What is Underpinning and When is It Needed?
January 10, 2022
Underpinning is a tactful construction approach to strengthen an existing foundation or place a new foundation at a greater depth below the old one. Since underpinning is a precise repair project, contractors must choose the right method to implement it. To determine the correct underpinning method, you need to understand and determine the soil strata, your building’s current position, and the issues concerning the building’s foundation, your desired depth, and the dimensions of the new foundation. An experienced building contractor can offer more insight on how to investigate the issues affecting your foundation. But first, here’s everything you need to know about underpinning and when it could be necessary for your building.
Underpinning is the process of strengthening or supporting the foundation of a building, house, or related structure. Building contractors achieve this by bolstering the existing foundation, reinforcing the soil by incorporating an expanding filler for optimal stability or extending the foundation to distribute the load of the building over a larger surface area.
Purposes of Underpinning
Building contractors can perform underpinning for any of the following purposes:
- To strengthen a foundation that has settled due to cracks in the walls.
- If a building’s existing foundation rests on poor strata, contractors can perform underpinning, so the foundation rests on soil with higher bearing capabilities (deeper soil strata).
- To build a basement in an existing building.
- Suppose you have adjoining buildings, one with a deeper foundation than the other; contractors can perform underpinning to lead the latter building’s shallow foundation to an optimal or similar depth.
When is Underpinning Necessary?
Your house may need underpinning if the initial foundation isn’t strong enough to support the building. This could be due to:
- Failure by the building contractors to properly understand the properties of the soil at the construction site during the initial designing of the foundation. If that’s the case, your house’s foundation may not resonate with the present soil conditions.
- Changes in the soil that supports the foundation of your building could necessitate underpinning. These changes include but aren’t limited to soil expansion and contraction as a reaction to moisture, unrepaired damaged plumbing, large trees growing near the house, and subsidence (the collapse or caving in of a portion of land)
- Changes in the building structure due to renovations.
- Excavation of soil nearby for the construction of new buildings can compromise the support of your existing foundation.
- The addition of uncontemplated storeys to an existing building.
- Droughts, floods, earthquakes, and other natural disasters could destabilize or cause a building to “shift.”
Pit Method/Mass Concrete Underpinning
This traditional underpinning technique involves extending the existing foundation to a stable soil stratum. Contractors use pins or stages to excavate the soil beneath the old foundation in bits. Once they have attained optimal strata, the contractors fill the excavation with concrete and leave it to cure before the next excavation phase. They also pack dry sand-cement to provide a new pin that facilitates the transfer of the building’s load from the existing foundation to the new one. It is crucial to note that while the pit method may be a cheaper underpinning method compared to the rest, it is best suited for shallow foundations.
Cantilever Needle Beam Underpinning
This is a more complex version of the pit method. Building contractors can use this underpinning method if the plan’s interior column is strong and the foundation only needs an extension on one side. Below are the advantages and disadvantages of the cantilever needle beam method:
- Only one side has access to the foundation.
- It’s faster than the pit method.
- High capability to support the building’s load.
- Access constraints make it challenging to use needle beams.
- Digging may be deemed uneconomical, especially if the building has a deep foundation.
Here, concrete piles are driven into adjacent sides of the wall supporting the failing foundation. A pin or needle is then driven through the wall to connect both concrete piles, acting like a pile cap. Building contractors can use this underpinning method to treat soil settlement due to water clogging.
This method is best suited for pad or strip foundation and can be used for buildings with between five and ten storeys. Before underpinning, contractors compress the subsoil to increase the soil’s load-bearing capability in the desired excavation level.
Now that you know what underpinning is all about, indicative signs could deem the procedure necessary for your building. For instance, you should reach out to a professional contractor when you notice large cracks in your walls and floors, misaligned doors and windows, or changes in your floor leveling.