Generation and propagation of acoustic emissions in buried steel infrastructure for monitoring soil–structure interactions

Soil–structure systems (e.g. pipelines, pile foundations, retaining structures) deteriorate with time and experience relative deformations between the soil and structural elements. Whether a result of age, working conditions, or environmental conditions, deformations have the potential to cause catastrophic social, economic, and environmental issues, including limit state failure (fatigue, serviceability, ultimate). The UK spends £100s of millions a year spent on infrastructural maintenance; the early detection of deterioration processes could reduce this spend by an order of magnitude.Techniques to monitor ground instability and deterioration are consequently increasing in use, with most conventional approaches providing localised information on deformation at discrete time intervals. Nascent technologies (e.g. ShapeAccelArray, fibre optics) are however beginning to provide continuous measurements, allowing for near real-time observations to be made, although none are without either technical limitation or prohibitive cost.A novel monitoring system is proposed, whereby pre-existing and newly built steel infrastructure (e.g. utility pipes, pile foundations) are employed as waveguides to measure soil-steel interaction-generated AE using piezoelectric sensors. With this, a two-stage quantitative framework for understanding soil-steel interaction-generated AE and its propagation through steel structures is also developed where (stage 1) informs the creation of an adaptable sensor network for a variety of infrastructure systems, and stage (2) informs interpretations of the collected AE data to allow for decision makers to take appropriate action. Timely actions made possible by such a framework is of great significance to practitioners, having the potential to reduce the direct and indirect impacts of deterioration and deformation, whether long- and short-term.Stage 1 used an extensive programme of computational models, alongside small- and large-scale physical models, to enable attenuation coefficients to be quantified for a range of soil types. It was shown that both the structure and bounding materials, i.e. the burial system, significantly influenced propagation and attenuation through steel structures. In free-systems, though, the frequency-thickness product was more influential; propagation distances of 100s of metres are obtained at products Stage 2 used a programme of large direct-shear box tests to allow for relationships between AE and normal effective stress, mobilised shearing resistance, and shearing velocity to be quantified. This enabled for quantitative interpretations of soil-steel interaction behaviours to be made using various AE parameters. Both the magnitude of values, and the rates of change of the parameters, could be used in the interpretation of behaviours. Shearing and stress conditions of sand could also be determined, increasing proportionally with AE activity, whilst the point at which full shear strength mobilisation occurs was also identifiable.</div

Monitoring buried infrastructure deformation using acoustic emissions

Deformation of soil bodies and buried infrastructure elements (i.e. soil-structure systems) generates acoustic emission (AE). Detecting this AE by coupling sensors to buried structural elements can provide information on asset condition and early warning of accelerating deformation behaviour. A novel approach for deformation monitoring of buried steel infrastructure (e.g. pipes and pile foundations) using AE is described in the paper. The monitoring concept employs pre-existing, or newly built, buried steel infrastructure assets as waveguides. The propagation of AE through example pipes acting as waveguides has been modelled computationally using the program Disperse. A parametric study has been used to investigate the influence of key variables such as burial depth, surrounding soil type, internal environment, pipe diameter, wall thickness, frequency and mode type upon AE propagation and attenuation. Understanding the propagation and attenuation of AE is of fundamental importance for development of a monitoring strategy and specifically to determine the spacing of sensors deployed along infrastructure elements. The generation of AE due to soil-structure interaction mechanisms has been investigated using a programme of large direct shear tests of soil against steel plates under a range of conditions (e.g. soil type, plate surface conditions, stress level, strain rate). New, fundamental understanding of AE generation and propagation in buried infrastructure is enabling a framework to be developed for interpreting asset condition from AE measurements. The paper will introduce the approach developed, describe the parametric study of AE propagation and attenuation presenting example results, and show typical AE behaviour for soil-structure interaction obtained in the large shear tests. The implications for design of a monitoring framework will be discussed

Acoustic emission generated by granular soil-steel structure interaction

This study has established quantitative interpretation of acoustic emission (AE) generated by granular soil/steel structure interaction. This new knowledge will enable the evolution of soil/structure interaction behaviour to be interpreted from AE measurements. AE monitoring could now provide early warning of soil/structure system (e.g. buried pipelines, pile foundations and retaining structures) limit state failures in the field and enhance insights into element and physical model tests in the laboratory. Results from a programme of large direct-shear tests performed on granular soil/steel interfaces show that AE generation is influenced by the normal effective stress, mobilised shearing resistance and shearing velocity. Compression-induced AE activity in granular soil/steel systems is negligible until the current stress conditions exceed the maximum that has been experienced in the past. Relationships have been quantified between AE and normal effective stress, mobilised shearing resistance and shearing velocity, enabling quantitative interpretation of cyclic and accelerating soil/structure interaction behaviour from AE measurements