Foreign countries began to apply geotechnical anchoring technology in mining and water conservancy construction as early as the 1920s. From the 1960s to the 1980s, with the application of high-strength low-relaxation steel strands and the development of construction technology, large-scale anchorage technology Prestressed anchor cables of various tonnage are widely used. The bearing capacity of a single anchor cable reaches more than 3000kN, and the largest one reaches 16500kN.
In 1964, my country used 2400-3200kN prestressed anchor cables to reinforce the dam foundation in Meishan Reservoir in Anhui Province. In the early 1980s, my country began to use prestressed anchoring technology for landslide prevention and control, and later developed the use of prestressed anchor cable frames (lattice anchoring) to control landslides, such as the control of the K14 landslide on the secondary highway from Taiyuan to Gujiao in Shanxi, and more The most effective method is to use prestressed anchor cable frames (ground beams or anchor piers) and anti-slide piles to control landslides, and to reinforce high slopes to prevent landslides. Today, anchoring technology has been widely used in roads, mines, water conservancy, urban construction and other construction.
The prestressed anchor cable used to stabilize landslides is to set the anchoring section in the stable stratum below the sliding surface (or potential sliding surface), and pass the reaction device (pile, frame, ground beam or Anchor pier) transmits the landslide thrust into the anchor section to stabilize the landslide. Therefore, the design of the prestressed anchor cable includes the design of the anchor cable itself and the design of the reaction device.
(1) Failure forms of anchor cables
1. Types of anchor cables
According to the load transmission mode, the types of anchor cables are divided into three types, namely Straight hole friction anchor cables (including tensile anchor cables and compression anchor cables), support anchor cables, and friction-support composite anchor cables. An anchor cable with only one force transmission mode and a single free segment is called a single anchor cable. The most common one is the friction-type tension anchor cable, which is the most widely used anchor cable at present. This type of anchor cable has a simple structure and is easy to construct; however, the force transmission mechanism in the stress state is not reasonable enough, causing stress concentration in the upper part of the anchoring section, and the friction resistance is unevenly distributed along the anchoring section. When the length of the anchoring section exceeds 10m, it is difficult to increase the anchoring force. It has no obvious effect and is not conducive to preventing rust. Therefore, single-hole composite anchor cables have appeared in recent years. Any anchor cable that has two or more force-transmitting modes or steel strands with different free sections is called a single-hole composite anchor cable.
The types of single-hole composite anchor cables include: tension-dispersed anchor cables, pressure-dispersed anchor cables, tension-pressure hybrid anchor cables, expansion-hole anchor cables, hole-bottom expansion anchor cables, hole-bottom expansion anchor cables, and hole bottom anchor cables. Set up mechanical inner anchor head anchor cable.
The advantage of the composite anchoring system is that the stress distribution is relatively uniform along the entire length of the anchoring section, and it can make full use of the friction resistance between the surrounding rock (soil) and the anchor cable mortar body and the bearing capacity of the formation, thus maximizing the Significantly improve the anchoring force of the anchor cable. Since the free sections of each unit body of the composite anchor cable are of different lengths, compensatory tensioning should be carried out during tension locking to make the steel strands evenly stressed. In principle, the prestress value applied to each steel strand should be equal to the free section. The length is directly proportional to the relationship.
2. Damage forms of anchor cables
The damage of anchor cables is generally divided into the following 7 forms:
1) The anchor cable mortar body and surrounding rock (soil) ) is not large enough, the anchor cable body is pulled out from the hole.
2) The surrounding rock (soil) has insufficient compressive strength or the anchor cable mortar body has insufficient strength, causing the anchor cable to fail.
3) The holding force between the cement mortar and the steel strand is not enough, and the steel strand is pulled out of the mortar body.
4) The free section of the steel strand was broken. The reasons include: insufficient length of the free section, unqualified material, mismatch between material safety factor and load safety factor, etc.
5) The anchor head clip is unqualified, causing the steel strand to slip or break the steel strand at the anchor head.
6) The anchor cable is dragged out with the surrounding rock (soil).
7) The bottom of the anchoring section of the group anchor falls outside the through-crack surface at the same time, and the rock mass loosens along the fissure surface after the anchor cable is stressed.
The possibility of the above two damage forms 6) and 7) is very small, and there is no precedent at home and abroad. Therefore, verification is generally not performed and the design is not controlled. The holding force of the cement mortar body on the steel strands is much greater than the ultimate bearing capacity of the steel strands and the frictional resistance between the mortar body and the surrounding rock (soil), so the 3) failure mode will not occur. Need to check.
Types 4) and 5) of damage are caused by design errors and poor anchor quality. Therefore, for a single tension type anchor cable, you only need to check the first), that is, the friction resistance between the anchor cable mortar body and the surrounding rock (soil) to control the design, while for a composite anchor cable, you should also check the first). ) type and 2) type of damage.
(2) Design of prestressed anchor cables
1. Design anchoring force of prestressed anchor cables
The determination of the design anchoring force of prestressed anchor cables can be divided into For two situations.
(1) Rock landslide
Calculation based on the limit equilibrium method requires consideration of the anti-sliding force exerted by the prestress along the sliding surface and the normal sliding resistance exerted by the vertical sliding surface. The recommended formula for calculating the stability coefficient is as follows:
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Correspondingly, the prestressed anchoring force is
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In equations (2-40) to (2-43): V is the hydrostatic pressure of the trailing edge crack, , γw is the bulk density of water (kN/m3); U is the uplift pressure along the sliding surface, , H is the slope height (m); φ is the internal friction angle (°); θ is the anchor cable inclination angle (°); β is the angle between the anchor cable and the landslide (°), which is the sum of the landslide inclination angle (α) and the anchor cable inclination angle (θ) The relationship between Cohesive force (kPa); L is the length of the sliding surface (m).
If the locking anchoring force is less than 50% of the design anchoring force, the normal anti-slip force generated by the prestressed anchor cable can be ignored, and the stability coefficient calculation formula is simplified as follows:
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Correspondingly, the prestressed anchoring force is
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The meanings of the symbols in the formula are the same as before.
(2) Accumulation layer (including soil) landslide
Calculated according to the transfer coefficient method, taking into account the anti-sliding force exerted by the prestressed anchor cable along the sliding surface, and does not need to consider the vertical sliding surface The normal sliding force generated. The required anchoring force is
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where: T is the design anchoring force (kN/m); P is the landslide thrust (kN/m); θ is the anchor Cable inclination angle (°).
In addition, when locking the prestressed anchor cable, the locking anchoring force should be determined based on the landslide structure and deformation conditions. There are three situations as follows:
1) When the structural integrity of the landslide body is good, the locking anchoring force can reach 100% of the design anchoring force.
2) When the landslide body creeps significantly and the prestressed anchor cables are combined with anti-slide piles, the locking anchoring force should be 50% to 80% of the design anchoring force.
3) When the landslide has collapse properties, the locking anchoring force should be 30% to 70% of the design anchoring force.
2. Calculate the number of anchor cables
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In the formula: N is the number of anchor cables; P is the resistance of a single anchor cable Pulling force (kN), obtained through field tests; E is the sliding force of the landslide (kN); φ is the internal friction angle of the sliding surface (°); α is the angle between the anchor cable and the sliding surface (°); K is The safety factor ranges from 2.0 to 4.0. Generally, it is recommended to take 2.0.
3. Effective anchoring length
The effective anchoring section length can be determined comprehensively according to the following three methods, among which the empirical analogy method is more important. The specification stipulates that the length of the effective anchoring section should not be greater than 10m.
(1) Theoretical calculation
1) When the anchor cable body is pulled out from the cement body, the formula for calculating the anchoring length is
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In the formula: Lm1 is the effective anchoring length (m) required to prevent the anchor cable body from being pulled out of the cement body; T is the design anchoring force (kN); K is the safety factor, with a value of 2.0 ~4.0, generally 2.0 is recommended; n is the number of steel strands; d is the diameter of the steel strand (mm); C1 is the allowable bonding strength between the mortar and the steel strand (MPa).
2) According to the cement body and the anchor cable body sliding along the hole wall, the formula for calculating the anchor length is
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In the formula: Lm2 is the effective anchoring length (m) required to prevent the cement body and the anchor cable body from sliding along the hole wall; d is the hole diameter (mm); C2 is the cementation coefficient between the mortar and the rock (MPa), which is 1/ of the mortar strength Divide 10 by the safety factor (safety factor is 1.75~3.0); other symbols have the same meaning as before.
(2) Analogy method
Based on the experience of Lianziya dangerous rock mass anchoring project, the recommended effective anchoring length is shown in Table 2-16.
(3) Pull-out test
When the geological conditions of the landslide are complex or the prevention and control engineering is important, the above two methods can be combined and a destructive test can be conducted on the anchor cable to Determine the effective anchorage length. The pullout test can be conducted in three conditions: 7 days, 14 days, and 28 days. The water-cement ratio is adjusted to 0.38 to 0.45.
Table 2-16 Recommended values ??for anchorage length
4. Prestressed anchor cable inclination angle
The prestressed anchor cable inclination angle is mainly determined by construction conditions. Assume that the design bearing capacity of a single-bundle anchor cable is P, and the anti-sliding force (F) it provides is Sliding force, but the anchor cable is too long, making construction difficult and uneconomical; if θ is too large, although the length of the anchor cable is reduced, the anti-sliding force provided is also reduced, which is also uneconomical, so there is an optimal inclination angle to choose. problem. The optimal inclination angle can be comprehensively considered according to the following two methods.
(1) Theoretical formula
Theoretical analysis shows that it is the most economical when the inclination angle of the anchor cable satisfies the following formula
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In the formula: θ is the inclination angle of the anchor cable (°); α is the inclination angle of the sliding surface (°); φ is the internal friction angle of the sliding surface (°).
(2) Practical experience
For free grouting anchor cables, the anchor cable inclination angle should be greater than 11°, otherwise a grout stop ring needs to be added for pressure grouting.
5. Anchor cable spacing and group anchor effect
The number of prestressed anchor cables depends on the thrust generated by the landslide and the safety factor of the prevention and control project. The distance between anchor cables should be greater than 4m; if the distance between anchor cables is less than 4m, group anchor effect analysis needs to be performed. The recommended formula is as follows:
1) Japan's "VSL Anchorage Design and Construction Specifications" adopts the formula:
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In the formula: D is the minimum anchor cable Spacing (m); d is the anchor cable drilling hole diameter (m); L is the anchor cable length (m).
2) The formula recommended by the "Technical Regulations for Design and Construction of Landslide Prevention and Control in the Reservoir Area of ??the Three Gorges Project of the Yangtze River":
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In the formula: T is Design anchoring force (kN); ρ is the correction coefficient (taken as 105kN2·m); other symbols have the same meaning as before.
6. Arrangement of inner ends of anchor cables
Adjacent anchor cables should not be designed with equal lengths. They can be arranged in a staggered manner according to the strength and integrity of the rock mass, with the length difference between 1 and 2m.
7. Prestress loss of anchor cable
The prestress loss of anchor cable generally consists of three parts:
1) When prestress is applied, at the top The loss caused when pressing the working anchor clip is inevitable. This part of the prestress loss can be calculated based on the increased value of the high-pressure oil pump pressure gauge when pressing the anchor clip. Generally, it is about 5%.
2) After the prestress lock is applied, the loss of prestress generated during the unloading process of the jack is also inevitable. After locking, at the moment when the jack is unloaded, the steel strand loses its balance and will inevitably retract into the hole with the clip, causing accelerated movement, which may cause slight slippage. This part of the loss can be calculated by measuring the retraction length of the anchor cable strand at the anchor and the displacement of the reaction pier.
3) In addition to the above, factors such as creep of the ground, relaxation of steel strands, looseness of anchor heads, etc. will all cause prestress loss.
8. Anti-corrosion of anchor cables
The corrosion of anchor cables is an important factor affecting the life of anchor cables.
The main factors causing anchor cable corrosion are erosion of the formation and groundwater, failure of the anchor cable protection system, bimetallic effects, and the presence of stray currents in formation water. They can cause different forms of corrosion, such as general corrosion, local corrosion and stress corrosion. In addition to corrosion caused by corrosive media, stress corrosion under the action of high tensile stress and the resulting damage can directly cause the breakage of steel wires and steel strands. For example, several prestressed steel wires of anchor cables with a bearing capacity of 1,300 kN in the Jukes Dam in France broke after only a few months of use. The stress used in the steel wires was 67% of the ultimate value. After many tests, it was concluded that corrosion under high tensile stress is the main cause of steel wire damage.
There are many anti-corrosion measures for anchor cables, but whether it is domestic or foreign, evenly wrapping the steel strands with cement mortar is still the most basic and most effective measure. There are also double-layer protection methods, that is, corrugated metal pipes are used to cover the outside of the steel strands, and mortar, resin cement slurry and corrugated pipe protective sleeves are poured together to form double-layer protection. However, the cost is relatively high, and it is generally used in important projects and has Used under strongly corrosive environmental conditions.
9. Design of external anchor head and pressure-bearing reaction device
Anchors are an important part of prestressed anchor cables. You must choose reliable and reliable quality supporting products. The following mainly explains the design of the pressure-bearing reaction device - anchor pier, ground beam and frame.
(1) Design of anchor piers
The specific size of the anchor pier is determined by the load size and the load-bearing capacity of the slope. When the sliding rock mass is complete, has high strength, and has a large bearing capacity, the anchor pier can be designed to a smaller size; conversely, when the surface of the sliding body is soil or broken loose rock, it should be controlled by its bearing capacity. The size of the bottom surface of the anchor pier is to avoid the loss of anchor cable prestress due to too small size and insufficient bearing capacity.
The size of the anchor pier should meet the requirements of the following formula:
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In the formula: P is the designed pull-out force of a single anchor cable ( kN); A is the bottom area of ??the anchor pier (m2); σ0 is the allowable bearing capacity of the rock and soil on the surface of the sliding body (kN/m2).
In addition, the bottom surface of the anchor pier is preferably perpendicular to the anchor cable to ensure uniform stress. If there is an angle, the possibility of uneven stress on the anchor pier and slippage along the slope after being stressed should be considered.
Anchor piers are generally set to have a trapezoidal section with a small top and a large bottom to disperse the pressure of the anchor cables on the slope and reduce the prestress loss caused by the compression deformation of the topsoil. They are generally reinforced concrete anchor piers. The reinforcement should be properly densely arranged under the steel pad of the anchor head, and a steel pressure-bearing plate or spiral reinforcement should be installed at the opening between the anchor pier and the anchor. For soil slopes, due to the small bearing capacity of the surface soil, large anchor piers are often required and the appearance is poor, so ground beams or frames are generally used as reaction devices.
(2) Design of ground beams
The cross-sectional size of ground beams is controlled by two factors: first, the design tension of the anchor cable; second, the bearing capacity of the slope rock and soil . When the slope rock and soil are weak and the anchor cable tension is large, the width of the beam should be increased to increase the bearing area and prevent prestress loss. Considering that the distance between anchoring sections cannot be too close, the distance between ground beams is generally 3 to 4m.
The calculation of the beam is relatively simple. It is still calculated according to the elastic foundation beam. The landslide thrust is uniformly distributed in a rectangle within the length of the beam. The anchor cable is used as the fulcrum. When two anchor cables are placed on a beam, the simple calculation is When calculating support beams, when more than three anchor cables are laid, they are calculated as continuous beams. The landslide thrust borne by each beam is the landslide thrust of the spacing width of adjacent beams. When the landslide thrust is large, the ground beams can be designed with multiple rows up and down. The design of the beam is the same as that of the reinforced concrete beam and will not be described in detail here. The issues worth noting are the following five aspects:
1) Ground beams are designed, calculated and reinforced according to two stress stages. The first is when the landslide is in a relatively stable state, with no or only a small landslide thrust acting on the ground beam. The ground beam mainly bears the prestress imposed on the anchor cable, that is, the prestressed stage. At this time, the outer bending moment in the middle of the beam is large. , with more reinforcement; the second is that when the landslide thrust reaches the design thrust after prestressing is applied, the landslide thrust becomes the main external load (when the landslide thrust does not reach the maximum value, sometimes the active earth pressure can also become the main external load), that is, the ground pressure During the working stage of the beam, the maximum bending moment occurs on the mountain side of the middle part of the beam, which controls the reinforcement. The old ground beam needs to be reinforced on both sides.
2) In order to prevent uneven settlement of beams, beams should be set up separately where the rock and soil layers change.
3) The reinforcement should be densely laid out where the stress of the anchor cable is concentrated.
4) When the ground is too gentle, for example, less than 1:1.5, in order to prevent the prestressed loss caused by the displacement of the stressed rear beam upwards on the hillside, the inclination of the anchor cable should be steepened or anti-climbing facilities should be added.
5) In order to prevent the beam from being damaged due to uneven stress when adding prestress, the anchor cables in each hole should be tensioned in stages and should not be stretched to the design tension at one time. For example, if there are two bundles of anchor cables on a beam, each tensions 50% of the designed tension for the first time, and the remaining 50% and the super-tensioned part for the second time. If there are three anchor cables on a beam, it is best to tension all three at the same time. However, during construction, it is often difficult to do this due to equipment limitations. You can first tension the middle one to 50% of the designed tension, and then tension the upper and lower cables. bundle. Repeat this method for the second time to reach the design tension and over-tensioning part to prevent the ground beam from cracking during the tensioning process.
Design calculations are always simplified to the ideal state of uniform stress, which often deviates from the actual project. Therefore, the reinforcement of the beam should be appropriately increased to ensure safety.
(3) Design of anchor cable frame
The anchor cable frame is to set prestressed anchor cables at the intersection of vertical and horizontal beams, and should be set continuously, as shown in Figure 2-16 .
Figure 2-16 Schematic diagram of anchor cable frame and ground beam
Theoretically, it is more reasonable to calculate the frame's design and calculation based on three-dimensional space stress. However, in actual engineering, it is often simplified to vertical calculation. Beams and cross beams are designed separately, and the design is controlled in two states: the prestress application stage and the landslide thrust action stage. The distribution of forces on vertical beams and cross beams usually has the following three methods:
1) The vertical beams are used to bear the landslide thrust, and the cross beams are only used as connecting components to expand the load-bearing area of ??the vertical beams. The design calculation is the same as that of ground beams, but the cross-section size of cross beams can be smaller.
2) Vertical beams and cross beams both bear the landslide thrust, but vertical beams are allocated more, accounting for about 60% to 70%, and are designed separately.
3) Vertical beams and cross beams bear the same landslide thrust. In order to simplify the calculation, each anchor cable is taken as a node, and 1/2 of the vertical and horizontal beams are designed as cantilever beams. This method is safer, but wastes more material.
(3) Prestressed anchor cable structure
1. Anchor cables generally use steel strands or high-strength steel wire bundles. The steel strand used for anchor cables should comply with national standards (GB/T 5223-95, GB/T 5224-95). The parameters of my country's national standard 7-wire standard steel strand are shown in Table 2-17.
Table 2-17 National standard 7-wire standard steel strand parameters
2. Centering bracket (wiring ring)
Prestressed anchor cables must be spaced at intervals A centering bracket is installed at 1.5~3.0m to avoid entanglement of steel strands and reduction of mortar holding effect. The centering bracket can be processed from steel plate or hard plastic.
3. Anchors
There are many types of prestressed cable anchors. The commonly used ones are XM, QM and OVM external anchor heads. The engineering design unit must list them on the engineering design and construction drawings. Indicate the model, mark and anchoring performance parameters of the anchor. The basic parameters of OVM anchorage are shown in Table 2-18.
Table 2-18 Basic parameters of OVM anchorage (unit: mm)
4. Pressure-bearing reaction device
The pressure-bearing reaction device includes anchor piers , ground beams and frame 3 categories, made of reinforced concrete. The anchor pier is a reaction device of a single-bundle anchor cable on the ground. It is a purely compressive component and is generally made into a trapezoidal cross-section. Its function is to spread the concentrated load of the anchor and transfer it to the sliding body. The ground beam is one or several rows of vertical beams arranged perpendicular to the main sliding direction on the surface of the landslide (or high slope). Two or three bundles of anchor cables are arranged on each beam. When the sliding body is soil or weathered and broken rock mass, in order to enable the anchor system to bear force as a whole to stabilize the landslide or reinforce the slope, a reinforced concrete frame should be used as a reaction device. The frame is generally composed of two vertical beams, two or three horizontal beams.
5. Guide tip shell
The front part of the anchor cable is made into a shape as shown in Figure 2-17.
When the steel strand is lowered to the bottom of the hole, increase the thrust so that the steel strand that is not welded to the pointed shell is ejected from the side hole into an anchor shape, thereby increasing the strength of the sphere and the holding force between the steel strand and the mortar. .
The currently commonly used anchor cable structure in China is shown in Figure 2-18.
Figure 2-17 Guide tip shell with side hole
Figure 2-18 Schematic diagram of friction anchor cable structure
(4) Prestressed anchor cable construction
The construction of prestressed anchor cables includes the following processes: drilling and cleaning of anchor cables; braiding and forming of steel strands; installation of anchor cables; grouting of inner anchorage sections; pouring of outer anchor piers; tensioning of anchor cables Pull and anchor force lock.
1. Drilling and clearing holes for anchor cables
Anchor drilling rigs are used for drilling. Fix the drilling rig according to the designed depression angle of the anchor cable (generally 15° to 30°), adjust the azimuth and inclination, check the drilling position, and then tighten all fasteners. When ready, you can start drilling operations. The actual depth of the borehole is 1.0m longer than the designed depth, leaving a sediment section.
The aperture of the prestressed anchor cable is related to the number of steel strands, the thickness of the mortar protective layer and the landslide structure. Generally, an anchor cable composed of 5 to 10 steel strands has a hole diameter of 75 to 115mm; an anchor cable composed of 11 to 15 steel strands has a hole diameter of 115 to 135mm; an anchor cable composed of 15 to 20 steel strands , the aperture is 135~175mm. When the landslide structure is loose or the diameter of the borehole is significantly reduced, the hole diameter can be increased. When the sliding body is a soil layer or a soft rock layer, and when the sliding bed is a hard rock layer, a three-cone drill bit should be used to drill the section from the hole opening to the sliding surface, and high-pressure air should be used to remove the slag. If the porosity of this section of the formation is good, open hole drilling can be performed; if the porosity of this section of the formation is poor, drilling with a pipe can be used, and a casing can be placed to protect the hole wall, or cement slurry can be used to reinforce the hole wall; From the surface to the bottom of the hole, impact drilling can be used.
After drilling, pull out the drill pipe and drilling tools. Use a polyethylene pipe with a ruler to check the depth of the hole, and blow the hole with high-pressure air or wash the hole with high-pressure water. When the dust in the hole is cleaned and the hole depth meets the requirements, pull out the polyethylene pipe and seal the hole. Cover and set aside.
Drilling accuracy requirements: After the hole is formed, use a hole inclinometer to measure the hole inclination. The hole inclination does not exceed 1/100; the drilling position error is less than 100mm; the drilling inclination and horizontal angle errors are within ±1°. ;The hole depth must ensure that the tension section passes through the sliding belt by 2m.
2. Steel strands are braided into bundles
For Level I landslide prevention and control projects, the design load of steel strands can be reduced by 65% ??of the failure load; for Level II and III For landslide prevention and control projects, the design load of steel strands can be reduced by 65% ??to 80%.
According to the designed length of the anchor cable and the number of steel strands per hole, use a grinding wheel cutting machine to cut the anchor cable. In addition to the free section and anchoring section of the anchor cable, the length should be extended by 1.5m as a tension. Pull section. The strand must be straight.
The anchor cables are braided and assembled on the workbench. Anchor cables that are too long can be assembled on a site with a scaffolding, and then transported and hoisted into the hole. Set up a workbench about 0.5m high and 1.5m wide on a flat site. Place the cut steel strands smoothly on the stand and inspect them one by one. Any damaged steel strands should be removed. Bind wire rings, tightening hoops, guide shells and grouting pipes as required. The free sections of steel strands are coated with anti-corrosion oil and put on plastic pipes respectively, and sealed at the bottom. Plastic pipes must not be damaged during weaving, transportation and installation.
Assembled anchor cables must be inspected and registered by a dedicated person. Check the length, centering frame installation, and whether the steel strands overlap. After passing the test, they will be numbered, marked, and ready for installation in the holes.
3. Anchor cable installation
Before entering the anchor cable into the hole, it is necessary to check whether the anchor cable number and the hole number are consistent. After confirming that the hole depth and anchor cable length are correct, use a guide probe to probe the hole. If there is no obstruction, the anchor cable can be inserted into the hole.
Put the braided anchor cable bundle into the hole manually or mechanically, and check whether it reaches the designed position at the bottom of the hole. Otherwise, it should be pulled out, cleaned and reinstalled.
4. Consolidation grouting of the inner anchoring section
Generally, cement mortar is used for cementation. The cement mortar mix ratio is water:cement:sand=0.4:1:1.
In order to speed up the progress, 0.3‰~0.5‰ early strength agent (accounting for cement mass) can be added to the slurry, and the 7-day compressive strength f is required to be ≥ 25~30MPa.
The cement grade should be no less than 32.5, the sand should be sieved with a hole diameter of 4mm and washed with water. If the sand particle size is too large, it will easily segregate and block the grouting pipe. The mixed mortar must also be sieved to prevent cement from clumping and blocking the grouting pipe. Pure cement slurry is also available, but it shrinks easily.
When grouting, use reverse grouting, that is, lower the grouting pipe to the bottom of the hole, and reverse grouting from the bottom of the hole toward the hole opening. Reverse grouting can ensure that the mortar completely fills the anchor cable hole, while forward grouting can easily cause the exhaust pipe to block the bottom of the hole and form compressed air, making it impossible for the mortar to be pressed in. The grouting pressure is generally 0.3~0.6MPa.
The grouting pipe in the hole is made of metal pipe or PVC pipe. When using metal pipes, use external couplings to connect them, and it is prohibited to use reducing joints to connect them. Wet the inner wall of the grouting pipe with clean water before grouting.
In order to ensure uniform grouting, the grouting speed should not be too fast. Use a milliammeter as the primary grouting indicator, but ensure that the distance between the two probes is more than 800mm, and the exposed part cannot come into contact with the steel strand. Use a polyethylene pipe with a ruler to recalibrate the grouting length of the inner anchorage section. If it does not meet the requirements, grouting needs to be added. The mortar used should be mixed evenly with a mixer until it reaches the specified index, and the mixing should not be stopped until the grouting is completed. Do not pull or move the anchor cable before the mortar is completely cured. After grouting is completed, pull out the first-stage grouting pipe. When the ground in the anchoring section is weak and the anchoring force is insufficient, secondary splitting grouting can be used.
5. Pouring of external anchor piers
The external anchor piers should generally be embedded 20cm into the slope, using a C25 or above cast-in-place reinforced concrete structure, preferably with a trapezoidal cross-section. The dimensions of the external anchor pier are shown in Table 2-19, and its structure is shown in Figure 2-19.
Table 2-19 Dimensions of external anchor piers
Note: Φ is the diameter.
Figure 2-19 Structural diagram of external anchor pier of 3000kN grade prestressed anchor cable (unit: mm)
6. Tension and anchoring force locking of anchor cable
Tensioning is carried out 7 days after grouting of the inner anchorage section. Before the tensioning operation, the tensioning equipment needs to be calibrated. When calibrating, connect the jack, oil pipe, pressure gauge and high-pressure pump. Use the jack's active force output method on the press to repeat three times, take the average value, and draw a curve between the jack output and the pressure indicated by the pressure gauge as the basis for tensioning the anchor cable. When calibrating, the maximum output of the jack should be higher than the value when the anchor cable is over-tensioned.
First pre-tension a single anchor cable twice to improve the stress uniformity of each steel strand of the anchor cable. For 3000kN grade anchor cables, the single tension force is 30kN; for 2000kN grade anchor cables, the single tension force is 20kN; for 1000kN grade anchor cables, the single tension force is 10kN.
The anchor cable is tensioned by applying load in stages, and the locking operation can only be carried out until the pressure gauge does not return. If the prestress loss is too large, overall tensioning and re-locking are required. After tensioning and locking, secondary grouting is performed. When the mortar reaches the outer anchor pier, grouting can be stopped. When sealing the hole, leave 100mm of steel strand measured from the anchor, cut off the excess section, and cover it with a cement mortar protective layer with a thickness of not less than 100mm.
The locking anchoring force can be determined by two methods: direct measurement by a load cell and calculation of the deformation of the prestressed steel strand during tension locking. The calculation formula is as follows:
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Where: Px is the prestress that can be obtained after locking (kN); P is the tension required for anchoring (kN); P0 is the maximum tensile load (kN); Pi is the initial tensile load (kN); ΔL is the retraction amount of the anchor cable (mm) when Pi is loaded to P0, and the retraction amount of the clip is 6mm.
(5) Quality inspection of prestressed anchor cables
1. Quality inspection content of prestressed anchor cables
Including the assembly of anchor holes and anchor rod bodies And placement, grouting, tensioning and locking, etc.
2. Actual measurement items
1) Anchor holes: hole position, hole diameter, anchoring angle, inner anchoring section length and other items.
2) Production and placement of anchor cable rods: steel strand strength, steel strand configuration, rod length, wire ring density, there should be no joints when using steel strands.
3) Grouting: mortar mix ratio, strength, insertion depth of grouting pipe, etc.
4) Tension and locking: the concrete strength of the outer anchor pier, the verticality of the steel pad plane and the hole axis, tension load, locking load, anchors, anchor protective layer and other items.
3. Each independent landslide prevention and control project should conduct anchor cable bearing capacity inspection. Randomly select 10% to 20% of the total number for over-tensioning inspection, and the tensioning force is 120% of the design anchoring force. If the project is important, all anchor cables can be tested for 120% over-tensioning of the designed anchoring force.
4. Qualification conditions for anchor cable quality
The anchoring force of the anchor cable should reach more than 120% of the designed anchoring force.
5. Quality assessment requirements
(1) Guarantee items
1) Hole diameter, internal anchor length, steel strand strength, steel strand configuration, rod body The length and mortar strength must meet the design requirements.
2) No broken wires are allowed in a single steel strand.
3) Jacks, oil gauges, steel rulers and other equipment used for bearing capacity testing should be inspected and calibrated, and the bearing capacity must meet the aforementioned requirements.
4) Anchors must be inspected and qualified before they can be used.
5) The locking load should meet the design requirements.
(2) Allowable deviation items
The allowable deviation items of prestressed anchor cables shall comply with the provisions of Table 2-20.
Table 2-20 Allowable deviation items of prestressed anchor cables