All structures including skyscrapers resist sinking into the soil beneath them despite their heavy load due to their well-designed foundation that takes into account the load of the superstructure and the bearing capacity of the soil available at a particular stratum at which the foundation rests.
Due to correct assumptions, predictions and design of foundation by geotechnical and structural experts no such failure of foundation occurs and really heavy structures can continue to stand for centuries.
To elaborate this, let me give a brief description about the bearing capacity of soil and other concepts of foundation design.
Bearing capacity of soil is the maximum amount of vertical stress it can bear before failing in either shear or excessive settlement.
This bearing capacity is estimated using experiments such as plate load test or ascertained using formulae involving bearing capacity factors.
Different types of structures and soil conditions require different types of foundations.
- Shallow foundations: These are used when sufficient bearing capacity is available at shallow depth.
- For normal few storey residential buildings that are not framed structures, continuous strip footings beneath the walls are used.
- For normal few storey residential framed structures isolated footings under columns are usually used.
- Raft/mat foundations are used in places where lack of bearing capacity is taken care of by increasing area of footing to a continuous raft or mat designed as an inverted slab.
- Deep foundations:
- When sufficient bearing capacity for a soil is not available at shallow depths, pile or well foundations are used to transfer load of structure via a stiff pile or rubble filled well to a deeper hard stratum.
- These foundations also advantageouly use the friction b/w pile and surrounding soil to carry part or whole of the load in addition to point bearing resistance available at large depths.
- Compensated raft/ Floating footing:
- It is observed that soil experiences settlement or shear failure only at loads over and above that which it is initially subjected to due to overburden.
- Hence, if overburden pressure is removed by excavating the soil upto a particular level and not backfilled, that much load equal to overburden pressure can be applied without stressing the soil at that level.
- Such technique helps in designing extremely stable and strong foundation that experiences little to no settlement and creates room for basements as well. Since the soil is completely unstressed the footing is said to simply float on the soil.
In case of skyscrapers a combination of traditional and/or innovative techniques are used for foundation design.
- Shallow foundations are almost never used except in rare cases where hard rock stratum is available at shallow depths.
- Usually a combination of compensated raft and pile foundation is used to ensure non sinking, non tilting foundation.
- Compensated raft at large depth provides good bearing without over stressing the soil and pile foundation provides increased stability against differential settlement and tilting by virtue of its skin friction. It is preferable to sink piles upto a hard rock stratum to maximise the available point bearing resistance.