Curing Soft Clay to Improve Its Strength with Composite Microbially Induced Carbonate Precipitation (MICP) and Coal Fly Ash

Owing to soft clay’s high water content and low strength, the large volumes of it that are excavated during coastal and waterway construction must be disposed of by being dumped on vacant land or used as fill, and results in increasingly serious environmental problems. Another important regional environmental problem is the large volumes of coal fly ash (CFA) produced by coal-fired power plants. Soft clay disposal methods that can also use CFA can have important practical significance for infrastructure construction projects and can also help the regional economies in areas with limited land resources. At present, microbially induced calcium carbonate precipitation (MICP) is used to strengthen sandy material with high porosity and permeability. Because of the special chemical compositions and complex mechanical properties of both soft clay and CFA, studies on the solidification of soft clay-CFA mixtures by microbial methods are rare. This paper describes the results of systematic MICP curing tests on composite soft clay samples with different amounts of added CFA. Changes in the strength of MICP/CFA-cured soft clay samples and X-ray diffraction analyses are used to study the MICP strengthening mechanisms. The results show the following conclusions. (1) Using composite MICP/CFA solidification can effectively improve the strength of soft clay. The clay unconfined compressive strength can be increased by a factor of 5.96. (2) MICP/CFA-aided solidification cements soft clay particles together and reduces water content by consuming the free water of soft clay during their respective reactions. (3) MICP and CFA synergistically enhance the strength of soft clay, and the solidification effect is more obvious for low water content soft clay. (4) There is an optimal composite ratio under the experimental conditions with a 0.75 mol/L bacterial nutrient concentration; the optimal composite ratio for curing is 30% water content of soft clay with 30% CFA.