A method for detecting the length of underground anchor rod

Abstract

The present invention provides a method for detecting the length of underground anchor rods. Specifically, the method applies an alternating current of a certain frequency to the target anchor rod to be detected, and then measures the distribution characteristics of the alternating electromagnetic field generated by the anchor rod in an auxiliary detection hole near the anchor rod to be detected and parallel to the anchor rod to be detected, and draws a curve H showing the change of the alternating magnetic field intensity of the anchor rod and anchor bar with the depth of the borehole where the probe is located. L , and draw a curve dH of the gradient of the alternating magnetic field intensity versus the depth of the borehole where the probe is located L / dL, the position where the inflection point of the two curves is distorted is determined as the bottom position of the anchor rod and anchor bar, and the anchor rod and anchor bar length L is determined by comparison and analysis. The determination method of the present invention is simple and accurate, and can use one borehole to detect multiple anchor rods around it within its effective range, which is a very practical anchor rod and anchor bar length detection method.

Description

Technical Field

[0001] The present invention belongs to a method for detecting the length of underground metal anchor rods by utilizing electromagnetic method. Specifically, an alternating current of a certain frequency is applied to the target anchor rod to be detected, and then the distribution characteristics of the alternating electromagnetic field generated by the anchor rod are measured in an auxiliary detection hole near the detected anchor rod and parallel to the detected anchor rod, thereby determining the length of the anchor rod.

[0002] Background

[0003] Anchor rods are widely used tension components in modern engineering construction. They are not only widely used in mine tunnels, slope reinforcement, tunnels, and water conservancy dams, but also widely used in the field of modern urban engineering construction due to their simple structure, convenient construction, low cost, and strong adaptability. However, in recent years, there have been many cases of cutting corners in the field of engineering construction, where the construction length of pull-out and tensile anchor rods does not meet the design requirements. Due to the insufficient length of the anchor rods, the anchoring force does not meet the design requirements, resulting in a series of problems such as slope collapse, damage to the floor structure of the building, and floating due to damage to the building structure. The question of how to efficiently, accurately, non-destructively, and randomly detect the length of the anchor rod has been raised. The research on non-destructive testing methods for the length of the anchor rod and the formulation of relevant testing specifications have been paid more and more attention. The detection of the length of the anchor rod has become one of the must-check items for project acceptance, which is of great significance to the control of the quality of engineering construction.

[0004] The existing detection technology for anchor rod length is the acoustic wave reflection method. This method is light, low-cost and fast, but is greatly affected by the wave velocity difference between the anchor body and the surrounding rock, and the bonding effect between the rod body and the bonding material. The effective detection length is generally less than 10 meters. TDR technology has begun to be used for anchor rod length detection at home and abroad in recent years, but the application premise of this detection technology is that a wire parallel to the anchor rod must be buried at the same time when each anchor rod is constructed, which increases the construction cost, and the construction personnel may still cut corners by increasing the wire length to reduce the anchor rod length. Therefore, a detection method that completely eliminates the length cheating in anchor rod construction is needed to carry out anchor rod construction acceptance inspection. Summary of the invention

[0005] In view of the deficiencies in the prior art, the present invention provides a method for detecting the length of underground anchor rods. The method applies an alternating current of a certain frequency to the target anchor rod to be detected, and then measures the distribution characteristics of the alternating electromagnetic field generated by the anchor rod in an auxiliary detection hole near the detected anchor rod and parallel to the detected anchor rod, thereby determining the length of the anchor rod. The judgment method is simple and accurate, and one borehole can be used to detect multiple anchor rods around it within its effective range.

[0006] In order to solve the above technical problems, the present invention provides a method for detecting the length of an underground anchor rod, characterized in that the method comprises the following steps:

[0007] (1) Arrange auxiliary inspection holes with a diameter of 60 mm to 110 mm within 3 meters of the anchor bolt and anchor bar to be inspected, keep the drill hole relatively parallel to the anchor bolt, and the drilling depth should be at least 5 meters greater than the designed construction length of the anchor bolt;

[0008] (2) Use a transmitter to load an alternating current of a certain frequency to the anchor bolts and anchor bars to be tested by charging or clamping, and use a receiver in the borehole to measure the alternating magnetic field strength H generated by the anchor bolts and anchor bars. First, place the receiver probe at the bottom of the hole to receive the magnetic field strength of the first sampling point, and then lift the receiver probe at a uniform speed of 0.1m / s to 0.15m / s, with a measurement point spacing of 0.05m to 0.1m. During the lifting process, record the depth L of the probe in the borehole and the alternating magnetic field strength H measured at the L. L , and draw a curve H showing the intensity of the alternating magnetic field of the anchor rod and anchor bar changing with the depth of the borehole where the probe is located L ;

[0009] (3) According to the curve H of the alternating magnetic field strength of the anchor rod and anchor bar changing with the drilling depth of the probe L , calculate the alternating magnetic field strength difference dH between two adjacent measuring points L The length difference dL between two adjacent measuring points can be used to obtain the alternating magnetic field intensity gradient dH of any two adjacent measuring points. L / dL, and draw a curve dH of the gradient of the alternating magnetic field intensity versus the depth of the borehole where the probe is located L / dL;

[0010] (4) Using the curve H drawn in step (2) showing the change of the alternating magnetic field strength of the anchor rod and anchor bar with the depth of the probe in the borehole L and the curve dH drawn in step (3) showing the gradient of the alternating magnetic field intensity as a function of the depth of the borehole where the probe is located L / dL determines the length of the anchor rod and anchor bar, and the curve H of the alternating magnetic field strength of the anchor rod and anchor bar changes with the depth of the borehole where the probe is located L The curve dH of the alternating magnetic field intensity gradient changing with the depth of the probe in the borehole L The position where the inflection point / dL appears is the bottom end of the anchor rod and anchor bar, and the corresponding depth at this time is the length L of the anchor rod and anchor bar.

[0011] A preferred technical solution of the present invention: the auxiliary detection borehole in step (1) is formed by drilling with a drilling rig, using a non-magnetic casing as the wall protection, the inner diameter of the pipe is 50mm to 100mm, and the electromagnetic magnetic field receiver probe can be freely raised and lowered in the borehole.

[0012] The preferred technical solution of the present invention is as follows: the charging method in step (2) is to connect one end of the AC power supply of the detector transmitter of a certain frequency to the exposed part of the anchor rod and anchor reinforcement, and the other end to the grounding electrode far away from the contact point, that is, single-end charging. At this time, a current loop is formed between the anchor rod and anchor reinforcement, the earth and the power supply, and an alternating magnetic field is generated around the anchor rod; the clamp method in step (2) is also called the coil induction method, which uses a special clamp to cover the anchor rod and anchor reinforcement to be detected, so that an induced current is generated in the anchor rod and anchor reinforcement, and then a secondary annular alternating magnetic field is generated around the anchor rod, and a receiver is used to receive the alternating magnetic field generated by the anchor rod and anchor reinforcement in the auxiliary detection borehole; in step (2), the receiver probe is placed at the bottom of the hole to receive the magnetic field strength of the first sampling point as zero and record the gain.

[0013] The preferred technical solution of the present invention is: dH in step (3) L =H i+1 -H i , where H i is the alternating magnetic field strength measured at the i-th measuring point, H i+1 is the alternating magnetic field intensity measured at the i+1th measuring point; the alternating magnetic field intensity gradient dH L / dL is the alternating magnetic field strength difference dH between two adjacent measuring points L The ratio of the distance between the measuring points to dL.

[0014] The preferred technical solution of the present invention is: the step (4) is to calculate the curve H of the alternating magnetic field strength of the anchor rod and anchor bar as the probe is located at the drilling depth. L Calculate its decibel value:

[0015]

[0016] Where: dB is H L The decibel value of the curve, that is, the L Find the 20-fold common logarithm value;

[0017] H L is the alternating magnetic field strength measured by the receiving probe at depth L;

[0018] Draw the dB curve based on the dB value calculated by the above formula, and combine it with the curve H of the alternating magnetic field intensity of the anchor rod and anchor bar changing with the drilling depth of the probe. L The curve dH of the alternating magnetic field intensity gradient changing with the depth of the probe in the borehole L / dL determines the bottom position L of the anchor rod and anchor bar; the position of the distortion point of the three curves is the bottom position of the anchor rod and anchor bar, and the corresponding depth at this time is the length L of the anchor rod and anchor bar.

[0019] A preferred technical solution of the present invention: the receiver in step (2) should have at least two mutually perpendicular receiving coils, and the alternating magnetic field strength H measured by the two mutually perpendicular receiving coils can be recorded independently and can be linearly superimposed.

[0020] A preferred technical solution of the present invention: the receiver in step (2) is provided with three mutually perpendicular receiving coils and an independent processing circuit, and the receiving probe is arranged in a sealed waterproof cavity.

[0021] A further technical solution of the present invention is as follows: In step (4), the curve H of the alternating magnetic field intensity varying with the drilling depth of the probe is L The curve dH of the alternating magnetic field intensity gradient changing with the depth of the probe in the borehole L When the position of the inflection point distortion is inconsistent and there is an error, when the error is less than the test step length, calculate the average value of the depth. When the error is greater than the test step length, reduce the test step length and encrypt the measuring points within 1 meter above and below the distortion point until the depth error is less than the step length, and calculate the average value of the two depths.

[0022] The receiver in the present invention should have at least two mutually perpendicular receiving coils, and the alternating magnetic field strength H measured by the two mutually perpendicular receiving coils can be recorded independently and can be linearly superimposed to prevent the deflection of the receiver in the borehole from causing the measured alternating magnetic field strength H to be too small. In the present invention, the receiver probe is placed at the bottom of the hole at one time, and the receiver gain is adjusted so that the magnetic field strength of the first sampling point received by the receiver is zero and the gain is recorded.

[0023] The present invention applies alternating current to the anchor rod and anchor bar to generate a stable alternating electromagnetic field, measures the alternating electromagnetic field strength H around the anchor rod and anchor bar, and analyzes the curve H of the alternating magnetic field strength changing with the drilling depth of the probe. L , the curve of the gradient of the alternating magnetic field intensity changing with the depth of the borehole where the probe is located dH L / dL and the curve of alternating magnetic field strength changing with the depth of the probe in the drilling hole L The dB curve is comprehensively analyzed to determine the anchor rod and anchor bar length. L , curve dH L The position where the inflection point of the / dL and dB curves appears is the bottom end position L of the anchor rod and anchor bar. The judgment method is simple and accurate, and a borehole can be used to detect multiple anchor rods around it within its effective range. It is a very practical method for detecting the length of anchor rods and anchor bars. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the magnetic field generated by the current in an infinitely long straight wire;

[0025] Figure 2This is the curve H of the anchor bar alternating magnetic field strength changing with the probe test height when the anchor bar model with a length of 20m and Φ28 is measured at a distance of 0.5m from the anchor bar at a voltage of 36V in the experiment. L ;

[0026] Figure 3 yes Figure 2 Middle curve H L 20 times logarithmic curve graph;

[0027] Figure 4 The anchor bar model with a length of 20m and a diameter of 28mm in the experiment is measured at a voltage of 36V at a distance of 0.5m from the anchor bar. L / dL curve graph with probe test height L;

[0028] Figure 5 H is a curve of the anchor rod alternating magnetic field strength measured in the embodiment as a function of the probe test height. L Graphs;

[0029] Figure 6 yes Figure 5 Middle curve H L 20 times logarithmic curve graph;

[0030] Figure 7 is the anchor rod alternating magnetic field intensity gradient dH measured in the embodiment L / dL curve graph with probe test height L;

[0031] Figure 8 This is a schematic diagram of the electromagnetic coil arrangement in the receiver.

[0032] In the figure: 1 - receiving coil, 2 - processing circuit board, 3 - waterproof sealed housing. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with the accompanying drawings and embodiments. The technical solutions shown in the accompanying drawings are specific solutions of the embodiments of the present invention and are not intended to limit the scope of the invention to be protected. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] The magnetic field generated by the anchor rod and anchor bar in the present invention is derived as follows: Assume that the direction of the current in the anchor rod and anchor bar at a certain moment is as follows: Figure 1 As shown, according to the Biot-Scharfar law, the magnetic field generated by the current element at a point P in space is dB. Unless otherwise explained in the following formula, the same parameters refer to the same quantity:

[0035]

[0036] In the formula: dB represents the tiny magnetic flux generated by the current element at point P;

[0037] I0 represents the current intensity;

[0038] dl represents the current element length;

[0039] r represents the length from the current element to point P;

[0040] θ represents the angle between the direction of the current element and the vector from the current element to point P;

[0041] μ represents magnetic permeability;

[0042] w represents the circular frequency of alternating current.

[0043] The magnetic field of the long wire corresponding to the dB integral of the above formula is:

[0044]

[0045] From the geometric relationship, we can know that:

[0046] l=r cos(π-θ)=-r cosθ

[0047] r0=r sin(π-θ)=r sinθ

[0048] l=-r0ctgθ

[0049]

[0050] Where: dl represents the length of the current element;

[0051] l represents the current length;

[0052] r represents the length from the current element to point P;

[0053] r0 represents the vertical distance from point P to the current element;

[0054] θ represents the angle between the direction of the current element and the vector from the current element to point P;

[0055] Substituting the above transformation relationship into formula ③, we get:

[0056]

[0057] When A1→-∞, A2→+∞, θ1=0, θ2=π (the condition is that the length of the anchor rod is greater than 5-6 times the vertical distance between the receiver observation hole and the anchor rod). Therefore, the magnetic field generated by the current in the infinitely long straight wire is:

[0058]

[0059] For non-magnetic media, μ is approximately equal to μ0 (vacuum magnetic permeability) in vacuum. Since what we measure in actual work is the magnitude of the magnetic field, for convenience, we rewrite equation ④ as follows:

[0060]

[0061] From formula ⑤, we can see that the magnetic coil generated by the current in the anchor bolt and anchor bar in space is a set of concentric circles with the anchor bolt and anchor bar as the center on the section perpendicular to the direction of the anchor bolt and anchor bar. Due to the leakage phenomenon of the anchor bolt and anchor body, the alternating magnetic field H P There is a negative correlation with the anchor length L, that is, it decreases with the increase of the anchor length. At the contact point between the bottom end of the anchor and the anchor body, the alternating magnetic field H P Attenuates sharply and continues to extend downward, the alternating magnetic field strength H P The distribution of is very complex, and the curve measured during the test is similar to the alternating magnetic field characteristics generated by an electric dipole (mutually coupled with an electromagnetic pole).

[0062] In order to verify the effect of the present invention, the inventors of the present application conducted the following experiments:

[0063] At the test site, a 20m long Φ28 (28mm diameter) anchor model was suspended with 10cm wooden planks, and one end of the anchor was charged with an alternating voltage of 36V 33K. The alternating magnetic field strength was tested at a step distance of 0.1m starting from 1m from the charging end on a straight line parallel to the anchor at a distance of 0.5m, and the test was ended at 22m from the charging end of the anchor. During the receiver receiving process, the gain was adjusted in real time according to the magnitude of the alternating magnetic field strength. The curve H of the measured anchor alternating magnetic field strength H versus the probe test height L in the test is shown in Figure 1. L like Figure 2 As shown. Figure 2 H L It can be seen from the curve that the alternating magnetic field intensity H decays rapidly (drops steeply) 20 meters from the end of the anchor bar. When H decays to zero, the receiver gain is adjusted to measure the alternating magnetic field 2 meters behind the end of the anchor bar. Figure 3 Yes Figure 2 Medium H L The curve is according to the formula Calculate the 20-fold logarithmic curve. Figure 4 Yes Figure 2 Medium H L The alternating magnetic field intensity gradient dH is calculated from the ratio of the alternating magnetic field intensity difference dH between each two adjacent measuring points of the curve and the measuring point spacing dL. L / dL, then plot the alternating magnetic field intensity gradient dH L / dL vs. probe test height L (i.e. dH L / dL curve).

[0064] from Figure 2 , Figure 3 and Figure 4 It can be seen that at 20 meters from the end of the anchor bar, the alternating electromagnetic field strength (H L ) curve, alternating magnetic field intensity gradient (dH L / dL) curve and alternating electromagnetic field strength (H L ) curve has undergone obvious changes, so the length of anchor rods and anchor bars can be determined by comprehensive analysis based on the curve changes.

[0065] The present invention is further described below in conjunction with an embodiment. The embodiment detects the length of a Φ32 anchor rod at a construction site in Shenzhen. The receiver used in the detection process should be modified according to the receiver of an existing underground metal pipeline detector, and a receiving coil and a receiving probe should be miniaturized and sealed and waterproof. Figure 8 As shown, three mutually perpendicular receiving coils 1 and independent processing circuits 2 are arranged in the receiver. The processing circuits 2 are electrically connected to the three receiving coils 1 respectively, and the received signals are processed by the processing circuits. The three receiving coils 1 and the processing circuits 2 are sealed in a waterproof sealed cavity 3, and the signal line is led out of the waterproof sealed cavity. The alternating magnetic field strength H measured by the three mutually perpendicular receiving coils can be independently recorded and can be linearly superimposed. The specific process is as follows:

[0066] (1) Arrange a 60mm to 110mm diameter auxiliary drilling hole at a distance of 0.5m from the anchor to be tested. Use a drilling rig to drill the hole. The auxiliary drilling hole should be protected by a non-magnetic material casing such as PVC pipe. The inner diameter of the pipe should be 50mm to 100mm. The drilling depth should exceed the bottom of the anchor by no less than 5m. The electromagnetic magnetic field receiver probe should be able to rise and fall freely in the hole. Keep the drill hole and the anchor relatively parallel.

[0067] (2) Using a transmitter to load an alternating current of a certain frequency to the anchor rod and anchor bar to be tested by a charging method or a clamping method. The charging method is to connect one end of the AC power supply of a detector transmitter of a certain frequency to the exposed part of the anchor rod and anchor bar, and the other end to a grounding electrode far away from the contact point, which is single-ended charging. At this time, a current loop is formed between the anchor rod and anchor bar, the earth and the power supply, and an alternating magnetic field is generated around the anchor rod; the clamping method is also called the coil induction method, which uses a special clamp to cover the anchor rod and anchor bar to generate an induced current in the anchor rod and anchor bar, thereby generating a two-phase magnetic field around the anchor rod. The alternating magnetic field generated by the anchor bolt and anchor bar is received by the receiver in the auxiliary detection borehole. The receiver probe is placed at the bottom of the hole at one time, and the receiver gain is adjusted so that the magnetic field intensity of the first sampling point received by the receiver is zero and the gain is recorded. Then the receiver probe is lifted at a uniform speed. The lifting rate should be 0.1m / s to 0.15m / s, and the distance between the measuring points is 0.1m. The smaller value should be taken near the bottom of the anchor bolt. During the lifting process of the probe, the anchor bolt alternating magnetic field intensity H is recorded, and a curve of the anchor bolt alternating magnetic field intensity H changing with the hole depth L is drawn (H L ),like Figure 5 ;

[0068] (3) According to the actual measured anchor bolt alternating magnetic field strength H 1 l , through the formula ΔH l 1 =H 1 l(i+1) -H 1 li Find the changing magnetic field strength H at adjacent point i 1 l Variable magnetic field strength H at points i and i+1 1 l The difference between i+1 is ΔH l 1 , and then calculate ΔH l 1 The ratio of the distance between the measuring points, i.e. the variable magnetic field strength H 1 l The rate of change H in the depth direction of the anchor rod 1 ll , and plot the rate of change H 1 ll The curve (dH L / dL), that is, the alternating magnetic field intensity gradient dH L / dL curve, such as Figure 7 As shown;

[0069] (4) The curve H of the anchor bolt alternating magnetic field strength changing with the drilling depth of the probe in step (2) L Calculate its decibel value and draw a curve H L The dB curve is as follows: Figure 6As shown; the curve H of the alternating magnetic field strength changing with the drilling depth of the probe L The decibel value is calculated as follows:

[0070] Where: dB is H L The decibel value of the curve, that is, the L Find the 20-fold common logarithm value;

[0071] H L is the alternating magnetic field strength measured by the receiving probe at depth L;

[0072] (5) The curve H of the anchor bolt alternating magnetic field intensity H versus hole depth L drawn in step (2) L It can be seen that the alternating magnetic field intensity decays rapidly (drops steeply) 15 meters from the end of the anchor bar. When the alternating magnetic field intensity decays to zero, the receiver gain is adjusted to measure the alternating magnetic field 2 meters after the end of the anchor bar. Figure 2 and Figure 5 , you will find Figure 5 H L The curve decays faster with increasing depth, which is due to the current loss between the anchor and the anchor inclusion. Figure 6 The curve H of the anchor bolt alternating magnetic field strength H measured in the experiment changes with the hole depth L L , Figure 7 The alternating magnetic field intensity gradient dH calculated in L / dL curve and Figure 8 The curve H of the anchor bolt alternating magnetic field intensity H calculated in the calculation with the hole depth L L From the dB curve, it can be seen that the curve has undergone obvious changes at 15 meters from the end of the anchor bar. Based on the comprehensive analysis of the curve change, it is determined that the anchor bar length is 15 meters.

[0073] For this project, the anchor rods were taken out for actual measurement, and the value was indeed 15m, proving that the detection accuracy of the method in this application is high.

[0074] The above is only one embodiment of the present invention, and its description is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the attached claims.

Claims

1. A method for detecting the length of an underground anchor rod, characterized in that The method comprises the following steps: (1) Arrange auxiliary inspection holes with a diameter of 60 mm to 110 mm within 3 meters of the anchor bolt and anchor bar to be inspected, keep the drill hole relatively parallel to the anchor bolt, and the drilling depth should be at least 5 meters greater than the designed construction length of the anchor bolt; (2) Use a transmitter to load an alternating current of a certain frequency to the anchor bolts and anchor bars to be tested by charging or clamping, and use a receiver in the borehole to measure the alternating magnetic field strength H generated by the anchor bolts and anchor bars. First, place the receiver probe at the bottom of the hole to receive the magnetic field strength of the first sampling point, and then lift the receiver probe at a uniform speed of 0.1m / s to 0.15m / s, with a measurement point spacing of 0.05m to 0.1m. During the lifting process, record the depth L of the probe in the borehole and the alternating magnetic field strength H of the anchor bolts and anchor bars measured at the L. L , and draw a curve H showing the intensity of the alternating magnetic field of the anchor rod and anchor bar changing with the depth of the borehole where the probe is located L ; (3) According to the curve H of the alternating magnetic field strength of the anchor rod and anchor bar changing with the drilling depth of the probe L , calculate the alternating magnetic field strength difference dH between two adjacent measuring points L The length difference dL between two adjacent measuring points can be used to obtain the alternating magnetic field intensity gradient dH of any two adjacent measuring points. L / dL, and draw a curve dH of the gradient of the alternating magnetic field intensity versus the depth of the borehole where the probe is located L / dL; dH L =H i+1 -H i , where H i is the alternating magnetic field strength measured at the i-th measuring point, H i+1 is the alternating magnetic field intensity measured at the i+1th measuring point; the alternating magnetic field intensity gradient dH L / dL is the ratio of the alternating magnetic field intensity difference dH between two adjacent measuring points to the distance between the measuring points; (4) Using the curve H drawn in step (2) showing the change of the alternating magnetic field strength of the anchor rod and anchor bar with the depth of the probe in the borehole L and the curve dH drawn in step (3) showing the gradient of the alternating magnetic field intensity as a function of the depth of the borehole where the probe is located L / dL determines the length of the anchor rod and anchor bar, and the curve H of the alternating magnetic field strength of the anchor rod and anchor bar changes with the depth of the borehole where the probe is located L The curve dH of the alternating magnetic field intensity gradient changing with the depth of the probe in the borehole L / dL The position where the inflection point distortion appears is the bottom end position of the anchor rod and anchor bar, and the corresponding depth at this time is the length L of the anchor rod and anchor bar; the curve H of the change of the alternating magnetic field intensity of the anchor rod and anchor bar with the drilling depth of the probe in step (4) L Calculate its decibel value: Where: dB is H L The decibel value of the curve, that is, the L Find the 20-fold common logarithm value; H L is the alternating magnetic field strength measured by the receiving probe at depth L; Draw the dB curve based on the dB value calculated by the above formula, and combine it with the curve H of the alternating magnetic field intensity of the anchor rod and anchor bar changing with the drilling depth of the probe. L The curve dH of the gradient of the alternating magnetic field intensity changing with the depth of the probe in the borehole L / dL determines the bottom position L of the anchor rod and anchor bar; the position of the distortion point of the three curves is the bottom position of the anchor rod and anchor bar, and the corresponding depth at this time is the length L of the anchor rod and anchor bar.

2. A method for detecting the length of an underground anchor rod according to claim 1, characterized in that: The detection auxiliary borehole in step (1) is formed by drilling with a drilling rig, using a non-magnetic material casing as the wall protection, the inner diameter of the pipe is 50mm to 100mm, and the electromagnetic magnetic field receiver probe can be freely raised and lowered in the borehole.

3. A method for detecting the length of an underground anchor rod according to claim 1, characterized in that: The charging method in step (2) is to connect one end of the detector transmitter AC power supply of a certain frequency to the exposed part of the anchor rod and anchor bar, and the other end to the grounding electrode far away from the contact point, which is single-ended charging. At this time, a current loop is formed between the anchor rod and anchor bar, the earth and the power supply, and an alternating magnetic field is generated around the anchor rod; the clamp method in step (2) is also called the coil induction method, which uses a special clamp to cover the anchor rod and anchor bar to be tested, so that an induced current is generated in the anchor rod and anchor bar, and then a secondary annular alternating magnetic field is generated around the anchor rod, and a receiver is used to receive the alternating magnetic field generated by the anchor rod and anchor bar in the auxiliary detection borehole; in step (2), the receiver probe is placed at the bottom of the hole to receive the magnetic field strength of the first sampling point as zero and record the gain.

4. A method for detecting the length of an underground anchor rod according to claim 1, characterized in that: The receiver in step (2) should have at least two mutually perpendicular receiving coils, and the alternating magnetic field strength H of the anchor rod and anchor reinforcement measured by the two mutually perpendicular receiving coils can be recorded independently and can be linearly superimposed.

5. The method for detecting the length of underground anchor rod according to claim 1, characterized in that: The receiver in step (2) is provided with three mutually perpendicular receiving coils and an independent processing circuit, and the receiving probe is arranged in a sealed waterproof cavity.

6. A method for detecting the length of an underground anchor rod according to claim 1 or 3, characterized in that: The curve H of the alternating magnetic field strength of the anchor rod and anchor bar in step (4) as a function of the drilling depth of the probe is L The curve dH of the alternating magnetic field intensity gradient changing with the depth of the probe in the borehole L When the position of the inflection point distortion is inconsistent and there is an error, when the error is less than the test step length, calculate the average value of the depth. When the error is greater than the test step length, reduce the test step length and encrypt the measuring points within 1 meter above and below the distortion point until the depth error is less than the step length, and calculate the average value of the two depths.

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