We've been gradually going through the steps in constructing a large building, with a slight detour last week to examine the difference between terracotta and terra cotta. In a recent Explainer about the building process though, we talked about the typical procedures that are employed to make excavation sites safe as a prerequisite to new high-rise construction. This week as we return to the building process, we do it with a slight detour to tackle something atypical — underpinning — looking at what can be done if there is an existing structure beside a building site where the excavation could cause instability due to the site's geology.
Dig close to an existing building, and the existing one's foundations may need added support in some circumstances. Earthquakes and floods may cause the foundations to move due to weakened soil conditions. Sometimes the existing building will be added onto itself; expansions and the addition of more floors will likely also necessitate increased foundation strength for that building. This is done through the careful excavation of sections of soil underneath the existing foundation, which is then usually replaced by concrete, a technique known as mass concrete underpinning.
There are several other different types of underpinning:
- Beam and base: A reinforced concrete beam is installed above the building footings or in replacement thereof, which transfers building loads to mass concrete bases.
- Mini-piled: Cylindrical supports — or piles — are driven deep into the ground to transfer foundation loads to more stable soils.
- Expanding resin injection: A relatively new approach to underpinning, this procedure involves the injection of a structural resin and hardener mix underground, which then creates a chemical reaction that expands and consolidates weaker soil, raising the structure above and healing cracks in the building.
As the world grows denser and land becomes scarcer, building new structures adjacent to and above existing ones will likely remain a common practice. Underpinning provides the stability demanded by these activities in a safe and thorough manner.
Have any other construction and development related terms that you would like to see featured on Explainer? Share your thoughts and questions in the comments section below!
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From 2015 to 2017, UrbanToronto and its sister publication, SkyriseCities, ran an occasional series of articles under the heading Explainer. Each one took a concept from Urban Planning, Architecture, Construction, or other topics that often wind up in our publications, and presented an in depth look at it. It's time to revisit (and update where necessary) those articles for readers who are unfamiliar with them. While you may already know what some of these terms mean, others may be new to you. We are publishing or updating and republishing Explainer on a weekly basis. This article is an update of one that first appeared in 2016.
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Do you have other planning terms that you would like to see featured on Explainer? Share your comments and questions in the comments section below!
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