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Most traditional soil improvement methods are time-consuming, expensive, require heavy machinery and environmentally detrimental. As a more ecofriendly ground improvement method, the biocementation of soil offers an alternative to traditional soil improvement techniques. This method is based on microbial precipitation of calcium carbonate serving as cementing or bonding agents between soil grains. To achieve this bonding, the method of in situ soil stabilization by surface percolation under free drainage of solutions containing either bacteria or calcium ions was chosen. This paper investigates the effects of this method on the strength improvement and microstructure of a dry loose poorly-graded sand after bioimprovement. To evaluate the success of treatment, a series of laboratory experiments was conducted, including, shear wave velocity, unconfined compressive strength, calcium carbonate content, triaxial shear test, scanning electron microscopy, etc. The study revealed that this newly emerging biomediated technique causes the improvement of soil strength as a result of the cementation of sand particles via crystal precipitation and the surface percolation method has the potential of stabilizing the loose sand with desirable depth. Results also showed that strength increase depends not only on calcium carbonate content but also on its precipitation location and particle-to-particle binding strength and numbers. In addition, scanning electron microscopy images and X-ray diffraction showed precipitation of biological calcium carbonate is different in polymorph type throughout the sand column. [ABSTRACT FROM AUTHOR]

The undrained shear behaviour of sand-silt mixture is studied in the loose state using triaxial test. To this end, besides common physical tests, samples were prepared with silt contents as 0%, 10%, 20%, 40%, 60% and 80% and tested under 50, 100, 150 and 300 kPa confining pressures. Test results showed that minimum void ratio indexes were achieved for the mixture having 20% to 30% silt content. Presence of silt content up to 40% changes the soil stress-strain and pore water pressure-strain behaviour pattern and further increasing of silt content has no important effect on the soil mixture behaviour. Also, presence of silt particles between the sand grains increased the effective internal friction angle of soil at critical state. At the end, regardless of soil mixture, there is a linear relationship between the final excess pore water pressure and shear strength of soil, so that, shear strength of soil decreases with increasing of final excess pore water pressure. [ABSTRACT FROM AUTHOR]

Geotechnical design for unsaturated soils requires an understanding of their fundamental characteristics. In this paper, in order to study the monotonic, cyclic and post-cyclic behaviour of an unsaturated clayey soil, monotonic and cyclic triaxial tests were carried out on Tehran oil refinery’s disturbed unsaturated clayey soil (CL). Tests were conducted on the specimens with different initial matric suctions and net confining pressures with the same dry density under constant pore-air pressure and undrained conditions for water. No reconsolidation was allowed after cyclic loading of specimens. Tests results showed that cyclic loading degrade the undrained shear strength and deformation modulus of specimens during the post-cyclic monotonic loading. It was observed that shear modulus degradation is more pronounced than that of the shear strength due to cyclic loading. Also, the shear strength and shear modulus of soil generally increased with increasing the net confining pressure and initial matric suction. [ABSTRACT FROM PUBLISHER]

The main goal of this study is to investigate the effect of soil properties such as permeability on the dispersion and movement of a water dissolved contaminant in three types of soil in saturated and 2D conditions. The experimental modeling was conducted using a constructed sand box. In order to evaluate the effect of soils particle size on the distribution and self-remediation of the contaminant, three types of soil, as coarse, medium, and fine-grained sand were used. Results of experiments showed that, at the first 25% of the test time, the contaminated area reduction rate in all three specimens varies significantly, so that for the medium and coarse sand, it is 2.2 and 3 times that of fine sand, respectively. The contaminant width reduction at the first 25% of the test time was 5%, 6%, and 35% for the fine, medium, and coarse sand, respectively, while the contaminant length reduction was 13%, 18%, and 37% for the fine, medium, and coarse sand, respectively. In addition, by comparing the contaminant movement in the saturated and semi-saturated areas, it was observed that the longitudinal and transverse movement of the contaminant under the water level are almost 2.5 times of the semi-saturated area. PRACTITIONER POINTS: Reduction rate of solution area in fine, medium and coarse-grained sample are nearly convex, linear and concave-shaped, respectively. The remediation process in saturated zones is implemented in both directions with higher intensity in a shorter time than unsaturated zones. In the strip formed plumes, the volume of the self-remediation is proportional to the time intervals during the test. In the elliptic masses the self-purification amount is lower at the beginning, due to the small cross-section ending of the contamination mass., (© 2020 Water Environment Federation.)