A method for constructing prefabricated foundations with enlarged excavation faces in small working areas

Abstract

A method for constructing a prefabricated foundation with a small working face and enlarged excavation includes the following steps: Step 1, foundation pit positioning and excavation; Step 2, installation of strand frame; Step 3, installation of fixed end anchor plate; Step 4, hoisting of prefabricated components of the enlarged head; Step 5, assembly of the truncated cone base of the enlarged head within the pit; Step 6, splicing of prestressed steel strands; Step 7, hoisting of prefabricated components of the straight column; Step 8, installation of the main foundation column; Step 9, installation of the tensioning end anchor plate and application of prestress to the foundation; Step 10, grouting and filling of the foundation perimeter and the gaps in the reinforcing bar holes. This invention addresses the current lack of advanced construction technology for excavated prefabricated foundations and the need for large-scale excavation in conventional prefabricated foundation construction. It designs a prefabricated foundation with an enlarged excavation for power transmission lines and proposes a method for constructing prefabricated foundations with a small working face. This method achieves in-pit installation of the enlarged head of the excavated foundation without additional pit widening, improving the construction efficiency of foundation engineering and reducing construction costs.

Description

Technical Field

[0001] This invention relates to the field of reinforced concrete foundation technology, and in particular to a method for constructing prefabricated foundations with enlarged excavation holes on a small working face. Background Technology

[0002] With underground cables not yet widely adopted, power transmission lines must traverse various complex terrains via above-ground corridors to deliver electricity. Unfavorable terrains such as mountains, forests, and rivers pose severe challenges to the construction of transmission lines. The foundation construction of transmission towers is a crucial component of transmission line projects, accounting for approximately 30% of the total cost, and repairing foundations after failure is particularly difficult. Therefore, selecting a technologically advanced, economically sound, easy-to-implement, and environmentally friendly foundation type, while ensuring safety and reliability, is a prerequisite for the healthy and sustainable development of power grid construction.

[0003] Currently, the foundations of transmission line towers mainly adopt cast-in-place foundations or slab / platform prefabricated foundations. All three types are fundamentally excavation-backfill foundations, which have the disadvantage of requiring large-scale excavation of the foundation pit. During construction, a large amount of earthwork needs to be excavated and transported, and the foundation needs to be filled and compacted after it is formed. Cast-in-place foundations also require multiple complex processes during the construction period, such as formwork construction, steel reinforcement binding, and concrete curing, increasing the time and labor costs of foundation construction. In mountainous areas with poor terrain, large-scale earthwork excavation can damage vegetation along the route, harming the local ecological environment. In cases of erosion, it can easily cause soil erosion, requiring the construction of protective measures to ensure the stability of the foundation. This increases the overall project cost and does not conform to the current development concepts of "environmentally friendly" and "resource-saving." Therefore, prefabricated foundations with enlarged heads for small working faces are the development trend of power transmission line foundation types under unfavorable terrain. However, most of the current traditional prefabricated foundation construction technology is aimed at large-scale excavation foundations. During the construction process, the large-diameter enlarged head of the excavation foundation is generally installed as a whole by enlarging the diameter of the foundation pit. There is no mature split construction method suitable for foundations with small working faces.

[0004] In response, some research has emerged in China on the construction methods of bored foundations for power transmission lines. For example, in their article "Promotion and Application of Straight-Column Excavated Foundations in the Design and Construction of High-Voltage Transmission Lines in the Panxi Region," Zhang Guobin et al. proposed using cast-in-place excavated foundations in the mountainous Panxi region. This approach, which fully utilizes the shear strength of the original soil, offers good economic and environmental benefits due to its small excavation, minimal waste soil, and low impact. However, the high altitude of the Panxi region means that the strong water absorption of the soil during foundation pouring can cause problems with the cast-in-place concrete. The presence of holes resulted in poor finished quality. In their paper "Research on Key Technologies for Excavation-Anchor Composite Foundation Design of Transmission Lines in Mountainous Areas," Zhang Wenxiang et al. proposed an excavation-anchor composite foundation suitable for geological conditions with overlying soil and underlying bedrock. When the foundation force is less than 4000kN, the cost is slightly lower than that of conventional plate-type anchor composite foundations. However, the construction of the composite foundation requires consideration of both drilling additional anchor holes and the installation conditions of the excavated foundation piles. This complicates the construction process while keeping the foundation's bearing capacity relatively stable.

[0005] For example, CN205171537U discloses a prefabricated flexible semi-excavation foundation for power transmission lines in mountainous areas, including an upper plate, a base, and anchor cables connecting the upper plate and the base. The base is assembled from several prefabricated plates into a frustum shape. The top surface of the base is fixedly connected to the lower anchor plate by bolts. A steel pad is provided on the top of the upper plate. Both the lower anchor plate and the upper plate have through holes for the anchor cables to pass through. The bottom end of the anchor cable is connected to the base through the lower anchor plate, and the top end of the anchor cable is fixed to the upper plate by the steel pad. A circular baffle is provided on the lower surface of the upper plate, and the center of the circular baffle is consistent with the center of the upper plate. This foundation requires the frustum-shaped base to be pre-assembled on the construction site. During foundation assembly, a large excavation pit cannot be avoided to allow the larger diameter frustum-shaped base to be stably hoisted into the pit.

[0006] For example, CN213867908U discloses a multi-section expanded-plate prefabricated hollow excavation foundation, including concrete assemblies and prestressed steel bars. The concrete assemblies have several through-holes along the height direction. Multiple concrete assemblies are stacked sequentially in the foundation pit. The prestressed steel bars pass through the corresponding through-holes on the multiple concrete assemblies to assemble the concrete assemblies into the foundation body. The concrete assemblies are non-cylindrical expanded-plate components with a hollow structure in the middle. The gap between the concrete assemblies and the foundation pit is filled with cement grout or fine aggregate concrete. Each section of this foundation adds an expanded plate portion to the cylindrical concrete base, resulting in a larger amount of earthwork excavation and concrete usage during foundation construction compared to cast-in-place excavation foundations.

[0007] In summary, prefabricated bored foundations with enlarged heads are better suited for power transmission line foundation engineering in adverse environments than cast-in-place foundations and composite foundations. However, current bored foundations cannot achieve separate installation of the enlarged head portion within the pit under small working surface conditions. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a construction method for prefabricated foundations with small working face enlarged excavation, filling the gap in the construction technology of excavation-type prefabricated foundations, and solving the problems that conventional prefabricated foundation construction requires large excavation, large earthwork transportation volume, large concrete consumption, and the inability to guarantee the ecological environment around the foundation.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for constructing prefabricated foundations with enlarged excavation heads in small working areas, comprising the following steps: Step 1: Positioning and excavation of the prefabricated foundation pit. Locate the foundation position of the power transmission tower according to the construction plan and complete the excavation of the small-scale foundation pit; Step 2: Install the stranding frame. Prepare a corresponding number of strand frames according to the number of steel strands required for the foundation. A certain length of steel strands for traction is reserved on the strand frames, and connectors are installed at the ends of the steel strands. Step 3: Install the fixed end anchor plate; Step 4: Hoisting of the precast head components; Step 5: Assemble the base of the expanded frustum in the pit; Step 6: Laying prestressed steel strands; Step 7: Hoisting of prefabricated straight column components; Step 8: Installation of the main foundation column; Step 9: Installation of tensioning end anchor plate and application of foundation prestress; Step 10: Grouting and filling of the foundation perimeter and the gaps in the reinforcement holes. The gaps between the foundation and the original soil, as well as the holes for prestressed steel strands and reinforcing bars, are filled with cement grout to ensure a tight bond.

[0010] In the preferred embodiment, the prefabricated foundation described in step 1 includes foundation columns composed of several straight precast concrete components stacked together, and a truncated cone base consisting of four truncated concrete components embedded and fixed together. The truncated cone base is fixed to the bottom of the foundation pit, and the foundation columns are stacked on the truncated cone base. They are connected and fastened together using prestressed steel strands, reinforcing bars, and anchor plates.

[0011] In the preferred embodiment, both the straight column precast concrete component and the enlarged head precast concrete component are reinforced concrete precast components, and each is individually provided with several hanging nail holes.

[0012] In the preferred embodiment, the specific steps for installing the fixed-end anchor plate in step 3 are as follows: after anchoring the prestressed steel strand to the fixed-end anchor plate, it is inserted from bottom to top into the prestressed anchor cable hole of the enlarged head precast component. A connector is installed at the end. The steel strand is temporarily secured with a steel strand clip just beyond the top surface of the enlarged head precast component to prevent the prestressed anchor from slipping out of the anchor cable hole during hoisting, ensuring that the anchor plate is tightly fitted to the bottom of the precast component without obvious gaps or gaps.

[0013] In the preferred embodiment, the specific steps for hoisting the expanded head precast component in step 4 are as follows: the underground construction personnel pre-position the first precast component in the well and prepare for auxiliary hoisting work; the ground construction personnel screw the lifting nail into the threaded hole of the precast nail; the hoisting rope of the hoisting equipment passes through the round head ring of the lifting nail, tightens the hoisting rope to lift the precast component, and after moving the expanded head precast component to the center of the foundation pit, the hoisting rope is loosened. During this process, the component is lowered at a uniform speed and smoothly. After reaching the bottom of the foundation pit, the underground construction personnel remove the lifting nail and steel strand clips and move the first precast component to the predetermined position.

[0014] In the preferred embodiment, the specific steps for assembling the truncated cone base in step 5 are as follows: repeat step 4 to hoist the remaining three precast truncated cone parts into the bottom of the pit in sequence. After placing them in the designated positions, the underground construction personnel manually move the precast truncated cone parts to overlap the steel structure embedded structure on the vertical surface of the precast parts. After aligning the insertion holes, insert the insertion bars and fill the gaps between the insertion bars and the holes with cement grout to ensure that the insertion bars are vertical. After all four precast truncated cone parts are assembled, a truncated cone base is formed. After the truncated cone construction is completed, the underground construction personnel return to the surface.

[0015] In the preferred embodiment, the specific steps for splicing the prestressed steel strands in step 6 are as follows: when hoisting the precast straight column, the hoisting method used in step 4 for hoisting the precast expanded head is used to hoist the precast to the center of the foundation pit. Then, the traction steel strands on the strand frame are passed through the anchor cable holes from top to bottom and connected to the prestressed anchor cables with connectors. The traction steel strands are gradually tightened until the prestressed steel strands are taut.

[0016] In the preferred embodiment, the specific steps for hoisting the precast straight column component in step 7 are as follows: the precast straight column component is positioned using the pre-arranged reinforcing bars, with each reinforcing bar passing through the reserved reinforcing bar hole in the precast straight column component; then the hoisting rope of the hoisting equipment is loosened to smoothly lower the precast straight column component to the top of the frustum-shaped expansion plate base.

[0017] In the preferred embodiment, the specific steps for installing the foundation main column in step 8 are as follows: repeat steps 6 and 7 to overlap the prefabricated straight column components until the last prefabricated straight column component is in place, ensuring that the last prefabricated straight column component can protrude 1 / 3 of its height above the ground surface to facilitate the installation of the tower foot plate. After all the prefabricated straight column components are stacked, a cylindrical foundation main column is formed.

[0018] In the preferred embodiment, the specific steps for installing the tensioning end anchor plate and applying foundation prestress in step 9 are as follows: tighten the tensioning device rod with internal threads at the end into the connector at the end of the steel strand for tensioning; after tensioning is completed, remove the end connector and anchor the prestressed steel strand with the anchor plate.

[0019] The present invention provides a method for constructing prefabricated foundations with enlarged excavation in small working areas, which has the following beneficial effects: 1. The prefabricated foundation construction method with enlarged head excavation proposed in this invention can be carried out in various adverse environments, and can be constructed without compromising the tensile bearing capacity of the foundation. It has a short cycle, and the construction progress and quality are less affected by environmental constraints. It has low requirements for terrain and geological conditions and has a considerable degree of universality. 2. The foundation construction method proposed in this invention does not require large-scale excavation of foundation pits. The amount of earth and stone excavation in the foundation pit during the construction process is small, which protects the ecological environment around the transmission line. It not only makes use of the high strength of the original soil, but also reduces soil erosion, which is conducive to the stability of the tower foundation and reduces the frequency of use of auxiliary protection measures such as retaining walls and slope protection. 3. Based on the embedded steel structure of the excavated foundation, the present invention realizes the separate assembly of the expanded truncated base of the excavated foundation in the foundation pit by embedding the large-size steel structure wing into the pre-reserved concave hole in the upper part of the small-size steel structure, thus truly realizing the small-scale construction environment of the prefabricated excavated foundation. Attached Figure Description

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the overall structure of the prefabricated excavation foundation of the present invention; Figure 2 This is a schematic diagram of the installation of the fixed end anchor plate of the present invention; Figure 3 This is a schematic diagram of the structure of the prefabricated head of the present invention; Figure 4 This is a schematic diagram of the hoisting of the prefabricated head component of the present invention; Figure 5 This is a schematic diagram of the assembly of the expanded-head frustum base in the present invention. Figure 6 This is a schematic diagram of the hoisting of the prefabricated straight column and the installation of the foundation column of the present invention; In the diagram: 1. Anchor plate; 2. Insert bar; 3. Steel strand; 4. Precast straight column; 5. Precast expanded head; 6. Connector; 7. Steel strand clip. Detailed Implementation

[0021] like Figures 1-6 As shown, a method for constructing a prefabricated foundation with enlarged excavation on a small working face includes the following steps: Step 1: Positioning and excavation of the prefabricated foundation pit. Locate the foundation position of the power transmission tower according to the construction plan and complete the excavation of the small-scale foundation pit; Step 2: Install the stranding frame. Prepare a corresponding number of strand frames according to the number of steel strands required for the foundation. A certain length of steel strands 3 for traction is reserved on the strand frames, and a connector 6 is installed at the end of the steel strands 3. Step 3: Install fixed end anchor plate 1; Step 4: Hoist the precast head component 5; Step 5: Assemble the base of the expanded frustum in the pit; Step 6: Overlap the prestressed steel strands 3 times; Step 7: Hoisting of precast straight column component 4; Step 8: Installation of the main foundation column; Step 9: Installation of tensioning end anchor plate 1 and application of foundation prestress; Step 10: Grouting and filling the gaps around the foundation and the two holes of the reinforcing bars. The gaps between the foundation and the original soil, as well as the three holes for the prestressed steel strands and the two holes for the reinforcing bars, were filled with cement grout to ensure a tight bond.

[0022] In this embodiment, the prefabricated foundation described in step 1 includes a foundation column composed of several straight precast concrete components 4 stacked together in sequence, and a truncated cone base with four truncated concrete components 5 embedded and fixed together. The truncated cone base is fixed to the bottom of the foundation pit, and the foundation column is stacked on the truncated cone base. It is connected and fastened into one piece by prestressed steel strands 3, reinforcing bars 2 and anchor plates 1. Both the straight column precast concrete component 4 and the enlarged head precast concrete component 5 are reinforced concrete precast components, and each is individually provided with several hanging nail holes. The specific steps for installing the fixed end anchor plate 1 in step 3 are as follows: after anchoring the prestressed steel strand 3 to the fixed end anchor plate 1, it is inserted from bottom to top into the prestressed anchor cable hole of the enlarged head precast component 5. The connector 6 is installed at the end. The steel strand 3 is temporarily secured with a steel strand clip 7 at the distance that just exceeds the top surface of the enlarged head precast component 5 to prevent the prestressed anchor from slipping out of the anchor cable hole during hoisting, and to ensure that the anchor plate 1 is tightly fitted to the bottom of the precast component without obvious gaps or gaps. The specific steps for hoisting the expanded head precast component 5 described in step 4 are as follows: the underground construction personnel pre-position the first precast component and prepare for auxiliary hoisting work. The ground construction personnel screw the lifting nail into the threaded hole of the precast nail. The hoisting rope of the hoisting equipment passes through the round head ring of the lifting nail. The hoisting rope is tightened to lift the precast component. After the expanded head precast component 5 is moved to the center of the foundation pit, the hoisting rope is loosened. During this period, the component is lowered at a uniform speed and smoothly. After reaching the bottom of the foundation pit, the underground construction personnel remove the lifting nail and steel strand clip 7 and move the first precast component to the predetermined position. The specific steps for assembling the truncated cone base in step 5 are as follows: Repeat step 4 to hoist the remaining three precast truncated cone parts 5 into the bottom of the pit in sequence. After placing them in the designated positions, the underground construction personnel manually move the precast truncated cone parts 5 to overlap the steel structure embedded structure on the vertical surface of the precast parts. After aligning the holes of the reinforcing bars 2, insert the reinforcing bars 2 and fill the gap between the reinforcing bars 2 and the holes with cement grout to ensure that the reinforcing bars 2 are vertical. After all four precast truncated cone parts 5 are assembled, a truncated cone base is formed. After the truncated cone construction is completed, the underground construction personnel return to the surface. The specific steps for splicing the prestressed steel strand 3 in step 6 are as follows: when hoisting the precast component 4 of the straight column, use the hoisting method in step 4 for hoisting the precast component 5 of the expanded head to hoist the precast component to the center of the foundation pit. Then, pass the traction steel strand 3 on the strand frame through the anchor cable hole from top to bottom and connect it to the prestressed anchor cable with the connector 6. Gradually tighten the traction steel strand 3 until the prestressed steel strand 3 is taut. The specific steps for hoisting the precast straight column 4 described in step 7 are as follows: the precast straight column 4 is positioned using the pre-arranged reinforcing bars 2. Each reinforcing bar 2 passes through the reserved reinforcing bar 2 hole of the precast straight column 4. Then, the hoisting rope of the hoisting equipment is loosened to lower the precast straight column 4 smoothly to the top of the frustum-shaped expansion plate base. The specific steps for installing the foundation main column described in step 8 are as follows: repeat steps 6 and 7 to overlap the precast straight column 4 layer by layer until the last precast straight column 4 is in place, ensuring that the last precast straight column 4 can protrude 1 / 3 of its height above the ground surface to facilitate the installation of the tower foot plate. After all the precast straight column 4 are stacked, a cylindrical foundation main column is formed. The specific steps for installing the tensioning end anchor plate 1 and applying foundation prestress in step 9 are as follows: tighten the tensioning device rod with internal threads at the end into the connector 6 at the end of the steel strand 3 for tensioning; after tensioning is completed, remove the end connector 6 and anchor the prestressed steel strand 3 with the anchor plate 1.

[0023] In the preferred embodiment, the prefabricated foundation described in step 1 includes foundation columns composed of several straight precast concrete components 4 stacked together, and a truncated cone base consisting of four truncated concrete components 5 embedded and fixed together. The truncated cone base is fixed to the bottom of the foundation pit, and the foundation columns are stacked on the truncated cone base. They are connected and fastened together by prestressed steel strands 3, reinforcing bars 2, and anchor plates 1. The above configuration realizes a simple modular structure and convenient assembly method, reducing the time and human resource costs consumed in the construction process.

[0024] In the preferred embodiment, both the straight column precast concrete component 4 and the enlarged head precast concrete component 5 are reinforced concrete precast components, and each is individually provided with several lifting nail holes; the provision of lifting nail holes facilitates the subsequent hoisting work of the straight column precast concrete component 4 and the enlarged head precast concrete component 5.

[0025] In the preferred embodiment, the specific steps for installing the fixed-end anchor plate 1 in step 3 are as follows: after anchoring the prestressed steel strand 3 to the fixed-end anchor plate 1, it is inserted from bottom to top into the prestressed anchor cable hole of the enlarged head precast component 5. The connector 6 is installed at the end, and the steel strand 3 is temporarily secured with a steel strand clip 7 at a distance just beyond the top surface of the enlarged head precast component 5 to ensure that the anchor plate 1 is tightly fitted to the bottom of the precast component without obvious gaps or gaps. The setting of the steel strand clip 7 effectively prevents the prestressed steel strand 3 from slipping out of the anchor cable hole during hoisting, thereby enhancing the stability and safety of the construction process.

[0026] In the preferred embodiment, the specific steps for hoisting the expanded head precast component 5 in step 4 are as follows: Underground construction personnel pre-position the first precast component and prepare for auxiliary hoisting work. Ground-based construction personnel screw the lifting nails into the pre-drilled threaded holes in the precast component. The hoisting rope passes through the round-headed ring of the lifting nail, and the rope is tightened to lift the precast component. After moving the expanded head precast component 5 to the center of the pit, the rope is loosened, maintaining a uniform and stable descent. Once at the bottom of the pit, the underground construction personnel remove the lifting nails and steel strand clips 7, and move the first precast component to the predetermined position. These steps further ensure that the expanded head precast component 5 is stably and accurately placed in the predetermined position, reducing the possibility of overlapping errors during the pit assembly process and further improving construction efficiency and quality.

[0027] In the preferred embodiment, the specific steps for assembling the truncated cone base in the pit in step 5 are as follows: Repeat step 4 to hoist the remaining three precast truncated cone parts 5 into the bottom of the pit in sequence. After placing them in the designated positions, the underground construction personnel manually move the precast truncated cone parts 5 to overlap the steel structure embedded structures on the vertical surface of the precast parts. After aligning the holes of the reinforcing bars 2 vertically, insert the reinforcing bars 2 and fill the gap between the reinforcing bars 2 and the holes with cement grout to ensure that the reinforcing bars 2 are vertical. After all four precast truncated cone parts 5 are assembled, a truncated cone base is formed. After the truncated cone construction is completed, the underground construction personnel return to the surface. The above operation process further strengthens the lateral connection strength of the truncated cone base, enabling it to better resist the soil pressure around the pit. The early installation of the reinforcing bars 2 plays a precise positioning role for the subsequent hoisting of the concrete precast concrete parts 4 of the straight column.

[0028] In the preferred embodiment, the specific steps for splicing the prestressed steel strand 3 in step 6 are as follows: when hoisting the precast straight column 4, the hoisting method in step 4 for hoisting the precast expanded head 5 is used to hoist the precast component to the center of the foundation pit. Then, the traction steel strand 3 on the strand frame is passed through the anchor cable hole from top to bottom and connected to the prestressed anchor cable with the connector 6. The traction steel strand 3 is gradually tightened until the prestressed steel strand 3 is taut. The use of the steel strand 3 and its connector 6 ensures that the flexible steel strand 3 remains taut throughout the construction process, preventing the steel strand 3 from slipping into the foundation pit and thus delaying the construction progress.

[0029] In the preferred embodiment, the specific steps for hoisting the precast straight column component 4 in step 7 are as follows: the pre-arranged reinforcing bars 2 are used to position the precast straight column component 4. Each reinforcing bar 2 passes through the reserved reinforcing bar 2 hole of the precast straight column component 4. Then, the hoisting rope of the hoisting equipment is loosened to smoothly lower the precast straight column component 4 to the top of the frustum-shaped expansion plate base. The above operation process makes the hoisting process of the precast straight column component 4 quick and accurate. The setting of pre-inserting the steel strands 3 ensures that the anchor cable holes of each precast straight column component 4 can be quickly aligned, further improving construction efficiency and construction quality.

[0030] In the preferred embodiment, the specific steps for installing the foundation main column in step 8 are as follows: repeat steps 6 and 7 to overlap the precast straight column 4 layer by layer until the last precast straight column 4 is in place, ensuring that the last precast straight column 4 can protrude 1 / 3 of its height above the ground surface. After all the precast straight column 4 are stacked, a cylindrical foundation main column is formed. The above operation process makes it easier to install the tower foot plate and its superstructure after the foundation is installed. The layered stacking design simplifies the construction process to the greatest extent and improves the overall construction efficiency and quality.

[0031] In the preferred embodiment, the specific steps for installing the tensioning end anchor plate 1 and applying foundation prestress in step 9 are as follows: tighten the tensioning device rod with internal threads at the end into the connector 6 at the end of the steel strand 3 for tensioning; after tensioning is completed, remove the end connector 6 and anchor the prestressed steel strand 3 with the anchor plate 1; the above operation process makes the application of prestress more convenient and further improves the accuracy, efficiency and safety of the construction process.

[0032] In summary, the prefabricated foundation construction method with enlarged excavation face provided by this invention solves the problems of high requirements for construction environment and geological conditions, large earthwork excavation volume, difficulty in ensuring construction quality, and high degree of damage to the ecological environment in conventional construction methods. It realizes the separate assembly of the enlarged head of the prefabricated foundation in the pit and the overall small-scale construction environment. It not only utilizes the high strength of the original soil to ensure the bearing capacity of the prefabricated foundation, but also stabilizes the tower base, thereby reducing the frequency of use of auxiliary protection devices and reducing the additional cost of construction projects. It has good economic and ecological benefits and provides a feasible way for large-scale excavation foundation construction projects with lower cost, higher efficiency, and less environmental damage.

Claims

1. A method for constructing prefabricated foundations with enlarged excavation faces in small working areas, characterized in that, Includes the following steps: Step 1: Positioning and excavation of prefabricated foundation pit. Position the power transmission tower foundation according to the construction plan and complete the small operation pit excavation. The prefabricated foundation includes a foundation column composed of several prefabricated straight column parts (4) stacked together in sequence, and a frustum base with four prefabricated expanded head parts (5) embedded and fixed together. The frustum base is fixed at the bottom of the foundation pit. The foundation column is stacked on the frustum base and connected and fastened into one piece by prestressed steel strands (3), reinforcing bars (2) and anchor plates (1). Step 2: Install the strand frame. Prepare the corresponding number of strand frames according to the number of steel strands (3) required for the foundation. A certain length of traction steel strands (3) is reserved on the strand frame, and a connector (6) is installed at the end of the steel strands (3). Step 3: Install the fixed end anchor plate (1); Step 4: Hoisting of the precast head component (5); Step 5: Assemble the base of the expanded frustum in the pit; Step 6: Lap joint of prestressed steel strands (3); Step 7: Hoisting of the precast straight column components (4); Step 8: Installation of the main foundation column; Step 9: Installation of tension end anchor plate (1) and application of foundation prestress; Step 10: Grouting and filling of the gaps around the foundation and the reinforcing bar (2) holes. The gaps between the foundation and the original soil, as well as the holes for the prestressed steel strands (3) and the insertion bars (2), are filled with cement grout to make them tightly bonded.

2. The method for constructing a prefabricated foundation with enlarged excavation on a small working face according to claim 1, characterized in that: Both the straight column precast component (4) and the enlarged head precast component (5) are reinforced concrete precast components, and each has several hanging nail holes.

3. The method for constructing a prefabricated foundation with enlarged excavation on a small working face according to claim 1, characterized in that: The specific steps for installing the fixed end anchor plate (1) in step 3 are as follows: after anchoring the prestressed steel strand (3) to the fixed end anchor plate (1), it is inserted from bottom to top into the prestressed anchor cable hole of the enlarged head precast part (5), and the connector (6) is installed at the end. The steel strand (3) is temporarily locked with a steel strand buckle (7) at the distance that just exceeds the top surface of the enlarged head precast part (5) to prevent the prestressed anchor from sliding out of the anchor cable hole during hoisting, and to ensure that the anchor plate (1) is tightly attached to the bottom of the precast part without obvious gaps or gaps.

4. The method for constructing a prefabricated foundation with enlarged excavation on a small working face according to claim 1, characterized in that: The specific steps for hoisting the precast part (5) with the enlarged head described in step 4 are as follows: the underground construction personnel go down into the well in advance to locate the position of the first precast part and prepare for the auxiliary hoisting work. The ground construction personnel screw the hoisting nail into the threaded hole of the hoisting nail reserved in the precast part. The hoisting rope of the hoisting equipment passes through the round head ring of the hoisting nail, tightens the hoisting rope to lift the precast part, moves the precast part (5) with the enlarged head to the center of the foundation pit and then loosens the hoisting rope. During this period, the precast part is lowered at a uniform speed and steadily. After reaching the bottom of the foundation pit, the underground construction personnel remove the hoisting nail and the steel strand buckle (7) and move the first precast part to the predetermined position.

5. The method for constructing a prefabricated foundation with enlarged excavation on a small working face according to claim 4, characterized in that: The specific steps for assembling the truncated cone base in step 5 are as follows: repeat step 4 to hoist the remaining three precast truncated cone parts (5) into the bottom of the pit in sequence. After placing them in the set position, the underground construction personnel manually move the precast truncated cone parts (5) to overlap the steel structure embedded structure on the vertical surface of the precast parts. After aligning the holes of the reinforcing bars (2) with the top and bottom, insert the reinforcing bars (2). Pour cement grout to fill the gap between the reinforcing bars (2) and the holes to ensure that the reinforcing bars (2) are vertical. After all four precast truncated cone parts (5) are assembled, a truncated cone base is formed. After the truncated cone construction is completed, the underground construction personnel return to the ground.

6. The method for constructing a prefabricated foundation with enlarged excavation on a small working face according to claim 1, characterized in that: The specific steps for splicing the prestressed steel strand (3) described in step 6 are as follows: When hoisting the precast component (4) of the straight column, the construction workers on the ground screw the hoisting nail into the threaded hole of the hoisting nail reserved in the precast component (4) of the straight column. The hoisting rope of the hoisting equipment passes through the round head ring of the hoisting nail, tightens the hoisting rope to lift the precast component (4) of the straight column. After the precast component (4) of the straight column is hoisted to the center of the foundation pit, the traction steel strand (3) on the strand frame passes through the anchor cable hole from top to bottom and is connected to the prestressed anchor cable with the connector (6). The traction steel strand (3) is gradually tightened until the prestressed steel strand (3) is taut.

7. The method for constructing a prefabricated foundation with enlarged excavation on a small working face according to claim 1, characterized in that: The specific steps for hoisting the precast straight column component (4) described in step 7 are as follows: the first precast expanded head component (5) is moved to the predetermined position, and the remaining three precast expanded head components (5) are hoisted into the bottom of the foundation pit in sequence. After being placed in the set position, the underground construction personnel manually move the precast expanded head component (5) to overlap the steel structure embedded structure on the vertical surface of the precast component. After the holes of the insert bars (2) are aligned up and down, insert bars (2) are inserted. The precast bars (2) arranged on the precast expanded head component (5) are used to realize the positioning of the precast straight column component (4). Each insert bar (2) passes through the reserved insert bar (2) hole of the precast straight column component (4). Then, the hoisting rope of the hoisting equipment is loosened and the precast straight column component (4) is smoothly lowered to the top of the frustum-shaped expanded plate base.

8. The method for constructing a prefabricated foundation with enlarged excavation on a small working face according to claim 1, characterized in that: The specific steps for installing the foundation main column described in step 8 are as follows: repeat steps 6 and 7 to overlap the precast straight column parts (4) layer by layer until the last precast straight column part (4) is in place, ensuring that the last precast straight column part (4) can be exposed 1 / 3 of the height above the ground surface, which is convenient for installing the tower foot plate. After all the precast straight column parts (4) are stacked, a cylindrical foundation main column is formed.

9. The method for constructing a prefabricated foundation with enlarged excavation on a small working face according to claim 1, characterized in that: The specific steps for installing the tensioning end anchor plate (1) and applying foundation prestress in step 9 are as follows: tighten the tensioning device rod with internal thread at the end into the connector (6) at the end of the steel strand (3) for tensioning. After tensioning is completed, remove the end connector (6) and anchor the prestressed steel strand (3) with the anchor plate (1).

Images