Detection and evaluation of unknown foundations of old construction, particularly buried piles can be a big challenge, because:
* No structural drawing is available.
* The access to the piles is difficult, if not impossible.
* The traffic or operation of (or over) the superstructure can not be interrupted.
Certain non-destructive methods have been developed over years to study these type of substructure facilities. The application of these methods can be as easy as detecting buried foundations in shallow depth, or detecting pile lengths of a group of piles with pile cap, where access is totally impossible. Non-Destructive Testing (NDT) methods provide an excellent alternative for evaluating the length of existing piles. Different Non-Destructive Evaluation methods have been developed and tested within the past two decades. A comprehensive study performed by the Bridge Maintenance Office of the Florida Department of Transportation and Federal Highway Administration investigates and researches methods to accurately and economically evaluate the potential risk of scour failure for unknown foundation bridges. In this article, we will focus on GRP and how it can help engineers to obtain useful information about the unknown foundations.
GPR for Evaluation of Concrete Piles
GPR consists of a transmitter antenna and a receiver antenna, and a signal processing unit. GPR emits electromagnetic pulses (radar pulses) with specific central frequency to scan the subsurface medium. The reflected waves from subsurface layers, and objects are captured by the receiver antenna. Depending on impulse frequency, GPR is able to detect the internal events and objects at different depths. When there is a need for high resolution scanning, antenna with high frequency is required; however, the depth of penetration will be limited to the very first few centimeters. Lower frequencies are required when the objective is to detect very deep subsurface events and objects.
According to the report prepared by Florida Department of Transportation, “this technique has been widely used in Florida as a method of karst feature investigation and mapping”. The technology offers 3D scanning of sub-surface objects. However, certain challenges should be addressed when the method is being used for pile investigation.