Sand-clay Mixtures Research Articles

Experimental investigation of the strength properties of sand–clay mixtures reinforced with randomly distributed discrete polypropylene fibers

ABSTRACT: This paper describes the findings of a series of laboratory unconfined compression tests and split tensile tests performed on specimens of varying fiber–sand–clay proportions. Sand was mixed with clay at six different proportions, namely 0.0, 1.0, 2.5, 5.0, 7.5, and 10.0% by dry weight of clay. Fiber was included in the sand–clay mixture at four different percentages, namely 0.0, 0.25, 0.5, and 1.0% by dry weight of clay. Compaction characteristics of the sand–clay mixtures were evaluated at standard compaction energy. According to fiber content, four groups of samples were prepared at maximum dry unit weight and optimum water content. For each of the four sample groups, unconfined compressive stress-axial strain and split tensile stress-diametrical strain behavior of the fiber–sand–clay mixtures were investigated. The test results showed that unconfined compressive strength (UCS) reached its maximum value at a fiber/clay ratio of 1.0% and sand/clay ratio of 7.5%. There was a reasonably strong linear correlation (R2≈ 1.0) between UCS and average initial tangent modulus values. The ratio of UCS to split tensile strength (STS) for all fiber–sand–clay mixtures varied between 3.2- and 5.8-fold.

Abiotic factors affecting the infectivity of Steinernema carpocapsae (Rhabditida: Steinernematidae) on larvae of Anastrepha obliqua (Diptera: Tephritidae)

The effects of soil depth, soil type and temperature on the activity of the nematode Steinernema carpocapsae (Filipjev) were examined using larvae of the West Indian fruit fly, Anastrepha obliqua (Macquart). Bioassays involved applying infective juveniles (IJs) to the surface of sterilized sand in PVC tubes previously inoculated with fly larvae of two ages. The 50% lethal concentration (LC50) values estimated for 6-day-old larvae were 9, 20 and 102 IJs/cm2 in tubes containing 2, 5 and 8 cm depth of sand, respectively, whereas for 8-day-old larvae, LC50 values were 16, 40 and 157 IJs/cm2, respectively. The effect of soil texture on the activity of S. carpocapsae was tested by applying the corresponding LC50 concentrations of nematodes to sand, sand–clay and loamy–sand soils. For 6-day-old larvae, soil type had a highly significant effect on infection with the highest percentages of infection observed in the sand–clay mixture (60–82% depending on depth) compared to 45–64% infection in sand and 23–30% infection in loamy–sand soil. A very similar pattern was observed in 8-day-old larvae except that infection rates were significantly lower than in younger larvae. There was a significant interaction between soil type and soil depth. The effect of three temperatures (19, 25 and 30°C) on infection was examined in sand–clay soil. The infectivity of S. carpocapsae was affected by temperature and soil depth and by the interaction of these two factors. Response surface analysis applied to second order multiple linear regression models indicated that the optimal temperature for infection of larvae of both ages was ~26°C, at a depth of 7.9 cm for 6-day-old larvae and <2 cm for 8-day-old larvae, resulting in a predicted 91.4% infection of 6-day-old larvae and 61.2% infection of 8-day-old larvae. These results suggest that S. carpocapsae may have the potential to control fruit fly pests in tropical ecosystems with warm temperatures and high soil moisture levels, although this assertion requires field testing.

Engineering characteristics of sand-clay mixtures used for clay cores of earth-fill dams

AbstractClay is the main construction material for clay cores of earth-fill dams. Clay minerals swell when they become wet and shrink when they dry out; cracks develop as they lose moisture. If precautions are not taken to prevent seepage through these cracks, dam failures may result. In this study, sand was added to montmorillonite-dominant clay soils to investigate the effect of sand-inclusion rates on the engineering characteristics of clay soils used in the construction of clay cores of earth-fill dams. Changes in the consistency limits, compaction characteristics, permeability, stress-strain relationships and swelling characteristics with increasing sand inclusion rates were evaluated. Based on the results from experimental trials, a 30% sand inclusion rate appears to be the optimum proportion; most of the swelling occurred in the voids of grains and led to permeability levels below the allowable limits for earth-fill dams.

Percolation Threshold of Sand-Clay Binary Mixtures

Many poorly graded granular materials of engineering importance can be characterized as gap-graded binary mixtures. Such mixtures display a volume-change response at a threshold value of the coarse fraction that is reminiscent of systems described by percolation theory. An experimental investigation on a sand-clay mixture is presented that clearly displays threshold behavior and sheds light on the role that each soil fraction plays in transferring loads through the medium. There are two key effects. First, an analysis of void ratio of the interpore clay fraction for varying compaction energies reveals an abrupt reduction in clay density at the threshold fraction of sand, whereby it is virtually impossible to impart compaction on the clay fraction at sand contents exceeding this threshold. Second, although force chains cannot be observed directly, analysis of the sand in terms of its component void ratio, computed based on treating the clay as part of the void space, shows that the sand carries a majority of the load at component void ratios that are too high to form stable force chains. The traditional interrelationship between mean stress and void ratio based on critical state theory breaks down when the sand content nears its threshold fraction. When the sand content is near the threshold limit, increasing mean stress results in a greater dilative tendency. Results are compared with findings on consolidation of sand-bentonite mixtures, and so-called reverse behavior of sand-silt mixtures.

Determination of Selected Geotechnical Properties of Soil Using Electrical Conductivity Testing

Abstract The use of electrical conductivity measurements, at relatively low frequencies, has been shown to be an effective tool for characterizing soils for hydrogeological studies. Many of the properties that affect the hydraulic and mechanical behavior of a soil also affect the electrical response. Thus, there is a likelihood that electrical measurements of soils will provide useful information for predicting geotechnical parameters. This paper presents the results of efforts to develop an electrical conductivity testing system that can be used specifically to evaluate geotechnical properties of soils. The testing system consisted of a robust data acquisition and control system that allowed for autonomous testing of various sand-clay mixtures and a testing apparatus that was rugged enough to allow soil samples to be compacted directly into the cell. The testing apparatus utilized a multi-electrode configuration which facilitated the investigation of anisotropic electrical measurements of the compacted soil samples. The data obtained during the evaluation phase of this research showed that low-frequency electrical conductivity measurements are viable for evaluating and predicting geotechnical properties of soils such as void ratio and volumetric water content, and the use of the multi-electrode configuration is very promising for evaluating anisotropy in soils.

Internal Flow Effects on Isotropic Confined Sand-Clay Mixtures

Under the effect of internal flows, a liner can undergo a washing out of particles, which modifies the particle size distribution and affects hydraulic, chemical and mechanical characteristics. This paper discusses the effects of internal flows on sand/kaolin mixture, in terms of rate of erosion and modification of the hydraulic conductivity. A parametric study is conducted with a specific device that consists of three modified triaxial cells. These cells allow isotropically consolidating and confining specimens, they prevent a parasitic flow and survey large deformations of specimen. The tests reveal that suffusion of clay is accompanied by a clogging in the specimen that induces a drop in hydraulic conductivity. For high gradients the erosion of clay is accompanied by the backward erosion of sand and finally the specimen collapses. The erosion rate then depends on the values of the different parameters considered (hydraulic gradient, clay content and filter pore opening size).

Dielectric characteristics of soils and categories of soil water

The complex permittivity of the Ap horizons of a leached chernozem, gray forest soil, and soddy-podzolic soil, as well as those of kaolin and its mixtures with sand, was studied by the capacitance method. The relationships between the characteristics of losses (the imaginary component ɛ″ and the loss tangent tan δ) and the water content (W) are more indicative of changes in the water categories than the analogous relationships of the real component (ɛ′), which characterizes the relaxation capacity of water molecules. The breakpoints on the curves tan ɛ′ = f(W) and ɛ″ = f(W) are more pronounced compared to the curve ɛ′ = f(W) and better agree with the soil-geological constants: tan δ = f(W) at the W value close to the wilting point (WP) for all soils and at the maximum water adsorption capacity (MWAC) and maximum capillary sorption capacity (MCSC) for kaolin and its mixtures; ɛ″ = f(W) at the MWAC, WP, and maximum molecular water capacity (MMWC) for the leached chernozem and soddy-podzolic soil. On the curve ɛ′ = f(W), breakpoints were observed only for the gray forest soil and sand-clay mixtures.

The casting core composition and provenance of the Goljamata Kosmatka (Bulgaria) bronze head

The composition of casting core residues from the interior of a 4th century BC bronze head, unearthed in a Thracian tomb of central Bulgaria, was studied by means of petrographic, SEM, chemical and XRD analyses. Results indicated that the sand–clay mixture is dominantly rich in rock and mineral fragments, mostly derived from metamorphic and plutonic silica-rich rocks. No significant compositional differences were noted between the five analysed samples, except for content of bronze impurities and oxidation products. On the basis of geological studies and of chemical comparison with potteries of various ages, speculations were made about the location of the bronze foundry. The results are compatible with a derivation of the casting core from northeast Greece and the southern areas of F.Y.R.O. Macedonia and Bulgaria have terrains compatible with the casting core composition

Undrained Behavior of Compacted Sand-Clay Mixtures Under Monotonic Loading Paths

Undrained Behavior of Compacted Sand-Clay Mixtures Under Monotonic Loading Paths

Effect of Penetration Rate on Cone Penetration Resistance in Saturated Clayey Soils

In this paper, the effects of penetration rate on cone resistance in saturated clayey soils are investigated. Shear strength rate effects in clayey soils are related to two physical processes: the increase of shear strength with increasing rate of loading and the increase of shear strength as the process transitions from undrained to drained. Special focus is placed on this second effect. Cone penetration tests were performed at various penetration rates both in the field and in a calibration chamber, and the resulting data were analyzed. The field cone penetration tests were performed at two test sites with fairly homogeneous clayey silt and silty clay layers located below the groundwater table. Additionally, tests with both cone and flat-tip penetrometers in sand-clay mixtures were performed in a calibration chamber to investigate the change in drainage conditions from undrained to partially drained and from partially drained to fully drained. A series of flexible-wall permeameter tests were conducted in the laboratory for various clayey sand mixtures prepared at various mixing ratios in order to obtain values of the coefficient of consolidation, which is required to estimate the penetration rates below which penetration is drained and above which penetration is undrained. A correlation between cone resistance and drainage conditions was established based on the results of the calibration chamber and field penetration tests.

Wetting Solution and Electrical Double Layer Contributions to Bulk Electrical Conductivity of Sand–Clay Mixtures

This study investigated the contributions of the wetting liquid and electrical double layers (EDLs) to the bulk electrical conductivity (σ) of sand–clay mixtures. The conductivity is small at small water content due to few conductive pathways and low ionic mobility. The fully expanded state of the EDLs provided the largest σ in distilled water but not in salt solutions. At small water content, σ increases following a logarithmic function, but changes linearly at large water content after reaching a transitional large value. This conductivity increases with increasing electrical conductivity of the wetting solution (σw) exponentially at small water content, but linearly at large water content. Existing bulk electrical conductivity models described the measured σ values well and resulted in stable values of the electrical formation factor (F) and dimensionless parameter. Due to considerable leach cations in the clay, the cations in the bulk solution exerted negligible influence on the surface conductivity (σs). Although F is independent of σw, the confounding effect of shrink–swell in the samples with changing σw shows an apparent dependency of F on σw The surface mobility of the counterions decreased nonlinearly with increasing water‐filled porosity (ϕ), and also with increasing σw The cementation exponent (m) decreased monotonically from −4.4 at ϕ = 0.22 to −94.1 at ϕ = 0.93 in the samples wetted with distilled water. With increasing σw, m also increased. The value of m for three samples wetted with salt solutions of σw = 0.878, 1.273, and 1.572 S m−1 ranged from 2.1 to 2.5 at ϕ = 0.22 and 4.1 to 5.3 at ϕ = 0.93.

Initial Shear Modulus of Remolded Sand-Clay Mixtures

This paper presents the dynamic properties of undisturbed and remolded clays and sand-clay mixtures based on the results of undrained hollow cylindrical torsional cyclic simple shear tests. The clays were obtained from different sites having a wide range of plasticity, covering fluvial and marine sediments. The main focus of this investigation is the dynamic properties of sand-clay mixtures whose properties are intermediate between those of clays and sands. The initial shear moduli are examined in terms of fines content, and conventional plasticity index. An equivalent plasticity index has been defined in the present investigation and a better correlation is obtained between the initial shear modulus of sand-clay mixtures and this parameter than with the conventional plasticity index. Further, a simple method has been suggested to determine the equivalent plasticity index based on the conventional consistency limit tests and grain size analysis. An empirical correlation for predicting the initial shear modulus is also proposed.

Strain-Dependent Dynamic Properties of Remolded Sand-Clay Mixtures

A series of undrained cyclic torsional simple shear tests using hollow cylindrical torsional shear apparatus was carried out to investigate the dynamic shear moduli and damping properties of clayey specimens with various sand contents and plasticity indices. The clayey soils used were collected from various sites along the coast of west Japan. Among these clayey soils, a clay sample with intermediate plasticity and another with high plasticity were mixed with silica sand at different proportions in order to examine the dynamic properties of sand-clay mixtures. In addition, experiments were carried out on undisturbed and remolded natural clay specimens with various plasticities. The effects of plasticity, loading frequency and confining pressure on the strain dependent normalized shear modulus and damping ratio were examined. Based on the results, empirical correlations for predicting the normalized shear modulus and damping ratio of remolded sand-clay mixtures at various shear strain levels were proposed.

Non-reactive solute diffusion in unconfined and confined specimens of a compacted soil

The effect of specimen confinement on the determination of the effective diffusion coefficients, D*, for chloride, a non-reactive (non-adsorbing) solute, diffusing in a compacted soil was evaluated. The diffusion tests were performed by placing an acetic acid/sodium acetate buffer solution containing ZnCl2 (pH approximately 4.8) in a reservoir in contact with unconfined and confined specimens of a compacted sand-clay mixture for test durations of 7 or 14 d. The concentrations of chloride in the reservoir were measured as a function of time during the test, as well as a function of depth within the specimen at the end of the test. The resulting concentration distributions were analyzed using two models to Fick's second law for non-reactive solute diffusion in porous media, viz., (1) an analytical model assuming the porosity distribution could be represented by a single, weighted mean porosity and (2) a commercially available model, POLLUTE, that directly accounted for the measured porosity distribution. The D* for unconfined specimens based on the analytical model tended to be overestimated by a factor ranging from 1.13 to 1.59 relative to the D* using POLLUTE, whereas the D* values based on both methods for confined specimens typically were more consistent. In addition, the D* for unconfined specimens was greater than the D* for confined specimens when soil concentrations were used for the analysis, presumably due to the higher porosity for the unconfined specimens relative to the confined specimens. Analyses based on reservoir concentrations were inconsistent and contradictory in some cases, suggesting that the D* values based on soil concentrations were more reliable.

Geostatistical analysis of arsenic concentration in the groundwater of Malda district of West Bengal, India

The problem of arsenic (As) poisoning in the upper deltaic plain of the Ganga-Bhagirathi river system in the Bengal Basin of West Bengal, India is an alarming issue. Four blocks (Kaliachak-1, 2, 3 and English Bazar) of Malda district, West Bengal were critically studied. Geomorphologically, the area exhibits three terraces: the present Youngest terrace (T0-terrace), the Older Shaugaon Surface (T1-terrace) and the Oldest Baikunthapur Surface (T2-terrace). On the basis of numerous measurements, including As-content, pH, DO, specific conductivity and salinity, it was observed that maximum As-content beyond the permissible limit (0.05 mg/L, Indian standard) occurs within a depth range of 10–30 m with a non-linear distribution pattern. Variance test also found that a block effect was highly significant in an As-distribution pattern. Mean arsenic level of Kaliachak block-1 is 0.2253 mg/L, followed by Kaliachak-2 with arsenic level 0.1923, Kaliachak-3 with arsenic level 0.1755 and English Bazar with arsenic level 0.1324. The arsenious belt lies mainly within the Older terrace (T1). The very recent flood plain deposits of silvery white, fine sands lying very close to the Ganga River margin do not contain significant amounts of As. Elevated As-concentration in the ground water was observed in alluvial sands, grayish white to brownish in color and occurring away from the Ganga margin. The Oldest terrace (T2) further away from the Ganga margin (e.g. English Bazar) and Barind surface contains less arsenic. Barind surface acts as a hard capping with ferruginous sands and lateritic concretions-chocolate, mottled and purple brown in color-occurring northeast of the studied area. Arsenic content of ground water in the same locality within a radius of ∼ 20 m varies within wide limits. Thus, it poses problem to delineate its distribution pattern. Such a patchy occurrence possibly could not be explained satisfactorily solely by geomorphology. Chemical analysis of aquifer clay samples of the cores shows a maximum Ascontent of up to 3 mg/kg, whereas the bulk samples (sandclay mixture) of the cores contain a maximum of 17 mg/kg As-value. Therefore, it is not always true that clay contains elevated As-value.

A hydrogeophysical synthetic model generator

HGmod is a computer program that builds on stochastic realizations of porosity fields to derive electrical conductivity, dielectric permittivity and hydraulic permeability models. The presence of clay, the influence of salinity as well as temperature of the fluid of imbibition are taken into account in the underlying formulations. The saturated and unsaturated zones are also considered through the application of a saturation profile on the porosity field. A micro-geometrical model is used to relate the porosity to the clay fraction. This model also used to derive an expression for the pore specific surface of the sand–clay mixture. The specific surface is subsequently used to compute the conduction at the pore surface when building electrical conductivity models. Dielectric permittivity fields are built by successive applications of either the Hanai–Bruggeman or Maxwell–Garnett mixing models, depending on the relative proportions of sand, clay, water or air. In addition, the dielectric permittivity of water and clay follow a Cole–Cole behavior. HGmod is therefore a versatile tool useful to generate synthetic datasets needed to anticipate the geophysical response under specific conditions and to study hydrogeophysical sensitivity or resolution analysis.

Effects of Clay Content on Liquefaction Characteristics of Gap-Graded Clayey Sands

A series of undrained, cyclic simple shear tests were performed on reconstituted specimens with various clay contents to study the effects of clay content on liquefaction characteristics of clayey sands based on a framework of an idealized binary packing model and intergrain state parameters. From observed liquefaction characteristics, clayey sands with different clay contents can be grouped as sand-like or clay-like soils depending on the clay content and the transitional fines content of the sand-clay mixture. A simple equation is derived and verified to correlate the transitional fines content with the void ratios of the clean sand and the pure clay consisting of the mixture. In addition, a new relationship for clay content correction is proposed based on the linear relationship between the cyclic resistance ratio and the clay content at the same intergranular void ratio. The cyclic resistance ratio of sand-like clayey sands can be divided into two components: (1) the resistance of the sand skeleton at the specific intergranular void ratio, and (2) the increment of cyclic resistance from clayey fines. The rate of increment for cyclic resistance varies with the properties of contained clay particles. Data from three independent studies have shown the proposed procedure is promising.

Cyclic behaviour of soil-structure interfaces associated with modular-block reinforced soil-retaining walls

This paper describes the interaction behaviour of soil/geogrid and soil/concrete interfaces of a reinforced soil-retaining wall system under monotonic and cyclic loading conditions. A sand-clay mixture was tested with geogrids, which were manufactured from polyester and polyvinyl alcohol, and with concrete mortar in a modified direct shear device. In the cyclic tests, a total of 15 loading cycles or more were applied under constant displacement and by controlling the shear stress at a level of 56–74% of the monotonic shear strength. The test results showed that the strength of soil/geogrid interfaces was not affected significantly by cyclic loadings. At the end of cyclic loading, a larger stiffness and stress was mobilised in the reloading curve owing to the quasi-preconsolidated behaviour of the sand–clay mixture. For the soil/concrete interfaces, the interaction behaviour was different between the fresh and used (polished) specimens: significant strength reduction of up to 30% occurred in the used concrete specimens compared with the corresponding fresh specimens. The limited test results suggest that interface shear strengths obtained from monotonic loading may be relevant for designing reinforced soil-retaining walls subjected to earthquake loading.

Microbial synthesis of organic and condensed forms of phosphorus in acid and calcareous soils

The potential for microorganisms to affect the quantity and quality of organic and condensed forms of phosphorus (P) in soils was investigated by repeated addition of different carbon sources (glucose, starch, cellulose; 2.5 g C kg −1) with or without inorganic P (50 mg P kg −1) to acid and calcareous soils which were either natural soils or clay–sand mixtures free of organic matter. Forms of P after five amendments and subsequent incubation periods of 5 weeks each were analyzed by 31P solution nuclear magnetic resonance (NMR) spectroscopy, and the microbial community composition was assessed by selective plate counts and fatty acid methyl ester (FAME) analysis. All carbon additions induced a redistribution of P from inorganic to organic and condensed forms, which was only little affected by the addition of inorganic P. Compared to non-carbon-amended controls, the greatest increase (7–38 mg P kg −1) in organic P was observed in the monoester region. In the acid clay–sand mixture, there was a large accumulation of pyrophosphate (101 mg P kg −1) after glucose addition and smaller accumulations (6–25 mg P kg −1) after addition of starch and cellulose. Carbon additions increased the microbial biomass in all cases and except in the natural calcareous soil also the proportion of fungi. Redundancy analysis with Monte Carlo permutation tests revealed that for carbon-amended soils, the microbial community composition was more strongly influenced by soil type than by carbon source. Pyrophosphate was positively related to fungi, and diester P was positively related to soil pH. A large proportion of organic and condensed forms of P may still have been in microbial cells at the time of extraction. We have shown that soil organic P consists of some discrete and simple compounds along with some more complex forms, and that organic P recently synthesized by microbes consists almost exclusively of and thus is a likely source for the simple compounds found in natural soils.

Effects of Maghnian Bentonite on Physical Properties of Sandy Soils Under Semi-Arid Mediterranean Climate

This research has for object to study the influence of clay addition, i.e., Maghnian bentonite, like deposit clay, in the physical properties of sandy materials from Mostaganem plateau (North-West Algeria) submitted to salinity and sodicity. The first result was to show that the clay content changes drastically the physical properties of clay-sand mixtures. Important differences were observed as a function of the sand particle size distribution. At given clay content, the saturated Hydraulic Conductivity (HCs) was lower when the sand size was small and spread. For the coarse sand the salinity was maintained, even for high clay contents, a significant hydraulic conductivity. One of the main characteristics of Maghnia clay is the presence of calcium carbonates in the natural material. In comparison to that of Mostaganem clay of other deposit, it appears less sensitive to sodicity. An important aspect is the initial state of the clay when used in addition to sands, i.e., disturbance, conditions of preparation of sand clay mixtures and presence of associated components such as carbonates. Maghnia clay appeared to be adapted to the improvement of sandy soils, not because its mineralogical characteristics, but for its natural cationic form and obviously the presence of calcite in it.