With the development of engineering construction,

building floors are gradually increasing in height,

and foundation engineering has become particularly important;

the quality of foundation engineering directly affects the engineering quality of the entire building.

Only by laying a solid foundation can the safety of subsequent construction and the stability of the building be effectively guaranteed.

With the vigorous development of infrastructure construction in China, reinforcing various problematic foundation soils has become an important issue in engineering construction.

Traditional foundation reinforcement methods have adverse effects on subsequent plant growth and the sustainable development of the environment.

Bio-geotechnical technology

Bio-geotechnical technology uses the decomposition capacity of specific microorganisms to trigger a series of reactions in soil that generate calcium carbonate crystals, binding loose soil particles into a whole; it is an interdisciplinary product of biotechnology and geotechnical engineering.

Due to its advantages of being environmentally friendly and low in energy consumption, bio-geotechnical technology has attracted academic attention since its inception. Scholars from various countries have continuously conducted pioneering experimental research, gradually forming the relatively mature MICP (microbially induced calcium carbonate precipitation) technology.

For silt and cohesive soil composed of finer particles, current research on MICP is still far from sufficient.

Therefore, Peng Bangyang, Zhao Zhifeng, and others from Nanjing Forestry University took the marine hydraulic fill silt from the "Dongtai Tiaozini Reclamation Project," the largest reclamation project along the eastern coast of Jiangsu, as the research object. On the basis of having achieved good reinforcement results using pressure grouting, they studied the feasibility of reinforcement by surface infiltration-induced calcium carbonate precipitation, aiming to find a new method for reinforcing silt using MICP technology and to further understand the mechanism of microbial cementation reinforcement of soil.

1 Materials and methods

1.1 Preparation of bacterial solution

1.2 Test soil

1.3 Preparation of cementation solution; the cementation solution is calcium chloride

1.4 Test apparatus

1.5 Determination of reinforcement effect

1.5.1 Evaluation of water stability

1.5.2 Determination of the percentage of CaCO3 generated

2 Feasibility study of surface infiltration reinforcement

Some researchers in China have used the method of cyclically alternating infiltration of bacterial solution and cementation solution to reinforce silt, but the results were far from satisfactory. The main reason is that the pore throat size of silt particles is small, so that when the bacterial solution infiltrates, it first reacts with the cementation solution in the upper part of the sample, and the generated calcium carbonate crystals block the downward infiltration of subsequent liquid, resulting in unsatisfactory reinforcement. To solve the above problem, two experimental schemes were adopted: 1) use the supernatant instead of the bacterial solution to avoid clogging (Fig. 2); 2) mix the bacterial solution with the silt to prepare samples, which not only avoids uneven distribution of microorganisms but also uses microorganisms as nucleation sites for CaCO3 crystals.

2.1 Cyclic alternating infiltration of supernatant and cementation solution

2.1.1 Test method

The supernatant was used instead of the bacterial solution, and the supernatant and cementation solution were alternately infiltrated for reinforcement. Specific method: drip 1 pore volume (Vv) of supernatant onto the soil sample surface, and after an interval of 12 h drip an equal volume of 1 mol/L cementation solution; this constitutes one round of infiltration testing; after an interval of 12 h, carry out the next round of infiltration testing... Three and six rounds of infiltration testing were performed on two soil samples, respectively. After the test, the soil samples were cured at room temperature (25 °C) for 24 h, and then water stability tests were carried out; for soil that had not disintegrated, the percentage of CaCO3 generated was determined.

2.1.2 Test results

The water stability results of the samples after different numbers of infiltration rounds are shown in Fig. 3. Among them, the sample after 3 rounds of infiltration basically completely disintegrated, while the sample after 6 rounds of infiltration formed a reinforcement layer about 1 cm thick at the top, with the rest disintegrating. Measurement showed that the amount of CaCO3 generated in the sample after 3 rounds of infiltration was extremely low (1% of the sample mass), making it difficult to form effective cementation, so the water stability was very poor. After 6 rounds of infiltration, the amount of CaCO3 generated in the sample reached 6% of the total mass, forming good cementation in the upper part. From the test results, it can be seen that using supernatant instead of bacterial solution can avoid microbial clogging of pore throats, thereby improving the reinforcement effect of soil samples, and when the number of infiltration rounds is large, a reinforcement layer of a certain thickness can be formed. However, this method also has great limitations: the urease rich in the supernatant remains in the soil, and CaCO3 crystals form first in the upper part of the sample with a relatively fast reaction, while effective cementation is still difficult to form in the middle and lower parts of the sample.

2.2 Mixing soil samples with bacteria and cyclically infiltrating cementation solution

To solve the problem of the limited reinforcement effect of the supernatant, the bacterial solution and silt were mixed together to prepare samples, so as to achieve uniform distribution of microorganisms in the sample; then the cementation solution was infiltrated multiple times from the top of the sample, so that the cementation solution reacted with the microorganisms in the soil sample to generate CaCO3 crystals.

2.2.1 Test method

Add 1.2 pore volumes (1.2Vv) of bacterial solution to oven-dried, sieved silt and mix evenly; place the mixed soil into a container and prepare samples. After sample preparation, two groups of comparative tests were conducted: Group 1, first drip 1Vv of nutrient solution onto the sample surface, and after an interval of 12 h drip an equal volume of 1 mol/L cementation solution; this constitutes one round of infiltration testing, and after an interval of 12 h carry out the next round of infiltration testing... Three rounds (Sample 1) and six rounds (Sample 2) of infiltration were performed on two samples, respectively. Group 2, without adding nutrient solution, only drip 1Vv of 1 mol/L cementation solution onto the sample surface, and after an interval of 12 h infiltrate the next round of cementation solution... Three rounds (Sample 3) and six rounds (Sample 4) of infiltration were performed on two samples, respectively. After reinforcement, the samples were placed at room temperature (25 °C) for 24 h, and then water stability tests were carried out; for samples that had not disintegrated, the amount of CaCO3 generated was determined.

2.2.2 Test results

The test results show that both methods can effectively improve the water stability of silt, with the reinforcement thickness after 3 rounds of infiltration being 80%–90% of the sample height, and the reinforcement thickness after 6 rounds of infiltration almost reaching the entire sample height (Fig. 4). From the comparison of the two groups of test results, it can be seen that the addition of nutrient solution did not achieve the purpose of supplementing nutrients for microorganisms and promoting the reinforcement effect. Samples treated with cementation solution alone had better water stability, which may be because the addition of nutrient solution affected the uniformity of reinforcement. To verify this, the mass percentage of CaCO3 generated at different locations was determined.

From the distribution of CaCO3 in the four samples (Fig. 5), regardless of whether nutrient solution was added, the CaCO3 generated by 6 rounds of surface infiltration (Samples 2 and 4) was significantly higher than that by 3 rounds of infiltration (Samples 1 and 3). In the samples with added nutrient solution (Samples 1 and 2), no more CaCO3 was generated, and after 6 rounds of infiltration the distribution of CaCO3 was more uneven. This indicates that the addition of nutrient solution cannot increase the amount of CaCO3 generated, but may instead accelerate the reaction rate in the upper part of the sample and affect the uniformity of reinforcement. Therefore, in subsequent tests, the addition of nutrient solution was canceled, and the method of mixing bacteria into the soil sample followed by surface infiltration of cementation solution was adopted to reinforce silt.

It should be noted that, since the initial stage of the test was an exploration of feasibility, various methods were attempted, and many methods did not meet expectations, so parallel tests were not carried out; however, in the subsequent tests after finding an effective method, parallel samples were used. In addition, in the determination of the amount of CaCO3 generated, there are errors in both the sampling and filtration steps at different locations, and because the sample size in this section was small, the relative error is more obvious.

3 Study on the scope of surface infiltration reinforcement

3.1 Tests on specimens of different sizes

Through the previous tests, a new method for reinforcing silt had already been found, but the sample size was small (diameter 5 cm, height 3 cm). To further verify the feasibility of this method, reinforcement was carried out on two samples of different diameters (diameters 5 and 16 cm, both 3 cm in height). Both samples were mixed with bacterial solution of the same pore volume and concentration, and after an interval of 12 h, 1Vv of 1 mol/L cementation solution was infiltrated on the surface; the cementation solution was infiltrated for a total of 5 passes. The samples after treatment and water stability testing are shown in Fig. 6. The results show that the water stability of the samples was good, and this method also achieved good cementation effects on silt samples of different sizes.

3.2 Study on reinforcement depth

To study the influence of different factors in the surface infiltration method on reinforcement depth, the tests in Table 3 were designed. The four samples had the same size (diameter 5 cm, height 10 cm), while factors such as cementation solution concentration differed. The water stability test results of each sample after treatment are shown in Fig. 7. Sample a and Sample b achieved effective cementation over the entire height, with very good water stability; the effective reinforcement thickness of Sample c was about 5 cm, the worst among the four samples, and the effective reinforcement thickness of Sample d was about 6 cm. The test results show that the concentration of bacterial solution had little effect on the reinforcement effect; the bacterial solution concentration of Sample b was twice that of Sample a, but the reinforcement effect was almost the same. From the mass percentage of CaCO3, Sample a with the lower bacterial solution concentration had 16.5% CaCO3, slightly higher than 15.6% in Sample b. However, the concentration of cementation solution had a significant effect on the reinforcement effect: after the cementation solution concentration was reduced to 0.5 mol/L, the reinforcement depth decreased significantly. It is worth noting that when the cementation solution concentration was low, increasing the number of infiltration passes of cementation solution could not increase the reinforcement depth. For example, Sample a was infiltrated 5 times with 1 mol/L cementation solution, and Sample d was infiltrated 10 times with 0.5 mol/L cementation solution; the total amount of cementation solution used for the two samples was the same, but the effects differed markedly. In this test, using a higher concentration and fewer passes of cementation solution achieved better results in silt reinforcement.

Conclusions

On the basis of previous work using grouting methods to reinforce marine silt with MICP technology, this study explored a new surface infiltration reinforcement method, and through a series of tests obtained the following conclusions:

1) The main difficulty in reinforcing silt by liquid surface infiltration lies in the compatibility between microorganisms and the pore throat size of silt. Using supernatant instead of bacterial solution for surface infiltration can avoid clogging and form effective cementation reinforcement on the surface layer of the soil sample, but the reinforcement depth is limited.

2) Mixing the bacterial solution with silt to prepare samples, and then infiltrating cementation solution from the surface for reinforcement, significantly improved the reinforcement depth, water stability, and calcium carbonate generation of silt samples; however, the addition of nutrient solution adversely affected the uniformity of reinforcement.

3) Tests with different sizes and different influencing factors further verified the effectiveness of the method of mixing bacteria and infiltrating cementation solution for reinforcing marine silt; when the parameters are reasonable, the entire sample can be effectively cemented. Reinforcement with a higher concentration (1 mol/L) and fewer passes of cementation solution achieved better results.