Deep and large foundation pit forward and reverse combined construction method model test device and test method

Abstract

The invention relates to the technical field of construction test devices, in particular to a deep and large foundation pit forward and reverse combined construction method model test device and test method. The device comprises a model box base and a model box arranged on the model box base, the model box comprises a supporting part, a soil body in the model box and monitoring equipment; the supporting component comprises an underground diaphragm wall model; the underground diaphragm wall model is installed in the model box, the structural bottom plate model is installed at the bottom of one end of the underground diaphragm wall model, and the method comprises the steps that the supporting structure, rainfall and soil body excavation processes in forward and reverse combined construction are highly restored through the model reduced in proportion and various materials, so that the test environment is closer to engineering practice, and the test efficiency is improved. Through accurately controlled test steps, key procedures in actual construction can be verified and optimized, possible instability risks can be predicted and avoided in advance, and a direct basis is provided for formulating a safe and efficient construction scheme.

Description

Technical Field

[0001] This invention relates to the field of construction testing equipment technology, and in particular to a model testing device and testing method for a combined forward and reverse construction method for deep foundation pits. Background Technology

[0002] With the acceleration of urbanization, urban construction land is becoming increasingly scarce. Urban buildings, including traditional houses and rail transit structures, are gradually trending towards being "higher and deeper," resulting in larger and deeper foundation pits. For general foundation pits, most projects employ open-cut and reverse construction methods. However, in recent years, due to the emergence of many ultra-large and ultra-deep foundation pits with significant depth and area, and stringent environmental requirements, many engineers and scholars have begun to use a combined open-cut and reverse construction method, combined with support structure design and construction, to ensure foundation pit safety. The characteristic of this combined method is that the main body of the foundation pit can be excavated and constructed using the open-cut and forward construction method, while the portion closer to the retaining structure can be constructed using the cut-and-cover reverse construction method, thereby improving efficiency, shortening the construction period, and saving costs.

[0003] However, for foundation pits constructed using a combination of open-cut and top-down construction, most engineers rely solely on existing standards for design calculations and supplement the construction process with monitoring methods to ensure the safety of the foundation pit and surrounding structures. At present, there is a lack of a more effective construction method model testing device and method that combines open-cut and top-down construction to better ensure construction safety.

[0004] Therefore, the main objective of this invention is to provide a model test device and test method for a combined forward and reverse construction method for deep foundation pits, aiming to better reveal the mechanism of changes in the safety status of foundation pits during the combined forward and reverse construction process, minimize irreversible personnel and property losses caused by actual construction, and solve the technical problem that existing foundation pits that are intended to use the combined forward and reverse construction method cannot effectively verify their own safety. Summary of the Invention

[0005] Therefore, it is necessary to provide a model test device and test method for the combined forward and reverse construction method of deep foundation pits to address the aforementioned technical problems.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A model test device for a combined forward and reverse construction method for deep foundation pits, the device comprising a model box base and a model box set on the model box base;

[0008] The model box includes support components, soil inside the model box, and monitoring equipment;

[0009] The support components include a diaphragm wall model, a structural base plate model, a wainscoting model, a PVC bracing model simulating two floors, a first-layer inclined bracing, a second-layer inclined bracing, and a steel pipe replacement bracing model. The diaphragm wall model is installed inside the model box, the structural base plate model is installed at the bottom of one end of the diaphragm wall model, the wainscoting model is installed at one end of the diaphragm wall model, and PVC bracing models simulating two floors are evenly distributed on the top of the structural base plate model. The first-layer inclined bracing, the second-layer inclined bracing, and the steel pipe replacement bracing model are installed between the diaphragm wall model and the structural base plate model for connection.

[0010] As a preferred embodiment of the deep foundation pit combined forward and reverse construction method model test device provided by the present invention, the first layer of inclined support, the second layer of inclined support and the steel pipe replacement support model all include an upper support, a pin, a segmented support rod, a sleeve, a bolt and a lower support. The upper support is fixed to the diaphragm wall model. The end of the upper support away from the diaphragm wall model is rotatably connected to the segmented support rod through the pin. A sleeve is fixed to the outer side of one end of the segmented support rod. The lower support is rotatably connected to one end of the sleeve through the bolt. The lower support is fixed to the structural base plate model.

[0011] As a preferred embodiment of the model test device for the combined forward and reverse construction method of deep foundation pit provided by the present invention, the corresponding monitoring equipment includes distributed optical fiber, earth pressure cell, dial gauge, strain gauge, and miniature water pressure gauge.

[0012] A portion of the distributed optical fiber is placed on the front and back sides of the diaphragm wall model to measure the lateral displacement of the diaphragm wall model, and another portion of the distributed optical fiber is placed in the soil behind the diaphragm wall model to measure the soil settlement.

[0013] Earth pressure cells were placed on the front and back sides of the diaphragm wall model to measure the soil pressure near the diaphragm wall model.

[0014] A dial gauge is placed on top of the diaphragm wall model to ensure that the lateral displacement of the diaphragm wall model for later distributed fiber optic output is the actual displacement.

[0015] Strain gauges were attached to the first layer of inclined propellers, the second layer of inclined propellers, and the steel pipe support model to measure the axial force of the supports.

[0016] A model of a dewatering well made of PVC pipe was set up outside the pit, and an observation well model and a miniature water pressure gauge were set up outside the pit to continuously monitor the water level changes inside and outside the pit during the dewatering excavation and support replacement, so as to keep the water level inside the pit below the excavation surface.

[0017] As a preferred embodiment of the test device for the combined forward and reverse construction method of deep foundation pit provided by the present invention, the diaphragm wall model, the structural base plate model, the waist beam model and the PVC bracing model simulating two floors are all made of acrylic sheet.

[0018] As a preferred embodiment of the test device for the combined forward and reverse construction method of deep foundation pit provided by the present invention, the first layer of inclined support, the second layer of inclined support, the steel pipe replacement support model, the dewatering well model and the observation well model are all made of PVC pipe.

[0019] A model test method for a combined forward and reverse construction method for deep foundation pits, the steps of which are as follows:

[0020] S1: Before the experiment, process the diaphragm wall model, structural base plate model and wainscoting model according to the requirements;

[0021] S2: Positioning and assembly of the diaphragm wall model and the wainscoting model;

[0022] S3: Layered backfilling of the designated area, followed by progressive dewatering and three-stage slope excavation;

[0023] S4: Install and constrain the normal displacement of the structural base plate model at its position;

[0024] S5: Perform the following steps in sequence: install the first layer of PVC support model, remove the first layer of counterweight soil, install the second layer of PVC support model, and remove the second layer of counterweight soil. After each step is completed, wait until the reading stops changing before reading the data.

[0025] S6: Perform the following steps in sequence: install the first layer of inclined bracing, remove the third layer of counter-pressure soil to the bottom slab depth elevation, place the second layer of structural bottom slab model near the diaphragm wall model, install the second layer of inclined bracing, remove the second layer of inclined bracing, erect the steel pipe replacement model, remove the first layer of inclined bracing, and dismantle the steel pipe replacement model. After each step is completed, wait until the readings no longer change before reading the data.

[0026] S7: Foundation pit construction completed; test ended.

[0027] As a preferred embodiment of the model test method for the combined forward and reverse construction method of deep foundation pits provided by the present invention, in step S2, two waist beam models are fixed at corresponding positions behind the diaphragm wall model, the diaphragm wall model is fixed inside the model box, distributed optical fibers are installed at designated positions on the diaphragm wall model, and optical fibers for measuring soil settlement are fixed at designated positions; axial force gauges are installed on the designated first-layer inclined support, second-layer inclined support, and steel pipe replacement support models.

[0028] As a preferred embodiment of the model test method for the combined forward and reverse construction method of deep foundation pit provided by the present invention, in step S3, the soil is filled in multiple times, and compaction is carried out after each filling is completed. The soil pressure cell is buried at a specific height behind the wall at a designated location. The dewatering well model, the observation well model and the miniature water pressure gauge are placed at the designated location behind the wall. After the filling is completed, it is compacted and left to stand for 1 month to wait for the soil to consolidate and stop settling.

[0029] As a preferred embodiment of the model test method for a combined forward and reverse construction method for deep foundation pits provided by the present invention, step S3, involving stepwise dewatering and three-stage slope excavation, is as follows:

[0030] Before excavating the soil of each foundation pit, a small water pump connected to the dewatering well model 14 behind the wall is used to carry out dewatering operations until the water level drops below the pre-excavation surface.

[0031] During this period, the water level changes in the foundation pit were observed using observation well model 15 and miniature water pressure gauge 16;

[0032] The excavation of the foundation pit was carried out in four stages, as shown in the diagram: one vertical excavation and three sloping excavations until the bottom slab depth was reached.

[0033] After each rainfall and excavation, relevant data is read, and the next excavation is carried out only after the data stops changing, until the foundation elevation is reached.

[0034] As a preferred embodiment of the model test method for the combined forward and reverse construction method of deep foundation pits provided by the present invention, between steps S3 and S4, a weight is placed in the middle of the foundation pit to simulate the forward application of the main structure, and the relevant data is read after the data no longer changes.

[0035] It is clear without a doubt that the technical solution described above in this application can solve the technical problem that this application aims to address.

[0036] Meanwhile, through the above technical solutions, the present invention has at least the following beneficial effects:

[0037] 1. The present invention provides a model test device and test method for a combined forward and reverse construction method for deep foundation pits. Through a scaled-down model and various materials, it highly replicates the support structure, dewatering, and soil excavation process in the combined forward and reverse construction, making the test environment closer to the actual engineering situation. At the same time, through precisely controlled test steps, it can verify and optimize key procedures in actual construction (such as support replacement and support removal), predict and avoid possible instability risks in advance, and provide direct basis for formulating safe and efficient construction plans.

[0038] 2. This invention integrates multiple sensors such as distributed optical fibers, strain gauges, and earth pressure cells, enabling synchronous and real-time monitoring of key safety indicators such as foundation pit deformation, support axial force, and earth pressure, and clearly revealing the response mechanism of the foundation pit during construction.

[0039] 3. The device of the present invention is low in cost and reusable. It can verify various design schemes without causing actual engineering losses, effectively reduce engineering trial and error costs, shorten the construction period, and has important engineering application value. Attached Figure Description

[0040] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is a top view of the model box of the support structure of the present invention, which includes the inclined propeller model and the steel pipe replacement model.

[0042] Figure 2 This is a cross-sectional side view of the model box portion of the support structure of the present invention, which includes the inclined propeller model and the steel pipe replacement model.

[0043] Figure 3 This is a schematic diagram of the three-stage counter-pressure soil inside the foundation pit of the present invention;

[0044] Figure 4 This is a schematic diagram showing the connection between the inclined bracing model, the structural base plate model, and the diaphragm wall model of the present invention;

[0045] Figure 5 This is a schematic diagram showing the connection between the steel pipe support replacement model, the structural base plate model, and the diaphragm wall model of the present invention;

[0046] Figure 6 This is a schematic diagram of the block arrangement of the base plate of the present invention;

[0047] Figure 7 This is a schematic diagram of the PVC support model of the present invention;

[0048] Figure 8 A top view showing the arrangement of the monitoring equipment of the present invention;

[0049] Figure 9 This is a side view showing the arrangement of the monitoring equipment of the present invention;

[0050] Figure 10 This is a flowchart of the test method of the present invention.

[0051] In the diagram: 1. Model box; 2. Model box base; 3. Diaphragm wall model; 4. Structural base plate model; 5. Waist beam model; 6. PVC bracing model; 7. First layer of inclined bracing; 8. Second layer of inclined bracing; 9. Steel pipe bracing model; 10. Distributed optical fiber; 11. Earth pressure cell; 12. Dial gauge; 13. Strain gauge; 14. Dewatering well model; 15. Observation well model; 16. Miniature water pressure gauge; 17. Upper support; 18. Pin; 19. Segmented support rod; 20. Sleeve; 21. Bolt; 22. Lower support. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0053] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0054] It should be noted that, unless otherwise specified, the embodiments and features and technical solutions in the present invention can be combined with each other.

[0055] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0056] Example 1

[0057] Reference Figures 1-9 A model test device for a combined forward and reverse construction method for deep foundation pits:

[0058] refer to Figure 2 The device includes a model box base 2 and a model box 1 disposed on the model box base 2;

[0059] Model box 1 includes support components, the soil inside model box 1, and corresponding monitoring equipment;

[0060] refer to Figures 1-3The support components include a diaphragm wall model 3, a structural base plate model 4, a waist beam model 5, a PVC bracing model 6 simulating two floors, a first-layer inclined bracing 7, a second-layer inclined bracing 8, and a steel pipe replacement bracing model 9. The diaphragm wall model 3 is installed inside the model box 1, the structural base plate model 4 is installed at the bottom of one end of the diaphragm wall model 3, the waist beam model 5 is installed at one end of the diaphragm wall model 3, the top of the structural base plate model 4 is evenly equipped with PVC bracing models 6 simulating two floors, and the first-layer inclined bracing 7, the second-layer inclined bracing 8, and the steel pipe replacement bracing model 9 are installed between the diaphragm wall model 3 and the structural base plate model 4 for connection.

[0061] refer to Figure 4 and Figure 5 The first-layer inclined bracing 7, the second-layer inclined bracing 8, and the steel pipe replacement bracing model 9 all include an upper support 17, a pin 18, a segmented support rod 19, a sleeve 20, a bolt 21, and a lower support 22. The upper support 17 is fixed to the diaphragm wall model 3. The end of the upper support 17 away from the diaphragm wall model 3 is rotatably connected to the segmented support rod 19 through the pin 18, so that the angle can be adaptively adjusted by rotating the segmented support rod 19 inside the pin 18. A sleeve 20 is fixed to the outside of one end of the segmented support rod 19. The lower support 22 is rotatably connected to one end of the sleeve 20 through the bolt 21, so that the lower support 22 can be adaptively adjusted. The lower support 22 is fixed to the structural base plate model 4.

[0062] refer to Figure 6 Near the diaphragm wall model 3, there is a second layer of structural base plate model 4. This base plate is made of 8 (or other numbers) long strips of acrylic plexiglass spliced ​​together, and the splicing joints are glued together.

[0063] refer to Figure 7 One end of the PVC bracing model 6 is assembled with the waist beam model 5;

[0064] The soil in the model box includes the soil inside the foundation pit and the soil outside the foundation pit. During the test, the soil inside the pit will first be excavated into a three-level slope counter-pressure soil to ensure the safety of the foundation pit, and then gradually excavated under the support of inclined bracing and replacement bracing.

[0065] The corresponding monitoring equipment includes distributed optical fiber 10, earth pressure cell 11, dial gauge 12, strain gauge 13, and miniature water pressure gauge 16.

[0066] Among them, model box 1 is a 1:50 scale model box with a length of 2.6 meters, a width of 1.8 meters, and a height of 1.25 meters, based on the foundation pit project.

[0067] Among them, the diaphragm wall model 3, the structural base plate model 4, the waist beam model 5, and the PVC bracing model 6 simulating two floors are all made of plexiglass acrylic sheets, while the first-layer inclined bracing 7, the second-layer inclined bracing 8, the steel pipe replacement bracing model 9, the dewatering well model 14, and the observation well model 15 are all made of PVC pipes.

[0068] In order to facilitate the installation of the first layer of inclined bracing 7, the second layer of inclined bracing 8 and the steel pipe replacement model 9, a model box base 2 is set at the connection between them and the structural base plate model 4 and the waist beam model 5 to facilitate connection during the test.

[0069] To facilitate the excavation and structural layout of the model test, the PVC support model 6 was approximated by splicing acrylic strips together according to the principle of similar compressive stiffness.

[0070] Furthermore, to facilitate the arrangement of the floor slab model, small openings were cut into the floor slab of the wainscoting model 5 near the diaphragm wall model 3 to facilitate subsequent connections.

[0071] refer to Figure 8 and Figure 9 The monitoring equipment includes distributed optical fibers 10, some of which are arranged on the front and back sides of the diaphragm wall model 3 to measure the lateral displacement of the diaphragm wall model 3, and some of which are placed in the soil behind the diaphragm wall model 3 (i.e. outside the foundation pit) to measure the soil settlement, and are supplemented with plastic coupling plates to enhance the coupling with the soil.

[0072] refer to Figure 8 and Figure 9 The monitoring equipment includes an earth pressure cell 11 placed on both the front and back sides of the diaphragm wall model 3 to measure the soil pressure near the diaphragm wall model 3.

[0073] refer to Figure 8 and Figure 9 The monitoring equipment includes strain gauges 13 that are attached to two layers of inclined propeller models (i.e., a combination of the first layer of inclined propeller 7 and the second layer of inclined propeller 8) and steel pipe replacement propeller model 9 to measure the axial force of the support.

[0074] refer to Figure 8 and Figure 9 The monitoring equipment, including the dial gauge 12, is placed on top of the diaphragm wall model 3 to ensure that the lateral displacement of the diaphragm wall model 3 output by the distributed optical fiber 10 in the later stage is the actual displacement.

[0075] refer to Figure 8 and Figure 9To better simulate the actual precipitation and seepage effect, a PVC pipe-made precipitation well model 14 was installed outside the pit, connected to a small water pump to simulate actual precipitation operations outside the pit. An observation well model 15 and a miniature water pressure gauge 16 were installed outside the pit to continuously monitor the water level changes inside and outside the pit during the precipitation excavation and support replacement to keep the water level inside the pit below the excavation face. The entire model box was placed in a water tank to simulate the water head at infinity.

[0076] In this embodiment, each structure has its own service life. In actual manufacturing and application, the corresponding structure made of different materials can be replaced according to the needs of use.

[0077] Example 2

[0078] refer to Figure 10 The above embodiment one discloses a test method, the steps of which are as follows:

[0079] A. Before the test begins, ensure that the diaphragm wall model 3, the structural base plate model 4, and the waist beam model 5 are welded with grooves or connection ports. Before the test begins, pre-install each component to determine whether the test can be carried out smoothly, so as to ensure that the PVC support model 6, the first layer of inclined support 7, the second layer of inclined support 8, and the steel pipe support model 9 can be installed smoothly after the test begins.

[0080] B. Fix the two waist beam models 5 to their corresponding positions after fixing the diaphragm wall model 3. Fix the diaphragm wall model 3 inside the model box 1. Install the distributed optical fiber 10 at the designated position of the diaphragm wall model 3. Fix the optical fiber used to measure soil settlement (deep soil displacement) at the designated position. Install axial force gauges on the designated first-layer inclined support 7, second-layer inclined support 8, and steel pipe support model 9.

[0081] C. Layered backfilling of the designated area. The soil is filled in multiple stages, with compaction performed after each stage. Earth pressure cells 11 are embedded at a specific height behind the wall at designated locations. Dewatering well models 14, observation well models 15, and miniature water pressure gauges 16 are placed at designated locations behind the wall. After filling, the soil is compacted and left to stand for one month to allow it to consolidate and stop settling.

[0082] D. Gradual Dewatering and Three-Stage Slope Excavation. Before excavating each layer of the foundation pit, a small water pump connected to the dewatering well model 14 behind the wall is used for dewatering until the water level drops below the pre-excavation surface. During this period, the water level changes in the foundation pit are observed through the observation well model 15 and the miniature water pressure gauge 16. The foundation pit excavation is carried out in four stages, as shown in the diagram: one vertical excavation and three slope excavations until the bottom slab depth is reached. Leveling instruments and other measuring tools are used to ensure that the excavation depth is accurate and consistent each time, and precise slope is applied. After each dewatering and excavation, relevant data is read, and the next excavation is carried out only after the data no longer changes, until the foundation elevation is reached.

[0083] E. Place weights in the middle of the foundation pit to simulate the sequential application of the main structure, and wait for the data to stop changing before reading the relevant data.

[0084] F. Place the part of the structural base plate model 4 near the side of the model box 1, and constrain the normal displacement of this part of the structural base plate model 4 relative to the inner wall of the model box 1.

[0085] G. Install the first layer of PVC support model 6 (i.e., install 5 PVCs in sequence), and read the relevant data after the reading stops changing.

[0086] H. Remove the first layer of counterweight soil, and after the reading stops changing, read the data.

[0087] I. Install the second layer of PVC support model 6 (install 5 PVCs in sequence), and read the relevant data after the reading stops changing.

[0088] J. Remove the second layer of counterweight soil, and after the reading stops changing, read the data.

[0089] K. Construct the first layer of inclined bracing 7 (partially grooved). The first layer of inclined bracing 7 should be connected to the base plate from the previous step. After the reading stops changing, read the data.

[0090] L. Excavate the third layer of counterweight soil down to the bottom slab depth elevation, and wait for the reading to stop changing before taking the data.

[0091] M. Place the second-layer structural base plate model 4 near the diaphragm wall model 3. This base plate is made of 8 (or other numbers) long strips of acrylic plexiglass spliced ​​together, with the splices glued together. After the second-layer structural base plate model 4 is completed and the reading no longer changes, read the data.

[0092] N. Apply the second layer of inclined support 8 (partially cut out), and after the reading stops changing, read the data.

[0093] O. Remove the second layer of inclined support 8. After the reading stops changing, read the data.

[0094] P. Set up the steel pipe support model 9, and after the readings stop changing, read the data.

[0095] Q. Remove the first layer of inclined support 7, and after the reading stops changing, read the data.

[0096] R. Finally, dismantle the steel pipe replacement support model 9. After the readings stop changing, read the data. The foundation pit construction is complete. The test is over.

[0097] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A model test device for a combined forward and reverse construction method for deep and large foundation pits, characterized in that, The device includes a model box base (2) and a model box (1) disposed on the model box base (2); The model box (1) includes support components, soil inside the model box (1), and monitoring equipment; The support components include a diaphragm wall model (3), a structural base plate model (4), a waist beam model (5), a PVC bracing model simulating two floors (6), a first-layer inclined bracing (7), a second-layer inclined bracing (8), and a steel pipe replacement bracing model (9). The diaphragm wall model (3) is installed inside the model box (1). The structural base plate model (4) is installed at the bottom of one end of the diaphragm wall model (3). The waist beam model (5) is installed at one end of the diaphragm wall model (3). The top of the structural base plate model (4) is evenly equipped with PVC bracing models simulating two floors (6). The first-layer inclined bracing (7), the second-layer inclined bracing (8), and the steel pipe replacement bracing model (9) are installed between the diaphragm wall model (3) and the structural base plate model (4) for connection.

2. The model test device for a combined forward and reverse construction method for deep foundation pits according to claim 1, characterized in that, The first layer of inclined bracing (7), the second layer of inclined bracing (8) and the steel pipe replacement model (9) all include an upper support (17), a pin (18), a segmented support rod (19), a sleeve (20), a bolt (21) and a lower support (22). The upper support (17) is fixed to the diaphragm wall model (3). The end of the upper support (17) away from the diaphragm wall model (3) is rotatably connected to the segmented support rod (19) through the pin (18). The sleeve (20) is fixed to the outside of one end of the segmented support rod (19). The lower support (22) is rotatably connected to one end of the sleeve (20) through the bolt (21). The lower support (22) is fixed to the structural base plate model (4).

3. The model test device for a combined forward and reverse construction method for deep foundation pits according to claim 1, characterized in that, The corresponding monitoring equipment includes distributed optical fiber (10), earth pressure cell (11), dial gauge (12), strain gauge (13), and miniature water pressure gauge (16). A portion of the distributed optical fiber (10) is arranged on both sides of the diaphragm wall model (3) to measure the lateral displacement of the diaphragm wall model (3), and a portion of the distributed optical fiber (10) is placed in the soil behind the diaphragm wall model (3) to measure the soil settlement. Earth pressure cells (11) are placed on the front and back sides of the diaphragm wall model (3) to measure the soil pressure near the diaphragm wall model (3); The dial gauge (12) is placed on top of the diaphragm wall model (3) to make the lateral displacement of the diaphragm wall model (3) output by the distributed optical fiber (10) in the later stage a true displacement; Strain gauges (13) are attached to the first layer of inclined bracing (7), the second layer of inclined bracing (8) and the steel pipe replacement model (9) to measure the axial force of the support; A PVC pipe model of a dewatering well (14) is set up outside the pit, and an observation well model (15) and a miniature water pressure gauge (16) are set up outside the pit to continuously observe the water level changes inside and outside the pit during the dewatering excavation and support replacement, so as to keep the water level inside the pit below the excavation surface.

4. The model test device for a combined forward and reverse construction method for deep foundation pits according to claim 3, characterized in that, The diaphragm wall model (3), structural base plate model (4), waist beam model (5), and PVC bracing model (6) simulating two floors are all made of acrylic sheets.

5. The model test device for a combined forward and reverse construction method for deep foundation pits according to claim 3, characterized in that, The first layer of inclined support (7), the second layer of inclined support (8), the steel pipe replacement support model (9), the dewatering well model (14) and the observation well model (15) are all made of PVC pipe.

6. A model test method for a combined forward and reverse construction method for deep and large foundation pits, used in the model test device for a combined forward and reverse construction method for deep and large foundation pits as described in any one of claims 1-5, characterized in that, The steps are as follows: S1: Before the experiment, the diaphragm wall model (3), the structural base plate model (4) and the waist beam model (5) are processed according to the requirements; S2: Assemble the positions of the diaphragm wall model (3) and the wainscoting model (5); S3: Layered backfilling of the designated area, followed by progressive dewatering and three-stage slope excavation; S4: Install and constrain the normal displacement of the structural base plate model (4) at its position; S5: Perform the steps of installing the first layer of PVC support model (6), removing the first layer of counterweight soil, installing the second layer of PVC support model (6), and removing the second layer of counterweight soil in sequence. After each step is completed, wait until the reading no longer changes before reading the data. S6: Perform the following steps in sequence: install the first layer of inclined bracing (7), remove the third layer of counter-pressure soil to the bottom plate depth elevation, place the second layer of structural bottom plate model (4) near the diaphragm wall model (3), install the second layer of inclined bracing (8), remove the second layer of inclined bracing (8), erect the steel pipe replacement model (9), remove the first layer of inclined bracing (7), and dismantle the steel pipe replacement model (9). After each step is completed, wait until the reading no longer changes before reading the data. S7: Foundation pit construction completed; test ended.

7. The model test method for a combined forward and reverse construction method for deep foundation pits according to claim 6, characterized in that, In step S2, two waist beam models (5) are fixed at the corresponding positions behind the diaphragm wall model (3), the diaphragm wall model (3) is fixed inside the model box (1), the distributed optical fiber (10) is installed at the designated position of the diaphragm wall model (3), and the optical fiber used to measure soil settlement is fixed at the designated position; axial force gauges are installed on the designated first layer inclined support (7), second layer inclined support (8), and steel pipe replacement support model (9).

8. The model test method for a combined forward and reverse construction method for deep foundation pits according to claim 6, characterized in that, In step S3, the soil is filled in multiple times. After each filling is completed, it is compacted. The soil pressure box (11) at a specific height behind the wall is buried at the designated location. The dewatering well model (14), the observation well model (15) and the miniature water pressure gauge (16) are placed at the designated location behind the wall. After the filling is completed, it is compacted and left to stand for 1 month to wait for the soil to consolidate and stop settling.

9. The model test method for a combined forward and reverse construction method for deep foundation pits according to claim 6, characterized in that, In step S3, the gradual dewatering and three-stage slope excavation are carried out as follows: Before excavating the soil of each foundation pit, a small water pump connected to the dewatering well model 14 behind the wall is used to carry out dewatering operations until the water level drops below the pre-excavation surface. During this period, the water level changes in the foundation pit were observed using observation well model 15 and miniature water pressure gauge 16; The excavation of the foundation pit was carried out in four stages, as shown in the diagram: one vertical excavation and three sloping excavations until the bottom slab depth was reached. After each rainfall and excavation, relevant data is read, and the next excavation is carried out only after the data stops changing, until the foundation elevation is reached.

10. The model test method for a combined forward and reverse construction method for deep foundation pits according to claim 6, characterized in that, Between steps S3 and S4, a weight is placed in the middle of the pit to simulate the application of the main structure, and the relevant data is read after the data stops changing.

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