Clay Specimens Research Articles

Stress dependence of the gas permeability of montmorillonite sediments in the presence of methane hydrate

The evolution of porosity and permeability of silty clay sediments in the South China Sea has significant effect on gas hydrate production due to the stress effect of submarine overburden rocks. The gas permeability of silty clay specimens under different effective stresses was measured in a cylindrical stainless steel reaction vessel. The experimental results in this paper show that application of effective stress destroys gas permeability of methane hydrate specimens. The power function relation between permeability and effective stress and exponential function relation between porosity and effective stress are given. The experimental data show that the formation of methane hydrate improves the compressible channel of montmorillonite deposits. The permeability damage rate increases with increase of effective stress, and stress sensitivity coefficient increases first and then decreases with increase of effective stress. Finally, an effective permeability equation of sediment containing hydrate saturation and effective stress parameters is established. The results are of guiding significance for the study of the stress dependence of porosity and permeability of gas hydrate silty clay reservoirs in the South China Sea. • Power law is superior to exponential law for showing permeability stress dependency. • Hydrate formation improves the compressible space of montmorillonite sediments. • Permeability damage rate increase with increasing effective stress in sediments. • Stress sensitivity coefficient show a trend of decreasing first and then increasing. • Relational expression between effective stress and permeability is proposed.

The effect of the addition of construction & demolition waste on the properties of clay-based ceramics

Waste glass and reclaimed brick are types of construction and demolition waste (C&DW) that could potentially be used as secondary raw materials in the production of ceramics. Ceramics based on clay, waste demolished brick (5-15 wt.%) and waste glass (5-20 wt.%) were produced by pressing (P = 68 MPa) and subsequently sintered at 900, 950, 1000, and 1050 oC for one hour. The physical and mechanical properties of the ceramics obtained were evaluated. The addition of demolished brick decreased the density and mechanical properties of the clay specimens and increased the water absorption. The incorporation of waste glass improved the sintering behavior and its mechanical properties. The addition of 20 wt.% waste glass and 10 wt.% waste demolished brick into the clay matrix improved the flexural strength by up to 20.6 % and decreased the water absorption by up to 22 %. The approach presented promotes an opportunity to recycle construction and demolition waste into alternative resource materials, and represents a positive contribution to the environment.

Primary and secondary compression behavior of dredged clay at low effective stresses

This study presents an experimental investigation on the time-dependent compression behavior of dredged clay samples at high initial water contents under lower effective stress. A series of laboratory one-dimensional consolidation tests with or without aging were conducted on dredged clay samples with various initial water contents. Test results showed that initial water content has a significant effect on the primary compression of dredged clay specimen. All dredged clay specimens follow the same decreasing trend in void ratio with time. Compared with the significant decrease in void ratio in the primary consolidation stage, void ratio decreases less significantly with time in the secondary consolidation stage and tends to level off. The aging strain will not continue increasing as the increase in initial water content more than two times the liquid limit. Aging strain of specimens at different initial water contents consistently increase linearly with the increasing time. The value of secondary compression index is influenced by both aging time and initial water content. It consistently reduces with increasing aging time yet increases with increasing initial water content. We should fully consider the effect of initial water content and aging time on the secondary compression index to predict the secondary compression settlement.

Development of swelling induced shear and slickensides in Vertisols

Vertisols shrink and crack as they dry, and swell as they wet. Shear surfaces, sometimes called slickensides, are often observed within the profile of such clay soils. A hypothesis for the appearance of these surfaces is in reaction to lateral swelling forces that induce shear within the soil mass. No direct proof supporting this hypothesis is found in the literature.Test data presented verify the development of lateral soil swelling pressure in response to wetting of a compacted clay soil. The data illustrate that the swell induced pressure can cause the soil to reach failure conditions and in essence, shear itself.During wetting, laboratory compacted specimens of clay were subjected to constant vertical pressure and allowed to swell in the vertical direction. Strain in the lateral direction was restrained and the horizontal soil pressures monitored as they approached a residual magnitude. From the series of tests it was found that the ratio between the residual horizontal swell pressure to the applied vertical pressure decreased monotonously with increase of applied vertical pressure.Mohr's circles representing the state of stress within the specimen at the residual state align along the Mohr-Coulomb failure envelope for tests performed at vertical pressures of 4 kPa through 27 kPa.These results provide credence to the hypothesis that slickensides are formed due to internal shear of the clay, and as such are expected to form at limited depths.

Pore water pressure responses of saturated sand and clay under undrained cyclic shearing

In this study, changes in the pore water pressure were observed for saturated specimens of a loose fined-grain sand (Nam O sand) and a soft silty clay (Hue clay) subjected to undrained cyclic shearing with different testing conditions. The cyclic shear tests were run for relatively wide range of shear strain amplitude (g = 0.05%-2%), different cycle numbers (n = 10, 50, 150 and 200) and various shear directions (uni-direction and two-direction with phase difference of q = 0o, 45o and 90o). It is indicated from the experimental results that under the same cyclic shearing condition, the pore water pressure accumulation in Hue clay is at a slower rate, suggesting a higher cyclic shear resistance of Hue clay than that of Nam O sand. Liquefaction is reached easily in nominally 50% relative density specimens of Nam O sand when g ³ 0.4%, meanwhile soft specimen of Hue clay is not liquefied regardless of the cyclic shearing conditions used in this study. The threshold number of cycles for the pore water pressure generation generally decreases with g meanwhile, the threshold cumulative shear strain for such a property mostly approaches 0.1%. In addition, by using this new strain path parameter, it becomes more advantageous when evaluating the pore water pressure accumulation in Nam O sand and Hue clay subjected to undrained uni-directional and two-directional cyclic shears.

Loading-unloading behavior of a clayey rock under thermo-hydro-mechanical conditions

Various investigations have been carried out on the hydro-mechanical behavior of geomaterials, but those of clayey rock are still limited. Among these existing studies, very few data are available related to the thermal effects, particularly the loading-unloading behavior of clayey rock at high temperatures. In this paper, undrained loading-unloading triaxial tests on undisturbed specimens of Boom Clay extracted from Mol, Belgium, were carried out with the aim to study (1) the influence of temperature and load path on the thermo-hydro-mechanical behavior of clayey rock, and (2) the formation mechanism of loading-unloading stress-strain hysteresis loops. The test results first revealed the stress paths–dependent stress-strain behavior of Boom Clay. The increase of initial mean effective stress p0′ has a positive effect on the mechanical behavior of Boom Clay. Secondly, temperature elevation induced the strength weakening of Boom Clay even under loading-unloading conditions. Thirdly, the p0′ and temperature had a bearing on the loading-unloading elastic modulus (defined as the peak-to-peak secant modulus of a hysteresis loop). Strain energy dissipation during the loading-unloading is then discussed from the perspective of thermodynamics. Finally, the study concludes that the stress-strain hysteresis loops for Boom Clay may be closely related to its high content of adsorbed water, and it may be driven by the combined effect of “friction resistance” and “viscous resistance.”

Drained triaxial testing of shales: insight from the Opalinus Clay

The investigation of the mechanical behaviour of swelling claystones and shales is challenging because of their very low permeability and of their high sensitivity to changes in water content. The former makes it difficult to carry out triaxial tests with controlled homogeneous water pore pressure fields, and the latter results in some possible effects of swelling when unsaturated extracted specimens are re-saturated prior to being tested. This work presents some data from drained triaxial tests performed on specimens of Opalinus Clay, designated as host rock for radioactive waste disposal in Switzerland, extracted at shallow depth close to the city of Lausen. The data are compared to those recently published from undrained triaxial tests on specimens from the same place, and also to two sets of recent data independently obtained on Opalinus Clay specimens sourced from greater depth at the Mt Terri Underground Research Laboratory. By comparing the data of our drained tests on Lausen specimens to those from specific undrained tests in which swelling has been prevented, quite a good comparability in shear strength is observed at confining effective stresses larger than 5–6 MPa. This is related to the small magnitude of the swelling occurring above this stress during the specimen hydration in drained tests. The question of the possible linearity of the shear strength criterion at low stress is also discussed with respect to both our data and other published ones. It is suggested that undrained tests be also carried out at low confining stresses to investigate whether some perturbations due to hydration swelling occur in this area that could result in a nonlinear shape of the criterion. The change in Young modulus and Poisson coefficient with stress is also discussed, and the data on Opalinus Clay are compared to those of the Callovo–Oxfordian claystone from France, evidencing interesting similarities in terms of shear strength properties and Young’s moduli.

Impact of Single and Multiple Specimen Suction Control Oedometer Testing on the Measurement of the Soil–Water Characteristic Curve

Abstract Devices that simultaneously facilitate controlling suction and applying a net stress on soil specimen provide soil–water characteristic curves (SWCCs) in terms of both the water content and degree of saturation, and volumetric deformations at various applied suctions. Such tests determine the water content of soil specimens based on the measured water volume changes at various applied suctions. However, studies have shown disagreements between the water volume–based calculated water content and the actual water content of soil specimens determined by the oven-drying method. Testing multiple soil specimens at predetermined suctions and measuring water content by the oven-drying method can overcome this but are a time-consuming approach. In this study, the impact of testing single and multiple soil specimens on the subsequently determined suction-water content and suction-degree of saturation SWCCs for the wetting process were studied. Statically compacted specimens of a sandy clay were used for establishing SWCCs using a suction control oedometer. Differences were noted between the calculated and measured water content and degree of saturation for an applied suction range of 0 to 95 kPa. Differences were noted between the SWCC fitting parameters obtained from the test results of single and multiple soil specimens. Statistical analysis suggested the differences between the results from single and multiple soil specimens testing were not significant. Corrections applied to the water volume change measurements were found to minimize these differences.

Evolution of Tensile Properties of Compacted Red Clay under Wet and Dry Cycles

Tensile strength is an important soil parameter that affects the stability of structures built on clayey soils. This paper presents an experimental study of the change of tensile properties of red clay during wet-dry cycles. Cyclic wet-dry tests were performed on compacted red clay specimens with different initial water contents and dry densities. Direct tensile tests and Brazilian split tests were then conducted on these specimens to determine the soil strengths. The effects of initial water content, initial dry density, number of wet-dry cycles, and crack rate on the tensile properties of red clay were investigated. The results demonstrate that the tensile strength of red clay is generally 1.566 times the splitting strength. Both the tensile strength and splitting strength are negatively correlated with the initial water content but are positively correlated with the initial dry density. Because of the presence of desiccation cracks, the tensile strength goes on reducing under wet-dry cycles. The tensile strength can be expressed by a power function of the initial water content, initial dry density, and crack rate. The proposed equation is useful to evaluate the tensile strength of cracked soils subjected to wet-dry cycles.

CONSOLIDATION CHARACTERISTICS OF A SILTY CLAY: CONCERNING THE EFFECT OF SOIL DISTURBANCE

In this paper, undisturbed specimen of a silty clay constituting of Phu Bai formation (ambQ21-2 pb) was collected from boreholes in Hue city and surrounding areas. The soil, under both undisturbed and disturbed conditions, was then subjected to standard one-dimensional consolidation tests with 7 loading increments. It is shown from the experimental results that the time to the end of primary consolidation (EOP), determined by Log Time method (tLT) and 3-t method (t3T), decreases with the load increment and under the same vertical stress, the primary consolidation of disturbed silty clay finish at a shorter time than those of the undisturbed one. The coefficient of secondary consolidation (Cα) increases with the vertical stress and reaches the maximum values before decreasing. The obtained values of Cα = 0.005 - 0.020 suggest a relatively low secondary compressibility of the silty clay constituting of Phu Bai formation.

Time-dependent strength behavior of dredged clays at low effective stresses

This paper presents an experimental investigation on the effects of initial water content and time during secondary compression on the strength behavior at low effective stresses of dredged clays obtained from two places in China. A suite of vane shear tests was conducted on clay samples consolidated at 16 kPa using the self-developed compression apparatus. Tests results showed that the shear angle corresponding to the peak shear stress almost increases with the decreasing time during secondary compression and the vane shear stress increases with decreasing initial water content at the same shear angle. A significant increase in peak strength with time during secondary compression was observed compared with the insignificant increase in residual strength. The normalized strength increases linearly with log time during secondary compression. The undrained strength of dredged clay specimens with different initial water contents consistently increases with time during secondary compression. The strength and time curve for specimens at lower initial water content lies above that at higher initial water content. A quantitative expression of normalized strength as functions of normalized water content and time during secondary compression was obtained. The proposed equation is verified by comparing the calculated results and measured results for other sources of samples.

Large-Scale Instrumented Triaxial Setup for Investigating the Response of Soft Clay Reinforced with Sand Column Groups

Granular columns are widely used as a soil improvement solution adopted in the case of soft clays in an effort to increase their bearing capacity and stiffness. Granular columns, whether sand drains or gravel columns, are designed and constructed in groups and not as individual elements, given that there is a clear need to study the group effects and load sharing under various drainage conditions. Toward this need, a new fully instrumented triaxial test setup was developed and utilized to investigate the overall response of the composite. Particular emphasis was placed on monitoring pore-water pressures and contact stresses over the sand columns and the surrounding clay. The results from clay specimens that were reinforced at area replacement ratios of 17.1% and 30.4% indicate that the test setup is capable of quantifying the distribution of the stresses in the columns and the surrounding clay at different levels of axial strain. As such, previous information about the dependency of the stress concentration ratio on the area replacement ratio, axial strain, rate of loading, and drainage conditions could be inferred. It is anticipated that the insights gained and reported in this paper will facilitate the development of design methodologies for soft clays reinforced with sand column groups.

Stabilised sulfate-rich clay with magnesium oxysulfate cement

ABSATRACT Magnesium oxysulfate cements are modern green air-hardening magnesia-based cementitious materials that were produced using the chemical reaction of magnesium sulphate (MgSO4), magnesium oxide (MgO), distilled water and a small number of additive materials. In this paper, the influences of thawing and freezing cycles on the chemical-mechanical and microstructural properties of stabilised sulphate-rich clay with MOSC using a triaxial cell set-up, scanning electron microscopy and X-ray diffraction were investigated. The sulphate-rich clay specimens with various dosages of Magnesium Oxysulfate Cement (MOSC) and different curing times were exposed to one, two and three thawing and freezing cycles in soaked and un-soaked conditions. The consequences showed that the compression strength and behaviour of the stress and strain samples significantly depend on the amount of MOSC. The durability index which was described as the ratio of the resistance before and after stabilising revealed that the durability augments by the increase of MOSC. In addition, increasing curing times in the MOSC stabilised specimens leads to an increase in their compression resistance.

A mechanical insight into the triggering mechanism of frequently occurred landslides along the contact between loess and red clay

The triggering mechanism and movement evolution of loess-red clay landslides, which occurred frequently along the contact between the loess and red clay on the Loess Plateau, are closely related to the mechanical properties of the contact surface. This work presents an experimental investigation on loess, clay and loess-red clay interlaminar (LRCI) samples obtained from a typical loess-red clay landslide in northern part of Shaanxi province of China, using a series of ring shear tests, microscopic observation and scanning electron microscopy tests, in an attempt to explore the mechanical behavior of loess, clay and LRCI samples with variation in moisture content, normal stress and shear rate. The results revealed that for all specimens, both the peak shear strength tau_{p} and the residual shear strength tau_{r} decreased with increasing moisture content, among which, moisture content has the greatest influence on the tau_{p} and tau_{r} of red clay, followed by the LRCI specimen, and the loess specimen is least affected by moisture content. Meanwhile, exponential functions describing the correlations between shear strength and moisture content of LRCI, red clay and loess specimens were proposed. Furthermore, the macroscopic morphological characteristics and the microstructure of shear surface obtained from the LRCI specimens showed that a localized water accumulation was built up within the shear surface as the water content increases to some extent, and a high degree of liquefaction developed within shear surface when the moisture content reached to the saturate degree. The microstructural observation on LRCI specimen suggested that the shear surface became smoother and the larger percentage of small-sized pores was observed with moisture content. Accordingly, the built-up excess pore water pressure during shearing is difficult to be dissipated due to a close structure of small-sized pores. Due to the low permeability, high pore-water pressure built up within the shear zone and the increase in the fine particle content, the LRCI soils with a high saturation degree shows the potential for the localized liquefaction within shear zone, which further provides a scientific explanation for the triggering mechanism of loess-red clay landslides with high-speed and long- run out.

Model pile behavior in calibration chamber under very large number of cycles of axial loading in saturated clay

ABSTRACT The results of a series of displacement-controlled cyclic axial loading tests of model pile in saturated kaolinite clay, carried out in a calibration chamber, are presented. An instrumented pile-probe was used to investigate the evolution of local skin friction and excess pore water pressure during cyclic loading. After a brief description of the experimental setup and the testing procedure, the tests results are presented concerning the response of the probe during installation, initial monotonic axial loading, cyclic loading up to large numbers of cycles (105 cycles) and post-cyclic monotonic axial loading. During cyclic loading, an initial degradation phase of local skin friction followed by a stabilization phase is observed. Generation of excess pore water pressure occurs during the 60 first cycles followed by a dissipation phase. Parametric study showed that, for initial monotonic loadings, maximum tip resistance and local skin friction are proportional to stress level. Furthermore, excess pore water pressure generated during cyclic loading depends both on initial stress level applied to the clay specimen and cyclic displacement amplitude. A peak value of local skin friction resistance has been observed during post-cyclic monotonic loadings.

Shrinkage behaviour of slurry-consolidated and compacted clay soil

This research investigates the shrinkage behaviour of clay specimens subjected to different hydraulic paths. The volume change and microstructure of desiccated clay specimen directly dried from reconstituted slurry and compacted states, and the desiccated clay specimens obtained by subjecting the as-compacted clay specimen to cyclic wetting and drying process were compared. The experimental results showed that the initial microstructural difference between reconstituted slurries and compacted specimens influences their shrinkage behaviour. The compacted clay specimens retained the bimodal structure even after subjecting to cyclic wetting and drying, unlike the reconstituted slurry, which showed a predominant unimodal structure at the end of drying process. This is due to the absence of large macropores in the desiccated clay specimen dried from reconstituted slurry state, which is evidenced from the mercury intrusion porosimetry studies, scanning electron microscopy and nano-focus X-ray-computed tomography imaging experiments.

A new simple and low-cost air permeameter for unsaturated soils

Air permeability, ka, plays an important role in understanding the air transport behavior in unsaturated soils. As for the measurement of ka, existing air permeameters using steady-state methods are relatively complicated and costly. Therefore, in this study, a new simple and low-cost air permeameter was developed, which can supply steady air pressure on soil specimen and measure the flow rate of air through soil specimen accurately. To verify the reliability of this new air permeameter, a comparison of ka of clay, silt loam, and sandy loam specimens measured using the new air permeameter and the one proposed in ASTM D6539 was conducted. One-way analysis of variance shows that the difference of the results from the two air permeameters is not significant (p > 0.05).

Statistical studies on the pore characteristic of Qinghai-Tibet Plateau silty clay modified by nano-silica

Abstract Nano-particles as additive materials have been widely used in geotechnical engineering, which can significantly improve the strength and suppress cracks formation of geo-materials. Nano-particle modified geo-materials consist of a ruleless distribution of soil particles, nano-particles, water, and gas. Thus, the micro-pore properties of modified geo-materials are uncertain and variable. In addition, the variability phenomenon is intensified by the strong water absorption and adhesive aggregation of nano-silica. However, the micro-pore characteristics of nano-material modified soil and the statistical properties are very limited in literature. To address those, large samples tests on Qinghai-Tibet Plateau silty clay ( ≥ 50 specimens) were conducted, and the median pore diameters and cumulative volume of pore diameters smaller than 0.1 μ m at four nano-silica contents (0, 0.75%, 2%, and 5%) were measured by the mercury intrusion porosimetry. The experimental data show that the two micro-pore parameters of fifty silty clay specimens with nano-silica additive show noticeable variability. To quantify these variablities, we assume the micro-pore diameter follows one of four probability distributions (normal, lognormal, Weibull, and gamma). The experimental curves were compared with four selected potential probability distributions and it was found that all distributions agreed well with the experimental data, approximatively. Nevertheless, only three distributions (normal, lognormal, and gamma) satisfy the Hypothesis testing. The results of chi-square test indicate that the normal distribution had the smallest effect evaluation index, and it could well describe the micro-pore parameters of nano-silica modified silty clay. The results will benefit a better understanding of the changeability of micro-pore characteristics of nano-particle modified geomaterials. Besides, these results are significant for interpreting the micromechanical mechanism of modified soils, and are helpful to guide the selection of additive materials for modified porous media.

A Bioinspired Technique for Improving the Interaction Between Cohesive Soil and Geotextile Reinforcements

Reinforced soil structures often cannot perform properly when there is a weak interface which is not capable of mobilizing the required shear strength between fine-grained soils and the reinforcing elements. The calcium carbonate precipitation (CCP) technique adopted in this research has proved to be extremely useful in removing this deficiency. In this study, plant-derived urease-induced calcium carbonate precipitation (PDUICCP) was used as a novel method to improve the performance of the interface. Instead of using commercial-grade purified urease, the extract of soybean seeds was mixed with calcium chloride and urea to prepare a precipitating solution to reduce the costs. In addition, xanthan gum was used as a benchmark to examine the efficiency of the calcium carbonate precipitation process. Modified direct shear tests were performed on the kaolin clay specimens with two different relative compactions, three concentrations of the precipitating solution, and three different geotextiles to assess the performance of the proposed method. The results revealed that the application of the proposed method improved the interfacial shear strength considerably with the interface efficiency increasing from 12 to 270% depending on the geotextile texture and roughness, the relative soil compaction, and the concentration of the precipitation solutions. The highest increase in the interfacial shear strength for all the concentrations of the precipitating solutions was obtained when the polypropylene nonwoven geotextile was used. In all the cases, increasing the relative soil compaction enhanced the interfacial shear strength and interface efficiency. The optimal concentration of the constituents in the precipitating solution was obtained when 0.64 molar (M) calcium chloride and 1 M urea were used. The results of this study can mark the beginning of a new type of geosynthetics with multiphase strength parameters after the addition of environmentally friendly materials. Moreover, they pave the way for using fine-grained soils as the bulk backfill material in reinforced soil structures where coarse-grained soil is not readily available.

The influence of cyclic loading on the response of soft subgrade soil in relation to heavy haul railways

The design of rail tracks is often challenged by the highly compressible behaviour of soft estuarine clays over which they must pass. It is prudent that the realistic long-term behaviour of subgrade materials under repeated loading applied by fast moving heavy haul trains is properly understood. One feature that should not be ignored when estimating the long-term performance of track foundations is the continuous principal stress rotation (PSR) induced by moving wheel loads. The main purpose of this research is to combine the traditional cyclic triaxial test results (fixed axes, no PSR) with those obtained from the dynamic hollow cylinder apparatus (allowing PSR) to examine the relative influence of cyclic stress ratio (CSR) and frequency on the behaviour of soft subgrade subjected to simulated heavy haul train loading. Employing these two types of equipment applying contrasting stress path regimes, a series of cyclic undrained laboratory tests was conducted on reconstituted sandy clay specimens at varying frequencies (f = 0.1–1 Hz) and cyclic stress ratios (CSR = 0.2–0.3). The hollow cylinder test results have shown that higher CSR values and lower frequencies induce greater permanent deformations and excess pore water pressures at a given number of loading cycles (N). For CSR = 0.2, pore pressures and axial strains were found to increase even after a large number of cycles (N = 50,000). However, when the higher CSR value of 0.3 was imposed, the soil failed in less than 300 cycles by reaching 5% of axial strain. Undoubtedly, PSR adversely affected the accumulation of axial strains and soil degradation, whereas in contrast, the development of pore water pressure was less influenced by PSR.