Pre Purchase Building Inspections Western Suburbs
- Posted by: John Rosa
- Category: Uncategorized
The problem arises in practice mainly with buildings, built either on shrinkable clay or in mining areas or extensive site works to new building subdivisions on and around Melbourne suburbs liable to subsidence. For average conditions, a spell of a few weeks of dry weather in the summer will cause certain clays to shrink for a depth of about 2400 mm from the surface.
The amount of shrinkage and the depth to which it occurs depends on the shelter that the ground has from the sun and the presence of vegetation which through its root system removes water from the clay./ The shrinkage is greater near the surface, involving a settlement often of the order of 25 mm and is usually of little importance at a depth of 1200-1500 mm.
Buildings built on the traditional shallow strip foundations on such clays are liable to serious distortion and consequent cracking since, as a result of the protection from climatic changes afforded to the soil within the periphery of the building there will be differential settlement between the clay on the inside and that on the outside of the external walls. The effects can be considerably worsened if trees are growing close to the
building. Although during the winter the clay will swell, the cracks are partially closed and there may be a progressive increase in the widths of the cracks each year.
The incidence of cracking in buildings can be avoided or reduced by using foundations so deep that the soil movements are unimportant, or by keeping trees so far from the building that the roots do not reach the foundations.
It is often cheaper, instead of using deep strip foundations, to support the buildings on bored piled foundations. The piles were designed to have adequate strength and sufficient resistance to settlement so that the relative movements of the piles are unimportant.
Where, as in new subdivisions large spoil settlements are liable to occur, damage to buildings can be minimized in three very different ways:
1. The building can be strengthened, by reinforcement or otherwise so that it can act as a single structural unit that may tilt but may not crack even though its support by the ground is reduced to a few localized areas.
2. As an alternative to this method, the foundation can be specifically designed, either as a strong reinforced concrete raft or as a structural system of beams and slabs with three-point support from the ground, so that soil movement cause tilting of the foundation and hence of the building but not relative movement of the building such as will cause cracking.
3. The building can be so constructed that it is very flexible, relative movements of the soil being accommodated in the structure in a predetermined manner, details of connections and cladding carefully worked out so that the movements can occur without damage to structural elements or finishes., Provision is then made for easy replacement of cladding units when the relative movements exceed a specified amount.
Whatever method is chosen, the risk of cracking is reduced making the individual structures as small as is convenient.