Foundation pit mixed supporting structure and construction method thereof

Abstract

The invention discloses a foundation pit mixed supporting structure and a construction method thereof, relates to the technical field of foundation pit supporting, and aims to solve the technical problems that in the prior art, all supporting parts are poor in collaboration, the due supporting effect is difficult to give full play, and the foundation pit collapse risk is high. Comprising a single-layer fulcrum structure in the form of adding a pre-stressed anchor cable to a common Larsen pile arranged on the side adjacent to a building. On the side adjacent to the structure, the Larsen steel sheet piles and the pre-stressed anchor cables achieve the synergistic effect, large lateral soil pressure can be borne, a solid foundation is provided for supporting, the foundation pit is prevented from deforming towards the structure, the soil nailing wall reinforces the soil body above the bottom elevation of the shallow foundation, and the integrity and the anti-sliding capacity of the side slope soil body are enhanced. By means of the improved construction technology, it can be ensured that all parts fully play roles, the foundation pit collapse risk is effectively reduced, and the surrounding structure and construction safety is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of foundation pit support, and more specifically, to a foundation pit mixed support structure and a construction method thereof. Background Art

[0002] Foundation pit engineering plays a vital role in various construction projects: creating space for foundation construction: in the construction of buildings, bridges, underground parking lots and other projects, it is necessary to excavate foundation pits to provide sufficient space for foundation construction, ensure that the foundation can be built according to the size, depth and position required by the design, and provide stable support for the superstructure. For example, in the construction of high-rise buildings, suitable space is excavated through foundation pit engineering to cast reinforced concrete foundations to bear the weight of the entire building. In modern engineering construction, the number and scale of foundation pit projects are constantly increasing, especially in the fields of urban construction and industrial facility construction. In many projects, the surrounding environment of the foundation pit is complex, and there are often adjacent structures, existing shallow foundations, roads, etc. For example, in some old urban area renovation projects and factory expansion projects in some cities, the distance between the foundation pit and surrounding buildings or facilities is relatively close.

[0003] However, in the prior art, there is often a problem of poor coordination of various support components. For example, some support structures simply set up Larsen steel sheet piles or soil nail walls without effective combination optimization. On the side adjacent to the structure, if only a single Larsen steel sheet pile is used, its ability to resist lateral earth pressure is limited, and it is difficult to provide sufficiently stable support through coordination with prestressed anchor cables, which can easily cause the foundation pit to deform significantly in the direction of the structure. In addition, during the construction of traditional soil nail walls, the arrangement of soil nails may lack scientificity, and the soil characteristics are not fully considered to make a reasonable plum blossom arrangement and determine the appropriate inclination angle, so that the soil nails are not tightly combined with the soil, and it is difficult to effectively enhance the integrity and anti-slip ability of the slope soil. At the same time, the construction process is not strict and standardized, and there is a lack of precise control in the production and installation of anchor cables, the insertion of steel sheet piles, etc., which leads to the inability of various components to fully play their due support role, thereby increasing the risk of foundation pit collapse and making it difficult to ensure the safety of surrounding structures and construction. In view of this, we propose a foundation pit mixed support structure and its construction method. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art, adapt to actual needs, and provide a foundation pit mixed support structure and a construction method thereof to solve the technical problems in the current technology that the various support components have poor coordination, it is difficult to fully play their due support role, and the risk of foundation pit collapse is high.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: a mixed foundation pit support structure, including a single-layer support structure in the form of an ordinary Larsen pile and a prestressed anchor cable arranged on the side adjacent to the structure, and a soil nail wall structure arranged above the bottom elevation of the shallow foundation; the Larsen steel sheet pile adopts a 12m or 15m long Larsen steel sheet pile, and a double-jointed channel steel waist beam and a prestressed anchor cable are arranged.

[0006] Preferably, the locking mouth of the Larsen steel sheet pile is coated with grease before construction.

[0007] Preferably, the soil nails are arranged in a plum blossom shape with an inclination angle of 20°.

[0008] Preferably, the steel strands of the prestressed anchor cable are cut according to the designed dimensions using a cutting machine, with each strand having a length error of no more than 50 mm, and a locator is provided every 1.0-1.5 m. The free section is wrapped with a plastic tube and sealed with a waterproof tape at the intersection with the anchoring section, and the grouting pipe is placed in the center of the locator and no more than 300 mm away from the bottom of the hole.

[0009] A construction method of a foundation pit mixed support structure comprises the following steps:

[0010] S1: The site is leveled to the designed elevation;

[0011] S2: Carry out soil nail wall support construction above shallow foundation, including excavation of working surface, slope correction, driving soil nails, tying steel mesh, installing drainage pipes, surface construction and maintenance;

[0012] S3: Carry out Larsen pile construction, including surveying, setting out, leveling the ground and clearing underground obstacles, driving steel sheet piles, anchor cable and steel waist beam construction, excavating earth to the bottom elevation of the foundation pit, backfilling earth, removing Larsen steel sheet piles and backfilling pile holes;

[0013] S4: Excavate the earth in layers until it is completed.

[0014] Preferably, in the excavation process of the soil nail wall support construction above the shallow foundation in step S2, the excavation depth shall not exceed the existing shallow foundation bottom elevation, and the excavation shall be carried out in layers and sections, using excavation equipment and methods that cause little disturbance to the slope soil. After the slope is mechanically excavated, small machinery or shovels are used to cut and clear the slope, and the transport horseway is designed in a diagonal direction.

[0015] Preferably, in the step of driving steel sheet piles in the Larsen pile construction in step S3, before driving the steel sheet piles, the conditions of underground pipelines and structures are familiarized and the center line of the supporting piles is accurately laid out, the steel sheet piles are checked one by one, and the steel sheet piles with rusted and severely deformed connecting locks are removed, the slope of the monitoring piles is measured and monitored during the driving process and does not exceed 2%, the vibration frequency of the vibrating hammer is greater than the natural vibration frequency of the steel piles during vibration pile sinking, the number of test piles is not less than 10, and the order of driving the steel sheet piles is from the midpoints of the four peripheral lines to the four corners in sequence, with two piles at the closing point, one high and one low, and the directions of the corner piles are adjusted to make the locks parallel.

[0016] Preferably, in the anchor cable and steel waist beam construction steps of the Larsen pile construction in step S3, after laying out the line and determining the hole position, a special anchor drilling rig is used to align the hole position and adjust the angle, the drilling is done by casing drilling, the depth meets the requirements, the anchor cable production and placement, grouting, waist beam and anchor construction, tensioning and locking are all carried out in accordance with the corresponding specifications and design requirements, wherein the anchor cable grouting adopts a two-time grouting process, the first normal pressure grouting, and the second high-pressure grouting is carried out after the first grouting body is initially set, and there are corresponding pressure and cement dosage control standards.

[0017] Preferably, during the earthwork construction in step S3, the next excavation is carried out when the strength of the mixed support structure reaches 75%, and the excavation is carried out in layers and sections by an excavator. The excavation depth of each layer is not more than 2m. After the foundation pit is excavated, drainage ditches and sump wells are set up in the pit in time, and the transport horseway is designed in a diagonal direction. It is strictly prohibited to pile up loads within 3m of the top line of the foundation pit slope and at the platform position, and it is strictly prohibited to overload outside 3m of the foundation pit.

[0018] Preferably, when the steel sheet pile is pulled out in step S3, the head of the steel sheet pile is first clamped with a pile driver and vibrated for 1-2 minutes to loosen the soil and then pulled out by vibration. When pile pulling is difficult, the prescribed operation is followed, and the pile hole is backfilled in time after the pile is pulled out. The pulling out is suspended and the pile is vibrated every time the pile is pulled out by 1m to fill the soil hole. After the pile is pulled out, the remaining gap is filled with medium-coarse sand.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. In the present invention, Larsen steel sheet piles and prestressed anchor cables work together on the side adjacent to the structure, which can withstand large lateral soil pressure. The length of 12m or 15m can penetrate into the stable soil layer, provide a solid foundation for support, and prevent the foundation pit from deforming toward the structure. The soil nail wall reinforces the soil above the shallow foundation bottom elevation. The plum blossom-shaped soil nails with an inclination of 20° are closely combined with the soil, which enhances the integrity and anti-slip ability of the slope soil. Strict construction technology such as accurate feeding, positioning and grouting of anchor cables, and standardized insertion and driving of steel sheet piles. The present invention can ensure that each component plays a full role through improved construction technology, effectively reduce the risk of foundation pit collapse, and ensure the safety of surrounding structures and construction.

[0021] 2. The Larsen steel sheet piles, anchor cables and other materials of the present invention are common and reasonably used, and the cost is lower than that of some complex support systems. In terms of construction, the steps of site leveling, layered and segmented excavation and other steps are orderly and efficient. For example, the layered excavation depth is controlled to be no more than 2m, which reduces the disturbance to the soil and the difficulty of large-scale earthwork processing. The steel sheet pile locks are greased, the soil nail wall is simply constructed, and a reasonable transportation horseway is designed. The construction process of the present invention is smooth, the construction period is shortened, the requirements for construction equipment and site are not harsh, and it can be successfully applied in various geological conditions and projects with limited space, reducing the overall construction cost and improving the economic benefits of the project. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the cross-sectional structure of the support structure of the present invention;

[0023] Figure 2 It is a schematic diagram of the longitudinal structure of the Larsen pile of the present invention. DETAILED DESCRIPTION

[0024] Embodiment 1, as Figure 1 to Figure 2 As shown, the present invention relates to a foundation pit mixed support structure, including a single-layer support structure in the form of an ordinary Larsen pile and a prestressed anchor cable set on the side of the adjacent structure, and a soil nail wall structure set above the bottom elevation of the shallow foundation; the Larsen steel sheet pile adopts a 12m or 15m long Larsen steel sheet pile, and a double-jointed channel steel waist beam and a prestressed anchor cable are set. The anchor section length of the prestressed anchor cable is 12000, the hole diameter D=160mm, the tension standard value is 125kN, and the locking value is 80kN; the slope of the soil nail wall is 1:1.00, the total length of the soil nail is 0.8m, and the horizontal spacing is 2m. The steel strands of the prestressed anchor cable are cut by a cutting machine according to the design size, and the length error of each strand is not more than 50mm, and a positioner is set every 1.0-1.5m, the free section is wrapped with a plastic tube and sealed with a waterproof tape at the intersection with the anchor section, and the grouting pipe is placed in the middle of the positioner and the distance from the bottom of the hole is not more than 300mm.

[0025] The design of various parameters of prestressed anchor cables also has its scientific basis. The length of the anchoring section is 12000mm, the hole diameter D=160mm, the standard value of tension is 125kN, and the locking value is 80kN. These parameters are determined by analyzing and calculating professional geotechnical engineering calculation software based on the geological conditions of the soil, the depth of the foundation pit, and the surrounding environmental load. In the design process, factors such as the side resistance coefficient of different soil layers, the anchoring performance of the anchor cable, and the safety factor of the support structure are considered to ensure that the anchor cable can provide sufficient tension to maintain the stability of the support structure. The steel strands are cut by a cutting machine according to the design size, and the length error of each strand is no more than 50mm. A locator is set every 1.0-1.5m. The free section is wrapped with a plastic tube and sealed with waterproof tape at the intersection with the anchor section. The grouting pipe is placed in the middle of the locator and the distance from the bottom of the hole is no more than 300mm. These measures are all to ensure the construction quality and mechanical properties of the anchor cable. Precise feeding length and positioner settings can ensure the accurate position of the steel strand in the hole and prevent it from shifting or twisting; protective measures for the free section can prevent the steel strand from being eroded and damaged by the surrounding soil, ensuring its long-term durability; reasonable grouting pipe position and grouting process can ensure that the slurry is evenly diffused in the hole, so that the anchoring section is fully bonded to the soil and the pull-out resistance of the anchor cable is improved.

[0026] The locking ends of the Larsen steel sheet piles are coated with grease before construction.

[0027] During the design process, we fully relied on the principles of geotechnical mechanics and relevant engineering experience. The length of the Larsen steel sheet pile was selected to be 12m or 15m, which was determined by comprehensively considering factors such as the depth of the foundation pit, the properties of the surrounding soil, and the possible loads. Through the force analysis and simulation calculation of steel sheet piles of different lengths under various working conditions, it was found that this length range can meet the strength and stability requirements of the support structure. A double-jointed channel steel waist beam and a prestressed anchor cable are set up to form an effective support system and enhance the bending and shear resistance of the support structure. The steel waist beam can evenly transfer the tension of the anchor cable to the steel sheet pile, thereby improving the collaborative working performance of the overall structure.

[0028] The soil nails are arranged in a plum blossom shape with an inclination angle of 20°.

[0029] The soil nail wall structure plays an important role above the shallow foundation bottom elevation. Its slope is 1:1.00, which is the optimal slope calculated through stability analysis based on the physical and mechanical parameters of the soil such as the internal friction angle and cohesion. Under this slope, the soil can maintain a certain stability under its own gravity, and at the same time, combined with the reinforcement effect of the soil nails, the anti-slip ability of the slope is further improved. The soil nails are 0.8m long in total, with a horizontal spacing of 2m. They are arranged in a plum blossom shape with an inclination of 20°. This arrangement allows the soil nails to fully contact the soil and form an effective anchoring system. The soil nails can restrain the soil, increase the shear strength of the soil, and prevent sliding damage to the soil.

[0030] A construction method of a foundation pit mixed support structure comprises the following steps:

[0031] S1: The site is leveled to the designed elevation;

[0032] S2: Carry out soil nail wall support construction above shallow foundation, including excavation of working surface, slope correction, driving soil nails, tying steel mesh, installing drainage pipes, surface construction and maintenance.

[0033] In the excavation process of the soil nail wall support construction above the shallow foundation in step S2, the excavation depth shall not exceed the existing shallow foundation bottom elevation, and the excavation shall be carried out in layers and sections, using excavation equipment and methods that cause little disturbance to the slope soil. After the slope is mechanically excavated, small machinery or shovels are used to cut and clear the slope, and the transport horseway is designed in a diagonal direction.

[0034] When excavating the working surface, the excavation depth and slope should be strictly controlled to avoid over-excavation or under-excavation. A combination of small excavators and manual excavation should be used to reduce disturbance to the slope soil. During the excavation process, the stability of the slope soil should be observed at any time. If the soil is found to be loose or cracked, the excavation should be stopped immediately and appropriate reinforcement measures should be taken, such as increasing the number of soil nails, shortening the distance between soil nails, or spraying concrete for temporary support.

[0035] When correcting the slope, a small machine or shovel should be used to carefully cut and clear the slope to ensure that the slope flatness meets the design requirements. The slope flatness deviation should be controlled within ±50mm, otherwise it will affect the laying of the steel mesh and the quality of shotcrete. For local unevenness on the slope, manual filling or trimming should be used to ensure the smoothness of the slope.

[0036] When driving soil nails, the position, depth and angle of the soil nails should be in accordance with the design requirements. If the soil nails encounter obstacles and cannot be driven in according to the design position, their position and angle can be adjusted appropriately within the allowable deviation range, but the anchoring length of the soil nails should be ensured not to be less than the design requirements. During the driving of soil nails, a dedicated soil nailing machine or excavator should be used in conjunction with manual operation to control the driving speed and pressure of the soil nails to avoid excessive impact on the soil.

[0037] When tying the steel mesh, the spacing of the steel mesh should be uniform and firm. The intersection of the steel bars should be tied with iron wire, and the wire head should be facing the inside of the concrete to avoid exposure and affecting the appearance quality of the concrete. The steel mesh should be kept at a certain distance from the slope, generally 30-50mm, and fixed by inserting steel support or pads into the soil to ensure that the steel mesh will not shake or shift during the spraying of concrete.

[0038] When installing the drain pipe, ensure that the position of the drain pipe is reasonable and the drainage is smooth. The drain pipe generally uses a PVC pipe with a diameter of 50-100mm, which is arranged in a plum blossom shape on the slope with a spacing of 2-3m. The drain pipe should be inserted into the soil to a certain depth, generally 300-500mm, and a filter screen should be wrapped outside the pipe to prevent the soil from blocking the drain pipe. Before spraying concrete, the drain pipe should be temporarily blocked to prevent concrete from entering the pipe.

[0039] When constructing the surface layer, the mix ratio and spraying process of shotcrete should be strictly controlled. The concrete mix ratio should be determined by trial mixing according to the strength grade required by the design, generally cement: crushed stone soil: river sand = 1:2:2 (weight ratio), and the water-cement ratio is 0.4-0.5. Before spraying concrete, the spraying equipment should be debugged to ensure that the spraying pressure is stable between 0.3-0.5MPa. During the spraying process, the spraying should be layered, and the thickness of each layer should be controlled between 50-80mm. The spraying sequence should be carried out from bottom to top to ensure the density and strength of the concrete. After the spraying concrete is completed, it should be maintained in time, and the maintenance time should be no less than 7 days. Covering watering maintenance or spraying curing agent can be used to keep the concrete surface moist and promote the growth of concrete strength.

[0040] S3: Carry out Larsen pile construction, including surveying, setting out, leveling the ground and clearing underground obstacles, driving steel sheet piles, anchor cable and steel waist beam construction, excavating earth to the bottom elevation of the foundation pit, backfilling earth, removing Larsen steel sheet piles and backfilling pile holes.

[0041] In the step S3 of driving steel sheet piles in the Larsen pile construction, before driving the steel sheet piles, the conditions of underground pipelines and structures are familiarized and the center line of the supporting piles is accurately laid out. The steel sheet piles are checked one by one, and the steel sheet piles with rusted or severely deformed connecting locks are removed. During the driving process, the slope of the monitoring pile is measured to be no more than 2%. During the vibration pile sinking, the vibration frequency of the vibration hammer is greater than the natural vibration frequency of the steel pile. The number of test piles is not less than 10. The order of driving the steel sheet piles is from the midpoint of the four perimeter lines to the four corners in sequence. The two piles at the closing point are one high and one low, and the directions of the corner piles are adjusted to make the locks parallel.

[0042] In the step S3, in the anchor cable and steel waist beam construction steps of the Larsen pile construction, after laying out the line and determining the hole position, a special anchor drilling rig is used to align the hole position and adjust the angle, the drilling is performed using casing drilling, and the depth meets the requirements. The production and placement of the anchor cable, grouting, waist beam and anchor construction, tensioning and locking are all carried out in accordance with the corresponding specifications and design requirements. The anchor cable grouting adopts a two-time grouting process, the first normal pressure grouting, and the second high-pressure grouting is carried out after the initial setting of the first grouting body, and there are corresponding pressure and cement dosage control standards.

[0043] During the earthwork construction in step S3, the next excavation is carried out when the strength of the mixed support structure reaches 75%. The excavator is used for layered and segmented excavation. The excavation depth of each layer is not more than 2m. After the foundation pit is excavated, drainage ditches and sump wells are set up in time. The transport horseway is designed in a diagonal direction. It is strictly prohibited to pile up loads within 3m of the top line of the foundation pit slope and at the platform position, and it is strictly prohibited to overload outside 3m of the foundation pit.

[0044] In step S3, when the steel sheet pile is pulled out, the pile driver is first used to clamp the head of the steel sheet pile and vibrate it for 1-2 minutes to loosen the soil, and then the pile is pulled out by vibration. When it is difficult to pull out the pile, the prescribed operation is followed, and the pile hole is backfilled in time after the pile is pulled out. The pulling out is stopped and the pile is vibrated every time the pile is pulled out by 1m to fill the soil hole. After the pile is pulled out, the remaining gap is filled with medium-coarse sand.

[0045] Before measuring and setting out, the design drawings and actual conditions on site should be carefully checked to ensure the accuracy of the setting out. Use high-precision measuring instruments such as total stations or theodolites to set out the center and side lines of the support piles, and set up obvious marker piles on site. The marker piles should be firm and reliable and not easily damaged so that they can be reviewed at any time during the construction process.

[0046] When leveling the ground, the construction site should be leveled according to the survey and layout results. For places with large height differences, a combination of excavation and filling should be used to ensure the verticality and position accuracy of the steel sheet piles when they are inserted. During the leveling process, care should be taken to protect underground pipelines and structures to avoid damage to them.

[0047] Clearing underground obstacles is one of the key links in construction. Before construction, the location and situation of obstacles such as underground pipelines, foundations, and ancient tombs should be fully understood by consulting geological survey reports, conducting on-site exploration, or communicating with relevant departments. For small obstacles, such as underground pipelines, etc., measures such as relocation, protection, or removal can be taken; for large obstacles, such as foundations and ancient tombs, special treatment plans should be formulated to ensure safe and smooth construction.

[0048] Before the steel sheet piles are driven, in addition to checking each steel sheet pile and removing unqualified piles, the lock mouth should be cleaned and greased. There should be no debris or rust in the lock mouth, and the grease should be applied evenly with a moderate thickness, generally 2-3mm. During the grease application process, care should be taken to protect the surrounding environment to avoid grease contamination of soil and water.

[0049] During the driving process, a dedicated person should be arranged to use instruments such as a total station or theodolite to measure the slope of the monitoring pile in real time. If the slope of the pile is found to exceed 2%, the driving should be stopped immediately, and the deviation should be corrected by adjusting the position of the pile driver, pulling the pile head with a wire rope, etc. The correction process should be slow and steady to avoid affecting the piles that have been driven.

[0050] When vibrating piles, the vibration frequency and amplitude of the vibrating hammer should be reasonably selected according to factors such as the material, length and depth of the steel pile. The vibration frequency of the vibrating hammer is generally 10-20Hz, and the amplitude is 5-10mm. During the pile sinking process, the sinking speed and verticality of the steel pile should be closely observed. If the sinking speed of the steel pile suddenly decreases or abnormal vibration occurs, the pile sinking should be stopped immediately, the cause should be analyzed and corresponding measures should be taken. If the sinking is difficult due to excessive soil resistance, auxiliary sinking methods can be used, such as injecting water around the steel pile, digging the soil or flushing the soil with a high-pressure water gun, but attention should be paid to controlling the construction scope and depth to avoid affecting the surrounding soil and structures.

[0051] The number of test piles shall not be less than 10, and the test pile process shall be carried out in strict accordance with the requirements of formal construction. Through the test piles, the parameters such as the soil penetration depth, sinking speed, verticality, final penetration, and the optimal vibration frequency and amplitude of the vibrating hammer shall be obtained to provide a scientific basis for formal construction. After the test piles are completed, the test pile results shall be analyzed and summarized, and the construction plan shall be optimized and adjusted according to the test pile conditions.

[0052] The order of driving the steel sheet piles is from the midpoint of the four perimeter lines to the four corners in sequence. During the closing process, the number and position of the steel sheet piles should be accurately calculated. When closing, the straight-line distance at the bottom of the steel sheet pile should be measured and calculated, and the required number of sheets should be determined according to the width of the steel sheet pile, and the order and angle of the steel sheet piles should be adjusted accordingly. The two piles at the closing point are one high and one low, and the height difference is generally 10-20cm, which is convenient for adjustment and closing. When adjusting the direction of the corner piles to make the lock mouth parallel, you can use methods such as inserting a guide plate into the lock mouth of the corner pile or using a jack to push it to ensure that the closing is tight and the lock mouth is firmly connected.

[0053] After laying out the lines and determining the hole position, when aligning the hole position and adjusting the angle with a special anchor drill, a level and theodolite should be used for precise measurement to ensure that the verticality and horizontality deviation of the drill are within the allowable range. During the drilling process, the position and angle of the drill should be checked at any time, and any deviation should be corrected in time.

[0054] The drilling adopts the casing drilling technology. The diameter and length of the casing should be selected according to the design requirements of the anchor cable and the geological conditions. The casing should have sufficient strength and rigidity to withstand the soil pressure and friction during the drilling process. During the drilling process, the drilling speed and pressure should be controlled to avoid deformation or damage of the casing. When encountering hard soil layers or obstacles, impact drilling or other suitable drilling methods should be used, but care should be taken to protect the surrounding soil and constructed structures.

[0055] During the production and placement of anchor cables, the design requirements should be strictly followed. The steel strands should be tightly and neatly bundled, and the positioners should be firmly installed on the steel strands to ensure that the steel strands are accurately positioned in the hole. When placing the anchor cables, manual or mechanical assistance should be used to slowly and steadily place the anchor cables in the hole to avoid twisting, bending or colliding with the hole wall. If the hole is found to have collapsed or shrunken, it should be handled in a timely manner, such as by expanding or cleaning the hole, to ensure that the anchor cables can be smoothly placed in the hole and reach the designed depth.

[0056] Grouting is one of the key links in anchor cable construction. Before grouting, the performance of the grouting equipment and the mix ratio of the slurry should be checked to ensure the smooth progress of the grouting process. The slurry should be stirred evenly without precipitation and agglomeration. During the grouting process, the grouting pressure and grouting volume should be controlled, and the grouting process should be operated according to the design requirements. During the first normal pressure grouting, the slurry should be injected slowly so that the slurry can fully fill the soil pores around the anchor hole and the anchoring section. When the slurry flows to the orifice, the grouting should be suspended to check whether there is slurry leakage at the orifice. If there is leakage, it should be blocked in time. The second high-pressure grouting should be carried out after the initial setting of the first grouting body, generally 16-24 hours after the first grouting (after the strength of the cement stone body formed by the first grouting reaches 5.0MPa). During high-pressure grouting, the grouting pressure should be strictly controlled to be no less than 2.5MPa. At the same time, attention should be paid to the condition of the orifice and the surrounding soil. If there is any abnormality, the grouting should be stopped immediately and corresponding measures should be taken. After grouting is completed, the grouting equipment and pipelines should be cleaned in time to prevent the slurry from solidifying and clogging the equipment and pipelines.

[0057] During the construction of the waist beam and anchor, the steel perimeter purlin is made of 22# channel steel. During the processing, the splicing quality of the channel steel should be ensured. The splicing should be welded or bolted. The weld should be full and firm, and the bolts should be tightened. When welding the corbels at the corresponding support positions according to the support spacing, the position of the corbels should be accurate and the welding should be firm. The height and angle of the corbels should meet the design requirements. The steel perimeter purlin is processed in sections, and the corners should be accurately processed according to the actual length to ensure that the steel perimeter purlin can fit the steel sheet piles and support structure closely during the installation process. When installing the steel perimeter purlin, it should be hoisted section by section according to the order of support erection, and the steel perimeter purlin should be placed on the steel bracket manually with the crane. During the installation process, care should be taken to protect the steel perimeter purlin and steel bracket to avoid collision and damage. After installation, check whether the steel bracket is loose due to impact, and weld the gap between the bracket and the steel perimeter purlin with a steel wedge to ensure that the steel perimeter purlin and the steel bracket are firmly connected. The space between the steel purlin and the construction pile shall be no less than 60mm (and the construction error reserve of 200mm shall be considered to eliminate the construction error of the surrounding cast-in-place pile.

[0058] S4: Excavate the earth in layers until it is completed.

[0059] The present invention selects the following construction equipment:

[0060] Construction equipment

[0061] Vibratory hammer: The Z550 vibratory hammer is selected, which has a large exciting force and a stable vibration frequency, and can meet the requirements of inserting and removing Larsen steel sheet piles. When selecting the vibratory hammer, factors such as the material, length, depth of the steel sheet piles into the soil, and the geological conditions of the construction site are considered to ensure that the vibratory hammer can effectively overcome the resistance of the soil to the steel sheet piles, so that the steel sheet piles can be smoothly sunk or pulled out.

[0062] Air compressor: 10m 3 Air compressors provide a stable source of compressed air for construction processes such as drilling and shotcrete spraying. The selection of air compressors is mainly based on the gas demand of construction equipment and the construction efficiency requirements, ensuring that sufficient air pressure and air volume can be continuously provided during the construction process to ensure the smooth progress of the construction.

[0063] Grouting machine: BW-150 grouting machine can accurately control the grouting pressure and grouting volume to meet the process requirements of anchor grouting. When selecting the grouting machine, factors such as the properties of the slurry, the grouting pressure range, and the grouting flow rate are considered to ensure that the grouting machine can evenly and stably inject the slurry into the anchor hole to ensure the grouting quality.

[0064] Shotcrete machine: PZ-5B shotcrete machine has good spraying performance and can spray concrete evenly onto the slope of the soil nail wall to form a dense concrete surface layer. The selection of the shotcrete machine mainly considers factors such as the thickness and strength requirements of the shotcrete and the construction efficiency, ensuring that the shotcrete machine can meet the needs of soil nail wall support construction.

[0065] Rebar straightening machine: TQ4-13 rebar straightening machine is used to straighten and cut rebars to ensure that the straightness and length of the rebars meet the design requirements. When selecting a rebar straightening machine, factors such as the diameter range, straightening accuracy and production efficiency of the rebar are considered to ensure that the rebar straightening machine can effectively process rebars of different specifications.

[0066] Mixer: JDY-350A mixer can mix raw materials such as cement, crushed stone, river sand into uniform concrete or mortar to meet the casting and grouting requirements during the construction process. The selection of the mixer is mainly based on the amount of concrete or mortar required for construction, the mixing quality requirements and the construction progress, etc., to ensure that the mixer can provide stable mixing capacity.

[0067] Electric welding machine: 13X1-500 electric welding machine is used for welding steel bars, steel beams and other steel structure components. It has high welding current and stable welding performance. When selecting the electric welding machine, factors such as the type of welding materials, the form and size of the welding joint, and the welding quality requirements are considered to ensure that the electric welding machine can meet the needs of steel structure welding construction.

[0068] Cutting machine: GJ5Y-32 cutting machine is used to cut steel bars, steel strands and other materials, and can ensure the flatness and dimensional accuracy of the cutting surface. When selecting the cutting machine, factors such as the hardness of the cutting material, diameter range and cutting accuracy requirements are considered to ensure that the cutting machine can accurately cut different materials.

[0069] Truck crane: 50t truck crane is used to lift materials and equipment such as steel sheet piles, steel beams, and steel bars. It has a large lifting weight and operating radius, and can meet the lifting needs of the construction site. When selecting a truck crane, factors such as the maximum weight of the lifting object, lifting height, and horizontal distance are considered to ensure that the truck crane can complete the lifting task safely and efficiently.

[0070] The present invention adopts the following materials:

[0071] Rebar: Select appropriate specifications of rebar according to the design requirements of structures such as soil nail walls and steel waist beams. Rebar should have good mechanical properties and weldability, and its quality should meet relevant national standards. When purchasing rebar, check the quality certification documents of the rebar, including product certificates, factory inspection reports, etc., and conduct sampling inspections on the rebar to ensure that the quality of the rebar meets the design requirements.

[0072] Cement: P.O42.5R silicate cement is used, which has high strength and good durability and can meet the strength requirements of concrete and grouting materials. When selecting cement, the stability, setting time and strength of cement should be checked to ensure that the quality of cement is stable and reliable. Cement should be stored in a dry and ventilated warehouse to avoid moisture and agglomeration, which will affect its performance.

[0073] Crushed stone soil (melon seed slices): Crushed stone soil, as an aggregate for concrete, should have good particle gradation and strength. Its mud content should meet the requirements of relevant standards, otherwise it will affect the strength and durability of concrete. When purchasing crushed stone soil, it should be screened and tested for mud content to ensure that its quality meets the requirements.

[0074] River sand: Medium-coarse sand is used as fine aggregate for concrete and mortar, and its fineness modulus should meet the design requirements. River sand should be hard and clean, and the mud and mica content should be controlled within the specified range. Before use, river sand should be inspected to ensure its quality.

[0075] Steel: 22# channel steel is used to make steel waist beams. Its material should comply with relevant national standards and have sufficient strength and rigidity. When purchasing channel steel, the appearance quality of the channel steel should be checked. There should be no defects such as cracks, folds, and delamination. Its quality certification documents should be checked to ensure that its mechanical properties meet the requirements.

[0076] Anchor cable: The steel strand of 2ΦS15.2 anchor cable should meet the strength and elastic modulus requirements specified in the design, and its surface should be free of defects such as rust and oil stains. When purchasing anchor cables, sampling inspection should be carried out to check the mechanical properties and appearance quality of the steel strand to ensure the quality of the anchor cables.

[0077] Steel sheet piles: SP-U400x170 Larsen steel sheet piles should have good bending and shear resistance, and their locking mouths should be tight and flat, without deformation and damage. When entering the site, the steel sheet piles should be checked one by one, unqualified steel sheet piles should be removed, and the length, width, thickness and other dimensions of the steel sheet piles should be measured to ensure that they meet the design requirements.

[0078] The embodiments of the present invention disclose preferred embodiments, but are not limited thereto. A person skilled in the art can easily understand the spirit of the present invention based on the above embodiments and make different extensions and changes. However, as long as they do not deviate from the spirit of the present invention, they are all within the protection scope of the present invention.

Claims

1. A foundation pit mixed support structure, characterized in that: It includes a single-layer support structure in the form of an ordinary Larsen pile plus a prestressed anchor cable set on the side adjacent to the structure, and a soil nail wall structure set above the bottom elevation of the shallow foundation; the Larsen steel sheet pile adopts a 12m or 15m long Larsen steel sheet pile, and is equipped with a double-jointed channel steel waist beam and a prestressed anchor cable.

2. A foundation pit mixed support structure according to claim 1, characterized in that: The locking ends of the Larsen steel sheet piles are coated with grease before construction.

3. A foundation pit mixed support structure according to claim 2, characterized in that: The soil nails are arranged in a plum blossom shape with an inclination angle of 20°.

4. A foundation pit mixed support structure according to claim 3, characterized in that: The steel strands of the prestressed anchor cable are cut with a cutting machine according to the designed dimensions, with a length error of each strand not exceeding 50 mm, and a locator is provided every 1.0-1.5 m. The free section is wrapped with a plastic tube and sealed with waterproof tape at the intersection with the anchoring section. The grouting pipe is placed in the center of the locator and is not more than 300 mm away from the bottom of the hole.

5. A construction method of a foundation pit mixed supporting structure, which is applicable to a foundation pit mixed supporting structure according to claim 4, characterized in that: The following steps are involved: S1: The site is leveled to the designed elevation; S2: Carry out soil nail wall support construction above shallow foundation, including excavation of working surface, slope correction, driving soil nails, tying steel mesh, installing drainage pipes, surface construction and maintenance; S3: Carry out Larsen pile construction, including surveying, setting out, leveling the ground and clearing underground obstacles, driving steel sheet piles, anchor cable and steel waist beam construction, excavating earth to the bottom elevation of the foundation pit, backfilling earth, removing Larsen steel sheet piles and backfilling pile holes; S4: Excavate the earth in layers until it is completed.

6. The construction method of a foundation pit mixed support structure according to claim 5, characterized in that: In the excavation process of the soil nail wall support construction above the shallow foundation in step S2, the excavation depth shall not exceed the existing shallow foundation bottom elevation, and the excavation shall be carried out in layers and sections, using excavation equipment and methods that cause little disturbance to the slope soil. After the slope is mechanically excavated, small machinery or shovels are used to cut and clear the slope, and the transport horseway is designed in a diagonal direction.

7. The construction method of a foundation pit mixed support structure according to claim 5, characterized in that: In the step S3 of driving steel sheet piles in the Larsen pile construction, before driving the steel sheet piles, the conditions of underground pipelines and structures are familiarized and the center line of the supporting piles is accurately laid out. The steel sheet piles are checked one by one, and the steel sheet piles with rusted or severely deformed connecting locks are removed. During the driving process, the slope of the monitoring pile is measured to be no more than 2%. During the vibration pile sinking, the vibration frequency of the vibration hammer is greater than the natural vibration frequency of the steel pile. The number of test piles is not less than 10. The order of driving the steel sheet piles is from the midpoint of the four perimeter lines to the four corners in sequence. The two piles at the closing point are one high and one low, and the directions of the corner piles are adjusted to make the locks parallel.

8. The construction method of a foundation pit mixed support structure according to claim 5, characterized in that: In the step S3, in the anchor cable and steel waist beam construction steps of the Larsen pile construction, after laying out the line and determining the hole position, a special anchor drilling rig is used to align the hole position and adjust the angle, the drilling is performed using casing drilling, and the depth meets the requirements. The production and placement of the anchor cable, grouting, waist beam and anchor construction, tensioning and locking are all carried out in accordance with the corresponding specifications and design requirements. The anchor cable grouting adopts a two-time grouting process, the first normal pressure grouting, and the second high-pressure grouting is carried out after the initial setting of the first grouting body, and there are corresponding pressure and cement dosage control standards.

9. The construction method of a foundation pit mixed support structure according to claim 5, characterized in that: During the earthwork construction in step S3, the next excavation is carried out when the strength of the mixed support structure reaches 75%. The excavator is used for layered and segmented excavation. The excavation depth of each layer is not more than 2m. After the foundation pit is excavated, drainage ditches and sump wells are set up in time. The transport horseway is designed in a diagonal direction. It is strictly prohibited to pile up loads within 3m of the top line of the foundation pit slope and at the platform position, and it is strictly prohibited to overload outside 3m of the foundation pit.

10. The construction method of a foundation pit mixed support structure according to claim 5, characterized in that: In step S3, when the steel sheet pile is pulled out, the pile driver is first used to clamp the head of the steel sheet pile and vibrate it for 1-2 minutes to loosen the soil, and then the pile is pulled out by vibration. When it is difficult to pull out the pile, the prescribed operation is followed, and the pile hole is backfilled in time after the pile is pulled out. The pulling out is stopped and the pile is vibrated every time the pile is pulled out by 1m to fill the soil hole. After the pile is pulled out, the remaining gap is filled with medium-coarse sand.

Images