The construction and operation of a new light rail transit or metro system in an urban landscape presents a series of corrosion risks to an often densely-packed buried infrastructure. Structures at risk from stray current include the rails, rail infrastructure, buried utility infrastructure such as pipelines, cables and building infrastructure such as steel sheet piles and structural steel work. Design issues that impact on stray current are often wrongly set with the initial design of the system - constrained by factors like electric power and land availability and the desire to limit the initial costs of a project. This has often led to severe stray current damage and serious transit systems management failures.
Developing stray current models of transit systems at the design stage can be an invaluable tool to optimise the design. The models allow for the calculation of stray current effects at a rail, rail infrastructure - tunnels bridges - and at third party infrastructure level and helps to drive design decisions. In this paper a series of stray current and corrosion software models were constructed to study the impact of stray current leakage from a proposed transit system on the surrounding infrastructure and used to test the effect of different design solutions and mitigation methods on the infrastructure. Transit operating data were used in conjunction with soil and structure data gathered from site measurements to predict the corrosion impact.
This paper is primary concerned with the risks associated from the construction of a proposed metro system extension onto neighboring metallic infrastructure including significant steel sheet pile structures at a large retail, leisure and office complex. A central component of the work is the development of stray current and corrosion models for the retail complex. Results of the model were used to identify the areas prone to stray current corrosion and to drive design decisions and recommendations for protective provisions to address the corrosion impact. While the impact of stray current on utility services is well documented and understood by transit designers, the risks to surrounding structures are given a lower emphasis in design assessments. For this work the size and importance of the at-risk structures was such that detailed studies were required.
The phenomenon and consequences of stray current leakage from light rail transit systems is well documented in the literature. 1-3 In brief, power is fed to the rail vehicle or streetcar from traction supply substations via an overhead line with a return circuit through the car wheels onto the running rails and back to the substations. Since the negative return circuit (i.e. the rails) has a finite longitudinal resistance and a poor insulation from earth, a proportion of the return current will leak to earth and flow along parallel circuits (either directly through the soil or through buried conductors) before returning onto the rail. Corrosion will occur at each point that current transfers from a metallic conductor, such as, in this case, steel sheet piles, to the electrolyte (i.e. the soil). Hence stray current leakage can cause corrosion damage to the rails of the system, the steel sheet piles in the soil and any other low resistance buried metalwork - typically buried utility services and other structural steel work.
The degree of corrosion damage will generally depend upon the magnitude and duration of the stray current flow and the surface area of steel over which it occurs. Calculating the total metal loss due to stray current is relatively straight forward (1 Amp per year will corrode approximately 9 Kg of steel); determining the distri
