Since the 21st century, most researches still focus on the process of bacterial influence of MICP, however, most samples are mineralized by injecting the binding solution into the plexiglass column or syringe through a peristaltic pump to prepare the sample [62–64]. Weaver [65] conducted a series of experiments by injecting a mixed solution of urea-calcium chloride into a syringe. Yasuhara [66] periodically injected urease-solidification samples with urea and calcium chloride solutions to prepare samples for unconfined compressive strength tests. These curing perfusion methods are easy to make the bacteria liquid and cementation liquid react first at the perfusion port, in addition, the sand body or loess near the water inlet is blocked by the calcium carbonate crystals formed earlier, and it is difficult for the subsequent bacterial solution to be injected into the sample, this results in uneven cementation of the bacteria and the resulting calcium carbonate in the sample. Therefore, this experiment also carried out a new exploration of the MICP curing method, using the stirring and mixing method to mix the bacterial liquid, the cementation reagent and the sample, avoid the high-intensity and concentrated injection of bacterial liquid leading to crystal blockage of the injection port, which can make the sample mix more uniform and the sample more fully cemented.
2.1 Bacterial liquid
The test strain is Sporosarcina pasteurii, the glycerol strains of the US National Culture Bank No. (ATCC11859) were inoculated in the sterilized LB liquid culture medium, After the expansion treatment, the cells were cultured in a shaking incubator at 30°C and 200 rpm for 30 h. The OD600 value of the activated and cultured bacterial liquid used in this paper was tested by UV spectrophotometer to 1.25, the original bacterial solution was diluted with physiological saline (0.9% NaCl aqueous solution) to meet the requirements of the experiment. According to the OD600 value of the bacterial solution and the bacterial concentration, referring to the conversion formula proposed by Ramachandran et al [67], the converted bacterial concentration is 11.64×107 cells/mL.
2.2 cementation reagent
The cementation reagent is the key to the reaction with the bacterial liquid, and the cementation reagent mainly contains Urea and CaCl2, among them, urea provides nitrogen and energy sources for microorganisms, and Ca2+ in CaCl2 provides calcium sources for the mineralization of microorganisms, the test cementation reagent used Urea-CaCl2 mixture, and the molar ratio of Urea and Ca2+ was 1:1. The concentration of the cementing solution used in this test is 0.25mol/L, that is, the same volume of 0.25mol/L Urea solution and 0.25mol/L CaCl2 solution are mixed to prepare 0.25mol/L cementing solution for later use.
2.3 Silt and rocks
In this experiment, silt and rocks were selected as the reinforcement objects of microbial grouting. The soil used for the test was excavated at a construction site in Beijing, and the soil samples were screened by a multi-functional vibrating ballast cleaning machine, the basic physical parameters of the silt are shown in Table 1. The particle size is 0-0.075 mm and 0.075-0.1 mm, the void ratio e is 0.54, the initial moisture content is 5%, and the plasticity index is 5.98.
Table 1
Basic physical parameters of silt
soil sample name
|
particle composition mm
|
void ratio e
|
Moisture content W/%
|
Liquid limit WL/%
|
Plastic limit WP/%
|
Plasticity Index IP
|
Soil specific gravity GS
|
dry density ρd
|
Silt
|
d ≤ 0.075
87%
|
0.075<d ≤ 0.1
23%
|
0.54
|
5%
|
21.52
|
15.54
|
5.98
|
2.7
|
1.7265
|
2.4 Test plan
In order to reduce the difference in mineralization caused by the unevenness of the stones, small-grained pebbles of uniform particle size were selected as the stones in the soil-rock mixture. Three kinds of boulders with different particle sizes were selected, 0.2-0.4cm, 0.4-0.6cm, 0.6-0.9cm, respectively, Fig. 1 shows the test silt and rocks. Five different stone content ratios were selected, 20%, 30%, 40%, 50%, 60%, respectively, a total of 15 groups of soil-rock mixing ratio tests were carried out, as shown in Table 2.
Table 2 Mixing ratio scheme of soil and rock
After the soil sample and the boulders were thoroughly mixed in proportions, the Urea solution 0.25mol/L, the CaCl2 solution 0.25mol/L, respectively, were prepared, mix Urea and CaCl2 solution to make 0.25mol/L cementation reagent. The moisture content of the soil-rock mixture is set to 12%, and the volume ratio of cementitious liquid to bacterial liquid is set to 1:1, pour the cementitious liquid into the soil-rock mixture in proportion, and place it in a moisturizing bag for 6 hours, in order to ensure that the water molecules of the cementing solution diffuse evenly in the soil and rock, to remove the air and seal the tank, after standing for 6 hours, microbial grouting was carried out at room temperature (25 ± 2°C).
The soil-rock mixture sample was prepared using a triaxial saturator, and the sample size was 39.1mm (diameter) × 80mm (height). Before sample preparation, a certain soil-rock mixture sample was weighed, and a certain quality of cementitious liquid was added according to the proportion of moisture content, after stirring evenly, seal it with a fresh-keeping bag and place it in a moisturizing tank for 6 hours to make the calcium ions in the soil distribute as evenly as possible. Before sample preparation, 1.25 mol/L bacterial solution was mixed with the soil and rock body after 6 hours of simmering in the tank. When using a triaxial saturator for sample preparation, apply a small amount of Vaseline in the copper mold, in order to reduce the friction between the sample and the cylinder wall, the sample soil-rock mass will be prepared according to the quality. Each group of three soil-rock mixture samples with the same configuration is prepared, each sample was hammered 30 times, and it was loaded into the mold in 5 layers, the surface of the sample was flattened with a ring knife, and finally the sample was demolded and placed in an ambient temperature of 30°C, 5 days of curing in a constant temperature and humidity curing box of 70 ± 2% ambient humidity. Among them, 30 ℃ provides the best growth temperature for bacteria, according to the relative humidity of the China Meteorological Science Data Sharing Service Network on the official website of the China Meteorological Administration, a humidity environment of 70 ± 2% was used to simulate the underground soil humidity environment in Beijing.
In this test, manual operation was used, and the soil-rock mixture sample that had been cured for 5 days was put into a strain-type unconfined compression instrument, adjust the instrument to conduct a uniaxial unconfined compressive strength test on the sample at a uniform strain rate and a loading rate of 0.5 mm/s, and record the data. When the sample is damaged, that is, after the inflection point of the test record data appears, continue the test depending on the damage of the sample, the test was stopped when the sample was damaged to a certain extent, and the corresponding data was calculated.
2.5 Microscopic analysis of soil-rock mixture solidified by microorganisms
Microbial mineralization is a biological reaction, and the bacteria itself has the characteristics of small size, large number and rapid response. When the microorganisms act on the soil-rock mixture, the bacterial liquid distribution acts on the gaps between the particles, and the resulting crystals are small in size, strong dispersibility, it is necessary to use scanning electron microscope, SEM and phase analysis of xray diffraction, XRD to conduct microscopic observation of soil-rock mixture, combined with the laws of mechanical properties of the soil-rock mixture after improvement, the energy spectrum analysis and phase analysis of the samples were carried out, explore the material structure, morphology, content, distribution and other laws of microbial mineralization. The SEM was a Zeiss ZEISS GeminiSEM 300 electron microscope, it can be magnified up to 1 million times, very tiny material structures can be observed; The model used by the EDS spectrometer is Smartedx, and the spectrometer uses the different characteristics of X-ray characteristic wavelengths of different elements to analyze the composition of the sample; XRD is a technique for analyzing the structure of materials by utilizing the diffraction effect of X-rays in crystalline materials. Take the broken block in the center of the soil stone column, grind the sample finely, grind the sample to less than 200 mesh, sieve, and test the chemical composition of the crystal in the sample by the tablet method.