Fibre-Microbial Curing Tests and Slope Stability Analysis

Abstract

In response to the deformation resistance deficiency and poor toughness characteristics of soil after microbial curing, a combination of fibre reinforcement technology and microbial curing technology was used to conduct microbial curing tests using basalt fibres and denitrifying bacteria. In this paper, the effects of fibre on the strength and toughness of soil consolidation were analysed by unconfined compressive strength test and direct shear test, and the stability of reinforced slope was analysed by numerical simulation. The results show the following. (1) Basalt fibre can effectively improve the characteristics of brittle damage of microbially consolidated soil while increasing the compressive and shear strength. (2) Fibre dosing and fibre length have important effects on the mechanical properties of microbially consolidated soil. (3) The appropriate amount of basalt fibre can promote the generation of calcium carbonate. (4) The plastic strain area of the slope decreases after microbial reinforcement and the maximum equivalent plastic stress decreases by 65 kPa.

1. Introduction

Microbially induced calcium carbonate deposition (MICP) is an emerging geotechnical improvement technology that uses calcium carbonate cemented soil particles produced by microbial metabolic activities to enhance the physical and mechanical characteristics of geotechnical bodies [1,2]. This has great potential for application in many fields including soil mechanical modification, embankment erosion resistance, and prevention of heavy metal ion pollution [3,4,5,6,7]. The use of microorganisms to reinforce the slope surface improves the stability of the slope while improving the soil, facilitating plant growth, and being ecologically friendly.

MICP technology has received wide attention from researchers at home and abroad because of its simplicity of operation, high efficiency of reinforcement, and lack of pollution to the environment [8], and much research has been conducted on the engineering properties of MICP-modified soils [9,10,11,12,13,14,15,16,17,18]. Chu et al. [19] used MICP technology to improve sandy soils to form a weakly permeable crust layer, which served the purpose of enhancing impermeability in engineering. Liu Lu et al. [20] used a microbial curing method to treat dikes, and the experimental results showed that the treated dikes improved the erosion resistance. Li Chi [21] used microbially induced calcium carbonate precipitation (MICP) technology combined with adsorbent materials to cure/stabilise the remediation of Zn-Pb composite heavy metal contaminated soil, revealing the remediation mechanism of MICP technology to treat Zn-Pb heavy metal contaminated soil. Liu Xiaojun [22] used MICP technology for soil site fracture remediation and curing. These studies showed that MICP, as an emerging soil consolidation technology, has potential practical engineering value and can be applied to various fields. However, the strength and toughness of materials are usually opposed to each other, and some studies have shown that microbially cured soils exhibit significant brittle characteristics [23], which to some extent inhibits the application of MICP technology in practical engineering. Therefore, it is necessary to investigate how to improve the toughness of microbially cured specimens.

Studies have demonstrated that by adding discrete short filament fibres as reinforcement to the soil, the soil becomes substantially less brittle when it fractures, increasing its strength [24]. Through indoor research, Yetimoglu et al. [25] observed that fibre reinforcement has no discernible impact on peak shear strength, but that it can ameliorate sandy soil shear brittle damage by increasing the residual shear strength of the soil sample by increasing the amount of fibre admixture. According to Shao et al. [26], adding fibres to sandy soil had an impact on its shear strength, lowering strength loss after the peak strength and improving the nature of brittle damage. Wei Li et al. [27] used wheat-straw-fibre-reinforced seaside saline soil and found that fibre reinforcement increased the cohesion c of saline soil substantially, and its resistance to deformation was greatly enhanced.

For this reason, this paper uses a combination of fibre reinforcement technology and microbial curing technology for soil curing tests. Based on the unconfined compressive strength test and direct shear test, the effects of basalt fibre on the mechanical properties of microbial soil consolidation are analysed, and a three-dimensional slope model is established by ABAQUS finite element software to analyse the stability of the slope after microbial consolidation.

2. Materials and Methods

2.1. Test Soil

The soil used for the test is taken from Longlang Expressway, Xinhua County, Loudi City, Hunan Province. This soil is clayey, and its physical parameters are listed in

Table 1. Physical parameters of clay.

Optimum Moisture Content/%	Maximum Dry Density/g·cm${}^{-3}$	Liquid Limit/%	Plasticity Index
16.8	1.68	35	18.6

.

2.2. Test Fibres

The fibres used for the test were basalt short-cut fibres. Compared with ordinary synthetic fibre, basalt fibre has obvious advantages in tensile strength, elastic modulus, impermeability, and freeze–thaw resistance, overcoming the shortcomings of synthetic fibres that are easily pulled off when cracks expand. Referring to Abd Al-kaream et al.’s [28] formulation study of polypropylene fibres for soft soil improvement, the fibres were added to the soil at 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 by mass. The fibre is produced by Zhejiang Haining Anjie Composite Material. A photograph is shown in Figure 1, and the physical and mechanical properties are listed in

Table 2. Physical and mechanical properties of fibre.

Physical and Mechanical Indicators	Monofilament Diameter/µm	Specific Gravity/g·cm${}^{-3}$	Modulus of Elasticity/GPa	Tensile Strength/MPa
Parameters	7∼15	2.63∼2.65	91∼110	3000∼4800

.

2.3. Bacterial Solution for Test

The microorganisms used were paracoccus, a type of denitrifying bacteria, purchased at the Shanghai Conservation Biotechnology Center (SHBCC). Denitrifying Bacillus is a heterotrophic, parthenogenic anaerobic bacterium that can survive in the medium of nitrate. It acts as a nitrogen source, reducing nitrate to nitrite and further reducing nitrite to ammonia and free nitrogen under insufficient soil oxygen. The medium consists of 5 g peptone, 3 g beef paste, 5 g sodium chloride, 15 g agar, and 1000 mL distilled water, and the specific method of activating the culture includes the following steps:

(1)

The strain is added to sterile water, gently shaken to dissolve, and inoculated on slant medium. After inoculation is complete, it is placed in an incubator for incubation, with the incubator temperature set at 30 °C and time set at 24 h, and finally placed in a refrigerator at 4 °C for backup.

(2)

The ingredients were weighed into triangular flasks according to the medium recipe, the agar was heated and melted, and the pH of the medium was adjusted to 7.0 using a solution of sodium hydroxide at a concentration of 1 mol/L. The triangular flask is added with a plunger and wrapped and placed in an autoclave for sterilisation, the sterilisation temperature is set at 120 °C, and the sterilisation time is set at 30 min.

(3)

After autoclaving, the triangular flasks were placed on a sterile operating table to cool. The cultured colonies were inoculated into the agar-free culture medium by aseptic operation and incubated for 36–48 h in an intelligent shaker set at an ambient temperature of 30 °C and a shaker speed of 150 r/min.

2.4. Test Cementing Solution

The cementing solution was a mixture of calcium chloride (CaCl₂), sodium nitrite (NaNO₂), and potassium nitrate (KNO₂). Among them, sodium nitrite and potassium nitrate provide nitrogen sources in the denitrification process of denitrifying bacteria, while calcium chloride provides calcium sources; calcium chloride is also the fixing solution of the bacterial solution, and calcium ions and bacterial cell walls have adsorption effects, which facilitate bacteria attachment to the surface of negatively nucleated soil particles and play the role of fixing bacteria. The sedimentation and cementation principle of denitrifying bacteria is the combination of carbonate ions and calcium ions to produce the cementation CaCO₂ precipitate. CaCO₂ adheres to the surface of soil particles and connects the loose particles by wrapping and filling the gaps between particles. The specific reaction equations are (1)–(3) [29].

$$5{\mathrm{CH}}_3{\mathrm{COO}}^{-}+8{\mathrm{NO}}_3^{-}+13{\mathrm{H}}^{+}\to 10{\mathrm{CO}}_2+4{\mathrm{N}}_2+14{\mathrm{H}}_2\mathrm{O}$$

(1)

$${\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{HCO}}_3^{-}+{\mathrm{H}}^{+}$$

(2)

$${\mathrm{Ca}}^{2+}+{\mathrm{HCO}}_3^{-}+{\mathrm{OH}}^{-}\to {\mathrm{CaCO}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}$$

(3)

2.5. Specimen Preparation Steps

(1)

The lower bedding block of the test mould was placed into the lower part of the test mould but exposed by approximately 2 cm.

(2)

According to the sample preparation standards, four equal portions of soil and basalt short-cut fibre were weighed, and the two were mixed and stirred, layered into the mould, and lightly compacted with a tamping rod to a predetermined height (20 mm per layer). After the sample was completed, the upper mat with the test mould was placed into the test mould, and the upper mat was exposed to approximately 2 cm.

(3)

A peristaltic pump was used to inject 50 mL of bacterial solution at a rate of 5 mL/min, and after standing for 4 h, the same volume of cementing solution was injected at a rate of 10 mL/min and allowed to stand for 6–8 h to ensure that the microorganisms reacted fully within the specimen.

(4)

A peristaltic pump was used to inject 50 mL of cementing solution into the specimen at a rate of 10 mL/min at an interval of 12 h. The injection was stopped after reaching a predetermined number of treatments, and water was continuously and slowly injected from the top of the specimen to clean the inside of the specimen to terminate the microbial curing process.

2.6. Test Method

2.6.1. Compressive Strength Test without Lateral Limit

A strain-controlled unconfined compression tester is used, and the processed specimen is tested according to the Highway Geotechnical Test Procedure JTG3430-2020, see [30] (pp. 213–217). The loading rate of unconfined compressive strength is kept at 1.0 mm/min until the specimen breaks the ring to end the test. The compression test schematic is shown in Figure 2.

2.6.2. Calcium Carbonate Content Determination Test

The acid washing method was used to measure the calcium carbonate content inside each specimen separately. The modified specimens were dried in an oven at 100 °C until constant weight, and the mass of the specimen and calcium carbonate was recorded as m1. A sufficient amount of dilute hydrochloric acid was added and soaked for a period of time, and when no bubbles were generated in the solution, the calcium carbonate in the specimen was considered to have reacted completely. After rinsing the sample with distilled water several times and then putting it into the oven to dry, the mass of the treated sample was m2. The difference between m1 and m2 is the mass of calcium carbonate produced.

2.6.3. Direct Shear Test

After the specimen is compacted, each layer is weighed and filled with 30 g of soil in the ring knife, and the test is carried out with quadruple direct shear in accordance with the Highway Geotechnical Test Procedure JTG3430-2020, see [30] (p. 185). The shear rate set to 0.8 mm/min and recorded every 10 s. After the shear is finished, the shear force and vertical pressure are removed, and the test block is taken out to end the test. The shear test schematic is shown in Figure 3.

3. Results and Discussion

3.1. Stress–Strain Curve

The stress–strain curves of denitrifying bacterial consolidated soil under different reinforcement conditions were basically the same. The specimens with 0.4 fibre dosing and a 12 mm fibre length were analysed as an example, as shown in Figure 4, where $\mathsf{\sigma }$ is the stress and $\mathsf{\epsilon }$ is the strain.

It is obvious from the figure that the stress peak of the 0.4 fibre-doped specimen is higher and the stress peak appears later compared with the non-fibre-doped specimen. This is because on the one hand, the presence of fibre promotes the generation of calcium carbonate, the integrity and strength of the specimens are improved, and the stress limit that the specimens can withstand is increased. On the other hand, when cracks appear inside the specimens, the tensile force generated by the fibre inhibits the further development of the cracks, and the stress of the specimens rises slowly and reaches its peak. At the same time, it can be seen that the stress of the specimens with 0.4 fibre dosing decreases more slowly. It is obvious that the addition of fibre can effectively enhance the strength and toughness of the specimens after curing with denitrifying bacteria and improve the brittle damage characteristics of the soil.

3.2. Effect of Fibre Admixture on Strength of Specimens

Figure 5 shows the variation in the unconfined compressive strength of the specimens with different fibre lengths with respect to the fibre admixture. From Figure 5, it can be seen that after the treatment of denitrifying bacteria, the originally loose soil can be effectively solidified and present a higher strength. When basalt fibres were added to the soil, the unconfined compressive strength of the specimens was further improved. For example, the unconfined compressive strengths of the corresponding specimens were 526 kPa, 612 kPa, 674 kPa, and 886 kPa when the fibre dosing amount was 0, 0.1, 0.2, and 0.3 at a 12 mm fibre length, respectively. Overall, the unconfined compressive strength of the specimens tended to increase and then decrease with increasing fibre dosing, reaching peak strengths of 886 kPa and 918 kPa at 0.3 and 0.4 fibre dosing, respectively. This is because the interaction between the fibres and soil particles in the soil sample can limit the relative sliding of the fibres, which provides the fibres with an ability to bear the external load and transfer the load and reduce the stress concentration in the soil sample. The addition of an appropriate amount of fibre in microbial soil consolidation can improve the strength of soil, and the optimal fibre admixture is between 0.3 and 0.4.

3.3. Effect of Fibre Length on Strength of Specimens

Figure 6 reflects the variation in the lateral limitless compressive strength of the specimens with fibre length for different fibre dosing amounts. It can be seen from the figure that when the fibre dose is too low (0.1, 0.2, 0.3, 0.4), the unconfined compressive strength of the specimen increases with increasing fibre length. When the fibre dose is too high (0.5, 0.6), the unconfined compressive strength of the specimen increases with increasing fibre length and then decreases. When the fibre length is 12 mm, the basalt fibre has the most obvious effect on denitrifying bacteria.

3.4. Calcium Carbonate Content

Figure 7 shows the calcium carbonate content in the specimens with different fibre doping and gives the unconfined compressive strength of each specimen. It can be seen from the figure that the lowest calcium carbonate content of 1.91 was found in the specimens without fibre dosing, while the highest calcium carbonate content of 6.56 and 6.34 was found in the specimens with fibre dosing of 0.3 and 0.4. The overall trend of calcium carbonate content increased and then decreased with increasing fibre dosing, but the decrease in calcium carbonate content after reaching the peak was not obvious. This occurred mainly because calcium carbonate was attached to the soil particles after generation, and the addition of fibre increased the ‘colonisation area’ available for microorganisms. However, the volume of pores inside the soil body is certain, and with the increase in fibre incorporation, the pores inside the soil body are gradually occupied by fibres, which compresses the growth environment of microorganisms and leads to the restriction of microorganism growth and has a negative effect on the amount of calcium carbonate production. Therefore, it can be inferred that the addition of fibres in the soil is beneficial to the production of calcium carbonate, and the highest content of calcium carbonate is achieved at 0.3 and 0.4 fibre admixture, but the addition of excessive fibres is detrimental to the production of calcium carbonate.

3.5. Shear Strength

Figure 8 shows the relationship between vertical pressure and shear strength for different fibre dosing. From the figure, it can be seen that the maximum shear strength of the plain soil is 96 kPa and the maximum shear strength of the soil is 114 kPa when the fibre admixture is 0. This indicates that the microbial curing technology can improve the shear strength of the clay soil with a maximum increase of 118.9. With an increase in fibre content, the shear strength increases and then decreases, and the maximum shear strength of 163 kPa is reached at 0.3 fibre admixture.

Figure 9 and Figure 10 show the relationship between fibre dosing and cohesion and internal friction angle. With an increase in fibre dosing, the cohesion and internal friction angle increase and then decrease, and they all reach their maxima when the fibre dosing is 0.3. This is because excessive fibre content will inhibit the formation of calcium carbonate and thus reduce the shear strength of soil. Therefore, the cohesiveness and internal friction angle of the soil mass will decrease. On the whole, the cohesion and internal friction angle of reinforced soil are generally higher than those of unreinforced soil.

4. Numerical Simulation

4.1. Finite Element Basic Principle of Strength Reduction Method

The strength reduction method is a widely used analysis method in slope stability analysis. It is calculated by substituting the cohesion and internal friction angle into the finite element model, with the reduction factor of the slope just reaching the damage state as the safety factor, the essence of which is that the cohesion and internal friction angle of the material gradually decrease. This results in the stress of a unit not being matched with the strength or beyond the yield surface, and the unbearable stress is gradually transferred to the surrounding soil units. When a continuous sliding surface appears, the soil is destabilised.

4.2. Model Construction

Using the Longlang Expressway in Loudi city, Hunan Province, as the base project, ABAQUS software was used to establish a three-dimensional slope model with a length of 25 m, width of 7 m, height of 13 m, and slope of 1:1.5. Microbial reinforcement treatment was carried out along the vertical slope direction, and the reinforcement thickness was 1 m. The model assumed that the soil was an ideal elastic–plastic body, and the Mohr–Coulomb model was used. According to the experimental results, the soil before microbial reinforcement is taken as density $\mathsf{\rho }$ = 1.78 g/cm${}^{-3}$, elastic modulus E = 10 MPa, Poisson’s ratio µ = 0.35, cohesion c = 13 kPa, and internal friction angle $\mathsf{\phi }$ = 11.7°, and after microbial reinforcement is taken as density $\mathsf{\rho }$ = 1.96 g/cm${}^{-3}$, elastic modulus E = 10 MPa, Poisson’s ratio µ = 0.35, cohesion c = 25.95 kPa, and internal friction angle $\mathsf{\phi }$ = 15.6°. The cohesion and internal friction angle vary with the field variables, and the range of field variables varies between 0.5 and 3. The boundary conditions are set in ABAQUS load to constrain the displacements in the x, y, and z directions at the bottom of the model, constrain the displacements in the y directions before and after the model, constrain the displacements in the x directions to the left and right of the model, and apply the gravity force in the z-axis direction to the model as a whole. The mesh division of the slope model adopts eight-node linear hexahedral cells (C3D8), and the calculation area is divided into a total of 2415 cells.

4.3. Stability Analysis

4.3.1. Plastic Zone Analysis

The plastic strain clouds before and after the finite element simulation analysis of microbial reinforcement are shown in Figure 11.

From the above figure, it can be seen that when t = 0.2, the plastic zone appears above the foot of the slope, and when t = 0.25, the plastic zone above the foot of the slope expands and extends to the inner part of the slope and gradually connects with the plastic zone inside the slope. When t = 0.34, the plastic zone basically penetrates, and the slope is destabilised. Comparing the plastic strain clouds before and after microbial reinforcement, it can be seen that at t = 0.2, the strain in the plastic zone at the foot of the slope after microbial reinforcement is lower and the strain inside the slope body is higher, but the value is smaller than the strain inside the slope body before microbial reinforcement. This may occur because after the slope surface is reinforced by microorganisms, the microorganisms have not completely penetrated into the interior of the soil body and have less influence on the interior of the slope body. The subsequent development of the plastic zone is roughly similar before and after microbial reinforcement. The analysis of the strain cloud diagram at the time of complete penetration shows that the maximum plastic strain before microbial reinforcement appears at the foot of the slope, and the corresponding stress reaches 239.3 kPa, while the maximum plastic strain after microbial reinforcement also appears at the foot of the slope, but its distribution range is greatly reduced, and the maximum stress value is lowered to 174.5 kPa.

4.3.2. Displacement Cloud Analysis

Figure 12 shows the total displacement, X-directional displacement, total displacement vector, and X-directional displacement vector of the slope after microbial reinforcement derived by the intensity reduction method. Analysis of Figure 12a shows that the maximum displacement of the slope is 1.36 cm, the area of the maximum displacement is located at the foot of the slope, and the displacement gradually decreases in a circular shape with this position as the centre downward. Figure 12b shows the X-direction displacement. From the X-direction displacement, we can see the location of the potential sliding surface of the slope and the range of the potential landslide area. The total displacement vector map in Figure 12c clearly shows the location and regional range of the sliding surface of the slope, which forms a circular arc-shaped sliding surface from the top of the slope to the foot of the slope. Figure 12d shows the X-direction displacement vector diagram. The X-direction specified by the calculation model points to the inside of the slope body in the positive direction. From the diagram, it can be seen that the maximum displacement in the X-direction is 1.34 cm, the displacement produced is the displacement of the landslide sliding outward, the sliding surface can be clearly seen, and the displacement of the soil below the sliding surface is basically zero.

4.3.3. Calculation of Safety Coefficient

Nodal 1622 at the top of the slope is selected as the characteristic point, and the graphs related to the safety coefficient and displacement in the process of discounting are obtained. From Figure 13 and Figure 14, two obvious inflection points can be seen. The appearance of the inflection points indicates that the slope is unstable or close to damage when the plastic stress of the slope increases and the displacement increases suddenly. If the inflection point of displacement is used as the stability index of the slope, then it can be concluded that the safety coefficient of the slope without microbial reinforcement Fs = 1.16, and the safety coefficient of the slope after microbial reinforcement Fs = 1.41, the safety coefficient after reinforcement is increased by 21.5 percent, and the reinforcement effect is obvious. If the nonconvergence of numerical analysis calculation is used as an index to evaluate the stability of slope, then it can be concluded that the safety factor of slope without microbial reinforcement Fs = 1.18, the safety factor of slope after microbial reinforcement Fs = 1.46, and the safety factor after reinforcement is increased by 23.7 percent.

5. Conclusions

In this paper, the mechanical properties of basalt fibre-reinforced microbially consolidated soil were studied by conducting unconfined compressive strength tests and direct shear tests and analysing the stability of the slope after microbial reinforcement in terms of plastic zone characteristics, displacement clouds, and safety factors through finite element numerical simulations. The following conclusions were obtained:

(1)

The combination of fibre reinforcement technology and MICP technology can significantly improve the compressive and shear strength of the soil and improve the characteristics of brittle soil after microbial curing.

(2)

The incorporation of basalt fibres has an important influence on the mechanical properties of denitrifying bacterial consolidated soil. When the fibre admixture is low, the unconfined compressive strength of the soil increases with increasing fibre length, and when the fibre admixture is high, the strength tends to increase and then decrease with increasing fibre length. The optimal fibre admixture is between 0.3 and 0.4, and the best fibre length is 12 mm.

(3)

The effect of increasing basalt fibre incorporation on the calcium carbonate content of the soil after microbial curing tends to increase and then decrease. This is because after the fibre is added to the soil, the area where calcium carbonate can be colonised increases, and when the fibre is increasingly added, the internal void of the soil is gradually occupied by the fibre, and the growth of microorganisms is restricted. This has a negative impact on the generation of calcium carbonate.

(4)

The development of the plastic zone of the slope before and after microbial reinforcement is generally similar, but the scope of the plastic zone after reinforcement is reduced as a whole, especially in the reinforced area of the slope, and the maximum equivalent plastic stress is decreased by 65 kPa.

(5)

The safety coefficient of the slope before microbial reinforcement was 1.16, and the safety coefficient after reinforcement was 1.41. The stability of the slope was obviously strengthened.