Abstract The contribution of indigenous bacteria and of some possible precipitation due to chemical reactions occurring during the treatment of sandy soils through microbially induced calcite precipitation (MICP) is investigated in this paper considering two different grading size distributions. This is a novelty, because the presence of the feeding solution alone in the soil (usually used as control test, and named here as CICP) is usually not quantified as it is assumed that the total amount of calcium carbonate is formed only due to ureolytic activity of the bacteria added to soils. However precipitation can occur because bacteria naturally exist in soils and also, even if less likely, due to CO2 sequestration in a process called chilled ammonia process. The justification is because the ions necessary for forming calcium carbonate, both by the indigenous bacteria or by CO2 sequestration are supplied in the feeding solution (solution with urea, ammonium and a calcium source). Experimental tests were performed on a set of sand samples subjected to biological treatment using bacteria and another set exposed to chemical treatment using only the feeding solution, after checking that natural bacteria present in the soil would not interfere with the process. The samples were prepared with similar dry density. They were analysed through mercury intrusion porosimetry tests, chemical analysis and unconfined compression strength tests performed after partially drying the samples. Although the hydrolysis of urea made by the added bacteria speeds up the precipitation of calcium carbonate, the results showed that CICP appears to be independent from sand grading size distribution and introduces significant strength increment. For this reason, the treatment will have effect even if the bacteria added die. Nevertheless, it is important to promote adequate conditions for these bacteria to survive because higher strength values were measured in the samples where MICP treatment was done, and therefore the effectiveness of the technique increases in significant manner.

中文翻译：

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关于砂生物胶结上碳酸钙的沉淀

摘要 本文考虑了两种不同的分级粒度分布，研究了本地细菌的贡献以及由于在通过微生物诱导的方解石沉淀 (MICP) 处理沙质土壤过程中发生的化学反应而导致的一些可能的沉淀。这是一个新奇事物，因为土壤中单独存在的补料溶液（通常用作对照试验，此处称为 CICP）通常无法量化，因为假设碳酸钙的总量仅由于尿素分解而形成添加到土壤中的细菌的活性。然而，由于细菌天然存在于土壤中，并且即使不太可能，由于在称为冷冻氨工艺的过程中二氧化碳封存，也会发生沉淀。理由是因为形成碳酸钙所需的离子，由本地细菌或通过 CO2 封存提供给进料溶液（含尿素、铵和钙源的溶液）。在检查土壤中存在的天然细菌不会干扰该过程后，对一组使用细菌进行生物处理的沙样和另一组仅使用进料溶液进行化学处理的沙样进行了实验测试。制备具有相似干密度的样品。通过压汞孔隙率测试、化学分析和部分干燥样品后进行的无侧限抗压强度测试对它们进行了分析。虽然添加细菌产生的尿素水解加速了碳酸钙的沉淀，结果表明，CICP 似乎与砂的级配粒度分布无关，并引入了显着的强度增量。因此，即使添加的细菌死亡，治疗也会有效果。然而，促进这些细菌存活的适当条件很重要，因为在进行 MICP 处理的样品中测量到更高的强度值，因此该技术的有效性显着增加。