【Excerpt】How to construct slope anchor cable?

1. Internal technical preparation

        (1) Before the commencement of construction, relevant technical personnel should be organized to study the implementation of the construction organization design carefully, read and review the construction drawings, clarify relevant technical problems, and be familiar with the specifications and technical standards.

        (2) According to the requirements of the technical disclosure model, the technical disclosure shall be prepared and the construction personnel shall be given technical disclosure. The construction personnel participating in the construction shall be given pre-job technical training and work with certificates after passing the examination.

        (3) Construction safety guarantee measures and emergency plans shall be formulated.

2. Field technical preparation

        (1) According to the material items and quantity in the material plan submitted by the project material department, the materials shall arrive at the site and pass the test, supervision sampling and inspection.

        (2) The site for the disposal and discharge of slurry has been implemented and the procedures have been completed.

        (3) The equipment (jacks and oil gauges) was delivered to the site and calibrated.

        (4) The site construction is 0.5 meters below the anchor cable anchoring line, and the anchor cable construction conditions are available.

        (5) The land acquisition of the construction site shall be completed, and the site shall be leveled. The living and production houses shall be completed, and the living and office facilities shall be fully equipped to meet the needs of the main management and technical personnel entering the site for living and office.

3. Technical requirement

        (1) To prepare and implement the construction organization design, including the construction sequence of pile holes, construction process design and main technical measures, safety and quality assurance measures, labor organization and material support, etc.

        (2) Calibrate the jack, high pressure oil pump and pressure gauge, and draw the pressure gauge reading-tension relationship curve to know the on-site strong pull operation.

        (3) Prepare enough steel strand and anchor according to the number of construction drawings, and conduct sampling mechanical property test.

        (4) Install air compressor, grouting pump, lay air supply pipe and grouting pipe.

        (5) Before the construction of anchor cables, anchoring tests should be carried out on site according to 3% of the working anchor cables, and no less than 3 anchor cables.

4. Construction procedure and process flow

4.1 Drill hole

        Using a crane, the geological drilling rig is positioned on the first row of anchor cable operation platforms. The position of the drilling holes and the inclination angle of the rig are adjusted. The rig is secured to the operation platform and the rock surface using fasteners. Construction proceeds from the bottom up, and as the drilling progresses, the next layer of the operation platform is promptly erected. The process involves: positioning, drilling, and high-pressure air cleaning.

        The anchor cable hole has a diameter of 130mm and is drilled using mechanical drilling methods to ensure that the depth meets the design specifications. Water flushing drilling is strictly prohibited. If the stratum is loose, fractured, or prone to collapse, pipe-following drilling technology should be used. If collapse occurs, drilling should be immediately halted, and grouting with a pressure of 0.1—0.2MPa should be performed to stabilize the wall. After initial setting, the hole should be re-drilled. After drilling is completed, high-pressure air must be used to clean the hole to remove rock powder and water accumulation. A polyethylene pipe should be used to verify the hole depth; if the depth is no less than the designed length of the anchor cable, the polyethylene pipe should be removed, and the hole should be sealed with fabric or cement bag paper for future use.

4.1.1 Anchor fabrication

        The anchor cable is designed to use 1x7 standard steel strand wire, with a nominal diameter of φ15.2mm and a standard strength of 1860Mpa. When manufacturing the anchor cable, the steel strand wire is cut to the following lengths: anchoring section + free section + 1.5m exposed tensioning section. The steel strand wire is cut using a toothless saw, and the materials for the anchor cable body's guide cone, expansion ring, and positioning plate are made from φ45mm steel pipes and φ8mm rebar. The anchor cable can be fabricated on-site while drilling, with the internal anchoring section shaped in a wavy form and the tensioning section in a straight line.

        The free section of the anchor cable should be coated with a strong anti-corrosion coating. The anchor cable bundle should be tied with fine iron wire every 1m, and then coated with rust-proof oil. A corrugated pipe should be fitted over it, filled with grease, sealed at both ends, and secured with engineering tape. Alternatively, a plastic protective pipe with a diameter of φ20—22mmPVC can be fitted over each steel strand, filled with grease within 10—20cm at both ends, and sealed with engineering tape. At the anchoring end of the anchor cable, a centering bracket (expansion ring) should be installed every 1.0m, made from a 20mm thick polyethylene plastic plate. The anchor cable should be tied between every two expansion rings with fine iron wire.

4.1.2 Anchor installation

        Before installing the anchor cable in the anchor hole, check whether the number of the anchor cable is consistent with the hole number. After confirming that there is no error, clean the hole once with high pressure air, and then start to install the anchor cable.

        The following four points should be paid attention to when installing anchor cables:

        (1) Check the position of the positioning and plugging rings. Replace any damaged rings according to technical specifications.

        (2) Inspect the position and unobstructed condition of the exhaust pipe.

        (3) Insert the anchor cable into the hole. When the positioning and plugging rings reach the hole opening, stop pushing and install the grouting pipe and one-way valve.

        (4) After the anchor cable is in place, recheck the exhaust pipe for unobstructed flow. If it is not unobstructed, remove the anchor cable, resolve the issue, and reinsert it.

4.1.3 Slip casting

        The anchor grouting is constructed by bottom injection, the mortar is injected from the bottom of the hole, the air is discharged from the exhaust hole, and the grouting position is controlled by the mortar position indicator.

        The anchor hole grouting is carried out by the grouting machine. The grouting material is M40 cement mortar, and the mix ratio is determined by test. The grouting pressure should not be lower than 0.6—0.8Mpa; the mortar must be full and compact. After the first grouting is completed and the cement mortar solidifies and shrinks, the hole opening should be replenished with grouting.

4.1.4 Construction anchor beam

        The anchor beam, a compression member, transfers the concentrated load from the anchor to the rock surface. Additionally, since the rock surface at the hole opening is usually not perpendicular to the anchor cable axis, the anchor beam also helps to adjust the direction of force on the rock surface. To ensure that the upper surface of the anchor beam is perpendicular to the anchor cable axis, a thin-walled steel pipe with an outer diameter matching the drill bit diameter and a backing plate are pre-orthogonally welded. Before pouring the anchor beam, the other end of the steel pipe is inserted into the drill hole.

4.1.5 Tensioning of anchor cables

        The anchorage cable tensioning is carried out after the grouting and anchor beam concrete have reached 70% of their design strength. Before tensioning the cable, the tensioning equipment must be calibrated to avoid stress errors. During calibration, connect the jack, oil pipe, pressure gauge, and high-pressure oil pump. On the press, use the jack to apply force three times in a repetitive manner, take the average value, and plot the curve of the jack's output (KN) versus the pressure gauge's reading (Kpa) to serve as the basis for cable tensioning. The initial starting pressures of domestic pressure gauges may vary, so the calibration curve must include the specific pressure gauge number used during calibration, which should not be altered during use. If the pressure gauge is damaged or the jack is removed and reinstalled, recalibration is required.

        The anchor cable tensioning is carried out in two stages, with each stage being 70% of the total tension. The interval between the two stages should be at least 3 to 5 days. To minimize prestress loss, the total tension should include an over-tension value. When the free section is soil, the over-tension value should be 15% to 25%, and when it is rock, it should be 10% to 15%. During tensioning, the elongation of the anchor cable should be recorded, and the elongation should be verified to match the applied force. Each stage of tensioning should increase by 25% of the design tension, with a 2 to 5-minute interval between stages, and a 30-minute interval for the final stage. To address prestress loss due to factors such as ground layer creep, a supplementary tensioning is performed, followed by locking, cutting off excess steel strands, and sealing the anchor with concrete.

4.1.6 Seal the hole and grout

        The grouting pipe is inserted from the reserved hole until the pipe mouth enters the top surface of the anchoring section for about 50cm grouting. The air in the hole is discharged through the exhaust pipe located at the positioning stop ring.

4.1.7 External protection

        After sealing and grouting, 50mm steel strand shall be left from the anchor, and the rest shall be cut off. C30 concrete with a thickness of not less than 50mm shall be wrapped on the outside of the anchor.

4.2 Construction technology process

        The construction process is shown in the prestressed anchor cable construction process flow chart

5. Construction requirements

        In the process of prestressed anchor cable construction, we must pay attention to the following points:

        (1) Drilling must strictly control the hole formation error, inclination and horizontal Angle error should be controlled below 1 degree, hole slope deviation less than 1/50, try not to disturb the surrounding rock layer of the hole, after drilling, should continue to blow the hole from the bottom to the outside for at least 10 minutes.

        (2) When making the anchor cable, it should be prevented from twisting and bending to minimize the generation of secondary stress; the anti-corrosion of the free section of the anchor cable should be carefully done, and the anti-corrosion oil should be applied according to the design and plastic casing should be added; for the anchoring section, the de-oiling treatment must be carried out to enhance the adhesion between the anchor cable and mortar.

        (3) The anchor beam is the direct force-bearing component of the anchor cable tension, so the bearing surface of the pad should be flat and perpendicular to the axis of the anchor cable.

——Source: Railway and bridge construction safety

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