Ecological Engineering 90 (2016) 498–509 Contents lists available at ScienceDirect Ecological Engineering jo ur nal home p ag e: www.elsevier.com/locate/ecoleng Residual life and degradation assessment of wood elements used in soil bioengineering structures for slope protection N. Romano a,∗ , G.P. Lignola b , M. Brigante b , L. Bosso c , G.B. Chirico a a Department of Agricultural Sciences, Division of Agricultural, Forest and Biosystems Engineering, University of Napoli Federico II, Portici (Napoli), Italy b Department of Structures for Engineering and Architecture, University of Napoli Federico II, Napoli, Italy c Department of Agricultural Sciences, Division of Biology and Protection of Agricultural and Forest Systems, University of Napoli Federico II, Portici (Napoli), Italy a r t i c l e i n f o Article history: Received 30 July 2015 Received in revised form 30 December 2015 Accepted 26 January 2016 Available online 9 March 2016 Keywords: Soil bioengineering Slope protection Erosion control Wood Mechanical properties Nondestructive testing Residual life Degradation a b s t r a c t Soil bioengineering techniques to repair slope failures and increase slope stability are often used, when possible, as alternatives to traditional structures in order to mitigate environmental impact without com- promising effectiveness. However, such techniques use live plants and entrust most of their structural resistance to wood members whose mechanical characteristics diminish with time. To date, the decay of mechanical characteristics of wood elements employed in soil bioengineering techniques and life- time predictions has been little explored. Such 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 ten years ago in the Vesuvius National Park (Naples, Southern Italy). To help interpret the behavior of a complex material such as wood, a novelty of this study lies not only in combining ultrasonic and sclerometric nondestructive testing (NDT) methods so as to reduce measurement errors, but also in framing the outcomes from mechanical tests within the biological anal- ysis of fungi detected on the timbers. Exploratory statistical analyses reveal that there exist hardly any correlations between moisture contents in the timber elements and both transversal and longitudinal ultrasonic velocities, although some clustering provides insights into 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. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Soil bioengineering methods are increasingly applied for slope protection, stabilization of shallow rapid landslides, erosion con- trol, stream-bank problems, and riparian restoration (USDA, 1995; Stokes et al., 2008). To mimic nature, the basic structural com- ponents of most soil bioengineering structures are living plants in combination with wood elements arranged in different ways. Typical examples are wattle fences to establish terraces in over- steepened slopes or timber crib retaining walls (live cribwalls, for short) whose inert structural parts are untreated log or timber ele- ments. The functionality of the system, i.e. living plants plus inert ∗ Corresponding author at: Department of Agricultural Sciences, Division of Agricultural, Forest and Biosystems Engineering, University of Napoli Federico II, Via Università, n. 100, 80055 Portici (Napoli), Italy. Tel.: +39 081 2539421; fax: +39 081 2539412. E-mail address: nunzio.romano@unina.it (N. Romano). parts, relies not only on the mechanical properties of the individ- ual elements, but also on the entire structural behavior originating from the lattice-like assembly and stacking arrangements of the various structural members. In this paper, we basically deal with the former problem, even though we also elucidate on the latter aspect regarding the effects exerted by the decay of the element mechanical characteristics on the overall structural behavior. Soil bioengineering works are generally designed under the assumption that the growth of plant roots, within and outside the inert wooden structure, gradually provides additional strength to the whole structure, hence in some way compensating the reduc- tion in the mechanical characteristics of the decaying wooden elements (USDA, 2010). Therefore, from a practical perspective, the design of a soil bioengineering structure can undoubtedly benefit from scientific studies aiming to assess, under different environ- mental conditions, the time-scales of the decay on wood strength to be compared with the time-scales of plant root development. As all civil engineering structures, soil bioengineering systems are preliminarily designed taking allowable stresses and design http://dx.doi.org/10.1016/j.ecoleng.2016.01.085 0925-8574/© 2016 Elsevier B.V. All rights reserved.