What is Foundation Settlement? Its Types and Causes

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What is foundation settlement?

Inevitably, soils deform under the load of foundation structures. The total vertical displacement that occur at foundation level is termed as settlement. The cause of foundation settlement is the reduction of volume air void ratio in the soil. Moreover, the magnitude of foundation settlement is controlled by many factors type of soil and foundation structure. Foundations on bedrock settle a negligible amount. In contrary, Foundations in other types of soil such as clay may settle much more. An example of this is Mexico City palace of fine arts has settled more than 15 feet (4.5m) into the clay soil on which it is founded since it was constructed in the early 1930s. However, building foundation settlement is normally limited to amounts measured in millimeter or fractions of an inch. Structures will suffer damages due to settlement of its foundation specifically when the settlement occur in quick manner. In this article, different types of foundation settlement along with their cases and expected effects on the structure will be discussed.

Types of foundation settlement

• Differential foundation settlement
• Uniform foundation settlement

Differential foundation settlement

• Settlement that occurs at differing rates between different portions of a building is termed differential settlement.
• Differential settlement occurs if there is difference in soils, loads, or structural systems between parts of a building. in this case, different parts of the building structure could settle by substantially different amounts.
• Consequently, the frame of the building may become distorted, floors may slope, walls and glass may crack, and doors and windows may not work properly.
• Uneven foundation settlement may force buildings to shift out of plumb which lead to crack initiation in foundation, structure, or finish.
• Majority of foundation failures are attributable to severe differential settlement.
• Lastly, for conventional buildings with isolated foundations, 20mm differential settlement is acceptable. And 50mm total settlement is tolerable for the same structures.

Uniform foundation settlement

• when foundation settlement occurs at nealy the same rate throughout all portions of a building, it is called uniform settlement.
• If all parts of a building rest on the same kind of soil, then uniform settlement the most probable type to take place.
• Similarly, when loads on the building and the design of its structural system are uniform throughout, the anticipated settlement would be uniform type.
• Commonly, uniform settlement has small detrimental influence on the building safety.
• However, it influences utility of the building for example damaging sewer; water supply; and mains and jamming doors and windows.

Fig.4: Difference between uniform and differential settlement

Foundation settlement causes

Direct causes

• The direct cause of foundation settlement is the weight of building including dead load and live load.

Indirect causes

• Failure of collapsible soil underground infiltration
• Yielding of excavation done adjacent to foundation
• Failure of underground tunnels and mines
• Collapse of cavities of limestones
• Undermining of foundation while flood
• Earthquake induced settlement
• Finally, due to extraction of ground water and oil.

Components of total settlement of foundations

Immediate settlement

• It is also called short term settlement.
• Immediate settlement take place mostly in coarse grained soils of high permeability and in unsaturated fine-grained soils of low permeability.
• Lastly, it occurs over short period of time which about 7 days. So, it ends during construction time.

Primary settlement

• It also termed as primary consolidation
• Take place over long period of time that ranges from 1 to 5 years or more
• Primary settlement frequently occurs in saturated inorganic fine grain soil.
• Expulsion of water from pores of saturated fine grain soil is the cause of primary settlement.

Secondary settlement

• Secondary settlement is the consolidation of soil under constant effective stress.
• Frequently, it occurs in organic fine grain soil.
• It continues over the life span of foundation structure similar to creep in concrete.