Water Retention Behaviour Of Soils Research Articles

Effects of vegetation growth on soil microstructure and hydro-mechanical behaviour

Literature studies are presenting counterposed effects of vegetation on soil hydraulic behaviour. Besides, root impacts at the micro-scale are rarely correlated to phenomenological observations due to the complexity of adopting up-scaling factors. In this study, the microstructural causes behind observed changes in vegetated compacted clayey sand’s hydraulic and volume change behaviour were explored. Laboratory experiments were carried out on compacted samples seeded with Cynodon dactylon. Significant changes in soil water-saturated permeability, water retention and shrinkage upon drying were observed after root growth. Laboratory measurements were complemented by the quantification of root morphological features and observations at the micro-scale for different soil hydraulic states. X-ray scans and mercury intrusion porosimetry allowed to cover seven orders of magnitude of pore sizes, shedding light on soil fabric changes at the soil–root interface and the clay aggregate scale. The effects of roots on the soil pore size distribution consisted of Multiphysics phenomena: fissure generation, void clogging and soil aggregation due to roots’ chemical interaction. Furthermore, a good correlation was found between the normalised volume of roots and the micro-pores volume. This new expression was included in a framework to predict soil water retention behaviour considering the aggregated structure and soil–root hydro-chemical interactions.

Wide-suction-range hysteretic water retention behaviour of compacted soils amended by decomposing hydrochar

Wide-suction-range hysteretic water retention behaviour of compacted soils amended by decomposing hydrochar

Influences of ink-bottle effect evolution on water retention hysteresis of unsaturated soils: An experimental investigation

The ink-bottle effect, resulting from the non-uniformity of pore structure, plays a significant role in the hydraulic behaviour of soils. In this paper, a series of laboratory tests including standard mercury intrusion porosimetry (MIP), intrusion-extrusion cyclic MIP (IEC-MIP), and scanning electron microscopy (SEM) were conducted on two soils (a Nanyang expansive clay and a Kaolin mixture) to examine the evolution pattern of ink-bottle effect in soils with different initial void ratios. To evaluate the impact of such evolution on the soil-water retention behaviour, the axis translation and vapour equilibrium techniques were utilised to measure the water retention curve of the tested soils (SWRC). A key part of the analysis involved a separation between pore bodies (or ink-bottle pores) and pore throats to facilitate a more thorough examination of the pore structure. The test results indicated that the reduction in soil volume during compression is mainly attributed to the reduction in pore body size, whereas the pore throat size remains relatively constant. Therefore, the ink-bottle effect would weaken with a decreasing initial void ratio, which will reduce the hysteresis potential of soils, manifesting as a contraction of the SWRC hysteresis loop. Additionally, under a fixed void ratio but with decreasing pore size, the ink-bottle effect will also weaken, which will reduce the distance between the drying and wetting curves with increasing matric suction. Finally, the mechanisms regarding how the evolution of ink-bottle effect during soil deformation influences water retention hysteresis were addressed.

Influence of biochar on the soil-water retention behavior of compacted loess in man-made earth structures in loess regions

Influence of biochar on the soil-water retention behavior of compacted loess in man-made earth structures in loess regions

Water-retention behaviour and microscopic analysis of two biopolymer-improved sandy soils

Soil erosion in the Yellow River basin in China has seriously affected the stability of the foundations of civil engineering structures. Traditional treated materials used for preventing the soil erosion are often harmful to the environment. Biopolymers have been widely used for soil improvement because of their low cost, environmental friendliness, and feasible application. However, most studies on the water-retention behaviour of treated soils conducted to date have focused on the low or high suction range, while few have examined the behaviour over a wide suction range. Converting the saturated state of treated soils into unsaturated state until it is completely dry will lead to changes in the water retention behaviour of biopolymer-treated soils. Therefore, this study investigated variations in the water-retention behaviour and micromechanism of biopolymer-treated sandy soils over a wide range of suction. Specifically, the water-retention behaviour of treated soil was tested using a pressure plate apparatus in the low suction range and a WP4C apparatus in the high suction range. Scanning electron microscopy was also used to evaluate water-retention mechanism of soils treated with different ratios of biopolymer. Biopolymers were found to absorb water to form hydrogels and biofilms, which reduced the pore spaces between soil particles and improved the water storage ability of sandy soils. The water-retention capacity of both Xanthan gum and Gellan gum-treated sandy soils was higher than that of untreated sandy soils, and increased as the biopolymer ratio increased. The water-retention behaviour of the Gellan gum-treated sandy soil were better than that of the Xanthan gum-treated sandy soil. Finally, a prediction model of the soil–water retention curve that considered the biopolymer ratio was proposed based on the Fredlund-Xing model. This study provides a scientific basis for the application of biopolymers to control soil erosion in the Yellow River basin in China.

Soil–water retention behavior of a loess–paleosol sequence and its significance for hydrology and paleoclimate: a case study from the Luochuan profile of the Loess Plateau, China

The soil water retention behavior of the loess–paleosol sequence has important significance for hydrology and paleoclimate. This report examines the soil–water retention curves and microstructures of the Luochuan loess–paleosol sequence, which has a thickness of around 43.7 m and includes the S0–L6 layers. The experimental results demonstrated that as burial depth increases, there is a significant change in the soil–water retention curve of loess–paleosol sequence. Simultaneously, the saturated volumetric water content and water loss rate gradually decrease, while the air entry value and residual water content increase. As a whole, the loess layer has a lower water-holding capacity than the adjacent paleosol layer. Accordingly, the microstructure of the loess–paleosol sequence changes considerably with the burial depth. The microstructure changes from overhead structure to matrix structure, the particle contact relationship changes from direct point contact to indirect surface contact, the degree of cementation changes from slight cementation to complete cementation, and the volumes of all pore types decrease. The results suggest that the loess layer deposited in cold and dry climate conditions can act as an aquifer, which is related to weak pedogenic weathering. In contrast, paleosol layers that were formed in warm, humid climates underwent significant pedogenic weathering and can behave as an aquiclude. Based on the previously mentioned results, the link between paleoclimate changes and the current hydrological system has been explored. This offers a theoretical foundation and guidance for future engineering and ecological construction operations, as well as water resources management in loess areas.

Drying and wetting soil-water retention behaviour of a highly expansive clay under varying initial density

Expansive (or swelling) clays are the most prolific problem soil across Southern Africa and many parts of the world. Large volume changes due to seasonal wetting and drying cause millions of dollars’ worth of damage to engineering infrastructure yearly. Soil-water retention behaviour is dependent ondensity, and determining the correct retention curve may be key for reliable design of infrastructure. Soilwater retention curves and shrinkage curves were measured for recompacted samples of a highly expansive bentonite clay from South Africa. Four samples were prepared at varying initial dry densities between approximately 1200 and 1500 kg/m3 and subjected to total suction testing in a dewpoint hygrometer. The water content of each sample was varied through wetting and drying. The sample volume was measured after each suction reading, allowing relationships between suction and void ratio, degree of saturation and water content to be determined. The relationship between gravimetric water content and suction was not significantly influenced by initial density. Degree of saturation at a given suction varied significantly according to initial density. Volume reductions of 25% to 36% from saturated to residual conditions were recorded, and samples tended to a residual void ratio of 0.35 to 0.4. The swelling clay showed propensity for maintaining high suctions (over 300 MPa at residual conditions) and hysteretic response between primary drying and wetting.

Enhancement of a hydro-mechanical hypoplastic model for unsaturated fine-grained soils accounting for small strain stiffness

Due to global climate change, larger extreme seasonal and daily moisture variations have been occurring more frequently in the last decades. This phenomenon can affect geotechnical structures by inducing cyclic coupled hydro-mechanical loads. However, reproducing this behaviour from a numerical point of view requires robust constitutive models that can predict the coupling between the hydraulic and mechanical behaviour of fine-grained soils, combined with predictions of history-dependent stiffness evolution at small strains. For this reason, in the present work the hypoplastic model for unsaturated finegrained soils was further modified to better predict the water retention behaviour of unsaturated soils incorporating a smoothed hysteretic Water Retention Curve (WRC). In addition, the constitutive model wascalibrated using experimental data available on the literature of a completely decomposed tuff (CDT) from Hong Kong. At the end, the capabilities of the extended model to predict cyclic behaviour of unsaturated soils were evaluated using cyclic constant water triaxial tests at different suctions The results indicate that the extended model is able to describe with more accuracy the cyclic hydro-mechanical behaviour of the decomposed tuff if additional suction-dependency of one of its small-strain parameters is considered. Without this, the model can be calibrated to data at a given suction but its cyclic predictions for different suctions are not reasonably accurate.

Pore scale investigation of unsaturated granular soil behaviour by means of in situ CT experiments

With continuing evolution of imaging techniques from medical applications and materials science, non-destructive imaging experiments have also become an important method to investigate soil specimens. Amongst other methods, computed tomography (CT) has developed to a tool to visualise and better understand the microstructure of different soils based on 3D image data. Furthermore, the acquisition of a temporal series of CT images allows to study processes in soils on the microscale, e. g., during mechanical loading. In order to study the hydraulic and mechanical behaviour of unsaturated granular soils, we combine different custom-built miniaturised experimental set-ups with geomechanics background with computed tomography, yielding so-called in situ CT experiments. By means of image reconstruction and further image analysis based on segmented CT images acquired during different hydraulic and mechanical experiments, we study the drainage and imbibition process as well as the shear process of unsaturated sand and glass bead specimens on the pore or grain scale. The analysis of data on the microscopic level, including the phase distribution, interfacial areas, contact lines as well as radii of curvature of capillary menisci, allows to obtain insights into the macroscopic water retention behaviour and shear behaviour of granular soils.

An analytical method to evaluate the capacity of laterally loaded piles in unsaturated soils

The growing pressures of climate change, increased usage and unprecedented geo-hazards impose a modification in the way civil engineering structures are designed and constructed. This isparticularly true for geotechnical works, which are very sensitive to changes of environmental conditions. For instance, the response of a pile under lateral loading is strongly influenced by the stiffness and strength of the first few metres of soil below the surface, which are often partly saturated. To consider this effect, the present paper describes an analytical method, which extends the well-known Broms approach to predict thelateral capacity of piles in unsaturated soils. More specifically, the proposed method considers the combined effects of the position of the ground water table and the extra strength of the partially saturated soil above it. Compared to Broms approach, the solution introduces four additional non-dimensional parameters that relate the soil-water retention behaviour to the geometry of the pile. The method provides a direct evaluation of the lateral pile capacity in partly saturated soils, which can be used as a basis for more accurate design.

Experimental basis for determining contact angle in the ideal model of unsaturated soils

In an ideal unsaturated soil model, the contact angle (CA) is often determined by the sessile drop method (SDM), or assumed without any test. However, both approaches can lead to errors in the model calculation results. In this study, methods that combine models of unsaturated soil with CA measurement methods is presented. The characteristics of soil CA were studied using the sessile drop method (SDM) and the direct observation method (DOB). The results demonstrated that the CA values of soils and CA variation characteristics measured with SDM were significantly different from those measured with DOB. SDM measures the CA of a surface, which is a “plane” formed by many particles, whereas DOB measures the CA between soil particles on a smaller scale. The measurement results of SDM can determine the degree of soil hydrophobicity but may cause large errors when used as a parameter of an ideal unsaturated soil model. The DOB can simulate the natural drying-wetting cycles of soils, and its measurement results are closer to the natural conditions. Using the mean CA as a model parameter is reasonable, except when the CA value is 0. The hysteresis phenomenon caused by using CA hysteresis for the soil-water characteristic curve (SWCC) is likely to overestimate the effect of CA hysteresis on soil water retention behavior.

Pore structure effects on the water retention behaviour of a compacted silty sand soil subjected to drying-wetting cycles

Most of the previous studies on the effects of pore structure on soil water retention behaviour are limited to a single drying and wetting cycle. This study evaluated the effects of pore structure on water retention behaviour for specimens prepared at different pore structures of similar density, subjected to drying-wetting cycles. Specimens were compacted at two density states equivalent to loosely compacted fill slopes (80% relative compaction, e = 0.78), and densely compacted slopes or subgrade compaction specifications (95% relative compaction, e = 0.50) at different water contents on the wet side (20%) and dry side (10%) of the optimum water content. The results show that loosely compacted specimens prepared on the wet side have a lower water retention ability due to the larger formed macropores. They also show a significantly higher degree of hysteresis which decreases with drying and wetting cycles. For loosely compacted specimens, there is a higher increase in water retention ability with cycles for specimens prepared on the dry side, due to more drying and wetting-induced pore evolution and volumetric changes. Increasing compaction density results in a higher water retention ability but reduces the number of cycles needed to achieve water retention equilibrium with drying-wetting cycles, subsequently behaving as scanning curves. This is attributed to the less pore evolution and smaller volumetric strain at lower void ratios. Pore structure effects on water retention behaviour subjected to drying-wetting cycles should be considered in civil engineering designs for earth structures, such as loosely compacted fill materials.

Cylindrical cavity expansion responses in anisotropic unsaturated soils under plane stress condition

In this paper, an anisotropic critical state model for saturated soils was extended to unsaturated conditions by introducing suction into its yield function. Combining this model with soil-water characteristic curves related to porosity ratio was employed to characterize the coupled hydromechanical behavior of unsaturated anisotropic soil. Based on the plane stress condition, the problem of the cylindrical cavity expansion in unsaturated anisotropic soils was transformed into first-order differential equations using the Lagrangian description. The equations were solved as an initial value problem using the Runge-Kutta algorithm, which can reflect the soil-water retention behavior during cavity expansion. Parametric analyses were conducted to investigate the influences of overconsolidation ratio (OCR), suction, and degree of saturation on the expansion responses of a cylindrical cavity in unsaturated anisotropic soil under plane stress condition. The results show that the above factors have obvious influences on the cavity responses, and the plane strain solution tends to overestimate expansion pressure and degree of saturation but underestimates suction around the cavity compared to the proposed plane stress solution. The theoretical model proposed in this paper provides a reasonable and effective method for simulating pile installation and soil pressure gauge tests near the ground surface of the unsaturated soils.

A Unified Approach for Establishing Soil Water Retention and Volume Change Behavior of Soft Soils

Abstract The assumption of treating the specimen volume as constant while experimentally determining the soil water retention curve (SWRC) of soils is valid only for nonplastic, granular soils. Fine-grained soils usually undergo significant volume change during dewatering and densification, and therefore such an assumption is misleading, even falsifying. The need for developing an easy-to-use lab-scale technique that can enable continuous monitoring of the evolutions in volume, suction, and moisture content of a progressively drying soft soil specimen is evident in the field of characterizing unsaturated soils. Such a method is relevant to establishing SWRC and soil shrinkage curve (SSC) of soft soils that exhibit an appreciable degree of deformation upon subjection to dewatering. To this end, a simple yet improvised method based on the balloon technique incorporating a commercially available high-capacity polymer tensiometer has been proposed to establish SWRC-SSC of soft soils. A comparison between the SWRC and SSC obtained through the proposed method and the conventional methods demonstrated a satisfactory degree of agreement. Densification of the materials realized under the influence of mechanical and hydraulic stresses has been discussed through a comparative analysis between the results from the proposed method and one-dimensional odometer test. For soft soils, the proposed method is particularly appropriate for establishing SWRC in terms of volumetric moisture content and degree of saturation through just a single test.

Soil-water retention behaviour of fine/coarse soil mixture with varying coarse grain contents and fine soil dry densities

The interlayer soil identified in conventional French rail track corresponds to a mixture of fine soil and coarse grains. To investigate the role of fines in the soil-water retention property of such a mixture, different coarse grain contents fv and dry densities of fine soil ρd–f were considered. The filter paper method was applied to measure matric suction. Mercury intrusion porosimetry tests were performed to observe the microstructure of the fine soil. In terms of the gravimetric water content of fine soil wf with matric suction ψ, the soil-water retention curve (SWRC) was significantly affected by ρd–f for ψ < 715 kPa, while independent of ρd–f for ψ > 715 kPa. Interestingly, this threshold ψ corresponded to a delimiting pore diameter of the bi-modal microstructure of fine soil, which separated micropores from macropores. In terms of degree of saturation Sr with ψ, the SWRC was significantly affected by ρd–f in the full suction range, while it was independent of fv. These findings help to better understand the results for samples where dry density of mixture ρd is kept constant: an increase in fv resulted in a decrease in ρd–f and the suction changed accordingly. In this case, both fv and ψ affected the mechanical behavior.

Modelling the water retention behaviour of anisotropic soils

Water retention curve (WRC) is an important parameter for unsaturated soils. It is greatly affected by the anisotropy of pore structure, as supported by experimental results in the literature. So far, however, the mechanism and theoretical modelling of anisotropy effects have not been investigated. These two issues were explored in this study based on two-dimensional analysis of soil pores, while were approximated as a series of ellipses for simplicity. According to experimental results in the literature, the pores of anisotropic specimen are more elongated than those of isotropic specimen on average. The elongated pore has a higher water retention ability than the round pore when they have the same area. As a consequence, the water retention ability of anisotropic specimen is higher than that of isotropic specimen. On the basis of this mechanism, a new WRC model was proposed for isotropic and anisotropic soils . To verify the new model, it was applied to simulate the WRCs of three soils with isotropic and anisotropic pore structures. Measured and calculated results were well matched with the coefficient of determination (R2) in the range of 0.89 to 0.99 and the root-mean-square error (RMSE) ranging from 0.009 to 0.073. It is convincingly demonstrated that the new model is able to capture the influence of anisotropy on WRC.

Effects of pyrolysis temperature, feedstock type and compaction on water retention of biochar amended soil

Recent studies on water retention behaviour of biochar amended soil rarely considers the effect of pyrolysis temperature and also feedstock type into account. It is well known that pyrolysis temperature and feedstock type influences the physical and chemical properties of biochar due to stagewise decomposition of structure and chemical bonds. Further, soil density, which is in a loose state (in agricultural applications) and dense (in geo-environmental engineering applications) can also influence water retention behaviour of biochar amended soils. The major objective of this study is to investigate the water retention properties of soil amended with three different biochars in both loose and dense state. The biochars, i.e. water hyacinth biochar (WHB), chicken manure biochar (CMB) and wood biochar (WB) were produced in-house at different pyrolysis temperature. After then, biochars at 5% and 10% (w/w%) were amended to the soil. Water retention behaviour (soil suction and gravimetric water content) was studied under drying and wetting cycle simulated by varying relative humidity (RH, 50–90%). Results show that 10% WHB produced at 300 °C were found to possess highest water retention. CMB is found to possess higher water retention than WB for 10% amendment ratio. In general, the addition of three biochars (at both 300 °C and 600 °C) at 10% (w/w) significantly improved the water retention at all suction ranges in both loose and dense compaction state as compared to that of the bare soil. The adsorption (wetting) and desorption (drying) capacity of biochar amended soils is constant at corresponding RH.

Application of Spatial Time Domain Reflectometry for Investigating Moisture Content Dynamics in Unsaturated Loamy Sand for Gravitational Drainage

The strength of unsaturated soil is defined by the soil water retention behavior and soil suction acting inside the soil matrix. In order to obtain the suction and moisture profile in the vadose zone, specific measuring techniques are needed. Time domain reflectometry (TDR) conventionally measures moisture at individual points only. Therefore, spatial time domain reflectometry (spatial TDR) was developed for characterizing the moisture content profile along the unsaturated soil strata. This paper introduces an experimental set-up used for measuring dynamic moisture profiles with high spatial and temporal resolution. The moisture measurement method is based on inverse modeling the telegraph equation with a capacitance model of soil/sensor environment using an optimization technique. With the addition of point-wise soil suction measurement using tensiometers, the soil water retention curve (SWRC) can be derived in the transient flow condition instead of the static or steady-state condition usually applied for conventional testing methodologies. The experiment was successfully set up and conducted with thorough validations to demonstrate the functionalities in terms of detecting dynamic moisture profiles, dynamic soil suction, and outflow seepage flux under transient flow condition. Furthermore, some TDR measurements are presented with a discussion referring to the inverse analysis of TDR traces for extracting the dielectric properties of soil. The detected static SWRC is finally compared to the static SWRC measured by the conventional method. The preliminary outcomes underpin the success of applying the spatial TDR technique and also demonstrate several advantages of this platform for investigating the unsaturated soil seepage issue under transient flow conditions.

Simple Testing Method for Measuring the Triaxial Stress-Strain Relations of Unsaturated Soils at High Suctions

Abstract Unsaturated soils at high suctions are widespread in arid and semiarid areas. Test data on the mechanical behavior of unsaturated soils are scarce at high suctions. In this article, the effect of void ratio or density on the soil-water retention behavior was discussed on the basis of measured data of various compacted soils at high suctions. According to the test data, the soil-water retention behavior is independent of the dry density or void ratio over a high suction range, namely, if the water content of a soil specimen is kept constant, the suction will be constant and independent of the void ratio (deformation) at high suctions. Therefore, a simple testing method for measuring the stress-strain relations of unsaturated triaxial specimens at high suctions is proposed by combining the vapor equilibrium technique (VET) with saturated salt solutions and the triaxial shear tests at constant water contents. First, a known initial high suction is imposed on the specimen by using the VET with a saturated salt solution. Then, the specimen is triaxially sheared under a constant water content, thereby resulting in a constant high suction during triaxial shearing. The proposed testing method can substantially shorten the testing period for one triaxial test. Finally, the results of triaxial shear tests on silt, silty clay, and expansive soil at high suctions obtained by the proposed testing method are presented.

A general mathematical framework for modelling soil-water retention behaviour

A new constitutive model for the soil-water retention behaviour of unsaturated soils is proposed, able to reproduce the main drying and wetting paths, the cyclic retention behaviour and its dependence on the specific volume. The most significant aspect is the inclusion of the evolution, with the specific volume, of the degree of saturation when suction tends to zero in wetting paths considering the presence of entrapped air bubbles. The model is used to reproduce with success the drying/wetting cycles of two Pearl clay samples.