Constructing in rock usually requires reinforcement measures. Installation of rock bolts are always carried out within the theoretically sealed (grouted) zone of rock tunnels. This should mean that they are installed in dry condition. Despite this, there are a significant number of bolt holes, which after drilling are leaking/dripping water. These leaky bolt holes must be handled efficiently to ensure the stability of the rock and durability of the rock bolts that is required. Today there is no uniform guideline in Sweden for treatment of leaky bolt holes in rock. The purpose of this research is to investigate the cause, current solution and proposal for handling, the possibility of reducing measures as well as providing suggestions for the regulation of leaky bolt holes in the construction of rock.
The investigation has been based on available data from technical specification at Bypass Stockholm, production data regarding geology, rock reinforcement (bolting) and grouting to map the reasons behind occurrence of leaky bolt holes. At the Bypass Stockholm project, it assumes that over 30 000 leaky bolt holes (5% of 600 000 pieces) must be fixed. The primary investigation by following up data from geological mapping, MWD data and performed treatment of leaky bolt holes shows that the main factors for occurrence of the leaky bolt holes are geology, shape of the grouting fan and the length of the bolts. Mapping and MWD data show the variation in rock mass in the investigated areas. In some areas there are structures parallel to the tunnel roof. Clay-filled cracks and dripping water has been observed in these areas. Investigation of about 1200 treated leaky bolt holes was performed to verify if there are some specific areas where the most of the leaky bolt hole occurs. This area has been identified as critical zone, including overlap and 6 meters beyond. The overlap area is where the deviation of grouting hole from the former fan is largest. On the other hand, the distance between tunnel contour and grouting hole from new fan is as lowest.
The result from the investigation also shows that longer bolt holes increase the risk for leaky bolt holes. Another reason for increased risk for leaking bolt holes is due to unsealed structures, which can occur because of deviation of grouting holes.
The proposals for reducing the occurrence of leaking bolt holes is including modification of fan geometry. Proposed fan geometry regarding inclination has two purposes. The first is to compensate for the deviation of grouting hole in the end of the former fan and to have longer distance to the tunnel contour for the new fan. Secondary, by having different inclination in the very same fan, the probability to hit more fractures in different orientations around the tunnel increases.
To minimize the number of leaky bolt holes, the leaky bolt hole should be grouted and re-drilled. The cement shall have a maximum grain size of 16 micrometres. Furthermore, a method statement has been established for how to treat the already leaking bolt holes in order to increase the productivity and to take advantage of the performed grouting work.