How Does Self-Drilling Rock Bolts Support Soft Soil in Deep Foundation?

With the development of urban underground space, there are more and more deep foundation pit projects. The traditional slope anchoring method encounters complex geological conditions, especially unfavorable geological conditions such as soft soil and sandy pebble strata, which cannot meet the requirements of slope stability and cannot meet the requirements of slope stability. The porosity is very low, which puts a lot of pressure on the grouting anchorage in the later stage, and the construction cost is greatly increased. Self-drilling bolt technology is a new type of anchoring technology used in geotechnical engineering, which can adapt to various complex geological conditions and construction environments.

Deep Foundation Support of An Iran Project

Self-drilling rock bolts use the hollow bolt body with a threaded surface as the drill pipe when the bolt is formed into a hole, connect a disposable drill bit at the end of the rod body, use the drilling rig to drive the rod body into the formation, and then pass the middle hole of the rod body to the ground. Ground grouting, so that the bolt body is wrapped with cement mortar or cement paste, and it contacts with the surrounding soil along the rod body and forms a combination to act as a group. Under the condition of soil deformation, the anchor is passively stressed by the bonding friction force on the contact surface with the soil, and the soil is restrained, strengthened, or stabilized mainly by tension. The setting direction of the anchor rod is generally consistent with the direction of the main tensile strain that may occur in the soil, which is close to the horizontal and has a small downward inclination angle.

The Self-drilling rock bolt is a kind of hollow bolt, the bolt is threaded through the whole body, the length of the bolt is connected by a coupling sleeve, and a disposable drill bit is installed at the front end. Its structure includes a hollow rod body, a backing plate, a nut, a coupler, and a drill bit. It has the three functions of drilling, grouting, and bolting, and realizes the unification of the functions of bolting, grouting, and anchoring.


Due to the special structure of the self-drilling hollow bolt, it has the following advantages for anchoring construction:

Sinorock Stainless Steel Self-Drilling Rock Bolt

1. High construction efficiency: the hollow bolt body is not only a drill pipe, but also a grouting pipe, and it is also a carrier of earth pressure. It integrates drilling, grouting, and anchoring. Each process of construction is completed in one process, saving energy and construction time, and easy to use.

2. It is suitable for formations that are difficult to form holes: such as soft soil layers and relatively soft surrounding rock, it is generally difficult to form holes. Self-drilling hollow bolt technology just avoids the shortcomings of conventional methods. After the drilling is completed, the rod body remains in the hole and does not need to be withdrawn, so it is not afraid of collapsing the hole, saving the time of penetrating other rod bodies and reducing the difficulty of construction.

3. Strong adaptability: the self-drilling bolt has threads on the whole rod body, which can be cut off and extended at any position, and can be arbitrarily cut and connected with a sleeve for lengthening. The drill bit can be applied to all kinds of formations.

4. High anchoring force: The self-drilling anchor rod body is a trapezoidal thread, which is firmly connected with the coupling sleeve and is well combined with the grouting body.

5. Good anchoring effect: When grouting, since the grout is grouted upward from the bottom of the hole, the grouting is full, the density is good, the grouting quality is good, and the anchoring strength is increased.

6. Simple construction: Compared with the conventional bolt construction, there is no need for casing wall protection, pre-grouting, and other measures when forming the hole, which reduces the procedures of drilling, bolt installation, grouting pipe installation, and the construction is quick and easy.

7. The high-strength coupler is processed by mechanical cutting technology, and the self-drilling bolt has the characteristics of lengthening while drilling, so that it can construct longer bolts in narrow construction space.

Construction Steps

The bolt construction is mainly composed of three parts: drilling, drill pipe extension, and grouting. Different cleaning liquids can be selected according to the surrounding rock conditions during drilling. The specific steps in the construction are as follows:

Excavation working face → initial shotcrete → anchor rod positioning → drilling rig in place → anchor rod drilling → grouting → shotcrete surface layer → anchor rod locking → excavation of the next layer of earthwork.

Construction Notes

1. Before installation, check whether the water hole of the anchor rod and the drill bit is unobstructed.

2. If the drill is stuck during the drilling process, the collapsed hole should retreat the anchor rod a little, the drilling rig should be less impacted, rotate more, sweep the hole repeatedly, and then feed slowly until the predetermined hole depth is drilled.

3. The bolt hole should be as straight as possible.

4. Pure cement slurry or 1:1 mortar should be used for bolt grouting. When using mortar, the particle size of sand should not be greater than 1.0mm, and the water-cement ratio should be controlled at 0.4-0.5.

5. The grouting material should be poured from the middle hole of the bolt body. When the anchor hole returns to the grout or the pressure of the grouting pump reaches the design value, it indicates that the anchor hole has been filled.

6. After the bolt body is grouted, the grouting material should avoid knocking, vibration, and hanging heavy objects before the grouting material reaches the design strength. When the grouting strength reaches 5.0MPa, the nut can be tightened to apply stress.

To sum up, the self-drilling bolt shows good advantages in simplifying the construction process, drilling quality, drilling efficiency, grouting process, grouting quality, etc.

The Foundation of Columbia University

The application of self-drilling rock bolts can better improve construction efficiency, production benefit, economic benefit and social benefit, which fully shows that self-drilling bolt has great wide applicability in slope support under adverse geological conditions.

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