Plug-in open caisson foundation for land large wind turbine generator and construction method of plug-in open caisson foundation

Abstract

The invention provides a plug-in open caisson foundation for a land large wind turbine generator. The open caisson sinks to the designed elevation, the reinforced concrete ring beam is arranged on the inner wall of an open caisson cylinder body of the open caisson, and the bottom of the concrete tower cylinder is inserted into the open caisson cylinder body and supported on the reinforced concrete ring beam. The prestressed tendons penetrate through the concrete tower drum in the axial direction and are anchored on the reinforced concrete ring beams; and the cement soil is formed outside the open caisson. The invention further provides a construction method of the insertion type open caisson foundation. The concrete tower drum and the open caisson are combined into a whole, the binding force of the open caisson and the external soil body, the side friction resistance and the end supporting force are improved through the cemented soil, the concrete tower drum and the open caisson are connected in an inserted connection mode, a direct force transmission path from the concrete tower drum to the deep portion of the open caisson is formed, the anti-overturning rigidity of the wind power tower is improved, and the wind power generation efficiency is improved. In a dense stratum, the anti-overturning safety coefficient can reach 2.13 or above, and the horizontal displacement is far lower than a standard limit value.

Description

Technical Field

[0001] This invention belongs to the field of wind power engineering technology, specifically relating to an insert-type caisson foundation for large onshore wind turbine units and its construction method. Background Technology

[0002] As wind turbine units develop towards a capacity of 10MW and above, the horizontal load and overturning moment borne by their towers increase dramatically, posing greater challenges to the overturning resistance, overall stiffness, and economy of their foundation structures.

[0003] Onshore wind turbine foundations can be categorized into beam-slab spread foundations, gravity spread foundations, pile foundations, cylindrical foundations, and rock anchor foundations. Among these, cylindrical foundations have gained widespread use due to their direct load-bearing capacity, excellent overturning load resistance, and high structural efficiency. Cylindrical foundations are typically constructed using traditional open-cut or caisson methods. However, open-cut methods are unsuitable for deep excavations due to the large volume of excavation and backfilling, which damages the original soil. Caisson construction involves lowering a prefabricated caisson into the ground by excavating soil from the inside of the caisson to allow it to sink slowly and evenly. The caisson is then backfilled. Because a gap forms between the caisson and the surrounding soil, the soil's constraint on the caisson is reduced. Furthermore, the caisson relies primarily on the soil directly beneath it for support, which decreases the wind turbine tower's resistance to overturning. Therefore, a solid foundation slab is often poured on top of the caisson to expand the foundation area and improve the wind turbine tower's resistance to overturning. However, this increases the foundation construction cost.

[0004] Because caissons have advantages such as large burial depth, strong bearing capacity, simple construction and multi-functionality, they can be used as foundations, retaining walls and support structures during construction. Therefore, if the existing advantages of caissons can be maintained and they can be improved and used as foundations for onshore wind power, the mature technology of caisson construction can be used to effectively improve the construction safety of onshore wind power foundations and thereby reduce construction costs. Summary of the Invention

[0005] To address the issue that existing technologies using caissons as onshore wind turbine foundations require pouring a solid foundation slab on top of the caisson to increase the foundation area and improve the overturning resistance of the wind turbine tower, thus increasing foundation construction costs, this application first proposes an insert-type caisson foundation for large onshore wind turbine units, comprising:

[0006] The caisson, which is lowered to the design elevation, includes a cylindrical caisson body and a bottom plate formed at the bottom of the caisson body;

[0007] A reinforced concrete ring beam is installed on the inner wall of the caisson cylinder;

[0008] A concrete tower is inserted into the caisson body at the bottom and supported on a reinforced concrete ring beam. There is an annular gap between the concrete tower and the caisson body, and grout is poured into the annular gap.

[0009] Prestressed tendons that penetrate the concrete tower along the axial direction and are anchored to the reinforced concrete ring beam;

[0010] Cement soil formed on the outside of the caisson.

[0011] The insert-type caisson foundation described in this application is applicable to the foundations of wind turbine units with a single unit capacity of 10MW and above.

[0012] The cement-soil outside the caisson includes the cement-soil in the radially outer region of the caisson and the cement-soil in the axial direction below the caisson, so that the cement-soil is extended and connected into one piece in both the radial and axial directions. The radial thickness of the cement-soil in the outer region is 0.5-0.8m in the horizontal direction, and the thickness of the cement-soil in the axial direction below the caisson is 0.5-1.2m in the vertical direction.

[0013] The caisson body can be formed by segmented casting or assembled from precast concrete segments. The reinforced concrete ring beam can be precast directly on the caisson body, or it can be constructed by post-casting on the inner wall of the caisson body after the caisson body has sunk and the foundation pit inside the caisson has been excavated. When constructing the reinforced concrete ring beam by post-casting, it is preferable to precast corbels on the inner wall of the caisson body to facilitate the binding of the steel mesh of the reinforced concrete ring beam.

[0014] In this application, the bottom of the concrete tower is inserted into the caisson, and prestressed tendons are used to lock the concrete tower onto the reinforced concrete ring beam. Grouting material is poured into the annular gap between the concrete tower and the caisson, integrating the concrete tower and the caisson into a single unit. The cement-soil surrounding the caisson effectively improves the bond between the caisson and the external soil, increases lateral friction, and utilizes the higher strength of the cement-soil compared to the soil itself, thereby enlarging the lateral cross-section of the caisson and increasing its end support. Because an interlocking connection is used to connect the concrete tower and the caisson, a direct force transmission path is formed from the concrete tower to the depth of the caisson, improving the overturning stiffness of the wind turbine tower. In dense strata such as loess, hard silty clay, dense sand, and hard silt, the overturning safety factor can reach over 2.13, and the horizontal displacement is far below the standard limit.

[0015] Specifically, to ensure an effective connection between the concrete tower and the caisson, the concrete tower is inserted into the caisson to a depth of 1.5-3.0m.

[0016] Furthermore, to increase the weight of the foundation itself and thus improve its stability, a grouting backfill body is incorporated within the caisson cylinder. This grouting backfill body is formed from cement-reinforced backfill soil, and a middle slab is located on top of the grouting backfill body, filling the space between the bottom slab and the middle slab. This middle slab is situated below the reinforced concrete ring beam. In this application, the grouting backfill soil is reinforced with cement to replace the traditional simple backfill method, effectively increasing the compaction of the backfill soil and further increasing the weight of the caisson.

[0017] Furthermore, to fully utilize the internal space of the caisson, a top slab is formed on the reinforced concrete ring beam, and a working area is formed between the top slab and the middle slab. The top slab has an entrance and exit for the working area. This working area can serve as a storage room or maintenance space for wind turbine towers and wind turbine units, or as a space for other purposes depending on specific needs.

[0018] Furthermore, a grouting pipe system is pre-embedded inside the caisson, which is used for sequential grouting of the inner side, outer side, and bottom of the caisson.

[0019] Specifically, the grouting piping system includes:

[0020] The sleeve valve pipe inside the caisson body has an inner grouting hole and an outer grouting hole. The inner grouting hole and the outer grouting hole are staggered in the vertical direction. The inner grouting hole penetrates the inner wall of the caisson body inward, and the outer grouting hole penetrates the outer wall of the caisson body outward.

[0021] An annular pipe is embedded in the base plate and connected to the sleeve valve pipe. A bottom grouting pipe extending downwards is connected to the annular pipe and extends downwards into the soil below the base plate.

[0022] A grouting pipe that can be detachably inserted into the sleeve valve tube.

[0023] This grouting pipe system allows for sequential grouting of the caisson's interior and exterior, particularly grouting axially downwards. Different grouting pressures can be applied to different areas based on varying needs. Since grouting in all areas is carried out via sleeve valve pipes, there is no need to install different grouting pipes for different grouting areas. Simply controlling the height of the grouting perforated pipe within the sleeve valve pipe is sufficient to grout different areas, eliminating the need for frequent disassembly and reassembly, thus improving grouting efficiency. Existing mature technology can be used for the grouting perforated pipes; this application does not impose any limitations on the type of grouting perforated pipe.

[0024] Secondly, this application also proposes a construction method for an insert-type caisson foundation for large onshore wind turbine units, so as to successfully implement any of the above-mentioned caisson foundations. The construction method includes the following steps:

[0025] (1) The caisson body is lowered to the design elevation and the earthwork excavation of the foundation pit inside the caisson is completed. A vertically extending sleeve valve pipe is pre-embedded in the caisson body. An inner grouting hole and an outer grouting hole are opened on the sleeve valve pipe, and the inner grouting hole and the outer grouting hole are staggered in the vertical direction. The inner grouting hole penetrates the inner wall of the caisson body inward, and the outer grouting hole penetrates the outer wall of the caisson body outward.

[0026] (2) Insert the bottom grouting pipe, lay the ring pipe, and connect the bottom grouting pipe to the ring pipe, and connect the ring pipe to the sleeve valve pipe. Pour the bottom plate at the bottom of the caisson body, and the bottom plate and the caisson body together form a caisson.

[0027] (3) Backfill the soil into the caisson until the set thickness is reached, and then pour the middle plate on top of the backfill soil; there is a reinforced concrete ring beam on the inner wall of the caisson cylinder, and the middle plate is located below the reinforced concrete ring beam.

[0028] The reinforced concrete ring beam is integrally formed on the caisson body or is poured after the caisson body has been sunk.

[0029] (4) Insert the grouting pipe into the sleeve valve pipe and perform grouting in sequence to form cement soil outside the caisson. The grouting in sequence includes the following steps:

[0030] (41) Grouting is performed on the radially outer side of the caisson, where the caisson is lower than the height of the bottom plate;

[0031] (42) Grouting is performed into the soil below the bottom slab through the ring distribution pipe and the connected bottom grouting pipe;

[0032] (43) Grouting is injected into the backfill soil inside the caisson to form a grouting backfill body; grouting is injected into the area on the radial outer side of the caisson that is higher than the bottom plate height;

[0033] (5) Hoist the concrete tower into the caisson and support it on the reinforced concrete ring beam. Tension the anchor cables to form prestressed tendons. The prestressed tendons pass through the concrete tower along the axial direction and are anchored on the reinforced concrete ring beam.

[0034] (6) Grouting material is poured into the annular gap between the concrete tower and the caisson.

[0035] The caisson body was lowered using the traditional method of excavating soil within the caisson pit and gradually lowering the caisson body to the design elevation.

[0036] This application first lowers the caisson cylinder to a predetermined elevation. After completing the excavation of the foundation pit within the caisson and grouting to form cement-soil, a concrete tower is inserted into the caisson. Prestressed tendons are used to anchor the concrete tower to a reinforced concrete ring beam. Finally, grout is poured into the annular gap between the concrete tower and the caisson cylinder, thus forming a unified structure between the concrete tower and the caisson. This plug-in connection between the concrete tower and the caisson creates a direct force transmission path from the concrete tower to the depths of the caisson, improving the overturning stiffness of the wind turbine tower. In dense strata such as loess, hard silty clay, dense sand, and hard silt, the overturning safety factor can reach over 2.13, and the horizontal displacement is far below the standard limits.

[0037] By utilizing the cement-soil surrounding the caisson, not only can the bonding force between the caisson and the external soil be effectively improved and the lateral friction resistance increased, but the higher strength of the cement-soil compared to the soil can also be used to expand the transverse cross section of the caisson and improve the end support force of the caisson.

[0038] Specifically, in step (4), the grouting pressure is 0.8-1.0 MPa when grouting into the soil below the base slab; 0.5-0.8 MPa when grouting into the backfill soil inside the caisson; and 0.3-0.5 MPa when grouting into the soil radially outside the caisson. The grouting pressure is highest below the base slab to effectively increase the compaction of the soil foundation below the caisson, reduce the overall foundation settlement, and improve the stability of the foundation.

[0039] Furthermore, to fully utilize the internal space of the caisson, a top slab is formed on the reinforced concrete ring beam, and a working area is formed between the top slab and the middle slab. The top slab has an entrance and exit for the working area. This working area can serve as a storage room or maintenance space for wind turbine towers and wind turbine units, or as a space for other purposes depending on specific needs. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of a caisson foundation on which a wind turbine tower is installed.

[0041] Figure 2 for Figure 1 Enlarged view of section A.

[0042] Figure 3 for Figure 2 Enlarged view of section B.

[0043] Figure 4 for Figure 1 A magnified view in the CC direction.

[0044] Figure 5 for Figure 1 A magnified view along the DD direction.

[0045] Figure 6 This is a diagram showing the grouting process in the area radially outer of the caisson that is lower than the height of the bottom plate.

[0046] Figure 7 This is a diagram showing the grouting process in the soil beneath the base plate.

[0047] Figure 8 This is a diagram showing the grouting process being carried out into the backfill soil inside the caisson. Detailed Implementation

[0048] The following describes the insert-type caisson foundation for the large onshore wind turbine 50 in this application. The geological conditions of the construction area in this embodiment are 2m thick hard plastic clay on the top and 6m thick soft sandstone with a saturated compressive strength of 10MPa on the bottom. The outer diameter of the caisson is 10m and the sinking depth is 8m. The wind turbine is 10MW.

[0049] Please see Figures 1-5 The caisson foundation includes:

[0050] A caisson 10 is lowered to the designed elevation. The caisson 10 includes a cylindrical caisson body 11 and a bottom plate 12 formed at the lower part of the caisson body. A groove 111 is provided on the inner side of the lower part of the caisson body 11, and the outer end of the bottom plate 12 is held within this groove. A bottom seal 112, formed of plain concrete, is located at the lower part of the bottom plate. Figure 5 For clarity, the base plate 12 is not shaded in cross-section. In this embodiment, the wall thickness of the caisson cylinder 11 is 0.8m.

[0051] The reinforced concrete ring beam 16 is installed on the inner wall of the caisson cylinder. In this embodiment, the reinforced concrete ring beam 16 is cast in a second pour after the caisson cylinder is lowered. The reinforced concrete ring beam 16 protrudes inward from the inner wall of the caisson cylinder. In order to improve the load-bearing capacity of the reinforced concrete ring beam 16, a corbel 17 is integrally formed on the inner side of the caisson cylinder, and the reinforced concrete ring beam 16 is supported on the corbel 17.

[0052] A concrete tower 51 is inserted into the bottom of the caisson and supported on a reinforced concrete ring beam 16. A steel tower 52 is installed on top of the concrete tower 51, and a wind turbine 53 is installed on top of the steel tower 52.

[0053] A prestressing tendon 54 is axially inserted through the concrete tower and anchored to the reinforced concrete ring beam. The top end of the prestressing tendon 54 extends upward through the concrete tower 51 and is anchored by an upper anchor. The bottom end of the prestressing tendon 54 extends downward through the reinforced concrete ring beam and is anchored by a lower anchor 541. To facilitate the installation of the lower anchor 541, a receiving cavity 171 extending vertically is provided on the corbel. This receiving cavity is a through hole. A receiving cavity is provided on the corbel for each prestressing tendon. The lower end of the prestressing tendon and the lower anchor are both accommodated in the corresponding receiving cavity. The prestressing tendon is formed by tensioning the anchor cable. After the anchor cable is tensioned, the receiving cavity is sealed with grout, so that the lower end of the prestressing tendon and the lower anchor are both wrapped in the grout.

[0054] In this embodiment, the distance from the upper surface of the reinforced concrete ring beam 16 to the top surface of the caisson cylinder is 2.5m, meaning the depth to which the concrete tower cylinder is inserted into the caisson cylinder is 2.5m. It is understood that in other embodiments, the depth to which the concrete tower cylinder is inserted into the caisson cylinder can also be 1.5m, 1.8m, 2m, 2.8m, or 3m, or any other depth between 1.5m and 3m. It is recommended that the higher the wind turbine tower, the greater the depth to which the concrete tower cylinder is inserted into the caisson cylinder. An annular gap 18 is formed between the concrete tower cylinder and the inner wall of the caisson cylinder, and within this annular gap is a ring-shaped adhesive body 181 formed by grout poured into the annular gap.

[0055] The caisson exterior is covered with cement-soil 40, formed by soil and cement grout. This cement-soil enhances the connection strength between the caisson and the surrounding soil, thereby increasing the lateral friction resistance of the cement-soil and improving the end bearing capacity of the caisson. Specifically, the cement-soil includes cement-soil in the radially outer region of the caisson and cement-soil axially below the caisson. In this embodiment, the radial thickness of the cement-soil in the outer region is 0.6m in the horizontal direction, and the thickness of the cement-soil axially below the caisson is 1.0m in the vertical direction.

[0056] To improve the stability of the concrete tower installation process, rebars 21 are pre-embedded in the reinforced concrete ring beam 16. Rebar holes 511 corresponding to the rebars are set at the lower end of the concrete tower. Rebar adhesive is filled into the rebar holes. The rebars are inserted into the corresponding rebar holes and bonded to the concrete tower by the rebar adhesive. After the rebar adhesive has solidified, the anchor cables are tensioned.

[0057] In this embodiment, the caisson cylinder is a precast component formed by segmented casting. It can be understood that in other embodiments, the caisson cylinder can also be assembled from precast concrete segments.

[0058] A middle slab 13 is poured inside the caisson cylinder. A grouting backfill body (not shown in the attached drawings) is located between the middle slab and the bottom slab 12. This grouting backfill body is formed by cement-reinforced backfill soil and fills the space between the bottom slab and the middle slab. The middle slab is located below the reinforced concrete ring beam. In this embodiment, the middle slab 13 is a plain concrete slab; however, it is understood that in other embodiments, the middle slab could also be a reinforced concrete slab.

[0059] A top slab 14 is formed on the reinforced concrete ring beam 16, and a work area 15 is formed between the top slab and the middle slab. An entrance and exit 141 for entering and exiting the work area is provided on the top slab. The work area can be used as a storage room or maintenance space for wind turbine towers and wind turbine units, or as a space for other purposes as needed.

[0060] A grouting pipe system is pre-embedded within the caisson, used for sequential grouting of the inner side, outer side, and bottom of the caisson. Please also refer to... Figure 6 Specifically, in this embodiment, the grouting pipe system includes a sleeve valve pipe 31, an annular pipe 32, and a grouting perforated pipe 60. The sleeve valve pipe 31 is pre-embedded inside the caisson body, and has an inner grouting hole and an outer grouting hole, which are vertically staggered. An inner connecting pipe 331 is connected to the inner grouting hole, which penetrates the inner wall of the caisson body, allowing the inner grouting hole to penetrate the inner wall of the caisson body. An outer connecting pipe 332 is connected to the outer grouting hole, which penetrates the outer wall of the caisson body, allowing the outer grouting hole to penetrate the outer wall of the caisson body. The grouting perforated pipe 60 is detachably inserted into the sleeve valve pipe. After grouting is completed, the grouting perforated pipe is pulled out of the sleeve valve pipe, cleaned, and then recycled.

[0061] An annular pipe 32 is pre-embedded in the bottom slab 12 of the caisson. The annular pipe is connected to the sleeve valve pipe via an intermediate pipe 311. A bottom grouting pipe 34 extends downwards from the annular pipe and inserts into the soil below the bottom slab. Grouting holes 341 are formed on the pipe wall of the bottom grouting pipe. (For simplicity, the attached drawings are omitted.) Figure 6 , Figure 7 and Figure 8 The image shows only one sleeve valve pipe, one bottom grouting pipe, and one intermediate pipe as an example. In specific embodiments, the corresponding number of sleeve valve pipes, bottom grouting pipes, and intermediate pipes can be set as needed.

[0062] Rubber plugs are installed at the inner end of the inner connector, the outer end of the outer connector, and the grouting hole on the pipe side. Under the corresponding grouting pressure, the rubber plugs can be completely pushed out of the inner connector, outer connector, and grouting hole. Specifically, in this embodiment, the rubber plug in the inner connector can be completely pushed out of the inner connector under a grouting pressure of 0.5-0.8 MPa; the rubber plug in the outer connector can be completely pushed out of the outer connector under a grouting pressure of 0.3-0.5 MPa; and the rubber plug in the grouting hole on the pipe side can be completely pushed out of the grouting hole under a grouting pressure of 0.8-1.0 MPa. It can be understood that in other embodiments, a rubber ring can be fitted onto the bottom grouting pipe to seal the grouting hole on the pipe side. The rubber ring expands under the grouting pressure, causing cement slurry to flow outwards. The rubber plug is used to prevent external soil from entering the inner pipe, outer pipe, or bottom grouting pipe, which would prevent grouting. Other existing mature technologies can also be used for the protective sealing of the inner pipe, outer pipe, and grouting holes on the pipe side. This application does not make any specific limitations.

[0063] The construction method for the aforementioned caisson foundation is described below. Please refer to [link / reference needed]. Figures 1-8 The specific steps of this construction method are as follows:

[0064] (1) The caisson body 11 is lowered to the design elevation, and the excavation of the foundation pit inside the caisson is completed. The inner wall of the caisson has an integrally formed corbel 17. Specifically, in this embodiment, the construction method of excavating the foundation pit inside the caisson and slowly lowering the caisson body to the design elevation is adopted to lower the caisson body to the design elevation.

[0065] A sleeve valve pipe 31 is pre-embedded inside the caisson body. An inner grouting hole and an outer grouting hole are opened on the sleeve valve pipe, and the inner grouting hole and the outer grouting hole are staggered in the vertical direction. An inner pipe 331 is connected to the inner grouting hole. The inner pipe 331 penetrates the inner wall of the caisson body inward, so that the inner grouting hole penetrates the inner wall of the caisson body inward. An outer pipe 332 is connected to the outer grouting hole. The outer pipe 332 penetrates the outer wall of the caisson body inward, so that the outer grouting hole penetrates the outer wall of the caisson body inward.

[0066] (2) Insert the bottom grouting pipe 34 and pour plain concrete at the bottom of the foundation pit until the set thickness is reached to form a bottom seal 112. Then lay the annular pipe 32 and the intermediate pipe 311 connecting the annular pipe and the sleeve valve pipe 31 on the upper surface of the bottom seal; pour the bottom plate 12, with the outer end of the bottom plate 12 inserted into the groove 111 at the bottom of the caisson cylinder 11, and the annular pipe and the intermediate pipe are poured into the floor. The bottom plate and the caisson cylinder together form the caisson 10.

[0067] (3) Backfill the soil into the caisson until the set thickness is reached, and then pour plain concrete on top of the backfill soil to form a middle plate 13, which is located below the corbel 17.

[0068] A reinforced concrete ring beam 16 and a top plate 14 integrally formed inside the reinforced concrete ring beam are poured, with an inlet / outlet 141 reserved on the top plate. This ensures that the middle plate is located below the reinforced concrete ring beam.

[0069] It is understood that, in another embodiment, the reinforced concrete ring beam can also be integrally formed on the inner wall of the caisson cylinder. When the reinforced concrete ring beam is integrally formed on the inner wall of the caisson cylinder, the top slab needs to be constructed after the middle slab is completed, and the corbel can be eliminated.

[0070] (4) Insert the grouting pipe 60 into the sleeve valve pipe 31 and perform grouting in sequence to form cement soil 40 outside the caisson. The grouting in sequence includes the following steps:

[0071] (41) Please refer to Figure 6 Grouting is performed on the radially outer side of the caisson, in areas lower than the bottom plate height, at a grouting pressure of 0.4 MPa.

[0072] (42) Please refer to Figure 7 Grout is injected into the soil below the base plate through the ring distribution pipe and the connected bottom grouting pipe, with a grouting pressure of 0.9 MPa.

[0073] (43) Please refer to Figure 8 Grouting is performed at a pressure of 0.7 MPa into the backfill soil inside the caisson to form a grouting backfill. Grouting is then performed at a pressure of 0.4 MPa into the radially outer area of ​​the caisson that is higher than the bottom slab. After grouting is completed, the grouting pipe is pulled out from the sleeve valve pipe, then cleaned and recycled.

[0074] (5) The concrete tower 51 is hoisted so that its bottom is inserted into the caisson and supported by the reinforced concrete ring beam 16. The anchoring bars 21 pre-embedded on the reinforced concrete ring beam are inserted into the anchoring holes 511 at the lower end of the concrete tower. Anchoring adhesive is pre-filled into the anchoring holes so that the anchoring bars are connected to the reinforced concrete ring beam. In this embodiment, epoxy anchoring adhesive is specifically used.

[0075] The anchor cable is tensioned to form a prestressed tendon 54, which penetrates the concrete tower along the axial direction and is anchored to the reinforced concrete ring beam. Grouting material is poured into the receiving cavity 171 to seal the receiving cavity, and the lower end of the prestressed tendon and the lower anchor are both wrapped in the grouting material.

[0076] In this embodiment, prestressed steel strands are used as prestressed tendons, and a total of 16 bundles of Φs15.2 prestressed steel strands are installed. During tensioning, they are tensioned symmetrically in four stages to 0.75fptk. Φs15.2 indicates that the nominal diameter of the steel strand is 15.2mm.

[0077] (6) Grouting material is poured into the annular gap 18 between the concrete tower and the caisson body to form an annular bond 181. The construction of the caisson foundation is completed, and then other structural components of the wind turbine are installed.

[0078] Calculations show that the foundation has a vertical bearing capacity safety factor of 3.98, a horizontal bearing capacity safety factor of 3.13, and an overturning resistance safety factor of 2.13, all of which far exceed the specifications. Compared with the solution requiring a 16m deep pure clay stratum under the same load, the concrete usage is reduced by about 50%, and the construction period is shortened by about 40%.

Claims

1. An insert-type caisson foundation for large onshore wind turbine units, characterized in that, include: The caisson, which is lowered to the design elevation, includes a cylindrical caisson body and a bottom plate formed at the bottom of the caisson body; A reinforced concrete ring beam is installed on the inner wall of the caisson cylinder; A concrete tower is inserted into the caisson body at the bottom and supported on a reinforced concrete ring beam. There is an annular gap between the concrete tower and the caisson body, and grout is poured into the annular gap. Prestressed tendons that penetrate the concrete tower along the axial direction and are anchored to the reinforced concrete ring beam; Cement soil formed on the outside of the caisson.

2. The caisson foundation according to claim 1, characterized in that, The concrete tower is inserted into the caisson to a depth of 1.5-3.0m.

3. The caisson foundation according to claim 1, characterized in that, The caisson contains a grouting backfill body formed by cement-reinforced backfill soil. A middle plate is located on top of the grouting backfill body, filling the space between the bottom plate and the middle plate. The middle plate is located below the reinforced concrete ring beam.

4. The caisson foundation according to claim 3, characterized in that, A top slab is formed on the reinforced concrete ring beam, and a working room is formed between the top slab and the middle slab. The top slab has an entrance and exit for entering and exiting the working room.

5. The caisson foundation according to claim 1, characterized in that, A grouting pipe system is pre-embedded inside the caisson, which is used to perform sequential grouting on the inner side, outer side, and bottom of the caisson.

6. The caisson foundation according to claim 5, characterized in that, The grouting piping system includes: The sleeve valve pipe inside the caisson body has an inner grouting hole and an outer grouting hole. The inner grouting hole and the outer grouting hole are staggered in the vertical direction. The inner grouting hole penetrates the inner wall of the caisson body inward, and the outer grouting hole penetrates the outer wall of the caisson body outward. An annular pipe is embedded in the base plate and connected to the sleeve valve pipe. A bottom grouting pipe extending downwards is connected to the annular pipe and extends downwards into the soil below the base plate. A grouting pipe that can be detachably inserted into the sleeve valve tube.

7. A construction method for an insert-type caisson foundation for a large onshore wind turbine, characterized in that, Includes the following steps: (1) The caisson body is lowered to the design elevation and the earthwork excavation of the foundation pit inside the caisson is completed. A vertically extending sleeve valve pipe is pre-embedded in the caisson body. An inner grouting hole and an outer grouting hole are opened on the sleeve valve pipe, and the inner grouting hole and the outer grouting hole are staggered in the vertical direction. The inner grouting hole penetrates the inner wall of the caisson body inward, and the outer grouting hole penetrates the outer wall of the caisson body outward. (2) Insert the bottom grouting pipe, lay the ring pipe, and connect the bottom grouting pipe to the ring pipe, and connect the ring pipe to the sleeve valve pipe. Pour the bottom plate at the bottom of the caisson body, and the bottom plate and the caisson body together form a caisson. (3) Backfill the soil into the caisson until the set thickness is reached, and then pour the middle plate on top of the backfill soil; there is a reinforced concrete ring beam on the inner wall of the caisson cylinder, and the middle plate is located below the reinforced concrete ring beam. The reinforced concrete ring beam is integrally formed on the caisson body or is poured after the caisson body has been sunk. (4) Insert the grouting pipe into the sleeve valve pipe and perform grouting in sequence to form cement soil outside the caisson. The grouting in sequence includes the following steps: (41) Grouting is performed on the radially outer side of the caisson, where the caisson is lower than the height of the bottom plate; (42) Grouting is performed into the soil below the bottom slab through the ring distribution pipe and the connected bottom grouting pipe; (43) Grouting is injected into the backfill soil inside the caisson to form a grouting backfill body; grouting is injected into the area on the radial outer side of the caisson that is higher than the bottom plate height; (5) Hoist the concrete tower so that its bottom is inserted into the caisson and supported on the reinforced concrete ring beam. Tension the anchor cables to form prestressed tendons. The prestressed tendons pass through the concrete tower along the axial direction and are anchored on the reinforced concrete ring beam. (6) Grouting material is poured into the annular gap between the concrete tower and the caisson.

8. The construction method according to claim 7, characterized in that, In step (4), when grouting into the soil below the bottom plate, the grouting pressure is 0.8-1.0 MPa; when grouting into the backfill soil inside the caisson, the grouting pressure is 0.5-0.8 MPa; when grouting into the soil on the radially outer side of the caisson, the grouting pressure is 0.3-0.5 MPa.

9. The construction method according to claim 7, characterized in that, A top slab is formed on the reinforced concrete ring beam, and a working room is formed between the top slab and the middle slab. The top slab has an entrance and exit for entering and exiting the working room.

Images