A foundation pit excavation method for a composite support system

Abstract

The present application discloses a foundation pit excavation method for a composite support system, which includes the following steps: excavation block division; construction steps for the first side area, first excavating the area on the far side of the subway in the first side area and interspersing the completion of the concrete support operation, and then completing the excavation of the area on the side adjacent to the subway in the first side area and the concrete collar beam and concrete support operation within a limited time; construction steps for the second side area, after the earth excavation in the first side area is completed, carrying out the construction of the second side area; the first stage of the middle area construction, the middle area is excavated after the concrete support in the first side area is formed; during the construction of the middle area, first carry out the excavation operation in the area on the far side of the subway in the middle area, and pre-assemble the steel collar beam and steel support in advance for each area on the far side of the subway in the middle area; the second stage of the middle area construction, carry out the excavation of the area on the side adjacent to the subway in the middle area and the steel collar beam and steel support operation. The present application can effectively support the foundation pit during the construction process as soon as possible to meet the anti-deformation protection requirements of the subway adjacent to the foundation pit.

Description

Technical Field

[0001] This application relates to the technical field of construction engineering, and particularly to a method for excavating a foundation pit with a composite support system. Background Art

[0002] A foundation pit is an earth pit excavated at the design position of the foundation according to the base elevation and the foundation plan dimensions. Among them, a deep foundation pit refers to a project with a excavation depth exceeding 5 meters (including 5 meters), or a project with a depth less than 5 meters but with particularly complex geological conditions, surrounding environment and underground pipelines.

[0003] For deep foundation pit projects close to the subway, the risk and difficulty of foundation pit excavation are greater, and the social attention is higher. For this type of deep foundation pit project, the excavation method corresponding to the foundation pit support layout system is crucial for the anti-deformation protection of the foundation pit and the anti-deformation protection of the subway adjacent to the foundation pit. In the prior art, the method of excavating from one side to the other side or from the middle to both sides is often used. During the foundation pit excavation, it is not conducive to effectively supporting the foundation pit as soon as possible, and it is difficult to meet the anti-deformation protection requirements of the subway adjacent to the foundation pit. Summary of the Invention

[0004] In order to solve the problem that it is not easy to form an effective support for the foundation pit during the construction of deep foundation pit projects close to the subway, this application provides a method for excavating a foundation pit with a composite support system.

[0005] A method for excavating a foundation pit with a composite support system provided by this application adopts the following technical solution:

[0006] A method for excavating a foundation pit with a composite support system includes the following steps:

[0007] Excavation block division: Define the side of the foundation pit close to the subway as the side adjacent to the subway, and the side of the foundation pit far from the subway as the side far from the subway. Define the side of the foundation pit along the side adjacent to the subway as the long side. Divide the excavation area of the foundation pit into a middle area and side areas on both sides of the middle area. Define one of the two side areas as the first side area and the other side area as the second side area. In the case where the excavation amounts of the two side areas are inconsistent, the side area with less excavation amount is defined as the first side area. The support structure in the middle area is a steel structure, and the support structures in the two side areas are concrete structures. The middle area and the two side areas are arranged in sequence along the long side of the foundation pit. The dimension of the middle area along the long side of the foundation pit is greater than the dimensions of the two side areas along the length direction of the foundation pit;

[0008] Construction steps of the first side area: First, excavate the area on the side far from the subway in the first side area, and complete the concrete support operation in an interspersed manner. Then, complete the excavation of the area on the side adjacent to the subway in the first side area and the concrete collar beam and concrete support operations within a limited time;

[0009] Construction steps of the second side area: After the earth excavation in the first side area is completed, carry out the construction of the second side area;

[0010] During the first construction stage of the middle area, excavation of the middle area is carried out after the concrete support in the first side area is formed; during the construction of the middle area, excavation operations in the area of the middle area far from the subway side are carried out first, and pre-assembly operations of the steel waling and steel supports in the area of the middle area far from the subway side are carried out in advance; the construction in the area of the middle area far from the subway side is carried out synchronously with part of the second side area during some periods.

[0011] During the second construction stage of the middle area, after the excavation and steel support operations in the area of the middle area far from the subway side are completed, excavation and steel waling and steel support operations in the area of the middle area adjacent to the subway side are carried out.

[0012] Repeat other operation processes except the above excavation block division. The above construction steps complete the construction operations of one layer, forming an overall planar composite support system. After the strength of the concrete support reaches the design requirements, the above operations are repeated layer by layer from top to bottom until the bottom layer of the foundation pit.

[0013] By adopting the above technical solutions, the two side areas and the middle area are generally constructed in sequence and are constructed in an interleaved manner during some periods. The steel supports can be installed and completed during the period when the concrete support is cured to reach the strength, so as to effectively support the foundation pit as soon as possible during the foundation pit excavation process, and can speed up the earthwork excavation and support construction processes of each layer, ensure that the support reaches the design strength requirements before the next layer of earthwork excavation, and reduce the overlapping time of earthwork excavation of each layer; for the foundation pit project with limited construction site, the completion of the concrete support operation on one side and the handover to the steel support operation are beneficial to reducing the influence of the overlapping construction of different types of work on the site occupation.

[0014] Optionally, in the construction steps of the second side area, before the concrete support operation in the first side area, a marking line for formwork support is first marked on the bottom of the second side area by using a marking auxiliary tool. The marking line extends along the long side direction of the foundation pit. The marking auxiliary tool includes a marking silk thread. When marking, the marking silk thread is stretched across the foundation pit along the long side direction of the foundation pit and straightened, so that the shadow line cast by the marking silk thread under sunlight or artificial light coincides with the lower edge of the concrete support in the first side area.

[0015] By adopting the above technical solutions, the straightened marking silk thread can project a straight shadow line under sunlight or artificial light, and the shadow of the marking silk thread is used instead of the marking silk thread itself as the marking reference. Even if the middle area in the foundation pit has not been excavated, a marking line aligned with the concrete support in the first side area can be marked on the bottom of the second side area; thus, it is beneficial to make the concrete supports formed in the construction of the two side areas along the long side direction of the foundation pit keep coaxial alignment as much as possible.

[0016] Optionally, the marking silk thread includes multiple parallel nylon monofilaments arranged side by side.

[0017] By adopting the above technical solution, the marking thread is formed by combining multiple nylon monofilaments arranged side by side, so that the marking thread can project a thicker shadow line under sunlight or artificial light, which is convenient for visual identification. The combination state of multiple nylon monofilaments arranged side by side is relatively loose, with a small mass, and it is not easy to sag under the action of gravity.

[0018] Optionally, the marking auxiliary tool also includes two pulling piles for pulling the marking silk thread, the two pulling piles correspond to the two ends of the marking silk thread respectively, the pulling piles are provided with threading holes for the marking silk thread to pass through, the two ends of the marking silk thread pass through the corresponding threading holes respectively, and are connected to heavy objects.

[0019] By adopting the above technical scheme, the two ends of the marking silk thread are pulled by the pulling piles inserted into the ground, which is beneficial to keep the marking silk thread stable during use. The two ends of the marking silk thread are straightened by the gravity of two weights. When the position of the marking silk thread is located, the staff who pulls the marking silk thread can leave the pulling piles, which is beneficial to reduce the waste of human resources.

[0020] Optionally, a limiting piece is provided on one end of the outer circumference of the wire pulling pile away from the wire threading hole, and the limiting piece is used to abut against the ground.

[0021] By adopting the above technical solution, when the distance between the threading hole and the limiting member is constant, after the wire pile is inserted into the ground, under the limiting action of the limiting member, the distance between the two ends of the marking wire and the ground is relatively close, so as to facilitate the control of the parallelism between the marking wire and the long side of the foundation pit.

[0022] Optionally, when constructing the second layer and below of steel supports and concrete supports, multiple plumb lines attached to the sides of the upper layer of steel supports or concrete supports are used as a reference to control the parallelism of the second layer and below of steel supports and concrete supports with the upper layer of steel supports or concrete supports.

[0023] By adopting the above technical solution, multiple plumb lines attached to the same steel support or concrete support are generally located in the same plane. Therefore, by controlling the distance between the lower steel support or concrete support and each plumb line, the parallelism between the lower steel support or concrete support and the corresponding steel support or concrete support of the upper layer can be controlled.

[0024] Optionally, the plumb line includes a wire body and plumb blocks located at two ends of the wire body, and the two ends of the plumb line span the upper surface of the corresponding steel support or concrete support.

[0025] By adopting the above technical solution, both ends of the silk thread body are respectively suspended by corresponding plumb blocks. By making the two plumb blocks pass from top to bottom through the steel support or concrete support to be installed in the lower layer, when there is a horizontal position deviation between the steel support or concrete support in the lower layer and the corresponding steel support or concrete support in the upper layer during the installation of the steel support or concrete support in the lower layer, a gap appears between one side of the steel support or concrete support in the lower layer and one end of the silk thread body, while the other side forces the other end of the silk thread body to displace.

[0026] Optionally, a rubber damping strip is fixedly arranged at the middle part of the silk thread body. The length direction of the rubber damping strip is arranged along the length direction of the silk thread body. The surface of the rubber damping strip is provided with anti-slip convex lines, and the texture direction of the anti-slip convex lines is perpendicular to the length direction of the rubber damping strip.

[0027] By adopting the above technical solution, the silk thread body contacts the surface of the steel support or concrete support through the rubber damping strip, making it difficult for the silk thread body to slide relative to the steel support or concrete support where it is located, which is beneficial to keeping the position of the silk thread body stable.

[0028] Optionally, the steel support includes a transverse steel support and a longitudinal steel support. The length direction of the longitudinal steel support is perpendicular to the extension direction of the long side of the foundation pit. The end face of the longitudinal steel support close to the side area abuts against the end of the steel waling and the end of the concrete waling at the same time.

[0029] By adopting the above technical solution, using the longitudinal steel support to abut against both ends of the concrete waling is beneficial to enabling the steel support to form a connection with the concrete support waling system in advance, enabling the integrity of the waling system to be formed in advance, and enabling the steel waling to effectively transfer a part of the shear force of the support, achieving the effect of limiting and anti-sliding.

[0030] Optionally, during the installation process of the steel support in the first construction stage of the middle area and the second construction stage of the middle area, the shadow cast by the marking silk thread on the surface of the transverse steel support is used as a reference, so that the length direction of the transverse steel support is kept parallel to the long side of the foundation pit.

[0031] By adopting the above technical solution, both ends of the steel support located in the middle area need to be coaxially connected with the corresponding coaxial concrete support located in the two side areas during installation. Using the shadow of the marking silk thread as a reference during the installation process of the steel support is convenient for controlling the coaxiality of the steel support and the corresponding concrete support on both sides.

[0032] In summary, the present application includes at least one of the following beneficial technical effects:

[0033] 1. The two side areas and the middle area are generally constructed in sequence, and the construction is carried out alternately during some periods, so that the steel supports can be installed during the period when the concrete supports are cured to reach the strength, so as to effectively support the foundation pit during the foundation pit excavation as soon as possible; for the foundation pit project with limited construction site, after the concrete support operation on one side is completed and handed over to the steel support operation, it is beneficial to reduce the influence of the overlapping construction of different types of work on the site occupation.

[0034] 2. Using the longitudinal steel supports to abut against both ends of the concrete collar beam is beneficial to enable the steel supports to form a connection with the concrete support collar beam system in advance, so as to form the integrity of the collar beam system in advance, and enable the steel collar beam to effectively transfer a part of the shear force of the support, achieving the function of limiting anti-slip. Description of the Drawings

[0035] Figure 1 It is a schematic diagram of the steps of the foundation pit excavation method of the composite support system in Embodiment 1.

[0036] Figure 2 It is a schematic diagram of the zoning of the foundation pit in Embodiment 1.

[0037] Figure 3 is Figure 2 The partial enlarged view at A in

[0038] Figure 4 It is a schematic diagram of Embodiment 2 for showing the use state of the scribing structure.

[0039] Figure 5 is Figure 4 The partial enlarged view at B in

[0040] Figure 6 The schematic diagram of Embodiment 2 for showing the use state of the plumb line.

[0041] Description of the Reference Numerals:

[0042] 1. Middle area; 2. Side area; 21. First side area; 22. Second side area; 4. Concrete collar beam; 41. Steel plate; 5. Concrete support; 51. Transverse concrete support; 52. Longitudinal concrete support; 6. Steel collar beam; 7. Steel support; 71. Transverse steel support; 72. Longitudinal steel support; 8. Scribing auxiliary tool; 81. Marking silk thread; 82. Pulling pile; 821. Thread-passing hole; 83. Heavy object; 84. Limiting part; 85. Adjusting part; 851. Cross bar; 852. First longitudinal bar; 853. Second longitudinal bar; 854. Nut; 9. Plumb line; 91. Silk thread main body; 92. Plumb bob; 93. Rubber damping strip. Detailed Embodiment

[0043] The following is a further detailed description of the present application in conjunction with the attached Figure 1-6 drawings.

[0044] Embodiment 1

[0045] The present application embodiment discloses a composite support system foundation pit excavation method. Figure 1 and Figure 2 The composite support system foundation pit excavation method includes the following steps:

[0046] Step 1: Divide the excavation blocks. Define the side of the foundation pit close to the subway as the subway-adjacent side, the side of the foundation pit far from the subway as the far subway side, and define the side of the foundation pit along the subway-adjacent side as the long side. Divide the excavation area of ​​the foundation pit into a middle zone 1 and side zones 2 located on both sides of the middle zone 1. The supporting structure of the middle zone 1 is a steel structure, and the supporting structures of the two side zones 2 are concrete structures. One of the two side zones 2 is defined as a first side zone 21, and the other side zone 2 is defined as a second side zone 22. When the excavation volumes of the two side zones 2 are inconsistent, the side zone 2 with less excavation volume is defined as the first side zone 21. The side middle zone 1 and the two side zones 2 are arranged in sequence along the long side direction of the foundation pit. The size of the middle zone 1 along the long side direction of the foundation pit is larger than the size of the two side zones 2 along the length direction of the foundation pit. The middle zone 1 is divided along the long side direction of the foundation pit to form multiple sub-divisions.

[0047] Step 2: The first side zone 21 construction step is to first excavate the area far from the subway of the first side zone 21, and promptly complete the concrete perimeter purlin 4 and concrete support 5 operations in the area far from the subway of the first side zone 21, and then complete the excavation and concrete perimeter purlin 4 and concrete support 5 operations in the area adjacent to the subway of the same side zone 2 within a limited time; in addition, in this step, if the excavation volume of the two side zones 2 is the same, one of the side zones 2 can be selected for first excavation;

[0048] Step 3: Construction step of the second side area 22, after the excavation of the first side area 21 in step 2 is completed, the construction of the second side area 22 with a larger excavation volume is carried out;

[0049] Step 4: In the first construction phase of the intermediate zone 1, after the concrete support 5 of the first side zone 21 in step 2 is formed, the excavation is started, and the site is handed over to the steel support unit; during the construction of the intermediate zone 1, the excavation work of the area far from the subway side of each sub-division is carried out first, and the steel purlin 6 and steel support 7 of the area far from the subway side of each sub-division are pre-assembled in advance; the construction of the far from subway side of each sub-division of the intermediate zone 1 is carried out synchronously with the second side zone 22 in part of the period;

[0050] Step 5: Construction phase 2 of the middle zone 1: after the excavation and steel support 7 operations of the far subway side areas of each sub-division of the middle zone 1 in step 4 are completed, the excavation and steel purlin 6 and steel support 7 operations of the subway side areas of each sub-division are carried out;

[0051] Step 6: Repeat the operations in Steps 2 - 5. The above construction steps complete the construction of one layer, forming an overall planar composite support system. After the strength of the concrete support 5 reaches the design requirements, repeat the above operations layer by layer from top to bottom until the bottom layer of the foundation pit.

[0052] In this embodiment, the dimension of the foundation pit along the long side direction is set as the length dimension, and the dimension of the foundation pit perpendicular to the long side direction is set as the width dimension. During the process of the first side area 21 in Step 2, when excavating first, the excavation dimension of the first side area 21 along the width direction of the foundation pit is equal to the width dimension of the foundation pit minus four times the layered excavation depth.

[0053] Refer to Figure 2 and Figure 3 and, the steel collar 6 and the concrete collar 4 on the same layer jointly form an annular structure, and during the construction process of the concrete collar 4, steel plates 41 for welding the steel collar 6 are pre - set at the ends, making the connection between the concrete collar 4 and the steel collar 6 stable.

[0054] Refer to Figure 2 and, the steel support 7 includes a transverse steel support 71 and a longitudinal steel support 72, the concrete support 5 includes a transverse concrete support 51 and a longitudinal concrete support 52. The length directions of both the transverse steel support 71 and the transverse concrete support 51 are parallel to the extension direction of the long side of the foundation pit, and the length directions of both the longitudinal steel support 72 and the longitudinal concrete support 52 are perpendicular to the extension direction of the long side of the foundation pit. The transverse steel support 71 in the middle area 1 is connected to the transverse concrete support 51 in each side area 2 one - to - one and is coaxially arranged.

[0055] The implementation principle of the foundation pit excavation method of the composite support system in this application embodiment is as follows: The two side areas 2 and the middle area 1 are generally constructed in sequence and are constructed in an interspersed manner during some periods. It is possible to install the steel support 7 during the period when the concrete support 5 is cured to reach the strength, ensuring that the support reaches the design strength requirements before the next layer of earth excavation, and reducing the overlapping time of each layer of earth excavation; for foundation pit projects with limited construction sites, after the operation of one side's concrete support 5 is completed and transferred to the steel support 7 operation, it is beneficial to reduce the influence of different types of work interspersed construction on the occupied site.

[0056] Embodiment 2

[0057] The difference between this embodiment and Embodiment 1 is that in Step 3, before the concrete support 5 operation in the side area 2 with more excavation volume, first use the marking - line auxiliary tool 8 to mark the bottom of the side area 2 with more excavation volume for formwork support. The marking line extends along the long side direction of the foundation pit. And this marking line and the side surface of a concrete support 5 in the side area 2 with less excavation volume are located in the same vertical plane.

[0058] Refer to Figure 3 and Figure 4The marking auxiliary tool 8 includes a marking silk thread 81 and two pulling wire piles 82. The marking silk thread 81 includes multiple nylon monofilament lines parallel to each other. The two pulling wire piles 82 correspond to the two ends of the marking silk thread 81 respectively. The pulling wire piles 82 are provided with threading holes 821 for the marking silk thread 81 to pass through. After the two ends of the marking silk thread 81 pass through the corresponding pulling wire piles 82 respectively, they are tied and fixed with heavy objects 83.

[0059] When marking, the marking wire 81 is stretched across the foundation pit along the long side of the foundation pit, and two wire piles 82 are inserted into the ground for positioning. The marking wire 81 is stretched straight under the gravity of two weights 83, so that the shadow line cast by the marking wire 81 in sunlight or artificial light coincides with the lower edge of the concrete support 5 in the side area 2 with less excavation. Then, lime, ink or other materials are used to make a marking line along the marking wire 81 at the bottom of the pit of the side area 2 with less excavation, so as to facilitate the support of the template for the construction of the horizontal concrete support 51, so that the horizontal concrete supports 51 of the two side areas 2 can be kept coaxial as much as possible.

[0060] The installation process of the steel support 7 in step 4 and step 5 uses the shadow cast by the marking thread 81 on the surface of the transverse steel support 71 as a reference, so that the length direction of the transverse steel support 71 remains parallel to the extension direction of the long side of the foundation pit.

[0061] Reference Figure 3 A stopper 84 is provided at one end of the outer circumference of the wire pile 82 away from the threading hole 821. The stopper 84 is a ring plate. The stopper 84 is fixedly connected to the pull pile sleeve. The stopper 84 is used to abut the ground to limit the insertion depth of the wire pile 82, thereby controlling the pulling height of the marking wire 81. After the stopper 84 abuts the ground, it can also be pressed by heavy objects such as stones and iron blocks.

[0062] Reference Figure 4 An adjusting piece 85 is provided at one end of the wire pulling pile 82 near the threading hole 821, and the adjusting piece 85 includes a cross bar 851, one end of the cross bar 851 is provided with a first longitudinal bar 852, and the other end of the cross bar 851 is provided with a second longitudinal bar 853. The first longitudinal bar 852 is coaxially rotatably connected to the wire pulling pile 82, and the first longitudinal bar 852 is threadedly connected with a nut 854 for abutting the end face of the wire pulling pile 82, and the second longitudinal bar 853 is used to move the marking wire 81.

[0063] The adjusting member 85 can rotate relative to the pull-wire pile 82 via the first longitudinal rod 852, so that the second longitudinal rod 853 can move the marking wire 81 to adjust the position of the marking wire 81, thereby adjusting the position of the shadow of the marking wire 81. When the position of the marking wire 81 is adjusted, the adjusting member 85 can be positioned by the locking nut 854.

[0064] Step 6: When constructing the second layer and below of steel supports 7 and concrete supports 5, use multiple plumb lines 9 attached to the side of the upper layer of steel supports 7 or concrete supports 5 as a reference. The plumb lines 9 attached to the same steel support 7 or concrete support 5 are parallel to each other, which can be used as a reference for controlling the parallelism of the second layer and below of steel supports 7 and concrete supports 5 with the upper layer of steel supports 7 or concrete supports 5.

[0065] Reference Figure 6 The plumb line includes a main body 91 and plumb blocks 92 at both ends of the main body 91. The plumb blocks 92 are conical. The main body 91 is connected to the center of the large end of the plumb block 92. The two ends of the plumb line span the upper surface of the corresponding steel support 7 or concrete support 5. The two ends of the main body 91 of the wire fall down through the corresponding plumb blocks 92. When the steel support 7 or concrete support 5 of the lower layer is installed, there is a position deviation in the horizontal direction from the corresponding steel support 7 or concrete support 5 of the upper layer. There is a deviation in the spacing between the two sides of the steel support 7 or concrete support 5 of the lower layer and the corresponding plumb blocks 92 at both ends of the same main body 91 of the wire.

[0066] Reference Figure 6 A rubber damping strip 93 is fixedly provided in the middle part of the wire body 91. The rubber damping strip 93 is flat. The wire body 91 and the rubber damping strip 93 are bonded and fixed by glue or tape. The length direction of the rubber damping strip 93 is arranged along the length direction of the wire body 91. The surface of the rubber damping strip 93 is provided with anti-skid convex patterns (not shown in the figure). The texture direction of the anti-skid convex patterns is perpendicular to the length direction of the rubber damping strip 93. The rubber damping strip 93 replaces the wire body 91 to contact the surface of the steel support 7 or the concrete support 5. The friction resistance between the rubber damping strip 93 and the steel support 7 or the concrete support 5 is relatively large, so that the relative position between the plumb line 9 and the steel support 7 or the concrete support 5 is relatively stable.

[0067] When the plumb line 9 is placed on the concrete support 5, the two ends of the rubber damping strip 93 extend to the two vertical sides of the concrete support 5. The two ends of the rubber damping strip 93 are respectively provided with raised portions 931. The raised portions 931 are used to force a spacing margin to be formed between the wire body 91 and the vertical side surfaces of the concrete support 5 to avoid the plumb block 92 and the template of the lower concrete support 5.

[0068] The above are all preferred embodiments of the present application, and the protection scope of the present application is not limited thereto. Therefore, any equivalent changes made according to the structure, shape, and principle of the present application should be included in the protection scope of the present application.

Claims

1. A foundation pit excavation method for a composite support system, characterized in that, It includes the following steps: Excavation block division: Define the side of the foundation pit close to the subway as the side adjacent to the subway, and the side of the foundation pit far from the subway as the side far from the subway. Define the side of the foundation pit along the side adjacent to the subway as the long side. Divide the excavation area of the foundation pit into the middle area (1) and the side areas (2) on both sides of the middle area (1). One of the two side areas (2) is defined as the first side area (21), and the other side area (2) is defined as the second side area (22). In the case where the excavation amounts of the two side areas (2) are inconsistent, the side area (2) with less excavation amount is defined as the first side area (21); the support structure of the middle area (1) is a steel structure, and the support structures of the two side areas (2) are concrete structures. The middle area (1) and the two side areas (2) are arranged in sequence along the long side direction of the foundation pit, and the dimension of the middle area (1) along the long side direction of the foundation pit is greater than the dimensions of the two side areas (2) along the length direction of the foundation pit; Construction steps of the first side area (21): First, excavate the area on the side far from the subway in the first side area (21), and intersperse and complete the operation of the concrete support (5). Then, complete the excavation of the area on the side adjacent to the subway in the first side area (21) and the operations of the concrete collar beam (4) and the concrete support (5) within a limited time; Construction steps of the second side area (22): After the earth excavation of the first side area (21) is completed, carry out the construction of the second side area (22); The first construction stage of the middle area (1): The construction of the middle area (1) is carried out after the concrete support (5) of the first side area (21) is formed; during the construction of the middle area (1), first carry out the excavation operation of the area on the side far from the subway in each middle area (1), and pre-assemble the steel collar beam (6) and the steel support (7) in the area on the side far from the subway in the middle area (1) in advance; the construction on the side far from the subway in the middle area (1) is synchronized with the second side area (22) during some periods; The second construction stage of the middle area (1): After the excavation and the operation of the steel support (7) in the area on the side far from the subway in the middle area (1) are completed, carry out the excavation of the area on the side adjacent to the subway in the middle area (1) and the operations of the steel collar beam (6) and the steel support (7); Repeat other operation processes except the above-mentioned excavation block division. The above construction steps complete the construction operation of one layer, forming an overall planar composite support system. After the strength of the concrete support (5) reaches the design requirements, repeat the above operations layer by layer from top to bottom until the bottom layer of the foundation pit.

2. The foundation pit excavation method of a composite support system according to claim 1, characterized in that: In the construction steps of the second side area (22), before the operation of the concrete support (5) in the first side area (21), first use the marking auxiliary tool (8) to draw marking lines for formwork support on the bottom of the second side area (22). The marking lines extend along the long side direction of the foundation pit. The marking auxiliary tool (8) includes a marking silk thread (81). When marking, the marking silk thread (81) is pulled along the long side direction of the foundation pit to span the foundation pit and be straightened, so that the shadow line cast by the marking silk thread (81) under sunlight or artificial light coincides with the lower edge of the concrete support (5) in the first side area (21).

3. The excavation method of a foundation pit with a composite support system according to claim 2, characterized in that: The marking silk thread (81) includes multiple parallel nylon single silk threads.

4. A method for excavating a foundation pit of a composite support system according to claim 2, characterized in that: The marking auxiliary tool (8) further comprises two pulling piles (82) for pulling the marking silk thread (81), the two pulling piles (82) respectively corresponding to the two ends of the marking silk thread (81), the pulling piles (82) are provided with threading holes (821) for the marking silk thread (81) to be threaded, the two ends of the marking silk thread (81) respectively pass through the corresponding threading holes (821), and are connected to weights (83).

5. A method for excavating a foundation pit of a composite support system according to claim 2, characterized in that: A stopper (84) is provided at one end of the outer peripheral surface of the wire pulling pile (82) away from the wire threading hole (821), and the stopper (84) is used to abut against the ground.

6. A method for excavating a foundation pit of a composite support system according to claim 1, characterized in that: When constructing the second layer and the following steel supports (7) and concrete supports (5), a plurality of plumb lines (9) attached to the side surfaces of the upper layer steel supports (7) or the side surfaces of the concrete supports (5) are used as a reference to control the parallelism between the second layer and the following steel supports (7) and the concrete supports (5) and the upper layer steel supports (7) or the concrete supports (5).

7. A method for excavating a foundation pit of a composite support system according to claim 6, characterized in that: The plumb line comprises a wire body (91) and plumb blocks (92) located at two ends of the wire body (91), and the two ends of the plumb line span the upper surface of the corresponding steel support (7) or concrete support (5).

8. A foundation pit excavation method for a composite support system according to claim 7, characterized in that: A rubber damping strip (93) is fixedly arranged in the middle part of the thread body (91), the length direction of the rubber damping strip (93) is arranged along the length direction of the thread body (91), and the surface of the rubber damping strip (93) is provided with anti-skid convex patterns, and the pattern direction of the anti-skid convex patterns is perpendicular to the length direction of the rubber damping strip (93).

9. A method for excavating a foundation pit of a composite support system according to claim 1, characterized in that: The steel support (7) comprises a transverse steel support (71) and a longitudinal steel support (72), wherein the length direction of the longitudinal steel support (72) is perpendicular to the extension direction of the long side of the foundation pit, wherein two ends of one of the longitudinal steel supports (72) respectively abut against the inner side walls of two end portions of a concrete purlin (4), and two ends of another longitudinal steel support (72) respectively abut against the inner side walls of two end portions of another concrete purlin (4).

10. A foundation pit excavation method for a composite support system according to claim 9, characterized in that: During the installation process of the steel support (7) in the intermediate area (1) construction phase 1 and the intermediate area (1) construction phase 2, the shadow cast by the marking thread (81) on the surface of the transverse steel support (71) is used as a reference, so that the length direction of the transverse steel support (71) remains parallel to the long side of the foundation pit.

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