A design method for the thrust ring at the end of buried pressure steel pipe in hydropower station

Abstract

The present invention provides a design method for a thrust ring at the end of a buried penstock of a hydropower station, the method comprising: determining a design object; establishing a water thrust transmission model; judging whether the thrust ring needs to be set at the downstream section of the penstock; formulating design parameters of the thrust ring and design parameters of the backfill concrete; judging whether the bearing capacity of the thrust ring, the penstock at the root of the thrust ring, and the weld at the root of the thrust ring meet the requirements; judging whether the local compressive bearing capacity and the shear bearing capacity of the backfill concrete meet the requirements; judging whether the overall anti-sliding stability between the backfill concrete and the surrounding rock meets the requirements; judging whether the bearing capacity of the surrounding rock meets the requirements. The present invention provides support for the design of the thrust ring, so that the force characteristics and interactions of each element are clearer and more specific, in line with mechanical properties and engineering practice, and more secure for ensuring the safety of the penstock and plant of the hydropower station.

Description

Technical Field

[0001] The invention belongs to the technical field of hydropower station engineering design, and in particular relates to a design method for a thrust ring at the end of a buried pressure steel pipe of a hydropower station. Background Art

[0002] With the development of hydropower industry, the safety of various structures in hydropower projects is becoming more and more important. The thrust ring is an important component before the end of the pressure steel pipe enters the powerhouse. When the hydropower station is suddenly shut down or unloaded, the thrust ring is used to eliminate the impact of water hammer pressure on the pressure steel pipe and the powerhouse, which is very important for the safe operation of the hydropower station. In the existing pressure steel pipe engineering design, the design of the thrust ring is mainly based on experience. The design is relatively subjective and it is difficult to form a thrust ring design method that can be promoted and used. Therefore, the demand for a design method to quantitatively determine the relevant parameters of the thrust ring is increasing. Summary of the invention

[0003] In view of the defects in the prior art, the present invention provides a design method for a thrust ring at the end of an embedded pressure steel pipe of a hydropower station, which can effectively solve the above problems.

[0004] In order to achieve the above object, the present invention adopts the following technical solution:

[0005] The present invention provides a design method for a thrust ring at the end of a buried pressure steel pipe of a hydropower station, comprising the following steps:

[0006] Step 1: Determine the design object:

[0007] The upstream pressure pipeline of the hydropower station power plant adopts a buried pressure steel pipe; the buried pressure steel pipe includes a pressure steel pipe arranged coaxially with the tunnel, and backfill concrete is provided between the outer surface of the pressure steel pipe and the surrounding rock of the tunnel;

[0008] The penstock includes, from upstream to downstream, an upstream section, a transition section and a downstream section; the downstream section of the penstock is in contact with and connected to the upstream surface of the upstream side wall of the plant building;

[0009] The area between the upstream starting point of the penstock transition section and the upstream side wall of the powerhouse is selected as the design object; the design object includes the following elements: the penstock transition section, the penstock downstream section, the thrust ring, the backfill concrete and the surrounding rock;

[0010] Step 2: Establish the water thrust transfer model:

[0011] A water thrust transmission model is established for the transition section of the penstock and the downstream section of the penstock; the water thrust transmission model describes the water thrust transmission mode; in the downstream section of the penstock, the water thrust transmission mode is: the water thrust is transmitted among the downstream section of the penstock, the thrust ring, the backfill concrete and the surrounding rock, and the surrounding rock finally bears the water thrust;

[0012] Step 3: Determine whether it is necessary to set the thrust ring at the downstream section of the penstock:

[0013] Specifically, when the water thrust acts on the downstream section of the penstock, the water thrust is transmitted through the gripping force between the downstream section of the penstock and the backfill concrete; therefore, the gripping force of the backfill concrete on the downstream section of the penstock is determined, and the calculated value of the gripping force safety factor is obtained according to the ratio of the gripping force of the backfill concrete on the downstream section of the penstock to the water thrust; if the calculated value of the gripping force safety factor is higher than the designed value of the gripping force safety factor, the water thrust can be effectively transmitted and a thrust ring is not set; otherwise, a thrust ring needs to be set and step 4 is executed;

[0014] Step 4: Formulate the design parameters of the thrust ring and the backfill concrete;

[0015] Step 5: Based on the design parameters of the thrust ring designed in step 4, determine whether the bearing capacity of the thrust ring, the thrust ring root pressure steel pipe, and the thrust ring root weld meets the requirements; if the requirements are met, proceed to step 6; otherwise, return to step 4 and re-draft the design parameters of the thrust ring;

[0016] Step 6: Based on the design parameters of the backfill concrete designed in step 4, determine whether the local compressive bearing capacity and the punching shear bearing capacity of the backfill concrete meet the requirements; if they meet the requirements, proceed to step 7; otherwise, return to step 4 and re-draft the design parameters of the backfill concrete;

[0017] Step 7: Based on the determined design parameters of the thrust ring and the design parameters of the backfill concrete, determine whether the overall anti-sliding stability between the backfill concrete and the surrounding rock meets the requirements; if the requirements are met, proceed to step 8;

[0018] Step 8: Determine whether the bearing capacity of the surrounding rock meets the requirements; if it does, the design process ends and the final design parameters of the thrust ring and the backfill concrete are obtained.

[0019] Preferably, step 3 is specifically:

[0020] Step 31, using formula (3-1), determine the water thrust F borne by the downstream section of the penstock:

[0021]

[0022] Where: F is the water thrust; γ Q is the action partial coefficient; ψ is the operating coefficient; P is the water hammer pressure in the downstream section of the pressure steel pipe; D is the inner diameter of the downstream section of the pressure steel pipe;

[0023] Step 32, using formula (3-2), determine the holding force F of the backfill concrete on the downstream section of the penstock τ :

[0024] F τ =c p lπD0 (3-2)

[0025] Among them: F τ is the grip strength; c p is the design value of the bond strength between the downstream section of the penstock and the backfill concrete; l is the length of the downstream section of the penstock; D0 is the outer diameter of the downstream section of the penstock;

[0026] Step 33, set the design value K of the safety factor of the backfill concrete holding force on the downstream section of the penstock τ ; Determine whether formula (3-3) is established:

[0027]

[0028] in: is the calculated value of the safety factor of the holding force;

[0029] If formula (3-3) holds true, it means that the backfill concrete has a strong grip F on the downstream section of the penstock. τ The safety margin is sufficient and the safety factor of the holding force meets the design requirements. Therefore, a thrust ring is not required; otherwise, a thrust ring is required.

[0030] Preferably, step 4 is specifically:

[0031] The design parameters of the thrust ring include the setting position, number n, height h, thickness t of the thrust ring, and the size w of the thrust ring root weld between the thrust ring and the downstream section of the pressure steel pipe; wherein the thrust rings are distributed at equal intervals and avoid the annular weld of the pressure steel pipe, and the height h and thickness t of each thrust ring are the same;

[0032] The design parameters of the backfill concrete include concrete strength and backfill concrete thickness.

[0033] Preferably, step 5 is specifically:

[0034] After the thrust ring is set, under the action of water thrust, the thrust ring, the pressure steel pipe at the root of the thrust ring, and the weld at the root of the thrust ring are all under stress; the setting of the thrust ring, that is, the design parameters of the thrust ring should make the bearing capacity of the above three meet the requirements, so as to ensure the safety and reliability of the thrust ring, and then ensure the effective transmission of water thrust from the downstream section of the pressure steel pipe to the backfill concrete.

[0035] Preferably, step 5 is specifically:

[0036] Step 51, determine the water thrust F borne by each thrust ring i i :

[0037] Specifically, assuming that n thrust rings are provided, the water thrust F borne by the downstream section of the penstock is borne entirely by n thrust rings, i.e., F τ =0; Considering the integrity of the penstock and the instantaneous nature of the water thrust, it is assumed that the water thrust F is borne by the n thrust rings set up on average. Therefore, through formula (5-1), the water thrust F borne by the thrust ring i is obtained i :

[0038]

[0039] Where: n is the number of thrust rings; F i The water thrust borne by the thrust ring i, i = 1, 2, ..., n;

[0040] Step 52, determine whether the bearing capacity of each thrust ring i meets the requirements:

[0041] Step 521, for the thrust ring i, assume that the water thrust F it bears i , along the height direction of the thrust ring i, it is distributed in a triangular state, the top force is 0, and the force at the root of the thrust ring, that is, the welding point with the downstream section of the pressure steel pipe, is the maximum value q0; the water thrust F i A bending moment is formed at the root of the thrust ring, which acts on the downstream section of the pressure steel pipe, causing bending stress to be generated at the root of the thrust ring. Using formula (5-2), the bending stress σ generated at the root of the thrust ring i due to the bearing water thrust is obtained: r :

[0042]

[0043] Where: I r is the moment of inertia of the root of thrust ring i; t is the thickness of thrust ring i;

[0044] M0 is the bending moment generated at the root of the thrust ring i due to the water thrust, which is determined by formula (5-3):

[0045]

[0046] Where: q0 is the water thrust F borne by the thrust ring i i The maximum force on the root when it is triangularly distributed along its height direction; h is the height of the thrust ring i;

[0047] Step 522, using formula (5-4) to determine the resistance limit σ of the pressure steel pipe and thrust ring R :

[0048]

[0049] Where: γ0 is the structural importance coefficient of the pressure steel pipe and thrust ring; γ pd is the structural coefficient of the pressure steel pipe and thrust ring; f is the design value of the material strength of the pressure steel pipe and thrust ring;

[0050] Step 523, determine whether formula (5-5) is established:

[0051] σ r ≤σ R (5-5)

[0052] If it is established, the bearing capacity of the thrust ring i meets the requirements; otherwise, the bearing capacity of the thrust ring i does not meet the requirements;

[0053] Step 53, determine whether the bearing capacity of the penstock at the root of each thrust ring i meets the requirements:

[0054] Step 531, taking the penstock at the root of the thrust ring i as a calculation element, and simplifying its stress state into a plane problem, and satisfying the Von Mises yield criterion, the penstock at the root of the thrust ring i bears water thrust in the axial direction and water pressure in the circumferential direction;

[0055] The bending moment M0 formed by the water thrust at the root of the thrust ring i needs to be balanced by the corresponding bending moment M1 generated by the pressure steel pipe at the root of the thrust ring i, and it is half on each side of the thrust ring i. Therefore, M1 is the bending moment on both sides of the penstock at the root of the thrust ring i;

[0056] The existence of the bending moment M1 causes bending stress in the penstock at the root of the thrust ring i along the axial direction;

[0057] Step 532, using equations (5-6), (5-7) and (5-8) respectively, obtain the axial principal stress σ1, the annular principal stress σ2 and the equivalent stress σ of the penstock at the root of thrust ring i. e :

[0058]

[0059]

[0060]

[0061] in:

[0062] T is the wall thickness of the penstock at the root of the thrust ring;

[0063] v is the Poisson’s ratio of steel;

[0064] Step 533, determine the axial principal stress σ1 of the pressure steel pipe, the annular principal stress σ2 of the pressure steel pipe, and the equivalent stress σ e , whether they satisfy equations (5-9), (5-9) and (5-11) respectively:

[0065] σ1≤σ R (5-9) σ2≤σ R (5-10)

[0066] σ e ≤σ R (5-11)

[0067] If equations (5-9), (5-9) and (5-11) hold, the bearing capacity of the root pressure steel pipe of the thrust ring i meets the requirements; otherwise, the bearing capacity of the root pressure steel pipe of the thrust ring i does not meet the requirements;

[0068] Step 54, determine whether the bearing capacity of the root weld of the thrust ring i meets the requirements:

[0069] Step 541, using equations (5-12), (5-13) and (5-14), respectively obtain the bending stress S1, shear stress S2 and equivalent stress S borne by the root weld of thrust ring i e :

[0070]

[0071]

[0072]

[0073] Where: w is the root weld size of the thrust ring; q i The force formed by resisting / transmitting the bending moment of the thrust ring on the weld per unit length at the root of the thrust ring i;

[0074] Step 542, determine the bending stress S1, shear stress S2 and equivalent stress S1 borne by the root weld of the thrust ring i e , whether they satisfy equations (5-15), (5-16) and (5-17) respectively:

[0075]

[0076]

[0077]

[0078] in: is the design value of fillet weld strength;

[0079] If equations (5-15), (5-16) and (5-17) hold true, the bearing capacity of the root weld of thrust ring i meets the requirements; otherwise, it does not meet the requirements.

[0080] Preferably, step 6 is specifically:

[0081] When the water thrust acts on the thrust ring, the water thrust is transmitted through the gripping force between the downstream section of the pressure steel pipe and the backfill concrete, and also directly acts on the backfill concrete through the thrust ring. The thrust ring acts on the backfill concrete and makes the backfill concrete bear local compression and shearing conditions. The proposed design parameters of the backfill concrete should make the local compression and shearing bearing capacity of the backfill concrete meet the requirements to ensure the safety and reliability of the backfill concrete, thereby ensuring that the water thrust is effectively transmitted under the overall action of the backfill concrete.

[0082] Preferably, step 6 is specifically:

[0083] Step 61, determine whether the local compressive bearing capacity of the backfill concrete meets the requirements:

[0084] Step 611, determine the local compressive bearing capacity F of the backfill concrete p :

[0085] After the thrust ring i bears the water thrust, it acts on the backfill concrete. The backfill concrete is in a local compressive state. Using formula (6-1), the local compressive bearing capacity F of the backfill concrete is obtained. p :

[0086]

[0087] Where: γ cd is the structural coefficient of backfill concrete; is the load distribution influence coefficient, which is 1 for uniform load and 0.75 for non-uniform load; c A is the design value of compressive strength of backfill concrete; i The local compressive area of ​​the backfill concrete is determined by formula (6-2):

[0088]

[0089] Where: D h is the outer diameter of the thrust ring i; D0 is the outer diameter of the downstream section of the penstock;

[0090] Step 612, determine whether equation (6-3) holds true:

[0091] F i ≤F p (6-3)

[0092] Among them: F i The water thrust F borne by the thrust ring i i ;

[0093] If it is established, the local compressive bearing capacity of the backfill concrete meets the requirements; otherwise, it does not meet the requirements;

[0094] Step 62, determine whether the shear bearing capacity of the backfill concrete meets the requirements:

[0095] Step 621, using formula (6-4), determine the shear bearing capacity F of the backfill concrete t :

[0096]

[0097] in:

[0098] Where: d i is the effective height of the punching failure cone; β d is the cross-section height influence coefficient, when d i When ≤0.8m, take β d =1; when d i ≥2m, take β d =0.9; the period is taken by linear interpolation; f t is the design value of tensile strength of backfill concrete; u i D is the perimeter of the punching failure cone at half the effective height; b The outer diameter of the backfill concrete;

[0099] Step 622, determine whether equation (6-5) is established:

[0100] F i ≤F t (6-5)

[0101] If so, the shear bearing capacity of the backfill concrete meets the requirements; otherwise, it does not meet the requirements.

[0102] Preferably, step 7 is specifically:

[0103] Under the overall action of backfill concrete, the hydraulic thrust is transmitted to the surrounding rock through the friction resistance on the contact surface with the surrounding rock. The proposed design parameters of the backfill concrete and the selected design object length should make the friction resistance between the backfill concrete and the surrounding rock meet the overall anti-sliding stability requirements of the backfill concrete to ensure the effective transmission of the hydraulic thrust from the backfill concrete to the surrounding rock.

[0104] Preferably, step 7 is specifically:

[0105] Step 71, determine the frictional resistance between the backfill concrete and the surrounding rock:

[0106] Specifically, assuming that the friction resistance between the surrounding rock and the backfill concrete is uniformly distributed, the anti-sliding bearing capacity F between the backfill concrete and the surrounding rock is obtained by formula (7-1): c :

[0107] F c =c c πD b L (7-1)

[0108] Among them: F c is the anti-sliding bearing capacity between backfill concrete and surrounding rock; c c is the cohesion between backfill concrete and surrounding rock; D b is the outer diameter of the backfill concrete; L is the distance from the upstream starting point of the transition section of the penstock to the upstream side wall of the plant, which is the length of the design object;

[0109] Step 72: Set the overall anti-sliding stability safety factor design value K between the backfill concrete and the surrounding rock c ; Determine whether formula (7-2) is established:

[0110]

[0111] in: is the calculated value of the overall anti-sliding stability safety factor between the backfill concrete and the surrounding rock; F is the water thrust;

[0112] If formula (7-2) is established, it means that the overall anti-sliding stability between the backfill concrete and the surrounding rock meets the requirements; if formula (7-2) is not established, other measures such as expanding the tunnel diameter, extending and ensuring the friction length should be taken to ensure the establishment of formula (7-2).

[0113] Preferably, step 8 is specifically as follows:

[0114] When the proposed design parameters of the thrust ring and the backfill concrete meet the requirements of the above steps, the water thrust is transmitted to the surrounding rock. When the bearing capacity of the surrounding rock meets the requirements, the water thrust is no longer transmitted.

[0115] The specific process of step 8 is as follows:

[0116] Step 81, determine the shear bearing capacity F of the surrounding rock cone r :

[0117] After the water thrust is transmitted to the surrounding rock, it is borne by the cone-shaped rock mass. Assuming that the strength of the surrounding rock is 0, the surrounding rock is borne only by cohesion. The shear bearing capacity F of the surrounding rock cone is obtained by formula (8-1):r :

[0118]

[0119] in:

[0120] F r is the shear bearing capacity of the surrounding rock cone; c r A is the cohesion of surrounding rock mass; r is the surface area of ​​the frustum;

[0121] r1 is the radius of the circular top of the frustum, D b The outer diameter of the backfill concrete;

[0122] r2 is the radius of the bottom circle of the cone,

[0123] l r is the height of the cone, that is, the effective friction section length between the backfill concrete and the surrounding rock; c c K is the cohesion between backfill concrete and surrounding rock; c is the design value of the overall anti-sliding stability safety factor between the backfill concrete and the surrounding rock; F is the water thrust;

[0124] Step 82: Set the design value of the safety factor of surrounding rock bearing capacity K r ; Determine whether formula (8-2) is established:

[0125]

[0126] in: is the calculated value of safety factor of surrounding rock bearing capacity;

[0127] If formula (8-2) is true, it means that the bearing capacity of the surrounding rock meets the requirements and the water thrust is ultimately borne by the surrounding rock; if formula (8-2) is not true, other measures to strengthen the rock mass should be taken to ensure the validity of formula (8-2).

[0128] The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station provided by the present invention has the following advantages:

[0129] The present invention establishes a basic model for the transmission of water thrust between the elements at the end of the penstock, which is simple and conforms to the load transfer logic; establishes a design process for the thrust ring at the end of the penstock, which, based on the water thrust transmission model, clarifies the interaction between the penstock, thrust ring, backfill concrete and surrounding rock, makes reasonable assumptions and derivation analysis on the self-strength and interaction between the elements, and gives a specific calculation formula, so that the force characteristics and interaction of each element are clearer and more specific, conforming to mechanical properties and engineering practice. The present invention provides a design method for the thrust ring at the end of a buried penstock of a hydropower station, which provides support for the design of the thrust ring and is more secure in ensuring the safety of the penstock and plant of the hydropower station. BRIEF DESCRIPTION OF THE DRAWINGS

[0130] Figure 1 A schematic flow chart of a design method for a thrust ring at the end of a buried penstock of a hydropower station provided by the present invention;

[0131] Figure 2 A schematic cross-sectional view of the end structure of the buried pressure steel pipe provided by the present invention;

[0132] Figure 3 A schematic plan view of the buried penstock terminal structure provided by the present invention;

[0133] Figure 4 A schematic diagram of the structure of the downstream section of the pressure steel pipe provided by the present invention;

[0134] Figure 5 A schematic diagram of the water thrust and holding force of the penstock provided by the present invention;

[0135] Figure 6 A schematic diagram of the hydraulic thrust and thrust force of the pressure steel pipe provided by the present invention;

[0136] Figure 7 A schematic diagram of the force acting on the thrust ring of a pressure steel pipe provided by the present invention;

[0137] Figure 8 A schematic diagram of the bearing capacity of the backfill concrete of the pressure steel pipe provided by the present invention;

[0138] Fig. 9 A schematic diagram of the friction resistance between the backfill concrete and the surrounding rock wall provided by the present invention;

[0139] Fig.10 A schematic diagram of the force on the surrounding rock cone frustum provided by the present invention.

[0140] Among them: 1---downstream section of the pressure steel pipe; 2---transition section of the pressure steel pipe; 3---backfill concrete; 4---surrounding rock; 5---upstream side wall of the plant building; 6---upstream section of the pressure steel pipe; 7---thrust ring; 8---annular weld of the pressure steel pipe; 9---the oblique section of the shear failure cone of the backfill concrete; 10---the weld between the thrust ring and the pressure steel pipe, that is, the weld at the root of the thrust ring; 11---contact surface of the effective friction resistance section of the backfill concrete; 12---the oblique section of the surrounding rock failure cone. DETAILED DESCRIPTION

[0141] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0142] The present invention establishes a basic model for the transmission of water thrust between the elements at the end of the penstock, which is simple and conforms to the load transfer logic; establishes a design process for the thrust ring at the end of the penstock, which, based on the water thrust transmission model, clarifies the interaction between the penstock, thrust ring, backfill concrete and surrounding rock, makes reasonable assumptions and derivation analysis on the self-strength and interaction between the elements, and gives a specific calculation formula, so that the force characteristics and interaction of each element are clearer and more specific, conforming to mechanical properties and engineering practice. The present invention provides a design method for the thrust ring at the end of a buried penstock of a hydropower station, which provides support for the design of the thrust ring and is more secure in ensuring the safety of the penstock and plant of the hydropower station.

[0143] See also Figure 1 The present invention provides a design method for a thrust ring at the end of a buried pressure steel pipe of a hydropower station, comprising the following steps:

[0144] Step 1: Determine the design object:

[0145] The upstream pressure pipeline of the hydropower station power plant adopts a buried pressure steel pipe; the buried pressure steel pipe includes a pressure steel pipe arranged coaxially with the tunnel, and backfill concrete is provided between the outer surface of the pressure steel pipe and the surrounding rock of the tunnel;

[0146] The penstock includes, from upstream to downstream, an upstream section, a transition section and a downstream section; the downstream section of the penstock is in contact with and connected to the upstream surface of the upstream side wall of the plant building;

[0147] The area between the upstream starting point of the penstock transition section and the upstream side wall of the powerhouse is selected as the design object; the design object includes the following elements: the penstock transition section, the penstock downstream section, the thrust ring, the backfill concrete and the surrounding rock;

[0148] As an example, in Figure 2 to Figure 10 The meanings of the numbers in the attached figure are as follows: 1---downstream section of the pressure steel pipe; 2---transition section of the pressure steel pipe; 3---backfill concrete; 4---surrounding rock; 5---upstream side wall of the plant building; 6---upstream section of the pressure steel pipe; 7---thrust ring; 8---annular weld of the pressure steel pipe; 9---the oblique section of the shear failure cone of the backfill concrete; 10---the weld between the thrust ring and the pressure steel pipe, that is, the weld at the root of the thrust ring; 11---contact surface of the effective friction resistance section of the backfill concrete; 12---the oblique section of the surrounding rock failure cone.

[0149] In the embodiment, the upstream pressure pipeline of a hydropower station power plant adopts buried pressure steel pipe, see Figure 2 , Figure 3 and Figure 4 , its main structure includes a pressure steel pipe, a thrust ring 7, backfill concrete 3 and surrounding rock 4; the pressure steel pipe includes a pressure steel pipe upstream section 6, a pressure steel pipe transition section 2 and a pressure steel pipe downstream section 1;

[0150] The inner diameter of the upstream starting point of the penstock transition section 2 is 4.0 m, and the inner diameter of the downstream end point is 2.42 m. The length, i.e. the length from the upstream starting point to the downstream end point of the penstock transition section 2 is 14.4 m.

[0151] The diameter of the downstream section 1 of the pressure steel pipe is 2.42m, the wall thickness is 32mm, and its length, that is, the length from the downstream end of the pressure steel pipe transition section 2 to the upstream surface of the upstream side wall 5 of the plant building is 5.5m. The steel material used is SD345SC.

[0152] Through the transition process, it is calculated that the maximum water hammer pressure when the power plant is unloaded is 3900kPa.

[0153] The tunnel from the upstream starting point of the transition section 2 of the penstock to the upstream face of the upstream side wall 5 of the powerhouse adopts a uniform cross section, which is a circular cross section. The penstock and the tunnel are arranged coaxially. The backfill concrete 3 outside the penstock adopts C25. The surrounding rock 4 of the tunnel chamber is VI granite, and its cohesion is 100kPa to 300kPa. In order to ensure the safety of the powerhouse and the penstock during load shedding, it is necessary to design the thrust ring at the end of the penstock using the method of the present invention.

[0154] Step 2: Establish the water thrust transfer model:

[0155] A water thrust transmission model is established for the transition section of the penstock and the downstream section of the penstock; the water thrust transmission model describes the water thrust transmission mode; in the downstream section of the penstock, the water thrust transmission mode is: the water thrust is transmitted among the downstream section of the penstock, the thrust ring, the backfill concrete and the surrounding rock, and the surrounding rock finally bears the water thrust;

[0156] Step 3: Determine whether it is necessary to set the thrust ring at the downstream section of the penstock:

[0157] Specifically, when the water thrust acts on the downstream section of the penstock, the water thrust is transmitted through the gripping force between the downstream section of the penstock and the backfill concrete; therefore, the gripping force of the backfill concrete on the downstream section of the penstock is determined, and the gripping force safety factor calculation value is obtained according to the ratio of the gripping force of the backfill concrete on the downstream section of the penstock to the water thrust; if the gripping force safety factor calculation value is higher than the gripping force safety factor design value, the water thrust can be effectively transmitted, and the thrust ring may not be set; otherwise, it means that the gripping force is insufficient, the water thrust cannot be effectively transmitted, and a thrust ring needs to be set, and step 4 is executed;

[0158] Step 3 is as follows:

[0159] Step 31, using formula (3-1), determine the water thrust F borne by the downstream section of the penstock:

[0160]

[0161] in:

[0162] F is the water thrust, kN; the calculated value in the example is 16114.62 kN;

[0163] γ Q is the action partial coefficient; in this example, it is taken as 1.0;

[0164] ψ is the operating coefficient; in this example, it is taken as 0.9;

[0165] P is the water hammer pressure in the downstream section of the penstock, kPa;

[0166] D is the inner diameter of the downstream section of the penstock, m; in this example, it is 2.42 m

[0167] Step 32, using formula (3-2), determine the holding force F of the backfill concrete on the downstream section of the penstock τ :

[0168] F τ =c p lπD0 (3-2)

[0169] In this step, the transition section of the penstock is used as the end expansion of the downstream section of the penstock, and it is assumed that only the downstream section of the penstock produces the holding force. Therefore, the holding force F of the backfill concrete on the downstream section of the penstock is obtained by (3-2). τ . See the diagram of the holding force Figure 5 .

[0170] in:

[0171] F τ is the holding force, i.e., the holding force of the backfill concrete on the downstream section of the penstock, kN; the calculated value in the example is 6272.19 kN;

[0172] c p is the design value of the bond strength between the downstream section of the penstock and the backfill concrete, kPa; 150 kPa is taken in this example;

[0173] l is the length of the downstream section of the penstock, m; the downstream section of the penstock is a straight section; in this example, it is 5.5 m;

[0174] D0 is the outer diameter of the downstream section of the penstock, m; in this example, it is taken as 2.484 m;

[0175] Step 33, set the design value K of the safety factor of the backfill concrete holding force on the downstream section of the penstock τ ; Determine whether formula (3-3) is established:

[0176]

[0177] in:

[0178] is the calculated value of the safety factor of the holding force;

[0179] K τ is the design value of the safety factor of the backfill concrete’s holding force on the downstream section of the penstock; in this example, it is taken as 1.8;

[0180] If formula (3-3) holds true, it means that the backfill concrete has a strong grip F on the downstream section of the penstock. τ The safety margin is sufficient and the safety factor of the holding force meets the design requirements. Therefore, a thrust ring is not required; otherwise, a thrust ring is required.

[0181] The calculated value of the safety factor of the holding force in the example is 0.389, and formula (3-3) does not hold. Therefore, a thrust ring should be installed.

[0182] Step 4: Formulate the design parameters of the thrust ring and the backfill concrete;

[0183] Step 4 is as follows:

[0184] The design parameters of the thrust ring include the setting position, number n, height h, thickness t of the thrust ring, and the size w of the thrust ring root weld between the thrust ring and the downstream section of the pressure steel pipe; wherein the thrust rings are distributed at equal intervals and avoid the annular weld of the pressure steel pipe, and the height h and thickness t of each thrust ring are the same;

[0185] The design parameters of the backfill concrete include concrete strength and backfill concrete thickness.

[0186] Step 5: Based on the design parameters of the thrust ring designed in step 4, determine whether the bearing capacity of the thrust ring, the thrust ring root pressure steel pipe, and the thrust ring root weld meets the requirements; if the requirements are met, proceed to step 6; otherwise, return to step 4 and re-draft the design parameters of the thrust ring;

[0187] Specifically, after the thrust ring is set, under the action of water thrust, the thrust ring, the pressure steel pipe at the root of the thrust ring, and the weld at the root of the thrust ring are all under stress; the setting of the thrust ring, that is, the design parameters of the thrust ring should make the bearing capacity of the above three meet the requirements, so as to ensure the safety and reliability of the thrust ring, and then ensure the effective transmission of water thrust from the downstream section of the pressure steel pipe to the backfill concrete.

[0188] Step 5 is as follows:

[0189] Step 51, determine the water thrust F borne by each thrust ring i i :

[0190] Specifically, considering the uncertainty of the mechanical model of the holding force between the backfill concrete and the penstock, in order to simplify the calculation, it is assumed that after setting n thrust rings, the water thrust F borne by the downstream section of the penstock is borne entirely by n thrust rings, that is, F τ =0; Considering the integrity of the penstock and the instantaneous nature of the water thrust, it is assumed that the water thrust F is borne by the n thrust rings set up on average. Therefore, through formula (5-1), the water thrust F borne by the thrust ring i is obtained i ; The water thrust F borne by each thrust ring i i See the schematic diagram Figure 6 ;

[0191]

[0192] Where: n is the number of thrust rings; in this example, it is 3;

[0193] F i The water thrust borne by the thrust ring i, i = 1, 2, ..., n, kN; the calculated value in the example is 16144.616 kN;

[0194] Step 52, determine whether the bearing capacity of each thrust ring i meets the requirements:

[0195] Step 521, for the thrust ring i, assume that the water thrust F it bears i , along the height direction of the thrust ring i, it is distributed in a triangular state, the top force is 0, and the force at the root of the thrust ring, that is, the welding point with the downstream section of the pressure steel pipe, is the maximum value q0; the water thrust F i A bending moment is formed at the root of the thrust ring, which acts on the downstream section of the pressure steel pipe, causing bending stress to be generated at the root of the thrust ring. Using formula (5-2), the bending stress σ generated at the root of the thrust ring i due to the bearing water thrust is obtained: r , force diagram see Figure 7 ;

[0196]

[0197] in:

[0198] σ r The bending stress generated at the root of the thrust ring i due to the water thrust, kPa; the calculated value in the example is 235950 kPa;

[0199] I r is the moment of inertia at the root of the thrust ring i, m 4 ; In this example, 2.25×10 -6 m 4 ;

[0200] t is the thickness of the thrust ring i, m; in this example, it is taken as 0.03 m;

[0201] M0 is the bending moment generated at the root of the thrust ring i due to the water thrust, which is determined by formula (5-3):

[0202]

[0203] in:

[0204] M0 is the bending moment generated at the root of the thrust ring i due to the water thrust, kN·m; the calculated value in the example is 35.39 kN·m;

[0205] q0 is the water thrust F borne by the thrust ring i i When the height is triangularly distributed, the maximum force at the root is kN / m; the calculated value is 9438 kN / m.

[0206] h is the height of the thrust ring i, m; in this example, it is 0.15 m;

[0207] Step 522, using formula (5-4) to determine the resistance limit σ of the pressure steel pipe and thrust ring R :

[0208]

[0209] in:

[0210] σ R is the resistance limit of the pressure steel pipe and thrust ring, kPa; in the example, it is 256198.35 kPa;

[0211] γ0 is the structural importance coefficient of the pressure steel pipe and thrust ring; it is taken as 1.1 in the example;

[0212] γ pd is the structural coefficient of the pressure steel pipe and thrust ring; in the example, it is taken as 1.1;

[0213] f is the design value of the material strength of the penstock and thrust ring, kPa; in this example, it is taken as 310000 kPa;

[0214] Step 523, determine whether formula (5-5) is established:

[0215] σ r ≤σ R (5-5)

[0216] If it is established, the bearing capacity of the thrust ring i meets the requirements; otherwise, the bearing capacity of the thrust ring i does not meet the requirements;

[0217] In the example, formula (5-5) holds true, that is, the thrust ring bearing capacity meets the requirements.

[0218] Step 53, determine whether the bearing capacity of the penstock at the root of each thrust ring i meets the requirements:

[0219] Step 531, taking the penstock at the root of the thrust ring i as a calculation element, and simplifying its stress state into a plane problem, and satisfying the Von Mises yield criterion, the penstock at the root of the thrust ring i bears water thrust in the axial direction and water pressure in the circumferential direction;

[0220] The bending moment M0 formed by the water thrust at the root of the thrust ring i needs to be balanced by the corresponding bending moment M1 generated by the pressure steel pipe at the root of the thrust ring i, and it is half on each side of the thrust ring i. Therefore, M1 is the bending moment on both sides of the penstock at the root of thrust ring i, kN·m; the existence of the bending moment M1 causes bending stress in the axial direction of the penstock at the root of thrust ring i;

[0221] Step 532, using equations (5-6), (5-7) and (5-8) respectively, obtain the axial principal stress σ1, the annular principal stress σ2 and the equivalent stress σ of the penstock at the root of thrust ring i. e :

[0222]

[0223]

[0224]

[0225] in:

[0226] In the example, the bending moment M1 is 17.696 kN·m;

[0227] T is the wall thickness of the penstock at the root of the thrust ring, m; in this example, it is 0.032 m;

[0228] v is the Poisson's ratio of steel; in this example, it is taken as 0.3;

[0229] Axial principal stress σ1, annular principal stress σ2 and equivalent stress σ of pressure steel pipe e , the calculated values ​​in the example are 1500878.965 kPa, 166040.596 kPa and 159002.856 kPa respectively;

[0230] Step 533, determine the axial principal stress σ1 of the pressure steel pipe, the annular principal stress σ2 of the pressure steel pipe, and the equivalent stress σ e , whether they satisfy equations (5-9), (5-9) and (5-11) respectively:

[0231] σ1≤σ R (5-9) σ2≤σ R (5-10) σ e ≤σ R (5-11)

[0232] If equations (5-9), (5-9) and (5-11) hold true, the bearing capacity of the root pressure steel pipe of the thrust ring i meets the requirements; otherwise, the bearing capacity of the root pressure steel pipe of the thrust ring i does not meet the requirements.

[0233] Therefore, the axial principal stress σ1 of the penstock is the sum of the stresses generated by the water thrust and the bending moment M1, which should meet the bearing capacity requirements; the annular principal stress σ2 of the penstock is the stress generated by the water pressure, which should meet the bearing capacity requirements; the equivalent stress σ e The bearing capacity requirements should also be met.

[0234] In the example, equations (5-9), (5-9) and (5-11) are all valid, that is, the bearing capacity of the pressure steel pipe at the root of the thrust ring meets the requirements.

[0235] Step 54, determine whether the bearing capacity of the root weld of the thrust ring i meets the requirements:

[0236] The root weld of the thrust ring i mainly bears the direct shearing action generated by the water thrust and generates shear stress S2, and at the same time bears the bending moment generated by the bending moment of the thrust ring root and generates bending stress S1.

[0237] Step 541, using equations (5-12), (5-13) and (5-14), respectively obtain the bending stress S1, shear stress S2 and equivalent stress S borne by the root weld of thrust ring i e ; Schematic diagram of the root weld of thrust ring i Figure 7 ;

[0238]

[0239]

[0240]

[0241] in:

[0242] w is the root weld size of the thrust ring, m; in this example, 0.012 m is taken;

[0243] q i The force formed by the weld per unit length at the root of the thrust ring i to resist / transmit the thrust ring bending moment, kN / m; the calculated value in the example is 842.679 kN / m;

[0244] In the embodiment, the bending stress S1, shear stress S2 and equivalent stress S1 borne by the weld at the root of the thrust ring i are e The calculated values ​​are: 28733.847 kPa, 70223.214 kPa and 75874.461 kPa;

[0245] Step 542, determine the bending stress S1, shear stress S2 and equivalent stress S1 borne by the root weld of the thrust ring i e , whether they satisfy equations (5-15), (5-16) and (5-17) respectively:

[0246]

[0247]

[0248]

[0249] in: is the design value of fillet weld strength, kPa; in this example, it is taken as 200000 kPa;

[0250] If equations (5-15), (5-16) and (5-17) hold true, the bearing capacity of the root weld of thrust ring i meets the requirements; otherwise, it does not meet the requirements.

[0251] The bending stress S1, shear stress S2 and equivalent stress S borne by the root weld of the thrust ring i e The strength requirements should be met.

[0252] In the example, equations (5-15), (5-16) and (5-17) are all valid, that is, the bearing capacity of the root weld of thrust ring i meets the requirements.

[0253] Step 6: Based on the design parameters of the backfill concrete designed in step 4, determine whether the local compressive bearing capacity and the punching shear bearing capacity of the backfill concrete meet the requirements; if they meet the requirements, proceed to step 7; otherwise, return to step 4 and re-draft the design parameters of the backfill concrete;

[0254] Specifically, when the water thrust acts on the thrust ring, in addition to being transmitted through the gripping force between the downstream section of the pressure steel pipe and the backfill concrete, the water thrust is also transmitted directly to the backfill concrete through the thrust ring; the thrust ring acts on the backfill concrete and causes the backfill concrete to bear local compression and shearing conditions. The proposed design parameters of the backfill concrete should ensure that the local compression and shearing bearing capacity of the backfill concrete meet the requirements, so as to ensure the safety and reliability of the backfill concrete, and further ensure that the water thrust is effectively transmitted under the overall action of the backfill concrete.

[0255] Step 6 is as follows:

[0256] Step 61, determine whether the local compressive bearing capacity of the backfill concrete meets the requirements:

[0257] Step 611, determine the local compressive bearing capacity F of the backfill concrete p :

[0258] After the thrust ring i bears the water thrust, it acts on the backfill concrete. The backfill concrete is in a local compressive state. Using formula (6-1), the local compressive bearing capacity F of the backfill concrete is obtained. p :

[0259]

[0260] in:

[0261] γ cd is the structural coefficient of backfill concrete; in this example, it is taken as 1.3;

[0262] is the load distribution influence coefficient, which is 1 for uniform load and 0.75 for non-uniform load; it is 0.75 in the example;

[0263] f c is the design value of compressive strength of backfill concrete, kPa; in this example, it is taken as 11900 kPa;

[0264] A i is the local compression area of ​​backfill concrete, m 2 , determined by formula (6-2):

[0265]

[0266] in:

[0267] A i The calculated value in the example is 1.241m 2 ;

[0268] D h is the outer diameter of the thrust ring i, m; in this example, 2.784 m is taken;

[0269] D0 is the outer diameter of the downstream section of the penstock; in this example, it is taken as 2.484m;

[0270] Step 612, determine whether equation (6-3) holds true:

[0271] F i ≤F p (6-3)

[0272] Among them: F i The water thrust F borne by the thrust ring i i ;

[0273] If it is established, the local compressive bearing capacity of the backfill concrete meets the requirements; otherwise, it does not meet the requirements;

[0274] In this example, formula (6-3) holds true and the local compressive bearing capacity of the backfill concrete meets the requirements.

[0275] Step 62, determine whether the shear bearing capacity of the backfill concrete meets the requirements:

[0276] When the local compressive bearing capacity of the backfill concrete meets the requirements, it should be further verified whether the shear bearing capacity of the backfill concrete meets the requirements.

[0277] Step 621, see Figure 8 , using formula (6-4), determine the shear bearing capacity F of the backfill concrete t :

[0278]

[0279] in:

[0280] in:

[0281] d i is the effective height of the shear failure cone, m; the calculated value in the example is 1.24m;

[0282] β d is the cross-section height influence coefficient, when d i When ≤0.8m, take β d =1; when d i ≥2m, take βd =0.9; the period is taken by linear interpolation; the calculated value in the example is 0.963;

[0283] f t is the design value of tensile strength of backfill concrete, kPa; in this example, 1270 kPa is taken;

[0284] u i is the circumference of the punching failure cone at half the effective height, m; the calculated value in the example is 12.642 m;

[0285] D b is the outer diameter of the backfill concrete, m; in this example, 5.264m is taken;

[0286] Step 622, determine whether equation (6-5) is established:

[0287] F i ≤F t (6-5)

[0288] If so, the shear bearing capacity of the backfill concrete meets the requirements; otherwise, it does not meet the requirements.

[0289] Specifically, when verifying the shear bearing capacity of backfill concrete, according to the principle of concrete shear failure, the shear surface is at an angle of 45° to the axis of the pressure steel pipe; at the same time, in order to ensure that the backfill concrete corresponding to each thrust ring has the same shear surface, the distance between the thrust ring on the most downstream side and the upstream side wall of the plant should be greater than the distance from the end of the thrust ring to the surrounding rock.

[0290] In this example, empirical calculation shows that formula (6-5) holds true and the shear bearing capacity of the backfill concrete meets the requirements.

[0291] Step 7: Based on the determined design parameters of the thrust ring and the design parameters of the backfill concrete, determine whether the overall anti-sliding stability between the backfill concrete and the surrounding rock meets the requirements; if the requirements are met, proceed to step 8;

[0292] Specifically, the hydraulic thrust is transmitted to the surrounding rock through the friction resistance on the contact surface between the backfill concrete and the surrounding rock under the overall action of the backfill concrete. The proposed design parameters of the backfill concrete and the selected design object length should make the friction resistance between the backfill concrete and the surrounding rock meet the overall anti-sliding stability requirements of the backfill concrete to ensure the effective transmission of the hydraulic thrust from the backfill concrete to the surrounding rock.

[0293] Step 7 is as follows:

[0294] Step 71, determine the frictional resistance between the backfill concrete and the surrounding rock:

[0295] Specifically, assuming that the friction resistance between the surrounding rock and the backfill concrete is uniformly distributed, the anti-sliding bearing capacity F between the backfill concrete and the surrounding rock is obtained by formula (7-1):c ; See the force diagram Fig. 9 ;

[0296] F c =c c πD b L (7-1)

[0297] in:

[0298] F c is the anti-sliding bearing capacity between backfill concrete and surrounding rock, kN; the calculated value in this example is 64495.641 kN;

[0299] c c is the cohesion between backfill concrete and surrounding rock, kPa; 200 kPa is taken in this example;

[0300] D b is the outer diameter of the backfill concrete; in this example, it is taken as 5.264m;

[0301] L is the distance from the upstream starting point of the transition section of the penstock to the upstream side wall of the plant building, which is the length of the design object, m; in this example, 19.5 m is taken;

[0302] Step 72: Set the overall anti-sliding stability safety factor design value K between the backfill concrete and the surrounding rock c ; Determine whether formula (7-2) is established:

[0303]

[0304] in:

[0305] K c is the design value of the overall anti-sliding stability safety factor between the backfill concrete and the surrounding rock; it is taken as 1.8 in this example;

[0306] is the calculated value of the overall anti-sliding stability safety factor between the backfill concrete and the surrounding rock; F is the water thrust; in the example It is 3.995.

[0307] If formula (7-2) is established, it means that the overall anti-sliding stability between the backfill concrete and the surrounding rock meets the requirements; if formula (7-2) is not established, other measures such as expanding the tunnel diameter, extending and ensuring the friction length should be taken to ensure the establishment of formula (7-2).

[0308] In this example, formula (7-2) holds true, and the overall anti-sliding stability between the backfill concrete and the surrounding rock meets the requirements.

[0309] Step 8: Determine whether the bearing capacity of the surrounding rock meets the requirements; if it does, the design process ends and the final design parameters of the thrust ring and the backfill concrete are obtained.

[0310] Specifically, when the proposed design parameters of the thrust ring and the design parameters of the backfill concrete meet the requirements of the above steps, the water thrust is transmitted to the surrounding rock, and when the bearing capacity of the surrounding rock meets the requirements, the water thrust is no longer transmitted;

[0311] The specific process of step 8 is as follows:

[0312] Step 81, determine the shear bearing capacity F of the surrounding rock cone r :

[0313] After the water thrust is transmitted to the surrounding rock, it is borne by the cone-shaped rock mass. Assuming that the strength of the surrounding rock is 0, the surrounding rock is borne only by cohesion. The shear bearing capacity F of the surrounding rock cone is obtained by formula (8-1): r ; See the force diagram Fig.10 ;

[0314]

[0315] in:

[0316] F r is the shear bearing capacity of the surrounding rock cone, kN; the calculated value in the example is 77565.570 kN;

[0317] c r is the cohesion of the surrounding rock mass, kPa; in this example, it is taken as 200 kPa;

[0318] A r is the surface area of ​​the cone, m 2 ; The calculated value in the example is 548.471m 2 ;

[0319] r1 is the circular radius of the top of the cone, m; D b is the outer diameter of the backfill concrete; in this example, the calculated value of r1 is 2.632m;

[0320] r2 is the circular radius of the bottom of the cone, m; The calculated value of r2 in the example is 11.418m;

[0321] l r is the height of the cone, m, which is the effective friction section length between the backfill concrete and the surrounding rock; c c is the cohesion between backfill concrete and surrounding rock, which is 200 kPa in this example; K c is the design value of the overall anti-sliding stability safety factor between the backfill concrete and the surrounding rock, which is taken as 1.8 in the example; F is the water thrust; in the example, l r The calculated value is 8.786m;

[0322] Step 82: Set the design value of the safety factor of surrounding rock bearing capacity K r ; Determine whether formula (8-2) is established:

[0323]

[0324] in:

[0325] K r In the example, take 1.8;

[0326] is the calculated value of the safety factor of surrounding rock bearing capacity; in this example, it is 4.804.

[0327] If formula (8-2) is true, it means that the bearing capacity of the surrounding rock meets the requirements and the water thrust is ultimately borne by the surrounding rock; if formula (8-2) is not true, other measures to strengthen the rock mass should be taken to ensure the validity of formula (8-2).

[0328] In this example, formula (8-2) holds true and the surrounding rock bearing capacity meets the requirements.

[0329] Through the design method of the thrust ring at the end of the buried penstock of a hydropower station disclosed in the present invention, a basic model of the transmission of water thrust between the elements at the end of the penstock is established. The model is simple and conforms to the load transfer logic; a design process of the thrust ring at the end of the penstock is established. Based on the water thrust transmission model, the process clarifies the interaction between the penstock, thrust ring, backfill concrete and surrounding rock, makes reasonable assumptions and derivation analysis on the self-strength and interaction between each element, and gives a specific calculation formula, so that the force characteristics and interaction of each element are clearer and more specific, which conforms to the mechanical properties and engineering practice. The design method of the thrust ring at the end of the buried penstock of a hydropower station provided by the present invention provides support for the design of the thrust ring, which is more secure for ensuring the safety of the penstock and plant of the hydropower station.

[0330] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be considered as the scope of protection of the present invention.

Claims

1. A design method for a thrust ring at the end of a buried pressure steel pipe of a hydropower station, characterized in that: The following steps are involved: Step 1: Determine the design object: The upstream pressure pipeline of the hydropower station power plant adopts a buried pressure steel pipe; the buried pressure steel pipe includes a pressure steel pipe arranged coaxially with the tunnel, and backfill concrete is provided between the outer surface of the pressure steel pipe and the surrounding rock of the tunnel; The penstock includes, from upstream to downstream, an upstream section, a transition section and a downstream section; the downstream section of the penstock is in contact with and connected to the upstream surface of the upstream side wall of the plant building; The area between the upstream starting point of the penstock transition section and the upstream side wall of the powerhouse is selected as the design object; the design object includes the following elements: the penstock transition section, the penstock downstream section, the thrust ring, the backfill concrete and the surrounding rock; Step 2: Establish the water thrust transfer model: A water thrust transmission model is established for the transition section of the penstock and the downstream section of the penstock; the water thrust transmission model describes the water thrust transmission mode; in the downstream section of the penstock, the water thrust transmission mode is: the water thrust is transmitted among the downstream section of the penstock, the thrust ring, the backfill concrete and the surrounding rock, and the surrounding rock finally bears the water thrust; Step 3: Determine whether it is necessary to set the thrust ring at the downstream section of the penstock: Specifically, when the water thrust acts on the downstream section of the penstock, the water thrust is transmitted through the gripping force between the downstream section of the penstock and the backfill concrete; therefore, the gripping force of the backfill concrete on the downstream section of the penstock is determined, and the calculated value of the gripping force safety factor is obtained according to the ratio of the gripping force of the backfill concrete on the downstream section of the penstock to the water thrust; if the calculated value of the gripping force safety factor is higher than the designed value of the gripping force safety factor, the water thrust can be effectively transmitted and a thrust ring is not set; otherwise, a thrust ring needs to be set and step 4 is executed; Step 4: Formulate the design parameters of the thrust ring and the backfill concrete; Step 5: Based on the design parameters of the thrust ring designed in step 4, determine whether the bearing capacity of the thrust ring, the thrust ring root pressure steel pipe, and the thrust ring root weld meets the requirements; if the requirements are met, proceed to step 6; otherwise, return to step 4 and re-draft the design parameters of the thrust ring; Step 6: Based on the design parameters of the backfill concrete designed in step 4, determine whether the local compressive bearing capacity and the punching shear bearing capacity of the backfill concrete meet the requirements; if they meet the requirements, proceed to step 7; otherwise, return to step 4 and re-draft the design parameters of the backfill concrete; Step 7: Based on the determined design parameters of the thrust ring and the design parameters of the backfill concrete, determine whether the overall anti-sliding stability between the backfill concrete and the surrounding rock meets the requirements; if the requirements are met, proceed to step 8; Step 8: Determine whether the bearing capacity of the surrounding rock meets the requirements; if it does, the design process ends and the final design parameters of the thrust ring and the backfill concrete are obtained.

2. The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station according to claim 1 is characterized in that: Step 3 is as follows: Step 31, using formula (3-1), determine the water thrust F borne by the downstream section of the penstock: Where: F is the water thrust; γ Q is the action partial coefficient; ψ is the operating coefficient; P is the water hammer pressure in the downstream section of the penstock; D is the inner diameter of the downstream section of the penstock; Step 32, using formula (3-2), determine the holding force F of the backfill concrete on the downstream section of the penstock τ : F τ =c p lπD0 (3-2) Among them: F τ is the grip strength; c p is the design value of the bond strength between the downstream section of the penstock and the backfill concrete; l is the length of the downstream section of the penstock; D0 is the outer diameter of the downstream section of the penstock; Step 33, set the design value K of the safety factor of the backfill concrete holding force on the downstream section of the penstock τ ; Determine whether formula (3-3) is established: in: is the calculated value of the safety factor of the holding force; If formula (3-3) holds true, it means that the backfill concrete has a strong grip F on the downstream section of the penstock. τ The safety margin is sufficient and the safety factor of the holding force meets the design requirements. Therefore, a thrust ring is not required; otherwise, a thrust ring is required.

3. The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station according to claim 2 is characterized in that: Step 4 is as follows: The design parameters of the thrust ring include the setting position, number n, height h, thickness t of the thrust ring, and the size w of the thrust ring root weld between the thrust ring and the downstream section of the pressure steel pipe; wherein the thrust rings are distributed at equal intervals and avoid the annular weld of the pressure steel pipe, and the height h and thickness t of each thrust ring are the same; The design parameters of the backfill concrete include concrete strength and backfill concrete thickness.

4. The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station according to claim 3 is characterized in that: Step 5 is as follows: After the thrust ring is set, under the action of water thrust, the thrust ring, the pressure steel pipe at the root of the thrust ring, and the weld at the root of the thrust ring are all under stress; the setting of the thrust ring, that is, the design parameters of the thrust ring should make the bearing capacity of the above three meet the requirements, so as to ensure the safety and reliability of the thrust ring, and then ensure the effective transmission of water thrust from the downstream section of the pressure steel pipe to the backfill concrete.

5. The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station according to claim 4 is characterized in that: Step 5 is as follows: Step 51, determine the water thrust F borne by each thrust ring i i : Specifically, assuming that n thrust rings are provided, the water thrust F borne by the downstream section of the penstock is borne entirely by n thrust rings, i.e., F τ =0; Considering the integrity of the penstock and the instantaneous nature of the water thrust, it is assumed that the water thrust F is borne by the n thrust rings set up on average. Therefore, through formula (5-1), the water thrust F borne by the thrust ring i is obtained i : Where: n is the number of thrust rings; F i The water thrust borne by the thrust ring i, i = 1, 2, ..., n; Step 52, determine whether the bearing capacity of each thrust ring i meets the requirements: Step 521, for the thrust ring i, assume that the water thrust F it bears i , along the height direction of the thrust ring i, it is distributed in a triangular state, the top force is 0, and the force at the root of the thrust ring, that is, the welding point with the downstream section of the pressure steel pipe, is the maximum value q0; the water thrust F i A bending moment is formed at the root of the thrust ring, which acts on the downstream section of the pressure steel pipe, causing bending stress to be generated at the root of the thrust ring. Using formula (5-2), the bending stress σ generated at the root of the thrust ring i due to the bearing water thrust is obtained: r : Where: I r is the moment of inertia of the root of thrust ring i; t is the thickness of thrust ring i; M0 is the bending moment generated at the root of the thrust ring i due to the water thrust, which is determined by formula (5-3): Where: q0 is the water thrust F borne by the thrust ring i i The maximum force on the root when it is triangularly distributed along its height direction; h is the height of the thrust ring i; Step 522, using formula (5-4) to determine the resistance limit σ of the pressure steel pipe and thrust ring R : Where: γ0 is the structural importance coefficient of the pressure steel pipe and thrust ring; γ pd is the structural coefficient of the pressure steel pipe and thrust ring; f is the design value of the material strength of the pressure steel pipe and thrust ring; Step 523, determine whether formula (5-5) is established: s r ≤σ R (5-5) If it is established, the bearing capacity of the thrust ring i meets the requirements; otherwise, the bearing capacity of the thrust ring i does not meet the requirements; Step 53, determine whether the bearing capacity of the penstock at the root of each thrust ring i meets the requirements: Step 531, taking the penstock at the root of the thrust ring i as a calculation element, and simplifying its stress state into a plane problem, and satisfying the Von Mises yield criterion, the penstock at the root of the thrust ring i bears water thrust in the axial direction and water pressure in the circumferential direction; The bending moment M0 formed by the water thrust at the root of the thrust ring i needs to be balanced by the corresponding bending moment M1 generated by the pressure steel pipe at the root of the thrust ring i, and it is half on each side of the thrust ring i. Therefore, M1 is the bending moment on both sides of the penstock at the root of the thrust ring i; The existence of the bending moment M1 causes bending stress in the penstock at the root of the thrust ring i along the axial direction; Step 532, using equations (5-6), (5-7) and (5-8) respectively, obtain the axial principal stress σ1, the annular principal stress σ2 and the equivalent stress σ of the penstock at the root of thrust ring i. e : in: T is the wall thickness of the penstock at the root of the thrust ring; v is the Poisson’s ratio of steel; Step 533, determine the axial principal stress σ1 of the pressure steel pipe, the annular principal stress σ2 of the pressure steel pipe, and the equivalent stress σ e , whether they satisfy equations (5-9), (5-10) and (5-11) respectively: σ1≤σ R (5-9) σ2≤σ R (5-10) s e ≤σ R (5-11) If equations (5-9), (5-10) and (5-11) hold, the bearing capacity of the root pressure steel pipe of the thrust ring i meets the requirements; otherwise, the bearing capacity of the root pressure steel pipe of the thrust ring i does not meet the requirements; Step 54, determine whether the bearing capacity of the root weld of the thrust ring i meets the requirements: Step 541, using equations (5-12), (5-13) and (5-14), respectively obtain the bending stress S1, shear stress S2 and equivalent stress S borne by the root weld of thrust ring i e : Where: w is the root weld size of the thrust ring; q i The force formed by resisting / transmitting the bending moment of the thrust ring on the weld per unit length at the root of the thrust ring i; Step 542, determine the bending stress S1, shear stress S2 and equivalent stress S1 borne by the root weld of the thrust ring i e , whether they satisfy equations (5-15), (5-16) and (5-17) respectively: in: is the design value of fillet weld strength; If equations (5-15), (5-16) and (5-17) hold true, the bearing capacity of the root weld of thrust ring i meets the requirements; otherwise, it does not meet the requirements.

6. The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station according to claim 1 is characterized in that: Step 6 is as follows: When the water thrust acts on the thrust ring, the water thrust is transmitted through the gripping force between the downstream section of the pressure steel pipe and the backfill concrete, and also directly acts on the backfill concrete through the thrust ring. The thrust ring acts on the backfill concrete and makes the backfill concrete bear local compression and shearing conditions. The proposed design parameters of the backfill concrete should make the local compression and shearing bearing capacity of the backfill concrete meet the requirements to ensure the safety and reliability of the backfill concrete, thereby ensuring that the water thrust is effectively transmitted under the overall action of the backfill concrete.

7. The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station according to claim 6 is characterized in that: Step 6 is as follows: Step 61, determine whether the local compressive bearing capacity of the backfill concrete meets the requirements: Step 611, determine the local compressive bearing capacity F of the backfill concrete p : After the thrust ring i bears the water thrust, it acts on the backfill concrete. The backfill concrete is in a local compressive state. Using formula (6-1), the local compressive bearing capacity F of the backfill concrete is obtained. p : Where: γ cd is the structural coefficient of backfill concrete; is the load distribution influence coefficient, which is 1 for uniform load and 0.75 for non-uniform load; c A is the design value of compressive strength of backfill concrete; i The local compressive area of ​​the backfill concrete is determined by formula (6-2): Where: D h is the outer diameter of the thrust ring i; D0 is the outer diameter of the downstream section of the penstock; Step 612, determine whether equation (6-3) holds true: F i ≤F p (6-3) Among them: F i The water thrust F borne by the thrust ring i i ; If it is established, the local compressive bearing capacity of the backfill concrete meets the requirements; otherwise, it does not meet the requirements; Step 62, determine whether the shear bearing capacity of the backfill concrete meets the requirements: Step 621, using formula (6-4), determine the shear bearing capacity F of the backfill concrete t : in: Where: d i is the effective height of the punching failure cone; β d is the cross-section height influence coefficient, when d i When ≤0.8m, take β d =1; when d i ≥2m, take β d =0.9; the period is taken by linear interpolation; f t is the design value of tensile strength of backfill concrete; u i D is the perimeter of the punching failure cone at half the effective height; b The outer diameter of the backfill concrete; Step 622, determine whether equation (6-5) is established: F i ≤F t (6-5) If so, the shear bearing capacity of the backfill concrete meets the requirements; otherwise, it does not meet the requirements.

8. The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station according to claim 1 is characterized in that: Step 7 is as follows: Under the overall action of backfill concrete, the hydraulic thrust is transmitted to the surrounding rock through the friction resistance on the contact surface with the surrounding rock. The proposed design parameters of the backfill concrete and the selected design object length should make the friction resistance between the backfill concrete and the surrounding rock meet the overall anti-sliding stability requirements of the backfill concrete to ensure the effective transmission of the hydraulic thrust from the backfill concrete to the surrounding rock.

9. The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station according to claim 8 is characterized in that: Step 7 is as follows: Step 71, determine the frictional resistance between the backfill concrete and the surrounding rock: Specifically, assuming that the friction resistance between the surrounding rock and the backfill concrete is uniformly distributed, the anti-sliding bearing capacity F between the backfill concrete and the surrounding rock is obtained by formula (7-1): c : F c =c c πD b L (7-1) Among them: F c is the anti-sliding bearing capacity between backfill concrete and surrounding rock; c c is the cohesion between backfill concrete and surrounding rock; D b is the outer diameter of the backfill concrete; L is the distance from the upstream starting point of the transition section of the penstock to the upstream side wall of the plant, which is the length of the design object; Step 72: Set the overall anti-sliding stability safety factor design value K between the backfill concrete and the surrounding rock c ; Determine whether formula (7-2) is established: in: is the calculated value of the overall anti-sliding stability safety factor between the backfill concrete and the surrounding rock; F is the water thrust; If formula (7-2) is established, it means that the overall anti-sliding stability between the backfill concrete and the surrounding rock meets the requirements; if formula (7-2) is not established, other measures such as expanding the tunnel diameter, extending and ensuring the friction length should be taken to ensure the establishment of formula (7-2).

10. The design method of the thrust ring at the end of the buried pressure steel pipe of a hydropower station according to claim 1, characterized in that: Step 8 is as follows: When the proposed design parameters of the thrust ring and the backfill concrete meet the requirements of the above steps, the water thrust is transmitted to the surrounding rock. When the bearing capacity of the surrounding rock meets the requirements, the water thrust is no longer transmitted. The specific process of step 8 is as follows: Step 81, determine the shear bearing capacity F of the surrounding rock cone r : After the water thrust is transmitted to the surrounding rock, it is borne by the cone-shaped rock mass. Assuming that the strength of the surrounding rock is 0, the surrounding rock is borne only by cohesion. The shear bearing capacity F of the surrounding rock cone is obtained by formula (8-1): r : in: F r is the shear bearing capacity of the surrounding rock cone; c r A is the cohesion of surrounding rock mass; r is the surface area of ​​the frustum; r1 is the radius of the circular top of the frustum, D b The outer diameter of the backfill concrete; r2 is the radius of the bottom circle of the cone, l r is the height of the cone, that is, the effective friction section length between the backfill concrete and the surrounding rock; c c K is the cohesion between backfill concrete and surrounding rock; c is the design value of the overall anti-sliding stability safety factor between the backfill concrete and the surrounding rock; F is the water thrust; Step 82: Set the design value of the safety factor of surrounding rock bearing capacity K r ; Determine whether formula (8-2) is established: in: is the calculated value of safety factor of surrounding rock bearing capacity; If formula (8-2) is true, it means that the bearing capacity of the surrounding rock meets the requirements and the water thrust is ultimately borne by the surrounding rock; if formula (8-2) is not true, other measures to strengthen the rock mass should be taken to ensure the validity of formula (8-2).

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