Method for predicting strength of soft and hard composite rock mass

Abstract

The invention discloses a strength prediction method for a soft and hard composite rock mass. The method comprises the following steps: acquiring rock mechanical parameters of the soft and hard composite rock mass and confining pressure of a bedding surface of the soft and hard composite rock mass in a target area; when the strength of the inclination angle theta of 45-75 degrees of the bedding surface of the soft and hard composite rock mass in the target area is calculated, the strength of the soft and hard composite rock mass in the target area is calculated according to the uniaxial compressive strength; when the inclined angle theta of the bedding surface of the target area is calculated to be 0-45 degrees and 75-90 degrees, the next step is directly carried out; the uniaxial compressive strength when the inclination angle theta of the target bedding surface is equal to 0 degree, the maximum principal stress when the inclination angle theta of the target bedding surface is equal to 15 degrees and the maximum principal stress when the inclination angle theta of the target bedding surface is equal to 30 degrees are obtained; calculating to obtain a correction coefficient; and calculating the strength of the bedding surface of the soft and hard composite rock mass in the target area according to the correction coefficient and the confining pressure. The method for determining the strength of the soft and hard composite rock mass is more accurate, wider in application and more in consideration factors.

Description

Technical Field

[0001] The invention relates to a strength prediction method for a soft-hard composite rock mass, and belongs to the technical field of geotechnical engineering. Background Art

[0002] Interlayered soft and hard rock is a common type of heterogeneous rock in engineering geological conditions. The presence of bedding planes and soft rock layers often leads to its destruction under relatively low external load conditions, which in turn causes geological disasters or engineering accidents. Exploring the influence of unfavorable factors such as soft rock layers and bedding planes on the mechanical properties of interlayered soft and hard rock, and being familiar with the load-bearing failure behavior of interlayered soft and hard rock will help to predict and evaluate the potential risks of related geological conditions in actual engineering projects and disaster prevention, so as to carry out targeted construction operations.

[0003] There are many factors that affect the mechanical properties and failure behavior of interlayered rock mass, such as the inclination of bedding plane, the thickness of rock layer and the strength of soft rock layer. The angle between bedding and load direction often determines whether the final failure mode of interlayered rock mass is tensile fracture, shear fracture or a composite fracture mode. Shear sliding failure along the bedding plane is the main failure feature unique to interlayered rock mass, which is common in interlayered rock mass with a bedding plane inclination of 15°-75°. In addition, the thickness and mechanical properties of the soft rock layer will also affect the final failure mode of the interlayered rock mass, which depends not only on the physical properties and geometric conditions of the soft rock itself, but also on the adjacent hard rock layer. As an important component of the layered structure, hard rock often has an important contribution to the final failure result, which is often easily overlooked in previous studies. Interlayered rock mass will produce various failure behaviors under adverse loads. With the change of the inclination of the bedding plane, its failure may be caused by the destruction of the rock matrix or the cracking of the bedding plane. For different types of failure, the causes of their internal correlation are different. Obviously, the cracking along the bedding plane depends on the bonding properties of the bedding plane, while the failure of the rock matrix depends on the strength of its own mechanical properties.

[0004] In summary, the development of an improved strength prediction method for soft-hard interlayered rock mass has important theoretical guiding significance for the stability assessment of soft-hard interlayered rock mass. The content of the present invention can also provide relevant guidance for the engineering of soft-hard composite rock mass. Summary of the invention

[0005] In order to overcome the defects in the prior art, the present invention aims to provide a strength prediction method for soft-hard composite rock mass.

[0006] The technical solution provided by the present invention to solve the above technical problems is: a strength prediction method for soft-hard composite rock mass, comprising the following steps:

[0007] Step 1: Obtain the rock mechanics parameters of the soft-hard composite rock mass and the confining pressure σ of the bedding plane of the soft-hard composite rock mass in the target area 3 ;

[0008] Step 2: Calculate the strength of the soft-hard composite rock mass bedding plane with an inclination angle of θ = 45° to 75° in the target area. Then, the sliding failure criterion is adopted. According to the confining pressure σ 3 Calculate the strength of the soft and hard composite rock mass in the target area; calculate the strength of the bedding plane in the target area at an inclination angle θ = 0° to 45° and 75° to 90°, and then proceed directly to the next step;

[0009] Step 3: Field test to obtain the uniaxial compressive strength σ when the bedding plane inclination angle θ = 0° in the target area soft and hard composite rock mass c(0°) , the maximum principal stress σ when the bedding plane dip angle θ = 15° 1(15°) and the maximum principal stress σ when the bedding plane dip angle θ = 30° 1(30°) ;

[0010] Step 4: According to the maximum principal stress σ when the bedding plane dip angle θ = 15° 1(15°) and the maximum principal stress σ when the bedding plane dip angle θ = 30° 1(30°) Calculate and obtain the correction coefficient A and n / k;

[0011] Step 5: According to the uniaxial compressive strength σ when the bedding plane inclination angle θ = 0° c(0°) , confining pressure σ 3 , correction coefficient A and n / k are used to calculate the strength of the bedding plane of the soft and hard composite rock mass in the target area.

[0012] A further technical solution is that the rock mechanics parameters include cohesion c w , friction angle

[0013] A further technical solution is that in step 1, the confining pressure σ of the underground target area is directly measured by using an underground pressure sensor. 3 .

[0014] A further technical solution is that the formula for calculating the strength of the soft-hard composite rock mass in the target area in step 2 is:

[0015]

[0016] Where: c w for cohesion; is the friction angle; θ is the tilt angle; σ 3 is the confining pressure; 1 is the maximum principal stress.

[0017] A further technical solution is that the specific process of step three is:

[0018] First, the samples are prepared by taking rock blocks from the bedding plane from the drill core or pit exploration trench from different directions and angles, so that the bedding plane inclination angles are θ = 0°, 15° and 30° respectively;

[0019] Then place the sample in the center of the pressure plate of the testing machine, ensure that both ends of the sample are in even contact with the upper and lower pressure plates of the testing machine, and load the sample at a loading speed of 0.5-1.0Mpa per second until the sample is destroyed, and record the destruction load;

[0020] The uniaxial compressive strength σ at the bedding plane inclination angle θ of 0, 15 and 30 degrees is calculated. c(0°) , σ c(15°) and σ c(30°) ;

[0021] Let the specific confining pressure σ 3 = 0, uniaxial compressive strength σ when bedding plane inclination angle θ = 15° c(15°) and uniaxial compressive strength σ when the bedding plane inclination angle θ = 30° c(30°) The magnitudes of are the maximum principal stress σ when the bedding plane dip angle θ = 15° 1(15°) and the maximum principal stress σ when the bedding plane dip angle θ = 30° 1(30°) .

[0022] A further technical solution is that the calculation formula of the uniaxial compressive strength is:

[0023]

[0024] Where: c is the uniaxial compressive strength of rock, P is the maximum failure load, and A is the cross-sectional area of ​​the specimen perpendicular to the loading direction;

[0025] A further technical solution is that the calculation formula in step 4 includes:

[0026]

[0027] Where: c(0°) is the uniaxial compressive strength when the bedding plane inclination angle θ = 0°; σ 1(15°) is the maximum principal stress when the bedding plane dip angle θ = 15°; σ 1(30°) is the maximum principal stress when the bedding plane dip angle θ = 30°; σ 3 is the confining pressure; A and n / k are correction factors.

[0028] A further technical solution is that the calculation formula in step 4 is:

[0029]

[0030] Where:1(θ) is the maximum principal stress when the bedding plane dips at an angle of θ; c(0°) is the uniaxial compressive strength when the bedding plane inclination angle θ = 0°; σ 3 is the confining pressure; A and n / k are correction factors.

[0031] The present invention has the following beneficial effects: the strength model of the present invention introduces the modification coefficient A to further correct the Tien-Kuo strength model, and the strength prediction error range of the soft and hard composite rock mass is 5%, which can well predict the strength of the soft and hard composite rock mass engineering. Therefore, the soft and hard composite rock mass strength method of the present invention is more accurate, more widely used, and takes more factors into consideration. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Coordinate system definition diagram for soft and hard interlayered rock mass. DETAILED DESCRIPTION

[0033] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. 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 calculation formula in the present invention is obtained by the following steps:

[0035] A. First, the Tien-Kuo strength model is established based on the defects of Jaeger strength criterion in determining the strength of layered rock mass;

[0036] This criterion is the strength of the sections according to the different failure modes of the rock specimens;

[0037] When rock undergoes shear sliding failure along the bedding plane, its strength expression is:

[0038]

[0039] Where: c w and They are the cohesion and friction angle of the bedding plane, respectively, and can be determined by compression tests.

[0040] When the destruction of rock is caused by the destruction of the rock matrix, its strength expression is:

[0041]

[0042] Where: 1(0°) and σ 1(θ) They are the maximum principal stress when the bedding plane dip angle θ = 0° and the maximum principal stress when the bedding plane dip angle θ = 0°, respectively.

[0043] k is the strength ratio of the vertical bedding specimen to the horizontal bedding specimen, expressed as:

[0044]

[0045] When the soft and hard interlayered specimen undergoes shear failure along the bedding plane, the strength expression shown in formula (1) can be obtained based on the single weak plane principle proposed by Jaeger. When the failure of the specimen is caused by the failure of the rock matrix, the elastic constitutive relation of the soft and hard interlayered rock mass can be expressed by the transversely isotropic constitutive model.

[0046] like Figure 1 As shown in the figure, the linear elastic constitutive relation of the soft-hard interlayer specimen in the local coordinate system is expressed in matrix form:

[0047]

[0048] Where: E and E' are the elastic moduli parallel and perpendicular to the transverse isotropic plane, G' is the shear modulus in the normal direction of the transverse isotropic plane, υ and υ' are the Poisson's ratios of the material when subjected to force in the corresponding directions. Correspondingly, the constitutive equation in the global coordinate system is expressed as:

[0049]

[0050] The relevant quantity K in the above flexibility matrix is 11 , K 12 ...K 66 It can be obtained by coordinate transformation of the corresponding quantities in the local coordinate system. Under compression, the stress matrix of the interlayer specimen can be written as follows:

[0051]

[0052] Where: S 1 is the deviatoric stress tensor, and;

[0053] S 1 =σ 1 -σ 3 (7)

[0054] Combined with formula (5), it can be seen that the axial strain related to the deviatoric stress can be expressed as:

[0055] ε yy =K 22 S 1 (8)

[0056] In the formula,

[0057]

[0058] Under compression, when the strain of the rock matrix exceeds its maximum strain, the failure of the soft and hard interlayered rock mass occurs.

[0059] According to the maximum principal strain criterion, the stress-strain relationship for interbedded rock samples with a bedding plane dip angle θ can be expressed as:

[0060] ε yf =K 22 S 1(β) (10)

[0061] The strain of the rock sample at failure ε yf Independent of the bedding plane dip angle, it is affected by the confining pressure. Therefore, when the strength of a rock sample at a certain bedding plane dip angle is determined, it is easy to obtain the strength of the rock sample at any bedding plane dip angle. The strength ratio is obtained by comparing the strength of the rock sample at the horizontal bedding plane dip angle;

[0062]

[0063] As ordered

[0064] E′=E (0°) ,E=E (90°) (12)

[0065] k=E (0°) / E (90°) =S 1(0°) / S 1(90°) (13)

[0066]

[0067] Then we can get the expression shown in formula (2), which is the strength expression of layered rock mass when matrix failure occurs in the Tien-Kuo strength model.

[0068] The Tien-Kuo strength model is a theoretical expression of strength based on elasticity theory. However, rock materials often have nonlinear characteristics. Based on this reality, many scholars have proposed strength criteria that can reflect the nonlinear changes in rock strength. For example, You Mingqing, based on a large amount of experimental data and theoretical research, proposed a parabolic criterion for predicting rock strength, namely;

[0069]

[0070] The display expression is:

[0071]

[0072] In the formula, σ c is the uniaxial compressive strength, the formula can be further written as follows:

[0073] σ1 -σ 3 =σ c +2(σ 3 σ c ) 0.5 (17)

[0074] When the matrix of the soft and hard interlayered rock mass is destroyed, the nonlinear characteristics of the matrix should be considered in the strength estimation. Therefore, formula (2) can be rewritten as:

[0075]

[0076] Since the soft and hard interlayered rock mass also has anisotropic characteristics, the coefficient A is also introduced in formula (17), which is a constant related to the anisotropic characteristics of the soft and hard interlayered rock mass.

[0077] Equation (17) is the improved strength criterion considering the nonlinear characteristics of rock, which is applicable to the failure type caused by cracking of rock matrix. The Tien-Kuo strength model is a segmented strength criterion derived according to different failure types, and the improved criterion still follows this basic criterion.

[0078] When the bedding plane inclination angle θ = 45° to 75°, the slip failure criterion, that is, formula (1), is used to obtain the cohesion c at a specific bedding plane inclination angle θ according to the experiment. w and friction angle Then obtain the confining pressure σ 3 Then, according to formula (1), the intensity values ​​at other angles between θ=45° and 75° can be calculated.

[0079] When the bedding plane inclination angle θ = 0°~45° and 75°~90°, the modified Tien-Kuo strength criterion, that is, equation (2), is used to obtain the uniaxial compressive strength σ when the bedding plane inclination angle θ = 0° in the soft-hard composite bedding plane. c(0°) When the typical θ=15° and 30° are selected, the maximum principal stress σ of the failure criterion obtained by field test is θ=15° and 30° 1(15°) and σ 1(30°) Assuming a certain confining pressure σ 3 Under the value, substitute into formula (18), then obtain according to formula (19) and formula (20), further according to formula (19) and formula (20) to obtain the correction coefficient A and n / k, then according to the correction coefficient A and n / k, the bedding plane inclination angle θ, the uniaxial compressive strength σ of the bedding plane inclination angle θ = 0° c(0°) Calculate a certain confining pressure σ 3 The compressive strength of soft-hard composite rock mass with other bedding plane inclination angles θ = 0°~45° and 75°~90°.

[0080] When the bedding plane inclination angle θ = 15° or 30°, the failure of the specimen is caused by the failure of the rock matrix. 4 15°=0.0045、sin 4 30°=0.0625, so sin 4 θ / k decreases exponentially, and equation (18) can be simplified to

[0081]

[0082] A strength prediction method for a soft-hard composite rock mass of the present invention comprises the following steps:

[0083] Step 1: Obtain rock mechanical parameters (cohesion c w , friction angle ) and the confining pressure σ of the bedding plane of the soft-hard composite rock mass in the target area 3 ;

[0084] The confining pressure σ of the underground target area is directly measured by using an underground pressure sensor 3 .

[0085] Step 2: When calculating the strength of the soft-hard composite rock mass in the target area with a bedding plane inclination angle of θ = 45° to 75°, the slip failure criterion is adopted. According to formula (1) and the confining pressure σ 3 Calculate the strength of the soft-hard composite rock mass in the target area;

[0086] When the intensity of the bedding plane angles θ = 0° to 45° and 75° to 90° in the target area is calculated, proceed directly to the next step;

[0087] Step 3: Obtain the uniaxial compressive strength σ of the soft-hard composite rock mass in the target area when the bedding plane inclination angle θ = 0° c(0°) , the maximum principal stress σ when the bedding plane dip angle θ = 15° 1(15°) and the maximum principal stress σ when the bedding plane dip angle θ = 30° 1(30°) ;

[0088] First, the samples are prepared. Rock blocks with bedding surfaces are collected from drill cores or pit exploration trenches from different directions and angles, so that the bedding surface inclination angles are θ=0°, 15° and 30° respectively. According to the regulations, the standard specimen is a cylinder with a diameter of 50mm and a height of 100mm. The processing accuracy of the specimen needs to meet the relevant requirements of the International Society of Rock Mechanics (ISRM).

[0089] Then the sample should be placed in the center of the pressure plate of the testing machine to ensure that both end faces of the sample are in uniform contact with the upper and lower pressure plates of the testing machine. Load the sample at a loading speed of 0.5-1.0Mpa per second until the sample is destroyed, and record the destruction load.

[0090] The uniaxial compressive strength of rock can be calculated according to the following formula.

[0091]

[0092] Where: c is the uniaxial compressive strength of rock (Mpa), P is the maximum failure load (N), and A is the cross-sectional area of ​​the specimen perpendicular to the loading direction (mm 2 );

[0093] The uniaxial compressive strength σ when the bedding plane inclination angle θ = 0°, 15° and 30° is calculated c(0°) , σ c(15°) and σ c(30°) In the failure criterion, let the specific confining pressure σ 3 = 0, when the bedding plane inclination angle θ = 15°, the uniaxial compressive strength σ c(15°) Uniaxial compressive strength σ when the bedding plane inclination angle θ = 30° c(30°) The magnitudes of are the maximum principal stress σ when the bedding plane dip angle θ = 15° 1(15°) and the maximum principal stress σ when the bedding plane dip angle θ = 30° 1(30°) .

[0094] Step 4: According to formula (19) and formula (20), at a specific σ 3 = 0, the maximum principal stress σ when the bedding plane dip angle θ = 15° 1(15°) and the maximum principal stress σ when the bedding plane dip angle θ = 30° 1(30°) Calculate and obtain the correction coefficient A and n / k;

[0095] Step 5: According to formula (18), the uniaxial compressive strength σ when the bedding plane inclination angle θ = 0° c(0°) , the confining pressure of the target area σ 3 , the correction coefficient A and n / k are used to calculate the strength of the bedding plane of the soft and hard composite rock mass in the target area.

[0096] The above description is not intended to limit the present invention in any form. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are within the scope of the technical solution of the present invention.

Claims

1. A strength prediction method for soft-hard composite rock mass, characterized in that: The following steps are involved: Step 1, obtaining the rock mechanical parameters of the soft-hard composite rock mass and the confining pressure σ3 of the bedding plane of the soft-hard composite rock mass in the target area; Step 2: Calculate the strength of the soft and hard composite rock mass in the target area when the inclination angle θ=45°~75° of the bedding plane. Then, the sliding failure criterion is adopted to calculate the strength of the soft and hard composite rock mass in the target area according to the confining pressure σ3; Calculate the strength of the bedding plane in the target area when the inclination angles θ=0°~45° and 75°~90°, and then proceed directly to the next step; Step 3: Field test to obtain the uniaxial compressive strength σ when the bedding plane inclination angle θ = 0° in the target area soft and hard composite rock mass c(0°) , the maximum principal stress σ when the bedding plane dip angle θ = 15° 1(15°) and the maximum principal stress σ when the bedding plane dip angle θ = 30° 1(30°) ; Step 4: According to the maximum principal stress σ when the bedding plane dip angle θ = 15° 1(15°) and the maximum principal stress σ when the bedding plane dip angle θ = 30° 1(30°) Calculate and obtain the correction coefficient A and n / k; Step 5: According to the uniaxial compressive strength σ when the bedding plane inclination angle θ = 0° c(0°) , confining pressure σ3, correction coefficient A and n / k are used to calculate the strength of the bedding plane of the soft and hard composite rock mass in the target area.

2. The strength prediction method of a soft-hard composite rock mass according to claim 1, characterized in that: The rock mechanics parameters include cohesion c w , friction angle 3. The strength prediction method of a soft-hard composite rock mass according to claim 1, characterized in that: In the step 1, the confining pressure σ3 of the underground target area is directly measured by using an underground pressure sensor.

4. The strength prediction method of a soft-hard composite rock mass according to claim 2, characterized in that: The formula for calculating the strength of the soft-hard composite rock mass in the target area in step 2 is: Where: c w for cohesion; is the friction angle; θ is the inclination angle; σ3 is the confining pressure; σ1 is the maximum principal stress.

5. The strength prediction method of a soft-hard composite rock mass according to claim 1, characterized in that: The specific process of step three is: First, the samples are prepared by taking rock blocks with bedding planes from drill cores or pit exploration trenches from different directions and angles, so that the bedding planes have inclination angles of θ = 0°, 15° and 30° respectively; Then place the sample in the center of the pressure plate of the testing machine, ensure that both ends of the sample are in even contact with the upper and lower pressure plates of the testing machine, and load the sample at a loading speed of 0.5-1.0Mpa per second until the sample is destroyed, and record the destruction load; The uniaxial compressive strength σ at the bedding plane inclination angle θ of 0, 15 and 30 degrees is calculated. c(0°) , σ c(15°) and σ c(30°) ; When the specific confining pressure σ3 = 0, the uniaxial compressive strength σ when the bedding plane dip angle θ = 15° c(15°) and uniaxial compressive strength σ when the bedding plane inclination angle θ = 30° c(30°) The magnitudes of are the maximum principal stress σ when the bedding plane inclination angle θ = 15° 1(15°) and the maximum principal stress σ when the bedding plane dip angle θ = 30° 1(30°) .

6. A strength prediction method for soft-hard composite rock mass according to claim 5, characterized in that: The calculation formula of the uniaxial compressive strength is: Where: c is the uniaxial compressive strength of rock, P is the maximum failure load, and A is the cross-sectional area of ​​the specimen perpendicular to the loading direction.

7. The strength prediction method of a soft-hard composite rock mass according to claim 1, characterized in that: The calculation formula in step 4 includes: Where: c(0°) is the uniaxial compressive strength when the bedding plane inclination angle θ = 0°; σ 1(15°) is the maximum principal stress when the bedding plane dip angle θ = 15°; σ 1(30°) is the maximum principal stress when the bedding plane dip angle θ = 30°; σ3 is the confining pressure; A and n / k are both correction coefficients.

8. The strength prediction method of a soft-hard composite rock mass according to claim 1, characterized in that: The calculation formula in step 4 is: Where: 1(θ) is the maximum principal stress when the bedding plane dips at an angle of θ; c(0°) is the uniaxial compressive strength when the bedding plane inclination angle θ = 0°; σ3 is the confining pressure; A and n / k are both correction factors.

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