Experimental Study On Solidification Of Sandy Clayey Purple Soil By Microbial Induced Calcium Carbonate Deposition

Hydraulic erosion and water-soil loss are serious since the sandy clayey purple soil and the concentrated rainfall in Chongqing region of shanxia reservoir.The main factor affecting soil erosion is inadequate adhesion between soil particles,and MICP technology takesadvantages of low pollution,low energy consumption and controllable reaction,W hich can bind soil particles and improve soil mechanical properties.Bacillus megaterium is an environmentally friendly,safe and harmless soil bacteria.Its size and strong tolerance are suitable for the high-temperature and rainy environment in Chongqing region.This paper takes sandy clayey purple soil as the research object.The MICP soil solidification tests using bacillus megaterium were conducted to investigate the change rules of solidification effect,in order to provide scientific basis and reference for the application of MICP technology in foundation,slope reinforcement and soil and water conservation in purple soil area.Mainly studied the following aspects:?1?The culture medium and culture conditions of bacillus megaterium were optimized by orthogonal tests.?2?The change rules of urease activity with time and diffirent concentration of curing fluid were investigated by the conductivity method and MICP tube test.?3?Low pressure grouting device,soft soaking device and hole-type grounting device were designed for soil solidification test,and the influence of bacterial solution concentration,curing fluid concentration,and grouting flow rate were considered in the hole-type grouting method.?4?The change rules of solidification effect on three solidification technology were studied by mechanical tests,micro-testes and solidification product measurement.And the correlation between the calcium carbonate uniformity and the unconfined compressive strength and elastic secant modulus of the soil was quantitatively analyzed.The main findings are as follows:?1?With the optimized culture medium and culture conditions,the number of viable bacteria of Bacillus megaterium was increased by 126%.Bacillus megaterium could not survive at a high temperature of 45°C,and its growth was inhibited and the number of live bacteria decreased by an order of magnitude at a low temperature of 20°C.?2?The urease activity of Bacillus megaterium showed a decreasing trend with time,which could maintain a high activity within 1d.Urease activity could be inhibited by CaCl₂ and decrease as the increase of CaCl₂ concentration,whereas urease activity could be promoted by urea in the concentration range of 0.5?1mol/L.But high concentration of Ca²⁺will promote the precipitation rate of CaCO₃ in the ion reaction.And the good urease activity of bacillus megaterium could be obtained with a curing fluid of 1mol/L CaCl₂ with 1?2mol/L Urea,which can deposit a lot of CaCO₃.?3?For low pressure grouting samples.As the pores were filled with calcium carbonate and the formation of upper and lower crusts,the permeability of sample treated by 10d low pressure grouting decreases continuously and eventually drops by 2 orders of magnitude.The unconfined compressive strength?UCS?,shear strength?cohesive force,c and internal friction angle,??,dry density and CaCO₃ production?C?increase continuously with the increase of treatment time,and the peak strength could appear due to constantly decreasing permeability.And the UCS,c and?of sample treated by 9d low pressure grouting was increased by 77%,204%and 22%,respectively.UCS positively correlates with C.MICP technology can improve corrosion resistance of soil.After 32h immersion,the collapse ratio?S?of sample treated by 7d low pressure grouting was reduced by 5%.The sample standard deviation of C in the upper,middle and lower layers of sample was used to define the CaCO₃ uniformity?s?,it is shown that the secant elastic modulus(E₅₀)rises fluctuatingly with the increase of C under the influence of s.When C is close or s differs greatly,the smaller the s is,the greater the E₅₀ is.?4?For soft soaking samples.Bonding points in sample cured by soft soaking are less than that in sample cured by low pressure grouting because curing fluid is hard to penetrate into soil in soft soaking method.The c and?of sample treated by 9d soft soaking were increased by 166%and 11%,respectively.After 32h immersion,the collapse ratio?S?of sample treated by 7d l soft soaking was reduced by 3%compared with nature sample.?5?For hole-type grouting samples.The uniformity can be improved by hole-type grouting,and the UCS and E₅₀ of sample treated by 7d hole-type grouting were increased by 106%and 464%,respectively.The higher the concentration of bacteria solution and curing fluid and the slower the grouting flow rate are,the bigger the C and the s.C has a effective interval of 2.5%?5.0%,the UCS significantly increases with the increase of C in the interval,whereas the growth of UCS becomes slow or even negative outside the interval.The sample standard deviation of C in six layers of sample was used to define the CaCO₃ uniformity?s?:The smaller the s is,the bigger the UCS and E₅₀0 are,and this effect is small when C<4%and is significant when C>4%.With the effect of s,the UCS and E₅₀ increase with different speed and then reduced as the increase of C.?6?Based on microstructure and mineral of hole-type grouting samples.The CaCO₃induced by bacillus megaterium are 1?5?m calcite crystal and some strains will serve as nucleation sites for calcite,which cemented and wrapped soil particles,and form the calcite space skeleton structure in the soil.?7?Based on curing effect,The priority of solidification effect is open-hole grouting method>low pressure grouting method>soft soaking method.In engineering applications,the low pressure grouting method and soft soaking method are suitable for weakening water resistance and wind erosion on the slopes,dams and other projects,and the open-hole grouting method is suitable for deep soil reinforcement such as foundation treatment.