Summary
Joints and fractures were weathered during a sub-tropical climate regime and thus may contain smectite and kaolinite. The presence of such minerals increases instability in fracture and fault zones. The clay-bearing zones may cause mechanical problems during both tunnel construction and later operation. Due to the chemical alteration of magnetic minerals during weathering, weak zones are characterized by negative magnetic anomalies. Such zones are also prone to topographic depressions. The recognition of this relationship has led to a method involving the combined analysis of magnetic and topographic data to predict zones of deep weathering referred as the AMAGER method (Olesen et al. 2007). We introduced AGC (Automatic Gain Correction) to the magnetic data to improve signals from areas having low/medium magnetic anomalies. The improved AMAGER method has produced a new map that is furthermore xtended to the west of the Oslofjord area relative to the Olesen (2006) version. The method has successfully mapped known weak zones in the Lieråsen and the Romeriksporten railway tunnels and the Hvaler road tunnel, as shown in the previous version by Olesen (2006). In addition, it has identified known weakness zones causing tunnel collapse reported in the Hanekleiv tunnel at E 18 and in the "Bygarasjen" in Skien area. This has resulted in an awareness map for tunnel planers ("aktsomhetskart for unnelplanlegging"), and was first published in 2006 (Olesen 2006), with a refurbished and expande version published in 2012 (Baranwal & Olesen 2012).
