Design and Introduction of Self Drilling Anchor Bolt in Geotechnical Engineering

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

1. Self Drilling Anchor Bolt Structure


The self-drilling bolt is mainly composed of 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 play the role of transmitting the force of the retaining structure to the tie rod. The hollow anchor bar is a fully threaded rod, which is mainly used as a drill pipe and a grouting pipe. After grouting, it is bonded and anchored with the surrounding rock and soil mass throughout the length, and the force value received by the bolt body is transmitted to the surrounding formation.


2. Application Types of Self Drilling Anchor Bolts


As a new type of construction technology, self drilling anchor bolt 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.

Prestressed anchor: used for slope, underground excavation, anti-floating and other scenarios to replace the anchor cable, the self drilling hollow anchor bar is drilled to the required depth, and then the end is grouted, and the tension is applied after solidification;

Micropiles: self drilling hollow anchor bar can be drilled down to form micropiles, which are often used in wind power station tower foundations, transmission tower foundations, building foundations, retaining wall pile foundations, bridge pile foundations, etc.;

Soil nails: often used for slope support, instead of conventional steel anchors, can also be used for deep foundation pit steep slope support;

Rock bolt: In some rock slopes or tunnels with severe surface weathering or joint development, self-drilling hollow anchor bars can be used for grouting after drilling to bond rock blocks together to improve their stability. The rocky side slope reinforcement of collapsed roads and railways can also replace conventional pipe umbrella for reinforcement at loose tunnel entrances.


3. Design of Self Drilling Anchor Bolt


Through calculation of deformation and stability, etc., the resistance provided by the rock bolt is obtained, and the standard value and design value of the bolt's pullout design bearing capacity are obtained;

Calculate the cross-sectional area of the rock bolt according to the design value of the tensile strength of the rock bolt (select the material of the rock bolt and the size of the cross-section);

According to the design value of the tensile strength of the rock bolt, calculate the anchorage area and anchorage length of the rock bolt;

Basic test and design parameter correction.

 slef drilling rock bolt design 1

4. Construction of Self Drilling Anchor Bolt


Before construction, it is necessary to investigate and verify the project site conditions, stratum properties, groundwater conditions, underground buried objects, various pipelines and the surrounding conditions of the site, etc. The construction organization design should be formulated in detail, the construction method should be determined, and the quality assurance system for bolt construction should be formulated. This is crucial to improving construction efficiency and achieving good economic benefits.

The appropriate drilling rig should be selected according to the rock and soil type, drilling scale, construction surface conditions, rock bolt type and drilling speed.

During the drilling process, verify the layered thickness of rock and soil in the anchorage section.

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.

When encountering loose strata that are prone to collapse and difficult to form, use self-drilling anchor bolts for construction. 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.




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. Choose suitable rock bolts should based on precise design. 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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