微生物诱导碳酸钙沉积固化三峡库区黏性紫色土试验研究

Performance of microbial induced carbonate precipitation(MICP) for reinforcing cohesive purple soil in the Three Gorges Reservoir Area

三峡库区自然灾害频发,微生物诱导碳酸钙沉积(MICP)技术是一种具有能耗低、无污染且可持续等优点的土体加固技术。黏性紫色土是三峡库区主要土壤类型,土壤孔隙较小,而MICP对其加固效果尚不明确。设置不同巴氏芽孢杆菌菌液浓度(OD_(600)为0、0.5、1.0、1.5)和胶结液浓度(0、0.5、1.0、1.5、2.0 mol/L)组合,对土壤试样进行MICP固化处理。开展不固结不排水三轴剪切试验,研究各试样的应力-应变关系、弹性模量和抗剪强度指标(黏聚力、内摩擦角),并利用扫描电镜测试分析其微观结构。结果表明:固定菌液浓度或胶结液浓度时,抗剪强度、弹性模量及黏聚力均随胶结液浓度或菌液浓度的增加呈先增后减的变化趋势,最适菌液与胶结液浓度组合为菌液OD_(600)=1.0和胶结液1.5 mol/L。平均内摩擦角随胶结液浓度的增加呈先增后减趋势,而胶结液浓度不变时,在菌液浓度OD_(600)=0.5或1.0时达到最高。固化后试样抗剪强度、弹性模量、黏聚力及内摩擦角最大分别提高62.59%、50.18%、119.50%和10.33%(226.00 kPa、6.44 MPa、48.30 kPa和26.70°)。通过扫描电镜观察发现MICP加固紫色土形成了大量球状碳酸钙晶体和片状碳酸钙晶体,分布于土壤颗粒表面和间隙中起胶结作用并增加土颗粒表面粗糙度,从而提升了土的黏聚力和内摩擦角。MICP可以有效提高紫色土的强度,在菌液浓度为OD_(600)=1.0和胶结液浓度1.5 mol/L组合时加固效果最优。

Three Gorges Reservoir Area is prone to natural disasters;microbial induced carbonate precipitation(MICP) is a soil consolidation technique with the advantages of low energy consumption,less pollution and great sustainability.The clayey purple soil is the main soil type in the Three Gorges Reservoir Area with small soil pores,and the effect of MICP on its reinforcement is unclear.in this study,the MICP-cured soil specimens,which conformed of different Bacillus bacterium concentrations(OD_(600)=0,0.5,1.0,1.5) and curing fluid concentrations(0,0.5,1.0,1.5 and 2.0 mol/L) were subjected to unconsolidated undrained triaxial shear tests to examine the stress-strain relationship,elastic modulus and shear strength index(cohesion and internal friction angle) while their microstructures were analyzed by SEM tests.The results indicated that the shear strength,elastic modulus and cohesion increased and then decreased with the increase of the bacterium concentration or the curing fluid concentration under the same curing fluid concentration or the same bacterium concentration.The best combination existed when bacterium concentration is OD_(600)=1.0 and the concentration of cement solution is 1.5 mol/L.The average internal friction angle was characterized as increase followed by decrease with the increase of curing fluid concentration,the highest value of internal friction angle was obtained at the bacterium concentration with OD_(600)=0.5 or OD_(600)=1.0 under the same curing fluid concentration.Compared with no treatment,the maximum values of the cured specimens were increased by 62.59%,50.18%,119.50%,and 10.33%(226.00 kPa,6.44 MPa,48.30 kPa and 26.70°),respectively.The SEM revealed that the MICP-reinforced purple soil formed a large number of spherical and flaky calcium carbonate crystals,which distributed on the surface and in the interstices of soil particles.The crystals present on the surface of soil particles increased the surface roughness and soil particle size,which in turn increased the friction angle of the soil,while the crystals present in the interstices of soil particles produced cementation,contributing to the cohesion.The MICP can effectively improve the strength of cohesive purple soil,and the optimal reinforce performance occurred at the combination of the bacterium concentration with OD_(600)=1.0 and curing fluid concentrations with 1.5 mol/L.