Hollow Grouting Anchor Bars in Tunnels: Unveiling Neglected Quality Concerns

Tunnel construction demands a delicate equilibrium between innovation, safety, and structural integrity. Amidst this intricate balance, self-drilling hollow grouting anchor bars emerge as crucial support structures, both for maintaining rock stability during construction and as enduring components of the permanent support system post-construction. However, the glaring reality reveals that a slew of quality concerns plague the effective implementation of these anchor bars, undermining the very essence of their purpose.


Common Quality Problems and Their Solutions

1. Incorrect Mix Ratio and Insufficient Cement

Misjudging the cement mix ratio or skimping on cement can yield disastrous consequences. The adhesive strength of the anchor bar to the rock depends significantly on the mix quality.
Solution: Stringent quality checks on the cement mix proportions and rigorous adherence to recommended ratios are non-negotiable. Any compromise in this regard undermines the anchor bar's effectiveness.

2. Anchor Bar Installation Angle is Horizontal

Most of the anchor installation adopts the drilling jumbo, and the air rock drill is used to form the hole. But the height direction of the jumbo can not be adjusted, and the general rock drill is long so that the anchor bars above the arch shoulder are easy to install into the horizontal angle, and can not be perpendicular to the excavation contour line, the phenomenon of the missing from the top of the arch part is very common. An anchor bar's installation angle directly influences its load-bearing capacity. An incorrect angle might render the anchor bar ineffectual, putting the entire structure at risk.
Solution: Implement comprehensive training for the workforce, ensuring that they possess the necessary knowledge and skills to achieve the proper installation angle.

3. Deviation in Anchor Bar Positioning and Insufficient Hole Depth

Inaccuracies in positioning anchor bars or drilling insufficiently deep holes cripple their ability to support the rock effectively. These situations may be due to the constructor's greed to save time, not according to the regulations for laying lines, marking, and pointing, but by the workers intuitively drilling holes, resulting in a large deviation from the hole position. Insufficient hole depth may be caused by unclear technical instructions or lazy workers.
Solution: Incorporate advanced positioning technologies and ensure drilling equipment meets the required specifications. Regular quality checks during installation can rectify deviations promptly.

4. Insufficient Number of Anchor Bars

Cutting corners by using fewer anchor bars than recommended compromises structural stability and undermines the safety of the entire tunnel.
Solution: Adhere strictly to the engineering design's recommended number of anchor bars. Site managers must be familiar with the design drawings and the number of anchors to be installed per cycle. It should be counted once before anchor grouting and again after all grouting is completed. Finally, it should also be counted once more before the initial spraying of concrete. Sacrificing safety for expedience is never an option.


5. Grouting Issues: None, False, or Insufficient

Anchor bars depend on proper grouting for a secure bond. Absence, false grouting, or insufficient grout density can lead to catastrophic failures. When checking, we should check whether there is an overflow of slurry around the stopper plug, and can also use the method of tapping the anchor to listen to the sound to determine whether the slurry has been grouted. Grouting is not full mainly because the stopper plug is not tight, resulting in too large a hole, insufficient pressure, or after the termination of the pressurized grout, the anchor mouth is not blocked in time, resulting in the backflow of cement slurry.
Solution: Implement meticulous monitoring during the grouting process. Each step must be scrutinized, and any deviations from the procedure should be addressed immediately.

Quality Control and Management Strategies

1. Staffing

Effective team composition is paramount. Assign a minimum of two construction workers and one tester per hole. This team should monitor hole positioning, drilling angles, inter-row spacing, drilling depth, anchor insertion, grouting ratios, and grouting on-site.

2. Inspection of Raw Materials

Raw material traceability ensures accountability. Register materials upon arrival, label their inspection status, and conduct necessary tests. Timely sampling and continuous tracking of material usage are essential for quality assurance.

3. On-Site Testing Equipment

Equipping site managers with the right tools facilitates effective inspections. From rechargeable searchlights to digital cameras, a well-equipped team enhances the accuracy of quality checks.

4. Whole-Process Checking

Regularly inspect every stage of construction. This not only identifies deviations but also prevents issues from escalating.

Safety Requirements: A Top Priority

1. Regular and Irregular Checks

Frequent inspection of hollow anchor bars is a safety imperative. Especially in challenging geological sections, specialized personnel should conduct inspections during every shift.

2. Suspended Work Safety

When work halts, proper support system placement against the excavation face and closure with spray concrete minimize potential hazards.

3. Secure Anchor Bar Construction Dollies

Anchor bar construction dollies must be securely placed to prevent accidents or mishaps.

4. Addressing Blockages or Bursts

In the event of wind, water, or material pipeline issues, halt their transportation sequentially to manage the situation effectively.

5. Deformation and Cracking Remedies

Monitor and measure the anchor support system consistently. Address any detected deformation or cracks promptly. In severe cases, evacuation from the danger zone is crucial.

6. Reinforcement for Critical Situations

Should significant deformation or anchor bar failure occur, prompt action is necessary. Install reinforcement hollow anchor bars with lengths exceeding 1.5 times that of the original anchor bar.


The realm of tunnel construction is unforgiving, and the implementation of self-drilling hollow grouting anchor bars is a pivotal component in ensuring structural integrity. By addressing the often-overlooked quality concerns and implementing rigorous quality control and management strategies, we can pave the way for safer, more reliable tunnels that stand the test of time. Vigilance, commitment, and a meticulous approach to construction are the cornerstones of a successful and secure tunneling venture.
Sinorock is a professional self-drilling anchor bolt manufacturer and has years of experience in tunneling. If you have any questions about ground support, welcome contact us at sinorock@sinorockco.com.

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