Introduction
In modern geotechnical engineering, prestressed anchor cable construction is highly favored for its high stability and strong bearing capacity. As an efficient drilling method, the quality and efficiency of down-the-hole hammer drilling with casing technology are directly related to the prestressing effect and overall stability of the anchor cable. This article will introduce the construction process of prestressed anchor cables using down-the-hole hammer drilling with casing technology to help readers understand and apply this method.
Construction process
The construction process of prestressed anchor cable construction with down-the-hole hammer drilling with casing is anchor hole positioning → drilling rig in place → down-the-hole hammer drilling with casing drilling → hole blowing → anchoring → anchor cable grouting → tensioning → compensating tensioning.
Anchor hole positioning
1. Anchor hole positioning
Measure and place the hole position, using the combination of theodolite and infrared instruments to measure and place the anchor hole. Strictly check that the drilling tool is consistent with the designed anchor hole axis position.
2. Marking
Use red paint to mark the anchor hole center point and number at the measured hole position.
Drill rig in place
To make the anchor hole in the construction process and after the completion of its axis of the pitch angle and azimuth angle in line with the design requirements, we must ensure the accuracy and stability of the drill rig in place. Therefore, the drill rig installation platform must be flat, solid, non-deformed, non-vibrating, etc. And strictly check whether the axis of the drilling tool is consistent with the axis position of the designed anchor hole.
1. Accuracy
- Adjust the elevation of the contact point between the vertical axis of the drill rig and the slope rock surface to be consistent with the calibrated hole position.
- Adjust the axis of the vertical axis of the drill rig to keep it consistent with the depression angle and azimuth of the designed center axis of the anchor hole.
The technicians measure and calibrate the axis of the drilling tool to make it consistent with the direction of the center axis of the anchor hole before drilling.
2. Stability
- Use clamping pipes to firmly clamp the drill rig.
- Test runs the drill rig, calibrate the axis of the drilling tool and the center axis of the anchor hole again to keep them consistent, and tighten the tightening screw.
- The stability of the drill rig must be maintained at all times during drilling.
- Install a hole guide pipe or a small guide pile to ensure that the straightness, inclination, and azimuth of the constructed anchor hole meet the design requirements.
Inspection before operation
- Before starting the operation, carefully check whether the air supply pipeline is firmly connected and no air leakage is allowed.
- Check whether the oil mist is full of oil.
- Check if the screws, nuts, and joints of each part are securely tightened, and ensure that the drill rig is installed firmly, straight, and level horizontally.
- Before drilling, repeatedly invert the DTH hammer to check if the piston movement is smooth and flexible. If the piston movement is obstructed, identify the cause and rectify the fault before use.
- Check whether there is debris in the air supply pipeline and the inner hole of the drill pipe, and clean them up in time to prevent clogging of the DTH hammer.
Drilling process parameters
Choosing drilling technical parameters is a critical issue in DTH hammer drilling. Its rationality will directly affect the drilling efficiency and economic benefits. The drilling technical parameters include air volume, air pressure, drilling pressure, and number of revolutions.
Air volume
According to the selected air compressor and DTH hammer performance, reasonable determination of air volume. For the DTH hammer to work well and remove rock powder, the minimum return air speed through the annular gap between the drill pipe and the inner wall of the casing should be 15 meters per second (m/s). If the air volume is small, it is difficult to remove the rock powder in the hole, thus affecting the drilling efficiency.
Air pressure
The impact frequency and impact function of the DTH hammer are closely related to the air pressure. The commonly used air pressure for DTH hammer drilling is one of the significant indicators of drilling parameters.
Impact Frequency
When the rated air volume and rated air pressure of the DTH hammer meet the requirements of the rated impact frequency, which is generally 800~1500 times/min.
Drilling pressure
In the principle of rock crushing by the DTH hammer, rocks are primarily crushed through the impact force. The drilling efficiency of a DTH hammer depends on the magnitude of impact energy and the frequency of impacts, and drilling pressure serves as an auxiliary force to ensure effective impact energy transmission. Each DTH hammer of a specific diameter has an optimal range of drilling pressure. Excessive pressure fails to enhance drilling efficiency but accelerates drill bit wear.
Number of revolutions
To effectively transmit the impact work of the DTH hammer to the bottom of the hole, the number of revolutions of the drilling tools should be determined according to the impact frequency of the DTH hammer and the properties of the rock being drilled, and it has an optimal impact interval. The coordination of the number of revolutions and the impact frequency of the drill should make the cutting tool work under the optimal impact interval. At this time, the drilling efficiency is the highest.
Conclusion
Down-the-hole hammer drilling with casing technology can efficiently and stably form holes in complex and changeable geological environments while significantly improving the stability and bearing capacity of geotechnical engineering through the prestressed anchor cables’ reinforcement. In actual applications, through precise construction preparation, reasonable drilling parameter adjustment, strict construction process, and reliable grouting and maintenance measures, we can realize the high efficiency and quality of anchor cable construction.
