We all know of sinking structures and whole cities. Sinking foundations of buildings is all too common, that is we come across such occurrences frequently. Normal settlements of foundations are the following.
However, the above mentioned settlements do not occur in isolation but all of them occur in order and in different proportions. The sum of all these natural settlement is generally negligible, known and expected by engineers and hence, will be accounted for in the building foundation design.
The purpose of this article, is actually the settlements of structures i.e., their foundations caused by other factors. As already explained only these settlements occur in considerable magnitude, therefore catch attention of general public. Some of the causes of settlement of foundations / subsurface soil is discussed below. Please note that rarely, a structure or a foundation pier appear to sink or shrink due to structural damage or deterioration of material but such cases are dealt with here, as they are rare and also not related to soil.
- Underground Erosion
Underground streams of ground water and leaching of soil material by solution in ground water may result in formation of cavities in the soil, which eventually collapse under the weight of structure leading to settlements or sink holes. Some times the cavities remain dormant till a structure arise above them and cave in after load on soil is increased.
- Structural Collapse of Soil
Dissolution of materials responsible for inter-granular bonding of soil may lead to structural collapse of soil mass. Saline soils, non-cohesive soils, gypsum soils, silts and loess are examples of such soils.
- Thermal Changes
Expansive soils tend to shrink on cooling.
- Frost Heave
As the frozen water takes more volume than in liquid state, the water in subsurface soil freezes in winter, the soils swells out, in what is called as frost heave. If sufficient caution is not taken in the design of buildings in cold climes, the foundations may settle when the frost thaws in summer.
- Vibration and Shock
This hardly needs an explanation! While filling bottles with flour or grains your mother always shook them after filling to the brim, to make more room to pour more into the bottle. Similarly mechanical vibrations and shocks emanating from the neighbourhood, may tend to settle loose and non-cohesive soils under foundations.
- Mining Subsidence
Subterraneous tunneling and cavities in mining activities when collapse under ground may lead to settlements in the neighbourhood.
- Landslides
If soil from under a building on hill slopes slides, it would results in disastrous settlements or collapse.
- Creep of Soils on Slopes
Gravity brings every thing down, right? It tries to do so even with soil on moderate slopes. If a building is built partly into the slope by cutting, the free foundations find the soil beneath them creeping down the slope, very slowly, though. On the other hand, the foundations on the rear side are relatively stable, since is creep is more on top layers of soil. This results in differential settlement of building.
- Constructions in the Vicinity
Variations in loads on soil from nearby structures affect the soil. For example, when a tall sky scraper is built besides one end of a long low level building, the end near tall building may experience more sinking due to excessive load from tall building.
- Liquefaction
Soils saturated with water tend to lose strength and stiffness when violently shook in earthquakes or rapid loadings. This would cause tremendous settlements or damage. When earthquake shaking causes the pressure of stable water filled between soil grains, to increase to the point where the soil particles can easily move among them, the liquefaction occurs.
- Quicksands
When there is upward pressure or flow of water in soil as on the toes of dams, the uplift pressure at some point neutralizes the soil grains’ weight and therefore causes it to boil. In such unstable quick sand conditions, settlements would occur.
