Desorption of zinc by kaolin from Suzhou, China

Heavy metals in soils may adversely affect environmental quality. This paper studied the influence of copper concentration, pH, temperature, and the ratio of solid to solution on the desorption of Zn in kaolin from Suzhou, China in a background solution of 0.01M CaC12 by batch extraction experiments. At 0, 5, 50, 100 mg/L Cu concentrations, for each 0.5, increase in pH between about 1.80 and 3.04 percent desorption decreasing by 3.80%, 13.87%, 9.97%, and 7.65%, respectively. The pH 50 （ pH at 50% Zn desorption） was found to follow the sequence of Cu （5mg/L, pH 50=2.60）.     

Effect of sand grain size and boundary condition on the swelling behavior of bentonite–sand mixtures

Deep geological repository is a favorable choice for the long-term disposal of nuclear wastes. Bentonite–sand mixtures have been proposed as the potential engineered barrier materials because of their suitable swelling properties and good ability to seal under hydrated repository conditions. To investigate the effects of sand grain size on the engineering performance of bentonite–sand mixtures, we prepare five types of bentonite–sand mixtures by mixing bentonite with sand of varying particle size ranges (0.075–0.25 mm, 0.25–0.5 mm, 0.5–1 mm, 1–2 mm and 2–5 mm, respectively). We carry out sequential oedometer tests under different simulated repository conditions, including constant vertical stress (CVS), constant stiffness (CS) and constant volume (CV) conditions. The microstructural heterogeneity and anisotropy of these soil mixtures are characterized through the quantitative analysis of micro-CT scanning results. Experimental results reveal that both sand grain size and boundary condition significantly influence the swelling of soil mixtures. Under three conditions, the temporal evolutions of swelling stress and strain follow similar trends that they increase faster at the beginning and gradually stabilize afterward. Comparing the ultimate values, swelling strains follow CVS?>?CS?>?CV, while swelling stresses follow CV?>?CS?>?CVS. Under CS boundary conditions, as the stiffness coefficient increases, the swelling pressure increases and the swelling strain decreases. CT results further indicate that mixtures with larger sand inclusions are more structurally heterogeneous and anisotropic, resulting in increased inter-particle friction and collision and a higher energy dissipation during the swelling process. Moreover, the non-uniform distribution of bentonite in local zones would be intensified, which plays an important role in compromising swelling behavior. Therefore, soil samples mixed with larger sand particles present a smaller swelling stress and strain values. This study may guide the choice of engineered barrier materials toward an improved design and assessment of geological repository facilities.

     

Swell–consolidation characteristics of artificial sand clay mixes

Many remedial measures have been devised to lessen the damage caused by expansive soils. Physical alteration, chemical stabilization, innovative foundation techniques like belled piers, drilled piers, under-reamed piles and granular pile anchors are some of these remedial measures. Mixing a non swelling material such as gravel or sand to expansive soil is one of the methods of physical alteration. This paper presents experimental data on artificially prepared sand-clay mixes. Swell and consolidation characteristics of these artificially prepared sand-clay mixes were studied in one dimensional consolidometer. Fine sand content and fines content in the expansive soil were arbitrarily varied in the investigation. The fines content was varied as 425–300 μm and 150–75 μm, separated from the same expansive soil based on the grain size. Swell potential and swelling pressure decreased with increasing fine sand content but increased with increasing fines content. Coefficient of compressibility, coefficient of volume compressibility and compression index of the samples decreased initially up to a sand content of 15% and thereafter increased at higher sand contents.     

Influence of the saffman force,lift force,and electric force on sand grain transport in a wind–sand flow

Quasi-horizontal trajectories of salting sand grains were found using high-speed video-recording in the desertified territory of the Astrakhan region. The sizes and displacement velocities of the saltating sand grains were determined. A piecewise logarithmic approximation of the wind profile in a quasi-stationary wind–sand flow is suggested, which is consistent with the data of observations and modeling. It was established that, in the regime of stationary saltation, the wind profile in the lower saltation layer of the wind–sand flow depends only slightly on the wind profile variations in the upper saltation layer. The vertical profiles of the horizontal wind component gradient in a quasi-stationary wind–sand flow were calculated and plotted. It was shown using high-speed video recording of the trajectory of a sand grain with an approximate diameter of 95 μm that the weightlessness condition in the desertified territory of the Astrakhan region in a stationary wind–sand flow is satisfied at a height of approximately 0.15 mm. The electric parameters of a wind–sand flow, which can provide for compensation of the force of gravity by the electric force, were estimated. In particular, if the specific charge of a sand grain is 100 μC/kg, the force of gravity applied to the sand grain can be compensated by the electric force if the vertical component of the electric field in a wind–sand flow reaches approximately 100 kV/m. It was shown that the quasi-horizontal transport of sand grains in the lower millimeter saltation layer observed in the desertified territory can be explained by the joint action of the aerodynamic drag, the force of gravity, the Saffman force, the lift force, and the electric force.     

Swelling pressure–suction relationship of heavily compacted bentonite–sand mixtures

This paper presents results of an experimental work to determine a relationship between swelling pressure and suction of heavily compacted bentonite–sand mixtures. For comparison, tests were also carried out on heavily compacted bentonite specimens. A series of swelling pressure tests were performed using multi-step constant-volume method where suction of the specimens tested was reduced in a stepwise manner toward a zero value. The suction reduction was induced using vapor equilibrium and axis-translation techniques. It is shown that compacted specimens did not exhibit any collapse upon suction decrease and exhibited maximum swelling pressures at zero-equilibrium suction. The development of swelling pressure with decreasing suction of the specimens showed threshold suctions below which a further reduction in suction yields an increase in the swelling pressure of the same magnitude. The magnitude of threshold suction was found to be a function of bentonite content in compacted specimens.     

A new generalized soil thermal conductivity model for sand–kaolin clay mixtures using thermo-time domain reflectometry probe test

As the demand of exploitation and utilization of geothermal energy increases, more geothermal-related earth structures occur recently. The design of the structures depends upon an accurate prediction of soil thermal conductivity. The existing soil thermal conductivity models were mostly developed by empirical fits to datasets of soil thermal conductivity measurements. Due to the gaps in measured thermal conductivities between any two tested natural soils, the models may not provide accurate prediction for other soils, and the predicted thermal conductivity might not be continuous over the entire range of soil type. In this research, a generalized soil thermal conductivity model was proposed based on a series of laboratory experiments on sand, kaolin clay and sand–kaolin clay mixtures using a newly designed thermo-time domain reflectometry probe. The model was then validated with respect to k _dry–n (thermal conductivity of dry soils and porosity) and k _r–S _r (normalized thermal conductivity and degree of saturation) relationships by comparing with previous experimental studies. The predicted thermal conductivities were found to be in a good agreement with the experimental data collected from both this study and the other literatures with at least 85% confidence interval. It is concluded that the proposed model accounts for the effects of both environmental factors (i.e., moisture content and dry density) and compositional factors (i.e., quartz content and soil type) on soil thermal conductivity, and it has a great potential in predicting soil thermal conductivity more accurately for geothermal applications.     

Shear strength and compressibility of oyster shell–sand mixtures

This paper investigates the fundamental characteristics of shear strength and deformation of crushed oyster shell–sand mixtures to stimulate recycling of waste oyster shells. Standard penetration tests (SPT) and large-scale direct-shear tests were carried out with different kinds of dry unit weight and mixing rate of oyster shell–sand mixture. Correlations between N-value, dry unit weight, and friction angle of mixtures were observed from the results of experimental tests, making it possible to estimate the in situ strength from SPT, and the coefficient of volume compressibility from the confined direct-shear compression test. These results also make it possible to compute the settlement of oyster shell–sand mixture when used in soft ground improvement.     

Effect of wetting–drying cycles on swelling behavior of lime stabilized sand–bentonite mixtures

In the present study, influence of wetting–drying cycles on swelling pressures of sand–bentonite mixtures used in the construction of sanitary landfills to have an impermeable liner was investigated before and after lime treatment of the mixtures. Swelling pressure tests were conducted to see if the swelling pressures were affected by wetting–drying cycles. First series of specimens were prepared as a mixture of sand and bentonite only. In the first series of specimens, sand was mixed with bentonite in various proportions with their optimum water contents and compacted by using standard proctor energy. In the second series of the specimens, lime in various proportions was added to the mixtures of sand–bentonite. Then, the sand–bentonite mixtures stabilized by lime were compacted with the standard proctor energy at their optimum moisture contents. Five wetting–drying cycles were performed on each specimen and values of swelling pressures were measured at the end of each cycle. Results of swelling pressure tests indicated that the swelling pressure is decreased when lime is added to the mixtures. In addition, decrements were observed on swelling pressures by wetting–drying cycles. The results of the experiments of this investigation showed that the beneficial effect of lime stabilization to control the swelling pressures was partly lost by the wetting–drying cycles. However, the test results indicated that the swelling pressures of the specimens made of sand–bentonite mixtures stabilized by lime were lower than the swelling pressures of the specimens made of only sand–bentonite mixtures.     

Undrained monotonic response of sand–silt mixtures: effect of nonplastic fines

The effect of non-plastic fines (silt) on the undrained monotonic response of saturated and isotropically consolidated sand specimens prepared to various measures of their density was studied in detail through various approaches namely gross void ratio approach, relative density approach, sand skeleton void ratio approach, and interfine void ratio approach. Specimens of 50 mm in diameter and 100 mm in height were tested at a rate of loading of 0.6 mm/min for this purpose. The limiting silt content and the relative density of a specimen were found to influence the undrained monotonic response of sand–silt mixtures to a great extent. Undrained monotonic response was observed to be independent of silt content at very high relative densities; however the presence of fines significantly influenced this response of loose to medium dense specimens. Individual and combined analyses of undrained monotonic peak strengths which are closely related to the liquefaction related problems have been done in this paper to assess the variation patterns.     

Laboratory investigation and constitutive modeling of the mechanical behavior of sand–GRP interfaces

Acta Geotechnica - The frictional behavior of soil–pipe interfaces plays a paramount role in the design and construction of underground pipes, particularly applying trenchless technologies...     

Impact of pore fluid chemistry on the thermal conductivity of bentonite–sand mixture

Thermal conductivity is an important parameter to consider when designing clay-based barriers for use in deep geological repositories (DGR). In the DGR environment, the infiltration of local saline groundwater can potentially change the pore fluid chemistry of a barrier over its lifetime. This change in chemistry is known to alter the thermal properties of the barrier materials. In order to examine the impact of pore fluid salinity on thermal conductivity, experiments were conducted under both distilled water and saline pore fluid conditions. The material mixtures were prepared at two different dry densities using two different salt types. Furthermore, five different thermal conductivity prediction models were selected and evaluated on their performance with respect to the experimental outcomes. In general, these results indicated that an increase in the constituent pore fluid’s salt concentration leads to a decrease in the thermal conductivity of the material. Additionally, the thermal conductivity values of the materials prepared at a high dry density were greater than of those compacted at a low dry density.     

A modification to dense sand dynamic simulation capability of Pastor–Zienkiewicz–Chan model

A modification to the nonlinear Pastor–Zienkiewicz–Chan (PZC) constitutive model without any change in the number of model parameters is introduced in order to simulate stiffness degradation of dense sands at dynamic loading. The PZC model is based on generalized plasticity and was verified by good prediction of liquefaction and undrained behavior of saturated sand. The PZC is a robust model that can predict drained dynamic behavior of sands, especially stiffness increase in loose sand at reloading of dynamic loading. Yet, this model does not show stiffness degradation of dense sand at reloading. The modification is made through modifying the stress memory factor, H _DM, which is multiplied by the plastic modulus, H _L. This modification does not influence reloading behavior of loose sand. The modified PZC model is verified via results of drained cyclic tests. Two cyclic triaxial tests on loose and dense specimens, along with two cyclic plane strain tests on dense sand are utilized for validation. The model simulation shows that the modified PZC model is able to predict the stiffness degradation of dense sand at reloading well.     

The results of application of the VES-IP method during the study of sand–gravel mixes in Mosalsk district,Kaluga oblast

Vertical electrical sounding (VES) is a traditional method of exploration of sand–gravel mix (SGM) deposits. Geophysicists are challenged not only to determine the boundaries of SGM deposits, but also to determine their basic properties, for example, to locate the zones with high gravel contents within SGM deposits or determine the content of clay particles in sands. In order to meet these challenges, it is necessary to use new methods, in particular, the method of induced polarization (IP). This paper presents the results of direct-current VES-IP surveys that were carried out in the territory of Mosalsk district, Kaluga oblast, in 2015. The measurements were made at both known SGM deposits that are under development and new areas under prospecting where drilling has not been performed as yet. The study results show that all SGM deposits are characterized by high values of induced polarization. The complex adjustment of the apparent resistivity and apparent polarizability curves significantly confine the limits of equivalent models when interpreting the data in complex geological situations when sand-and-gravel deposits weakly differ from the host rocks or are superimposed with a thick layer of conducting clay loams.     

Interpretation of grouting characteristics in unsaturated sand from the perspective of water–air interface

The design of grouting engineering in practice is either based on conventional soil mechanics or empirical procedures ignoring the effect of degree of saturation (water content). In this study, a series of laboratory-pressurized grouting tests were conducted on unsaturated sand to reveal the influence of soil water content on the grouting characteristics. With combination of direct shear tests at constant water content, water retention tests as well as microscopy observations, the mechanisms that controlling the strength and in turn the grouting characteristics in unsaturated sand were interpreted from the perspective of water–air interface. It was found that the non-monotonic phenomena of grouting characteristics (injectability and diffusion characteristics) with increasing water content were strongly dependent on the shear strength, which is influenced by the apparent cohesion induced by capillary mechanisms relating to the water–air interface. The threshold value of the injectability and diffusion pattern is corresponding to the boundary of the two transition zones (two different desaturation mechanisms) in the water retention curve. In the primary transition zone, the water phase is interconnected with air bulbs entrapped. With the drainage of bulk water in the large pores, the amount of water menisci increases, generating larger and larger surface tension force between particles. Therefore, less and less grout was injected as the bearing capacity and shear strength increase. However, in the second transition zone, with the drainage of menisci water, the menisci area of each pores decreases, inducing less and less surface tension force. Thus, more and more grout was injected as the bearing capacity and shear strength decrease. It is hoped that the work in this study will facilitate researching the grouting mechanisms in unsaturated soil, thus optimizing the grouting parameters in engineering practice.

     

A laboratory study of the self sealing behaviour of a compacted sand–bentonite mixture

Bricks made of compacted sand–bentonite mixture are considered as a possible engineered barrier to isolate high-level radioactive waste at great depth. This work is aimed at investigating some specific effects related to the presence of discontinuities at the contact between the bricks and the excavation wall. In order to do this, an experimental device was developed in the laboratory. The model is made up of a specially designed infiltration cylinder which allows the precise definition of a planar discontinuity between the compacted specimen (a sand–bentonite mixture made up of sand and Kunigel clay from Japan) and a metal wall. During hydration and subsequent specimen swelling, the planar wall is filled, resulting in a healing process. Three total pressure gauges placed along the wall allow a detailed observation of the increase in total stress against the wall. After different periods of swelling, the maximum resistance of the specimen–wall interface to pressure was tested by imposing a pressure increase through a porous stone placed at one end of the cylinder. It was found that the maximum pressure supported by the interface is a function of the initial thickness of the discontinuity and the initial density of the specimen. It was also found that the maximum sustainable pressure depends linearly on the elapsed time. These results are of interest for optimizing water infiltration procedures in either mock-up tests or real disposal systems. If the maximum sustainable pressure at the interface is known, it is possible either to ensure homogeneous hydration of a mass of bricks by respecting the maximum injection pressure limit or to accelerate hydration by forcing water paths along the discontinuities.     

Mesoscopic pullout behavior of geosynthetics–sand–clay layered reinforced structures using discrete element method

Acta Geotechnica - The geosynthetics–sand–clay layered reinforced (GSCLR) structure has wide application prospects due to its massive adoption of the clay. To date, the coordination...     

The microstructure and internal architecture of shear bands in sand–clay sequences

The microstructures of cm-scale displacement faults offsetting unlithified sequences of finely interbedded sands, silts and clays from outcrops in Denmark have been examined. A variety of shear band types are recognised based on their grain-scale deformation mechanism and internal structure. Shear bands in a Jurassic sequence exposed along the coastline of Bornholm are characterised by intense cataclasis of both sand and clay layers. This deformation mechanism is accompanied by extensive grain scale mixing along discrete shear bands to give a fault rock composition that reflects the relative amount of sand and clay within the faulted sequence. In contrast, shear bands at Nr. Lyngby and Jensgaard, both on the Jutland coast, are characterised by granular flow within the sand units. Grain scale mixing is subdued at these locations so that layers maintain their integrity across the shear band to form a layered internal structure of sand, silt and clay smears. In some instances, particularly at Nr. Lyngby, clays have deformed in a brittle manner so that they do not contribute material to the shear band, which is then comprised exclusively of coarser-grained components. The different deformation mechanisms and internal structures of shear bands are thought to be controlled by burial depth at the time of faulting.     

Impact of wetting–drying cycles on the hydraulic conductivity of liners made of lime-stabilized sand–bentonite mixtures for sanitary landfills

In this study, an investigation was performed to determine if lime-stabilized sand–bentonite mixtures are appropriate for the construction of sanitary landfills liners. For this aim, the hydraulic conductivity tests were conducted in the laboratory on sand–bentonite mixtures and lime-stabilized sand–bentonite mixtures to evaluate the effect of wetting–drying cycles. The hydraulic conductivity tests were performed to see if their hydraulic conductivities are affected by wetting–drying cycles. First series of specimens have been prepared as a mixture of sand and bentonite only. In the first series of specimens, sand was mixed with bentonite in proportions of 20, 30, 40, and 50 %. In the second series of the specimens, lime in proportions of 1, 2 and 3 % by weight was added to the mixtures of sand–bentonite in proportions of 20, 30, 40, and 50 %. From the results of the tests, it was observed that while optimum water content increased, maximum dry density decreased with addition of lime to the sand–bentonite mixtures. Generally, the hydraulic conductivity increased with the addition of lime to the mixtures but at low percentages of lime (1–2 %), however, slight decreases in k were recorded. It was also observed that the wetting–drying cycles on the permeability test indicate cure effect on specimens with addition of lime which resulted in decreased the hydraulic conductivity.