Permeability Experimental Study On Bio-Cemented Calcareous Sand By Mixing Method

With the further development of national defense construction and the need for marine resources,the construction of artificial marine islands and reefs in China is in full swing.Calcareous sand,as the most common sand material in marine environment,should be the best choice for civil engineering materials of island construction.Except natural reefs,the development of offshore oilfields and air traffic transit has led to the rise of artificial island projects.In offshore artificial island construction,the most commonly used method is the blow-filling method.But the mechanical properties of the initial blow-filled foundation are weak,and there is still much room for improvement for application and design.Microbial induced carbonate precipitation technology(MICP)is a new type of biological technology,proved to be effective for foundation treatment mainly for the terrigenous sand.The particles cemented by MICP technology have better mechanical and hydraulic properties.As a new foundation treatment method,MICP technology provides a new way for the treatment of soft blow-filling foundation.In order to combine the MICP technology with the blow-filling method,this paper puts forward the method of microbial mixing instead of t he traditional grouting treatment.Through a series of tests on material scale and model scale,the results of the microbial mixing method on calcareous sand is explored.First of all,calcareous sand existed in the actual project was selected,of which the particle size distribution may lead to a potential engineering problem.This paper discusses the possibility of using high concentration of calcium ions for MICP.After that,the formula of microbial mixing on calcareous sand was optimized and selected through a series of direct shear tests,permeability tests and tests on dry density and water absorption,where the proportions of bacteria and reaction solution are variable as well as the concentration of calcium ions.A calcareous sand column with a height of 20 cm and a diameter of 18 cm was treated,and the permeability coefficient and critical hydraulic gradient of different relative density of calcareous sand before and after were compared through the penetration test.The effect of compaction method and microbial mixing method on improving the hydraulic performance was compared.Finally,a model test coupling with stress and seepage was carried out.The test was designed to study the internal erosion of the calcareous sand under certain vertical stress and certain pressure seepage.The variation of the permeability and settlement of the samples during the process is also observed.