Improvement of Unconfined Compressive Strength in Granite Residual Soil by Indigenous Microorganisms

Abstract

In order to study how indigenous microorganisms can enhance the strength properties of granite residual soil in the Hanzhong area, two Bacillus species that produce urease were isolated from the local soil. The two Bacillus species are Bacillus subtilis and Bacillus tequilensis, and they were used for the solidification and improvement of the granite residual soil. Unconfined compressive strength tests, scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses were systematically used to analyze the influence and mechanism of different cementation solution concentrations on the improvement effect. It has been found that with the growth of cementing fluid concentration, the unconfined compressive strength of improved soil specimens shows an increasing tendency, reaching its highest value when the cementing solution concentration is 2.0 mol/L. Among different bacterial species, curing results vary; Bacillus tequilensis demonstrates better performance across various cementing solution concentrations. The examination of failure strain in improved soil samples indicates that brittleness has been successfully alleviated, with optimal outcomes obtained at a cementing solution concentration of 1.0 mol/L. SEM and XRD analyses show that calcium carbonate precipitates (CaCO₃) are formed in soil samples treated by both strains. These precipitates effectively bond soil particles, verifying improvement effects on a microscopic level. The present study proposes an environmentally friendly and economical method for enhancing engineering applications of granite residual soil in Hanzhong area, which holds significant importance for projects such as artificial slope filling, subgrade filling, and foundation pit backfilling.

1. Introduction

Granite residual soil is widely distributed in the southeast and southwest regions of China (accounting for about 5% to 8% of the country’s land area). It is a special soil with a large proportion of sand, a small proportion of clay, and complete weathering. When it comes into contact with water, it is likely to soften and break apart easily. This results in a decrease in the soil’s strength and may cause engineering problems like instability of roadbeds and collapse of artificial slopes. These problems bring great risks to the safety of construction projects [1,2,3] Therefore, granite residual soil, which is an important engineering fill material, needs proper improvement measures to satisfy the strict requirements of engineering applications.

At present, mechanical vibration, lime improvement, cement improvement, reinforcement improvement and fiber improvement [4,5,6,7,8,9,10] are among the main ways to increase the strength of granite residual soil. These methods can make the treated granite residual soil meet engineering requirements, but they come with high energy consumption, a high level of labor and material costs, and large engineering expenses [11,12]. Furthermore, the improvement with lime and cement may bring about non-sustainable negative effects on the ecological environment [13]. Over the past few years, an emerging soil enhancement technique known as microbial-induced calcium carbonate precipitation (MICP) technology has attracted more and more attention because it is friendly to the environment and cost-efficient [14,15]. MICP is a biomineralization technique that induces calcium carbonate precipitation through the metabolic activities of microorganisms. The core process involves the hydrolysis of urea to produce ammonia and carbon dioxide. Ammonia hydrolysis releases hydroxide ions, significantly increasing the local pH value, which in turn promotes the conversion of carbon dioxide to carbonate ions. In an alkaline environment, calcium ions combine with carbonate ions to precipitate calcium carbonate. Calcium carbonate crystals nucleate and grow at the contact points of soil particles, forming a cementing structure; simultaneously, they cover the particle surfaces, increasing roughness, and deposit in the pores to reduce porosity and increase density. These effects collectively enhance the physical and mechanical properties of the soil. The chemical reaction principles are shown in Equations (1)–(5).

$$\mathrm{CO}({\mathrm{NH}}_2{)}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Urease}}{\to }\mathrm{C}{\mathrm{O}}_2+2 \mathrm{N}{\mathrm{H}}_3$$

(1)

2 NH₃ + 2 H₂O↔2 NH₄⁺ +2 OH⁻

(2)

CO₂ + OH⁻↔HCO₃⁻

(3)

HCO₃^- + OH⁻↔CO₃²⁻ + H₂O

(4)

Ca²⁺ +CO₃²⁻→CaCO₃↓

(5)

In recent years, a great number of researchers have utilized MICP technology to boost the strength of different kinds of soil. For instance, Gowthaman [16] and Cardoso [17] both reached the conclusion that the formation of calcium carbonate crystals enhances the roughness of soil particles, which is an important aspect in strengthening soil. Expansive soil treated by MICP, as Tiwari et al. [18] have shown, had a more than 205% increase in compressive strength compared to untreated samples. In the study by Liang et al. [19], the strength of granite residual soil was improved by using MICP technology. It was found that after this treatment, the shear strength, internal friction angle, and cohesion of the soil were significantly enhanced. Peng Jie et al. [20] carried out experiments regarding MICP pressure grouting reinforcement of organic clay with different cementing solution concentrations and found that raising the urea concentration in the cementing solution to 0 could enhance the soil’s unconfined compressive strength substantially. The unconfined compressive strength of the soil could be significantly improved when the urea concentration in the cementing solution was raised to 0.25 M. Cui Mingjuan et al. [21] investigated the effects of particle size on the compressive strength of microbially. It was found in their research that the compressive strength of samples improved by microbes was significantly affected by particle size, and the influence on the content of calcium carbonate and porosity was relatively small. The selection of calcium sources and different improvement methods also plays a crucial role in increasing soil strength via MICP. For instance, Tobler et al. [22] investigated the influence of different injection methods on sand strengthening. They discovered that the cyclic injection of bacterial solution and cementitious mixture enhanced the uniformity, thus increasing the strength of the samples. They looked into the influence of different injection methods on sand reinforcement and discovered that the cyclic injection of bacterial solution and cementing mixture increased uniformity, thus increasing the strength of the samples. Zhao Zhifeng and Chen Wenjie [23] explored how drying curing, immersion curing, and dry–wet cycle curing influenced the compressive strength of soil specimens. Their study showed that the compressive strength of the samples with immersion curing or dry–wet cycle curing was much lower than that of the samples with drying curing. Huang Wandong et al. [24] developed an indoor testing device to conduct microbial solidification experiments on granite residual soil with varying injection times. Their study demonstrated that the number of injections significantly influenced the unconfined compressive strength of the solidified samples. Zhang et al. [25] performed grouting experiments using three calcium sources—calcium chloride, calcium acetate, and calcium nitrate—to investigate the effects of different calcium sources on the durability and mechanical properties of sand columns. The results demonstrated that samples treated with calcium acetate exhibited higher strength and better uniformity. Hu et al. [26] integrated coconut shell fiber with microbial improvement, established different fiber content gradients, and comprehensively demonstrated through triaxial testing, calcium carbonate content analysis, wet disintegration tests, and SEM observation that the combined approach can effectively enhance the shear strength of clay.

Based on the previous literature review, researchers have extensively applied MICP technology from multiple perspectives to enhance the properties of various soil types, achieving significant improvements. Among the microbial strains employed, Sporosarcina pasteurii has been widely utilized due to its high urease activity. Most related studies typically rely on commercially available freeze-dried powders or liquid cultures. However, the high cost of commercial strains limits the scalability and widespread application of MICP in practical engineering projects. In contrast, indigenous microorganisms native to the local environment exhibit inherent adaptability and are expected to have higher survival rates. Moreover, their use can substantially reduce microbial cultivation costs, thereby facilitating broader implementation in field applications. Notably, no existing literature has reported the application of indigenous microorganisms (isolated directly from local soil) for the improvement of granite residual soils. To address this research gap, the present study selected two urease-producing indigenous bacterial strains—Bacillus subtilis and Bacillus tequilensis—isolated from the Hanzhong region of Shaanxi Province, as the bio-curing agents. The feasibility of using these strains to improve local granite residual soils was systematically evaluated through unconfined compressive strength tests, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses. The findings of this research provide a novel approach for the reinforcement and stabilization of granite residual soils in artificial fill slopes and subgrade engineering projects in the Hanzhong area, offering significant practical value and promising prospects for future engineering applications.

2. Materials

2.1. Physical Properties of Granite Residual Soil

The granite residual soil utilized in the present study was collected from the slope of a highway in Chenggu County, Hanzhong City, Shaanxi Province. According to the “Geotechnical Test Method Standard” (GBT50123-2019) [27], the collected soil samples were dried and then subjected to particle size analysis, standard compaction tests, and limit moisture content tests. Figure 1 shows the particle size distribution, and

Table 1. Basic physical indicators of granite residual soil (referenced standard: GB/T50123-2019 [27]).

Materials	Maximum Dry Density (g·cm−3)	Dry Density (g·cm−3)	Natural Moisture Content (%)	Optimal Moisture Content (%)	Liquid Limit (%)	Plastic Limit (%)	Plasticity Index	Void Ratio	Specific Gravity
Granite residual soil	2	1.8	8.2	9.8	16.8	10.5	6.3	0.5	2.7

summarizes the basic physical properties.

2.2. Culture Medium and Cementing Solution

2.2.1. Culture Medium

For screening and purification of bacterial strains, respective culture media are needed. The composition of screening and separation medium is as follows: peptone 1 g/L, sodium chloride 5 g/L, potassium dihydrogen phosphate 2 g/L, glucose 1 g/L, urea 20 g/L, phenol red 0.012 g/L, agar 15 g/L, and pH 7.0 ± 0.2. Urea and glucose should be dissolved separately, sterilized by filtration, and added to the sterilized culture medium [23]. The composition of Luria–Bertani (LB) medium, which is used for preserving purified bacterial strains, is as follows: pancreatic peptone 10 g/L, yeast extract 5 g/L, sodium chloride 10 g/L, agar 15 g/L, and pH 7.0 ± 0.2.

2.2.2. Cementing Solution

The experiment utilized a mixed solution of calcium chloride and urea as the cementing solution. Specifically, calcium chloride served as the calcium source for the curing reaction, while urea was hydrolyzed by urease to generate carbonate ions. The preparation method of the cementing solution is as follows: calcium chloride, urea, and distilled water were mixed evenly in proportion. The specific ratios are as follows: 0.5 mol/L cementing solution (1000 mL of distilled water, 55.5 g of calcium chloride, 30.03 g of urea), 1.0 mol/L cementing solution (1000 mL of distilled water, 111 g of calcium chloride, 60.06 g of urea), and 2.0 mol/L cementing solution (1000 mL of distilled water, 222 g of calcium chloride, 120.12 g of urea) [28].

3. Procedures

3.1. Screening and Purification of Urease-Producing Strains

In the local soil of Hanzhong, urease-producing strains were screened. After removing large particles such as sand and stones from the soil samples, the samples were passed through a 60-mesh sieve. The soil samples were then diluted 10-fold with distilled water and incubated in an 80 °C water bath for 15 min to eliminate heat-sensitive contaminants. Subsequently, the samples were placed in a shaker (30 °C, 150 r/min) and shaken for 20 to 30 min to ensure the uniform distribution of microorganisms within the soil samples. We added sterile water to the thoroughly shaken soil sample solution, mixed it uniformly, and allowed it to stand for 20 to 30 s. We collected the supernatant and performed gradient dilutions up to 10⁻⁶. We selected 100 μL aliquots from each of the three dilutions (10⁻⁴, 10⁻⁵, and 10⁻⁶) and spread them onto the screening and separation medium plates. We incubated the plates upside down at 30 °C for 24 to 48 h.

The screening and separation medium contains urea and phenol red indicator. The urease-producing strains hydrolyze urea to generate alkaline ammonia, which causes the area around the colonies to turn purple–red under the action of the phenol red indicator. The strains were further purified on this medium until single colonies appeared. The purified strains were then stored on LB slant medium. The screened strains were subjected to liquid culture, and their urease activity was measured at their optimal growth concentration. Through molecular biological identification, the two strains with higher urease activity were identified as Bacillus subtilis and Bacillus tequilensis, respectively. The detailed procedure is illustrated in Figure 2.

3.2. Bacterial Concentration and Urease Activity Test

To determine the optimal growth characteristics of bacteria, the bacterial concentration was measured using a spectrophotometer. The bacterial concentration was expressed as the absorbance (OD₆₀₀) at a wavelength of 600 nm, with the OD₆₀₀ value being positively correlated with the bacterial concentration. The Bacillus subtilis and Bacillus tequilensis strains grown on LB slant medium were inoculated into LB liquid medium under the following conditions: pH 7.0 ± 0.2, temperature 30 °C (optimum growth temperature for most bacterial strains), and oscillation for 24 h to prepare the seed solution, which was stored for subsequent use. The seed solution was then inoculated into fresh LB liquid medium at an inoculation volume of 1% and cultured at 30 °C with shaking at 150 r/min for 72 h. Samples were collected at 0, 4, 8, 12, 16, 20, 24, 36, 48, 60, and 72 h, and the absorbance of the culture medium at 600 nm was measured at each time point. Based on these data, growth curves of the urease-producing bacterial strains were constructed. Both Bacillus subtilis and Bacillus tequilensis entered the stationary phase between 24 and 48 h (Figure 3).

Urease activity is a crucial factor influencing the enhancement effect of granite residual soil. Among the various urease-producing strains isolated from the soil, Bacillus subtilis and Bacillus tequilensis exhibited particularly prominent urease activities. The determination of urease activity was conducted using the conductivity method. By measuring the change in conductivity of the mixture containing urease solution and urea solution, we were able to indirectly assess urease activity. The specific steps are as follows. (1) Take the bacterial culture that has been incubated for 24 h and subject it to ultrasonic shaking in an ultrasonic instrument for 10 min, followed by a cooling period of 5 min. After repeating this process six times, transfer the bacterial liquid into a centrifuge and centrifuge at 5500 r/min for 20 min. Collect the supernatant and filter it to obtain the urease solution. (2) Prepare a mixed solution containing urea and calcium chloride, both at a concentration of 1.1 mol/L. (3) Add 10 mL of the urease solution to a beaker, then introduce 90 mL of the mixed urea–calcium chloride solution to dilute the original urease concentration by a factor of ten. Place this mixture on a magnetic stirrer, ensuring that both the magnetic rotor and conductivity meter probe are immersed in the beaker. (4) Initiate stirring with the magnetic stirrer for 30 s before taking an initial reading from the conductivity meter; record this value as N₁. After allowing an interval of five minutes, take another reading from the conductivity meter; measure and record this value as N₂. Repeat this measurement process three times. (5) According to Whiffin’s calculation method [29], these results can be utilized to quantify urease activity. The calculation formula is presented in Equation (6).

$$U={\displaystyle \frac{N_2-N_1}{t}}\times 11.11\times n$$

(6)

In this formula, U denotes the unit urease activity [mmol/(L·min)]; N₂ − N₁ represents the change in conductivity (mS/cm); t refers to the time interval (min); n indicates the dilution factor; 11.11 signifies the quantity of urea hydrolyzed per unit change in conductivity (μmol/L), which was determined through extensive calibration experiments.

The urease activities of Bacillus subtilis and Bacillus tequilensis were measured at 24 h. The results indicated that the urease activity of Bacillus tequilensis was significantly higher than that of Bacillus subtilis, as presented in

Table 2. Urease activity of different bacterial strains.

Bacterial Strain	Urease Activity [(mmol/(L·min)]
Bacillus subtilis	6.657
Bacillus tequilensis	8.275

.

3.3. Microbial Improvement of the Granite Residual Soil Test

For the improved experiments, researchers typically prepare samples using methods such as mixing, soaking, and grouting [28,30,31]. In the present study, the mixing method was employed for sample preparation. After thoroughly drying the retrieved granite residual soil, it was sieved to obtain soil particles with a size of less than 2 mm for further processing. The soil samples, cementing solution, and bacterial solution were prepared in a mass-to-volume ratio of 200 g: 20 mL: 15 mL. To ensure uniform distribution of the bacterial solution and cementing solution, a stepwise mixing procedure was employed. Initially, the bacterial solution was added to the soil sample and mixed for 5 min until homogeneously incorporated. Subsequently, the cementing solution was introduced and mixed for an additional 5 min. The cementing solution concentrations were established based on a commonly used bacterial strain for soil improvement—Sporosarcina pasteurii—which typically achieves optimal stabilization performance at a concentration of 1.0 mol/L. When the concentration exceeds this threshold, excessive calcium ions may inhibit microbial activity, thereby negatively impacting the improvement efficiency [27]. Accordingly, three cementing solution concentrations—0.5 mol/L, 1.0 mol/L, and 2.0 mol/L—were selected for the present study; meanwhile, the bacterial solution was derived from cultures that had reached a stable stage after 24 h of incubation. Additionally, in the blank control group, both the cementing solution and bacterial solution were prepared using distilled water instead. Three parallel samples were prepared for each group, respectively. The specific testing plan is detailed in

Table 3. Test protocols for the present study.

Bacterial Strain	Concentration of Cementing Solution/(mol/L)	Number
Blank control group	0	GRS-0
Bacillus subtilis	0.5	BS-0.5
	1	BS-1.0
	2	BS-2.0
Bacillus tequilensis	0.5	BT-0.5
	1	BT-1.0
	2	BT-2.0

. Subsequently, the prepared samples were placed in a self-constructed constant temperature (30 °C) and constant humidity curing device for a duration of 14 days, as illustrated in Figure 4.

3.4. Unconfined Compressive Strength Analyses

After the drying process of the cured granite residual soil samples, an Unconfined Compression Strength (UCS) analysis was performed. The samples had a diameter of 3.91 cm and a height of 8.00 cm, with a dry density measured at 1.8 g/cm³. The specific testing equipment and data acquisition procedures are illustrated in Figure 5.

3.5. SEM and XRD Analyses

The SEM analysis was conducted using a German ZEISS Sigma 300 scanning electron microscope. The XRD analysis was conducted using an X-ray diffractometer (Rigaku SmartLab SE of Japan) with diffraction angles between 5° and 90°.

4. Analysis of the Test Results of Unconfined Compressive Strength

4.1. Analysis of Stress–Strain Curves of Granite Residual Soil Improved by Microorganisms

The unconfined compressive stress–strain curves for both the improved soil samples and the untreated soil samples are presented in Figure 6.

It can be observed from Figure 6 that the stress–strain curves of both the improved and unimproved soil samples are categorized into four distinct stages: initial compaction, elastic deformation, plastic yield, and post-peak failure. These curves exhibit a strain softening characteristic, initially increasing before subsequently decreasing. As illustrated in Figure 6a,b, the stress–strain curve for the improved soil sample demonstrates an overall upward shift; this is accompanied by an increase in the stress peak while maintaining a consistent variation pattern throughout the curve. Consequently, we selected the stress–strain curves of Bacillus subtilis-treated soil samples under a cementing solution concentration of 2.0 mol/L for further analysis (Figure 6c,d). In the initial testing phase, it is evident that the curve for improved soil samples displays a gradual upward trend, whereas that of unimproved soil samples shows a linear ascent with greater stiffness. During the elastic deformation stage, both sets of samples exhibit approximately linear growth trends; however, at this stage, improved soil samples demonstrate superior stiffness compared to their unimproved counterparts. In the stage of plastic yielding, both types of soil samples experience a gradual reduction in stiffness as they approach their peak stress levels. During the post-peak failure stage, there is a decline in stresses for both improved and unimproved soil samples as strain increases. Notably at this stage, improved soil samples retain relatively high residual stresses compared to unimproved ones.

4.2. Analysis of Compressive Strength of Granite Residual Soil Improved by Microorganisms

The relationship between the UCS of the improved soil samples and varying cementation concentrations is illustrated in Figure 7. To ensure the accuracy and scientific validity of the analysis results, a one-way ANOVA was performed on the experimental data to investigate the differences in strength among the two types of bacteria-treated soil samples under varying concentration levels, as presented in

Table 4. Results of one-way ANOVA.

		Sum of Squares	Degrees of Freedom	Mean Square	F-Statistic	p-Value
Bacillus subtilis	Between groups	$1.21\times {10}^6$	3	$4.03\times {10}^5$	$3.78\times {10}^2$	$5.88\times {10}^{-9}$
	Within groups	$8.52\times {10}^3$	8	$1.07\times {10}^3$
	Total		11
Bacillus tequilensis	Between groups	$1.61\times {10}^6$	3	$5.36\times {10}^5$	$5.25\times {10}^2$	$1.60\times {10}^{-9}$
	Within groups	$8.17\times {10}^6$	8	$1.02\times {10}^3$
	Total	$1.62\times {10}^6$	11

.

It can be observed from Figure 7 that the unconfined compressive strength of the samples exhibits a significant increase with higher concentrations of the cementing solution. When the concentration of the cementing agent reached 2 mol/L, the unconfined compressive strength of the improved soil samples achieved values of 1511.1 kPa (an increase of 141.4%) and 1649.7 kPa (an increase of 163.7%), respectively, indicating a superior improvement effect attributed to Bacillus tequilensis. This enhancement is primarily due to an increased precipitation of calcium carbonate within the sample as the concentration of the cementing solution rises; consequently, the strength of the improved soil samples correlates positively with elevated calcium carbonate content, which is in agreement with the similar conclusion reached by Chu et al. [32]. As presented in Table 4, the p-values for the strength of soil samples treated with Bacillus subtilis and Bacillus tequilensis under varying concentrations of the cementing solution were $5.88\times {10}^{-9}$ and $1.60\times {10}^{-9}$, respectively. Both values were substantially lower than the significance level α = 0.05, indicating that the concentration of the cementing solution exerted a highly significant influence on the strength of the bacterial-treated soil samples.

4.3. Analysis of the Failure Strain in Granite Residual Soil Improved by Microorganisms

The relationship between the variation of failure strain in the improved soil samples and different concentrations of cementing solution is illustrated in Figure 8.

It can be observed from Figure 8 that the failure strain of the improved soil sample is significantly higher than that of the unimproved soil sample, indicating a reduction in the brittle characteristics of the improved soil. As the concentration of the cementing solution increases, the failure strain of the improved soil sample initially rises and then declines, reaching its peak at a concentration of 1.0 mol/L. Notably, at a cementing solution concentration of 0.5 mol/L, the failure strain for Bacillus subtilis-improved soil samples was greater than that for Bacillus tequilensis-improved samples; however, this trend reversed under other improvement conditions. Interestingly, when Cui et al. [33] utilized Sporosarcina pasteurii for MICP enhancement, they found that brittleness increased significantly with rising strength in improved soils—contradicting our findings in the present study. This suggests that both strains examined herein possess distinct advantages in mitigating brittle deformation of soils.

Analyzing the macroscopic test phenomena, the failure modes of the soil samples before and after improvement are illustrated in Figure 9. The unimproved sample (Figure 9b) displayed typical characteristics of brittle failure, characterized by extensive, numerous, and irregular cracks; consequently, this sample was prone to fragmentation into blocks. In contrast, the improved sample (Figure 9c,d) exhibited a significant reduction in both the number and width of cracks.

5. Analysis of Microscopic Test Results

As shown in Figure 10, in the improvement experiment, microorganisms decomposed urea in the cementing solution through the urease they produced, generating ammonium ions and bicarbonate ions. Calcium ions in the cementing solution combine with bicarbonate ions to form calcium carbonate precipitates. These calcium carbonate precipitates encapsulate and bond soil particles together, thereby creating stable aggregates that enhance the overall effectiveness of soil improvement. To further elucidate its solidification mechanism, the present study conducted X-ray diffraction tests and scanning electron microscopy analyses to investigate both the mineral composition and microscopic morphology of soil samples before and after treatment.

5.1. XRD Results

The soil samples improved with a cementing solution concentration of 2.0 mol/L were selected for X-ray diffraction analysis. By comparing the mineral PDF cards, the specific changes in mineral composition of the soil samples before and after treatment can be determined, as illustrated in Figure 11.

It can be observed from Figure 11a that the primary mineral components of the unimproved soil sample include quartz, albite, muscovite, illite, and montmorillonite. In contrast, Figure 11b,c illustrate that the mineral composition of the improved soil samples has undergone significant changes, notably with the introduction of calcite. This suggests that Bacillus subtilis and Bacillus tequilensis facilitated the formation of calcium carbonate precipitates, which predominantly exist in the soil as calcite.

5.2. SEM Results

The microstructural characteristics of the soil samples, both before and after improvement, were analyzed through scanning electron microscopy experiments, as depicted in Figure 12.

It can be observed from Figure 12 that the particle microstructure of the unimproved soil sample predominantly exhibits a lamellar structure. The particles are relatively dispersed, lacking a distinct aggregate formation, and there is a considerable amount of pore space (Figure 12a). In contrast, among the soil particles treated with Bacillus tequilensis, numerous aggregates are widely distributed. The soil pores have significantly decreased in size, accompanied by noticeable calcium carbonate deposition, which has markedly improved particle adhesion (Figure 12b). Although some aggregates are present among the soil particles improved by Bacillus subtilis, they tend to be smaller in size and less uniformly distributed; however, there is still a certain amount of calcium carbonate deposition (Figure 12c). The MICP enhancements facilitated by both types of bacteria resulted in the formation of calcium carbonate “cementing bodies,” thereby increasing the compactness of the soil samples and enhancing inter-particle bonding forces. This process significantly improves their compressive strength.

6. Discussion

Although the present study preliminarily validated the feasibility of utilizing indigenous microorganisms and cementing solutions for the improvement of granite residual soil in the Hanzhong region, several limitations remain, as follows.

• Although the effects of varying cementing solution concentrations were examined, scenarios involving concentrations exceeding 2.0 mol/L were not investigated. This is because high concentrations of calcium ions may inhibit urease activity, thereby negatively impacting the soil improvement process. In future research, higher concentrations will be tested to validate this hypothesis. Additionally, the curing process in the present study was conducted under constant temperature conditions; therefore, temperature was not considered as a variable factor. Furthermore, parameters such as the pH value of the cementing solution, the organic content in the bacterial solution, and the cation exchange capacity (CEC) of the soil were not included in the current analysis. These factors will be further examined in subsequent studies.
• The current study exhibits limitations in the quantitative characterization of the underlying mechanism. While XRD analysis was primarily employed for qualitative confirmation of calcite formation, the absence of peak area statistics in the experimental design phase hinders the establishment of a quantitative correlation between calcium carbonate content and macroscopic strength at this stage. Although SEM observations provided insights into the distribution of cementing substances, the variation patterns of porosity and calcium carbonate precipitation were not systematically analyzed. Future research will focus on conducting quantitative assessments at these microscopic scales.

7. Conclusions

In the present study, the feasibility of solidifying and enhancing granite residual soil in the Hanzhong area using two self-extracted indigenous microorganisms (Bacillus subtilis and Bacillus tequilensis) through MICP technology was investigated. This research provides a novel technical approach for the reinforcement and improvement of granite residual soil. The main conclusions drawn from the present study are as follows:

• The stress–strain curves of both the improved and unimproved soil samples exhibit strain-softening behavior; however, the residual stress of the improved soil sample is markedly higher than that of the unimproved sample.
• Both Bacillus subtilis and Bacillus tequilensis demonstrate significant efficacy in enhancing the unconfined compressive strength of granite residual soil from the Hanzhong area. The improvement effect increases with the concentration of the cementing solution, reaching a maximum at a concentration of 2.0 mol/L. Among the two bacterial strains, Bacillus tequilensis exhibits superior performance, achieving a peak strength of 1649.7 kPa, representing an increase of 163.7%.
• The failure strain of the granite residual soil is significantly improved through the application of Bacillus subtilis and Bacillus tequilensis, effectively mitigating the brittle failure characteristics of the soil. At low concentrations of cementing solution, Bacillus subtilis-treated soil exhibits a higher failure strain than that treated with Bacillus tequilensis. However, the latter performs better at higher concentrations. Both bacterial strains significantly enhance the failure morphology of the soil, markedly reducing the width and number of cracks.
• SEM and XRD analyses indicate that the improved granite residual soil exhibits evident calcium carbonate cementation, which reduces interparticle porosity. The bonding force and density between soil particles are significantly enhanced, thereby contributing to the increased unconfined compressive strength.