In this article, we’ll be discussing underpinning foundation methods and help you get a proper understanding of the topic.
A weakened building foundation is bad for many reasons, the worst of which is that it compromises the structure of your home and reduces its value.
Thankfully, there are methods with which you can repair and solidify the foundation of your building. One of such methods is known as underpinning.
This approach in itself has its own sub-methods which we believe that all homeowners should be aware of.
What Is Underpinning?
Simply put, underpinning is an approach used in the repair and fortification of a building’s foundation.
This is done in cases where the foundation begins to fail or is damaged (usually after the sub and superstructure of the building has already been finished).
This is considered an emergency, therefore it has to be treated with all urgency, so the building can regain its former stability.
Basement underpinning involves the installation of temporary or permanent support in the foundation for the sake of adding extra depth and load-bearing capacity.
This process is not a DIY matter, it is a task that has to be done by only experienced contractors!
Factors That Should Determine The Right Underpinning Method
To determine the right underpinning method, you will have to consider some very important factors. These include the age of the building and the type of structural problem the foundation has.
Any building that is above 150 years old is grouped under “ancient structures”. For buildings that are between 50 to 150 years old, they are grouped under “recent structures”.
Buildings that are not up to 50 years old are considered “modern structures”.
Type of Works That Determine Underpinning Method
As we have already mentioned, the type of work (or the unique problem the foundation is facing) should also determine the underpinning method to be used.
If you plan to convert the structure for the sake of functionality (better load-bearing capacity), then the foundation should be fortified so it is stronger than the already-existing one.
An underpinning method can also be employed if the structure’s foundation needs to be protected from specific instances.
- The already-existing foundation isn’t strong or stable enough (underpinning will protect the building from collapsing)
- Protection from the nearby excavation that can affect the soil supporting the existing footing
- To stabilize the foundation as a means of protecting the building against natural disasters such as earthquakes
If there were mistakes made during the original foundation design, then underpinning can be used to remedy the errors.
Structural Conditions Which Require Underpinning
Different structural conditions can also determine the kind of underpinning method to be used.
A professional contractor will first examine the building to determine what the exact problem with the foundation is before suggesting the right approach.
One of such instances could be the degradation of timber piles that have been used as the foundation. These can cause unwanted settlements due to water table fluctuations.
These fluctuations can lead to a decrease in the bearing capacity of the soil, hence causing the building to gradually sink in.
Any building that is erected above the soil that cannot bear its capacity would be prone to settlement.
Identifying The Need For Underpinning
Based on careful observation, a decision can be made to carry out an underpinning repair. When the building starts to show signs of settlement, it is required that vertical level readings be carried out.
Before excavation can begin, the contractor has to undergo a close examination and identify the soil’s ability to resist the structure from pressing in.
The need for underpinning will be determined based on the results of the structural examination.
Examining the soil’s capacity before the structure is built will spare the homeowner the stress of underpinning in the future.
Methods Of Underpinning
Depending on the specific problem, many underpinning methods can be used to fix a structural problem.
Below are the most commonly used.
- Mass concrete underpinning method (pit method)
- Underpinning by cantilever needle beam method
- Pier and beam underpinning method
- Mini piled underpinning
- Pile method of underpinning
- Pre-test method of underpinning
Regardless of the underpinning method that has been chosen, the existing foundation will be extended either in length or breadth, and the soil stratum would be made stronger.
The purpose of doing this is to allow a more even distribution of load over a wider area.
Let us take a closer look at the different underpinning methods mentioned above one by one. Once again, the method chosen should depend on the condition of the ground and the foundation depth you hope to achieve.
Mass Concrete Underpinning Method (Pit Method)
This is the traditional method of underpinning, and it’s probably the oldest. It has been used for hundreds of years to fix foundation problems, and it is still being used up until this day.
This approach involves the extension of the old foundation until it has reached a stable stratum. Here, the bottom soil (the soil under the existing foundation) is excavated delicately with the use of stages or pins.
When the suitable strata have been achieved, the excavated area will then be filled with concrete and left to cure before the next excavation begins.
To move the load from the old foundation to the new one, a new pin is provided with the use of a dry sand cement pack.
This method of underpinning is not expensive, and it is ideal for treating shallow foundations. If the foundation has more complicated problems, then more thorough underpinning methods should be employed.
Underpinning By Cantilever Needle Beam Method
This method of underpinning is a more complicated version of the traditional pit method. It can be used if the foundation needs to be extended only to a certain side and the plan has a stronger interior column.
The cantilever needle beam method has the following advantages –
- It is much faster than the traditional method
- One side access only
- Higher load-bearing capacity
It also comes with some downsides, which we will list out below.
- If the already-existing foundation is deep, then it tends to be a lot more expensive than the traditional method
- Constraint in access limits the use of needle beams
Pier and Beam Underpinning Method
This is also known as the base and beam method, and it came to prominence after World War 2. This approach became successful after it was discovered that the mass concrete method wasn’t ideal for foundations with great depth.
One of the major advantages of this approach is that it works well on most ground conditions.
With this method, reinforced concrete beams are installed so that the load is carried over to the mass concrete bases or piers. The depth and size of the beams used are determined by the ground conditions load.
It is very economical for depths that are shallower than 6m.
Mini Piled Underpinning
The mini piled underpinning method can be used when you want to transfer the load from the foundation to a stratum located at a distance of about 5m or greater. It is also ideal for different soil types, and in cases where access is restrictive.
Here, piles with diameters of about 150mm to 300mm are used. They could be steel cased or augured.
Pile Method of Underpinning
This method of underpinning involves driving piles on adjacent sides of the wall to offer support to the weakened foundation. The pin or needle pushes through the wall which is in turn connected to the piles.
This method can be used to fix settlements in the soil that have been caused by water clogging. It can also be used if the settlement is a result of the structure being built on soil that is clayey in nature.
Pre-test Method of Underpinning
This method of underpinning can be used to fix deteriorating strip or pad foundations. It is also ideal for buildings that are as tall as 5 to 10 stories high.
During the process, the subsoil is compressed and made to be compact following the predetermined load on the soil.
The main disadvantage of this process is that it cannot be used for raft foundations.
Is There An Alternative For Underpinning?
As far as correcting or fixing foundation errors are concerned, underpinning isn’t the only route you can take. One of the most common alternatives is resin injections.
This is considered to be a quicker, less invasive solution to foundational problems.
In the modern-day, resin injection solutions can be used to save homes that have been affected by subsidence in a very good time. It is also less tedious than the traditional underpinning methods.
Underpinning has been used to fix foundation problems for centuries, and the method that should be employed should depend on the uniqueness of the situation.
Ask a professional contractor to examine the building before deciding on which method to employ.