Compacted Silty Sand Research Articles

Investigating the Impacts of Biochar Amendment and Soil Compaction on Unsaturated Hydraulic Properties of Silty Sand

The application of biochar as an environmentally friendly additive for agricultural soils has recently gained significant attention. However, the influence of biochar addition on unsaturated hydraulic behavior at high suction ranges (i.e., exceeding 100 kPa) remains largely understudied. This study investigates the impact of biochar addition on the unsaturated hydraulic properties of biochar amended soil (BAS). The effects of biochar content, particle size, and soil compaction on the unsaturated hydraulic properties of BAS were also considered. Peanut shell biochar was utilized in this investigation and was amended into a compacted silty sand with distinct particle size groups. Soil water retention curves and unsaturated permeability were measured through a series of evaporation tests. Results demonstrate that the impact of soil compaction on the unsaturated hydraulic properties of BAS diminishes at high suction range, regardless of biochar particle size and content. A high degree of compaction reduces the saturated permeability of BAS by minimising soil macropores. On the other hand, incorporating high biochar contents with fine particles into the soil enhances the reduction of unsaturated permeability and the improvement of water holding capacity, thereby making biochar an effective application in soil for sustainability of the agroecological environment.

Influence of aggregate structure on the compressibility of an unsaturated compacted silty sand

It is well recognised that the compressibility of unsaturated soils is affected by soil structure. However, the effects of soil aggregation and pore structure on the compressibility behaviour of unsaturated soil are still not well understood. In this study, the effects of soil aggregation and pore structure on the compressibility of unsaturated silty sand were investigated. Specimens were purposely prepared at two compaction water contents to similar void ratios. They were saturated after compaction and dried to suction values of 0, 50, 150, or 300 kPa following a similar hydraulic loading path, after which they were subjected to isotropic loading and unloading. Mercury intrusion porosimetry (MIP) and micro-X-ray computed tomography (μ-XCT) were used for microstructural studies. At a given suction, specimens compacted on the dry side are less aggregated (smaller aggregate sizes) showing higher compressibility, whiles specimens compacted on the wet side are highly aggregated (larger aggregate sizes) revealing lower compressibility. The low compressibility of the highly aggregated soil is attributed to the additional drying-induced intra-aggregate strength due to the suction stiffening effects from meniscus water within the aggregates.

The Amazing Mechanism of the Standing for 700years of the Tall Masonry Stone Tower of 32m in height on Manmade Sandy High Mound by Shallow Direct Foundation, Bayon Temple, Angkor, Cambodia

Soils and foundation of Bayon temple of Angkor Thom has been studied since 1994 by Japanese Government Team for Safeguarding Angkor (JSA). The main tower of Bayon of 32m in height from the base foundation mound consists of manmade fill of 14 m in thickness. The foundation was studied and found as a simple shallow direct foundation. This is just like a 10 story RC building standing upon thick manmade sand fill without such a deep foundation as piling. At present, such a structure based upon thick sandy fill shall lose the foundation stability in the dry season under the monsoon climate of South-eastern Asia. The amazing mechanism, standing for 700 years, has been identified as the unsaturated characteristics of well compacted silty sand.

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.

Laboratory study of water infiltration and evaporation in biochar-amended landfill covers under extreme climate

Biochar has been used as an environment-friendly enhancer to improve the soil hydraulic properties. Previous studies focused on the effect of biochar addition for irrigation in agricultural soils. However, the understanding of the influence of biochar addition on water infiltration in compacted soils as used in landfill covers is limited. This study investigated the effects of peanut shell biochar addition on soil water infiltration with consideration of soil microstructure variations. The performance of biochar-amended soil was also explored under extreme rainfall and drought conditions. In this experiment, peanut shell biochar with particles finer than 0.25 mm was amended into compacted silty sand. Index soil properties and microstructure were observed. One-dimension (1-D) column tests and corresponding numerical modelling were carried out to investigate the performance of this cover material under different climate scenarios. The results suggested that the application of biochar can increase soil porosity, but a significant number of large pores (i.e., larger than 20 μm) was minimized. With the application of biochar, the soil covers thus become more efficient in preventing infiltration and percolation. This is also crucial to minimize the need for a relatively large thickness of soil cover. With an increase in porosity, the biochar can improve the soil water retention. Under extreme drought, the application of biochar can reduce the very low pore-water pressure (PWP) in soils by more than 50%. From all of these, peanut shell biochar can potentially be an eco-friendly and more sustainable solution for soil covers, even under extreme climate conditions.

Effect of Suction and Confining Pressure on Shear Strength and Dilatancy of Highly Compacted Silty Sand: Single-Stage versus Multistage Triaxial Testing

Abstract Evaluating short- and long-term performance of engineered earthen structures and slopes requires proper understanding of the behavior of compacted and highly compacted soils. However, limited studies exist in the literature of unsaturated soil mechanics experimentally testing highly compacted soils, which often possess complex shear strength and dilatancy characteristics. The main objective of this study is to investigate the effect of suction and confining pressure on shear strength and dilatancy of a highly compacted silty sand through multistage and single-stage triaxial testing. Multistage triaxial testing offers a time and cost efficient approach that can reasonably represent field conditions where soils experience fluctuations in external loading conditions (e.g., flood loading/unloading). This article presents the results of consolidated drained multistage and single-stage triaxial tests on highly compacted silty sand under saturated and unsaturated conditions. Soil specimens were isotropically consolidated at a constant matric suction of 0 (saturated), 20, 50, and 95 kPa under net confining pressures of 50, 100, and 200 kPa. Unsaturated specimens consistently exhibited higher shear strength in multistage and single-stage tests compared to saturated specimens. Peak state lines for all unsaturated tests were plotted and found to be near parallel to the saturated peak state line. All tested specimens exhibited dilatancy following a small initial contraction. Although the dilatancy during shearing in multistage tests was comparable to that in the single-stage tests, the cumulative volumetric strain resulted in a net increase in the specimen volume in the case of unsaturated tests, and a net decrease in the case of saturated tests. This difference in volumetric strain behavior resulted in a lower peak shear strength of the saturated specimens tested using multistage testing compared to single stage but a higher peak shear strength in the case of the unsaturated specimens.

Impact of biochar produced from hardwood of mesquite on the hydraulic and physical properties of compacted soils for potential application in engineered structures

Biochar is a carbon-rich porous material obtained after pyrolysis of labile biomass in oxygen deficit condition. The production of biochar from invasive weed mesquite biomass could be beneficial of managing waste and preventing the emission of CO2 gas by open combustion, and amending them in soil could sequester carbon in soil and improve soil properties. The effect of biochar on soil hydraulic properties was studied for loose soils suitable for agriculture. However, soils in engineered/bioengineered structures often compacted for achieving structural stability. Moreover, biochar of different feedstock poses variable effect on soil hydro-physical properties and a combined study of hydro-physical properties of soil amended with mesquite biochar lacks in the existing literature. In the present study, the effect of mesquite biochar on the hydro-physical properties, such as Atterberg limits, compaction characteristics, infiltration rate (I) and saturated hydraulic conductivity (Ksat) of compacted silty sand and pure sand is investigated for potential application in bioengineered structures. The results show that the amendment of 5%, 10% and 15% biochar increased the atterberg limits and water absorption capacity while decreased the plasticity index, specific gravity, dry density and I of the soils. Moreover, the Ksat is found to be increased in silty sand whereas decreased in pure sand due to biochar amended. These changes in soil properties are attributed to the altered physical properties, mainly porosity (inter-pores) of the soil by the biochar amendment and the presence of chemical compound in biochar surface. The reduced I and Ksat in the present study suggest application of biochar amended soil in bioengineered structures; however, further field-scale studies are needed before implication.

Effects of plant growth and spacing on soil hydrological changes: a field study

Effects of plant growth on soil hydrological changes need to be considered for long-term vegetation management of geotechnical infrastructure. Most existing studies focused on one particular plant age. This study quantifies the effects of plant growth on the evolution of soil hydraulic properties and matric suction over time, through field monitoring and numerical soil–plant–atmosphere interaction modelling. A full-scale flat landfill cover in China was monitored for more than a year. Four 6 m × 6 m grass plots made of compacted silty sand were formed, three of which were vegetated with shrubs having different plant spacing (0·5, 0·8 and 1·1 m), while the fourth one was left grassed. In addition to monitoring matric suction changes, multiple soil cores were sampled to determine the root length density (RLD) and saturated hydraulic conductivity (ks) after 2, 4, 7 and 13 months of transplantation. Regardless of plant spacing, ks always reduced during the first 2–3 months of growth. When a threshold RLD of 2 cm/cm3 was reached, ks increased substantially. This was especially the case for closely spaced shrubs, whose roots were decayed. Suction preserved upon rainfall depended on the plant growth-induced changes in ks. Although closely spaced (0·5 m) shrubs preserved the most suction during the first 6 months of growth, the beneficial hydrological effects vanished after growing for 9 months because of the shrub growth-induced increase in ks. Instead, the widely spaced (1·1 m) shrubs, which showed the least increase in ks, preserved the most suction in later stages. Nonetheless, the spacing of shrubs did not affect the annual cumulative percolation of the landfill cover very significantly, which narrowly ranged between 3·1 and 4·7% of the annual precipitation during the monitoring period.

Effect of biochar type on infiltration, water retention and desiccation crack potential of a silty sand

Biochar is a carbon-rich material obtained after thermochemical conversion of biomass under no oxygen environment. The effect of biochar amendment on soil properties, such as water retention, infiltration and desiccation crack potential was studied in the recent years. However, the effect of biochar or feedstock type on these properties is not explicit. This study investigates the effect of two different (in terms of feedstock) types of biochar on the water retention, infiltration and desiccation cracking behavior of compacted silty sand. Water retention characteristics, infiltration rate and the progression of desiccation cracks were measured after compacting soil amended with 5–10% (w/w) biochar produced from water hyacinth (WHB) and mesquite. Measurements were also taken for an unpyrolyzed material coir pith (CP, sourced from coconut husk)-amended soil for comparing the results of biochar-amended soil. The results show that the amendment of 5% to 10% biochar increased the maximum water holding capacity (θs), air entry value (AEV) and water content at 1500 kPa (θ1500) of the soil, whereas decreased the infiltration rate and peak crack intensity factor (CIF) of the soil. Moreover, the application of CP increased the infiltration rate. The amendment of WHB showed the highest increment in AEV and θ1500 and the highest decrement in infiltration rate and CIF compared to the other amendments. Based on the results, it is advisable to use the WHB-amended soil in bioengineered structures that could promote the growth of vegetation by higher water retention and could reduce the potential of leachate formation by decreasing water infiltration and desiccation crack potential.

Overtopping erosion of model earthen dams analysed using digital image-processing method

Overtopping erosion is one of the main factors responsible for the destruction of earthen structures due to floods. Considering the shortage of existing studies and the need for further research to properly understand the processes leading to breaching failure, this study investigates the effect of compaction level (dry density) on the overtopping erosion response of model compacted silty sand dams using a laboratory flume apparatus. From side-view video recordings of the homogeneous earthen dam cross-section, digital image-processing techniques are employed to track and forensically analyse and interpret the initiation and progression of the erosion edge. The results indicate that the overall pattern of initiation and development of overtopping erosion depends on the compaction level, with greater compactive effort (higher shear strength of soil) reducing its progression speed, thereby increasing the time period for the erosion edge to reach the dam's upstream crest (tB) and breaching failure. Artificial neural network and response surface methodology (RSM) approaches are investigated for estimating the experimental tB values, with the RSM-derived third-order polynomial found to produce good predictions for the three compaction levels investigated. Finally, recommendations are given for further research, including employing the experimental set-up presented for investigating other types of dam failure.

Determination of soil permeability coefficient following an updated grading entropy method

This paper presents a critical review of the grading entropy approach of permeability-coefficient predictions (k P) for coarse-grained soils. The approach applies the grading entropy theory to particle-size distributions (PSDs), such that the entirety of each gradation curve can be interpreted as a single point on a grading entropy chart, plotting its normalised entropy increment (B) against relative base grading entropy (A) values. Published data sets of measured permeability-coefficient (k M) values for saturated compacted silty sand, sand and gravel materials are examined to understand the dependence of A and B on various gradation parameters and the void ratio (e). In particular, log k M negatively correlates with log B and positively correlates with log A and e (log e). As such, power functions of the form [Formula: see text] prove statistically superior, noting that the fitting coefficient C 1 to C 4 values are specific to the PSD range and densification (compaction) states investigated for the permeability tests. Recommendations are given for increasing the predictive power, including separate models for well-graded and poorly graded materials and the addition of a particle shape factor and specific surface parameters in the regression correlation.

Calibration of a generalized plasticity model for compacted silty sand under constant-suction shearing tests

The stress-strain response of compacted silty sand with over-consolidated stress history often exhibit distinct peak stress before reaching the critical stress type of response when subjected to suction-controlled triaxial shearing. Such heavily consolidated soil also tends to simultaneously manifest initial compression which transitions into dilational type volumetric response. Modelling such strain-softening type response, especially emulating the smooth transition from peak to critical state is a challenge. In this paper a previously developed generalized plasticity constitutive model, called MPZ (Modified Pastor-Zienkiewicz) is fine-tuned and calibrated using a set of suction-controlled consolidated drained triaxial tests conducted on compacted silty sand specimens. Firstly, the saturated and unsaturated silty sand characteristics and the experimental test program are briefly introduced. Secondly, the calibration of each component of the constitutive model, namely critical state, dilatancy, peak state, loading direction, water retention curve and bounding function are briefly explained. Furthermore, the material parameters are estimated, model performance is displayed, and finally discussed. Preliminary simulations show that the MPZ model is able to mimic overall suction controlled triaxial test response of compacted silty sand decently well by taking into account the changes in density, pressure and suction. However, the peak states are not accurately modelled for low-high suction levels which needs further modifications in proposed model.

Modelling Unsaturated Soil Response Beyond Residual Suction State via Vapor-Pressure Controlled Triaxial Testing

Most of the previous research has been focused on developing and validating constitutive models to predict response of unsaturated soils in low-medium suction range. However, there is a scarcity of efforts in developing soil models to simulate its mechanical response in high suction range, particularly above the residual suction. This article presents a new constitutive model introducing net stress and suction as two independent variables. Furthermore, non-associative flow rule incorporating modified stress-dilatancy relationship to take unsaturated state into account is introduced to improve the model results in low-medium to high suction range. The essential soil model parameters are calibrated using suction-controlled triaxial test results for predictions of compacted silty sand response at high values of total suction above residual suction. Preliminary simulations show that proposed model can reasonably simulate the post-peak strain softening response obtained from suction-controlled CTC tests above residual suction value with reasonable accuracy. Although, the proposed model captures initial compression followed by dilation volumetric response with reasonable accuracy, it needs some improvements to be able to capture volumetric response accurately over entire suction range.

Erodibility assessment of compacted biochar amended soil for geo-environmental applications

Biochar amended soil (BAS) has been explored as a cover material for geo-environmental applications such as landfill cover due to its vegetation potential. Soil erosion in these infrastructures can progressively lead to failure and hamper the workability of the system. BAS is compacted for geo-environmental applications, unlike agricultural soil, which are loose in nature. Furthermore, the love-hate relationship of biochar with water can potentially affect the functioning of compacted cover system. Thus, the performance of compacted BAS in the context of erosion potential is not well understood. The major objective of this technical note was to explore the erosion potential of compacted BAS sourced from four distinct biochars. Biochar were produced in-house and mixed with soil at 5% and 10% by weight. In total, 81 pinhole erosion tests were performed to gauge the erosion rate of bare soil and BAS at three different compaction states at same compaction energy. It was revealed that the erosion rate decreased with gradual increment in water content for BAS, which was mainly attributed to the change of particle orientation from flocculated to dispersed along the compaction curve. Addition of biochar to soil resulted in decrease of erosion along the dry state whereas the opposite was observed for wet state. This was attributed to the surface functional groups as well as particle gradation of biochar. Erodibility coefficient and critical shear stress plot of soil and BAS revealed that addition of biochar had minimal effect on erosion of compacted silty sand.

Coupled effects of atmospheric CO2 concentration and nutrients on plant-induced soil suction

Some studies have shown that an increasing atmospheric CO2 concentration reduces plant transpiration while others have demonstrated that it interacts with nutrients in soil to enhance plant transpiration and plant growth including an increase in leaf area. However, none of these studies has focused on plant-induced suction in vegetated soils. The objective of this study is to quantify transpiration induced soil suction by Schefflera heptaphylla in NPK (Nitrogen, Phosphorus, Potassium) nutrient supplied, heavily compacted silty sand under two different atmospheric CO2 concentrations (400 ppm and 1000 ppm). Three replicates of the plant were grown in NPK nutrient supplied silty sand and their plant characteristics and soil matric suction were measured under 400 and 1000 ppm atmospheric CO2 for three months. Due to the supply of nitrogen-rich nutrient in silty sand, leaf area index (LAI) of plant increased by 22% under 1000 ppm CO2 compared to 400 ppm CO2. Thus, the larger LAI induced higher soil suction substantially since no significant difference in soil suction was found between current and elevated atmospheric CO2. LAI of Schefflera heptaphylla could be a reliable parameter to understand and predict soil suction as verified by strong correlation coefficient (R2 = 0.85–0.98; P value < 0.1) of peak induced suction and atmospheric CO2 concentration. This study reveals that the use of vegetation in soil structures needs proper management with additional supply of nutrients to thrive under future atmospheric condition and to induce suction hence improve shear strength in vegetated soil structures.

Influences of plant spacing on root tensile strength of Schefflera arboricola and soil shear strength

Mechanical root reinforcement depends not only on root biomechanical properties but also on root biomass. Although it is known that plant spacing can affect root growth, it is not clear how it affects root tensile strength. We interpreted a set of field data to study the effects of spacing of Schefflera arboricola on root area ratio (RAR), root tensile strength and their combined effects on soil shear strength (also termed root cohesion). S. arboricola was transplanted into compacted silty sand at a spacing of 0.5 m, 0.8 m and 1.1 m. After 20 months of growth in the field, the root systems were excavated for root tensile testing and post-test trait measurements. Plant spacing affected the growth and tensile strength of roots. More closely spaced plants had higher RAR but lower root tensile strength, especially for roots 0.5–2 mm in diameter. According to the existing root breakage and fibre bundle models used in this study, which calculate root cohesion as the product of RAR and root tensile strength, the effects of plant spacing on root cohesion were minimal for most soil depths apart from 0.4- to 0.5-m depth.

A study on effects of leaf and root characteristics on plant root water uptake

Water uptake intensity (WUI), which depicts the relationship between the rate of plant root water uptake per unit soil volume and soil matric suction, has been commonly used to analyse the transpiration-induced soil hydrological changes in civil infrastructure. However, the effects of plant characteristics on WUI are not clear, causing difficulties when estimating the effects of vegetation on soil moisture/suction changes. The aim of this study is to examine any correlations between plant characteristics and WUIs. Changes in suction and volumetric water content due to transpiration of eight tree individuals (Schefflera heptaphylla) vegetated in compacted silty sand were monitored to estimate the distribution of WUI within the root zone. Results show that the WUI remains a maximum and constant when suction is less than a threshold value of suction, beyond which the WUI decreases approximately linearly. The threshold suction is similar within the root zone of each tree individual, but it has a negative correlation with the ratio of leaf area (LA) and root length (RL). The reduction rate of WUI is positively correlated with root length density (RLD). This suggests that plants with a lower LA/RL ratio and lower RLD may be more capable of maintaining their water uptake ability, as high suction is induced.

Experimental and numerical investigations of the behaviour of footing on geosynthetic reinforced fill slope under cyclic loading

This paper investigates the cyclic loading responses of a strip footing supported by a geosynthetic reinforced fill embankment. A series of large-scale model footing tests were conducted first to investigate the accumulation of permanent footing displacement and residual vertical soil stress over large number of load cycles. The embankment fill was a heavily compacted silty sand and the reinforcement was a flexible geogrid, so that the model test configurations were representative of actual field conditions. Both permanent displacement and residual stress accumulated asymptotically with load cycles and majority of the build-up occurred over the first few hundred cycles. The potential effect of load interruptions was part of the study. Depending on how cyclic load interruption was implemented, it may or may not induce a trailing effect on subsequent cyclic loading responses. To have more in-depth understanding, these footing tests were also investigated numerically based on a soil model that can capture the unload-reload stress-strain loop over large number of load cycles. Reasonably good agreement between experimental observations and numerical predictions was also achieved.

Modeling Stress–Dilatancy Behavior of Compacted Silty Sand Under Suction-Controlled Axisymmetric Shearing

A thorough stress–dilatancy analysis of statically compacted silty sand is presented based on results from a comprehensive suction-controlled (drained) triaxial test program. Induced soil suction values, via axis-translation and relative humidity techniques, ranged between 0.05 to 300 MPa, with net confining pressures varying from 100 to 300 kPa. A first companion paper documented the strength, stiffness, and stress-induced anisotropy of the same soil under similar test conditions and variables. This paper focuses on the dilatancy rate, stress–strain and volume change responses, shear band formations, and peak as well as critical strength characteristics of isotropically consolidated silty sand under suction-controlled axisymmetric shearing. Augmentation in the peak stress ratio is found to be heavily dependent on the level of matric or total suction imposed, the maximum induced dilatancy, and the critical stress ratio. Experimental observations are also used to substantiate, or further refine, existing stress–dilatancy theories, which has led to the postulation of a simplified new formulation for predicting peak friction angle over the entire test suction range applied.

Bounding surface‐based modeling of compacted silty sand exhibiting suction dependent postpeak strain softening

SummaryThis article focuses on modeling the strain hardening‐softening response of statically compacted silty sand as observed from a comprehensive series of suction‐controlled, consolidated‐drained triaxial tests accomplished in a fully automated, double‐walled triaxial test system via the axis‐translation technique. The constitutive model used in this work is based on the theory of Bounding Surface (BS) plasticity and is formulated within a critical state framework. The essential BS model parameters are calibrated using the full set of triaxial test results and then used for predictions of compacted silty sand response at matric suction states varying from 50 to 750 kPa. Complementary simulations using the Barcelona Basic Model have also been included, alongside BS model predictions, in order to get further enlightening insights into some of the main limitations and challenges facing both frameworks within the context of the experimental evidence resulting from the present research effort. In general, irrespective of the value of matric suction applied, the Barcelona Basic Model performs relatively well in predicting response at peak and critical state failure under low net confining pressure while the Bounding Surface Model performs relatively well under high net confining pressures.