induced by the increase of cohesion is assumed to be equal to that induced by the increase of mean principle stress with same values, the frost expansion induced by the water-ice phase transition is also linear to the volumetric ice content of unsaturated soil. Such linear relations seem to be unique and are applicable to unsaturated soil samples with any initial void ratio and water content.

MECHANICAL MODEL OF UNSATURATED LOESS-CONCRETE INTERFACE DUE TO FREEZE-THAW ACTION

In the seasonal frozen soil area, periodic freeze-thaw action has continually changed micro-structure and physical-mechanical properties of shallow unsaturated soil, which has directly affected the interaction between soil and structure. The stress-strain relation and strength characteristics of the interface between soil and structure are the bases and keys to determine the bearing capacity and security of foundation engineering, and to analyze the interaction between structure and soil in seasonal frozen soil regions. In order to provide the practical engineering with better services, the stress-strain curve and the interface strength parameter changing with the number of freeze-thaw cycles under different experimental conditions are obtained from the direct shear test of unsaturated loess-concrete samples with different water contents and different freeze-thaw cycles. Correspondingly, the matrix suction of the loess under the same experimental conditions is tested using filter paper method. Moreover, the constitutive model of shear stress and displacement as well as freeze-thaw cycle in the loess-concrete interface is established with experimental data, which has a good description of the relationship between shear stress and displacement of the loess-concrete interface under different pressure after several freeze-thaw cycles. Also, the research results will provide reference for the numerical calculation of the foundation bearing capacity in the seasonal frozen soil area.