Sinorock self drilling hollow bar in tunneling and underground mining, steel rod inserted in a hole drilled into the roof or walls of a rock formation to provide support to the roof or sides of the cavity. self drilling hollow bar reinforcement can be used in any excavation geometry, is simple and quick to apply, and is relatively inexpensive. The installation can be fully mechanized. The length of the bolts and their spacing can be varied, depending on the reinforcement requirements.
There are three major ways of anchoring the self drilling hollow bars: mechanical, grouted, and friction. The most common form of mechanically anchored self drilling hollow bar uses an expansion shell. A wedge attached to the bolt shank is pulled into a conical expansion shell as the bar is rotated. This forces the shell to expand against the wall of the borehole. The two mechanisms by which the shell is anchored against the borehole wall are friction and interlock. A preload can be applied to the rock surface by tensioning the bolt with an attached hanger or face plate, which are designed to distribute the load uniformly onto the surrounding rock.
The quality of roof support achieved with self drilling hollow bars depends more on the quality of installation, than on the theoretical strength of the bolts.
The cost of installing self drilling hollow bars is high. How high depends on many things: the method of mining, the method of installing bolts, the accessories used with the bolts, how many bolts must be installed to guarantee adequate support of the rock and, most of all, how well they are installed.
The c are as follows:
The cost of the fixture or self drilling hollow bar.
The cost of drilling a hole; the length and diameter may vary for different types of bolts.
The most important cost is the cost of unsuccessful installation. This is also the most difficult to estimate. But it is this that determines the number of bolts required.
The regulations often require that representative samples of the bolts installed be pulled and torque tested.
Servicing of self drilling hollow bars is also costly. Mechanical bolts loose their tension and must be retorqued.
If screening is used and new holes are required to overlap the screen, this adds greatly to the bolting cost.