Preliminary Investigation On Improving Engineering Properties Of Peaty Soil Based On Biogeotechnology

Peaty soil is the soil with the highest content of organic matter in nature,and it has unique engineering properties.The organic matter mainly comes from the residual of degradation of branches,leaves,roots,secretions of plant and animals,and it is the source of nutrition for microorganisms and the most active components of matter in soil,which has a great influence on the engineering properties of peaty soil.There are a large number of microorganisms in rocks and soil,their activities of life will change the content and composition of organic matter in peaty soil,thus changing the engineering properties of soil.It is called biogeotechnology to control and utilize microbiological processes to solve specific geotechnical problems.In recent years,more and more researchers have paid attention to the problem of improving the physical and mechanical properties of rocks and soil by using biogeotechnology,and have carried out relevant researches.In the natural environment,all the factors that can affect the quanti ty,activity and physiological function of microorganisms will affect the decomposition and transformation of organic matter in the soil,which mainly include:temperature,characteristics of nutrients,characteristics of soil,moisture and ventilation of soil,etc.The eutrophication of the water in Dianchi Lake will inevitably affect the composition of nutrients in the soil around the lake through the circulation of lake-groundwater,resulting in the increase of nitrogen and other elements in peaty soil,which has the possibility of accelerating the decomposition of organic matter in peaty soil,thus causing the problem of long-term settlement of peaty foundation in the area around Dianchi Lake is more serious.Inspired by biogeotechnology,it is suggested that the microbial enrichment technology should be used to increase the number and activity of primary bacteria in peaty soil,so as to accelerate the degradation rate of organic matters in soil,and achieve the purpose of significantly reducing the content of organic matters and improving the engineering properties of soil in a short time,so as to explore the treatment of peaty foundation.The specific research contents and conclusions are as follows:（1）The process of decomposition of organic matter in peaty soil with different ratio of carbon to nitrogen（C:N）leading to biogas production under anaerobic conditions was analyzed by the model test in the laboratory;The amount of microorganism in peaty soil with different ratio of carbon to nitrogen was determined by spread plate method;The influence of the change of ratio of carbon to nitrogen on the characteristics of long-term deformation of peaty soil was analyzed by the test of one-dimensional consolidation.The experimental results show that:with the decrease of ratio of carbon to nitrogen,the volume of biogas and the rate of biogas production increase,and the process of biogas generation is roughly divided into four stages;The number of microorganisms in peaty soil increased with the ratio of c arbon to nitrogen decreased;The deformation of one-dimensional consolidation of peaty soil with different ratio of carbon to nitrogen includes instantaneous settlement,settlement of primary compression,settlement of secondary compression and settlement of tertiary compression.With the decrease of ratio of carbon to nitrogen,the coefficient of tertiary compression(C_{a II})tends to increase.In general,the decomposition of organic matter in peaty soil due to the increase of nitrogen may cause significant problems of additional settlement of foundation.（2）Two sets of model devices are developed to simulate the degradation process of organic matters of peaty soil under anaerobic and aerobic environments,respectively.The combustion loss,limit moisture content and one-dimensional consolidation deformation characteristics of peaty soil after being decomposed are tested.The results show that under the anaerobic environment,the biogas yield of peaty soil soaked in the enriched bacteria solution is significantly higher than that of pure water immersion,and its gas production kinetic characteristics are in line with the modified Gompertz model.Under the aerobic environment,when the microbial degradation lasts for about 30 days,the burning loss of peaty soil in the two sites decreases by 10.28%and 15.58%,respectively,which is larger than that under the anaerobic environment.The experimental results show that the liquid limit of peaty soil degraded by microorganism decreases with the increase of the reaction time,and the plastic limit does not change much.The one-dimensional consolidation tests show that the secondary consolidation coefficient of peaty soil after degradation of organic matters decreases,and the longer the degraded time,the more significant the reduction of the secondary consolidation coefficient.The characteristics of the new technology are analyzed,and its theory and application are prospected.This technique is expected to be an ecological friendly new treatment method f or peaty soil foundation,and improving peaty soil foundation is expected to have a potential application field of biogeotechnology.（3）A model test device was developed to decompose the peaty soil with enriched bacteria solution for 90 days.Then the physical and mechanical properties of the peaty soil were analyzed and tested during the decomposition process.The results showed that the organic matter content of peaty soil decreased from 67%to 61.5%within 90 days.As the decomposition time increased,the relative density（G_s）and saturation(γ_{s at})of peaty soil increased,the liquid limit（w _L）decreased significantly,and the plastic limit（w _P）almost maintains constant.The compression coefficient(a_1-2)of the decomposed peaty soil decreased,while the compression modulus（E_s）increased.The fast shear strength of the consolidated peaty soil generally increased by 15%～25%after decomposition.The cohesion(c_cq)decreased from the initial 18.3k Pa to 15.6 k Pa,while the internal friction angle(φ_{c q})increased from 13.9°to 18.8°.The decomposition time increased,and the permeability coefficient of the pre-compressed peaty soil sample decreased.The mechanism analysis shows that the change of engineering properties of peaty soil is not only related to the decrease of organic matter content,but also related to the change of composition.