Underpinning Methods, Process, and Applications
Underpinning is the process of repairing and strengthening an existing foundation of a building. There are several Melbourne underpinning methods used today.
Different underpinning methods are applied through different processes. However, they’re all meant to strengthen a damaged or weak foundation.
There are several situations that lead to the failure of the footing or foundation of a structure. These situations happen unexpectedly in the life of a building.
In case the footing or foundation fails, you should treat it as an emergency and have it repaired soonest possible. A structural engineer will suggest the best solution to fix the problem for the building to regain its structural stability.
Underpinning assists in strengthening an existing foundation. It involves installing temporary or permanent support to a foundation to give it more bearing capacity and depth. That way, it’s stabilized to support the structure.
Choosing the Right Underpinning Method
The right underpinning method for fixing a foundation is selected depending on the kind of work involved and the age of the structure.
Structures are categorized based on their age as follows:
- Modern structures- At most 50 years old
- Recent structures- Aged between 50 to 150
- Ancient structures- At least 150 years old
The kinds of work involved when choosing an underpinning method include:
When converting a structure to serve another function, it’ll require a stronger foundation than the existing one.
Protection works are required if a structure has one of these problems:
- The foundation is weakened or destabilized
- Nearby excavation has affected the soil supporting the existing footing
- Adding a basement underneath an existing structure
- Stabilizing the soil to make it resist movement resulting from natural calamities
- Working on an existing building
- Mistakes in the original foundation design that causes the structure to subside
Structure Conditions That Require Underpinning
There are several reasons that might make a structural engineer recommend underpinning as the best solution to stabilize a substructure. These reasons include:
- Wooden piles used in making a foundation can degrade over time and cause the supported building to settle. The structure may also degrade as a result of fluctuations in the underlying water table.
- If the underlying water table is rising and lowering frequently, the soil supporting a building may lose its bearing capacity, thereby causing the building to settle.
- A building that’s built on a soil that has a weak bearing capacity for supporting it may settle.
Why Underpinning May Be Required
Certain observations on a structure may necessitate underpinning. When a building starts showing certain changes as a result of settlement or other forms of distress, it’s necessary to take the building’s vertical level measurements and its offset level.
Before any excavation is done in preparation for underpinning, a professional has to examine the soil closely and determine its capability in resisting the structure it’s supporting from settling. The report of the soil test and examination may recommend underpinning.
Sometimes, underpinning might not be recommended. Thus, it’s necessary to have the building inspected by a professional before hiring an underpinning contractor.
Here are the different techniques used in underpinning to strengthen a foundation:
- Pile underpinning
- Pre-test underpinning
- Piers and beams underpinning
- Mini-piled method
- Cantilever needle beams method
- Mass concrete method (also known as pit method)
Regardless of the underpinning method to be used, they all work by either extending the foundation breadthwise or lengthwise.
Underpins are typically laid on stronger soil. Extending an existing foundation distributes the weight of a building over a wider area for enhanced support.
We’ve discussed different underpinning techniques with details below. The underpinning method to be used on a structure is determined by the required depth of the foundation and soil conditions.
In this technique, piles are inserted into either side of a wall to support the weakened foundation. Each pin or needle is driven through a wall and attached to the inserted piles.
Each needle works like a pile cap. Pile underpinning is recommended for treating soil settlement resulting from clayey nature or water clogging.
Pre-test underpinning is used for pad or strip foundation. It’s ideal for use on buildings that have 5-10 stories. The subsoil on which the foundation sits is compressed or compacted upon excavation to make it more solid.
Once done, underpinning is carried out. Disruption and some little noise are expected. This underpinning method is not recommended for raft foundations.
Piers and Beams Underpinning
The piers and beams method is also referred to as the base and beams method of underpinning. It’s preferred over mass concrete since mass concrete is not reliable with deep foundations.
It’s ideal for most issues resulting from various ground conditions. In this method, beams reinforced with concrete are placed strategically to take the load of an existing structure.
The depth and size of the piers and beams depend on the applied loads and ground conditions. This method is economical for depths not exceeding 6 meters.
The mini-piled method is ideal for use in situations where the load on a foundation has shifted to strata situated more than 5 meters away.
This technique is recommended for soil with variable nature and where there’s restrictive access to the base of the foundation. In this case, piles with a diameter of between 150-300 mm are driven into the ground.
Cantilever Needle Beams Method
The cantilever method is like an extension of the pit technique. This method is recommended in situations where a foundation is to be extended to just one side to strengthen the interior column.
The advantages of this method include:
- Access is only needed from one side
- Has the ability to carry a large load
- It’s faster than traditional underpinning
Its disadvantages include:
- Challenging to use needle beams due to constrained access
- It’s uneconomical to excavate in deep foundation
Mass Concrete Method
Mass concrete is also known as pit underpinning. It’s a traditional underpinning method. The method has been used for several centuries now. It involves extending an existing foundation until it’s stabilized on a solid stratum.
When using mass concrete, the soil beneath the foundation in question is excavated carefully through pins or stages. Once the suitable stratum is reached, concrete is poured into the excavation and allowed to cure before starting the next excavation process.
New pins are placed on a dry pack of sand and cement to relocate the load of the existing foundation to the new one. Mass concrete is economical and ideal for use on shallow foundations.
If the foundation is experiencing more complicated issues, then other superior foundation reinforcement methods would be recommended.