Ground settlement
Lowering of the ground surface, known as subsidence or settling, often occurs during an earthquake. Common causes of ground subsidence during an earthquake include consolidation or failure of the ground under a foundation, densification of sand and gravel layers due to the ground shaking and liquefaction.
Settlement caused by ground failure can cause buildings to displace, tilt, stretch, twist, buckle or a combination of all five. How badly the building is damaged depends on:
- the severity of the settlement (related to ground performance and behaviour)
- the type and strength of the building’s foundations and structure
- the geometry and complexity of the overall design.
Uniform settlement
Uniform settlement occurs when a building foundation settles by the same amount over its entire footprint area, effectively lowering the structure in place. A related effect called tilt settlement occurs when the whole building tilts as a solid box.
Uniform (left) and tilt settlement (right) due to liquefaction.
Differential settlement
In some cases, only part of the foundation is affected by ground failure, or part of the foundation is affected to a greater extent than other parts. This kind of effect is called differential settlement and can cause much more severe damage to a building than uniform or tilt settlement.
For instance, one corner of a slab foundation may experience settlement while the rest of the foundation does not. The resulting change in foundation settlement can induce higher levels of stress in the floor, wall and roof members.
Differential settlement that causes the edges of the foundation to settle further than the centre is known as hogging (left). Where it causes the centre to settle further than the edges, it is known as sagging or dishing (right).
Differential settlement can cause an abrupt change in level or cause all corners of the foundation to settle by different amounts.