Bacterial degradation of nine wooden foundation piles from Gothenburg historic city center and correlation to wood quality, environment, and time in service

Nine wooden foundation piles of spruce and pine supporting historic buildings in the old town of Gothenburg, Sweden, were examined for fungal and bacterial degradation. The aim was to assess the type and degree of decay of the pile heads and correlate variations to the local environment, time in service and the wood quality. Soil and water samples were measured in the field for basic hydrogeological analyses, and pH, redox, O2 and groundwater level. The piles showed strong similarities in decay. Detailed light microscopy examination revealed that solely erosion bacteria caused degradation; both in outermost soft layers of the sapwood and further inwards. Most severe decay was present in the outermost layers, 1-3 cm, thereafter decay generally decreased and stopped at varying depths. Most piles had sound interior wood tissue, but a few were superficially to moderately degraded throughout. Some piles showed pronounced variation in degree of decay at the north respectively south side of the stem. This might be related to local water flow directions in the soil. In two piles, minor decay by white rot and soft rot were observed. Soft rot attacks were concluded to be of older date, most likely related to the time of construction, whereas white rot either infected the wood during an aerobic period; e.g. extreme drainage of unknown date, or alternatively the attack developed already in the living tree before felling. Despite that all pile heads (except one) were exposed below groundwater level and mainly in compact clay soil with low redox values, it was not possible to correlate any specific environmental parameter from soil and pore water analyses to the observed decay rates. Diameter of the pile and time in ground were the only two factors clearly correlated to the long-term performance of the piles in this specific urban environment.