Aespoe Hard Rock Laboratory. The TASS-tunnel. Geological mapping

The project entitled 'Sealing of tunnel at great depth' (Fintaetning av tunnel paa stort djup) needed a new tunnel in an area as undisturbed as possible and with cross-cutting water-bearing structures. The new tunnel, which was given the name TASS, was excavated on the -450 m level of SKB's Aespoe Hard Rock Laboratory (Aespoe HRL). The length of the tunnel is approximately 80 m and the theoretical tunnel area 19 m2. As is the case with all the other tunnels of the Aespoe HRL, the new tunnel has been geologically mapped. In addition, laser scanning combined with digital photography has been carried out. The tunnel was also used to test various types of explosives, borehole layouts and drilling techniques. The geological mapping of tunnel floor, walls and roof took place on four major occasions when a halt was made in tunnel excavation to allow for various tests. Before the mapping started on these occasions, laser scanning took place. The tunnel faces were mapped after each round (drilling, blasting and unloading). The present report describes the geological features of the tunnel and briefly how the laser scanning was performed. Water-bearing structures have been compared to similar structures in the neighbouring tunnels. The rock type names used here follow the old established Aespoe HRL nomenclature. Narrow (<0.1 m wide) dykes are normally mapped as fracture fillings. The dominating rock type is Aespoe diorite, which constitutes some 90 % of the rock mass. It is mostly mapped as fresh rock. . Minor constituents of the rock mass are fine-grained granite, hybrid rock, pegmatite, quartz veins/lenses and undifferentiated mafic rock. The mapping of fractures and deformation zones considers a number of parameters such as number of fractures, open/healed, width, length, description of fracture surfaces (roughness, planarity, etc), fracture filling, alteration and water. The deformation zones are discriminated into two main categories ('increased fracturing' and 'deformation zones proper'). The orientation of the structures is measured with a compass, and magnetic north is used for reference purposes. Two main fracture sets appear in the TASS-tunnel. 1. East-west striking and steeply dipping. This fracture set dominates with a mean orientation of 097/86. 2. Sub-horizontal to gently dipping with a more varying strike. This set may be divided into two subsets with the mean orientations 037/03 and 280/18 respectively. The observed categories of deformation zones appear to fall within two major groups of orientation. 1. Increased fracturing is oriented approximately ENE-WSW with a moderate dip to the north. 2. Deformation zones proper (brittle/ductile) have a general orientation of approximately ESE-WNW strike with steep dip mostly to the south. These zones generally cross-cut the tunnel and their widths vary from 0.1 - 0.3 m. The fractures and deformation zones normally contain secondary minerals. The most common fracture-filling mineral in the tunnel is chlorite, followed by calcite, epidote and prehnite. Quartz, pyrite and red feldspar are also quite common, while biotite, iron oxide and unconsolidated filling materials (grout and clay) have only been observed a few times. Occurrences of water are of great interest since one of the major tasks of the project was to prove that the grouting compounds used could seal tunnels at the tentative final repository depth. The orientations of water-bearing fractures for the tunnel are dominated by sub-horizontal to gently dipping fractures, with mean orientations of 000/00 and 297/28. Two less prominent water-bearing fracture sets of 287/70 and 099/89 also occur