A5.1.3 Dome-saucer effect
Dome and saucer effects occur when there are long term changes in the moisture content of the soil under a building.
The dome effect is more likely to occur in areas that are subject to hot wet summers and warm dry winters. Moisture tends to migrate under a house, causing the plastic soil to expand in a dome shape, see Figure 3(a). The effect in most cases is permanent and will only be affected to any practical extent by extremes in seasonal changes, e.g a dry winter followed by a drought
The symptoms from the dome effect usually occur in the form of the external walls tending to rotate outwards at the top causing general cracking, especially around openings. Internal partition walls may develop cracks which will opcn up at the top of the wall and narrow down at the bottom. A slight bulging of the floor may occur as the supports rise.
The saucer effect is likely to occur as a result of long dry summers and cold wet winters. The soil under the house is dried out extensively in summer and never regains an equilibrium moisture content in winter when the soil surrounding the house is continually wet. This results in shrinkage of the plastic soil under the house causing a slight saucering effect, this tends to be a long term condition fluctuating only slightly with normal seasonal changes, see Figure 3(b).
The symptoms from the saucer effect usually occur in the form of the external walls tending to rotate inwards at the top causing general cracking. Internal walls may develop cracks that open at the bottom of the wall and narrow towards the top. The effect on the floor is that the internal supports drop below the bottom of the bearers and the floor becomes springy and sags slightly.
