A Study on the Load-Bearing Characteristics and Load Transfer Mechanism of Bag Grouting Pile in Soft Soil Areas

Abstract

In soft soil areas, to compare the load-bearing characteristics of bag grouting piles and cement mixing piles and study the load-bearing mechanism of bag grouting piles, field tests are conducted in this study, including the comparative compressive test of bag grouting piles and cement mixing piles, and the analysis of pile axial force, pile side friction resistance, and pile end resistance. Moreover, a numerical simulation is developed using ABAQUS 2020 (finite element analysis software) for three-dimensional modeling. The numerical simulation results are compared with the field test results to verify the reliability of the numerical simulation. Furthermore, the influences of five factors are studied; namely, pile length, pile diameter, pile spacing, the thickness of the bedding layer, and grouting pressure are studied for their effects on the compressive bearing characteristics of the bag grouting pile. The results show the following: (1) For composite foundations, bag grouting piles are more effective than cement mixing piles in soft soil areas, and the former provide an 8.8% increase in the bearing characteristics. (2) With an increase in the load, the bag grouting pile experiences greater compression in the middle of the pile body, and the pile side friction resistance is increased; therefore, the pile side friction resistance can be fully developed, and the bag grouting piles have the ability to transfer the load from the top of the pile to the soil at the bottom of the pile. (3) When the external load is maximized, the sharing ratio of pile side friction resistance reaches 96.3%, which shows the excellent frictional performance of bag grouting piles. (4) Among the five factors mentioned above, the most important one is the pile diameter, followed by the pile length and pile spacing, the thickness of the bedding layer, and finally the grouting pressure. The optimal combination in this paper is a pile length of 18 m, pile diameter of 0.4 m, pile spacing of 1.0 m, bedding thickness of 0.3 m, and grouting pressure of 0.6 MPa. Therefore, changing the pile diameter can be given priority during the construction design. The findings in this paper can provide valuable insights and practical experience for the design of similar engineering projects.

1. Introduction

Bag grouting pile reinforcement technology is an advanced foundation reinforcement technique derived from the original grouting reinforcement technology. It innovatively restrains cement mortar inside a geotextile bag, thus achieving the same reinforcement effects as the original grouting technology and preventing the cement mortar from polluting the surrounding soil. Moreover, the column formed by the solidified cement mortar integrates with the surrounding soil, leading to the formation of a composite foundation. This innovative technology significantly improves the foundation bearing capacity and reduces foundation settlement [1,2]. It has great potential to reduce disturbances and pollution on an existing line during pile foundation construction.

In recent years, most studies on bag grouting piles have focused on field tests, theoretical analysis, numerical simulation, and indoor tests. In terms of field tests, Li [3] conducted a study on the reinforcement of a saturated soft clay foundation project using bag grouting piles, analyzed the soil displacement around the pile and super static pore water pressure during the pile forming process through a field test, and studied the soil-crowding effect of bag grouting piles under the condition of a saturated soft clay foundation. Wang [4] pointed out that the use of bag grouting piles can effectively control foundation settlement by observing the internal settlement of the roadbed and the horizontal displacement of the foundation soil. Zuo [5] investigated the working characteristics of a composite foundation of bag grouting piles by combining the traditional cement mixing pile and bag grouting pile reinforcement technologies and studied the changes in lateral displacement, settlement, and pile–soil stress of a composite foundation by testing the Ningbo–Taizhou–Wenzhou ballast rail passenger railroad. In terms of theoretical analysis, Zhou [6] discussed the foundation settlement calculation method, carried out research on the calculation method for a bag grouting pile foundation, proposed a set of calculation methods for bag grouting pile settlement for the special case of a sandwich composite foundation, and verified the calculation results with each other and the measured data; Tang [7] discussed the settlement calculation method for a bag grouting pile composite foundation, supplemented with calculation examples, and then conducted an analysis of the bag grouting pile composite. The traits of the composite foundation of bag grouting piles were analyzed and some conclusions were drawn. In terms of numerical simulation, Zheng [8] used Plaxis finite element software to build two numerical simulation models: with and without bag grouting pile treatment. Through consolidation analysis, settlement, superhydrostatic pressure, and stress were compared and analyzed to qualitatively derive the partial action mechanism of bag grouting piles; Du [9] used Flac3D finite difference software to build a numerical membrane bag grouting pile action model. Through software simulation, the expansion process of the membrane bag grouting pile was studied and compared with a theoretical solution to verify the applicability of the theoretical model. As for model tests, Shi [10] investigated the construction process, pile formation law, and reinforcement effect of polymer bag grouting piles through model tests of pile formation by grouting in soils with different densities and different layering conditions; Zhang [11] used model tests as a research tool to investigate the pile formation mechanism on the basis of experimental research on the feasibility of pile formation by bag grouting piles.

In conclusion, most previous studies have used a single method to study the bearing characteristics, without considering a comprehensive use of multiple methods. This paper takes Ningbo Hub’s Zhuangqiao–Ningbo section, consisting of additional three- and four-line projects as an example. In order to reduce disturbances and pollution on the existing line during pile foundation construction, bag grouting piles are widely used, partly supplemented with cement mixing piles. In this paper, a comprehensive approach is adopted by integrating field tests with numerical simulations. First, single-pile composite foundation load tests of bag grouting piles and cement mixing piles are conducted, which aim to compare the compressive load-bearing characteristics between these two types of piles. Static load tests are performed on bag grouting piles, and earth pressure gauges are embedded in the pile bottom and strain gauges are placed within the pile body, which aim to analyze the load transfer mechanism of bag grouting piles. Then, a numerical simulation is conducted using the finite element analysis software ABAQUS to simulate a compressive static load test of the bag grouting piles. This allows for an in-depth analysis of the load transfer mechanism of bag grouting piles. Finally, an orthogonal test is designed to find the optimal design solution by considering the influences of various factors on the bearing characteristics of the composite foundation of the bag grouting pile.

2. Single-Pile Composite Foundation Load Test

2.1. Experimental Overview

To study the bearing characteristics of bag grouting piles and compare them with cement mixing piles, a single-pile composite foundation load test was conducted, randomly selecting four composite foundations of bag grouting piles and cement mixing piles, respectively, at the same site. For the bag grouting piles, the pile diameter was 0.4 m, the pile length was 12 m, the pile spacing was 1.3 m, and the design value of composite foundation bearing capacity was 150 kPa, the number of bag grouting piles are 1#–4#. For the cement mixing piles, the pile diameter was 0.5 m, the pile length was 12 m, the pile spacing was 1.0 m, and the design value of composite foundation bearing capacity was 150 kPa, the number of cement mixing piles are 5#–8#. The distribution of the test piles is shown in Figure 1, and the on-site view of the bag grouting piles is shown in Figure 2.

2.2. Bag Grouting Pile Construction Process

Bag grouting piles are a foundation reinforcement method that involves injecting cement mortar into geotextile bags, forming reinforced piles together with geotextile bags. The reinforcement principle of bag grouting piles is shown in Figure 3. The pile formation process is as follows [12,13].

(1)

Drilling: drill to the designed hole depth with a drilling rig and complete the soil clearing work to prevent the pile hole from collapsing.

(2)

Pipe placement: put the geotextile bag on the grouting pipe and place them together at the bottom of the hole.

(3)

Grouting: Inject cement mortar into the geotextile bag. Under the influence of grouting pressure, the geotextile bag undergoes expansion and shaping, achieving the desired squeezing effect.

(4)

Hardening: after a certain period of time, the cement mortar is gradually solidified, and the bag grouting pile is formed.

2.3. Single-Pile Composite Foundation Load Test

The test methods and data analysis followed the specification [14]. The pressure and displacement values under different load levels were recorded using pressure transducers and displacement transducers through graded loading with an oil pressure jack. The load–settlement curve (P–s curve) was generated, as shown in Figure 4, and the on-site view of loading test is shown in Figure 5. The settlement of the composite foundation at each location and its corresponding allowable bearing capacity are illustrated in

Table 1. Summary table of settlement and allowable bearing capacity of composite foundation.

Pile Number	Settling Volume/mm	Allowable Load Capacity/kPa
1#	13.29	208
2#	17.19	190
3#	12.64	203
4#	14.11	195
5#	9.64	192
6#	11.99	172
7#	11.41	184
8#	13.11	178

.

In Figure 4, the eight curves are all slow-varying. When the 4 composite foundations with bag grouting piles were loaded to the maximum test load of 300 kPa, the settlements were stable. From Table 1, the accumulated settlements of the four composite foundations with bag grouting piles were 12.64–17.19 mm. The pressure (190–208 kPa) corresponding to the deformation value s = 0.006b (7.8 mm) can be taken as the allowable bearing capacity of the composite foundation of the bag grouting pile, according to the specification [14] C.0.12, paragraph 3, clause 4. Similarly, the settlements of the four cement mixing pile composite foundations were stabilized when loaded to the maximum test load of 300 kPa, and the accumulated settlements of the four composite foundations were 9.64–13.11 mm. The test did not show the situation described in paragraph 1, 2, and 3 of Article C.0.10 from the specification [14], the bearing capacity did not reach the limit, and the pressure–settlement (P–s) curve does not show obvious proportional limits. The pressure (172–192 kPa) corresponding to the deformation value s = 0.006b (6 mm) can be taken as the allowable bearing capacity of the composite foundation of the cement mixing pile, according to the specification [14] C.0.12, paragraph 3, clause 4.

The comparison analysis demonstrates that the bag grouting pile exhibited a slightly smaller pile diameter (0.4 m), a slightly larger pile spacing (1.3 m), and the former provided an 8.8% increase in the bearing characteristics, compared to the cement mixing pile with a pile diameter of 0.5 m and a pile spacing of 1.0 m. Moreover, the construction period of bag grouting pile is shorter, the cost is lower, and the process requirement is simpler, so the treatment effect of bag grouting pile can be considered to be slightly better than that of cement mixing pile.

3. Experiments on the Bearing Characteristics of Bag Grouting Piles

3.1. Test Overview

As the effectiveness of the vibrating-string-type steel concrete strain gauge is related to the strength of the solidified and hardened cement mortar grouted by the bag, three bag grouting piles were randomly selected at the site for investigation. After 28 days of construction, it was completed, and core sampling tests were conducted by drilling holes and core samples were taken from the upper, middle, and lower sections within the range of the tested pile length for unconfined compressive strength testing. A summary of the unconfined compressive strength of the three bag grouting piles is presented in

Table 2. Unconfined compressive strength test results of core samples after 28 days of pile formation.

Test Number	Pile Diameter/m	Age /d	Coring Section	Damage Load /kN	Compressive Strength /MPa	Compressive Strength Average Value/MPa
1	0.4	≥28	top	89.32	11.73	11.03
			middle	89.5	11.67
			bottom	73.78	9.69
2	0.4	≥28	top	83.46	10.86	11.08
			middle	82.6	10.68
			bottom	90.12	11.69
3	0.4	≥28	top	95.37	12.52	11.50
			middle	89.5	11.75
			bottom	77.76	10.23

. An unconfined compressive strength diagram of the field core sample is shown in Figure 6. The test results show that the unconfined compressive strength of the core samples, aged beyond 28 days, exceeded 5 MPa, which is in accordance with the relevant specification and design requirements [14].

Subsequently, a single-pile composite foundation load test was conducted on a selected bag grouting pile with a pile length of 12 m and a pile diameter of 0.4 m. The test aimed to measure the pile side friction resistance and pile end resistance during graded loading, using vibrating-string-type steel concrete strain gauges and vibrating-string-type earth pressure gauges, as depicted in Figure 7 and Figure 8.

3.2. Instrument Installation

Considering the special characteristics of the material used in the bag grouting pile, which involves injecting cement mortar into the geotextile bag without the use of steel reinforcement, no steel reinforcement was employed during the installation of the instruments in order to reflect the stress on the raw materials as accurately as possible.

3.2.1. Earth Pressure Meter Installation

In the piling process of bag grouting pile, first, a hole was drilled to the designed depth with a drilling machine. Then, the grouting pipe and bag were placed, and the grouting pressure was used to make the bag expand, completing the piling process. Therefore, after drilling, only a small amount of cement mortar was grouted into the bottom of the pile, and after a day of hardening, the bottom of the pile was leveled and prepared for the installation of the earth pressure gauge. The diameter of the pile hole was 400 mm and the depth was 12 m, while the diameter of the earth pressure gauge was only 120 mm and it was lightweight. Hence, in order to prevent the earth pressure gauge from overturning when it was placed downward, a prism-shaped bracket was prepared in advance, and some stones were placed evenly to increase the self-weight of the earth pressure gauge. The upper side of the bracket and the lower surface of the earth pressure gauge were glued with epoxy resin and fixed with steel wire ties. During the placement, a rope with scale markings was attached to the upper side of the bracket. Two individuals simultaneously lowered it from the same reference height, paying close attention to the markings on the rope throughout the process. When a significant deviation occurred, they repeatedly lifted and pulled so that it was at the same level. To prevent the blockage, consolidation was carried out using the grouting pipe. After the earth pressure gauge was completely placed at the bottom of the hole, the leads were carefully moved around the pile using the grouting pipe. A schematic diagram of the earth pressure gauge installation is shown in Figure 9 and Figure 10.

3.2.2. Strain Gauge Installation

The pile length was 12 m and each pile was divided into three test profiles, where the strain gauges were installed at 2 m, 6 m, and 10 m of the pile, respectively. Two strain gauges were placed symmetrically at each test profile. To ensure the secure installation of the strain gauges, the concrete on the outside of the strain gauges was polished smoothly and then the strain gauges were glued to the geotextile bag using epoxy resin glue. After the strain gauges were installed, the leads were pulled smoothly and placed in a neat position. The schematic diagram of strain gauge installation is shown in Figure 9.

3.3. Load-Bearing Characteristics Test Analysis

3.3.1. Pile Axial Force

The values of the strain gauges were read during the graded loading. The strain at each pile section was measured with the strain gauge and the pile axial force was obtained using Equation (1) [15]. The distribution of pile axial force along the pile length during graded loading is shown in Figure 11.

$$Q_i=EA{\epsilon }_i$$

(1)

where E is the pile modulus of elasticity, A is the pile cross-sectional area, and ${\epsilon }_i$ is the strain value at section i.

As can be seen from Figure 11, during the application of load at all levels, the maximum value of pile axial force was located at the top of the pile, and the pile axial force in the middle and upper part of the pile decreased rapidly, indicating that the pile side frictional resistance in the middle and upper part of the pile was more significant. Subsequently, the load was gradually transferred to the lower part of the pile. At the early stage of loading, the side friction resistance in the middle and upper part of the pile body was gradually stabilized. When the applied load was 180 kN, the pile axial force in the upper and middle section of the pile decreased significantly at a faster rate. When the applied load reached 288 kN, it was observed that in the middle section of the pile body, the pile axial force exhibited a notable increase compared to the previous load level, indicating that the pile side friction resistance was further exerted. At the bottom of the pile, the pile axial force approached 0, indicating that the soil was further compressed and the pile side friction resistance played a significant role, which reduced the pile axial force.

3.3.2. Pile Side Friction Resistance and Pile End Resistance

The pile side friction resistance can be obtained by subtracting the pile axial force at the adjacent section. The pile end resistance was obtained from Equation (2) [15] by measuring the micro-strain at the bottom of the pile through the earth pressure gauge. According to the analyzed data, the relationship between the pile side friction resistance, pile end resistance, and external load was obtained and is shown in Figure 12.

$$Q_p=K\times \mu \epsilon$$

(2)

where K is the earth pressure meter calibration factor and $\mu \epsilon$ is the strain measured by the earth pressure meter.

From Figure 11, it can be seen that the pile side friction resistance can be transferred gradually from top to bottom along the pile body under all loading levels. Moreover, the upper part of the pile body experienced a higher magnitude of pile side friction resistance compared to the lower part. At the beginning of loading, the pile side resistance in the upper part of the pile experienced a faster increase, and it bore a larger external load. As the load increased, the side friction resistance in the lower part of the pile also experienced a faster increase, and its side friction resistance was fully utilized. Throughout the entire loading process, there was no apparent decrease in the growth trend of the pile side friction resistance, indicating that the pile side friction resistance continued to play a significant role in this process.

The performance of the pile side friction resistance and pile end resistance under all loading levels is shown in Figure 13.

As can be seen from Figure 13, under all loading levels, the pile end resistance gradually increased with the increase in the load but always played a relatively minor role. When the load reached 288 kN, the pile end resistance accounted for a significantly higher proportion of the load than that at the pre-loading stage, which was 3.1%. When the load reached 360 kN, the sharing ratio of pile side friction resistance was 96.3%, reflecting the great frictional performance.

4. ABAQUS Finite Element Simulation

Considering the limitations in the number of instruments available for field installation and the uncertainties associated with field operations, it is essential to conduct a more comprehensive investigation into the bearing mechanism of the bag grouting pile and analyze the pile axial force, pile side friction resistance, and relative displacement of the pile and soil. Therefore, a numerical simulation was carried out using ABAQUS 2020 (finite element analysis software) for three-dimensional modelling, and the numerical simulation results were compared with the field test analysis results. The conclusions derived from both approaches can effectively complement and enhance each other.

4.1. Model Establishment

In this paper, a three-dimensional model of a composite foundation for bag grouting pile is established, and the essential components include the bag grouting pile, bedding layer, and soil layer. Considering the boundary effect caused by the construction of the bag grouting pile, the model height was taken as twice the pile length, i.e., 24 m, and the width was taken as 10 m. Nine piles were arranged in a square configuration, and graded loading was applied to the pile in the middle with a maximum loading of 360 kN. The pile spacing was 1.3 m, the pile length was 12 m, the pile diameter was 0.4 m, and the thickness of the bedding layer was 0.6 m.

In the composite foundation model of the bag grouting pile, the simulation was carried out by employing three eight-node solid units, while the principal model of both the pile body and bedding layer was represented using a linear elastic model. As the project involved foundation reinforcement treatment in a soft soil area, the influence of principal stresses on the calculation results was relatively insignificant. The parameters were easily determined and can reflect the actual deformation under load. Therefore, the Mohr–Coulomb model could be chosen in combination with the calculation procedure [6,8,9,16]. The parameters of this model were determined according to the static touching and geological drilling data of this project, combined with the commonly accepted range of soil and pile parameters found in the literature [6,17,18]. After undergoing multiple trial calculations, the specific parameter values were finalized and are presented in

Table 3. Soil layer material parameter value table.

Name of Soil Layer	Thickness of Soil Layer/m	Mass Density/kN/m3	Young’s Modulus/MPa	Poisson’s Ratio [18]	Friction Angle/°	Cohesive Yield Stress/kPa
Manual filling	1.34	19	3.0	0.3	35	10
Silty powdery clay	20.96	17.7	2.45	0.45	14.79	13
Powdery clay	17.8	19	6.43	0.35	18.8	26.46

and

Table 4. Pile body and cushion parameter value table.

Type	Mass Density/kN/m3	Young’s Modulus/MPa [17]	Poisson’s Ratio	Expansion Coefficient
Bedding layer	21	4.5	0.25	-
Pile	23	20,000	0.20	10−5

. The interaction between the contact surfaces consists of two parts: normal action and tangential action [15], and two pairs of contact surfaces were set in this model. The normal action between the bag grouting pile and the soil around the pile is “hard contact”, and the tangential action of the friction formula is “penalty”. The friction coefficient between the geotextile bag and the soil is 0.5–0.6 according to the literature [18], and it was set as 0.6 in this paper. The normal action is “hard contact”, and the tangential action is frictionless. Considering the effect of grouting pressure, the expansion coefficient of the pile body was set and the grouting pressure was simulated using the heating method. We set displacement constraints in the X and Y directions (U1 = 0, U2 = 0) and fixed constraints at the bottom of the model (U1 = U2 = U3 = 0). During the grid partitioning, special attention was given to locally refining the soil elements around the pile to analyze the characteristics of the pile–soil contact. During the model’s establishment, careful consideration was given to the factors that contribute to soil deformation, such as the self-weight stress of the soil and other influencing factors. Consequently, ground stress equilibrium was required before graded loading. After the equilibrium, nine analysis steps were set, and the contact action of the pile–soil interface was defined in the “interaction” module. Finally, the graded loading was conducted. The model schematic diagram is shown in Figure 14 and Figure 15, and the settlement cloud of the model at maximum loading is shown in Figure 16.

4.2. Validation of the Model

To verify the accuracy of the numerical analysis method in this paper, a comparative analysis was conducted between the simulated values and the measured values obtained from the field experiments, as shown in Figure 17. From Figure 17, it can be seen that the simulated curve trend closely corresponds to the measured curve trend. Both curves exhibit a gradual deformation pattern, and the maximum discrepancy between them is found to be 3.8%, which indicates that the numerical simulation model demonstrates a high degree of realism in replicating the actual scenario and exhibits a remarkable level of reliability [19,20].

4.3. Analysis of Calculation Results

In this section, the calculation results from ABAQUS are used to analyze the changes in the pile axial force, pile side friction resistance, and relative displacement of the pile and soil during the loading process, thereby further verifying the reliability of the model.

4.3.1. Pile Axial Force

The pile axial force under all loading levels is shown in Figure 18. In general, under all loading levels, the pile axial force gradually decreases from the top to the bottom, which indicates that the upper load is gradually transferred to the soil between the piles. Moreover, during this process, the soil surrounding the pile exerts upward frictional resistance against the pile body, and the axial force near the lower part of the pile body is relatively smaller. Within this range, the pile side frictional resistance plays a significant role in the load transfer mechanism. At the early stage of loading, the pile body bears the majority of the upper load. As the load increases, the soil between the piles gradually transitions from the elastic state to the plastic state, and the axial force at the bottom of the pile also gradually increases. This indicates that the bearing capacity of the soil at the pile end starts to take effect. The bag grouting pile exhibits a certain level of rigidity and can transfer the load from the top to the bottom.

4.3.2. Pile Side Friction Resistance

As rigid piles, bag grouting piles play a significant role in bearing the upper load in composite foundations within soft soil areas. Consequently, it becomes necessary to investigate the working characteristics of bag grouting piles. The distribution of pile side friction resistance along the pile length is shown in Figure 19.

From Figure 19, it can be seen that the magnitude of pile side friction resistance varies with the layered position of the soil between the piles and the applied load at the top of the pile. The elastic modulus of the bedding layer is greater than that of the soil between the piles. Therefore, when each loading level is applied, the pile side frictional resistance within the bedding layer comes into play earlier. Additionally, the pile side frictional resistance within the bedding layer is relatively greater compared to that of the first layer of soil, where the first layer of soil is relatively shallow and its pile side friction resistance is uniformly distributed along the pile body. In the second layer, the pile side friction resistance reaches the maximum in the middle of the pile body, gradually decreases thereafter, and eventually approaches zero near the bottom of the pile. At the interfaces between different soil layers, there is an abrupt change in the pile side frictional resistance due to the differences between the soil parameters. With the increase in the applied load on the top of the pile, the pile side friction resistance gradually increases at each position, indicating that the pile side friction resistance gradually comes into play.

4.3.3. Relative Displacement of Pile and Soil

The relative displacement of pile–soil under each level of load is shown in Figure 20.

As can be seen from Figure 20, the relative displacement of the pile and soil generally increases with the increase in load. Under each loading level, the relative displacement of the pile and soil gradually decreases from the top to the bottom. At a lower loading level, the relative displacement of the pile and soil increases slowly. However, as the load reaches 144 kN, the relative displacement of the pile and soil increases faster, and the increasing speed keeps growing with the increase in the load. When the external load is 288 kN, the relative displacement of the pile and soil increases significantly. This aligns with the conclusions in Section 3.2.2, where the pile end resistance at this loading level accounts for a significantly higher proportion of the load than that in the earlier loading stage, verifying the consistency between the field test results and the ABAQUS finite element model results.

4.3.4. Pile Side Friction Resistance and Relative Displacement of the Pile and Soil

The relationship between the pile side friction resistance and the relative displacement of the pile and soil at different soil layers is shown in Figure 21.

As can be seen from Figure 21, as the relative displacement of pile and soil increases, the pile side friction resistance provided by each soil layer also increases. In the upper and lower parts of the pile body, when the relative displacement of the pile and soil reaches a certain value, the pile side friction resistance progressively reaches its full potential. As the relative displacement of the pile and soil continues to increase, the pile side friction resistance remains relatively constant. In the middle of the pile body (5~8 m), the pile side friction resistance plays a significant role, and the pile side friction resistance increases rapidly with the increase in the relative displacement of the pile and soil.

5. Orthogonal Experiment

The compressive load-bearing capacity of bag grouting piles can be determined using the relationship between the pressure and settlement [14]. Therefore, the settlement was taken as the evaluation index and the orthogonal test was designed to study the effect of different design parameters on the compressive load-bearing capacity. In this paper, the orthogonal test was conducted using ABAQUS to study the effect of five factors, namely the pile length, pile diameter, pile spacing, thickness of the bedding layer, and grouting pressure, on the compressive bearing characteristics of a single bag grouting pile, thereby obtaining the optimal design scheme for this project.

5.1. Test Scheme

The factors were indexed as A, B, C, D, and E, respectively, and four levels were considered for each factor. The orthogonal table for the experiment is L16 (4⁵), containing a total of 16 experimental groups, without considering the interaction among the factors. The factors and corresponding levels in the tests are shown in

Table 5. L₁₆ (4⁵) orthogonal test design.

Level	Factor
	A Pile Length /m	B Pile Diameter/m	C Pile Spacing/m	D Thickness of the Bedding Layer/m	E Grouting Pressure/MPa
1	12	0.4	1	0.3	0.3
2	14	0.5	1.3	0.4	0.4
3	16	0.6	1.4	0.5	0.5
4	18	0.7	1.5	0.6	0.6

, and the parameters for the pile and the materials of each soil layer in the ABAQUS finite element model were set to the same as in Section 4.1.

5.2. Analysis of Orthogonal Test Results

From the numerical simulation in ABAQUS, the settlement cloud maps of the experimental groups at the maximum load are shown in Figure 22, and a summary of the settlement values is provided in

Table 6. Orthogonal test results of composite foundation settlement.

Factor	A Pile Length/m	B Pile Diameter/m	C Pile Spacing/m	D Thickness of the Bedding Layer/m	E Grouting Pressure/MPa	Settling Volume /mm
Test Number
1	12	0.4	1.0	0.3	0.3	33.93
2	12	0.5	1.3	0.4	0.4	21.41
3	12	0.6	1.4	0.5	0.5	16.02
4	12	0.7	1.5	0.6	0.6	12.58
5	14	0.4	1.3	0.5	0.6	32.06
6	14	0.5	1.0	0.6	0.5	24.05
7	14	0.6	1.5	0.3	0.4	16.31
8	14	0.7	1.4	0.4	0.3	13.32
9	16	0.4	1.4	0.6	0.4	32.24
10	16	0.5	1.5	0.5	0.3	22.0
11	16	0.6	1.0	0.4	0.6	19.91
12	16	0.7	1.3	0.3	0.5	14.32
13	18	0.4	1.5	0.4	0.5	32.81
14	18	0.5	1.4	0.3	0.6	23.29
15	18	0.6	1.3	0.6	0.3	18.20
16	18	0.7	1.0	0.5	0.4	17.38

.

From Table 6, it is apparent that the settlement in group 4 is minimal, while the settlement in group 1 reaches the maximum. The settlement of a single pile is affected by multiple factors, so a significance analysis should be conducted for the sensitivity of each factor. The results of orthogonal tests are often processed through a range analysis. Then, the primary and secondary factors are divided and the optimal design combination is obtained. The results of the range analysis are shown in

Table 7. The results of the range analysis.

Factor	A	B	C	D	E
K1	20.985	23.8175	21.9625	21.9625	21.8625
K2	21.435	22.6875	21.4975	21.8625	21.835
K3	22.1175	17.61	21.2175	21.865	21.8
K4	22.92	14.4	20.925	21.7675	21.96
Rj	1.935	9.4175	1.0375	0.195	0.16
Excellent level	A4	B1	C1	D1	E4

.

From Table 7, it can be observed that the order is: B1 > A4 > C1 > D1 > E4. Therefore, the factors ranked in descending order of sensitivity are as follows: pile diameter, pile length, pile spacing, bedding thickness, and grouting pressure. The optimal combination is a pile length of 18 m, pile diameter of 0.4 m, pile spacing of 1.0 m, bedding thickness of 0.3 m, and grouting pressure of 0.6 MPa.

In this project, the sensitivity of pile diameter is much greater than that of other factors, which means that changing the pile diameter has a great impact on the bearing capacity of the project. Therefore, it is advisable to prioritize an adjustment to the pile diameter during the construction design. There is no significant difference between the sensitivity of bedding thickness and grouting pressure. Considering the higher material consumption associated with altering the bedding thickness, it is recommended to prioritize the adjustment to the grouting pressure during construction.

6. Conclusions

In this paper, a field static load test was conducted, and the results of the field test were compared with those of the finite element model in ABAQUS. The conclusions are as follows:

(1)

In soft soil areas, the bag grouting pile presents superior effectiveness compared to the cement mixing pile in the treatment of composite foundations. And the former provides an 8.8% increase in the bearing characteristics.

(2)

With an increase in the load, the bag grouting pile experiences greater compression in the middle of the pile body, and the pile side friction resistance is increased. This allows the pile side frictional resistance to be effectively exerted.

(3)

When the external load is maximum, the sharing ratio of pile side friction resistance reaches 96.3%, demonstrating the excellent frictional performance of a bag grouting pile.

(4)

In the compressive test of a single bag grouting pile, the most important factor affecting the settlement of bag grouting pile is the pile diameter, followed by the pile length and pile spacing, the bedding thickness, and finally the grouting pressure. The optimal combination in this paper is a pile length of 18 m, pile diameter of 0.4 m, pile spacing of 1.0 m, bedding thickness of 0.3 m, and grouting pressure of 0.6 MPa Therefore, it is recommended to prioritize an adjustment to the pile diameter during construction.