AN ANALYSIS OF SEASONAL SUBSIDENCE AND HEAVE IN DEFENCE OF OUR URBAN TREES

Summary Conclusions are drawn from a case study of seasonal subsidence extending over a period of some forty years on a private dwelling house built in an area underlain by the Upper Tunbridge Wells Series. The cause of the subsidence has always been associated with the presence of the surrounding trees. In this paper the writer asserts that the degree of subsidence is directly related to the local annual rainfall profile which, if it is insufficient to make up the loss to ground water from the clay layers in which the foundations are founded, subsidence can occur even in the absence of trees. A model is described which predicts, for the subject case, the degree of subsidence for a given local annual rainfall profile. The predictions are compared with the precise level measurements and rainfall profiles recorded over a number of years. Mitigating measures such as tree reduction appear to have had little influence on the movement.