An increase in urban population and the reduced number of suitable lands for construction projects have necessitated the need for ground improvement methods with no environmentally detrimental effects. Microbial-induced calcite precipitation (MICP) is a relatively environmentally friendly method for soil regeneration. In the present paper, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed to investigate the type and method of cement formation in loose quartz sand with different densities stabilized using bacteria. The results showed that the highest content of calcium carbonate cement was produced in the sample with the lowest density. Moreover, after microbial stabilization, the shear strength measured in the direct shear test was increased in the specimen with a lower density. We observed an increase in shear strength from 0.63 kg/cm² before injection for loose sand (γ = 1.5 g/cm³) to 3.92 kg/cm² after injection. In addition, the effect of injection time was investigated and found that shear strength in the two-stage injection per day is greater than that in one- and three-stage injections per day. With prolonging the injection time from one to five days, shear strength was significantly increased from 2.07 to 4.54 kg/cm². Furthermore, prolonging the bacterial treatment period led to a significant increase in the produced carbonate cement and, consequently, enhanced soil shear strength.

The Menard pressuremeter test is a relatively expensive in situ test, which generates useful information about the strength and deformation properties of any soil and weak rock, which is carried out in some projects. On the other hand, the SPT test is a rather inexpensive, simple and typical in situ test used to determine the engineering properties of silt, clay, sand, and fine gravel which is utilised in almost all projects. In this study SPT blow counts (N₆₀) were correlated with pressuremeter modulus (E_PMT) and limit pressure (P_L) and empirical equations were proposed to estimate P_L and E_PMT from N₆₀ in silty sand and silty clay soils separately. These tests have been conducted during subway geotechnical investigation in Mashhad, Iran. Moreover, in order to verify these empirical equations, they were compared with similar equations that have been proposed by other researchers. These comparisons display that in all equations a linear relationship exists between N₆₀ - E_PMT and N₆₀ - P_L. However, the line slopes are different so it can be concluded the line slopes are related to soil type and geological condition of an area. Thus, for each area a separate empirical equation must be presented.

Today, with regard to land shortage and ever-increasing growth of the world’s population, construction on loose soil has been inevitable. Therefore, to deal with this issue, it is necessary to improve project site materials. The conventional soil improvement techniques are limited due to high costs and environment unfriendliness. In this regard, microbial-induced calcite precipitation (MICP) is a relatively environment-friendly method that is efficient for soil improvement. In this method, hydrolysis of urea by bacteria containing urease enzyme and in the presence of soluble calcium ions leads to the precipitation and attachment of CaCO₃ among the soil grains. In this study, the effects of grain size, a method of infiltration, infiltrated bacteria suspension (BS) volume and infiltrated fixation solution (FS), and cement solution (CS) on Quartz sand shear strength are investigated. The results of the conducted tests showed that among four different infiltration methods, continuous infiltration of BS and CS, have the strongest effect on shear strength. Studying shear strength two different sand sizes based on direct shear test showed that sand with larger effective size and higher permeability has a shear strength 6.2 times compared to non-treated sand. The results showed that soils with less volume of infiltrated bacteria have a higher shear strength. Adding a FS to three different BS volumes showed the one with the lowest amount has the highest shear strength. According to the X-ray diffraction and scanning electron microscopy analyses, the efficient CaCO₃ precipitation between the soil particles is confirmed.