News


October 2017
The Terrace has won a GOLD MEDAL in the Bristol in Bloom competition

June 2017
GREEN SQUARES AND SECRET GARDENS

The Garden took part in the Green Spaces and Secret Gardens event on 10-11 June 2017

 

May 2016 : the large copper beech at the south east corner of the garden may be infected with brittle cinder fungus, kretzsmaria deusta. This can affect the trunk and lead to sudden collapse. Stress test report summary below: It is safe to park near the tree.

Stress Test Report Summary July 2016
You commissioned us to perform a static load test on a mature copper beech (Fagus sylvatica’Purpurea’) at Worcester Terrace Gardens, due to concern about its condition given the presence of the decay fungus Kretschzmaria deusta, and to use the data collected during the site visit to provide an assessment of the likelihood of the structural failure of the whole tree through either stem fracture or through failure of the root plate.

(Not) attached are the results of the test, which we conducted on 6th July 2016. We tested the tree in two directions (WSW and NW) and performed 3 tests in each direction.

We applied a load to the tree and measured its reaction to this load in terms of root plate stiffness and stem bending. We then conducted a wind load analysis based on wind speed data, the tree’s dimensions and a number of parameters related to the surrounding environment and how they are likely to affect the wind loading using a model based on the Eurocode applied by engineers and architects when designing structures. We assess the tree using a design wind load that would be likely to occur during a 1 in 50 year storm. Consequently, the wind loads to which the tree is exposed annually are likely to be considerably lower. This is a conservative approach, taking into account a worst case scenario. We then apply this wind load analysis to the tree’s basic dimensions, assuming no defect or decay, to establish a theoretical safety factor for the tree. We then compare the theoretical safety factor with the measured data to calculate a calculated safety factor for the tree that represents its actual condition.

The safety factors at each position measured, in terms of root plate stability and stem fracture, were all found to be acceptable. A safety factor of 1.5 is the lowest safety factor that would still give a degree of confidence that the tree has an acceptably low likelihood of failure. A safety factor of 1 would mean that the tree is just able to withstand the forces to which it is assumed to be subjected. A safety factor of 1.5 gives a margin that allows for the natural variability in the test model.

The lowest safety factor for root plate stability for the beech was 2.7; that for stem fracture was 2.3, therefore both above the threshold of 1.5. However, although the absolute safety factors are acceptable, they are significantly lower than the theoretical safety factors of 6.7 (WSW) and 7.9 (NW), indicating strength loss that may well be due to the observed decay. The lowest safety factors for the stem were all measured below 1 m, which corresponds to the observed position of the decay.

So, in summary, the tree currently has acceptable safety factors for both root plate failure and stem fracture. However, this might change as both fungus and tree develop. It is not possible to determine how quickly, if at all, the fungus will progress through the tree. For this reason, we would recommend re-testing the tree in 3 years’ time to determine whether the safety factors are still acceptable. The next test may show little change, in which case future test intervals could be extended and then suspended if we keep showing no change, until some visual observation indicates a possible deterioration in condition.

Although we noted some dieback in the upper crown of the tree and somewhat small and sparse foliage, we would not advise any crown pruning work at this stage, as this is likely to be detrimental to the tree’s ability to respond to decay (a tree relies on the resources it produces from photosynthesis to withstand decay. Loss of leaf area reduces photosynthesis and causes stress; both weaken the tree’s ability to withstand decay). To help the tree to better withstand the decay, we would recommend improving the soil environment around the tree by spreading an area of woodchip in a circle around the tree out as far as the dripline (where this crosses the lawn you would need to carefully remove the turf layer first). The woodchip would preferably be from broadleaved trees and should be spread to a depth of around 10 cm, leaving a small ring (c. 10 cm wide) clear immediately around the stem. This would improve drainage, suppress competing weeds and restore good soil conditions, which may well have been compromised by poor drainage and the composting of grass clippings near the tree. We can advise further and liaise with the contractor if you commission this work to be carried out to avoid unintended damage being caused to the roots of the tree.

As I have explained, the wind load analysis that we apply to calculate the safety factors is pretty conservative, and is based on a model rather than actual wind conditions, which would need to be measured. One alternative to retesting the tree using a static load would be to use tree motion sensors to monitor the reaction of the tree, at the base, to real wind events over time. This would provide data that could be incorporated into the wind load analysis and may serve to show that the tree is afforded a degree of shelter that we cannot currently be confident to include within the wind load analysis. If the tree is sheltered the design wind load falls and all safety factors would increase proportionally. The cost of fitting the sensors and monitoring the results over a 1-year period would be £200 per annum. This is a nominal fee, has been heavily discounted and does not reflect the costs of the instruments or of the time required to monitor the tree. Do let me know if you are interested in this service.

Claire Harbinson
Senior Arboricultural Consultant
Treework Environmental Practice

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