Anchor cable piles and anchor cable construction applications
Anchor cable piles: This is the most commonly used type of anchor cable structure. When the anti-slide piles are deep, the landslide thrust force and the bending moment承受ed by the piles are large, and cantilever piles cannot meet the requirements, using anchor-pulled anti-slide piles is a reasonable choice. The pile head of the anti-slide pile serves as the anchorage head, with the anchorage segment acting as the sliding bed of the landslide. The length of the anchorage segment is determined by calculating the anchorage force and the bond strength of the rock, while the free segment represents the length of the anchor cable as it traverses the landslide body.
Lattice anchor cables: Used for landslide and unstable embankment management, the lattice serves as the anchor head's securing head. The lattice uses reinforced concrete beams with larger cross-sections, and the cross-section of the lattice can be enlarged and thickened at the anchor head.
Single anchor structure: Used for the management of unstable rock masses on high slopes, where the rock mass is typically hard rock. The free segment is the unstable rock mass segment, and the anchorage segment is the stable bedrock. The anchor head uses an anchor plate, which is a reinforced concrete thick plate with a side length of 0.5–1.0 m.
Anchor cable construction: Anchor cables typically use 7 φ5 steel strands. The number of steel strands in the anchor cable is determined based on the anchor pull force. The anchor hole diameter is generally φ100–φ150, with the hole diameter depending on the number of steel strands and the rock mass properties of the anchorage segment. Typically, 7 steel strands use a hole diameter of φ100, 11 strands require a hole diameter of φ120, and 15 strands require a hole diameter of φ150.
The free segment of the anchor cable can move freely. The anchor cable is enclosed in a PVC pipe filled with corrosion-resistant grease. The anchorage segment of the anchor cable is secured by positioning rings, with a spacing of approximately 2 meters between rings. Expansion rings and contraction rings are arranged alternately.
Anchor cable construction procedures and site preparation: Anchor cable construction includes site preparation, drilling, hole cleaning, cable assembly, cable installation, grouting, secondary grouting, and tensioning and locking procedures.
If the slope of the construction site is relatively gentle, excavation and backfilling can be used to level the anchor cable construction site. The site must allow for the positioning and operation of drilling equipment, with sufficient space for cable lowering and convenient grouting access. For steeper slopes, such as those exceeding 25°, a construction platform must be erected. Typically, steel pipes are used for construction, and vertical scaffolding may be employed for steep slopes. The platform must be stable and secure, and facilitate construction operations.
Anchor cable drilling: Currently, there is almost no use of circulating fluid rotary drilling, and instead, pneumatic down-the-hole hammers are used for drilling. The power head is a pneumatic down-the-hole hammer, with the drill bit diameter matching the hole diameter. The air compressor has an airflow rate of 10–20 m³. Ingersoll Rand's 20 m³ diesel air compressor performs excellently and can typically power two anchor cable drilling machines. Drilling machines are increasingly becoming smaller and lighter, with the control panel and drill frame separated. The drill rod sections are 1.5–2 m in length. Drilling efficiency is 1 m/4–6 minutes for shallow holes and 1 m/10–20 minutes for deep holes.
Anchor cable assembly: Anchor cables are assembled on a dedicated platform. Assembly should be performed after the anchor hole is completed, otherwise the final hole depth may not match the design specifications, leading to waste or rejection of the anchor cable. The free segment and anchorage segment are determined based on design requirements and construction hole conditions. The anchorage segment is fixed in place using positioning rings, while the free segment anchor cables should be able to move freely and have reliable corrosion protection. The anchor head should have sufficient tensioning length, typically 2 meters.
Anchor cable fabrication and installation: Anchor cables are made of high-strength, low-relaxation prestressed steel strands with a diameter of 15.2 mm and a ultimate tensile strength of 1860 MPa. They are available in bonded and unbonded types. The free segment uses unbonded steel strands enclosed in corrosion-resistant corrugated tubes. Before bundling the anchor cables, the steel strands must be kept straight, arranged uniformly, and free of rust and oil contamination. Steel strands with kinks or mechanical damage must not be used. Anchorage segment support rings are spaced at 1-meter intervals, while free segment support rings are spaced at 2-meter intervals to ensure the steel strands remain straight.
Hole cleaning and cable lowering: If the hole shape is intact and the hole walls do not collapse, the hole cleaning process can be omitted. Hole cleaning is performed using pneumatic cleaning until all debris is removed from the hole. When the anchor cable is longer, a guide device should be installed at the anchor cable head. The anchor cable should remain straight during the hole entry section, with a uniform insertion rate, and the anchor end should be lowered to the bottom of the hole.
Anchor cable grouting: Grouting materials include cement slurry and cement mortar. Cement slurry is easier to apply but has the drawback of being prone to shrinkage. Cement mortar has higher strength and better volume stability.
The grouting pipe is lowered with the anchor cable to the bottom of the hole. Grouting is performed in sections from the bottom of the hole upward at low pressure and low speed until the hole opening is filled with grout. During the curing process of the cement slurry, shrinkage may occur, so timely grout supplementation at the hole opening is required.
To enhance the anchoring effectiveness of the anchor cables and increase the anchor pull force, some anchor cables require secondary grouting. Secondary grouting is performed after the initial setting of the first grout, typically 2–5 days after the first grouting. Secondary grouting must overcome the initial pressure of the previous grout, so high-pressure grouting is required, with the grouting pump pressure exceeding 5 MPa.
Grouting cement typically uses high-strength 525# ordinary Portland cement with a water-cement ratio of 0.45, supplemented with water-reducing agents, etc. The cement consumption is 10–30 kg per meter. In areas with well-developed cracks or karst cavities, cement consumption increases significantly. In such cases, the water-cement ratio should be reduced to increase the concentration of the cement slurry.
Tensioning and locking: Design specifications typically define the tensioning stress and locking stress. The tensioning force is generally 100–120% of the design pull-out force, while the locking force is typically 80–90% of the tensioning force.
The tensioning method adopts a stepwise tensioning method, first tensioning individual strands and then tensioning the entire system. Simplified tensioning methods that only use individual strand tensioning or one-time overall tensioning are also available. If the individual strand tensioning method is used, it is important to tension and lock each strand individually to ensure uniform stress distribution across the entire anchor cable.
Some anchor cables require supplementary tensioning after the initial tensioning is completed, with a certain interval. After all tensioning is completed, the excess anchor cables should be cut off, and the concrete cover layer protecting the anchor heads should be poured.
Tensioning operations can only be carried out when the strength of the grout body and the pier foundation concrete reaches 80% or more of the design strength. Before formal tensioning, apply 30% of the design tensioning load for 1–2 preliminary tensioning cycles to ensure tight contact between all parts and complete straightening of the steel strands. The tensioning load may be applied in four stages according to relevant standards. During the final tensioning load application, maintain the load for 10–15 minutes, observe stability, and then lock the anchor.
During tensioning, the pier foundation must provide sufficient reaction force. After anchor cable tensioning is completed, the anchor heads should be promptly grouted and sealed.
Anchor cable construction in complex rock and soil layers: Anchor cables are typically 15–40 m in length, with a diameter of 100–150 mm. The commonly used construction machinery is the MD-50. Anchor cable drilling requires dry drilling and must not use mud circulation drilling. The deviation rate of the drill hole axis should not exceed 2% of the anchor cable length. The anchor cable hole depth should be 30–50 cm greater than the anchor cable length. After drilling to the designed hole depth, the drill should be stabilized for 3–5 minutes.
In highly fractured rock and soil layers or karst-developed strata, borehole drilling is extremely challenging. One issue is unstable borehole walls leading to collapse, and the other is non-sealed rock and soil layers causing loss of drilling power gas, or even inability to advance. In highly fractured strata sections, casing drilling can be adopted, or wall stabilization treatment can be performed by injecting cement slurry and re-drilling after curing. After the anchor hole is completed, anchor cable installation and anchor hole grouting should be carried out promptly to prevent hole collapse due to prolonged exposure.
Grid-type slope protection and anchor-reinforced concrete grids are common structural forms for grid-type slope protection, used on steep slopes. They consist of three parts: anchor bolts, reinforced concrete grids, and grid-to-grid filling. Anchor rod diameter: φ100, rod body: φ18–φ36 reinforcing bars, full-hole grouting, with positioning rings installed at intervals. Anchor rod depth: 4–20 m, typically 6–12 m. The truss cross-section height is 30–60 cm, width 20–40 cm, reinforced according to continuous beam design, with concrete strength C20–C30. The truss spacing is 2–4 m. The inter-truss filling primarily uses dry-laid stone; for steeper slopes, mortar-laid stone or precast concrete components may also be used, with a stone thickness of 25–45 cm.
Anchor-reinforced concrete grid construction process:
1. Clear and shape the slope surface to form a straight slope; install access paths when the slope is long;
2. Anchor rod construction: Use pneumatic down-the-hole hammers and lightweight drilling rigs to construct according to the following sequence: positioning, drilling holes, installing anchors, and grouting;
3. Excavate the grid foundation trench;
4. Fabricate grid reinforcing bars;
5. Truss concrete pouring: Since concrete is poured on the slope surface, the formwork type and pouring method are critical. Pouring should proceed from bottom to top. On steeper slopes, segmented beam-top formwork should be installed. The poured concrete must be vibrated and cured;
6. Truss inter-space filling: Use dry-laid or mortar-laid stone. Construction should be carried out after the truss concrete has hardened.
Masonry lattice construction: The lattice is square or diamond-shaped, with a spacing of 2–4 m between lattices. The lattice cross-section height is 30–60 cm, width 20–40 cm, and the lower part is buried 10–20 cm into the soil. Dry-laid or mortar-laid stone is laid between the lattices. The stones used are fresh, unweathered stones with a diameter of 20 cm or more and a strength of M30 or higher. The mortar used for masonry is M5–M10. Construction procedures:
1. Slope shaping and cleaning: First, the original slope is shaped and cleaned, excavated into a straight slope. If the slope is long, access paths should be set up in sections, with a width of 1–3 m. Drainage ditches are generally installed on the access paths.
2. Excavation of foundation trenches: The foundation trench depth is 10–20 cm, with a regular excavation cross-section and accurate layout.
3. Mortar-jointed stone grids: The外观 quality requirements include full mortar coverage, mortar strength meeting specifications, large block sizes, intact shapes, no weathering or erosion, and grid cross-sectional dimensions meeting design requirements.
4. Dry-laid stone grid structure: Dry-laid stone construction is similar to mortar-laid stone construction, with comparable construction processes and quality requirements. However, since dry-laid stone does not use cement mortar filling, it has higher requirements for block size and masonry quality. Larger-sized stones should be used whenever possible, with the stones having a prismatic shape, and the self-stabilization of the dry-laid stone blocks should be good.
5. Filling between lattice sections: Fill the spaces between lattice sections with dry-laid stone or mortar-laid stone as required.
Selection of grid-type slope protection methods: There are dozens of grid-type slope protection methods available, and selecting the appropriate method is of great importance. Commonly used methods include: dry-laid stone slope protection, mortar-laid stone slope protection, stone-concrete grid-type slope protection, dry-laid stone anchored concrete grid-type slope protection, and mortar-laid stone anchored concrete grid-type slope protection. The natural slope angle for dry-laid stone slope protection is generally less than 20 degrees. For slopes between 20 and 25 degrees, mortar-laid stone slope protection or stone-concrete grid-type slope protection is typically used. For slopes between 25 and 30 degrees, dry-laid stone anchored concrete grid-type slope protection can be employed. For slopes exceeding 30 degrees, mortar-laid stone anchored concrete grid-type slope protection is generally selected.
Anchor rod construction: The anchor rod body is made of thick reinforcing bars, mostly single-rod, with a few double-rod or triple-rod configurations. There are two types: rock anchors and soil anchors. Short anchors are approximately 2 meters long, while long anchors exceed 15 meters. When selecting anchor rod drilling equipment, emphasis should be placed on compactness to facilitate construction. Commonly used equipment includes small-sized rotary percussion drills, such as the MD-30 model. For very short rock anchors, handheld percussion drills can also be used to create holes. For anchor rods longer than 10 meters, the general procedure is to install the anchor first and then grout. For shorter anchor rods, grouting can be done before installation. The rod body should be straight before insertion into the hole and should be free of rust and oil contamination. The grouting pipe should be inserted to a depth of 5–10 cm from the bottom of the hole. As the grout is injected, the pipe should be slowly and evenly withdrawn. If grouting is interrupted for more than 30 minutes, the pipeline should be flushed with water or diluted cement slurry.
Slope filling and block stone construction: The slope surface of the slope protection should generally be a natural slope or a slope formed by excavation. It is generally not advisable to construct slope protection on a filled slope. Slope protection structures on loose fill soil are difficult to stabilize. Therefore, when the slope surface has significant undulations, do not pursue planar integrity but instead follow the natural contours of the slope. When aesthetics and stability conflict, prioritize stability. Masonry grid slope protection requires a large amount of block stone and concrete materials, resulting in significant slope surface transportation volumes and high construction labor intensity. When a gravel filter layer is required in the reservoir water level fluctuation zone, the filter layer thickness must be controlled.
For grid-type slope protection, the following should be controlled: structural dimensions, strength of blocks and concrete, appearance of the slope protection surface, and drainage hole placement. The size of the blocks and the thickness of the stone layers should meet requirements. The blocks should be fresh and intact, free of weathered rock. The formed slope surface should be sturdy and aesthetically pleasing. Anchor bolts should undergo segmented pull-out tests, and mortar-jointed grid structures should have drainage holes spaced at intervals.
