Underpinning is the reinforcement of an existing building foundation. It is done when the original foundation is not strong enough to support the house. This happens due to some changes in the soil structure or some external influence on the soil, the type of the soil is changed. The Underpinning Sussex strengthens the ground by expanding the foundation, distributing the load over a greater surface area.
What is the necessity of underpinning the foundation?
- Underpinning is required when the existing or original foundation is not stable.
- When the purpose of the structure is changed.
- The foundation’s strengthening or supporting soil property has altered.
- During the design process, the foundation was mischaracterized.
- The existing foundation is disturbed due to construction in a nearby structure.
Types of Foundations
There are two broad types of foundations used in construction.
- Shallow foundation
- Deep foundation
SHALLOW FOUNDATION:
Individual footing or isolated footing:
this foundation is the most commonly used in building construction that is also known as a pad foundation. Constructed in a single column, it is designed to bear the loads, and the size is calculated based on the bearing capacity of the soil. Furthermore, the rectangular footing is intended to experience moments caused by horizontal forces.
Combined footing:
They are constructed when two or more columns are close enough and overlap. It is a combination of isolated footing, but its structural design varies.
- The columns are placed next to each other
- When the columns are closed to the property lines
- Dimensions of one-sided the footing is restricted
The base of spread footing is larger than a typical load-bearing wall foundation. The wider base of this footing provides more area and better stability. Furthermore, the wall and spread footings are used for individual columns, bridges and wall piers where the soil layer is 10cm from the ground surface. Moreover, the soil bearing capacity must support the structure’s load. This type should not be used on soils where there is a possibility of water flow because it will result in liquefaction. They are economical when:
- Loads to be transmitted are of small magnitude.
- It is placed on dense sand and gravel.
Raft or mat foundations:
These foundations are spread all over the entire building area to support a heavy structural load of walls and columns where the loads from the structure are extremely high. It is designed by underpinning services to stop differential settlement of individual footings; hence designed as a single mat. This foundation is suitable for expansive soils where bearing capacity is relatively less. Furthermore, a raft foundation is cheaper when the structure is covered with individual footing, and only wall footings are provided.
It is economical when:
- The soil is weak and the load has to be spread over the large area
- The structure includes a basement
- Columns are closely placed
- Other kinds of foundations are feasible
- Differential settlement is to be prevented,
DEEP FOUNDATION
Pile foundations:
It is a type of deep foundation which is used to allocate heavy loads from the structure by construction a hard rock stratum deep below the ground level. These are used when the soil is not suitable for heavy loads. The depth of the hard rock stratum ranges from 5cm to 50 cm deep from the ground level. It is economical when:
- soil with great bearing capacity is at greater depth
- when there are chances of construction of irrigation canals
- when it is extremely expensive to provide a raft
- in marshy places
- when the topsoil layer is squeezable in nature
Caissons or drilled shafts foundation:
Drilled shaft foundation, also known as caissons, is a type of deep foundation and qualities similar to the pile foundation mentioned above. It resists loads through toe resistance or shaft resistance, or both of these. It is used where the depth of the stratum below ground level is 10m to 100m. Several types of caisson foundations include box caisson, floating caisson, pneumatic caisson, open caisson, sheeted caisson, excavated caisson.
They are economical when:
- The pile cap requirement is low
- Noise and vibration need to be reduced
- Has to be placed beneath water bodies
- Highly lateral and axial loading capacity is required
