Design method for pressing bearing capacity of variable cross-section anchor group foundation

Abstract

The invention discloses a novel design method for pressing bearing capacity of a variable cross-section anchor group foundation. The design method comprises the following steps: calculating a pressing bearing capacity characteristic value Na of a single variable cross-section anchor rod; taking the variable cross-section anchor rod as a reinforcement body, and calculating a composite foundation bearing capacity characteristic value fspk of the anchor rod and the foundation soil according to the Na value; calculating the maximum pressure stress pmax of the bottom surface of the bearing platform; the composite foundation bearing capacity characteristic value fspk is compared with the maximum pressure stress pmax of the bottom face of the bearing platform, and if pmax is smaller than or equal to fspk, design of the variable cross-section group anchor foundation is completed; if pmax is greater than fspk, repeatedly iterating by adjusting the diameter of the anchor rod, increasing the number of the anchor rod or increasing the size of the anchor rod and the like, and stopping calculation until pmax is less than or equal to fspk; according to the calculation method, the formula is simple and convenient, the coupling bearing characteristics of the foundation and the anchoring body can be brought into full play, the calculation precision and the safety margin are both achieved, and the bearing capacity of the anchor rod foundation can be evaluated more accurately and reasonably. By adopting the calculation method, the foundation size can be optimized, the engineering cost can be effectively reduced, and the calculation method has remarkable engineering significance on environmental protection and adaptability to topographic conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of power transmission line foundation design, and in particular to a method for designing the downward pressure bearing capacity of a variable-section group anchor foundation. Background Art

[0002] Large-scale hydropower and wind power UHV transmission and their supporting transmission line projects are entering a construction boom. However, the intensive grid construction is leading to increasingly tight transmission corridors, with an increasing proportion of lines passing through steep mountainous areas.

[0003] In recent years, State Grid Corporation of China and China Southern Power Grid Corporation have been actively promoting the use of fully mechanized construction in power grid infrastructure. While mechanized construction is relatively high in plain and hilly areas due to the convenient access of machinery, mountainous areas have a relatively low level of mechanization due to factors such as the high cost of building access roads for large machinery and the significant damage to the ecological environment. Given the current dual requirements for mechanization and environmental and water conservation in mountainous transmission line construction, anchor foundations are a resource-saving and environmentally friendly foundation type, as they fully utilize the subsoil's pull-out bearing capacity, offer a high degree of mechanization, require compact construction equipment, and minimize environmental damage. In recent years, they have become a popular choice for overhead transmission line foundations.

[0004] However, due to the small cross-section of the anchor rods, the contribution of the anchor rods to the foundation's downward bearing capacity is ignored during the design process of the traditional anchor foundation. Only the effect of the foundation reaction force at the bottom of the pedestal is considered, and the bearing capacity of the covering soil layer is relatively low. When the pedestal is placed in the covering layer, in order to meet the bearing capacity requirements, the pedestal size is usually large. In order to reduce the excavation volume, the anchor foundation pedestal needs to be rock-embedded to meet the downward bearing capacity requirements. However, under the conditions of thick covering layers, the pedestal depth and excavation volume increase significantly, which makes its environmental protection advantage no longer available, limiting the application of anchor foundations in areas with thick covering layers.

[0005] To solve the above problems, engineering technicians proposed a new type of variable-section anchor solution. The variable-section anchor foundation increases the diameter of the anchor section in the upper covering layer, improves the anchor stiffness and downward bearing capacity, and achieves the effect of jointly resisting the downward load with the pedestal, thereby eliminating the requirement for the pedestal to be embedded in rock, reducing the burial depth and excavation volume of the pedestal, and can be used under geological conditions with a covering layer thickness of 3 to 5 meters, greatly expanding the application range of the anchor foundation.

[0006] However, the calculation method of the downward pressure bearing capacity of variable-section anchor foundations is still in the exploratory stage, and the existing calculation methods cannot accurately evaluate the bearing capacity of anchor foundations. Therefore, it is necessary to propose an accurate downward pressure design method for variable-section group anchor foundations to promote the engineering application of new variable-section anchor foundations. Summary of the Invention

[0007] In view of the shortcomings of the existing technology, the present invention proposes a method for calculating the downward pressure bearing capacity of a variable-section group anchor foundation. The present invention takes into account the contribution of anchor rods to the downward pressure bearing capacity and has good calculation accuracy and practicality.

[0008] To achieve the above objectives, the technical solution of the present invention is:

[0009] The present invention provides a method for calculating the downward pressure bearing capacity of a variable-section anchor group foundation, comprising the following steps:

[0010] S1: Calculate the characteristic value N of the compressive bearing capacity of a single variable cross-section anchor a ;

[0011] S2: Consider the variable cross-section anchor as a reinforcement and calculate the composite foundation bearing capacity characteristic value f of the anchor and foundation soil spk ;

[0012] S3: Calculate the maximum compressive stress p on the bottom surface of the pedestal max ;

[0013] S4: Comparison of the bearing capacity characteristic value f of composite foundation spk The maximum compressive stress p on the bottom surface of the pedestal max , if p max ≤f spk , then the design of variable-section group anchor foundation is completed;

[0014] If p max >f spk , then adjust the anchor diameter, increase the number of anchors or increase the size of anchors, and repeat S1 to S3 until p max ≤f spk Stop calculation when .

[0015] The bearing capacity of a single variable cross-section anchor is borne by the lateral friction resistance of the rock-embedded section of the anchor and the end resistance at the variable cross-section. During the calculation process, it is assumed that the lateral friction resistance between each rock layer and the anchor is evenly distributed. The calculation formula is as follows:

[0016]

[0017] Where:

[0018] N a — Characteristic value of the compressive bearing capacity of a single variable-section anchor (kN);

[0019] d i —diameter of anchor rod in the i-th rock layer (m);

[0020] L i — length of anchor rod in the i-th rock layer (m);

[0021] τ mi—The lateral friction resistance between the rock layer i and the anchor interface, which is determined based on on-site geological survey tests (kPa);

[0022] S—annular area at variable cross section (m 2 )

[0023] p u —Ultimate compressive strength of rock mass, determined based on on-site geological survey tests (kPa).

[0024] The composite foundation bearing capacity characteristic value f of the anchor rod and foundation soil is spk The calculation formula is as follows:

[0025]

[0026] Where:

[0027] f spk —Eigenvalue of bearing capacity of composite foundation after considering reinforcement with variable cross-section anchor rods (kPa);

[0028] f sk — Bearing capacity characteristic value of soil layer at the bottom of the cap (kPa), which is determined based on the on-site geological survey report;

[0029] N a — Characteristic value of the compressive bearing capacity of a single variable-section anchor (kN);

[0030] A p — cross-sectional area of anchor bolt variable section (m 2 );

[0031] β—the bearing capacity coefficient of soil between anchors, which can be 1.0 to 1.1;

[0032] m—area replacement rate, the sum of the cross-sectional areas of the anchor rods in the overburden (nA p ) and the bottom area of the pedestal (A c ),

[0033] The maximum compressive stress p of the foundation soil at the bottom of the cap max The calculation method is shown in the following formula:

[0034]

[0035] If p min ≤0, p max It should be calculated as follows:

[0036]

[0037] The calculation meets the following requirements:

[0038]

[0039] Where:

[0040] p max — Maximum compressive stress of foundation soil at the bottom of the cap (kPa);

[0041] p min —Minimum compressive stress of foundation soil at the bottom of the cap (kPa);

[0042] F k —Standard value of vertical pressure transmitted from the superstructure to the bottom surface of the foundation (kN);

[0043] G k — Self-weight of foundation and weight of soil on foundation (kN);

[0044] A c —Bottom area of pedestal (m 2 );

[0045] M kx 、M ky —Standard values of the moments in the x and y directions acting on the bottom surface of the pedestal (kN·m);

[0046] W x 、W y —Resistance moment of foundation bottom around x and y axis (m 3 );

[0047] e x 、e y —Eccentricity of the axial forces acting on the bottom surface of the foundation in the x-direction and y-direction (m);

[0048] b—M ky Side length of the bottom surface of the bearing platform in the direction of action (m);

[0049] l—M kx Side length of the bottom surface of the pedestal in the direction of action (m).

[0050] The beneficial effects of the present invention are:

[0051] 1. The method of the present invention calculates the composite foundation bearing capacity of the anchor rod group and foundation soil by considering the anchor rods as reinforcement bodies. The present invention takes into account the contribution of the anchor rods to the foundation's downward pressure bearing capacity, solves the problem that traditional design methods are too conservative, reduces redundant designs, optimizes structural economy, reduces engineering costs, and has significant engineering significance for environmental protection, soil and water conservation, and adaptation to terrain conditions.

[0052] 2. The method of the present invention supports the design of variable-section anchor foundations, promotes the engineering application of new anchor foundations, and can expand the application scenarios of anchor foundations under complex terrain and soil conditions. It has significant engineering significance for improving the mechanized construction rate of transmission line foundations.

[0053] 3. The novel design method for the downward compressive bearing capacity of a variable-section group anchor foundation proposed in this invention can accurately evaluate the downward compressive bearing capacity of the group anchor foundation. The error compared with the numerical simulation analysis does not exceed 15%, and it has good calculation accuracy and safety margin. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 This is a flow chart of the calculation method of the present invention.

[0055] Figure 2 This is a schematic diagram of the variable-section anchor group foundation structure of the present invention.

[0056] Figure 3 It is a schematic diagram of the compression resistance of a variable-section single anchor according to the present invention.

[0057] Among them: foundation column 1; anchor rod variable section 2; anchor rod constant section 3; ground line 4; rock surface line 5. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solutions and advantages of the invention clearer, the present invention is further described below with reference to the accompanying drawings.

[0059] The novel variable cross-section anchor group foundation of the present invention is as follows Figure 2 As shown, the new variable-section anchor foundation includes a pedestal column 1, an anchor variable-section segment 2 and an anchor equal-section segment 3. The diameter of the anchor variable-section segment 2 is larger than the diameter of the anchor equal-section segment 3. The anchor equal-section segment 3 is placed in the rock stratum, and the anchor equal-section segment 3 is located below the rock surface line 5. Most of the variable-section segment anchors are placed in the covering layer, and a small part is embedded in the rock. Most of the pedestal column 1 is placed in the covering layer, and the top of the pedestal column 1 protrudes from the ground line 4. Figure 2 A represents the foundation reaction force.

[0060] The variable-section anchor group foundation compressive bearing capacity design method regards the variable-section anchor rods as reinforcement bodies. The variable-section anchor rods and the weak covering layer form a composite foundation to jointly resist the compressive load transmitted by the pedestal columns.

[0061] The bearing capacity of a variable cross-section single anchor is composed of three parts: the lateral friction between the stratum and the anchor rod, the end resistance at the variable cross-section, and the end resistance at the bottom of the anchor rod. Studies have shown that the lateral friction in the overburden and the end resistance at the bottom of the anchor rod are relatively small, so the contributions of the above two parts can be ignored in the calculation and used as safety redundancy. Therefore, when calculating the characteristic value of the bearing capacity of a variable cross-section single anchor, only the contributions of the lateral friction between the rock stratum and the anchor rod in the rock-embedded section and the end resistance at the variable cross-section are considered. For example, Figure 3 As shown, Figure 3 Where B is the end resistance at the variable cross-section, and C is the side friction resistance of the rock-embedded section. It is assumed that the side friction resistance is evenly distributed in each rock layer. The formula for calculating the characteristic value of the compressive bearing capacity of a single anchor with a variable cross-section is as follows:

[0062]

[0063] Where:

[0064] N a — Characteristic value of the compressive bearing capacity of a single variable-section anchor (kN);

[0065] d i —diameter of anchor rod in the i-th rock layer (m);

[0066] L i — length of anchor rod in the i-th rock layer (m);

[0067] τ mi —The lateral friction resistance between the rock layer i and the anchor interface, which is determined based on on-site geological survey tests (kPa);

[0068] S—annular area at variable cross section (m 2 )

[0069] p u —Ultimate compressive strength of rock mass, determined based on on-site geological survey tests (kN).

[0070] After calculating the bearing capacity of a variable-section single anchor, the variable-section anchor is considered as a reinforcement in the overburden layer. The strength and bearing area of the foundation soil and anchor are used to calculate the characteristic bearing capacity of the composite foundation. The calculation method is as follows:

[0071]

[0072] Where:

[0073] f spk —Eigenvalue of bearing capacity of composite foundation after considering reinforcement with variable cross-section anchor rods (kPa);

[0074] f sk — Bearing capacity characteristic value of soil layer at the bottom of the cap (kPa), which is determined based on the on-site geological survey report;

[0075] N a — Characteristic value of the compressive bearing capacity of a single variable-section anchor (kN);

[0076] A p — cross-sectional area of anchor bolt variable section (m 2 );

[0077] β—the bearing capacity coefficient of soil between anchors, which can be 1.0 to 1.1;

[0078] m—area replacement rate, the sum of the cross-sectional areas of the anchor rods in the overburden (nA p ) and the bottom area of the pedestal (A c ),

[0079] Consider the cap columns as extended foundations placed on the composite foundation and calculate the maximum stress p of the foundation soil at the bottom of the cap. max , the calculation formula is as follows:

[0080]

[0081] If p min ≤0, p max It should be calculated as follows:

[0082]

[0083] The calculation meets the following requirements:

[0084]

[0085] Where:

[0086] p max — Maximum compressive stress of foundation soil at the bottom of the cap (kPa);

[0087] p min —Minimum compressive stress of foundation soil at the bottom of the cap (kPa);

[0088] F k —Standard value of vertical pressure transmitted from the superstructure to the bottom surface of the foundation (kN);

[0089] G k — Self-weight of foundation and weight of soil on foundation (kN);

[0090] A c —Bottom area of pedestal (m 2 );

[0091] M kx 、M ky —Standard values of the moments in the x and y directions acting on the bottom surface of the pedestal (kN·m);

[0092] W x 、W y —Resistance moment of foundation bottom around x and y axis (m 3 );

[0093] e x 、e y —Eccentricity of the axial forces acting on the bottom surface of the foundation in the x-direction and y-direction (m);

[0094] b—M ky Side length of the bottom surface of the bearing platform in the direction of action (m);

[0095] l—M kx Side length of the bottom surface of the pedestal in the direction of action (m).

[0096] Finally, the bearing capacity of the composite foundation reinforced with variable cross-section anchor rods was compared. spk The maximum compressive stress p of the foundation soil at the bottom of the cap max , when f spk ≥p max When f spk <p max When f is calculated, the size of cross-section anchor rods, the number of anchor rods, or the area of the bottom of the cap can be modified. spk , until the bearing capacity of the anchor foundation reaches the requirement.

[0097] It can also be calculated by the maximum compressive stress p of the foundation soil at the bottom of the pedestal. max The maximum downward pressure load that the foundation can withstand is calculated by the calculation method, and compared with the actual load to verify whether the bearing capacity of the anchor foundation meets the requirements.

[0098] Taking a certain UHV transmission line project as an example, the overburden is silty clay with thicknesses of 2m, 2.5m, and 3m; the burial depth of the base is 1m; the total length of the anchor rods is 6m, and the lengths of the variable-section sections are 1.25, 1.75, and 2.25m (0.25m embedded in rock), respectively. The uniform-section sections are all embedded in the rock stratum; the rock stratum is moderately weathered sandy conglomerate.

[0099] Nine variable-section anchors were installed. The anchor bars were made of 36mm diameter HRB400 rebar with an elastic modulus of 200GPa. The grouting material used was a high-performance grouting material with an elastic modulus of 40GPa and a Poisson's ratio of 0.2. The diameter of the variable-section anchors was 250mm, and the diameter of the uniform-section anchors was 110mm. The anchor spacing was 750mm. The platform width was 2.1m.

[0100] The novel variable-section anchor group foundation compression bearing capacity design method proposed in this invention can predict the compression bearing capacity of a single variable-section anchor rod and an anchor group foundation. The accuracy of the design method is verified by comparing the calculation results with the numerical simulation results. The results are shown in Table 1:

[0101] Table 1 Comparison of design results of bearing capacity of variable cross-section anchor group foundation

[0102]

[0103] In the above table: N1 and N2 represent the compressive bearing capacity of variable-section anchor groups obtained by theoretical calculation and numerical simulation, respectively, in kN.

[0104] As can be seen from Table 1, under the above geological conditions, the design method provided by the present invention calculates the bearing capacity of the variable cross-section anchor group foundation, which differs from the finite element results by about 15%. This error is due to the fact that the calculation process of the variable cross-section single anchor neglects the side friction resistance of the overburden and the end resistance of the anchor rod bottom. Therefore, it is believed that the design method for the bearing capacity of the variable cross-section anchor group proposed by the present invention can meet the accuracy requirements of actual engineering applications and has a certain safety redundancy.

[0105] In summary, the new variable-section group anchor compression bearing capacity design method proposed in the present invention has a clear mechanical concept. It is the first time that a variable-section single anchor and group anchor foundation compression bearing capacity design method is proposed. It also takes into account the contribution of anchor rods to the foundation compression bearing capacity for the first time. The formula is simple and has both calculation accuracy and safety margin.

[0106] Finally, it should be noted that the contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field. The above description is only the preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to replace some of the technical features therein with equivalents. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A new method for calculating the downward bearing capacity of a variable-section anchor group foundation, characterized in that: The method comprises: S1: Calculate the characteristic value N of the compressive bearing capacity of a single variable cross-section anchor a ; S2: Consider the variable cross-section anchor as a reinforcement, according to the N a Calculation of the bearing capacity characteristic value f of composite foundation considering the reinforcement of variable cross-section anchors spk ; S3: Calculate the maximum compressive stress p on the bottom surface of the pedestal max ; S4: Comparison of composite foundation bearing capacity characteristic values f spk The maximum compressive stress p on the bottom surface of the pedestal max , if p max ≤f spk , then the design of variable-section group anchor foundation is completed; If p max >f spk , then adjust the anchor diameter, increase the number of anchors or increase the size of anchors, and repeat S1 to S3 until p max ≤f spk Stop calculation when .

2. The method for calculating the downward bearing capacity of a new type of variable-section anchor group foundation according to claim 1 is characterized in that: The bearing capacity of a single variable cross-section anchor is borne by the side friction resistance of the anchor rock section and the end resistance of the variable cross-section. During the calculation process, it is assumed that the side friction resistance between each rock layer and the anchor is evenly distributed. The calculation formula is as follows: Where: N a — Characteristic value of the compressive bearing capacity of a single variable-section anchor (kN); d i —diameter of anchor rod in the i-th rock layer (m); L i — length of anchor rod in the i-th rock layer (m); τ mi —lateral friction between the i-th rock layer and the anchor interface (kPa); S—annular area at variable cross section (m 2 ); p u —Ultimate compressive strength of rock mass (kPa).

3. The method for calculating the downward bearing capacity of a new type of variable-section anchor group foundation according to claim 1 is characterized in that: The bearing capacity characteristic value f of the composite foundation considering the variable cross-section anchor reinforcement spk The calculation formula is as follows: Where: f spk —Emative value of bearing capacity of composite foundation after considering reinforcement with variable cross-section anchor rods (kPa); f sk — Bearing capacity characteristic value of soil layer at the bottom of the cap (kPa); N a — Characteristic value of the compressive bearing capacity of a single variable-section anchor (kN); A p —Cross-sectional area of anchor bolt variable section (m 2 ); β— bearing capacity coefficient of soil between anchors; m—area replacement rate, the sum of the cross-sectional areas of the anchor rods in the overburden (nA p ) and the bottom area of the pedestal (A c ), 4. The method for calculating the downward bearing capacity of a new type of variable-section anchor group foundation according to claim 1 is characterized in that: The maximum compressive stress p of the foundation soil at the bottom of the cap max The calculation method is shown in the following formula: If p min ≤0, p max It should be calculated as follows: The calculation meets the following requirements: Where: p max — Maximum compressive stress of foundation soil at the bottom of the cap (kPa); p min —Minimum compressive stress of foundation soil at the bottom of the cap (kPa); F k —Standard value of vertical pressure transmitted from the superstructure to the bottom surface of the foundation (kN); G k — Self-weight of foundation and weight of soil on foundation (kN); A c —Bottom area of pedestal (m 2 ); M kx 、M ky —Standard value of the moment in the x and y directions acting on the bottom surface of the pedestal (kN·m); W x 、W y —Resistance moment of foundation bottom around x and y axis (m 3 ); e x 、e y —Eccentricity of the axial forces acting on the bottom surface of the foundation in the x-direction and y-direction (m); b—M ky Side length of the bottom surface of the bearing platform in the direction of action (m); l—M kx Side length of the bottom surface of the pedestal in the direction of action (m).

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