Research on the Reinforcement Characteristics of Thick Cushion Layer and Rigid Pile Composite Foundation

Abstract

The rigid pile composite foundation method is the most commonly used method for strengthening weak soil foundations. In this method, piles usually need to pass through weak soil layers, and the pile end falls on a bearing layer with good bearing capacity. Under existing technical conditions, the thicker the weak soil layer, the longer the pile body, and the more difficult it is to ensure the construction quality of the pile. In response to this issue, some scholars have adopted the rigid pile composite foundation method with a thick cushion layer for reinforcement treatment. This article uses PLAXIS 3D (V20.04.00.790) software to establish a finite element model of rigid pile composite foundation with a thick cushion layer and simulate the process of foundation reinforcement. The influence of parameters such as thickness, compression modulus, and shear strength index of the cushion layer on foundation settlement and pile–soil stress distribution is studied, and the reasonable range of these parameters is analyzed under the condition of considering reinforcement effect. Through comparative analysis, it can be concluded that for deep and weak soil areas, the thickness of the cushion layer can range from 0.5 to 2.6. The thickness and compressive modulus of the cushion layer have a significant impact on the settlement of the foundation, the pile–soil stress ratio, and the stress of the pile body, while the shear strength index of the cushion layer has a relatively small impact on these parameters. Reasonably selecting the geometric and mechanical parameters of the cushion layer can effectively reduce stress concentration at the pile top and better play the role of the soil between piles.

1. Introduction

When constructing structures on weak soil layers, serious problems such as soil deformation and even overall foundation instability are likely to occur under natural foundation conditions. In order to meet the engineering requirements, relevant measures must be taken to reinforce the weak foundation. The rigid pile composite foundation method has a high stiffness of the pile body and strong particle bonding, which can transfer loads to deeper soil layers. Compared with flexible piles, it has obvious advantages in improving the bearing capacity of the foundation, so it is widely used in the treatment of soft soil foundations [1,2].

Tradigo, Pisano, di Prisco, and Mussi [3] used finite element numerical simulation analysis to study the interaction between piles and soil in composite foundations. Ye, Wu, and Li [4] proposed a new type of pile cap beam (rigid pile) composite foundation system for the reinforcement of the coastal soft soil foundation. The new system reduces lateral displacement, reduces total settlement between piles and surrounding soil, and performs better in terms of uneven settlement. Fu and Li [5] studied the vertical load transfer behavior of the rigid pile composite foundation and its response to adjacent excavation through centrifuge model experiments and explored the variation laws of induced axial force, pile skin friction force, and pile soil stress ratio with depth.

Due to the widespread distribution of soft soil in China, some cross-river and cross-sea bridge projects often encounter conditions where the soft soil layer is very thick and it is difficult for rigid piles to reach the bearing layer. In response to this challenge, some scholars have improved the existing rigid pile composite foundation method by increasing the thickness of the cushion layer, which can effectively reinforce deep soft soil foundations and save economic costs [6].

Many scholars use finite element numerical simulation methods to explore the role of the cushion layer in the reinforcement treatment of CFG pile composite foundation. The research results show that the cushion layer can reduce the stress concentration at the top of the pile, transfer some of the load to the soil between the piles, and ensure that the pile and soil jointly bear the load, thereby reducing foundation settlement [2,7,8]. Shallow foundations have been investigated in a deterministic and stochastic point of view [9,10]. This indicates that the cushion layer is an important guarantee for the reinforcement of CFG pile composite foundation.

According to the research results of a large number of scholars, the reasonable thickness range of cushion for the composite foundation is 300–500 mm, but the value range is mostly based on the research that the pile end falls on the holding layer with a certain bearing capacity. Due to the varying engineering geological conditions and thickness of weak soil layers in different regions, the thickness of the cushion layer should be determined based on the actual situation.

In actual bridge engineering construction, many bridge engineering projects need to cross deep and weak soil layers. When considering seismic requirements comprehensively, a thick cushion layer larger than the thickness range of the cushion layer obtained by the specifications or existing research will be selected to reinforce the weak soil foundation. The large thickness cushion has been successfully applied to engineering examples, and the applicability and influence mechanism of the existing theoretical research results of rigid pile composite foundations in the reinforcement of deep and weak soil foundations have yet to be studied.

In this study, the foundation settlement and pile–soil stress distribution before and after reinforcement of rigid pile composite foundation with thick mattress bedding were obtained by centrifuge test. The PLAXIS 3D software was used to establish the finite element model, and the feasibility of the finite element model was evaluated by comparing the test data with the centrifuge test results.

On this basis, the effects of changes in bedding layer thickness, compression modulus, and shear strength index on foundation settlement and pile–soil stress distribution were investigated to determine the reasonable range of values for the relevant parameters. In addition, the applicability of the rigid pile composite foundation method with thick bedding layer in the reinforcement of deep and weak foundation and the reinforcement mechanism are also evaluated.

In the current research results on the action mechanism of the bedding layer, the influence of the shear strength index of the bedding layer on the action mechanism of the rigid pile composite foundation is seldom considered. However, the shear strength index of the bedding layer affects its deformation and damage pattern under the upper load, which in turn affects the overall stability and bearing capacity of the rigid pile composite foundation. In this study, the effect of shear strength index of bedding layer on foundation settlement and pile–soil stress will be investigated.

2. Centrifuge Test

2.1. Test Equipment

The centrifugal test adopts the TLJ-2-100g-t geotechnical centrifuge from the centrifuge laboratory of Southwest Jiaotong University, and the main components include a soil pressure box, a displacement sensor, and its strain gauges. According to the theory of magnitude analysis, on the basis of ensuring that the mechanical properties (stress and strain) of the soil body are consistent with the prototype, the similarity ratio of different physical quantities in the centrifuge test can be deduced, and the dimensions of the model box used in the test are finally determined to be 80 cm long, 60 cm wide, and 60 cm high [11,12]. the composition of the model box includes an organic glass material plate and an aluminum alloy plate with a thickness of 20 mm. The back wall of the model box is made of a Plexiglas plate with a thickness of 30 mm, which is fixed with 18 mm bolts to ensure the stability of the model box, while the front wall, left wall and right wall are made of aluminum alloy plates with a thickness of 20 mm.

2.2. Test Material

The foundation soil used in this test was the remolded Chengdu clay that had been prepared manually. The preparation process includes air-drying, crushing, sieving, mixing, and homogenizing the test clay with water, and then putting it into the model box. The model box was placed into the corresponding position in the centrifuge to perform the self-weight consolidation test of the foundation soil, and the desired foundation soil was obtained under a centrifugal field of 100 g. The model box was placed into the centrifuge at the corresponding position in the centrifuge to perform the self-weight consolidation test of the foundation soil. The liquid limit and plastic limit of the soil were 52.4% and 21.0%, respectively, and the optimum moisture content was 21.2%. The moisture content of the soil used in this test was 28% and its quality met the test requirements.

To avoid the particle size effect as much as possible, the thick bedding material was fine sand instead of gravel bedding, which is often used in engineering. A 14.7 cm diameter steel cylinder was used to simulate the upper foundation, steel sheets of the same material were added to the cylinder to replace the upper load, and the upper foundation and upper members were converted to an external load of 295 kPa.

Aluminum alloy tubes were in this test instead of rigid piles. Aluminum alloy tubes can withstand larger loads during the test and have high strength and stiffness to provide similar reinforcement as the actual rigid piles. The pile diameter of the model pile is 8 mm, the pile length is 18 cm, the wall thickness is 1 mm, the modulus of elasticity is 68.9 GPa, and the number of model piles is 57, as shown in Figure 1. The relevant parameters of all test materials used in the test are shown in

Table 1. Parameters related to the test material.

Material Name	Clay	Fine Sand (Cushion)	Steel Cylinder	Model Pile
Layer thickness/size	45 cm	2 cm	D = 15 cm
Moisture content ω (%)	28
Weight γ (KN/m3)	17.4	18.5	25	27
Deformation modulus Es (MPa)	6	10	2 × 105	6.89 × 104
Internal friction angle φ (°)	15	35
Cohesive-shear stress c (kPa)	30	0~2
Permeability coefficient k (cm/s)	10−7	8 × 10−4

.

2.3. Test Program

The test was divided into two cases: natural foundation without piles and rigid pile composite foundation with thick cushion for the centrifuge test. The size of the model soil body was 80 cm × 60 cm × 45 cm (length × width × height). The composite foundation model reinforced by thick bedding layer rigid piles was used; the pile diameter d was 8 mm, the pile length was 18 cm, the pile spacing was 3 d (d is the diameter of the pile, the same below), and the pile arrangement was the equilateral triangular distribution of piles. The thickness of the weak soil layer under the pile end was 27 cm, which is 1.5 times the test pile length. The test model is shown in Figure 2.

2.4. Arrangement Scheme of Test Components

The same displacement transducer arrangement scheme was adopted for both groups of models. The displacement sensors were installed on both sides of the steel cylinder and at 5 cm, 15 cm, and 25 cm from the right side of the cylinder, and the numbers were Z1, Z2, Z3, Z4, and Z5 from right to left, respectively. To obtain the stress distribution change of the pile, some model piles were selected as the test piles from the composite foundation model of the thick bedding layer rigid piles, and the relevant data were obtained by sticking strain gauges on the piles of the test piles. The strain gauges of the test piles were spaced equally from the top of the pile to the end of the pile, and the spacing of the strain gauges was 0.045 mm. The location of the test piles is shown in Figure 3.

2.5. Centrifuge Test Results and Analysis

1.

Foundation settlement

Due to the influence of the inner wall of the model box on the position of the displacement sensor Z-1, the difference between the values of Z4 and Z5 and Z2 and Z3 was used as the settlement of the foundation caused by the upper load. The calculation results were converted according to a 1:100 experimental model ratio, and the upper load was 295 kPa. The experimental results show that after foundation reinforcement treatment, the settlement of the foundation decreased by 37.7%, indicating that the rigid pile composite foundation with a thick cushion layer has a significant effect on strengthening deep soft soil natural foundation and reducing its foundation settlement. The settlement of the foundation at different positions of the natural foundation model and the rigid pile composite foundation model are shown in Figure 4.

2.

Additional stress of soil

The stress measured by the soil pressure box in the experiment is the sum of the self-weight stress of the overlying soil and the stress caused by the upper load. It can be inferred that the additional stress caused by the upper load should be the difference between the stress of the soil within the load range and the stress of the soil outside the load range (or the calculated self-weight stress of the soil). The trend diagram of the variation of additional stress depth of natural foundation and rigid pile composite foundation soil with a thick cushion layer is shown in Figure 5.

From Figure 5a, it can be seen that under the condition of upper load, the additional stress of the naturally weak soil foundation gradually decreases along the depth direction. Near the bottom of the soil layer, the additional stress of the soil approaches zero. In the natural foundation state, the depth affected by the upper load can reach about 28 m below the bottom of the cushion layer.

According to Figure 5b, it can be seen that under the action of upper loads, the additional stress on the surface soil of the naturally weak soil foundation is reduced by 55.68% by using a thick cushion layer rigid pile composite foundation for reinforcement; When reaching the pile end plane, the additional stress of the soil decreases to 62.5 kPa, but at a depth of 2 m below the pile end, the additional stress of the soil increases to 118.1 kPa and gradually decreases with increasing depth. This trend of change is consistent with existing research results [13,14,15,16,17]. In contrast, after using a thick cushion rigid pile composite foundation to reinforce a naturally weak soil foundation, the pile body will bear a certain load, which reduces the pressure on the soil between the piles, and therefore, the additional stress of the soil will also be correspondingly reduced. In addition, through the action of a thick cushion layer, the pile can transfer some of the load downwards, so the depth of load influence increases. Meanwhile, during the process of transmitting the upper load downwards, its changing trend also shows a different pattern from that without piles. This indicates that after using a thick cushion rigid pile composite foundation to reinforce deep and weak soil, the piles and soil can jointly bear more upper loads, reduce foundation deformation, and improve foundation bearing capacity.

3.

Axial stress of pile body

Based on the data collected from strain gauges at different positions in the centrifuge model test, the axial stress value of the pile body was obtained through sorting and conversion. The axial stress depth curve of the pile body for different test piles in the rigid pile composite foundation with a thick cushion layer is shown in Figure 6.

According to Figure 6, the stress on the central pile is the highest, while the stress on the outer pile gradually decreases. The stress distribution of the pile body shows a trend of first increasing and then decreasing. In addition, within a certain depth range below the pile top, the pile will experience negative frictional resistance from the soil between the piles.

The distribution of stress on the pile body is almost parallel throughout the entire pile length range. The axial stress on the pile body first increases and then decreases in the downward direction of the pile body length, indicating that the pile body is subjected to negative frictional resistance from the soil between the piles within a certain depth range from the pile top downwards. The maximum value of axial stress on the pile body is about 4.5 m below the pile top. These findings are closely related to the role of the cushion layer in composite foundations. The cushion layer creates a stress balance layer between the pile and the upper foundation. After being subjected to the upper load, the pile will thrust upwards into the thick cushion layer, causing negative frictional resistance on the pile body. Through the negative frictional force, the load is transmitted to the soil between the piles, reducing stress concentration at the top of the pile, lowering the stress level of the pile body, and extending the service life of the pile.

3. Establishment of Finite Element Model

PLAXIS 3D is a commonly used 3D geological modeling and numerical analysis software, which is widely used for the calculation and analysis of geotechnical engineering, tunneling, underground structures and groundwater problems. With a friendly user interface and rich documentation and examples, the software is suitable for workers in related industries [18].

3.1. Model Parameters

The dimensions of the PLAXIS 3D model are based on the size of the centrifuge model and enlarged proportionally at a scale of 1:100. That is, the size model of the foundation soil is length × width × height = 80 m × 60 m × 45 m. The basic soil element in PLAXIS 3D is a 10-node tetrahedral element, as shown in Figure 7. This model adopts ultra-fine grid density, with a refinement coefficient of 0.5, and is divided into 92,023 units and 131,936 nodes in total. The experimental model and grid division are shown in Figure 8. The boundary conditions of the model are set so that the horizontal and vertical displacement of the bottom boundary of the soil is fixed, the horizontal displacement around the soil is fixed, and the vertical displacement is free.

The distance from the lateral boundary of the model and the distance between the lower bound of the model from the top should be taken sufficiently so that the effects of the boundaries in the numerical model on the results are minimized. The displacement and the stress contours in the finite element software indicate that this distance is sufficient [19].

3.2. Model Constitutive Relationship and Material Properties

When conducting numerical simulation analysis of soil, the primary issue is which constitutive model to choose. For many geotechnical problems with deformation control as the core, the stress level in most areas of the soil is low, so the soil is basically in an elastic–plastic state. Therefore, elastic–plastic models or other advanced models can be chosen for numerical simulation calculations [20]. This model adopts the same soil layer material settings as the centrifuge test, and the foundation soil and cushion layer use the Mohr–Coulomb elastoplastic model, with drainage material as the drainage type. The model parameters are mainly determined by reshaping the specimen in simple indoor experiments, and the model parameters are usually checked and corrected based on the test results. In addition, in the establishment of numerical simulation models, the determination of model parameters will also refer to some empirical values [20]. The rigid pile and upper foundation adopt a linear elastic model, and the drainage type is a nonporous material. The rigid piles in this model are set using the embedded pile elements unique to the software. The material parameters of the finite element analysis model are detailed in

Table 2. Model material parameters table.

Material Name	Size (m)	Constitutive Type	Weight (N/cm3)	Elastic Modulus (MPa)	Poisson’s Ratio	Internal Friction Angle (°)	Cohesion (kPa)	Drainage Type
Clay layer	80 × 60 × 45	Mohr–Coulomb	17.4	6	0.30	15	30	Drainage
Cushion	Thickness: 2	Mohr–Coulomb	18.5	10	0.25	35	0.001	Drainage
Rigid pile	Diameter: 0.8	Linear elastic	30	6.89 × 104	0.20	-	-	Non-porous
	pile length: 18
Foundation platform	radius: 7.5	Linear elastic	25	2.00 × 105	0.20	-	-	Non-porous
	Thickness: 4

below.

3.3. Model Load and Step-by-Step Construction Mode

In this study, the step-by-step construction mode of PLAXIS 3D was used for numerical simulation. In this mode, the geometric model and loads can be changed for each calculation stage by activating or freezing the soil and structures created in the previous step. At each calculation stage of the model, changes in the geometric model often cause a significant amount of unbalanced forces. To solve this problem, this article adopts the automatic load step method, gradually applying unbalanced forces to the finite element mesh. Specifically, in this model, the load is applied uniformly on the surface of the foundation cap. This load will be applied to the model at each calculation stage to reflect the actual situation in the project.

3.4. Numerical Model Validation

To verify the reliability of the numerical model, the calculation results of the finite element model were compared with the monitoring results of the centrifuge model test.

As shown in

Table 3. Comparison table of foundation settlement.

Foundation Type	Experimental Data (m)	Numerical Simulation (m)	Relative Error (%)
Natural foundation	0.890	0.9327	4.8
Thick cushion layer–rigid pile composite foundation	0.548	0.5410	1.28

and

Table 4. Comparison table of pile–soil stress ratio for rigid pile composite foundation with thick cushion layer.

	Pile Top Stress (kPa)	Soil Stress between Piles (kPa)	Pile Soil Stress Ratio	Relative Error (%)
Experimental data	1895.30	125.50	15.102
Formula calculation result			13.938	7.71
Numerical simulation	1801.55	121.14	14.884	1.44

, the relative error of the natural foundation settlement is 4.8%, and the relative error of the composite foundation settlement is 1.28%. Secondly, the relative error between the magnitude of the pile–soil stress ratio obtained from numerical simulation and the experimental value is 1.44%. This verifies that the numerical model and parameter settings mentioned above are reasonable and reliable.

4. Result and Analysis

4.1. The Influence of Cushion Layer Thickness on Foundation Settlement and Pile Soil Stress Distribution

For the limitations of the current cushion layer setting in engineering practice, this article proposes that the thickness range of the cushion layer should also be different under different degrees of soil softness and hardness, rather than using a single range given by the specification for design. In addition, this article mainly studies the influence of thick cushion layers. Therefore, the thickness values of the cushion layer are 0.5 m, 1.0 m, 1.5 m, 2.0 m, 2.6 m, 3.5 m, 4.0 m.

In Figure 9, the total foundation settlement increases gradually with the increase in bedding layer thickness, and when the bedding layer thickness is within the range of 0.5 m~2.6 m, the increase in total foundation settlement is not large, but when the thickness reaches 2.6 m, continuing to increase the thickness of the bedding layer increases the trend of the foundation settlement, which produces a larger settlement and affects the stability of the foundation.

Figure 10 shows that with the increase in bedding layer thickness, part of the load carried by the pile body is transferred to the soil layer around the pile, and the load shared by the pile body becomes less, which leads to a reduction in axial stress of the pile body. However, when the bedding layer thickness exceeds 2.6 m, the pile axial force starts to rebound. It can also be seen that the axial stress of the pile body starts to generate from the top of the pile, gradually increases with the increase in the length of the pile body, and gradually decreases along the length direction of the pile body after reaching the maximum value at about 5.7–6.0 m below the top of the pile.

From Figure 11, it can be seen that as the thickness of the cushion layer increases, the range of negative frictional resistance on the pile side continues to expand, and the position of the neutral point also moves downwards. At this point, the area of positive frictional resistance of the pile body shrinks, indicating that the foundation reinforcement area has good bearing capacity due to the coordinated effect of the pile and soil, and the load transmitted to the underlying layer is relatively small, reflecting the coordinated role of the cushion layer in the pile–soil load transmission process in the rigid pile composite foundation.

From Figure 12, it can be seen that as the thickness increases, the additional stress value increases. On the one hand, this is due to the increase in the thickness of the cushion layer, which also leads to an increase in the total load on the surface of the soil between the piles, increasing the additional stress. On the other hand, due to the increase in the thickness of the cushion layer, the maximum point of the additional stress on the soil between the piles also changes, and the depth of influence of the soil between the composite foundation piles gradually increases. When the thickness of the cushion layer is less than 2 m, the depth of influence at the pile end varies more significantly with increasing thickness.

As can be seen from Figure 13, the pile–soil stress ratio gradually decreases and then increases with the increase in bedding layer thickness. The pile–soil stress ratio decreases from 9.28% to 6.45% when the bedding layer thickness increases from 0.5 m to 2.0 m, which is a total decrease of 2.83%, and increases from 6.45% to about 10.58% when the bedding layer increases from 2.0 m to 2.6 m, which is an increase of 4.13%. Starting to increase, the effect of bedding layer action will be significantly weakened, which will increase the upper load of the foundation and adversely affect the foundation bearing.

4.2. The Influence of Compression Modulus of Cushion Layer on Foundation Settlement and Pile Soil Stress Distribution

As shown in Figure 14, as the compression modulus of the cushion layer increases, the settlement of the foundation gradually decreases. The compression modulus increases from 10 MPa to 50 MPa, and the settlement decreases from 0.5410 m to 0.4419 m, a decrease of 0.0991 m. The decrease in foundation settlement is more significant. When the compression modulus increases from 50 MPa to 200 MPa, the settlement changes from 0.4419 m to 0.4187 m, a decrease of 0.0232 m, and the change in foundation settlement is not significant.

From Figure 15, it can be seen that as the compressive modulus of the cushion layer gradually increases, the axial stress of the pile at the top of the pile also increases. When the compression modulus of the cushion layer is small, the cushion layer is prone to significant deformation. Due to the small stiffness of the cushion layer at this time, the rigid pile is prone to upward stabbing, resulting in a smaller axial stress on the pile body. When the compression modulus of the cushion layer is large, the stiffness of the cushion layer increases. Compared to when the compression modulus is small, it is more difficult for the rigid pile to penetrate the cushion layer, and the axial stress of the pile body also increases.

In Figure 16, when the compression modulus of the bedding layer is 10~200 MPa, the neutral point of the pile body is at about 6.142 m, 5.782 m, 5.555 m, 5.468 m, 5.455 m, 5.431 m, 5.393 m, 5.224 m below the top of the pile, respectively. The position of the neutral point of the pile body gradually moves upward with the increase in the compression modulus of the bedding layer, and the change amplitude is gradually flattened.

According to Figure 17, it can be seen that the increase in the compressive modulus of the cushion layer has a certain impact on the additional stress value of the upper half of the soil between piles in the reinforcement area of the foundation. As the depth increases, this effect becomes smaller and smaller. When the additional stress value reaches the minimum value, the additional stress value hardly changes with the change of the compressive modulus.

As can be seen from Figure 18, the pile–soil stress ratio increases with the increase in bedding layer compression modulus Es. When the bedding layer compression modulus is 10 MPa and 200 MPa, the pile–soil stress ratio is 13.23 and 48.94, respectively, and it can be seen that the increase in compression modulus from 10 MPa to 200 MPa enlarges the pile–soil stress ratio nearly threefold.

4.3. Impact of Shear Strength Index of Cushion Layer on Foundation Settlement and Pile–Soil Stress Distribution

4.3.1. Friction Angle Inside the Cushion Layer

According to Figure 19, it can be seen that under the same upper load, the settlement of the rigid pile composite foundation decreases with the increase in the internal friction angle of the cushion layer, and the magnitude of the decrease becomes smaller. The settlement curves of the foundation under different internal friction angle conditions overlap. The settlement of the foundation gradually decreases along the depth direction of the foundation and reaches zero at the boundary of the foundation.

As shown in Figure 20, when the internal friction angle increases from 20° to 35°, the axial stress of the pile body significantly increases. However, as the internal friction angle increases from 35° to 50°, the increase in axial stress of the pile body slows down, and the trend of curve change gradually tends to flatten. This indicates that as the internal friction angle increases to a certain extent, the influence on the axial stress of the pile body gradually decreases. In addition, as the internal friction angle increases, the stress at the pile top also increases. The maximum stress point of the pile body is located about 0.53 m below the pile top, indicating that most of the load is borne by the pile body, while the load shared on the soil between the piles is less.

From Figure 21, it can be seen that as the internal friction angle of the bedding layer increases, it makes the shear strength and bearing capacity of the bedding layer increase, and the restraining capacity of the pile top of the rigid piles slightly increases, which makes the pile–soil stress ratio increase to some extent.

4.3.2. Cohesive Force of Cushion Layer

According to Figure 22, as the depth of the soil layer deepens, the settlement of the foundation first decreases rapidly and then tends to be gentle. When it reaches the pile end, the rate of decrease accelerates again, and when it reaches the boundary of the soil layer, the settlement approaches zero. This trend of change is similar to the friction angle inside the cushion layer, which may be due to the increase in cohesion of the cushion layer, which improves the shear strength of the cushion layer. The depth of pile penetration into the cushion layer decreases, and the amount of load transferred to the soil between piles through the cushion layer also decreases accordingly, resulting in a decrease in foundation settlement.

From Figure 23, it can be seen that as the cohesion of the cushion layer gradually increases, the axial force of the pile body and the stress at the pile top also increase. The maximum point of the pile body stress is located at about 0.54 m of the pile length. Under the same cohesive force, the axial force of the pile increases first and then decreases along the length of the pile, reaching its minimum value near the pile end. In addition, within the range of increasing cohesion from 0 kPa to 15 kPa, there is a significant increase in the axial stress of the pile body, indicating that within this range, the axial stress of the pile body is greatly affected by the cohesion of the cushion layer. When the cohesion exceeds 15 kPa, the amplitude of the increase in axial stress of the pile body decreases, and the change curve gradually overlaps, indicating that the influence of cohesion on the axial stress of the pile body decreases.

From Figure 24, it can be seen that the stress at the top of the pile increases with the increase in cohesive force, while the additional stress in the soil between the piles is exactly the opposite, increasing the pile–soil stress ratio with the increase in cohesive force of the cushion layer. In rigid pile composite foundations, the pile body always plays the main role, and the influence of the soil between the piles gradually decreases. When the cohesion increases from 0 kPa to 15 kPa, the proportion of the increase in pile–soil stress ratio gradually decreases. When the cohesion is greater than 15 kPa, the proportion of the increase in pile–soil stress ratio begins to gradually increase again. This indicates that the change in the cohesion of the cushion layer will have a certain degree of impact on the pile–soil stress ratio, making it difficult for the cushion layer to effectively coordinate the load sharing between the pile and soil.

5. Conclusions and Recommendations

This article uses a combination of centrifuge model testing and numerical simulation to analyze the influence of geometric and mechanical parameters such as the thickness, compressive modulus, and shear strength of the cushion layer of rigid pile composite foundation in deep and weak soil areas on foundation settlement and pile–soil stress distribution. The feasibility of using the thick cushion layer rigid pile composite foundation method to reinforce deep and weak soil layers is verified, and suggestions for the values of geometric and mechanical parameters of the cushion layer are provided.

Thick bedding layer–rigid pile composite foundation can effectively reinforce the deep and weak land foundation, so that the foundation settlement is significantly reduced, the upper load is transmitted to the shallow pile soil by bedding layer, and the stress is spread to the deeper soil layer through the pile body, which effectively reduces the foundation settlement, and the bearing capacity of the foundation is significantly improved. And under the action of the thick bedding layer, it ensures that the pile and soil bear the load together, and improves the overall stability of the foundation.

The thickness of the bedding layer has less influence on the foundation settlement and more influence on pile–soil stress distribution, and it can be concluded through comparative analysis that the range of bedding layer compression modulus should be 0.5~2.6 m for deep and weak soil areas. Through comparative analysis, it can be concluded that the thickness of the bedding layer in the area of deep and weak soil can be taken as 0.5~2.6 m. It is recommended that the compression modulus of the bedding layer should be taken as 10~50 MPa, the angle of internal friction should be taken as 20°~35°, and the cohesion should be taken as 0~15 kPa.

In actual engineering, the nature of the soil is complicated, and the foundation model with multiple soil layers should be used for testing or numerical simulation in combination with the actual engineering geological conditions, and the effect will be closer to the reality. In addition, the weak soil foundation may be located in the fault zone or earthquake-prone zone area, and it is necessary to further investigate the seismic performance of rigid pile composite foundation with thick mattress bedding on the basis of the existing research.