John E Cheney. This paper was first published in GE’s July 1988 edition.
Precise levelling observations have recorded long-term heave of a single-storey building founded on London Clay after removal of trees. Observations were started when heave was suspected and extend from 8 to 32 years after construction.
The observations are presented in graphs and contour plots which show amounts and rates of up-lift, the effect on various parts of the building, and indicate the long time scales involved before movements slowed sufficiently for permanent repair work to be carried out.
From contours of recorded heave, even though for a period starting years after tree removal, it is shown that the most probable position of the removed trees can be deduced and their zone of influence defined.
Two methods of estimating initial unrecorded heave are discussed and it is evident that the maximum total heave was over 160mm (6.5in).
The results of soil swelling-potential and moisture content tests at a nearby site close to remaining trees are used to assess swelling potential of the clay.