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When Is Underpinning Required?

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Underpinning is the reinforcement of the foundations of buildings. It is needed in the event that the foundation itself is not sufficient to support the structure. It is typically the result of a change in the soil’s structure because of the type of soil or an external factor that affects the soil. Check out the following article for more in-depth details.

What are the underpinnings (of the building)?

Underpinning refers to the process of strengthening or supporting the structure of a foundation, home or other structure. It is done by strengthening the foundation that is already in place, by strengthening the soil by adding an expanding filler or expanding the foundation to ensure it is spread across a larger area.

What is the best time to use underpinning?

In the majority of homes it is necessary to underpin their home when the foundation of the house is not sufficient to support the structure. This usually happens as caused by:

the soil that supports the foundation has been altered in the way that supports the foundation e.g. through subsidence, expansion/contraction due to moisture, large trees nearby, damaged plumbing left unrepaired.

the soil’s characteristics weren’t fully understood in the initial design of the foundation. This means that the foundation isn’t suitable to the soil conditions.

In less typical cases the need for underpinning may be necessary in the following circumstances:

The way in which the structure is utilized changes e.g. after a major remodel

New construction is happening nearby, which results from the digging of soil to support foundations that are already in place

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To improve the capacity of foundations already in place e.g. for supporting a second storey of the structure

Natural disasters, such as floods, earthquakes, or droughts, have caused structures to shift or become unstable.

To better understand when it is necessary to underpin we will take a closer look at the most important components that influence the foundation.

Site Classifications and Types of Soils

It is the soil’s type that plays an important aspect in the foundation’s stability. Certain types of soil are susceptible to more severe structural changes that occur in environment (e.g. in prolonged periods of dry or dry climate) and can cause foundation issues that are structural. These soils are referred to as “reactive”.

The soil type under your home can affect the degree of damage your home will suffer and the method of underpinning that is best is suited for stabilising the structure.

Class A

“Acceptable” between 0 and 10mm, mostly rock sites and sand, with minimal or no ground movement due to the moisture change .
Class S

“Satisfactory” 10-20mm A little reactive clay site. Very little ground movement due to changes in moisture are anticipated.
Class M/M-D

“Moderate” 20-40mm moderately reactive silt or clay sites, which may have moderate movement of the ground due to the changes in moisture.
Class H1/H1-D

“Highly reactive” 40-60mm highly active clays. The ground can move a lot due to changes in moisture.
Class H2 or H2-D

“Highly reactive” 60-75mm highly active clays. It is possible to experience extreme ground movement due to changes in moisture.
Class E/E-D

“Extreme” 75mm+ Highly responsive sites. The ground can move dramatically due to changes in moisture.
Class P

“Problem” Sites that include soft soils like silt or soft clay, or loose sand. They can also include varying in depth of the fill mining subsistence and collapsing soils sandy soils susceptible to erosion reacting sites that are subject to unusual conditions of moisture or that are not classified as such.

“D” in the above classifications refers to “D” included in these classifications is referring to deep movements in soil caused by deep variations in the amount of moisture. These classifications are typically located in dry regions.

Different types of foundations for buildings and foundations

Technically speaking”foundation” refers to”foundation” technically means the “foundation” refers to the ground or strata on that the “footings” for a structure are built. But the term “foundation” is frequently used to mean”the “footing system” as well as”flooring systems” or the “flooring system” which together form the foundation.

Slab on Ground

There are a variety of slabs on the ground like an raft slab, a waffle pod slab or slab featuring dropped beams, or the strengthened slab on fill.

Suspended Floors

They are usually covered with piers or stumps they are supported with joists and bearers.

The common footing systems in residential construction comprise of:

Continuous footings

For example, an concrete slab or strip that is used to support evenly weights that are evenly distributed.

Pad feet

Like the round or square concrete pad designed to support a heavy load. Commonly used with stumps.


Made of hollow steel, timber poles or posts.

Piles and Piers

Like stumps, however they are made of wood and drilled in the earth. Typically, they are used when more support is needed. This includes poured concrete piers bored pilings driven piles (timber steel, concrete, and steel) along with screws made of steel.

Most often, underpinning is done using “Slab on Ground” type foundations.

What causes foundations to fail?

There are many reasons the foundations of a building could fail.

Reactive Soils

The most common cause of concern is due to the shifting of soils with high reactivity. This can cause shrinkage (which results in settle) as well as expansion (which can cause the soil to heave). If dry conditions continue soils lose water slowly and begin to shrink. If the moisture levels are high like during prolonged times of rainy weather soils can expand up to several hundred percent.

The expansion and shrinkage of soil could weaken the structural integrity in the structure of the foundation resulting subsidence, heaving, and visible cracks in walls and foundations.

Poorly Printed Fill

If a location was filled often, the filler material isn’t compact enough to be able to support the load of the structure that is over it. In these situations foundation issues are common. The issue could stem from poor compacted fill, application of multiple filling materials, or the combination of both.

Site Erosion

Erosion may erode the foundations’ soil to the point that foundations are structurally damaged. Erosion can come from many causes, including the burst of a water pipe, any other water flow that is not controlled or drainage issues or similar.

Slope Failure

Failure of a slope can be attributed to the motion of the earth downhill. It can be caused by the slow breakdown, commonly referred to as “creep” or sudden failures, which is known as “landslides”. If a slope is failing because of creep, underpinning may be utilized to fix the issue. But, this is local to the area and requires a thorough assessment.

Transpiration (aka Trees)

Trees play a major role in foundation collapse. Every plant removes moisture from soil. This is called transpiration. Large trees that drain water from soils can dramatically accelerate the shrinkage of soil. If trees are placed too close to structures it can result in the expansion or shrinkage of soils to the point that it compromises the foundation.

Foundation Design

In lesser amounts it is possible that the initial design of the foundation might have been flawed. This could be due to soil’s properties not being fully recognized during the initial conception of the foundation, which means that the foundation was not suitable for the soil conditions. However, thanks to contemporary building codes, this becomes less of a problem.

Different types of underpinning

As we discussed earlier in the article, underpinning is strengthening the foundation already in place.

When it comes to fixing the foundations of buildings that have stumps, the procedure employed is known as restumping or Reblocking. This is basically replacing the stumps of the foundation when they have been damaged or cracked. This isn’t considered to be underpinning.

In this definition of underpinning There are three different methods used today:

Concrete slab

Screw pile

Resin injection or grout

There have been historically two primary underpinning methods that have been used. This is the concrete slab method (also called slab jacking) and screw pile underpinning (also called Pier underpinning (also known as piering). Recently, a third technique is being employed, which is known as grouting or injection.

Concrete slab

Historically concrete underpinning was utilized to boost the dimensions of foundations and to consolidate them. It’s still being used frequently in the present.

Screw pile

It is a popular method that combines concrete footings and steel piers to support the structure and ensure that the lifts are returned to their place of origin, closing cracks and gaps. Piers are considered to be a permanent solution – one that is not changed by changes to the soil around the building – this is the reason we choose this method.

Grout / Resin injection

This is the most recent method of underpinning, but it’s not underpinning. It involves injecting grout or resin into the ground , which creates voids underneath the slab. The slab expands and then compresing the ground. It’s not the most quantifiable method in terms of the durability of repairs and end-to-end cost (the size of the grout needed cannot be precisely calculated and frequently exceeds the initial estimates) and isn’t suitable for all soil conditions.

Do I need underpinning?

There are some indicators to look out for when performing an assessment on your own home. While reading this list, it’s crucial to realize that subsidence is a common feature of properties at different levels. The need for underpinning only occurs when subsidence is taking place. When subsidence occurs, it is possible that and the structure is in equilibrium which means there’s no risk. If you aren’t sure you are not sure, seek out an expert.

Cracks in floors and walls

Cracks don’t have to be scary. Sometimes, they’re just superficial cracks, like minor or hairline cracks in cornices, plaster or skirting boards. Cracks that are more extensive and often point to larger root causes, like uneven weight distribution caused by weak foundations.

Cracks to look out for can be either interior (plaster wall and floor tiles) or external (brickwork or concrete slab, render).

It is recommended to examine the cracks over the course of time. This can be done over a period of weeks or even months. find out if the cracks appear to get larger in size, width or length and if any new cracks start appearing. If they aren’t changed for a long time it is likely that the subsidence has taken place and the house is now become settled.

Floor isn’t at a level

A problem that’s not always as apparent as cracks is uneven floors. If you can spot them you notice a slight lean towards one or the other sides of your home is a clear indication that there are serious foundation issues to be considered.

In some serious instances we’ve witnessed that you could stand at the end of a hall and observe the house fall down as you gaze down the hallway. Sometimes, floors that aren’t level could cause misaligned doors. In general, you can make use of a spirit level to gauge the degree of unevenness in a room. You can also place a ball into the room to see whether it is in place or moves in a certain direction. However, to comprehend how important this is , it’s best to consult an expert.

Another thing to be aware of are the irregularly-shaped trenches that form around the edges of the slab or building or slabs, within the top layers of soil. Another sign of subsidence.

Windows and doors that are out of alignment

Windows and doors can be an indicator of foundation issues. The gaps are appearing and becoming larger around your doors and windows. It is becoming difficult to close (or open) your windows or doors or to secure them.

In more advanced instances there may be more obvious doors that are leaning towards them windows or doors, and the door frames can begin to pull away from the walls around them.

How do I get help?

None of the indicators above do not guarantee that you’ll require underpinning. However the last thing you want to be waiting around until your symptoms start to become serious. If you’ve done a self inspection of your house, there are concerns then the best thing to do is to remain relaxed – there’s lots of assistance readily available.