# Effect of Particle Size on Mechanical Property of Bio-Treated Sand Foundation
## 3 Results and Discussion
### The Spatial Distribution of Calcium Carbonate Precipitation in the Columns  
The uniformity of the material is a nonnegligible factor in the application of bio-treated sand foundation as an improvement technology [32], as it can affect the targeted strength. The distribution of calcium carbonate precipitation along the height of bio-treated granulometric columns for the current study is shown in Figure 7. It was obvious that the calcium carbonate precipitated more in the upper portion of sand column than any other part of these column. This is because the areas closest to bio-treatment were more easily exposed to mineralized cementation reagent than the other region of the cylinder [22]. Furthermore, this increased precipitation deposition on the upper surface may relate to the fact that the newly injected microbial cells were adsorbed by the upper sand particles, which led to a reduction in concentration of bacteria along the injection path [34].  
The calcium carbonate crystals in the whole sand column were unevenly distributed along the vertical direction. On the whole, the content of calcium carbonate in the sand column decreased from top to bottom, and the plugged phenomenon at the upper part of the column did exist during the bio-treated process (mentioned in Section 3.1). Quantitative analysis of each bio-treated column showed that the maximum range R of calcium carbonate content in the column along different heights was 4.6, 4.4, and 2.1%, respectively, accounting for 40.4, 30.1, and 12.9% of the average value in three groups of tests with different particle size ranges of Figure 7a-c. By the sand column of particle size of 300-600 \(\upmu\)m, the maximum range R of calcium carbonate content was decreased by approximately 54.3% compared with the particle size of 75-150 \(\upmu\)m.  
The coefficient of variation (_C-V_) of calcium carbonate contents was found to be, respectively, 10.3, 13.6, 11.9, and 15.4% for sample 1, 2, 3, and 4 in the Figure 7a. The _C-V_ was found to be, respectively, 11.2, 13.3, 10.2, and 13.1% for sample 5, 6, 7, and 8 in the Figure 7b. The average _C-V_ decreased by 6.7% compared with the particle size of 75-150 \(\upmu\)m, where the _C-V_ was found to be, respectively, 7.7, 5.1, 6.1, and 5.7% for sample 9, 10, 11, and 12 in the Figure 7c, and the average _C-V_ decreased by 52.0% compared with the particle size of 75-150 \(\upmu\)m.  
In the present study, the calcium carbonate distribution in consolidated column is more uniform for groups with larger particle size, which is expected to meet the requirements of soil foundation treatment technology. This is because it has the largest pore volume which allows easier flow for reagent solution and bacterial cells to occur. Previous studies had demonstrated that uneven deposition of calcium carbonate resulted in heterogeneity in strength distribution of engineering properties of improved sand foundation [35]. Therefore, the breakthroughs should be made in technology and injection methods. It is suggested that vertical growing technology should be improved to horizontal growing technology.  
Figure 7: Quantification calcium carbonate contents along the height of solidified sand specimens: (a) 75–150 \(\upmu\)m; (b) 150–300 \(\upmu\)m; (c) 300–600 \(\upmu\)m.