Experimental Study on the Shear Properties of Soil around Piles with Permeation Grouting

Abstract

While post-grouting is frequently reported to improve the engineering performance of piles, the shear strength of the soil around piles is not well understood. To investigate the strengthening mechanism of soil around piles with permeation grouting, laboratory tests were carried out on homemade cement soil from the aspects of shear strength and microstructure characteristics. The evolution rule of the shear stress–shear displacement curve of grouting soil with different cement contents was analyzed, and the influence of grouting parameters on shear strength was explained. A composite exponential model describing the shear stress and shear displacement of permeation grouting silty clay was established. Furthermore, the influence of different cement content on changes in the microstructural characteristic parameters of the soil around piles was studied. The results show that penetration grouting has a positive effect on improving the shear strength of the soil around piles, and the failure mode of silty clay changed from elastic–plastic failure to brittle failure after grouting. Permeation grouting makes the particle structure denser, which limits the changes in pore arrangement and distribution. The shear failure of the soil around piles under permeation grouting obeys the Mohr–Coulomb criterion of failure. It is recommended to increase the grout diffusion radius during construction, considering the reinforcement effect of permeation grouting on the soil around piles.

1. Introduction

As the scale of infrastructure projects expands in a deeper and larger direction, the number of pile foundations built is rapidly increasing. The bored pile foundation has been applied to engineering widely due to its good performance. However, the soil around bored piles is severely disturbed, and the original structural form is destroyed during drilling, resulting in a significant decrease in soil strength. Post-grouting technology provides a reasonable way to improve pile-bearing capacity as a compensation method [1]. Post-grouting technology is a combination of soil reinforcement technology and pile foundation engineering technology, which is carried out within a certain time after the completion of the bored piles. It involves three types of grouting at the pile tip, pile side, or pile tip and side simultaneously.

In 1958, the post-grouting technology was used for the first time on the Maracaibo Bridge in Venezuela [2]. Researchers have conducted extensive studies and investigations over the last several decades, and post-grouting technology has been consistently improved and developed [3,4,5]. Post-grouting technology is still a hot issue in geotechnical engineering due to the uniqueness of diverse geological features and the unpredictability of the grouting process [6,7]. The post-grouting of bored piles significantly increases the pile-bearing capacity and load transfer characteristics compared with non-grouted piles [8], while the quantity of injected cement slurry and the mechanical qualities of the soil around piles also affects the piles’ bearing capabilities and load transmission mechanisms [9]. Based on a full-scale field test, Zhang et al. [10] pointed out that combining pile-side and pile-tip grouting may significantly enhance the mechanical properties of the soil around piles. For piles with a length of more than 15 m and high requirements for increasing bearing capacity, combined grouting at the pile tip and pile side should be adopted. In combined post-grouting, pile-side grouting first can effectively increase the pressure during pile-tip grouting, which is helpful to form an enlarged solid at the pile tip [11]. Many engineering practices have identified post-grouting as an exceptionally successful approach for improving the engineering performance of bored piles [12,13,14,15]. This can increase the loading capacity of the pile tip, improve the frictional resistance of the pile side, and reduce uneven settlement.

While post-grouting technology is widely used, researchers have also studied the shear resistance of post-grouting piles through a variety of methods. For example, Wang et al. [16] researched the shear performance of the interface between post-grouting cohesive soil and concrete piles with different roughnesses. The interface of the cement soil–pile is different from that of the soil–pile, and the cement soil produces a cohesive force at the interface after grouting. For the former, some researchers have studied the shear properties of the soil–concrete interface, cement grout, loess of cement-treated, cemented soil–concrete interface, and clay–cement soil interface [17,18,19,20,21]. Furthermore, the influence of the unloading effect on the shear strength of the post-grouting sand–concrete interface was also considered [22,23]. Chen et al. [24] investigated the influence of roughness on the shear performance of the soil–concrete interface and found that there was a good correlation between the tangential deformation at failure and roughness. The location of shear failure surfaces for soil and concrete can be classified into three types based on the differential between normal pressure and the roughness of the contact surface. It contains the concrete surface, the interface between the soil and concrete, and the internal shear failure of the soil. The so-called internal shear failure of soil refers to the failure caused by the insufficient resistance of the soil around the pile to deformation in the actual project. As indicated in Figure 1, the soil around post-grouting piles is classified into two types [25]. The first type is the soil around the compaction grouting piles, which is close to the grouting port and closely connected with the pile interface. The second type is the soil around permeation grouting piles far away from the grouting port. However, the two types are assumed to be a continuous whole in current engineering practice, which ignores the soil around permeation grouting piles beyond the pile–soil interface.

As the slurry wall protection technology is adopted for bored piles, the mud cake existing on the pile–soil interface also influences the loading performance of the piles [26]. For the thin layer of weak mud cakes formed between the pile and the bore wall during pile formation, post-grouting can greatly enhance the thin layer’s shear resistance, hence improving the pile’s bearing capacity [27]. Maalej et al. [28] conducted a triaxial undrained shear test on mud cakes surrounding a pile and found that the water content, void ratio, and compressibility of the mud cakes after grouting were all significantly reduced, and the shear strength was dramatically enhanced. However, there is little research on the shear behavior of the soil around the pile strengthened by permeation grouting outside the pile–soil interface, and the strengthening mechanism of this soil under permeation grouting is not well understood. Most of the present studies about the shear properties of post-grouting cast-in-place bored piles are focused on the soil–pile interface, which is the first type of soil around piles. The bearing capacity of piles is affected by both the pile-cemented soil interface and the strength of the cement soil around piles [29]. In order to explore the shear property of the soil around piles with permeation grouting, analyze its strengthening mechanism, and provide guidance for specific engineering construction, this paper intends to conduct research through indoor tests.

In this article, the soil around permeation grouting piles was simulated by homemade cement-soil samples with different cement content to carry out the shear strength test, aiming to study the shear property of the soil around piles from the following investigations. Direct shear tests were carried out, including the evolution rule of the shear stress–shear displacement curve of the soil around the pile under different cement content and the influence of different cement content on the shear strength parameters. A composite exponential model describing the shear stress and shear displacement of permeation grouting silty clay was established. A microstructure test was carried out to investigate the variation rules of the structural characteristic parameters of the soil around piles with permeation grouting, and the strength formation process with the increase of cement content was analyzed. The macro mechanical behavior and microstructural characteristics of grouting soil were investigated to reveal the strengthening mechanism of soil around piles by permeation grouting. The results are of great significance to the optimization of grouting parameters and the evaluation of shear properties of soil around piles.

2. Materials and Methods

2.1. Materials

The soil was taken from a cast-in-place bored pile construction scene of an expressway project, and in situ soil samples were collected with a thin-wall sampler at a specific depth and position. According to ASTM [30], the soil samples were named muddy clay and silty clay, respectively. The physical and mechanical properties of soil samples are analyzed according to the methods of Deb et al. [31,32], their physical and mechanical parameters are listed in

Table 1. Physical and mechanical parameters of soil.

Soil Sample	Water Content (%)	Dry Density (g/cm3)	Liquid Limit (%)	Plastic Limit (%)	Plasticity Index Ip
Muddy clay	40.7	1.28	33.9	18.6	15.3
Silty clay	19.6	1.71	30.4	17.5	12.9

. After the undisturbed soil was dried naturally, it was rolled and dispersed, screened through a 2 mm geotechnical sieve, and the test soil samples were prepared.

The cement slurry is made up of cement and water. The 42.5R ordinary Portland cement used was produced at the Nanjing Zhonglian Cement Plant, China. The specific gravity of cement is 3.12 g/cm³, and the specific surface area is 256.0 m²/kg. The basic physical parameters of cement are shown in

Table 2. Basic parameters of 42.5R ordinary Portland cement.

Fineness (80 μm,%)	Standard Consistency Water Consumption	Initial Setting Time	Final Setting Time	Flexural Strength (MPa)		Compressive Strength (MPa)
	(%)	(min)	(min)	3 Days	28 Days	3 Days	28 Days
1.18	25.50	160	230	5.23	7.55	28.75	45.33

, and the content of the main compounds is listed in

Table 3. Contents of main compounds in cement.

Component	SiO2	K2O	SO3	CaO	Al2O3	Fe2O3	MgO	Na2O
Content (%)	20.33	0.42	2.15	65.50	4.83	4.90	1.30	0.10

.

2.2. Sample Preparation

The cement slurry forms into a large slurry solidified body around the piles after grouting, which has the same strength as cement soil. To ensure the comparability of the test results, grouting cement soil can be used to replace the soil around piles with permeation grouting [33]. Usually, the value range of the water–cement ratio of unsaturated soil should be 0.7–0.9. Considering the fluidity and setting time of the cement slurry, the water–cement ratio in this test was taken as 0.7, and the test water was tap water [22].

First, the cement and water were mixed evenly according to a predetermined proportion to prepare the cement slurry. Then, the soil sample was prepared according to the percentage of the weight of the mixed soil sample. The amount of cement = the weight of the mixed cement/the weight of the mixed soil. Considering the complexity of the soil layer and the influence of water content, the concentration of the grout that penetrates from the grouting hole to the soil around the pile may be diluted, so the range of cement content is selected from 0% to 15%. As a comparison, 0% indicates that the soil around the piles is not penetrated by grouting. To ensure the uniformity of the soil moisture content, the mixed soil sample was placed into a moisturizer, consolidated and moisturized for 48 h, and then cut into a soil sample. In each type of soil sample, eight samples were tested for each cement percentage, including testing at four different normal pressures.

2.3. Experimental Method

2.3.1. Direct Shear Test

To measure the shear strength of the soil samples, the shear apparatus uses a strain-controlled direct shear instrument. The normal stress used in the test is 50 kPa, 100 kPa, 200 kPa, and 400 kPa, and the horizontal load is 1.2 kN. The leverage ratio is 1:12, and the area of the specimen inside the shear box is 30 cm².

According to the soil layer category, the shear tests of permeation grouting muddy clay and permeation grouting silty clay were carried out. In theory, at least two samples of each cement content should be prepared in case the cement slurry in one of the samples is not sufficiently mixed with the soil, resulting in a large deviation in the experimental results. A total of 64 sets of experiments were carried out according to different soil types and cement content, as shown in

Table 4. Samples of different soils and cement content.

Soil Category	Group	Cement Content				Normal Stress
		0%	5%	10%	15%
Muddy clay	M1	M1-0	M1-5	M1-10	M1-15	50 kPa; 100 kPa; 200 kPa; 400 kPa
	M2	M2-0	M2-5	M2-10	M2-15
Silty clay	S1	S1-0	S1-5	S1-10	S1-15
	S2	S2-0	S2-5	S2-10	S2-15

.

After the test equipment was debugged, the prepared samples were subjected to shear testing as follows:

• After cleaning the shear box, the porous stone and filter paper were placed, the upper and lower boxes were aligned, and pins were inserted to connect them.
• Then, the edge of the ring knife was made with the sample upward, and the sample was carefully pushed into the shearing box. Filter paper, porous stone, and pressurized cover plate were placed on the sample in sequence.
• The transmission device is moved to make the steel ball on the upper box contact with the force ring, and the left tip of the lower box is just in contact with the transmission rod. The displacement measuring device is installed, and the dial indicator is adjusted to the zero position.
• Adjust the pressure frame to make it fully in contact with the cover plate, and then apply normal stress to the cover plate in sequence.
• The speed during shear is set to 0.08 mm/min; every time the specimen has a strain of 0.3–0.5 mm, the dynamometer and displacement measurements are recorded until the sample is sheared.

Following the shear test, the sample is removed and used for the microstructure test. The preceding steps were repeated to shear the other samples after cleaning the shear box. Figure 2 depicts the testing procedure.

2.3.2. Microstructure Test

The microscopic parameters of the soil can be obtained by image acquisition equipment. The image acquisition equipment adopts the soil microstructure test system independently developed by Hohai University. This system is composed of a long-distance microscope, three-dimensional control platform, charge couple device (CCD) camera, coaxial cold light source, built-in image acquisition card, and computer, as shown in Figure 3. The long-distance microscope is connected to a CCD camera, and the fine structure image captured by the CCD camera is transmitted to the computer. The three-dimensional control platform can precisely adjust the test position of the sample with an accuracy of 0.01 mm. The digital image acquisition card is the Matrox Meteor II/Standard type, and the image acquisition rate is greater than 25 frames/second. The CCD camera is a Uniq UM-201 type with an asynchronous acquisition function, and the signal-to-noise ratio is 56 dB.

Specimens after direct shear failure were used for microstructure testing. With the help of finger force, the sheared sample is broken apart to expose the fresh surface, representative small pieces are selected, and the size of the sample is adjusted and controlled for microstructure testing. The specimen was placed on the three-dimensional control platform in front of the camera, and the focal length was roughly adjusted to make the image more clearly displayed on the computer screen. Then, the focal length is finely adjusted to achieve the best display effect of the image accuracy. Different magnifications do not have obvious effects on the soil sample structural parameters. The image data collected by the test system were saved to the computer, and the mesoscopic images were binarized by the image segmentation method. The binarized image is used to analyze the change of soil structure characteristic parameters. In the binarized image, white represents soil particles, and black represents pores, as shown in Figure 3.

2.3.3. Microstructure Characteristic Parameters

In this paper, the size, shape, arrangement, and distribution of soil particles and pores are taken for quantitative analysis of the microstructure characteristics of soil around piles.

• Roundness. Soil particle roundness is defined as the ratio of the area of the soil particle to the area of the circle defined by its perimeter. The morphological size of the particles is represented by the roundness value, which characterizes the degree to which the particles are close to circular, as shown in Equation (1).

$$R_0 = \frac{4\pi S}{L^2}$$

(1)

where R₀ is the roundness value, S is the area of the soil particle, and L is the perimeter of the soil particle. Then, the area of the circle determined by the perimeter is $L^2/4\pi$.

2.

Orientation. The degree of orientation is described by probability entropy. The greater the probability entropy, the more disordered the orientation distribution, as shown in Equation (2).

$$H=-{\displaystyle \sum _{i=1}^n{p}_i(\alpha )}{\log }_n{p}_i(\alpha )$$

(2)

where $p_i(\alpha )$ is the orientation probability that the ith $\alpha$ is a particle or pore within 0°~180°, and $\alpha$ represents the angle of each position after it is divided into n equal positions within 0°~180°, $\alpha ={180}^{\circ }/n$.

3.

Particle distribution fractal dimension. Here, the more common sandbox method is used to calculate the distribution fractal dimension of the particles. The fractal dimension refers to the slope of the linear relationship between $\ln \alpha$ and −H and is represented by D_s. The pore distribution fractal dimension (D_h) characterizes the degree of dispersion of pores, and it can still be calculated by Equation (3).

$$D_s=({\displaystyle \sum _{i=1}^n{p}_i(\alpha )}{\log }_n{p}_i(\alpha ))/\ln \alpha$$

(3)

4.

Porosity. The concept of porosity is used to describe the relative area of pores and particles, that is, the ratio of the pore area to the total area in the microstructure image, as shown in Equation (4).

$$\mathsf{\Phi }=\frac{S_h}{S_h+S_s}$$

(4)

where $\mathsf{\Phi }$ is the porosity, $S_h$ is the pore area in the image, and $S_s$ is the soil particle area in the image. Since the gray value of pores in the binarized image is 0 and that of particles is 255, the area occupied by particles can also be obtained.

3. Results and Discussion

3.1. Analysis of Direct Shear Test Results

3.1.1. Evolution Rule of Shear Stress

The variation curve of the shear stress–shear displacement of permeation grouting muddy clay under different normal stresses is shown in Figure 4.

Figure 4 shows that the shear stress of the soil around piles after grouting is significantly higher than that without grouting. As the shear displacement increases, the shear stress of muddy clay without grouting first increases approximately linearly, then slows down, and finally tends to a stable value. In addition, the soil around the pile produces an obvious peak shear stress after grouting, and its value is slightly greater than the residual stress. For the permeation grouting muddy clay, the variation rules of shear stress–shear displacement under different cement content are consistent and can be divided into a rapid growth stage (0 < δ < 1 mm), peak stress stage (1 < δ < 3 mm), and stabilization stage (3 < δ < 7 mm). The cement content has a positive effect on increasing the shear stress of the soil around piles with permeation grouting. The peak shear stress of the soil around piles rises significantly with the cement content, and the shear displacement required to reach the peak strength gradually decreases. This shows that the soil around piles with high cement content is developing toward brittle failure. The shear strength of muddy clay and the mechanical properties of the soil around piles are improved by the permeated cement slurry. The shear stress–shear displacement curves of the soil around piles have obvious peak shear stress differences between 0% to 5% and 5% to 10%. This is because, as the cement content increases, more hydration products fill the pores in the soil, and the greater cementation strength enhances the bonding force between particles. The soil skeleton is more compact under normal stress, and the shear strength is significantly improved.

For the softening curve of muddy clay in Figure 4, the shear stress corresponding to the peak point is taken as the representative peak shear stress. Figure 5 shows the relation between the peak shear stress and grouting parameters of permeation grouting muddy clay under different cement content. Here, the peak shear stress of the soil around permeation grouting piles increases approximately linearly when the cement content is less than 5%. However, the rate of peak shear stress development slows significantly when the cement content is between 5 and 10%. The peak shear stress no longer increases basically under lower normal stress when the cement content is greater than 10% but still increases slowly under higher normal stress. This is because the greater the cement content, the more obvious the hydrolysis and hydration between it and the soil particles [34]. The generated cementing substance filled in the soil particles, which greatly improved the strength of the cement soil.

Figure 6 shows the variation curve of permeation grouting silty clay with different normal stresses. For the silty clay without grouting, its curve has a short linear rapid growth initially and then grows slowly. The shear stress basically tends to be stable when the shear displacement approaches the maximum shear strain. It is shown that the silty clay has elastic deformation at the beginning and then gradually changes to plastic deformation. Finally, the sample experiences shear failure, the shear strength reaches the maximum, and the shear stress tends to be stable and no longer increases. Under the same normal stress, the slope of the initial shear curve gradually increases, and the increase in cement content obviously improves the shear strength of the soil around the piles.

The variation trend of permeation grouting silty clay is generally consistent when the cement content is less than 15%. Each curve reaches or approaches its stable state before the maximum shear displacement, and the shear stress does not increase at this time. However, the shear stress–shear displacement curve increases approximately linearly when the cement content is 15%, and then the growth rate decreases gradually. The shear stress decreases after reaching the peak and finally remains at the stress value after specimen failure. This result indicates that the cement content of the slurry can change the failure mode of silty clay, and the original elastic–plastic failure will gradually change to brittle failure after grouting.

3.1.2. Effect of Grouting Parameters on Shear Strength

The fitting curve of the shear strength of the soil around the permeation grouting piles under different cement content is shown in Figure 7.

The shear strength of the soil around the permeation grouting piles increases with the normal stress and cement content and has positive linear correlations with the normal stress. This demonstrates that the shear failure of the soil around permeation grouting piles also obeys the Mohr–Coulomb criterion of failure, as shown in Equation (5).

$${\tau }_{\mathrm{f}}=c+\sigma \tan \phi$$

(5)

where c is the cohesion of the soil around the piles, and φ is the friction angle.

For muddy clay, the shear strength is significantly improved after adding cement slurry, and the maximum increase is at 5% cement content. However, the shear strength of silty clay is slightly less sensitive to cement. With the increase of cement content, the shear strength increases slightly, and the shear strength curves of different cement content are approximately parallel. The shear strength parameters of the soil around permeation grouting piles under different cement content are obtained by fitting curves (Figure 7), as shown in

Table 5. Shear strength parameters of soil around piles with permeation grouting.

Cement Content	Muddy Clay			Silty Clay
	φ (°)	c (kPa)	R2	φ (°)	c (kPa)	R2
0%	5.0	9.7	0.996	14.3	16.9	0.997
5%	9.8	42.6	0.964	15.4	31.5	0.998
10%	13.4	57.3	0.996	17.0	40.1	0.999
15%	16.3	60.9	0.997	18.5	46.1	0.999

, in which c and φ represent cohesion and friction angle, respectively.

The shear strength parameters of the soil around piles under permeation grouting are significantly higher than those of the non-grouted soil. Taking the muddy clay with 15% cement content as an example, its cohesion increased by 527.8%, and the friction angle increased by 226%. The cohesion of grouted muddy clay is greater than that of grouted silty clay at the same cement content. Concretely, the cohesion of the soil around the permeation grouting piles increases from 9.7 kPa to 60.91 kPa when the cement content is from 0 to 15%, and the friction angle increases from 5.0° to 16.3°. Permeation grouting significantly increases the cohesion of the soil around piles and improves the shear strength.

3.1.3. Effect of Grouting Parameters on Shear Strength Index

An increase in shear strength is reflected in the change in c and φ. To make the variation in the shear strength index of the soil around piles more intuitive, the variation curve of the shear strength parameter with the cement content is drawn in Figure 8.

The shear strength indices of the two soils increase with the cement content from the variation characteristics of the curve, and the muddy clay is more sensitive to the cement. The silty clay’s value of friction angle fluctuates within a narrow range, but its friction angle under different grouting parameters is greater than that of muddy clay. The cohesion of silty clay increases slowly with the cement content. The cohesion of muddy clay is greater than that of silty clay after grouting, but it begins to decline when the cement content reaches more than 13%. This is because there is an optimal cement content in engineering practice. Cement soil with high content shows brittle failure, which is similar to the research results of Ruan et al. [35]. The slurry of permeation grouting contributes to the shear strength index of the soil around piles. For the soil around the pile with a low cement content ($w\le 15\%$), the fitting formula given in Figure 8 can be used to reveal the strengthening mechanism of the soil around piles with permeation grouting. Therefore, the grouting parameters can be adjusted appropriately according to different types of soft soil on site to achieve the best strengthening effect.

3.2. Composite Exponential Model of Shear Stress and Shear Displacement

From Figure 4 and Figure 6, it can be seen that the variation in the shear stress–shear displacement curves of the two soils are slightly different. The muddy clay with permeation grouting has an obvious peak shear stress and stress decline stage, while grouted silty clay only has peak shear stress at a high content of cement (15%), and the attenuation trend of shear stress is relatively slow. In order to find a corresponding relationship between the shear stress and shear displacement of the soil around piles with permeation grouting, for the curves in Figure 4 and Figure 6, four curve models were used for fitting, including compound exponential, hyperbolic, polynomial, and exponential models. It was found that the compound exponential model had the best fit. The formula is as follows:

$$\tau =a(1-e^{-b\delta })$$

(6)

According to Equation (6), when the shear displacement $\delta$ tends to infinity, the shear stress $\tau =a$. Since the shear failure of the soil around permeation grouting piles also follows the Mohr–Coulomb criterion of failure, the parameter $a$ in Equation (6) can be expressed as $a=\sigma \tan {\phi }_1+c_1$. The model parameter $b$ represents the development rate of shear stress with shear displacement, and its value is related to grouting parameters. The values of $b$ under different grouting parameters were obtained by fitting, as shown in

Table 6. Model parameter b under different grouting conditions.

Cement Content	Normal Stress (kPa)				Average
	50	100	200	400
0%	0.923	0.927	0.518	0.395	0.691
5%	0.829	0.841	0.736	0.428	0.709
10%	1.192	1.277	1.406	0.868	1.186
15%	1.903	1.679	1.588	1.214	1.596

.

From Table 6, it can be seen that the model parameter $b$ is positively correlated with the cement content and negatively correlated with the normal stress. The fluctuation range of parameter $b$ under the same cement content and different normal stresses is large and has no obvious regularity. Using its average value for analysis, it is found that the parameter $b$ and grouting parameter $w$ have a positive linear correlation (R² = 0.907), as shown in Equation (7).

$$b=0.064w+0.566$$

(7)

Parameters $a$ and $b$ are substituted into Equation (6), and the composite exponential model of shear stress and shear displacement can be obtained, as shown in Equation (8).

$$\tau =\left(\sigma \tan {\phi }_1+c_1\right)\left(1-e^{-(0.064w+0.566)\delta }\right)$$

(8)

The compound exponential model in Equation (8) was used to simulate two soil samples with different cement content. The fitted curve of muddy clay deviates slightly from the test curve, while the fitting curve of silty clay is basically the same as the test curve. Among them, the R-squared of the silty clay when the cement content is 10% is the best, as shown in Figure 9. The fitting curves under other normal stresses are in good agreement with the test curves except for the s-shaped fluctuations in the curve at 200 kPa. This result indicates that the model can be used to describe the relationship between the shear stress and shear displacement of silty clay with permeation grouting.

3.3. Analysis of Microstructure Characteristic Test Results

The macroscopic engineering characteristics of soil are influenced by the state and variation of the microstructure, and its complex mechanical behavior is a specific expression of microstructure characteristics. To quantitatively analyze the influence of different cement content on the shear strength of the soil around piles, the image information of permeation grouting soil was obtained through a soil microstructure test system on the basis of the direct shear test. The changes in the internal characteristic parameters of grouting soils with different cement content are further analyzed, to reveal the strengthening mechanism of permeation grouting to the soil around piles from the mesoscopic level.

Figure 10 shows the binarized image of the microstructure of permeation grouting muddy clay with different grouting parameters. Through the analysis of image information, the variation law of the microstructural characteristic parameters of muddy clay with permeation grouting can be obtained, as shown in Figure 11.

From Figure 11, it can be seen that the proportion of particle area in the muddy clay of permeation grouting increased when compared with the non-grouted muddy clay, and the ratio of pore area decreased. This is because the cement in the soil around piles is hydrated after grouting, and the generated cementing substance fills the pores between particles. In Figure 11a, the particle roundness of muddy clay showed an upward trend after grouting. Roundness is used to represent the shape of the particles, and the higher the value, the closer the spatial arrangement. The compressibility of particles is negatively correlated with roundness. The compressibility is reduced, and thus the ability of the soil skeleton to resist deformation increases. In addition, the fractal dimension of the particle distribution indicates the orientation degree of the particles. The higher the fractal dimension value of distribution, the worse the orientation of particles, the more chaotic and dispersed the particles, and the greater the density of soils.

Comparing Figure 11a,b, the fractal dimension value of the particle distribution tends to increase overall after grouting, while the change in pore orientation is opposite to that of the particles. After grouting, the pore orientation of the soil improves and the pore density decreases, while the fractal dimension of the pore distribution does not have a relatively clear change law. This is because grouting makes the particle structure denser, which limits the change in pore arrangement and distribution, resulting in a reduction in the change in pore orientation and fractal dimension of pore distribution. Reflected at the macroscopic level, this is manifested as an obvious stress peak on the shear stress–shear displacement curve.

Figure 12 shows the binarized image of the microstructure of permeation grouting silty clay with different grouting parameters. Through the analysis of image information, the variation law of the microstructural characteristic parameters of silty clay with permeation grouting can be obtained, as shown in Figure 13.

Figure 13 shows that the particle area ratio of silty clay decreases and the pore area ratio increases after grouting. Since the cement hydration products grow along the outer edge of the particles, the equivalent diameter of the particles increases continuously, resulting in an increase in the particle roundness. Thus, the particles are arranged more closely, and the compressibility is reduced. However, the distribution fractal dimension of the particles showed a decreasing trend as a whole. This indicates that the degree of soil particle agglomeration increases and the pore connectivity decreases. Under different cement content, the particle distribution fractal dimension of the silty clay with permeation grouting has little difference, ranging from 1.66 to 1.74. This development trend is consistent with the evolution law of the soil particle area ratio.

In Figure 13b, the degree of orientation and distribution fractal dimension of pores have similar changing trends and both increase overall. After grouting, the pore distribution fractal dimension of silty clay fluctuates within a narrow range, and the distribution and arrangement of pores become slightly chaotic. Due to the randomness of the extension direction of the cement hydration products, the long axis of the pores of the silty clay is constantly changing, and the pore orientation is constantly adjusted without obvious regularity. This is because the cohesion between silty clay particles is lower, and the added cement slurry is wrapped by silty clay particles and firmly bonded. After fully mixing, the particles change from large particles to small particles, which are dispersed in silty clay and continue to be connected with the surrounding particles. The interconnected soil particle pores improve the soil structure skeleton, hence increasing the soil’s shear strength. Reflected on the macroscopic level, the slope of the shear stress–shear displacement curve gradually increases with the cement content in the initial shear stage.

In other words, the cement slurry added to the soil underwent a hydration reaction, and the generated cementing substance extended to the pores between the particles and particle aggregates. The mutual bonding between particles improved the roundness of the particles, changing the original deposition direction of the soil particles, making the soil skeleton more stable, and increasing the strength of the soil around the piles. Compared with silty clay, the structural connection of muddy clay is slightly worse, but the engineering properties of both clays are generally enhanced after grouting. During on-site construction, the grouting parameters can be appropriately adjusted according to the actual situation.

4. Conclusions

In this paper, laboratory tests of simulated penetration grouting were carried out for soil around piles. The macro mechanical behavior and microstructural characteristics of grouting soil were investigated. The conclusions are as follows:

• The shear stress of the soil around piles was found to be obviously affected by permeation grouting. With the increase of grouting cement content, the peak shear stress of the soil around the pile increases significantly, and the corresponding shear displacement decreases gradually. For the permeation grouting muddy clay, the variation rules of shear stress–shear displacement can be divided into a rapid growth stage (0 < δ < 1 mm), peak stress stage (1 < δ < 3 mm), and stabilization stage (3 < δ < 7 mm).
• For the permeation grouting silty clay, the curve of shear stress and shear displacement shows hardening development when the cement content is less than 15%. The shear stress and shear displacement curve increase approximately linearly first when the cement content is 15%, and the shear stress decreases gradually after reaching the peak, tending to a stable finally. The cement content of the slurry could change the failure mode of silty clay, and the original elastic–plastic failure gradually changed to brittle failure after grouting.
• The shear failure of the soil around piles with permeation grouting obeys the Mohr–Coulomb criterion of failure, the shear strength of which increases with the normal stress and cement content, and has positive linear correlations with the normal stress. When the cement content increased from 0 to 15%, the cohesion of muddy clay increased from 9.7 kPa without grouting to 60.91 kPa, and the internal friction angle increased from 5.0° to 16.3°.
• A composite exponential model was established to describe the silty clay with permeation grouting. This model included the main mechanical parameters such as shear stress, normal stress, friction angle, cohesion, cement content, and shear displacement. The variation in the shear stress of permeation grouting silty clay with the shear displacement can be well simulated.
• The variations in the structural characteristic parameters of the two types of soil around piles are different, and the structural connectivity of the muddy clay is slightly worse than that of the silty clay. Permeation grouting makes the particle structure denser, which limits changes in pore arrangement and distribution. This makes the particle skeleton arranged more closely, thus improving the shear strength. During on-site construction, the grouting parameters can be adjusted appropriately according to different soft soils to achieve the best strengthening effect.

Due to the limited experimental conditions, some factors cannot be considered. This study only analyzed the change of soil sample microstructure characteristic parameters at the mesoscopic level, which can be further investigated through SEM testing in the future. In addition, more interesting and important conclusions can be obtained by improving test methods or conducting field tests.