Foundations must be designed to support the loads from a structure, taking proper account of ground conditions. The design of foundations and the calculations involved will not be considered in detail, but only the diagnosis of foundation defects which may be observed in the course of investigations into defects in buildings.
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| Image courtesy: Sinai construction |
Normal foundations comprise concrete strips on which the walls are erected, together with prepared foundations to support any solid floors. It is well recognized that strip foundations must have sufficient width to provide an adequate bearing area to support the imposed load on the ground support concerned; the depth of the foundation strip is important in terms of the ability of the foundations to resist local fluctuations in the bearing capacity of the ground without developing fractures, and the total depth of the foundations is important if it is necessary to excavate sufficiently deeply to provide bearings on better compacted soil or perhaps even rock at a greater depth. The importance of careful preparation of foundations for solid floors is not so widely recognized, as floor loads are much less and the requirements much less severe, but problems commonly arise if the floor slab support is poorly compacted, or support varies on a sloping site between excavated ground at one end and inadequately compacted fill at the other. If the load bearing capacity of the ground is poor, construction on a substantial reinforced concrete raft may provide the best means for spreading the foundation load, and various piling systems may be preferred to excavating deep foundation trenches.
Settlement and subsidence
The differences between settlement and subsidence failures are not widely recognized, although the different causes are obviously important in relation to remedial works. The causes may also be critical should damage result in a dispute for negligence or breach of contract in design or construction.
Settlement is due to normal compaction of the supporting ground as the building loads are imposed on the foundations. Some settlement always occurs during construction, but there is usually some further settlement or creep following completion. Settlement is normal and must be anticipated in design, so that damage due to settlement is an indication of inadequate design in relation to ground conditions or failure to observe the design during construction. General settlement usually occurs on loose ground such as sand or shingle or on readily compressed ground with a high organic and moisture content, such as peat. It is only troublesome because the external walls, the internal walls and solid floors impose different loads and therefore settle to a different extent, the most obvious damage being doming or an increase in height of the floors, although actually it is the walls that are settling around the floors, or settlement of the heavily loaded peripheral walls in relation to the internal partition walls. In one extensive housing development on sandy soil in the south of England, extensive damage was caused by the preferential settlement of the partition walls, and this was found to be due to inadequate foundations. Inadequately compacted landfill is also a serious problem, particularly if it contains biodegradable materials, such as wood, which will cause further settlement over a prolonged period. Certainly the worst settlement problems are associated with sites in which the load bearing capacity of the ground varies so that part of the foundations settle, sometimes causing massive structural fractures. In extreme cases, diagonal fractures may be identified as resulting from the settlement of a distinct part of the structure; in other cases fractures, generally through openings which represent the weakest parts of walls, will be wider at the tops of the walls than at the bottoms, indicating that local settlement has occurred to one side of the fracture, such damage being common on inadequately compacted fill, on land with patches of fill, or on sloping sites where walls are on excavated ground at one end but fill or inadequately compacted ground at the other. Once settlement has been fully relieved a damaged building may become structurally stable and no remedial works are necessary, other than the repair of the settlement fractures, unless structural weakening has occurred which requires the fractures to be re-bonded to restore structural integrity, as when vertical fractures occur between gables and adjacent bracing walls. However, if foundations are seriously inadequate and settlement is continuing, it may be necessary to provide additional support by inserting deeper foundations or perhaps supporting piles.
A special situation arises when buildings are constructed on shrinkable clay soils. Normally clay soils will be moist and in their expanded condition, but abnormal drying may result in shrinkage. Such problems are not usual in the British Isles, but serious damage occurred in this way through the exceptionally dry weather in 1976.
In addition, many of the houses constructed on shrinkable clay during the exceptionally dry weather have since suffered damage due to expansion of the clay on subsequent wetting. Buildings themselves and associated patios, paths, drives and roads all reduce rain penetration into the ground and can result in clay shrinkage damage following construction. For these reasons deeper foundations must always be used on clay soils to provide support below the clay or at a depth within the clay which will not be affected by such moisture content changes.
Subsidence follows from some unexpected event following construction. Ground water percolation may result in removal of material and loss of support. Water percolation can occur in this way through natural groundwater movement, but usually some event prompts subsidence, such as the diversion of a stream, the overflow of a drainage system, or even a fracture in the rainwater or sewage drains associated with the building itself, although one of the most common causes of subsidence is the fracture of a water main due to frost or traffic damage. Landslip is also another example of subsidence, usually because apparently stable ground has been fluidised through the accumulation of an exceptional amount of rainwater, sometimes causing severe damage to houses and gardens constructed on sloping sites.
Tree root damage
Tree root damage results most obviously from the penetration of tree roots into masonry and beneath foundations, and rupturing due to progressive root growth. Such damage can usually be readily identified by excavation and does not justify special comment, except in relation to safe separation between trees and buildings.
A more serious problem is the presence of trees in conjunction with shrinkable clay. Deciduous trees will remove water from the clay during the summer and the clay will shrink, but the tree will have no demand for water during the winter so that the clay moisture content will increase and it will expand. This seasonal movement can only be avoided by ensuring that buildings are constructed a sufficient distance from established trees, or new trees are planted a sufficient distance from a building. If these requirements cannot be satisfied deeper foundations are necessary to penetrate below the clay, or sufficiently deeply in the clay for moisture content fluctuations to be minimal. If a new building is constructed in the summer on a site from which trees have been recently removed, clay soil may have an unusually low moisture content; subsequent wetting may cause expansion and damage to the new building, unless the foundations are sufficiently deep to avoid the effect as previously explained.
Safe separations between trees and buildings depend upon the tree species and are summarized in Table 1.
Table 1 Recommended minimum distances between trees and buildings
Tree species
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Minimum separation on
shrinkable clay soil
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Normal maximum
tree heIght (H) (m)
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Oak
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1H
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16-23
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Poplar
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1H
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24
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Lime
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(1/2)H
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16-24
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Ash
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(1/2)H
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23
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Plane
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(1/2)H
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25-30
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Willow
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1H
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15
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Elm
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(1/2)H
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20-25
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Hawthorn
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(1/2)H
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10
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Maple/sycamore
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(1/2)H
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17-24
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Cherry/plum
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1H
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8
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Beech
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(1/2)H
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20
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Birch
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(1/2)H
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12-14
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White beam/rowan
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1H
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8-12
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Cypress
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(1/2)H
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18-25
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Further reading:
☞Book: Settlement Calculation on High Rise Buildings by Xiangfu Chen
☞Foundation Engineering Handbook by Robert W.Day
☞Causes of Failures of Foundations and Preventive Measures