Sand-clay Mixtures Research Articles

DEM study on the micromechanical behaviour of sand-clay mixtures

A DEM model incorporating realistic sand shape was developed to simulate sand-clay mixture (SCM) behaviour based on CT images acquired during uniaxial compression of a SCM containing quartz sand (SCM_R) and a SCM containing glass beads (SCM_S) with in-situ X-ray micro-tomography scanning. The influences of shape, size and content of sand grains on the uniaxial behaviour of SCM were studied. SCM_R revealed a higher peak strength and elastic modulus than SCM_S with the same sand content, sand size and initial void ratio. Increasing sand content led to significantly reduced peak strength for both SCM, and remarkably increased and unnoticeably changed elastic modulus for SCM_R and SCM_S, respectively. Meanwhile, increasing sand size resulted in dramatically decreased yet insignificantly varied peak strength and elastic modulus for SCM_R and SCM_S, respectively. The different phenomenon between SCM_R and SCM_S were attributed to the significantly different sand-clay bond intensities and interface interlocking effects between them.

Laboratory Investigation of the Unsaturated Shear Strength Characteristics of Sand–Clay Mixtures

Sandy‐clay mixtures are commonly used in civil engineering projects, such as transportation and water resources, due to their unique construction properties. While the macromechanical characteristics of these mixtures under saturated conditions are well understood, their behavior in unsaturated states is still not fully elucidated. This study aims to investigate the mechanical behavior of sand–clay mixtures under unsaturated conditions by examining the influence of varying sand content and matric suction on their shear strength. The research includes unsaturated shear strength tests and nuclear magnetic resonance (NMR) experiments to analyze the pore characteristics within the mixtures at different sand content levels. The correlation between changes in pore structure and shear strength is systematically analyzed. The unsaturated shear strength of the mixtures is predicted using the Fredlund shear strength formula and compared with the measured values. The results indicate that the unsaturated shear strength of the mixture decreases as the sand content increases but stabilizes once the sand content reaches 20%. Below 20% sand content, the changes in micropore distribution are not significant with increasing sand content. However, beyond 20% sand content, there is a noticeable increase in the ratio of large pores and a decrease in the proportion of small pores. The sand content of 20% represents a critical threshold for the strength and porosity variations in the mixture. Under constant matric suction, the unsaturated shear strength of the mixture improves with the increase in sand content. However, the predictive accuracy of the Fredlund shear strength formula gradually diminishes as the sand content increases.

Changing law of permeability coefficient during compression for reconstituted sandy clays

Sandy clay is defined as a sand–clay mixture with the mechanical behavior dominated by fine clay but affected by coarse sand. Twenty sets of one-dimensional incremental load consolidation and falling head permeability coefficient tests were performed on sandy clays. The samples of sandy clays were prepared by mixing two hosted clays with different sand fractions from 0% to 60%. The interrelation between the change of permeability coefficient with void ratio and the compression curves is established. The results indicate that the effect of sand fractions on permeability coefficient can be attributed to the variation of void ratio at liquid limit induced by sand fractions. The initial void ratio affects permeability coefficient by changing the void ratio under the same effective vertical stress. These two findings highlight that the permeability coefficient-void ratio relationship of sandy clays is a function of void ratio at liquid limit and void ratio. An empirical method of estimating permeability coefficient is also proposed using two physical indices of void ratio and void ratio at liquid limit for sandy clays having void ratio at liquid limits ranging from 0.63 to 1.73 and initial void ratios varying from 0.72 to 2.67.

Thermo-mechanical behavior of sand–clay mixtures—Part II: constitutive modeling

Thermo-mechanical behavior of sand–clay mixtures—Part II: constitutive modeling

Improvement of Subsurface Thermal Characteristics for Green Parks

The ground surface of green parks in arid and semi-arid areas may not be comfortable at specific times during the day and night due to the sun and the rate at which the subsoil gains or loses heat. Knowledge of the subsurface soil’s thermal properties can provide designers with convenient and comfortable settings. Design focus is generally directed toward stability, density, and hydraulic conductivity. An assessment of the thermal properties of clay–sand mixtures of 10%, 20%, and 30% clay content is conducted. The proposed clay–sand layers are subjected to three different thermal gradients of 30, 20, and 10 degrees of magnitude. The profile of temperature changes was monitored using 5TE sensors and data loggers. The mixtures were also subjected to cooling at room temperature. The results indicate that the clay type and the clay content govern the response of subsurface clay–sand liners to temperature gain and loss. Two field sections with clay–sand layers of 15% and 20% clay were examined for temperature changes over an extended period. In winter, green areas rich in clays were found to keep heat for several hours and provide relatively warm evenings. In summer, the mixture retains a cool temperature for some time during the day.

Thermo-mechanical behavior of sand-clay mixtures. Part I: experimental study

Thermo-mechanical behavior of sand-clay mixtures. Part I: experimental study

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.

Predicting Swell in Clay-Sand Mixtures Used in Liners

A method to predict the swellability of clay-sand mixtures is proposed. This model is a modified form of the Studds (1997) prediction model for clay-sand mixtures. The new proposed model uses the laboratory fall cone penetration technique to produce a characterization chart. This chart presents slope levels that can be used to obtain an equation for the final clay void ratio versus the vertical effective stress for clays. The porosity of the clay-sand mixtures was worked out based on a correction factor obtained from compression and porosity measurements in the laboratory. The porosity of the mixture was merged into the clay profile equation to compute the final clay void ratio at a specified stress level, which made it possible to predict the swelling behavior for different and variable stress levels. The swellability slope was obtained using fall cone tests conducted on the fine portion. Mixtures of kaolinite and bentonite were introduced to represent soils with different swell potentials. The fall cone measurements of a few points can be used to establish the swellability relationships for natural clay. Merging fall cone points with the swellability slope chart can define the profile of the vertical effective stress versus the clay void ratio.

Impact of clay particle reattachment on suffusion of sand-clay mixtures

The detached clay particles directly filtrated through the sand–clay mixture lead to suffusion; however, if the detached clay particles are subjected to reattachment, the degree of suffusion may be less significant. This study investigates the impact of clay particle reattachment on suffusion of sand–clay mixtures through laboratory soil-column experiments. The observed breakthrough curves (BTCs) of kaolinite, illite, and montmorillonite for 5 different column lengths (3 in, 6 in, 9 in, 12 in, and 18in) indicated that a higher breakthrough concentration was observed as the column length (L) decreased for kaolinite and illite, whereas a reverse trend was observed for montmorillonite. In addition, the increase in the fraction of filtrated clay particles (Me) with an increase in L (Me = 10.42% for L = 3 in and Me = 3.59% for L = 18 in) for the sand-illite mixture indicated that the reattachment effect became more significant as the travel length of detached clay particles increased. The observed BTCs, retention profiles after injection, and fraction of filtrated clay presented herein suggest the need to incorporate the reattachment effect when assessing the suffusion of clay-containing soils.

Cracking and erosion behaviors of sand–clay mixtures stabilized with microbial biopolymer and palm fiber

Drying-induced cracks and precipitation-induced erosion negatively impact the performance of soils in the context of extreme weather events. This study introduces two effective and sustainable materials, microbial biopolymer (MB) and palm fibers (PF), for cracking and erosion control in the sand–clay mixtures. A series of desiccation cracking tests, erosion tests, and SEM tests were conducted to evaluate the effectiveness of the treatment. The results showed that MB could significantly improve the resistance of the soil to cracking and scouring, and the improvement increased with increasing MB content. The optimum MB content was 0.15 % to achieve the maximum cracking and erosion resistance. For samples with varying sand contents, 0.15 % MB addition reduced the crack ratio, total crack length, and accumulative erosion ratio by 19.55 %–96.91 %, 4.22 %–99.58 %, and 57.88 %–89.53 %, respectively. In addition, PF positively affected the anti-crack and anti-erosion properties of the soil, and the application of 0.60 % PF had the best performance for both improvements. The cracks in the soils were mostly fine and shallow with the addition of 0.60 % PF, and therefore, the accumulative erosion ratio decreased by 44.18 %–62.76 % for samples with varying sand contents. Compared to the untreated soil, the degree of cracking and erosion was less due to the formation of a structure with more macropores and a sand skeleton in the treated samples with higher sand content. MB addition provides strong inter-particle bonding connections and a hydrophilic crust structure to improve the soils' resistance to cracking and erosion, while the fiber reinforcement effect benefits from interfacial friction and spatial restriction effects. This study provides mechanistic interpretations of desiccation cracking and erosion behavior in sand–clay mixtures under different treatments. It may guide the design of low-carbon technologies for geotechnical engineering applications.

Predicting propeller jet scour in silty and sandy marine environments

The propeller jet generated by a ship's propeller during berthing and unberthing from quays initiates sediment transport and causes scouring around the quay structures. Investigating the flow area caused by ship propeller jets at quays and piers is important in terms of determining the local scour at the seabed. In this study, the water jet formed by ship's propellers and sediment movements on the seabed were investigated. While medium sand is typically used as the sediment type in literature, fine-grained sand and silt were used as base materials in this study. As a result of the experiments conducted with a three-dimensional flow velocity measuring device (ADV), the measured three-dimensional velocity components and the propeller efflux velocity coefficients were redefined. By examining the time dependent scour graphs of silt and sand, their scour rates were compared, and time scales were obtained from graphs. Silt was observed to have a significantly slower scour rate compared to sand. Contrary to the findings in the literature, despite silt having a smaller median particle diameter than sand, the maximum scour depths of silt were found to be less than those of sand under the same experimental conditions. Based on the results of this study and previous studies, two equations were proposed to quantify the scour depth for sand material, and silt and sand-clay mixtures.

Small and Intermediate Strain Characteristics of a Partially Saturated Sand–Clay Mixture

Small and Intermediate Strain Characteristics of a Partially Saturated Sand–Clay Mixture

Dewatering rate of vacuum tube filter (VTF)-system-treated sand–clay mixed slurry

Vacuum dewatering methods are widely used to treat high-moisture dredged sludge; however, significant particle migration problems exist during treatment, particularly for sand–clay mixture slurries. A dewatering technology called a vacuum tube filter (VTF) system is proposed herein to alleviate the particle migration problem of sand–clay mixture slurries. First, four sets of model tests were performed using the VTF system and conventional vacuum (dewatering) method (CVM, i.e., a prefabricated strip-shaped drain assisted by vacuum) with two types of sand–clay mixed slurries. The water content distribution, settlement, and discharged water from the VTF system were compared with those from the CVM. The VTF system was superior to the CVM in terms of the dewatering rate and effect. Moreover, the particle migration problem was theoretically analyzed using a consolidation solution. With both slurries, the VTF system exhibited a lower particle-migration-problem-induced clogging effect. Thus, the VTF system can effectively alleviate the particle migration problem and enhance the dewatering of high-moisture sand–clay slurries.

REGULATION OF THE PROPERTIES OF SAND-CLAY MOLDING MIXTURES BY SODIUM PHOSPHATE ADDITIONS

Purpose. Study of the effect of sodium phosphate additives on the properties of a reversible sand-clay mixture in the green and dry state to improve the quality of castings from iron-carbon and aluminum alloys.
 Research methods. A reversible molding mixture based on quartz sand and kaolin clay was used. The effect of three additives (sodium tripolyphosphate, sodium hexametaphosphate, sodium pyrophosphate) in amounts from 0.5 to 2.0% was studied. Properties of mixture were determined using standard methods: clay component content, strength, gas permeability, crumbleness, flowability, formability, packing.
 Results. Effective additives have been studied to ensure the possibility of multiple use of reversible molding sand-clay mixtures. Special attention is paid to the regulation of the properties of the mixture in the dry state, since this issue has been much less researched today. For this purpose, sodium phosphates, which are produced by the chemical industry in sufficient quantities, were used for the first time. As materials that affect the properties of the mixture as a whole, they have not been considered before.
 Scientific novelty. For the first time, the positive effect of sodium phosphate additives on the physical and mechanical properties such as strength and crumbleness of reversible sand-clay mixtures was established. This effect is manifested in the fact that additives contribute to a slight improvement of properties of the green mixture and a very significant improvement of these properties of the dry mixture. There is practically no effect on important technological properties (flowability, formability, packing).
 Practical value. Based on the results of the experiments, it is recommended to use sodium tripolyphosphate or hexametaphosphate additives in the amount of 0.5 to 1.0 % to significantly improve the properties of dry sand-clay foundry molds. The use of these additives eliminates the appearance of surface defects in cast parts from aluminum and iron-carbon alloys.

Increasing performance efficiency of reconstructed oil fields

In the oil-gathering stations of most OGPDs, oil water sand clay mixtures that enter the settling equipment after passing through the separators are initially separated and collected in appropriate tanks. Prior to the production of commercial oil, more labor and energy is utilized to separate water and sand-clay mixtures from oil. Tanks are quickly contaminated with bottom sediments consisting of sand-clay and cleaning of them is difficult. Mixtures of sand-clay-water from settler, formation water from oil tanksa are drained into open oil traps, as a result, the environment is polluted with oil wastes and oil losses occur. It is more efficient to use a horizontal oil and gas separator to overcome shortcomings identified in the reconstruction of the tank farm with a capacity of more than 1.500 m3/day. Sand-clay separator should be installed at the inlet of it to protect the separator and tanks from sand-clay mixed sediments. Sand-clay separators should be installed inside the overflow tanks for better separation of formation water and sand-clay mixtures from oil, oil suspensions and sand-clay mixtures from water in formation water storage tanks. In order to reduce evaporation losses in the tanks, an auxiliary palte should be used under the PSV and the gas phase of the technological tanks and commercial oil tanks should be connected via pipes.

Influence of Sand Grain Size and Clay Mineralogy on Suffusion of Sand-Clay Mixtures

Suffusion is a type of internal erosion caused by the detachment of fine particles. Additionally, continuous suffusion reduces the stability of geotechnical infrastructures. In this study, the impact of clay mineralogy and sand grain size on the suffusion of sand-clay mixtures was investigated using two-dimensional laboratory experiments on nine different sand-clay mixtures made of three types of sands (K3, K4, and K5) and clays (kaolinite, illite, and bentonite). The breakthrough curve for each sample was obtained for the top/middle/bottom outlets of the designed cell to measure and analyze the amount of detached clay. It was observed that as the sand grain size decreased, the amount of detached clay was more and clays with kaolinite exhibited greater suffusion than those with illite. Bentonite exhibited the lowest amount of detached clay owing to its high swelling potential. The influence of clay mineralogy and sand grain size on the suffusion has been discussed based on the obtained breakthrough curves.

Flow index-liquid limit relationship by fall-cone tests in clay-sand mixtures

The Fall-cone test is a useful method which can be used in evaluation of many soil index and strength identifiers. In this regard, the flow index is calculated as the slope of the plot of water content against natural logarithm of the penetration depth. This is also a fruitful method used to determine the liquid limit of soils, and evaluation of test results may lead to many parameters related to soil plasticity, i.e. using the flow index. In this study, the changes in the liquid limit and plasticity index of a soil as a function of the flow index were investigated. Later, a simple correlation between the plasticity index and the flow index was obtained, and as a result, the plasticity index was directly calculated. The Fallcone test results show that there is an excellent relationship between the plasticity index and the flow index in soils within a wide range of plasticity level and this relationship is significantly affected from mineralogical properties of components of sand-clay mixtures. A linear relationship with R2 = 0.97 (n = 44) was obtained. As a result, both the liquid limit and the plasticity index of the soils are calculated directly based on the results of the Fall-cone test. In addition, this study compared the results of the standard plasticity chart with the new plasticity chart proposed by Vardanega et al. (2022) for the classifying fine-grained soils. It was shown that the new plasticity chart proposed by Vardanega et al. (2022) can also be used in the classification of both fine-grained soils and clay-sand mixtures. The new plasticity chart by Vardanega et al. (2022) provided modifications on standard plasticity chart. The usefulness of the new chart is also verified by the results of this study, experimental results showed that soil classes -for 115 specimens of 125 total- obtained using Fall-cone tests are in accordance with classifications proposed by Vardanega et al. (2022). According to experimental results, the highest flow index was obtained in %100 bentonite soil at 146.47, while the lowest flow index at 7.20 was obtained in a mixture of 90% sand (SW) −10% bentonite. In 100% pure kaolin clay, the liquid limit was 56.09%, the plasticity index was 27.52% and the flow index was 34.55.

Effect of Dynamic Loads on the Long-Term Efficiency of Liner Layers

The liner layers of natural sand-clay mixtures are extensively used in a range of geotechnical and geoenvironmental projects. In many of these projects, these layers are exposed to dynamic loads or waves due to natural seismic earthquakes or due to human activities, such as machine vibrations, traffic repeated loads, and other impact loads. The permeability of liners is a key property and should be adequately designed to sustain these loads during their lifetime of serviceability. This study sought to evaluate the possible effects of dynamic loads on the efficiency of two different sand-expansive clay (SCL) liners during their lifetime. This was achieved through experimental tests for two series; the first series was subjected to dynamic loads (up to 500 cycles) using the triaxial dynamic system and then tested for permeability for a long period of up to 100 days. The permeability results were compared with the second series tested without being subject to dynamic loads. The dynamic properties for both liners, such as the shear modulus (G), damping ratio (D), and degradation index (δ) were determined and discussed. The results indicated that both materials showed significant degradation with an increase in cycles of dynamic loads; most of the degradation took place during the first 100 cycles. In consequence, the dynamic loads induced a significant effect on the performance of the liners during their lifetime (reducing the permeability by more than six times). These effects were time-dependent and should be taken into account during the design phases.

Quantitative clay mineralogy predicts radiocesium bioavailability to ryegrass grown on reconstituted soils

Current radiocesium (137Cs) models to evaluate the risk of 137Cs transfer from soil to plants are based on the clay and exchangeable potassium (K) contents in soil. These models disregard the mineralogy of the clay fraction and are likely not capable of accurately predicting the 137Cs transfer factor (TF) in soils of contrasting parent rocks and weathering stages. The objectives of this study were to test that hypothesis and to identify whether quantitative information on mineralogy can improve the predictions. A pot cultivation experiment was set up with clay-sand mixtures in single and double clay doses that were fertilized, spiked with 137Cs and grown with ryegrass for 30 days. Four clays (illite, biotite, smectite and vermiculite) along with six deposits from clay-rich geological units were compared. The TF generally decreased with increasing clay dose for each of these ten different clay groups, however, the TF varied two orders of magnitude across clay groups and doses. The TF was highest for clays with little 137Cs specific sites such as bentonite and/or where the exchangeable K content was low compared to the other clays. The TF was well predicted from the soil solution 137Cs and K concentrations (R2 = 0.72 for log transformed TF), corroborating earlier findings in natural soils. The TF (log transformed) was statistically unrelated to total phyllosilicate content or 1:1 and 2:1:1 type phyllosilicate content while it significantly decreased with increasing 2:1 phyllosilicate content (R2 = 0.32). A multiple regression model with four different X-ray diffraction (XRD) based phyllosilicate groups yielded the strongest predictive power (R2 = 0.74). We conclude that XRD quantification is valuable for describing 137Cs bioavailability in plant substrates. These findings now await confirmation for natural soils.

Equivalent Compression Curve for Clay–Sand Mixtures Using Equivalent Void-Ratio Concept

Clay–sand mixtures are deposited worldwide, such as naturally sedimentary soils and dredged clays in human activities. These mixtures usually have a fine fraction above the transitional fine content, and the compressibility is dominated by clay matrix and affected by sand fraction. In this paper, an explicit equivalent compression curve is proposed for clay–sand mixtures. This is done by directly incorporating the equivalent void ratio into the intrinsic compression model of the clay matrix. The roles of initial and evolving soil structure developed during the compression process are described by a structural variable, which can be directly computed from the experimental data. Experimental data compiled from various literature are used for evaluating the accuracy of the suggested model. They indicate that the proposed relationship of stress versus equivalent void ratio is simple yet effective to evaluate the compressibility of clay–sand mixtures with a wide spectrum of sand fraction. The model is also practically useful for estimating the compressibility of other gap-graded soils, such as clay–gravel mixtures. The proposed stress versus equivalent void-ratio equation is an elementary function and can be readily implemented into general elastoplastic models.