Separation of bi-directional stress waves for the non-destructive condition assessment of in-service timber utility poles

Timber utility poles represent a significant part of Australia's infrastructure for power distribution and communication networks. Due to the advanced age of Australia's timber pole infrastructure, significant efforts are undertaken by state authorities on maintenance and asset management to prevent utility lines from failure. However, the lack of reliable tools for assessing the condition of in-service poles seriously jeopardizes the maintenance and asset management. For example, each year approximately 300,000 poles are replaced in the Eastern States of Australia with up to 80% of them still being in a very good serviceable condition, resulting in significant waste of natural resources and money. Non-destructive testing (NDT) methods based on stress wave propagation can potentially offer simple and cost-effective tools for identifying the in-service condition of timber poles. However, most of the currently available methods are not suitable for condition assessment of timber poles in-service due to complicating factors such as orthotropic material properties, soil embedment, complex wave behaviour and other uncertainties. An additional challenge for the assessment of poles is the simultaneous presence of upward and downward travelling waves resulting from the impact excitation at the lower section of the pole instead of the top due to practical constraints; i.e. in field testing, access to the top section of timber utility poles is prohibitive. The presence of the bi-directional travelling waves with their reflections from two ends of the pole produces complex wave propagation patterns that lead to difficulties in wave interpretation and analysis. This paper attempts to solve this problem by proposing a signal processing method based on low pass Finite Impulse Response (FIR) digital filtering, predictive deconvolution, and frequency wavenumber (F-K) phase velocity filtering algorithms to separate upward and downward travelling waves from the stress wave testing signals, and as a result detect a clear reflection from the bottom of the pole for embedment length estimation. In the proposed technique, first, low pass FIR digital filtering is applied to remove noise and unwanted high frequency components. Then, a predictive deconvolution algorithm is employed for the separation of the bi-directional wave of a pole with isotropic material properties. Finally, and F-K filter is designed and applied for better reflection detection of an orthotropic pole. The method is tested on numerical data of a timber pole modelled with both isotropic and orthotropic material properties.