Triaxial shear strength and immersion compressive strength of sandy clayey purple soil treated by microbial induced calcite precipitation

Abstract: The sandy clayey purple soil is widely distributed in the Yangtze River basin，the southwestern part of China， which is a kind of erosive high-productive soil. In the abundant rainfall environment， the road， slope， water conservancy and other engineering facilities always have been broken due to the low strength， inadequate adhesion and poor corrosion resistance of sandy clayed purple soil. The main factor affecting soil erosion is inadequate adhesion between soil particles， microbial induced calcite precipitation (MICP) technology can bind soil particles and improve soil mechanical properties. Bio-mediated soil has recently emerged as a new sustainable technique of soil improvement，and MICP technology takes advantages of low pollution，low energy consumption and controllable reaction. In this paper，the sandy clayey purple soil was treated by bacillus megaterium which is more suitable than sporosarcina pasteurii in the sandy clayey purple soil area. Bacillus megaterium is difficult to be washed away by seriflux and heavy rainfall due to its large size and thin rod shape. Besides this，it also has environmental protection effects such as soil phosphorus-solubilizing and water purification. At first，MICP tube tests were conducted to investigate urease activity influenced by different concentrations of urea and CaCl2 in curing fluid，which was conducted to select the approximate curing concentration. Low-pressure grouting curing device and soft soaking device were manufactured for soil curing test. Based on triaxial shear test and immersion unconfined compressive strength test (0~32h)，the influence rule of curing process including low-pressure grouting and soft soaking，curing time and immersion duration on the triaxial strength and immersion strength were studied. The results showed that urease activity could be inhibited by CaCl2 and decrease as the increase of CaCl2 concentration，whereas urease activity could be promoted by urea. And the good urease activity of bacillus megaterium could be obtained with a curing fluid of 1 mol/L CaCl2 with 1-2 mol/L Urea，which can deposit a lot of CaCO3. After treating by low-pressure grouting，the triaxial shear strength and CaCO3 production of soil sample increased with curing time，the increase has become smaller and smaller，and the curing peak strength could appear due to constantly decreasing permeability. The cohesion and internal friction angle of the 9 days’ low-pressure grouting curing sample increased by 203.9% and 21.5% (81kPa，31.59°) compared with nature sample，respectively. MICP technology can improve corrosion resistance of soil. After 32 h immersion，the collapse ratio，immersion unconfined compressive strength and strength loss rate of 7 days’ low-pressure grouting curing sample were reduced by 5.1 percentage points (3.54%)，increased by 223.7% (108.1 kPa) and reduced by 36.5 percentage points (8%) compared with nature sample，respectively. The curing effect of soft soaking is significantly weaker than that of low-pressure grouting. Bonding points in sample cured by soft soaking are less than that in sample cured by low-pressure grouting because seriflux is hard to penetrate into soil in soft soaking method. It provides scientific basis and reference for the application of MICP technology in foundation，slope reinforcement and soil and water conservation in purple soil area.