What is An Expandable Rock Bolt?

Table of Contents

In numerous engineering projects, Ensuring the stability of structures demands the utilization of diverse anchoring technologies. Among them, expandable rock bolts have increasingly gained prominence, owing to their distinctive principles and outstanding performance. So, what precisely constitutes an expandable rock bolt, and how does it wield its remarkable influence?

What is an expandable rock bolt?

expandable rock bolt

Expandable rock bolts represent a prevalent form of support in rock mechanics engineering, finding extensive application in tunnels, mining, hydropower projects, and various other fields. The rock bolt is made out of a thin-walled steel tube folded on itself and achieves its functionality through expansion within the drilled borehole. This expansion creates friction and an interlocking anchor, delivering instantaneous reinforcement and support to the structure.

The composition of expandable rock bolts

expandable rock bolt

The expandable rock bolt is crafted from a double-layer concave tubular rod using an A3 seamless steel pipe. This tube possesses an outer diameter larger than the anchor hole and is processed into a double-layer concave tubular rod smaller than the anchor hole’s diameter. The components include end sleeves, retaining rings, liquid injection ends, trays, and other accessories. The construction procedure is: drilling → installing the rod body on the tray and sending it to the bottom of the hole → connecting the liquid injector and the liquid injection nozzle → injecting high-pressure water (pressure 15-20 MPa) → close contact between the rod body and the surrounding rock → completion.

Working Principle of expandable rock bolts

expandable rock bolts

Install the anchor rod in the anchor hole and inject high-pressure water. After the water pressure exceeds the elastic limit of the pipe wall material, the rod will undergo permanent plastic expansion deformation with the geometry of the anchor rod hole, making it firmly embedded in the surrounding rock. In addition, when the rod expands, the anchor rod exerts a higher pressure on the surrounding rock mass, forcing the surrounding rock to generate strain, increasing the surrounding rock stress, and in turn, the surrounding rock on the anchor rod body also produces corresponding extrusion, and hydraulic expansion type anchor in the process of water expansion, because of its diameter from thin to thick, along the longitudinal direction there is a certain amount of contraction so that the tray of the anchor rod is tightly pressed against the surface of the surrounding rock to generate an upward supporting force, to apply to prestress to the surrounding rock.

Characteristics of expandable rock bolts

expandable rock bolt

Compared with ordinary rock bolts, the main features of expandable rock bolts are summarized as follows:

1. It is easy to bury and master, and the installation quality depends less on the judgment or proficiency of the installer.

2. The embedding of anchor rods can be carried out by manual, semi-mechanized, or fully mechanized methods. The embedding quality of different methods is the same.

3. The burial speed is fast. Under existing drilling conditions, one worker can bury 40 to 50 anchor rods per hour.

4. Once the water pump pressure attains the predetermined expansion pressure value, the pump automatically ceases operation. This automated shutdown ensures the bearing capacity of each anchor rod, and the anchor rod becomes operational promptly upon installation.

5. The rock bolt demonstrates adaptability to a range of stratigraphic conditions, spanning from softer sand and clay to the most resilient granite.

6. The bearing capacity of the anchor rod is not affected by the joints of the surrounding rock. When the joints deform, the anchoring strength increases instead.

7. Anchor rods of various lengths up to 12m can be manufactured. The burying procedure of the anchor rod does not change with the length of the anchor rod.

8. While the rock bolt may incur a higher initial cost, the overall installation expenses are notably low.

Types of expandable rock bolts

1. Standard type, used for general rock mass support.

2. Yield type, used for large deformation rock mass support.

3. Anti-erosion type, used for erosive rock mass support.

4. Super type, used for rock mass support requiring large rock bolt bearing capacity and large drilling diameter.

5. Intermediate expandable rock bolts are mainly designed for rock masses with large drilling diameters and low bearing capacity.

How to give full play to the advantages of expandable rock bolts?

Achieving the full potential of expandable rock bolts requires rigorous quality control throughout the entire process, involving design, production, and installation. Engineering and technical personnel play a crucial role in this process. Here are key considerations:

Quality control in design, production, and installation:

  • Enforce rigorous quality control measures during the design phase to guarantee that the specifications align with engineering requirements.
  • Supervise the production process to ensure the utilization of top-notch materials and strict adherence to design specifications.
  • While installing, make sure to meticulously follow procedures, ensuring that each rock bolt is securely placed according to design parameters.

Tailoring design to actual needs and geological conditions:

  • Conduct a thorough assessment of geological conditions to inform the rock bolt design process.
  • Tailor the design to meet specific engineering needs, considering factors such as load requirements, deformations, and structural stability.

Appropriate rock bolt specifications:

  • Select expandable rock bolts with specifications that match the varying diameters and lengths of different boreholes.
  • Ensure compatibility between the chosen rock bolts and the geological conditions at the site.

Adjusting parameters for optimal performance:

  • Modify parameters, such as water injection volume and pressure, based on the specific geological conditions and engineering requirements.
  • Optimize these parameters to achieve the best anchoring effect in different situations.

Continuous monitoring and adjustment:

  • Establish ongoing monitoring throughout the project’s lifecycle to promptly identify any deviations or issues.
  • Be prepared to make adjustments in real-time based on monitoring data, ensuring that the expandable rock bolts continue to perform effectively.

Regular evaluation and improvement:

  • Conduct regular evaluations of the rock bolt system’s performance.
  • Utilize experience gained from previous projects to enhance the design, production, and installation process.

Training and skill development:

  • Conduct training sessions for engineering and technical personnel to elevate their skills in designing, producing, and installing expandable rock bolts.
  • Stay updated on industry best practices and incorporate new knowledge into project execution.

By incorporating these measures, engineering and technical teams can uphold the highest standards in the design, production, and installation of expandable rock bolts, ensuring their effectiveness in various geological and engineering contexts.

Construction process of expandable rock bolts

Construction preparation

Before construction, a comprehensive survey of the construction site is necessary to assess critical factors such as geological conditions and rock mass characteristics. Simultaneously, the required construction equipment and materials are prepared according to the engineering design specifications. Additionally,  it is necessary to train the construction personnel to ensure they are familiar with the construction process and operation specifications of expandable rock bolts.


  • Accurate positioning: according to the construction plan, use measuring tools to mark the predetermined location to ensure the drilling location is correct. During positioning, we should avoid potential obstacles or weak strata that could compromise drilling quality and construction progress.
  • Drilling method: choose an appropriate drilling method according to geological conditions and hole diameter requirements. For hard strata such as rock, impact or rotary drilling can be employed; for soft strata such as soil, spiral or rotary drilling can be used. During drilling, keep the drill bit stable and avoid offsetting or shaking.
  • Control the drilling depth: strictly adhere to the drilling depth as per the design requirements. Once the predetermined depth is reached, promptly cease drilling to prevent over-penetration or inadequate depth. Additionally, ensure the verticality of the drill hole to facilitate smooth insertion of the rock bolt.
  • Clean the debris in the hole: after drilling is completed, clean the soil, gravel, and other debris in the hole in time to keep the hole clean. It facilitates subsequent anchor installation and water filling.

Insert expandable rock bolt

Insert the expandable rock bolt into the hole, ensuring an appropriate gap between the bolt and the hole wall to facilitate the subsequent expansion process. At the same time, we also need to pay attention to the verticality and horizontality of the rock bolts to ensure their stability in the rock mass.

Water injection expansion

Once the rock bolt is pre-embedded, the water injection and expansion operation commences. First, connect the water injection pipe through the hole in the rock bolt. During the water injection process, it is crucial to strictly control the injection speed and volume to prevent over-injection or under-injection. After completing the water injection, allow for a certain period to make it fully expanded.

Quality inspection and acceptance

After the water injection expansion stage is completed, the construction quality needs to be inspected and accepted. It includes checking the expansion of the rock bolt, the connection with the rock mass, etc. If any quality issues or safety hazards are identified, they must be promptly addressed and rectified. At the same time, construction records and related materials also need to be organized to provide a basis for subsequent project acceptance and file management.


In conclusion, as an efficient, straightforward, and environmentally friendly anchoring technology, expandable rock bolts have showcased distinctive advantages and significant potential across various fields. With continuous advancements in science and technology and the deepening of engineering practices, there is a strong belief that expandable rock bolts will assume a more pivotal role in future engineering constructions, contributing to remarkable achievements for humankind.

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