Residual life and degradation assessment of wood elements used in soil bioengineering structures for slope protection

Soil bioengineering techniques to repair slope failures and increase slope stability are often used, if possible, as alternatives to traditional structures in order to mitigate the environmental impact without losing the effectiveness of the work. These techniques use live plants and entrust most of their structural resistance to wood members whose mechanical characteristics, however, decrease with time. Very few investigations have dealt with the decay of mechanical characteristics of wood elements employed in soil bioengineering techniques and lifetime predictions. This information is also important for durability forecasting. In this paper we present the results of experimental analyses carried out to evaluate the degradation of mechanical properties of untreated timber elements, which were collected from single and double live cribwalls or check-dams built about 10 years ago in the Vesuvius National Park (Naples, Southern Italy). To help in adequately interpreting the behavior of a complex material such as wood, a novelty of this study is not only the coupling of the ultrasonic and sclerometric nondestructive testing (NDT) methods so as to reduce measurement errors, but also to frame the outcomes from mechanical tests within the biological analysis of fungi detected on the timbers. Exploratory statistical analyses have revealed that there exist nearly no correlations between moisture contents in the timber elements and both transversal and longitudinal ultrasonic velocities, but some clustering provides insights in the results obtained. Comparisons between results using data measured on "old" and "new" timber elements enable some evaluations of residual safety factor assessment to be carried out.