Structure inspection manuals such as the Ontario Structure Inspection Manual (OSIM) mostly emphasizes on the visual inspection and physical condition of bridge structures. The visual inspection methodology in this guideline consists of close up visual assessment of defects in the superstructure and substructure. This methodology is limited to inspection of elements that are at the arms length. For elements with difficult access (i.e. long girders mid-span or deck over river or highways), the methodology is not very practical or accurate. Also, the methodology does not provide any information on how to evaluate concrete piles or foundations.
Stein and Sedmera (2006) reported that approximately 60,000 bridges throughout the US has been identified as having unknown foundations. As the author’s knowledge, this condition might be same in Canada. This is crucial because Canada owns a huge number of bridge and civil infrastructure facilities built between 1950 and 1980. These bridges and infrastructure are widely distributed across Canada, owned by counties, towns, municipalities, and provincial and federal governments. Most of these bridges and infrastructure need to be upgraded in accordance with the new code and standard requirements. To do so, it is essential to have enough information about the present condition of (unknown) foundations and piles.
How to Evaluate Concrete Piles?
A reliable condition assessment of substructures (foundations and piles) needs a basic information about type, material and as-built dimension. When foundations and piles are accessible, visual inspection, and mechanical loading of the piles is an effective option. This involves identifying the location of piles (or group of piles), and in-place static or dynamic loading tests to evaluate the reliability of piles and foundations. However, when the access to the piles is not possible, other techniques should be used. NDT methods provide a very reliable and practical alternative for testing and evaluating this group of piles.
Non Destructive Evaluation of Piles and Foundations
Different NDT methods exist for surveying foundations and piles. To select the best methodology for condition assessment of substructure, following factors should be considered:
- Type and material of foundation (e.g. timber, reinforced concrete pile, steel pile, composite system)
- Structural system of substructure (e.g. pile, foundation, or a combined system of foundation and pile)
- Pile condition (exposed or covered up by pile cap)
- Pile surrounding area
- Pile configuration in the layout
- Ground water level
- Foundation geometry
The NDT methods for unknown foundation and pile survey are in two main groups as of:
A. Surface Methods; and
B. Sub-Surface Methods.
Surface methods include NDT methods in which both acoustic wave source and receiver are placed at the surface. A probe signal propagates into unknown foundations, reflected off the foundation ends and boundaries. The received signal at the surface is used for post analyses.
The subsurface methods mostly include a probe borehole near to unknown foundation. A borehole or a group of borehole are drilled within surrounding areas. In these methods, the probe signal emitted into unknown foundation, receiving by an adapted transducer placed within borehole.
Other NDT methods
Acoustic surface methods can obtain simultaneously the following information:
1. Pile dimension (pile length)
2. Pile stiffness variations: this parameter can be correlated to cross section variation of pile in cast in-place concrete piles
3. Crack and defect location