Rock-soil cohesive force measuring device and arching effect calculating method

Abstract

The invention discloses a rock-soil cohesive force measuring device and an arching effect calculating method. The rock-soil cohesive force measuring device comprises the following steps: acquiring a columnar soil sample of on-site undisturbed rock-soil; the soil sample is installed in a rock-soil cohesive force direct measuring device, axial tension is applied to the soil sample until the soil sample is broken, the total tension during breaking and friction force generated by a moving part of the device are measured, and the rock-soil cohesive force is directly calculated according to the cross section area of the soil sample; and substituting the directly measured cohesive force, the rock-soil volume weight and the preset safety coefficient into a semispherical soil arch limit radius formula, and calculating to obtain the suggested safe excavation radius of the tunnel vault. When the rock-soil body is a homogeneous medium and the normal stress at the potential slip crack surface is close to zero, the shear strength is degraded into the contribution of pure cohesive force, the device for directly measuring the breaking force of the broken rock-soil sample directly measures the cohesive force of the rock-soil through the relation between the breaking force and the area of the rock-soil sample, and finally a more accurate rock-soil cohesive force value is obtained.

Description

Technical Field

[0001] This invention relates to the field of geotechnical testing and measurement technology, and in particular to a soil cohesion measuring device and an arching effect calculation method. The device can directly measure the cohesion of soil samples, and the method can calculate the soil arching effect through soil cohesion parameters. Background Technology

[0002] On the technical front, innovative achievements such as intelligent shield tunneling with simultaneous pushing and assembling, multi-stage dust suppression systems, parallel construction of three tunnels, and geological CT scanning have enabled efficient and safe tunneling under ultra-long, ultra-deep, and complex geological conditions. Tunnel construction safety is of paramount importance.

[0003] The "soil arching effect" in geotechnical mechanics reveals that when relative displacement exists within soil, an arch-like stress path forms between soil particles, transferring the load to relatively stationary areas. In tunnel excavation, the surrounding rock arching effect is the core of the load-bearing mechanism. Classical calculation methods include Protodyakonov's theory based on the limit equilibrium of loose media, Terzaghi's theory considering lateral pressure transfer, and the elastoplastic theory (Kastner formula) deriving surrounding rock pressure from the plastic zone radius. Modern methods widely employ the finite element method (FEM), discrete element method (DEM), and PFC numerical simulation, combined with the "Tan's arch" three-dimensional soil arch model, to accurately predict stratum deformation and support stress. The existence and application of numerous classical and modern methods indicate that each method has its advantages, disadvantages, and applicable scope; a highly unified and simple method for calculating the soil arching effect has yet to be established. Most calculation methods for the arching effect of soil require the use of two soil parameters: cohesion and internal friction angle. Traditional test methods for cohesion mainly include direct shear test, triaxial compression test, unconfined compressive strength test, and in-situ vane shear test. However, regardless of the test, it is derived from the calculation formula. For soils with a certain strength, it cannot be directly measured through the test. This leads to possible deviations in the calculated cohesion value, which in turn causes deviations in the calculation of the arching effect.

[0004] In summary, for soil and rock with certain strength and cohesion, there is an urgent need to find a method that can directly and accurately measure its cohesion and accurately derive the calculation method of soil arching effect through existing theorems and parameters such as cohesion to effectively guide tunnel construction and improve tunnel construction safety. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing soil and rock cohesion measuring devices and soil arching effect calculation methods, and to provide a soil and rock cohesion measuring device and an arching effect calculation method. Under certain assumptions about the soil and rock conditions, the isoperimetric inequality theorem is used to deduce that the soil arching effect is only related to cohesion and soil unit weight. Furthermore, a device for directly measuring the cohesion of soil and rock samples in the field is invented, ensuring the accuracy of the soil arching effect calculation.

[0006] The objective of this invention is achieved through the following technical solution: A method for calculating the arching effect of soil and rock based on directly measured cohesion includes the following steps: Obtain columnar soil samples of the original soil and rock at the site; The soil sample is installed in the direct soil-rock cohesion measuring device, and an axial tensile force is applied to the soil sample until it breaks. The total tensile force F2 at the time of breakage and the frictional force F1 generated by the moving parts of the device are measured, and the soil-rock cohesion c is directly calculated based on the cross-sectional area As of the soil sample. Substituting the directly measured cohesion c, soil unit weight γ, and preset safety factor A into the formula for the ultimate radius of the hemispherical soil arch, the recommended safe excavation radius R of the tunnel arch is calculated, which is used to evaluate the stability of the soil arch and guide the tunnel design. Wherein, the cohesive force c is calculated according to the following formula: ; The formula for the ultimate radius of the hemispherical soil arch is: ; In the formula, It is the cohesion of soil and rock; This represents the total tensile force measured when the soil sample breaks. The frictional force generated by the movement of the moving parts of the measuring device; This represents the cross-sectional area of ​​the soil sample. Recommend a safe excavation radius for the tunnel; This refers to the unit weight of soil and rock, expressed in kN / m³. 3 ; For safety factor; The formula for the ultimate radius of the hemispherical soil arch is applicable to homogeneous, cohesion-dominant soil strata, and to the case where the collapsed body is hemispherical and the normal stress at the collapse interface can be ignored when the arch crown is locally unstable.

[0007] By applying axial tension to undisturbed columnar soil samples using a direct soil cohesion measurement device until fracture, the breaking force is directly obtained. After deducting the device's frictional force, the soil cohesion *c* is accurately calculated based on the cross-sectional area of ​​the soil sample, avoiding errors caused by theoretical assumptions or parameter inversion in traditional indirect experimental methods. Based on this, and using geotechnical analysis, under homogeneous, cohesion-dominated conditions with negligible normal stress at the collapse interface, the tunnel arch instability morphology tends to be hemispherical. Using this physical model, the limit equilibrium relationship between the total cohesion at the fracture surface and the self-weight of the collapsed body is established, deriving the "hemispherical soil arch limit radius formula." This method combines high-precision measured cohesion with analytical formulas with clear physical meaning, significantly improving the accuracy and engineering applicability of soil arch effect assessment. It can quickly determine the safe excavation radius of the tunnel arch, providing a scientific basis for the design of unsupported section lengths and timely support, effectively ensuring the safety and economy of tunnel construction under complex geological conditions.

[0008] As a preferred embodiment, the columnar soil sample is cylindrical, and its cross-sectional area is [missing information]. ,in The radius of the soil sample is given.

[0009] As a preferred method, the frictional force F1 is obtained by driving the direct measurement device of soil-rock cohesion to the same displacement stroke as the actual test when the soil sample is not installed, and the reading is obtained from the tension gauge.

[0010] As a preferred method, the unit weight γ of the soil and rock is determined by on-site weighing or by using the measured value in the geological survey report.

[0011] As a preferred method, cohesion measurements are repeated at the same depth location at no less than three times. The maximum and minimum values ​​are discarded, and the average value is taken as the final c value for calculating R.

[0012] As a preferred approach, the recommended safe excavation radius R is used to guide the design of the unsupported excavation span of a circular arch tunnel. During construction, the excavation progress should be controlled and initial support should be applied in a timely manner to maintain the stability of the natural soil arch.

[0013] A device for directly measuring the cohesion of soil and rock, used to implement the direct cohesion measurement step in the above method, the device comprising: Equipment platform; The first fixed end and the second fixed end are fixed on the equipment platform; The first clamp is installed on the first fixed end and is used to hold one end of the soil sample. The upper movable end is connected to a second clamp at one end to hold the soil sample at the other end, which is connected to a tension gauge. The force transmission device is connected to the tension gauge via a steel wire rope; An adjustable screw, one end of which is connected to a manual turntable, and the other end is threaded into a force transmission device; A fixed horizontal shaft runs through the force transmission device and the lower movable end, allowing the force transmission device to slide axially. The rotating manual turntable can drive the force transmission device to move, and apply a controllable axial tension to the soil sample via a steel wire rope and a tension gauge.

[0014] As a preferred embodiment, the clamping surfaces of the first and second clamps are provided with anti-slip pads to prevent the soil sample from slipping or being damaged during loading.

[0015] As a preferred embodiment, the upper movable end and the lower movable end are rigidly connected by a connecting pin, and the lower movable end is sleeved on a fixed horizontal shaft to ensure a stable loading path.

[0016] As a preferred embodiment, the device is a detachable and assembleable structure, which facilitates transportation to the tunnel face or drilling site for on-site cohesion testing.

[0017] This invention has at least the following beneficial effects: When the soil and rock mass is a homogeneous medium and the normal stress at the potential slip surface approaches zero, its shear strength degenerates into a contribution from pure cohesion. In this case, the ultimate breaking force per unit area can be considered a direct measure of cohesion. Simultaneously, by extending the mathematical theorem of isoperimetric inequalities to the conclusion that the surface area of ​​a sphere is the smallest among all closed curved surfaces of equal volume in three-dimensional space, it is analyzed that the collapse shape above the excavated tunnel should be approximately a hemisphere. By comparing the breaking force of the soil and rock on the surface area of ​​the hemisphere with the weight of the collapsed soil and rock, a calculation formula for the soil arching effect is derived, showing that it is directly proportional to cohesion and inversely proportional to the unit weight of the soil and rock. Furthermore, a device for directly measuring the breaking force of a soil and rock sample is invented. By analyzing the relationship between the breaking force and the area of ​​the soil and rock sample, the cohesion of the soil and rock can be directly measured, ultimately obtaining a more accurate value for soil and rock cohesion. The soil arch effect calculation formula derived from the isoperiodic inequality mathematical theorem is used to calculate the limit value of the soil excavation radius, that is, the limit excavation span and shape under natural soil conditions. Attached Figure Description

[0018] To reveal the technical details of the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below. It should be emphasized that these drawings only present several embodiments of the present invention and should not be considered as defining the scope of the invention. For those skilled in the art, other related drawings can still be derived based on these drawings without inventive effort.

[0019] Figure 1 A schematic diagram of the tunnel arch collapse during excavation; Figure 2 A three-dimensional schematic diagram of a soil and rock cohesion measuring device; Figure 3This is a front view of a soil and rock cohesion measuring device.

[0020] In the diagram, 1-original rock and soil, 2-collapse interface, 3-collapsed rock and soil, 4-tunnel, 5-first fixed end, 6-first clamp, 7-rock and soil sample, 8-second clamp, 9-upper movable end, 10-tension gauge, 11-steel wire rope, 12-force transmission device, 13-adjustable screw, 14-second fixed end, 15-manual turntable, 16-equipment platform, 17-fixed horizontal shaft, 18-connecting pin, 19-lower movable end, 20-anti-slip pad. Detailed Implementation

[0021] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings, but the scope of protection of the present invention is not limited to the following description.

[0022] In the following description, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. However, it should be understood that the present disclosure is not limited to the specific forms shown herein. Rather, it should be understood to encompass various variations, equivalents, and / or alternatives to the embodiments of the present disclosure. In illustrating the drawings, the same reference numerals will be used to denote similar components.

[0023] In the various embodiments of this disclosure, the terms "first," "second," "the first," or "the second" are intended to modify different components and not to indicate order and / or importance, nor do they constitute a limitation on the respective components. For example, a first user equipment and a second user equipment represent different user equipments, although they both fall under the category of user equipment. Similarly, a first component may be named a second component, and a second component may be named a first component, without changing their essential attributes within the scope of this disclosure.

[0024] In this disclosure, terminology is used to describe specific embodiments and does not constitute a limitation thereof. In this context, the use of the singular form also encompasses the plural form, unless otherwise expressly stated herein. In the course of description, terms such as “comprising” or “having” are intended to indicate the presence of features, quantities, steps, operations, structural components, parts, or combinations thereof, and do not preclude the possibility or addition of one or more other features, quantities, steps, operations, structural components, parts, or combinations thereof.

[0025] It should be clarified that while the following description provides detailed specific information to aid in a comprehensive understanding of the exemplary embodiments, those skilled in the art will recognize that the exemplary embodiments can be implemented even without these specific details. For example, the system may be illustrated using block diagrams to avoid excessive detail that could obscure the clarity of the example. In other cases, to maintain the clarity of the example, unnecessary details of well-known processes, structures, and techniques may be omitted.

[0026] During tunnel excavation, it is desirable for the soil and rock above to remain stable for an extended period, allowing construction workers time for initial support and secondary lining. For soil and rock with a certain level of cohesion and internal friction angle, an arching effect is observed during actual excavation. Therefore, many technicians have been researching methods for calculating the soil arching effect, hoping to find the most critical soil and rock parameters to accurately calculate the soil arching effect under different parameter values. However, traditional methods rely on indirect inversion to obtain cohesion, which fails to accurately reflect the tensile breaking characteristics of soil and rock under stress-free conditions, leading to uncertainty in soil arch stability assessment. Therefore, it is necessary to establish a soil arching effect calculation model based on directly measured cohesion, starting from the instability mechanism.

[0027] This paper analyzes the formation process of a soil arch in rock and soil. A square tunnel 4 is excavated within undisturbed rock and soil 1. Due to the lack of support at the excavation face during excavation, the stress at the excavation face changes. According to previous research, due to the influence of parameters such as cohesion and internal friction angle, the stress above the tunnel arch is redistributed. As the rock and soil at the excavation face deforms into the tunnel, the normal stress of the rock and soil outside the excavation face transfers outward, eventually forming a soil arch effect at a certain location. The rock and soil inside that has not formed a soil arch effect cannot transfer gravity to the nearby rock and soil, and thus collapses into rock and soil 3, eventually forming a collapse interface 2. Outside the collapse interface 2, a soil arch effect forms and no longer collapses. During the downward deformation of the collapsed rock and soil 3, the normal stress of the undisturbed rock and soil 1 outside the collapse interface 2 transfers outward, forming an arch effect, and finally the entire collapsed rock and soil 3 falls. The paper now analyzes why the collapsed rock and soil 3 falls.

[0028] The collapse of the collapsed rock and soil 3 was likely caused by a disruption of the force balance at the collapse interface 2 between the undisturbed rock and soil 1 and the collapsed rock and soil 3. More specifically, the weight of the collapsed rock and soil 3 exceeded the collapse-resistant breaking force of the rock and soil at the collapse interface 2. The calculation formula is based on Mohr-Coulomb's law. The cohesive force is The internal friction angle is Shear strength is Rock and soil fracture and cohesion Normal stress internal friction angle Regarding the above analysis of the soil arching effect formation process, during the downward deformation of the collapsed rock and soil 3, the normal stress σ gradually decreases. When the normal stress... When the force of the rock and soil at the collapse interface 2 is zero, the breaking force is minimal, and the gravity of the collapsed rock and soil 3 remains unchanged. At this point, collapse is most likely to occur. Therefore, the breaking and falling of the collapsed rock and soil 3 is only related to cohesion, and the breaking force is the product of cohesion c and the area of ​​the collapse interface 2.

[0029] Now that we have the method for calculating the breaking force of collapsed rock and soil 3, we need to analyze under what shape the collapsed interface 2 is most likely to break. Given that the unit weight γ, cohesion c, and volume of collapsed rock and soil 3 remain constant, the smaller the area of ​​the collapsed interface 2, the easier it is to break. The isoperimetric inequality theorem rigorously proves that among all three-dimensional geometric solids of the same volume, the sphere has the smallest surface area. Simply put, this theorem can be written as: S 3 ≥36πV 2 Where S is the surface area and V is the volume. When the equality holds, this geometric solid is a sphere. For the collapsed rock and soil 3, it cannot be a sphere, but rather a hemisphere. For a three-dimensional figure, under the condition of constant volume, the area of ​​the collapsed interface 2 is minimized when it is a hemisphere; in this case, the collapsed rock and soil 3 is most prone to fracture. Therefore, according to the isoperimetric inequality theorem, it can be analyzed that the rock and soil are most prone to fracture when the collapsed interface 2 is a hemisphere.

[0030] A method for calculating the arch effect of soil and rock based on directly measured cohesion is proposed. This method combines direct field testing with formulas to achieve rapid quantitative assessment of the stability of the soil arch at the tunnel crown. The specific implementation steps are as follows: Obtain columnar soil samples of the original soil and rock at the site; The soil sample is installed in the direct soil-rock cohesion measuring device, and an axial tensile force is applied to the soil sample until it breaks. The total tensile force F2 at the time of breakage and the frictional force F1 generated by the moving parts of the device are measured, and the soil-rock cohesion c is directly calculated based on the cross-sectional area As of the soil sample. First, use geological drilling equipment to collect circular soil samples on site. Cut the soil sample at the required length L (L=L1+L2+L3, where L1 is the length of the soil sample that can be placed in the first clamp 6; L2 is the length of the soil sample that can be placed in the second clamp 8; L3 is the distance between the two clamps when measuring cohesion, generally taken as 10-20mm) and install the cohesion measuring device on site. Rotate the manual turntable 15, push the force transmission device 12 to the leftmost position, manually move the upper movable end 9 so that the distance between the first clamp 6 and the second clamp 8 is 10-20mm, open the upper end cover of the first clamp 6 and the second clamp 8, and place anti-slip pads 20 on the inner surface of the lower cover of the first clamp 6 and the second clamp 8 respectively, and put the soil sample 7 into the clamp. Then, the anti-slip pad 20 covers the upper surface of the soil sample 7, and the upper and lower covers of the first clamp 6 and the second clamp 8 are clamped tightly respectively (Note: the soil sample should not be damaged, but it should be ensured that the soil sample 7 will not slip when the cross section is broken. The clamping force can be determined by test).

[0031] Substituting the directly measured cohesion c, soil unit weight γ, and preset safety factor A into the formula for the ultimate radius of the hemispherical soil arch, the recommended safe excavation radius R of the tunnel arch is calculated, which is used to evaluate the stability of the soil arch and guide the tunnel design. Wherein, the cohesive force c is calculated according to the following formula: ; The formula for the ultimate radius of the hemispherical soil arch is: ; In the formula, The cohesion of soil and rock is expressed in kPa. The total tensile force measured when the soil sample breaks is expressed in kN. The frictional force generated by the movement of the moving parts of the measuring device, expressed in kN; The cross-sectional area of ​​the soil sample is expressed in m². 2 ; Recommended safe excavation radius for tunnels, in meters; This refers to the unit weight of soil and rock, expressed in kN / m³. 3 ; For safety factors, the value ranges from 1.5 to 2.0; The formula for the ultimate radius of the hemispherical soil arch is applicable to homogeneous, cohesion-dominant soil strata, and to the case where the collapsed body is hemispherical and the normal stress at the collapse interface can be ignored when the arch crown is locally unstable.

[0032] In a preferred embodiment, the columnar soil sample is cylindrical, and its cross-sectional area is [missing information]. ,in The radius of the soil sample is in meters (m).

[0033] The theoretical basis of this invention stems from an in-depth analysis of the instability mechanism of tunnel arches. For example... Figure 1 As shown, after tunnel excavation in homogeneous rock and soil, the rock and soil above the arch deform due to loss of support, and stress redistribution forms a soil arch effect. The rock and soil that did not participate in the arch formation (i.e., collapsed rock and soil 3) moves downward under its own weight, and the normal stress at the collapse interface 2 between it and the stable rock mass gradually decreases; when this normal stress approaches zero, according to the Mohr-Coulomb strength criterion ( The cohesive force is The internal friction angle is Shear strength is The shear strength degenerates to be solely determined by cohesion. Control. At this point, the stability of the collapsed body depends on the balance between the total breaking force provided by cohesion and its own weight. Furthermore, based on the isoperimetric inequality theorem, when the volume is constant, a sphere has the minimum surface area, which can be mathematically expressed as: ,in For surface area, For volume, the equality holds if and only if the geometric body is a sphere. It can be deduced that when the collapsed body is hemispherical, its contact area with the stable rock mass is minimized, the required total cohesion to maintain stability is lowest, and therefore it is most prone to instability. Based on this, a hemispherical collapse model is established, where the total cohesion at the fracture surface (…) ) equals the weight of the collapsed body ( Substitute the surface area of ​​the hemisphere into the equation. With volume The critical radius formula can then be derived. Introducing a safety factor Then, practical engineering formulas were obtained. This is used to determine the recommended safe excavation radius. Based on the above theoretical model, this invention proposes a method for calculating the arching effect of soil and rock based on directly measured cohesion.

[0034] Below is the derivation of the formula for calculating the arching effect of soil and rock. Let the radius of the hemisphere be R, the area of ​​the collapsed interface 2 be S, and the volume of the collapsed rock and soil 3 be V, then: S=4πR 2 / 2=2πR 2 (1) V=4πR 3 / 3 / 2=2πR 3 / 3(2) The conditions for the collapse of rock and soil 3 to fall are: S×c<γ×V(3) Substituting equations (1) and (2) into equation (3), we get: 2πR 2 ×c<2πR 3 / 3×γ(4) By simplifying the calculation, we can obtain: R>3c / γ(5) Equation (5) calculates that when the radius of the sphere is greater than this value, the collapsed rock and soil 3 will collapse, and when it is equal to this value, it is in a critical state.

[0035] In actual tunnel excavation, we hope that the excavated tunnel will have minimal deformation and that the top of the arch will not collapse. Equation (5) represents the conditions for collapse. Tunnel excavation cannot be spherical; therefore, a circular arch is optimal for the tunnel excavation face. The tunnel will only form an arch when R ≤ 3c / γ, and no collapse will occur above it. According to Equation (5), when the excavation radius calculated by the theoretical soil arch effect cannot meet the requirements, the soil-rock cohesion should be increased or the soil-rock unit weight should be reduced. The soil-rock unit weight is generally unchangeable; the soil-rock cohesion can only be increased through pre-reinforcement and other measures to improve tunnel construction safety.

[0036] Of course, soil and rock cannot be completely homogeneous. This formula is a limit calculation method. The actual value of R should be increased by a certain safety factor. The radius R should be the theoretical calculation value divided by a safety factor of 1.5 to 2.

[0037] Let the safety factor be A, then the actual value of R for the soil arching effect should be: R=3c / (Aγ)(6) Equation (6) is the formula for calculating the soil arching effect.

[0038] Based on the calculation formulas for soil and rock parameters and soil arching effect, when the soil and rock unit weight is 25 kN / m³ 3 For a safety factor of 2, the recommended values ​​for the radius of the semi-circular tunnel excavation face at the crown under different cohesion conditions are shown in Table 1 below:

[0039] Based on the geotechnical parameters and the soil arching effect calculation formula, when the soil cohesion is 100 kPa and the safety factor is 2, the suggested radius of the semi-circular tunnel opening face at the arch crown under different unit weights is as follows: Table 2

[0040] As shown in the table above, the soil arching effect (with a semi-circular tunnel at the arch crown) mainly depends on the soil cohesion and unit weight, with soil cohesion being the primary factor. Traditional testing methods for cohesion include direct shear tests, triaxial compression tests, unconfined compressive strength tests, and in-situ vane shear tests. However, all of these tests rely on calculation formulas, and for soils with certain strength and cohesion, the calculated cohesion value may be inaccurate, leading to deviations in the calculation of the soil arching effect and potentially causing safety risks. To address this issue, a direct cohesion measurement device has been invented, effectively improving the accuracy of soil cohesion measurement.

[0041] However, the above formula is based on the assumption that the collapsed body is an ideal hemisphere. In actual tunnel engineering, due to the anisotropy of ground stress, the attitude of rock strata, or the development of joints, the collapsed body formed by the instability of the arch crown often takes the shape of a semi-rotating ellipsoid (i.e., the upper half of a complete rotating ellipsoid cut along the equatorial plane). Let the vertical semi-axis (sagitta) of this semi-rotating ellipsoid be... (Unit: m), the horizontal half-axis (half-span) is (Unit: m), the flatness ratio of the soil arch is defined as: ; in It is a dimensionless parameter, and ;when At that time, the collapsed body degenerated into a hemispherical shape.

[0042] Under this geometric model, the fracture surface area of ​​the collapsed body (i.e., the contact surface with the surrounding stable rock and soil) and volume These are half the quantities corresponding to the complete rotating ellipsoid. According to the limit equilibrium condition, the total cohesion on the fracture surface is equal to the weight of the collapsed object, i.e. After geometric simplification, the equivalent safe excavation radius correction formula considering the non-circularity of the shape can be obtained: ; in, The equivalent safe excavation radius (referring to the horizontal half-span) after considering the flattening effect of the soil arch is expressed in meters. The soil-rock cohesion is directly measured using the device of this invention, and the unit is kPa (i.e., kN / m). 2 ); The preset safety factor has a range of values. ; This refers to the unit weight of soil and rock, expressed in kN / m³. 3 ; The geometric correction function for the earth arch is defined as follows: And satisfy When soil and rock masses tend to flatten due to tectonic or sedimentary processes (such as...), ),but It is recommended that the excavation radius be reduced by about 6% accordingly. This correction will allow the original hemispherical model to adapt to a wider range of geological formations.

[0043] In a preferred embodiment, the frictional force F1 is obtained from the reading of a tension gauge by driving the direct measurement device for soil-rock cohesion to the same displacement stroke as the actual test when the soil sample is not installed.

[0044] Rotate the manual turntable 15, the force transmission device 12 moves to the right, the wire rope 11 tightens, and the tension gauge 10 begins to display a tension value, and the soil sample 7 begins to be under tension. Slowly rotate the manual turntable 15 to increase the tension, carefully observe the soil sample 7 between the first clamp 6 and the second clamp 8 and the value displayed by the tension gauge 10, and accurately record the value of the tension gauge 10 when the soil sample 7 between the first clamp 6 and the second clamp 8 breaks.

[0045] In a preferred embodiment, the unit weight γ of the soil and rock is determined by on-site weighing or by using the measured value in the geological survey report.

[0046] In a preferred embodiment, the cohesion measurement is repeated at the same depth position at no less than three times, and the average value is taken after discarding the maximum and minimum values ​​as the final c value for calculating R.

[0047] In a preferred embodiment, the recommended safe excavation radius R is used to guide the design of the unsupported excavation span of a circular arch tunnel. During construction, the excavation progress should be controlled and initial support should be applied in a timely manner to maintain the stability of the natural soil arch.

[0048] In a preferred embodiment, the method is applicable to soil and rock formations with a cohesion greater than 10 kPa and where complete columnar samples can be obtained.

[0049] In a preferred embodiment, when the safety factor A=1, R represents the theoretical limit excavation radius; in engineering applications, A>1 is taken to introduce a safety margin.

[0050] In a preferred embodiment, the axial tensile fracture test is completed within 2 hours after the soil sample is taken out to avoid cohesion distortion caused by changes in moisture content.

[0051] In a preferred embodiment, the soil sample breaks at the free segment between the two clamps, and the fracture surface is perpendicular to the soil sample axis, indicating that the sample is in a state of pure tension.

[0052] A device for directly measuring the cohesion of soil and rock, used to implement the direct cohesion measurement step in the method described above, the device comprising: Equipment platform 16; The first fixed end 5 and the second fixed end 14 are fixed on the equipment platform; The first clamp 6 is installed on the first fixed end and is used to clamp one end of the soil sample. The upper movable end 9 is connected to the second clamp 8 at one end to hold the soil sample at the other end, and the other end is connected to the tension gauge 10. The force transmission device 12 is connected to the tension gauge 10 via the steel wire rope 11; The adjustable screw 13 is connected to the manual turntable 15 at one end and threaded into the force transmission device 12 at the other end. A fixed horizontal shaft 17 passes through the force transmission device 12 and the lower movable end 19, allowing the force transmission device 12 to slide axially. The rotating manual turntable 15 drives the force transmission device 12 to move, applying a controllable axial tensile force (such as...) to the soil sample via the wire rope 11 and the tension gauge 10. Figure 2 , Figure 3 (As shown).

[0053] The following points should be noted when using a cohesion measuring device: (1) The strength and rigidity of the device must meet the requirements of the maximum breaking force of the soil sample and have a certain safety factor; (2) The force transmission device 12, the lower movable end 19 and the fixed horizontal shaft 17 are clearance fit to ensure that the force transmission device 12 and the lower movable end 19 slide freely and the friction is small; (3) The centers of the first clamp 6, the second clamp 8, the upper movable end 9, the tension gauge 10, the wire rope 11, the adjustable screw 13, the manual turntable 15, etc. are all in a straight line to ensure that the soil sample is in a tensile state.

[0054] In a preferred embodiment, the clamping surfaces of the first clamp 6 and the second clamp 8 are provided with anti-slip pads 20 to prevent the soil sample from slipping or being damaged on the surface during loading.

[0055] In a preferred embodiment, the upper movable end 9 and the lower movable end 19 are rigidly connected by a connecting pin 18, and the lower movable end 19 is sleeved on the fixed horizontal shaft 17 to ensure a stable loading path.

[0056] In a preferred embodiment, the device is made entirely of high-strength metal material, and its structural strength meets a safety margin of more than 1.5 times the maximum expected breaking force.

[0057] In a preferred embodiment, the device is detachable and assembleable, facilitating its transport to the tunnel face or drilling site for on-site cohesion testing.

[0058] In summary, as Figures 2-3As shown, the cohesion measuring device includes components such as a platform 16, a first fixed end 5, an upper movable end 9, a lower movable end 19, a tension gauge 10, a force transmission device 12, an adjustable screw 13, a second fixed end 14, a manual turntable 15, and a fixed horizontal shaft 17.

[0059] The upper surface of the equipment platform 16 is fixed to the lower surface of the first fixed end 5 and the lower surface of the second fixed end 14. The fixed horizontal shaft 17 is fixed to the lower part of the first fixed end 5 and the lower part of the second fixed end 14, and passes through the circular hole on the lower movable end 19 and the force transmission device 12 with a certain gap, ensuring that the lower movable end 19 and the force transmission device 12 can slide freely on the fixed horizontal shaft 17. The upper part of the first fixed end 5 is fixed to the first clamp 6, one side of the upper movable end 9 is fixed to the second clamp 8, and the other side is fixed to the tension gauge 10. The other side of the tension gauge 10 is fixed to the force transmission device 12 through the wire rope 11. The upper movable end 9 and the lower movable end 19 are fixed by the connecting pin 18. The adjustable screw 13 is connected to the manual turntable 15, and passes through the circular hole on the second fixed end 14 with a certain gap. The other side of the adjustable screw 13 passes through the threaded hole on the force transmission device 12. The distance of the adjustable screw 13 into the threaded hole on the force transmission device 12 is achieved by rotating the manual turntable 15 in the forward or reverse direction, thereby realizing the left and right movement of the force transmission device 12.

[0060] The following points should be noted when using a cohesion measuring device: (1) The strength and rigidity of the device must meet the requirements of the maximum breaking force of the soil sample and have a certain safety factor; (2) The force transmission device 12, the lower movable end 19 and the fixed horizontal shaft 17 are clearance fit to ensure that the force transmission device 12 and the lower movable end 19 slide freely and the friction is small; (3) The centers of the first clamp 6, the second clamp 8, the upper movable end 9, the tension gauge 10, the wire rope 11, the adjustable screw 13, the manual turntable 15, etc. are all in a straight line to ensure that the soil sample is in a tensile state.

[0061] To facilitate operation by engineering personnel, the complete cohesion measurement process is summarized as follows: Procedure for measuring the cohesion of soil samples (1) Installation of soil samples First, use geological drilling equipment to take circular soil samples on site. Cut the soil sample at the cohesion depth location to the required length L (L=L1+L2+L3, where L1 is the length of the soil sample that can be placed in the first clamp 6; L2 is the length of the soil sample that can be placed in the second clamp 8; L3 is the distance between the two clamps when measuring cohesion, generally taken as 10-20mm). Install the cohesion measuring device on site, rotate the manual turntable 15, push the force transmission device 12 to the leftmost position, manually move the upper movable end 9 so that the distance between the first clamp 6 and the second clamp 8 is 10-20mm, open the upper end cover of the first clamp 6 and the second clamp 8, place anti-slip pads 20 on the inner surface of the lower cover of the first clamp 6 and the second clamp 8 respectively, and put the soil sample 7 into the clamp. Then, the anti-slip pad 20 covers the upper surface of the soil sample 7, and the upper and lower covers of the first clamp 6 and the second clamp 8 are clamped tightly respectively (Note: the soil sample should not be damaged, but it should be ensured that the soil sample 7 will not slip when the cross section is broken. The clamping force can be determined by test).

[0062] (2) Fracture of soil samples Rotate the manual turntable 15, the force transmission device 12 moves to the right, the wire rope 11 tightens, and the tension gauge 10 begins to display a tension value, and the soil sample 7 begins to be under tension. Slowly rotate the manual turntable 15 to increase the tension, carefully observe the soil sample 7 between the first clamp 6 and the second clamp 8 and the value displayed by the tension gauge 10, and accurately record the value of the tension gauge 10 when the soil sample 7 between the first clamp 6 and the second clamp 8 breaks.

[0063] (3) Calculation of cohesion Let the radius of the circular soil sample be r. By rotating the manual turntable 15, the tension gauge 10 separately measures the frictional force F1 as the upper movable end 9 and the lower movable end 19 move to the right. When the soil sample 7 breaks, the value of the tension gauge 10 is F2. The cohesion of the soil sample 7 is calculated as follows: F2-F1=πr 2 c(7) The cohesion can be calculated using equation (7): c = (F2 - F1) / (πr) 2 (8) Equation (8) is the cohesion value obtained by directly measuring the soil sample 7 using a cohesion measuring device. To ensure the accuracy of the cohesion of the soil sample 7, multiple sets of data should be measured, and the maximum and minimum values ​​should be deleted and the average value taken. Then, the excavation radius of the soil is calculated using the soil arching effect calculation formula in Equation (6), which effectively guides the tunnel design and construction.

[0064] It should be noted that when a real tunnel arch fails, the potential slip surface is in a near-zero normal stress state, and its resistance is mainly provided by cohesion. The failure mode is either opening-type or tension-shear combined rupture. However, when a rigid clamp is used to axially stretch a soil sample in the laboratory, the strong constraint of the clamp on the end of the sample inhibits local deformation, resulting in a measured breaking force higher than the intrinsic strength of the soil and rock in the free fracture state.

[0065] To eliminate the influence of end-constraint effects, this invention proposes a cohesion reduction correction method based on the elastic Poisson effect. This method assumes that the ideal unconstrained tensile breaking strength... Compared with measured cohesion The following relationship exists between them: ; Among them, equivalent cohesion Calculate using the following formula: ; The meanings of each symbol are as follows: The equivalent cohesion after end constraint correction represents the intrinsic cohesion of soil and rock under near-free fracture conditions, and the unit is kPa (i.e., kN / m²). The total tensile force measured when the soil sample breaks is expressed in kN. Friction force generated by the moving parts of the device, measured in kN; This represents the cross-sectional area of ​​the soil sample, in m². The end constraint correction factor is derived from the Poisson's ratio of soil and rock. Confirmed, the calculation formula is: ; The Poisson's ratio for soil and rock typically ranges from 0.20 to 0.40 and can be obtained through small-scale compression tests, wave velocity tests, or geological survey reports. The correction factor originates from the transverse strain constraint effect caused by axial tension in elasticity theory: when the specimen is fully clamped by a rigid fixture, transverse deformation at the ends is suppressed, generating additional radial stress, thereby increasing the apparent breaking strength. This is achieved by introducing... This allows the measured values ​​to be reduced to a level closer to the ideal cohesion without lateral constraints. For example, if the Poisson's ratio of soil and rock... ,but This means that the measured cohesion needs to be reduced by approximately 16.3% as the input parameter for calculating the stability of the soil arch. This correction ensures that the cohesion value is closer to the actual mechanical conditions of arch crown instability, avoiding an overestimation of the safe excavation radius due to experimental boundary effects.

[0066] Compared with traditional indirect experimental methods, this invention has the following significant advantages: (1) The soil arching effect during tunnel excavation is affected by a variety of factors. This invention simplifies the complex and identifies the most important soil and rock parameters. It accurately analyzes that during the redistribution of soil arching stress, the original soil and rock 1 outside the collapse interface 3 exerts no normal stress on the collapsed soil and rock 3 when the collapsed soil and rock 3 collapses. Furthermore, it is analyzed that the collapsed soil and rock 3 is only affected by cohesion when the collapse interface 2 breaks. (2) The calculation formula for the soil arching effect of rock and soil (6) is derived from the mathematical theorem of isoperiodic inequality, which has a theoretical basis and a better calculation method; (3) A device for direct measurement of cohesion was invented, which is more accurate than the cohesion calculated by the Mohr-Coulomb law formula; (4) The cohesion measuring device can directly measure the freshly taken natural soil sample on site, avoiding the impact of long-term transportation and waiting on the soil sample 7 on the measurement of cohesion value.

[0067] (5) The cohesion measuring device of the present invention is suitable for measuring the cohesion of strata with a certain cohesion where a certain length of complete soil sample can be extracted through geological exploration; (6) Anti-slip pads 20 are installed between the outer surface of the soil sample 7 and the inner surfaces of the first clamp 6 and the second clamp 8, which greatly reduces the probability of damage to the soil sample 7; (7) It is impossible to excavate the tunnel into a spherical or hemispherical shape during the excavation process. The actual excavated arch is semi-circular. Therefore, in order to ensure the safety of tunnel excavation, the excavation step distance should be as short as possible, and timely support and secondary lining should be carried out.

[0068] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the invention. The above descriptions are merely preferred embodiments of the invention and are not intended to limit the invention. It should be noted that any modifications, equivalent substitutions, and improvements made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for calculating the arching effect of soil and rock based on directly measured cohesion, characterized in that, Includes the following steps: Obtain columnar soil samples of the original soil and rock at the site; The soil sample is installed in the direct soil-rock cohesion measuring device, and an axial tensile force is applied to the soil sample until it breaks. The total tensile force F2 at the time of breakage and the frictional force F1 generated by the moving parts of the device are measured, and the soil-rock cohesion c is directly calculated based on the cross-sectional area As of the soil sample. Substituting the directly measured cohesion c, soil unit weight γ, and preset safety factor A into the formula for the ultimate radius of the hemispherical soil arch, the recommended safe excavation radius R of the tunnel arch is calculated, which is used to evaluate the stability of the soil arch and guide the tunnel design. Wherein, the cohesive force c is calculated according to the following formula: ； The formula for the ultimate radius of the hemispherical soil arch is: ； In the formula, It is the cohesion of soil and rock; This represents the total tensile force measured when the soil sample breaks. The frictional force generated by the movement of the moving parts of the measuring device; This represents the cross-sectional area of ​​the soil sample. Recommend a safe excavation radius for the tunnel; This refers to the unit weight of soil and rock, expressed in kN / m³. 3 ; For safety factor; The formula for the ultimate radius of the hemispherical soil arch is applicable to homogeneous, cohesion-dominant soil strata, and to the case where the collapsed body is hemispherical and the normal stress at the collapse interface can be ignored when the arch crown is locally unstable.

2. The method for calculating the arching effect of soil and rock based on directly measured cohesion according to claim 1, characterized in that, The columnar soil sample was cylindrical, and its cross-sectional area was... ,in The radius of the soil sample is given.

3. The method for calculating the arching effect of soil and rock based on directly measured cohesion according to claim 1, characterized in that, The frictional force F1 is obtained by driving the direct measurement device of soil-rock cohesion to the same displacement stroke as the actual test when the soil sample is not installed, and is obtained by reading the force gauge.

4. The method for calculating the arching effect of soil and rock based on directly measured cohesion according to claim 1, characterized in that, The unit weight γ of the soil and rock was determined by on-site weighing or by using the measured value in the geological survey report.

5. The method for calculating the arching effect of soil and rock based on directly measured cohesion according to claim 1, characterized in that, Perform cohesion measurements at the same depth location at least three times, discard the maximum and minimum values, and take the average value as the final c value for calculating R.

6. The method for calculating the arching effect of soil and rock based on directly measured cohesion according to claim 1, characterized in that, The recommended safe excavation radius R is used to guide the design of the unsupported excavation span of circular arch tunnels. During construction, the excavation progress should be controlled and initial support should be applied in a timely manner to maintain the stability of the natural soil arch.

7. A device for directly measuring the cohesion of soil and rock, characterized in that, The apparatus for performing the direct cohesion measurement step in the method according to any one of claims 1 to 6, the apparatus comprising: Equipment platform; The first fixed end and the second fixed end are fixed on the equipment platform; The first clamp is installed on the first fixed end and is used to hold one end of the soil sample. The upper movable end is connected to a second clamp at one end to hold the soil sample at the other end, which is connected to a tension gauge. The force transmission device is connected to the tension gauge via a steel wire rope; An adjustable screw, one end of which is connected to a manual turntable, and the other end is threaded into a force transmission device; A fixed horizontal shaft runs through the force transmission device and the lower movable end, allowing the force transmission device to slide axially. The rotating manual turntable can drive the force transmission device to move, and apply a controllable axial tension to the soil sample via a steel wire rope and a tension gauge.

8. The direct measuring device for soil and rock cohesion according to claim 7, characterized in that, The clamping surfaces of the first and second clamps are provided with anti-slip pads to prevent the soil sample from slipping or being damaged during loading.

9. The direct measuring device for soil and rock cohesion according to claim 7, characterized in that, The upper movable end and the lower movable end are rigidly connected by a connecting pin. The lower movable end is sleeved on a fixed horizontal shaft to ensure a stable loading path.

10. The direct measuring device for soil and rock cohesion according to any one of claims 7-9, characterized in that, The device has a detachable and assembleable structure, which facilitates transportation to the tunnel face or drilling site for on-site cohesion testing.

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