What is Underpinning? 

If you are living in an older building or home, you might notice signs of weak foundations. These indicators serve as critical warnings that your foundation requires additional support. In such conditions, underpinning becomes essential to enhance stability and provide necessary reinforcement.

While underpinning is a comprehensive and intricate process, understanding its specifics can be challenging. However, our blog provides a detailed explanation of the common signs, extensively used methods, and benefits to guide you through it.

What is Underpinning?

Uneven soil compression beneath a foundation can result in soil deformation and subsequent settlement. Changes in neighbouring structures can also alter soil conditions, leading to subsidence. Such settlement and subsidence pose significant risks, including potential building collapse, necessitating proactive measures like underpinning.

Underpinning stands as a widely used foundation enhancement technique. It addresses subsidence issues and enhances soil load-bearing capacities by redistributing loads to deeper, more stable soil layers. This method involves strategically adding reinforcements along the width or depth of an existing foundation, thereby ensuring its durability and stability.

Moreover, underpinning proves indispensable when expanding structures with basements or loft conversions. Increased foundation loads due to altered building usage or additional storeys can exceed initial design parameters, causing excessive settlement or potential future instability. In such scenarios, underpinning not only enhances the soil’s load-bearing capacity but also reinforces the foundation to withstand augmented structural demands.

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Signs Your Foundation Needs Underpinning

If you are confused about whether your foundation needs underpinning or not, look for the following visible signs or contact the foundation experts like Alpha Piling Ltd.

  • Bowing or protruding walls
  • Foundation subsidence
  • Gaps between bricks, walls, doors, and ceilings or floors
  • Increasing cracks in walls, doorframes, and window frames
  • Problems in opening and closing doors and windows
  • Sloping or uneven floors  
  • Soil displacement around the foundation
  • Visible structural instability or leaning
  • Water damage

Commonly Used Underpinning Methods

Underpinning encompasses different methods which are different in terms of the installation process and depths. Each type has its distinct features and is selected according to the unique requirements of different residential and commercial projects.

Here are a few most widely used underpinning methods.

Mass Concrete Underpinning

Mass concrete underpinning, commonly known as the pit method, is a time-tested and cost-effective technique ideal for structures with shallow or thin foundations. This method involves extending the current footing of a vulnerable section of the building to establish a new, more robust foundation.

To execute this technique, controlled and predetermined segments of the existing base—typically measuring 1200mm—are excavated extensively to reach the deeper, more stable soil strata. Once excavation is complete, the space is filled with concrete, which is then allowed to cure. During the concrete pouring stage, a thin gap is left between the top of the new base and the underside of the existing footing.

After the concrete is fully cured, this gap is filled with dry pack mortar, facilitating the transfer of loads from the old footing to the new foundation. The connection between the old and new bases is typically reinforced with joggle joints. To further enhance stability and strength, reinforcement cages are installed, ensuring continuity between the foundations.

Beam and Base Underpinning

For residential and commercial structures experiencing settlement issues, the beam and base underpinning method is a highly effective solution. This technique is an advanced version of the traditional mass concrete underpinning.

It entails the strategic installation of concrete beams either above, below, or as replacements for the existing foundation footings. These beams redistribute the structural load onto newly constructed concrete bases.

The beams are positioned down the length of the foundation, resisting the structure’s heavy loads by offering strong lateral resistance. This method not only fortifies the building’s structural integrity but also ensures its long-term durability and safety, making it an essential approach for maintaining the stability of settlements-prone constructions.

Mini-piled Underpinning

This method is crucial for projects where noise and disruptions are significant concerns, particularly on constrained construction sites. It effectively transfers loads to deeper soil layers, especially where a depth of more than 5 meters is required.

The procedure involves driving or augering piles with diameters ranging from 150mm to 300mm into the ground. In the driven method, the ground is excavated, and short steel tubes are installed to the desired depth, which are then filled with high-quality concrete or grout. The augered method, on the other hand, employs precast reinforced concrete piles, which are installed at more stable soil layers for enhanced support and strength.

This technique ensures greater accuracy and increased safety compared to other underpinning methods due to its minimal disruptions, low noise levels, reduced labour, and precise load targeting. However, unlike other alternatives, this method is more costly. It necessitates a higher degree of precision and expertise, which is provided by licensed specialists such as Alpha Piling Ltd.

5 Advantages of Underpinning Foundations

Here are a few significant benefits of investing in underpinning your older foundations:

  • The foremost and primary benefit it offers is increased structural integrityand stability.
  • A properly underpinned structure promises increased resale value, making them worth the investment.
  • Underpinning prepares the foundation to withstand the building’s scalability.
  • Underpinning gives access to underground wiring, heating and plumbing systems so you can either upgrade or repair them if required.
  • It is less messy than renovatinga building and is more cost-effective.