Underpinning is a method for the repair and strengthening of building foundations. Underpinning methods, procedures and their applications in the strengthening of different types of foundations are discussed in this article. There are situations where a failure in foundation or footing happens unexpectedly after the completion of the whole structure (both sub and superstructure). Under such an emergency situation, a remedial method has to be suggested to regain structural stability. The method of underpinning helps to strengthen the foundation of an existing building or any other infrastructure. These involve the installation of permanent or temporary support to an already held foundation so that added depth and bearing capacity are achieved.
Underpinning, in terms of structural & architectural engineering, has been around for well before the 15th century. In fact, the great Italian Renaissance humanist and architect Leon Battista Alberti outlined the extensive benefits of underpinning his comprehensive treatise on theory and practice of architecture, De Re Aedificatoria (On the Art of Building), published in 1452.
It has long been a tradition to restrengthen bridges, tunnels, skyscrapers, and homes by use of underpinning. A more modern example of underpinning is the story of the antiquated Hotel Commodore (designed by the highly celebrated Homer G Balcom, who also designed the Empire State Building).
The Hotel Commodore, built-in 1917, was erected right above (and, consequently, supported by) the Grand Central Station subway structure. In 1979, the Hotel Commodore underwent an extensive refurbishment before resuming business under the new appellation Grand Hyatt.
This renovation proved extreme and required space to be opened up for an increase in headroom at the subway mezzanine level (intermediate floor) to improve passenger outflow.
This required extensive underpinning and significant load transfer of the hotel above by temporary support, sequencing, and jacking.
Types of Underpinning
There is a general consensus among architects, engineers, and construction workers that there are three primary types of underpinning:
Mass Concrete Underpinning/Mass Pour
Mass Concrete underpinning is perhaps the oldest, most frequently utilized foundation repair method on the planet.
This involves digging holes/voids underneath a weakened foundation and pouring new mud/concrete/filler below the original foundation.
This extends the depth and breadth of the foundation, providing extra inherent support.
This is simple. Basic. Gets the job done. You can chalk it up to – digging a hole underneath the existing foundation and filling it up with supporting material.
Beam and Base Underpinning
This is a newer form of underpinning where a reinforced concrete/steel beam is constructed below, above, or in replacement of the existing footing.
The beam then transfers the weight of the building to these concrete/metal bases, which have been positioned in strategic load-bearing locations.
This is slightly more sophisticated than mass concrete underpinning in that it's essentially adding a new concrete/steel base to an older one, not replacing it.
Mini-piled Underpinning
This type of underpinning allows for the greatest flexibility in regard to where access is limited/restricted and environmental pollution is significant.
Mini-piled underpinning is typically when the load of the structure needs to be transferred to more competent load-bearing soil at a greater depth (occasionally as deep as 50 feet).
This originated in Italy back in 1952 and has undergone many different variations under numerous names and patents.
Push Piers
Hollow cylinder tubes are typically 30 inches long and comprised of hot-dipped-galvanized steel, with a maximum load capacity of 68 thousand pounds.
These are pushed via drive head to the load-bearing stratum and attached to the footing of the structure with a remedial bracket.
Helical Piers
Square or round tubes are typically 30 inches long and comprised of hot-dipped-galvanized steel, with helices welded to the bottom to provide extra support and a maximum load capacity of 74 thousand pounds.
These are screwed into the earth until they reach a competent, load-bearing stratum and then attached to the footing of the structure with a remedial bracket.
Helical Tiebacks
Square tubes that vary in size, typically comprised of hot-dipped-galvanized steel, have helices/flights and are driven into the earth at an angle to act as an anchor.
Helical tiebacks are rods implemented to support, strengthen, anchor, and straighten retaining walls or bowed basements and can have a maximum load capacity of up to 200,000 pounds.
These are screwed into the earth until they reach a competent, load-bearing stratum and then secured via a whaling plate.
This is the most complex version of underpinning, as it involves a wide variety of tools and resources for an application.
In simplistic terms, imagine putting your house on underground stilts.
These stilts are driven into load-competent soil or bedrock, providing deep-rooted support for your foundation.
Alternatives to Underpinning
As the definition of underpinning pertains specifically to the reinforcement/strengthening of the foundation itself, alternatives attempt to strengthen the soil.
We see this by the introduction of grout and other urethane-based chemicals that expand – such as Polyurethane.
Mudjacking
The raising of pavement/concrete/slabs by means of pumping mud/slurry (concrete) under it through drilled holes. The pressure of the expanding mud raises the pavement in elevation.
Grout injection
This method of foundation repair is where grout is injected directly into the soil, changing the chemical makeup of the adjacent native soils so that they develop low viscosity.
The grout/chemical is typically injected via a sleeve port pipe which is inserted into a pre-dug hole.
The grout is henceforth pumped out and allowed to permeate the soil, essentially calcifying the affected area so that the soil is stronger, thicker, and less permeable.
Just remember Kayne's number one hit single, "Stronger." The goal is to make the ground harder, better...stronger.
These are not great alternatives to underpinning.
Mudjacking and using polyurethane material is not a long-lasting solution to your problem.
And, considering homes stick around for quite a while, it's better to go the more permanent route.
Frequently Asked Questions
The cost of restumping can vary depending on the size of the house, whether the whole house needs restumping and the accessibility to the stumps themselves but as a guide the cost will typically range from $10,000 to $30,000.
Stumps were traditionally made of hardwood, but over time these rot away leaving the floor unsupported. These have now been replaced by treated timber, cast concrete or galvanised steel columns. These must be designed by an engineer to ensure correct sizing and spacing.
What is involved in a subsidence survey? A specialist will come to your property to carry out an investigation into the existence and extent of subsidence. This will begin with a visual inspection to look at cracks in the walls, sticking doors and windows and other obvious signs of subsidence.
Severe subsidence can be a very serious problem. Subsidence can lead to severe structural problems which can make a house uninhabitable. Severe subsidence can be dangerous and eventually lead to structural instability and collapse. Subsidence can make a house unmortgageable and so, perhaps, unsaleable.
When is underpinning needed?
For most homeowners, underpinning is required when the original foundation is not strong enough to support the house. This is usually a result of:
The soil supporting the foundation has changed in some way, e.g. through subsidence, expansion/contraction due to moisture, large trees nearby, and damaged plumbing left unrepaired.
The properties of the soil were not adequately understood during the original design of the foundation - meaning the foundation is not adequate for the conditions.
In less common cases, underpinning is also required for the following reasons:
The way the structure is used has changed, e.g. following a major renovation.
New construction nearby results in the excavation of soil supporting existing foundations.
To increase the capability of existing foundations, e.g. to support another storey to the building.
Natural disasters such as earthquakes, floods or droughts have caused the structure to move or become unstable.
To help understand if and when underpinning is needed, let's take a closer look at the key elements that impact a foundation.
Soil Types and Site Classifications
The soil type plays a key role in the stability of foundations. Certain types are prone to more significant structural changes in the soil conditions (e.g. during extended periods of wet or dry weather) and thus contribute to structural foundation problems. We call these soils "reactive".
The types of soil you have beneath your house will play a part in the extent of damage to your home and the method of underpinning best suited to stabilizing the building.
Soils can be classified in a number of ways. When it comes to building foundations, we characterize the soil by the Site Classification (in accordance with Australian Standard AS 2870/2011, Residential slabs and footings). This allows us to understand the soil's potential to support a structure.
Class A
"Acceptable" 0-10mm Mostly sand and rock sites, with little or no ground movement due to moisture change expected.
Class S
"Satisfactory" 10-20mm Slightly reactive clay sites. Only slight ground movement from moisture changes is expected.
Class M / M-D
"Moderate" 20-40mm Moderately reactive clay or silt sites can experience moderate ground movement from moisture changes.
Class H1 / H1-D
"Highly Reactive" 40-60mm Highly reactive clay sites. Can experience high ground movement from moisture changes.
Class H2 / H2-D
"Highly Reactive" 60-75mm Highly reactive clay sites. Can experience very high ground movement from moisture changes.
Class E / E-D
"Extreme" 75mm+ Extremely reactive sites. Can experience extreme ground movement from moisture changes.
Class P
"Problem" Sites which include soft soils, such as soft clay or silt or loose sand, varying depths of fill, landslips, mine subsistence, collapsing soils, soils subject to erosion, reactive sites subject to abnormal moisture conditions, or sites that cannot be classified otherwise.
The "D" inclusion in the above classifications refers to 'deep' movements in soil due to deep variances in moisture. These classifications are mostly found in dry areas.
Types of building foundations and footings
Technically speaking, the "foundation" is the earth or strata upon which the "footings" for a building are constructed. However, the word "foundation" is regularly used today in Australia to refer to the "footing system" and the "flooring system" that together make up the foundation.
In residential construction, there are two common flooring systems used:
Slab on Ground
There are several types of slab on the ground foundation, such as a raft slab, waffle pod slab, slab with dropped edge beams or a reinforced slab on fill. These are the typical foundation systems that have been in use for many years in Australia, particularly in QLD and NSW.
Suspended Floors
These foundations are most commonly framed with stumps or piers and are supported by bearers and joists.
The footing systems used commonly in residential construction consisting of:
Continuous footings
Such as a concrete strip or slab used to support uniformly distributed loads.
Pad footings
Such as a square or round concrete pad used to support a concentrated load. Most commonly used in conjunction with stumps.
Stumps
Designed in hollow steel or timber poles or machined timber posts.
Piles and Piers
Like stumps but driven or bored into the ground, commonly used where additional support is required. Includes poured concrete piers, bored piers, driven piles (timber, steel, concrete), and steel screw piles.
Underpinning is most commonly carried out on "Slab on Ground" type foundations.
Are There Any Drawbacks to Underpinning?
It's possible that you'll be advised by friends or relatives that underpinning your home is a bad idea. The two arguments they're likely to make are that it's expensive and that your home will be harder to insure.
Underpinning is certainly an expensive procedure. If it's as a result of subsidence, however, your building insurance may well cover the cost. In any case, the bottom line is that it's far less expensive than having to rebuild your home after the collapse.
Traditionally, there's been a perception in the insurance industry that a house that's needed to be underpinned is a greater risk. This doesn't really make sense since underpinning actually makes the building more secure, and a number of insurance providers now accept that. Although you may have to hunt a bit further for a good policy, it's certainly possible to find one.
Importance of carrying reblocking service for your house:
A house built of stumps or in a flood-prone is always exposed to moisture, especially if built of woods. These homes are more likely to deteriorate as an action of pests and moisture. So it is important to carry out restumping service to avoid any accidents. Other importance is as follows:
Dreaded dry rots remain at bay: Houses sitting on the same wooden stumps may have grown dreaded dry rot over a long period. These dry rot often make their way up to the house, causing damage. However, these days when you carry out reblocking, the professionals use materials that prevent this condition from happening. They use technology that was not there a few years back. Moreover, the professionals will give you an option between hardwood, concrete, and galvanized steel, the latter being stronger. It will give you protection again dry rot and help you keep them at bay.
Security against flood: Now, while restumping, you also get the option of residing above flood level. Thus, there is an assurance that you don't need to fear for your life in the event of heavy rain and flood. This also significantly increases the value of the property as the residents feel safe and flood-free. Moreover, the burden of higher insurance premiums against floods also decreases for the same purpose.
Make extra room for you: Being flood-free is not the only type of advantage you get from reblocking. It also gives you the advantage of extra room. If you are a person that likes to work alone with no one beside you disturbing you, then restumping the stumps under your houses is a great idea. It makes extra room for you in the low-set house. Here you can carry out any work you want or enjoy some alone time with yourself. It also saves you from the loss of yard space which you would have to compromise otherwise.
Increase the worth of your house: If you have a long-term plan to sell the house in the future, it is always advisable to maintain your house's condition through reblocking. It not only improves the beauty of the house but also gives the buyers a sense of safety. Moreover, when people look to buy a house, they always look at the expense they have to incur in house maintenance. So when the buyers see that the house is freshly stumped, it significantly increases the worth of your property. However, when you ignore the maintenance side of the stumps, it hurts the overall price of the house.
Reblocking
Conclusion:
It is not safe to be reckless towards your house. It reduces the value of the property you live in and exposes you to a lot of threats. By taking proper professional reblocking service, you reduce the chances of loss of life or resources significantly. Plus, you make the place better by levelling it and allowing the wind to pass through it smoothly. Thus, it makes it a perfect living place.
