Aging transportation infrastructure in North America is a serious threat to both the fabric of society and its economy. Many of these critical structures, such as bridges, wind turbines, transmission lines and telecommunication towers require proper inspection and maintenance. The most important issues with regard to ageing structures are the present condition of these assets and identifying maintenance priorities among a wide range of infrastructure facilities.1
Foundations are one of the key elements to look after when dealing with existing structures. Any structural upgrade and/or replacement, upgrade of structural loads and load paths requires careful examination of foundations. However, unlike structural components in the superstructure, foundation elements are often inaccessible, making characterization of them somewhat challenging.
Conventional inspection procedures such as excavation, coring and boring are intrusive and expensive. Moreover, the application of these methods for identifying the shape, dimensions and properties of existing foundations is often very limited.
Non-destructive evaluation of unknown foundations has gained interest among engineers and researchers over the past few years. Several inspection techniques have been developed and used to help engineers with the characterization of existing foundations.
Unknown foundations: the challenges
When inspecting existing structures, it is common to see that the original pile and foundation design information, as well as installation records, are no longer available. For example, the National Bridge Inventory (NBI) has identified more than 86,000 bridges across the United States with no foundation data on record. Bridge scour susceptibility has been a major concern for bridges around the globe.2
The same set of challenges exists in the inspection and testing of the foundations of existing telecommunication and transmission line towers. The introduction of new communication systems (i.e. 5G), requires a change in the design of antennas. Engineers may require additional information on the foundation of these towers to evaluate performance and structural reliability.
Another group of unknown foundations belong to street/highway lighting monopoles and street poles. In the absence of such information, engineers rely on field inspection results to evaluate the present condition of the foundation.
Depending on the type of the superstructure, the foundation system may vary. Foundations can either be shallow (individual spread footing), or deep (pile foundations). In the case of deep foundations, the arrangement of piles and pile cap adds to the complexity of the situation. Moreover, different soil conditions also affect the selection of proper investigation technique. A combination of non-destructive testing solutions and intrusive methods can be used to collect information about the unknown foundations. These methods aim to address two of the most important questions regarding existing foundations:
- Physical shape and dimensions (i.e. pile depth)
- Pile Integrity
The following describes the most common non-destructive testing methods for evaluating the present condition of unknown foundations.
Non-destructive evaluation of unknown foundations
Non-Destructive Testing (NDT) methods provide an excellent tool for evaluating the length and integrity of existing piles. Different NDT methods have been developed and tested within the past two decades. A comprehensive study was performed by the Bridge Maintenance Office of the Florida Department of Transportation and FHWA3 on the application of NDT methods for a cost effective and accurate evaluation of existing unknown foundations. The following describes some of the most widely recognized testing solutions:
Surface NDT methods
Surface NDT methods do not require any soil boring or probe. The main advantage with this group is that they are typically fast with minimal intrusion. The test equipment can easily be moved around the top of the substructure. The traffic disruption caused by this group of tests is often minimal. A major disadvantage to surface NDT methods is their inability to provide data below a subsurface pile cap.3 The following NDT methods are widely used:
- Low strain pile integrity test (PIT)
- Bending waves
- Ultra-seismic
- Surface wave spectral analysis
- Ground penetrating radar (GPR)
- Dynamic foundation response
Sonic-echo or low strain pile integrity test
Low strain pile integrity testing, known as PIT, is the most widely used NDT method for the evaluation of deep foundations. PIT provides a cost-effective and easy to deploy test for rapid assessment of integrity in piles and deep foundations. PIT was developed based on the concept of impact-echo and is customized for slender structural elements.
PIT uses stress waves generated by a hand-held hammer strike over the pile head. A motion transducer placed on the pile head records echoes (reflections) from the pile toe or other internal defects and anomalies. The recorded signal is amplified, digitalized and used for data interpretation and analysis.
Ground penetrating radar
GPR can be used to evaluate the sub-surface condition and configuration of the foundations. It provides a rapid and cost-effective tool for evaluation of sub-surface soil. It can be used to identify the location of spread footings and pile foundations. The selection of GPR antenna frequency depends on the estimated depth of foundation blocks.
Sub-surface methods
Unlike surface NDT methods, sub-surface methods involve putting in at least one soil boring or probe on or around the existing foundation to analyze the foundation. This group of testing solutions are often more time consuming, more expensive and often more difficult to execute in the field. However, the big advantage is higher versatility and reliability.3 The most widely used NDT methods in this group are:
- Parallel seismic
- Borehole radar
- Borehole sonic
- Cross hole sonic
- Induction field
- Borehole magnetic
How to choose the right test?
In order to select the most reliable and cost-effective testing solutions, engineers and project managers need to set the objectives right. The selection of the proper NDT method relies on accessibility, traffic and complexity of the foundation. The following general information helps engineers select the most appropriate test methodology:
- Pile material
- Pile configurations in the foundation layout
- Pile surrounding area (material)
- Pile condition (exposed or covered by pile cap)
- Ground water level
References
- Department of Homeland Security. (2010) “Aging Infrastructure: Issues, Research, and Technology,”
BIPS 01, 2010, available at
https://www.wbdg.org/FFC/DHS/bips_01.pdf
(last visited 8 Feb 2020) - Rashidyan, Saman. “CHARACTERIZATION OF UNKNOWN BRIDGE FOUNDATIONS.” (2017).
https://digitalrepository.unm.edu/ce_etds/177 - Federal Highway Administration & Florida Department of Transportation, (2010) “Unknown Foundation Bridges Pilot Study.”