Construction method of prestressed diaphragm wall as foundation pit partition wall
By adopting post-tensioned prestressed diaphragm walls in foundation pit construction and dynamically adjusting the prestress, the problem of complex stress and deformation of foundation pit partition walls was solved, and the deformation and internal forces of the wall body were effectively controlled, thereby improving construction safety and economic benefits.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- CSCEC INT CONSTR
- Filing Date
- 2023-03-21
- Publication Date
- 2026-05-26
Smart Images
Figure CN116145684B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of foundation pit excavation, and more specifically to a foundation pit construction method using prestressed diaphragm walls as foundation pit partition walls. Background Technology
[0002] For foundation pits, unlike lateral retaining structures, the stress on foundation pit partition walls is more complex.
[0003] For lateral retaining structures, the stress is the earth pressure behind the wall. For partition walls, however, the stress is closely related to the construction sequence. (See attached diagram) Figure 1 As shown, a partition wall is set between the first-phase excavation pit and the second-phase excavation pit.
[0004] Its stress state can be divided into the following situations:
[0005] First, when the first phase of excavation has begun but the second phase has not, the retaining structure of the partition wall and the foundation pit are the same, both being subjected to the earth pressure behind the wall and the axial support force within the first phase pit.
[0006] Secondly, during the first and second phases of excavation, the partition wall is subjected to the following forces: axial support force within the first phase pit and axial support force within the second phase pit. At this point, because the second phase excavation causes partial unloading of the soil behind the wall, the concentrated load on the horizontal support of the first phase excavation side is adversely affected.
[0007] The literature “An Jingbo. Deformation behavior analysis of retaining structure for deep foundation pit zonal excavation [J]. Engineering Construction and Design, 2017(11):2” has already stated that the deformation of the partition wall changes continuously with the progress of construction.
[0008] Prestressed technology, due to its excellent performance in controlling structural deformation and fully utilizing the load-bearing capacity of steel reinforcement and concrete, has been widely used in municipal bridges, large-span steel structures, and high-rise buildings. Applying prestressed technology to diaphragm wall structures in foundation pit engineering, by applying vertical prestress, can improve the stiffness and crack resistance of the diaphragm wall, reduce the amount of steel reinforcement and deformation under soil and water pressure, and ultimately improve the safety of the foundation pit. For example, literature CN114575331A, "Prestressed Irregular Diaphragm Wall and Its Construction Method," proposes that by setting prestressed steel reinforcement in a curved bundle shape, the horizontal displacement of the wall and the amount of concrete used can be effectively reduced.
[0009] However, the application of prestressing to diaphragm walls in existing technologies is essentially just an improvement in structural form.
[0010] Because the stress and deformation of the foundation pit partition wall are more complex, it is impossible to fully simulate the construction phase during the design phase. Therefore, the applicant proposed the "construction concept of dynamically adjusting the prestress of the partition wall during the construction phase." After reviewing relevant literature, no relevant construction experience has been found in existing projects. In other words, there is a lack of practical experience in how to implement the "construction concept of dynamically adjusting the prestress of the partition wall during the construction phase." Summary of the Invention
[0011] This invention addresses the problem that different vertical arrangements of supports on both sides of the partition wall in different foundation pit sections lead to unloading of the partition wall in the excavated and exposed sections and the adverse effect of concentrated loads from the horizontal supports on the first excavated side. It proposes a post-tensioned prestressed diaphragm wall partition wall method to solve the problem of excessive internal forces and water level deformation in the partition wall section, which leads to wall damage and instability. Furthermore, it proposes a foundation pit construction method using prestressed diaphragm walls as foundation pit partition walls.
[0012] The proposed solution is as follows:
[0013] The foundation pit construction method of using prestressed diaphragm walls as foundation pit partition walls involves excavating the foundation pit in two phases, with the bottom depth of the foundation pit in phase one being greater than that in phase two. A foundation pit partition wall is set between the foundation pit excavated in phase one and the foundation pit excavated in phase two, and the foundation pit partition wall is a prestressed diaphragm wall.
[0014] Prestressed diaphragm walls are post-tensioned diaphragm walls;
[0015] The prestressed diaphragm wall is tensioned at one end and anchored at the other end, wherein the tensioning end and the anchoring section are selected from the top of the capping beam or the bottom of the first-phase foundation pit; or, the prestressed diaphragm wall is tensioned at both ends, wherein the tensioning end is the top of the capping beam or the bottom of the first-phase foundation pit.
[0016] After the concrete for the foundation pit partition wall is poured and formed, prestressing tendons are not pre-embedded at this time. The construction is carried out in the following sequence:
[0017] 1) After the first phase of the foundation pit has been excavated to the bottom and the supporting structure has been completed, the first layer of support for the second phase of the foundation pit shall be constructed.
[0018] 2) Apply initial prestress to the foundation pit partition wall. After the prestressing tendons are completed and the holes are drilled, inject drag-reducing grease into the gaps between the holes and initially tension them to 20% of the control stress, i.e., 0.2σcon.
[0019] 3) Continue excavating the second-phase foundation pit to the top surface of the second support of the first-phase foundation pit, and continue to tension the prestressed tendons to 30% to 60% of the control stress, i.e. (0.3 to 0.6)σcon;
[0020] 4) The remaining excavation is based on the deformation of the partition wall to dynamically adjust the tensile stress:
[0021] The deformation of the partition wall is closely monitored by displacement monitoring. When the maximum horizontal deformation of the wall reaches the warning value of 0.7[Sh], the excavation is stopped and the prestress is increased step by step, with each increment being 0.05σcon. When the maximum horizontal deformation of the wall recovers to 0.5[Sh], the application of prestress is stopped and the excavation of the foundation pit continues.
[0022] Furthermore, the second-phase foundation pit was excavated step by step according to the above method. When the pit bottom was reached, the tension of the prestressed tendons of the partition wall did not exceed 100% of the control stress, and the horizontal deformation of the wall did not exceed the allowable value [Sh].
[0023] Furthermore, during construction, the lateral deformation of the foundation pit and the partition wall is closely monitored. Based on the monitoring feedback, the tension stress of the prestressing tendons can be adjusted in real time, and the maximum tension stress can reach 120% of the control stress, i.e., 1.2σcon.
[0024] Furthermore, steel strands are selected as prestressing tendons in prestressed diaphragm walls.
[0025] Furthermore, inclinometers are installed in the foundation pit partition wall to obtain the wall deformation.
[0026] Furthermore, the pre-embedded ducts of the post-tensioned prestressed diaphragm wall have the following characteristics: the top of the duct is located on the top surface of the capping beam of the wall, and a steel plate is installed at the top opening to anchor it to the reinforcing steel skeleton of the capping beam. The duct opening is reinforced with spiral stirrups and temporarily sealed with plastic plugs. The bottom of the duct is located on the wall surface above the bottom of the deeper excavation pit that has been excavated earlier. A steel plate is installed at the bottom opening to anchor it to the reinforcing steel skeleton of the wall. The duct opening is reinforced with spiral stirrups and temporarily sealed with plastic plugs. No prestressing tendons are pre-embedded in the ducts.
[0027] The advantages of this invention are:
[0028] First, this invention addresses the problem of the partial unloading of the soil behind the wall and the concentrated load of the horizontal support on the side of the earlier excavation caused by the successive excavation of the partition wall on both sides of the foundation pit at different depths. It proposes a post-tensioned prestressed underground continuous wall partition wall structure and construction method to solve the problem of excessive internal force and water level deformation of the partition wall section, which leads to wall damage and instability, and has significant direct economic benefits.
[0029] Secondly, to solve the aforementioned problems, this invention employs pre-embedded prestressed tendon ducts in the wall body of the partition wall (diaphragm wall) (attached). Figure 1The method involves applying vertical prestress to the wall at the top of the capping beam or the bottom of the initial excavation side, and using a post-tensioning technique with adjustable tension based on wall deformation monitoring feedback. In other words, the construction scheme of this application dynamically adjusts the prestress during construction; therefore, the top of the capping beam and the bottom of the first-stage foundation pit serve as the tensioning and anchoring ends, respectively, which aligns with the construction concept.
[0030] Third, the post-tensioned prestressed diaphragm wall partition wall used in this invention is constructed using the cast-in-place method. Prestressed tendon ducts are pre-embedded within the wall, and the ducts are corrugated pipes. The curvature and positioning of the ducts are determined based on the vertical arrangement of the foundation pit supports and the bending moment distribution of the wall, and should be staggered from other ordinary vertical main reinforcement bars (as shown in the attached diagram). Figure 4 , 5 (As shown).
[0031] Fourth, in the post-tensioned prestressed underground continuous wall partition wall adopted in this invention, the prestressing tendon tensioning process is carried out synchronously and in stages with the subsequent second-stage foundation pit excavation process. Based on the monitoring feedback of the lateral deformation of the foundation pit and partition wall during the second-stage foundation pit excavation process, the prestressing tendon tensioning stress is adjusted in real time. Attached Figure Description
[0032] The present invention will be further described in detail below with reference to the embodiments shown in the accompanying drawings, but this does not constitute any limitation on the present invention.
[0033] Figure 1 This is a schematic cross-sectional view of the foundation pit partition wall (diaphragm wall) in Example 1.
[0034] Figure 2 This is a detailed view of the channel port in Embodiment 1.
[0035] Figure 3 This is a schematic diagram of the structural load of the partition wall (diaphragm wall) after the foundation pit is excavated to the bottom in Example 1.
[0036] Figure 4 yes Figure 3 A schematic diagram of section 1-1.
[0037] Figure 5 yes Figure 3 Schematic diagram of section 2-2.
[0038] Figure label:
[0039] 1. Outer vertical main reinforcement; 2. Inner vertical main reinforcement; 3. Outer distribution reinforcement; 4. Inner distribution reinforcement; 5. Truss reinforcement; 6. I-beam joint; 7. Post-tensioned prestressed duct; 8. Duct positioning reinforcement. Detailed Implementation
[0040] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. Technical features in the various embodiments of the present invention can be combined accordingly without mutual conflict.
[0041] Example 1, in conjunction with Appendix Figure 1 As shown, the foundation pit construction method using prestressed diaphragm walls as foundation pit partition walls involves two phases of foundation pit excavation. A foundation pit partition wall is installed between the foundation pit excavated in the first phase and the foundation pit excavated in the second phase. The foundation pit partition wall is a prestressed diaphragm wall.
[0042] The depth of the first-phase foundation pit is greater than the depth of the second-phase foundation pit;
[0043] Prestressed diaphragm walls are post-tensioned diaphragm walls.
[0044] After the concrete for the foundation pit partition wall is poured and formed, the following construction sequence shall be carried out:
[0045] 1) After the first phase of the foundation pit has been excavated to the bottom and the supporting structure has been completed, the first layer of support for the second phase of the foundation pit shall be constructed.
[0046] 2) Apply initial prestressing force to the foundation pit partition wall. The construction unit may freely choose the tensioning end and anchoring end (top of the capping beam or wall body at the bottom of the first-phase foundation pit) or tension both ends simultaneously, depending on the site conditions and its own circumstances. Flexible steel strands are used for the prestressing tendons. After the prestressing tendons are drilled, drag-reducing grease is injected into the gaps between the ducts, and the tendons are initially tensioned to 20% of the control stress, i.e., 0.2σcon.
[0047] 3) Continue excavating the second-phase foundation pit to the top surface of the second support of the first-phase foundation pit, and continue to tension the prestressed tendons to 30% to 60% of the control stress, i.e. (0.3 to 0.6)σcon;
[0048] 4) The remaining excavation is based on the deformation of the partition wall to dynamically adjust the tensile stress:
[0049] During construction, the deformation of the partition wall was closely monitored by displacement monitoring. When the maximum horizontal deformation of the wall reached the warning value of 0.7[Sh], the excavation was stopped and the prestress was increased step by step, with each increment being 0.05σcon. When the maximum horizontal deformation of the wall recovered to 0.5[Sh], the application of prestress was stopped and the excavation of the foundation pit continued.
[0050] 5) Excavate the second-phase foundation pit step by step according to the above method. When the pit bottom is reached, the tension of the prestressed tendons of the partition wall shall not exceed 100% of the control stress, and the horizontal deformation of the wall shall not exceed the allowable value [Sh].
[0051] 7) During construction, closely monitor the lateral deformation of the foundation pit and the partition wall. Based on the monitoring feedback, the tension stress of the prestressed tendons can be adjusted in real time. The maximum tension stress can reach 120% of the control stress, i.e., 1.2σcon.
[0052] The calculation method for the maximum control stress σcon of the prestressing tendon is as follows:
[0053] (1) Based on the calculation results of the retaining wall structure of the first-phase foundation pit, the envelope value of the axial force of the support in the first-phase foundation pit, Fhi (i is the number of the support from top to bottom);
[0054] (2) Determine the maximum allowable horizontal deformation value of the enclosure wall [Sh] according to the design documents or relevant specifications;
[0055] (3) Based on the axial force Fhi of the support in the foundation pit and the allowable value of horizontal deformation of the wall [Sh], and in conjunction with the existing ordinary steel reinforcement design of the wall, determine the prestressed tendon reinforcement design and determine the maximum control stress σcon of the prestressed tendon.
[0056] The basic principle of the post-tensioned prestressed diaphragm wall partition wall adopted in this invention is as follows:
[0057] 1) The first phase of the deeper foundation pit is implemented first. When the first phase foundation pit is excavated to the bottom and the second phase foundation pit has not yet been implemented, the partition wall bears the active soil and water pressure ea outside the pit (side of the second phase foundation pit), the passive soil and water pressure ep inside the pit (bottom of the pit side of the first phase foundation pit) and the concentrated force Fh of the support inside the pit.
[0058] 2) After the second phase of shallow foundation pit excavation is completed, the water and soil pressure in the active zone outside the original pit of the excavated exposed section of the partition wall is partially unloaded. Due to the different depths of the foundation pit, different vertical supports are used on both sides of the partition wall. As a result, the partition wall in the excavated exposed section bears the concentrated load of the first phase of foundation pit horizontal support, which is unfavorable. The partition wall body will generate large cross-sectional internal forces (bending moments) and water level deformation.
[0059] 3) By applying vertical prestress to the wall, the prestressing tendons arranged in a curved pattern inside the wall will generate horizontal compressive stress qe on the wall (see attached diagram). Figure 3 This compensates for the unloading of soil and water pressure in the active zone caused by the second-phase foundation pit excavation, improves the internal force of the wall section, and controls the deformation of the wall water level.
[0060] The above-described embodiments are preferred embodiments of the present invention and are only used to facilitate the illustration of the present invention. They are not intended to limit the present invention in any way. Any person skilled in the art who makes local modifications or alterations to the technical content disclosed in the present invention without departing from the scope of the technical features of the present invention shall still fall within the scope of the technical features of the present invention.
Claims
1. A method for constructing a foundation pit using a prestressed diaphragm wall as a foundation pit partition wall, characterized in that, The foundation pit excavation is divided into two phases. The bottom depth of the foundation pit in phase one is greater than that in phase two. A foundation pit partition wall is set between the foundation pit excavated in phase one and the foundation pit excavated in phase two. The foundation pit partition wall is a prestressed underground continuous wall. Prestressed diaphragm walls are post-tensioned diaphragm walls; The prestressed diaphragm wall is tensioned at one end and anchored at the other end, wherein the tensioning end and the anchoring section are selected from the top of the capping beam or the bottom of the first-phase foundation pit; or, the prestressed diaphragm wall is tensioned at both ends, wherein the tensioning end is the top of the capping beam or the bottom of the first-phase foundation pit. After the concrete for the foundation pit partition wall is poured and formed, prestressing tendons are not pre-embedded at this time. The construction is carried out in the following sequence: 1) After the first phase of the foundation pit has been excavated to the bottom and the supporting structure has been completed, the first layer of support for the second phase of the foundation pit shall be constructed. 2) Apply initial prestress to the foundation pit partition wall. After the prestressing tendons are completed and the holes are drilled, inject drag-reducing grease into the gaps between the holes and initially tension them to 20% of the control stress, i.e., 0.2σcon. 3) Continue excavating the second-phase foundation pit to the top surface of the second support of the first-phase foundation pit, and continue to tension the prestressed tendons to 30% to 60% of the control stress, i.e. 0.3σcon to 0.6σcon; 4) The remaining excavation is based on the deformation of the partition wall to dynamically adjust the tensile stress: The deformation of the partition wall is closely monitored by displacement monitoring. When the maximum horizontal deformation of the wall reaches the warning value of 0.7[Sh], the excavation is stopped and the prestress is increased step by step, with each increment being 0.05σcon. When the maximum horizontal deformation of the wall recovers to 0.5[Sh], the application of prestress is stopped and the excavation of the foundation pit continues.
2. The foundation pit construction method using prestressed diaphragm wall as a foundation pit partition wall according to claim 1, characterized in that, The second-phase foundation pit is excavated step by step according to the above method. When the pit is reached, the prestressing tendons of the partition wall are tensioned to no more than 100% of the control stress, and the horizontal deformation of the wall does not exceed the allowable value [Sh].
3. The foundation pit construction method using prestressed diaphragm wall as a foundation pit partition wall according to claim 1, characterized in that, During construction, the transverse deformation of the foundation pit and the partition wall is closely monitored. Based on the monitoring feedback, the tension stress of the prestressing tendons can be adjusted in real time. The maximum tension stress can reach 120% of the control stress, i.e., 1.2σcon.
4. The foundation pit construction method using prestressed diaphragm wall as a foundation pit partition wall according to claim 1, characterized in that, Steel strands are used as prestressing tendons in prestressed diaphragm walls.
5. The foundation pit construction method using prestressed diaphragm wall as a foundation pit partition wall according to claim 1, characterized in that, Inclinometers are installed in the foundation pit partition wall to obtain the wall deformation.
6. The foundation pit construction method using prestressed diaphragm wall as a foundation pit partition wall according to claim 1, characterized in that, The pre-embedded ducts of the post-tensioned prestressed diaphragm wall are located at the top of the capping beam of the wall. A steel pad is installed at the top opening and anchored to the reinforcing steel skeleton of the capping beam. The duct is reinforced with spiral stirrups at the opening and the opening is temporarily sealed with a plastic plug. The bottom of the duct is located on the wall surface above the bottom of the first-phase foundation pit. A steel pad is installed at the bottom of the duct and anchored to the wall reinforcement skeleton. The duct is reinforced with spiral stirrups at the duct opening and the duct opening is temporarily sealed with a plastic plug. No prestressing tendons are pre-embedded in the duct.