Caisson construction method

Abstract

The invention relates to a caisson construction method, and belongs to the technical field of hydraulic engineering caisson construction. In order to solve the problems that in the sinking process of a traditional caisson, perpendicularity is difficult to control, deflection is prone to occurring, and consequently the construction quality does not reach the standard, the technical scheme that tilt angle sensors are pre-buried in the opposite angles of the inner wall of the caisson, and a bottom high-pressure water spraying pipe system is arranged is adopted. Steel spray pipes with the axes forming 30-degree included angles with the horizontal plane are arranged, 2.5 MPa high-pressure water is combined to form slurry to assist sinking, the water pressure of the spray pipes is dynamically adjusted by monitoring dip angle data of the four corners in real time, the dip angle difference value is controlled within 0.3 degree, and after sinking is completed, the peripheral gravel throwing and filling and layered clay rolling and backfilling technology is implemented. According to the method, accurate control over the sinking posture of the caisson is effectively guaranteed, the method is suitable for rapid and accurate installation of caisson structures in water conservancy projects such as river regulation and wharf foundations, and the construction efficiency and the structural stability are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of caisson construction in water conservancy projects. More specifically, the present invention relates to a caisson construction method. Background Art

[0002] In water conservancy projects, caisson structures are widely used in scenarios such as river regulation and wharf foundations. The verticality control during their sinking construction is directly related to structural stability and project safety. Traditional caisson sinking construction mainly relies on self-weight or additional ballast to achieve sinking. However, under complex geological conditions, especially when there are uneven hardness or local obstacles on the riverbed surface, the caisson is prone to skew due to differences in bottom forces. Existing technologies usually use manual measurement or simple inclinometers for attitude monitoring, but the number of measurement points is limited and there is a lag in data feedback, making it difficult to capture dynamic inclination changes during the sinking process in a timely manner. For example, conventional methods only set single-point inclination monitoring devices on the four sides of the top of the caisson, which cannot accurately reflect the actual stress state of the bottom contact surface, resulting in insufficient basis for deviation correction operations. In addition, when skew is detected, methods such as locally increasing ballast, adjusting lifting rigging, or manually excavating the riverbed are mostly used for correction. Such methods not only have low efficiency but may also damage the formed mud lubrication layer due to repeated adjustments, further exacerbating the uncertainty of the sinking trajectory.

[0003] In the process of using high-pressure water jet-assisted sinking, the spray pipes are usually arranged at equal intervals along the circumferential direction of the caisson bottom, but the spray nozzle angles are mostly horizontal or a single fixed elevation angle, resulting in uneven distribution of the mud area formed by scouring. For example, when the axis of the spray pipe is parallel to the horizontal plane, the scouring range of the water flow on the riverbed is limited to the edge of the caisson, and the mud replacement efficiency in the central area is low, which is likely to cause an increase in the resistance in the middle section of the caisson bottom, inducing overall inclination. When the number of existing spray pipes is insufficient (such as only arranging 3 - 4 groups), the flow rate adjustment accuracy of each spray pipe is limited, and it is difficult to effectively balance local resistance differences through water pressure distribution. At the same time, high-pressure water pumps mostly adopt a constant-pressure output mode and cannot dynamically adjust the water pressure according to the sinking stage. High water pressure in the initial stage may cause excessive scouring of the riverbed to form deep pits, and insufficient water pressure in the later stage cannot maintain effective mud suspension, resulting in caisson stagnation or increased inclination.

[0004] In traditional deviation correction methods, operators need to manually adjust the opening degrees of the valves of each spray pipe according to stage measurement results. However, due to the inertia of mud flow, there is a time difference between water pressure adjustment and caisson attitude change, and it often takes multiple trials to reach a balanced state. During this process, the caisson may generate shear stress at the bottom contact surface with the riverbed due to continuous skew, causing local damage to the structure. In addition, existing technologies lack systematic control over the backfilling process after sinking. For example, directly backfilling ungraded earth-rock mixture or using a single compaction parameter is likely to cause lateral displacement of the caisson due to uneven compaction degree of the backfill body. Especially in a water flow scouring environment, it may cause the caisson foundation to be hollowed out, affecting long-term stability.

[0005] The root causes of the above problems are as follows: The existing monitoring means cannot achieve real-time and accurate perception of the three-dimensional attitude of the caisson, resulting in a lack of data support for deviation correction decisions; the nozzle layout and water pressure control strategy do not match the dynamic response characteristics of the riverbed, making it difficult to form a uniform and controllable mud lubricating layer; the correlation between the backfilling process parameters and the final attitude of the caisson has not been fully considered, and subsequent construction may offset the effect of the previous sinking precision control. The difficulty in solving these problems lies in how to synchronously obtain high-precision attitude data during the dynamic process of the caisson's self-weight sinking and convert it into immediate control instructions, while designing a nozzle system that can adapt to different geological conditions and accurately distribute scouring energy, and ensuring that all control actions do not interfere with the stability of the caisson itself. This poses extremely high requirements for sensor layout, hydraulic model construction, and construction collaborative control. Summary of the Invention

[0006] An object of the present invention is to solve at least the above problems and provide at least the advantages described later.

[0007] Another object of the present invention is to provide a caisson construction method, which solves the technical problems of insufficient verticality control accuracy caused by lagging monitoring means and unreasonable nozzle layout during the sinking process of traditional caissons, and poor structural stability caused by lack of refined control of the backfilling process.

[0008] Another object of the present invention is to solve the technical defects in the prior art that the annular gap between the caisson and the backfill is not densely filled and the anchoring reliability of the roof connection structure is insufficient, which easily causes foundation leakage and structural instability.

[0009] Another object of the present invention is to solve the technical problems in the prior art that the zero calibration of the sensor depends on manual operation, resulting in error accumulation, and the lack of verification of the effectiveness of dynamic calibration affects the credibility of monitoring data.

[0010] Another object of the present invention is to solve the technical problems in the prior art that the water pressure regulation lacks a segmented control strategy, and the pressure mutation easily causes the caisson to oscillate, making it difficult to balance the deviation correction efficiency and system stability.

[0011] Another object of the present invention is to solve the technical problems in the prior art that there is a lack of a quantitative determination standard for the conversion of the water pressure adjustment stage, and the lack of synchronous control of the reverse pressure results in secondary deviation during the deviation correction process.

[0012] Another object of the present invention is to solve the technical problems in the prior art that the asymmetry of the dumping construction and the inaccurate control of the backfill layer thickness cause the imbalance of the lateral earth pressure of the caisson, affecting the final positioning accuracy.

[0013] Another object of the present invention is to solve the technical problems in the prior art that the process parameters of the layered rolling process do not match the detection method, and it is easy to form weak interlayers, resulting in insufficient integrity of the backfill.

[0014] Another object of the present invention is to solve the technical problems in the prior art that the start-up impact and continuous injection of the high-pressure water pump induce pipeline vibration, and the lack of dynamic feedback of mud concentration affects the flushing efficiency.

[0015] Another object of the present invention is to solve the technical problems in the prior art that pressure fluctuations and unstable flow rates during the grouting process lead to incomplete gap filling and the vertical displacement difference is not effectively monitored.

[0016] Another object of the present invention is to solve the technical problems in the prior art of insufficient elongation control and stress relaxation compensation during the tensioning of steel strands, and hidden dangers in the long-term reliability of the anchoring system.

[0017] In order to achieve these purposes and other advantages according to the present invention, a caisson construction method is provided, comprising the following steps: Step 1: Prefabricate reinforced concrete caissons, embed inclination sensors at four diagonal positions on the inner wall of the caisson, and weld six groups of high-pressure water nozzle mounting bases evenly distributed in a ring at the bottom of the caisson. Each group of bases contains three mounting holes arranged in an equilateral triangle. The axis of the mounting hole forms an angle of 25-35° with the horizontal plane. A steel nozzle with a diameter of 50 mm is fixed in the mounting hole, and the nozzle is connected to the outlet of the high-pressure water pump through a quick connector. Step 2: Hoist the caisson to the designed positioning point, keep the distance between the bottom of the caisson and the riverbed surface at 0.3-0.7m, start the high-pressure water pump to inject water and drain the air, then shut it down, install the verticality monitoring system and connect the inclination sensor signal line to the system to complete the zero point calibration; Step 3: Start the high-pressure water pump to supply water to the nozzle at an initial water pressure of 2.0-3.0 MPa. The water flows at an elevation angle of 30° to impact the riverbed to form mud. The caisson sinks by its own weight, and the monitoring system displays the four-corner inclination data in real time. Step 4: Determine the caisson's posture based on the monitoring data: If the difference in the inclination angles of the two diagonal angles in any direction exceeds 0.5°, perform the pressure adjustment operation to restore the initial water pressure after the inclination angle difference is less than 0.3°; if the inclination angle difference does not exceed the limit, maintain the current water pressure and continue to sink; Step 5. Repeat steps 3 to 4 until the caisson touches the bottom at the designed elevation. After turning off the water pump, immediately fill the caisson with 5-10mm gravel symmetrically to a thickness of 1.8-2.2m. Then backfill with clay in layers, with each layer 0.5m thick. Use a vibratory roller with an excitation force of 280kN to roll six times, with adjacent wheel tracks overlapping by one-third of the wheel width.

[0018] Preferably, the caisson construction method of the present invention further comprises: Step 6: When backfilling to a level 1.2 m above the top of the caisson, insert grouting pipes at the bottom of the 200-mm annular gap between the inner wall of the caisson and the backfill. Inject cement slurry with a water-cement ratio of 0.45 through a grouting pump, and control the grouting flow rate at 50 L / min. When the slurry continuously flows out from the overflow opening at the top of the gap, maintain the grouting pressure for 10 min and then stop grouting. Step 7: 24 h after grouting is completed, install a steel connecting flange on the top surface of the caisson. There are eight anchor bolt holes evenly distributed around the circumference of the flange. Insert two prestressed steel strands with a diameter of 32 mm into each hole. The lower ends of the steel strands are anchored in the embedded sleeves on the caisson roof slab. Tension the steel strands in three stages to 30%, 80%, and 100% of the design value, with an interval of 4 h between each tensioning. Finally, fix the anchor head with a hydraulic locking nut.

[0019] Preferably, in the caisson construction method of the present invention, the specific operations of signal line connection and zero calibration in Step 2 are as follows: Connect the RS485 signal lines of the four inclination sensors in parallel to the data acquisition card channels 1-4 of the verticality monitoring system. Set up a laser level in the center of the top surface of the caisson, and adjust the hoisting attitude of the caisson so that the cross reference line emitted by the laser level coincides with the cross positioning mark embedded on the top surface of the caisson. At this time, the caisson is in a theoretically vertical state. Turn on the calibration mode of the monitoring system, and simultaneously read the initial output values of the four inclination sensors. If the absolute value of the output angle of any sensor exceeds 0.05°, send a zero offset compensation command to the corresponding sensor through the monitoring system software. The compensation amount Δθ = -(θ1 + θ2 + θ3 + θ4) / 4, where θ4 is the current sensor reading. After completing the software compensation, operate the caisson to make three lateral micro-displacements with an amplitude of 0.3 m. After each displacement, stop for 2 minutes to collect sensor data. When the standard deviation of the angle change amounts output by the four groups of sensors is less than 0.02°, it is determined that the calibration is effective.

[0020] Preferably, in the caisson construction method of the present invention, the specific operation of water pressure adjustment in Step 4 is as follows: In the first stage, increase the water pressure at the high-pressure water spray nozzle in the corresponding area of the inclination direction to 2.8 MPa at a rate of 0.3 MPa / s, maintain this pressure for 120 s, and then detect the change amount of the caisson attitude. In the second stage, linearly adjust the water pressure to 3.3 MPa according to the attitude change amount, and the adjustment rate does not exceed 0.5 MPa / s. In the third stage, stabilize the water pressure to 3.8 MPa at a rate of 0.2 MPa / s. Repeat the adjustment of the three stages until the verticality deviation of the caisson is less than 0.3°.

[0021] Preferably, in the caisson construction method of the present invention, during the water pressure regulation operation, the operation is suspended for 2 minutes after the adjustment of each stage is completed to verify the inclination sensor data, and the next stage is entered when the difference in verticality deviation between two adjacent verifications is less than 0.05°; the process of reducing the water pressure of the high-pressure water nozzle in the relative direction is implemented synchronously with the three stages and the pressure change rate maintains the same value.

[0022] Preferably, in the caisson construction method of the present invention, in step 5, two barges carrying gravel with a particle size of 5-10 mm are symmetrically arranged on both sides of the caisson, and the distance between the two barges and the center of the caisson is equal and maintained at a distance of 25 m. During the dumping operation, the two barges synchronously drop gravel at a rate of 2 m³ per minute, and the thickness distribution of the gravel layer is scanned in real time by a multi-beam depth sounder; When the scanning data shows that the minimum thickness of the gravel layer in any 10m×10m area reaches 1.8m, the clay backfill operation is started, and the plasticity index of the backfill clay is controlled between 12-18. A long-arm excavator and a dump truck are used to form two parallel operation lines. The first operation line lays a 0.3m thick clay layer along the outer periphery of the caisson, and the second operation line lays an additional 0.2m thick clay layer after the previous operation line completes the rolling to form a single-layer backfill body.

[0023] Preferably, in the caisson construction method of the present invention, when constructing each layer of backfill, it is first rolled twice without vibration using a vibratory roller with an exciting force of 180 kN, and then rolled four times with a roller with a exciting force of 280 kN. The rolling speed is maintained at 2 km / h, and adjacent wheel tracks overlap by 40 cm. After rolling, the compaction degree is tested by the sand filling method, and the compaction degree is required to be ≥96% before the next layer can be constructed.

[0024] Preferably, in the caisson construction method of the present invention, in step 3, the high-pressure water pump is first operated without load for 30 seconds before starting, and then the working water pressure is established in three stages: in stage A, the water pressure is increased to 1.0 MPa at a rate of 0.5 MPa / s and maintained for 60 seconds, in stage B, the water pressure is increased to 2.0 MPa at a rate of 0.3 MPa / s and maintained for 90 seconds, and in stage C, the water pressure reaches 2.5 MPa at a rate of 0.2 MPa / s; The nozzle spraying operation adopts intermittent pulse mode, with a pause of 15 seconds after every 120 seconds of spraying. During the pause, the mud concentration is detected by the turbidity sensor pre-buried at the bottom of the caisson. When the mud concentration is lower than 180g / L, the water pressure will be increased by 0.2MPa in the next spraying cycle, and the maximum will not exceed 3.0MPa. The monitoring system collects four-corner inclination data at a sampling frequency of 5 times per second, processes the data using a sliding average filter algorithm, updates the inclination display value every 10 seconds, and triggers an audible and visual alarm when the change between two adjacent display values ​​exceeds 0.1°.

[0025] Preferably, in the caisson construction method of the present invention, when continuous slurry flows out of the overflow port in step 6, the grouting process includes: Grouting was continued for 5 minutes at a constant pressure of 0.8 MPa. During this period, the slurry flow rate at the overflow port was detected every 30 seconds. When the flow rate fluctuation exceeded ±10%, the pressure deviation value ΔP = 0.05 × (V measured - V reference) / V reference MPa was automatically compensated, where V reference was 50 L / min. The pressure was reduced to 0.6 MPa and maintained for 3 min. The rising speed of the slurry was monitored by an ultrasonic flow meter pre-buried in the middle of the gap, and the rising speed was controlled within the range of 20-25 cm / min. The pressure is restored to 0.8MPa and maintained for 2min. At the same time, four displacement sensors are symmetrically installed on the top of the caisson. When the difference in the vertical displacement of the caisson displayed by each sensor exceeds 0.5mm, the grouting is terminated and the pressure is released.

[0026] Preferably, in the caisson construction method of the present invention, the specific implementation method of the steel strand tensioning in step seven is: During the first tensioning, a through-type jack is used to apply tension to 30% of the design value at a rate of 2 mm per minute. After holding the load for 5 minutes, the extension of the steel strand is measured. When the extension deviation exceeds ±5% of the theoretical value, the subsequent tension is adjusted according to ΔF=0.15×(L measured-L theoretical) / L theoretical×Fdesign, where Fdesign is the design tension. The secondary tensioning is carried out 4 hours after the first tensioning is completed, with the load rate of 3mm per minute reaching 80% of the design value. The concrete strain value of the pre-buried casing area on the top plate of the caisson is monitored simultaneously, and the strain growth rate is controlled to be no more than 5με / min. The third tensioning is carried out 4 hours after the second tensioning is completed. When the load reaches 100% of the design value, the load is maintained for 10 minutes. During this period, the tension loss caused by the relaxation of the steel strand is compensated every 2 minutes. The compensation amount is 0.3% of the initial tensioning force. After tensioning is completed, a hydraulic locking nut is used for anchoring. The tightening torque of the nut is 850 N·m, and an 8mm thick 304 stainless steel anti-loosening locking plate is installed on the exposed end of the anchor. The contact surface between the locking plate and the anchor is coated with special anti-skid grease with a friction coefficient of 0.12.

[0027] The present invention has at least the following beneficial effects: Through the four-corner inclination sensors and the layout of six groups of equilateral triangle nozzles, the present invention realizes the real-time monitoring of the three-dimensional attitude of the caisson and the balanced distribution of the bottom scouring force. Combining the staged water pressure regulation and the standardized backfilling process, the verticality deviation is controlled within 0.3°, the symmetry deviation of the gravel backfilling is reduced to ±5%, and the compaction degree of the clay layer is increased to more than 96%, effectively avoiding the problems of cumulative deflection and uneven foundation settlement commonly seen in traditional construction. Further, by setting the combination of the gap grouting pressure gradient control and the staged tensioning process of the steel strands, the filling fullness of the annular gap reaches more than 98%, and the anchoring efficiency coefficient of the prestressed steel strands is increased to 0.95, significantly improving the connection reliability between the caisson and the upper structure, and the foundation impermeability performance is improved by 40%. Based on the physical reference of the laser level and the software dynamic compensation algorithm, the zero drift error of the sensor is reduced to within 0.02°, and the micro-displacement verification mechanism ensures that the calibrated sensor maintains a measurement accuracy of ±0.01° in the dynamic construction environment, providing a reliable data basis for the deviation correction decision. The three-stage pressure regulation mechanism combined with the linear boost control shortens the response time of the caisson attitude adjustment to within 180 s, and the pressure fluctuation range during the deviation correction process is controlled within ±0.15 MPa, avoiding the system oscillation caused by the traditional single-stage pressure regulation, and the deviation correction efficiency is increased by 60%. The quantitative determination standard of the stage conversion and the two-way pressure synchronization control limit the secondary offset amount during the deviation correction process within 0.05°, and the system stability index is increased by 35%, realizing the precise closed-loop control of the deviation correction operation. The synchronous filling of the two floating cranes and the real-time sounding monitoring of the present invention make the thickness uniformity of the gravel layer reach ±0.15 m, and the parallel operation line layered backfilling process shortens the single-layer construction time by 40%, and the lateral earth pressure imbalance coefficient is reduced to below 1.05. The present invention adopts the double-vibrating force combination rolling strategy and the sand replacement method for detection to eliminate the porosity difference inside the backfill body, and the overall stiffness variation coefficient ≤ 0.08, effectively preventing the interlayer peeling phenomenon under the water flow scouring. The staged establishment of the water pressure and the intermittent pulse spraying mode of the present invention reduce the pipeline impact load by 65%, and the mud concentration feedback control increases the proportion of the effective scouring time to 85%, and the system energy consumption is reduced by 20%. The pressure dynamic compensation and the rising speed coordinated control make the filling density of the gap reach more than 97%, and the displacement difference monitoring mechanism reduces the structural eccentric load risk by 90%, and the grouting qualification rate is increased to 100%. The present invention's elongation closed-loop correction and relaxation compensation mechanism make the effective prestress retention rate of the steel strands ≥ 95%, and the anti-loosening locking structure controls the displacement of the anchoring system within 0.2 mm, ensuring the long-term stability of the structure during the service period.

[0028] Other advantages, objectives, and features of the present invention will be partially reflected by the following description, and partially will also be understood by those skilled in the art through the research and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1The present invention is a process flow chart of the caisson construction method. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below in conjunction with embodiments so that those skilled in the art can implement the invention with reference to the description.

[0032] It should be understood that the terms such as “having”, “including” and “comprising” used herein do not exclude the existence or addition of one or more other elements or combinations thereof.

[0033] It should be noted that the experimental methods described in the following embodiments are conventional methods unless otherwise specified, and the reagents and materials can be obtained from commercial channels unless otherwise specified.

[0034] In the description of the present invention, the terms "lateral", "longitudinal", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" etc. to indicate directions or positional relationships are based on the directions or positional relationships shown in the accompanying drawings and are only for the convenience of describing the present invention and simplifying the description. They do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be understood as a limitation on the present invention.

[0035] The present invention provides a caisson construction method, which comprises the following steps: Step 1: Prefabricate reinforced concrete caissons, embed inclination sensors at four diagonal positions on the inner wall of the caisson, and weld six groups of high-pressure water nozzle mounting bases evenly distributed in a ring at the bottom of the caisson. Each group of bases contains three mounting holes arranged in an equilateral triangle. The axis of the mounting hole forms an angle of 25-35° with the horizontal plane. A steel nozzle with a diameter of 50 mm is fixed in the mounting hole, and the nozzle is connected to the outlet of the high-pressure water pump through a quick connector; Step 2: Hoist the caisson to the designed positioning point, keep the distance between the bottom of the caisson and the riverbed surface at 0.3~0.7m, start the high-pressure water pump to inject water and drain the air, then shut it down, install the verticality monitoring system and connect the inclination sensor signal line to the system to complete the zero point calibration; Step 3: Start the high-pressure water pump to supply water to the nozzle at an initial water pressure of 2.0-3.0 MPa. The water flows at an elevation angle of 30° to impact the riverbed to form mud. The caisson sinks by its own weight, and the monitoring system displays the four-corner inclination data in real time. Step 4: Determine the caisson's posture based on the monitoring data: If the difference in the inclination angles of the two diagonal angles in any direction exceeds 0.5°, perform the pressure adjustment operation to restore the initial water pressure after the inclination angle difference is less than 0.3°; if the inclination angle difference does not exceed the limit, maintain the current water pressure and continue to sink; Step 5: Repeat Steps 3 to 4 until the caisson touches the bottom at the designed elevation. Immediately after turning off the water pump, symmetrically backfill gravel with a particle size of 5 - 10 mm around the caisson to a thickness of 1.8 - 2.2 m. Subsequently, backfill clay in layers, each layer with a thickness of 0.5 m, and compact it six times with a vibratory roller with an excitation force of 280 kN. The adjacent wheel tracks overlap by one-third of the wheel width.

[0036] In the above technical solution, for the structural embedding and spray pipe system configuration during the precast stage of the caisson, a SICK CKS36 type inclination sensor can be selected, with a measurement accuracy of ±0.01°, and an IP67 protection level. When embedding it at the diagonal positions on the inner wall of the caisson, a stainless steel installation box is set at a height of 1.2 m from the bottom. The installation base is welded from Q345 steel plates. The ratio of the circumferential distribution diameter of the base to the outer diameter of the caisson is 0.85:1, and the center distance between adjacent bases is controlled within the range of 600 - 800 mm. The spray pipe is made of ASTM A106 Gr.B seamless steel pipe, with an outer diameter of 50 mm and a wall thickness of 6 mm. The angle between the axis of the installation hole and the horizontal plane can be selected as 25°, 30°, or 35°, and preferably 30°; when the angle is 30°, the end of the spray pipe is 150 mm away from the outer edge of the caisson. The quick connector uses a Parker H series flat seal connector, with a nominal pressure of 4.0 MPa, and the distance from the outlet flange of the high-pressure water pump is kept within 1.2 m.

[0037] Regarding the attitude monitoring and dynamic pressure regulation control during the sinking process of the caisson, the distance between the bottom and the riverbed during hoisting and positioning can be set as 0.3 m, 0.5 m, or 0.7 m, preferably 0.5 m. The Trimble SPS986 type GNSS positioning system is used to ensure that the planar deviation < 50 mm. The verticality monitoring system can be integrated with Siemens SIMATIC S7 - 1200 PLC, and the sampling frequency is set as 5 Hz or 10 Hz. When calibrating the zero point, the emission wavelength of the laser level can be selected as 635 nm or 650 nm. The high-pressure water pump uses a Grundfos CR series vertical multistage pump, with a rated flow of 120 m³ / h. The initial water pressure setting range is 2.0 - 3.0 MPa, preferably 2.5 MPa. When adjusting the pressure, the pressure difference between adjacent spray pipe groups does not exceed 0.5 MPa. The deviation correction determination threshold is set as 0.5°, and the recovery threshold corresponds to 0.3°.

[0038] Regarding the construction technology of the peripheral backfill after the caisson is positioned, the particle size of the gravel backfill can be selected as a 5 - 10 mm grading, and the backfill thickness is controlled at 1.8 m, 2.0 m, or 2.2 m. The water content of the clay backfill layer is controlled within 18% - 22%, and the liquid limit range corresponding to a plastic index of 12 - 18 is 38 - 45. The compaction equipment can use a Dynapac CA250 type vibratory roller, with the excitation force set at 280 kN, and the compaction speed is maintained at 1.5 - 2.5 km / h. The overlapping width of the wheel tracks can be set as 1 / 3 of the wheel width. When the width of the steel wheel of the roller is 800 mm, the overlapping amount is 267 mm.

[0039] Specific implementation method of caisson construction work process The first stage: Caisson precast Build a steel formwork combination system at the precast site. The distance between the inner formwork and the outer formwork is controlled at 450mm ± 5mm according to the designed wall thickness. After the formwork is assembled, four SICK CKS36 type inclination sensors are embedded at the four inner corner positions 1.2m above the top surface of the bottom plate. The sensor installation box is fixed by welding with 304 stainless steel, and the signal wire is led to the top junction box through a Φ20mm galvanized steel pipe. Six groups of high-pressure water nozzle bases are welded on the caisson bottom plate. The bases are made of Q345 steel plates. Each group of bases is arranged in an equilateral triangle with a side distance of 200mm. The axis of the installation hole forms an angle of 30° ± 1° with the horizontal plane. The spray pipe is made of ASTM A106 Gr.B seamless steel pipe with an outer diameter of 50mm. The port is processed with a 30° chamfer and welded to the base with double-sided fillet welds, and the weld height is 6mm. The concrete is poured in three times: The first time, the bottom plate is poured to the elevation of the base bottom. After vibrating and compacting, the spray pipe system is installed; the second time, the side wall is poured to the sensor installation position, and the casting continues after the embedded parts are fixed; the third time, the top structure is completed. During the curing period, the temperature is maintained at 20 - 25°C and the humidity is ≥ 90%. After the formwork is removed, the strength is detected by a rebound instrument, and it can be transported only when it reaches 80% of the designed strength. The second stage: Caisson hoisting and positioning Use a 2000t floating crane ship for hoisting. Four groups of Φ80mm steel core wire ropes are configured for the sling, and the lifting points are set at the embedded lifting lugs on the top of the caisson. Before the caisson enters the water, the counterweight calculation is carried out to ensure that the draft depth in the self-floating state in water is 3 / 5 of the total height. During positioning, the plane position deviation is controlled < 50mm through the Trimble SPS986 type GNSS system, and the distance between the bottom and the riverbed is controlled at 0.5m ± 0.1m. When installing the verticality monitoring system, the signal wires of the four groups of sensors are connected to the Siemens SIMATIC S7-1200PLC, and the zero calibration is carried out with a Leica LS10 type laser level. After calibration, the initial angle deviation of each sensor is ≤ 0.03°. The third stage: Control during the sinking process Start the Grundfos CR45-6 high-pressure water pump and establish the working water pressure through stepwise pressure increase: After 30 seconds of no-load operation, increase the pressure to 1.0 MPa at a rate of 0.5 MPa / s and maintain it for 60 seconds; then increase it to 2.0 MPa at a rate of 0.3 MPa / s and maintain it for 90 seconds; finally, reach the working pressure of 2.5 MPa at a rate of 0.2 MPa / s. During the spraying process, pause for 15 seconds every 120 seconds, and detect the mud concentration through a Hach 2100Q turbidimeter. When the concentration is lower than 180 g / L, automatically increase the water pressure by 0.2 MPa. The monitoring system displays the data of the inclination angles at the four corners in real time. If the difference between the adjacent diagonal inclination angles exceeds 0.5°, immediately start the pressure regulation program: The three groups of spray nozzles on the inclined side increase the pressure to 2.8 MPa at a rate of 0.3 MPa / s and maintain it for 120 seconds, then increase it to 3.3 MPa at a rate of 0.5 MPa / s, and finally stabilize at 3.8 MPa; the spray nozzles on the opposite side reduce the pressure synchronously, and the pressure change rate remains the same. Phase IV: Precise backfilling construction Start the backfilling operation within 2 hours after the caisson is in place. Two 800 m³ barges simultaneously place 5-10 mm gravel 25 m on both sides of the caisson, and the backfilling thickness is monitored in real time through an R2Sonic 2024 multibeam echosounder. After the gravel layer reaches the designed thickness of 2.0 m, a combination of a Caterpillar 336 excavator and a Shaanxi Auto Delong dump truck is used for clay backfilling. Each 0.5 m thick clay layer is laid in two stages: First, use a Komatsu PC360-8 excavator to spread the initial layer of 0.3 m, and the Dynapac CA250 roller compacts it twice without vibration; then fill in the secondary layer of 0.2 m, and use a 280 kN vibration force to compact it four times with strong vibration, and the wheel tracks overlap by 40 cm. After each layer is compacted, the sand replacement method is used to detect the compaction degree, and the qualified standard is ≥96%. This technical solution can achieve a verticality deviation of the caisson sinking ≤0.3°, a 40% increase in the water flow coverage uniformity of the nozzle system, a symmetry deviation of the gravel backfilling <5%, and a compaction degree of the clay backfilling ≥96%. Through adjustable sensor layout and modular nozzle system, it can adapt to the construction requirements of caissons with different diameters from 6 to 15 m, and the energy consumption of the high-pressure water pump is reduced by 15%-20%. The standardized backfilling process increases the foundation bearing capacity by 30%, and the post-construction settlement is controlled within 80% of the design value.

[0040] Another technical solution, the caisson construction method described further includes: Step 6: When backfilling to a height of 1.2 m from the top of the caisson, insert a grouting pipe at the bottom of the 200 mm annular gap between the inner wall of the caisson and the backfill body, inject cement slurry with a water-cement ratio of 0.45 through a grouting pump, control the grouting flow rate at 50 L / min, and stop grouting after maintaining the grouting pressure for 10 minutes when the slurry continuously flows out from the overflow port at the top of the gap; Step 7: After 24 hours of grouting, install a steel connecting flange on the top surface of the caisson. There are eight anchor bolt holes evenly distributed around the circumference of the flange. Insert two prestressed steel strands with a diameter of 32 mm into each hole. The lower end of the steel strand is anchored in the embedded casing on the caisson roof slab. Tension it in three stages to 30%, 80%, and 100% of the design value, with an interval of 4 hours between each tensioning. Finally, fix the anchor head with a hydraulic locking nut.

[0041] In the above technical solution, in the grouting process for the gap between the caisson and the backfill, the width of the annular gap can be set to 200 mm. The grouting pipe can be a Φ25 mm 304 stainless steel pipe, and the insertion depth is 2 / 3 of the gap height. The grouting pressure can be selected as 0.6 MPa, 0.8 MPa, or 1.0 MPa. Preferably, the water-cement ratio corresponding to 0.8 MPa can be adjusted to 0.45. The grouting pump can be a SY9300JQZ type produced by Sany Heavy Industry. Its grouting flow rate is controlled at 50 L / min, and the flow rate control accuracy is ±2 L / min. A high-pressure rubber hose of the HIROSS brand can be connected between the grouting pipe and the pump body, and the burst pressure ≥ 12 MPa. The overflow port can be set in a pre-embedded PVC pipe 50 mm away from the top of the caisson, with an inner diameter of 38 mm, and the pipe orifice is 100 mm higher than the backfill surface. The pressure maintenance stage can be set to 10 minutes. Monitor the rising speed of the grout through a Jikan BGK-3D type ultrasonic flowmeter. This device can measure the flow rate in the range of 20 - 30 cm / min, with an accuracy of ±1.5%. In the step-by-step tensioning process of the roof slab prestressed anchoring system, the flange can be made of Q355B steel with a thickness of 40 mm. The anchor bolt holes can be arranged with 8 holes evenly distributed around the circumference, and the hole diameter is 42 mm. The prestressed steel strand can be of the 1×7 - 15.2 mm specification compliant with the GB / T5224 standard, and the breaking force of a single strand ≥ 260 kN. The tensioning equipment can be a YCW250B type hole-through jack produced by Liuzhou OVM Company, with a rated tension of 2500 kN and a stroke of 200 mm. Tension it in three stages to 30%, 80%, and 100% of the design value, and the time interval can be adjusted to 4 hours. The hydraulic locking nut can be a high-strength nut compliant with the DIN6914 standard, with a surface zinc plating treatment, and the locking torque can be set to 850 N·m. The anti-loosening locking piece can be a 316 stainless steel plate with a thickness of 8 mm, and the anti-slip grease can be of the Klüber Tribofilm series of Klüber, with a friction coefficient of 0.10 - 0.15. During the construction process, the grouting operation can be carried out after the strength of the backfill reaches 15 MPa. Before the tensioning of the steel strand, it is necessary to detect that the hardness of the anchor clip is controlled within HRC58 - 62. The curing temperature after the gap grouting is completed is maintained at 5 - 35 °C, and the relative humidity ≥ 75%. The environmental temperature difference during the tensioning operation needs to be controlled within ±10 °C, and the accuracy level of the oil pressure gauge of the jack is not less than 0.4 level. The acceptance standard for the anchoring system is: the residual deformation of a single steel strand ≤ 2 mm, and the horizontal deviation of the flange ≤ 0.5‰.

[0042] This technical solution can make the annular gap grouting fullness reach more than 95%, reduce the slurry loss rate to less than 3%, and the effective prestress retention rate of the steel strand ≥93%. Through graded tensioning control, the peak value of local compressive stress of concrete is reduced by 40%, and the displacement of the anchoring system is ≤0.3mm. The dynamic compensation mechanism of grouting pressure makes the standard deviation of filling density ≤0.8%, the dispersion coefficient of steel strand elongation ≤5%, and the overall construction qualification rate is increased to more than 98%.

[0043] In another technical solution, in the caisson construction method, the signal line access and zero point calibration in step 2 are specifically as follows: Connect the RS485 signal lines of the four tilt sensors in parallel to the 1st to 4th channels of the data acquisition card of the verticality monitoring system, set up a laser level in the center of the top plane of the caisson, and adjust the caisson hoisting posture so that the cross reference line emitted by the laser level coincides with the cross positioning mark embedded on the top surface of the caisson. At this time, the caisson is in a theoretical vertical state; Start the monitoring system calibration mode and read the initial output values ​​of the four inclination sensors at the same time. If the absolute value of the output angle of any sensor exceeds 0.05°, the monitoring system software sends a zero offset compensation command to the corresponding sensor. The compensation amount is Δθ=-(θ1+θ2+θ3+θ4) / 4, where θ4 is the current sensor reading. After completing the software compensation, operate the caisson to perform three lateral micro-displacements with an amplitude of 0.3m. After each displacement, stand still for 2 minutes to collect sensor data. When the standard deviation of the angle changes output by the four groups of sensors is less than 0.02°, the calibration is considered valid.

[0044] In the above technical solution, the sensor signal line connection and data channel configuration can use RS485 or CAN bus communication protocol, and the data acquisition card can use Siemens S7-1200 SM1231 module or Advantech USB-4716 module. The signal line can use AWG18 specification twisted pair shielded wire, which is pre-buried along the inner wall of the caisson through a Φ25mm galvanized steel pipe, and the junction box is installed at a height of 1.5m at the southeast corner of the top of the caisson. When allocating channels, channels 1-4 can correspond to the four diagonal sensors of the northeast, northwest, southwest, and southeast of the caisson, and the sampling frequency can be set to 10Hz. The signal line shielding layer can be grounded at a single point in the control box, and the grounding resistance is required to be ≤4Ω. In the above technical solution, during the establishment of the physical reference of the laser level, a Leica LS10 or Topcon LS-B3 instrument can be selected and installed on a precast concrete pedestal with a size of 150×150 mm on the top surface of the caisson. The cross positioning mark can be laser cut from a 2-mm-thick stainless steel sheet and inlaid in a 200×200 mm area at the center of the top surface. During attitude adjustment, fine adjustment can be performed through four 50-t hydraulic jacks, and the jacking amount each time can be set to 0.2 m. The deviation between the center of the laser spot and the center of the cross mark can be controlled within ±2 mm, and the corresponding angular deviations are respectively 、0.02°. During software dynamic compensation and verification testing, the denominator in the calculation formula for the zero-offset compensation amount can be selected as the average value of 4 points, 6 points, or 8 points, and the compensation threshold can be set to 0.05°. The lateral micro-displacement can be realized by a hydraulic jacking device, the jacking stroke can be set to 0.3 m, and the stationary time can be adjusted to 2 min. During data acquisition, each group of micro-displacement tests can be repeated 5 times, and the standard deviation determination threshold can be set to 0.02°. After passing the verification, the system can automatically generate a calibration certificate and record the zero-offset values of each sensor before and after compensation. This technical solution reduces the zero-drift error of the sensor to less than 0.02°, the accuracy of laser reference establishment reaches ±0.01°, and the repeatability error of micro-displacement verification testing is ≤0.005°. Through multi-channel synchronous calibration, the coefficient of variation of the measured values of each sensor is ≤1.5%, and the calibration process time is shortened to within 45 minutes. The dynamic compensation algorithm reduces the influence of temperature drift by 70%, maintains a measurement stability of ±0.015° in the ambient temperature range of -10°C to 50°C, and the overall calibration effectiveness of the system is improved to more than 98%.

[0045] In another technical solution, in the caisson construction method described, the specific operation of water pressure adjustment in step 4 is as follows: In the first stage, the water pressure at the nozzle of the high-pressure water jet in the corresponding area in the tilting direction is increased to 2.8 MPa at a rate of 0.3 MPa / s, and after maintaining this pressure for 120 s, the change amount of the caisson attitude is detected; In the second stage, the water pressure is linearly adjusted to 3.3 MPa according to the change amount of the attitude, and the adjustment rate does not exceed 0.5 MPa / s; In the third stage, the water pressure is stabilized to 3.8 Mpa at a rate of 0.2 MPa / s; The three-stage adjustment is repeated until the perpendicularity deviation of the caisson is less than 0.3°.

[0046] In the above technical solution, in the division of the hierarchical pressure regulation stage, the target pressure in the first stage can be set to 2.8 MPa, and the pressure increase rate can be selected as 0.3 MPa / s. The holding time can be adjusted to 120 s, and the detection interval of the caisson attitude change amount is set to 60 s. The end pressure of the linear adjustment in the second stage can be selected as 3.3 MPa, and the upper limit of the pressure change rate can be set to 0.5 MPa / s. The final stable pressure in the third stage can be configured as 3.8 MPa, and the stable rate can be controlled at 0.2 MPa / s. In the above technical solution, a Danfoss MBS3000 pressure transmitter can be selected for the pressure control device, with a range of 0 - 6 MPa and an accuracy of 0.5% FS, installed 1.2 m downstream of the outlet flange of the high-pressure water pump. An electric control valve of Wuzhong Instrument ZDLP type can be selected, with a nominal diameter of DN50. The pipeline connecting to the nozzle uses ASTM A106 Gr.B seamless steel pipe with a wall thickness of 6 mm. The pressure regulation system can be integrated with Siemens S7-1500 PLC. The analog output module is selected as SM1232, and the signal cable can be selected as BELDEN 8761 double-shielded twisted pair wire, which is laid along the outer wall of the caisson through a Φ32 mm galvanized steel pipe. In the above technical solution, during the dynamic monitoring process, the operation can be paused for 2 min for data verification before each pressure regulation stage conversion. The determination threshold of the verticality deviation difference can be set to 0.05°, and the time interval between adjacent two verifications can be set to 60 s. The pressure synchronization control system can be configured with a proportional-integral regulation algorithm, the integral time constant can be set to 3 s, and the proportional band can be adjusted to 15%. When implementing the reverse pressure regulation, the pressure decrease rate can maintain a synchronous accuracy of ±0.05 MPa / s with the pressure increase rate. This technical solution controls the pressure overshoot in the pressure regulation process within ±0.15 MPa, and shortens the response time of the caisson attitude adjustment to less than 150 s. The three-stage pressure regulation mechanism reduces the system oscillation amplitude by 60%, and compresses the fluctuation range of the verticality deviation difference to 0.02° - 0.04°. The dynamic feedback control improves the pressure synchronization accuracy to ±0.8%, increases the success rate of the deviation correction operation to over 95%, and reduces the overall system energy consumption by 18% - 22%.

[0047] In another technical solution, in the caisson construction method, during the water pressure regulation operation, after each stage adjustment is completed, the operation is paused for 2 min for inclination sensor data verification. When the verticality deviation difference between adjacent two verifications is less than 0.05°, the next stage is entered; the reduction process of the water pressure at the high-pressure water nozzle in the opposite direction is implemented synchronously with the three stages and the pressure change rate maintains the same value.

[0048] In the above technical solution, in the setting of the phase conversion quantization determination criterion, the pause time after the voltage regulation phase is completed can be set to 2 min. The threshold value of the difference in perpendicularity deviation between two adjacent verifications can be selected as 0.05°, the data acquisition interval can be configured as 60 seconds. The number of verification times can be set to 3 consecutive acquisitions, and the standard deviation calculation window can be set to a 7-point moving average. When the difference in deviation is less than 0.05° for two consecutive times, it is allowed to enter the next phase. In the above technical solution, when implementing the reverse pressure synchronization control, the initial pressure of the nozzle group in the opposite direction can be set to 2.2 MPa, and the pressure reduction rate can be the same as the pressure increase rate, which is 0.4 MPa / s. For the pressure balance detection, a Yokogawa EJA430A differential pressure transmitter with a range of 0 - 2 MPa can be selected and installed between the main pipelines of the inclined side and the opposite side nozzle group, 1.8 meters away from the outlet of the regulating valve. The synchronous control algorithm can be configured with a feedforward-feedback composite control mode, the feedforward compensation coefficient can be set to 01.0, and the feedback adjustment period can be set to 3 seconds. In the above technical solution, when the system is linked and executed, the pressure regulation command can be sent to the four groups of nozzle controllers simultaneously, and the response time difference is controlled within 0.8 seconds. The electric actuator can be of the ATOK AUMA SAR type, and the stroke time can be set to 20 seconds / full stroke. The pressure fluctuation suppression module can be integrated into the Siemens S7-1500 PLC, and the filtering time constant can be set to 1.0 second. The dynamic balance detection frequency can be configured as 2 times per second, and the pressure matching tolerance can be set to ±0.08 MPa. This technical solution reduces the phase conversion determination error to within 0.02°, and shortens the system response delay to less than 0.7 seconds. The reverse pressure synchronization control limits the secondary offset within the range of 0.05°, and reduces the pressure balance time by 40%. The composite control algorithm reduces the pressure fluctuation amplitude by 55%, increases the system stability index by 35%, and reduces the hydraulic impact energy during the operation process to below 120 kJ / m³.

[0049] In another technical solution, in the caisson construction method, in step five, two barges carrying gravel with a particle size of 5 - 10 mm are symmetrically arranged on both sides of the caisson. The distances between the two barges and the center of the caisson are equal, and a spacing of 25 m is maintained. During the dumping operation, the two barges synchronously release gravel at a rate of 2 m³ per minute, and the thickness distribution of the gravel layer is scanned in real time by a multibeam sounding instrument. When the scanning data shows that the minimum value of the gravel layer thickness in any 10 m × 10 m area reaches 1.8 m, start the clay backfilling operation. The plastic index of the backfilled clay is controlled between 12 - 18. A long-arm excavator and a dump truck are used to form two parallel operation lines. The first operation line lays a 0.3 m thick clay layer along the periphery of the caisson, and the second operation line supplements and lays a 0.2 m thick clay layer after the previous operation line is compacted to form a single-layer backfill body.

[0050] In the above technical solution, in the control of symmetric dumping operation, a 800-ton or 1000-ton self-unloading barge can be selected. The hull length can be set to 45 meters, and the distance from the caisson center can be controlled within 25 meters. The distance between barges can be set to 50 meters, and the dumping rate can be configured to 2 cubic meters per minute. The synchronous delivery control system can select the Beidoustar BD982 positioning terminal with a planar positioning accuracy of ±0.1 meter, which is installed at a height of 1.8 meters on the top of the barge cab. During the dumping operation, the difference in the draft of the ship needs to be controlled within the range of ±0.5 meter. In the above technical solution, for the monitoring of the thickness of the gravel layer, an R2Sonic 2024 or Kongsberg EM2040 multi-beam sounding instrument can be selected. The transducer can be installed in the middle of the bottom of the survey ship at a depth of 0.6 meter from the water surface. The scanning area can be divided into 10-meter × 10-meter grids, and the thickness determination threshold can be set to 1.8 meters. When the minimum thickness in a single grid reaches the threshold, the control system can automatically trigger an audible and visual alarm, and the alarm signal delay does not exceed 3 seconds. In the above technical solution, when implementing the layered backfilling process, the plasticity index of the clay can be controlled in the range of 12 - 18, and the corresponding optimal moisture content range is 18% - 24%. A Caterpillar 336 or Komatsu PC360-8 long-arm excavator can be selected, and the arm span can be set to 18 meters. A Shaanxi Auto Delong X3000 or Jiefang J6P dump truck can be selected, and the load capacity can be configured to 25 tons. The laying thickness of the first operation line can be set to 0.3 meter, and the thickness of the second supplementary layer can be selected to be 0.2 meter, forming a single-layer total thickness of 0.5 meter. This technical solution controls the symmetry deviation of gravel dumping within ±4%, and the thickness uniformity reaches an accuracy of ±0.12 meter. The multi-beam real-time monitoring reduces the misjudgment rate to less than 3%, and shortens the backfilling start response time to within 15 seconds. The layered backfilling process makes the standard deviation of the clay compaction degree ≤1.2%, compresses the single-layer construction period to 2.5 hours, reduces the lateral earth pressure imbalance coefficient to below 1.03, and reduces the overall foundation settlement by 35%.

[0051] In another technical solution, for the caisson construction method described, when constructing each layer of backfill, first use a vibratory roller with a vibration force of 180 kN to roll without vibration twice, and then use a roller with a vibration force of 280 kN to roll strongly four times. The rolling speed is maintained at 2 km / h, and the adjacent wheel tracks overlap by 40 cm. After rolling, the sand replacement method is used to detect the compaction degree. It is required that the compaction degree ≥96% before the next layer of construction can be carried out.

[0052] In the above technical solution, in the configuration of layered rolling process parameters, a vibratory roller with an exciting force of 180kN can be used in the initial compaction stage, the number of non-vibration rolling passes can be set to 2, and the rolling speed can be controlled at 2.0km / h. A roller with an exciting force of 280kN can be configured in the re-compacting stage, the number of strong vibration rolling passes can be selected to 4, and the overlapping width of adjacent wheel tracks can be set to 40cm. A 1.5m wide unrolled strip can be reserved in the transitional rolling area for joint treatment. In the above technical scheme, the compaction degree test can be carried out by sand filling method or nuclear density meter method. The diameter of the sand filling tube can be 200mm, and the standard quartz sand particle size of 0.25-0.5mm can be used for measuring sand. The detection point arrangement can be 3 measuring points per 800㎡, and the measurement point positions can be distributed in plum blossom shape or grid shape. The compaction degree qualification standard can be set to 96%, and the allowable deviation value of a single measuring point is -1.0%. The detection data can be uploaded to the project management platform in real time, and the over-limit data triggers the red warning mark. In the above technical scheme, during the quality control of the construction process, the thickness of each backfill layer can be checked by the insertion method or laser rangefinder, and the checking frequency can be set to one section every 30m. A rapid measuring instrument can be used for clay moisture content detection, and the number of detections per work shift can be set to 5 times. The rolling track can be recorded by the GPS positioning system, and the track coverage rate is required to reach 97%. The abnormal working condition treatment plan includes 2 times of pressure replenishment, replacement or addition of lime for improvement, and the treatment range can be limited to 1.5m outside the defect area. This technical solution has increased the compaction rate of clay backfill to more than 98.5%, and the shear strength of the interlayer interface has increased by 40%. The two-stage rolling process has increased the construction efficiency by 30%, and the wheel track overlap control accuracy has reached ±5cm. The discrete coefficient of quality inspection data is ≤3.5%, the defect handling response time is shortened to within 20 minutes, and the overall backfill permeability coefficient is reduced to 1×10⁻ 6 cm / s order of magnitude.

[0053] In another technical solution, in the caisson construction method, in step 3, the high-pressure water pump is first run at no load for 30 seconds before starting, and then the working water pressure is established in three stages: in stage A, the water pressure is increased to 1.0 MPa at a rate of 0.5 MPa / s and maintained for 60 seconds, in stage B, the water pressure is increased to 2.0 MPa at a rate of 0.3 MPa / s and maintained for 90 seconds, and in stage C, the water pressure reaches 2.5 MPa at a rate of 0.2 MPa / s; The nozzle spraying operation adopts intermittent pulse mode, with a pause of 15 seconds after every 120 seconds of spraying. During the pause, the mud concentration is detected by the turbidity sensor pre-buried at the bottom of the caisson. When the mud concentration is lower than 180g / L, the water pressure will be increased by 0.2MPa in the next spraying cycle, and the maximum will not exceed 3.0MPa. The monitoring system collects the inclination data at the four corners with a sampling frequency of 5 times per second, processes the data using a sliding average filtering algorithm, updates the inclination display value every 10 seconds, and triggers an audible and visual alarm when the change in the display value between two adjacent times exceeds 0.1°.

[0054] In the above technical solution, in the start-up control of the high-pressure water pump, the no-load running time can be set to 30 seconds, and the pressure increase rates in the three stages can be configured to be 0.5 MPa / s respectively. The target pressure in stage A can be selected as 1.0 MPa, and the holding time can be adjusted to 60 seconds. The pressure increase end point in stage B can be set to 2.0 MPa, and the holding time can be configured to 90 seconds. The final working pressure in stage C can be controlled at 2.5 MPa or 2.7 MPa, and the pressure stabilization time can be set to 150 seconds. In the above technical solution, the spraying operation mode can be selected as a cyclic combination of spraying for 150 seconds and pausing for 20 seconds. The turbidity sensor can be selected as the Hach 2100Q type or the E+H CUS71 type, with a measuring range of 0 - 500 g / L, installed at the central position of the bottom of the caisson, 1.2 meters away from the outlet of the nearest spray pipe. The water pressure increase step can be set to 0.2 MPa, and the maximum pressure limit can be configured to 3.0 MPa. The determination threshold of the mud concentration can be set to 180 g / L, and the response delay for triggering the water pressure adjustment does not exceed 8 seconds. In the above technical solution, the data acquisition of the monitoring system can be configured with a sampling frequency of 5 Hz, and the sliding average filtering window can be set to 10 points. The update interval of the inclination display value can be selected as 10 seconds, and the audible and visual alarm trigger threshold can be set to 0.1°. The data storage module can be selected as the Advantech USB-4716 type acquisition card, and the storage interval can be configured to 2 seconds. The signal transmission line can be selected as the BELDEN 8761 double twisted shielded wire, and is laid along the maintenance passage of the caisson to the control room through a Φ25 mm galvanized steel pipe. This technical solution reduces the start-up impact load of the high-pressure water pump by 65%, and controls the pipeline vibration amplitude within ±0.12 mm. The feedback control of the mud concentration increases the proportion of the effective scouring time to 82% - 85%, and shortens the water pressure adjustment response time within 7 seconds. The data drift of the monitoring system ≤0.015°, the alarm trigger accuracy rate reaches 97%, and the overall energy consumption of the system decreases by 18% - 22%.

[0055] In another technical solution, in the caisson construction method described, when continuous slurry flows out of the overflow port in step six, the grouting process includes: Continuously grout at a constant pressure of 0.8 MPa for 5 minutes. During this period, the slurry flow rate at the overflow port is detected every 30 seconds. When the fluctuation amplitude of the flow rate exceeds ±10%, automatically compensate the pressure deviation value ΔP = 0.05×(V 实测 -V 基准 ) / V 基准 MPa, where V 基准is 50 L / min; Reduce the pressure step to 0.6 MPa and maintain for 3 min. Synchronously monitor the rising speed of the grout through an ultrasonic flowmeter embedded in the middle of the gap, and control the rising speed within the range of 20 - 25 cm / min; The pressure is restored to 0.8 MPa and maintained for 2 min. At the same time, four displacement sensors are symmetrically installed on the top of the caisson. When the vertical displacement difference of each sensor exceeds 0.5 mm, stop grouting and release the pressure.

[0056] In the above technical solution, in the dynamic compensation control of the grouting pressure, the constant pressure stage can be set to 0.8 MPa, and the duration can be selected as 5 min. The threshold of the flow rate fluctuation range can be configured as ±10%, and the corresponding pressure compensation coefficient can be set to 0.05. The reference flow rate can be set to 50 L / min. For real-time flow rate detection, an Emerson Rosemount 8712 electromagnetic flowmeter can be selected and installed 0.8 meters downstream of the outlet flange of the grouting pump. The denominator in the pressure deviation compensation formula can also select 90% or 110% of the reference flow rate as the calculation base. In the above technical solution, when the grout rising speed is coordinated and adjusted, the target pressure in the pressure reduction stage can be selected as 0.6 MPa, and the maintenance time can be adjusted to 3 min. An ultrasonic flowmeter of Geokon BGK-3D type can be selected, with a range of 15 - 30 cm / min, and installed in a buried casing at the mid-height of the gap. The casing material can be selected as PVC-U pipe. The speed control algorithm can be configured in a proportional-integral regulation mode, the integral time constant can be set to 2 s, and the proportional gain can be adjusted to 0.8. The pressure correction step when the grout rising speed exceeds the limit can be set to 0.03 MPa. In the above technical solution, in the displacement difference monitoring and grouting termination mechanism, an Omron ZX-LD40 laser displacement meter can be selected as the displacement sensor, with a range of ±10 mm and an accuracy of ±0.1%. It is symmetrically installed on the 100×100 mm steel plate bases embedded at the four corners of the top surface of the caisson. The vertical displacement difference threshold can be set to 0.5 mm, and the data acquisition frequency can be configured as 3 times per second. An electric ball valve with a nominal diameter of DN50 can be selected for the pressure release process, and the pressure release rate can be controlled at 0.2 MPa / s. After stopping grouting, the pressure needs to be kept stable for 20 min, and the displacement rebound amount during this period shall not exceed 0.2 mm. This technical solution makes the filling density of the grouting gap reach 96% - 98%, and the grout loss is controlled within 2.5 m³. The dynamic pressure compensation compresses the flow rate fluctuation range to below ±7%, and the rising speed control accuracy reaches ±1.2 cm / min. The displacement difference monitoring reduces the structural eccentric load risk by 85%, improves the grouting termination judgment accuracy to over 98%, and the overall construction qualification rate reaches 99.2%.

[0057] In another technical solution, for the caisson construction method, the specific implementation of the steel strand tensioning in step seven is as follows: During the first tensioning, a hole-through jack is used to apply the tensile force at a rate of 2 mm per minute until 30% of the design value is reached. After holding the load for 5 minutes, the elongation of the steel strand is measured. When the deviation of the elongation exceeds ±5% of the theoretical value, the subsequent tensile force is adjusted according to ΔF = 0.15×(L 实测 -L 理论 ) / L 理论 ×F design , where F design is the design tensile force; The second tensioning is carried out 4 hours after the first one, and the load is increased at a rate of 3 mm per minute to 80% of the design value. At the same time, the concrete strain value in the area of the embedded casing on the caisson top plate is monitored, and the strain growth rate is controlled not to exceed 5 με / min; The third tensioning is implemented 4 hours after the second one. When the load is increased to 100% of the design value, the load is maintained for 10 minutes. During this period, the tension loss caused by the relaxation of the steel strand is compensated every 2 minutes, and the compensation amount is 0.3% of the initial tensile force; After the tensioning is completed, a hydraulic locking nut is used for anchoring. The tightening torque of the nut is 850 N·m, and a 304 stainless steel anti-loosening locking piece with a thickness of 8 mm is installed at the exposed end of the anchor. A special anti-slip grease with a friction coefficient of 0.12 is applied to the contact surface between the locking piece and the anchor.

[0058] In the above technical solution, in the control of the step-by-step tensioning parameters, the proportion of the first tensioning design value can be set to 30%, the loading rate can be selected as 2 mm per minute. The holding time can be configured as 5 minutes, and the allowable range of the elongation deviation can be set to ±5%. The loading rate of the second tensioning can be adjusted to 3 mm per minute, and the threshold value of the concrete strain growth rate monitored synchronously can be set to 5 με / min. The holding time of the third tensioning can be selected as 10 minutes, and the tension loss compensation frequency can be configured as once every 2 minutes. In the above technical solution, for the monitoring during the tensioning process, a Geokon BGK-4850 vibrating wire strain gauge can be selected, with a measuring range of ±1500 με, and it is installed within 150 mm around the embedded casing on the caisson top plate. For the measurement of the elongation, an Omron ZW-7000 laser length gauge can be selected, with an accuracy of ±0.05 mm, and it is installed outside the piston rod of the jack. The adjustment coefficient in the compensation amount calculation formula can be set to 0.15, and the reference value of the design tensile force can be taken as 70% of the breaking strength of the steel strand. In the above technical solution, for the anti-loosening treatment of the anchoring system, a hydraulic locknut conforming to DIN6914 standard with M36 specification can be selected, and the tightening torque can be set at 850 N·m. The anti-loosening locking piece can be a 304 stainless steel plate with a thickness of 8 mm, and the outer diameter can be set at 110 mm. The special anti-slip grease can be Klüber Tribofilm NW12 type of Klüber, with a friction coefficient range of 0.10 - 0.15, and the applied thickness can be controlled at 0.3 mm. For the anti-corrosion treatment of the exposed end of the anchor, hot-dip galvanizing or Dacromet process can be adopted, and the coating thickness can be selected as 60 μm. In this technical solution, the control accuracy of the steel strand elongation reaches ±1.8 mm, and the effective prestress retention rate ≥ 95%. The step-by-step tensioning process reduces the peak value of the local compressive stress of the concrete by 30%, and the strain monitoring response delay ≤ 0.5 s. The anti-loosening structure treatment controls the displacement of the anchoring system within the range of 0.15 - 0.25 mm, and after 10 6 times of fatigue load tests, the prestress loss rate ≤ 2.5%, and the slippage of the anchor clip ≤ 0.08 mm.

[0059] Application example of the caisson construction method In a river regulation project, 12 reinforced concrete caissons need to be installed, with a single weight of 800 t and a designed sinking depth of 9 m. The riverbed geology is silty clay interbedded with sand layer, and the underground water flow velocity is 0.8 m / s. As Figure 1 shown, the following operations are carried out using this technical solution: S100: Caisson prefabrication and equipment installation When fabricating the caisson in the prefabrication plant, SICK CKS36 tilt sensors (accuracy ±0.01°) are embedded at four diagonal corners of the inner wall, 1.2 m from the bottom plate, and the signal wires are led to the top junction box through Φ20 galvanized steel pipes. Six groups of high-pressure water nozzle bases are welded on the bottom plate, with three Φ50 nozzles in each group arranged in an equilateral triangle (side length 200 mm), and the axis forms a 30° angle with the horizontal plane. The nozzles are made of ASTM A106 Gr.B seamless steel pipes and are connected to Grundfos CR45-6 high-pressure water pumps (flow rate 120 m³ / h) through Parker H series quick connectors.

[0060] S200: Precise positioning and system calibration A 2000 t floating crane is used to hoist the caisson. The Trimble SPS986 positioning system controls the plane deviation ≤ 40 mm, and the bottom is kept at a distance of 0.5 m from the riverbed. After connecting the Siemens S7-1200 monitoring system, a Leica LS10 laser level is used for zero calibration: adjust four 50 t hydraulic jacks to make the laser crosshair coincide with the prefabricated mark, and the software automatically compensates for the zero offset of the sensor (maximum compensation amount 0.03°). After three 0.3 m lateral micro-displacement tests, the standard deviation of the corner inclination data is stable at 0.018°.

[0061] S300: Controlled Sinking Operation The high-pressure water pump builds up water pressure in stages: rising from 0.5 MPa / s to 1.0 MPa (60 s) → 0.3 MPa / s to 2.0 MPa (90 s) → 0.2 MPa / s to 2.5 MPa. The jetting adopts a 120 s working / 15 s intermittent mode, and the Hach 2100Q turbidimeter detects the mud concentration in real time. When the inclination difference between the northeast-southwest diagonals reaches 0.52°, the system starts the pressure regulating program: the 3 groups of nozzle pipes on the inclined side rise from 0.3 MPa / s to 2.8 MPa (120 s) → 0.5 MPa / s to 3.3 MPa → stabilize at 3.8 MPa; the opposite side reduces the pressure synchronously, and after 180 s, the inclination difference drops to 0.27°. The total sinking time is 6.5 h, and the verticality deviation is 0.28°.

[0062] S400: Intelligent Backfilling Construction After the caisson is in place, two 800 m³ barges simultaneously backfill 5-10 mm gravel at a 25 m spacing, and the R2Sonic 2024 depth sounder generates a thickness cloud map in real time. After 2.2 h, a 2.0 m gravel layer is completed (thickness deviation ±0.12 m). The clay backfilling uses a combination of a Caterpillar 336 excavator and a Dynapac CA250 roller: each layer is laid in two times of 0.3 m + 0.2 m, and is compacted without vibration 2 times with 180 kN + 4 times with 280 kN strong vibration (speed 2 km / h, overlap 40 cm). The compaction degree of 10 points is detected by the sand replacement method, and the results are 96.2% - 97.8%.

[0063] S500: Interstitial Grouting and Anchoring When backfilling to 1.2 m from the top, insert a Φ25 grouting pipe and inject cement slurry with a water-cement ratio of 0.45. The Jikon BGK-3D monitors the rising speed of the slurry at 23 cm / min, and the grouting pressure is dynamically compensated 3 times (maximum ΔP = 0.04 MPa). The displacement sensor shows that when the maximum vertical difference is 0.42 mm, the grouting is terminated. After 24 h, install a Q355B flange, and the OVM YCW250B jack tension the steel strands in three stages of 30% (2 mm / min), 80% (3 mm / min), and 100%, and compensate for the tension loss 0.3% × 3 times during this period. Finally, tighten the 850 N·m hydraulic nut and install an 8 mm 304 locking piece.

[0064] Relevant parameter data during the entire construction process 1. Verticality control: The maximum inclination difference during the whole process is 0.52° → after adjustment is 0.27°, and the final sinking deviation is 0.28° (traditional process 1.2° - 2.5°) 2. Backfilling quality: The thickness range of the gravel layer is 0.24 m (specification allows 0.5 m); the average clay compaction degree is 97.1% (specification ≥95%) 3. Grouting effect: Ultrasonic detection shows that the interstitial filling degree is 98.2% (traditional process 85 - 90%) 4. Anchoring performance: The prestress retention rate after tensioning is 96.3% at 72 h (89 - 92% for the traditional process). 5. Construction efficiency: The construction period for a single caisson is 78 h (120 - 150 h for the traditional process). This example verifies the significant improvement of the caisson construction method provided by the present invention in aspects such as verticality control, backfill compactness, and structural connection reliability. All indicators are superior to industry standards, effectively solving key technical problems such as cumulative deflection and foundation leakage in traditional caisson construction.

[0065] The equipment quantities and processing scales described here are used to simplify the description of the present invention. Applications, modifications, and variations of the present invention will be apparent to those skilled in the art.

[0066] Although the embodiments of the present invention have been disclosed as above, they are not limited to only the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the field, additional modifications can be easily achieved. Therefore, without departing from the general concept defined by the claims and the equivalent scope, the present invention is not limited to specific details and the illustrated and described examples here.

Claims

1. A caisson construction method, characterized in that: The following steps are involved: Step 1: Prefabricate reinforced concrete caissons, embed inclination sensors at four diagonal positions on the inner wall of the caisson, and weld six groups of high-pressure water nozzle mounting bases evenly distributed in a ring at the bottom of the caisson. Each group of bases contains three mounting holes arranged in an equilateral triangle. The axis of the mounting hole forms an angle of 25-35° with the horizontal plane. A steel nozzle with a diameter of 50 mm is fixed in the mounting hole, and the nozzle is connected to the outlet of the high-pressure water pump through a quick connector. Step 2: Hoist the caisson to the designed positioning point, keep the distance between the bottom of the caisson and the riverbed surface at 0.3-0.7m, start the high-pressure water pump to inject water and drain the air, then shut it down, install the verticality monitoring system and connect the inclination sensor signal line to the system to complete the zero point calibration; Step 3: Start the high-pressure water pump to supply water to the nozzle at an initial water pressure of 2.0-3.0 MPa. The water flows at an elevation angle of 30° to impact the riverbed to form mud. The caisson sinks by its own weight, and the monitoring system displays the four-corner inclination data in real time. Step 4: Determine the caisson's posture based on the monitoring data: If the difference in the inclination angles of the two diagonal angles in any direction exceeds 0.5°, perform the pressure adjustment operation to restore the initial water pressure after the inclination angle difference is less than 0.3°; if the inclination angle difference does not exceed the limit, maintain the current water pressure and continue to sink; Step 5. Repeat steps 3 to 4 until the caisson touches the bottom at the designed elevation. After turning off the water pump, immediately fill the caisson with 5-10mm gravel symmetrically to a thickness of 1.8-2.2m. Then backfill with clay in layers, with each layer 0.5m thick. Use a vibratory roller with an excitation force of 280kN to roll six times, with adjacent wheel tracks overlapping by one-third of the wheel width.

2. The caisson construction method according to claim 1, characterized in that: Also includes: Step 6: When the backfill reaches an elevation of 1.2m from the top of the caisson, insert a grouting pipe at the bottom of the 200mm annular gap between the inner wall of the caisson and the backfill body, and inject cement slurry with a water-cement ratio of 0.45 through a grouting pump. The grouting flow rate is controlled at 50L / min. When the slurry flows continuously from the overflow port at the top of the gap, maintain the grouting pressure for 10 minutes and then stop grouting; Step 7. 24 hours after the grouting is completed, install the steel connecting flange on the top surface of the caisson. Eight anchor holes are evenly distributed around the circumference of the flange. Two prestressed steel strands with a diameter of 32 mm are inserted into each hole. The lower ends of the steel strands are anchored in the embedded casing of the caisson top plate. They are tensioned three times to 30%, 80% and 100% of the design value. Each tensioning interval is 4 hours. Finally, the anchor head is fixed with a hydraulic locking nut.

3. The caisson construction method according to claim 2, characterized in that: The details of signal line access and zero point calibration in step 2 are as follows: Connect the RS485 signal lines of the four tilt sensors in parallel to the 1st to 4th channels of the data acquisition card of the verticality monitoring system, set up a laser level in the center of the top plane of the caisson, and adjust the caisson hoisting posture so that the cross reference line emitted by the laser level coincides with the cross positioning mark embedded on the top surface of the caisson. At this time, the caisson is in a theoretical vertical state; Start the monitoring system calibration mode and read the initial output values ​​of the four inclination sensors at the same time. If the absolute value of the output angle of any sensor exceeds 0.05°, the monitoring system software sends a zero offset compensation command to the corresponding sensor. The compensation amount is Δθ=-(θ1+θ2+θ3+θ4) / 4, where θ4 is the current sensor reading. After completing the software compensation, the caisson was operated to perform three lateral micro-displacements with an amplitude of 0.3 m. After each displacement, it was kept still for 2 minutes to collect sensor data. The calibration was considered valid when the standard deviation of the angle changes output by the four groups of sensors was less than 0.02°.

4. The caisson construction method according to claim 1, characterized in that: The water pressure adjustment operation in step 4 is as follows: In the first stage, the water pressure of the high-pressure water nozzle in the corresponding area of ​​the tilt direction is increased to 2.8MPa at a rate of 0.3MPa / s, and the pressure is maintained for 120s before the change in the caisson's posture is detected; In the second stage, the water pressure is linearly adjusted to 3.3MPa according to the change in posture, and the adjustment rate does not exceed 0.5MPa / s; In the third stage, the water pressure is stabilized to 3.8 MPa at a rate of 0.2 MPa / s; Repeat the three-stage adjustment until the verticality deviation of the caisson is less than 0.3°.

5. The caisson construction method according to claim 4, characterized in that: During the water pressure regulation operation, the operation is suspended for 2 minutes after each stage of adjustment to verify the inclination sensor data. When the difference in verticality deviation between two adjacent verifications is less than 0.05°, the next stage is entered. The process of reducing the water pressure of the high-pressure water nozzle in the relative direction is implemented synchronously with the three stages and the pressure change rate remains at the same value.

6. The caisson construction method according to claim 1, characterized in that: In step 5, two barges loaded with gravel with a particle size of 5-10 mm are symmetrically arranged on both sides of the caisson. The distance between the two barges and the center of the caisson is equal and maintained at a distance of 25 m. During the dumping operation, the two barges simultaneously drop gravel at a rate of 2 m³ per minute, and the thickness distribution of the gravel layer is scanned in real time by a multi-beam echo sounder; When the scanning data shows that the minimum thickness of the gravel layer in any 10m×10m area reaches 1.8m, the clay backfill operation is started, and the plasticity index of the backfill clay is controlled between 12-18. A long-arm excavator and a dump truck are used to form two parallel operation lines. The first operation line lays a 0.3m thick clay layer along the outer periphery of the caisson, and the second operation line lays an additional 0.2m thick clay layer after the previous operation line completes the rolling to form a single-layer backfill body.

7. The caisson construction method according to claim 6, characterized in that: During the construction of each layer of backfill, first use a vibratory roller with an exciting force of 180kN to roll it without vibration for two times, and then use a roller with an exciting force of 280kN to roll it four times with strong vibration. The rolling speed is maintained at 2km / h, and adjacent wheel tracks overlap by 40cm. After rolling, the compaction degree is tested by the sand injection method. The compaction degree must be ≥96% before the next layer can be constructed.

8. The caisson construction method according to claim 1, characterized in that: In step 3, the high-pressure water pump is first run at no load for 30 seconds before starting, and then the working water pressure is established in three stages: in stage A, the water pressure is increased to 1.0 MPa at a rate of 0.5 MPa / s and maintained for 60 seconds, in stage B, the water pressure is increased to 2.0 MPa at a rate of 0.3 MPa / s and maintained for 90 seconds, and in stage C, the water pressure reaches 2.5 MPa at a rate of 0.2 MPa / s; The nozzle spraying operation adopts intermittent pulse mode, with a pause of 15 seconds after every 120 seconds of spraying. During the pause, the mud concentration is detected by the turbidity sensor pre-buried at the bottom of the caisson. When the mud concentration is lower than 180g / L, the water pressure will be increased by 0.2MPa in the next spraying cycle, and the maximum will not exceed 3.0MPa. The monitoring system collects four-corner inclination data at a sampling frequency of 5 times per second, processes the data using a sliding average filter algorithm, updates the inclination display value every 10 seconds, and triggers an audible and visual alarm when the change between two adjacent display values ​​exceeds 0.1°.

9. The caisson construction method according to claim 1, characterized in that: When continuous slurry flows out of the overflow port in step 6, the grouting process includes: The grouting was continued for 5 minutes at a constant pressure of 0.8 MPa. The slurry flow rate at the overflow port was detected every 30 seconds. When the flow rate fluctuation exceeded ±10%, the pressure deviation value ΔP = 0.05 × (V 实测 -V 基准 ) / V 基准 MPa, where V 基准 50L / min; The pressure was reduced to 0.6 MPa and maintained for 3 min. The rising speed of the slurry was monitored by an ultrasonic flow meter pre-buried in the middle of the gap, and the rising speed was controlled within the range of 20-25 cm / min. The pressure is restored to 0.8MPa and maintained for 2min. At the same time, four displacement sensors are symmetrically installed on the top of the caisson. When the difference in the vertical displacement of the caisson displayed by each sensor exceeds 0.5mm, the grouting is terminated and the pressure is released.

10. The caisson construction method according to claim 1, characterized in that: The specific implementation method of the steel strand tensioning in step seven is: During the first tensioning, a through-type jack is used to apply tension to 30% of the design value at a rate of 2 mm per minute. After holding the load for 5 minutes, the extension of the steel strand is measured. When the extension deviation exceeds the theoretical value by ±5%, the value is calculated according to ΔF=0.15×(L 实测 -L 理论 ) / L 理论 ×F design Adjust the subsequent tension force, where F design To design the tension force; The secondary tensioning is carried out 4 hours after the first tensioning is completed, and the load is loaded to 80% of the design value at a rate of 3mm per minute. The concrete strain value of the pre-buried casing area of ​​the caisson top plate is monitored simultaneously, and the strain growth rate is controlled to be no more than 5με / min; The third tensioning is carried out 4 hours after the second tensioning is completed. When the load reaches 100% of the design value, the load is maintained at 10 mmin. During this period, the tension loss caused by the relaxation of the steel strand is compensated every 2 minutes. The compensation amount is 0.3% of the initial tensioning force. After tensioning is completed, a hydraulic locking nut is used for anchoring. The tightening torque of the nut is 850 N·m, and an 8mm thick 304 stainless steel anti-loosening locking plate is installed on the exposed end of the anchor. The contact surface between the locking plate and the anchor is coated with special anti-skid grease with a friction coefficient of 0.12.

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