Safety, reliability and satisfactory performance of structures require a well-planned maintenance program, as well as remedial actions in short and mid-term. Existing structures that are designed in accordance with older design codes or were built by older construction practices may need to be extensively repaired, rehabilitated, or replaced within the next few years. The remedial actions may include structural and/or foundation upgrade or replacement. Reuse of existing piles and foundations is a sustainable, cost-effective, and time saving solution in the repair or replacement of existing poor infrastructure. In this article, we will briefly discuss how NDT can provide useful information about existing piles and deep foundations.
Reuse of Existing Piles – Challenges
When it comes to choose between replacing existing piles, or reusing them for the new in the project, certain questions should be asked. Does the capacity of existing piles meet the requirements of new projects? Or, what would be the consequences of removing existing piles? For some projects, replace existing piles and foundations is a challenging task in terms of project timelines, technical issues and budget. A great example is replacement of existing substructures in urban areas, surrounded by high-rise buildings and infrastructure facilities. Reuse of existing piles provide a more cost-effective, sustainable, and efficient solution. However, the reliability of piles and foundations should be evaluated prior to reuse of existing piles.
Other considerations would be:
+ The locations of applied loads and the existing foundations should be compatible in new structure with that of old structure;
+ Sufficient verification should be performed on the reliability of existing foundations and piles (e.g. pile length, pile integrity and pile capacity testing);
+ Pile and foundation remaining service life will fulfill the new structure requirement.
How Can NDT Help?
The access to existing pile elements is often challenging. Existing piles or foundations are generally covered up by cap or superstructure (not exposed). In overpass structures (e.g. bridges, viaducts), substructure is placed underwater that makes the inspection and evaluation process very challenging. Suitable solutions should be adopted for pile length and integrity testing. Nondestructive testing methods provide a great alternative for evaluating the length and integrity of existing piles. NDT methods are extensively used for evaluating pile length and integrity testing. However, there is no unique NDT solution that covers answers all questions. A combination of NDT methods will provide useful information on the re-usability of piles. The testing method may be selected based on the following criteria:
+ 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
A Spotlight on Pile Integrity Testing – PIT
The test method based on ASTM D5882 (Standard Test Method for Low Strain Impact Integrity Testing of Deep Foundations), generally referred to as Pile Integrity Testing (PIT), is mostly used for concrete piles with access to pile head and top surface. The test results can be used for evaluation of the pile cross-sectional area and length, the pile integrity and continuity, as well as consistency of the pile material; It is noted that this evaluation practice is approximate.