Pile foundation for small clear distance tunnel and construction method thereof

Abstract

This invention relates to the field of pile foundation construction, and discloses a pile foundation for existing tunnels with small clearance and its construction method. The existing tunnels with small clearance are equipped with steel casings on both sides, and a steel casing is also installed between two existing tunnels. Each steel casing comprises at least two annular casing sections, all of which are arranged vertically and coaxially, with adjacent casing sections connected sequentially. The top of the steel casing is above ground level, and the bottom is driven into the rock layer as a permanent casing. Concrete piles are also installed inside the steel casing, forming the pile foundation. This invention, by driving the bottom of the steel casing into the rock layer below the ground level of the existing tunnel as a permanent casing, replaces traditional temporary or partial casings. This fundamentally blocks the disturbance transmission path of pile foundation construction to existing tunnels with small clearance, avoiding the problem in existing technologies where mud pressure fluctuations and mechanical vibrations can still be transmitted to the tunnel structure through the bottom of the steel casing, thus reducing the possibility of tunnel deformation or settlement.

Description

Technical Field

[0001] This invention relates to the field of pile foundation construction, specifically to a pile foundation for existing tunnels with small clearance and its construction method. Background Technology

[0002] With the acceleration of global urbanization and the booming development of rail transit networks, subways have become an indispensable transportation artery in modern large and medium-sized cities. In urban renewal and renovation projects, bridges crossing existing subway lines are increasingly common. However, the pile foundation construction process inevitably affects these existing tunnels, leading to ground disturbance, structural deformation, and even settlement, posing a potential threat to the stability of subway operations.

[0003] Tunnels with small clearance refer to tunnels where the clearance between two adjacent tunnels is less than the minimum clearance specified in the code, typically less than one tunnel diameter. During construction, these tunnels significantly influence each other, requiring extremely high levels of control over ground disturbance. For tunnels with small clearance, the risks are even higher due to the significant mutual influence between existing tunnels and the complex ground stress.

[0004] In traditional pile foundation construction methods, when the soil depth is relatively deep (usually greater than 30m) and the steel casing is relatively long, it is difficult to ensure the verticality of the steel casing when it is lowered. The steel casing will not be lowered below the bottom of the existing tunnel. Therefore, a full-depth isolation from the ground surface to the bottom of the existing tunnel is not formed. This allows the fluctuation of mud pressure and mechanical vibration during drilling and excavation to be transmitted to the existing tunnel through the bottom of the steel casing, making it difficult to control the settlement and deformation of the existing tunnel. Summary of the Invention

[0005] This invention provides a pile foundation for existing tunnels with small clearance and a construction method thereof to solve the above-mentioned technical problems.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a pile foundation for an existing small-clearance tunnel, wherein the existing small-clearance tunnel includes two existing tunnels with a spacing smaller than the inner diameter of the tunnel; steel casings are provided on both sides of the existing small-clearance tunnel, and a steel casing is also provided between the two existing tunnels.

[0007] The steel casing includes at least two annular casing sections, all of which are arranged vertically and coaxially, with adjacent casing sections connected in sequence.

[0008] The top of the steel casing is above the ground, and the bottom is driven into the rock layer as a permanent casing.

[0009] The rock layer is located beneath the existing tunnel;

[0010] The steel casing contains cast-in-place concrete piles, which together form the pile foundation.

[0011] This invention also provides a construction method for pile foundations of existing tunnels with small clearance, comprising the following steps:

[0012] Step S1, measurement and layout;

[0013] Step S2: The rotary drilling rig begins its initial borehole excavation and lowers the first casing section.

[0014] Step S3: Repeat the process of connecting a new casing section to the top of the already lowered casing section, excavating the secondary pilot hole with the rotary drilling rig, and lowering all the connected casing sections as a whole until the bottom surface of the lowest casing section is at the same height as the bottom surface of the existing tunnel.

[0015] Step S4: Replace the rotary drilling rig with a reverse circulation drilling rig, and cycle through connecting new casing sections to the top of the lowered casing sections, excavating with the reverse circulation drilling rig, and lowering all the connected casing sections as a whole until the bottom casing section is drilled into the rock layer.

[0016] Step S5: Clean the hole and lower the reinforcing cage;

[0017] Step S6: Pour concrete to form a concrete pile.

[0018] Specifically, before step S1, site leveling and excavation of the foundation pit are required, with the foundation pit serving as the site for subsequent excavation and lowering of the steel casing.

[0019] Site leveling specifically involves removing the preloaded soil in layers and sections until it reaches the ground elevation;

[0020] The excavation of the foundation pit is as follows: the foundation pit is excavated by sloping excavation, and water collection wells are set at the four corners of the foundation pit. Drainage ditches are excavated at the toe of the slope of the foundation pit, and plastic film is laid in the drainage ditches.

[0021] Specifically, step S1 involves: nailing a central stake at the center point of the designed location of the steel casing, and using the central stake as a reference, releasing four control stakes by drawing cross lines. Using the four control stakes as references, a temporary casing is installed, with the inner diameter of the temporary casing being larger than the outer diameter of the casing section, serving as a protective structure for the orifice.

[0022] Specifically, in step S2, the initial drilling depth of the rotary drilling rig is less than the height of the casing section, and after the initial drilling is completed, the first casing section is lowered by a drilling and pulling machine.

[0023] Specifically, in step S3, the secondary borehole excavation depth of the rotary drilling rig is equal to the height of the casing section;

[0024] The entire process of connecting all the casing sections is as follows: using a puller at low speed, all the connected casing sections are driven downwards and inserted into the bottom of the excavated hole.

[0025] Furthermore, when all the casing sections are connected by the downward insertion and insertion machine, the verticality of the lowered casing sections is also checked regularly.

[0026] Furthermore, in step S5, before lowering the reinforcing cage, sonic logging tubes are tied inside the reinforcing cage along the axial direction of the cage, and the ends of the sonic logging tubes are sealed with plugs during tying; after the concrete pile is formed, ultrasonic testing is performed on the concrete pile.

[0027] Furthermore, in step S6, the concrete is poured using a duct pouring method and in batches.

[0028] After each batch of concrete is poured, the guide pipe is slowly and steadily pulled up a certain distance before the next batch of concrete is poured.

[0029] The amount of concrete poured in each batch must meet the requirements for continuous and dense concrete pouring. The specific formula for calculating the amount of concrete poured in each batch is as follows:

[0030] ;

[0031] ;

[0032] In the formula, The volume of concrete poured per batch, in units of ; The diameter of the cast-in-place pile is expressed in units of 1. ; For the first batch of concrete pouring, the distance is the distance between the bottom of the pile hole and the bottom of the guide pipe; for subsequent batches of concrete pouring, the distance is the distance between the concrete pouring surface of the previous batch and the bottom of the guide pipe, in units of [unit missing]. ; The initial insertion depth of the catheter, in units of ; The inner diameter of the catheter is expressed in units of... ; To ensure the concrete in the pile hole reaches the depth required for the guide pipe to be embedded. The height required for the concrete inside the guide pipe to balance the mud pressure outside the guide pipe, in units of ; Mud depth, in units of ; This refers to the specific gravity of the mud, in units of... ; This refers to the specific gravity of concrete, in units of... .

[0033] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0034] 1. By placing steel casings on both sides of existing tunnels with small clearance, and between two existing tunnels, as permanent casings, and using the steel casings as guide tubes for subsequent drilling and excavation and as limiting tubes for concrete pouring, the cast-in-place concrete piles and steel casings together form the pile foundation of the existing tunnels with small clearance, providing sufficient support for the building structures subsequently constructed above the existing tunnels with small clearance. In this process, by driving the bottom end of the steel casing into the rock layer below the surface of the existing tunnel as a permanent casing, replacing the traditional temporary or partial casing, the disturbance transmission path of the pile foundation construction to the existing tunnels with small clearance can be fundamentally blocked. This shields the mud pressure fluctuations and mechanical vibrations during the drilling process and transmits them directly to the rock layer, thus avoiding the problem in existing technologies where mud pressure fluctuations and mechanical vibrations can still be transmitted to the tunnel structure through the bottom of the steel casing, reducing the possibility of tunnel deformation or settlement.

[0035] 2. By using a steel casing that reaches directly to the rock layer, a concrete pouring space is formed that reaches directly to the rock layer, so that the concrete piles can also reach the rock layer. This allows the upper load to be directly transferred to the rock layer, avoiding the load from being transferred to the soil around the existing tunnel and affecting the existing tunnel, causing tunnel deformation or settlement.

[0036] 3. By setting the steel casing as a combined structure of multiple casing sections connected in sequence, and by using a pulling machine to lower the casing sections in sections, and by regularly checking the sag of the casing sections, the verticality of the steel casing can be ensured during the lowering process. Attached Figure Description

[0037] Figure 1 This is a schematic diagram showing the positional relationship between the steel casing of the present invention and an existing tunnel.

[0038] Figure 2 This is a flowchart illustrating the principle of the present invention.

[0039] The meanings of the labels in the diagram are as follows:

[0040] Existing Tunnel 1;

[0041] Steel casing - 2. Detailed Implementation

[0042] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, so as to provide a better understanding of the concept of the present invention, the technical problem solved, the technical features constituting the technical solution, and the technical effects brought about.

[0043] like Figure 1As shown, a pile foundation for an existing small-clearance tunnel is provided. The existing small-clearance tunnel includes two existing tunnels 1 with a spacing smaller than the inner diameter of the tunnel. Steel casings 2 are provided on both sides of the existing small-clearance tunnel and a steel casing 2 is also provided between the two existing tunnels 1. The steel casing 2 includes at least two annular casing sections. All casing sections are arranged vertically and coaxially, and adjacent casing sections are connected in sequence.

[0044] The top of the steel casing 2 is higher than the ground, and the bottom is driven into the rock layer as a permanent casing.

[0045] The rock layer is located below the existing tunnel 1;

[0046] The steel casing 2 is equipped with cast-in-place concrete piles, and the steel casing 2 and the cast-in-place concrete piles serve as the pile foundation.

[0047] In this invention, the existing narrow-clearance tunnel consists of two existing tunnels 1 with a spacing smaller than the inner diameter of the tunnel; the steel casing 2 is a hollow cylindrical structure with its top end above the ground to accommodate the pile head; the pile head is clamped by a driving and pulling machine, and the steel casing 2 is driven downward into the soil until the bottom end of the steel casing 2 is inserted into the rock layer, thus serving as a permanent casing; during the process of driving the steel casing 2 into the soil, the soil inside the steel casing 2 is drilled and excavated (the two processes of driving the steel casing 2 and excavating the soil inside the steel casing 2 are repeated cyclically) to create a grouting space for the concrete pile, and concrete is poured to form a cast-in-place concrete pile; all the steel casings and cast-in-place concrete piles together constitute the pile foundation.

[0048] By inserting the bottom end of the steel casing 2 into the rock layer below the existing tunnel 1 as a permanent casing, it can replace the traditional temporary or partial casing. It does not need to be removed during subsequent construction. On the one hand, it can block the disturbance transmission path of the pile foundation construction to the existing tunnel 1, thereby shielding the mud pressure fluctuations and mechanical vibrations during the drilling process and avoiding the impact of the pile foundation construction on the surrounding soil. On the other hand, the steel casing 2 can also form a grouting space that penetrates into the rock layer, allowing the concrete pile to directly transfer the load above to the rock layer, thereby avoiding the problem of the pile foundation load affecting the soil and causing deformation or settlement of the existing tunnel 1 in the existing technology.

[0049] It should be noted that the provision of steel casings 2 on both sides of the existing narrow-clearance tunnel and between the two existing tunnels 1 means that steel casings 2 are installed on the left and right sides of the two existing tunnels 1, and also between the two existing tunnels 1. The number of steel casings 2 at each location can be one or more. When there are multiple steel casings 2, they are arranged along the axial direction of the existing tunnels 1. The specific number of steel casings 2 needs to be determined based on the soil conditions at the construction site. Furthermore, when there are more than two existing tunnels 1, the same construction method can be used.

[0050] The steel casing 2 is formed by vertically splicing and welding at least two casing sections in sequence. The casing section is a ring-shaped part formed by rolling and welding steel plates. The process of driving the steel casing 2 downward is actually a segmented driving of the casing sections in multiple stages. That is, first, a casing section is driven downward, leaving the top surface of the casing section above the ground (i.e., the driving depth is less than the height of the casing section); then, a coaxial casing section is welded on top of the casing section, and driven downward by the height of one casing section. Then, a coaxial casing section is welded on top, and driven downward by the height of one casing section again. This process is repeated until the bottom casing section reaches the rock layer below the existing tunnel 1. Thus, on the one hand, according to the actual rock layer depth, more casing sections can be added to meet the length requirements of the steel casing 2. On the other hand, the overall driving of the steel casing 2 can be replaced by segmented driving to solve the problem of difficulty in accurately controlling the verticality due to excessive single driving depth.

[0051] This invention also includes a construction method for the pile foundation of the aforementioned small-clearance tunnel, such as... Figure 2 As shown, it includes the following steps:

[0052] Step S1, measurement and layout;

[0053] Step S2: The rotary drilling rig begins its initial borehole excavation and lowers the first casing section.

[0054] Step S3: Repeat the process of connecting a new casing section to the top of the already lowered casing section, excavating the secondary pilot hole with the rotary drilling rig, and lowering all the connected casing sections as a whole until the bottom surface of the bottom casing section is at the same height as the bottom surface of the existing tunnel 1.

[0055] Step S4: Replace the rotary drilling rig with a reverse circulation drilling rig, and cycle through connecting new casing sections to the top of the lowered casing sections, excavating with the reverse circulation drilling rig, and lowering all the connected casing sections as a whole until the bottom casing section is drilled into the rock layer.

[0056] Step S5: Clean the hole and lower the reinforcing cage;

[0057] Step S6: Pour concrete to form a concrete pile.

[0058] In actual construction, the site is first surveyed and marked out to determine the actual construction position of the steel casing 2. Then, a rotary drilling rig is used for initial pre-drilling. The first casing section is lowered into the excavated hole using a jacking machine. A new casing section is then coaxially welded to the top of the first casing section to extend the length of the steel casing 2. A second pre-drilling is then performed using the rotary drilling rig, and the two existing casing sections are lowered as a whole using the jacking machine. This process is repeated until the bottom surface of the lowest casing section is level with the bottom surface of the existing tunnel 1. During this process, the soil is typically soft. Although the rotary drilling rig causes significant soil disturbance, the disturbance is effectively contained within the steel casing 2, thus maintaining high excavation efficiency. When the rotary drilling rig reaches the bottom elevation of the existing tunnel 1, it will be replaced with a reverse circulation drilling rig. The reason is that the subsequent soil is usually a sandy layer (the existing tunnel 1 is usually set on a sandy layer to ensure sufficient support and avoid settlement). Therefore, the excavation method of the reverse circulation drilling rig is adopted to meet the needs of slag removal and hole cleaning.

[0059] The excavation of the soil inside the steel casing 2 is carried out using a combination of rotary drilling rigs and reverse circulation drilling rigs until the rock layer below the existing tunnel 1 is reached. During the excavation process of the reverse circulation drilling rig, the inside of the steel casing 2 needs to be filled with mud, and the mud is continuously replaced through a mud circulation system (usually including a mud circulation tank and a mud sedimentation tank) to clean the borehole excavated by the reverse circulation drilling rig. After excavation and cleaning are completed, a reinforcing cage is lowered into the inside of the steel casing 2. The reinforcing cage is lowered by hoisting, and the hoisting process should be slow and gentle to avoid the reinforcing cage swaying and hitting the wall. Finally, concrete is poured to form concrete piles, and all the steel casings and concrete piles together constitute the pile foundation of the existing small clearance tunnel.

[0060] As a preferred embodiment, before step S1, site leveling and excavation of the foundation pit are required, with the foundation pit serving as the site for subsequent excavation and lowering of the steel casing.

[0061] Site leveling specifically involves removing the preloaded soil in layers and sections until it reaches the ground elevation;

[0062] The excavation of the foundation pit is as follows: the foundation pit is excavated by sloping excavation, and water collection wells are set at the four corners of the foundation pit. Drainage ditches are excavated at the toe of the slope of the foundation pit, and plastic film is laid in the drainage ditches.

[0063] In this embodiment, site leveling is mainly carried out by removing the preloaded soil in layers and sections. The excavation of the foundation pit is carried out by sloping excavation with a slope ratio of 1:1.5. Drainage ditches are set at the toe of the slope and covered with plastic film to prevent water seepage, thereby diverting and draining the water accumulated at the toe of the slope. Water collection wells are excavated at the four corners of the foundation pit to collect groundwater seeping around the foundation pit. The water in the water collection wells is pumped out by drainage pumps to avoid affecting the excavation of the foundation pit. After the foundation pit is excavated, the foundation pit is used as the site for subsequent excavation and the lowering of the steel casing 2.

[0064] As a further embodiment, step S1 specifically involves: driving a center pile at the center point of the designed location of the steel casing 2, and using the center pile as a reference, laying out four control piles by drawing cross lines. Using the four control piles as references, a temporary casing is installed, with the inner diameter of the temporary casing being larger than the outer diameter of the casing section, serving as the orifice protection structure. In this embodiment, the temporary casing is installed by measurement and layout. Specifically, a total station is used to accurately lay out the pile position on-site, and a center pile is driven at the center point of the designed location of the steel casing 2. Using the center pile as a reference, cross lines are drawn to lay out four control piles, and a temporary casing is installed using the four control piles as references. The inner diameter of the temporary casing is larger than the outer diameter of the casing section, and the casing section is placed inside the temporary casing. The temporary casing serves as the orifice protection structure, protecting and reinforcing the soil around the steel casing 2.

[0065] The temporary casing is 2.5-3m high and 40-60cm larger in diameter than the pile diameter. After the temporary casing is laid out by surveying and setting out, the first casing section (coaxial with the temporary casing) is installed inside the temporary casing.

[0066] In a preferred embodiment, in step S2, the initial drilling depth of the rotary drilling rig is less than the height of the casing section, and after the initial drilling is completed, the first casing section is lowered by a drilling and pulling machine.

[0067] In this embodiment, the initial drilling depth of the rotary drilling rig is less than the height of the casing section. After the first casing section is placed in the hole, a part of the casing section is still above the ground, serving as the pile head. This facilitates the welding of the next casing section to the pile head and can also serve as the working position of the driving and pulling machine.

[0068] The casing section is 6m high, the initial borehole depth is 5m, and the diameter is the same as the pile diameter, both being 2m. After the casing section is lowered into the 5m hole, there is still 1m above the ground, which makes it easier to weld the next casing section onto the first casing section.

[0069] As a further embodiment, in step S3, the secondary borehole excavation depth of the rotary drilling rig is equal to the height of the casing section;

[0070] The process of lowering all the connected casing sections as a whole involves using a puller at low speed to drive all the connected casing sections down to the bottom of the excavated hole.

[0071] In this embodiment, the secondary pre-drilling depth of the rotary drilling rig is equal to the height of the casing section. Therefore, it can be ensured that the top of each casing section excavated and lowered through the secondary pre-drilling of the rotary drilling rig has a pre-reserved pile head during lowering, which facilitates the cyclic connection to the next casing section. The lowering of all connected casing sections as a whole is specifically carried out by using a driving and pulling machine at a low speed to drive all connected casing sections down to the bottom of the excavated hole. The driving and pulling machine uses a low speed (i.e., first or second gear) for driving and pulling operations, thereby reducing the vibration generated during the driving and pulling process and avoiding impact on the soil, thus avoiding impact on the existing tunnel. In this way, until the bottom surface of the bottom casing section is at the same height as the bottom surface of the existing tunnel, the excavation construction of the rotary drilling rig is completed.

[0072] The secondary borehole depth is 6m; the lowering depth of the drilling and pulling machine is the same as the depth of a single casing section, i.e., 6m; welding, secondary borehole and lowering of casing sections are repeated in this way until the bottom surface of the bottom casing section is at the same height as the bottom surface of the existing tunnel 1, thus completing the installation of the steel casing 2 of the rotary drilling rig.

[0073] In a preferred embodiment, when connecting all the casing sections after the new casing section is driven down using a hammer and puller, the verticality of the lowered casing sections is periodically checked. The reason for periodically checking the verticality of the casing sections in this embodiment is to ensure that the casing sections are lowered vertically. A laser plumb line is a commonly used testing device on construction sites. In this embodiment, at regular intervals or each time a casing section is welded and lowered, the verticality is checked at four points on the circumference of the casing section using a laser plumb line to ensure that the verticality meets the requirements. When the verticality does not meet the design requirements (usually a deviation of ≤1%), it is necessary to adjust the contact position between the hammer of the hammer and puller and the casing section, or to use a guide chain, to ensure the verticality is maintained.

[0074] In a preferred embodiment, in step S5, before lowering the reinforcing cage, sonic logging tubes are tied along the axial direction inside the reinforcing cage, with the ends of the sonic logging tubes sealed with plugs during tying; after the concrete pile is formed, ultrasonic testing is performed on the concrete pile.

[0075] In this embodiment, after the concrete piles are formed, ultrasonic testing is performed on them to determine the quality of the pile formation. The sonic logging tubes involved in the ultrasonic testing need to be tied inside the reinforcing cage in step S5, before it is lowered. During tying, both ends of the sonic logging tubes are sealed with plugs to prevent concrete from entering and blocking the tubes during subsequent concrete pouring. The number of sonic logging tubes needs to be calculated according to specifications, and they are evenly distributed around the circumference of the concrete piles. In actual ultrasonic testing, all concrete piles need to be tested. The sonic logging tube, also known as a sonic detection embedded tube, is a core component of the ultrasonic testing system for cast-in-place piles, primarily serving as the channel for the ultrasonic probe to enter the pile body and detect structural defects.

[0076] In a preferred embodiment, in step S6, the concrete is poured using a tremie pipe method in batches; after each batch of concrete is poured, the tremie pipe is slowly and steadily pulled up a certain distance before the next batch of concrete is poured.

[0077] The amount of concrete poured in each batch should meet the requirements for continuous and dense concrete pouring. The specific calculation formula is as follows:

[0078] ;

[0079] ;

[0080] In the formula, The volume of concrete poured per batch, in units of ; The diameter of the cast-in-place pile is expressed in units of 1. ; For the first batch of concrete pouring, the distance is the distance between the bottom of the pile hole and the bottom of the guide pipe; for subsequent batches of concrete pouring, the distance is the distance between the concrete pouring surface of the previous batch and the bottom of the guide pipe, in units of [unit missing]. ; The initial insertion depth of the catheter, in units of ; The inner diameter of the catheter is expressed in units of... ; To ensure the concrete in the pile hole reaches the depth required for the guide pipe to be embedded. The height required for the concrete inside the guide pipe to balance the mud pressure outside the guide pipe, in units of ; Mud depth, in units of ; This refers to the specific gravity of the mud, in units of... ; This refers to the specific gravity of concrete, in units of... .

[0081] In this embodiment, the concrete pouring adopts the tremie pipe pouring method, which involves pouring concrete in batches. This means that the tremie pipe is first inserted into the rock layer below the existing tunnel 1, and then lifted upwards to a height of H1. At this time, the first batch of concrete is poured. After the pouring is completed, the surface of the concrete (i.e., the surface of the concrete slurry) is basically flush with the bottom of the tremie pipe. Then, the tremie pipe is lifted upwards again by H1 to pour the next batch of concrete. By repeating the above process, the concrete pouring procedure can be completed.

[0082] Because concrete has inherent cohesive properties, at the end of each batch of concrete pouring, there may be instances where concrete cannot properly drain from the tremie pipe (i.e., insufficient remaining concrete in the tremie pipe to counteract the mud pressure, preventing the remaining concrete from draining). The mud is injected into the steel casing 2 during reverse circulation drilling, primarily serving to protect the casing and remove debris and soil produced during excavation. This results in the concrete pouring volume for that batch (i.e., the amount of concrete entering the tremie pipe in each batch) not meeting requirements. Therefore, to ensure continuous and dense concrete pouring, the pouring volume for each batch should be calculated using a formula. This ensures that after most of the concrete has been poured, the tremie pipe contains not only the concrete needed for the remaining pouring but also additional concrete to overcome the current mud pressure. This additional concrete, by its own weight, overcomes the mud pressure, ensuring that all concrete that should be poured into the steel casing 2 is poured, thus avoiding the problem of some concrete failing to drain, leading to a concrete pouring volume that does not meet design requirements.

[0083] The conduit is made of steel pipe with a wall thickness of not less than 3mm and a diameter of 20-25cm. Multiple sections of steel pipe are quickly connected by double-threaded square threads at the joints to form the conduit as a whole.

[0084] It should be noted that the tremie pipe must undergo watertightness, pressure resistance, and joint tensile strength tests before it can be used for concrete pouring. During concrete pouring, the pump pipe cannot be directly inserted into the tremie pipe; instead, it must be transferred through a hopper to ensure the tremie pipe remains full throughout the concrete pouring process, thus guaranteeing the quality of the concrete pile.

[0085] The terms "connection" and "fixing" appearing in the description of this invention can refer to fixed connection, processing and forming, welding, or mechanical connection. The specific meaning of the above terms in this invention should be understood according to the specific circumstances.

[0086] In the description of this invention, the terms "center," "upper," "lower," "horizontal," "inner," and "outer," etc., are used only to indicate the orientation or positional relationship for the convenience of describing this invention and to simplify the description, and do not indicate or imply a specific orientation that the device or element referred to must have, and therefore should not be construed as a limitation of this invention.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A pile foundation for an existing tunnel with a small clearance, wherein the existing tunnel with a small clearance comprises two existing tunnels (1) with a spacing smaller than the inner diameter of the tunnel; characterized in that, The existing small-clearance tunnel is equipped with steel casings (2) on both sides, and steel casings (2) are also provided between the two existing tunnels (1). The steel casing (2) includes at least two annular casing sections, all of which are arranged vertically and coaxially, and adjacent casing sections are connected in sequence. The top of the steel casing (2) is higher than the ground, and the bottom is driven into the rock layer as a permanent casing. The rock layer is located below the existing tunnel (1); The steel casing (2) is equipped with concrete piles, and the steel casing (2) and concrete piles serve as the pile foundation.

2. A construction method for pile foundations of existing tunnels with small clearance as described in claim 1, characterized in that, Includes the following steps: Step S1, measurement and layout; Step S2: The rotary drilling rig begins its initial borehole excavation and lowers the first casing section. Step S3, repeat the process of connecting a new casing section to the top of the already lowered casing section, excavating the secondary pilot hole with the rotary drilling rig, and lowering all the connected casing sections as a whole until the bottom surface of the lowest casing section is at the same height as the bottom surface of the existing tunnel (1); Step S4: Replace the rotary drilling rig with a reverse circulation drilling rig, and cycle through connecting new casing sections to the top of the lowered casing sections, excavating with the reverse circulation drilling rig, and lowering all the connected casing sections as a whole until the bottom casing section is drilled into the rock layer. Step S5: Clean the hole and lower the reinforcing cage; Step S6: Pour concrete to form a concrete pile.

3. The construction method for pile foundations of existing tunnels with small clearance according to claim 2, characterized in that, Before step S1, site leveling and excavation of the foundation pit are required, with the foundation pit serving as the site for subsequent excavation and lowering of the steel casing. Site leveling specifically involves removing the preloaded soil in layers and sections until it reaches the ground elevation; The excavation of the foundation pit is carried out by sloping excavation. Water collection wells are set at the four corners of the foundation pit, and drainage ditches are excavated at the toe of the slope of the foundation pit. Plastic membranes are laid in the drainage ditches.

4. The construction method of a pile foundation for a small clear distance tunnel according to claim 2, characterized in that, The specific steps of step S1 are as follows: nail a central pile at the center point of the designed position of the steel casing (2), and use the central pile as a reference to release four control piles by pulling cross lines. Use the four control piles as a reference to bury a temporary casing. The inner diameter of the temporary casing is larger than the outer diameter of the casing section, which serves as the opening protection structure.

5. The construction method of a pile foundation for a small clear distance tunnel according to claim 2, characterized in that, In step S2, the initial drilling depth of the rotary drilling rig is less than the height of the casing section. After the initial drilling is completed, the first casing section is lowered by a drilling and pulling machine.

6. The construction method of a pile foundation for a small clear distance tunnel according to claim 5, characterized in that, In step S3, the secondary borehole excavation depth of the rotary drilling rig is equal to the height of the casing section. The process of lowering all the connected casing sections as a whole involves using a puller at low speed to drive all the connected casing sections down to the bottom of the excavated hole.

7. The method according to claim 5, wherein, When all the casing sections are connected by driving and pulling the machine downwards, the verticality of the lowered casing sections is also checked regularly.

8. The construction method of a pile foundation for a small clear distance tunnel according to claim 2, characterized in that, In step S5, before lowering the reinforcing cage, sonic logging tubes are tied along the axial direction inside the reinforcing cage, and the ends of the sonic logging tubes are sealed with plugs during tying; after the concrete pile is formed, ultrasonic testing is performed on the concrete pile.

9. The method according to claim 2, wherein, In step S6, the concrete is poured using the tremie pipe pouring method and in batches. After each batch of concrete is poured, the guide pipe is slowly and steadily pulled up a certain distance before the next batch of concrete is poured. The amount of concrete poured in each batch must meet the requirements for continuous and dense concrete pouring. The specific formula for calculating the amount of concrete poured in each batch is as follows: ； ； In the formula, The volume of concrete poured per batch, in units of ; The diameter of the cast-in-place pile is given in units of 1. ; For the first batch of concrete pouring, the distance is the distance between the bottom of the pile hole and the bottom of the guide pipe; for subsequent batches of concrete pouring, the distance is the distance between the concrete pouring surface of the previous batch and the bottom of the guide pipe, in units of [unit missing]. ; The initial insertion depth of the catheter, in units of ; The inner diameter of the catheter is expressed in units of 1000 mm. ; To ensure the concrete in the pile hole reaches the depth required for the tremie pipe to be embedded. The height required for the concrete inside the guide pipe to balance the mud pressure outside the guide pipe, in units of ; Mud depth, in units of ; This refers to the specific gravity of the mud, in units of... ; This refers to the specific gravity of concrete, in units of... .

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