Rock bolt failure and grout quality

rock bolt

Rock bolt systems are widely used in mining and geotechnical engineering as ground anchorage. Many bolts are usually installed by grouting in pre-drilled holes in the rock mass. The supporting capacity of the grouted bolts depends greatly on the grout quality.

For grouted rock bolts, it is understood that a rock bolt system may fail in any of the following components (Fig. 1): the plate at the collar if any, the bolt itself, the anchorage (or grout for grouted rock bolts), the interface between the bolt and the anchorage, and the interface between the host rock and the anchorage.

The load bearing capacity (LBC), which is defined as the maximum tension tolerable by the rock bolt system, is thus the smallest bolt tension tolerable by the corresponding components of the system.

Plate failure is usually in the form of bending, causing partial tension relief in the bolt (a yielding effect) and may not necessarily render the bolt system completely losing its supporting capacity.
A thicker plate may easily solve the problem if required. The failure of the interface between the host rock and anchorage often occurs in soft ground. This is not an engineering factor and will not be considered here. In hard rocks, the LBC
will mostly depend on the bolt breaking force, the anchorage capacity and friction at the interface between the bolt and the anchorage. Bolt failure itself can however be avoided by selecting larger diameter or higher quality bolts, or by increasing the number of bolts in an area. The friction at the interface between the bolt and the anchorage can be and has been increased significantly by using corrugated bolt. The anchorage capacity is then the single important engineering factor to consider.

The anchorage capacity is defined as the maximum anchoring force of a bolt before the anchorage fails. The anchoring force in a grouted bolt is the total tension mobilized along the grouted segment starting at the grout end near the collar and progresses towards the hole bottom as the bolt tension increases. The anchorage capacity increases with the length of the grouted segment and can exceed the bolt breaking force when the grouted length reaches certain limit. Anchorage failure is possible only when the bolt tension reaches the anchorage capacity.

The amount of anchoring force mobilized at a segment of the grouted Bolt will depend on the grout quality. From the practical point of vie win the field, the grout quality will be most suitably measured by the total anchoring force, which the anchorage is able to hold against pull-out before it fails. However as mentioned earlier, this would require testing of the actual bolt tension at failure and would not be a desired method in practice. In this regard, an alternative and practical method is suggested here as a measurement of the grout quality by considering the compressive strength and the uniformity of the grout.

The uniformity is measured in this research by the air content in the grout.

fig:1
plate

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