Sand-kaolin Mixtures Research Articles

Shear Strength of Unsaturated Soils under Multiple Drying-Wetting Cycles

AbstractUnder the same stress state, a soil behaves differently when it follows drying and wetting processes because of hysteresis. The purpose of this study was to understand the shear strength characteristics of unsaturated soil under multiple cycles of drying and wetting. A series of unsaturated consolidated drained (CD) triaxial tests were conducted using three different sand-kaolin mixtures. All sand-kaolin mixtures experienced hysteresis during multiple cycles of drying and wetting processes. The CD test results indicated that the specimens on the drying paths have higher shear strengths than those on the wetting paths owing to the hysteresis effects on the soil during drying and wetting processes. The differences between the shear strengths on the drying and wetting paths of the first cycle were found to be more significant than the differences between the shear strengths on the drying and wetting paths of the subsequent cycles. The specimens on the first-cycle wetting path and the subsequent cycle...

Interpretation of the stiffness and permeability of Sand-Kaolin mixtures in the framework of homogenization

This study deals with the behaviour of mixtures of sand and saturated kaolin paste considered as composite materials made of permeable and deformable (with non-linear behaviour) matrix (the kaolin paste) with rigid and impervious inclusions (the sand grains). Oedometric and permeability tests conducted on such mixtures highlight the key role of the state of the clay paste, and show the existence of a threshold of sand grain concentration above which a structuring effect influences both modulus and permeability. At the light of these experiments, the usual and tangent homogenization process (with simplifying assumptions to make the problem manageable) has been applied to estimate the mixture permeability and tangent compressibility. Qualitative and quantitative comparisons with experimental data point out the domain of interest and the limitations of such approaches.

Decrease in thermal conductivity with increasing temperature in nearly dry sandy soil

SummaryTo clarify the role of the water bridges between soil particles on the transfer of heat we studied the dependence of thermal conductivity (λ) and electrical conductivity (E) on temperature between 278 and 338 K of sand and sand mixed with kaolin in the nearly dry state. The thermal conductivity decreased as temperature increased in the sand at volumetric water contents less than 0.07 m3 m−3, but it increased in the sand–kaolin mixture over the measured range of water content. In the sand, the ratio of E in the soil solution to the electrical conductivity of pure water increased gradually with increasing water content at the water contents less than 0.05 m3 m−3 and was almost constant at larger water contents. The ratio of E of the sand–kaolin mixture increased with increasing water content, particularly at the lower temperature. For both samples the ratio of E decreased as temperature increased, which suggested that the conduction of heat decreased through the decrease in the water bridges as temperature increased. Because the decrease in λwith increasing temperature could not be explained by the transfer of latent heat transfer, we considered that the temperature dependence of λwas due not only to the transfer of latent heat but also to the thermal bridge of water. We conclude that the condensation, conduction and evaporation in series involved in the latent heat transfer take place mainly through the water films. Our experimental results will help to understand the mechanism of the latent heat transfer in soil with the water films surrounding the soil particles.

Performance of TDR calibration models as affected by soil texture

Time domain reflectometry (TDR) is widely used to measure the soil water contents. The effect of soil composition on TDR calibration has to be quantified to diminish the need in re-calibration of TDR. Our objectives were: (a) to evaluate the ability of published models to fit TDR calibration data for the soils of different texture, and (b) to explore using a piecewise-constant distribution to approximate the continuous dependence of the dielectric constant on the position of water within soil pore space. We calibrated TDR in sand–kaolin mixtures, dernovo-podzolic soil, gray forest soil, and leached chernozem. Distinct changes in slopes of the calibration curves were observed for the fine-textured samples in the range of volumetric water contents from 0.13 to 0.27. The ratio of the lack-of-fit mean square error and the mean square pure error was used as a statistic for the error comparison to conclude whether a model provided a satisfactory fit of data. Published models that did not have parameters to adjust fitted well data for coarse-textured mixtures and soils but failed to fit data for fine textured mixtures and soils. The piecewise-constant distribution model with three adjustable parameters fitted well all data. Parameters of this model were related to clay content in the samples.

Unsaturated Hydraulic Conductivity of Compacted Sand-Kaolin Mixtures

The unsaturated hydraulic conductivity of compacted sand-kaolin mixtures containing 0, 5, 10, and 30 kaolin (by dry weight) is measured for matric suctions, ψm<∼6.0 m. The measured unsaturated hydraulic conductivity (kmeasured) values are compared with predicted unsaturated hydraulic conductivity (kpredicted) values using the Brooks-Corey-Burdine and van Genuchten-Mualem relative hydraulic conductivity functions. In general, the accuracy of kpredicted decreases with an increase in kaolin content or an increase in ψm. In addition, kmeasured tends to be underpredicted for kaolin contents of 10 and 30 at relatively high suctions (1.0 m ≤ψm≤ 6.0 m) and overpredicted for kaolin contents of 0 and 5 at relatively low suctions (0.1 m ≤ψm< 1.0 m). For a given kaolin content and ψm, kpredicted based on the Brooks-Corey-Burdine function tends to be more accurate than kpredicted based on the van Genuchten-Mualem function. Finally, for 1.0 m ≤ψm≤ 6.0 m, kpredicted based on analysis using the maximum volumetric water content (θm) attained under steady-state flow conditions typically is more accurate than kpredicted based on analysis using the saturated volumetric water content, θs, where θm∼ 84–90 of θs in this study.

Compaction and shrinkage of sands and sand-kaolin mixtures

A comparison was made between the porosities of sands and of mixtures of sand and kaolin paste which had either been allowed to air-dry or had been consolidated under a load of 1.5 MPa. For all the mixtures of sand and kaolin, air-drying resulted in porosities which were from 0.1 to 0.05 (v/v) lower than those produced by consolidation. In contrast, two sands had porosities 0.01 and 0.05 (v/v) higher when air-dried than when consolidated. This demonstrates that some sandy-textured soils may compact significantly on drying.