Device and method for measuring pile body stress of rotary excavating implanted pile

Abstract

The invention discloses a device and a method for measuring the pile body stress of a rotary excavating implanted pile. The device comprises a composite pile inner core, a composite pile outer core, an inner core monitoring unit, an outer core monitoring unit and a bearing platform, the inner core monitoring unit is a first optical fiber string which is formed by connecting a plurality of first high-precision strain sensors in series in a segmented encryption manner, and the first optical fiber string is arranged in a channel formed in the inner core pile body of the composite pile; the outer core detection unit is a second optical fiber string formed by connecting a plurality of second high-precision strain sensors in series in a segmented encryption manner and is mounted in the composite pile outer core, and after mounting is completed, the first optical fiber string and the second optical fiber string are in one-to-one correspondence; the inner core monitoring unit and the outer core monitoring unit are both connected with the fiber bragg grating acquisition instrument and transmit acquired sensor data to a control center for pile body stress measurement; the bearing platform is simultaneously connected with the top of the inner core and the top of the outer core to eliminate stress lag of the outer core of the composite pile; a hole is formed in the center of the bearing platform and used for allowing a pile end settlement testing pipe to penetrate through and providing a pile end monitoring channel.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical and foundation engineering technology, specifically relating to a device and method for measuring the stress of rotary drilling piles. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] As a traditional high-bearing-capacity structure, pile foundations undergo continuous technological innovation to meet complex engineering challenges. From a technological perspective, piles can be broadly classified into precast piles and cast-in-place piles. While cast-in-place piles are highly adaptable, they suffer from drawbacks such as difficulty in removing sediment, easy necking and mud inclusion, complex processes, and severe pollution, which prevent them from fully realizing their bearing capacity. Precast piles, on the other hand, offer advantages such as stable pile quality, high construction efficiency, and environmental friendliness, but they are prone to soil squeezing when penetrating hard soil layers, affecting the stability of the pile foundation's bearing capacity. Against this backdrop, rotary-drilled composite piles combine the advantages of both through an innovative process: a hole is drilled using rotary drilling, and after cleaning, low-grade concrete or cement mortar is poured in, followed by the insertion of precast pipe piles, forming a composite pile where both share the load. This pile type effectively avoids the bearing capacity loss caused by pile driving, significantly increasing the pile's bearing capacity. Simultaneously, the inserted pile maintains its own high strength and reinforces the surrounding soil, requiring little or no mud wall protection, greatly reducing mud discharge during construction, improving resource utilization, and aligning with green construction standards. To accurately quantify the mechanisms by which rotary-drilled composite piles achieve these advantages, pile stress testing is particularly crucial. By monitoring the stress distribution and transmission patterns within the pile, the load sharing and collaborative stress-bearing process between the precast inner core and the cast-in-place outer core can be clarified, enabling precise control over its bearing potential and in-depth optimization of application scenarios.

[0004] Rotary-dug composite piles consist of a precast inner core pile (PHC pipe pile) and an outer core cast-in-place material (usually mortar or concrete). These two components do not act in isolation during stress distribution but rather involve a complex load distribution and stress transfer mechanism. In the composite pile structure, the inner core component bears the main load due to its high strength, while the outer core material achieves synergistic stress distribution through interfacial interaction with the surrounding soil, thus reinforcing the soil. However, without a systematic understanding of the stress distribution patterns of the inner and outer cores at different load stages and depths, it will be difficult to accurately quantify their load-sharing ratio and assess stress transfer efficiency, thereby affecting the accurate assessment of the overall structural bearing capacity. Only by simultaneously revealing the stress distribution of both the inner and outer cores can the bearing mechanism of rotary-dug composite piles be fully analyzed, providing a scientific basis for their rational application under different geological conditions, the construction of bearing capacity calculation models, and further optimization of construction techniques. Selecting appropriate pile stress testing methods is the prerequisite and foundation for achieving this goal.

[0005] However, current methods for stress monitoring of rotary-dug composite piles present challenges. Existing technologies primarily focus on monitoring the internal forces of the inner core pile, such as using distributed fiber optic sensing technology to monitor the axial force, side friction, and bending moment of the inner core pile in stiffened composite piles, thereby revealing its stress patterns. However, the bearing capacity of rotary-dug composite piles depends on the synergistic effect of the inner and outer core piles. Focusing solely on the stress state of the inner core pile while neglecting the stress conditions of the outer core, the stress distribution within both the inner and outer cores, and the stress at the pile ends fails to fully elucidate the overall bearing mechanism of the composite pile. It is difficult to accurately quantify the load-sharing ratio between the two piles, assess stress transfer efficiency, and consequently, affect the accurate assessment of the overall structural bearing potential. This hinders the rational application of this pile type under different geological conditions, the construction of bearing capacity calculation models, and further optimization of construction techniques. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a device and method for measuring the stress of rotary-dug piles. By using distributed optical fiber monitoring technology, the device quantifies the load distribution and stress transmission of the inner and outer cores of rotary-dug piles in mudstone strata, reveals the bearing mechanism of piles under mudstone foundation conditions, and effectively eliminates the engineering risk of abnormal pile bearing capacity in mudstone foundations. This provides reliable technical support for the design and construction of pile foundation projects under similar geological conditions.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution: On one hand, the technical solution of the present invention provides a device for measuring the stress of rotary-drilled piles, including: an inner core of a composite pile, an outer core of a composite pile, an inner core monitoring unit, an outer core monitoring unit, and a pile cap; wherein, the inner core monitoring unit is a first optical fiber string composed of multiple first high-precision strain sensors connected in a segmented and encrypted manner, the first optical fiber string being disposed in a channel opened in the inner core of the composite pile; the outer core monitoring unit is a second optical fiber string composed of multiple second high-precision strain sensors connected in a segmented and encrypted manner, the second optical fiber string being installed inside the outer core of the composite pile, and after installation, the first high-precision strain sensors correspond one-to-one with the second high-precision strain sensors; both the inner core monitoring unit and the outer core monitoring unit are connected to a fiber optic grating acquisition instrument, which transmits the acquired sensor data to a control center for pile stress measurement; the pile cap simultaneously connects the top of the inner core of the composite pile and the top of the outer core of the composite pile, eliminating the stress lag of the outer core of the composite pile; the center of the pile cap is provided with a hole for inserting a pile end settlement test tube, providing a pile end monitoring channel.

[0008] In at least one embodiment, the channel is a rectangular channel that is parallel to the axis of the core pile of the composite pile.

[0009] In at least one embodiment, the segmented and encrypted deployment method is as follows: within a set distance from the top of the pile, high-precision stress sensors are arranged at a first set interval to accurately capture the stress changes in the stress concentration area on the upper part of the pile; the remaining parts are arranged at a second set interval to balance detection efficiency and data integrity.

[0010] In at least one embodiment, the first predetermined spacing is smaller than the second predetermined spacing.

[0011] In at least one embodiment, the first high-precision stress sensor and the second high-precision stress sensor employ a range of [missing information]. High-precision fiber optic strain sensor.

[0012] In at least one embodiment, the first optical fiber string is parallel to the pile axis of the composite pile core; both ends of the first high-precision stress sensor are fixed by fixed supports, which are connected to the pile body of the composite pile core by expansion bolts. The first high-precision stress sensor is connected in series via a sensor armored optical cable, which is laid along the channel and kept in a naturally relaxed state.

[0013] In at least one embodiment, the composite pile outer core is cast onto the outside of the composite pile inner core by means of casting; the second optical fiber string is inserted into the composite pile outer core before the initial solidification of the composite pile outer core and corresponds one-to-one with the first optical fiber string.

[0014] In at least one embodiment, a first high-precision stress sensor at the bottom of the first fiber optic string is close to the bottom of the inner core of the composite pile; a second high-precision stress sensor at the bottom of the second fiber optic string is close to the bottom of the outer core of the composite pile.

[0015] On the other hand, the technical solution of the present invention also provides a method for measuring the stress in the shaft of a rotary-drilled pile, comprising: S1. A rectangular channel parallel to the pile axis is opened on the pile body of the composite pile core, and the installation position of each first high-precision stress sensor is precisely marked according to the segmented and densified layout method. S2. Clean the channel to ensure that there is no dust or debris residue inside; after cleaning, level the bottom of the channel and the installation position of the first high-precision stress sensor. S3. According to the marked installation position, first place the first optical fiber string in the channel and adjust it to be parallel to the pile axis of the composite pile core. Then, use a fixed support to fix both ends of the first high-precision stress sensor. The fixed support is connected to the pile body of the composite pile core through expansion bolts to ensure that the fixed support is tightly fitted to the pile body. Then, lay the sensor armored optical cable along the channel and ensure that it remains in a natural slack state. S4. After the first fiber string is laid, epoxy resin encapsulation is carried out; after encapsulation, plastic film is covered on the surface of the channel for curing for a set time; after the epoxy resin is fully cured, check the encapsulation layer for cracks and peeling and ensure that the set hardness has been reached. S5. Conduct a survival rate test on the first fiber optic string. The survival rate must reach more than 95%. For the first high-precision stress sensor that fails to meet the requirements, find out the cause and rework and replace it to ensure the stability and reliability of the core monitoring unit. S6. Precast pipe piles are prepared by casting an outer core of the composite pile on the outside of the inner core of the composite pile. Before the outer core of the composite pile initially sets, the second fiber string is slowly inserted into the set position of the outer core of the composite pile. After the insertion is completed, the second fiber string corresponds one-to-one with the first fiber string. After installation, the survival rate of the second fiber string is tested. S7. Piling and drilling are carried out, and cement mortar is injected into the hole after drilling is completed. Then, the precast pipe pile is inserted and driven by static pressure method. After the pile is planted, the composite pile is cured to ensure that the concrete strength meets the standard. The top of the inner core and the top of the outer core of the composite pile are connected by the pile cap, and the pile end settlement test tube is inserted in the hole in the center of the pile cap. S8. Static load test based on slow sustained loading method, set maximum load, load in multiple stages until pile failure, inner core monitoring unit and outer core monitoring unit monitor stress changes of inner core and outer core of composite pile in real time, fiber optic grating acquisition instrument transmits the collected sensor data to control center to complete pile stress measurement.

[0016] In at least one embodiment, the integrity of the pile body is tested using a low-strain dynamic testing method within a set number of days before the static load test and within a set time after the test.

[0017] The beneficial effects of the above-described technical solution of the present invention are as follows: 1) The rotary drilling and implanted pile stress measuring device of the present invention adopts a dual-sensor system design, and the two sensors correspond one-to-one, realizing the synchronous acquisition of stress data of the inner core and the outer core under the same load conditions and at the same depth. Through the sensor near the pile bottom, the stress value of the pile end can be directly measured, providing key data for analyzing the bearing contribution of the pile end. It can accurately determine the stress distribution characteristics and transmission law of the rotary drilling and implanted composite pile, and deeply reveal the bearing mechanism of the implanted pile under mudstone foundation conditions, thereby effectively eliminating the engineering risk of abnormal pile bearing capacity in mudstone foundation, and providing reliable technical support for the design and construction of pile foundation engineering under similar geological conditions.

[0018] 2) The method for determining the stress of rotary-drilled piles of the present invention uses a rotary-drilled pile stress measuring device. During the static load test, the entire cross-section of the inner and outer cores of the composite pile is subjected to pressure. Based on the stress measured by the optical fiber in the pile body, the stress distribution of the inner and outer cores at the same depth can be obtained, and the stress distribution characteristics and transmission law of the rotary-drilled composite pile can be accurately determined. Attached Figure Description

[0019] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0020] Figure 1 This is a schematic diagram of the installation of a rotary drilling pile stress measuring device disclosed in Embodiment 1 of the present invention; Figure 2 This is a cross-sectional view of the installation of a rotary drilling pile stress measuring device disclosed in Embodiment 1 of the present invention; Figure 3 This is a top view of the installation of a rotary drilling pile stress measuring device disclosed in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the static load test disclosed in Embodiment 2 of the present invention.

[0021] In the diagram: 1. Inner core of composite pile; 2. Outer core of composite pile; 3. First fiber optic string; 4. Pier; 5. Fiber optic grating acquisition instrument; 6. Control center; 7. Second fiber optic string; 8. High-strength bolt; 9. Hole. Detailed Implementation

[0022] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0023] As described in the background section, the purpose of this invention is to overcome the shortcomings of the prior art and provide a device and method for measuring the stress of rotary-dug piles. By using distributed optical fiber monitoring technology, the device quantifies the load distribution and stress transmission of the inner and outer cores of rotary-dug piles in mudstone strata, reveals the bearing mechanism of piles under mudstone foundation conditions, and effectively eliminates the engineering risk of abnormal pile bearing capacity in mudstone foundations. This provides reliable technical support for the design and construction of pile foundation projects under similar geological conditions.

[0024] Example 1 In a typical embodiment of the present invention, such as Figures 1-4As shown in the figure, this embodiment discloses a device for measuring the stress of rotary drilling piles, including: a composite pile inner core 1, a composite pile outer core 2, an inner core monitoring unit, an outer core monitoring unit, and a pile cap 4.

[0025] In this embodiment, the composite pile core 1 (i.e., precast pipe pile) is used as the test pile body. The outer diameter and length of the pile body can be determined according to the test requirements. Before use, a comprehensive inspection is required to ensure that the pipe pile is free of cracks and honeycomb defects, and to ensure that the surface flatness deviation of the pile body is controlled within 5mm. In this embodiment, a precast concrete pipe pile with an outer diameter of 500mm is selected, and the pile body length is determined according to the engineering design requirements.

[0026] In this embodiment, the precast pipe pile is one of the core load-bearing components of the composite pile, used to bear part of the load and transfer stress. Its top is connected to the pile cap 4 by high-strength bolts 8. Rectangular grooves are opened on the surface of the pile body to install the core monitoring units for monitoring the stress changes of the core 1 of the composite pile. Specifically, firstly, multiple groove center lines parallel to the pile axis are marked along the surface of the core 1 of the composite pile using a chalk line. The accuracy error of the chalk line must be controlled within the design requirements (e.g., ±2mm). Then, according to the layout design of the core monitoring units, the rectangular grooves are marked according to the designed groove dimensions (e.g., width 4cm × depth 2cm), and the installation position of each monitoring device in the core monitoring unit is precisely marked. Subsequently, the grooving construction is carried out. A special stone cutting machine is used to cut the contour along the center line. During the cutting process, the cutting speed must be controlled (e.g., 30cm / min) to avoid vibration caused by high-speed cutting that could damage the pile body. After the contour cutting is completed, use relevant equipment (such as a 16mm diameter impact drill) to excavate the trench. During excavation, keep the drill bit perpendicular to the pile body to avoid cracking at the edge of the trench. After the trench is completed, first use a high-pressure air gun to blow away the floating slag in the trench, and then use a stiff brush dipped in clean water to repeatedly clean the surface of the trench to ensure that there is no dust or debris residue in the trench. After cleaning, apply AB glue evenly to the bottom of the trench and the installation position of each monitoring device in the inner core monitoring unit for leveling treatment. The leveling thickness is controlled at 1-2mm. After the AB glue has initially cured (about 30 minutes), the inner core monitoring unit can be installed.

[0027] In this embodiment, the core monitoring unit is a first optical fiber string 3 composed of multiple first high-precision strain sensors connected in a segmented and encrypted manner. Specifically, the first high-precision strain sensors adopt a range of... The high-precision fiber optic strain sensor consists of multiple first high-precision strain sensors connected in series via sensor armored optical cables (e.g., using armored optical cables with a diameter of 4mm). The deployment method adopts a segmented and encrypted approach. Within a set distance (e.g., 3m) from the top of the inner core 1 of the composite pile, the first high-precision stress sensors are arranged at a first set interval (e.g., 500mm) to accurately capture the stress changes in the stress concentration area on the upper part of the pile. Outside the set distance from the top of the inner core 1 of the composite pile, the sensors are arranged at a second set interval (e.g., 1000mm) to balance detection efficiency and data integrity.

[0028] In this embodiment, the first predetermined spacing is smaller than the second predetermined spacing. When the composite pile is under load (e.g., vertical compression or horizontal shear), the upper part of the pile (especially within 3m of the pile top) is the area with the most significant stress concentration and the most drastic stress changes. Therefore, by setting a smaller first predetermined spacing, the density of the first high-precision strain sensors on the upper part of the pile is increased, allowing the high-precision strain sensors to cover as many stress change nodes as possible. As the load is transmitted downwards, the stress gradually disperses and the gradient decreases, resulting in a relatively stable stress state. Therefore, by using a second predetermined spacing that is larger than the first predetermined spacing, the density of the first high-precision strain sensors on the lower part of the pile is reduced, effectively avoiding data redundancy.

[0029] The first optical fiber string 3 is installed within a channel carved into the core 1 of the composite pile and secured by a fixed support. The fixed support is connected to the core 1 via expansion bolts and is used to monitor stress changes in the core 1. Specifically, after determining the installation position, the first optical fiber string 3 is first positioned and adjusted to be parallel to the axis of the core 1. Then, a fixed support (such as a 50mm×30mm×10mm stainless steel fixed support) is used to secure both ends of the sensor. The fixed support is connected to the pile body via expansion bolts (such as M6 expansion bolts), ensuring a tight fit between the fixed support and the pile body. Subsequently, the sensor armored optical cable is laid along the channel. The sensor armored optical cable must be kept in a naturally slack state with a bending radius of not less than 30cm to avoid excessive bending that could damage the optical fiber. As a further implementation method, the first optical fiber string 3 needs to be temporarily secured with high-temperature resistant tape at predetermined intervals (such as 50cm). This tape can be removed later, or an epoxy resin seal can be applied directly.

[0030] Considering that temperature changes at the engineering site may interfere with the test data, a fiber Bragg grating (FBG) temperature sensor is synchronously deployed in the core monitoring unit. By collecting ambient temperature data in real time, the monitoring results of the fiber Bragg grating strain sensor are accurately compensated for temperature, ensuring the accuracy of the stress test data.

[0031] In this embodiment, the outer core 2 of the composite pile is wrapped around the outer side of the inner core 1 of the composite pile by casting material, and together with the inner core 1, they constitute the composite pile body. The top of the outer core 2 is connected to the pile cap 4 by high-strength bolts 8. After the pile is driven into the hole, it is formed by grouting and other methods. Its performance directly affects the overall bearing capacity of the composite pile, and the quality must be ensured during the casting process. It works together with the inner core to bear the load.

[0032] In this embodiment, the outer core detection unit is a second optical fiber string 7, which is composed of multiple second high-precision strain sensors connected in a segmented and encrypted manner. The second optical fiber string 7 is installed inside the outer core 2 of the composite pile. After installation, the first high-precision strain sensor and the second high-precision strain sensor correspond one-to-one. Specifically, the second high-precision strain sensor adopts a range of... The high-precision fiber optic strain sensor, and multiple second high-precision strain sensors are connected in series through sensor armored optical cables (such as armored optical cables with a diameter of 4mm). Their segmented and encrypted layout is the same as that of the first optical fiber string 3. They are implanted in the outer core 2 of the composite pile before the initial solidification and correspond one-to-one with the first optical fiber string 3.

[0033] Specifically, the second optical fiber string 7 is bound to the outer wall of an implantation tube of a specified specification (such as a PVC pipe with a diameter of 50mm, a wall thickness of 3.5mm, and a compressive strength ≥16MPa). The binding position is marked with the same segmented encryption method as the first optical fiber string 3, and fixed with cable ties (such as 304 stainless steel cable ties with a width of 10mm). The tightness of the cable ties should be such that the sensor does not slip, avoiding damage to the sensor due to excessive tightness. 30 minutes before the initial setting of the composite pile outer core 2, the implantation tube and the second optical fiber string 7 are slowly sent into the designed position of the composite pile outer core 2 using a scaffolding pipe (50cm deeper than the pile length). A positioning bracket is installed on the top of the scaffolding pipe to ensure that the centering deviation of the implantation tube is ≤50mm. After positioning, the scaffolding pipe is slowly rotated to separate it from the implantation tube and then pulled out. During the pulling process, disturbance to the pouring material is avoided to ensure the stability of the second high-precision stress sensor position. After installation, the connection of the second high-precision stress sensor is checked, and then the stress change of the composite pile outer core 2 is tracked and monitored at the monitoring frequency using a fiber optic strain sensor.

[0034] In this embodiment, the first high-precision stress sensor at the bottom of the first fiber optic string 3 is close to the bottom of the inner core 1 of the composite pile (e.g., 50cm away from the bottom of the pile), and the second high-precision stress sensor at the bottom of the second fiber optic string 7 is close to the bottom of the outer core 2 of the composite pile (e.g., 50cm away from the bottom of the pile). By deploying sensors near the bottom of the pile, the stress value at the pile end (i.e., the bottom of the pile) can be directly measured, that is, the bearing force of the mudstone at the pile end, providing key data for analyzing the bearing capacity contribution of the pile end.

[0035] In this embodiment, both the inner core monitoring unit and the outer core detection unit are connected to the fiber Bragg grating acquisition unit 5. The fiber Bragg grating acquisition unit 5 transmits the acquired sensor data to the control center 6 for pile stress measurement. Specifically, the fiber Bragg grating acquisition unit 5 has a sampling frequency of 1Hz and is used to receive strain signals from the high-precision stress sensors in the inner core monitoring unit and the outer core detection unit. It transmits the acquired stress data to the control center 6 for storage. The control center 6 can process and analyze the stress data to determine the pile stress and provide data support for subsequent research. As a further implementation, the fiber Bragg grating (FBG) temperature sensor synchronously deployed in the inner core monitoring unit is also connected to the fiber Bragg grating acquisition unit 5 and transmits the temperature data to the control center 6 for storage, processing, and analysis.

[0036] In this embodiment, the pile cap 4 connects the top of the inner core 1 and the top of the outer core 2 of the composite pile simultaneously. It can be made of Q235 steel with a thickness of 50mm and dimensions of 1.2m × 1.2m. High-strength bolts 8 are used to connect both the inner core 1 and the outer core 2 to the bearing plate simultaneously, avoiding the problem of delayed stress on the outer core 2 caused by only embedding the inner core into the pile cap 4 in conventional methods. Furthermore, a hole 9 is provided in the center of the pile cap 4 for inserting a pile end settlement test tube, serving to bear and transfer the load, ensuring that the load is evenly distributed on the composite pile.

[0037] Furthermore, the high-strength bolt 8 is of model M20, which is used to connect the inner core 1 and the outer core 2 of the composite pile to the bearing plate of the pile cap 4 at the same time, to ensure a firm connection, avoid the problem of delayed force on the outer core caused by only embedding the inner core into the pile cap 4 in the conventional way, and ensure the overall synergy of the composite pile's force.

[0038] Furthermore, the pile end settlement test tube uses a 50mm diameter galvanized steel pipe with a displacement sensor installed at the bottom to provide a channel for monitoring pile end settlement.

[0039] Furthermore, displacement sensors are installed at the pile ends of the inner core 1 and the outer core 2 of the composite pile. The settlement changes are recorded synchronously by the displacement sensors, and load-displacement curves are plotted. By analyzing the load and displacement, it can be determined whether the inner and outer cores are under stress in coordination, thereby analyzing the load transmission law in the pile body.

[0040] This embodiment of a rotary-drilled pile stress measuring device employs a dual-sensor system design, with each sensor corresponding to the other. This enables synchronous acquisition of stress data from the inner and outer cores under the same load conditions and at the same depth. Through sensors near the pile bottom, the force value at the pile tip can be directly measured, providing crucial data for analyzing the pile tip bearing capacity contribution. This device can accurately determine the stress distribution characteristics and transmission patterns of rotary-drilled composite piles, deeply revealing the bearing mechanism of piles in mudstone foundations. This effectively eliminates the engineering risk of abnormal pile bearing capacity in mudstone foundations and provides reliable technical support for the design and construction of pile foundation projects under similar geological conditions.

[0041] Example 2 In a typical embodiment of the present invention, this embodiment discloses a method for determining the stress in the shaft of a rotary-drilled pile, comprising: S1. A rectangular groove parallel to the pile axis is opened on the pile body of the composite pile core 1, and the installation position of each first high-precision stress sensor is precisely marked according to the segmented and densified layout. Before this, a comprehensive inspection of the precast concrete pipe pile serving as the composite pile core 1 is required to ensure that the pile body is free of defects such as cracks and honeycombing, and that the surface flatness deviation is controlled within 5mm. In this embodiment, a precast concrete pipe pile with an outer diameter of 500mm is selected as the composite pile core 1, and the pile length is determined according to the engineering design requirements.

[0042] When creating the trench, first use a chalk line to mark the center line of the trench parallel to the pile axis along the surface of the precast pipe pile 1. The accuracy of the marking should be controlled within ±2 mm. According to the sensor layout design, mark the rectangular trench with a width of 4cm and a depth of 2cm, and accurately mark the installation position of each FBG sensor. The trenching construction uses a special stone cutting machine to cut the outline along the center line. During the cutting process, the cutting speed must be controlled (30cm / min is recommended) to avoid cracking of the pile body due to vibration caused by high-speed cutting. After the outline is cut, use a 16mm diameter impact drill to excavate the trench. During excavation, the drill bit should be kept perpendicular to the pile body to avoid chipping at the edge of the trench.

[0043] S2. Clean the grooves to ensure there is no dust or debris residue. After cleaning, level the bottom of the grooves and the installation location of the first high-precision stress sensor. Specifically, after grooving, first use a high-pressure air gun to blow away any floating debris, then use a stiff brush dipped in clean water to repeatedly clean the surface of the grooves to ensure there is no dust or debris residue. After cleaning, evenly apply AB glue to the bottom of the grooves and the sensor installation location for leveling. The leveling thickness should be controlled at 1-2 mm. After the AB glue has initially cured (approximately 30 minutes), proceed to the next process.

[0044] S3. According to the marked installation position, first place the first optical fiber string 3 in the channel and adjust it to be parallel to the pile axis of the composite pile inner core 1. Then, use a fixed support to fix both ends of the first high-precision stress sensor. The fixed support is connected to the pile body of the composite pile inner core 1 through expansion bolts to ensure that the fixed support is tightly fitted to the pile body. Then, lay the sensor armored optical cable along the channel and ensure that it remains in a natural relaxed state.

[0045] Specifically, the first fiber optic string 3 is deployed in a segmented and densified manner. Within 3 meters of the pile top, each high-precision stress sensor is arranged at a 500 mm interval to accurately capture stress changes in the stress concentration area on the upper part of the pile. Below 3 meters, the sensors are arranged at a 1000 mm interval to balance monitoring efficiency and data integrity. After determining the installation location, the FBG sensor string 3 is first positioned and adjusted to be parallel to the pile axis. Then, stainless steel fixing brackets (50mm×30mm×10mm) are used to fix both ends of the sensor. The brackets are connected to the pile body with M6 expansion bolts to ensure a tight fit between the brackets and the pile body. Subsequently, the sensor armored optical cable (4 mm in diameter) is laid along the channel. The optical cable must be kept in a naturally slack state with a bending radius of not less than 30cm to avoid excessive bending and damage to the optical fiber. The first fiber optic string 3 needs to be temporarily fixed every 50cm with high-temperature resistant tape.

[0046] S4. After the first fiber optic string 3 is laid, epoxy resin encapsulation is carried out; after encapsulation, plastic film is covered on the surface of the channel for curing for a set time; after the epoxy resin is fully cured, check the encapsulation layer for cracking and peeling and ensure that the set hardness has been reached.

[0047] Specifically, after the first fiber optic cable string 3 is laid, epoxy resin encapsulation is performed. The epoxy resin used is a special structural adhesive with components A and B in a 1:1 mass ratio, and the mixing time is no less than 3 minutes to ensure uniform mixing without air bubbles. During encapsulation, the epoxy resin is evenly filled into the channel using a scraper, making the adhesive surface flush with the pile surface, and repeatedly compacted with a trowel to avoid voids. After encapsulation, a plastic film is covered on the surface of the channel for curing, and it is left to stand for 48 hours until the epoxy resin is completely cured (insulation measures are required when the ambient temperature is below 15℃). After curing, the encapsulation layer must be checked for cracks and peeling, and the hardness must reach Shore D85 or higher before pile planting can proceed.

[0048] S5. Perform a survival rate test on the first fiber string 3. The survival rate must reach more than 95%. For the unqualified first high-precision stress sensor, find out the cause and rework and replace it to ensure the stability and reliability of the inner core monitoring unit.

[0049] Specifically, before the test, the sensor armored optical cable is connected to the fiber optic grating acquisition instrument 5 (sampling frequency 1Hz) and the control center 6. The survival rate is tested using dedicated calibration software: the survival rate must reach more than 95%. For unqualified sensors, the cause must be identified and they must be reworked and replaced to ensure the stability and reliability of the core pile stress testing system.

[0050] S6. A precast pipe pile is prepared by casting an outer core 2 on the outside of the inner core 1 of the composite pile. Before the outer core 2 of the composite pile initially sets, the second optical fiber string 7 is slowly inserted into the set position of the outer core 2 of the composite pile. After the insertion is completed, the second optical fiber string 7 corresponds one-to-one with the first optical fiber string 3. After the installation is completed, the survival rate test of the second optical fiber string 7 is carried out.

[0051] Specifically, a PVC pipe with a diameter of 50mm and a wall thickness of 3.5mm (compressive strength ≥16MPa) was selected as the implantation tube, and a pressure test was conducted on the pipe upon arrival at the site (water pressure 0.6MPa, maintained for 30 minutes without leakage). The sensor binding positions were marked on the outer wall of the PVC pipe using the same segmented and encrypted method as the inner core. The second fiber optic string 7 was then secured with 304 stainless steel cable ties (10mm wide), with the cable ties tightened only to prevent the sensor from slipping, avoiding excessive tightness that could damage the sensor. 30 minutes before the initial setting of the composite pile outer core 2, the PVC measuring tube and the second fiber optic string 7 were slowly inserted into the designed position of the outer core using a scaffolding pipe (50cm deeper than the pile length). A positioning bracket was installed on the top of the scaffolding pipe to ensure the PVC pipe's centering deviation was ≤50mm. After positioning, the scaffolding pipe was slowly rotated to separate it from the PVC pipe and then pulled out. During the pulling process, disturbance to the pouring material was avoided to ensure the sensor's stable position. After installation, the sensor connection was checked, and then the stress changes in the pile body were tracked and monitored at the monitoring frequency using a fiber optic strain sensor.

[0052] S7. Piling and drilling are carried out, and cement mortar is injected into the hole after drilling is completed. Then, the precast pipe pile is installed and driven by static pressure method. After the pile is installed, the composite pile is cured to ensure that the concrete strength meets the standard. The top of the inner core 1 and the top of the outer core 2 of the composite pile are connected by the pile cap 4, and the pile end settlement test tube is installed in the hole 9 in the center of the pile cap 4.

[0053] Specifically, rotary drilling rigs are used for pile driving, with the hole diameter controlled at 800mm (300mm larger than that of the inner core pipe pile) as per design requirements. Mud slurry is used for wall protection during the drilling process, and the sediment thickness is controlled within 50mm. After the hole is completed, cement mortar is injected into the hole, and then the precast pipe pile is inserted. Static pressure method is used for pile driving, with the driving rate controlled at 1-2m / min, ensuring that the verticality deviation of the pile body is ≤1%.

[0054] After the piles are installed, the composite piles need to be cured for 28 days. During the curing period, the pile tops are covered with geotextile and watered regularly to ensure that the concrete strength meets the standards. The load test adopts a full-section compression scheme for the integral pile with inner and outer cores: the pile cap 4 is made of Q235 steel structure (thickness 50 mm, size 1.2m×1.2m), and the contact surface with the pile top is leveled with 1:2 cement mortar (thickness 20 mm). The inner core and outer core are connected to the bearing plate simultaneously with M20 high-strength bolts 8, avoiding the problem of delayed stress on the outer core caused by only embedding the inner core into the pile cap 4 in the conventional method. The hole 9 in the center of the pile cap 4 has a diameter of 100 mm and is used to install the pile end settlement test tube (made of galvanized steel pipe with a diameter of 50 mm and a displacement sensor installed at the bottom).

[0055] S8. A static load test was conducted based on the slow sustained loading method. The maximum load was set and the load was applied in multiple stages until the pile failed. The inner core monitoring unit and the outer core monitoring unit monitored the stress changes of the inner core 1 and the outer core 2 of the composite pile in real time. The fiber optic grating acquisition instrument 5 transmitted the collected sensor data to the control center 6 to complete the pile stress measurement.

[0056] Specifically, the static load test adopted the slow sustained loading method, with a maximum load of 6000 kN, divided into 12 loading levels (500 kN per level), and loaded until the pile failed. When the settlement rate at the top of the pile was ≥0.1 mm / h for 2 consecutive hours and did not converge, the ultimate load was recorded to provide complete data for the study of the bearing mechanism.

[0057] Three days before and within 24 hours after the static load test, the integrity of the pile was assessed using a low-strain dynamic testing method. The testing equipment used was a pile dynamic testing instrument (sampling frequency 20 kHz), and the sensor was a piezoelectric accelerometer (sensitivity 100 mV / g). Before testing, loose soil was removed from the pile top and a coupling agent (grease) was applied to ensure a tight fit between the sensor and the pile top. During testing, a 5 kg hammer was used to strike the pile top center three times and three times at a distance of 20 cm from the center. The integrity of the pile was determined by analyzing the reflected wave waveform: a normal pile wave velocity should be 3500-4500 m / s with no obvious reflected waves; for piles with defects, the location was marked and the cause analyzed.

[0058] During the static load test, FBG sensors monitored the stress changes of the inner core (precast pipe pile) and outer core (cast material) of the composite pile in real time. Sampling intervals were 10 consecutive samples taken 10 minutes after each loading stage stabilized, and the average value was taken. Since sensors were installed 50cm above the pile bottom, the pile tip stress value, i.e., the bearing force of the mudstone at the pile tip, could be directly measured, providing crucial data for analyzing the pile tip bearing capacity contribution. Simultaneously, settlement changes were recorded synchronously using displacement sensors at the pile top and pile tip (accuracy 0.01mm), and load-displacement curves were plotted.

[0059] This embodiment of the method for determining the stress in a rotary-drilled pile body uses a rotary-drilled pile body stress measuring device. During the static load test, the entire cross-section of the inner and outer cores of the composite pile is subjected to pressure. Based on the stress measured by the optical fiber inside the pile body, the stress distribution of the inner and outer cores at the same depth can be obtained, and the stress distribution characteristics and transmission law of the rotary-drilled composite pile body can be accurately determined.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A device for measuring the stress in the shaft of a rotary-drilled pile, characterized in that, include: The composite pile comprises an inner core, an outer core, an inner core monitoring unit, an outer core monitoring unit, and a pile cap. The inner core monitoring unit is a first optical fiber string formed by multiple high-precision strain sensors connected in a segmented and densely arranged configuration. This first optical fiber string is installed within a channel carved into the inner core of the composite pile. The outer core monitoring unit is a second optical fiber string formed by multiple high-precision strain sensors connected in a segmented and densely arranged configuration. This second optical fiber string is installed inside the outer core of the composite pile. After installation, each high-precision strain sensor corresponds one-to-one with the first and second high-precision strain sensors. Both the inner core monitoring unit and the outer core monitoring unit are connected to a fiber optic grating acquisition instrument, which transmits the acquired sensor data to a control center for pile stress measurement. The pile cap connects the top of both the inner and outer cores of the composite pile, eliminating stress lag in the outer core. A hole is provided in the center of the pile cap for inserting a pile end settlement test tube, providing a monitoring channel for the pile end.

2. The device for measuring the stress in a rotary drilling pile as described in claim 1, characterized in that, The channel is rectangular and parallel to the axis of the core pile inside the composite pile.

3. The device for measuring the stress in a rotary drilling pile as described in claim 1, characterized in that, The segmented and encrypted deployment method is as follows: within a set distance from the top of the pile, high-precision stress sensors are arranged at a first set interval to accurately capture the stress changes in the stress concentration area on the upper part of the pile; the remaining parts are arranged at a second set interval to balance detection efficiency and data integrity.

4. The device for measuring the stress in a rotary drilling pile as described in claim 3, characterized in that, The first set spacing is less than the second set spacing.

5. The device for measuring the stress in a rotary drilling pile as described in claim 1, characterized in that, The first and second high-precision stress sensors use a measurement range of High-precision fiber optic strain sensor.

6. The device for measuring the stress in a rotary-drilled pile as described in claim 1, characterized in that, The first optical fiber string is parallel to the pile axis of the composite pile core; the two ends of the first high-precision stress sensor are fixed by fixed supports, and the fixed supports are connected to the pile body of the composite pile core by expansion bolts. The first high-precision stress sensor is connected in series via a sensor armored optical cable, which is laid along the channel and kept in a naturally relaxed state.

7. The device for measuring the stress in a rotary-drilled pile as described in claim 1, characterized in that, The outer core of the composite pile is cast on the outside of the inner core of the composite pile; the second optical fiber string is inserted into the outer core of the composite pile before the initial solidification of the outer core and corresponds one-to-one with the first optical fiber string.

8. The device for measuring the stress in a rotary-drilled pile as described in claim 1, characterized in that, The first high-precision stress sensor at the bottom of the first fiber optic string is close to the bottom of the inner core of the composite pile; the second high-precision stress sensor at the bottom of the second fiber optic string is close to the bottom of the outer core of the composite pile.

9. A method for determining the stress in the shaft of a rotary-drilled pile, characterized in that, include: S1. A rectangular channel parallel to the pile axis is opened on the pile body of the composite pile core, and the installation position of each first high-precision stress sensor is precisely marked according to the segmented and densified layout. S2. Clean the channel to ensure that there is no dust or debris residue inside; after cleaning, level the bottom of the channel and the installation position of the first high-precision stress sensor. S3. According to the marked installation position, first place the first optical fiber string in the channel and adjust it to be parallel to the pile axis of the composite pile core. Then, use a fixed support to fix both ends of the first high-precision stress sensor. The fixed support is connected to the pile body of the composite pile core through expansion bolts to ensure that the fixed support is tightly fitted to the pile body. Then, lay the sensor armored optical cable along the channel and ensure that it remains in a natural slack state. S4. After the first fiber optic string is laid, epoxy resin encapsulation is carried out; after encapsulation, plastic film is covered on the surface of the channel for curing for a set time. After the epoxy resin has fully cured, check the encapsulation layer for cracks and peeling, and ensure that the set hardness has been reached. S5. Conduct a survival rate test on the first fiber optic string. The survival rate must reach more than 95%. For the unqualified first high-precision stress sensor, find out the cause and rework and replace it to ensure the stability and reliability of the core monitoring unit. S6. Precast pipe piles are prepared by casting an outer core of the composite pile on the outside of the inner core of the composite pile. Before the outer core of the composite pile initially sets, the second fiber string is slowly inserted into the set position of the outer core of the composite pile. After the insertion is completed, the second fiber string corresponds one-to-one with the first fiber string. After installation, the survival rate of the second fiber string is tested. S7. Piling and drilling are carried out, and cement mortar is injected into the hole after drilling is completed. Then, the precast pipe pile is inserted and driven by static pressure method. After the pile is planted, the composite pile is cured to ensure that the concrete strength meets the standard. The top of the inner core and the top of the outer core of the composite pile are connected by the pile cap, and the pile end settlement test tube is inserted in the hole in the center of the pile cap. S8. Static load test based on slow sustained loading method, set maximum load, load in multiple stages until pile failure, inner core monitoring unit and outer core monitoring unit monitor stress changes of inner core and outer core of composite pile in real time, fiber optic grating acquisition instrument transmits the collected sensor data to control center to complete pile stress measurement.

10. The method for determining the stress in a rotary-drilled pile as described in claim 9, characterized in that, Before the static load test, a set number of days were set, and within a set time after the test, the integrity of the pile body was tested using the low-strain dynamic measurement method.

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