Foundation settlement causes and underpinning

Causes of foundation settlement

1. Consolidation of compressible clay or silt under load.
2. Weak, loose, or filled soil, poorly compacted.
3. Non-uniform soil or unequal loading.
4. Change of water table; shrinkage and swelling of clays.
5. Overloading or addition of floors.
6. Adjacent excavation, vibrations, or construction.
7. Tree roots, erosion, leakage and underground cavities.
8. Earthquake and liquefaction.
9. Faulty design: footing too small or too shallow.

Underpinning

Underpinning is the process of strengthening and deepening an existing foundation by extending it down to a stronger stratum, or enlarging its base, so that it supports the structure safely. It is used to stop settlement, to add floors, to lower basements, or when deep excavation is done nearby.

Methods of underpinning

1. Pit (needle) method: The wall is divided into short sections (1 to 1.5 m). Pits are dug under alternate sections down to the firm layer, one at a time, and filled with concrete or masonry up to the base of the old footing, packed tight with dry mortar and wedges. Then the adjacent sections are done. This is the simplest method, for shallow depths.
2. Pile underpinning: Piles are driven or bored on both sides of the wall (or through it), and a needle beam or cap transfers the load to them; used when the firm layer is deep.
3. Pretest method: The new piles or piers are pre-loaded with hydraulic jacks before the load is transferred to them, so that their settlement takes place in advance.
4. Jet grouting / chemical grouting: Grout is injected to strengthen the soil.
5. Miniature (micro) piles: Small-diameter piles drilled and grouted through the existing footing.
6. Beam and needle method: Needle beams under the wall pass the loads to piers on either side.

 wall
 |==== old footing ====|
   ||pit 1||    ||pit 2|| 
   (concrete filled pit, alternate sections)

During underpinning, the structure is shored, and cracks are monitored.