Landslide Mitigation through Biocementation

Landslide and other geo-disasters are causing a great damage to people and the resources all over the world. An environment friendly countermeasure of landslide disasters is necessary. Microbially induced calcite precipitation (MICP) is a bio-cementation process that can improve the geotechnical properties of granular soils through the precipitation of calcium carbonate (calcite) at soil particle contacts. This MICP can be an environment friendly solution for the biocementation of soil. In this study, an evaluation of biocemented soil has been carried out through direct shear test and direct simple shear test. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS) and X-ray Computed Tomography (X-ray CT) tests were conducted to analysis the calcite precipitation inside the biotreated soil by bacteria by using Toyoura sand and silica sand no. 4. It was observed that the amount of calcite generated in silica sand was larger than Toyoura sand. The particle shape influences the result of calcite precipitation and consequent strength of the bio-cemented sand. The amount of strength which was obtained by direct shear test and direct simple shear test indicated the granular soil became bio-stabilized within 7 days of application of nutrients from the surface. However, the amount of generated calcite was not uniformed in different layers while applying the nutrients and bacterial from the surface which was revealed by X-ray CT scan test.

Consolidated drained (CID) behavior of fibre reinforced cemented Toyoura sand in triaxial loading conditions

The effects of fibre (0–3 %) and cement (0–3 %) additives, on Toyoura sand were examined under consolidated drained compression and extension loading conditions. All samples were prepared to a target dry density value (e.g., $${\rho }_{d}$$ ρ d = 1.489 g/cm3) of Toyoura sand using under-compaction moist tamping technique. In compression, the unreinforced specimens exhibited a behavior of medium dense sand and reached a peak deviator stress (qp) at approximately 4 % axial strain ($${\epsilon }_{a}$$ ϵ a ) for the varying mean effective stresses, pʹ (i.e. 50–400 kPa). The peak drained strength increases in fibre reinforced cemented specimens were found to be up to 132 % (lower effective stresses) and 243 % (higher effective stresses), while, the drained strength increases at critical state for the fibre reinforced cemented specimens were found to be up to 105 % (lower effective stresses) and 245 % (higher effective stresses). Overall, the fibre and cement additives increased the stiffness, peak and strength at critical state of pure Toyoura sand but were found to be least effective in extension loading. Moreover, the stress ratio, peak and critical state stress ratios increase with the addition of fibres and cement. The secant modulus shows limited increases for the fibre reinforced specimens. However, a significant improvement in the secant modulus is observed for the fibre reinforced cemented specimens. For both unreinforced and reinforced specimens there is a decrease in volumetric strain with greater effective stresses or in other words, the rate of dilation decreases with increases in effective stresses. The fibre and cement additives also increased the strength parameters (frictional angle, cohesion), dilatancy angle, slope of the critical state line, and decreased the state parameter of pure Toyoura sand.

Effect of Fiber and Cement Additives on the Small-Strain Stiffness Behavior of Toyoura Sand

The disposal of 2011 Japan earthquake waste has become an important issue in Japan and it is not realistic or economical to send all of these wastes to landfill sites, due to limited space, high costs, and related environmental issues. In sustainable geotechnical applications, mixing of the separated soils from disaster wastes with additives (e.g., cement and fiber) is required to improve their strength and stiffness characteristics. In this study, monotonic triaxial drained compression tests are performed on medium dense specimens of Toyoura sand-cement-fiber mixtures with different percentages of fiber and cement (e.g., 0–3%) additives. The experimental results indicate that behavior of the mixtures is significantly affected by the concentration of fiber and cement additives. Based on a comprehensive set of test results, modifications to the series of equations were developed that can be used to evaluate the shear modulus and mobilized stress curves at small-strain levels. The experimental results and model comparison show that the elastic threshold strain (γe), reference strain (γr), increases with fiber and cement additives. In addition, the range of curvature parameter, from 0.88 to 1.0, provides a good comparison with the results of small-strain measurements. Overall, the comparison of the results and model shows that the small-strain measurements obtained using local strain transducers fall within the range of model upper and lower bound curves. The results of the unreinforced, fiber, and cemented sand shows a close agreement with the model mean curve, but fiber-reinforced cemented sand shows a good comparison with model upper bound.