In a bridge system, bridge superstructure is defined as all structure above bridge bearing elevation and bridge substructure is defined as everything below the superstructure. Therefore, bridge substructure includes all foundation elements such as columns, wall piers, footings, pile caps, precast or auger-cast concrete piles, drilled shafts, etc. The substructure can be generalized as an abutment or pier, which can be made of concrete, masonry, stone, steel and/or timber.

Bridges play a key role in the national transportation system, and the ability to assess their condition is vital for safe transportation operations. Some of the bridges that comprise the system traverse waterways, where bridge failures most commonly occur due to scour damage. The loss of support from scour damage can result in settlement, which, in turn, can affect the superstructure and lead to subsequent failure. These problems are not always easily detected. Furthermore, many older bridges in the United States have no plans of the foundation system.

There are approximately 580,000 highway bridges in the National Bridge Inventory. The type and/or depth of the foundations of about 104,000 of these bridges are unknown. For a large number of older non-federal-aid bridges, and to a lesser extent federal-aid bridges, there are no design or as-built bridge plans available to web manual the type, depth, geometry, or materials incorporated in the foundations.

These unknown bridge foundations pose a significant problem to State Department of Transportations from scour volunerability concerns. Because of the risk of scour undermining bridge foundations and the threat to public safety, there is a need to screen and evaluate all bridges over rivers, streams, and in coastal areas, both on and off state systems. To determine the susceptibility of a bridge to scour, information on the foundation type and depth is needed to perform an accurate scour evaluation of each bridge. It would be quite beneficial to have a method, or methods, that can detect and evaluate the structural conditions beneath bridge piers. One promising approach is nondestructive testing.

In this section, nondestructive test (NDT) methods used for condition evaluation of bridge substructure are described. The term “condition evaluation” includes determination of the unknown depth and geometry of the foundations as well as assessing the integrity of bridge substructure elements and bridge scour. In Bridge System Superstructure, NDT methods used for evaluating bridge superstructures are discussed.