Due to uneven compression of the foundation soil, the soil under the structure deforms and foundation settlement takes place. After construction, some settlement is usually natural, but uneven or differential settlement might lead to serious issues.
Therefore, we use the underpinning foundation to recover from a situation where the foundation has failed because of settlement. Underpinning is the process of strengthening a building’s or structure’s foundation during construction or renovation.
What is the Underpinning Foundation?
Underpinning is a technique used to fix and reinforce the foundation of a building. Underpinning is done by adding reinforcements along the width or depth of an existing foundation. There are many underpinning foundation methods that ensure a strong base for a structure. It rests on strong soil layers and spreads its weight across a greater region. For underpinning, the micro-piling and jet grouting processes are used, but they are both expensive and time-consuming.
When foundation loads increase due to a change in building use, existing buildings sometimes experience excessive settlement under their design loads or might face excessive settlement in the future. There are a variety of foundation enhancement techniques available to stop settlements or improve the performance of current foundations in the future, such as underpinning foundations, according to M. Makarchian and H. G. Poulos (authors of the journal article – Simplified Method for Design of Underpinning Piles).
Continue reading to find out how to underpin a foundation.
For the underpinning foundation process, the soil moving away from the building is dug out, and its support is removed. It makes the existing foundations weaker and causes the building to move. New, stronger materials and deeper footings are laid on firmer ground to secure the property’s foundations and strengthen its structure.
Underpinning the Foundation is Important
Before digging for a new project, professionals have to look closely at the soil and figure out how well it can hold up the structure on top of it. This report determines the need for underpinning and helps decide on the underpinning foundation methods. Sometimes, these tests would eliminate the need for underpinning after the building is built.
Underpinning is necessary when the original foundation is weak and the properties of the soil supporting the foundation have changed. Observations can help decide the kind of foundation needed. When an already-built structure starts to change because of settlement or other damage, it is important to take readings at both the vertical and offset levels as soon as possible. How long it takes depends on how bad the settlement is.
Various Work Forms for Underpinning Method Selection
The following work is carried out before installing the underpinning foundation:
- Conversion Works
- Protection works
- Remedial Works
01. Conversion Works
The building has to be changed to serve a different purpose, which requires a stronger foundation than what is already there.
02. Protection Works
Protective measures are necessary for the following problems with a structure:
- The current foundation is neither solid nor reliable.
- Due to excavation nearby, the soil supporting the current footing is compromised.
- Soil stabilization for protection against earthquakes, floods, and other disasters
- The need for an underground space below an existing building
03. Remedial Works
- The structure sinks as a result of errors in the initial foundation design.
- Instead of constructing a new structure, improve the one you have.
Structural Conditions That Require Underpinning
Engineers often suggest underpinning foundations as a substructure stabilising method for many reasons, including:
- Normal buildings that use wood piles as their foundation would settle as the piles spoiled. Changes in the water table cause this damage to structures.
- When the water table rises and falls, it can make the soil less stable, which can cause a building to sink.
- Structures built on soil that can’t support the structure’s weight will settle over time.
Methods of Underpinning Foundation
The following six techniques are used as the underpinning foundation methods:
- Mass Concrete Underpinning or Pit Method
- Pile Method of Underpinning
- Jack Pile Method of Underpinning
- Underpinning: Root Pile or Angle Piling
- Pynford’s Stool Method for Underpinning
- Underpinning Columns
Let’s have a detailed look at each one of these foundation-underpinning designs.
01. Mass Concrete Underpinning or Pit Method
The mass concrete approach, or pit foundation, is one of the oldest types of underpinning foundations. Here, the current base is enlarged until a solid layer is reached. Using pegs, the earth beneath the existing foundation is gradually exposed. On reaching a proper strata, the excavation is filled with concrete and set aside to cure before the next removal phase can begin.
Another pin is provided by establishing a dry sand-cement pack, which transfers the load from the old foundation to the new foundation. This low-cost technique works well with a thin base. There are better alternatives to tackle more complicated challenges at the foundation.
02. Pile Method of Underpinning
The driving technique is used to install piles in the piling method for supporting a foundation. These driving piles are positioned on either side of the wall to add support. Needles made of steel or concrete are driven into the wall and then linked to the piles. These steel or concrete piles operate as a beam and a pile cap, respectively. Clay soils, weak-bearing strata, and water logging are all conditions where underpinning piles are used.
03. Jack Pile Method of Underpinning
When the depth of the suitable bearing subsoil is deeper and the traditional underpinning method is therefore ineffective, one of the underpinning foundation methods, called Jack Pile, comes into play. The main benefit of the Jack pile foundation underpinning design is that it is quiet and doesn’t cause vibrations. It is flexible too and hence the pile depth can be changed to fit the subsoil conditions. The pile caps, which are cast onto the jack pile heads after the hydraulic jacks have been removed, must fit over the heads of the existing foundations; therefore, they must be in good condition.
04. Underpinning: Root Pile or Angle Pile
Modern concrete drilling applications are used in the root piling or angle piling method to make concrete, which is cost-effective and saves time on construction. Line Reinforced concrete piles are set up or driven in pairs at right angles to each other. Walls are predrilled with an air-flushed percussion drill, making the brick wall stronger and not damaging the existing floors. Angle piling can be hard to put on the back side of a wall. The piles become stable when put close to each other.
05. Pynford Stool Method for Underpinning
The Pynford stool method can be used when the foundations of the existing structure are in bad shape and the bearing capacity of the soil is poor and unstable. This method allows the wall to be underpinned continuously with the needles. The holes formed in the wall receive steel or precast concrete stools. The reinforced concrete beam made by this method might be enough to spread the load of the existing wall, or it could be used along with other types of underpinning, like traditional and jack piles.
06. Underpinning Columns
Columns can be supported similarly to walls when the loads from columns have been removed, using either the conventional technique or jack piles. Foundation underpinning designs are used, as in most cases, dead beaches can be utilized to transfer beam loads away from the columns, and a pair of beams working on a collar at the foot of the column shaft can transfer the real column weight.
Advantages of Underpinning
- Strengthening Structural Integrity: The main advantage of underpinning is that it strengthens the foundation and protects it from all kinds of disasters.
- Improved Lighting and Ceiling Height: With an underpinning foundation, your building will be able to have better lighting. You expand your building by adding extra rooms or offices as well as more windows and doors to let natural light in.
- Updating the Plumbing, Electrical, and Insulation Systems: By underpinning your basement, you’ll be able to get to your home’s plumbing, insulation, and wiring, so you can check to see if they need to be fixed.
- Increased Valuation of Property: One of the best advantages of underpinning is that it makes your property worth more. If you want to sell your property in the future, a well-built structure with finished underpinnings will make it worth more and give you a good return.
- To Sum Up, underpinning is a great way to make more space in a less expensive and disruptive structure. If you need extra space, buying a new home could be expensive; however, underpinning can help you save that expense. Also, foundation underpinning costs will save you from having to pay for expensive repairs in the future.
Disadvantages of Underpinning
- Time-Consuming and Expensive: One of the major disadvantages of underpinning is that the foundation underpinning cost is expensive.
- Difficult to Work: It is quite difficult to sustain structures on waterlogged or unstable ground.
- Difficult to Discard: On the working site, a significant volume of excavated material needs to be dumped or disposed of.
Conclusion
A process called underpinning foundation is used to strengthen and repair a building’s foundation. By underpinning existing buildings and infrastructure, weak foundations can be strengthened. One such technique is to extend the depth and load capacity of an existing base by adding permanent or temporary support.
Author Bio
Sikandar Choudhury – Sikandar Monwar Huda Choudhury is a freelance article writer who is passionate about sharing his knowledge and experience with others through writing. With several years of experience in the engineering field and having written 100+ articles related to construction, Sikandar is a skilled writer with a talent for breaking down complex concepts and making them accessible to a wider audience. Sikandar is always looking for new opportunities to share his knowledge and experience with others through writing and is available for hire as a freelance civil engineering article writer. He is easily reachable on LinkedIn- https://www.linkedin.com/in/sikandar-monwar-huda-choudhury-2b3a1a20a/.