Long-term assessment of chestnut live cribwalls deterioration using drilling resistance measurements

Live cribwalls, a common Soil and Water Bioengineering (SWB) intervention, are typically built using timber, in combination with soil and plant materials. Timber, due to prolonged exposure to environmental and biological factors, inevitably undergoes progressive deterioration. This process is a fundamental element of live cribwall design that needs to be considered, as the simultaneous growth of live plants gradually provides soil reinforcement and compensates for the loss of wood resistance. Despite the importance of this dual dynamic, long-term monitoring data and standardized diagnostic tools to assess the deterioration of timber are still lacking. Among various diagnostic tools used to evaluate timber deterioration, drilling resistance (DR) is a semi￾destructive technique capable of providing rapid and reliable assessments of wood density and mechanical properties. A 15-year monitoring campaign on 15 chestnut (Castanea sativa Mill.) cribwalls, aged from 2 to 31 years, produced 794 perforations. Because DR measurements are influenced by operational parameters such as feed and drill speeds, we developed a correction model to standardize data collected over the years with different instrument settings. Results revealed a consistent linear decrease of DR values with structure age, corresponding to a reduction of approximately 2% per year, directly reflecting the progressive loss of density and mechanical strength of the wooden member. Distinct degradation stages were identified among cribwalls of different ages, confirming the gradual deterioration of structural timber over time. Within single structures, additional differences emerged depending on log position and microclimatic exposure. DR techniques proved valid and practical for operational use, offering a quick and reliable tool suitable for monitoring programs and research studies. The standardized approach developed here enhances consistency in the monitoring of cribwalls. It opens new opportunities for integrated research that combines wood degradation monitoring with the assessment of root reinforcement from vegetation growth, leading to a better understanding of the life cycle and functional lifespan of live cribwalls.