# Mechanical Property of Biomodified Geogrid and Reinforced Calcareous Sand

Authors: Qiang Ou; Yifu Li; Yang Yang; Zhaogang Luo; Shaokang Han; Tan Zou
Journal: Geofluids
 (2022)

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## Abstract

The strength and deformation properties of maritime geotechnical structures made primarily of calcareous sand are critical for project safety. The geogrid reinforcement is developed as a promising approach to improve the mechanical properties of calcareous sand. This study investigates the mechanical property of biomodified geogrid via a microbially induced calcite precipitation (MICP) process to improve the effectiveness of geogrid for reinforcement of calcareous sand. A series of unconsolidated undrained triaxial experiments were conducted to evaluate the mechanical property and deformation behaviors of biomodified geogrid and reinforced calcareous sand (BGRCS), taking into consideration the impacts of the geogrid layer, times of biotreatment, and confining pressure. Compared to the untreated geogrid, the strength of the BGRCS is distinctly changed due to the increase roughness, and the deviatoric stress-strain curves are evidently hardening. Strength and pseudocohesive force can be further enhanced by raising the geogrid layer of the reinforced specimens, while internal friction angle also increases the amplitude of variation with the times of biotreatment. The geogrid, times of biotreatment, and confining pressure are all intimately related to the strength and the deformation of the reinforced specimens. The interactions of geogrid ribs and calcareous sand particles are analyzed and friction using scanning electron microscope tests that could provide a reference for revealing the mechanical mechanism of BGRCS.


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## Body

## 1. Introduction

Calcareous sands are formed by the remains of marine organisms deposited over a long period of geological action and are characterized by porosity, high calcium carbonate content, and easy fragmentation [1–3]. Between 30°N and 30°S, these widely distributed calcareous sands are a potentially important material source for marine engineering construction. Under a certain stress level, large deformation or even grain breakage could be generated in calcareous sand, posing a major threat to the structural integrity of geotechnical constructions [4–7]. In recent years, large-scale offshore wind turbines [8, 9], subsea energy mining platforms [10, 11], and artificial reefs [12, 13] have all been erected, and their safety must be ensured completely.Mastering the strength and deformation characteristics of calcareous sand is a basic requirement for the design and safe construction of offshore geotechnical engineering. It is generally believed that the strength and deformation of calcareous sand are affected by particle size gradation, density, and stress state [14–18]. Under the high stress, calcareous sand would undergo obvious particle fragmentation [5]. The fine particles produced by the crushing cause a significant transformation of the mechanical properties. Wei et al. [19] investigated the influence of particle breakage on the shear strength and found that the increase in shear stress was limited due to the intense particle breakage and rearrangements. In addition, particle size distribution, particle shape, and particle surface roughness also have significant effects on particle crushing and stress-strain responses [6]. Ground settlement and the associated structural stability can be potentially affected by these fundamental mechanical properties in calcareous sand zone (e.g., the penetration of large diameter piles and the construction of underground pipes) [3, 20–22].Adopting reinforcement methods, including chemical, biological, and physical methods, can improve the strength and deformation properties of calcareous sands; thus, the safety and stability of geotechnical structures will be significantly improved. For chemical reinforcement [23], chemical substances are difficult to recycle and may pollute the marine environment, causing irreversible environmental damage. Therefore, chemical reinforcement methods are not suitable for large-scale promotion. Physical reinforcement methods usually involve adding some stable materials to the soil, such as common fibers, geotextiles, and geogrids [24–31]. These materials are easy to construct and recycle with obvious effects. Thus geomaterials have the potential for large-scale applications in traditional road foundations, slopes, and retaining walls [29, 32, 33]. Out of the aforementioned geomaterials, geogrids are easy-operated and relatively cheaper. However, their application of geogrids in marine engineering construction is still limited. Recently, an innovative biomodification technique, so-called microbial induced carbonate precipitation (MICP) [15, 34–36], has attracted wide attention on enhancing friction between geomaterials and construction material. For example, Hao et al. 2018 [37] (paper: Enhancing fiber/matrix bonding in polypropylene fiber reinforced cementitious composites by microbially induced calcite precipitation pre-treatment) have studied the improved fiber/matrix bonding of cementitious composites via MICP-modified fiber. Zhang et al. [31] also investigated the effect of MICP-modified steel fiber on the bonding properties of ultrahigh performance concrete.In this paper, a biomodified small-size geotechnical geogrid via MICP treatment was used to reinforce calcareous sand, enhancing the friction performance of geogrids reinforced calcareous sand. Through a series of unconsolidated undrained triaxial tests (UU), the mechanical and deformation properties of geogrids reinforced calcareous sand with biomodified geogrids and untreated geogrids under different confining pressures were compared. For calcareous sand reinforced by biomodified geogrids, the influence of confining pressures, the layer of the geogrid, and times of biotreatment on the mechanical and deformation performance were also considered. Microscopic interfacial interaction characteristics of biomodified geogrid were analyzed through scanning electron microscope (SEM) tests. The mechanical characteristic of biomodified geogrid reinforcement was also discussed.


## 2. Materials and Methods

### 2.1. Materials

The calcareous sand used in the test originated from the South China Sea, which is also typical coral sand [38, 39]. The calcareous sand particles are off-white, irregular in shape, and with a large number of microscopic pores [16, 39]. The dried calcareous sand particles were passed through a 2 mm sieve to remove oversized impurities for triaxial tests. Figure 1 shows the sieved calcareous sand and its particle size distribution curve, and the basic physical property indexes of the calcareous sand are shown in Table 1. Additionally, the specially customized polyethylene geogrid with a small mesh size manifests similar properties to the geogrid used in actual engineering projects. Therefore, it can be effectively used for these laboratory tests. The basic tensile performance of the geogrid was obtained from the unidirectional tensile tests in Figure 2, and its ultimate tensile strength exceeds 2.4 kN/m.Figure 1
Initial grain distribution curve of calcareous sand.Table 1
Physical properties of calcareous sand.
ItemsValueSpecific gravity2.78Median particle diameter (d50) (mm)0.82Uniformity coefficient (Cu)2.50Curvature coefficient (Cc)1.30Minimum void ratio (emin)0.79Maximum void ratio (emax)1.24Figure 2
The curve of tension-strain for the geogrid.


### 2.2. Biomodification Treatment

Commercial Sporosarcina pasteurii (DSM33) was used for the biomodification process. The bacteria cultivation process was followed by the same procure mentioned by Yang et al. [40]. The OD600 of the harvested bacterial solution was 2 ~ 2.3, and the urease activity was 10±0.5U/ml (1U=1molureahydrolyzedperminute). The cementation solution used in this study included 1 M of equimolar of urea (60 g/L) and calcium chloride (111 g/L).Three acrylic tray boxes were used for the MICP treatment of geogrids. The geogrids were soaked into the solution that consisted of bacterial culture (2.5 U/ml diluted by deionized water) and cementation solution with a volume ratio of 1 : 1. The tray box was kept at room temperature (25±2°C) for 24 h to allow the calcite generated by the bacteria to be deposited on the surface of geogrids. The volume of biological reagent solution was about 5 times the volume of geogrids. Repeated treatment was carried out every 24 hours by replacing the biological reagent solution. After treatment, geogrids were rinsed with 5 times of tap water and air-dried prior to use. Different times of biotreatment varied from 1 to 3 were conducted to achieve different biomodification by varying the amounts of coating. The calcium carbonate content (CCC) measurement was also conducted using the acid washing method, reported by Choi et al. (2017) [41].


### 2.3. Triaxial Tests

In order to investigate the basic mechanical properties of biomodified geogrid and reinforced calcareous sand, unconsolidated undrained triaxial tests (UU) were conducted in the laboratory. The effects of geogrid, times of biotreatment, and confining pressure were considered. The specific testing scheme is presented in Table2. The sample is 39.1 mm in diameter and 80 mm in height. The geogrid was processed into a circular piece with a diameter of slightly less than 39.1 mm, which was divided into 1/3/5 layers of horizontal layout in the sample. Sample sizes and geogrid layout are shown in Figure 3.Table 2
Test schemes and initial parameters.
NumberParticle size range (mm)Geogrid layersRelative density (%)Moisture content (%)Confining pressure (kPa)Times of biotreatmentShearing rate (mm·min-1)T10-20500100/200/300/40000.2T20-21500100/200/300/4000/1/2/30.2T30-23500100/200/300/4000/1/2/30.2T40-25500100/200/300/4000/1/2/30.2Figure 3
Geogrid and layout diagram.During the test, the sample was divided into two/four/six parts equally according to the set quality, and the density was strictly controlled. After that, a shear rate of 0.2 mm/min was used for the test. When the axial strain reached 20%, the test was stopped, and the data was sorted out. When the deviated stress-strain curve is softening, the peak strength is taken as the shear strength, whereas the stress corresponding to 15% axial strain is taken as the shear strength when the deviated stress-strain curve is hardening.


## 2.1. Materials

The calcareous sand used in the test originated from the South China Sea, which is also typical coral sand [38, 39]. The calcareous sand particles are off-white, irregular in shape, and with a large number of microscopic pores [16, 39]. The dried calcareous sand particles were passed through a 2 mm sieve to remove oversized impurities for triaxial tests. Figure 1 shows the sieved calcareous sand and its particle size distribution curve, and the basic physical property indexes of the calcareous sand are shown in Table 1. Additionally, the specially customized polyethylene geogrid with a small mesh size manifests similar properties to the geogrid used in actual engineering projects. Therefore, it can be effectively used for these laboratory tests. The basic tensile performance of the geogrid was obtained from the unidirectional tensile tests in Figure 2, and its ultimate tensile strength exceeds 2.4 kN/m.Figure 1
Initial grain distribution curve of calcareous sand.Table 1
Physical properties of calcareous sand.
ItemsValueSpecific gravity2.78Median particle diameter (d50) (mm)0.82Uniformity coefficient (Cu)2.50Curvature coefficient (Cc)1.30Minimum void ratio (emin)0.79Maximum void ratio (emax)1.24Figure 2
The curve of tension-strain for the geogrid.


## 2.2. Biomodification Treatment

Commercial Sporosarcina pasteurii (DSM33) was used for the biomodification process. The bacteria cultivation process was followed by the same procure mentioned by Yang et al. [40]. The OD600 of the harvested bacterial solution was 2 ~ 2.3, and the urease activity was 10±0.5U/ml (1U=1molureahydrolyzedperminute). The cementation solution used in this study included 1 M of equimolar of urea (60 g/L) and calcium chloride (111 g/L).Three acrylic tray boxes were used for the MICP treatment of geogrids. The geogrids were soaked into the solution that consisted of bacterial culture (2.5 U/ml diluted by deionized water) and cementation solution with a volume ratio of 1 : 1. The tray box was kept at room temperature (25±2°C) for 24 h to allow the calcite generated by the bacteria to be deposited on the surface of geogrids. The volume of biological reagent solution was about 5 times the volume of geogrids. Repeated treatment was carried out every 24 hours by replacing the biological reagent solution. After treatment, geogrids were rinsed with 5 times of tap water and air-dried prior to use. Different times of biotreatment varied from 1 to 3 were conducted to achieve different biomodification by varying the amounts of coating. The calcium carbonate content (CCC) measurement was also conducted using the acid washing method, reported by Choi et al. (2017) [41].


## 2.3. Triaxial Tests

In order to investigate the basic mechanical properties of biomodified geogrid and reinforced calcareous sand, unconsolidated undrained triaxial tests (UU) were conducted in the laboratory. The effects of geogrid, times of biotreatment, and confining pressure were considered. The specific testing scheme is presented in Table2. The sample is 39.1 mm in diameter and 80 mm in height. The geogrid was processed into a circular piece with a diameter of slightly less than 39.1 mm, which was divided into 1/3/5 layers of horizontal layout in the sample. Sample sizes and geogrid layout are shown in Figure 3.Table 2
Test schemes and initial parameters.
NumberParticle size range (mm)Geogrid layersRelative density (%)Moisture content (%)Confining pressure (kPa)Times of biotreatmentShearing rate (mm·min-1)T10-20500100/200/300/40000.2T20-21500100/200/300/4000/1/2/30.2T30-23500100/200/300/4000/1/2/30.2T40-25500100/200/300/4000/1/2/30.2Figure 3
Geogrid and layout diagram.During the test, the sample was divided into two/four/six parts equally according to the set quality, and the density was strictly controlled. After that, a shear rate of 0.2 mm/min was used for the test. When the axial strain reached 20%, the test was stopped, and the data was sorted out. When the deviated stress-strain curve is softening, the peak strength is taken as the shear strength, whereas the stress corresponding to 15% axial strain is taken as the shear strength when the deviated stress-strain curve is hardening.


## 3. Results and Analysis

### 3.1. Failure Patterns

During the test, the loading failure process of each sample was observed, and the typical failure characteristics of some samples were recorded. The test results indicate that the expansion of calcareous sand is quite obvious in the triaxial tests, and most of the samples are bulging during the damage process without any obvious shear surface. Generally, the shearing expansion of calcareous sands is pronounced [19, 39], especially at low confining pressures. Figure 4 compares the failure modes of the unreinforced and the reinforced calcareous sand samples under 200 kPa confining pressure. The lateral bulge of the sample without reinforcement is obvious (Figure 4(a)), and there is no shear surface when it is destroyed. However, the lateral bulge of the reinforced specimens is relatively limited. It can be observed that the reinforced specimens manifest obvious “restraint marks,” limiting the lateral movement of the nearby calcareous sand and thus reducing the deformation. As can be seen, the effect of restraint is more obvious when the layer of geogrid increases. Goodarzi and Shahnazari [26] and Rezvani [29] investigated the mechanical properties of geotextile-reinforced calcareous sand and found that the strength could also be significantly increased compared to the unreinforced specimens.Figure 4
Failure modes of specimens at 200 kPa confining pressure: (a) nonreinforced, (b) reinforced by 1 geogrid layer, (c) reinforced by 3 geogrid layers, and (d) reinforced by 5 geogrid layers.
(a)(b)(c)(d)The above results also suggest that increasing the layers of the geogrid reinforcement may be more likely to reduce volumetric expansion tendency leading to significant strength enhancement.


### 3.2. Deviatoric Stress-Strain Relationship

The strength and deformation characteristics of calcareous sand can be reflected by the stress-strain relationship. The relationships between deviatoric stress and axial strain for reinforced and unreinforced specimens, as well as reinforced specimens with different times of biotreatment, were experimentally compiled, as shown in Figure5. The deviatoric stress-strain curves of medium-dense calcareous sand (Dr=0.5) show an obvious intensify under the confining pressure at 100 kPa~400 kPa compared the Figures 5(a) and 5(b). The peak stress increased 8.2% compared to the unreinforced sample when the strains corresponded to the peak strengths. Meanwhile, Figures 5(b)–5(e) show the deviatoric stress-strain curves that the sample reinforced by 1 layer of geogrid which treated by different times of biotreatment. With the biomodified geogrid using the MICP process, the deviatoric stress-strain relationships of the calcareous sand specimens in medium-dense states gradually harden, and the strengths are also significantly improved (Figures 5(b)–5(e)). The strength of the geogrid-reinforced calcareous sand was improved with biomodified geogrid, i.e., the strength of the sample with one time biotreatment (CCC=0.81%w/w) increased 26.4%, 18.0%, 12.5%, and 10.8% when the confining pressure increased from 100 kPa to 400 kPa, respectively. With the two times of biotreatment, the peak intensity of the sample slightly increases. However, the peak intensity decreases when the geogrid was treated 3 times by MICP. More detailed data can be found in Figure 6. It can be seen that the peak intensity increasing rate shows a decreasing trend with the increase of confining pressure. In other words, the biomodified geogrid and its reinforcement effect work relatively well at low confining pressure. This phenomenon can be explained through Figure 7. Figure 7(a) is the SEM of the geogrid without biotreatment, and the surface of untreated geogrid is relatively smooth compared to the geogrid treated by MICP (Figures 7(b)–7(d)). Figures 7(b)–7(d) demonstrate that the geogrids were treated by the MICP at different times, and many CaCO3 crystals were precipitated on the surface of geogrid. The interface of the biomodified geogrid and calcareous sand becomes rough compared to the sample without biotreatment; thus, the strength of the triaxial test will significantly increase. A similar observation was observed by Zhang et al. [31]. When the geogrid was treated twice (CCC=1.49%w/w), the interface produced more CaCO3 grain rough in Figure 7(c), and the interface friction was further enhanced. However, the intensity of the sample was decreased when the geogrid was treated 3 times (CCC=2.13%w/w). It can be accounted for the excess CaCO3 coarse grain affect the interface interaction, which makes the contact interface between geogrid and calcareous sand particles changed into that between CaCO3 particles and calcareous sand. So, the peak intensity of the sample that was treated 3 times decreased.Figure 5
Deviatoric stress-strain curves in T1 and T2: (a) 0 time of biotreatment without reinforced layer, (b) 0 time of biotreatment with 1 reinforced layer, (c) 1 time of biotreatment with 1 reinforced layer, (d) 2 times of biotreatment with 1 reinforced layer, and (e) 3 times of biotreatment with 1 reinforced layer.
(a)(b)(c)(d)(e)Figure 6
The relationship between the peak intensity increasing rate and times of biotreatment in T1 and T2.Figure 7
SEM of geogrid treated by MICP process.
(a)
0 times(b)
1 times(c)
2 times(d)
3 timesThese results reveal that using biomodified geogrids to reinforce calcareous sands is an effective way to increase the strength and improve the deformation behavior of GRCS.Figure8 shows the stress-strain relationship of the samples reinforced by 3 layers of geogrids which are treated with MICP. Compared with Figure 8(a), the peak intensity of the samples apparently increases with the introduction of biomodified geogrids in Figures 8(b)–8(d). Specifically, the peak intensity increased from 1958 kPa to 2158 kPa, about 10.2% when the confining pressure is 300 kPa as shown in Figure 8(b). And the increase rates are 24.3% and 17.4% when the geogrids were treated 2 times and 3 times in Figures 8(c) and 8(d), respectively. It can be concluded that using biomodified geogrids can enhance the strength of the geogrid reinforced calcareous sand to a certain extent due to the strengthened interface friction by increased roughness. However, the enhancement of the interaction could be diminished when the precipitation is used more than a certain amount. On the other hand, the strength of three layers of geogrids reinforced calcareous sand is generally higher than that of one layer of geogrid reinforced calcareous sand.Figure 8
Deviatoric stress-strain curves in T3: (a) 0 time of biotreatment with 3 reinforced layers, (b) 1 time of biotreatment with 3 reinforced layers, (c) 2 times of biotreatment with 3 reinforced layers, and (d) 3 times of biotreatment with 3 reinforced layer.
(a)(b)(c)(d)Figure9 shows the stress-strain curves with different times of biotreatment when the calcareous sand was reinforced by 5 layer geogrids. In general, the peak intensity is more than that reinforced by 1 layer geogrid and 3 layer geogrids. It says that the increase in the number of layers of the geogrid will increase the strength of the sample. Meanwhile, taking the σ3=400kPa for example, the peak stress increased from 2592 kPa, to 2908 kPa, 2991 kPa, and 2840 kPa about 12.2%, 15.4%, and 9.6% in Figure 9, respectively. When the geogrids were treated once or twice, the intensity will continue to enhance as the number of treatments increases. However, the effect of the intensity increase will weaken as the geogrids are treated three times.Figure 9
Deviatoric stress-strain curves in T4: (a) 0 time of biotreatment, (b) 1 time of biotreatment, (c) 2 times of biotreatment, and (d) 3 times of biotreatment.
(a)(b)(c)(d)


### 3.3. Strength Indexes

In order to evaluate the strength enhancement effect of the BGRCS, the strength of the T1 specimens is used as a benchmark to calculate the strength enhancement rate of the remaining specimens,Rs. The Rs is defined as follows.
(1)Rs=σif−σ0fσ0f×%,where the σ0f is the shear strength of the T1 specimen under a specific confining pressure, and the σif is the shear strength of a biomodified geogrid and reinforced specimen under the corresponding confining pressure. The strength enhancement rates of all BGRCS are presented in Figure 6. The Rs tends to be lower with the increasing confining pressure, implying that the reinforcing effect of the geogrids is more obvious at lower confining pressure. For the four groups of reinforced specimens with different times of biotreatment, the Rs also increases with the increasing number of treatments when the treatments are less than twice. After that, the Rs trends to be a lower state. The reason can be the same as the above sections.The cohesion of calcareous sand is generally considered to be zero. Natural sandy soils also manifest a certain pseudocohesive force [42–44] related to the intergranular occlusion and suction in the case of unsaturated sands [45, 46]. When geogrids are used to reinforce calcareous sands, the mesh of the geogrids generates a restraining effect, which results in a significant pseudocohesion. In addition, biomodification can further improve the intensity of the reinforced calcareous sands. As shown in Figure 10, the pseudocohesions are calculated by compiling the experimental data for different times of biotreatment and the layer of the geogrids. The results show that the pseudocohesion is significantly affected by the layer of the geogrids, and it increases with increasing the layer of the geogrid. At the same time, the pseudocohesion is significantly affected by the times of biotreatment, and the pseudocohesion gradually increases when the biotreatment times are less than 2, while the pseudocohesion would decrease when the geogrids were treated 3 times.Figure 10
The pseudocohesive force with different times of biotreatment.Figure11 displays the variation of internal friction angle with the layer of biomodified geogrids. It can be seen from Figure 11 that the change range is wider when the geogrid is more than 1 layer. The results show that the internal friction angle will firstly increase and then decrease with increasing times that the geogrid was treated. The internal friction angle follows a common similar pattern with the pseudocohesion. The results discussed above indicated that the coarse particles reinforced by the biomodified geogrids would form a larger pseudocohesion and a little change for internal friction angle.Figure 11
Variation of internal friction angle with the layer of biomodified geogrid.


## 3.1. Failure Patterns

During the test, the loading failure process of each sample was observed, and the typical failure characteristics of some samples were recorded. The test results indicate that the expansion of calcareous sand is quite obvious in the triaxial tests, and most of the samples are bulging during the damage process without any obvious shear surface. Generally, the shearing expansion of calcareous sands is pronounced [19, 39], especially at low confining pressures. Figure 4 compares the failure modes of the unreinforced and the reinforced calcareous sand samples under 200 kPa confining pressure. The lateral bulge of the sample without reinforcement is obvious (Figure 4(a)), and there is no shear surface when it is destroyed. However, the lateral bulge of the reinforced specimens is relatively limited. It can be observed that the reinforced specimens manifest obvious “restraint marks,” limiting the lateral movement of the nearby calcareous sand and thus reducing the deformation. As can be seen, the effect of restraint is more obvious when the layer of geogrid increases. Goodarzi and Shahnazari [26] and Rezvani [29] investigated the mechanical properties of geotextile-reinforced calcareous sand and found that the strength could also be significantly increased compared to the unreinforced specimens.Figure 4
Failure modes of specimens at 200 kPa confining pressure: (a) nonreinforced, (b) reinforced by 1 geogrid layer, (c) reinforced by 3 geogrid layers, and (d) reinforced by 5 geogrid layers.
(a)(b)(c)(d)The above results also suggest that increasing the layers of the geogrid reinforcement may be more likely to reduce volumetric expansion tendency leading to significant strength enhancement.


## 3.2. Deviatoric Stress-Strain Relationship

The strength and deformation characteristics of calcareous sand can be reflected by the stress-strain relationship. The relationships between deviatoric stress and axial strain for reinforced and unreinforced specimens, as well as reinforced specimens with different times of biotreatment, were experimentally compiled, as shown in Figure5. The deviatoric stress-strain curves of medium-dense calcareous sand (Dr=0.5) show an obvious intensify under the confining pressure at 100 kPa~400 kPa compared the Figures 5(a) and 5(b). The peak stress increased 8.2% compared to the unreinforced sample when the strains corresponded to the peak strengths. Meanwhile, Figures 5(b)–5(e) show the deviatoric stress-strain curves that the sample reinforced by 1 layer of geogrid which treated by different times of biotreatment. With the biomodified geogrid using the MICP process, the deviatoric stress-strain relationships of the calcareous sand specimens in medium-dense states gradually harden, and the strengths are also significantly improved (Figures 5(b)–5(e)). The strength of the geogrid-reinforced calcareous sand was improved with biomodified geogrid, i.e., the strength of the sample with one time biotreatment (CCC=0.81%w/w) increased 26.4%, 18.0%, 12.5%, and 10.8% when the confining pressure increased from 100 kPa to 400 kPa, respectively. With the two times of biotreatment, the peak intensity of the sample slightly increases. However, the peak intensity decreases when the geogrid was treated 3 times by MICP. More detailed data can be found in Figure 6. It can be seen that the peak intensity increasing rate shows a decreasing trend with the increase of confining pressure. In other words, the biomodified geogrid and its reinforcement effect work relatively well at low confining pressure. This phenomenon can be explained through Figure 7. Figure 7(a) is the SEM of the geogrid without biotreatment, and the surface of untreated geogrid is relatively smooth compared to the geogrid treated by MICP (Figures 7(b)–7(d)). Figures 7(b)–7(d) demonstrate that the geogrids were treated by the MICP at different times, and many CaCO3 crystals were precipitated on the surface of geogrid. The interface of the biomodified geogrid and calcareous sand becomes rough compared to the sample without biotreatment; thus, the strength of the triaxial test will significantly increase. A similar observation was observed by Zhang et al. [31]. When the geogrid was treated twice (CCC=1.49%w/w), the interface produced more CaCO3 grain rough in Figure 7(c), and the interface friction was further enhanced. However, the intensity of the sample was decreased when the geogrid was treated 3 times (CCC=2.13%w/w). It can be accounted for the excess CaCO3 coarse grain affect the interface interaction, which makes the contact interface between geogrid and calcareous sand particles changed into that between CaCO3 particles and calcareous sand. So, the peak intensity of the sample that was treated 3 times decreased.Figure 5
Deviatoric stress-strain curves in T1 and T2: (a) 0 time of biotreatment without reinforced layer, (b) 0 time of biotreatment with 1 reinforced layer, (c) 1 time of biotreatment with 1 reinforced layer, (d) 2 times of biotreatment with 1 reinforced layer, and (e) 3 times of biotreatment with 1 reinforced layer.
(a)(b)(c)(d)(e)Figure 6
The relationship between the peak intensity increasing rate and times of biotreatment in T1 and T2.Figure 7
SEM of geogrid treated by MICP process.
(a)
0 times(b)
1 times(c)
2 times(d)
3 timesThese results reveal that using biomodified geogrids to reinforce calcareous sands is an effective way to increase the strength and improve the deformation behavior of GRCS.Figure8 shows the stress-strain relationship of the samples reinforced by 3 layers of geogrids which are treated with MICP. Compared with Figure 8(a), the peak intensity of the samples apparently increases with the introduction of biomodified geogrids in Figures 8(b)–8(d). Specifically, the peak intensity increased from 1958 kPa to 2158 kPa, about 10.2% when the confining pressure is 300 kPa as shown in Figure 8(b). And the increase rates are 24.3% and 17.4% when the geogrids were treated 2 times and 3 times in Figures 8(c) and 8(d), respectively. It can be concluded that using biomodified geogrids can enhance the strength of the geogrid reinforced calcareous sand to a certain extent due to the strengthened interface friction by increased roughness. However, the enhancement of the interaction could be diminished when the precipitation is used more than a certain amount. On the other hand, the strength of three layers of geogrids reinforced calcareous sand is generally higher than that of one layer of geogrid reinforced calcareous sand.Figure 8
Deviatoric stress-strain curves in T3: (a) 0 time of biotreatment with 3 reinforced layers, (b) 1 time of biotreatment with 3 reinforced layers, (c) 2 times of biotreatment with 3 reinforced layers, and (d) 3 times of biotreatment with 3 reinforced layer.
(a)(b)(c)(d)Figure9 shows the stress-strain curves with different times of biotreatment when the calcareous sand was reinforced by 5 layer geogrids. In general, the peak intensity is more than that reinforced by 1 layer geogrid and 3 layer geogrids. It says that the increase in the number of layers of the geogrid will increase the strength of the sample. Meanwhile, taking the σ3=400kPa for example, the peak stress increased from 2592 kPa, to 2908 kPa, 2991 kPa, and 2840 kPa about 12.2%, 15.4%, and 9.6% in Figure 9, respectively. When the geogrids were treated once or twice, the intensity will continue to enhance as the number of treatments increases. However, the effect of the intensity increase will weaken as the geogrids are treated three times.Figure 9
Deviatoric stress-strain curves in T4: (a) 0 time of biotreatment, (b) 1 time of biotreatment, (c) 2 times of biotreatment, and (d) 3 times of biotreatment.
(a)(b)(c)(d)


## 3.3. Strength Indexes

In order to evaluate the strength enhancement effect of the BGRCS, the strength of the T1 specimens is used as a benchmark to calculate the strength enhancement rate of the remaining specimens,Rs. The Rs is defined as follows.
(1)Rs=σif−σ0fσ0f×%,where the σ0f is the shear strength of the T1 specimen under a specific confining pressure, and the σif is the shear strength of a biomodified geogrid and reinforced specimen under the corresponding confining pressure. The strength enhancement rates of all BGRCS are presented in Figure 6. The Rs tends to be lower with the increasing confining pressure, implying that the reinforcing effect of the geogrids is more obvious at lower confining pressure. For the four groups of reinforced specimens with different times of biotreatment, the Rs also increases with the increasing number of treatments when the treatments are less than twice. After that, the Rs trends to be a lower state. The reason can be the same as the above sections.The cohesion of calcareous sand is generally considered to be zero. Natural sandy soils also manifest a certain pseudocohesive force [42–44] related to the intergranular occlusion and suction in the case of unsaturated sands [45, 46]. When geogrids are used to reinforce calcareous sands, the mesh of the geogrids generates a restraining effect, which results in a significant pseudocohesion. In addition, biomodification can further improve the intensity of the reinforced calcareous sands. As shown in Figure 10, the pseudocohesions are calculated by compiling the experimental data for different times of biotreatment and the layer of the geogrids. The results show that the pseudocohesion is significantly affected by the layer of the geogrids, and it increases with increasing the layer of the geogrid. At the same time, the pseudocohesion is significantly affected by the times of biotreatment, and the pseudocohesion gradually increases when the biotreatment times are less than 2, while the pseudocohesion would decrease when the geogrids were treated 3 times.Figure 10
The pseudocohesive force with different times of biotreatment.Figure11 displays the variation of internal friction angle with the layer of biomodified geogrids. It can be seen from Figure 11 that the change range is wider when the geogrid is more than 1 layer. The results show that the internal friction angle will firstly increase and then decrease with increasing times that the geogrid was treated. The internal friction angle follows a common similar pattern with the pseudocohesion. The results discussed above indicated that the coarse particles reinforced by the biomodified geogrids would form a larger pseudocohesion and a little change for internal friction angle.Figure 11
Variation of internal friction angle with the layer of biomodified geogrid.


## 4. Conclusions

This study focuses on the strength and deformation properties of the biomodified geogrid and reinforced calcareous sand (BGRCS). Through a series of unconsolidated undrained triaxial tests and scanning electron microscope tests, the effects of the layer of the geogrid, times of biotreatment, and confining pressure on the stress-strain response, strength indexes, and failure patterns of biomodified and reinforced calcareous sand were investigated. Combining the presented experimental results and microscopic interface analysis, the mechanical property of BGRCS has been discussed. The following conclusions can be obtained:(1)
From the failure patterns of the reinforced samples, increasing the layers of the geogrid reinforcement can reduce volumetric expansion tendency, consequently leading to significant strength enhancement(2)
In comparison with the unreinforced calcareous sand specimens, the strength of the reinforced specimens can be significantly enhanced by about 33.0%, 86.4%, and 135.5% for 1 layer geogrid, 3 layer geogrids, and 5 layer geogrids in this test. The overall strength is improved with increasing the layer of the geogrids(3)
The strength of the biomodified and reinforced specimens can be significantly improved by about 24.9%, 34.7%, and 6.5% after 1 time, 2 times, and 3 times of biotreatment. As such, reasonable use of biomodified geogrids can improve the strength of geogrid reinforced calcareous sand by increasing the roughness of geogrids so as to the interface friction properties, but the excessive coating resulted from additional biotreatment could reduce the strength of the geogrids reinforced calcareous sand(4)
The strength enhancement rate in geogrid reinforced specimens is relatively large at low confining pressure and tends to decrease with the increasing confining pressure. The pseudocohesion also improved by the MICP process, reaching a maximum value when the samples were treated twice, then slightly decreasing with additional biotreatment



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*Source: 3768967-2022-04-05.xml*