EXPERIMENTAL STUDY ON BACKFILLING OF MICROBIAL MODIFIED FERTILIZER TANK

In modern urban construction, deep and large foundation pits, and narrow and ultra-deep fertilizer tanks are becoming more and more common. Due to the limited working space, the backfill quality of such fertilizer tanks is generally low, resulting in uneven settlement, water seepage, and anti-floating problems of soil around the fertilizer tanks in the later stage. New and effective measures are urgently needed to improve the backfill quality of fertilizer tanks. Microbial induced calcium carbonate deposition (MICP), as a new reinforcement technique, can be used to improve the strength of backfill and reduce the permeability of backfill. In this paper, laboratory experiments were conducted to discuss the modification effect of the two modification methods under different cell concentrations of the bacterial solution, the proportion of cementing dry powder applied, and the proportion of cementing liquid concentration. Then, the modification method with the best ratio was applied to practical projects for verification. The results showed that, compared with the unmodified sample, the permeability coefficient of the mixing method was significantly reduced by 2 to 4 orders of magnitude. Both the seepage method and the mixing method can effectively improve the quality of the backfill. Compared with the mixing method, the seepage method had a better modification effect and controllable cementation quality. The optimal additive ratio of the percolation method was 10 times diluted bacterial solution and low concentration 0.5 mol/L∶0.5 mol/L (concentration ratio of urea to Call₂)of cemented solution. Field verification test showed that the unconfined compressive strength of the modified fertilizer tank backfill was increased by 174.2% to 772.7%, compared with the conventional measure. The compressibility coefficient of the modified fertilizer tank backfill was smaller than that of the conventional backfill, and the medium compressible soil became low compressible soil. The porosity ratio and permeability coefficient of modified fertilizer tank backfill were obviously lower than those of conventional backfill. After the test, the non-uniform settlement of the backfill in the fertilizer tank was basically eliminated, the ability to block groundwater seepage was enhanced, and the quality of the backfill was significantly improved.