Construction method for foundation pit excavation of bridge foundation

Abstract

The application discloses a kind of bridge foundation foundation pit excavation construction methods, belong to bridge engineering construction technical field.The application obtains the layered distribution of soil layer and rock layer by geological survey and determines the gradient of benching respectively, uses total station and level to measure lofting, uses excavating equipment along excavation contour excavation, so that foundation pit side wall forms the ladder-like benching structure of the junction of soil layer gentle slope and rock layer steep slope, displacement monitoring is carried out to foundation pit slope during excavation and stops excavation reinforcement when over-limit, excavation face is set distance from base, stop mechanical excavation and use artificial base cleaning when, detect the bearing capacity of base, finally clean base and pour concrete cushion to form foundation bottom mould.The application realizes the consideration of foundation pit slope stability and excavation efficiency under soil-rock composite stratum condition, improves the safety and foundation construction quality of deep foundation pit excavation, and is suitable for bridge enlarged foundation deep foundation pit excavation construction.

Description

Technical Field

[0001] This invention relates to the field of bridge engineering construction technology, specifically to a construction method for excavating bridge foundation pits. Background Technology

[0002] As a crucial component of the bridge substructure, the quality and safety of the excavation of the bridge foundation pit directly affect the stability and service life of the overall bridge structure. In bridge foundation construction, slope excavation is commonly employed. This involves excavating the pit slope into a gradient based on the mechanical properties of the soil or rock mass, relying on the soil's own strength to maintain slope stability. For deep pits, slope excavation is widely used due to its simplicity, ease of construction, and lower cost, eliminating the need for complex support structures.

[0003] In soil-rock composite strata, the strata within the excavation area of ​​a foundation pit often contain both soil and rock layers, with significant differences in their mechanical properties. Soil layers have low shear strength and are easily softened by water, requiring gentler slopes to ensure slope stability. Rock layers, on the other hand, have high strength and good integrity, allowing for steeper slopes to reduce excavation work. Current technologies typically employ a uniform slope for foundation pit excavation, making it difficult to simultaneously ensure the stability of the soil slope and the excavation efficiency of the rock layer. This can easily lead to slope instability in the soil layer or unnecessary over-excavation in the rock layer.

[0004] Chinese invention patent application CN103898919A discloses a method for controlling slope ratio during deep foundation pit excavation. The technical solution involves using lime powder to mark the positions of the upper and lower edges of the foundation pit, employing an excavator for layered excavation, with each layer excavated to the depth determined by the deep foundation pit support design, and controlling the slope ratio to ensure the quality of slope formation. While this method proposes the basic idea of ​​layered excavation and slope ratio control, it does not address graded slope schemes for soil and rock layers under soil-rock composite strata conditions, nor does it propose technical means to smoothly transition between different slopes at the interface between soil and rock layers to avoid stress concentration at the interface.

[0005] In their paper "Optimization and Deformation Analysis of Excavation Procedures for Deep and Large Foundation Pits in Soil-Rock Composite Strata," published in the first issue of *Shandong Transportation Science and Technology* in 2026, Chen Diankai and Zhang Wanzhi used the foundation pit excavation at Qingdao Dongcheng Station as an engineering background. Employing a combination of numerical simulation and on-site monitoring, they compared different excavation procedures for deep and large foundation pits in soil-rock composite strata. They proposed a segmented, vertically layered, and progressively sloped excavation method to improve the safety and construction efficiency of foundation pit excavation in soil-rock composite strata. While the paper focuses on optimizing excavation procedures and analyzing deformation characteristics, it does not address the method for determining the graded slope of soil and rock sections driven by geological exploration parameters, nor does it propose a control strategy that links displacement monitoring and early warning with the comparison of numerical model prediction results during the excavation process.

[0006] Therefore, the existing technology lacks a construction method for bridge foundation pit excavation that can determine the slope gradient according to the differences in mechanical parameters of soil-rock composite strata, achieve a smooth transition between slope gradient and stepped slope structure to balance slope stability and excavation efficiency, and combine displacement monitoring and early warning for dynamic control. Summary of the Invention

[0007] The technical problem to be solved by this invention is that in the prior art, the slope excavation of foundation pits under the condition of soil-rock composite strata adopts a uniform slope, which makes it difficult to simultaneously take into account the stability of the soil slope and the excavation efficiency of the rock section, and is prone to causing the soil slope to become unstable or the rock section to be unnecessarily over-excavated; at the same time, the existing slope excavation method lacks dynamic monitoring and early warning during the excavation process and graded slope design driven by geological parameters, and cannot adapt to the complex conditions of soil-rock composite strata.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0009] A construction method for bridge foundation pit excavation includes: conducting geological surveys of the strata within the excavation area to obtain the layered distribution of soil and rock layers, and determining the slope gradients for the soil and rock layers respectively; using a total station to lay out the foundation axis, using a level to measure the ground elevation of the foundation location to determine the excavation depth, and widening the excavation width by a predetermined distance on each side of the foundation according to the foundation design dimensions, marking the excavation outline with lime lines; using excavation equipment to excavate along the excavation outline, with the soil layer excavated according to the soil layer slope gradient, and the rock layer excavated according to the rock layer slope gradient, forming a stepped slope structure on the pit sidewall where the gentle slope of the soil layer connects to the steep slope of the rock layer; when the excavation depth exceeds the predetermined depth, covering the slope surface with protective netting, and setting up a safety barrier on the wider side of the pit. A walkway for personnel and machinery is provided, with guardrails erected on both sides. During excavation, the horizontal and vertical displacements of the pit slope are continuously monitored, and cumulative and daily early warning values ​​for displacement are set. When the monitored displacement exceeds the early warning value, excavation is stopped and slope reinforcement measures are taken. When the excavation surface is a set distance from the bottom of the pit, mechanical excavation is stopped, and manual excavation is used to clear the foundation to the design elevation in one go. The bearing capacity of the foundation is tested. If the test results do not meet the design requirements, excavation continues downward. If the test results still do not meet the design requirements after continued excavation, construction is stopped, and the test results are informed to the design unit. After removing accumulated water, debris, and loose soil from the pit and making the foundation clean and flat, a concrete cushion layer is poured to form the foundation bottom formwork. Foundation reinforcement is tied to the foundation bottom formwork, and after the formwork is installed, foundation concrete is poured.

[0010] Preferably, a segmented, vertically layered, and progressively sloped excavation method is adopted. The foundation pit is divided longitudinally into several excavation segments, and each excavation segment is further divided vertically into several excavation layers. Excavation is carried out segment by segment and layer by layer alternately. After each segment is excavated, the slope surface of that segment is sealed and slope protection measures are implemented before moving on to the next segment. Before excavation, a numerical calculation model is established for the entire foundation pit excavation process to simulate the distribution of ground settlement and lateral displacement of the slope near the foundation pit under different excavation conditions. Based on this, the excavation sequence of each segment, the excavation thickness of each layer, and the slope are determined. The following measures were implemented: Displacement monitoring points were set up at the top of the foundation pit slope and on each level of the slope platform. The on-site monitoring data was compared and analyzed with the prediction results of the numerical calculation model. When the deviation between the on-site monitoring data and the prediction results exceeded the set range, the excavation parameters for subsequent sections were adjusted. Anti-sliding reinforcement measures were taken at the toe of the foundation pit slope, and the groundwater level was dynamically monitored. When the observed water level rose to a level that affected the stability of the slope, dewatering measures were taken to lower the groundwater level. Excavation continued only after the groundwater level had decreased. At the junction of the soil layer and the rock layer, the slope gradient of the soil layer was smoothly transitioned to that of the rock layer.

[0011] Preferably, the foundation formwork is installed after the base treatment is completed. During installation, the formwork is assembled tightly against the side of the foundation, and the joints of the formwork are sealed to prevent grout leakage. A first layer of support members is set on the outside of the formwork at a set interval, and a second layer of support members is set on the outside of the first layer of support members at a set interval, so that the formwork and each layer of support members form a layered support from the outside to the inside. Tie connectors are used to tie and fix the support system on opposite sides. After the formwork is installed, the position of the formwork is measured, corrected and fixed with the center point of the designed pile as the reference point. A release agent is applied to the surface of the formwork. Before pouring concrete, the stability of the support and formwork is checked. If loosening, deformation or displacement is found, it is dealt with in time. Vertical formwork that does not support concrete is removed after pouring for no less than a set time or when the concrete strength is not lower than a set value. The surface of the formwork to be reused is cleaned and repaired, and cracked, worn or delaminated formwork surface materials are not used.

[0012] Preferably, after the formwork is installed, the foundation concrete is poured. Before pouring the foundation concrete, the mix proportion is optimized, low heat of hydration cement is selected and mineral admixtures are added, and the setting time and workability of the concrete are adjusted by admixtures. During pouring, the foundation is divided into several pouring layers vertically, and each pouring layer is poured continuously in sequence and at set intervals. The thickness of each pouring layer and the pouring interval between adjacent pouring layers are controlled so that the previous layer of concrete covers the upper layer of concrete before the initial setting of the previous layer. The internal temperature, surface temperature and ambient temperature of the concrete are measured in real time, the temperature difference between the inside and outside of the concrete and the rate of temperature rise are calculated, and a temperature difference control threshold is set. When the concrete reaches its initial setting... When the temperature difference between the inside and outside of the soil reaches the threshold, the thickness of the insulation covering layer on the concrete surface is increased. When the temperature difference drops back to a safe range, the insulation covering layer is gradually reduced. The concrete surface is covered with insulation and moisture-retaining materials. The number of covering layers and the frequency of watering are dynamically adjusted according to changes in ambient temperature and concrete temperature measurement results. When the concrete temperature peak occurs and the cooling stage begins, a zoned formwork removal method is adopted. The formwork in the area with lower temperature stress is removed first, and the removed area is immediately covered with insulation. Then, the formwork in the remaining areas is removed in sequence. The entire curing process is recorded and managed, including the covering material, covering time, watering frequency, formwork removal time, and temperature measurement data.

[0013] Preferably, after the concrete is poured and the surface is sealed, a layer of plastic film is first covered. After the concrete has initially set, the plastic film is removed and geotextile is covered and water is sprinkled for moisture retention and curing. During the curing period, the temperature is measured regularly, and the curing time is not less than the set number of days. Temperature measuring points are arranged at a set interval in the foundation plane, and the temperature measuring points are set at a set distance from the edge of the foundation. A manual thermometer is used to measure the temperature regularly, and the temperature difference between the inside and outside of the concrete is monitored based on the temperature measurement results, and the curing measures are adjusted accordingly.

[0014] Preferably, before excavation, a hydrogeological survey of the site should be conducted to determine the distribution of aquifers, the height of confined water head, and the seepage path. Water interception measures should be implemented around the excavation pit, extending into the impermeable layer below the pit's bottom. Drainage and collection measures should be installed within the pit to collect and discharge seepage water. If a confined water layer exists at the bottom of the pit, pressure reduction and dewatering measures should be implemented to lower the confined water head to a safe height below the pit's bottom before continuing excavation. During excavation, the bottom and sidewalls of the pit should be inspected for leaks. If localized leaks are found, the seepage water should be guided to a collection area, and the area around the leak point should be backfilled and sealed with filter material. In the event of sudden piping or gushing, excavation should be stopped, and a filter weight should be piled around the gushing point to increase drainage capacity. After the gushing water stabilizes, underwater concrete or grouting should be used to seal the gushing water channel.

[0015] Preferably, the foundation reinforcement is lapped and tied, the splicing of reinforcement in any section does not exceed the set ratio, the clear distance between reinforcements in one layer is not less than a set multiple of the nominal diameter of the reinforcement and not less than a set multiple of the maximum size of the coarse aggregate, and the clear distance between layers is not less than the set value; the spacing between reinforcements and formwork is controlled by setting protective layer control components, the protective layer control components are staggered and tied firmly to the reinforcement, and the tying wire ends do not enter the concrete protective layer; the pre-embedded reinforcement of the pier body extends into the foundation, and the exposed ends of adjacent pre-embedded reinforcements are staggered.

[0016] Preferably, after the concrete has been cured, the contact area between the foundation and the soil is waterproofed, a waterproof membrane is laid on the base before the foundation is poured, and emulsified waterproof asphalt is applied to the exposed surface of the foundation after the foundation concrete has been poured.

[0017] Preferably, after the foundation concrete has been cured to the set strength, the foundation pit is backfilled according to the design requirements. The backfill soil is compacted in layers by manual labor or small machinery, the thickness of each layer is controlled, the backfill compaction degree is not lower than the set value, the filling height is higher than the set distance of the foundation, and the compaction is carried out to ensure that it does not affect the foundation structure.

[0018] Preferably, the monitoring frequency of the foundation pit is dynamically adjusted according to the construction stage. During the excavation of the foundation pit, the monitoring frequency is higher, and after the foundation is poured, the monitoring frequency is set. The monitoring frequency is gradually reduced as the age increases until the foundation pit is backfilled and passes the acceptance test.

[0019] The present invention has the following beneficial effects:

[0020] First, by obtaining the layered distribution of the soil-rock composite strata through geological exploration, the slope of the soil layer and the rock layer is determined respectively, so that the sidewall of the foundation pit forms a stepped slope structure in which the gentle slope of the soil layer connects with the steep slope of the rock layer. This not only ensures the stability of the slope of the soil layer, but also reduces the amount of excavation work in the rock layer, thus achieving a balance between slope stability and excavation efficiency.

[0021] Secondly, during the excavation process, the horizontal and vertical displacements of the foundation pit slopes were continuously monitored. Cumulative and daily displacement warning values ​​were set. When the monitored displacement exceeded the warning value, the excavation was stopped and reinforcement measures were taken, which effectively reduced the risk of slope instability in the dangerous foundation pit project.

[0022] Third, mechanical excavation was stopped and manual excavation was carried out when the excavation surface was a set distance from the bottom of the foundation pit. This avoided disturbing the foundation with mechanical excavation. The foundation quality was ensured through foundation bearing capacity testing and a step-by-step handling process for unqualified foundations, thus providing reliable foundation conditions for subsequent foundation construction.

[0023] Fourth, a segmented and layered vertical slope excavation method was adopted, and the excavation parameters were dynamically adjusted by combining numerical calculation model prediction with on-site monitoring data. This optimized the excavation process in the soil-rock composite strata, further improving the safety and construction efficiency of the foundation pit excavation. At the junction of the soil and rock layers, different slopes were smoothly connected to avoid stress concentration at the interface.

[0024] Fifth, the formwork installation adopts a layered support structure formed by the formwork and multi-layer support components, combined with measurement correction and tie fixing, which improves the rigidity and stability of the formwork system and ensures the quality of foundation concrete molding.

[0025] Sixth, the large-volume concrete pouring adopts a temperature control scheme that combines optimized mix proportions, layered continuous pouring, real-time temperature measurement, temperature difference threshold control, dynamic adjustment of the insulation covering layer, and zoned formwork removal. This effectively prevents the generation of temperature cracks, and the record management of the entire curing process forms a traceable temperature control archive, which improves the construction quality of the concrete foundation.

[0026] Seventh, through water interception measures, drainage and water collection measures, pressure reduction and dewatering, reverse filtration and sealing, and sudden surge sealing, the water hazard problems such as groundwater seepage, sudden surge of the foundation and slope instability during the foundation pit excavation process were systematically solved, ensuring the safety of foundation pit construction. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to specific embodiments. This embodiment takes the excavation of a bridge foundation pit as an application scenario. The strata in the construction area are a composite soil-rock stratum with an upper soil layer and an underlying rock layer. The excavation depth of the pit is determined according to the design. For example, in this embodiment, the excavation depth of the pit is 3 to 5 meters. The construction area is located in a high-temperature desert climate environment with high summer temperatures and low rainfall. For example, the annual rainfall in the area where this embodiment is located is only 81.3 mm. All values ​​recorded in the following embodiments are exemplary values ​​and are only used to illustrate the implementation process of the present invention. They are not intended to limit the present invention. In actual construction, they should be reasonably determined based on engineering design documents, geological survey data, and on-site construction conditions.

[0028] Before the excavation of the foundation pit begins, a geological survey is conducted on the strata within the excavation area. This survey combines drilling sampling with laboratory geotechnical testing. Drilling is carried out along the foundation axis and around the perimeter of the foundation pit, with the drilling depth exceeding the designed excavation depth, to determine the strata distribution within the excavation area. Laboratory geotechnical tests are used to determine the unit weight, cohesion, and internal friction angle of each soil layer. For example, in this embodiment, the unit weight of weathered limestone is 24 kN / m³ to 25 kN / m³, the cohesion is 5750 kPa to 7230 kPa, and the internal friction angle is 40 degrees to 42 degrees. Based on the survey results, the layered distribution of soil and rock layers is obtained, and the interface location and depth between soil and rock layers are determined. Based on the differences in geotechnical parameters between the soil and rock layers, the slope gradients for each layer are determined separately. The soil layer uses a gentler slope, while the rock layer uses a steeper slope. For example, in this embodiment, the soil layer uses a 1:1 slope, and the rock layer uses a 1:0.1 slope. These values ​​are exemplary; the actual slope gradient should be determined based on slope stability calculations. The slope gradient is determined according to slope stability calculations, which consider soil unit weight, cohesion, internal friction angle, and potential slope loads to ensure that the safety factor of each slope layer meets relevant specifications. Near the interface between the soil and rock layers, the slope gradients are smoothly transitioned to avoid abrupt slope changes and stress concentrations at the interface.

[0029] After the geological survey is completed, surveying and setting out are carried out. Surveying and setting out are conducted using a total station and a level. The total station is used to set out the foundation's axis, determining its accurate position on the plane. The level is used to measure the ground elevation of the foundation's location. Combined with the designed foundation bottom elevation, the excavation depth is calculated. The excavation width is determined by widening each side of the foundation according to the design dimensions. This widening provides working space for formwork installation and on-site workers. The widening distance is chosen to ensure sufficient working space for construction needs; for example, in this embodiment, each side is widened by 1.5 meters. After determining the excavation width, the excavation outline is marked with lime lines as the working boundary for mechanical excavation. After the surveying and setting out are completed, the results are checked to ensure the accuracy of the axis position and elevation data.

[0030] After surveying and setting out, the foundation pit is excavated in stages with sloping. Excavation equipment is used along the excavation outline. When the rock layer in the stratum is hard, excavation equipment with crushing capabilities is selected to improve crushing efficiency. During excavation, soil sections are excavated according to their respective slope gradients, and rock sections are excavated according to their respective slope gradients, creating a stepped slope structure on the foundation pit sidewalls where gentle slopes of the soil layer connect with steep slopes of the rock layer. At the interface between the soil and rock layers, the slope gradients of the soil and rock layers are smoothly transitioned to form a continuous slope, avoiding stress concentration at the interface. When the foundation pit excavation depth is large, a protective net is covered on the slope surface. The depth of the protective net is determined based on the slope height and the degree of looseness. For example, in this embodiment, when the foundation pit excavation depth exceeds 3 meters, a protective net is covered on the slope surface to prevent loose soil or gravel from rolling down and injuring people. A walkway is constructed on the wider side of the excavation pit for personnel and machinery to pass through. The width and slope of the walkway are designed to ensure safe passage for excavating equipment and workers. For example, in this embodiment, the walkway is 3 meters wide and has a slope of 30%. Guardrails are erected on both sides of the walkway to ensure safe passage. The walkway extends synchronously with the increase in excavation depth and is sloped synchronously on both sides with the same slope as the elevation decreases, ensuring that personnel and machinery can safely enter and exit the excavation pit during construction.

[0031] For foundation pits with significant excavation depth, a segmented longitudinal and layered vertical excavation method with progressive sloping is adopted. The foundation pit is divided longitudinally into several excavation segments, and each segment is further divided vertically into several excavation layers. Excavation is carried out segment by segment and layer by layer alternately. After each segment is excavated, the slope surface of that segment is sealed. Only after the slope protection measures for that segment are in place can the next segment be excavated, thus avoiding prolonged exposure of the foundation pit slope. Before excavation, a numerical calculation model is established for the entire foundation pit excavation process to simulate the distribution of ground settlement and lateral displacement of the slope near the foundation pit under different excavation procedures. Based on the numerical simulation results, the excavation sequence of each segment, the excavation thickness of each layer, and the slope gradient are determined, ensuring that the arrangement of excavation procedures guarantees slope stability while improving construction efficiency.

[0032] During the excavation of the foundation pit, the horizontal and vertical displacements of the pit slope are continuously monitored. Displacement monitoring points are set at the top of the pit slope and on each level of the slope protection platform. The monitoring points are arranged at predetermined intervals along the perimeter of the pit, with the spacing ensuring that the monitoring data reflects the overall deformation of the slope. Measuring instruments are used to periodically observe the monitoring points and record the horizontal and vertical displacement data of the slope. Cumulative and daily displacement warning values ​​are set. The warning values ​​are determined based on the slope stability and the engineering risk level. For example, in this embodiment, the cumulative displacement warning value is set to 20 mm and the daily displacement warning value is set to 3 mm. When the monitored displacement exceeds the warning value, excavation is immediately stopped, the cause of the displacement exceeding the limit is analyzed, and slope reinforcement measures are taken. Construction resumes only after the slope stabilizes and the monitoring data returns to normal. The monitoring frequency of the foundation pit is dynamically adjusted according to the construction stage. During the excavation period, monitoring is conducted at a higher frequency, such as once a day. After the foundation is poured, monitoring is conducted at the set frequency, gradually decreasing as the concrete ages. For example, monitoring is conducted every 2 days for the first 7 days after pouring, every 3 days for 7 to 14 days, every 7 days for 14 to 28 days, and every half month after 28 days, until the foundation pit is backfilled and passes inspection. During the monitoring process, the monitoring data is recorded and organized to form monitoring data, which is used to analyze the deformation development trend of the foundation pit slope.

[0033] When the excavation of the foundation pit reaches near the design foundation elevation, mechanical excavation is stopped when the excavation surface is a predetermined distance from the bottom of the pit. Manual excavation is then employed to clear the foundation to the design elevation in one go. The transition distance between mechanical excavation and manual clearing is determined to ensure that the machinery does not disturb the foundation soil. For example, in this embodiment, mechanical excavation is stopped and manual clearing is used when the excavation surface is 20 cm from the bottom of the pit. Manual clearing avoids disturbance to the foundation soil caused by mechanical excavation, protecting the original structure of the foundation. After clearing is completed, the bearing capacity of the foundation is tested. The testing process is witnessed and signed off by the supervision unit. If the test results do not meet the design requirements, excavation continues until the bearing capacity meets the requirements. If continued excavation still does not meet the design requirements, construction is stopped, and the design unit is informed of the situation and the matter is handled according to the design unit's instructions.

[0034] After the foundation bearing capacity test is passed, remove accumulated water, debris, and loose soil from the foundation pit to ensure a clean and level foundation. After cleaning, pour a concrete cushion layer to form the foundation formwork. The thickness and concrete strength grade of the cushion layer are determined according to design requirements; for example, in this embodiment, a 10 cm thick C16 concrete cushion layer is poured. The cushion layer provides a flat working surface for subsequent reinforcement binding and formwork installation. After pouring, the cushion layer is cured. Once it reaches a certain strength, foundation reinforcement is bound to the foundation formwork, and the formwork is installed. Subsequently, foundation concrete is poured. The strength grade of the foundation concrete is determined according to design requirements; for example, in this embodiment, the cushion layer concrete strength grade is C16, and the foundation concrete strength grade is C30.

[0035] The foundation reinforcement is centrally processed and shaped at the reinforcement processing yard. After inspection and approval, the processed reinforcement is transported to the foundation pit for binding. The foundation reinforcement uses a lap-binding method, and the splicing ratio of reinforcement at any cross-section does not exceed a set proportion to avoid excessive reinforcement joints at the same cross-section weakening the structural load-bearing capacity. For example, in this embodiment, the splicing ratio of reinforcement at any cross-section does not exceed 50%. The clear distance between reinforcements within a layer is not less than a set multiple of the nominal diameter of the reinforcement, and also not less than a set multiple of the maximum size of the coarse aggregate, to ensure that the concrete can smoothly fill the gaps between the reinforcements. For example, in this embodiment, the clear distance is not less than 1.5 times the nominal diameter of the reinforcement and not less than 1.5 times the maximum size of the coarse aggregate. The clear distance between layers is not less than a set value; for example, in this embodiment, the clear distance between layers is not less than 25 mm or one reinforcement diameter. The spacing between the reinforcing bars and the formwork is controlled by setting protective layer control components. These components are staggered and securely tied to the reinforcing bars, ensuring the tying wires do not penetrate the concrete protective layer. This ensures the protective layer thickness meets design requirements. The density of the protective layer control components is designed to ensure uniform protective layer thickness; for example, in this embodiment, they are spaced approximately 1 meter apart. The pre-embedded reinforcing bars in the pier extend into the foundation at the designed locations, with pre-embedded lengths ranging from 0.86 meters to 1.945 meters. The exposed ends of adjacent pre-embedded reinforcing bars are staggered, for example, by 1 meter, to ensure reliable connection between the pier and the foundation.

[0036] After the base treatment is completed, the foundation formwork is installed. During installation, the formwork is assembled tightly against the sides of the foundation. The formwork thickness is determined according to the foundation dimensions and lateral pressure; for example, in this embodiment, a 15mm thick formwork is used. Double-sided tape is used to seal the formwork joints to prevent grout leakage. A first layer of support members is installed on the outside of the formwork at predetermined intervals. In this embodiment, the first layer of support members uses I-beams, spaced at 30cm intervals. A second layer of support members is installed on the outside of the first layer of support members at predetermined intervals. In this embodiment, the second layer of support members uses double-channel steel back ribs, spaced at intervals not exceeding 1000mm. This creates a layered support structure from the outside in, with each layer of support members sharing the lateral pressure during concrete pouring. Tie-couplers are used to connect and fix the support systems on opposite sides. In this embodiment, 22mm diameter round steel is used for the tie-couplers, spaced at intervals not exceeding 1212mm, ensuring that the forces on opposite sides of the formwork are balanced and guaranteeing the overall stability of the formwork system. After the formwork is installed, the position of the formwork is measured, corrected, and fixed using the center point of the designed pile as the reference point to ensure the accurate position of the top plane of the formwork. A release agent is applied to the surface of the formwork to facilitate subsequent demolding. Before pouring concrete, the stability of the supports and formwork is checked, and any loosening, deformation, or displacement is addressed promptly. Vertical formwork not supporting concrete is removed at least after a set time or when the concrete strength is not lower than a set value. For example, in this embodiment, it is removed at least 24 hours after pouring or when the concrete strength is not lower than 2.5 MPa, to avoid premature demolding and damage to the concrete. The surface of the formwork to be reused is cleaned and repaired; cracked, worn, or delaminated formwork surface materials are not used to ensure the appearance quality of the concrete.

[0037] After the template is installed and accepted, the foundation concrete is poured. Before pouring the foundation concrete, the mix proportions are optimized, using low-heat-of-hydration cement and adding mineral admixtures to reduce the total heat of hydration. Admixtures are used to adjust the concrete's setting time and workability, ensuring good workability during transportation and pouring. The pouring height is controlled; if the height difference is too large, a tremie pipe is used. For example, in this embodiment, the height difference should not exceed 1.5 meters; if it exceeds 1.5 meters, a tremie pipe is used. The foundation is vertically divided into several pouring layers, each layer's thickness meeting the compaction requirements. For example, in this embodiment, each layer's thickness should not exceed 30 centimeters. Each pouring layer is poured sequentially and continuously according to a set order and interval, ensuring the previous layer covers the next layer before initial setting, preventing cold joints. The total time from mixing to compaction of each batch of concrete is controlled to ensure the concrete's workability does not deteriorate due to excessive time; for example, in this embodiment, it should not exceed 180 minutes. The concrete pouring temperature is controlled; for example, in this embodiment, the pouring temperature does not exceed 30 degrees Celsius. During pouring, the internal and surface temperatures of the concrete, as well as the ambient temperature, are measured in real time to calculate the temperature difference between the inside and outside of the concrete and the rate of temperature rise. A temperature difference control threshold is set. When the temperature difference between the inside and outside of the concrete approaches the threshold, the thickness of the insulation layer on the concrete surface is increased to slow down surface heat dissipation and reduce the temperature difference. Once the temperature difference returns to a safe range, the insulation layer is gradually reduced to prevent excessively rapid cooling and resulting temperature stress. The concrete surface is covered with thermal insulation and moisture-retaining material. The amount of insulation and the frequency of water spraying are dynamically adjusted based on changes in ambient temperature and concrete temperature measurements to ensure a slow and uniform change in the concrete surface temperature.

[0038] When the concrete enters the cooling phase after the temperature peak, a zoned formwork removal method is adopted. First, the formwork in areas with lower temperature stress is removed, and these areas are immediately insulated and covered. Then, the formwork in the remaining areas is removed sequentially to avoid sudden temperature drops during formwork removal that could cause concrete cracking. The entire curing process is recorded and managed, including the covering material, covering time, watering frequency, formwork removal time, and temperature measurement data, creating a traceable temperature control record to provide a basis for construction quality assessment.

[0039] After the concrete is poured and the surface is sealed, a layer of plastic film is first applied to prevent the surface moisture from evaporating too quickly. Once the concrete has initially set, the plastic film is removed, and geotextile is applied and watered to maintain moisture. The concrete surface is kept moist, and the temperature is measured regularly during the curing period. The curing time is determined by ensuring the normal development of the concrete strength; for example, in this embodiment, the temperature is measured every two hours, and the curing time is no less than 21 days. Temperature measuring points are arranged at predetermined intervals within the foundation plane, with a set distance from the foundation edge. For example, in this embodiment, the measuring points are arranged at 3-meter intervals and 1.5 meters from the foundation edge. A manual thermometer is used to measure the temperature regularly. Based on the measurement results, the temperature difference between the inside and outside of the concrete is monitored. When the temperature difference exceeds the set range, the insulation covering measures and watering frequency are adjusted to ensure that the temperature difference between the inside and outside of the concrete is always controlled within the allowable range.

[0040] Before excavation of the foundation pit, a hydrogeological survey of the site is conducted to determine the distribution of aquifers, the height of the confined aquifer head, and the seepage path, providing a basis for the formulation of water hazard prevention measures. Water interception measures are implemented around the foundation pit, extending into the impermeable layer below the bottom of the pit to block the recharge channels of groundwater from outside the pit into the pit. Drainage and water collection measures are installed within the foundation pit to collect and discharge seepage water outside the pit. The drainage capacity is designed to ensure a dry working environment within the pit; for example, in this embodiment, the drainage capacity is not less than 1.5 to 2.0 times the total seepage volume. When a confined aquifer exists at the bottom of the foundation pit, depressurization and dewatering measures are implemented to lower the height of the confined aquifer head to a safe height below the bottom of the pit before continuing excavation to prevent sudden heave at the foundation. During excavation, the bottom and sidewalls of the foundation pit are inspected for leaks. When local leaks are found, the seepage water is guided to the catchment area, and the area around the leak is backfilled and sealed with filter material to prevent soil particles from being carried out by the seepage water, causing soil erosion and slope instability. In the event of sudden piping or gushing, excavation is stopped immediately, and a filter weight is piled up around the gushing point to increase drainage capacity. After the gushing water stabilizes, the gushing water channel is sealed with underwater concrete or grouting. After sealing is completed and inspection confirms that there is no leakage, subsequent construction can proceed.

[0041] After the concrete has cured, the areas where the foundation contacts the soil are waterproofed. Before pouring the foundation pad, a waterproof membrane is laid on the base to block the upward seepage of groundwater. After the foundation concrete is poured, emulsified waterproof asphalt is applied to the exposed surface of the foundation. The number of layers is determined according to the waterproofing grade requirements. For example, in this embodiment, two layers of emulsified waterproof asphalt are applied to form a waterproof protective layer, preventing moisture and corrosive substances in the soil from eroding the foundation concrete and improving the durability of the foundation structure.

[0042] After the foundation concrete has cured to the set strength, the foundation pit is backfilled according to the design requirements. For example, in this embodiment, backfilling is carried out after curing to 80% of the design strength. The backfill soil is compacted in layers manually or with small machinery. The thickness of each layer is determined to ensure compaction effectiveness; for example, in this embodiment, the thickness of each layer does not exceed 25 cm. The compaction degree is not lower than the set value, for example, not lower than 95%, ensuring the density and bearing capacity of the backfill soil. The backfill height is higher than the set distance of the foundation, for example, at least 50 cm higher than the foundation in this embodiment, to provide conditions for subsequent superstructure construction. During compaction, care is taken to ensure that it does not affect the foundation structure, avoiding collisions between compaction machinery and the foundation or excessive lateral pressure on the foundation. After backfilling is completed, the area around the foundation pit is leveled to restore the original terrain.

[0043] The aforementioned construction steps are carried out sequentially: geological investigation, surveying and setting out, graded slope excavation, slope monitoring, manual foundation clearing, foundation treatment, rebar tying, formwork installation, concrete pouring, curing and temperature monitoring, waterproofing, and foundation pit backfilling. Each step is closely integrated with the others. Slope displacement monitoring is conducted throughout the entire process from excavation to backfilling, with the monitoring frequency dynamically adjusted according to the construction stage. Water hazard prevention measures are implemented before excavation and continuously monitored during the excavation process. Concrete temperature control begins with mix design optimization and continues throughout the pouring and curing processes. The technical parameters for each step are determined based on design documents and site conditions, and are dynamically adjusted during construction based on monitoring data and actual conditions to ensure the safety and quality of foundation pit excavation and foundation construction.

[0044] The parts not described in detail in this embodiment are well-known in the art. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A construction method for excavating a bridge foundation pit, characterized in that, Includes the following steps: A geological survey was conducted on the strata within the excavation area of ​​the foundation pit to obtain the layered distribution of soil and rock layers, and the slope of the soil and rock layers was determined respectively. The axis of the foundation is laid out using a total station, the ground elevation of the foundation location is measured using a level to determine the excavation depth of the foundation pit, and the excavation width is determined by widening each side of the foundation by a set distance according to the foundation design dimensions. The excavation outline is marked with lime lines. Excavation equipment is used to excavate along the excavation outline. The soil section is excavated according to the slope of the soil section, and the rock section is excavated according to the slope of the rock section, so that the side wall of the foundation pit forms a stepped slope structure that connects the gentle slope of the soil layer and the steep slope of the rock layer. When the excavation depth of the foundation pit exceeds the set depth, the slope surface is covered with protective netting. A horse walkway for personnel and machinery to pass through is set up on the wider side of the foundation pit, and guardrails are erected on both sides of the horse walkway. During the excavation process, the horizontal and vertical displacements of the foundation pit slope are continuously monitored. Accumulated displacement warning values ​​and daily displacement warning values ​​are set. When the monitored displacement exceeds the warning value, the excavation is stopped and the slope is reinforced. When the excavation surface is at a set distance from the bottom of the foundation pit, mechanical excavation is stopped, and manual excavation is used to clear the foundation to the design elevation in one go; the bearing capacity of the foundation is tested, and if the test results do not meet the design requirements, excavation continues downward. If the test results still do not meet the design requirements after continued excavation, construction is stopped, and the test results are informed to the design unit. After removing accumulated water, debris, and loose soil from the foundation pit to make the base clean and level, a concrete cushion layer is poured to form the foundation bottom formwork. Foundation reinforcement is tied to the foundation bottom formwork and the formwork is installed before pouring foundation concrete.

2. The construction method for bridge foundation pit excavation as described in claim 1, characterized in that, The excavation method adopts a longitudinal segmented and vertically layered approach, dividing the foundation pit into several excavation segments along the longitudinal direction, and each excavation segment into several excavation layers along the vertical direction. The excavation is carried out alternately segment by segment and layer by layer. After each segment is excavated, the slope surface of the segment is sealed and the slope protection measures for the segment are set up before moving on to the next segment. Before excavation, a numerical calculation model was established for the entire process of foundation pit excavation to simulate the distribution of surface settlement and lateral displacement of slope near the foundation pit under different excavation procedures. Based on this, the excavation sequence of each section, the excavation thickness of each layer, and the slope gradient were determined. Displacement monitoring points are set at the top of the foundation pit slope and on each level of the slope platform. The on-site monitoring data is compared and analyzed with the prediction results of the numerical calculation model. When the deviation between the on-site monitoring data and the prediction results exceeds the set range, the excavation parameters of the subsequent sections are adjusted. Anti-sliding reinforcement measures were taken at the toe of the foundation pit slope, and the groundwater level was dynamically monitored. When the monitored water level rose to a level that affected the stability of the slope, dewatering measures were taken to lower the groundwater level. Excavation was continued only after the groundwater level had decreased. At the junction of the soil and rock layers, the slope of the soil layer is smoothly transitioned to the slope of the rock layer.

3. The construction method for bridge foundation pit excavation as described in claim 1 or 2, characterized in that, After the base treatment is completed, the base formwork is installed. During installation, the formwork is assembled tightly against the side of the base, and the joints of the formwork are sealed to prevent grout leakage. A first layer of support members is set on the outside of the template at a set interval, and a second layer of support members is set on the outside of the first layer of support members at a set interval, so that the template and each layer of support members form a layered support from the outside to the inside. The support system on the opposite sides is tied and fixed by tie rod connectors. After the template is installed, the position of the template is measured, corrected and fixed using the center point of the designed pile as the reference point; Apply release agent to the formwork surface. Before pouring concrete, check the stability of the support and formwork. If any loosening, deformation or displacement is found, deal with it in time. Vertical formwork that is not supported by concrete should be removed after pouring for no less than a set time or when the concrete strength is not lower than a set value. The template surface should be cleaned and repaired before reuse, and cracked, worn, or delaminated template surface materials should not be used.

4. The construction method for bridge foundation pit excavation as described in claim 3, characterized in that, After the template is installed, the foundation concrete is poured. Before pouring the foundation concrete, the mix proportion is optimized, low heat of hydration cement is selected and mineral admixtures are added, and the setting time and workability of the concrete are adjusted by admixtures. During pouring, the foundation is divided into several pouring layers vertically. Each pouring layer is poured continuously in sequence according to the set order and interval. The thickness of each pouring layer and the pouring interval between adjacent pouring layers are controlled so that the previous layer of concrete can cover the upper layer of concrete before the initial setting of the previous layer. Real-time measurement of concrete internal temperature, surface temperature and ambient temperature; calculation of temperature difference and temperature rise rate between the inside and outside of concrete; setting of temperature difference control threshold; when the temperature difference between the inside and outside of concrete reaches the threshold, increase the thickness of the insulation covering layer on the concrete surface; when the temperature difference falls back to a safe range, gradually reduce the insulation covering layer. The concrete surface is covered with thermal insulation and moisture-retaining materials, and the number of covering layers and the frequency of watering are dynamically adjusted according to changes in ambient temperature and concrete temperature measurement results. When the concrete enters the cooling stage after the temperature peak, a zoned formwork removal method is adopted. First, the formwork in the area with less temperature stress is removed, and the removed area is immediately covered with insulation. Then, the formwork in the remaining areas is removed in sequence. The entire curing process is recorded and managed, including the covering material, covering time, watering frequency, formwork removal time, and temperature measurement data.

5. The construction method for bridge foundation pit excavation as described in claim 4, characterized in that, After the concrete is poured and the surface is sealed, first cover it with a layer of plastic film. After the concrete has initially set, remove the plastic film and cover it with geotextile and sprinkle water to keep it moist and curing. During the curing period, the temperature should be measured regularly and the curing time should not be less than the set number of days. Temperature measurement points are arranged at a set interval within the foundation plane, with a set distance between the measurement points and the edge of the foundation. A manual thermometer is used to measure the temperature at regular intervals. Based on the temperature measurement results, the temperature difference between the inside and outside of the concrete is monitored and the curing measures are adjusted accordingly.

6. The construction method for bridge foundation pit excavation as described in claim 1 or 2, characterized in that, Before excavating the foundation pit, the hydrogeological conditions of the site should be investigated to determine the distribution of aquifers, the height of confined water head, and the seepage path. Water interception measures are taken around the foundation pit, extending into the waterproof layer below the bottom of the foundation pit. Drainage and water collection measures are set up inside the foundation pit to collect the seepage water and discharge it outside the pit. When there is a confined aquifer at the bottom of the foundation pit, pressure reduction and dewatering measures are taken to lower the confined aquifer head height. Excavation can continue after the confined aquifer head height is reduced to a safe height below the bottom of the foundation pit. During the excavation process, the bottom and side walls of the foundation pit are inspected for leakage. When a local leakage point is found, the seepage water is guided to the water collection area and the area around the leakage point is backfilled and sealed with filter material. When a sudden piping or gushing occurs, excavation should be stopped and a filter press should be piled up around the gushing point to increase drainage capacity. After the gushing water stabilizes, the gushing water channel should be sealed with underwater concrete or grouting.

7. The construction method for bridge foundation pit excavation as described in claim 3, characterized in that, The basic reinforcement is lapped and tied. The splicing of reinforcement in any section shall not exceed the set ratio. The clear distance between reinforcements in one layer shall not be less than the set multiple of the nominal diameter of the reinforcement and not less than the set multiple of the maximum size of the coarse aggregate. The clear distance between layers shall not be less than the set value. The spacing between the reinforcing bars and the formwork is controlled by setting protective layer control components. The protective layer control components are staggered and firmly tied to the reinforcing bars, and the tying wire ends do not enter the concrete protective layer. The pre-embedded steel bars in the pier body extend into the foundation, and the exposed ends of adjacent pre-embedded steel bars are staggered.

8. The construction method for bridge foundation pit excavation as described in claim 5, characterized in that, After the concrete has cured, the contact area between the foundation and the soil is waterproofed. Before pouring the foundation pad, a waterproof membrane is laid on the base. After the foundation concrete is poured, emulsified waterproof asphalt is applied to the exposed surface of the foundation.

9. The construction method for bridge foundation pit excavation as described in claim 8, characterized in that, After the foundation concrete has been cured to the set strength, the foundation pit is backfilled according to the design requirements. The backfill soil is compacted in layers by manual labor or small machinery, the thickness of each layer is controlled, the compaction degree of the backfill is not lower than the set value, the filling height is higher than the set distance of the foundation, and the compaction is carried out to ensure that it does not affect the foundation structure.

10. The construction method for bridge foundation pit excavation as described in claim 2, characterized in that, The monitoring frequency of the foundation pit is dynamically adjusted according to the construction stage. During the excavation of the foundation pit, the monitoring frequency is higher. After the foundation is poured, the monitoring frequency is set. As the age increases, the monitoring frequency is gradually reduced until the foundation pit is backfilled and passes the acceptance test.