Gravity anchor design method and gravity anchor

Abstract

The invention provides a gravity anchor design method and a gravity anchor, and the gravity anchor design method comprises the following steps: carrying out field investigation on a rock mass at the mounting position of a pre-stressed anchor cable, and determining the type of the rock mass and an adopted grouting body; obtaining basic data according to the type of the rock mass and the grouting body, and measuring the limit bonding strength between the rock mass and the grouting body; establishing a design model according to the basic data and the limit bonding strength; the ultimate tension of the main cable and the self-weight of the gravity anchor are obtained, the ultimate tension and the self-weight of the gravity anchor are imported into the design model, design parameters are obtained, and the pre-stressed anchor cable is selected according to the design parameters. The problems that in the prior art, a gravity type anchorage foundation is large in pouring amount, large in size, high in pouring and excavation engineering strength and long in consumed time are solved.

Description

Technical Field

[0001] The invention relates to the technical field of bridge engineering, and in particular to a gravity anchor design method and a gravity anchor. Background Art

[0002] Anchors are important components of suspension bridges and cable-stayed bridges. As an important part of suspension bridges, anchors transfer the tension of the main cable to the foundation. Their stability is directly related to the safety of the entire suspension bridge. Therefore, studying anchors is an indispensable part.

[0003] Traditionally, gravity anchors are mainly used. Gravity anchors are large in size and take a long time to construct. Although different types of anchors have emerged based on traditional gravity anchors, such as toothed ridge gravity anchors and ground-connected wall gravity anchors, which stabilize the overall structure by adding foundations or changing the foundation structure of the anchors to reduce the amount of foundation concrete pouring, in today's projects, the foundation pouring volume is still too large, and the corresponding rock excavation volume is also large. The pouring and excavation projects are high in intensity, time-consuming, and have poor timeliness. Therefore, how to reduce the dead weight of gravity anchors, save costs, and at the same time ensure the overall stability and performance of the anchors and improve the timeliness of the project is a problem that needs to be solved urgently. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a gravity anchor design method and a gravity anchor, which solve the problems in the prior art of large pouring volume and volume of the foundation of the gravity anchor, high intensity of pouring and excavation works, and long time consumption.

[0005] According to an embodiment of the present invention, a gravity anchor design method includes:

[0006] Conduct a field survey of the rock mass at the location where the prestressed anchor cables are installed to determine the rock mass type and the grouting material to be used;

[0007] Obtain basic data based on rock mass type and grouting body, and determine the ultimate bond strength between rock mass and grouting body;

[0008] Establish a design model based on basic data and ultimate bonding strength;

[0009] The ultimate tension of the main cable and the deadweight of the gravity anchor are obtained, and the ultimate tension and the deadweight of the gravity anchor are imported into the design model to obtain the design parameters, and the prestressed anchor cable is selected according to the design parameters.

[0010] Preferably, the basic data includes anchor hole diameter, safety factor and anchor cable strength coefficient, and the anchor cable strength coefficient is 0.55-0.65;

[0011] The design parameters include an anchor cable combination, an anchoring section length and an anchoring depth, and the anchor cable combination includes the number of steel strands and the diameter of the steel strands.

[0012] Preferably, the process of importing the ultimate tension and the deadweight of the gravity anchor into the design model to obtain the design parameters includes:

[0013] The ultimate tension minus the deadweight of the gravity anchor is used to obtain the working anchoring force of the anchor cable. The length of the anchoring section is obtained based on the safety factor and the working anchoring force of the anchor cable.

[0014] Determine the rock mass strength and the contact surface strength between the grouting body and the rock mass;

[0015] Compare the contact surface strength with the rock mass strength, determine the rock mass failure mode based on the comparison results and calculate the corresponding ultimate pull-out force;

[0016] The cross-sectional area of ​​the anchor cable is obtained according to the strength coefficient of the anchor cable and the ultimate pull-out force, and the anchor cable combination is selected according to the cross-sectional area of ​​the anchor cable;

[0017] The rock shear strength at the installation location is measured, and the anchoring depth is obtained based on the rock shear strength and the anchor cable combination.

[0018] Preferably, the method for determining the rock mass failure mode according to the comparison results includes:

[0019] If the contact surface strength is less than the rock mass strength, it is determined to be the first failure mode;

[0020] If the contact surface strength is greater than the rock mass strength, it is determined to be the second failure mode;

[0021] If the contact surface strength is equal to the rock mass strength, it is determined to be the third failure mode.

[0022] Preferably, after selecting the prestressed anchor cable, it is also necessary to calculate the theoretical shear stress between the anchor cable and the grouting body, and then measure the actual shear stress between the prestressed anchor cable and the grouting body through a direct tension-shear test. If the difference between the theoretical shear stress and the actual shear stress is within the error range, it is determined that the prestressed anchor cable can be used directly. If it is not within the error range, the anchor cable combination is reselected and subsequent design is carried out.

[0023] On the other hand, according to an embodiment of the present invention, there is also provided a gravity anchor, which uses the above-mentioned gravity anchor design method, including:

[0024] A front anchor chamber, a rear anchor chamber, a loose-cable saddle pier, a foundation and a plurality of prestressed anchor cables, wherein the rear anchor chamber and the loose-cable saddle pier are both fixed on the foundation, one end of the front anchor chamber is fixedly connected to the rear anchor chamber, and the other end is fixedly connected to the upper end of the loose-cable saddle pier, one end of all anchor cables is fixedly connected to the rear anchor chamber, and the other end passes through the front anchor chamber and is fixedly connected to the main cable, and the prestressed anchor cables are evenly distributed and fixed on the lower side of the foundation.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] Reduce the volume and deadweight of the foundation, set prestressed anchor cables under the foundation to connect with the rock mass, use the pull-out force generated by the prestressed anchor cables to replace the reduced weight of the foundation to offset the main cable tension, thereby reducing the foundation pouring volume of the gravity anchor, reducing the volume and the intensity of the excavation and pouring projects. At the same time, the design parameters of the prestressed anchor cables are adaptively designed so that the prestressed anchor cables can meet the stability of the composite anchor structure after installation and will not cause the structure to overturn. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a structural diagram of a gravity anchor according to an embodiment of the present invention.

[0028] Figure 2 The present invention is a flowchart of prestressed anchor cable design according to an embodiment of the present invention.

[0029] Figure 3 Schematic diagram of three rupture modes according to an embodiment of the present invention.

[0030] In the above drawings: front anchor chamber 1, rear anchor chamber 2, loose cable saddle pier 3, foundation 4, prestressed anchor cable 5, rock mass 6. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0032] like Figure 1 As shown, an embodiment of the present invention proposes a gravity anchor, a front anchor chamber 1, a rear anchor chamber 2, a loose cable saddle pier 3, a foundation 4 and a plurality of prestressed anchor cables 5, the rear anchor chamber 2 and the loose cable saddle pier 3 are both fixed on the foundation 4, one end of the front anchor chamber 1 is fixedly connected to the rear anchor chamber 2, and the other end is fixedly connected to the upper end of the loose cable saddle pier 3, one end of all anchor cables is fixedly connected to the rear anchor chamber 2, and the other end passes through the front anchor chamber 1 and is fixedly connected to the main cable, and the prestressed anchor cables 5 are evenly distributed and fixed on the lower side of the foundation 4.

[0033] The point of action of the tension generated by the main cable on the gravity anchor is generally at the connection between the loose cable saddle pier 3 and the front anchor chamber 1 (just at the top of the entire gravity anchor). In this embodiment, taking the direction of the tension toward the upper left as an example, the tension of the main cable can be decomposed into a horizontal component directed horizontally to the left and a vertical component directed vertically upward. The horizontal component is offset by the foundation reaction force and base friction force provided by the embedded resistance between the foundation 4 and the rock mass 6. Since the material strength is high enough, there is almost no deformation and damage compared with the force exerted by the structure on the foundation.

[0034] Therefore, in the general environment, the horizontal slip of the structure can be ignored, and only the vertical displacement can be considered, that is, the deadweight of the gravity anchor plus the pull-out resistance provided by the prestressed anchor cable 5 can offset the vertical component of the main cable tension so that it can meet the stability of the composite anchor structure and will not cause the structure to overturn.

[0035] On the other hand, Figure 2 In order to ensure that the prestressed anchor cable used can just offset (or be greater than) the vertical component of the main cable tension together with the gravity anchor deadweight during installation, and reduce the deadweight of the foundation under the premise that the deadweight of other structures outside the gravity anchor foundation remains unchanged, it is necessary to perform parameter design on the prestressed anchor cable. The embodiment of the present invention also provides a gravity anchor design method, including the following steps:

[0036] Conduct a field survey of the rock mass at the location where the prestressed anchor cables are installed to determine the rock mass type and the grouting material to be used;

[0037] The selection of grouting body is determined according to the type of rock formation. There are mainly two types of grouting bodies:

[0038] Cement grouting fluid: It is composed of cement, sand and water. It is suitable for repairing cracks in concrete structures, enhancing structural strength and sealing performance. In rock formations, if the pores and particles of the rock formations are relatively small, ultrafine cement slurry or pure cement slurry can be used directly.

[0039] Polymer grouting fluid: It is composed of polymer resin and active agent, suitable for reinforcing soil, rock or mud, improving foundation bearing capacity and stability. For gravel formations or other rock formations with large pores, polymer grouting fluid or other suitable grouting materials can be selected.

[0040] Obtain basic data based on rock mass type and grouting body, and determine the ultimate bond strength between rock mass and grouting body;

[0041] Basic data include anchor hole diameter, safety factor, and anchor cable strength coefficient.

[0042] Generally speaking, the anchor hole diameter should be 10 to 15 mm larger than the outer diameter of the prestressed steel bar tendon, the outer diameter of the steel bar butt weld joint, or the outer diameter of the anchor that needs to pass through the hole. For example, if the diameter of the anchor is 25 mm, then the anchor hole diameter should be 50 to 75 mm.

[0043] In soil layers, the diameter of the hole for prestressed anchor cables is usually 130mm, while in rock layers it should not be less than 110mm, and the allowable deviation of the hole position should not be greater than 50mm.

[0044] The safety factor of the prestressed anchor cable is generally above 1.5. In this embodiment, the safety factor is 1.5.

[0045] The anchor cable strength coefficient is a certain proportion of the material strength of the anchor cable manufacturing material, which is generally 0.55-0.65. Prestressed anchor cables are generally made of steel strands. The material strength of steel strands has various specifications, such as 1470MPa, 1570MPa, 1670MPa, 1770MPa, 1860MPa and 1960MPa, etc. Therefore, it can be obtained that the anchor cable rupture strength = anchor cable material strength x anchor cable strength coefficient.

[0046] The ultimate bond strength between rock mass and grouting body can be determined by direct shear test.

[0047] Establish a design model based on basic data and ultimate bonding strength;

[0048] The ultimate tension of the main cable and the deadweight of the gravity anchor are obtained, and the ultimate tension and the deadweight of the gravity anchor are imported into the design model to obtain the design parameters, and the prestressed anchor cable is selected according to the design parameters.

[0049] The design parameters include anchor cable combination, anchor section length, and anchor depth. The anchor cable combination includes the number of steel strands and the diameter of the steel strands. The anchor section length is the length of the prestressed anchor cable wrapped by the penguin cup grouting in the anchor hole. The anchor length is the length of the prestressed anchor cable in the anchor hole.

[0050] The process of importing the ultimate tension and the deadweight of the gravity anchor into the design model and obtaining the design parameters includes:

[0051] The ultimate tension minus the deadweight of the gravity anchor is used to obtain the working anchoring force of the anchor cable, and the anchoring section length is obtained based on the safety factor and the ultimate tension.

[0052] The vertical component of the ultimate tension, after deducting the deadweight of the gravity anchor, can give the minimum pull-out force required for the prestressed anchor cable. At this time, we regard the grouting body and the prestressed anchor cable as a whole, then the minimum pull-out force is the minimum anchor cable working anchoring force, and the anchoring section length is calculated by the following formula:

[0053]

[0054] Where L0 is the length of the anchoring section, T w is the anchoring force of the anchor cable, S f is the safety factor, D is the anchor hole diameter, τ s is the ultimate bond strength between rock mass and grouting body.

[0055] Since the calculated anchor section length is a limit value and may be affected by other factors during actual installation, the anchor section length will be slightly larger than the calculated limit value during actual installation.

[0056] Determine the rock mass strength and the contact surface strength between the grouting body and the rock mass;

[0057] Compare the contact surface strength with the rock mass strength, determine the rock mass failure mode based on the comparison results and calculate the corresponding ultimate pull-out force;

[0058] The ultimate pull-out force is the maximum force that the prestressed anchor cable and the grouting body can separate when the prestressed anchor cable is subjected to pull-out force. The ultimate pull-out force of the prestressed anchor cable depends on the failure form of the anchor cable anchor body, such as Figure 3 shown.

[0059] (1) If the contact surface strength is less than the rock mass strength, it is determined to be the first failure mode.

[0060] like Figure 3 As shown in the rupture mode on the left, in this case, the hoop tensile strength of the grouting body is low, and cracks have occurred locally. The presence of cracks causes stress redistribution in the grouting material, and the hoop stress σ of the cracked / uncracked section is obtained in the cracked area. θ The compressive stress P1 at the anchor / grout interface at the crack extension is calculated as follows;

[0061]

[0062] Where r1 is the inner diameter of the cracking zone, and r2 is the outer diameter of the cracking zone.

[0063] The crack length and the corresponding interfacial compressive stress when the crack begins to expand unstably are determined by the maximum value of the formula:

[0064]

[0065] Using the maximum tensile stress criterion, σ θ =σ Tg , σ Tg is the tensile strength of the grouting body. Substituting formula (3) into formula (2) yields:

[0066]

[0067] The ultimate pull-out force of the anchor cable occurs at the peak point of unstable crack expansion, so the ultimate pull-out force of the anchor cable can be expressed as:

[0068]

[0069] In the formula, is the internal friction angle of the grouting body, and D is the anchor hole diameter.

[0070] Formula (5) finds the derivative of l The ultimate pull-out force T of the anchor cable can be obtained u .

[0071] (2) If the contact surface strength is greater than the rock mass strength, it is determined to be the second failure mode.

[0072] like Figure 3 The fracture pattern in the middle shows that this failure occurs when the anchor length is small. The failure mechanism is shear failure of rock under a composite stress state, and its ultimate pull-out force can be deduced from the static equilibrium condition and the Mohr-Coulomb condition:

[0073]

[0074] Where l is the length of the anchoring section, c is the ultimate bonding strength between the rock mass and the grouting body, is the internal friction angle of the grouting body.

[0075] Formula (6) finds the derivative of l The ultimate pull-out force T of the anchor cable can be obtained u .

[0076] (3) If the contact surface strength is equal to the rock mass strength, it is determined to be the third failure mode.

[0077] like Figure 3 As shown in the rupture mode on the right, this type of damage is a combination of the above two types of damage, and the ultimate pull-out force is also the superposition of the first two ultimate pull-out forces:

[0078]

[0079] Find the derivative of l for formula (7) The ultimate pull-out force T of the anchor cable can be obtained u .

[0080] The cross-sectional area of ​​the anchor cable is obtained according to the strength coefficient of the anchor cable and the ultimate pull-out force, and the anchor cable combination is selected according to the cross-sectional area of ​​the anchor cable;

[0081] The calculation formula of the cross-sectional area of ​​the anchor cable is as follows:

[0082]

[0083] Where A is the cross-sectional area of ​​the anchor cable, F ptk is the breaking strength of the anchor cable.

[0084] Since each prestressed anchor cable is formed by winding multiple steel strands, the cross-sectional area of ​​each prestressed anchor cable = the number of steel strands * the cross-sectional area of ​​the steel strand = the number of steel strands * the diameter of the steel strand * π * 1 / 2.

[0085] An anchor cable combination is selected based on the cross-sectional area of ​​the anchor cable, which includes the number of steel strands and the diameter of the steel strands.

[0086] The rock shear strength at the installation location is measured, and the anchoring depth is obtained based on the rock shear strength and the anchor cable combination.

[0087] Since there are multiple prestressed anchor cables under the foundation and they are evenly distributed, the anchor depth is calculated using the following formula, taking into account the restraint between the prestressed anchor cables:

[0088]

[0089] Where a is the spacing between prestressed anchor cables and τ is the shear strength of the rock mass.

[0090] In summary, the design parameters of each prestressed anchor cable have been designed, and the prestressed anchor cable can be selected or manufactured according to the design parameters.

[0091] After the prestressed anchor cable is selected or manufactured, it is necessary to verify it to determine whether the selected or manufactured prestressed anchor cable meets the engineering requirements. The best way is to judge by the shear stress between the prestressed anchor cable and the grouting body.

[0092] First, the theoretical shear stress between the prestressed anchor cable and the grouting body is calculated under the design parameters. The calculation formula is as follows:

[0093]

[0094] Where n is the number of steel strands and d is the diameter of the steel strand.

[0095] Afterwards, the actual shear stress between the selected or manufactured prestressed anchor cable and the grouting body is measured through direct tension-shear test, and the difference between the theoretical shear stress and the actual shear stress is calculated. If the difference is within the error range, it is judged that the prestressed anchor cable meets the engineering requirements and can be used directly. If it is not within the error range, a new anchor cable combination is selected and subsequent design is carried out until the new prestressed anchor cable meets the engineering requirements.

[0096] In summary, the design parameters of the prestressed anchor cable are adaptively designed so that the prestressed anchor cable can meet the stability of the composite anchor structure after installation and will not cause the structure to overturn.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A gravity anchor design method, characterized in that: include: Conduct a field survey of the rock mass at the location where the prestressed anchor cables are installed to determine the rock mass type and the grouting material to be used; Obtain basic data based on rock mass type and grouting body, and determine the ultimate bond strength between rock mass and grouting body; Establish a design model based on basic data and ultimate bonding strength; The ultimate tension of the main cable and the deadweight of the gravity anchor are obtained, and the ultimate tension and the deadweight of the gravity anchor are imported into the design model to obtain the design parameters, and the prestressed anchor cable is selected according to the design parameters.

2. A gravity anchor design method as claimed in claim 1, characterized in that: The basic data include anchor hole diameter, safety factor and anchor cable strength coefficient, and the anchor cable strength coefficient is 0.55-0.65; The design parameters include an anchor cable combination, an anchoring section length and an anchoring depth, and the anchor cable combination includes the number of steel strands and the diameter of the steel strands.

3. A gravity anchor design method as claimed in claim 2, characterized in that: The process of importing the ultimate tension and the deadweight of the gravity anchor into the design model and obtaining the design parameters includes: The ultimate tension minus the deadweight of the gravity anchor is used to obtain the working anchoring force of the anchor cable. The length of the anchoring section is obtained based on the safety factor and the working anchoring force of the anchor cable. Determine the rock mass strength and the contact surface strength between the grouting body and the rock mass; Compare the contact surface strength with the rock mass strength, determine the rock mass failure mode based on the comparison results and calculate the corresponding ultimate pull-out force; The cross-sectional area of ​​the anchor cable is obtained according to the strength coefficient of the anchor cable and the ultimate pull-out force, and the anchor cable combination is selected according to the cross-sectional area of ​​the anchor cable; The rock shear strength at the installation location is measured, and the anchoring depth is obtained based on the rock shear strength and the anchor cable combination.

4. A gravity anchor design method as claimed in claim 3, characterized in that: Methods for determining rock mass failure modes based on comparison results include: If the contact surface strength is less than the rock mass strength, it is determined to be the first failure mode; If the contact surface strength is greater than the rock mass strength, it is determined to be the second failure mode; If the contact surface strength is equal to the rock mass strength, it is determined to be the third failure mode.

5. A gravity anchor design method as claimed in claim 4, characterized in that: When the first failure mode is determined, the ultimate pull-out force is calculated according to the following formula: Among them, T u is the ultimate pull-out force, is the internal friction angle of the grouting body, D is the anchor hole diameter, r1 is the inner diameter of the cracking zone, r2 is the outer diameter of the cracking zone, and l is the length of the anchoring section.

6. A gravity anchor design method as claimed in claim 4, characterized in that: When the second failure mode is determined, the ultimate pull-out force is calculated according to the following formula: Among them, T u is the ultimate pull-out force, l is the length of the anchoring section, c is the ultimate bonding strength between the rock mass and the grouting body, is the internal friction angle of the grouting body.

7. A gravity anchor design method as claimed in claim 3, characterized in that: After selecting the prestressed anchor cable, it is also necessary to calculate the theoretical shear stress between the anchor cable and the grouting body, and then measure the actual shear stress between the prestressed anchor cable and the grouting body through a direct tension-shear test. If the difference between the theoretical shear stress and the actual shear stress is within the error range, it is determined that the prestressed anchor cable can be used directly. If it is not within the error range, the anchor cable combination is reselected and subsequent design is carried out.

8. A gravity anchor, characterized in that: The gravity anchor uses the gravity anchor design method according to any one of claims 1 to 7, comprising: A front anchor chamber, a rear anchor chamber, a loose-cable saddle pier, a foundation and a plurality of prestressed anchor cables, wherein the rear anchor chamber and the loose-cable saddle pier are both fixed on the foundation, one end of the front anchor chamber is fixedly connected to the rear anchor chamber, and the other end is fixedly connected to the upper end of the loose-cable saddle pier, one end of all anchor cables is fixedly connected to the rear anchor chamber, and the other end passes through the front anchor chamber and is fixedly connected to the main cable, and the prestressed anchor cables are evenly distributed and fixed on the lower side of the foundation.

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