Primary rock stress measurement method suitable for weakly cemented soft rock disintegrated in case of cementation

Abstract

The invention relates to the technical field of ground stress measurement, and discloses a protolith stress measurement method suitable for weakly cemented soft rock disintegrated when meeting cementized.The protolith stress measurement method comprises the steps that compressed air is used for replacing water to serve as a powder discharging medium, and drilling construction is conducted; in the drilling process, the drilling area is cooled through a cooling medium, and the temperature is monitored to keep the rock mass temperature constant; installing a hollow inclusion strain gauge in the drill hole; drilling a rock core under a water-free condition, and completely taking out the rock core; according to the strain change measured by the hollow inclusion strain gauge before and after coring, the stress value of the protolith is calculated. According to the method, a dry drilling process of discharging powder by compressed air and controlling temperature by liquid nitrogen is adopted, so that the contact between water and weakly cemented soft rock is thoroughly avoided, and drilling and coring can be smoothly implemented on a stratum which cannot be subjected to primary rock stress measurement originally and is disintegrated when meeting cement.

Description

Technical Field

[0001] This invention relates to the field of geostress measurement technology, specifically to a method for measuring the original rock stress of weakly cemented soft rock that has encountered cementation and disintegration. Background Technology

[0002] In-situ stress is a fundamental parameter for the design, construction, and stability analysis of underground engineering projects, and its measurement accuracy is directly related to project safety. Currently, the core stress relief method is one of the main methods for measuring in-situ stress. This method involves drilling a hole in the rock mass, installing strain gauges, and drilling a core containing the strain gauges to relieve the stress between the core and the surrounding rock mass. The in-situ stress is then calculated by measuring the strain changes before and after the stress relief.

[0003] Existing core drilling stress relief methods require the use of water flow for drilling dust removal and cooling. Specifically, during drilling and core sampling, circulating water carries rock dust out of the hole while simultaneously cooling the drill bit to prevent overheating from affecting drill bit life and rock mass properties. The hollow inclusion strain gauge is typically installed manually using a steel threaded guide rod, which is then inserted into the small hole. After the guide rod is removed and the adhesive has solidified, core sampling can proceed. The core sleeve employs a one-piece hollow cylindrical structure; after core drilling, it is removed from the sleeve by tapping or jacking.

[0004] When existing methods are applied to weakly cemented soft rock formations that are prone to softening, mudding, or even disintegration upon contact with water, the rock structure is damaged due to the water flow used for dust removal and cooling. This results in the inability to obtain complete core samples and alters the mechanical properties of the rock mass, failing to reflect the true stress state of the original rock. If dry drilling is used to avoid water-related problems, a large amount of dust will accumulate in the borehole. At the same time, the friction of the drill bit will generate heat, causing the rock mass temperature to rise. The thermal expansion and contraction effect will introduce thermal stress interference, resulting in severely distorted measurement results.

[0005] To address the problems in related technologies, this invention proposes a method for measuring the original rock stress of weakly cemented soft rock that is prone to cementation and disintegration. Summary of the Invention

[0006] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a method for measuring in-situ stress in weakly cemented soft rock that is prone to cementation and disintegration. It has advantages such as dry drilling, active temperature control, precise installation, and complete core sampling, and solves the problem that existing core drilling stress relief methods cannot be effectively applied to weakly cemented soft rock that is prone to cementation and disintegration.

[0007] (II) Technical Solution To address the technical problem that existing borehole core stress relief methods cannot be effectively applied in soft rock with weak cementation and disintegration, this invention provides the following technical solution: A method for measuring in-situ stress in weakly cemented soft rock that has encountered cementation and disintegration includes the following steps: using compressed air instead of water as the dust removal medium for drilling; cooling the drilling area with a cooling medium during drilling and monitoring the temperature to maintain a constant rock mass temperature; installing hollow inclusion strain gauges inside the borehole; drilling and extracting rock cores under waterless conditions; and calculating the in-situ stress value based on the strain changes measured by the hollow inclusion strain gauges before and after core extraction.

[0008] Preferably, the cooling medium is liquid nitrogen, which is delivered to the bottom of the hole through a hollow drill pipe.

[0009] Preferably, the temperature of the rock mass inside the borehole is monitored in real time by a wireless temperature sensor installed on the drill pipe, and the amount of liquid nitrogen delivered is adjusted according to the monitoring data.

[0010] Preferably, during drilling, a dust collection box is installed at the borehole opening to collect the dust emitted by the compressed air.

[0011] Preferably, after drilling is completed, a guide rod is used to automatically insert the hollow package into the small hole inside the borehole; the guide rod is a telescopic structure and is equipped with a scale, angle sensor and visual guide.

[0012] Preferably, the guide rod is also provided with openable blades, which are spaced apart along the axial direction of the guide rod; when the hollow package is placed and the guide rod is pulled out, the blades are opened to remove rock fragments that have fallen into the borehole.

[0013] Preferably, the steps of drilling and extracting rock cores under waterless conditions include: drilling rock cores using a longitudinally detachable core sleeve, and extracting the rock cores completely by disassembling the core sleeve.

[0014] Preferably, the outer wall of the core-taking sleeve is provided with a spiral chip removal groove.

[0015] (III) Beneficial Effects Compared with existing technologies, this invention provides a method for measuring in-situ stress in weakly cemented soft rock that is prone to cementation and disintegration, and has the following advantages: 1. This invention abandons the traditional method of relying on water flow for dust removal and cooling, and adopts a dry drilling process with compressed air dust removal and liquid nitrogen temperature control, which completely avoids the contact between water and weakly cemented soft rock, so that drilling and core sampling can be successfully carried out in cemented and disintegrated strata where it was originally impossible to measure the original rock stress.

[0016] 2. This invention monitors the borehole temperature in real time by setting a wireless temperature sensor on the drill rod and adjusts the liquid nitrogen delivery rate according to the temperature, so that the rock mass maintains a constant temperature during drilling and core sampling. This effectively eliminates the thermal expansion of the rock mass caused by frictional heat generated by dry drilling, avoids the interference of thermal stress on strain measurement, and improves the accuracy of the original rock stress data.

[0017] 3. This invention uses a telescopic guide rod with a scale, angle sensor and visual guide, which can accurately send the hollow package into the center of the small hole, solving the defects of traditional manual placement method which is difficult to center in deep or inclined holes and is time-consuming and laborious; at the same time, the openable blades on the guide rod can help remove the pieces falling into the hole when pulling out, avoiding guide rod jamming and improving work efficiency and success rate.

[0018] 4. By employing a longitudinally detachable core sleeve, the present invention allows for the direct extraction of a complete core sample after drilling, avoiding secondary damage to weak core samples caused by the traditional integrated sleeve hammering or jacking process. This provides a complete core sample for subsequent stress calculations and further ensures the reliability of the measurement results. Attached Figure Description

[0019] Figure 1 This is a structural diagram of the device for placing hollow packages according to the present invention; Figure 2 This is a diagram showing the installation position of the dust collection box in this invention; Figure 3 This is a flowchart of the method of the present invention.

[0020] In the diagram: 1. Dust collection box; 2. Drilling hole; 3. Guide rod; 4. Blade. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] As described in the background section, there are shortcomings in the existing technology. In order to solve the above-mentioned technical problems, this application proposes a method for measuring the original rock stress of weakly cemented soft rock that has encountered cementation and disintegration.

[0023] Please see Figure 1-3 A method for measuring in-situ stress in weakly cemented soft rock that has encountered cementation and disintegration includes the following steps: S1. Preparation and large-hole drilling; The dust collection box 1 is securely fixed to the rock wall at the predetermined measurement location. A drilling hole 2 is pre-drilled in the middle of the dust collection box 1 for inserting the drill bit and drill rod. In this embodiment, both the drill rod and drill bit are specially designed hollow structures resistant to liquid nitrogen cryogenic temperatures. Liquid nitrogen can be introduced into the drill rod, and a wireless temperature sensor is installed on the first drill rod.

[0024] Connect the compressed air supply pipe to the drill rod to blow out rock dust during drilling; simultaneously connect the liquid nitrogen supply pipe to the drill rod to supply liquid nitrogen to the bottom of the hole for cooling. Start the drilling rig to begin large-hole drilling. During drilling, compressed air blows the rock dust generated during drilling out of the hole, where it is collected and stored by a dust collection box to prevent dust generation. A wireless temperature sensor monitors the temperature inside the hole in real time and sends the data to the operator. The operator adjusts the liquid nitrogen supply based on the temperature data to keep the rock mass temperature constant and avoid thermal stress interference caused by increased rock mass temperature due to dry drilling friction heat. After drilling to the designed depth, remove the drill rod and drill bit.

[0025] S2, Drilling concentric small holes; Replace the drill bit and drill rod with those suitable for the smaller hole diameter, using the same hollow structure resistant to liquid nitrogen cryogenicity, and connect compressed air and liquid nitrogen pipelines. Following the same dry drilling process as the larger hole, construct concentric small holes at the bottom of the larger hole, using a dust collection box for dust removal and liquid nitrogen for temperature control during the drilling process. After the small holes are completed, remove the drill rod and drill bit.

[0026] S3. Automatic placement of hollow packaging; The hollow package strain gauge is mounted on the front end of a specially designed guide rod. This guide rod 3 is a mechanically telescopic structure, equipped with a scale for displaying the insertion depth, an angle sensor for displaying the installation angle of the hollow package in real time, and a visual guide. The visual guide is used to prevent deflection caused by gravity due to the excessive length of the guide rod, ensuring that the hollow package can be accurately aligned with the center of the small hole.

[0027] Insert the guide rod into the borehole, and use a visual guide and scale to precisely position the hollow package into the predetermined location within the borehole. Once the hollow package is in place, operate the openable blades 4, which are installed every 0.5m on the guide rod, to open them. During the removal of the guide rod, the blades can help scrape away any large rocks that may fall into the borehole, preventing the guide rod from getting stuck, and simultaneously cleaning the borehole.

[0028] S4. Core removal and stress relief; After the glue inside the hollow cavity has completely solidified, bonding the cavity tightly to the surrounding rock mass, the cavity is connected to the geostress testing instrument. A detachable core-sampling sleeve is used for core sampling. This sleeve consists of two longitudinally separable half-cylinders, with a spiral-shaped chip removal groove on the outer wall of the half-cylinder.

[0029] The coring sleeve is connected to the drill pipe, and the same dry drilling process (compressed air dust removal, liquid nitrogen temperature control) is used to begin drilling for rock cores containing hollow inclusions. During drilling, rock dust is discharged from the borehole through the spiral dust removal groove and compressed air. After drilling to the predetermined depth, the coring sleeve, along with the rock core, is pulled out of the borehole. By longitudinally disassembling the coring sleeve, the soft rock core can be completely extracted, avoiding the damage to the rock core caused by hammering and pushing out the core using traditional one-piece sleeves.

[0030] S5. Stress calculation; The extracted intact rock core was placed in a stable indoor environment, and strain data before and after stress relief were read using a geostress testing instrument and hollow inclusion strain gauges. Based on the conventional calculation principles of the borehole core stress relief method, and combined with the rock elastic modulus and Poisson's ratio measured in indoor tests, the original rock stress value at the measuring point was calculated according to well-known standard calculation methods in the field. This completes the entire original rock stress measurement process.

[0031] The preferred standard calculation method is to obtain the magnitude and direction of the three-dimensional principal stresses by solving a system of equations. Specifically: The extracted intact rock core was placed in a stable indoor environment, and the strain data from 12 channels recorded by the hollow inclusion strain gauge was read using a geostress testing instrument. The rock core with strain gauges was then placed in a confining pressure calibrator for a confining pressure test to determine the rock's elastic modulus and Poisson's ratio. Based on elasticity theory, an analytical relationship between borehole wall strain and far-field stress components was established, and an overdetermined equation set was constructed using 12 strain observations. The least squares method was used to solve this overdetermined equation set, yielding six stress components in the borehole coordinate system. Furthermore, the eigenvalues ​​and eigenvectors of the stress tensor were solved, and the magnitudes and spatial orientations of the three principal stresses were calculated.

[0032] Furthermore, the dust collection box is fixed to the opening and can be equipped with suction material or a dust collection chamber to capture dust carried out by the compressed air. The guide rod body has spaced-opening blades that retract in their natural state and can be opened mechanically or electrically when needed. The detachable core-taking sleeve consists of two half-cylinders connected by a flange or clips for easy assembly and disassembly.

[0033] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for measuring the original rock stress of weakly cemented soft rock that has encountered cementation and disintegration, comprising the following steps: Compressed air is used instead of water as the dust removal medium for drilling operations; During the drilling process, the drilling area is cooled by a cooling medium, and the temperature is monitored to maintain a constant rock mass temperature; hollow inclusion strain gauges are installed inside the borehole; rock cores are drilled under waterless conditions and removed intact; the original rock stress value is calculated based on the strain changes measured by the hollow inclusion strain gauges before and after core extraction.

2. The method for measuring the original rock stress of weakly cemented soft rock that disintegrates upon cementing, as described in claim 1, is characterized in that... The cooling medium is liquid nitrogen, which is delivered to the bottom of the hole through the hollow drill pipe.

3. The method for measuring the original rock stress of weakly cemented soft rock that disintegrates upon contact with cementing, as described in claim 2, is characterized in that... The temperature of the rock mass inside the borehole is monitored in real time by a wireless temperature sensor installed on the drill pipe, and the amount of liquid nitrogen delivered is adjusted according to the monitoring data.

4. The method for measuring the original rock stress of weakly cemented soft rock that disintegrates upon contact with cementing, as described in claim 1, is characterized in that... When drilling, a dust collection box is installed at the borehole opening to collect the dust emitted by the compressed air.

5. The method for measuring the original rock stress of weakly cemented soft rock that disintegrates upon cementing, as described in claim 1, is characterized in that... After drilling is completed, a guide rod is used to automatically insert the hollow package into the small hole inside the borehole; the guide rod is a telescopic structure and is equipped with a scale, angle sensor and visual guide.

6. The method for measuring the original rock stress of weakly cemented soft rock that disintegrates upon cementing, as described in claim 5, is characterized in that... The guide rod is also equipped with openable blades, which are spaced apart along the axial direction of the guide rod. When the hollow package is placed and the guide rod is pulled out, the blades are opened to remove rock fragments that have fallen into the borehole.

7. The method for measuring the original rock stress of weakly cemented soft rock that disintegrates upon contact with cementing, as described in claim 1, is characterized in that... The steps for drilling and extracting rock cores under waterless conditions include: drilling the rock core using a longitudinally detachable core sleeve, and extracting the rock core completely by disassembling the core sleeve.

8. The method for measuring the original rock stress of weakly cemented soft rock that disintegrates upon cementing, as described in claim 7, is characterized in that... The outer wall of the core-taking sleeve is provided with a spiral chip removal groove.

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