Lemcon was nearing completion of its drill and blast excavation this month on the Nygard project in Sweden, having achieved an average advance rate of 20m per week.
The project involves construction of a 3km long, 125m² section, double-track single bore tunnel through mostly granitic gneiss. There is also a parallel, 2km long, 35m², maintenance tunnel plus five cross passages 500m apart.
Drilling started in late 2005 with a 500m adit to the common drives, north and south on the running tunnel and also on the adjacent maintenance tube. The south drives on each tunnel were the last to hold through
The north heading proved the more difficult of the two drives on the main tunnel, progress being more around 17m per week compared to 20m on the south face. Approximately two-thirds of the additional shotcreting and bolting works has been completed.
Over the majority of the programme the contractor has been driving four faces employing a combination of drill rigs from Atlas Copco and Tamrock – three of the four-boom rigs, a three-boomer and a two-boom unit. Only three rigs were in use at any single time.
The typical pull on the average round at each face was almost 6m though there were some closer to 5m initially. There was wide variability in the type of weaknesses in the rock at different locations along the drives, and systematic pre-grouting was employed for the main tunnel over groundwater concerns.
