In addition, non-destructive testing
approaches are usually employed to detect the location and potential
harm of hidden defects, such as void, delaminating behind
coating, thickness of concrete cover, or corrosive reinforcing bars.
Furthermore, chemical ingredients of structural materials and
environmental substances should be sampled from the tunnel
and tested in laboratory to check their potential detriment to
structural performance. In a few cases on-site structural experiment
may be conducted to judge the safety level of a tunnel structure.