A half-cover digging cover plate support structure and stability early warning method

Abstract

The present application relates to a kind of half cover excavates cover plate support structure, including half width cover plate, foundation pit concrete support, center column, contact beam, limiting block, column top support, fixed bolt, connecting rod, external steel plate, buffer spring, vertical connecting rod, cushion block and telescopic steel wedge, more involves a foundation pit stability evaluation method;The present application is connected with foundation pit concrete support by external steel plate, guarantee the supporting force of horizontal support;Buffer spring support device is installed in the lower side of limiting block and column top support, guarantee that the overall structure can occur vertical displacement within certain safety range, reduce the influence that foundation pit concrete support structure is subjected to center column vertical displacement;Telescopic steel wedge is set in external steel plate and column top, guarantee the support effect of center column to cover plate structure when center column occurs excessive vertical displacement of settlement;Cushion block is installed at the connecting place of limiting block and external steel plate, and the overall structure can smoothly occur vertical displacement;Improve construction efficiency by comprehensive three-dimensional evaluation method.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering technology, specifically to a semi-cut-and-cover cover plate support structure and a stability early warning method. Background Technology

[0002] With the acceleration of urbanization and the continuous development of rail transit construction in my country, a nationwide surge in underground space construction has emerged, and various engineering foundation pit support projects are also developing towards larger and deeper dimensions. The construction quality of deep foundation pits, as the cornerstone of engineering construction, directly determines the stability and safety of subsequent projects. Currently, due to terrain limitations and the influence of surface transportation networks, traditional open-cut foundation pit construction is no longer suitable for most urban areas. Furthermore, considering construction progress and project costs, the cut-and-cover method is not as suitable as the emerging semi-cut-and-cover method. The semi-cut-and-cover method, as a new construction technology, is a construction method that combines the advantages of both open-cut and cut-and-cover methods, making it very suitable for large-section underground construction in busy urban areas.

[0003] During the construction phase, foundation pit deformation is a key factor affecting construction safety and even the safety of the entire project. Predicting foundation pit deformation in advance, combined with on-site monitoring, and addressing both factors to prevent accidents is one effective way to ensure the safety of foundation pit projects. For the semi-cut-and-cover method, the lower part of the half-cover slab is supported by columns in the foundation pit. These columns settle under top loads (including vehicle loads and their own weight), and bulge as the soil rebounds during excavation. Horizontal displacement also occurs under the influence of support loads, making the situation complex and variable. However, the vertical displacement caused by the bulging of the columns has the greatest impact overall, forcing displacement of the entire support structure. The resulting additional bending moment can damage various nodes, even leading to instability of the entire support structure and potentially causing the foundation pit to overturn.

[0004] According to current academic research on central columns in foundation pits, the presence of these columns restricts the lateral displacement of the retaining piles affected by soil excavation. Simultaneously, by connecting beams to support the internal supports of the foundation pit, they improve the overall vertical stability of the support structure, making them indispensable in most cases. To minimize the impact of vertical displacement of the central columns on the overall stability of the support, the pile length and diameter of the central columns need to be considered during the design phase. Increasing the cross-sectional dimensions and improving material strength can ensure construction safety. The rebound of the central columns during support removal can negatively impact the stability of the top cover structure, which is a problem that urgently needs to be addressed in this field.

[0005] Currently, technologies for stability monitoring of semi-cut-and-cover foundation pits are constantly being researched and updated both domestically and internationally. However, there is still no unified standard for how to limit the displacement and deformation of the central column and how to achieve efficient monitoring and effective early warning. Most current foundation pit projects use traditional monitoring point layouts. These points and the information collected can only meet the basic requirements stipulated in the specifications. Once the foundation pit project encounters problems such as large spans, complex terrain, intricate support systems, or transportation hub locations, the data collected based on the traditional monitoring point layout is not representative, which, from another perspective, poses potential safety hazards to the foundation pit project.

[0006] Therefore, finding a new and effective method for evaluating the stability of semi-covered excavation pits is of great significance for ensuring the safety of excavation projects. Summary of the Invention

[0007] The purpose of this invention is to provide a support structure for a semi-covered excavation pit cover plate, and based on this structure, to provide a stability early warning method for semi-covered excavation pits, so as to solve the problems existing in the prior art.

[0008] To achieve the objective of this invention, the technical solution adopted is as follows:

[0009] A semi-cut-and-cover cover plate support structure includes a half-cover plate, a foundation pit concrete support, a central column, a connecting beam, limiting blocks, column top supports, fixing bolts, connecting rods, an external steel plate, a buffer spring, a vertical connecting rod, pads, and a retractable steel wedge. An external steel plate is installed at the lower part of the half-cover plate, and the lower end of the external steel plate is directly connected to the central column via connecting rods passing through it. Both sides of the external steel plate are connected to the foundation pit concrete support. A connecting beam is installed inside the external steel plate, with its top connected to the half-cover plate. A column top support is installed at the lower part of the connecting beam, and limiting blocks are installed on both sides of the lower part of the connecting beam. Vertical connecting rods are installed on the limiting blocks. A buffer spring is fitted onto the lower vertical connecting rod; the connecting beam and the column top support are fixedly connected by fixing bolts; the connecting rod below the column top support passes through the outer steel plate and is directly connected to the central column; a pad is installed at the connection between the limiting block and the outer steel plate; a retractable steel wedge is installed and inserted in the gap between the connecting beam and the half-cover plate; the limiting block is movably fitted onto the vertical connecting rod, and the lower surface of the limiting block is directly connected to the buffer spring; the pad between the limiting block and the outer steel plate is movably connected to the outer steel plate; the outer steel plate has a beam mounting groove through an opening on its upper surface, in which the connecting beam is installed and fixedly connected to the column top support below it.

[0010] A method for evaluating the stability of a foundation pit, as described above, includes the following steps:

[0011] Step 1: Select the supporting structure around the foundation pit, and select multiple key locations as monitoring points at the concrete support, steel support and column ends on both sides of the foundation pit.

[0012] Step 2: Compare the measured cover plate deflection and support axial force with the allowable values ​​set in the respective engineering designs, and conduct a stability assessment of the foundation pit if they meet the safety standards.

[0013] Step 3, based on the measured horizontal displacement d at each of the monitoring points of the supporting structure. i and the span of the foundation pit D i Calculate the rate of change of horizontal displacement k within each of the foundation pit planes. 1i ;

[0014] Step 4, based on the measured vertical displacement l at each of the monitoring points of the supporting structure. i The distance L between the monitoring point of the foundation pit and the initial excavation surface of the foundation pit. i The vertical displacement rate γ of each of the foundation pit support planes is calculated by dividing the foundation pit into three parts: top (concrete support), middle (steel support), and bottom. 1i γ 2i and γ 3i Simultaneously, weighting coefficients are allocated proportionally to obtain the overall vertical displacement rate of change k. 2i ;

[0015] Step 5: Based on the horizontal displacement change rate k calculated in the previous steps... 1i Vertical displacement rate of change k 2i By proportionally allocating the weighting coefficients, the displacement stability parameter K2 is obtained.

[0016] Step 6: Based on the selected multiple sets of foundation pit planes, determine measurement points at constant distances on the surrounding ground in the direction away from the excavation face of the foundation pit, and record the elevation H of the soil layer around the pit. i And during the excavation stage of the foundation pit, the settlement h of the soil around the pit was measured regularly. i Calculate the external settlement change rate k at each of the measurement points. 3i , namely, the environmental stability parameter K3;

[0017] Step 7: Based on the displacement stability parameter K2 and environmental stability parameter K3 obtained in Steps 5 and 6, allocate weight coefficients proportionally to obtain the overall quantitative evaluation value K, which is used to evaluate the stability of the foundation pit in the project.

[0018] Step 8: Compare the quantitative evaluation value K with K0, where K0 is the evaluation standard value;

[0019] Step 9: When K > K0, the monitored evaluation value is greater than the standard value, then the foundation pit support is strengthened and foundation pit instability early warning measures are taken; when K ≤ K0, the monitored evaluation value is less than or equal to the standard value, then the original plan remains unchanged.

[0020] As a further aspect of the present invention, in step 1, by monitoring each support sampling point, comprehensive coverage of the foundation pit stability monitoring is achieved, making the entire monitoring results more systematic, accurate, and effective.

[0021] As a further aspect of the present invention, the span D of the foundation pit in step 3... i The data was obtained by measuring with a laser rangefinder installed at the monitoring point, and the difference between several monitoring data points was compared to obtain the horizontal displacement d of the retaining pile. i .

[0022] As a further aspect of the present invention, the vertical displacement l in step 4 i The initial distance L between the monitoring point and the initial excavation face of the foundation pit i The top displacement can be directly obtained by reading the data on the scale on the connecting rod below the column top support, the middle displacement can be measured by a laser rangefinder, and the bottom displacement can be obtained by a level instrument using the leveling method.

[0023] As a further aspect of the present invention, the rate of change of horizontal displacement k at any support monitoring point in step 3 is... 1i Calculated according to formula (1), formula (1) is as follows:

[0024]

[0025] In formula (1), k 1i d is the rate of change of horizontal displacement. i and D i These are the horizontal displacement at the monitoring point of the supporting structure and the span of the foundation pit, respectively.

[0026] As a further aspect of the present invention, the vertical displacement change rate k at any support monitoring point in step 4... 2i Calculated according to formula (2), which is as follows:

[0027]

[0028] In formula (2), k 2i l is the rate of change of vertical displacement. i To support the vertical displacement at the monitoring points of the supporting structure, L i This refers to the distance between the monitoring point of the foundation pit and the initial excavation surface of the foundation pit.

[0029] As a further aspect of the present invention, the displacement stability parameter K2 in step 5 is calculated according to formula (3), which is as follows:

[0030]

[0031] In formula (3), K2 is the displacement stability parameter, γi is the distribution coefficient, γi=0-1 and ∑γ i =1.

[0032] As a further aspect of the present invention, the external settlement change rate k at any support monitoring point in step 6 3i That is, the environmental stability parameter K3 is calculated according to formula (4), which is as follows:

[0033]

[0034] In formula (4), K3 is the environmental stability parameter, k3 is the external settlement rate, and H i h is the elevation of the soil layer around the pit. i The soil around the pit has settled.

[0035] As a further aspect of the present invention, the overall quantitative evaluation value K in step 9 is calculated according to formula (5), which is as follows:

[0036] K = ε1K2 + ε2K3 (5)

[0037] In formula (5), K is the quantitative evaluation value, and ε i ε is the distribution coefficient. i =0-1 and ∑ε i =1.

[0038] The beneficial effects of this invention are as follows:

[0039] 1. In this invention, the horizontal support is ensured by connecting the external steel plate to the concrete support of the foundation pit;

[0040] 2. In this invention, by installing a buffer spring support device on the lower side of the limiting block and the column top support, the overall structure can be vertically displaced within a certain safe range, thereby reducing the impact of the vertical displacement of the central column on the concrete support structure of the foundation pit.

[0041] 3. In this invention, by setting retractable steel wedges on the outer steel plate and the top of the column, the supporting effect of the middle column on the cover plate structure is ensured when the vertical displacement of the middle column is too large due to settlement.

[0042] 4. In this invention, the pads installed at the connection between the limiting block and the external steel plate ensure that the overall structure can smoothly undergo vertical displacement.

[0043] 5. In this invention, the vertical displacement of the central column can be accurately determined by the scale on the connecting rod below the column top support;

[0044] 6. The evaluation method provided by this invention comprehensively considers multiple factors, which is different from the traditional evaluation method that uses a single factor as a standard for overall evaluation, making the evaluation method more reasonable.

[0045] 7. This invention makes the evaluation method more reasonable by separately calculating the top concrete support and the middle steel support in the foundation pit support;

[0046] 8. This invention measures the settlement at different distances around the road surface after the initial excavation of the foundation pit, calculates the external settlement change rate at each measurement point, and monitors the impact of foundation pit excavation on the external road surface environment in real time, making the evaluation method more comprehensive and three-dimensional.

[0047] 9. This invention can immediately evaluate the overall stability of the foundation pit after on-site measurement data, and use this as a basis for scientific guidance, thereby improving construction efficiency. Attached Figure Description

[0048] Figure 1 This is a cross-sectional view of the improved elastic support structure of the present invention;

[0049] Figure 2 This is an external illustration of the improved elastic support structure of the present invention;

[0050] Figure 3 This is a schematic diagram of the internal structure of the improved elastic support structure of the present invention.

[0051] Figure 4 This is a diagram showing the connection between the limiting block and the external steel plate in this invention;

[0052] Figure 5 This is a diagram showing the connection between the cover plate and the top of the column in this invention;

[0053] Figure 6 This is a schematic diagram of the foundation pit stability evaluation method according to an embodiment of the present invention;

[0054] Figure 7 This is a schematic diagram of the foundation pit stability evaluation method according to an embodiment of the present invention;

[0055] In the diagram: 1. Half-width cover plate; 2. Foundation pit concrete support; 3. Central column; 4. Connecting beam; 5. Limiting block; 6. Column top support; 7. Fixing bolt; 8. Connecting rod; 9. External steel plate; 10. Buffer spring; 11. Vertical connecting rod; 12. Pad block; 13. Telescopic steel wedge. Detailed Implementation

[0056] The technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the technical content of the embodiments described herein. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0057] Example 1

[0058] like Figure 1-5 As shown, a semi-cut-and-cover cover plate support structure includes a half-cover plate 1, a foundation pit concrete support 2, a central column 3, a connecting beam 4, a limiting block 5, a column top support 6, a fixing bolt 7, a connecting rod 8, an external steel plate 9, a buffer spring 10, a vertical connecting rod 11, a pad 12, and a retractable steel wedge 13. An external steel plate 9 is provided at the lower part of the half-cover plate 1. The lower end of the external steel plate 9 is directly connected to the central column 3 through the connecting rod 8. The two sides of the external steel plate 9 are connected to the foundation pit concrete support 2. A connecting beam 4 is installed inside the external steel plate 9. The top of the connecting beam 4 is connected to the half-cover plate 1. A column top support 6 is provided at the lower part of the connecting beam 4. Limiting blocks 5 are provided on both sides of the lower part of the connecting beam 4. A vertical connecting rod 11 is installed on the limiting block 5. A buffer spring 10 is fitted onto the vertical connecting rod 11 at the lower part of the limiting block 5.

[0059] like Figure 1-5 As shown, a semi-cut-off cover plate support structure is provided. The connecting beam 4 and the column top support 6 are fixedly connected by fixing bolts 7. The connecting rod 8 below the column top support 6 passes through the outer steel plate 9 and is directly connected to the central column 3. A pad 12 is installed at the connection between the limiting block 5 and the outer steel plate 9. A retractable steel wedge 13 is installed and inserted in the gap between the connecting beam 4 and the half-cover plate 1. The limiting block 5 is movably fitted on the vertical connecting rod 11. The lower surface of the limiting block 5 is directly connected to the buffer spring 10. The pad 12 between the limiting block 5 and the outer steel plate 9 is movably connected to the outer steel plate 9. The outer steel plate 9 is provided with a beam installation groove through an opening on its upper surface. The connecting beam 4 is installed in the beam installation groove and is fixedly connected to the column top support 6 below it.

[0060] like Figure 6-7 As shown, a method for evaluating the stability of a foundation pit includes the following steps:

[0061] Step 1: Select multiple sets of foundation pit planes with constant distances according to different spans, so that each set of foundation pit planes has a corresponding support structure. Select multiple key parts as monitoring points at the concrete supports, steel supports and column ends on both sides of the foundation pit for data collection.

[0062] Step 2: Compare the measured cover plate deflection and support axial force with the allowable values ​​set in the respective engineering designs. If they meet the safety standards, conduct a stability assessment of the foundation pit. According to relevant regulations, the maximum allowable value of cover plate deflection is one four-hundredth of the cover plate width, and the allowable value of support axial force generally does not exceed 70% of the design value of support axial force.

[0063] Step 3: Measure the horizontal displacement d of the surrounding retaining piles around the foundation pit at each of the monitoring points of the supporting structure. i and the span of the foundation pit D i Calculate the rate of change of horizontal displacement k within each of the foundation pit planes. 1i Where i = 1, 2...n; n is a positive integer, representing the number of key parts at the foundation pit support structure;

[0064] Step 4, based on the measured vertical displacement l at each of the monitoring points of the supporting structure. i The distance L between the monitoring point of the foundation pit and the initial excavation surface of the foundation pit. i The vertical displacement rate γ of each of the foundation pit support planes is calculated by dividing the foundation pit into three parts: top (concrete support), middle (steel support), and bottom. 1i γ 2i and γ 3i Simultaneously, weighting coefficients are allocated proportionally to obtain the overall vertical displacement rate of change k. 2i Where i = 1, 2...n; n is a positive integer, representing the number of key parts at the foundation pit support structure;

[0065] Step 5: Based on the horizontal displacement change rate k calculated in the previous steps... 1i Vertical displacement rate of change k 2i By proportionally allocating the weighting coefficients, the displacement stability parameter K2 is obtained.

[0066] Step 6: Based on the selected multiple sets of foundation pit planes, determine measurement points at constant distances on the surrounding ground in the direction away from the excavation face of the foundation pit, and record the elevation H of the soil layer around the pit. i And during the excavation stage of the foundation pit, the settlement h of the soil around the pit was measured regularly. i Calculate the external settlement change rate k at each of the measurement points. 3i , i.e., environmental stability parameter K3; where i = 1, 2...n; n is a positive integer, representing the number of key parts at the foundation pit support structure;

[0067] Step 7: Based on the displacement stability parameter K2 and environmental stability parameter K3 obtained in Steps 5 and 6, allocate weight coefficients proportionally to obtain the overall quantitative evaluation value K, which is used to evaluate the stability of the foundation pit in the project.

[0068] Step 8: Compare the quantitative evaluation value K with K0, where K0 is the evaluation standard value, and K0 = 0.1%;

[0069] Step 9: When K > K0, the monitored evaluation value is greater than the standard value, then the foundation pit support is strengthened and foundation pit instability early warning measures are taken; when K ≤ K0, the monitored evaluation value is less than or equal to the standard value, then the original plan remains unchanged.

[0070] In the above-described method for assessing the stability of a foundation pit, preferably, step 1 involves monitoring each support sampling point to achieve comprehensive coverage of the foundation pit stability monitoring, making the overall monitoring results more systematic, accurate, and effective.

[0071] In the foundation pit stability assessment method described above, preferably, the foundation pit span D in step 3... i The data was obtained by measuring with a laser rangefinder installed at the monitoring point, and the difference between several monitoring data points was compared to obtain the horizontal displacement d of the retaining pile. i .

[0072] In the foundation pit stability assessment method described above, preferably, the vertical displacement l in step 4... i The initial distance L between the monitoring point and the initial excavation face of the foundation pit i The top displacement can be directly obtained by reading the data on the scale on the connecting rod below the column top support, the middle displacement can be measured by a laser rangefinder, and the bottom displacement can be obtained by a level instrument using the leveling method.

[0073] In the foundation pit stability assessment method described above, preferably, the rate of change of horizontal displacement k at any support monitoring point in step 3 is... 1i Calculated according to formula (1), formula (1) is as follows:

[0074]

[0075] In formula (1), k 1i d is the rate of change of horizontal displacement. i and D i These are the horizontal displacement at the monitoring point of the supporting structure and the span of the foundation pit, respectively.

[0076] In the foundation pit stability assessment method described above, preferably, the vertical displacement change rate k at any support monitoring point in step 4 is... 2i Calculated according to formula (2), which is as follows:

[0077]

[0078] In formula (2), k 2i l is the rate of change of vertical displacement. iTo support the vertical displacement at the monitoring points of the supporting structure, L i This refers to the distance between the monitoring point of the foundation pit and the initial excavation surface of the foundation pit.

[0079] In the above-described method for evaluating the stability of a foundation pit, preferably, the displacement stability parameter K2 in step 5 is calculated according to formula (3), which is as follows:

[0080]

[0081] In formula (3), K2 is the displacement stability parameter, γi is the distribution coefficient, γi=0-1 and ∑γ i =1.

[0082] In the foundation pit stability assessment method described above, preferably, the external settlement change rate k at any support monitoring point in step 6 is... 3i That is, the environmental stability parameter K3 is calculated according to formula (4), which is as follows:

[0083]

[0084] In formula (4), K3 is the environmental stability parameter, k3 is the external settlement rate, and H i h is the elevation of the soil layer around the pit. i The soil around the pit has settled.

[0085] In the above-described method for evaluating the stability of a foundation pit, preferably, the overall quantitative evaluation value K in step 9 is calculated according to formula (5), which is as follows:

[0086] K = ε1K2 + ε2K3 (5)

[0087] In formula (5), K is the quantitative evaluation value, and ε i ε is the distribution coefficient. i =0-1 and ∑ε i =1.

[0088] The stability early warning method was verified using the foundation pit project of Hefei Metro Line 6 Science Avenue Metro Station. Based on the engineering data, the standard section width of the foundation pit is 23m, the cover plate width is 12m, and the allowable deflection value of the cover plate is... The axial force control value is 2000KN, and the actual measured values ​​are all within a reasonable range.

[0089] The maximum displacement of the foundation pit retaining structure is: 19.27 mm on the open-cut side and 15.95 mm on the cut-and-cover side;

[0090] Substituting into formula (1), we get: k 11 =0.084%, k 12 =0.069%;

[0091] The maximum vertical displacement of the top of the column is -3.56mm, which is the fifth step of the foundation pit construction, with an excavation depth of 8m; after construction, the uplift is 5.1mm, and the excavation depth is 18m.

[0092] Substituting into formula (2), we get: k 21 =0.045%;

[0093] If all the distribution coefficients are equal, and we substitute them into formula (3), we get: K2 = 0.061%;

[0094] The maximum surface settlement on both sides of the excavation pit is: 8.23 ​​mm on the open-cut side and 7.93 mm on the cut-and-cover side.

[0095] If all the distribution coefficients are equal, substituting them into formula (4) yields: k 31 =0.046%, k 32 =0.044%, K3=0.046%;

[0096] If all the distribution coefficients are equal, and we substitute them into formula (5), we get: K = 0.054% < K0;

[0097] The overall stability of the foundation pit met expectations, and the original construction plan will remain unchanged.

[0098] In this invention, the external steel plate is connected to the foundation pit concrete support, ensuring the support strength of the horizontal support; by installing a buffer spring support device on the lower side of the limiting block and the column top support, the overall structure can undergo vertical displacement within a certain safe range, reducing the impact of the vertical displacement of the central column on the foundation pit concrete support structure; by setting a retractable steel wedge on the external steel plate and the top of the column, the supporting effect of the central column on the cover plate structure is ensured when the central column settles and the vertical displacement is too large.

[0099] The pads installed at the connection between the limiting block and the external steel plate ensure that the overall structure can smoothly undergo vertical displacement; the scale on the connecting rod below the column top support can accurately indicate the vertical displacement of the central column; the provided evaluation method comprehensively considers multiple factors, unlike traditional evaluation methods that use a single factor as a standard for overall evaluation, making the evaluation method more reasonable; by separately measuring the top concrete support and the middle steel support in the foundation pit support, the evaluation method becomes more reasonable;

[0100] By measuring the settlement at different distances around the road surface at the initial excavation of the foundation pit, and calculating the external settlement change rate at each measurement point, the impact of the foundation pit excavation on the external road surface environment can be monitored in real time, making the evaluation method more comprehensive and three-dimensional. The overall stability of the foundation pit can be evaluated immediately after the on-site measurement data is collected, which can be used as a basis for scientific guidance and improve construction efficiency.

[0101] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A semi-cut-off cover plate support structure, characterized in that, It includes a half-width cover plate (1), a foundation pit concrete support (2), a central column (3), a connecting beam (4), a limiting block (5), a column top support (6), a fixing bolt (7), a connecting rod (8), an external steel plate (9), a buffer spring (10), a vertical connecting rod (11), a pad (12), and a retractable steel wedge (13); The lower part of the half-cover plate (1) is provided with an external steel plate (9). The lower end of the external steel plate (9) is directly connected to the central column (3) through the connecting rod (8). The two sides of the external steel plate (9) are connected to the foundation pit concrete support (2). The external steel plate (9) is equipped with a connecting beam (4). The top of the connecting beam (4) is connected to the half cover plate (1). The lower part of the connecting beam (4) is equipped with a column top support (6). Limiting blocks (5) are provided on both sides of the lower part of the connecting beam (4). A vertical connecting rod (11) is installed on the limiting block (5). A buffer spring (10) is fitted on the lower part of the limiting block (5) and the vertical connecting rod (11). The connecting beam (4) and the column top support (6) are fixedly connected by fixing bolts (7). The connecting rod (8) below the column top support (6) passes through the outer steel plate (9) and is directly connected to the middle column (3). A pad (12) is installed at the connection between the limiting block (5) and the outer steel plate (9). A retractable steel wedge (13) is installed and inserted in the gap between the connecting beam (4) and the half cover plate (1). The limiting block (5) is movably mounted on the vertical connecting rod (11). The lower surface of the limiting block (5) is directly connected to the buffer spring (10). The pad (12) between the limiting block (5) and the outer steel plate (9) is movably connected to the outer steel plate (9). The outer steel plate (9) has a beam mounting groove through an opening on its upper surface. A connecting beam (4) is installed in the beam mounting groove. The connecting beam (4) is fixedly connected to the column top support (6) below it.

2. A method for evaluating the stability of a foundation pit using a semi-cut-and-cover support structure as described in claim 1, characterized in that, Includes the following steps: Step 1: Select the supporting structure around the foundation pit, and select multiple key parts as monitoring points at the foundation pit concrete support (2), steel support and column ends on both sides of the foundation pit. Step 2: Compare the measured cover plate deflection and support axial force with the allowable values ​​set in the respective engineering designs, and conduct a stability assessment of the foundation pit if they meet the safety standards. Step 3, based on the measured horizontal displacement d at each of the monitoring points of the supporting structure. i and the span of the foundation pit D i Calculate the rate of change of horizontal displacement k within each of the foundation pit planes. 1i ; Step 4, based on the measured vertical displacement l at each of the monitoring points of the supporting structure. i The distance L between the monitoring point of the foundation pit and the initial excavation surface of the foundation pit. i The vertical displacement rate γ of each of the foundation pit support planes is calculated, dividing the foundation pit into three parts: top concrete support, middle steel support, and bottom support. 1i γ 2i and γ 3i Simultaneously, weighting coefficients are allocated proportionally to obtain the overall vertical displacement rate of change k. 2i ; Step 5: Based on the horizontal displacement change rate k calculated in the previous steps... 1i Vertical displacement rate of change k 2i By proportionally allocating the weighting coefficients, the displacement stability parameter K2 is obtained. Step 6: Based on the selected multiple sets of foundation pit planes, determine measurement points at constant distances on the surrounding ground in the direction away from the excavation face of the foundation pit, and record the elevation H of the soil layer around the pit. i And during the excavation stage of the foundation pit, the settlement h of the soil around the pit was measured regularly. i Calculate the external settlement change rate k at each of the measurement points. 3i , namely, the environmental stability parameter K3; Step 7: Based on the displacement stability parameter K2 and environmental stability parameter K3 obtained in Steps 5 and 6, allocate weight coefficients proportionally to obtain the overall quantitative evaluation value K, which is used to evaluate the stability of the foundation pit in the project. Step 8: Compare the quantitative evaluation value K with K0, where K0 is the evaluation standard value; Step 9: When K > K0, the monitored evaluation value is greater than the standard value, then the foundation pit support is strengthened and foundation pit instability early warning measures are taken; when K ≤ K0, the monitored evaluation value is less than or equal to the standard value, then the original plan remains unchanged.

3. The method for evaluating the stability of a foundation pit according to claim 2, characterized in that, The span D of the foundation pit in step 3 i The data was obtained by measuring with a laser rangefinder installed at the monitoring point, and the difference between several monitoring data points was compared to obtain the horizontal displacement d of the retaining pile. i .

4. The method for evaluating the stability of a foundation pit according to claim 2, characterized in that, The vertical displacement l in step 4 i The initial distance L between the monitoring point and the initial excavation face of the foundation pit i The top displacement can be directly obtained by reading the data on the scale on the connecting rod below the column top support, the middle displacement can be measured by a laser rangefinder, and the bottom displacement can be obtained by a level instrument using the leveling method.

5. The method for evaluating the stability of a foundation pit according to claim 2, characterized in that, The rate of change of horizontal displacement k at any support monitoring point in step 3 1i Calculated according to formula (1), formula (1) is as follows: In formula (1), k 1i d is the rate of change of horizontal displacement. i and D i These are the horizontal displacement at the monitoring point of the supporting structure and the span of the foundation pit, respectively.

6. The method for evaluating the stability of a foundation pit according to claim 2, characterized in that, The vertical displacement change rate k at any support monitoring point in step 4 2i Calculated according to formula (2), which is as follows: In formula (2), k 2i l is the rate of change of vertical displacement. i To support the vertical displacement at the monitoring points of the supporting structure, L i This refers to the distance between the monitoring point of the foundation pit and the initial excavation surface of the foundation pit.

7. The method for evaluating the stability of a foundation pit according to claim 2, characterized in that, In step 5, the displacement stability parameter K2 is calculated according to formula (3), which is as follows: In formula (3), K2 is the displacement stability parameter, and γ i γ is the distribution coefficient. i =0-1 and ∑γ i =1.

8. The method for evaluating the stability of a foundation pit according to claim 2, characterized in that, The external settlement change rate k at any support monitoring point in step 6 3i That is, the environmental stability parameter K3 is calculated according to formula (4), which is as follows: In formula (4), K3 is the environmental stability parameter, k3 is the external settlement rate, and H i h is the elevation of the soil layer around the pit. i The soil around the pit has settled.

9. The method for evaluating the stability of a foundation pit according to claim 2, characterized in that, In step 9, the overall quantitative evaluation value K is calculated according to formula (5), which is as follows: K = ε1K2 + ε2K3 (5) In formula (5), K is a quantitative evaluation value, and ε i ε is the distribution coefficient. i =0-1 and ∑ε i =1.

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