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Determining the Unknown Depth of Foundations
Bridge foundations can be divided into shallow footings or deep foundations. Footings are mostly square or rectangular in shape. They may also be pedestal masonry stone footings or massive cofferdam footings in shape. Piles might be present with or without pile caps and may be battered or vertical. Piles can be made of concrete (round, square, or octagonal), steel (H-piles or round pipe sections), or timber. Deep foundations can be pre-cast concrete piles, or more recently, drilled shafts and auger-cast concrete piles. The top of footings or pile caps may be buried underneath riprap, backfill mud and/or channel soils.

Nondestructive tests (NDT) that are used for the determination of the unknown depth of foundations usually involve seismic methods and are often called small strain tests. The term "small strain test" is used to describe tests where a small seismic energy source, such as a hammer, is used to generate the seismic waves.

These tests can be divided into two groups: Surface NDT, if access is required only at the surface of a foundation, or Borehole NDT, if a borehole is drilled close to the foundation structure and extends along its length.

Five potential surface NDT methods are used to determine foundation depths. These are Sonic Echo, Bending Waves, Ultraseismic Vertical Profiling, Seismic Wave Reflection Survey, and Transient Forced Vibration Survey. Borehole NDT methods include Parallel Seismic, Borehole Radar, Magnetic Methods, Dynamic Foundation Response, Borehole Sonic, cross-borehole tomography, and, where the substructures contain steel, Induction Field. The application of each method to specific engineering problems, the basic concept for each method, field data acquisition, processing and interpretation are described below. Advantages and limitations are also discussed.