Experimental investigation into the effects of cast-iron pipe corrosion on GPR detection performance in clay soils

Cast iron water distribution pipes are used widely in the UK and worldwide. Corrosion of these cast iron pipes often occurs due to an electrochemical process where the pipe is buried directly in a chemically aggressive ground (as is the case for some clays). The electrochemical process changes the pH environment and releases iron ions into the clay. This can cause chemical alteration of the clay minerals and corrosion products, such as iron oxide, hydroxide, and aqueous salts, to form in the soil. These chemical interactions are complex and time dependent, and can potentially result in pipe failure, and thus the conditions under which they occur need to be understood. Ground penetrating radar (GPR) has been proposed for routinely detecting, assessing, and monitoring buried cast iron pipes, and thus it is important to know how these chemical changes affect the electromagnetic properties of soil. A bespoke set of laboratory experiments was devised to simulate and accelerate cast iron corrosion (using electrokinetics) and ion migration processes in two types of clay, namely Kaolin clay and Oxford clay. Tests were conducted for periods of up to 3 months using both inert electrodes and a cast iron disc as the anode. The changes in the geotechnical properties (undrained shear strength, moisture content, and Atterberg limits), the geophysical properties (permittivity), and the geochemical properties (iron content, pH, and conductivity) were monitored. The results indicated that the Oxford clay was much more aggressive in terms of the corrosion activity compared to the Kaolin clay. The laboratory results were used in GPR simulations in relation to the detection of a buried cast iron pipe. The results showed that the chemically induced changes to the Kaolin clay did not materially affect the performance of GPR to detect the cast iron pipe, whereas for a pipe buried in Oxford clay the (greatly accelerated) chemically-induced changes were sufficiently advanced after approximately 7-8 weeks to cause the GPR to be unable to detect the corroded pipe.