Under Soft Covering Layer and Hard Rock Layer, How to Construct Self-drilling Hollow Bolt?

In projects using self-drilling hollow bolts, it is inevitable to encounter geological conditions with soft and broken covering layers and hard rock layers. Under such geological conditions, the construction of self-drilling hollow bolts is definitely not the same. So, how is it constructed? Today, Sinorock leads everyone to learn together with specific project cases.

In the left bank shaft project of a hydropower station, the geological conditions are very complicated. The geological conditions of the overburden from top to bottom are: 24~33m of alluvial deposits, mainly composed of crushed gravel and clay, loose, broken and prone to landslides; 42~55m of glacier deposits, mainly composed of basalt fragments It is composed of breccia sand, which has poor self-stabilizing ability and is easy to collapse. The ancient landslide deposits of 36-51 m are mainly composed of sand shale and limestone fragments, and the rock is relatively hard.

In the early stage of the project, ordinary mortar bolts were used for spraying protection, pipe shed grouting and self-drilling hollow bolt construction. Construction; using the pipe shed grouting construction scheme, although it can solve the above problems, but the construction efficiency is low and the cost is high; the self-drilling hollow bolt construction, the drilling of the drill pipe and the placement of the bolt are completed simultaneously, and the grouting effect can meet the construction needs. Although the cost is high, the construction progress is relatively fast. In addition, a certain downward inclination angle is designed during the construction of the bolt, which can play the role of advanced consolidation grouting, which is beneficial to the next cycle of construction, and can meet the construction quality and safety requirements. Decided to choose self-drilling anchor construction.

Although it is determined to use self-drilling hollow bolts for construction, the selection of drilling rigs and drill bits is still a tricky thing.

Since the construction work surface is uneven after the slag is discharged, and the height and drilling position of each cycle are different, it is necessary to choose a drilling rig that is convenient for construction, easy to operate, and can carry out construction in a narrow space. After discussion, the construction party decided to use a crawler-type bolting rig. The drilling diameter is 50~70mm, and the maximum drilling depth is 25m.

Regarding the selection of drill bits, according to the test results of the project, ordinary cast in-line drill bits were used in the initial stage of construction. However, when the self-drilling hollow bolt is drilled into glaciers and ice-water accumulations, due to its high rock strength, the in-line drill bit is severely worn. To replace the drill bit, the bolt needs to be reversed out of the hole, which seriously affects the construction progress. Therefore, the subsequent construction adopts the cross-shaped drill bit of high-strength alloy, which improves the construction efficiency.

In this project, there are two main advantages of self-drilling hollow bolts: one is that drilling and grouting are carried out simultaneously, so there is no need to worry about the problem of hole collapse; the other is that the cross alloy bit has high strength and wear resistance, avoiding frequent replacement. Drill bit and increase the engineering cost. It can be seen that, as a new type of support material, self-drilling hollow bolt has the irreplaceable advantage of ordinary mortar bolt.

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