Mechanical properties of timber foundation piles derived from small-scale compression tests

Abstract

Timber pile foundation piles are important structural components of historic buildings and constructions throughout the Netherlands. Many of these piles have been in service for a long time and start showing serious signs of biological degradation that affects their load-bearing capacity. It is essential to preserve these aged timber piles and, in order to make an estimate of the remaining functionality, it is essential to compute their wet mechanical properties. The conventional method for determining the wet compressive strength is by performing large-scale compression tests on segments from the pile. In this project it is researched whether it is also possible to determine the wet compressive strength by performing small-scale compression tests on discs taken from the pile, which could possibly save both time and resources during future investigations.

For this experimental study a total of 6 spruce foundation piles were selected that originated from 1727, 1886, 1922 and 2019. These piles brought forth 45 round wooden discs with each a height of 15 centimeters extracted from the pile head, middle and tip. The discs contained different amounts of bacterial degradation and high concentrations of knots in order to investigate their effects on the strength. The degradation was quantified using micro-drill measurements and the presence of the knots was specified as a knot ratio. From the results of the small-scale compression tests on the discs it can be concluded that the strength of the discs is well correlated with the strength from the large-scale tests. It was observed that the degraded wood hardly contributes to the strength of the pile and that the section with the highest knot ratio governs the strength of the pile. All in all, it was found possible to obtain accurate values for the wet compressive strength of aged timber piles by performing small-scale tests on discs from the piles, as long as the effects of the biological degradation and the knots are taken into account.