According to swelling-shrinkage characteristics and modified mechanism, the strain A,, for improving the swelling-shrinkage characteristics of expansive soil, is separated from the farmland soil and the expansive soil by waving-tube method and mixing-plate method, and then made into solid inoculant. By direct shear test and swelling-shrinkage test, swell-shrink index and strength index of expansive soil, inoculated with inoculant of A, and cultured for different periods of time, are tested. The results show that, after adding inoculant of A,, the cohesive force of the expansive soil is lower than the original soil, but the internal friction angle is larger. However, both of the two factors increase with time. After 30 days, the cohesive force would become the same value as the original soil. After 30 days standard culturing, the swelling-shrinkage characteristics of the expansive soil are obviously reduced than the original expansive soil and the total swell-shrink ratio is reduced from 9.45% to 3. 35% , a drop of 65%. Through the X-ray diffraction analysis, the mineral compositions of the expansive soil make no obvious change by the action of microorganism. But from the SEM images, the microorganism effect results can be clearly seen in aberrance of the expansive soil micro-and meso-structure.%从膨胀土的胀缩性和改性机理出发,采用摇管分离法和混合平板法从农田土壤和膨胀土中分离出对膨胀土胀缩性改良效果较好的菌株A9,并制成菌剂.利用直接剪切试验和胀缩性试验,对接入菌剂A9并经不同时间培养的膨胀土进行胀缩性指标和强度指标测试.结果表明,加入菌剂A9后膨胀土的粘聚力小于原土,内摩擦角则大于原土,但二者均随培养时间的延长而增长,30天后粘聚力可恢复至原土的粘聚力值；经标准培养30天后膨胀土的胀缩性较原膨胀土显著降低,胀缩总率由9.45％降低至3.35％,降幅达65％.X射线衍射分析发现,在微生物作用下膨胀土的矿物成分并没有大的变化,但从SEM图可以清楚看出,微生物作用引起了膨胀土微细观结构的变异.
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