Method for monitoring internal force of super-long pile foundation based on FBG (Fiber Bragg Grating) sensor

Abstract

The invention discloses a super-long pile foundation internal force monitoring method based on FBG sensors. The specific main process of the monitoring method comprises the steps of point fixing, sensor installation, cable sleeving, demodulator reading and reading analysis. The fiber bragg grating (FBG) sensor is called an FBG sensor for short, and has the characteristics of simple structure, stable performance, narrow bandwidth, weak reflection, no need of a loop and the like, so that the FBG sensor is widely applied to various sensors. According to the FBG sensor-based ultra-long pile foundation internal force monitoring method, the FBG sensor is installed on a pile foundation reinforcement cage and permanently embedded in a concrete pile, an outgoing line and a connector sleeved with a protection pipe are reserved, optical fiber strain data are obtained through reading of an optical fiber demodulator, and then the pile foundation internal force is calculated. The FBG sensor is adopted to successfully achieve long-period monitoring of the internal force of the super-long foundation pile, the survival rate is high, stability is good, a loop does not need to be formed, the FBG sensor has wide application prospects in foundation engineering construction, and reference can be provided for performance monitoring of the super-long foundation pile.

Description

Technical Field

[0001] The present invention relates to the technical field of geological monitoring, and more specifically, to a method for monitoring the internal force of an ultra-long pile foundation based on an optical fiber Bragg sensor. Background Art

[0002] When the underlying karst cave is buried deep, super-long piles are needed to ensure the bearing capacity of the foundation. Due to the complex geological conditions in the karst area, it has great heterogeneity. At the same time, the open karst cave area is often accompanied by drainage groundwater, which may cause the karst cavity to continue to develop, and the bearing capacity and stability are difficult to accurately estimate. The use of pile internal force monitoring with high timeliness and accuracy is one of the key means to ensure the stability of the bridge foundation.

[0003] The determination of pile foundation bearing capacity is currently focused on pile inspection after the completion of pile construction. In order to determine the strain of the pile body under various loads and understand the bearing mechanism of the pile in the static load test, various sensing elements such as steel bar stress gauges and strain gauges are drilled or buried in the pile body. This traditional point sensor is prone to missed detection when the number of buried sensors is small, and the junction between the sensor and the transmission line is prone to poor contact and breakage, which makes the detection point invalid, and the survival rate is low. It cannot play a good role in long-term monitoring, and these detection methods can only obtain phased results, making it difficult to monitor the bearing performance of the key pile position in the next stage or even the entire life. Summary of the invention

[0004] The purpose of the present invention is to provide an ultra-long pile foundation internal force monitoring method based on FBG sensors, which is easy to construct, has a higher survival rate and is easy to obtain data. The method has a long sensing distance, high positioning accuracy, can carry out full-life monitoring, has a large amount of data, and can provide deformation and stress information of the pile foundation in a timely and accurate manner.

[0005] The present invention provides a super-long pile foundation internal force monitoring method based on fiber Bragg sensor, and the materials required for implementation include: FBG-S01B high-durability fiber Bragg grating embedded GFRP pile foundation strain gauge and temperature sensor, the number of optical fibers in each cable is 1, the range is -1500 to 3000, the resolution is ±1, the measurement accuracy is 0.5% FS, the strain coefficient is about 1.1pm / , the central reference wavelength range is 1528 to 1568nm, the operating temperature range is -30℃ to +80℃, the strain gauge temperature coefficient is 14.3pm / ℃, and the temperature sensor temperature coefficient is 10.3pm / ℃. The data acquisition instrument adopts FP-M01 fiber Bragg grating portable demodulator, which has four channels, a sampling frequency of 3Hz, a wavelength demodulation accuracy of ±2pm, a resolution of 1pm, and a detection wavelength range of 1528 to 1568nm. Self-locking nylon cable ties, PVC 6-point water pipes, PVC 4-point corrugated pipes, UV-resistant waterproof PE square boxes, spray paint, waterproof tape, optical fiber fusion splicing machines, etc.

[0006] Among them, the fiber grating embedded GFRP pile foundation strain gauge and temperature sensor are embedded and installed on the main reinforcement of the steel cage using nylon tie, the PVC6-point water pipe and PVC4-point corrugated pipe are used to wrap and cover the top of the lead-out line, and the FP-M01 fiber grating portable demodulator is used for data collection.

[0007] The N sensors are connected in series using optical fiber cables. The cable connected in series with the N optical fiber sensors is a monitoring line group, and is arranged longitudinally along the pile body, so that each foundation pile has N sensing optical fiber line groups.

[0008] Further, the method is deployed in a friction pile or an end bearing pile that is pre-set by the user and is located above or through the solution cavity.

[0009] Furthermore, the sensing arm optical fiber and the reference arm optical fiber are single-mode optical fibers.

[0010] The patent of the present invention provides a method for monitoring the internal force of an ultra-long pile foundation based on a fiber optic Bragg sensor. Compared with the traditional experimental test method of pile foundation, the technical solution of the present invention uses FBG as a sensor to monitor the strain of the pile foundation. The strain signals of reinforced concrete along the optical fiber can be detected. Due to the buried layout, the survival rate is high and the durability is good, which realizes true full-life monitoring. Moreover, it is deployed in areas with high karst development to carry out key monitoring of key pile foundation points, with high positioning accuracy. The acquired monitoring data is continuous and reliable, and can provide monitoring information in a timely, effective and accurate manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a structural schematic diagram of a method for monitoring the internal force of an ultra-long pile foundation based on a fiber optic Bragg sensor in the patent of the present invention;

[0012] Figure 2 This is a schematic diagram of the structure of the optical fiber Bragg sensor deployed in the pile foundation of the present invention. DETAILED DESCRIPTION

[0013] The technical solution in the patent of the present invention will be described clearly, completely and in detail below in conjunction with the drawings in the patent of the present invention.

[0014] See also Figure 1 , is a schematic diagram of the structure of a method for monitoring the internal force of an ultra-long pile foundation based on a fiber optic Bragg sensor in the present invention. Figure 1As shown, the pile monitoring system includes: N groups of fiber Bragg grating strain sensors and temperature sensor lines (the quantity is determined according to user needs), fiber Bragg grating demodulator, self-locking nylon cable ties, PVC 6-way water pipe, PVC 4-way corrugated pipe, UV-resistant waterproof PE square box, spray paint, waterproof tape, and fiber optic fusion splicer.

[0015] Determine the location of monitoring points according to the specifications and the experience of internal force monitoring in similar projects, and reasonably set the number and spacing of sensors. For friction piles, one strain monitoring point can be arranged every 5m along the vertical direction of the pile body. The monitoring points can be arranged throughout the pile. For end-bearing piles, they can be arranged only in the lower part of the pile body. Figure 2 Several strain monitoring points on the same cable form a group. Four groups of monitoring points should be arranged on one pile, and they should be arranged at the edge of the pile along the transverse and longitudinal directions of the bridge. The monitoring point on the pile top should take into account the impact of subsequent pile head breaking operations, and the sensor should be set 2m below the pile top.

[0016] When the sensor is lowered with the steel cage, use nylon tie to tie the sensor and the optical fiber cable to the main reinforcement of the steel cage. The sensor should be placed inside the cage to prevent damage from scratching the hole wall during lowering, and lubricating oil can be applied on the cable surface for protection. The cable should be stretched as straight as possible to avoid loosening. Generally, it should be tied and fixed every 1.5m. The lowering speed of the steel cage should not be too fast to avoid breaking the cable.

[0017] Use PVC water pipe and hard corrugated pipe to double-wrap the cables at the lead-out end of the pile top. Select pipes of appropriate diameter according to the number of cables, pass the cables from the lead-out end into the corrugated pipe, and then pass the corrugated pipe into the PVC water pipe.

[0018] The strain spectrum data along the depth direction of the pile is obtained using a portable fiber Bragg grating data acquisition demodulator.

[0019] After denoising and smoothing the frequency spectrum, accurate axial force distribution diagram, pile side friction resistance distribution diagram, etc. are calculated.

[0020] To prevent the fiber optic connector from rusting and getting dusty, clean the FC / APC connector at the lead end with an alcohol cotton ball after collecting data.

[0021] Fiber Bragg gratings have the characteristics of simple structure, stable performance, narrow bandwidth, weak reflection, and no need for loops. The installation and protection scheme of FBG sensors in ultra-long piles can realize long-term monitoring of the negative friction resistance of the foundation piles caused by the upper load. This example technical scheme can improve the forecasting and early warning capabilities and prevention and control level of foundation engineering in karst areas, minimize casualties caused by geological disasters, and provide strong protection for people's property.

[0021] The above is a preferred implementation of the patent of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the patent of the present invention. These improvements and modifications are also regarded as the protection scope of the patent of the present invention.

Claims

1. A method for monitoring the internal force of an ultra-long pile foundation based on an FBG sensor, characterized in that: The following steps are involved: S1. Fixed point: According to the specifications and the experience of internal force monitoring in similar projects, the number and spacing of sensors should be reasonably set. One strain monitoring point should be arranged every 5m along the vertical direction of the pile body. Several strain monitoring points on the same cable form a group. Four groups of monitoring points should be arranged on one pile, and they should be arranged at the edge of the pile along the transverse and longitudinal directions of the bridge. S2.Sensor installation: When the sensor is lowered with the steel cage, use nylon tie to tie the sensor and the optical fiber cable to the main reinforcement of the steel cage. S3. Cable casing: Use PVC water pipe and hard corrugated pipe to double-wrap the cables at the lead-out end of the pile top. Select pipes of appropriate diameter according to the number of cables, pass the cables from the lead-out end into the corrugated pipe, and then pass the corrugated pipe into the PVC water pipe. S4. Demodulator readings: Use a portable fiber Bragg grating data acquisition demodulator to obtain strain spectrum data along the depth direction of the pile. S5. Reading analysis: After noise reduction and smoothing of the frequency spectrum, accurate axial force distribution diagram, pile side friction resistance distribution diagram, etc. are calculated.

2. The method for monitoring the internal force of an ultra-long pile foundation based on a FBG sensor as claimed in claim 1, characterized in that: In S1, the sensor groups are arranged in a cross shape along the transverse and longitudinal directions of the bridge.

3. The method for monitoring the internal force of an ultra-long pile foundation based on a FBG sensor as claimed in claim 1, characterized in that: In S1, several strain monitoring points on the same cable are grouped together, and each monitoring point is spaced 5 m apart.

4. The method for monitoring the internal force of an ultra-long pile foundation based on a FBG sensor as claimed in claim 1, characterized in that: In S3, a double layer of PVC water pipe and hard corrugated pipe is used to wrap part of the lead-out cables at the top of the pile.

Images