WHAT IS UNDERPINNING? DIFFERENT METHODS OF UNDERPINNING.

Underpinning is the process of strengthening the foundation of an existing building or other structure or It is a process of arrangement of supporting an existing structure's foundation by providing supports underneath it to strengthen the existing structure.

Underpinning may be necessary for a variety of reasons which are given below.
  • The original foundation isn't strong or stable enough.
  • The usage of the structure has changed.
  • The properties of the soil supporting the foundation may have changed (possibly through subsidence) or were mischaracterized during design.
  • The construction of nearby structures necessitates the excavation of soil supporting existing foundations.
  • To increase the depth or load capacity of existing foundations to support the addition of another storey to the building.
  • It is more economical, due to land worth or otherwise, to work on the present structure's foundation than to build a new one.
  • Earthquake, flood, drought or other natural causes have caused the structure to move, requiring stabilization of foundation soils and/or footings.

Methods of Underpinning

1. Mass Concrete (Pit) Method of Underpinning

This is the traditional method of underpinning and has been in use since centuries. It involves stretching the old foundation until it reaches a stable stratum.
The soil under the current foundation is excavated in a controlled way through pins or stages. Upon reaching the suitable strata, the excavation is filled with concrete and set for curing prior to the next excavation.
A new pin is provided by placing a dry sand-cement pack so as to transfer the load from the old foundation to the new one. This technique is low-cost and is suitable for shallow foundations. For more complicated problems, other superior techniques need to be chosen.

2. Cantilever Needle Beam Method

This is an extension of pit method. It can be used when the foundation needs to be extended only to one side and the structure has a stronger interior column.
Benefits of this method are:
  • One side access only
  • Faster than the traditional method
  • Capability of carrying high load
There are some disadvantages too, including being unaffordable digging when current foundation is deep and limitation of access restricts the use of needle beams.

3. Underpinning Using Brackets and Screw Piles

The method of underpinning using brackets and screw pile is usually used in particular instances where regular underpinning cannot be done. Thus, certain buildings might need excavating to huge depths or maybe it’s impracticable for them to use a piling rig thus making it ideal to use brackets and screw piles technique. Only a two man crew can install the brackets and screw piles by hand or with small equipment like a mini excavator.
Because of their capacity to work in compression and tension, and withstand vibration and shear forces, and lateral and vertical wind forces, screw piles can be installed in foundations. They are perfect when used with underpinning support brackets. Then one can lift the structure back to a level position and load of the foundation is transferred to the pier and bracket system.
There are many benefits of screw piles over traditional pilings, including the speed of installation, minimal vibration and little noise, thus causing no damage to the surrounding area.

4. Pile and Beam Method

Another great and popular method to improve the foundation is underpinning with piles and beams. It requires installation of a min-pile on either side of the affected wall. After installation, brickwork below the wall is removed and then piles are connected and wall is supported with reinforced concrete needle beam. Decreasing the distance between needle beams can facilitate very high loads. The number, depth, diameter and spacing of piles used can be determined by the bearing capacity of the underlying strata. Case driven piles or augured piles can be used with this technique of underpinning. The benefits of this method are:
  • Facilitates restricted access
  • High load capacity
  • Faster than traditional underpinning
  • Less spoil generated, less disruption and completed fast.

5. Piled Raft Method

Piled raft method should be used for underpinning when the entire structure requires to be underpinned. When foundations are too deep to use other underpinning methods or where the soil is so tough that small equipment is not able to excavate up to the needed depth.
In this technique, piles are placed at planned locations by loading conditions. Next the pockets blow foundations are broken and reinforced needle beams are added to bear the load of the wall. To connect all needles, a ring beam is built and concrete is poured in the structure.
Benefits of this technique are:
  • Offers lateral and traverse ties all through the structure
  • No need of external access
  • Economical at depths more than 1.5 m
  • Decreases disruption to drainage systems.

6. Mini Piled Underpinning

This technique can be used when the loads from the foundation need to be shifted to strata located at a distance more than 5 m. This technique is usable for soil having variable nature, there is restricted access and environmental pollution problems exist.
Piles of diameter from 150 to 300 mm are driven which either may be driven steel cased or augured.

7. Pre-test Technique of Underpinning

This method is used for strip or pad foundation. It can be used for structures with 5 to 10 storeys. In this method, the subsoil is compressed and made compact, in the new excavation level that provides predetermined loads to the soil. This is done prior to performing underpinning.
This technique cannot be used for raft foundation and less disruption and noise are expected.

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