Application of Self-Drilling Rock Anchor Bolts in Deep Foundation

With the development of the world economy, the degree of development and utilization of underground space is increasing, and the scale and depth of various deep foundation pit projects are increasing. At present, new technologies and techniques for foundation pit support emerge in an endless stream, but the form of pile-anchor support is still one of the most widely used forms of support. After long-term engineering practice, ordinary prestressed bolts are widely used with mature technology, reliable technology, convenient construction, and low cost. However, under some complex geological conditions, such as filling and soft soil, there may be collapse holes and necking. Insufficient bolt length, poor grouting effect, etc., make construction more difficult and even affect the safety of the supporting structure and the stability of the slope. In order to solve the above problems, self-drilling rock bolts have been gradually applied.


rock anchor bolt


Construction Difficulties


A deep foundation pit project is covered with deep gravel fill, and the internal prestressed anchor rod has serious hole collapse phenomenon during the construction process, and even stuck or cannot be drilled, resulting in the length of the anchor rod not meeting the design requirements. When using casing pipe instead, the drilling process is still slow, and it is difficult to drill. After the casing is pulled out, the hole will collapse and blocks the grouting channel. In order to ensure the smooth progress of the project, it was decided to switch to self-drilling rock bolt construction technology.


Self-Drilling Rock Bolt Structure


Self-drilling bolt is a new type of bolt that has been used in foundation pit engineering in recent years. It is composed of a sacrificial drill bit, a hollow bar, a plate, a nut, a coupler, and other accessories. The rod body is generally a threaded high-strength seamless steel pipe. The difference between ordinary bolts is that they have multiple functions of drill pipe, grouting pipe, and bolt body at the same time. During the drilling process, the rod body is used as the drill rod. After the drilling is completed, the drill bit and the rod body are left in the soil, and the hollow rod body is grouted.


Construction Process of Self-Drilling Rock Bolt


self drilling rock anchor bolt


The construction process of the self-drilling rock bolt is mainly divided into layout positioning, installation of drill pipe and drill bit, rod body drilling, stop drilling to connect long drill pipe, grouting, tensioning, and locking.


1) Pay-off positioning: according to the designed vertical and horizontal spacing of the anchor rod, the anchor rod positioning is carried out.


2) Installation of drill pipe and drill bit: Generally, the KW180 series rock bolt drilling machine is used for drilling, and the rock bolt body is directly used as drill pipe when drilling. Section 1 Install an alloy drill bit with a grouting hole at the end of the drill pipe.


3) Rod drilling: In order to ensure the direction, slope, and accuracy of the anchor, before drilling, the surveyor will stake out the position of the anchor. After the stake is completed, adjust the angle of the drilling rig to start drilling.


4) Stop drilling and extend the hollow bar: After the first section of the rod is drilled in place, use the coupler to connect the second section of the rod body and continue drilling. The free section of the anchor rod needs to be covered with a 40mm PVC corrugated pipe, and the two ends of the corrugated pipe are wrapped and sealed with a film smeared with anti-corrosion oil.


5) Grouting: The grouting material is pure cement slurry, the slurry should be stirred evenly and used as needed, and the mixing of stones and sundries should be strictly prevented. When the grouting operation starts or stops for more than 10 minutes in the middle, it is advisable to lubricate the slurry pump and pipeline with water or thin grout. The grouting pipe adopts the high-pressure grouting pipe, the grouting pressure is ≥1.5MPa, and the grouting is stopped when the grouting pressure is reached.


6) Tensioning and locking: After the grouting body reaches the design strength, a waist beam is added and tensioned and locked.


Why Using the Self-Drilling Rock Bolt?


rock anchor bolts


It combines drilling, grouting, and anchoring together, and is suitable for broken rock and conditions which are difficult to drill holes. Casing pipe is omitted in construction, which can greatly improve construction efficiency.


It has a rich variety of drill bits, and drill bit can be selected according to the properties of rock and soil, which can improve the drilling efficiency.


It can fill cracks, consolidate rock mass and soil layer through pressure grouting, and has good grouting spreading radius and reliable anchoring quality.


The hollow bar can be cut arbitrarily and lengthened by the coupler. Therefore, it is suitable for construction in a narrow space where the large equipment can’t enter.


Summarize


Self-drilling rock bolt, "one rod for multiple purposes", can not only be used as a drill rod and a grouting pipe, but also as an anchor rod body. It can effectively overcome problems such as hole collapse and stuck drills in deep filling layers in foundation support construction. The grouting effect is good, and the waste of materials is largely avoided. On the premise of ensuring safety, the construction operability and economy have greater advantages than conventional bolts.

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