Stem and root system response of a Norway spruce tree (Picea abies L.) under static loading

The mechanical response of a well-instrumented Norway spruce (Picea abies L.) tree under controlled winch loading was monitored. The main aims of this study were to understand the tree-root-soil response to lateral pull loads, to examine the applicability of simple engineering principles to the tree-root-soil response and to introduce a soil component into the tree stability analysis. The stem response was recorded with tilt sensors at three different heights; two sensors at each height tracked the response in transverse directions. These data were used to derive deflection and bending moment profiles of the stem for different lateral loads. A root on the windward side and one on the leeward side were instrumented using strain gauges. The tree was winched to failure and the data were collected from the strain gauges during winching. Using the data obtained from strain gauges on the roots, strain, bending moment, shear force and deflection profiles the roots with increasing load were calculated. Using the soil reaction force-deflection profiles of the roots, equivalent spring stiffness constants were determined. An estimate of the anchorage strength and moment-rotation relationship was made using a simple characteristic curve equation normalized by the failure moment and rotation that can be modified for different soil conditions.