21 May 2012

Landscape Planning Group Ltd.

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Subsidence and trees

It is a commonly held belief that subsidence cases are dependent on at least four primary contributory factors being present at the same time:

A property with shallow foundations
Shrinkable clay soils
Dry summer conditions
Trees and other vegetation abstracting soil water

It is perfectly true that many subsidence cases are due to the inadequate depth of foundations, it is also true that shrinkable clay soils represent the single most important group of soil types in respect of the likelihood of damage occurring.

However the role of the climate and of vegetation other than trees in the potential for building damage is continually misunderstood.

Dry Summer Conditions

The enclosed chart demonstrates conclusively the limited role climate can play as an isolated causative factor of ground movements. The data is taken from the Meteorological Office, Rainfall and Evapotranspiration Calculating System (MORECS*) database. The data recreates the resulting soil moisture deficit loss from clay soils as they are subjected to:

The effects of sunlight on bare soil
The effects of grass on the soil
The effects of trees on the soil

The data illustrates clearly that sunlight (climate) can only generate relatively small soil moisture deficits without the presence of vegetation to actively â€œpumpâ€ water from the soil. Evaporation is a very slow and poor method of drying soil. No matter how dry the summer(s) sunlight is not a likely source of subsidence in low rise buildings. The effect of sunlight contrasts sharply with the effect of trees and grass on soil moisture levels.

VEGETATION OTHER THAN TREES

An assessment of the MORECS data comparing the relative effectiveness of grass v trees would naturally tend to suggest that the impact is quite considerable for both vegetation types. At soil moisture deficits of 100mm+ subsidence and damaging ground movements become increasingly likely, therefore the impact of grass and other minor vegetation, shrubs, climbers etc, might be thought to be significant, especially if they are competing for soil moisture reserves, this is a deceptive conclusion, which we treat with great caution.

Experience of many thousands of vegetation related subsidence cases indicates a statistically insignificant role for grass associated with subsidence of low rise buildings and yet grass can clearly dry soils very effectively.

How can we explain this anomaly? Whilst the MORECS data is entirely correct that grass can dry soil across its rootzone in a dramatic way. The key observation is the depth of rooting of the two plant types. Grass and other minor vegetation have very shallow and localised rooting patterns and whilst the impact locally and in the initial soil levels can appear dramatic the plants are unable to generate additional root tissue at depth, this is why the above ground leaves of grass within the garden grown as lawns die early in a dry summer. Trees however dominate the soil environment and particularly in the deeper engineering soil, below 1m, have no competitors for transpirational requirements.

MORECS TECHNICAL PAPER FINAL SEP07

Filed under: Insurance Claims and Trees on October 17, 2011
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