Permaculture Designers Manual
CHAPTER 8 – WATER IN PERMACULTURE
Section 8.19 –
Soils in House Foundations in Permaculture
Soils cause perhaps 60-80% of all house cracks and insurance claims for faulty construction and “tree damage”.
About 20% of the soils we build on will subside or heave depending on water content.
Specifically, black cracking clay, surface clays, and red-brown clay loams are subject to swelling and shrinking.
Solid stone and brick houses are most subject to structural failure, with wood-frame and veneer less so.
Over-irrigation of gardens, causing the water table to rise, is a primary cause of soil swelling.
The removal of trees assists this process, as do paved areas, and burst or leaking sewage and water pipes.
Some notorious white or yellow clays collapse as dam walls when wetted. It is as well to consult your local soil expert for large constructions as trials can be expensive.
While the effects are most noticed to 2m deep, probes to 10m deep need to be monitored for ground water levels.
Soils subside and shrink with excessive drying (too many trees too near the house) and swell and heave with excessive watering and no trees.
Adelaide (Australia) is an area where most damaged houses are on black soil clays, but several other areas also suffer these effects, and in some, large buildings need to be built on foundations capping deep piles (to 20 m) sunk to the bedrock or deep into the permanent water table (Figure 8.13).
Most Australian native trees have an efficient water removal via roots, so that eucalypts remove 2-3 times the water of pines or pasture (to 10 m radius).
Generally, householders should keep large trees at least one half the mature height from the house when building in high clay-fraction soils; sands and sand-loams are usually stable, as are rotten or fractured rock and sandstones.