The TNO Centre for Timber Research has executed an extensive research programme into the rate of decay in foundation beams, as a result of lowering of ground water tables. The aim of the research was to develop a method, predicting the decrease in strength and stiffness in beams, due to wood decay during drytime of foundation beams. With the calculated extremes in decay, the damage caused by local settlements of the foundation can be estimated. Though this research deals with untreated timber only, the results yield information which may be used for methods of testing treated timber in ground contact. From 1987 to 1989 literature studies and field inspections have been evaluated, and the most important influencing factors were described. In the following laboratory research the aim was, to quantify the influence of these factors (wood species, dimensions, age, soil wetness, temperature, fluctuation of water) on the decay rate. The rate of decay was measured by mass loss after one year of exposure in a wet but aerated soil, using pine and spruce in different dimensions. For part of the specimens, decrease in strength properties was also measured. Results indicate, that for timber with high moisture contents (over 80%), the decay type is dominated by softrot (95%) and is mainly determined by the surface/volume ratio. The relation between mass loss and loss of strength and stiffness was determined. These results will be used for calculating the deformation of foundation.

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