Mechanism Study On Mitigation Of Liquefaction By Desaturation Method Using Biogas

Desaturation is a new method to mitigate liquefaction of sand.Desaturation aims to prevent liquefaction by generating gas/air in the pores of fully saturated sands.The advantage of desaturation over other mitigation techniques will be its cost effective implementation for new as well as existing structures.In biogas bubbles method,microbial denitrification is used for the generation of nitrogen bubbles.Because the stable chemical properties and low solubility of nitrogen,the biogas bubbles method is suitable for desaturation.At present,the development of mitigation of liquefaction using biogas is still in the preliminary stage,which is difficulty to guide the applying in practical projects.Therefore,to have a comprehensive understanding of this method,combining the research results existed about sand liquefaction treatments,the reinforcement of liquefiable sand foundation by microbial bubbles was studied through using laboratory tests,theoretical analysis and numerical simulation.This study was supported by National Natural Science foundation of China?51878158?and the Research Innovation Program funds for Jiangsu College Graduates?KYLX15₀158?.The main research results of this paper are as follows:?1?Pseudomonas stutzeri is selected to be the strain for biogas generation through denitrification tests.To explore the feasibility of desaturation by nitrogen produced by pseudomonas stutzeri,the denitrification condition and the gas production efficiency of soil are studied.The influences of different carbon sources on the reduction rates of natrate and nitrite as well as the impacts of the temperature and the initial pH value on the final saturation,the average rate of gas production,the initial stagnation period are analyzed.The optimum temperature range of this method was from 20°C to 30°C,and the optimum pH value range was from 7 to 9.Carbon-nitrogen ratio?C/N?of the bacterium suspension can be used as the index to control the amount of desaturation,the amount of desaturation was determined to have a good empirical linear relationship with the C/N.Compared with the existing biogas methods,the application of this bacteria in IPS has advantages such as the fast average rate of gas production,short initial stagnation period,simple technology and so on.?2?The effect of saturation on the strength characteristics of sand with high degree of saturation condition was analyzed through triaxial tests.For static characteristics,as degree of saturation decreased,the stress-strain curve changed from strain softening behavior to strain hardening manner under undrained conditions,peak of stress-strain curve increased and the excess pore pressure decreased,the absolute value of slope of instability line increased,and liquefaction resistance ratio increased significantly.Static liquefaction potential index was very sensitive in the high degree of saturation range,and could be used as an evaluation index to evaluate the static liquefaction potential of the sand mitigated by bigas.For dynamic characteristic,dynamic strength curve shift up as degree of saturation decreased.With the high saturation rang being from 100.0%to 94.3%,liquefaction resistance ratio decreased with the decrease of degree of saturation and it is showd a good linear relationship between them.At the same strain level,as the degree of saturation decreased,dynamic elastic modulus reduced and the damping ratio increased.As the strain increased,the saturation influences were weakened.Finally,critical degree of saturation is determined based on theory of degree of saturation control.?3?To explore effect of degree of saturation on the dynamic response of model-foundation,a serial of shaking table tests were conducted.The maximum excess pore water pressure decreased with the decrease of the sample initial degree of saturation and the maximum pore pressure ratio decreased with depth.The settlement and settlement rate of samples decreased as degree of saturation decreased.The acceleration amplification coefficient decreased with the decrease of the sample saturation,which showed the obvious vibration attenuation effect.The lateral deformation of the sample at different depths decreased with the decrease of saturation.It should be notised that the increase of acceleration would weaken the reinforcement effect of desaturation.The sample volumetric strains increased with the increase of average pore pressure ratio and showed a power function relationship,which changed with degree of saturation.?4?The stability of biogas bubbles in pore of soil under different external conditions was studied.Under hydrostatic condition,degree of saturation samples was increased from 84.5%to 85.1%with only being increased by 0.63%through 72-week observations.Under hydraulic gradient flow condition,when the hydraulic gradient was constant,degree of saturation and permeability coefficient of the soil increased and decreased in the initial period respectively,and change to be a constant value in finally.There was a good linear relationship between seepage quantity and degree of saturation.Combining with experimental phenomena,it was inferred that the bubbles was discharged from the soil in dissolved phase.Through analyzing the change of permeability coefficient and void ratio under the seepage condition,it was inferred that the main reasons which affected the decrease of the sample permeability were the migrations of biogas bubbles and cells.Under the vibration condition,after 40000cycles,(1₍?6?/(1₍?60? was decreased from 1.0 to 0.95,and a small amount of bubbles were overflowed the soil.?5?The process of injecting bacterial suspension and desaturation was simulated using the software of TOUGH2.Due to low biogas generation rate and a certain amount of production,the desaturation region was“drop-shaped”when the biogas production was completed.Therefore,the desaturation area should be described and analyzed through horizontal and vertical impact range.From the sensitivity analysis of various factors on the horizontal and vertical influence ranges,it could be known that bacterial suspension volume and porosity was most sensitive to the lateral influence range and the vertical respectively.