A composite foundation of open-cut tunnel concrete pile and construction method

Abstract

The present application belongs to the field of open cut tunnel, and particularly relates to a kind of open cut tunnel plain concrete pile composite foundation and construction method, the composite foundation includes the stratum reinforced by a plurality of plain concrete piles and the broken stone cushion and C20 concrete cushion laid on the top of plain concrete pile;The plain concrete piles are arranged in matrix, and the width of the plain concrete pile matrix is greater than the width of the tunnel structure;The uniform transition of foundation stiffness between the filling section and the excavation section is realized by changing the pile length of the plain concrete pile;The construction method is the construction method of the above-mentioned composite foundation, wherein the plain concrete pile is backfilled after hole forming by rotary drilling, and then long spiral drilling machine is matched with ground pump for pouring.The composite foundation of the present application is simple in structure, reasonable in design, and reduces the difference of tunnel structure foundation;Compared with the traditional underwater steel guide pipe pouring method, the construction speed of the plain concrete pile in the present application is faster, the turnover material consumption is less, the cost is low, and the pile body integrity is better.

Description

Technical Field

[0001] This invention belongs to the field of open-cut tunnel construction technology, specifically relating to a plain concrete pile composite foundation for open-cut tunnels and its construction method. Background Technology

[0002] Cut-and-cover tunnels are tunnels constructed using the cut-and-cover method. This method involves excavating the surface, constructing the lining in the open, and then backfilling. It is commonly used for shallow tunnels and is the most basic and frequently used construction method in soft soil underground engineering. Generally, cut-and-cover tunnels do not require special foundation treatment; they typically use foundation pit support or diaphragm walls, followed by the main structure construction, and finally backfilling.

[0003] A project involves constructing an underpass tunnel for an airport. The tunnel will be constructed using the open-cut method, followed by backfilling to build the airport. The backfill layer will be approximately 20-30 meters thick. If the foundation of this underpass tunnel is not treated, settlement will occur, affecting the tunnel's structural safety. The underpass tunnel is 2489 meters long, located in a hilly area with significant undulations. The ground elevation ranges from 25 to 130 meters, and the maximum tunnel depth is approximately 80 meters. The longitudinal slope of the tunnel entrance and exit ramps ranges from 15° to 25°. Based on the relationship between the tunnel's elevation and the planned airport ground level elevation, the tunnel is divided into several excavation and backfill sections. Furthermore, the tunnel passes through multiple geological strata, including stratum W3 (strongly weathered metamorphic sandstone, soft rock), stratum W4 (completely weathered metamorphic sandstone, medium-hard soil), and Quaternary strata. For the foundation treatment of open-cut tunnels with both excavation and embankment sections in this project, since there are few similar projects and existing open-cut tunnel foundation treatment methods are difficult to apply directly, a foundation treatment technology for deep-buried open-cut tunnels in the above situation is needed. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a plain concrete pile composite foundation for open-cut tunnels and a construction method thereof.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A composite foundation of plain concrete piles for open-cut tunnels is applied to the fill-cut transition section of deeply buried open-cut tunnels. It includes a stratum reinforced with several plain concrete piles and a crushed stone cushion layer and a C20 concrete cushion layer laid on top of the plain concrete piles. The plain concrete piles are arranged in a matrix, and the width of the plain concrete pile matrix is ​​greater than the width of the tunnel structure. The uniform transition of foundation stiffness between the fill section and the cut section is achieved by changing the pile length of the plain concrete piles.

[0007] Furthermore, the plain concrete piles have a diameter of 0.6m, a length of not less than 3.0m, and a depth of not less than 1m into the bearing stratum at the pile tip; the transverse pile spacing in the plain concrete pile matrix is ​​1.70–2.35m, and the longitudinal pile spacing is 2–2.5m; the maximum particle size of the crushed stone cushion layer at the pile top does not exceed 50mm, the mud content is not greater than 5%, and the dynamic deformation modulus E after compaction is [not specified]. vd Not less than 40MPa.

[0008] Furthermore, plain concrete piles are arranged in sections every 10 meters along the mileage direction. Depending on the geological conditions, the length of plain concrete piles in each section may be the same or different. Depending on the geological conditions, the length of the fill-cut transition section is 10 to 50 meters.

[0009] Furthermore, for the excavation section, plain concrete piles, CFG piles, steel pipe grouting, and sealing cushion layer reinforcement measures are adopted according to the geological conditions; for the filling section, reinforced concrete bored pile measures are adopted for foundation treatment.

[0010] Furthermore, expansion joints with a width of 2cm are set in the transition sections of segments where different foundation treatment measures are adopted; the length of foundation treatment piles and the thickness of the cushion layer in the transition section gradually change to reduce the longitudinal differential settlement of the tunnel structure; in the cross section direction, the same foundation treatment measures are adopted within the influence range of the tunnel structure to reduce the lateral differential settlement of the structure.

[0011] Furthermore, the diameter of the drilled concrete piles is determined comprehensively based on the pile foundation type, stress analysis, and construction conditions. For sections with a soil cover of less than 10m, 1.25m diameter piles are used; for sections with a soil cover of more than 10m, 1.5m diameter piles are used. The pile type selection at the tunnel section in the fill section is the same as that of the open tunnel structure in the same section. Four drilled piles are arranged in the cross-sectional direction, respectively under the side wall of the open tunnel structure. The center-to-center distance of the drilled piles is twice the pile diameter, and the net distance from the edge of the structural base plate to the outermost row of piles is 0.5 times the pile diameter. The arrangement of the drilled piles along the line direction should be consistent with the casting module of the open tunnel structure. The longitudinal spacing of the piles is 5m. The single casting length of the open tunnel structure is 10m. The piles are arranged in the center, with the center distance from the structural construction joint 2.5m. The top of the pile is fixed to the open tunnel base plate structure and extends 100mm into the structural base plate.

[0012] Furthermore, the steel pipes used for grouting have an outer diameter of 76mm and a wall thickness of 3.5mm. They are arranged at 100cm x 100cm intervals, driven vertically to the foundation, and are 2.5m deep. The steel pipes are made of seamless hot-rolled steel pipes. Grouting holes are drilled at the front, with a diameter of 6mm and a spacing of 20-30cm, arranged in a quincunx pattern. The front end is processed into a cone shape, and the un-drilled length at the rear is not less than 100cm, serving as a grout-stopping section. Cement grout is used for grouting, with a water-cement ratio of 1:1 (by weight) or determined according to the site conditions. The grouting pressure is 0.5-1.0MPa, and the grout viscosity is 80s-90s.

[0013] Furthermore, the excavation section is reinforced with CFG piles, each with a diameter of 500mm and a depth of at least 0.5m into the bearing stratum. A 0.3m thick crushed stone cushion layer and a 0.15m thick C20 concrete cushion layer are installed on top of the piles, with the upper layer of the C20 concrete cushion layer serving as the bottom surface of the tunnel lining structure. The filling section is reinforced with drilled reinforced concrete piles, each with a 0.2m thick C20 concrete cushion layer. The fill-excavation transition section is reinforced with plain concrete piles, each with a diameter of 600mm and a 0.2m thick crushed stone cushion layer and a 0.15m thick C20 concrete cushion layer on top. Temporary drainage is implemented during the construction of the cushion layer in the excavation section, and C20 concrete is used to backfill the foundation pits at locations with unequal structural heights.

[0014] This invention also proposes a construction method for a plain concrete pile composite foundation for open-cut tunnels, comprising the following steps:

[0015] S1. Based on the relationship between the tunnel elevation, the current ground elevation and the airport planned apron elevation, and taking into account the influence of the thickness of the soft soil layer at the bottom of the tunnel, the boundary between the fill and cut sections is determined, and the fill section, cut section and fill-cut transition section are divided.

[0016] S2. Based on the tunnel geological conditions, select appropriate foundation treatment measures to reduce differential settlement of the structure;

[0017] Different foundation treatment measures are selected for the excavation section according to different strata; for the filling section, the foundation treatment adopts reinforced concrete bored pile measures, and the pile diameter, pile length, reinforcement and other parameters are determined by calculation; for the filling-excavation transition section, the foundation treatment adopts plain concrete pile measures, and the foundation stiffness between the filling section and the excavation section is uniformly transitioned by varying the pile length.

[0018] Furthermore, the plain concrete pile is made by drilling holes using rotary drilling, backfilling with soil, and then using a long spiral drilling rig with a ground pump to inject plain concrete.

[0019] After the rotary drilling is completed, the existing plain concrete piles with holes are backfilled to prevent the holes from collapsing. The backfill material is the clay produced during the drilling process.

[0020] The drilling speed of the long spiral drilling rig is 1.8–2.2 m / min. When drilling, observe the depth of the drill rod. When the maximum depth is reached, continue drilling for 3 minutes to ensure that the borehole reaches the designed depth. After drilling to the designed depth, lift the drill rod to open the drill bit flap and start pouring concrete through the ground pump. Pour concrete while lifting the drill rod, with a lifting speed of 1.2–1.5 m / min, and ensure that the drill bit tip is always buried in the grout mixture by 0.8–1.1 m to prevent pile breakage. When pouring concrete to the top of the pile, it should exceed the designed elevation of the pile top by no less than 50 cm.

[0021] The beneficial effects of this invention are:

[0022] The composite foundation structure of the present invention is simple and reasonably designed. The foundation of the fill-cut transition section is reinforced by dry plain concrete piles and a crushed stone cushion layer is provided. By changing the pile length of the plain concrete piles, a uniform transition of foundation stiffness between the fill section and the cut section is achieved, reducing the difference in foundation between the tunnel structure.

[0023] The method of the present invention provides reasonable foundation reinforcement treatment for the excavation section, the filling section and the excavation-filling transition section according to the tunnel geological conditions, thereby reducing the differential settlement of the tunnel structure caused by different geological conditions and ensuring the long-term stability of the open-cut tunnel under the airport.

[0024] In this invention, the plain concrete piles are constructed by rotary drilling, backfilling with soil, and then grouting with a long spiral drilling rig and a ground pump. Compared with the traditional underwater steel pipe grouting method, the plain concrete piles of this invention have a faster construction speed, consume less turnover materials, have lower costs, and have better pile integrity. Attached Figure Description

[0025] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein:

[0026] Figure 1 This is a cross-sectional layout diagram of CFG pile reinforcement according to an embodiment of the present invention.

[0027] Figure 2 This is a cross-sectional layout diagram of reinforced concrete bored pile reinforcement according to an embodiment of the present invention.

[0028] Figure 3 This is an elevation view of the reinforcement layout of a reinforced concrete bored pile according to an embodiment of the present invention.

[0029] Figure 4 This is a cross-sectional layout diagram of the steel perforated pipe grouting according to an embodiment of the present invention.

[0030] Figure 5 This is a diagram showing the grouting arrangement of the steel perforated pipe according to an embodiment of the present invention.

[0031] Figure 6 This is a cross-sectional layout diagram of the plain concrete pile foundation treatment according to an embodiment of the present invention.

[0032] Figure 7 This is a plan view of the plain concrete piles according to an embodiment of the present invention.

[0033] Figure 8 This is a schematic diagram of the longitudinal section of the fill-cut transition section according to an embodiment of the present invention.

[0034] Figure 9 This is a schematic diagram illustrating the reinforcement application of the fill-cut transition section in an embodiment of the present invention.

[0035] Figure 10 for Figure 9 Plan view of the foundation of the fill-cut transition section.

[0036] Figure 11 This is a schematic longitudinal section of the transition section between fully weathered and strongly weathered according to an embodiment of the present invention. Detailed Implementation

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art are within the scope of protection of the present invention.

[0038] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0039] like Figures 1 to 10 As shown, this invention proposes a plain concrete pile composite foundation for open-cut tunnels, applied to the fill-cut transition section of deeply buried open-cut tunnels. It includes a stratum reinforced with several plain concrete piles, and a crushed stone cushion layer and a C20 concrete cushion layer laid on top of the plain concrete piles. The plain concrete piles are arranged in a matrix, with the width of the plain concrete pile matrix greater than the width of the tunnel structure. A uniform transition of foundation stiffness between the fill and cut sections is achieved by changing the pile length of the plain concrete piles.

[0040] This invention also proposes a construction method for a plain concrete pile composite foundation for open-cut tunnels, used for the construction of the aforementioned plain concrete pile composite foundation for open-cut tunnels, comprising the following steps:

[0041] S1. Based on the relationship between the tunnel elevation, the current ground elevation and the airport planned apron elevation, and taking into account the influence of the thickness of the soft soil layer at the bottom of the tunnel, the boundary between the fill and cut sections is determined, and the fill section, cut section and fill-cut transition section are divided.

[0042] S2. Based on the tunnel geological conditions, select appropriate foundation treatment measures to reduce differential settlement of the structure;

[0043] Different foundation treatment measures are selected for excavation sections based on different geological strata. For embankment sections, reinforced concrete bored piles are used for foundation treatment, with pile diameter, length, and reinforcement parameters determined according to the requirements for composite foundation bearing capacity. For embankment-excavation transition sections, plain concrete piles are used for foundation treatment. By varying pile lengths, a uniform transition in foundation stiffness between the embankment and excavation sections is achieved, ensuring that the embankment-excavation transition section meets the composite foundation bearing capacity requirements in the relevant specifications ("Technical Specification for Foundation Treatment of Railway Engineering" (TB 10106-2023)). The principle of foundation treatment in this invention is as follows: along the railway line, foundation treatment measures are selected according to the changes in the geological strata at the bottom of the tunnel. Expansion joints are set in the transition sections where different foundation treatment measures are used. At the same time, the pile length and cushion layer thickness of the foundation treatment in the transition section gradually change to reduce longitudinal differential settlement of the tunnel structure. In the transverse direction, the same foundation treatment measures are used within the influence range of the tunnel structure to reduce lateral differential settlement of the structure.

[0044] Furthermore, depending on the geological conditions, plain concrete piles, CFG piles, steel pipe grouting, and sealing cushion layers are used in the excavation section. Specifically, for the open-cut tunnel construction under the airport in the background art of this invention; for the excavation section, when the tunnel bottom is located in the W3 stratum, the foundation treatment adopts sealing cushion measures; when the tunnel bottom is located in the W4 stratum, steel pipe grouting reinforcement (unloading section) or CFG pile composite foundation (loading section) is adopted; when the tunnel bottom is located in the Quaternary stratum, CFG pile composite foundation is adopted.

[0045] Furthermore, the diameter of the reinforced concrete bored piles is determined comprehensively based on the pile foundation type, stress analysis, and construction conditions; for sections with a soil cover of less than 10m, 1.25m diameter piles are used, and for sections with a soil cover of more than 10m, 1.5m diameter piles are used. The pile type selection for the tunnel section in the fill section is the same as that for the open tunnel structure in the same section. Figure 2 As shown, four bored piles are arranged in the cross-section, located under the sidewalls of the open-cut tunnel structure. The center-to-center distance between the bored piles is twice the pile diameter, and the net distance from the edge of the structural base slab to the outermost row of piles is 0.5 times the pile diameter. The arrangement of the bored piles along the tunnel route should be consistent with the casting module of the open-cut tunnel structure. The longitudinal spacing of the piles is 5m, the single casting length of the open-cut tunnel structure is 10m, the piles are centrally located, the center distance from the structural construction joint is 2.5m, and the top of the piles is fixed to the open-cut tunnel base slab structure, extending 100mm into the structural base slab. Figure 3As shown, the reinforced concrete bored pile has a continuous reinforcing cage. The main reinforcement bars extend into the structural base slab at a 15° angle, with an anchorage length of 1.5m, and are reinforced with stirrups. The reinforcing cage is equipped with spiral stirrups, which are spot-welded to the main reinforcement bars. The vertical spacing of the spiral stirrups is 200mm, with increased spacing of 100mm within a 5D range from the pile top and bottom. A stiffening stirrup is installed every 2m along the direction of the reinforcing cage; this stiffening stirrup is a closed stirrup and welded to the main reinforcement bars. The reinforced concrete bored pile body uses C40 reinforced concrete (P10 impermeability grade), with a 70mm thick concrete cover. The reinforcing bars used are HRB400 and HPB300 type.

[0046] Furthermore, such as Figure 6 , Figure 7 As shown, the plain concrete piles have a diameter of 0.6m. The pile diameter was determined according to the requirements for solid plain concrete pile diameter in the "Technical Specification for Foundation Treatment of Railway Engineering" (TB 10106-2023) and obtained through bearing capacity calculations, while also considering economic efficiency. The cross-sectional pile layout range is not less than 2m outside the tunnel floor structure, the pile length is not less than 3.0m, and the pile tip penetrates the bearing layer of W3 stratum to a depth of not less than 1.0m. Considering economic efficiency, the pile tip penetration depth into W3 stratum is 1m, which meets the bearing capacity requirements. The transverse pile spacing is 1.70~2.35m, the longitudinal pile spacing is 2~2.5m, the treatment on the excavation side extends 2m to the outside of the structure, and the treatment on the fill side extends 5m to the outside of the structure. The plain concrete pile body is made of C35 concrete, with a cubic compressive strength of not less than 35 MPa at 28 days. A 0.2m crushed stone cushion layer is set on the pile top. The crushed stone is hard, not easily weathered, and well-graded gravel or crushed stone with a maximum particle size of not more than 50mm and a mud content of not more than 5%. The dynamic deformation modulus E after compaction is [not specified]. vd Not less than 40MPa. When the pumpability of plain concrete mixture meets the construction requirements, pumping agent may not be added. When the cement content is relatively low and the temperature is high, appropriate pumping agent may be added to the plain concrete raw materials.

[0047] Furthermore, such as Figure 1 As shown, the CFG pile has a diameter of 0.5m, a length of not less than 3.0m, and a depth of not less than 0.5m into the W3 bearing stratum. The excavation side is treated to 2m beyond the outer edge of the structure, and the fill side is treated to 5m beyond the outer edge of the structure. The pile body uses C35 concrete, and the mix proportion should be determined through indoor tests before construction. The 28-day cubic compressive strength should not be less than 35MPa. A 0.3m thick crushed stone cushion layer is provided at the top of the pile, using hard, non-weatherable, and well-graded gravel or crushed stone with a maximum particle size not exceeding 50mm and a mud content not exceeding 5%. The dynamic deformation modulus E after compaction is [not specified]. vdThe pressure should not be less than 40 MPa. When constructing CFG piles, follow the standard CFG pile construction procedures. Note that the drill rod should be lifted statically, and the lifting speed should generally be controlled between 1.2 m and 1.5 m / min, ensuring continuous lifting. Overspeed lifting is strictly prohibited during construction.

[0048] Furthermore, such as Figure 4 , Figure 5 As shown, the steel pipes used for grouting have an outer diameter of 76mm, a wall thickness of 3.5mm, and are arranged at 100cm x 100cm intervals, driven vertically to the foundation to a depth of 2.5m; the grouting extends 2m to the outer side of the structure. Cement grout is used, with a water-cement ratio of 1:1 (by weight) or determined on-site, and a grouting pressure of 0.5–1.0MPa. The steel pipes are made of seamless hot-rolled steel pipes, with 6mm diameter grouting holes drilled at the front, spaced 20–30cm apart in a quincunx pattern. The front end is tapered, and the un-drilled section at the rear is at least 100cm long, serving as a grout stop. The grout viscosity should be 80s–90s. After 7 days of sealing with grout, the compressive strength of a 70.7mm x 70.7mm x 70.7mm cube specimen should be 0.3MPa–0.5MPa. Ordinary Portland cement is recommended as the grout. Fly ash can be partially added during grouting, with an addition amount of 20% to 50% of the cement weight. Depending on project requirements, accelerators, water-reducing agents, and anti-segregation agents can be added during grout mixing. Steel pipe installation generally employs a drilling method, with the borehole diameter 3-5mm larger than the steel pipe diameter. Small guide pipes are then passed through the steel frame and driven in using a hammer or drilling rig, with a driving length not less than 90% of the steel pipe length. High-pressure air is then used to blow out any sand or gravel inside the steel pipe. A water pressure test should be conducted before grouting the steel pipes to check the mechanical equipment and pipeline connections. To accelerate grouting and maximize equipment efficiency, group grouting (3-5 pipes at a time) can be used. Grouting should continue for 30 minutes after the grout volume reaches the designed grout volume or the grouting pressure reaches the maximum designed grouting pressure, and the grouting flow rate does not exceed 1L / min. During grouting, the changes in grouting pressure and grout pump discharge should be constantly monitored to analyze the grouting situation and prevent pipe blockage, grout leakage, and grout failure. Keep accurate records of the grouting process to facilitate analysis of its effectiveness.

[0049] like Figures 8 to 10The diagram shows the foundation treatment for the cut-fill transition section. The cut section is reinforced with CFG piles (500mm diameter), topped with a 0.3m thick crushed stone cushion layer and a 0.15m thick C20 concrete cushion layer. The upper layer of the C20 concrete cushion layer forms the bottom surface of the tunnel lining structure. The fill section is reinforced with drilled reinforced concrete piles, topped with a 0.2m thick C20 concrete cushion layer. The cut-fill transition section is reinforced with plain concrete piles (600mm diameter), topped with a 0.2m thick crushed stone cushion layer and a 0.15m thick C20 concrete cushion layer. The number of piles in the diagram is for illustrative purposes only and should be selected according to the lining type during construction. Expansion joints are provided in the lining structure of the transition section. The lining structure is constructed in a low-to-high sequence, with the cut section constructed first, followed by the fill section. Temporary drainage is implemented during the construction of the cushion layer in the cut section. C20 concrete is used to backfill the foundation pits at points of unequal structural height.

[0050] Furthermore, such as Figures 8 to 10 As shown, expansion joints are installed at locations where the cross-section of the open-cut tunnel changes, where foundation treatment measures change, at the boundary between the cut-and-cover tunnel and the roadbed structure, and at locations with significant changes in strata. The width of the expansion joints is 2cm, and conventional design is sufficient. Construction joints (excluding expansion joints) are installed at 10 or 20m intervals on the bottom slab of the open-cut tunnel structure, and construction joints (excluding expansion joints) are installed at 10m intervals on the arch wall of the open-cut tunnel.

[0051] Furthermore, the site should be leveled before pile foundation construction. If there are plants, their roots should be removed and the site leveled. The leveling elevation should be equal to the design pile top elevation plus 1.0m.

[0052] The following section uses the open-cut tunnel under the airport in the background project as an example to specifically illustrate the pile foundation construction process of this invention. The overburden layer within the design area of ​​this open-cut tunnel under the airport consists of Quaternary Holocene (Q4) and Pleistocene (Q1+2+3); underlying are Upper Tertiary Miocene (N1), Lower Tertiary (E), Cretaceous (K), Jurassic (J), Triassic (T), Carboniferous (C), Devonian (D), Ordovician (O), and Cambrian (∈) strata. The intrusive rocks are mainly Yanshanian (γ5) granites. The lithology of each rock and soil layer, from top to bottom and from newest to oldest, is as follows: metamorphic sandstone W4 stratum and metamorphic sandstone W3 stratum; metamorphic sandstone W4 stratum is a completely weathered stratum with a basic bearing capacity of 180 kPa; metamorphic sandstone W3 stratum is a strongly weathered stratum with a basic bearing capacity of 450 kPa. The concrete materials, mix proportions, and durability indicators used in foundation treatment should meet the requirement of a design service life of 100 years.

[0053] Specifically, the foundation treatment procedures for the tunnel embankment section are as follows:

[0054] The first step is to construct construction barriers, level the site, and construct drainage ditches around the foundation pit; if there is a pond, the pond in the tunnel area will be pumped out, and the surface silt will be treated.

[0055] The second step is to carry out the first phase of earthwork backfilling, filling the soil to the bottom elevation of the tunnel and then compacting it.

[0056] The third step is to construct the reinforced concrete bored pile foundation;

[0057] The fourth step is to construct the tunnel floor cushion layer, lay the waterproof layer for the bottom slab, and pour the concrete for the open-cut tunnel floor slab; once the concrete for the bottom slab reaches the design strength, the arch wall structure of the open-cut tunnel can be constructed.

[0058] In this embodiment, the length of the reinforced concrete bored pile is 16m to 86m.

[0059] Specifically, the foundation treatment procedures for the tunnel excavation section are as follows:

[0060] The first step is to construct construction barriers, level the ground, and excavate the foundation pit. A drainage ditch is set up at the top of the slope, and a brick retaining wall is set up on the outside of the slope line. The soil layer is excavated on a slope, with each excavation height not exceeding 1.0m. The mesh is then promptly installed and shotcrete is applied, and soil nailing is used for support. Excavation can only continue after these steps are completed.

[0061] The second step is to excavate the foundation pit to the first-level slope platform position, and then carry out the second-level slope excavation. Following the method in the first step, excavate to the design elevation of the bottom of the foundation pit, implement slope protection measures, and construct drainage ditches at the bottom of the pit.

[0062] The third step is to implement foundation treatment measures at the bottom of the excavation pit. For the excavation section, when the tunnel bottom is located in the W3 stratum, a sealing pad is used for foundation treatment; when the tunnel bottom is located in the W4 stratum, steel pipe grouting reinforcement (unloading section) or CFG pile composite foundation (loading section) is used; when the tunnel bottom is located in the Quaternary stratum...

[0063] CFG pile composite foundation is adopted;

[0064] The fourth step is to construct the tunnel floor cushion layer, lay the waterproof layer for the bottom slab, and pour the concrete for the open-cut tunnel floor slab. Once the concrete for the bottom slab reaches the design strength, the arch wall structure of the open-cut tunnel can be constructed.

[0065] The foundation treatment procedures for the fill-cut transition section are the same as those for the fill section and the cut section.

[0066] In this invention, CFG piles and reinforced concrete bored piles are constructed using conventional procedures. However, the plain concrete piles do not employ the conventional underwater steel guide pipe casting method; instead, an innovative construction scheme is used: rotary drilling and a long spiral drilling rig combined with a ground pump for casting. Specifically, the construction method for plain concrete piles in this embodiment includes the following steps:

[0067] (1) Construction preparation: On the basis of three connections and one leveling, the drilling platform site shall be leveled, and it shall be required to be flat and compacted. Before the drilling rig enters the site, the original ground elevation shall be 1m higher than the top elevation of the pile.

[0068] (2) Determining the borehole position: Based on the control points provided in the design drawings, a total station is used to set up and verify the control network on site. According to the coordinate values ​​of the center axis of the pile foundation, the center line and center point of the pile foundation are laid out using the coordinate method, and marker stakes are driven in. The layout error of the center line is controlled within 1cm. Cross-shaped control stakes are set in a safe area about 2m away from the center of the pile for easy verification. The stake number is marked on the stake.

[0069] (3) Excavation and Installation of the Casing: The casing is made of 10mm thick steel plate; its bottom is buried 3.5m below the ground surface, and the top of the casing is 0.5m above the ground surface. The casing is installed using the excavation method, ensuring accurate, horizontal, vertical, and stable installation. Clay is backfilled around the casing and compacted. The center line of the drill bit and the center line of the casing are kept on the same straight line, with a deviation not exceeding 50mm. The verticality deviation of the steel casing is less than 1%, ensuring that the drilling rig can work smoothly along the vertical direction of the pile position. After the casing is in place, the excavation joint on its outer side is backfilled and compacted in layers.

[0070] (4) Drilling rig positioning: The drilling rig can only be positioned after the pile position has been verified to be correct; after the rotary drilling rig is in position, adjust the verticality of the drilling frame, align the center of the drill bit with the center of the cross line of the pile protection, and inject mud.

[0071] Drilling mud is prepared using a mixer, where clay is first soaked in a mud-making tank and then stirred to form mud. The mud is then circulated and purified through sedimentation tanks and storage tanks. The mud circulation sequence is: freshly prepared mud → mud tank → pile hole → sedimentation tank → mud tank → pile hole. Laboratory tests show the relative density of the mud to be 1.05–1.15 g / m³. 3 Viscosity 16-22s, sand content not exceeding 4%.

[0072] (5) Drilling (Borehole Construction): Different drill bits are selected according to the soil conditions and drilling method; rotary drill bits can be used for cohesive soil, silt, fill, and medium-dense or denser sandy soil; rock-socketed drill bits can be used for gravelly soil, medium-hard rock, and weathered layers. The diameter of the rotary drilling rig drill bit must be selected to ensure that the hole diameter is not less than the designed pile diameter. When starting a rotary drilling rig, light pressure and slow advance should be used, and the drill bit speed should not exceed 10 r / min. Normal drilling can only be gradually accelerated after the active drill rod has entered the hole.

[0073] Rotary drilling rigs can use either mud slurry drilling or dry drilling, depending on the operation method and geological conditions. During drilling, the drill bit speed should be selected based on the soil geological conditions and operation method. The lifting and lowering speed should be strictly controlled during drilling to minimize disturbance to the borehole wall and prevent borehole collapse. The borehole position must be accurate, and the initial borehole wall should be vertical, smooth, and firm.

[0074] During drilling, geological changes should be frequently checked and recorded, and compared with the geological columnar section. Each drilling rig on site must be equipped with a cuttings sample box, and drilling records must be filled out promptly. Samples should be taken every 1 meter of drilling depth, and cuttings samples should also be retained when changing between different rock strata. The retention of cuttings samples is a major basis for determining the final borehole depth. In this embodiment, the cuttings sample is used to determine whether the W3 bearing layer has been reached. At the same time, drilling records should be kept carefully, generally recording the drilling depth every 2 meters. During drilling, the hole position, diameter, and inclination should be checked frequently, and deviations should be corrected promptly. After the borehole reaches the designed depth, a borehole inspection should be carried out promptly. The hole position, diameter, depth, and shape should be checked and recorded.

[0075] When constructing plain concrete piles, a verified steel measuring rope is used, and the construction team is required to exceed the required depth by about 20cm to ensure that the hole depth is qualified and to control the length of the plain concrete piles to be qualified.

[0076] In this embodiment, after the rotary drilling drill penetrated 1.0m into the W3 stratum, the drilling was completed, and the existing plain concrete piles were backfilled to prevent borehole collapse. The backfill material was clay produced during the drilling process.

[0077] (6) Grouting with a ground pump in conjunction with a long spiral drilling rig:

[0078] Due to the inclination of the existing ground line or weathered surface, plain concrete piles are laid out in 10m sections along the tunnel mileage. The pile lengths of different sections may be the same or different, and the pile lengths within each section may also be the same or different. The specific pile length is determined based on the location of the W3 stratum. In this embodiment, the composite foundation of the open-cut tunnel under the airport has multiple fill-cut transition sections, with lengths ranging from 10 to 50 meters. After each section of drilling is completed, the backfilled plain concrete piles are then grouted. A long spiral drilling rig is used in conjunction with a ground pump for grooving.

[0079] Before the grouting operation, the existing pile positions need to be laid out on the plain concrete pile working surface that has been backfilled. After the drilling rig is in place, the position should be corrected with the help of the vertical markers on the front, back, left and right sides of the drilling rig tower so that the drill rod is vertically aligned with the center of the pile position.

[0080] When starting drilling with a long spiral drilling rig, close the drill bit valve and move the drill rod downwards until the drill bit touches the ground, then start the motor to begin drilling. Start slowly and gradually increase the speed, while simultaneously checking for and correcting any deviations in the drilling. If the drill rod becomes wobbly or difficult to drill during the process, slow down the advance to prevent the pile hole from deviating, shifting, or being damaged by the drill rod or drilling tools. The long spiral drilling rig should drill at a constant speed, with a normal drilling speed of approximately 2 m / min, while controlling the drilling depth according to the markings on the drilling rig.

[0081] After the long spiral drilling rig reaches the designed depth, the site commander should instruct the rig to stop drilling and raise the drill rod by 30cm to open the drill bit's sliding flaps. Simultaneously, the pump operator should be notified to begin pouring concrete and maintain continuous pouring. When pouring concrete to the top of the pile, it should exceed the designed pile top elevation by at least 50cm to ensure that both the pile top elevation and the quality of the concrete meet design requirements. The driller and pump operator should work closely together to control the drilling speed according to the amount of mixed material pumped in, maintaining a speed of approximately 1.2–1.5 m / min, ensuring the drill bit tip remains submerged in the mixed material for about 1m to prevent pile breakage. If the mixed material does not reach the designed pile head elevation, it should be addressed promptly by inserting the pump pipe 50cm below the mixed material to replenish it. Before pouring concrete, the pipeline should be checked for smoothness and stability; before pouring the first pile of each shift, the pipeline should be moistened with cement mortar. When grouting concrete, the drill bit lifting height should be less than 25cm at a time, and the concrete embedment height should be greater than 1.0m. A designated person should be on-site to inspect the concrete grouting quality and handle any unexpected situations. Concrete should be poured immediately upon arrival on site (within 2 hours), and prolonged placement is strictly prohibited. Ensure the pile concrete undergoes at least 24 hours of curing to avoid disturbance. Construction records should be meticulously filled out during the construction process, with records kept for each shift and every 100m. 3 Three concrete mix specimens (150×150×150mm) were prepared for 28-day standard curing and their compressive strength was tested. After the piles were cast in place, the tops of the piles were sealed with wet clay for protection. The amount of concrete mixture poured for each pile must not be less than the designed pouring volume.

[0082] Near the end of the pouring process, the amount of concrete poured should be checked to ensure the pouring height is correct. After pouring, the top of the bored pile should be at least 50cm higher than the designed top to ensure good concrete quality at the cross-section.

[0083] In this embodiment, the diameter of the plain concrete piles in the cut-and-cover tunnel under the airport is 600mm. The pile length is adjusted according to the working conditions, requiring an penetration of 1m into the W3 bearing layer. The specific length is determined based on the rock debris samples obtained on site. Due to the inclination of the existing ground line or weathered surface, the length of the plain concrete piles in the cut-and-cover tunnel project under the airport ranges from 3 to 26m. The table below shows the pile lengths of some plain concrete piles in the cut-and-fill transition section.

[0084] Table 1 shows the length of plain concrete piles in the excavation-fill transition section.

[0085]

[0086] Note that when drilling with a long spiral drilling rig, observe the depth of the drill rod. When the maximum depth is reached, continue drilling for 3 minutes to ensure that the borehole reaches the designed depth. When starting to pour concrete, maintain continuous pouring and strictly control the concrete pouring speed. The lifting speed of the long spiral drilling rig should be controlled at 1.2 to 1.5 m / min to reduce any minor defects that may exist in the pile body after pile completion.

[0087] To ensure concrete quality, cement, sand, and aggregate raw materials must pass all required tests before use. The concrete mix should have good workability and maintain sufficient fluidity during pouring. Slump should be measured in real time and ideally controlled between 180 and 220 mm, taking into full account the slump loss due to temperature, transport distance, and construction time. The water-cement ratio must be strictly controlled to guarantee the concrete's strength. Slump and air content tests must be conducted before pouring, and pouring can only proceed if they meet the specified requirements.

[0088] (7) Chiseling the pile head: After the plain concrete pile is poured, the pile head is chiseled using the circumferential cutting method. After chiseling, the pile head should be dense and free of loose layers.

[0089] Compared with the underwater steel pipe grouting method, the plain concrete pile construction method of the present invention has a faster construction speed, less consumption of turnover materials, lower cost, and better pile integrity.

[0090] For the cut-and-cover tunnel under the airport in this embodiment, due to geological factors, the transition section includes not only the fill-cut transition section within the same rock stratum, but also a completely weathered-strongly weathered transition section. For example... Figure 11 The diagram shows a longitudinal profile of the transition section from fully weathered to strongly weathered. In the W4 stratum section, CFG piles are used for reinforcement, with a 0.3m thick crushed stone cushion layer and a 0.15m thick C20 concrete cushion layer on top of the piles. In the W3 / W4 stratum transition section, plain concrete piles are used for reinforcement. The plain concrete piles have a diameter of 600mm and a length of not less than 3m. A 0.2m thick crushed stone cushion layer and a 0.15m thick C20 concrete cushion layer are placed on top of the plain concrete piles. In the W3 stratum section, a 0.15m thick C20 concrete cushion layer is used for reinforcement. By gradually changing the thickness of the cushion layer, the longitudinal differential settlement of the structure is reduced.

[0091] The beneficial effects of this invention include:

[0092] 1. Ensuring Project Quality: Proper foundation treatment ensures the foundation meets bearing capacity and stability requirements, providing stable support for railway engineering and other structures, ensuring the load-bearing capacity and stability of the composite foundation, and guaranteeing safe use. Effective control of settlement deformation, as verified by relevant calculations, can reduce track and structural problems caused by settlement, ensuring the normal performance of the project.

[0093] 2. Optimize construction: The design follows the principle of adapting to local conditions, considering and comparing multiple factors to select the best solution, determine the economically reasonable and feasible solution, avoid unreasonable investment, and ensure the economic efficiency of the project.

[0094] 3. Reduce uneven settlement: Take transitional measures such as reinforce the strata with plain concrete piles in relevant sections and at the connection points to ensure a smooth transition of foundation characteristics, ensure close connection and coordinated stress of the project, reduce uneven settlement of the tunnel foundation caused by different geological conditions, and improve the integrity and continuity.

[0095] 4. Improve construction quality: Strengthen quality control and effect testing, carry out settlement and deformation observation and evaluation according to specifications, and make timely remedial measures. Strictly control key construction points to ensure the quality and long-term effect of foundation treatment.

[0096] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention shall be within the scope of protection of the pending claims of the present invention.

Claims

1. A plain concrete pile composite foundation for open-cut tunnels, applied to the fill-cut transition section of deeply buried open-cut tunnels, characterized in that: The structure includes a stratum reinforced with several plain concrete piles, and a crushed stone cushion layer and a C20 concrete cushion layer laid on top of the plain concrete piles. The plain concrete piles are arranged in a matrix, with the width of the plain concrete pile matrix being greater than the width of the tunnel structure. A uniform transition of foundation stiffness between the fill and cut sections is achieved by changing the pile length of the plain concrete piles. The plain concrete piles are arranged in sections every 10 meters along the mileage direction, and the pile lengths within each section may be the same or different depending on the geological conditions. The length of the fill-cut transition section is 10 to 50 meters, depending on the geological conditions. The excavation section is reinforced with CFG piles, each 500mm in diameter, with the pile tip penetrating at least 0.5m into the bearing stratum. A 0.3m thick crushed stone cushion layer and a 0.15m thick C20 concrete cushion layer are laid on top of the piles, with the upper layer of the C20 concrete cushion layer forming the bottom surface of the tunnel lining structure. The embankment section is reinforced with drilled reinforced concrete piles, each 0.2m thick C20 concrete cushion layer is laid on top of the piles. The embankment-excavation transition section is reinforced with plain concrete piles, each 600mm in diameter and at least 3.0m long, with the pile tip penetrating at least 1m into the bearing stratum. A 0.2m thick crushed stone cushion layer and a 0.15m thick C20 concrete cushion layer are laid on top of the piles. Temporary drainage is implemented during the construction of the cushion layer in the excavation section, and C20 concrete is used to backfill the foundation pits at locations with unequal structural heights. In the transition section between fill and excavation, the length of the foundation treatment piles and the thickness of the cushion layer gradually change to reduce the longitudinal differential settlement of the tunnel structure. In the cross-sectional direction, the same foundation treatment measures are used within the influence range of the tunnel structure to reduce the lateral differential settlement of the structure.

2. The open-cut tunnel plain concrete pile composite foundation according to claim 1, characterized in that: In the plain concrete pile matrix, the transverse pile spacing is 1.70~2.35m, and the longitudinal pile spacing is 2~2.5m; the maximum particle size of the crushed stone cushion layer at the pile top does not exceed 50mm, the mud content is not greater than 5%, and the dynamic deformation modulus E after compaction is... vd Not less than 40MPa.

3. The open-cut tunnel plain concrete pile composite foundation according to claim 1, characterized in that: Expansion joints with a width of 2cm are installed in the transition sections of segments where different foundation treatment measures are adopted.

4. The open-cut tunnel plain concrete pile composite foundation according to claim 1, characterized in that: The diameter of the drilled concrete piles is determined comprehensively based on the pile foundation type, stress analysis, and construction conditions. For sections with a soil cover of less than 10m, piles with a diameter of 1.25m are used. The pile type at the cavern in the fill section is the same as that of the open tunnel structure in the same section. Four piles are arranged in the cross section of the drilled piles, respectively under the side wall of the open tunnel structure. The center-to-center distance of the drilled piles is twice the pile diameter, and the net distance from the edge of the structural bottom plate to the outermost row of piles is 0.5 times the pile diameter. The arrangement of the drilled piles along the line should be consistent with the casting module of the open tunnel structure. The longitudinal spacing of the piles is 5m. The single casting length of the open tunnel structure is 10m. The piles are arranged in the center, with the center distance from the structural construction joint 2.5m. The top of the piles is fixed to the open tunnel bottom plate structure and extends 100mm into the structural bottom plate.

5. A construction method for a plain concrete pile composite foundation for an open-cut tunnel as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Based on the relationship between the tunnel elevation, the current ground elevation and the airport planned apron elevation, and taking into account the influence of the thickness of the soft soil layer at the bottom of the tunnel, the boundary between the fill and cut sections is determined, and the fill section, cut section and fill-cut transition section are divided. S2. Based on the tunnel geological conditions, select appropriate foundation treatment measures to reduce differential settlement of the structure; Different foundation treatment measures are selected for the excavation section according to different strata; for the filling section, the foundation treatment adopts reinforced concrete bored pile measures, and the pile diameter, pile length and reinforcement parameters are determined by calculation; for the filling-excavation transition section, the foundation treatment adopts plain concrete pile measures, and the uniform transition of foundation stiffness between the filling section and the excavation section is achieved by varying the pile length.

6. The construction method for the plain concrete pile composite foundation of an open-cut tunnel according to claim 5, characterized in that: The plain concrete piles are made by drilling holes using rotary drilling, backfilling with soil, and then injecting plain concrete using a long spiral drilling rig and a ground pump. After the rotary drilling is completed, the existing plain concrete piles with holes are backfilled to prevent the holes from collapsing. The backfill material is the clay produced during the drilling process. The drilling speed of the long spiral drilling rig is 1.8~2.2m / min. When drilling, observe the depth of the drill rod. When the maximum depth is reached, continue drilling for 3 minutes to ensure that the borehole reaches the designed depth. After drilling to the designed depth, lift the drill rod to open the drill bit flap and start pouring concrete through the ground pump. Pour concrete while lifting the drill rod. The lifting speed is 1.2~1.5m / min, and the drill bit tip is always buried in the grout mixture by 0.8~1.1m to prevent pile breakage. When pouring concrete to the top of the pile, it should be appropriately higher than the designed elevation of the pile top by no less than 50cm.

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