It has been observed that the corrosion of iron pipes in soil can produce variations in ground conductivity around the pipe, and that the visibility of such pipes to GPR can be greatly reduced. This new investigation and measurement of the permittivity and conductivity of soil contaminated by iron pipe corrosion products produces more accurate knowledge of permittivity and conductivity data and their likely spatial vari ation with distance from the corroding pipe. The experimental data are the result of monitoring accelerated corrosion over a period of several weeks and using TDR and direct conductivity measurement schemes. FDTD simulations of GPR signals show how the corrosion induced variation in the visibility of the pipe varies with the thickness and shape of the new spatial variations permittivity and conductivity.The results indicate that in the earlier stages of pipe corrosion use of lower GPR frequencies will still detect the pipe, although at lower spatial resolution.

The ground in which utility service pipelines are buried inevitably controls, to a large degree, the structural performance and progressive deterioration of the pipelines themselves. In a parallel programme of research to the UK Mapping the Underworld (MTU) project, a study of the fundamental properties of the ground, and how they change with the seasons and local physical and chemical contexts, is being conducted at the University of Birmingham, UK. While the results of this study feed into both the operational protocols for the MTU multi-sensor location device and the associated knowledge based system (KBS) that is being created to aid its deployment (both topics being the subjects of separate papers to this conference), the suite of complementary research projects on the ground and its properties provide valuable insights to the pipeline engineer. Geophysics is being used by the research team to explore the state of the ground with the aim of highlighting areas of concern for the structural health of pipelines buried in the ground. Studies of cast iron pipeline corrosion mechanisms have focussed on the changes that the reaction products cause to the surrounding soils, with a particular emphasis on clay soils, and one interesting finding is that these clay-iron reaction products can make the pipelines 'invisible' to standard geophysical location devices. Moreover there are other features in the ground that are being targeted (voids, ground wetting and softening due to leakage, ground weakening due to progressive erosion), and these features effectively make the ground more or less 'visible' to geophysical technologies. Alongside this work, bespoke tests have been developed for use on site to 'calibrate' the geophysics, thereby enhancing the signatures of the features. This paper introduces these parallel research projects and draws out the important findings for pipeline engineers charged with establishing the condition of existing buried assets.

This paper describes research into the effects of corroding cast iron pipes on the properties of surrounding clay soil. Accelerated corrosion tests, employing electrokinetics to simulate the galvanic cell that is set up naturally when cast iron comes into contact with clay, have shown that the chemical changes (e.g. pH) are more marked than for cases in which inert electrodes were used. These chemical changes result in both cast iron and clay mineral dissolution, ion migration and the precipitation of reaction products close to, and to a lesser degree away from, the pipe wall. The paper describes a laboratory study of the effects on a pure form of kaolinite after different lengths of electrokinetic treatment and draws conclusions on the potential effects that are likely to be found for old cast iron pipes in clays.

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 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.