Construction method of low-clearance cast-in-place piles for supporting beams in deep foundation pits in complex strata

Abstract

This invention relates to a construction method for cast-in-place piles with low clearance at the bottom of the support beam in the deep foundation pit with trestle slab area in complex strata. The method includes the following steps: 1) Construction preparation, including drilling the trestle slab and positioning the fully rotating equipment at the bottom of the pit; 2) Drilling the hole in the soil layer, lowering the steel casing into the soil layer and removing the soil; 3) Drilling the hole in the rock layer, including breaking the rock, removing slag, and cleaning the hole inside the steel casing; 4) Pouring concrete into the pile body. This invention can replace existing construction methods for cast-in-place piles in the low clearance area at the bottom of the support beam in the deep foundation pit with trestle slab area, achieving higher economic benefits and improved construction efficiency.

Description

Technical Field

[0001] This invention relates to the field of cast-in-place pile technology, specifically to a construction method for cast-in-place piles with low clearance at the bottom of the support beam in the trestle slab area of ​​a deep foundation pit in complex strata. Background Technology

[0002] In foundation pit engineering with pile support, due to the influence of the support structure, the engineering piles are often constructed simultaneously with the support piles before the foundation pit is excavated. At the same time, some foundation pits are equipped with trestle decks as temporary storage areas during foundation pit and foundation construction, which also facilitates the vertical removal of soil from the foundation pit. When the foundation pit is excavated to the bottom, the pile foundation is tested. Due to various factors, sometimes the engineering piles fail the test and additional piles need to be added. For engineering piles with complex strata and long pile lengths, how to carry out construction under the low clearance under the support beam and the constraints of trestle decks is often a difficult problem.

[0003] Currently, for the use of low-headroom cast-in-place piles at the bottom of the support beams in deep foundation pits in complex strata, traditional manual excavation can be used due to the low headroom operation. However, the excavation depth is generally no more than 30 meters, and it is not suitable for adverse strata conditions with thick silt and sand layers. Impact drilling can be used, and by making appropriate modifications to the frame, it can meet the construction requirements under low headroom conditions. However, impact drilling is inefficient, requires the use of mud slurry for wall protection, has a large volume of circulating mud, and is prone to hole collapse under the high water head pressure of the pit wall when drilling at the bottom of the pit, making it impossible to guarantee the quality of the pile. In addition, in addition to the limited frame height, the torque of low-headroom rotary drilling rigs is also difficult to meet the requirements for deep hole drilling into rock. Summary of the Invention

[0004] The purpose of this invention is to provide a construction method for cast-in-place piles with low clearance at the bottom of the support beam in the trestle slab area of ​​a deep foundation pit in complex strata, thereby solving the aforementioned problems existing in the current construction of cast-in-place piles with low clearance at the bottom of the support beam in the trestle slab area of ​​a deep foundation pit in complex strata.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: The construction method for low-clearance cast-in-place piles at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata includes the following steps. 1) Construction preparation, including drilling holes in the trestle panels and positioning the fully rotating equipment at the bottom of the pit; 2) Drill holes in the soil layer, lower the steel casing into the soil layer, and remove the soil; 3) Drilling in the rock strata section, and carrying out the processes of breaking rock, removing slag and cleaning the hole inside the casing; 4) Pile body concrete pouring.

[0006] Further optimization involves, in step 1), before opening the trestle plate, performing composite reinforcement and strengthening of the foundation pit safety, and simultaneously completing the measurement and marking of the pit bottom and trestle plate pile positions.

[0007] Further optimization involves reinforcing the bottom of the pit and the trestle pile positions with full-span bracing after completing step 1) of the pit bottom and trestle pile position measurement.

[0008] Further preferred, in step 1), the full-rotation equipment includes a hydraulic power station and a tracked self-propelled full-rotation drilling rig.

[0009] In a further preferred embodiment, during the process of drilling holes in the trestle plate in step 1), a water-cooled drill is used to drill core holes at the four corners of the area to be cut, and then a wire saw is used to perform static cutting inside the core holes.

[0010] Further optimization involves first performing advance drilling of the soil casing in step 2), then removing slag with a grab bucket on the trestle plate, and performing the casing extension process on the trestle plate until the casing and borehole are drilled to the rock surface.

[0011] Further optimization involves, in step 3), firstly, performing the rock-breaking process inside the casing with a hammer, then performing the slag-removing process inside the casing with a grab bucket, until the final hole is completed and the hole is cleaned once.

[0012] Further preferred, in step 3), when performing the advanced drilling process of the soil casing, water is injected into the casing to maintain a high water head.

[0013] Further optimization involves the following steps in step 4): first, the reinforcing cage is fabricated and placed; then, the grouting conduit is placed; and finally, the pile body concrete is poured using an air-lift reverse circulation secondary cleaning method and a pump truck at the top of the pit, until the design elevation is reached.

[0014] Further optimization involved pouring concrete to the designed elevation, then relocating the drilling rig, followed by concrete repair work on the working opening on the trestle.

[0015] The beneficial effects of this invention are: The present invention discloses a construction method for cast-in-place piles with low clearance at the bottom of the support beam in the deep foundation pit of complex strata. By drilling holes on the trestle and setting up a full-rotation device at the bottom of the pit, it is convenient to use steel casing for wall protection, so as to ensure the drilling of soil and rock sections and the concrete pouring of pile body. This method replaces the existing construction method of cast-in-place piles in the low clearance area at the bottom of the support beam in the deep foundation pit of trestle and bridge section, and achieves higher economic benefits and improved construction efficiency.

[0016] Furthermore, by adopting a tracked, fully rotating device, free movement is achieved. In addition, a suspension device is added, and the main unit of the equipment can carry a hydraulic power station to form an integrated machine, which effectively solves the problem of moving drilling equipment in low-headroom environments.

[0017] Furthermore, in this process, the self-propelled full-casing rotary drilling rig rotates and drills under the support beam at the bottom of the foundation pit. By extending the steel casing above the trestle deck, the crawler crane grab bucket is used to remove soil from the trestle deck. The full-rotation drilling operation is completed through vertical and vertical coordination.

[0018] Furthermore, to minimize the impact of the trestle deck on vertical operations, partial dismantling of the deck is necessary. This invention employs a water-cooled drill to create core holes at the four corners of the trestle deck to be dismantled, followed by static non-destructive cutting using a wire saw. After pile construction is completed, the openings in the dismantled trestle deck are repaired, thus minimizing the impact of construction on the structural safety of the trestle deck.

[0019] Furthermore, a specialized underwater grab bucket is used for soil drilling in soil layers, while a rock-breaking hammer is used for impact crushing in rock layers, and then the crushed rock mass is removed using an underwater grab bucket until the designed depth is reached. Considering the characteristics of the full casing process, there is no risk of borehole collapse due to the casing wall protection, and a high-power air compressor is used for air-lift reverse circulation borehole cleaning, which greatly improves the cleaning efficiency. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the construction method for low-clearance cast-in-place piles at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata according to the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0022] The construction method for cast-in-place piles with low clearance at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata, as described in this invention, is mainly applied in construction scenarios where additional cast-in-place piles are required in the low clearance area at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata. It enables the construction of cast-in-place piles with a clearance of not less than 5m at the bottom of the support beam in the foundation pit trestle area, and the pile positions are located in the gaps of the support beam. This replaces the existing construction method for cast-in-place piles in the low clearance area at the bottom of the support beam in the deep foundation pit trestle area, achieving higher economic benefits and improving construction efficiency.

[0023] like Figure 1 The diagram shown illustrates the principle of the construction method for low-clearance cast-in-place piles at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata. It mainly includes the following steps: 1) Construction preparation, including drilling holes in the trestle panels and positioning the fully rotating equipment at the bottom of the pit; 2) Drill holes in the soil layer, lower the steel casing into the soil layer, and remove the soil; 3) Drilling in the rock strata section, and carrying out the processes of breaking rock, removing slag and cleaning the hole inside the casing; 4) Pile body concrete pouring.

[0024] Step 1) includes the following steps: 1.1 Safety verification and reinforcement of the foundation pit; To minimize the impact of large crawler cranes on the foundation pit support structure during the drilling of cast-in-place piles, the original foundation pit support design unit reviewed the safety and stability of the supporting beams, slabs, and the overall foundation pit support system under the trestle platform before construction, based on the design loads of the trestle platform and column piles in the original design scheme, and considering the actual working conditions of the cast-in-place pile construction. They also proposed a foundation pit reinforcement design scheme to ensure the safety of the foundation pit during the cast-in-place pile construction. According to the foundation pit support reinforcement design requirements, the foundation slab in the passive zone around the bottom of the foundation pit was constructed first, with a width of 3-6m and a thickness of 1.5m, depending on the location of the supplementary piles. Since the bottom of the pit is located in soft soil, a continuous reinforced concrete slab was installed at the bottom of the pit to harden the construction site and ensure the safety of mechanical construction. For operations on the trestle platform at the top of the foundation pit, measures such as laying steel plates and timely removal of soil were taken. Temporary additional loads were strictly controlled according to requirements, with the maximum additional load not exceeding 30 kPa, to reduce the impact of construction operations on the support structure.

[0025] 1.2 Measurement and layout of the pit bottom and trestle platform positions; At the bottom of the foundation pit, a total station was used to measure and mark the center position of the piles. The cross-shaped method was used for positioning, the center of the piles was marked, and four protective piles were installed. The full-casing, fully rotating equipment was moved to the pile position, and then the center position of the equipment was checked and adjusted to be on the same vertical line as the center of the piles. The center position of the piles was marked on the trestle deck at the top of the foundation pit using a total station. Then, the edge line of the trestle deck pile hole to be cut was determined according to the pile diameter and appropriately expanded outwards, and the corresponding markings were made.

[0026] 1.3 Reinforcement of the bottom of the trestle deck with full-span bracing; Considering the significant load on the crawler crane operating on the foundation pit trestle, including its own weight, dynamic load from crane grabbing operations, and loads from other supporting equipment, a full-span scaffolding system was adopted to reinforce the construction area of ​​the foundation pit trestle to ensure safety. The reinforcement plan was implemented after review by the foundation pit design unit. According to the reinforcement plan, professional scaffolders erected the full-span support frame. To ensure the stability of the full-span support frame, steel pipe couplers were used to secure the support frame to the support beams using clamps. After the full-span scaffolding was erected and inspected, work on the trestle was carried out only after it passed inspection.

[0027] 1.4 Static cutting of holes in the trestle plate; To address the impact of the trestle deck on the vertical construction space of the cast-in-place piles, holes were cut into the trestle deck within the pile hole area to facilitate the lowering and lowering of the casing from the top of the pit to the bottom, allowing for the use of a full-casing rotary drilling rig. To minimize damage and impact on the trestle deck structure, core holes were drilled at the four corners of the area to be cut using a water-cooled drill. Static cutting was then performed using a wire saw inserted into the core holes, creating working holes with a diameter 300mm larger than the pile diameter. For safety, horizontal guardrails were installed around the openings as required. Additionally, to prevent injuries from falling objects during operations, vertical protective measures were implemented around the casing at the openings.

[0028] 1.5 Full-rotation equipment positioned at the bottom of the pit The main equipment used in the construction of the tracked full-rotation drilling rig includes: the drilling rig main unit, hydraulic power station, reaction fork, etc. The dimensions of the drilling rig working device together with the hydraulic power station are 8709mm × 4980mm × 4503mm (length × width × height). After disassembling the full-rotation equipment, it is hoisted to the bottom of the foundation pit for reassembly. The weight of a single disassembled unit does not exceed 25 tons to ensure the safety of the foundation pit trestle. Before the equipment is positioned, the center position of the pile is checked using a total station. The tracked full-casing full-rotation drilling rig is automatically moved and positioned using a walking wireless remote control system. The orientation of the equipment is considered in advance during positioning. At the same time, after the equipment moves to the pile position, the center position of the casing is determined by cross-section method. Then, the pile center position is checked using a plumb bob. The moving drilling rig is adjusted appropriately to make the center of the casing and the center of the pile on the same vertical line. Finally, the hydraulic balance support plate under the drilling rig chassis is supported, the drilling rig is adjusted to be in a horizontal state, and the pile center position is checked again. The equipment positioning at the bottom of the pit is then completed.

[0029] It should be noted that the key to low-clearance operation under the support beams in the foundation pit trestle area lies in solving the problem of how to move the pile driver quickly in the plane while ensuring normal and safe operation under the limited space under the support beams at the bottom of the foundation pit. This construction method modifies the traditional full-casing full-rotation equipment by adding tracks, using the DTR2106HZ tracked self-propelled drilling rig. This solves the problem of traditional equipment requiring crane assistance for movement. The equipment is 4053mm high, which is 5m below the clearance height of the support beam bottom, meeting the construction requirements under low clearance conditions. In addition, the rotation torque of the equipment used is 3085KN·m, which has a strong drilling capacity, with a maximum hole diameter of 2100mm and a hole depth of 80m, which can fully meet the design requirements for large-diameter and ultra-deep pile construction.

[0030] Furthermore, the drilling principle of the full-casing rotary drilling rig is as follows: relying on the powerful torque of the drilling rig to drive the steel casing to rotate 360° for drilling, the high-strength cutter head at the bottom of the casing cuts the soil, and the downward pressing function of the full-rotation drilling rig presses the casing into the strata. Then, a grab bucket is used to excavate and remove the excavated soil from the casing. Due to the poor soil conditions and the presence of pressurized water, the bottom of the casing is kept at least 6m above the excavation surface during drilling in the soil layer. The casing is continuously drilled and pressed into the soil layer until it reaches the rock surface. The steel casing achieves full-process drilling wall protection, effectively blocking the influence of adverse geological conditions during drilling. In addition, due to the good rigidity of the casing wall, the verticality control accuracy is high during drilling. When drilling reaches the bearing stratum rock surface, a hammer is used to impact and break the rock inside the casing. After breaking a section, the excavated soil is removed and cleaned with a grab bucket. This process of breaking and cleaning is repeated until the designed pile bottom elevation is reached, completing the pile hole drilling operation.

[0031] The principle of vertical operation on the support trestle plate of the foundation pit is as follows: Due to the influence of the support beams and the trestle plate, this method improves the conventional process of operating a full-casing full-rotation drilling rig on the same plane. The main unit of the full-casing full-rotation equipment is placed under the support beam at the bottom of the pit, and the crawler crane grab bucket is placed on the trestle plate. Through the central area between the support beams under the support beams at the top of the foundation pit and the support beams at the bottom of the foundation pit, after the pile hole is positioned on the trestle plate, static cutting is used to open the hole to ensure unobstructed space in the vertical direction. After the pile is completed, the hole is repaired. The full-casing full-rotation drilling rig drills under the trestle plate at the bottom of the foundation pit. The steel casing of the drilling rig extends above the surface of the trestle plate through the holes in the trestle plate. Therefore, the crawler crane grab bucket can cooperate with the full-rotation drilling rig on the trestle plate to carry out soil drilling.

[0032] Step 2) includes the following procedures; 2.1 Casing advance drilling; The grab crane is positioned on the trestle platform, placed within the reinforced area supported by the platform's full span, and steel plates are laid at the crane's location for safety. Before using the casing, its verticality is checked and corrected. After inspection and correction, the steel casing is installed section by section according to its number using a full-casing full-rotation equipment. When pressing the bottom steel casing in, its verticality is checked with a level. Generally, after the casing has been pressed to a certain depth (approximately 3m), the verticality is checked again. Then, during drilling, a plumb bob is used simultaneously in both the X and Y directions to check and adjust the casing's verticality. Due to the presence of deep sand layers and pressurized water in the strata, the overpressure depth of the casing is increased during drilling. Conventional full-casing drilling typically involves an overpressure depth of 2-3m; this method maintains an overpressure depth of no less than 6m. Considering the presence of pressurized water on site, water is injected into the casing to maintain a high head and balance the groundwater pressure.

[0033] 2.2 Slag removal using a grab bucket on the trestle deck; Because the casing is being operated with water inside, a special underwater grab bucket is used for soil removal and drilling to ensure soil removal and drilling efficiency. The excavated soil is transported by a loader to a designated area on the trestle for unified stacking, and then loaded onto trucks by an excavator for timely transportation.

[0034] 2.3 Extend the sleeve on the trestle plate; Since the crawler crane grab operates on the trestle plate, the casing needs to be connected to the working surface of the trestle plate first.

[0035] Through the working holes cut into the trestle panel, the steel casing is lowered to the bottom of the pit using a full-casing rotary device. Then, the casing is pressed into the soil at the bottom of the pit using the same device. Depending on the distance between the trestle panel working surface and the pit bottom, four casing sections need to be pressed in consecutively. The casing is then pulled back to the trestle panel working surface, allowing for casing extension operations on the trestle panel. Each time the casing protrudes approximately 0.5m from the trestle panel, it is extended on the trestle panel. Simultaneously, the verticality of the casing is monitored according to the aforementioned requirements during the extension and pressing process.

[0036] 2.4 The casing and borehole are drilled to the rock surface; The casing is pre-drilled to protect the borehole wall and prevent collapse. Then, an underwater grab bucket is used to continuously remove cuttings and continue drilling until the casing and borehole reach the rock surface. During casing drilling, a plumb bob is used to monitor the casing's verticality in two perpendicular directions.

[0037] After completing the drilling step in the soil layer section, the drilling step in the rock layer section begins, and the steps are as follows. 3.1 Rock breaking inside the casing of the impact hammer; After drilling to the bearing rock surface, the rock-breaking hammer is replaced for impact crushing. When encountering inclined rock surfaces, in order to ensure verticality, the hammer uses a small stroke and slow drilling speed. After the entire cross-section is penetrated into the rock, the rock surface is confirmed, and the impact rock penetration construction is completed according to the designed penetration depth.

[0038] 3.2 Slag removal from inside the grab bucket casing; The rock debris broken by the rock-breaking hammer is removed from the casing using a rock-breaking grab bucket. The removed rock debris is then piled on a trestle for off-site disposal. After removing a section of rock debris, the rock-breaking hammer is used repeatedly to impact and drill deeper, and then the grab bucket is used again to remove the debris. This process is repeated until the drilling depth meets the design requirements.

[0039] 3.3 Final hole cleaning and primary hole cleaning; Upon reaching the designed depth, final borehole measurement and acceptance are conducted. After final drilling, an underwater grab bucket is used for a final cleaning of the borehole. After cleaning, the sediment at the bottom of the borehole is measured to ensure that it meets the design requirements.

[0040] After completing the drilling process in the rock strata section, the pile body concrete can be poured. The pile body concrete pouring includes the following steps. 4.1 Fabrication and placement of the reinforcing cage; The reinforcing cage is prefabricated in a processing area set up at the top of the foundation pit, and the supervising engineer is notified for concealed acceptance before placement. The reinforcing cage is placed promptly after the borehole is cleaned. Due to the overall length of the reinforcing cage, a process of lifting it in sections from the top of the foundation pit using a crane and welding it to the openings of the trestle plate sleeves is employed. During placement, attention is paid to the lifting points to prevent deformation or accidents.

[0041] 4.2 Placement of the infusion catheter; After the reinforcing cage is in place, the grouting conduit should be installed promptly. The conduit should be made of seamless steel pipe with a wall thickness of 10mm and a diameter of 300mm, and the joint should be a flange connection. Before use, the conduit should be trial-assembled and pressure-tested, with a test pressure of not less than 0.6MPa. When connecting, a sealing ring should be installed to ensure a tight connection. After the conduit is installed, the bottom of the conduit should be kept 30-50cm away from the bottom of the hole.

[0042] 4.3 Air lift reverse circulation secondary cleaning; To ensure pile quality and strictly control the thickness of sediment at the bottom of the borehole, a secondary cleaning is performed after the reinforcement cage is installed. Due to the deep borehole, a reverse circulation air-lift process is used for the secondary cleaning. A 55kW screw air compressor with a rated exhaust pressure of 0.8MPa is selected, combined with a 1m³ air tank to provide a safe and stable airflow, ensuring good cleaning results. Since a full casing casing is used for wall protection, no special mud slurry preparation is required during drilling. However, to ensure normal circulation of the slurry within the borehole, water needs to be continuously injected into the casing during the secondary cleaning to maintain sufficient circulating water for the secondary cleaning operation. After the secondary cleaning via reverse circulation within the casing, the thickness of sediment at the bottom of the borehole is measured, achieving the standard of "zero sediment." After the cleaning is completed, the sediment thickness is measured and accepted together with the supervising engineer.

[0043] 4.4 The concrete for the pile body is poured using a pump truck at the top of the pit; After the secondary cleaning of the borehole, the pile body concrete is poured in a timely manner. Since the concrete volume for each pile exceeds 100m³, the initial pouring volume is large, and the efficiency of using a hopper for hoisting is low, with a single pile pouring time of approximately 18 hours, slow-setting concrete is used, with an initial setting time of no less than 20 hours. The pile body concrete is poured using a pump truck on a trestle platform. Using a boom pump can greatly improve the efficiency of concrete pouring. A 3m³ large hopper is used for the initial pouring, and the hopper is moistened with clean water before pouring. A spherical bladder is used as a water-stop plug. Before the initial pouring, the water-stop plug is placed inside the guide pipe, and the stopper plate at the bottom of the pouring hopper is pressed down. Then, concrete is poured into the hopper through the pump truck at the top of the pit. When the concrete in the pouring hopper meets the initial pouring volume, the stopper plate of the hopper is lifted, at which point the concrete presses against the spherical bladder and rushes into the bottom of the hole. During normal pouring, to facilitate pipe pulling, a smaller hopper is used, and concrete pouring is continuously carried out through the pump truck's feed pipe. During the grouting process, the concrete surface position should be measured regularly, and the pipes should be pulled out or dismantled in a timely manner. The burial depth of the guide pipe should be controlled between 2 and 4 meters. Finally, when the grouting is completed, ensure that the over-grouting height at the top of the pile meets the design requirements.

[0044] 4.5 Simultaneously inject and pull out the casing; During concrete pouring, the steel casing is pulled out in sections. Considering the influence of unfavorable strata such as sand, in order to ensure smooth pile formation, the height of the concrete inside the casing above the bottom of the steel casing is increased during the process, generally controlled to be no less than 20m. The casing is pulled out while pouring, until all the steel casing is pulled out after the pouring is completed.

[0045] 4.6 After the drilling rig is filled to the design elevation, it is moved. After filling to the design elevation, remove all steel casings, retract the chassis hydraulic balance support plate, restore the tracked walking mode, and dismantle the reaction fork. When the main unit and power station are integrated, the tracked full-rotation drilling rig can be directly moved to the next pile hole position using the walking wireless remote control system. Otherwise, the full-rotation drilling rig must be moved only after the hydraulic oil pipe system of the power station is disconnected.

[0046] Through the above steps, the construction of the low-clearance cast-in-place piles at the bottom of the support beam in the deep foundation pit of the complex stratum can be completed. After the construction is completed, the reinforced concrete of the working opening of the trestle should be repaired as soon as possible. First, the bottom formwork of the opening is erected, and the bottom formwork is fixed with bolts and I-beams. Finally, the reinforcing bars are tied, and the opening concrete is poured with a higher strength grade for repair.

[0047] The present invention relates to a method for constructing cast-in-place piles with low clearance at the bottom of the support beam in the deep foundation pit of complex strata. This method utilizes the low profile and convenient movement of a crawler-type self-propelled full-casing full-rotation drilling rig to drill holes under the support beam. Combined with a series of technical measures, including hydraulic full-casing pre-wall protection, water pressure balancing within the casing, placement of the reinforcing cage within the gap of the support beam at the top of the pit, air-lift reverse circulation secondary hole cleaning, and concrete pouring into the pile body using a pump truck at the top of the pit, this method solves the problem of pile construction in complex strata under conditions of low clearance at the bottom of the support beam in deep foundation pits. It achieves reliable quality, safety, and low overall cost, demonstrating significant social and economic benefits.

Claims

1. A construction method for low-clearance cast-in-place piles at the bottom of the supporting beam in the deep foundation pit trestle area of ​​complex strata, characterized by: Includes the following steps, 1) Construction preparation, including drilling holes in the trestle panels and positioning the fully rotating equipment at the bottom of the pit; 2) Drill holes in the soil layer, lower the steel casing into the soil layer, and remove the soil; 3) Drilling in the rock strata section, and carrying out the processes of rock breaking, slag removal and hole cleaning inside the steel casing; 4) Pile body concrete pouring; In step 2), the soil casing is first advanced drilled, and then the slag is removed by grab bucket on the trestle plate. The steel casing on the trestle plate is then extended until the steel casing is drilled to the rock surface. In step 1), when the trestle plate is opened, a water-cooled drill is used to drill core holes at the four corners of the area to be cut. Then, a wire saw is used to perform static cutting in the core holes. In step 4), the steel cage is first made and placed, then the grouting pipe is placed, and then the air-lift reverse circulation secondary cleaning is performed. The pile body concrete is then poured by the top pump truck until the design elevation is reached. After the design elevation is reached, the drilling rig is moved, and then the concrete repair process of the working opening is carried out on the trestle plate.

2. The construction method for low-clearance cast-in-place piles at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata as described in claim 1, characterized in that: In step 1), before drilling holes in the trestle deck, the safety of the foundation pit is checked and reinforced, and the bottom of the pit and the pile positions of the trestle deck are measured and marked.

3. The construction method for low-clearance cast-in-place piles at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata as described in claim 2, characterized in that: In step 1), after the bottom of the pit and the pile positions of the trestle bridge are measured and laid out, the bottom of the pit of the trestle bridge is reinforced with full-span bracing.

4. The construction method for low-clearance cast-in-place piles at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata as described in claim 3, characterized in that: In step 1), the full-rotation equipment includes a hydraulic power station and a tracked self-propelled full-rotation drilling rig.

5. The construction method for low-clearance cast-in-place piles at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata as described in claim 1, characterized in that: In step 3), the rock breaking process inside the casing is carried out first, followed by the slag removal process inside the casing of the grab bucket, until the final hole is completed and the hole is cleaned once.

6. The construction method for low-clearance cast-in-place piles at the bottom of the support beam in the deep foundation pit trestle area of ​​complex strata as described in claim 5, characterized in that: In step 3), during the advance drilling of the soil casing, water is injected into the steel casing to maintain a high water head.

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