Keywords

1 Introduction

In recent years, with the increasingly complex construction environment of large infrastructure projects, such as water conservancy ship locks and urban subways, the difficulty of foundation reinforcement during the construction process has gradually increased. The foundation reinforcement technology has achieved rapid development and breakthroughs, and the selection of reinforcement structures pays more attention to matching the actual characteristics of the projects [1]. The double-row piles supporting structure formed by bored piles and high-pressure rotary jet grouting piles has the advantages of small space occupation, short construction period, and large bearing capacity, and is widely used in foundation reinforcement engineering [2]. Considering that the contact area between the bored pile and the adjacent high-pressure rotary jet grouting pile is a weak area, which is prone to cracking under complex upper loads and causing groundwater leakage, this article proposes adding a cutoff wall with reliable anti-seepage performance between the double-row piles to form a composite structure for foundation reinforcement.

Driven by large-scale foundation engineering constructions, extensive studies on the bearing characteristics of double-row piles have been conducted by many researchers through the methods of model tests, theoretical analyses, and numerical simulations. Peng et al. [3] studied the influence of row spacing and excavation depth on the bearing behaviors of four groups of double-row pile supporting systems by carrying out large-scale indoor model tests and obtained the optimal value of row spacing of the double-row pile support method. Zhou et al. [4] analyzed the pile deformation during the construction of double-row-pile support engineering in a foundation pit by conducting a large-scale model test based on the similarity principle. Cao et al. [5] had proposed a new calculation model for double-row-pile structure in foundation pit by improving the equivalent model of soil between double-row piles, and studied the stress and deformation characteristics of the double-row piles retaining structure in a practical case. Xu et al. [6] used the finite element numerical software ABAQUS to analyze the spatial effect of the bearing deformation of the special double-row pile support system, and proposed the optimization design measure for the support system near the corner of the foundation pit. Zhang et al. [7] studied the load transfer characteristics of pile-group foundations in collapsible loess stratum by conducting small pile-group model tests. Wu et al. [8] used the model test method to compare the bearing characteristic of the pile-group and diaphragm wall with similar material dosages and found that the ultimate bearing capacity of the diaphragm wall foundation was about 1.1 times of the pile-group foundation. Shi et al. [9] proposed a new numerical calculation model for double-row sheet piles considering the actual conditions of weak strata, and analyzed the horizontal displacement of double-row sheet pile cofferdams arranged in weak strata. Wang et al. [10] analyzed the bearing characteristics of double-row piles by building an indoor test model for deep foundation pit engineering, and found that soil reinforcement between piles, passive area on the side of piles, and soil reinforcement at the end of piles could effectively improve the bearing performance of double-row piles. Hassen et al. [11] proposed to transform the pile reinforcement area into equivalent homogeneous anisotropic continuous material, and adopted the Moore-Coulomb strength criterion to estimate the upper limit of the ultimate bearing capacity of the pile foundation. Shivashankar et al. [12] proposed that in a soft foundation, the properties and layer thickness of soft soil in the middle and upper strata played a key role in the strengthening effect of piles. Wang et al. [13] used finite element software PLAXIS 3D FOUNDATION to numerical study the bearing characteristics of double-row piles, considering the influences of the excavation depth, the embedded depth of piles, the distance between rows, and the diameter of piles. Wang et al. [14] studied the surface settlement law of a new prefabricated circular double-row pile support system suitable for silty clay foundations upon excavation through indoor model tests and numerical simulations.

Although the above studies could well reveal the bearing characteristics of the double-row pile structure under different circumstances, there is still a lack of research on the bearing characteristic of the new type of composite foundation formed by adding a cutoff wall between the double-row piles. Therefore, this article systematically studied the combined bearing characteristics of the cutoff wall-double row piles composite foundation, by taking a case of the composite foundation structure, formed by the double-row of bored piles and high-pressure jet grouting piles and the intermediate concrete cutoff wall, in a large ship lock project as the research background. Based on three-dimensional finite element refined modeling, the influence of the elastic modulus of materials in the linking region between the cutoff wall and the upper structure, as well as the size of the upper load on the bearing characteristics of the composite foundation are simulated and analyzed.

2 Cutoff Wall-Piles Composite Foundation

The new type of composite foundation formed by the cutoff wall and the double-row piles in this study is derived from a large double-track ship lock project under construction, located at the junction of Huoqiu County and Yingshang County in the middle reaches of the Huaihe River in Lu’an City, Anhui Province, China. The downstream longitudinal cofferdam of this project is closely adjacent to the first-line ship lock channel to the north, as a result, the construction site is narrow. During the construction process, the downstream longitudinal cofferdam also needs to be in place of the original traffic roads for the passage of numerous vehicles. The strata in the engineering area are the Quaternary Holocene artificial accumulation layer (\({\text{Q}}_{4}^{ml}\)) and the Quaternary Upper Pleistocene alluvium layer (\({\text{Q}}_{3}^{al}\)). The specific stratigraphic information is shown in Table 1. Although using double-row piles formed by the combination of bored piles and high-pressure jet grouting piles for cofferdam foundation reinforcement can solve site limitations and meet bearing requirements, cracks may occur at the contact area between the bored pile and the high-pressure jet grouting pile during traffic load, leading to groundwater leakage problems. To improve the seepage-proofing reliability of the foundation reinforcement structure, a reinforcement system combining the cutoff wall and the double-row piles is proposed for the downstream longitudinal cofferdam foundation of the project, as shown in Fig. 1. The upper part of the foundation is a reinforced concrete box cofferdam, and the double-row piles are rigidly connected to the upper box structure. The cutoff wall located in the middle of the double-row piles is connected with the upper box structure by setting an additional linking region for the purpose of preventing damage to the cutoff wall that affects the seepage-proofing effect under the action of upper loading. Based on the regional geological characteristics and practical engineering experience, the dimensions of the composite foundation reinforcement structure are as follows: The diameter of the bored pile in the composite foundation is 1.6 m and the depth is 40 m, the spacing between neighboring bored piles in the same side row is 2 m; The high-pressure jet grouting piles are arranged in the middle of neighboring bored piles, with a diameter of 0.8 m and a depth of 22.9 m; The cutoff wall arranged in the middle has a width of 0.4 m and a depth of 22.9 m; The distance between the rows of bored piles is 8 m. The double-row pile structure composed of bored piles and high-pressure jet grouting piles can also exert a certain seepage-proofing effect, which is conducive to improving the anti-seepage reliability of the whole reinforcement structure.

Table 1. Stratigraphic information description
Full size table
Fig. 1.
figure 1

Schematic diagram of the combined foundation

Full size image

3 Modeling Analysis of Composite Foundation’s Joint-Bearing

3.1 Numerical Calculation Model

To reveal the joint-bearing characteristics of the combined foundation formed by the cutoff wall and the double-row pile, a three-dimensional finite element numerical model is established according to the surrounding strata information and the geometric size of the combined foundation structure, as shown in Fig. 2. The Bored piles, high-pressure jet grouting piles, cutoff wall, and linking region are finely modelled in the numerical model. The boundary size of the calculation model is 90 m (length) × 4 m (width) × 80 m (height), with a total of 40032 grid elements and 45862 nodes. The FLAC3D software is adopted to simulate the bearing characteristics of composite foundations, in which the soil mass and high-pressure jet grouting piles are simulated by the Mohr-Coulomb model. The elastic constitutive model is utilized for the composite reinforcement structure due to its high stiffness. Considering the significant difference in stiffness between the composite reinforcement structure and the surrounding soils, the thickness-free contact surface elements are set up in their contact area. Note that the joint-bearing of the composite foundation is mainly studied by the simulation method in this paper, the results are affected by the reliability of the simulation model, and more efforts will be made with the model test method to further verify the simulated results in our future study.

Fig. 2.
figure 2

Numerical calculation model for the cutoff wall-double row piles composite foundation

Full size image

3.2 Calculation Parameters and Conditions

According to the engineering geological survey report, the calculation of each material in the numerical calculation model is determined as shown in Table 2. To ensure the accurate application of load, a solid element simulation is conducted on the bottom plate of the pile-top concrete box structure, and the upper load acts on the bottom plate of the box structure during the loading process. The simulation study on the bearing characteristics of the composite foundation is conducted using a graded loading method with a load stage difference of 500 kN until the ultimate load state is reached. The boundary conditions of the model are set as below: zero normal displacement constraints are set at the bottom and lateral boundaries; At the top boundary, a uniformly distributed stress boundary is applied at the box structure area based on the upper loading, and the other regions are free. The normal stiffness and tangential stiffness of the contact surface elements between the composite reinforcement structure and the surrounding soils are set as 1 × 108Pa according to engineering experience.

Table 2. Physical and mechanical parameters of the strata and structural materials
Full size table

4 Result Analysis

4.1 Simulation Verification of Single Pile Test Results

To verify the reliability of the calculation parameters and simulation process selected in this article, simulation validation is first conducted on the single pile test results at the engineering site. The comparison between the numerical simulated vertical displacements of the pile top center under different vertical loads and the test results is shown in Fig. 3. One can find from Fig. 3 that with the increase of pile top vertical loading, the vertical displacement of the pile top center increases linearly at first and then increases sharply after exceeding about 7500 kN. The numerical simulation results are in good agreement with the test pile results, indicating the effectiveness of the numerical research method used in this study.

Fig. 3.
figure 3

Calculated and measured displacement-load curves of single pile test

Full size image

4.2 Analysis of Combined Bearing Characteristics for Composite Foundation

Figure 4 shows the variation relationship between the displacement at the center of the upper structure plate and the upper vertical load for the cutoff wall-double row piles composite foundation. The loading-displacement curve of the composite foundation overall shows a linear variation and belongs to a slowly increasing type. According to the standard code JGJ 94–2008 [15], for the slowly increasing type loading-displacement curve, the settlement corresponding to 0.04 times the diameter of the bored piles can be selected to determine the vertical ultimate bearing capacity. Therefore, the vertical ultimate bearing capacity of the composite foundation corresponding to the settlement of 64mm is determined as 15000 kN.

Fig. 4.
figure 4

Simulated displacement-load curve of composite foundation

Full size image

Under the action of ultimate loading, the distributions of the vertical stress, the maximum shear stress, and the plastic zone for the composite foundation are respectively shown in Figs. 5, 6 and 7. It can be seen that the vertical stress and maximum shear stress distributed in the reinforced structure are significantly greater than those in the surrounding soil strata. The vertical compressive stress distributed in the bored piles is obviously higher than that in the cutoff wall, indicating the bored piles play a major bearing role in the composite foundation. Under ultimate vertical loading, the plastic zone in the soil stratum concentrates occurring at the bottom area of the bored piles, and the shear failure region below the two rows of piles is connected.

Fig. 5.
figure 5

Vertical stress distribution of the composite foundation under ultimate loading (Pa)

Full size image
Fig. 6.
figure 6

Maximum shear stress distribution of the composite foundation under ultimate loading (Pa)

Full size image
Fig. 7.
figure 7

Plastic zone of the composite foundation under ultimate loading

Full size image

4.3 Influence of Linking Region’s Elastic Modulus on the Joint-Bearing Behavior

Considering that the cutoff wall may cause damage after sharing a large vertical load, which greatly reduces the seepage-proofing effect of the composite foundation, the ways to reduce the load sharing of the cutoff wall should be studied. In this section, the impact of different connection methods between the concrete cutoff wall and the upper box structure on the bearing characteristics of the composite foundation is analyzed. It is considered that different connection methods will result in different elastic moduli of the linking region. Five elastic moduli, 0.02 GPa, 0.2 GPa, 2 GPa, 10 GPa, and 20 GPa, are selected to study to influence of the linking region’s elastic modulus on the joint-bearing behavior of the composite foundation under the vertical loading of 15000 kN.

The vertical stress and displacement distributions in the cutoff wall under the linking region’s elastic moduli of 0.2 GPa and 2 GPa are shown in Figs. 8and 9. With the increase of the linking region’s elastic modulus, the stress and displacement of the cut-off wall increase. As the linking region’s elastic modulus increases from 0.2 GPa to 2 GPa, the maximum compressive stress distributed in the cut-off wall increases from 0.90 MPa to 1.03 MPa, the maximum vertical displacement of the cut-off wall increases from 7.16 cm to 7.26 cm. It can be foreseen that the damage risk of the cutoff wall will increase with the increase of the linking region’s elastic modulus, using a low elastic modulus connection form in the linking region is beneficial for reducing the compressive stress acting on the top of the cut-off wall.

Fig. 8.
figure 8

Vertical stress distribution of cutoff wall under different linking region’s elastic moduli (Pa)

Full size image
Fig. 9.
figure 9

Vertical displacement distribution of cutoff wall under different linking region’s elastic moduli (m)

Full size image

The influence of the elastic modulus of the linking region on the load sharing of the cutoff wall is shown in Fig. 10. It can be seen from Fig. 10 that the load-sharing ratio of the cutoff wall for the whole upper loading increases obviously with the increase of the elastic modulus of the material at the connection area between the cutoff wall and the upper structure. When the elastic modulus of the linking region increases from 0.2 GPa to 20 GPa, the load-sharing ratio contributed by the cutoff wall increases significantly from 3.8% to 22%. The upper load is mainly born by the double-row piles, and its load-sharing ratio maintains at about 70% under the vertical loading of 15000 kN. The elastic modulus of the linking region between the cutoff wall and the upper structure has little influence on the load sharing of the double-row piles. It can be seen that the degree of softness and hardness of the connecting area between the cutoff wall and the upper structure mainly influences the load sharing of the cutoff wall.

Fig. 10.
figure 10

Variation of the load-sharing ratio for the cutoff wall with the linking region’s elastic modulus

Full size image

4.4 Influence of the Upper Loading Level on the Joint-Bearing Behavior

The influence of the upper loading level on the load sharing of the composite reinforcement structure is studied in this section. Let the elastic modulus of the linking region between the cutoff wall and the upper structure be 2 GPa, the load-sharing ratio of each structure in the composite foundation under different load levels is calculated, as shown in Fig. 11. With the increase of loading level imposed on the bottom plate of the upper box structure, the load-sharing ratio of the cutoff wall and the double-row piles increases gradually. The influence of load level change on the load sharing of the cutoff wall and double-row piles in the composite foundation is obvious when the load is small. The load sharing of the cutoff wall is affected greater by the change in the upper loading level than the double-row piles. For the cutoff wall, its load-sharing ratio increases rapidly with the loading level when the upper load is less than 6000 kN, and after that, the load-sharing ratio basically remains unchanged. Therefore, controlling the level of upper loading can reduce the load sharing of the cutoff wall, which is beneficial for protecting the cutoff wall.

Fig. 11.
figure 11

Variation of load-sharing ratio for the composite reinforcement structure with the upper loading level

Full size image

5 Conclusions

This article uses a three-dimensional finite element numerical simulation method to study the joint-bearing characteristics of a new type of reinforced foundation with a combination of cutoff wall and double-row piles. The main research conclusions are as follows:

  1. (1)

    Under the action of the upper load, the load-bearing characteristics of the combined foundation of the cutoff wall and double-row pile are significantly affected by the combined structure system. The vertical stress in the area of the double-row piles and cutoff wall is greater than that in the surrounding soil, and the largest value is in the bored pile.

  2. (2)

    The combined load-bearing characteristics of the composite reinforced foundation are greatly affected by the elastic modulus of the connection area above the cutoff wall. The smaller the linking region’s elastic modulus, the smaller the vertical stress, settlement, and load-sharing ratio of the cutoff wall, when the elastic modulus decreases from 20GPa to 0.02GPa, the load-sharing ratio contributed by the cutoff wall decreases from 22% to 3.8%. Adopting a flexible connection between the cutoff wall and the upper structure can effectively reduce the damage of the cutoff wall under the action of upper loadings.

  3. (3)

    The upper loading level has a significant impact on the joint-bearing characteristics of the composite foundation reinforcement system. With the increase of the upper load, the load-sharing ratio of the cutoff wall increases significantly, and the load-sharing ratio of the double-row piles increases slightly. Controlling the upper load grade is also conducive to protecting the cutoff wall and better utilizing its seepage-proofing performance.

It is worth noting that this article has not fully considered the impact of factors such as groundwater seepage, foundation pit excavation and upper horizontal load, on the combined bearing characteristics of the composite foundation, and only utilizes the simulation method the explore the law, in our future work, relevant research will be improved by combining model tests or other methods.