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<strong>2.1 Materials</strong> <em>Sporosarcina pasteurii</em>, a type of urease-producing bacteria, was used in this study. The sterilized growth medium included 20g/l yeast extract, 15g/l NH4Cl, and 0.1 Mm NiCl2, at pH=9.25. The bacterial culture was then inoculated into a conical flask and cultured in a shaker at 30℃ at 150 rmp for 24 hours. The cementation solution was 1.0 M calcium chloride and 1.0 M urea (Whiffin, Van Paassen, and Harkes 2007). The concentration of the bacteria was determined by measuring the optical density via an ultraviolet spectrophotometer at a wavelength of 600 nm. After culturing for about 36 hours, the bacteria were harvested, and the <em>OD600</em> value was 1.2 with urease activity of 11 U/ml. The properties of soft soil were as followed: nature moisture content 54%, porosity ratio 1.483, liquid limit 60.2%, plastic limit 33.5%, organic matter content 5.6% and it was oven-dried with 60℃ for 48 h before the experiment. The spilt modules with 39.1 mm diameter and 80 mm height were used to prepare the samples. <strong>2.2 Testing program and sample preparation</strong> <strong>2.2.1 Preparation of remolded soft soil sample</strong> The soft soil was first mashed with a rubber mallet, and then the soil slurry with a certain moisture content of 200% was produced by mixing the soft soil with a predetermined quantity of deionized water in a homogeneous state in a mixer for 20 min. Next, the soil slurry passed through a 2 mm sieve to remove the shell and gravel. After fully immersing for 7 days, the soil slurry was slowly poured into a cylindrical barrel (height 60 cm and inner diameter 9 cm) made of polymethyl methacrylate (PMMA) along the barrel wall. Notably, a porous stone was placed on the bottom of the barrel with a piece of filter paper placed on the surface. The height of the sludge slurry was 50 cm, and a porous stone with filter paper, which was on the bottom of the porous stone, was also placed on the top surface of the sludge slurry. Next, six rings made of PMMA and weights were placed on the porous stone one by one in order to obtain the total pressure of 60 kPa (<strong>Fig. 1)</strong>. The standard samples (height 80 mm and diameter 39.1 mm) were prepared after the sludge slurry was consolidated for 6 months. <strong>Fig. 1</strong> Remolded soft soil preparation <strong>2.2.2 Application of bio-cementitious mortar treatment in soft soil</strong> The soft soil passing through a 2 mm sieve was divided into two parts which were used to prepare the MICP-S soft soil samples and C-MICP-S soft soil samples according to <strong>Table 1</strong> and <strong>Table 2,</strong> respectively. Notably, the deionized water was used to replace the bacteria solution and cementation solution. The procedures of sample preparation are as follows: (1) The bacteria solution and cementation solution (adding cement for C-MICP-S sample) was mixed first and then poured into the soft soil for 10 min to achieve uniformity. (2) Then, the mixture was transferred into the spilt modules. Two detachable plastic lids were used to seal both ends. (3) After 24-48 hours of curing, the stabilized soil specimens were removed from the molds and wrapped using plastic bags, cured in the controlled environment (25±2℃ and 95% relative humidity). <strong>Table 1</strong> Sample arrangement for the experiment of MICP-S soft soil <br> No. <br> <em>OD600</em> <br> Cementation solution concentration (mol/L) <br> Moisture content (%) <br> Curing time (days) <br> Confining pressure (kPa) Control Group <br> 1 <br> 0 <br> Deionized water <br> 50 <br> 3, 7, 14, 28 <br> 100, 200, 300 Test Group <br> 2 <br> 0.3 <br> 0.5 <br> 50 <br> 7, 14, 28 Test Group <br> 3 <br> 0.3 <br> 1.0 <br> 50 <br> 3, 7, 14, 28 Test Group <br> 4 <br> 0.3 <br> 1.5 <br> 50 <br> 7, 14, 28 Test Group <br> 5 <br> 0.3 <br> 2.0 <br> 50 <br> 7, 14, 28 Test Group <br> 6 <br> 0.6 <br> 0.5 <br> 50 <br> 7, 14, 28 Test Group <br> 7 <br> 0.6 <br> 1.0 <br> 50 <br> 3, 7, 14, 28 Test Group <br> 8 <br> 0.6 <br> 1.5 <br> 50 <br> 7, 14, 28 Test Group <br> 9 <br> 0.6 <br> 2.0 <br> 50 <br> 7, 14, 28 Test Group <br> 10 <br> 0.9 <br> 0.5 <br> 50 <br> 7, 14, 28 Test Group <br> 11 <br> 0.9 <br> 1.0 <br> 50 <br> 3, 7, 14, 28 Test Group <br> 12 <br> 0.9 <br> 1.5 <br> 50 <br> 7, 14, 28 Test Group <br> 13 <br> 0.9 <br> 2.0 <br> 50 <br> 7, 14, 28 <strong>Table 2 </strong>Sample arrangement for the experiment of C-MICP-S soft soil <br> No. <br> Water content(%) <br> Cement mixing ratio(%) <br> Water-cement ratio <br> Treatment method Test Group <br> I <br> 50 <br> 10 <br> 0.5 <br> MICP Test Group <br> II <br> 60 <br> 10 <br> 0.5 <br> MICP Test Group <br> III <br> 70 <br> 10 <br> 0.5 <br> MICP Test Group <br> IV <br> 80 <br> 10 <br> 0.5 <br> MICP Control Group <br> V <br> 50 <br> 10 <br> 0.5 <br> Deionized water Control Group <br> VI <br> 60 <br> 10 <br> 0.5 <br> Deionized water Control Group <br> VII <br> 70 <br> 10 <br> 0.5 <br> Deionized water Control Group <br> VIII <br> 80 <br> 10 <br> 0.5 <br> Deionized water <strong>2.3 Testing methods</strong> The MICP-S samples were applied to a triaxial compression test (unconsolidated and undrained, UU) with confined pressure of 100, 200, and 300 kPa. The shear rate was 0.08 mm/min (Lo and Wardani 2002; Zhao et al. 2019) and the experiment ended when the strain reached 20%. Separately, the uniaxial compression tests of C-MICP-S soft soil were conducted at a strain rate of 2.5% per hour (Mortensen and DeJong 2011; Montoya and DeJong 2015a) until the samples were destroyed. The internal friction angles φ and cohesion c of MICP-S samples were calculated from the Mohr circle envelope. For the C-MICP-S samples, the UCS was calculated by Eq. 8, and the elastic modulus and toughness were obtained from the stress-strain curves, as shown in <strong>Fig.2</strong>. After the UU test and UC test, SEM analysis was conducted on the remolded soil, MICP-S and C-MICP-S samples. Where σ= the value of UCS, MPa; F= the ultimate load, kN; A= the compressive area of the sample, mm2. <strong>Fig. 2 </strong>Schematic diagrams for calculating elastic modulus and toughness <strong>a. </strong>elastic modulus <strong>b.</strong> toughness