Determination of space behind pre-cast concrete elements in tunnels using GPR

Many tunnels in Nordic countries are lined with prefabricated concrete elements to protect against frost and leakages. This protective lining is fixed to the rock face in a few points, but when installed renders the rock surface inaccessible, owing to the lack of predesigned inspection hatches. Safety inspections have hitherto consisted of random drilling into the concrete lining. However, such random inspection is both unreliable and expensive. Therefore Ground Penetrating Radar has been introduced in the vault walls to map the contours of the gap more systematically. Such scanning technology provides satisfactory data, given optimal location of apertures. The scanning technique is therefore extended to the vault roof to pinpoint potential rockfalls.