A construction method for steel frame support in excavation of underground pipeline restricted foundation pits

Abstract

This invention provides a construction method for steel frame support in excavation of underground pipeline-constrained foundation pits, belonging to the field of underground pipeline construction technology. The method includes the following steps: investigating the foundation pit environment, measuring and marking critical lines, and excavating exploratory pits to identify underground pipelines; designing a support system based on the foundation pit depth, placing I-beam sidewall supports on both sides at 50 cm intervals, and adding 116 I-beams for transverse connection; constructing an external drainage ditch at the top of the slope with a slope exceeding 3%; excavating in layers using a mini excavator, installing vertical I-beam supports and anchor bolts, and fixing them with connecting steel plates; installing two longitudinal reinforcing bars and mesh on the outer side, welded on one side; spraying concrete with a wet shotcrete machine at an air pressure of 0.5-0.7 MPa; removing temporary supports after the main structure reaches the required strength; monitoring the foundation pit, setting cross-sections every 15 meters, and adjusting the monitoring frequency according to the depth. This invention, while meeting the requirements of foundation pit construction, improves pipeline protection, reduces construction difficulty, and enhances support performance.

Description

Technical Field

[0001] This invention belongs to the field of underground pipeline construction technology, and more specifically, relates to a construction method for steel frame support in the excavation of restricted foundation pits for underground pipelines. Background Technology

[0002] Urban infrastructure construction is a crucial support for national economic development, and underground pipeline engineering is an important component of urban underground space development and utilization. Underground pipelines, including water supply pipes, drainage pipes, gas pipes, and power cables, are crisscrossed underground, providing basic public services to cities. With the continuous advancement of urbanization, underground pipeline construction is characterized by large scale, dense distribution, and high construction difficulty, posing numerous challenges to foundation pit excavation and construction.

[0003] First, the location of underground pipelines is complex and varied, making it difficult to accurately determine their distribution. Information such as the burial depth, direction, and intersection locations of pipelines often lacks accurate survey data. If a pipeline is breached during the excavation of the foundation pit, it can cause serious consequences, such as pipeline damage and pollution of the surrounding environment, posing a serious threat to construction safety and the stability of surrounding buildings.

[0004] Secondly, the soil around pipelines is usually relatively loose, resulting in poor stability of the excavation pit. The laying of underground pipelines alters the original geological structure, causing changes in soil strength and deformation characteristics, which greatly increases the difficulty of support during the excavation process. If the support structure is not properly designed, serious safety accidents such as pit deformation and collapse can easily occur.

[0005] Furthermore, underground pipelines occupy a significant amount of underground space, making the excavation workspace extremely limited. Traditional large machinery struggles to access such confined spaces, necessitating manual excavation, which is highly inefficient and prone to damaging the pipelines.

[0006] Furthermore, the weakness of the soil near the pipeline makes the foundation pit support more complex. Once the foundation pit deforms or the support structure is damaged, the pipeline will also be affected, leading to a decrease in the stability of the pipeline itself and causing serious consequences such as pipeline leakage and bursting.

[0007] Currently, in foundation pit construction in areas with dense underground pipelines, traditional support methods such as sheet pile support, reinforced concrete support, and rod support are mainly used. However, these methods have many shortcomings in practical applications:

[0008] 1. Although sheet pile support can form a closed support system, sheet piles are heavy and difficult to drive into the ground. At the same time, the internal space is small and not convenient for pipeline protection.

[0009] 2. Reinforced concrete support requires retaining walls to be set up at the edge of the foundation pit. In the pipeline area, manual excavation and local reinforcement are required. The process is complicated and the construction is difficult.

[0010] 3. Although needle rod support can effectively suppress the deformation of the foundation pit, the needle rod passing through the pipeline will cause pipeline damage, and the needle rod anchoring requires a lot of auxiliary equipment.

[0011] In general, existing support technologies for foundation pit excavation in areas with dense underground pipelines suffer from inadequate pipeline protection, high construction difficulty, and insufficient support structure performance, making it difficult to meet the needs of modern urban underground space development. Summary of the Invention

[0012] In view of this, the present invention provides a construction method for steel frame support in excavation of underground pipeline restricted foundation pits, which can improve pipeline protection, reduce construction difficulty, and improve support performance while meeting the requirements of foundation pit construction.

[0013] This invention is implemented as follows:

[0014] This invention provides a method for constructing a steel frame support structure for excavation of underground pipeline confined foundation pits, comprising the following steps:

[0015] S10. Investigate the surrounding environment of the foundation pit, measure and mark the centerline of the structure and the edge of the foundation pit, and use the method of excavating exploratory pits to determine the burial depth, direction and location of underground pipelines and manholes.

[0016] S20. Design a steel frame support system according to the depth of the foundation pit. Set 120 I-beams on both sides of the foundation pit as side wall supports. The spacing between the I-beams is 50 cm. Set 116 I-beams on both sides as lateral connection supports.

[0017] S30. Excavate a water interception ditch 2 meters outside the top of the slope on both sides of the foundation pit. The cross-section of the water interception ditch is 50 cm × 50 cm, and ensure that the slope of the water interception ditch is more than 3%.

[0018] S40. A mini excavator is used for layered excavation. The first layer is 3 meters deep, and each excavation is 1 meter long. The longitudinal slope is 1:0.5. The trench sidewalls are excavated vertically.

[0019] S50. Install vertical I-beam supports, and install anchor bolts at the arch foot to weld to the I-beams. The I-beams are fixed together with 20 cm × 26 cm × 1.2 cm connecting steel plates by bolts.

[0020] S60. Install longitudinal connecting bars and steel mesh on the outside of the I-beam. The longitudinal connecting bars are HRB400φ22 steel bars, arranged alternately with a vertical spacing of 50 cm, and connected to the I-beam by single-sided welding.

[0021] S70. A TK-961 wet spraying machine is used for concrete spraying. The working air pressure for concrete spraying is controlled within the range of 0.5 MPa to 0.7 MPa, and the distance between the nozzle and the side wall of the foundation pit is controlled within the range of 0.8 meters to 1.0 meters.

[0022] S80. Construction of the main structure: After the main structure reaches 70% of its design strength, the temporary top support shall be removed.

[0023] S90. Conduct foundation pit monitoring. Set up a monitoring section every 15 meters in the foundation pit. When the foundation pit depth is no more than 5 meters, the monitoring frequency is once every 2 days. When the foundation pit depth is between 5 meters and 10 meters, the monitoring frequency is once a day.

[0024] Based on the above technical solution, the construction method of steel frame support for underground pipeline confined pit excavation according to the present invention can be further improved as follows:

[0025] The underground pipeline confined excavation pit steel frame support includes: a main support assembly, a locking assembly, and a connecting assembly; the main support assembly includes a vertical I-beam frame, a horizontal reinforcing plate, and a bottom support base; the vertical I-beam frame is formed by connecting multiple 120 I-beams through the horizontal reinforcing plate, the horizontal reinforcing plate has a rectangular structure with arc-shaped notches at the four corners, and a reinforcing rib is provided in the middle of the horizontal reinforcing plate, the reinforcing rib is arranged in a star shape and welded to the horizontal reinforcing plate; the bottom support base includes a base plate, an adjusting screw, and a support pad, the base plate has multiple adjusting screw holes, the adjusting screw passes through the adjusting screw holes and is fixed by a nut, the top of the adjusting screw is welded to the support pad, and the support pad has a concave groove for installing the vertical I-beam frame.

[0026] Furthermore, the locking assembly includes an arch foot locking plate, a locking foot anchor rod, and a reinforcing connecting plate; the arch foot locking plate has an L-shaped structure, with its vertical portion welded to the vertical I-beam frame, and its horizontal portion having multiple anchor rod mounting holes, each containing an anchor rod sleeve; one end of the locking foot anchor rod is threaded to the anchor rod sleeve, and the other end is an enlarged diameter section; the reinforcing connecting plate is located between the arch foot locking plate and the vertical I-beam frame, has a triangular structure, and has reinforcing ribs on its edges.

[0027] Furthermore, the connecting assembly includes an I-beam connector, a limiting block, and a stiffening plate; the I-beam connector consists of two symmetrical connecting steel plates, with multiple bolt through holes evenly distributed on the connecting steel plates; the limiting block is welded to the inner side of the connecting steel plates to limit the position of the I-beam; the connecting steel plates are used to fix adjacent vertical I-beam frames together with fastening bolts; a stiffening plate is provided on the outer side of the connecting steel plates, and the stiffening plate is arranged in a herringbone pattern.

[0028] Furthermore, rubber buffer pads are provided at the four arc-shaped notches of the transverse reinforcing plate. The rubber buffer pads are embedded in the arc-shaped notches through annular grooves. Sealing strips are provided at the edges of the transverse reinforcing plate to prevent concrete from entering the interior of the I-beam during spraying.

[0029] Furthermore, the adjusting screw of the bottom support is a two-way threaded rod, with an adjusting ring in its middle. The lifting and lowering of the support pad is achieved by rotating the adjusting ring; the inner wall of the concave groove of the support pad is provided with anti-slip texture.

[0030] Furthermore, the connecting steel plate of the I-beam connector is provided with anti-loosening teeth, and the anti-loosening teeth are distributed in a ring with the contact surface of the fastening bolt; the weld between the stiffening plate and the connecting steel plate is double-sided welded, and the weld length is not less than 80% of the height of the stiffening plate.

[0031] Furthermore, the appropriate area ratio between the transverse reinforcing plate and the I-beam is defined by the principles of structural mechanics.

[0032] Furthermore, the angle of the star-shaped reinforcing ribs is optimized by using trigonometric functions to achieve the optimal angle distribution of the reinforcing ribs, thereby enhancing the support effect.

[0033] Furthermore, the height adjustment of the bottom support is limited by introducing an exponential decay to reflect the deformation characteristics of the support during use.

[0034] Step S10 specifically includes:

[0035] Step 101: Use a GPS total station to conduct three-dimensional coordinate measurement of the construction site. The measurement accuracy should be controlled within 2 mm in the horizontal direction and within 1 mm in the vertical direction. The distance between measurement points should not exceed 5 meters. For areas with large terrain changes, the measurement points should be densified. Based on the measurement data, a three-dimensional surface fitting algorithm should be used to construct a digital elevation model of the site.

[0036] Step 102: Ground penetrating radar (GPR) technology is used to detect underground pipelines. The detection depth reaches 1.5 times the excavation depth of the foundation pit, and the detection accuracy is controlled within 5% of the pipeline burial depth. An orthogonal grid scanning method is used, with a grid spacing of no more than 1 meter. Wavelet denoising algorithm is used to process the detection signal. For metal pipelines, electromagnetic induction method is used for re-measurement, with the re-measurement accuracy controlled within 10 centimeters. For non-metallic pipelines, ultrasonic detection method is used for re-measurement, with the transmission frequency controlled between 50 kHz and 200 kHz. Based on the detection data, a three-dimensional model of the underground pipeline is established using the Kriging interpolation algorithm, with an interpolation grid spacing of no more than 20 centimeters.

[0037] Step 103: Mark the control lines of the structure according to the measurement and layout data. Use fluorescent paint to spray the lines, and control the line width to within 3 cm. Set permanent marker stakes on site. The marker stakes are made of concrete and the top is pre-embedded with positioning steel nails.

[0038] The specific steps of step S20 include:

[0039] Step 201: Determine soil parameters based on the geological survey report, including soil unit weight, internal friction angle, cohesion, etc., and use the Delwin fuzzy comprehensive evaluation method to assess the stability of the strata. The evaluation indicators include soil strength, groundwater level, strata lithology, etc.

[0040] Step 202: Establish a numerical model for foundation pit excavation based on the finite element method, calculate the deformation of the foundation pit and the internal forces of the support structure. In the model, the soil adopts the Mohr-Coulomb constitutive model, the support structure adopts the elastic model, and the contact surface adopts the Coulomb friction model. The calculation conditions include foundation pit excavation, support structure construction, dewatering, etc. The calculation step length is controlled at 10% of the excavation depth.

[0041] Step 203: Determine the specifications and spacing of the I-beams based on the calculation results. The selection of I-beams must meet the strength and stability requirements. The strength verification adopts the limit state method with a safety factor of not less than 1.5. The stability verification adopts the energy method with a critical load factor of not less than 2. The transverse connection supports are optimized by a grid search algorithm. The support spacing is not greater than 3 meters in both the vertical and horizontal directions. The gap between the support and the main structure is not less than 10 centimeters. The overall stability of the support structure is verified by the simplified Bishop method with a safety factor of not less than 1.3.

[0042] The specific steps of step S30 include:

[0043] Step 301: Use a laser level to measure the site topography, determine the catchment area and flow direction, and control the level accuracy to 1 mm per meter; determine the cross-sectional dimensions of the intercepting ditch based on hydraulic calculations, calculate the design flow rate using the Manning formula, the longitudinal slope of the ditch bottom is not less than 3%, the roughness coefficient is taken as 0.015, and the design return period is taken as 5 years.

[0044] Step 302: The intercepting ditch adopts a trapezoidal cross section with a bottom width of 50 cm, a depth of 50 cm, and a side slope ratio of 1:0.5. It is made of C20 concrete cast in place, and the concrete strength acceptance standard is not less than 25 MPa. A sedimentation tank is set every 20 meters in the intercepting ditch. The tank is 70 cm deep and has a volume of not less than 1 cubic meter for sedimentation.

[0045] Step 303: Install energy dissipation facilities at the connection between the intercepting ditch and the drainage system, using drop wells with a depth of not less than 1 meter and equipped with energy dissipation teeth.

[0046] The specific steps of step S40 include:

[0047] Step 401: Use the layered excavation method, with each layer having an excavation depth of no more than 3 meters and an excavation surface slope of 1:0.5; use a mini tracked excavator for excavation, with a bucket capacity of 0.3 cubic meters and a maximum excavation depth of no less than 7 meters.

[0048] Step 402: The length of the segmented excavation is controlled within 1 meter. The construction method is staggered and the distance between adjacent excavation segments is not less than 3 meters. When encountering underground pipelines, a 50-centimeter protection zone is reserved on both sides of the pipeline for manual excavation. When the excavation depth reaches 20 centimeters below the bottom of the pipeline, manual cleaning is used.

[0049] Step 403: Small dump trucks are used for earthwork transportation, with the transportation distance controlled within 2 kilometers. The transportation route avoids residential areas and main roads. During the excavation process, a total station is used to monitor the deformation of the foundation pit in real time. The distance between monitoring points is no more than 5 meters, the monitoring frequency is twice a day, and the deformation warning value is 20 millimeters.

[0050] The specific steps of step S50 include:

[0051] Step 501: Before installation, the I-beams should be treated with anti-corrosion coating using epoxy zinc-rich primer with a coating thickness of not less than 80 micrometers. The I-beams should be positioned using a laser line projector, with the verticality deviation controlled within three per thousand and the spacing error between adjacent I-beams not exceeding 10 millimeters.

[0052] Step 502: The I-beam and the connecting steel plate are connected with high-strength 10.9 grade bolts, with a tightening torque of not less than 100 Nm, and tightened in stages using a box wrench; the anchor rod is made of Φ48 mm seamless steel pipe with a wall thickness of 4 mm, the anchor rod length is not less than 2 meters, the inclination angle is 15 to 20 degrees, and the anchoring end is anchored with cement mortar with a mortar strength grade of not less than M30;

[0053] Step 503: The I-beam and the anchor rod are welded on both sides. The weld height is not less than 8 mm. The welding is done by manual electric arc welding with E50 welding rod. The preheating temperature before welding is not less than 100 degrees Celsius.

[0054] The specific steps of step S60 include:

[0055] Step 601: The longitudinal connecting steel bars are HRB400 grade steel bars with a diameter of 22 mm and a longitudinal spacing of 50 cm, arranged in a staggered manner; the bending angle of the end hook of the steel bar is 135 degrees, and the inner diameter of the bend is not less than 5 times the diameter of the steel bar; the steel bars are welded to the I-beams using fillet welds with a specification of 8 mm and a weld length of not less than 10 times the diameter of the steel bar.

[0056] Step 602: The steel mesh uses Φ8 mm steel bars, with a mesh size of 20 cm x 20 cm. The lap length is not less than 35 times the diameter of the steel bar, and it is fixed with wire. The spacing between the binding points is not greater than 50 cm.

[0057] The specific steps of step S70 include:

[0058] Step 701: The strength grade of the shotcrete shall not be lower than C25. The mix proportion shall be optimized by orthogonal test method. The water-cement ratio shall be controlled within 0.45. The dosage of early strength agent shall be 2% of the cement dosage, and the dosage of water-reducing agent shall be 1% of the cement dosage.

[0059] Step 702: Use wet spraying process, control the concrete slump to 10 cm to 14 cm, spray pressure to 0.5 MPa to 0.7 MPa, nozzle distance from the pit sidewall to 0.8 m to 1.0 m, spray angle perpendicular to the sidewall; single layer spray thickness not exceeding 5 cm, layer spraying time interval not exceeding 2 hours, total thickness not less than 12 cm;

[0060] Step 703: During the spraying process, rebound amount detection is used to control the spraying quality. The rebound amount shall not exceed 25%, and the rebound material shall be cleaned up in time.

[0061] The specific steps of step S80 include:

[0062] Step 801: The main structure construction adopts the bottom-up method, starting from the bottom of the foundation pit. The concrete strength grade is not lower than C30, the impermeability grade is not lower than P6, and the reinforcement ratio is not less than 0.2%.

[0063] Step 802: Concrete is poured in layers, with each layer not exceeding 50 cm in thickness. Vibration is performed using an immersion vibrator for 15 to 20 seconds. Curing is carried out by covering with geotextile and sprinkling with water for at least 14 days.

[0064] Step 803: Temporary supports may be removed only after the main structure has reached 70% of its design strength. The removal sequence is the reverse of the construction sequence, proceeding from top to bottom.

[0065] The specific steps of step S90 include:

[0066] Step 901: Use a total station to monitor the horizontal displacement of the top of the foundation pit. Set up a monitoring section every 15 meters in the foundation pit, with no less than 3 monitoring points in each section. When the depth of the foundation pit is no more than 5 meters, the monitoring frequency is once every 2 days. When the depth of the foundation pit is between 5 and 10 meters, the monitoring frequency is once a day.

[0067] Step 902: Use a level to monitor the surface settlement around the foundation pit. The monitoring points are located at the edge of the foundation pit and within a range of twice the depth of the foundation pit from the edge of the foundation pit.

[0068] Step 903: The monitoring data is processed using the Kalman filter algorithm to remove outliers and calculate the deformation rate and acceleration. When the monitoring data reaches the warning value, reinforcement measures must be taken immediately. The warning values ​​include: horizontal displacement of 20 mm, surface settlement of 30 mm, deformation rate of 2 mm per day, and acceleration of 0.1 mm per day.

[0069] The optimal area ratio between the transverse stiffening plate and the I-beam is determined using structural mechanics principles. Specifically, it is determined by a geometric relationship equation between the transverse stiffening plate and the I-beam, as described below:

[0070]

[0071] In the formula, A p Area of ​​the transverse reinforcement plate (square millimeters); A b α is the area of ​​the I-beam flange (square millimeters); h is the height of the I-beam (millimeters); b is the width of the I-beam flange (millimeters); k1 is the area ratio coefficient, ranging from 0.4 to 0.6; α1 is the correction coefficient, ranging from 0.1 to 0.2.

[0072] This equation is based on stress analysis and considers the optimal ratio between the area of ​​the transverse stiffening plate and the area of ​​the I-beam flange. The 1.5 power is used because the growth rate of the required transverse stiffening plate area gradually decreases as the height of the I-beam increases.

[0073] The optimal angle distribution of the star-shaped reinforcing ribs is achieved through trigonometric functions. Specifically, it is calculated using the star-shaped reinforcing rib angle optimization equation, as described below:

[0074]

[0075] In the formula, θ is the angle (in radians) between the reinforcing rib and the horizontal line; L is the length of the transverse reinforcing plate (mm); W is the width of the transverse reinforcing plate (mm); β is the angle adjustment coefficient, with a value range of 0.2-0.3; and n is the number of the reinforcing rib (1-8).

[0076] This equation is used to determine the optimal arrangement angle of the cross-shaped reinforcing ribs, taking into account the aspect ratio of the transverse reinforcing plate, and achieving a uniform distribution of the angle through a sine function.

[0077] The height adjustment of the bottom support is limited by introducing an exponential decay to reflect the deformation characteristics of the support during use. Specifically, it is limited by the bottom support height adjustment relationship equation, as described below:

[0078]

[0079] In the formula, H aH0 is the height of the adjusted support (mm); P is the actual bearing capacity (kN); P0 is the design bearing capacity (kN); γ is the deformation correction coefficient, ranging from 0.05 to 0.15; λ is the time decay coefficient, ranging from 0.001 to 0.003; t is the usage time (days).

[0080] This equation takes into account the settlement and deformation of the support during use, uses an exponential decay term to reflect the deformation characteristics of the support over time, and introduces a time factor to reflect the long-term performance of the structure.

[0081] In addition, this invention also provides a strength verification equation for I-beam connectors, based on the combined stress theory of materials mechanics, to ensure the strength of the connectors, as described in detail below:

[0082]

[0083] In the formula, σ max M is the maximum stress (MPa); M is the bending moment (kN·m); W x σ is the section modulus (cubic centimeters); N is the axial force (kilonewtons); A is the cross-sectional area of ​​the connector (square centimeters); k2 is the stress concentration factor, ranging from 1.2 to 1.5; [σ] is the allowable stress (megapascals).

[0084] Furthermore, this invention also provides an optimization equation for the geometric dimensions of the arch foot locking plate, considering the stress characteristics of the locking plate to achieve the optimal thickness ratio of the vertical and horizontal parts, as described in detail below:

[0085]

[0086] In the formula, t f The thickness (mm) of the vertical portion of the locking plate; t w h represents the thickness (mm) of the horizontal portion of the locking plate. f The vertical height is shown in millimeters; b f η is the width of the horizontal portion (mm); η is the thickness proportionality coefficient, ranging from 0.8 to 1.2; τ is the actual shear stress (MPa); τ0 is the reference shear stress (MPa).

[0087] To address the aforementioned technical challenges in excavation of foundation pits in areas with dense underground pipelines, this invention proposes a steel frame support construction method for foundation pit excavation with confined underground pipelines. This method, through optimized support structure design, effectively addresses the construction difficulties in environments with dense underground pipelines while ensuring the safety and stability of foundation pit excavation, demonstrating significant technological innovation.

[0088] Firstly, in the early stages of foundation pit excavation, this invention employs advanced technologies such as GPS total stations and ground-penetrating radar to conduct a comprehensive survey of the construction site, accurately obtaining key information such as the location, depth, and direction of underground pipelines. This provides a reliable basis for the subsequent design optimization of the support structure. This not only effectively reduces the risk of pipeline damage but also provides important references for the design parameters of the steel support system.

[0089] Secondly, this invention uses 120 H-beams as the main support, supplemented by 116 H-beams for lateral reinforcement, forming a reliable support framework. The H-beams are securely connected by connecting steel plates and high-strength bolts, ensuring overall rigidity and stability. Longitudinal reinforcing bars and steel mesh are also arranged on the outside of the H-beams, further enhancing the overall performance of the support structure. Simultaneously, adjustable support pads are installed at the bottom of the excavation pit to adapt to deformation characteristics under different geological conditions. This support structure not only meets the strength and stability requirements during the excavation process but also possesses sufficient rigidity and deformation capacity, effectively protecting adjacent underground pipelines.

[0090] Furthermore, this invention employs a layered, segmented construction method for the foundation pit excavation process, controlling the excavation depth to within 3 meters each time, and utilizing a mini excavator to significantly reduce the construction space. In areas near pipelines, manual excavation is used to ensure pipeline safety. Simultaneously, intercepting ditches and sedimentation tanks are installed around the foundation pit to effectively prevent surface water from entering, improving the controllability of the construction environment.

[0091] Furthermore, this invention also employs a series of quality control measures in key construction stages such as the installation of the support structure and concrete spraying, including anti-corrosion treatment of the I-beams, laser positioning to ensure installation accuracy, and optimization of spraying parameters. Simultaneously, a comprehensive monitoring system has been established to monitor the deformation of the foundation pit in real time, and reinforcement measures are immediately implemented once a warning value is detected, ensuring the safety and controllability of the entire construction process.

[0092] In summary, compared with traditional support methods, the steel frame support construction method for underground pipeline confined pit excavation of the present invention has the following significant technical advancements:

[0093] 1. Strong pipeline protection capability: By accurately surveying the pipeline location and using an adjustable support base, damage to the pipeline is effectively avoided.

[0094] 2. Reduced construction difficulty: By adopting measures such as layered and segmented excavation and small-scale machinery operation, the construction space has been greatly reduced and the operation efficiency has been improved.

[0095] 3. Superior support performance: The support structure has high rigidity and good overall stability, which can meet the requirements of foundation pit support in underground pipeline environments.

[0096] 4. Comprehensive quality control measures: From material selection to construction monitoring, targeted quality control measures have been adopted at every stage to ensure construction quality.

[0097] Therefore, this invention provides a safe and efficient solution for excavation of foundation pits in areas with dense underground pipelines, which has important practical significance for the development of urban underground space. Attached Figure Description

[0098] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0099] Figure 1 A flowchart of a construction method for steel frame support in the excavation of a restricted underground pipeline pit;

[0100] Figure 2 A schematic diagram of a steel frame support construction method for excavation of underground pipeline restricted foundation pits;

[0101] Figure 3 A cross-sectional view of a transverse reinforcement plate in a construction method for steel frame support in excavation of underground pipeline restricted foundation pits;

[0102] The attached diagram lists the components represented by each number as follows:

[0103] 1. Main support components; 11. Vertical I-beam frame; 12. Horizontal reinforcing plate; 13. Bottom support base; 2. Locking components; 21. Arch foot locking plate; 22. Lock foot anchor rod; 23. Reinforcing connecting plate; 3. Connecting components; 31. I-beam connector; 32. Limiting block; 33. Stiffening plate. Detailed Implementation

[0104] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0105] like Figure 1 The image shown is a first embodiment of a steel frame support construction method for underground pipeline confined excavation pits provided by the present invention. This embodiment includes the following steps:

[0106] S10. Investigate the surrounding environment of the foundation pit, measure and mark the centerline of the structure and the edge of the foundation pit, and use the method of excavating exploratory pits to determine the burial depth, direction and location of underground pipelines and manholes.

[0107] S20. Design a steel frame support system according to the depth of the foundation pit. Set 120 I-beams on both sides of the foundation pit as side wall supports. The spacing between the I-beams is 50 cm. Set 116 I-beams on both sides as lateral connection supports.

[0108] S30. Excavate a water interception ditch 2 meters outside the top of the slope on both sides of the foundation pit. The cross-section of the water interception ditch is 50 cm × 50 cm, and ensure that the slope of the water interception ditch is more than 3%.

[0109] S40. A mini excavator is used for layered excavation. The first layer is 3 meters deep, and each excavation is 1 meter long. The longitudinal slope is 1:0.5. The trench sidewalls are excavated vertically.

[0110] S50. Install vertical I-beam supports, and install anchor bolts at the arch foot to weld to the I-beams. The I-beams are fixed together with 20 cm × 26 cm × 1.2 cm connecting steel plates by bolts.

[0111] S60. Install longitudinal connecting bars and steel mesh on the outside of the I-beam. The longitudinal connecting bars adopt... The reinforcing bars are arranged vertically at 50-centimeter intervals, alternating between the inner and outer sides, and are connected to the I-beams by single-sided welding.

[0112] S70. Use a TK-961 wet spraying machine for concrete spraying. The working air pressure for concrete spraying is controlled within the range of 0.5 MPa to 0.7 MPa, and the distance between the nozzle and the side wall of the foundation pit is controlled within the range of 0.8 meters to 1.0 meters.

[0113] S80. Construction of the main structure: After the main structure reaches 70% of its design strength, the temporary top support shall be removed.

[0114] S90. Conduct foundation pit monitoring. Set up a monitoring section every 15 meters in the foundation pit. When the foundation pit depth is no more than 5 meters, the monitoring frequency is once every 2 days. When the foundation pit depth is between 5 meters and 10 meters, the monitoring frequency is once a day.

[0115] Regarding step S10, the purpose of this step is to comprehensively collect information from the construction site to provide accurate data support for subsequent construction. First, a GPS total station is used to conduct on-site three-dimensional coordinate measurements, with measurement accuracy controlled within 2 mm horizontally and 1 mm vertically. The distance between measurement points is no greater than 5 meters, and the number of measurement points is increased in areas with significant terrain variations. After the measurements are completed, a three-dimensional surface fitting algorithm is used to construct a digital elevation model of the site. This algorithm uses the least squares method to determine the surface undulations. Then, ground-penetrating radar technology is used to detect underground pipelines. The detection depth needs to reach 1.5 times the excavation depth of the foundation pit, and the detection accuracy is controlled within 5% of the pipeline burial depth. An orthogonal grid scanning method is used during detection, with a grid spacing no greater than 1 meter. Wavelet denoising algorithms are used to process the detection signals to improve signal quality. For metal pipelines, electromagnetic induction is used for re-measurement. An alternating magnetic field is generated by a transmitting coil, and the induced current is detected by a receiving coil to determine the pipeline location. The re-measurement accuracy is controlled within 10 centimeters. For non-metallic pipelines, ultrasonic testing was used for re-measurement, with the transmission frequency controlled between 50 kHz and 200 kHz. The received echo signal was processed using an adaptive filtering algorithm. A three-dimensional model of the underground pipeline was established based on the detection data, and the pipeline route was fitted using the Kriging interpolation algorithm, with the interpolation grid spacing not exceeding 20 cm to ensure model accuracy. Finally, control lines for the structures were marked according to the measurement and layout data. The control lines were painted with fluorescent paint, with the line width controlled within 3 cm, and permanent marker stakes were set on site. The marker stakes were made of poured concrete, with positioning steel nails pre-embedded at the top.

[0116] Regarding step S20, the purpose of this step is to design a safe and reliable support structure system. First, soil parameters, including soil unit weight, internal friction angle, and cohesion, are determined based on the geological survey report. The Delwin fuzzy comprehensive evaluation method is used to assess the stability of the strata, with evaluation indicators including soil strength, groundwater level, and stratum lithology. Then, a numerical model of the foundation pit excavation is established based on the finite element method to calculate the foundation pit deformation and the internal forces of the support structure. In the model, the soil adopts the Mohr-Coulomb constitutive model, the support structure adopts an elastic model, and the contact surface adopts the Coulomb friction model. Calculation conditions include foundation pit excavation, support structure construction, and dewatering, with the calculation step controlled at 10% of the excavation depth. Based on the calculation results, the specifications and spacing of the I-beams are determined. The selection of I-beams must meet strength and stability requirements. Strength verification uses the limit state method with a safety factor of not less than 1.5, and stability verification uses the energy method with a critical load factor of not less than 2. The transverse connecting supports are optimized using a grid search algorithm, with support spacing not exceeding 3 meters in both the vertical and horizontal directions, and a minimum clearance of 10 centimeters between the supports and the main structure. The overall stability of the support structure is verified using a simplified Bishop method, with a safety factor of not less than 1.3.

[0117] Regarding step S30, the purpose of this step is to prevent surface water from entering the foundation pit. First, a laser level is used to measure the site topography, determining the catchment area and flow direction. The level's accuracy is controlled to 1 mm per meter. The cross-sectional dimensions of the intercepting ditch are determined based on hydraulic calculations. The design flow rate is calculated using the Manning formula, with a longitudinal slope of not less than 3% at the bottom, a roughness coefficient of 0.015, and a design return period of 5 years. The intercepting ditch adopts a trapezoidal cross-section, with a bottom width of 50 cm, a depth of 50 cm, and a side slope ratio of 1:0.5. It is constructed using C20 cast-in-place concrete, with a concrete strength acceptance standard of not less than 25 MPa. A sedimentation tank is installed every 20 meters along the intercepting ditch, with a depth of 70 cm and a volume of not less than 1 cubic meter, for sedimentation. Energy dissipation facilities are installed at the connection between the intercepting ditch and the drainage system, using a drop well design, with a depth of not less than 1 meter and internal stilling teeth.

[0118] Regarding step S40, the purpose of this step is to safely and efficiently complete the foundation pit excavation. A layered excavation method is adopted, with each layer's excavation depth not exceeding 3 meters, and the excavation surface slope at 1:0.5. Mini excavators are used for excavation; the selection of equipment must consider the limitations of the working space, and tracked excavators are recommended, with a bucket capacity of 0.3 cubic meters and a maximum excavation depth of not less than 7 meters. The length of each excavation segment is controlled to 1 meter, and a staggered construction method is used, with an offset between adjacent excavation segments of not less than 3 meters. When encountering underground pipelines, a 50-centimeter protection zone is reserved on both sides of the pipeline for manual excavation; when the excavation depth reaches 20 centimeters below the bottom of the pipeline, manual clearing is used. Small dump trucks are used for earthmoving, with the transport distance controlled within 2 kilometers, and the transport route avoiding residential areas and main roads. During the excavation process, a total station is used to monitor the foundation pit deformation in real time, with monitoring points spaced no more than 5 meters apart, monitored twice daily, and a deformation warning value of 20 millimeters.

[0119] Regarding step S50, the purpose of this step is to ensure the integrity and stability of the support structure. Before installation, the I-beams undergo anti-corrosion treatment using epoxy zinc-rich primer, with a coating thickness of not less than 80 micrometers. I-beam positioning is achieved using a laser line projector, with verticality deviation controlled within 0.3%, and the spacing error between adjacent I-beams not exceeding 10 mm. The I-beams are connected to the connecting steel plates using high-strength bolts, with a bolt grade not lower than 10.9, a tightening torque not less than 100 Nm, and graded tightening using a box wrench. The anchor bolts are made of Φ48 mm seamless steel pipe with a wall thickness of 4 mm, an anchor bolt length not less than 2 meters, an inclination angle of 15 to 20 degrees, and cement mortar anchoring at the anchoring end, with a mortar strength grade not lower than M30. The I-beams and anchor bolts are double-sided welded, with a weld height not less than 8 mm, using manual arc welding with E50 welding rods, and a preheating temperature not lower than 100 degrees Celsius.

[0120] Regarding step S60, the purpose of this step is to enhance the overall integrity of the support structure. The longitudinal connecting reinforcement uses HRB400 grade steel bars, 22 mm in diameter, spaced 50 cm apart, and staggered to increase the spatial stress-bearing effect. The end hooks of the steel bars are bent at a 135-degree angle, with an inner diameter of not less than 5 times the steel bar diameter. The steel bars are welded to the I-beams using fillet welds, 8 mm in diameter, with a weld length not less than 10 times the steel bar diameter. The reinforcing mesh uses Φ8 mm steel bars, with a mesh size of 20 cm x 20 cm, an overlap length not less than 35 times the steel bar diameter, and is secured with wire ties, with tying points spaced no more than 50 cm apart.

[0121] Regarding step S70, the purpose of this step is to form the concrete support surface. The shotcrete strength grade shall not be lower than C25, the mix proportion shall be optimized using orthogonal experimental design, the water-cement ratio shall be controlled within 0.45, the early-strength agent dosage shall be 2% of the cement weight, and the water-reducing agent dosage shall be 1% of the cement weight. A wet spraying process shall be adopted, the concrete slump shall be controlled between 10 cm and 14 cm, the spraying pressure shall be between 0.5 MPa and 0.7 MPa, the nozzle distance from the pit sidewall shall be between 0.8 m and 1.0 m, and the spraying angle shall be perpendicular to the sidewall. The thickness of a single layer of shotcrete shall not exceed 5 cm, the time interval between layers of shotcrete shall not exceed 2 hours, and the total thickness shall not be less than 12 cm. During the shotcreting process, rebound measurement shall be used to control the shotcreting quality, the rebound amount shall not exceed 25%, and the rebound material shall be cleaned up in a timely manner.

[0122] Regarding step S80, the purpose of this step is to achieve coordination between the main structure and the supporting structure. The main structure is constructed using a bottom-up method, starting from the bottom of the foundation pit. The concrete strength grade is not lower than C30, the impermeability grade is not lower than P6, and the reinforcement ratio is not less than 0.2%. Concrete pouring is done in layers, with each layer not exceeding 50 cm in thickness. An immersion vibrator is used for compaction, with a compaction time of 15 to 20 seconds. Curing is achieved by covering with geotextile and sprinkling water, with a curing time of not less than 14 days. Temporary supports can only be removed after the main structure reaches 70% of its design strength, and the removal sequence is the reverse of the construction sequence, proceeding from top to bottom.

[0123] Regarding step S90, the purpose of this step is to achieve dynamic monitoring of the construction process. A total station is used to monitor the horizontal displacement of the top of the foundation pit, with a monitoring section placed every 15 meters in the pit, and each section having no fewer than 3 monitoring points. When the pit depth is no more than 5 meters, the monitoring frequency is once every two days; when the pit depth is between 5 and 10 meters, the monitoring frequency is once a day. A level is used to monitor the ground settlement around the pit, with monitoring points placed at the edge of the pit and within a range of twice the pit depth from the edge. The monitoring data is processed using a Kalman filter algorithm to remove outliers, and the deformation rate and acceleration are calculated. When the monitoring data reaches the warning values, immediate reinforcement measures must be taken. The warning values ​​include: horizontal displacement of 20 mm, ground settlement of 30 mm, deformation rate of 2 mm per day, and acceleration of 0.1 mm per day.

[0124] The meanings of the formula symbols used in this invention are shown in Table 1:

[0125]

[0126]

[0127] like Figure 2 , Figure 3 As shown, in the above technical solution, the steel frame support for the excavation of the underground pipeline confined foundation pit includes:

[0128] The main support assembly 1, the locking assembly 2, and the connecting assembly 3 are provided. The main support assembly 1 is used to support the foundation pit. The locking assembly 2 is located on the side wall of the main support assembly 1 and is used to support the side wall of the main support assembly 1. The connecting assembly 3 is located inside an adjacent main support assembly 1 and is used to connect and support the adjacent main support assembly 1.

[0129] The main support assembly 1 includes a vertical I-beam frame 11, a horizontal reinforcing plate 12, and a bottom support base 13. The vertical I-beam frame 11 is formed by connecting multiple 120 I-beams through the horizontal reinforcing plate 12. The horizontal reinforcing plate 12 has a rectangular structure and arc-shaped notches at the four corners. A reinforcing rib is provided in the middle of the horizontal reinforcing plate 12. The reinforcing rib is arranged in a star shape and welded to the horizontal reinforcing plate 12. The bottom support base 13 includes a base plate, an adjusting screw, and a support pad. The base plate has multiple adjusting screw holes. The adjusting screw passes through the adjusting screw holes and is fixed by a nut. The top of the adjusting screw is welded to the support pad. The support pad has a concave groove for installing the vertical I-beam frame 11.

[0130] Furthermore, in the above technical solution, the locking assembly 2 includes an arch foot locking plate 21, a locking foot anchor rod 22, and a reinforcing connecting plate 23; the arch foot locking plate 21 has an L-shaped structure, its vertical part is welded to the vertical I-beam frame 11, and its horizontal part has multiple anchor rod installation holes, with anchor rod sleeves installed in the anchor rod installation holes; one end of the locking foot anchor rod 22 is threaded to the anchor rod sleeve, and the other end is an enlarged diameter section; the reinforcing connecting plate 23 is disposed between the arch foot locking plate 21 and the vertical I-beam frame 11, has a triangular structure, and has reinforcing ribs on its edges.

[0131] Furthermore, in the above technical solution, the connecting component 3 includes an I-beam connector 31, a limiting block 32, and a stiffening plate; the I-beam connector 31 is composed of two symmetrical connecting steel plates, with multiple bolt through holes evenly provided on the connecting steel plates; the limiting block 32 is welded to the inner side of the connecting steel plates to limit the position of the I-beam; the connecting steel plates are fixedly connected to adjacent vertical I-beam frames 11 through stiffening plates 33, and stiffening plates are provided on the outer side of the connecting steel plates, with the stiffening plates arranged in a herringbone shape.

[0132] Furthermore, in the above technical solution, rubber buffer pads are provided at the four arc-shaped notches of the transverse reinforcing plate 12. The rubber buffer pads are embedded in the arc-shaped notches through annular grooves. Sealing strips are provided at the edges of the transverse reinforcing plate 12 to prevent concrete from entering the interior of the I-beam during spraying.

[0133] Furthermore, in the above technical solution, the adjusting screw of the bottom support 13 is a two-way threaded rod, with an adjusting ring in the middle. The lifting and lowering adjustment of the support pad is achieved by rotating the adjusting ring; the inner wall of the concave groove of the support pad is provided with anti-slip texture.

[0134] Furthermore, in the above technical solution, the connecting steel plate of the I-beam connector 31 is provided with anti-loosening teeth, and the anti-loosening teeth and the contact surface of the stiffening plate 33 are distributed in a ring; the weld between the stiffening plate and the connecting steel plate is double-sided welded, and the weld length is not less than 80% of the height of the stiffening plate.

[0135] Furthermore, in the above technical solution, the reasonable area ratio between the transverse reinforcing plate 12 and the vertical I-beam frame 11 is limited by the principles of structural mechanics.

[0136] Furthermore, in the above technical solution, the angle of the star-shaped reinforcing ribs is optimized by using trigonometric functions to achieve the optimal angle distribution of the reinforcing ribs, thereby enhancing the support effect.

[0137] Furthermore, in the above technical solution, the height adjustment of the bottom support 13 is limited by introducing an exponential decay to reflect the deformation characteristics of the support during use.

[0138] This invention provides a second embodiment of a steel frame support construction method for excavation of underground pipeline confined foundation pits. In this embodiment, a specific application scenario of the invention is provided:

[0139] In a large-scale infrastructure project in a certain city, the excavation of a foundation pit with dense underground pipelines was involved. The pit was 8 meters deep, and various pipelines, including water supply pipes, drainage pipes, and gas pipes, were located nearby, with burial depths ranging from 3 to 5 meters and a highly complex distribution. Existing traditional support methods, such as sheet pile support and reinforced concrete support, are insufficient to meet the requirements of both protecting the pipelines and ensuring the safety and stability of the foundation pit construction in this underground pipeline environment. Therefore, the steel frame support construction method for foundation pit excavation with confined underground pipelines proposed in this invention is a more suitable solution.

[0140] First, in the early stages of construction, the project team conducted a detailed site survey using a GPS total station. The measurement accuracy was controlled within 2 mm horizontally and 1 mm vertically, with a spacing of no more than 5 meters between measurement points. For areas with significant terrain variations, additional measurement points were added. A digital elevation model of the site was constructed using a 3D surface fitting algorithm, providing crucial topographic information for subsequent excavation design.

[0141] Next, ground-penetrating radar (GPR) technology was used to comprehensively detect underground pipelines. The detection depth reached 1.5 times the excavation depth of the foundation pit, and the detection accuracy was controlled within 5% of the pipeline burial depth. An orthogonal grid scanning method was used during detection, with a grid spacing of no more than 1 meter. Wavelet denoising algorithms were applied to the detection signals to improve signal quality. For metallic pipelines, electromagnetic induction was used for re-detection. An alternating magnetic field was generated by a transmitting coil, and the induced current was detected by a receiving coil to determine the pipeline location. The re-detection accuracy was controlled within 10 centimeters. For non-metallic pipelines, ultrasonic detection was used for re-detection, with the transmission frequency controlled between 50-200 kHz. Based on the detection data, a Kriging interpolation algorithm was used to fit the pipeline route, with an interpolation grid spacing of no more than 20 centimeters to ensure the accuracy of the model. Finally, based on the measurement and layout data, control lines for the structure were marked on-site with fluorescent paint, with a line width controlled within 3 centimeters, and permanent marker stakes were installed.

[0142] Based on a thorough understanding of the site's geology and pipeline distribution, the project team subsequently conducted design optimization work for the support structure. First, based on the geological survey report, the site's soil parameters were determined, including a soil unit weight of 18 kN / m³, an internal friction angle of 30 degrees, and a cohesion of 20 kPa. The Delwin fuzzy comprehensive evaluation method was used to assess the ground stability, with evaluation indicators including soil strength, groundwater level, and lithology.

[0143] Then, a numerical model for the foundation pit excavation was established based on the finite element method. In the model, the soil adopted the Mohr-Coulomb constitutive model, the support structure adopted the elastic model, and the contact surface adopted the Coulomb friction model. The calculation conditions included foundation pit excavation, support structure construction, and dewatering, with the calculation step size controlled at 10% of the excavation depth. Based on the calculation results, No. 120 I-beams were selected as the main support, with a spacing controlled at 50 cm. The strength of the I-beams was verified using the limit state method, with a safety factor of not less than 1.5, and the stability was verified using the energy method, with a critical load factor of not less than 2. No. 116 I-beams were arranged between the I-beams for lateral reinforcement. The support spacing was optimized using a grid search algorithm, with both vertical and horizontal spacing not exceeding 3 meters, and the reserved gap between the support and the main structure not less than 10 cm. The stability of the overall support structure was verified using the simplified Bishop method, with a safety factor of not less than 1.3.

[0144] To prevent surface water from entering the foundation pit, the project team excavated intercepting ditches on both sides of the pit. First, a laser level was used to measure the site topography, determining the catchment area and flow direction; the level's accuracy was controlled to 1 mm per meter. Based on hydraulic calculations, the cross-sectional dimensions of the intercepting ditches were determined. The design flow rate, calculated using Manning's formula, was 0.8 cubic meters per second, with a longitudinal slope of no less than 3% at the bottom, a roughness coefficient of 0.015, and a design return period of 5 years. The intercepting ditches adopted a trapezoidal cross-section, with a bottom width of 50 cm, a depth of 50 cm, and a side slope ratio of 1:0.5. C20 concrete was used for cast-in-place construction, with a concrete strength acceptance standard of no less than 25 MPa. A sedimentation tank, 70 cm deep and with a volume of no less than 1 cubic meter, was installed every 20 meters along the intercepting ditches to settle sediment. Energy dissipation facilities, using drop wells, were installed at the connection between the intercepting ditches and the drainage system, with a depth of no less than 1 meter and internal stilling teeth.

[0145] The excavation of the foundation pit adopted a layered, segmented construction method, with each layer not exceeding 3 meters in depth and the excavation surface sloped at 1:0.5. A mini tracked excavator was used for excavation, with a bucket capacity of 0.3 cubic meters and a maximum excavation depth of 7 meters. The length of each excavation segment was controlled to 1 meter, with adjacent excavation segments staggered by at least 3 meters, using a staggered construction method. When encountering underground pipelines, a 50-centimeter protection zone was reserved on both sides of the pipeline for manual excavation; when the excavation depth reached 20 centimeters below the bottom of the pipeline, manual clearing was used. Small dump trucks were used for earthmoving, with the transport distance controlled within 2 kilometers, and the transport route avoiding residential areas and major roads. During the excavation process, a total station was used to monitor the deformation of the foundation pit in real time, with monitoring points spaced no more than 5 meters apart, monitored twice daily, and a deformation warning value of 20 millimeters.

[0146] On both sides of the foundation pit, No. 120 I-beams were installed according to the design spacing, and the following measures were taken:

[0147] (1) The I-beams were treated with anti-corrosion coating using epoxy zinc-rich primer with a coating thickness of not less than 80 micrometers.

[0148] (2) The positioning of the I-beams is achieved using a laser line projector, with the verticality deviation controlled within three per thousand and the spacing error between adjacent I-beams not exceeding 10 mm.

[0149] (3) The I-beam and the connecting steel plate are connected by high-strength 10.9 grade bolts with a tightening torque of not less than 100 Nm and tightened in stages using a box wrench.

[0150] (4) The anchor rod is made of Φ48 mm seamless steel pipe with a wall thickness of 4 mm, a length of not less than 2 meters, an inclination angle of 15-20 degrees, and the anchoring end is anchored with M30 strength cement mortar.

[0151] (5) The I-beam and the anchor rod shall be welded on both sides, with a weld height of not less than 8 mm. The welding shall be done by manual electric arc welding, with E50 welding rods and a preheating temperature of not less than 100 degrees Celsius.

[0152] On the outside of the I-beams, the project team installed longitudinal connecting steel bars and steel mesh, as detailed below:

[0153] (1) The longitudinal connecting bars are made of HRB400 grade Φ22 mm steel bars, with a longitudinal spacing of 50 cm and staggered arrangement. The bending angle of the end hook of the steel bar is 135 degrees, and the inner diameter of the bend is not less than 5 times the diameter of the steel bar.

[0154] (2) The reinforcement bars and I-beams are welded by fillet welds with a specification of 8 mm and a weld length of not less than 10 times the diameter of the reinforcement bars.

[0155] (3) The steel mesh is made of Φ8 mm steel bars, with a mesh size of 20×20 cm and an overlap length of not less than 35 times the diameter of the steel bars. It is fixed by binding with iron wire, and the spacing between binding points is not greater than 50 cm.

[0156] After the support structure was installed, the project team used a wet-spraying process for concrete spraying. The concrete used was grade C25, and the mix design was optimized using orthogonal experimental design. The water-cement ratio was controlled below 0.45, the early-strength agent dosage was 2% of the cement weight, and the water-reducing agent dosage was 1%. The spraying pressure was controlled at 0.5-0.7 MPa, the nozzle distance from the pit sidewall was 0.8-1.0 meters, and the spraying angle was perpendicular to the sidewall. The thickness of a single layer of spraying did not exceed 5 cm, the time interval between layers did not exceed 2 hours, and the total thickness was not less than 12 cm. During the spraying process, the project team used rebound testing to control the spraying quality; the rebound rate was not allowed to exceed 25%, and rebound material was promptly cleaned up.

[0157] After the support structure was completed, the project team began construction of the main structure, adopting a bottom-up pouring sequence. The concrete strength grade of the main structure was no less than C30, the impermeability grade was no less than P6, and the reinforcement ratio was no less than 0.2%. Concrete was poured in layers, each layer not exceeding 50 cm in thickness, and compacted for 15-20 seconds using an immersion vibrator. Curing involved covering the concrete with geotextile and sprinkling water for at least 14 days. Once the main structure reached 70% of its design strength, the project team systematically removed the temporary supports, proceeding in the reverse order of construction, from top to bottom.

[0158] To monitor the construction safety of the foundation pit in real time, the project team established a comprehensive monitoring system. A total station was used to monitor the horizontal displacement at the top of the pit, with a monitoring section every 15 meters and at least three monitoring points at each section. When the pit depth was no more than 5 meters, the monitoring frequency was once every two days; when the pit depth was 5-10 meters, the monitoring frequency was once a day. A level was used to monitor the ground settlement around the pit, with monitoring points located at the edge of the pit and within a range of twice the pit depth. The monitoring data was processed using a Kalman filter algorithm to remove outliers, and the deformation rate and acceleration were calculated. When the monitoring data reached warning values, such as a horizontal displacement of 20 mm, ground settlement of 30 mm, deformation rate of 2 mm / day, and acceleration of 0.1 mm / day, the project team immediately implemented reinforcement measures.

[0159] By employing the steel frame support construction method for underground pipeline confined foundation pit excavation according to the present invention, the foundation pit excavation project achieved good construction results:

[0160] 1. Significant results were achieved in pipeline protection. By accurately surveying the pipeline locations and using adjustable support bases, damage to the pipelines was effectively avoided, and no pipeline accidents occurred.

[0161] 2. Construction difficulty is significantly reduced. By adopting measures such as layered and segmented excavation and small-scale machinery operation, the construction space is greatly reduced, and the work efficiency is significantly improved.

[0162] 3. The support structure exhibits superior performance. Through optimized design, the support structure boasts high rigidity and excellent overall stability, meeting the requirements for foundation pit support in underground pipeline environments, without exhibiting any deformation or damage.

[0163] 4. Comprehensive quality control measures. From material selection to construction monitoring, targeted quality control measures were implemented at every stage to ensure construction quality and prevent any quality incidents.

[0164] The core technical principle of the steel frame support construction method for underground pipeline confined pit excavation in this invention can be summarized as follows:

[0165] 1. Accurately survey pipeline locations to provide a basis for support structure design:

[0166] Before excavation, this invention employs advanced technologies such as GPS total station and ground-penetrating radar to conduct a comprehensive survey and 3D modeling of the location, depth, and direction of underground pipelines. This not only accurately determines the distribution of pipelines, providing crucial reference for the design and optimization of subsequent support structures, but also allows for the marking of pipeline locations using measurement data, significantly reducing the risk of pipeline damage during construction.

[0167] 2. Optimize the steel support structure to balance strength, stability, and deformation capacity:

[0168] This invention uses 120mm I-beams as the main support, and their dimensions and spacing are determined through finite element analysis to meet the strength and stability requirements during the excavation process. Simultaneously, 116mm I-beams are installed between the I-beams for lateral reinforcement, enhancing overall rigidity. The I-beams are connected with high-strength bolts, and longitudinal reinforcing bars and steel mesh are arranged on the outer sides to further improve the integrity and deformation coordination of the support structure. This not only ensures the safety and stability of the excavation but also effectively protects adjacent underground pipelines.

[0169] 3. A layered, segmented excavation process was adopted, combined with optimized support structures:

[0170] This invention employs a layered, segmented construction process during foundation pit excavation, controlling the excavation depth to within 3 meters for each stage. This not only improves excavation efficiency but also reduces the risk of deformation and damage during single-layer excavation. Simultaneously, small manual excavation tools are used near pipelines to ensure pipeline safety. The support structure design also complements this approach, incorporating adjustable support pads at the bottom to accommodate the deformation characteristics at different excavation depths.

[0171] 4. Comprehensive quality control measures to ensure construction safety and pipeline protection:

[0172] In key construction phases such as the installation of the support structure and concrete spraying, this invention employs a series of quality control measures. For example, anti-corrosion treatment is applied to the I-beams to ensure their durability; laser positioning is used to guarantee the accuracy of the support installation; and spraying parameters are optimized to improve the quality of the concrete. Furthermore, a comprehensive monitoring system is established to monitor the deformation of the foundation pit in real time, and reinforcement measures are immediately taken once the warning value is reached, ensuring the safety and controllability of the entire construction process.

[0173] In summary, the core technical principle of the steel frame support construction method for underground pipeline confined pit excavation in this invention is:

[0174] (1) Conduct a thorough investigation of the distribution of underground pipelines to provide a reliable basis for the design of the support structure;

[0175] (2) Optimize the steel support structure to ensure the strength and stability of the foundation pit while taking into account its deformation capacity, thereby effectively protecting the pipeline.

[0176] (3) Adopt the layered and segmented excavation process, combined with the support structure design, to reduce the deformation risk during the single-layer excavation process;

[0177] (4) Establish a sound quality control system to ensure construction safety and pipeline protection.

Claims

1. A construction method for steel frame support in excavation of underground pipeline confined foundation pits, characterized in that, Includes the following steps: S10. Investigate the surrounding environment of the foundation pit, measure and mark the centerline of the structure and the edge of the foundation pit, and use the method of excavating exploratory pits to determine the burial depth, direction and location of underground pipelines and manholes. S20. Design a steel frame support system according to the depth of the foundation pit. Set 120 I-beams on both sides of the foundation pit as side wall supports. The spacing between the I-beams is 50 cm. Set 116 I-beams on both sides as lateral connection supports. S30. Excavate a water interception ditch 2 meters outside the top of the slope on both sides of the foundation pit. The cross-section of the water interception ditch is 50 cm × 50 cm, and ensure that the slope of the water interception ditch is more than 3%. S40. A mini excavator is used for layered excavation. The first layer is 3 meters deep, and each excavation is 1 meter long. The longitudinal slope is 1:0.

5. The trench sidewalls are excavated vertically. S50. Install vertical I-beam supports, and install anchor bolts at the arch foot to weld to the I-beams. The I-beams are fixed together with 20 cm × 26 cm × 1.2 cm connecting steel plates by bolts. S60. Install longitudinal connecting bars and steel mesh on the outside of the I-beam. The longitudinal connecting bars are HRB400φ22 steel bars, arranged alternately with a vertical spacing of 50 cm, and connected to the I-beam by single-sided welding. S70. A TK-961 wet spraying machine is used for concrete spraying. The working air pressure for concrete spraying is controlled within the range of 0.5 MPa to 0.7 MPa, and the distance between the nozzle and the side wall of the foundation pit is controlled within the range of 0.8 meters to 1.0 meters. S80. Construction of the main structure: After the main structure reaches 70% of its design strength, the temporary top support shall be removed. S90. Conduct foundation pit monitoring. Set up a monitoring section every 15 meters in the foundation pit. When the foundation pit depth is no more than 5 meters, the monitoring frequency is once every 2 days. When the foundation pit depth is between 5 meters and 10 meters, the monitoring frequency is once a day. The underground pipeline confined excavation pit steel frame support includes: a main support assembly, a locking assembly, and a connecting assembly; the main support assembly includes a vertical I-beam frame, a horizontal reinforcing plate, and a bottom support base; the vertical I-beam frame is formed by connecting multiple 120 I-beams through the horizontal reinforcing plate, the horizontal reinforcing plate has a rectangular structure with arc-shaped notches at the four corners, and a reinforcing rib is provided in the middle of the horizontal reinforcing plate, the reinforcing rib is arranged in a star shape and welded to the horizontal reinforcing plate; the bottom support base includes a base plate, an adjusting screw, and a support pad, the base plate has multiple adjusting screw holes, the adjusting screw passes through the adjusting screw holes and is fixed by a nut, the top of the adjusting screw is welded to the support pad, and the support pad has a concave groove for installing the vertical I-beam frame; The locking assembly includes an arch foot locking plate, a locking foot anchor rod, and a reinforcing connecting plate. The arch foot locking plate has an L-shaped structure, with its vertical portion welded to a vertical I-beam frame and its horizontal portion having multiple anchor rod mounting holes. An anchor rod sleeves are installed in the anchor rod mounting holes. One end of the locking foot anchor rod is threaded to the anchor rod sleeve, and the other end is an enlarged diameter section. The reinforcing connecting plate is located between the arch foot locking plate and the vertical I-beam frame, and has a triangular structure with reinforcing ribs on its edges.

2. The construction method for steel frame support in the excavation of a confined underground pipeline pit according to claim 1, characterized in that, The connecting assembly includes an I-beam connector, a limiting block, and a stiffening plate. The I-beam connector consists of two symmetrical connecting steel plates, with multiple bolt through holes evenly distributed on the connecting steel plates. The limiting block is welded to the inner side of the connecting steel plates to limit the position of the I-beam. The connecting steel plates are used to fix adjacent vertical I-beam frames together with fastening bolts. A stiffening plate is provided on the outer side of the connecting steel plates, and the stiffening plate is arranged in a herringbone pattern.

3. The construction method for steel frame support in the excavation of a confined underground pipeline pit according to claim 2, characterized in that, Rubber buffer pads are provided at the four arc-shaped notches of the transverse reinforcing plate. The rubber buffer pads are embedded in the arc-shaped notches through annular grooves. Sealing strips are provided at the edges of the transverse reinforcing plate to prevent concrete from entering the interior of the I-beam during spraying.

4. The construction method for steel frame support in the excavation of a confined underground pipeline pit according to claim 3, characterized in that, The adjusting screw of the bottom support is a two-way threaded rod, with an adjusting ring in the middle. The height adjustment of the support pad is achieved by rotating the adjusting ring. The inner wall of the concave groove of the support pad is provided with anti-slip texture.

5. The construction method for steel frame support in the excavation of a confined underground pipeline pit according to claim 4, characterized in that, The connecting steel plate of the I-beam connector is provided with anti-loosening teeth, and the anti-loosening teeth are distributed in a ring with the contact surface of the fastening bolt; the weld between the stiffening plate and the connecting steel plate is double-sided welded, and the weld length is not less than 80% of the height of the stiffening plate.

6. The construction method for steel frame support in the excavation of a confined underground pipeline pit according to claim 5, characterized in that, The appropriate area ratio between the transverse reinforcing plate and the I-beam is determined by the principles of structural mechanics.

7. The construction method for steel frame support in the excavation of a confined underground pipeline pit according to claim 6, characterized in that, The angles of the star-shaped reinforcing ribs are optimized using trigonometric functions to achieve the best angle distribution, thus enhancing the support effect.

8. The construction method for steel frame support in the excavation of a confined underground pipeline pit according to claim 7, characterized in that, The height adjustment of the bottom support is limited by introducing an exponential decay to reflect the deformation characteristics of the support during use.

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