Initial Dry Density Research Articles

Factors affecting the swelling pressure for Jerash expansive soil

In this study for factors effecting the swelling pressure of jerash expansive soils were investigated in this study, effect of initial dry density and effect of initial water content on the jerash expansive soil were investigated.It show that as the initial dry density decrease from 1.85 gm/cm3 to1.25 gm/cm3 , the swelling pressure also decrease are from 3.1 to 0.25gm/cm2 also it show that as the initial water content increase from 0%to 15% , the swelling pressure of jerash expansive soil decrease from 2.65 gm/cm2 to 1.35 gm/cm2 .

Study of the volumetric behavior of a residual tropical clay

This paper focuses on the volumetric behavior of residual tropical soils. Two residual tropical soils belonging to the same geological formation and located in the coastal sedimentary basin of Gabon are studied. These soils are known as pelitic soils. As the hydromechanical behavior of residual tropical soils is still relatively unknown, it is interesting to study the influence of suction on the volumetric behavior of these soils through shrinkage curves. The purpose of this study is twofold. First, to study the volume variations in the two studied residual tropical soils with suction changes. Secondly, to propose a modelling of soil-water retention curves (SWRCs) of these soils. For this purpose, shrinkage tests are carried out to study the volume variations of the studied soils. These tests are supported by determining a bilinear approximation model to enable the variation in suction during drying to be estimated. Tests to determine SWRCs at different initial dry densities are used to calibrate the bilinear approximation model between void ratio and suction. The modified Kovács model (MK model) has been adapted to model SWRCs of the studied soils.

Root traits of crop species contributing to soil shear strength

Roots of annual species would be able to conserve soil properties during traffic induced compaction. The objective of this study was to determine and compare root traits and soil shear strength for three crop species with contrasted root types and morphological traits to see if roots of annual species are able to increase soil shear strength and thus explain the soil properties conservation. The experiment was performed under controlled conditions in steel cylinders containing a repacked loamy sand, with initial dry bulk density of 1.2 g cm−3 and that was kept moist. Soil shear strength parameters, i.e. cohesion and angle of internal friction, were determined from direct shear tests for soil cores at a soil matric potential of −10 kPa (i.e. water content at field capacity). The direct shear tests were performed with six external normal stress levels (25, 34, 44, 63, 83 and 93 kPa) and at a constant shear rate of 3 mm.min−1, and applying Mohr Coulomb equation. The difference of root type, root length density, root density, specific root length and root dry matter content among crop species was not related to a difference in soil shear strength. The root volume density was the main trait involved in both soil cohesion and the angle of internal friction. This study highlights the effect of roots of annual crop species on soil shear strength by comparing their root traits to the apparent cohesion of soil core where they grow. Vicia faba would be a good candidate to improve soil shear strength for soil conservation. This study constitutes a step towards improving the understanding of plants’ effects on soil shear strength with regards to selecting species and designing cropping systems for soil conservation.

An experimental investigation on the swelling behavior of compacted B75 bentonite

Compacted bentonite is often utilized as a buffer material between the canister containing radioactive waste and the host rock in deep geological repositories. Extremely low permeability and a potential to swell and develop large swelling pressures upon hydration are desirable characteristics for such material. This study focused on the B75 bentonite, a Ca-Mg-montmorillonite-rich clay selected as a potential buffer material for the planned repository in the Czech Republic. The effects of the initial dry density, vertical load, and pore fluid salinity on the swelling behavior of the clay were investigated. One-dimensional swelling tests were performed on compacted samples prepared at various initial dry densities (1.25–1.95 g/cm3). Conventional oedometer tests with various hydration and stress paths were also performed. An empirically corrected diffuse double layer model proved successful in predicting the swelling pressure during the tests. Swelling under constant load, swelling pressure under constant volume, swelling upon unloading, and unconfined swelling upon hydration were compared, and a unique relationship was found between the effective stress and the dry density at saturation.

Effects of plastic film residue and emitter flow rate on soil water infiltration and redistribution under different initial moisture content and dry bulk density

Drip irrigation technology combined with film mulching has expanded rapidly in arid and semi-arid areas. Without sufficient mulch film recovery, large amounts of plastic film remain in the field, changing the original water infiltration movement, which is not well understood. In this study, structural equation modeling was used to study the impact of residual plastic film (RPF) and emitter flow rate (FR) on the migration time of wetting front (MTWF), soil infiltration aspect ratio (AR) and accumulative infiltration (AI) under different initial moisture content (IMC) and dry bulk density (DBD). The results showed that RPF prevented the downward movement of water, which led to increased MTWF, AI and AR. However, RPF had no direct effect on the AI and infiltration AR, and the effects that it did have on these factors were indirectly influenced by the MTWF. When the RPF content was greater than 480 kg/hm2 (with a mulching history of 26 years), the infiltration AR was greater than 1.0. Additionally, there was a parabolic relationship between the emitter FR and the MTWF. When the FR was 0.7 L/h, the MTWF reached its minimum value. Overall, this study explored the process of water movement under drip irrigation infiltration of RPF farmland and provided a theoretical basis for the design of drip irrigation systems for RPF farmland.

Evolution of Tensile Properties of Compacted Red Clay under Wet and Dry Cycles

Tensile strength is an important soil parameter that affects the stability of structures built on clayey soils. This paper presents an experimental study of the change of tensile properties of red clay during wet-dry cycles. Cyclic wet-dry tests were performed on compacted red clay specimens with different initial water contents and dry densities. Direct tensile tests and Brazilian split tests were then conducted on these specimens to determine the soil strengths. The effects of initial water content, initial dry density, number of wet-dry cycles, and crack rate on the tensile properties of red clay were investigated. The results demonstrate that the tensile strength of red clay is generally 1.566 times the splitting strength. Both the tensile strength and splitting strength are negatively correlated with the initial water content but are positively correlated with the initial dry density. Because of the presence of desiccation cracks, the tensile strength goes on reducing under wet-dry cycles. The tensile strength can be expressed by a power function of the initial water content, initial dry density, and crack rate. The proposed equation is useful to evaluate the tensile strength of cracked soils subjected to wet-dry cycles.

Effect of drying–wetting cycles on the hydromechanical behaviour of compacted coal gangue

Compacted coal gangue is often used as subgrade soils in South Central China. To understand further the effects of drying–wetting (D-W) cycles on the hydromechanical behaviour of compacted coal gangue subgrade, this paper presents a series of laboratory test results on reconstituted coal gangue subjected to multiple D-W cycles. The effects of vertical load and compaction parameters on the soil deformation behaviours during hydraulic loading are fully discussed. Based on the laboratory investigations, the shrinkage strain was found to decrease with the increase in the initial dry density; however, it increased with the increase in vertical load. Experimental results also revealed that the variation in soil water content was more pronounced in the first D-W cycle. The characteristics of the void ratio and water content change during D-W cycles were investigated, and the shrinkage behaviour (e–w) was obtained. Significant hysteresis was detected during the D-W cycles, and the size of the hysteresis loop was found to decrease with the increase in D-W cycles, while it increased with the increase in vertical load. Besides, the D-W cycles were found to influence the pore volume at the microscopic scale, where both the volumes of inter-aggregate pores and intra-aggregate pores were found to decrease as the hydraulic loading increased.

Experimental Study on the Effect of Key Factors on the Soil–Water Characteristic Curves of Fine-Grained Tailings

The soil–water characteristic curve (SWCC) is an essential parameter for studying the mechanical properties of unsaturated tailings, and it plays an important role in stability assessment and prediction of unsaturated tailings dams. In this paper, the matrix suction was measured indirectly by a filter paper-based method to investigate the effects of key factors (gradation, temperature, and initial dry density) on the soil–water characteristic curves of fine-grained tailings, and the Van Genuchten model was adopted to obtain the empirical equation of SWCC and to verify the accuracy of experimental results. The results showed the following: 1) the Van Genuchten model fits well the relationship between matric suction and volumetric moisture content of fine-grained tailings, indicating that experimental data determined by filter paper-based method is accurate and appropriate; 2) at the same volumetric moisture content, the matrix suction increased with decreasing average particle size, and the decrease in temperature will increase the matrix suction and water-holding capacity of fine-grained tailings, and the matrix suction of the tailings in summer is 38.3% lower than that in winter under the natural volumetric moisture content (14.0%); 3) when the saturation degree of fine-grained tailings is less than 60%, the greater the initial dry density, the better the water-holding capacity and matrix suction.

Response of compacted bentonite to hyperalkalinity and thermal history

The use of compacted bentonite around the high-level nuclear waste canister (HLW) inside the deep geological repository (DGR) ensures the prevention of entry of active radionuclides in the atmosphere due to its noteworthy large swelling ability. In the eventual repository, the waste canister has a high (100 °C–200 °C) temperature initially, and it reduces over a vast period, which induces a thermal history over the compacted bentonite layer. The cement/concrete layer is constructed as a bulkhead or in the vaults or to support the access of galleries between a buffer and the host rock, and it degrades over the period. The hyperalkaline fluid is created when it percolates through the cement/concrete layer and comes in contact with the compacted bentonite. The contact of hyperalkaline fluid to compacted bentonite induced with thermal history can hamper the swell pressure characteristic of the bentonite. Therefore to determine the combined effect of hyperalkalinity to the thermal history induced compacted bentonite, swell pressure testing has been conducted on two compacted Barmer bentonites (B1 and B2) specimens with an initial dry density of 1.5 Mg/m3, 1.75 Mg/m3, and 2.0 Mg/m3 and saturated with distilled water as well as with hyperalkaline cement water (W/C = 1 und pH = 12.5) and heated to 110 °C and 200 °C. When the specimens were saturated with hyperalkaline cement water, the swell pressure exerted by both bentonites was noticeably reduced compared to specimens saturated with distilled water. Nevertheless, the time taken to full saturation was longer than distilled water for samples saturated with hyperalkaline cement water. Also, the decrease in swell pressure was observed in the samples subjected to thermal history than samples, which were tested without inducing thermal history in both the cases of hyperalkaline cement water and distilled water. The microstructural observations through XRD, FESEM and EDX revealed the clogging of pores due to the presence of non-swelling minerals.

Swelling behaviours of compacted lime-softening sludge for application in landfill liners

The objective of this study was to investigate the swelling potential of compacted lime-softening sludge for application in landfill liners. The study involved the assessment of the effect of compaction and moulding moisture content (30–40%), corresponding to the Proctor standard compaction test. One-dimensional oedometer swell tests were conducted using distilled water, tap water, and municipal landfill leachate, resulting in the determination of the expansion indices. Moreover, changes in the moisture content and dry density during the swelling process were investigated. The expansion index was significantly influenced by the initial moisture content and liquid chemistry. Subsequently, these factors also affected the sludge dry density decrease, and its moisture content increase, whereas the impact of the initial dry density on expansion was of low importance. An increase in the sludge moulding moisture content, limited swelling in all liquids used. The highest expansion, dry density, and moisture content changes due to swelling were identified for leachate at w < wopt. It should be underlined that the effect of liquid on the swelling potential faded away along with a further increase in the moisture content w > wopt. The novelty of the work lies in identifying a significant plunge of the expansion index at w ≈ wopt for the leachate swelling test. The lime-softening sludge non-swelling moisture content was defined as wnon ≈ (wopt + 4.0%) − (wopt + 4.5%). For practical engineering implications, the moisture content between (wopt + 2.0%) and (wopt + 4.0%) was provided for the most suitable sludge application in landfill liners.

Hydro-mechanical path dependency of claystone/bentonite mixture samples characterized by different initial dry densities

In the context of the French Cigéo-project, a mixture composed of 70% processed Callovo-Oxfordian claystone spoil and 30% MX80-bentonite could be a potential backfill material, whose installation aims to stabilize the surrounding rock formation and to limit the propagation of the excavation damaged zone. The backfill material must sustain the overburden pressure, despite that it might be exposed to different hydraulic and mechanical paths. The reference concept considers employing conventional compaction techniques, although their employment involves spatial variations in the dry density after compaction. In general, as the initial dry density has a significant impact on the hydro-mechanical behavior of backfill materials, it is of major importance to relate the variations in the initial dry density to differences in the behavior. This experimental laboratory study aimed to analyze how variations in the initial dry density affect the swelling and compression behavior of the claystone/bentonite mixture, in particular in unsaturated state. Further, it evaluated whether those variations affected possible hydro-mechanical path dependences. The experimental program comprised suction-controlled oedometer and constant-volume swelling pressure experiments, in which samples characterized by different initial dry densities were exposed to different hydro-mechanical paths. The analysis of microstructural and water retention characteristics complemented the program. Major results indicated that the magnitude of swelling pressure at a given suction depends considerably on the initial dry density, but it is independent of the imposed hydro-mechanical path. Interestingly, the dependency of the yield behavior on the hydro-mechanical path appears to be more pronounced as the initial dry density increases.

Effects of salt and solidification treatment on the oil-contaminated soil: A case study in the coastal region of Tianjin, China

Presence of salt increases the system complexity of multi-phase oil-contaminated soil. The coastal area of Tianjin, China, is a representative zone full of oil-contaminated soil with salt. By considering initial water content, oil content and dry density, this paper focuses on soil-water reaction and mechanical strength to investigate effect of salt on oil-contaminated soil. Application prospect of solidified oil-contaminated saline soil is also analysed. Results indicate that there is a reverse moisture absorption competition between oil and salt, and the liquid limit and plasticity index of the oil-contaminated soil with salt are lower than those without salt. Salt increases the water repellency and leads to a higher water content of low-density soil in the desorption process. The soil-water characteristic curve of the oil-contaminated soil increases in a quadratic-parabola under the salt saturation and supersaturation. With salt, the curves of the maximum dry density and the optimal water content show poor symmetrical distributions. When the oil content is more than 8%, salt on the strength of oil-contaminated soil shows a significant effect. Based on solidification of lime and fly ash, the strength of oil-contaminated soil with salt increases by 17.9-5.7 times, which meets requirements for engineering applications (like subgrade filler). It is concluded that solidification is an effective treatment to clean contamination for oil-contaminated soil with salt.

Thermal conductivity prediction model for compacted bentonites considering temperature variations

An engineered barrier system (EBS) for the deep geological disposal of high-level radioactive waste (HLW) is composed of a disposal canister, buffer material, gap-filling material, and backfill material. As the buffer fills the empty space between the disposal canisters and the near-field rock mass, heat energy from the canisters is released to the surrounding buffer material. It is vital that this heat energy is rapidly dissipated to the near-field rock mass, and thus the thermal conductivity of the buffer is a key parameter to consider when evaluating the safety of the overall disposal system. Therefore, to take into consideration the sizeable amount of heat being released from such canisters, this study investigated the thermal conductivity of Korean compacted bentonites and its variation within a temperature range of 25 °C to 80–90 °C. As a result, thermal conductivity increased by 5–20% as the temperature increased. Furthermore, temperature had a greater effect under higher degrees of saturation and a lower impact under higher dry densities. This study also conducted a regression analysis with 147 sets of data to estimate the thermal conductivity of the compacted bentonite considering the initial dry density, water content, and variations in temperature. Furthermore, the Kriging method was adopted to establish an uncertainty metamodel of thermal conductivity to verify the regression model. The R2 value of the regression model was 0.925, and the regression model and metamodel showed similar results.

Anisotropic swelling behaviour of unsaturated compacted GMZ bentonite hydrated under vertical stresses

Bentonite has been considered as a potential buffer/backfill material for construction of engineering barriers in deep geological repository for disposal of high-level nuclear waste. Anisotropy will be inevitably generated during static compaction during preparation of bentonite blocks, leading to induce possible influences on buffering function formation of the engineering barrier system and even the safety of the whole repository. In the present work, swelling deformation tests along the directions both perpendicular and parallel to the compaction plane were conducted on unsaturated compacted GMZ bentonite specimens using a self-developed lateral confined swelling test setup. Influences of initial dry density and vertical pressure on the final swelling strain and its anisotropy, as well as the mechanism of evolution of anisotropy coefficient on hydration, were analysed. Results demonstrate that the swelling time, average strain rate and final swelling strain increase with increasing initial dry density and decrease with increasing vertical pressure. Meanwhile, the swelling strain along the perpendicular direction is much higher than that along the parallel direction, exhibiting an obvious anisotropic swelling characteristic. The anisotropy coefficient is relatively large at the initial swelling stage, and then gradually decreases to be stabilised. The final anisotropy coefficient increases with initial dry density and decreases with vertical pressure.

Effects of Contact Angle on the Hysteresis Effect of Soil-Water Characteristic Curves during Dry-Wet Cycles

The hysteresis characteristics of soil-water characteristic curves (SWCCs) under dry-wet cycling conditions are very important for understanding unsaturated soil properties, so it is crucial to propose an accurate and efficient method for predicting the hysteretic behaviors of SWCCs. To this end, this paper investigates the hysteresis characteristics of SWCCs in the full suction range of seven kinds of Hunan red clay with different initial dry densities by combination of the pressure plate method, the paper filter method, and the saturated salt solution method. It is found that there are, respectively, strong and weak hysteresis zones in the drying and wetting SWCCs under dry-wet cycling conditions. By combining this feature and based on the drying curve, the soil volume and contact angle changes during the drying and wetting processes are employed to predict the hysteretic behaviors of SWCCs. To verify the validity of the prediction method, the predicted curves of the samples with different initial dry densities are compared with the measured curves. The results show that in the strong hysteresis zone, the hysteresis characteristics of the drying and wetting SWCCs are mainly resulted from the changes in the soil pore structure; in the weak hysteresis zone, the hysteresis characteristics are mainly influenced by the changes in the receding and advancing contact angles corresponding to the drying and wetting processes. The Young–Laplace theory is used to transform the changes of contact angle during the drying and wetting processes into the proportional relationship k of matric suction, and the corresponding wetting curve is obtained by smoothing the drying curve. It is found that the prediction effect in the high suction part (the strong hysteresis zone) is better than that in the weak hysteresis zone, which confirms that the hysteresis effect of SWCCs in the high suction part is influenced by the contact angle. Our proposed method can greatly reduce the test period and has a significant practical application value, which provides a new idea for the prediction of SWCCs under dry-wet cycling conditions.

Measurement and Uniform Formulation of Soil-Water Characteristic Curve for Compacted Loess Soil with Different Dry Densities

To investigate the effect of dry density on the soil-water characteristics of compacted soil, loess used as filling in the land-making project of the Yan’an new district was collected and compacted to five initial dry densities of 1.40, 1.50, 1.60, 1.70, and 1.80 g/cm3, respectively. The soil-water characteristic curves (SWCCs) of all specimens in the range of 0–105 kPa were measured using the filter paper method. The measured data were fitted using the Fredlund and Xing equation for each initial dry density. The SWCCs have obvious differences in a suction range below 100 kPa and overlap when the suction range is higher. This suggests that the SWCC of compacted soil is independent of the initial dry density in the high suction range, but the correlation with the initial dry density exists in the low suction range. Therefore, the correlation functions of the parameters in the Fredlund and Xing equation with respect to the initial dry density were regressed, respectively. By substituting these functions into the Fredlund and Xing equation, the state surface function of θ w − ψ − ρ d was obtained and can reflect the SWCCs of all densities of the filled soil to support the further investigation of the unsaturated behavior of compacted soil.

A swelling pressure cell for X-ray diffraction test

A swelling pressure cell is introduced to measure apparent swelling pressure (ps) and basal spacing of montmorillonite (d001) simultaneously for compacted bentonite during water absorption. Specimens with an initial dry density of up to 1·7 Mg/m3 were prepared with initially oven-dried bentonite powder. Results show that the ps time history curve has four stages: a sharp increase to the peak swelling pressure (pp); a drop to a valley swelling pressure (pv); another increase to the initial swelling pressure for equilibrium (pei); and equilibrium swelling pressure (peq). Meanwhile, d001 changes from 0·98 nm to 1·26 nm, 1·58 nm, 1·90 nm and 4·0 nm gradually during water absorption. Results imply that crystalline swelling of montmorillonite serves an important role for interpretation of ps behaviour of tested specimens. Another testing programme to measure ps compacted bentonite only was conducted using the swelling pressure cell. Results suggest that specimen dry density closely correlates with feature points of ps time history (i.e. pp, pv, pei and peq), which is consistent with results of earlier studies. Importantly, the 2 mm specimen thickness was used, which reduced the test duration to less than 24 h. Evidence from experiments suggests that the swelling pressure cell is a powerful and inexpensive tool for studying the swelling behaviour of compacted bentonite.

Experimental Study on the Relationship between Three-dimensional Stress Attenuation Value and Strain of Remolded Expansive Soil

This paper studies the characteristic of remolded expansive clay in Hefei. A series of three-dimensional swelling tests were conducted for cubic expansive soil of different initial moisture contents and dry density as well as the swelling pressure tests under the control of the vertical strain and the lateral strain. The research results show that: (1) Under the same initial water content, the vertical displacement of the soil sample is released. At this time, the attenuation value of both the horizontal and vertical expansion force of the soil sample increases with the increase of the initial dry density. (2) The horizontal displacement of the soil sample is released. At this time, the attenuation value of the horizontal expansion force of the soil sample increases with the increase of the initial dry density. Moreover, the horizontal displacement of the soil sample is released in the case of the same initial dry density. And the attenuation value of the horizontal expansion force of the soil sample decreases with the increase of the initial water content.

Experimental Study on Relationship between Free Load Swelling Rate and Water Content of Medium Expansive Soil

Expansive soil has the property of water swelling, which is related not only to the hydrophilic mineral composition of soil particles and the microstructure of soil, but also to the initial moisture content, dry density, and overburden condition of soil. Based on the typical expansive soil in a certain area, the samples were sampled and remodeled at the site. Extensive experimental tests were conducted to investigate the relationship between the hygroscopic expansion rate and the water content of the expansive soil under different initial moisture content, dry density, and free load. The results showed that, under the condition of natural initial water content and dry density, although the hygroscopic expansion rate of the medium expansive soil was nonlinear with the subsequent water content, in the range of large water content (within about 50%), the expansive soil swelled linearly. There was a linear relationship between the rate and the water content. With the increase of the initial water content, the hygroscopic expansion rate and expansion rate of the expansive soil decreased. With the increase of the dry density, the hygroscopic expansion rate and the expansion rate of the expansive soil increased. The water absorption performance did not decrease, and the soil continued to maintain the previous moisture absorption rate and expansion rate after the soil reached saturation, while after the water content reached 1.5∼2.0 times the saturated water content, the soil moisture absorption expansion rate gradually decreased until it finally stabilized. The slope k of the expansion rate increased with the initial dry density and decreased with the initial moisture content. As dry density was increased, the slope k was increased at an increased rate. Moreover, as the initial moisture content was decreased, the slope k was increased at an increased rate.

Influence of Freeze‐Thaw Cycles on Mechanical Response of Levee Pavement

The freeze‐thaw cycles cause deterioration in mechanical properties of levee soil and further endanger the pavement structure on the embankment. This study attempts to comprehensively understand the mechanical response of pavement after freeze‐thaw cycles. In this paper, the freeze‐thaw cycles test under an open system was carried out, and then the triaxial compression test was conducted. Based on the test results, the effects of freeze‐thaw cycles, temperature range, initial dry density, and initial moisture content of embankment soil on the mechanical response of road structure after freeze‐thaw were calculated and analyzed. Finally, the stability of the slope of the levee was evaluated. The results show that the number of freeze‐thaw cycles has the most significant impact on the mechanical response of pavement, the stress and strain of the structural layers vary in different ranges, and the pavement deflection increases by 5 times after 7 freeze‐thaw cycles. However, the initial dry density and initial moisture content of the soil have little influence on the pavement structure, and the temperature range will exert an influence when it exceeds a certain threshold.