Next Article in Journal
An Investigation of Recharging Groundwater Levels through River Ponding: New Strategy for Water Management in Sutlej River
Next Article in Special Issue
Assessment of New Bio-Cement Method for Sand Foundation Reinforcement
Previous Article in Journal
The Concept of Ocean Sustainability in High School: Measuring the Ocean Literacy of Vocational High School Students in Indonesia
Previous Article in Special Issue
Comparison between MICP-Based Bio-Cementation Versus Traditional Portland Cementation for Oil-Contaminated Soil Stabilisation
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Sustainability
Volume 15
Issue 2
10.3390/su15021044
sustainability-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Reinforcement Mechanism and Erosion Resistance of Loess Slope Using Enzyme Induced Calcite Precipitation Technique

by
Danyi Shen
1,2,
Zhenyuan Liu
1,
Zhichao Song
1 and
Chuangzhou Wu
1,*
1
Institute of Port, Coastal and Offshore Engineering, Ocean College, Zhejiang University, Zhoushan 316021, China
2
Institute of Geotechnical Engineering, College of Civil Engineering and Architecture, Zhejiang University, Hangzhou 310058, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(2), 1044; https://doi.org/10.3390/su15021044
Submission received: 8 December 2022 / Revised: 29 December 2022 / Accepted: 4 January 2023 / Published: 6 January 2023
(This article belongs to the Special Issue Microbial Bio-Grouting for Underground Constructions: Emerging Technologies for Sustainable Submarine Engineering)
Browse Figures
Review Reports Versions Notes

Abstract

:
The disaster of loess slope seriously threatened the safety of people and property. Enzyme Induced Calcite Precipitation (EICP) was demonstrated as an environmentally friendly soil improvement method. However, few studies have focused on the improvement effect of EICP on loess slopes. In this study, a series of tests were conducted to investigate the effect of EICP and added either basalt fiber (BF) to the loess or polyvinyl acetate emulsion (PVAC) to the solution on the erosion resistance of loess slopes. The results showed that all of the EICP, EICP-BF, and EICP-PVAC treatments could improve surface strength (SS). The addition of 50 g/L PVAC achieved high SS because the network structure formed by PVAC promoted the affixation of CaCO3. The thickness of the crust layer decreased with the increasing BF content or PVAC concentration. With the increasing number of EICP treatment cycles, the CaCO3 content increased progressively, but the increase rate decreased. For rainfall erosion, the time until erosion occurred was delayed and the stability was improved for loess slopes treated with EICP, EICP-BF, and EICP-PVAC. The high erosion resistance of loess slopes treated with EICP-0.5% BF, EICP-30 g/L PVAC, and EICP-50 g/L PVAC was attributed to the stable spatial structure formed by CaCO3 precipitation and the additional cementation provided by high BF content and PVAC concentration. The addition of 0.5% BF effectively inhibited the development of surface cracks in loess slope after dry–wet cycles. With the increasing number of dry–wet cycles, the accumulative loess loss weight of slopes treated with various methods increased gradually. Among all treatment methods, the number of dry–wet cycles had less effect on EICP-30 g/L PVAC treated loess slopes. This study provided guidance for loess slopes prevention.

1. Introduction

Loess is an aeolian silt, and is widely distributed in the Loess Plateau of China [1]. Loess has characteristics of high permeability, high compressibility, large porosity, low strength and strong collapsibility [2]. The untreated loess slope is very sensitive to water and prone to cause severe geohazards such as surface erosion, landslide, and collapse under rainfall conditions or dry–wet conditions [3,4,5]. Throughout history, disasters relating to loess slopes have posed great threats to lives and property [6,7]. For instance, in July 2003, a loess landslide occurred in Zigui County, Hubei Province, with a total volume of 1.542 × 104 m3, resulting in 14 deaths and the blockage of Qinggan river [8]. Another example is the loess landslide caused by concentrated rainstorms in Bailuyuan, Shanxi Province, in September 2011, leading to 32 deaths and 5 missing people [9]. A series of traditional treatment measures, such as supporting structure, ecological intervention, and cement grouting, is widely used in loess slope engineering to alleviate the instability of slopes [10,11,12]. However, these treatments do not solve the problem of surface erosion and normally have limitations in convenience, effectiveness, and environmental protection. Therefore, an efficient and eco-friendly method is essential to prevent surface erosion of loess slopes.
Enzyme Induced Calcite Precipitation (EICP) is a bioinspired technique employed to improve the strength and erosion resistance of soils in recent years [13,14,15,16]. In EICP, urea (CO(NH2)2) is decomposed into carbonate ions (CO32−) and ammonium (NH4+) by urease enzyme, and then the CO32− bond with Ca2+ and achieves carbonate precipitation (CaCO3) (Equations (1)–(5)). The urase is usually isolated from plants, such as soybeans, jack beans, melons, squash, and plants of the pine family [17]. Thus, the CaCO3 produced by EICP technology has good compatibility with the environment [18].
CO NH 2 2 + 2 H 2 O → urease H 2 CO 3 + 2 NH 3
H 2 CO 3 ⇌ HCO 3 − + H +
NH 3 + H 2 O ⇌ NH 4 + + OH −
HCO 3 − + OH − ⇌ CO 3 2 − + H 2 O
Ca 2 + + CO 3 2 − → CaCO 3
The implementation potential of EICP in soil treatment has been widely studied. Hamdan et al. [19] conducted wind tunnel tests on sand soil treated with EICP and proved that the dust suppression effect of EICP technology was better than that of traditional methods. Almajed et al. [20] claimed that the unconfined compressive strength of sands treated using EICP was higher than sands stabilized using 10% ordinary Portland cement. Sun et al. [21] proposed that the EICP solution of 0.25 M reagent concentration and 0.8 ratios of urease solution to the cementation solution improved the wind-erosion resistance and rainfall-erosion resistance of desert sand efficiently. Bang et al. [22] revealed that soils treated with EICP alone produced a higher increase in strength and resistance against wind erosion than those treated with Microbially induced calcite precipitation (MICP) or a mix of EICP and MICP. Zhao et al. [23] and Almajed et al. [24] used biopolymers to assist EICP to enhance soil water retention and wind-induced erosion resistance by retaining the carbonate around the soil particles. The improvement of soil properties was mainly due to the particle cementation and pore filling of CaCO3 precipitation [25]. However, there were few studies on resistance to rainfall of the surficial solidified loess. Sun et al. [21] found water erosion was harmful to EICP-treated soil. Thus, further study is needed to investigate the water erosion resistance of EICP-treated loess, especially under the conditions of rainfall and dry–wet circulation.
Basalt fiber (BF) and polyvinyl acetate emulsion (PVAC) have the potential to improve the water stability of aggregates [26,27]. In this study, BF and PVAC are used to enhance the performance of EICP. The feasibility of using EICP, and EICP assisted by BF and PVAC in the surficial treatment of loess slope are analyzed based on a series of small-scale laboratory tests. The structure of this study is as follows. First, the property of loess, BF, and PVAC, preparation of urease and EICP solution, and experimental design are described. Then, the surface strength (SS), thickness, and CaCO3 content of the crust layer that formed due to treatment techniques are measured. The micro-structure of crust layer is presented using scanning electron microscopy testing. Finally, the erosion resistance improvements of loess slopes treated with EICP, EICP-BF, and EICP-PVAC are discussed based on the rainfall simulation test and dry–wet cycle test.

2. Materials and Methods

2.1. Materials

2.1.1. Loess

The loess used in this study was obtained from the loess hilly area of Xi’an City in Shanxi Province in China. The grading curve of the loess obtained from sieve analysis is shown in Figure 1. The loess mainly consisted of 12.0% clay particles (<0.005 mm), 77.6% silt particles (0.005–0.075 mm), and 10.4% sand particles (>0.075 mm), with a specific gravity of 2.66. The plastic limit was 20.61%, the liquid limit was 30.96%, and the plasticity index was 10.35. The initial CaCO3 content of loess used here was about 8.83%.

2.1.2. BF and PVAC

BF is extracted from basalt rock, and no chemical additives, pigments, or hazardous materials are added during the production process [28]. Apart from sustainability, BF is non-combustible, non-explosive, and non-toxic, even after contact with water, air, and other chemicals compounds [29]. Hence, BF is environmental-friendly. BF used in this study, with a density of 2.63 g/cm3 and length of 9 mm, was purchased from Shanghai Chenqi Chemical Technology Co., Ltd. (Shanghai, China) (Figure S2a). The price of BF was 35 CNY/kg.
PVAC emulsion is a type of water-based and environment-friendly adhesive, which is mainly composed of a vinyl acetate (VAc) monomer [30]. In this study, PVAC emulsion was purchased from the Henkel Adhesives Co., Ltd. in Shanghai (China), with a content of 45% (Figure S2b). The price of PVAC was 25 CNY/kg.

2.1.3. Urease and EICP Solution

The urease used in this study was extracted from soybeans. Dry soybeans were ground into powder using a pulverizer (800C, Dongwan Fangtai Electric Appliance Co., Ltd., Dongwan, China). The soybean powders were placed in a 4 °C refrigerator before use. An amount of 50 g soybean powders was added per 1 L deionized water and mixed using a magnetic stirrer for 30 min to obtain a homogeneous suspension. After storage in a refrigerator at 4 °C for 24 h, the suspension was centrifuged at 3000 rpm by a centrifuge (H-1650R, Shanghai Lixinjian Centrifuge Co., Ltd., Shanghai, China) for 15 min. The clean supernatant liquid obtained was the crude urease solution (Figure 2). The urease activity measured by electrical conductivity method at room temperature was 7.1 mM/min, which was sufficient to be used in soil treatments [31,32,33].
EICP solutions were prepared by urease, dissolving urea, and calcium chloride (CaCl2). The extracted crude urease was mixed with 1 mol/L CS solution (1 mol/L CaCl2 and 1 mol/L urea) in a ratio of 1:1 to obtain the EICP solution. The residual calcium ion in the solution was about 6.29% after 24 h reaction at 20 °C.

2.2. Slope Sample Preparation and Treatment

The setup used was a cube-shaped container with a length of 0.20 m, a width of 0.15 m, and a height of 0.04 m. Thirty-three seepage holes with a diameter of 2 mm were opened at the lower part of the boundary to simulate the seepage conditions (Figure S1). A layer of non-woven fabric was laid at the bottom of the container to avoid loess leakage and provide drainage.
Loess (1800 g) was placed into the container in three equal portions and slight compaction was applied to each layer of loess to ensure the uniformity of the sample. The initial dry density of slope sample was identical at 1.50 g/cm3. The experimental slope gradient was set at 25° to simulate the nature loess slope. The water that flowed from loess slope was collected by a plastic container during tests (Figure 3).
All slope samples were prepared under identical conditions and divided into four groups according to the various treatment methods (Table S1). In the first group (G1-1–G1-3), the slope samples were sprayed uniformly with EICP solution alone for 1 to 3 treatment cycles. In the second group (G2-1–G2-3), 0.1%, 0.3%, and 0.5% of BF were uniformity mixed with loess before spraying EICP solution to the slope samples (Figure S2a). In the third group (G3-1–G3-3), 10 g/L, 20 g/L, and 30 g/L of PVAC emulsion were added to the EICP solution to form a new cementation solution and then sprayed to the slope samples. In addition, slope sample G4-1 was sprayed with distilled water 3 times for comparison. All slope samples were treated at room temperature of 25 °C.

2.3. Surface Strength, Crust Layer Thickness, and CaCO3 Content Determination

After treatment, a crust layer was formed by the cementation of loess. The SS and thickness of the crust layer are important indexes to evaluate the treatment effect [2]. A soil penetrometer (WXGR-4.0, Changzhou 80 Future Intelligent Technology Co., Ltd., Changzhou, China) was used to measure the SS by inserting a 0.3 cm3 probe into the soil (Figure S3). The inserting direction was perpendicular to the slope surface. Six measuring points were chosen to measure the SS, and the average value was taken as the final result. After surface strength testing, the thickness of sampled crust layer was measured perpendicular to the loess surface via caliper.
The CaCO3 content of the crust layer with different numbers of EICP treatment cycles was measured by neutralization titration method. During the process, 5 g loess was sampled from crust layer and added to 20.00 mL HCl with a concentration of 1 mol/L. After fully stirring, phenolphthalein was added and the remaining HCl was titrated with 1mol/L NaOH. Finally, the CaCO3 content of samples was calculated. The lost dry weight caused by acid digestion represented the weight of precipitated CaCO3.

2.4. Scanning Electron Microscopy Test

The scanning electron microscopy tests (SEM) provides information about the microstructure of CaCO3 crystals and how CaCO3 crystals interact with loess particles. SEM images were conducted on three series of samples: the first series was of samples treated with EICP and the second series was treated with EICP-50 g/L PVAC, whereas the third series was treated with distilled water. Before using SEM on samples, the crust layer was dried at 105 °C for 12 h to ensure the loess was completely dried. The crust layer was then crushed and coated with a conductive metal to avoid electron scattering. According to Zhang et al. [34], the mechanical properties of loess would not notably change below 400 °C, so the 105 °C drying method did not influence the compositional, structural, and physicochemical properties of loess in this study.

2.5. Rainfall Simulation Test

Loess rainfall erosion causes the detachment and transport of soil grains, and finally increases the uncertainties of slope stability [35]. A performance assessment of EICP technology with respect to rainfall erosion resistance is, therefore, necessary and recommended prior to the field applications.
Loess slope samples were dried at 20 °C for 7 days after preparation. The slope gradient was fixed at 25° because 79.6% of landslides in Loess Plateau occurred for a slope angle range from 25° to 45°. The rainfall water was controlled by a kettle. The kettle with a nozzle at the base was installed on the iron frame at the height of 1.0 m from the loess slope sample to ensure the uniformity of sprayed water (Figure 4). The nozzle had 10 holes, each hole with a diameter of 1 mm. During rainfall simulation tests, the rainfall intensity was adjusted by rotating the nozzle. The simulated rainfall intensity was 2.5 mm/min (150 mm/h) because accelerated soil erosion was frequently derived from erosive rainfall events with this intensity [36]. The observed diameter of raindrops was 3 mm. The pH of the rainfall water was about 7.5. The time duration for each simulated rainfall event was 1 h. Over the course of tests, the surface erosion pattern of the loess slope samples was recorded by a 3D scanner (G5-630, Guangzhou Tianxun Electromechanical Equipment Co., Ltd., Guangzhou, China) with 6.3 million pixels (Figure S4). The loess washed out from slope samples was collected by a container and the weight was measured every 4 min. A total of 8 rainfall simulation tests were performed.

2.6. Dry–Wet Cycle Test

The loess slopes undergo dry–wet cycles during diurnal changes in rainy–sunny weather and subsequent processes of rainfall evapotranspiration due to the variable climate in Loess Plateau [37]. The repeated dry–wet process causes surface erosion, desiccation cracking and shrinkage in loess, resulting in the deterioration in shear strength and compressibility [38]. Hence, evaluating the dry–wet cycle performance of loess slopes treated by various methods is necessary.
In the dry–wet cycle test, loess slope samples treated with EICP, EICP-BF, EICP-PVAC, and distilled water were immersed in distilled water at 20 °C for 12 h. Then, samples were removed from distilled water and weighed after wiping the surface water. After most of water was separated, samples were dried at 105 °C in the oven (DHG-9076A, Shanghai Jinghong Experimental Equipment Co., Ltd., Shanghai, China) for 12 h, followed by weighing again (Figure S5). One such treatment represented one dry–wet cycle. The surface morphology after each dry–wet cycle of loess slope samples was recorded by a digital camera (GZ-R10BAC, JVC, Yokohama, Japan). The dry–wet responses of samples were then studied after various numbers of dry–wet cycles (0–6 cycles).

3. Results and Discussion

3.1. Surface Strength, Crust Layer Thickness, and CaCO3 Content Determination

The SS of EICP solidified loess, which can be used instead of unconfined compressive strength, is an important indicator to evaluate the cementation effect [2,39]. The SS of sample G1-1 was 0.222 kPa higher than that of sample G4-1, indicating that cementing properties of CaCO3 precipitation cemented loose loess particles into a strong compound and increased the strength (Table 1). With the increasing number of treatment cycles of EICP solution, the SS increased from 1.469 kPa (G1-1) to 1.742 kPa (G1-3), and the thickness increased from 5.83 mm to 6.98 mm. However, the SS increased in the third treatment cycle (0.102 kPa) was less than that in the second treatment cycle (0.171 kPa). The surface of the sample gradually hardened after the first two treatment cycles. Therefore, it was difficult for EICP solution to penetrate into the loess again. The EICP solution flowed down along the sample surface after the third treatment cycle also indicated this reason.
The cementing effect of EICP-BF was better than that of EICP under the same treatment cycles. However, the thickness of crust layers of G2-1, G2-2, and G2-3 was smaller than that of G1-3 (Table 1). The BF distributed in loess wrapped with carbonate crystals restricted the relative displacement of loess and held the soil particles together, which enhanced the SS but reduced the infiltration of EICP solution. Compared with sample G1-3, the SS increased by 15.0%, 30.3%, and 27.5% when 0.1%, 0.3%, and 0.5% of BF (by weight of loess) was added, respectively. This result showed that the BF played an important role in increasing the SS of loess slope sample treated with EICP and around 0.3% BF was the most suitable content. The reason was that more CaCO3 precipitation occurred around BF rather than loess particles at a higher BF content. The decreasing bonding between loess particles resulted in a weaker SS [27]. These findings were consistent with research on fiber-reinforced soils treated with MICP with similar fiber content [27,40,41].
PVAC addition further increased the SS of loess slope samples and provided an additional cementation effect, eventually resulting in a denser and thinner treated surface than that of sample G1-3. The SS increased by 10.9% and 36.1%, and crust layer thickness decreased by 15.9% and 31.2% with the concentration of added PVAC increasing from 10 g/L to 50 g/L (Table 1). This result meant that a higher concentration of added PVAC resulted in a larger SS but a smaller thickness. The microstructure formed a network structure with the increase in PVAC concentration, which promoted the affixation of CaCO3 and a more stable spatial structure [42]. Moreover, near-surface loess pores were easily clogged with increasing PVAC concentration, resulting in the harder penetration of EICP-PVAC solution into deeper soil. Zhao et al. [23] and Sun et al. [2] also obtained similar results by adding hydrogel and polyacrylamide (PAM) into soils.
In addition to SS and crust layer thickness, the CaCO3 content was also measured. The contents of CaCO3 increased progressively with increasing number of EICP treatment cycles, as shown in Figure 5. The increase rate of CaCO3 contents could be divided into two stages according to the number of treatment cycles: obviously increase stage and slightly increase stage. After the first treatment cycle, the CaCO3 content increased by 0.83%, while the value was only 0.08% after the third treatment cycle. The reason was that the formation of crust layer prevented the infiltration of EICP solution, thus reducing the increase rate of CaCO3 content. This result also demonstrated that optimal treatment cycle could achieve a better treatment result and avoid a waste of EICP solution.

3.2. Microscopic Characteristics

The SEM images of samples treated using the distilled water, EICP, and EICP-50 g/L PVAC solution are shown in Figure 6. Under the magnification = 800×, the loess of the sample treated with distilled water were loose particles, while the loess particles of the EICP-treated sample were aggregated (Figure 6a,b). The CaCO3 crystals with irregular structures had formed on the surface of loess particles and provided bridging contacts between particles (Figure 6c,d). This phenomenon further indicated that the CaCO3 crystals generated by EICP played cementing role among loess particles. The size of CaCO3 crystals was about 2 μm, which was smaller than that reported by Omoregie et al. [43]. With PVAC addition, the loess particles were coated by PVAC and CaCO3 crystals, thus forming layer-like films and rough edge on the particles surface. Moreover, net structures were formed by PVAC between loess particles and provided both bonding force and tension force (Figure 6e,f). This was the reason for the observed improvement of SS in EICP-PVAC treated samples.

3.3. Rainfall Simulation Test

3.3.1. 3D Scanner Observation

A three-dimensional model of surface topography was established to analyze the improvement effect of different treatments on the rainfall surface erosion of loess slope samples. The surface erosion of loess slope sample treated with distilled water was clearly observed after 2 min of simulated rainfall. After 6 min, the loess slope was destroyed and about 1/4 of the loess in the container was lost (Figure 7a). This phenomenon indicated that the distilled water did not improve the rainfall erosion resistance of the loess slope.
After EICP treatment, the stability of loess slopes improved and water flow hardly formed channels at the slope surface. Loess was slowly washed away until 8 min after rainfall (Figure 7b). The residual crust layer was observed at the edge of the scour pit, indicating that crust layer generated by EICP treatment played an important role in improving erosion resistance. After 16 min, the loss of loess was still much less than that of slope sample treated with distilled water. EICP-0.3% BF treatment achieved better erosion mitigation than EICP treatment (Figure 7c). At 12 min, only several small scour pits appeared on the down slope. At 20 min, the crust layer began to collapse because the water entered the core of the sample through scour pits and destroyed the stable internal structure. Moreover, the high erosion resistance of EICP-30 g/L PVAC treated sample could be attributed to the stable spatial structure formed by CaCO3 precipitation and the additional cementation provided by PVAC, which was also confirmed by SEM tests. The layer-like film formed by PVAC avoided the increase in runoff and erosion in the slope [44]. The slope began to be destroyed until 44 min and the amount of loess soil loss was still much less than that of other slope samples after 52 min.

3.3.2. Quantification of Surface Erosion

The accumulative rainfall erosion weight of loess slope treated with various methods was shown in Figure 8. The percentage of accumulative erosion weight of sample treated with distilled water was rapidly increased to 81% within the first 24 min after the rainfall simulation test. Then, the accumulative erosion weight percentage increased slowly and finally remained constant at a level of around 90%. Compared with sample G4-1, the erosion time was delayed for samples treated with EICP, EICP-BF, and EICP-PVAC solution because the crust layer prevented rainfall infiltration. However, the total percentage of eroded loess of samples treated with EICP, EICP-0.1% BF, EICP-0.3% BF, and EICP-10 g/L PVAC solution still reached nearly 90%, indicating that these treatments did not significantly decrease the total amount of washed-out soil after long-term rainfall. The reason was that the crust layer was destroyed by prolonged rainfall and the strength of loess was reduced by water infiltration. Additionally, the percentages of finial accumulated loess erosion weight were 64%, 43%, and 12%, respectively, as samples treated with EICP-0.5% BF, EICP-30 g/L PVAC, and EICP-50 g/L PVAC. It was found that accumulative erosion weight decreased with increasing of BF or PVAC concentration due to the additional cementation provided by BF or PVAC. Consequently, the use of EICP technique could effectively enhance the surface erosion resistance of loess slope, and such improvement was proportional to the additional BF and PVAC.

3.4. Dry–Wet Cycle Test

3.4.1. Visual Observation

The surface morphology photos of loess slope samples treated with different treatments after dry–wet cycle test were shown in Figure 9. The sample treated with distilled water expanded and the loess at surface diffused during immersion. After EICP treatment, both horizontal and vertical cracks penetrated on the surface of sample after dry–wet cycles, but the width of cracks was small and unevenly distributed. However, the number of cracks of EICP-0.5% BF treated sample was lower and crack width was smaller than those treated with EICP because the BF addition effectively alleviated dry–wet weathering. For the sample treated with EICP-30 g/L PVAC, several long cracks emerged, and a piece of crust layer at the top slope was damaged and washed out. The width of cracks was obviously larger than those of samples treated with EICP and EICP-0.5% BF. The reason was that the PVAC addition promoted the cementation of loess into clumps.

3.4.2. Quantification of Surface Erosion

Figure 10 showed the percentage of accumulative loess loss weight of samples after various numbers of dry–wet cycles. The soil loss weight of samples with different treatments increased with the increasing number of dry–wet cycles due to the loose soil structure caused by water. The sample treated with distilled water was more sensitive to dry–wet cycles, especially to the initial dry–wet cycle. The percentage of accumulative loess loss weight reached 9% after the first dry–wet cycle and finally reached nearly 30% within the considered dry–wet cycles. This was mainly caused by the loess particle detachment.
After EICP treatment, the total percentage of loess loss was reduced to 41% of that with distilled water treatment. However, the percentage increased by about 6% from the fifth cycle to sixth cycle. The crust layer was destroyed with the increasing number of dry–wet cycles due to the repeated disturbance to soil structure. With BF increased from 0.1% to 0.5%, the finial percentage of loess loss weight decreased by 1.3%. It implied that the EICP-BF reinforcement interface of loess with high BF content was more firmly cemented, providing additional connection for loess particles. In addition, the optimal treatment effect of EICP-PVAC was to add PVAC at the concentration of 30 g/L. The finial loess loss weight percentage of the sample treated with EICP-50 g/L PVAC reached 13.2%, which was more than twice of the sample treated with EICP-30g/L PVAC. The main reason was that the surface of loess slope sample treated with EICP-50 g/L PVAC was destroyed by pieces under dry–wet cycles.

4. Conclusions

In this study, physical properties measurement tests, scanning electron microscopy test, rainfall simulation tests, and dry–wet cycle tests were performed to determine the influence of various treatment methods on the erosion resistance of reinforced loess slopes. The main conclusions are as follow.
  • The EICP, EICP-BF, and EICP-PVAC treatments improved surface strength (SS) of loess slope. The SS increased with increasing the content of basalt fiber (BF) or the concentration of polyvinyl acetate emulsion (PVAC), which was attributed to the more stable spatial structure of CaCO3 precipitation caused by additional BF or PVAC. However, the thickness of crust layer decreased with increasing the BF content or PVAC concentration because the crust layer prevented the further infiltration of solution. The CaCO3 content increased progressively with the increasing number of EICP treatment cycles;
  • Under the rainfall conditions, the time until erosion was delayed and the stability was improved for loess slope samples treated with EICP, EICP-BF, and EICP-PVAC. The percentage of accumulative erosion weight of the distilled water treated sample was increased rapidly to 81% within the first 24 min, whereas it remained at 0% for samples treated with EICP-0.5% BF, EICP-30 g/L PVAC, or EICP-50 g/L PVAC. The high content of BF and PVAC achieved superior erosion control on loess slopes during rainfall.
  • The addition of BF effectively inhibited the development of surface cracks in loess slope samples, especially at a BF content of 0.5%. With the increasing number of dry–wet cycles, the accumulative loess loss weight of samples treated with various methods increased gradually. The loess loss in EICP-30 g/L PVAC treated slopes subjected 6 dry–wet cycles remained smaller than 5.5%. In addition, loess loss weight percentage reached 13.2% for the sample treated with EICP-50 g/L PVAC as the surface loess was destroyed by pieces.
The results suggested that the EICP, EICP-BF, and EICP-PVAC methods had a significant influence on the improvement of SS and erosion resistance of loess slopes. These experimental results could provide guidance for loess prevention. Further research will focus on the influence of rainfall intensity, the reaction temperature, and the permeability of treated loess, as these also affect the long-term application potential of treatments.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su15021044/s1, Figure S1: Photo of setup; Figure S2: Photos of basalt fiber and polyvinyl acetate emulsion. (a) Basalt fiber; (b) Polyvinyl acetate emulsion; Figure S3: Soil penetrometer (WXGR-4.0); Figure S4: 3D scanner (G5-630); Figure S5: Drying oven (DHG-9076A); Table S1: Sample arrangements for laboratory tests.

Author Contributions

Methodology, C.W.; Software, Z.S.; Investigation, Z.L.; Data curation, Z.L.; Writing–original draft, D.S.; Writing—review & editing, C.W.; Visualization, Z.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 42177141).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Juang, C.H.; Dijkstra, T.; Wasowski, J.; Meng, X.M. Loess geohazards research in China: Advances and challenges for mega engineering projects. Eng. Geol. 2019, 251, 1–10. [Google Scholar] [CrossRef]
  2. Sun, X.H.; Miao, L.C.; Chen, R.F.; Wang, H.X.; Xia, J.X. Surface rainfall erosion resistance and freeze-thaw durability of bio-cemented and polymer-modified loess slopes. J. Environ. Manag. 2022, 301, 113883. [Google Scholar] [CrossRef] [PubMed]
  3. Wei, W.; Chen, L.D.; Fu, B.J.; Huang, Z.L.; Wu, D.P.; Gui, L.D. The effect of land uses and rainfall regimes on runoff and soil erosion in the semi-arid loess hilly area, China. J. Hydrol. 2007, 335, 247–258. [Google Scholar] [CrossRef]
  4. Chen, H.; Zhang, X.P.; Ablaa, M.; Lü, D.; Yan, R.; Ren, Q.F.; Ren, Z.Y.; Yang, Y.H.; Zhao, W.H.; Lin, P.F.; et al. Effects of vegetation and rainfall types on surface runoff and soil erosion on steep slopes on the Loess Plateau, China. Catena 2018, 170, 141–149. [Google Scholar] [CrossRef]
  5. Xin, W.; Lin, Z. Soil erosion control practices in the Chinese Loess Plateau: A systematic review. Environ. Dev. 2020, 34, 100493. [Google Scholar]
  6. Wang, J.-J.; Liang, Y.; Zhang, H.-P.; Wu, Y.; Lin, X. A loess landslide induced by excavation and rainfall. Landslides 2014, 11, 141–152. [Google Scholar] [CrossRef]
  7. Zhuang, J.Q.; Peng, J.B.; Wang, G.H.; Javed, I.; Wang, Y.; Li, W. Distribution and characteristics of landslide in Loess Plateau: A case study in Shaanxi province. Eng. Geol. 2018, 236, 89–96. [Google Scholar] [CrossRef]
  8. Cui, Y.; Kong, J.M.; Ni, Z.Q. Key dominant mechanism of heavy rain in landslide development and analysis of a typical case. J. Catastrophol. 2011, 26, 13–17. [Google Scholar]
  9. Zhang, Z. Field Test of Rainfall Infiltration of Loess Slopes and Its Disaster Warning in Northern Shanxi Province. Master’s Thesis, Xi’an University of Technology, Xi’an, China, 2018. [Google Scholar]
  10. Wu, J.-J.; Cheng, Q.-G.; Liang, X.; Cao, J.-L. Stability analysis of a high loess slope reinforced by the combination system of soil nails and stabilization piles. Front. Struct. Civ. Eng. 2014, 8, 252–259. [Google Scholar] [CrossRef]
  11. Zhao, Z.F.; Zhu, Y.P.; Ye, S.H. Calculation of horizontal displacement of loess fill slope supported by frame prestressed anchors based on minimum potential energy method. Sci. Rep. 2022, 12, 11281. [Google Scholar] [CrossRef]
  12. Wang, Y.D.; Liu, X.F.; Li, J.; Wang, Y.D.; Bai, J.Z.; Zhou, Z.X. Quantifying the spatial flow of soil conservation service to optimize land-use pattern under ecological protection scenarios. Front. Earth Sci. 2022, 10, 957520. [Google Scholar] [CrossRef]
  13. Neupane, D.; Yasuhara, H.; Kinoshita, N.; Unno, T. Applicability of Enzymatic Calcium Carbonate Precipitation as a Soil-Strengthening Technique. J. Geotech. Geoenviron. Eng. 2013, 139, 2201–2211. [Google Scholar] [CrossRef]
  14. Wu, C.Z.; Chu, J.; Wu, S.F.; Hong, Y. 3D characterization of microbially induced carbonate precipitation in rock fracture and the resulted permeability reduction. Eng. Geol. 2019, 249, 23–30. [Google Scholar] [CrossRef]
  15. Kavazanjian, E.; Hamdan, N. Enzyme Induced Carbonate Precipitation (EICP) Columns for Ground Improvement. In Proceedings of the International Foundations Congress and Equipment Expo 2015, San Antonio, TX, USA, 17–21 March 2015; pp. 2252–2261. [Google Scholar] [CrossRef]
  16. Ahenkorah, I.; Rahman, M.; Karim, R.; Beecham, S.; Saint, C. A Review of Enzyme Induced Carbonate Precipitation (EICP): The Role of Enzyme Kinetics. Sustain. Chem. 2021, 2, 92–114. [Google Scholar] [CrossRef]
  17. Dilrukshi, R.; Nakashima, K.; Kawasaki, S. Soil improvement using plant-derived urease-induced calcium carbonate precipitation. Soils Found. 2018, 58, 894–910. [Google Scholar] [CrossRef]
  18. Wu, M.; Hu, X.; Zhang, Q.; Zhao, Y.; Cheng, W.; Xue, D. Preparation and performance of a biological dust suppressant based on the synergistic effect of enzyme-induced carbonate precipitation and surfactant. Environ. Sci. Pollut. Res. 2022, 29, 8423–8437. [Google Scholar] [CrossRef] [PubMed]
  19. Hamdan, N.; Zhao, Z.; Mujica, M.; Kavazanjian, E.; He, X. Hydrogel-Assisted Enzyme-Induced Carbonate Mineral Precipitation. J. Mater. Civ. Eng. 2016, 28, 04016089. [Google Scholar] [CrossRef]
  20. Almajed, A.; Abbas, H.; Arab, M.; Alsabhan, A.; Hamid, W.; Al-Salloum, Y. Enzyme-Induced Carbonate Precipitation (EICP)-Based methods for ecofriendly stabilization of different types of natural sands. J. Clean. Prod. 2020, 274, 122627. [Google Scholar] [CrossRef]
  21. Sun, X.; Miao, L.; Wang, H.; Yin, W.; Wu, L. Mineralization crust field experiment for desert sand solidification based on enzymatic calcification. J. Environ. Manag. 2021, 287, 112315. [Google Scholar] [CrossRef]
  22. Bang, S.S.; Bang, S.; Frutiger, S.; Nehl, L.M.; Comes, B.L. Application of Novel Biological Technique in Dust Suppression. In Proceedings of the Transportation Research Board 88th Annual Meeting Transportation Research Board, Washington, DC, USA, 11–15 January 2009. [Google Scholar]
  23. Zhao, Z.; Hamdan, N.; Shen, L.; Nan, H.; Almajed, A.; Kavazanjian, E.; He, X. Biomimetic hydrogel composites for soil stabilization and contaminant mitigation. Environ. Sci. Technol. 2016, 50, 12401–12410. [Google Scholar] [CrossRef]
  24. Almajed, A.; Tirkolaei, H.K.; Kavazanjian, E.; Hamdan, N. Enzyme Induced Biocementated Sand with High Strength at Low Carbonate Content. Sci. Rep. 2019, 9, 1135. [Google Scholar] [CrossRef]
  25. Cui, M.J.; Lai, H.J.; Hoang, T.; Chu, J. One-phase-low-pH enzyme induced carbonate precipitation (EICP) method for soil improvement. Acta Geotech. 2020, 16, 481–489. [Google Scholar] [CrossRef]
  26. Bu, F.; Liu, J.; Bai, Y.; Prasanna Kanungo, D.; Song, Z.; Kong, F.; Pan, C. Effects of the Preparation Conditions and Reinforcement Mechanism of Polyvinyl Acetate Soil Stabilizer. Polymers 2019, 11, 506. [Google Scholar] [CrossRef] [Green Version]
  27. Zhao, Y.; Xiao, Z.; Fan, C.; Shen, W.; Wang, Q.; Liu, P. Comparative mechanical behaviors of four fiber-reinforced sand cemented by microbially induced carbonate precipitation. Bull. Eng. Geol. Environ. 2020, 79, 3075–3086. [Google Scholar] [CrossRef]
  28. Militký, J.; Mishra, R.; Jamshaid, H. Basalt fibers. In Handbook of Properties of Textile and Technical Fibres; Woodhead Publishing: Cambridge, UK, 2018; pp. 805–840. [Google Scholar]
  29. Khan, M.; Cao, M.; Chu, S.; Ali, M. Properties of hybrid steel-basalt fiber reinforced concrete exposed to different surrounding conditions. Constr. Build. Mater. 2022, 322, 126340. [Google Scholar] [CrossRef]
  30. Zhang, Y.; Pang, B.; Yang, S.; Fang, W.; Yang, S.; Yuan, T.-Q.; Sun, R.-C. Improvement in Wood Bonding Strength of Poly (Vinyl Acetate-Butyl Acrylate) Emulsion by Controlling the Amount of Redox Initiator. Materials 2018, 11, 89. [Google Scholar] [CrossRef] [Green Version]
  31. Chu, J.; Ivanov, V.; Stabnikov, V.; Li, B. Microbial method for construction of an aquaculture pond in sand. Géotechnique 2013, 63, 871–875. [Google Scholar] [CrossRef]
  32. Jiang, N.J.; Soga, K.; Kuo, M. Microbially induced carbonate precipitation for seepage-induced internal erosion control in sand-clay mixtures. J. Geotech. Geoenviron. Eng. 2017, 143, 04016100. [Google Scholar] [CrossRef]
  33. Gao, Y.; He, J.; Tang, X.; Chu, J. Calcium carbonate precipitation catalyzed by soybean urease as an improvement method for fine-grained soil. Soils Found. 2019, 59, 1631–1637. [Google Scholar] [CrossRef]
  34. Zhang, F.; Kong, R.; Peng, J. Effects of heating on compositional, structural, and physicochemical properties of loess under laboratory conditions. Appl. Clay Sci. 2018, 152, 259–266. [Google Scholar] [CrossRef]
  35. Alavinia, M.; Saleh, F.N.; Asadi, H. Effects of rainfall patterns on runoff and rainfall-induced erosion. Int. J. Sediment Res. 2019, 34, 270–278. [Google Scholar] [CrossRef]
  36. Zhang, Q.; Wang, Z.; Wu, B.; Shen, N.; Liu, J. Identifying sediment transport capacity of raindrop-impacted overland flow within transport-limited system of interrill erosion processes on steep loess hillslopes of China. Soil Tillage Res. 2018, 184, 109–117. [Google Scholar] [CrossRef]
  37. Tang, C.-S.; Cui, Y.-J.; Shi, B.; Tang, A.-M.; Liu, C. Desiccation and cracking behaviour of clay layer from slurry state under wetting–drying cycles. Geoderma 2011, 166, 111–118. [Google Scholar] [CrossRef] [Green Version]
  38. Tang, C.S.; Wang, D.Y.; Shi, B.; Li, J. Efect of wetting-drying cycles on profle mechanical behavior of soils with different initial conditions. Catena 2016, 139, 105–116. [Google Scholar] [CrossRef]
  39. Wu, C.Z.; Chu, J. Biogrouting method for stronger bond strength for aggregates. J. Geotech Geoenviron. 2020, 146, 06020021. [Google Scholar] [CrossRef]
  40. Jiang, H.; Cai, Y.; Liu, J. Engineering Properties of Soils Reinforced by Short Discrete Polypropylene Fiber. J. Mater. Civ. Eng. 2010, 22, 1315–1322. [Google Scholar] [CrossRef]
  41. Wu, C.Z.; Chu, J.; Cheng, L.; Wu, S.F. Biogrouting of Aggregates Using Premixed Injection Method with or without pH Adjustment. J. Mater. Civ. Eng. 2019, 31, 06019008. [Google Scholar] [CrossRef]
  42. Jazi, M.A.; Ramezani, S.A.A.; Haddadi, S.A.; Ghaderi, S.; Azamian, F. In situ emulsion polymerization and characterization of PVAc nanocomposites including colloidal silica nanoparticles for wood specimens bonding. J. Appl. Polym. Sci. 2020, 137, 48570. [Google Scholar] [CrossRef]
  43. Omoregie, A.I.; Palombo, E.A.; Ong, D.E.; Nissom, P.M. A feasible scale-up production of Sporosarcina pasteurii using custom-built stirred tank reactor for in-situ soil biocementation. Biocatal. Agric. Biotechnol. 2020, 24, 101544. [Google Scholar] [CrossRef]
  44. Sun, X.H.; Miao, L.C.; Wang, H.X.; Chen, R.F.; Wu, L.Y. Bio-cementation for the mitigation of surface erosion in loess slopes based on simulation experiment. J. Soils Sediments 2022, 22, 1804–1818. [Google Scholar] [CrossRef]
Sustainability 15 01044 g001 550
Figure 1. Grading curve of loess.
Figure 1. Grading curve of loess.
Sustainability 15 01044 g001
Sustainability 15 01044 g002 550
Figure 2. Extraction process of crude urease.
Figure 2. Extraction process of crude urease.
Sustainability 15 01044 g002
Sustainability 15 01044 g003 550
Figure 3. Schematic setup of tests.
Figure 3. Schematic setup of tests.
Sustainability 15 01044 g003
Sustainability 15 01044 g004 550
Figure 4. Rainfall simulation test device.
Figure 4. Rainfall simulation test device.
Sustainability 15 01044 g004
Sustainability 15 01044 g005 550
Figure 5. CaCO3 contents of various numbers of EICP treatment cycles.
Figure 5. CaCO3 contents of various numbers of EICP treatment cycles.
Sustainability 15 01044 g005
Sustainability 15 01044 g006 550
Figure 6. SEM images of loess slope samples treated with distilled water, EICP, and EICP-50 g/L PVAC: (a) sample G4-1, magnification = 800×; (b) sample G1-3, magnification = 800×; (c) sample G1-3, magnification = 2500×; (d) sample G1-3, magnification = 20,000×; (e) sample G3-3, magnification = 2500×; (f) sample G3-3, magnification = 20,000×.
Figure 6. SEM images of loess slope samples treated with distilled water, EICP, and EICP-50 g/L PVAC: (a) sample G4-1, magnification = 800×; (b) sample G1-3, magnification = 800×; (c) sample G1-3, magnification = 2500×; (d) sample G1-3, magnification = 20,000×; (e) sample G3-3, magnification = 2500×; (f) sample G3-3, magnification = 20,000×.
Sustainability 15 01044 g006
Sustainability 15 01044 g007 550
Figure 7. Three dimensional models of reinforced loess slope samples with various treatments in rainfall simulation test: (a) sample G4-1; (b) sample G1-3; (c) sample G2-2; (d) sample G3-2.
Figure 7. Three dimensional models of reinforced loess slope samples with various treatments in rainfall simulation test: (a) sample G4-1; (b) sample G1-3; (c) sample G2-2; (d) sample G3-2.
Sustainability 15 01044 g007
Sustainability 15 01044 g008 550
Figure 8. Accumulative erosion weight of loess slope samples treated with different methods.
Figure 8. Accumulative erosion weight of loess slope samples treated with different methods.
Sustainability 15 01044 g008
Sustainability 15 01044 g009 550
Figure 9. Sample photos after dry–wet cycle test: (a) G4-1; (b) G1-3; (c) G2-3; (d) G3-2.
Figure 9. Sample photos after dry–wet cycle test: (a) G4-1; (b) G1-3; (c) G2-3; (d) G3-2.
Sustainability 15 01044 g009
Sustainability 15 01044 g010 550
Figure 10. Accumulative loss weight of loess slope samples after various numbers of dry–wet cycles.
Figure 10. Accumulative loss weight of loess slope samples after various numbers of dry–wet cycles.
Sustainability 15 01044 g010
Table
Table 1. The surface strength and thickness of crust layers treated with different methods.
Table 1. The surface strength and thickness of crust layers treated with different methods.
Sample No.TreatmentTreatment CyclesSS (kPa)Thickness (mm)
G1-1EICP11.4695.83
G1-221.6406.22
G1-331.7426.98
G2-1EICP-0.1% BF32.0045.54
G2-2EICP-0.3% BF32.2695.34
G2-3EICP-0.5% BF32.2215.07
G3-1EICP-10 g/L PVAC31.9325.87
G3-2EICP-30 g/L PVAC32.2385.53
G3-3EICP-50 g/L PVAC32.3174.80
G4-1Distilled water31.247/
Note: EICP = Enzyme Induced Calcite Precipitation, BF = basalt fiber, PVAC = Polyvinyl acetate emulsion, SS = surface strength.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Shen, D.; Liu, Z.; Song, Z.; Wu, C. Reinforcement Mechanism and Erosion Resistance of Loess Slope Using Enzyme Induced Calcite Precipitation Technique. Sustainability 2023, 15, 1044. https://doi.org/10.3390/su15021044

AMA Style

Shen D, Liu Z, Song Z, Wu C. Reinforcement Mechanism and Erosion Resistance of Loess Slope Using Enzyme Induced Calcite Precipitation Technique. Sustainability. 2023; 15(2):1044. https://doi.org/10.3390/su15021044

Chicago/Turabian Style

Shen, Danyi, Zhenyuan Liu, Zhichao Song, and Chuangzhou Wu. 2023. "Reinforcement Mechanism and Erosion Resistance of Loess Slope Using Enzyme Induced Calcite Precipitation Technique" Sustainability 15, no. 2: 1044. https://doi.org/10.3390/su15021044

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop