Construction method for improving expansive soil embankment using phosphogypsum and microbe

A design and construction method for improving an expansive soil embankment using phosphogypsum and microbes, comprising the following steps:

• step 1, preparing a culture medium and a microbial solution, wherein a mixed solute in the culture medium comprises soybean meal, (NH4)2SO4, Na2HPO4, and NaOH, specifically,
• weighing 40 parts by weight of soybean meal, 10 parts by weight of (NH4)2SO4, 3.55 parts by weight of Na2HPO4, and 40 parts by weight of NaOH to prepare into the mixed solute of the culture medium, and mixing the mixed solute of the culture medium with water to prepare a liquid medium, wherein the content of the soybean meal in the liquid medium is controlled at 20-60 g/L;
• autoclaving the liquid medium in a 121° C. autoclave for 25 min, cooling to 30±2° C. for use;
• adding 1 part by volume of Bacillus pasteurii suspension to 100 parts by volume of liquid medium to prepare a microbial solution, placing the microbial solution into a constant temperature shaker incubator at 30° C., culturing at 200 rpm under shaking for more than 24 h until OD600 of Bacillus pasteurii in the microbial solution is 1.5±0.2;
• step 2, preparing a cementing fluid, wherein a mixed solute in the cementing fluid comprises urea and calcium chloride, specifically comprising:
• weighing 2 parts by weight of urea and 1 part by weight of calcium chloride, wherein the urea is in granular, and the calcium chloride is powdery;
• mixing the mixed solute with water to prepare a cementing fluid with a concentration of 0.5±0.1 mol/L;
• step 3, preparing a mixture, wherein the mixture comprises phosphogypsum, fly ash and expansive soil, specifically comprising:
• on a dry basis, weighing 3 parts by weight of phosphogypsum, 7 parts by weight of fly ash and 90 parts by weight of expansive soil, mixing and stirring well to prepare a mixture;
• step 4, preparing an improved mixture, specifically comprising:
• weighing 50-60 parts by volume of mixture, 2 parts by volume of microbial solution, 10 parts by volume of cementing fluid, and 8 parts by volume of water, mixing the 50-60 parts by volume of mixture and the 2 parts by volume of microbial solution with 4 parts by volume of water, stirring well, adding the 10 parts by volume of cementing fluid and 4 parts by volume of water, and stirring well to prepare an improved mixture, wherein the microbial solution and the cementing liquid are only added once throughout the process;
• step 5, leveling and compacting original ground, laying a first layer of geomembrane 1, and laying a first layer of improved mixture 2 over the first layer of geomembrane 1;
• after compacting in accordance with the requirements of compaction degree, laying a second layer of geomembrane 3 on the top surface of the first layer of improved mixture 2, laying a second layer of improved mixture 4 over the second layer of geomembrane 3, and compacting in accordance with the requirements of compaction degree, wherein it is required that laying and rolling compaction of well-prepared improved mixture are completed within 6 h, each layer of improved mixture has a thickness of uncompacted layer of 30-40 cm and is compacted by a road roller, and the geomembrane is a high-density polyethylene anti-seepage geomembrane;
• step 6, laying a first layer of geogrid 5 on the top surface of the second layer of improved mixture 4, laying a third layer of improved mixture over the first layer of geogrid 5, and compacting in accordance with the requirements of compaction degree;
• laying one layer of geogrid every three layers of improved mixture laid, wherein the geogrid is a polypropylene biaxially stretched geogrid, and transverse-longitudinal tensile yield stress is required to be not less than 30 kN/m;
• and
• step 7, after pavement of the embankment, laying a last layer of geomembrane on the top surface of the embankment, and paving a roadbed.