A deep foundation pit inverted shaft wall excavation construction method under eccentric compression working condition

Abstract

The application discloses a deep foundation pit inverted shaft wall excavation construction method under eccentric compression working condition, and is suitable for an asymmetric load scene in which the earth covering height of one side of a shaft is significantly greater than that of the other side. The method first carries out grouting on the outer periphery of the shaft wall below the present ground surface to form a soil reinforcement area; then a lock mouth ring beam is constructed at the shaft mouth, and an upper structure including a retaining wall, a counter pressure plate, a retaining wall ring beam and a plate brace is constructed upwardly on the ring beam; during the construction process, the earth covering side is simultaneously backfilled to the grading ground surface elevation; then the inverted shaft wall method is adopted to excavate downward in sections, after each excavation section, a circumferential arch frame, an angle brace and an anchor rod support are constructed, and upper and lower ring beams are arranged at the position of a horse head door and are connected with adjacent arch frames through vertical supports; finally, a concrete bottom sealing is constructed at the shaft completion surface. The application forms an anti-overturning moment through the counter pressure plate and the synchronous backfilling, and combines a multi-stage support system, so that the shaft wall instability problem caused by eccentric load is effectively solved.

Description

Technical Field

[0001] This application relates to the field of underground space construction technology, specifically to a method for excavating deep foundation pits with inverted well walls under eccentric pressure conditions. Background Technology

[0002] Urban underground space development can effectively alleviate the problem of land scarcity during urban development. The inverted shaft wall method is one of the construction methods for urban underground engineering, suitable for deep foundation pit construction scenarios with small pit areas and limited construction space. When using the inverted shaft wall method, excavation is carried out in sections from top to bottom, with each section forming a support structure.

[0003] The inverted shaft construction method is commonly used in underground engineering projects such as subway entrances and tunnel shafts. When constructing in confined urban spaces, asymmetrical backfill conditions are frequently encountered. For example, when one side of the shaft is adjacent to an existing roadbed, existing underground structure, or fill area, there is a significant difference in the backfill height on both sides of the shaft wall. This results in a significantly greater active earth pressure on the side with higher backfill than on the other side, causing the shaft wall structure to bear a larger additional bending moment and shear force, leading to stability risks. Eccentric loads may induce the shaft wall to translate or rotate towards the lower pressure side. In soft soil strata, when the bottom soil resistance is insufficient, it may even cause the shaft to tilt, the support structure to crack, and ultimately, local instability.

[0004] The portal frame is the interface between the tunnel or passageway and the main tunnel, and it is a weak point in terms of stress. Under eccentric loads, the stress concentration effect in the portal frame area is particularly prominent. Traditional construction methods often involve temporarily adding steel supports or grouting reinforcement before breaking down the portal frame, but as a passive remedial structure, it cannot work in tandem with the main support structure to form an integrated load-bearing system, and therefore still carries the risk of large deformation when the portal frame is broken down. Summary of the Invention

[0005] This application provides a construction method for excavating a deep foundation pit with an inverted well wall under eccentric compression conditions. This method solves the problem of local instability that may occur due to excessive pressure on the overburden side during the excavation of a deep foundation pit under eccentric compression conditions, as well as the problem that the reinforcement structure of the weak part cannot cooperate with the main support structure to bear the force.

[0006] The technical solution of this application is as follows:

[0007] A method for excavating a deep foundation pit with an inverted well wall under eccentric compression conditions includes the following steps:

[0008] Step 1: Grouting is carried out on the outer perimeter of the shaft wall below the existing ground elevation to form a soil reinforcement zone;

[0009] Step 2: Conduct partial excavation on the soil-covered side of the shaft opening, and construct a lock ring beam at the existing ground level of the shaft opening. The lock ring beam is reserved with upper structural reinforcement bars above and circumferential arch frame connecting bars below.

[0010] Step 3: Construct the superstructure upwards at the lock ring beam on the soil-covered side. The superstructure includes a retaining wall constructed based on the superstructure's reinforcing bars, a counter-pressure plate extending towards the soil-covered side, a retaining wall ring beam, and its matching plate supports. After the superstructure construction is completed, backfill the soil on the soil-covered side to the leveled ground elevation.

[0011] Step 4: Excavate in sections from the current ground elevation to the finished surface of the shaft. After each section is excavated, construct a circumferential arch frame on the inner wall of the shaft and install corner braces and anchor bolts for support. During the excavation, construct an upper ring beam and a lower ring beam at the top and bottom of the opening at the horse-head gate, respectively, and connect the upper ring beam and the lower ring beam to their adjacent circumferential arch frames through vertical supports. Connect the remaining circumferential arch frames with vertical supports in series.

[0012] Step 5: Concrete is applied to seal the bottom of the shaft from the top.

[0013] Furthermore, the soil reinforcement zone extends below the finished surface of the shaft.

[0014] Furthermore, in step three, the retaining wall is divided into two sections. The first section of the retaining wall extends from the lock ring beam to the bottom elevation of the counter-pressure plate, and the second section of the retaining wall connects to the first section of the retaining wall and is constructed to the level ground elevation.

[0015] Furthermore, in step three, the superstructure is constructed in three steps;

[0016] Step 1: Construct the first section of the retaining wall. After construction is completed, backfill the earth to the bottom elevation of the counterweight plate.

[0017] Step 2: Construct the counter-pressure plate and the first retaining wall ring beam. The first retaining wall ring beam is located above the counter-pressure plate, connecting the two sections of the retaining wall.

[0018] Step 3: Construct the second retaining wall section upwards from the first retaining wall ring beam, construct the second retaining wall ring beam at the top of the second retaining wall section, and backfill the soil to the level ground elevation after construction is completed.

[0019] Furthermore, in step three, the counterweight plate extends to the edge of the soil reinforcement zone at one end towards the overburden side, and a haunch is provided at the connection between the counterweight plate and the first retaining wall.

[0020] Furthermore, in step four, the upper and lower ring beams are connected to the circumferential arch frame on the same side as the horse-head gate through vertical supports.

[0021] Furthermore, at the end of each excavation section, a concrete spraying surface is applied to the shaft wall, which covers the vertical supports and circumferential arches.

[0022] Furthermore, in the fifth step, the height of the concrete seal is lower than the top elevation of the lower ring beam.

[0023] Due to the adoption of the above technical solution, the beneficial effects of this application are as follows:

[0024] 1. In step one, this application grouts the outer perimeter of the well wall to improve the properties of the soil around the shaft by forming a soil reinforcement zone, thereby enhancing the uniformity, cohesion and strength of the surrounding soil. At the same time, by reducing the permeability coefficient, the lateral pressure of the soil during the downward excavation stage of the shaft is reduced.

[0025] 2. This application creatively introduces a counterweight plate into the deep foundation pit inverted well wall construction system, specifically designed to address eccentric load conditions caused by asymmetric overburden height. Specifically, the high-side overburden exerts significant active earth pressure on the well wall, generating an overturning moment pointing towards the free side, which is the root cause of well wall cracking, convergence, and even local instability. To address this problem, this application embeds a horizontal counterweight plate in the lower part of the retaining wall, forming a lever-type anti-overturning structure with the retaining wall and the locking ring beam as the fulcrum. After construction, backfilling and compaction are performed simultaneously above the counterweight plate, allowing it to bear the vertical overburden load. This overburden load is converted into a reverse anti-overturning moment acting on the base of the retaining wall through the cantilever section of the counterweight plate, which is opposite in direction to and cancels out the clockwise overturning moment generated by the high-side earth pressure. Simultaneously, the bottom surface of the counterweight plate contacts the backfill soil, forming a passive anti-sliding constraint, further suppressing overall horizontal displacement. In addition, a concrete haunch is set at the junction of the counter-pressure plate and the retaining wall to effectively alleviate stress concentration at the joint, significantly improve the bending stiffness and ductility of this key connection part, and ensure the structural integrity of the counter-pressure system under long-term eccentric loading.

[0026] 3. Addressing the high-risk process of breaching the tunnel entrance, this application proposes a multi-level, spatially coordinated, locally reinforced support system to fundamentally ensure the structural stability of the entrance area. Specifically, reinforced concrete upper and lower ring beams are installed at the top and bottom of the tunnel entrance, respectively. These beams are rigidly connected to the circumferential steel arch frame of the shaft wall, forming a closed "portal frame" load-bearing ring. The circumferential arch frames are longitudinally connected via vertical steel supports, integrating the originally discrete support units into a truss-frame composite structure with overall spatial stiffness. Based on this, a shotcrete surface layer is applied across the entire cross-section, providing not only surface protection but also, through its combination with the steel frame, forming a composite support shell with shear, bending, and local spalling resistance. This multi-redundant reinforcement effectively disperses the stress redistribution during tunnel entrance breaching, significantly improving the load-bearing robustness and deformation control capacity of key nodes under dynamic excavation disturbances. Attached Figure Description

[0027] The accompanying drawings, which are provided to further illustrate this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application.

[0028] Figure 1 This is a plan view of the soil reinforcement around the shaft lock ring beam in an embodiment of this application;

[0029] Figure 2 This is a plan view of the upper structure of an embodiment of this application;

[0030] Figure 3 This is a plan view of the internal layout of the shaft according to an embodiment of this application;

[0031] Figure 4 This is a cross-sectional view of the shaft support structure in an embodiment of this application.

[0032] In the attached diagram:

[0033] 1. Soil reinforcement zone; 2. Lock ring beam; 3. Retaining wall; 4. Retaining wall ring beam; 5. Plate bracing; 6. Counterweight plate; 7. Circumferential arch frame; 8. Corner bracing; 9. Anchor bolt support; 10. Shotcrete surface; 11. Ring beam; 12. Vertical support; 13. Concrete bottom sealing; 14. Armhole corner. Detailed Implementation

[0034] As described in the background art, urban underground spaces, such as subway entrances and tunnel shafts, often encounter asymmetrical backfill conditions. For example, when one side of a shaft is adjacent to an existing roadbed or a constructed underground structure or fill area, there is a significant difference in the backfill height on both sides of the shaft wall. The side with higher backfill is called the covered side, and the side without backfill is called the free side. Shaft construction may involve eccentric conditions in multiple directions. For shafts with covered sides, the deep foundation pit inverted shaft wall excavation construction method under eccentric compression conditions provided in this application can be adopted, including the following steps:

[0035] Step 1: Grouting is carried out on the outer perimeter of the shaft wall below the existing ground elevation to form soil reinforcement zone 1.

[0036] As attached Figure 4 As shown, the current ground elevation refers to the ground elevation on the free side, which is the top of the shaft. The soil-covered side needs to be leveled during construction, and the height of the top after leveling is recorded as the leveling point elevation. In step one, ground grouting is used to grout the outer perimeter of the shaft wall. In specific implementation, the soil reinforcement zone 1 extends 1m below the finished surface of the shaft. Cement grout is used as the grout, which improves the properties of the soil around the shaft through the formed soil reinforcement zone 1, enhancing the uniformity, cohesion, and strength of the surrounding soil. Simultaneously, it reduces the lateral soil pressure during the downward excavation stage of the shaft by lowering the permeability coefficient. During grouting, grouting is carried out within a 5m radius around the shaft wall below the current ground level.

[0037] Step Two: Partial excavation is carried out on the soil-covered side of the shaft opening. Partial excavation refers to excavating a site for the construction of the superstructure. A locking ring beam 2 is constructed at the existing ground level above the shaft opening. The locking ring beam 2 has pre-installed reinforcing bars for the superstructure above and pre-installed connecting bars for the circumferential arch frame 7 below. (See attached...) Figure 1 and attached Figure 4 As shown, the lock ring beam 2 surrounds the top of the shaft, and at the same time, the upper structure spur bars and arch frame connecting bars are reserved, so that the lock ring beam 2 and the retaining wall 3 and circumferential arch frame 7 constructed in the subsequent construction form an integral structure, which, together with the counter pressure plate 6, resists the soil pressure.

[0038] Step 3: Construct the superstructure upwards at the lock ring beam 2 on the soil-covered side. The superstructure includes a retaining wall 3 constructed based on the superstructure's reinforcing bars, a counter-pressure plate 6 extending towards the soil-covered side, a retaining wall ring beam 4, and its matching plate bracing 5. After the superstructure construction is completed, backfill the soil on the soil-covered side to the leveled ground elevation.

[0039] As attached Figure 4 As shown, in step three, the retaining wall 3 is divided into two sections. The first section of the retaining wall 3 extends from the lock ring beam 2 to the bottom elevation of the counter-pressure plate 6. The second section of the retaining wall 3 connects to the first section of the retaining wall 3 and is constructed to the level ground elevation.

[0040] The superstructure was constructed in three steps:

[0041] Step 1: Construct the first section of retaining wall 3. After construction is completed, backfill the soil to the bottom elevation of the counterweight plate 6.

[0042] Step 2: Construct the counterweight plate 6 and the first retaining wall ring beam 4 (the middle retaining wall ring beam). The first retaining wall ring beam 4 is located at a height above the counterweight plate 6, connecting the two sections of retaining wall 3. The counterweight plate 6 extends to the edge of the soil reinforcement zone 1 at one end towards the soil cover side, and a haunch angle 14 is set at the connection between the counterweight plate 6 and the first section of retaining wall 3.

[0043] Step 3: Construct the second retaining wall 3 upwards from the first retaining wall ring beam 4, and construct the second retaining wall ring beam 4 at the top of the second retaining wall 3. After the construction is completed, backfill the soil to the level ground elevation.

[0044] Step 4: Excavate in sections from the existing ground elevation to the finished surface of the shaft. After each section is excavated, construct a circumferential arch frame 7 on the inner wall of the shaft and install corner braces 8 and anchor bolts 9. During the excavation, construct an upper ring beam 11 and a lower ring beam 11 at the top and bottom of the opening at the horse head gate, respectively. Connect the upper ring beam 11 and the lower ring beam 11 to their adjacent circumferential arch frames 7 through vertical supports 12. Connect the remaining circumferential arch frames 7 with vertical supports 12 in series.

[0045] As attached Figure 2 ~Appendix Figure 4As shown, in specific implementation, the excavation advance of the shaft is determined and cyclical excavation is carried out until the shaft is completed. During excavation, mechanical excavation is first used to excavate the core soil in the middle of the shaft. After the core soil within the corresponding advance range is excavated, manual excavation is used to excavate the area supported by the annular arch frame. Then, the circumferential arch frame 7, corner braces 8, and anchor bolt support 9 are constructed. In particular, at the opening of the shaft's gable, an upper ring beam 11 and a lower ring beam 11 need to be constructed. After the ring beam 11 is constructed, it is connected to the circumferential arch frame 7 through vertical supports 12. Finally, the sprayed concrete surface 10 forms a protective surface, and the next section of excavation is carried out. The next section of excavation repeats the above steps. When the lower ring beam 11 needs to be constructed, it is connected to the lowest circumferential arch frame 7 through vertical supports 12. It should be noted that vertical supports 12 are only constructed on both sides of the gable to ensure the safe demolition of the opening.

[0046] Step 5: Concrete seal 13 is applied to the shaft facing upwards. In this step, the height of concrete seal 13 is lower than the top elevation of the lower ring beam 11. Concrete seal 13 forms a permanent concrete structural system, after which the horse-head gate demolition process is carried out.

[0047] For any parts not mentioned in this application, existing technologies may be used or referenced.

[0048] The above description is merely an embodiment of this application and is not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A method for excavating a deep foundation pit with an inverted well wall under eccentric compression conditions, characterized in that, Includes the following steps: Step 1: Grouting is carried out on the outer perimeter of the shaft wall below the existing ground elevation to form a soil reinforcement zone, which extends below the finished surface of the shaft; Step 2: Conduct partial excavation on the soil-covered side of the shaft opening, and construct a lock ring beam at the existing ground level of the shaft opening. The lock ring beam is reserved with upper structural reinforcement bars above and circumferential arch frame connecting bars below. Step 3: Construct the superstructure upwards at the lock ring beam on the soil-covered side. The superstructure includes a retaining wall constructed based on the superstructure's reinforcing bars, a counter-pressure plate extending towards the soil-covered side, a retaining wall ring beam, and its matching plate supports. After the superstructure construction is completed, backfill the soil along the sides to the level ground elevation; The retaining wall is divided into two sections. The first section extends from the lock ring beam to the bottom elevation of the counter-pressure plate. The second section connects to the first section and is constructed to the level ground elevation. The superstructure was constructed in three steps; Step 1: Construct the first section of the retaining wall. After construction is completed, backfill the earth to the bottom elevation of the counterweight plate. Step 2: Construct the counter-pressure plate and the first retaining wall ring beam. The first retaining wall ring beam is located above the counter-pressure plate, connecting the two sections of the retaining wall. Step 3: Construct the second retaining wall section upwards from the first retaining wall ring beam, construct the second retaining wall ring beam at the top of the second retaining wall section, and backfill the earth to the level ground elevation after construction is completed; Step 4: Excavate in sections from the current ground elevation to the finished surface of the shaft. After each section is excavated, construct a circumferential arch frame on the inner wall of the shaft and install corner braces and anchor bolts for support. During the excavation, construct an upper ring beam and a lower ring beam at the top and bottom of the opening at the horse-head gate, respectively, and connect the upper ring beam and the lower ring beam to their adjacent circumferential arch frames through vertical supports. Connect the remaining circumferential arch frames with vertical supports in series. Step 5: Concrete is applied to seal the bottom of the shaft from the top.

2. The method for excavating a deep foundation pit with an inverted well wall under eccentric compression conditions according to claim 1, characterized in that, In step three, the counterweight plate extends to the edge of the soil reinforcement zone at one end towards the overburden side, and a haunch is provided at the connection between the counterweight plate and the first retaining wall.

3. The method for excavating a deep foundation pit with an inverted well wall under eccentric compression conditions according to claim 2, characterized in that, In step four, the upper and lower ring beams are connected to the circumferential arch frame on the same side as the horse-head gate through vertical supports.

4. The method for excavating a deep foundation pit with an inverted well wall under eccentric compression conditions according to claim 3, characterized in that, At the end of each excavation section, a concrete spraying surface is applied to the shaft wall, which covers the vertical supports and circumferential arches.

5. The method for excavating a deep foundation pit with an inverted well wall under eccentric compression conditions according to claim 4, characterized in that, In the fifth step, the height of the concrete seal is lower than the top elevation of the lower ring beam.

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