Unsaturated Sand Research Articles

Threshold of Saturation Degree of Unsaturated Sand to Cause Liquefaction Considering the Sample Volumetric Strain: Modelization and Experimental Laboratory Investigation

ABSTRACT This paper aims to determine a non-liquefiable domain corresponding to the threshold of saturation degree where liquefaction does not occur under cyclic loading. To determine this threshold, sample volumetric strain at liquefaction state of unsaturated soil was modeled considering the suction. In low saturation degree zones, capillary suction is taken into account. In a high saturation degree zone, spherical suction caused by surface tension of air bubbles is considered by using its microscopic analysis and its equivalent size. Finally, three series of experimental tests have been made to give the data and verify the presented model.

Soil-Pile Interaction of Laterally Loaded Fixed-Head Piles and Pile Groups in Unsaturated Sand

This study investigates the lateral behavior of piles in unsaturated cohesionless sandy soil using centrifuge modelling. The lateral loading was conducted on a single fixed-head pile and 2×2 fixed-head pile groups with 3D and 5D spacings. The results showed that the single fixed-head pile demonstrated greater magnitudes of lateral resistance response compared to the leading and trailing piles of the pile groups. Moreover, the piles experienced greater lateral loads when the water table was about a quarter of the embedded pile depth in the soil layer for the single fixed-head pile and leading piles in the group. The group efficiencies of the pile increased as the water table reduced up to half of the embedded pile length for the 3D pile spacing. However, this effect was negligible for 5D pile spacing. Considering the mixed unsaturated-saturated ground condition can lead to a more resilient foundation design under climate-induced and seasonal fluctuations of water table level and also pave the path for the inclusion of future climatic scenarios.

Thermally Induced Moisture Flow in a Silty Sand under a 1-D Thermal Gradient

Thermally induced moisture flow in unsaturated soils involves complex coupled thermal–hydro processes with the moisture flow in both the vapor and liquid phases. The accurate measurement of the moisture flow in unsaturated sands remains a challenging task due to low moisture migration, the temperature effect on moisture sensors, and the gravity effect on moisture flow. This study aims to accurately measure transient moisture flow, heat transfer, and thermal conductivity in a silty sand with 35% non-plastic fines in a closed heat cell with a controlled 1-D temperature gradient. The heat cell consists of two temperature-controlled heat exchanger plates, heat flux sensors, moisture sensors, thermocouples, and thermal conductivity sensors. The soil moisture sensors were calibrated in the test soil at room temperature and then at elevated incremental temperatures. Soil samples compacted at various initial moisture contents were tested under a constant 1-D temperature gradient of 4 °C/cm. Soil moisture redistribution, temperature, and thermal conductivity profiles were determined from the test results. Transient temperature responses indicated that a lower initial moisture content led to a higher temperature drop after reaching the peak, or a more concaved temperature profile in a steady state due to enhanced moisture migration driven by the temperature gradients. Dry soils exhibited uniform thermal properties, while moist soils showed varying thermal conductivity profiles. A critical moisture content was identified when the maximum moisture migration occurred. Thermal conductivity in soils increased with the distance from the heat source due to thermally induced moisture migration. These findings provide valuable insights into coupled moisture–heat flow dynamics in unsaturated sands.

Lateral Response of a Single Pile in Unsaturated Sand

Lateral Response of a Single Pile in Unsaturated Sand

Analytical and experimental evaluation of two-layered unsaturated sand bearing capacity

Pavement design methods based on principles of unsaturated soil mechanics take into account high soil shear strength due to matric suction resulting in more economical design especially in long roads. In this study, the bearing capacity of two-layer unsaturated sand was investigated using both analytical and experimental methods. At first, using the limit equilibrium method an analytical formula was proposed to determine the bearing capacity of two-layer unsaturated sand in which linear suction profile was considered in soil layers. It should be considered that the constant matric suction distribution assumed by the previous researchers does not show the real profile of matric suction within the soil, sometimes resulting in miscalculated unsaturated bearing capacity. Also, the bearing capacity of two-layer unsaturated poorly graded sand was investigated experimentally in different suctions by a special unsaturated chamber apparatus (UCA) designed for this purpose. The results show more than double increase of unsaturated soil bearing capacity with Sr = 25% compared to saturated soil. The formation of failure wedges in all experiments was investigated by image processing. An acceptable agreement was obtained between the theoretical and experimental bearing capacity results.

Sand Supply Affects Wind Erosion Efficiency and Sand Transport on Sand-Cemented Body Mulch Bed

Sand-cemented bodies (SCBs) are naturally distributed in some interdune corridors in the central Taklimakan Desert, northwest China. In this study, field-collected SCB particles were used as the experimental material, and wind tunnel experiments were conducted with different sand supplies, wind velocities, and SCB coverages to evaluate SCB wind erosion efficiency and vertical mass flux. The results showed that wind erosion efficiency decreased as SCB coverage increased. When the SCB coverage was above 40%, sand deposition processes occurred only under saturated sand flow, while sand transport remained unaffected by increases in SCB coverage under unsaturated sand flow. Under saturated flow, the highest concentrations of transported sand were found at 0–6 cm above the surface, and the main sand bed process was deposition. The sand bed process changed from aeolian erosion to deposition with increasing SCB coverage and tended to remain stable until the SCB coverage exceeded 40%. By contrast, under unsaturated sand flow, the sand bed process was primarily aeolian erosion, and the highest concentrations of transported sand were found at 0–4 cm above the surface. At high SCB coverage levels (more than 40%), a general balance between aeolian erosion and deposition processes was reached. In summary, increasing SCB coverage had a significant impact on surface wind erosion processes. Thus, SCBs can be used as a novel sand retention material.

Estimating the Thermal Conductivity of Unsaturated Sand

A modified parallel model for estimating the thermal conductivity of unsaturated sand was proposed in this study. The heat conduction in the solid phase of sand depends mainly on the form of contacts between solid particles, while water bridges at the particle contacts increase the contact areas and remarkably enlarge the transfer paths of heat conduction in sandy soils. However, the thermal conductivity of the solid particle itself (λs) cannot describe the influence of the form of contacts and water bridges on heat conduction through the solid phase. In this study, the equivalent thermal conductivity of the solid particle (λes) was presented which reflected the influence of the form of contacts and water bridges between particles under dry conditions or a low degree of saturation, respectively. The relationship between λes and degree of saturation was described by hyperbolic expression. The modified model was calibrated using measured values of the thermal conductivity from published datasets, including those for 41 types of sand from 15 studies. Numerical analyses of the temperature field of the energy pile were performed and validated against laboratory measurements. The results illustrated that the modified model was more applicable than the original model for predictions of sand thermal conductivity.

Experimental Investigation on Scouring v.s. Mass Failure of Unsaturated Soil Bed: Implications for Debris Flow Initiation and Erosion

AbstractScouring and mass failure are two common mechanisms used to describe soil bed erosion, but their combined effects are often not considered. To better understand how these mechanisms compete and under what conditions they prevail, it is essential to consider infiltration and a more realistic unsaturated soil bed. This study investigates soil bed erosion by considering unsaturated soil mechanics, a wetting front, and both erosion mechanisms of scouring and mass failure. Physical experiments were conducted on model water runoff over an unsaturated sand bed to investigate the effects of soil water content and flow velocity on erosion. Experimental results show that current understanding of soil bed erosion can be enhanced by adopting unsaturated soil mechanics and considering the combined effects of scouring and mass failure. The scouring rate is found to be independent of the bed water content because it only affects the uppermost soil particles, which immediately become saturated once water flows over them. Mass failure, on the other hand, is initiated at the wetting front when the rate of infiltration exceeds that of scouring. The depth of mass failure can be described by the net infiltration depth, which is defined as the difference between the infiltration and scouring depths. The net infiltration depth is jointly governed by the soil water content and flow velocity. The crucial role of the coupled effects of the hydro‐mechanical behavior of unsaturated soil in the realistic modeling of soil bed erosion is demonstrated. Outcomes present advancement toward improved hazard assessments of debris flows.

Analysis of permeability anisotropy of marine hydrate-bearing sediments using fractal theory combined with X-CT

Permeability anisotropy plays a crucial role in accurately predicting fluid productions during hydrate exploitation. However, the mechanisms governing permeability anisotropy have yet to be fully elucidated. This study conducted experiments on methane hydrate formation/dissociation in both unsaturated and saturated quartz sands, followed by X-ray computed tomography scanning and pore structure characterizations. Fractal parameters, encompassing fractal dimension, tortuosity fractal dimension, and maximum pore diameter, were employed to characterize the evolution of pore structure and changes in permeability anisotropy. Results show that the fractal dimension and maximum pore diameter decreased, while the tortuosity fractal dimension increased with the increase of hydrate saturation. Larger maximum pore diameter and smaller tortuosity fractal dimension in the horizontal direction become more pronounced with increasing hydrate saturation, resulting in heightened permeability anisotropy (kh/kv). The permeability anisotropy increases from 1.77 to 3.20 with hydrate saturation increasing from 0 to 0.38 in unsaturated sands, and from 1.47 to 4.70 with hydrate saturation increasing from 0 to 0.49 in saturated sands. Gas and water relative permeability anisotropy also amplifies with increasing hydrate saturation. Extrapolating to field tests, the findings suggest that horizontal wells present a more favorable option for gas production, especially in reservoirs with high hydrate saturation.

KURVA T-X

This seismic method practicum meeting is expected that practitioners are able to meet the objectives of knowing the process of forming the T-X curve in refraction seismic. The practicum was held on Monday, December 5, 2022 at the Faculty of Science and Technology, Jambi University. Based on the T-X curve attached to T-X 1, the reverse direct will be inverted from the forward. This value will be searched for an equation to get the result of rock velocity based on forward and reverse waves. Velocity is the level of rock hardness that will determine the speed of a wave in a rock layer. For interpretation, it can be seen from the table of seismic wave propagation velocity in the rock above. Where for V1 of 333.3333 m/s is included in layered weathered rock. For V2 where the speed value is 1000 m/s is unsaturated sand and gravel. For V3 where the velocity value is 1666,67 m/s, it is sand and shale. The interpretation of these rocks actually requires information about the formation where the data was collected.

An Experimental Study on the Lateral Behavior of Piles in Unsaturated Sand Under Monotonic, Cyclic and Post Cyclic Loading

An Experimental Study on the Lateral Behavior of Piles in Unsaturated Sand Under Monotonic, Cyclic and Post Cyclic Loading

Stress Release and Deviatoric Stress Behavior in an Unsaturated Sand During Inundation: Experimental Investigation and Insights from Triaxial Testing

Stress Release and Deviatoric Stress Behavior in an Unsaturated Sand During Inundation: Experimental Investigation and Insights from Triaxial Testing

Behaviour of saturated sand under cyclic loading: Model approach with experimental validation

This study presents a comparison between triaxial test results and constitutive model simulations for saturated sand subjected to a wide range of stress, relative density and both monotonic and cyclic loadings. This is a preliminary part of a project which is focussed on developing models for the behaviour of unsaturated soils under cyclic loading. There are currently limited experimental data of the cyclic behaviour of unsaturated soils due in part to the difficulty of controlling stress state, degree of initial saturation and uniformity of samples in experiments and their interpretation is challenging due to the mathematical complexity in constitutive models. This study attempts to bridge the abovementioned gap by creating an experimentally validated constitutive model for dynamic loading on saturated sand and then to extend this for unsaturated sands. The paper reports results from a series of monotonic triaxial tests and cyclic triaxial tests on saturated sand. Specimens of 100mm diameter, 200mm height and 54mm diameter, 110mm height have been prepared with different void ratios. These samples have been subjected to a wide range of deviatoric stress in both monotonic and cyclic tests. The results from these tests on saturated Sydney sand are presented and used to demonstrate the ability of the constitutive model. As the constitutive model has been developed for variable saturation demonstration of its success with saturated sand for which considerable data is available is the first stage in simulating the cyclic behaviour of the unsaturated sand.

Reliability of Soil Water Characteristics Curve under Different Normal Stresses for Unsaturated Sand

The soil water characteristics curve (SWCC) demonstrates a link between soil moisture content and suction. The SWCC is almost treated as an index parameter in unsaturated soil. The soil permeability and shear strength can be linked to SWCC. SWCC is established using filter paper for the wetting path. This paper compares SWCC for both cases stress-dependent and no stress (reference) in the wetting path. The samples of soil are brought from Al-Najaf City with a gypsum content of 29%, Iraq. The stress-SWCC is studied using a modified Oedometer cell with controlled water and air entrance to apply a specific matric suction. While the reference SWCC is estimated using the filter paper method. Matric suction is conducted at a range of pressures from 90 kPa to 0 kPa. The tests use three net normal stresses of 100, 200, and 400 kPa. The results show that there are decreasing values of the SWCC with increasing normal stresses. An interesting result is that this decrease is high and has zero matric suction. The water entry value (WEV) with the most significant value of the water entry change is represented by the matric suction of 50 kPa.

Experimental investigation of gas migration behaviour in unsaturated sand–clay mixtures

Understanding gas migration behaviours in unsaturated sand–clay mixtures is of great importance to design and construction involving applications of shield-cutting-tool replacement using pressure. A gas permeability test apparatus for unsaturated soil was upgraded to investigate gas migration behaviours further. A series of gas, liquid and solid three-phase coupling gas injection tests were performed on unsaturated clay under different moisture contents and sand–clay ratios. There are three stages of gas migration in unsaturated sand–clay: steep drop stage, transition stage and stability stage. In unsaturated sand–clay with low moisture contents and gas pressures, gas migration is affected primarily by the slippage effect. When the gas pressure is greater than 300 kPa, the permeability remains nearly unchanged. In this case, the gas permeability of the soil sample is affected primarily by the internal structural characteristics of the unsaturated soil sample and those of the shrinkage film. The ideal gas permeability of the soil used in actual engineering and the empirical formula for calculating the pressure of the shield pressure-holding system are summarised.

Experimental Investigation for Shear Wave Velocity and Dynamic Characteristics of Unsaturated Sand–Clay Liners

This study aims to investigate the shear wave velocity and dynamic characteristics of unsaturated sand–expansive clay liners (SECLs) over a wide range of suctions. Liner layers have gained significant interest as environmentally friendly materials for several geotechnical and geoenvironmental applications. These materials are typically found in an unsaturated state as compacted layers and are exposed to dynamic loads from natural phenomena or manmade activities. In such circumstances, sustainable and stable performance should be ensured during the operation and lifetime of these layers by addressing the dynamic characteristics of these materials and possible degradation. Several specimens belonging to different liners of sand and expansive clay were prepared at different suction levels. The shear wave velocity was determined using the bender element technique (BEls). The specimens were then subjected to extensive cycles of dynamic loads up to 500 cycles in the triaxial dynamic loading system. The shear wave velocity and dynamic characteristics of both liners, such as shear modulus (G), damping ratio (D), and degradation index (δ), were determined on the basis of soil suction and loading cycles. Results indicated a descending trend of shear wave velocity with an increase in suction up to 130 MPa, and a significant reduction in shear modulus was detected. Meanwhile, the damping ratio demonstrated a significant increase with the increase in the suction levels of both liners. The reported results are of great significance for sustainable design and modeling of the unsaturated behavior of liner layers in several applications of geotechnical and geoenvironmental problems.

Stability of tunnel face in unsaturated sand possessing apparent cohesion: A micro-macro analytical approach

Although the stability of tunnel face in the dry and saturated sandy ground is widely studied, the unsaturated sandy ground which possesses apparent cohesion is more common in engineering. For remedying this deficiency, the theoretical association between apparent cohesion and the saturation degree is firstly established in microscopic prospective. Then, the formation mechanism of the self-stabilized arch and the limit support pressure (LSP) of the tunnel face are derived by incorporating apparent cohesion into the macroscopic limit equilibrium analysis of the multi-arches model. Subsequently, the validities of the proposed approach in estimating apparent cohesion, loosening zone height and LSP are well confirmed (the average error rates of LSP are within 12 %) via comparisons with direct shear tests, model tests and other existing methods. Finally, as revealed by the parametric discussion, under the effect of apparent cohesion, LSP is negatively correlated with compactness, internal friction angle, and contact angle while decreases firstly (to a minimum value of 0.09γD ∼ 0.15γD) and then increase with the rise of saturation degree. Besides, the LSP has a parabolic distribution along the depth with its peak value emerges between 0.3D and 0.45D.

A hypoplastic model for unsaturated sand accounting for drying and wetting cycles

Existing constitutive models for unsaturated sand cannot capture the irreversible volumetric strain in the drying and wetting (DW) cycles. This study develops a novel hypoplastic model that aims to capture the behaviour of unsaturated soils, with a specific emphasis on the DW cycles experienced by unsaturated sand. The formulation integrates a hypoplastic model proposed by von Wolffersdorff for saturated sand with Bishop's effective stress. The simulation of soil water characteristic curve hysteresis is achieved through the implementation of the scanning rule for the main drying/wetting curve and scanning curve. The incorporation of a new tensorial term, Hs, into the hypoplastic equations, captures the irreversible volumetric strain in the DW cycle. A novel pyknotropy factor, referred to as fus, is proposed to replicate the rate at which irreversible volumetric strain accumulates during the DW cycle. Furthermore, the influence of suction effects on the critical state line is taken into account to represent the shear behaviour of unsaturated sand. The evaluation of the model performance is conducted through existing element tests on unsaturated sand. The comparison illustrates that the proposed model has the capability to capture certain essential characteristics, such as irreversible volumetric strain in the DW cycles and critical state behaviour of unsaturated sand. Due to its simplicity, the proposed model lends itself to straightforward implementation and potential utilization in future engineering applications, particularly in the analysis of slope stability in the presence of rainfall-induced conditions.

Investigation of the Water-Retention Characteristics and Mechanical Behavior of Fibre-Reinforced Unsaturated Sand

The introduction of fibres into soil can effectively improve its engineering properties. Systematically understanding the unsaturated mechanical properties of fibre-reinforced soil is highly significant. Moreover, there is currently no suitable model for describing the water-retention characteristics of unsaturated fibre-reinforced soil. The purpose of this study is to propose a model for the soil–water characteristic curve (SWCC) that can accurately describe unsaturated fibre-reinforced soil. The research focuses on unsaturated sand reinforced with PP fibres. A series of compression and direct shear tests were performed to investigate the mechanical behaviour. In addition, the SWCC was measured using the axis-translation technique. Based on the van Genuchten (VG) model, a modified model considering fibre reinforcement (VG-CFR) is developed for quantitatively analysing the influence of fibre content on the SWCC. The results showed that the established VG-CFR model can reflect the water-retention characteristics of fibre-reinforced sand.

Effectiveness of rolling dynamic compaction with a three-sided compactor on unsaturated sand

As a widely applied ground improvement technique, rolling dynamic compaction (RDC) densifies the soil by imparting a combination of kinetic and potential energy into the ground. In previous studies on RDC, attention has been paid to the stress distribution, soil particle movements, depth of influence, density increases etc. beneath the soil surface. However, the influence of the moisture contents and water table in unsaturated soils on the effectiveness of RDC remains uncertain. To better understand the performance of RDC on unsaturated soils, 6 laboratory experiments were conducted on Sydney sand with a 1:10 scale 3-sided impact roller. The roller has a mass of 46 kg and was towed at a constant velocity of 1.5 m/s in each test. Settlements at different depths beneath the surface were measured by buried plastic plates and particle image velocimetry (PIV) was applied to track the movements of the soil particles. Penetration tests were conducted before and after each experiment to assess the increases in the soil density. Measurements show that the depths of influence (DOI) for unsaturated sand were from 200 mm to 300 mm, however, density increases of less than 2 % were reported in all cases below the depth of 100 mm. These results, after compared with results of dry sand, demonstrates a negative influence of the moisture and water table on the effectiveness of RDC. The results from PIV suggest that the translational motion of the roller is transferred to a deeper level due to the tensile strength developed in unsaturated sand.