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How Does Underpinning Come Into Play, And When Is It Required?

Nov 17

Underpinning is the process of bolstering an existing building's foundation. The existing foundation needs to be replaced when it can no longer support the house. This is typically brought on by a change in the soil's structure, which may be brought on by the soil's type or an outside factor. For more information, keep reading.


What Is The Foundation Of A Building?

Underpinning is the process of supporting or strengthening the foundation of an existing house, building, or other similar structure. This may be accomplished in one of three ways: either extending the foundation or reinforcing the current one to distribute the weight across a wider surface area.


When Should Underpinning Be Used?

The majority of homeowners will need underpinning when the home's original foundation is no longer capable of supporting it. This commonly happens as a result of:

  • The soil that supports the foundation has changed in some way, for example, due to surrounding large trees, unrepaired plumbing issues, moisture expansion and contraction, or subsidence.
  • The original design of the foundation did not adequately account for the characteristics of the soil, resulting in a foundation that is unsuitable for the situation.

Additionally, underpinning is essential in unusual situations for the following reasons:

  • The way the building is used has changed as a result of a significant renovation.
  • Because of the nearby construction, the ground under the old foundations has been dug up.
  • Boost the building's foundations' ability to hold more floors, for instance.
  • Natural disasters like earthquakes, floods, or droughts that have caused the structure to move or become unstable.

Site Groupings And Soil Types

The stability of the foundation is significantly influenced by the soil type. Certain types are more prone to extreme structural changes in the soil conditions, which might cause structural foundation problems (for instance, during extended periods of wet or dry weather). They are referred to as "reactive" soils.

Your property's foundation's soil types will affect the extent of the damage and the kind of underpinning that is most effective at stabilizing the building.

There are many different subcategories of soil. When building foundations, the soil is classified using the Site Classification (in accordance with Australian Standard AS 2870/2011, Residential slabs and footings). This improves our understanding of the soil's capacity to support a building.

Class A 0-10mm is "acceptable." The bulk of the areas are sand and rock, and due to variations in moisture, little to no ground movement is anticipated.

"Satisfactory" (Class S) (Class S) 10-20mm areas where clay is a little reactive. Changes in moisture are only expected to cause a little amount of ground movement.

Class M / M-D 20-40mm, "Moderate" On sites with clay or silt that is fairly reactive to moisture changes, there may be some modest ground movement.

H1 / H1-D class "Highly Reactive" 40-60mm sites made of reactive clay. Variations in moisture content may lead to significant ground movement.

Class H2 / H2-D 60-75mm, "Highly Reactive" sites made of reactive clay. Changes in moisture may result in considerable earth displacement.

Class E / E-D "Extreme" Sites with a 75mm+ thickness and significant reactivity. Changes in moisture content may cause extreme ground movement.

Class P "Problem" Soft soils such soft clay, silt, or loose sand, shifting fill levels, landslips, mining waste, erodible soils, reactive sites exposed to abnormal moisture levels, or locations that cannot be classified in any other way

In the classes that follow, the letter "D" stands for "deep" soil motions brought on by significant moisture fluctuations. These classes may mostly be found in dry areas.


Building Footings And Foundations Are Available In A Wide Range Of Sizes And Shapes

Technically speaking, the ground or stratum upon which the "footings" of a structure are constructed is the "foundation." The "flooring system" and "footing system" that make up the foundation are now often referred to as the "footing system" in Australia.

In residential building, two flooring types are most often used:

Slab of Concrete

Examples of slab on ground foundations include raft slabs, waffle pod slabs, slabs with dropped edge beams, and reinforced slabs on fill. These are the standard foundation systems that have been in use across most of Australia, particularly in Queensland and New South Wales.

A kind of flooring that hangs from the ceiling is called a suspended floor.
These foundations are often built with stumps or piers, and they are supported by bearers and joists.

Some of the most common footing systems used in residential building are the ones listed below:

Regular Footings

Consider a slab or strip of concrete that sustains loads that are distributed equally.

Footings with Cushions

You may use a concrete pad that is square or round, for instance, to hold a huge weight. The most frequent use is for stumps.

Stump materials include hollow steel or wood poles and machined wood posts.

Stump-like structures that are pushed or drilled into the ground are called pilings and piers. This is a typical remedy when further assistance is required. This kind of pier includes steel screw piles, driven piles (concrete, steel, or wood), poured concrete piers, and drilled piers.

"Slab on Ground" foundations are the most common kind of underpinning foundations.


Why Do Building Foundations Crack?

A building's foundations might fail for a number of reasons.

Activated and Reactive Soils

The problem is often brought on by the movement of highly reactive soils. Both shrinkage and expansion, which result in settlement, are possible outcomes of this activity (which causes heaving). As dry conditions persist, soils gradually lose moisture and contract. When soil moisture levels are high, as they are during protracted periods of wet weather, soils grow - often by hundreds of percent.

The stability of the foundation may be compromised by heaving, sinking, and visible breaking in foundations and walls due to soil shrinkage and expansion.

Inappropriately Compacted Fill

The material used may not have been sufficiently compacted to support the weight of the structure above it after a site has been filled. In these circumstances, foundation problems are typical. The problem might be brought on by poorly compacted fill, the use of various fill materials, or perhaps both.

Erosion at the Location

The soil around foundations may become so eroded by erosion that it compromises their structural integrity. A broken water pipe or other uncontrolled water flow, inadequate drainage, and other factors may all contribute to erosion.

Unsuccessful Slope

The collapse of a slope is associated with the downward flow of earth. It could involve "landslides" or "creep," both of which are instances of delayed collapse. Underpinning may be used to fix a slope when creep causes it to start to erode. However, since this is a site-specific problem, a professional assessment is required.

Perspiration (aka Trees) 

A significant factor in foundation collapse are trees. All plants draw moisture from the earth. This is the main idea of transpiration. Large trees may drastically speed up soil shrinkage by absorbing rainwater from the ground. The soils may expand or contract to the point that the foundation is in danger when trees are placed too close to buildings.

Building Foundation Design

It's possible that the original foundation's design was flawed to some degree. This could be the case because the foundation's original design did not adequately account for the soil conditions, resulting in a foundation that is unsuitable for the situation. Thanks to updated building regulations, this is now less of an issue.


There Are Many Different Sizes And Shapes Of Underpinnings

As was previously said in the article, underpinning is the process of bolstering an existing foundation.

The procedure of repairing a structure's foundation using stumps is known as restumping or reblocking. This requires repairing the foundation stumps when they are broken or otherwise damaged. This is not thought of as a foundation.

There are three types of foundation currently in use that fit this definition:

  • a concrete slab
  • a stack of screws
  • resin or grout injection

Traditionally, there have been two main foundation approaches used. Underpinning techniques include screw pile underpinning and concrete slab underpinning (also called slab jacking) (also known as pier underpinning or piering). Grout or resin injection, a third method, has recently gained popularity.

A Concrete Slab

Historically, foundations have been strengthened and expanded in size using concrete underpinning. Today, it is still commonly used.

Steel piers and concrete footings are used in the Pile of Screws Foundation Solutions method to anchor the building, move it back to its original location, and repair any gaps or cracks. We choose piers since they are thought of as a long-term fix that won't be impacted by future ground changes around the property.

Injection Of Resin And Grout

Although it's not really a foundation, this is the most modern method. It requires injecting grout or resin into the ground, which expands and compacts the dirt by filling cracks beneath the slab. The amount of grout required is difficult to predict and sometimes surpasses initial estimates, making it the least quantifiable approach in terms of repair permanency and overall cost. It is also not appropriate for all types of ground conditions.