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Double-row pile supporting systems are mainly used in temporary engineering as a protection measure to ensure the smooth construction of the underground section of the main part of the structures and the safety of the environment around the foundation pit [1–4]. During the process of urbanization, the density of high-rise, super-high-rise, and underground buildings in cities has increased, and the utilization rate of underground space has been increasing. Influenced by the foundation of adjacent structures and the complex underground environment, the double-row pile supporting structure has attracted wide attention [5–8]. Based on the existing single-row pile, and by adding a row of piles behind it, the double-row pile supporting structure evolves from this. The double-row pile structure is mainly composed of the front pile, the back pile, the crown beam, and the connecting beam [9–14].
- double-row piles
- force characteristics
- deformation characteristics
1.介绍
In recent years, research on double-row pile supporting structures has achieved significant results. For example: (1) The control effect of the pile diameter, the pile length, the crown beam, the connecting beam, and other basic structural dimensions, as well as pile spacing and row spacing, on the horizontal deformation and vertical settlement of the whole double-row pile structure system and the cost control of foundation pit engineering [26,27,28,29,30]; (2) The influence of the development of the soil arch effect on the deformation of the double-row pile supporting structure during the excavation of the foundation pit [31,32,33]; (3) Based on the numerical simulation analysis, the deformation mechanism and deformation characteristics of the overall double-row pile structure, the front pile, the back pile, the crown beam, the connecting beam, etc. [34,35,36]; (4) Optimization and innovation of the pile-soil interaction calculation model [37,38,39,40]. Most of the above studies have been based on numerical simulation analysis, experimental research, and other methods. Generally, they researched the dimension parameters of double-row pile supporting structures, pile-soil stress and deformation characteristics, and calculation models.
According to the available data, in the same foundation pit wall protection project, the size parameters of the components, such as pile diameter and pile length, and the design parameters, such as pile spacing and row spacing, are often variable. However, there is a node near which the structural parameters can give full play to their supporting role in controlling the deformation of the foundation pit. When it is far away from this node, the supporting effect of the double-row pile supporting structure will not only be greatly reduced but also increase the project cost in terms of cost control [41,42,43,44]. For example, Wang et al. [29] found through numerical modeling that the control effect of a double-row pile supporting structure on the stability of the foundation pit is similar to that of a single-row pile when the spacing of the double-row pile supporting structure is too small. Appropriately increasing the double-row pile gap can have a good control effect on reducing the level shift of the supporting structure.
2. Study of Double-Row Pile Structures
The design of basic parameters such as the pile diameter, the pile length, the size of the crown beam and connecting beam, the pile spacing, and the row spacing of double-row pile supporting systems is very important. A double-row pile structure model is shown in Figure 1. The change of these basic structural design parameters will often have a great impact on the whole pull pile supporting structure. Using reasonable structural design parameters will play an important role in the deformation control of the whole system. The use of reasonable structural design parameters will play a significant role in the deformation control of the entire system [55,56].
Figure 1. Schematic diagram of a double-row pile supporting structure model.
In the design of a double-row pile supporting system, the design of optimal row spacing plays an important role, and the change in row spacing directly affects the horizontal bearing capacity of the front pile. For example, Ilyas et al. [57], Ooi et al. [58], and Qian et al. [59] studied the horizontal displacement of double-row pile supporting structures under different row spacings and found that the appropriate increase in row spacing had a good control effect in reducing the horizontal displacement of the double-row pile structure. However, the internal force of the double-row pile supporting structure will increase with the increase in row distance between the front and back piles, and the retaining effect of the back pile will also decline [60]. The maximum horizontal displacement of piles can be effectively reduced by reducing the row spacing of double-row piles. However, when the row spacing is too small, the supporting role of the back row pile may be greatly reduced, and its anti-lateral movement ability cannot be fully brought into play. At this time, the double-row pile can be regarded as a single-row pile in terms of its supporting effect [38]. Because it is difficult to obtain actual engineering monitoring data, finite element software can be used to conduct numerical simulation research and qualitative analysis of the optimal row and pile spacing. Through numerical modeling analysis, optimal structural design parameters with certain reliability can be obtained, and the structural parameters with reasonable economy can be given in combination with engineering practice [61,62,63].
When the supporting structure is in the optimal row and pile spacing, the control effect of the connecting beam becomes a new research object. When there is no connecting beam, the supporting structure system only acts as the single-row pile supporting structure, and the deformation characteristics of the structure are not significantly different from those of the single-row pile supporting structure. For the pile row support structure with connected beams, connected beams can not only reduce the range of soil stress concentration between two groups of double-row piles, but also increase the overall mechanical performance of the load section and the overall stiffness of the structure [71,72,73].
Drawing from a large amount of the literature and field monitoring data, it can be understood that the soil pressure of the front pile and the back pile in the double-row pile support system is different. For this reason, many scholars have carried out a lot of research on the characteristics of double-row pile support systems. Through the establishment of a numerical model to carry out numerical simulation analysis, it was found that the different embedment depths of the rear pile have an effect on the internal force distribution of the front and back piles, which further leads to the conclusion that the combination of long and short piles can give full play to the overall performance of the row pile structure [77,78,79].
The selection of the pile section size is also one of the parameters that cannot be ignored in the control of foundation pit deformation and engineering costs. With the increase in the height-diameter ratio of the supporting pile, the horizontal displacement of the pile top of a double-row pile also increases. The excessive height-diameter ratio will pose a serious threat to the stability of the double-row pile retaining structure itself [82]. If the pile length is too short, resulting in an insufficient embedment depth and not meeting the requirements of the foundation pit anti-uplift, it will lead to a large horizontal displacement of the pile and even to the stability of the whole structural system declining. If the pile length is too long, it can meet the control requirements for pile displacement, but because the structure design is too conservative, the performance of the supporting pile will not only not be fully utilized but will also waste the project’s investment. The same principle applies to the increase in pile diameter, which has little effect on controlling the horizontal displacement of the pile top. The same is true for increasing the pile diameter. Excessive pile diameter has little effect on the horizontal displacement control of the pile top [83,84]. In recent years, due to the limitations of the site, a double-row pile supporting structure with an arched beam has been gradually developed, as shown in Figure 2.
Figure 2. Schematic diagram of a plane model with an arched beam.
3. Influence of the Soil Arch Effect on Double-Row Pile Supporting Structures
According to the available data, Karl [88] first named the arch effect phenomenon, which means that the load in the unstable soil will be transmitted to the surrounding rigid boundary. Wang [89] believed that the soil arching effect is a phenomenon of soil shear stress transfer. In the process of foundation pit excavation, the double-row pile supporting structure will produce displacement and flexural deformation in the direction of the pit when it bears the earth pressure generated by the soil mass under the action of dead weight or overload. Then, the relative displacement between the supporting pile and the soil behind the pile will occur, and the soil will also deform due to the shear friction between the soil particles. At this time, the stress of the deformed soil will be released and transferred to the undeformed soil; that is, the earth arch effect occurs, and the collapse area before the arch is formed [90,91,92], as shown in Figure 3.
Figure 3. (a) Schematic diagram of the soil arching effect between piles on a horizontal surface; (b) Schematic diagram of the soil arching effect between piles on a vertical section.
Different from single-row pile supporting structures, double-row pile supporting structures will produce a soil arch effect around the front and back piles, and the supporting system will be affected by the superimposed soil arch effect of these two parts. Yang et al. [94] found through research that properly increasing the horizontal pile spacing in rows of piles could reduce the area of multiple soil arches as a whole, thus reducing the stress of the multiple soil arch effect as a whole. However, if the soil arch effect zones of the front pile and the back pile are compared and analyzed, the soil arch effect zone around the back pile is reduced, while the soil arch effect zone of the front pile is increased. At this time, the residual load of the front pile will increase, which is unfavorable to the horizontal bearing performance of the front pile [31,32,33].
In foundation pit wall protection engineering, there is soil pressure distribution transfer and balance at every stage of the development of the soil arch effect, which will always accompany the engineering. To simplify the calculation and facilitate analysis, the soil arching effect on a two-dimensional model is usually studied with emphasis; that is, a vertical section is taken out for analysis. However, the three-dimensional effect of soil cannot be ignored. The three-dimensional soil arching effect has a great influence on the supporting effect of the supporting structure [53,84].
4. Deformation Characteristics of Double-Row Pile Supporting Structures
In the process of foundation pit excavation, the deformation of the double-row pile structure will run through the whole project. Due to the disharmony between pile displacement and soil deformation, the displacement of the pile body is greater than that of the soil, which leads to the phenomenon of pile-soil separation and further causes the bending and horizontal displacement of the pile end [60,92].
He et al. [96], Zhao et al. [97], and Han et al. [98] found that double-row pile supporting structures can give better play to their overall rigidity characteristics and have a better deformation control effect than single-row pile supporting structures through comparative analysis. Based on the excavation project of the square in front of Huaian East Station, Zhou et al. [99] selected different excavation sequences for different pit locations and found that the excavation sequence from far to near could reduce the surface settlement and reduce the deformation of the foundation pit supporting and retaining structure. However, it is not sufficient to control the horizontal displacement of the foundation pit only using a reasonable excavation sequence, which should be combined with other influencing factors. The specific application effect needs to be further studied [100,101]. Based on the analysis of practical engineering examples, the deformation and stress of the double-row pile structure in the excavation process can be numerically simulated by using three-dimensional finite element theory to analyze the stress and deformation characteristics of the double-row pile structure [102,103,104].
Xiong et al. [112] studied the influence of different embedment depths of supporting piles on the deformation characteristics of double-row pile support structures and the stability of foundation pits through numerical modeling and other methods. The results showed that the deeper the embedment depth of supporting piles, or the harder the soil or rock in the embedment area of the foundation pit, the higher the stability of the double-row pile support structure and foundation pit.
5. Conclusions
The continuous development of underground space has attracted more attention in terms of the design and construction of foundation pit wall protection engineering. Among them, double-row pile supporting structures are widely used in the infrastructure construction process of wharves, bridges, subways, tunnels, and high-rise and super-high-rise buildings due to their advantages of large stiffness, small lateral stiffness, and no need for internal support [166]. However, the mechanical and deformation characteristics and mechanisms of double-row pile supporting structures are extremely complex, and the mechanical analyses of the pile-soil interaction and corresponding calculation models are constantly optimized; the system is also relatively complex.
The double-row pile supporting structure is a complex spatial integration framework [167,168], which presents huge challenges for researchers in exploring pile-soil, pile-pile interaction, and deformation. With the development of engineering technology, different forms of double-row pile supporting structures are also being upgraded, such as the newly emerging prefabricated double-row pile supporting structure and multi-row pile supporting structure in recent years, which necessitates higher requirements for researchers in the study of deformation rules and mechanisms of foundation pit supporting structures and the establishment of force analysis and calculation models. Rooted in environmental protection and sustainable development, the new materials used for foundation pit engineering [169,170,171,172] and the construction method based on artificial intelligence are also constantly being updated and iterated [173,174,175], which will also promote the development of the industry and greater impetus to reduce carbon emissions.
This entry is adapted from the peer-reviewed paper 10.3390/app13137715
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