Factors Affecting the Anchoring Effect of Rock Bolts

Table of Contents

As a vital means of engineering structure reinforcement, rock bolts are applicable in geotechnical, tunnel, bridge engineering, and other fields. The anchoring effect of rock bolts directly affects the safety and stability of the project. This article will introduce the factors that affect the anchoring effect of rock bolts and help you better understand and deal with related problems.

Rock bolts material

The anchor rod is the most fundamental condition for the rock bolt to generate anchoring force. Insufficient anchoring capacity in the anchor system, such as inadequate strength of the rod or insufficient strength of the adhesive, will significantly impact the anchoring effectiveness of the anchor system. The materials used as rock bolts in actual projects generally have steel bars, high-strength steel wires, and other alternative materials (such as bamboo, glass fiber, and carbon fiber.). Considering both performance and price, steel bars are widely used. Different strength grades, cross-section sizes, and construction measures of steel or various alternative materials can constitute the corresponding reinforced concrete structure, and its force properties are bound to change accordingly. The size of the anchorage capacity depends on the bonding force between the steel bar and the grouting body, and the bonding force between the steel bars and the grouting body of different cross-sections is very different.

self-drilling anchor bolt

Rock bolt structure

Based on different structures, rock bolts encompass prestressed and non-prestressed rock bolts, tension and pressure rock bolts, split sets, self-drilling anchor bolts, and hollow bar anchors. The mechanical properties and methods of rock bolts of different structures are different. For various rock masses, their anchoring effects vary greatly. Therefore, when reinforcing rock mass, it is crucial to choose the appropriate type of rock bolt for reinforcement based on the actual geological conditions, ensuring an effective reinforcement outcome.

Anchoring method

To transmit the tensile force of the rock bolt into the depth of the rock formation, effective integration with the rock mass is essential. How to maximize the effect is crucial for rock bolt reinforcement. Generally, three methods are utilized for securing the rock bolt: mechanical fixation, cement fixation, and expansion base fixation.

  • The mechanical fixing method is mainly suitable for temporary short rock bolts in rock formations.
  • The cementing material fixing method involves using cementing material (such as cement slurry or synthetic resin) to bond the anchor rod body with the surrounding formation. This method is widely employed for fixing rock bolts, particularly suitable for weak rock strata and soil. It effectively transmits greater pulling force into hard rock formations.
  • The expansion base fixation method can effectively improve the bearing capacity of the anchor.
expandable rock bolt

Rock mass properties

The mechanical properties of rock bolts depend on their mechanical parameters, as well as the mechanical properties of the surrounding rock. The fundamental factors that affect the mechanical properties of rock mass include rock mass hardness, rock mass integrity, the initial stress state of rock mass, and groundwater conditions.

Anchoring parameters

Matching of hole diameter and rod diameter

The relationship between the rock bolt’s diameter and the drill hole’s diameter affects the rock bolt’s anchoring strength and failure mode. When the relationship between the diameter of the rock bolt and the diameter of the drill hole is different, the location of the failure surface is also different. It could be the interface between the reinforcement and the mortar or the mortar and the rock.

Split Set

Anchor length

Experiments show that the ultimate pull-out force of rock bolts does not increase linearly with the increase in anchoring length. There is a critical anchoring length. When the anchor length exceeds the critical value, the increase in the ultimate pull-out force is not visible.

Inclination angle

The inclination angle of the rock bolt is an essential parameter in the anchoring structure. The correct selection of this parameter has a significant impact on the stress condition of the retaining structure, the length of the anchor cable, and the ease of construction. It is most advantageous for the force direction of the rock bolt to be consistent with the axis of the rock bolt, but this is often not always possible. Generally, as the angle between the rock bolt axis and the horizontal plane increases, a higher vertical component force is generated, leading to a reduction in the required horizontal support force. Therefore, it is required to design the rock bolt with an inclination angle less than or equal to 45°.

Rock bolt spacing

Under normal circumstances, rock bolt supports are arranged in a square pattern, meaning the distance between rock bolts equals the row spacing of rock bolts. When the distance between rock bolts in the anchor group is too small, the bearing capacity of the anchor group will be lower than the sum of the bearing capacities of each anchor. It is the so-called anchor effect. Therefore, when designing the spacing for rock bolts, it is essential to consider various factors comprehensively to determine the appropriate spacing. This approach will be beneficial for the anchoring effectiveness of the rock bolts.

Environmental conditions

Changes in ambient temperature will affect the temperature stress distribution of the soil and the stress state of rock bolts; the rise and fall of the groundwater level will affect the moisture content and compressibility of the soil, thereby affecting the load-bearing capacity of rock bolts; the occurrence of natural disasters such as earthquakes will disturb the soil and reduce the bearing capacity of rock bolts. Therefore, it is necessary to consider the influence of environmental conditions during the design and construction process and take corresponding measures to improve the adaptability and reliability of rock bolts.


Many factors affect the anchoring effect of rock bolts. To improve the anchoring effect of rock bolts, it is necessary to comprehensively consider these factors according to the project’s actual situation, select appropriate materials and construction techniques, and strengthen quality control and environmental monitoring during construction. At the same time, for important projects or projects under complex geological conditions, on-site tests or simulation analyses are needed to verify the feasibility and reliability of the design. By implementing scientific and rational design and construction measures, the load-bearing capacity and durability of rock bolts can be enhanced, ensuring the safety and stability of the project.

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