Design and Introduction of Self Drilling Anchor Bolt in Geotechnical Engineering

I. Introduction

Self drilling anchor bolt is an important part of the support system. Anchor bolts are the general term for tension rods. Geotechnical anchoring is a technology that embeds tension rods into the ground. Relying on the force provided by the ground around the anchor rod, the anchor bars and cement grout consolidation body makes the retaining structure and the rock and soil form a complex working together. The combination of the anchor rod, the cement grout and the rock and soil not only improves the strength and self-stability of the rock and soil, but also ensures the stability and safety of retaining structures. So how to design the self drilling anchor bolt?

self drilling rock bolt design

II. Self Drilling Anchor Bolt Structure

The self-drilling bolt mainly comprises a hollow anchor bar, a coupler, a nut, a bearing plate, a centralizer, and a drill bit. In the system, the bearing plate and the nut are used as the head of the anchor rod to firmly connect with the soil retaining structure and transmit the force of the retaining structure to the tie rod. The hollow anchor bar is a fully threaded rod, that serves as both a drill pipe and a grouting pipe, allowing for simultaneous drilling and grouting. This integration system enhances the bond between the anchor rod and the surrounding rock and soil. After grouting, it is bonded and anchored with the surrounding rock and soil mass throughout the length, ensuring the force value received by the bolt body is transmitted to the surrounding formation effectively.

III. Design of Self Drilling Anchor Bolt

Designing a self-drilling anchor bolt involves a meticulous process to guarantee its effectiveness.

1. Deformation and Stability Calculation: Through calculations involving deformation and stability, the resistance provided by the rock bolt is determined, establishing the standard and design values for pullout design bearing capacity.

2. Cross-Sectional Area Calculation: The cross-sectional area of the rock bolt is calculated based on the design value of its tensile strength, taking into consideration the material and size of the cross-section.

3. Anchorage Area and Length Calculation: Using the design value of tensile strength, the anchorage area and length of the rock bolt are calculated.

4. Basic Test and Parameter Correction: Essential tests and adjustments to design parameters ensure the reliability and effectiveness of the self-drilling anchor bolt.

self drilling rock bolt design

IV. Application Types of Self-Drilling Anchor Bolts

As a new type of construction technology, self-drilling anchor bolts can realize simultaneous drilling and grouting, especially suitable for loose formations such as difficult to form holes and collapsed holes. Self-drilling anchor bolts are widely used and can be used as micropiles, soil nails, and rock bolts.

1. Prestressed Anchor: Ideal for slopes, underground excavation, and anti-floating scenarios, replacing traditional anchor cables. The self-drilling hollow anchor bar is drilled to the required depth, then the end is grouted, and the tension is applied after solidification.

2. Micropiles: Used to form micropiles in diverse settings such as wind power station tower foundations, transmission tower foundations, building foundations, retaining wall pile foundations, bridge pile foundations, etc.

3. Soil Nails: Suited for slope support, replacing conventional steel anchors, and also applicable in deep foundation pit steep slope support.

4. Rock Bolt: Effective in reinforcing rock slopes or tunnels with severe surface weathering or joint development. The self-drilling hollow anchor bars facilitate grouting to bond rock blocks, enhancing stability. The rocky side slope reinforcement of collapsed roads and railways can also replace conventional pipe umbrellas for reinforcement at loose tunnel entrances.

V. Construction of Self Drilling Anchor Bolt

Efficient construction of self-drilling anchor bolts is imperative for the success of geotechnical projects:

1. Site Investigation and Verification: Prior to construction, a thorough investigation of project site conditions, stratum properties, groundwater conditions, and surrounding factors must be conducted. Formulated detailed construction organization design and quality assurance systems for bolts are essential for improving construction and achieving good economic benefits.

2. Drilling Rig Selection: Selection of an appropriate drilling rig based on rock and soil type, drilling scale, construction surface conditions, rock bolt type, and drilling speed is crucial.

3. Drilling Process Considerations: During drilling, verifying layered thickness of rock and soil in the anchorage section is essential. The drilling inclination angle is preferably 15°~35°. If the inclination angle is large, drilling is difficult and the horizontal component force is reduced. The depth should exceed the design length of the rock bolt by 0.3~0.5m. The thickness of the overlying stratum of the bolt hole should not be less than 4m, and the horizontal and vertical spacing should not be less than 1.5m in general, so as to avoid the group anchor effect and reduce the anchoring force.

4. Handling Loose Strata: When dealing with loose strata prone to collapse, self-drilling anchor bolts equipped for simultaneous drilling and grouting are advantageous. Connect the rotary grouting adapter to the hollow anchor bar and the output end of the drilling rig respectively, and the grouting while drilling function can be realized during drilling. The water-cement ratio of the cement grout is generally 0.4-0.6, which can be adjusted according to the actual situation or a second grout replenishment can be carried out.

VI. Conclusion

Self-drilling anchor bolt has the advantages of simple construction, fast installation, and low cost. It is widely used as soil nails, rock bolts, micropiles, and ground anchors in geotechnical engineering all around the world. As you embark on geotechnical projects, the precise design and careful construction of self-drilling anchor systems are paramount. Sinorock team has many years of experience in providing complete geotechnical engineering services. If you have any needs or questions about self-drilling anchor systems and their applications, please feel free to contact us at

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