Investigation of voids and cavities in bored tunnels using GPR

Tunneling by an Earth Pressure Balance Machine (EPB) encounters a range of ground conditions ranging from hard rock to soft soil. To maintain the stability of routed ground, grouting is required both before and during the advance of the machine. Due to difficult geological conditions and incomplete grouting, cavities and voids can be created around the tunnel excavation. These can cause unpredictable settlements of the ground and peak stresses in the tunnel lining. So far, these hollow /water filled spaces have only been detected manually by boreholes driven through the crown of the tunneling lining. GPR can be used as an alternative method to detect these spaces. Experimental and numerical investigations were conducted to prove the applicability of GPR for this task.