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The stabilization of soil through the addition of fly ash has been shown to be an effective alternative for improving the strength and stiffness of soil through the resulting chemical reactions. The chemical reaction that occurs dissociates the lime (CaO) in the fly ash, and the establishment of cementitious and pozzolanic gels (consisting of calcium silicate hydrate (CSH) gel and calcium aluminate hydrate (CAH) gel) binds the soil particles and increases the strength and stiffness of the soil. Investigations into the mechanical properties of sands stabilized with fly ash (fly-ash-stabilized sands) were conducted through a series of unconfined compressive strength (UCS) and direct shear strength tests for various fly ash percentages, curing times, grain sizes, degrees of saturation during sample preparation, and content of fines. It was found that the mechanical properties—UCS and direct shear strength (DSS)—of fly-ash-stabilized sands increased with both increasing fly ash content in the specimen and curing time, but decreased with increasing grain size, degree of saturation during sample preparation, and content of fines. The results indicated that fly-ash-stabilized sands required more than a month to attain their optimum performance with regard to binding sand particles.

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Organic soil has a high content of water and compressibility. Besides that, it has a low specific gravity, density, and shear strength. This study evaluates the applicability of the soybean crude urease for calcite precipitation (SCU-CP) method and its effectiveness in organic soil as a soil-amelioration technique. Various soybean concentrations were mixed with a reagent composed of urea and calcium chloride to produce the treatment solution. Its effect on the hydrolysis rate, pH, and amount of precipitated calcite was evaluated through test-tube experiments. SEM-EDS tests were performed to observe the mineralogy and morphology of the untreated and treated samples. The treatment solution composed of the reagent and various concentrations of soybeans was applied to organic soil. The increasing strength of the organic soil was evaluated using direct shear (DS) and unconfined compression (UCS) tests. The test-tube results show that a hydrolysis rate of 1600 u/g was obtained when using 50 g/L of soybeans with a precipitation ratio of 100%. The mechanical tests show a significant enhancement in the parameters of the organic soil’s shear strength. A shear strength improvement of 50% was achieved in this study. A UCS of 148 kPa and cohesion of 50 kPa was obtained in the treated samples of organic soil. This research elucidates that the SCU-CP is an effective technique for improving organic soil’s shear strength.

Enzymatically induced calcite precipitation (EICP) is a new breakthrough for liquefaction prevention. This approach uses urease enzyme directly instead of bacteria to hydrolyze urea that precipitated as calcite crystal by the availability of calcium ion. Cyclic triaxial shear test under undrained condition incorporated with bender element test were conducted conscientiously on the EICP-treated sands. The parameters study reviewed includes: size of the sand particle, confining pressure, calcite contents, and saturation degrees along curing. The effects of those factors on the shear modulus are systematically investigated and compared. It was revealed that the precipitated calcite wraps the sand particles, which supports simultaneously to the mechanical properties’ improvement. The sum of materials needed of urea and CaCl2 to reach a target of maximum shear modulus can be diminished prominently by reducing the saturation degree along curing. It is also revealed that the formation of the precipitated calcite is more significant than its amount on the strength improvement. The maximum shear modulus (Gmax ) of the sands treated with EICP increases with increasing in the calcite content, confining pressure, and decreasing in the saturation degrees during curing but the influence of the sands grain size is insignificant.