| In recent years, double-row pile structure has emerged as a new type of supporting structure for deep foundation pits. Because such structure has high lateral stiffness, which can effectively control its deformation, and exhibit good load conditions and overall stability with ease of construction, it has gradually become one of the optimal solutions for supporting structures of deep foundation pits. Scholars worldwide have conducted considerable research on double-row pile structure, and the results have provided significant guidance to actual engineering applications in many aspects; however, because the mechanic behavior of double-row pile structure is complex, practice has advanced ahead of theory with respect to some aspects, and thus the development of such supporting structures has been restricted. At present, research is lacking in various areas, including the calculation of the actual out-of-plane stiffness of the pile top beam, the affected length by top beam under spatial effects, the numerical integral solving for safety factors of different potential slip surfaces using the limit equilibrium method, the distribution rules of slip zones, and reinforcement of the foundation pit. After summarizing the existing research results worldwide of double-row pile structure, we offer a beneficial supplement to and improvement upon the above weaknesses existing in research on double-row pile structure for deep foundation pits by means of theoretical derivation, analysis, and numerical simulation, in the context of the geological conditions of homogeneous cohesive soil layers.The main works and achievements of this research are as follows:1. A theoretical algorithm was proposed to calculate the stresses and deformation of a double-row pile structure with the actual out-of-plane stiffness of the top beam being taken into account, and the influences of assumed unlimited out-of-plane stiffness and actual limited out-of-plane stiffness of the top beam on the stresses and displacement of the pile body were compared. This method is based on the Winkle assumption, and influential factors of the actual out-of-plane stiffness of the top beam were introduced to conduct a theoretical analysis of double-row pile structure. Thereby, the deflection differential equation of every section of the pile body was established and then solved by using power series by mathematical approximation according to the continuity of geometrical deformation and internal force at the ends of the pile section and corresponding boundary conditions. Afterward, the solution of the deflection differential equation of every section was derived to obtain the deformation and internal force at every point of the double-row piles. Through analysis of the calculation results in the context of assumed unlimited out-of-plane stiffness and actual limited out-of-plane stiffness of the top beam, the following can be obtained:1. The increase in stiffness of the top beam can reduce displacements of the front-and rear-row piles but will increase the bending moments of the pile bodies of the front-and rear-row piles.2. The influence of the variation in top beam stiffness on the bending moments of the pile bodies of the rear-row piles is a little greater than that on the front-row piles.3. The influence of the variation in top beam stiffness on the bending moments of the pile bodies is close to that on the pile top displacement. Compared with the current theoretical calculation method under the assumption that the top beam has unlimited stiffness, this method better conforms to practical engineering situations, providing a feasible theoretical calculation method for solving for deformation and stresses of double-row piles with the actual stiffness of the top beam being taken into account;2. Three-dimensional numerical simulations were conducted to study the stresses and deformation characteristics of double-row pile structure under spatial effects. On this basis, we studied the relationships between the affected length by top beam under spatial effects and major design parameters, such as the rigid or hinge joint mode between the top beam and the supporting pile, the long side length, the short side length and excavation depth of the foundation pit, the top beam stiffness, the cohesive force and friction angle of the soil layer, and the row spacing and pile spacing of the supporting structure. Through analysis of the rules governing variations in the affected length by top beam with different parameters, the following results were obtained:Variations in the short side length and excavation depth of the foundation pit and in the row spacing of the supporting structure have a significant influence on the affected length by top beam under spatial effects; in contrast, variations in the long side length, in the cohesive force and friction angle, in the top beam stiffness, and in the pile spacing of the supporting structure have a very small influence on the affected length by top beam under spatial effects.3. An integral method for calculating the safety factor for simplified integral stability was proposed for a foundation pit supported by double-row piles at different potential slip surfaces. In this method, based on the Bishop method, a simple plane rectangular coordinate system suitable for stability analysis of double-row pile structure in a deep foundation pit was established, and we derived the integral expressions of the safety factor for a simplified integral stability analysis of the potential slip surface when passing the arc at the intersection point between the foundation pit bottom and the front-row piles, etc. With possible reinforcement of the active zone, interpile soil, and passive zone being considered, we improved the integral expressions of the safety factor in different integral intervals and developed a corresponding computing program to calculate the stability safety factor of the foundation pit. Analysis of the calculation results shows that, if the safety factor of a slip surface passing an arc at the intersection point between the foundation pit bottom and the front-row piles is expressed as F1, that of the slip surface passing an arc at the bottom of the front-row piles is expressed as F2, and that of the slip surface passing an arc at the bottom of the rear-row piles is expressed as F3, then F1is slightly greater than F2, and F2is slightly greater than F3. Comparative analysis of these results with finite-element calculations verifies the feasibility of the proposed method, and the method improves the application of numerical integral solving for safety factors using the limit equilibrium method in double-row pile structure of deep foundation pits.4. After an analysis of the stability of foundation pits with double-row pile structure and a systematic study on the relationships between foundation pit stability and major design parameters such as the cohesive force and friction angle of the soil layer and the pile length and row spacing of the supporting structure, we concluded that the stability factor of the foundation pit increases with increases in the cohesive force and friction angle of the soil layer and in the pile length and row spacing of the supporting structure, with the influence of cohesive force and friction angle being the largest, followed by that of the pile length of the supporting structure, and finally by that of the row spacing of the supporting structure. Furthermore, the distribution characteristics and rules of the slip zones were also summarized.5. After analysis of reinforcement of a foundation pit supported by double-row piles through systematic study on the influences of the reinforcement of the active zone, interpile soil, and passive zone on the internal force and deformation characteristics of the pile body, we concluded that:1) For reinforcement of the active zone, when the reinforcement depth is larger than the foundation pit depth, the reinforcement effect is not evident;2) For reinforcement of the passive zone, the reinforcement is most effective when the reinforcement depth or width is about half of the foundation pit depth.6. On the basis of the above work, suggestions are given for conducting further relevant research. |
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