Tree Stability in Relation to Cyclic Loading

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Abstract
The mechanical behaviour of the soil-root systems of Sitka spruce trees growing on a peaty gley was measured under cyclic loading, using a specially designed apparatus. The trees were felled at a height of about 1.5 m above the soil surface before testing so that effects due to the stems and the crowns of the trees were eliminated. The cyclic loading apparatus repeatedly pulled the top of a stump by a set amount, between 5 mm and 100 mm, at a frequency of about 0.2 Hz. The force exerted on the stump and the corresponding vertical displacement of the root plate were measured. During a cyclic loading episode, the peak load in each cycle was less than the peak load in the previous cycle. The size of this difference became smaller with time and was negligible after about 25 cycles. The difference in load between the start and end of each cyclic loading episode tended to increase with stem displacement. A trench was dug around some trees so that soil failure could be isolated from other effects and some trees were tested without cyclic loading. Average maximum overturning moment for the untrenched trees was 15 kN m. Trenching decreased this by an average of 10 kN m. Cyclic loading decreased the soil resistance component of anchorage from 2.3 kN m to about 1.7 kN m. The reduction in peak overturning moment with repeated loading was associated with progressive failure of the anchorage, whereas the differences in anchorage strength between cyclic and static loading were associated mainly with energy dissipation.

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