What Is Basement Underpinning And When Is It Needed?

What Is Basement Underpinning And When Is It Needed?


Underpinning is a crucial construction process used to reinforce the foundation of an existing building or structure. This might be needed if the original foundation isn’t strong enough to support the building anymore. Typically, this involves either strengthening the existing foundation, enhancing the soil beneath it with a stabilizing agent, or extending the foundation to spread the structural load more widely.

When is Underpinning Necessary?

Underpinning becomes essential under a few key circumstances:

  • Soil Issues: Changes in the soil supporting the foundation, such as subsidence (sinking) or expansion and contraction due to moisture variations, can undermine stability.
  • Original Foundation Design Flaws: Sometimes, the foundation might not have been designed to accommodate current soil conditions correctly.
  • Change in Building Use: Modifying how a structure is used, or adding significant weight (like another story) can demand an underpinning process.
  • Nearby Construction: Excavation for new buildings nearby can affect the soil under existing foundations.
  • Natural Events: Earthquakes, floods, or extended droughts can cause movement or instability requiring underpinning.
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Signs You Might Need Underpinning


Identifying foundation issues early is crucial for maintaining your home’s structural integrity. Be alert to these signs, which might indicate the need for underpinning:

  • Visible Cracks: Significant cracks in exterior or interior walls, especially horizontal or stair-step cracks in brickwork, suggest foundational shifts.
  • Uneven Floors: Noticeable sloping or dipping in floors can signal foundation sinking or shifting.
  • Doors and Windows Sticking: If doors and windows begin to stick or won’t close properly, it could indicate that the foundation is moving.
  • Gaps in Wall Junctions: Watch for expanding gaps where walls meet ceilings or floors, showing potential foundation movement.
  • Floor Cracks: Unexplained cracks in tiles or other flooring materials may point to underlying foundation problems.
  • Water Intrusion: Persistent moisture or water in basements or crawl spaces could be due to foundation cracks.
  • Leaning Chimneys: Chimneys pulling away from the house are clear indicators of foundational concerns.

Signs You Might Need Underpinning

The stability of a building’s foundation is heavily influenced by the type of soil it rests upon. Soils that undergo significant changes under varying weather conditions are known as “reactive soils” and pose a greater risk for foundation issues due to their tendency to expand during wet seasons and shrink during dry ones.

The soil beneath your structure not only determines the potential risk to your foundation but also influences the best underpinning method to stabilize your property effectively.

Soils are categorized into several classifications based on their reactions to moisture changes and their bearing capacity, according to the Australian Standard AS 2870/2011 for residential slabs and footings. Here’s a brief overview of these classifications:

  • Class A (Non-reactive): Comprising mostly sand and rock, these sites exhibit minimal ground movement from moisture changes, making them the least problematic for foundations.
  • Class S (Slightly Reactive): Characterized by slightly reactive clay that exhibits only minor movement from moisture changes.
  • Class M / M-D (Moderately Reactive): These sites have moderately reactive clay or silt and can experience moderate levels of ground movement.
  • Class H1 / H1-D (Highly Reactive): Highly reactive clay sites in this classification can undergo significant ground movement.
  • Class H2 / H2-D (Very Highly Reactive): Similar to H1 but with even greater potential for ground movement under changing moisture conditions.
  • Class E / E-D (Extreme): Sites with extremely reactive soil compositions, prone to the most severe ground movements.
  • Class P (Problem Sites): This category includes sites with problematic soil conditions such as soft clay, loose sand, varying fill depths, landslips, mine subsidence, or any condition that poses severe challenges to standard foundation designs.

The additional “D” denotation refers to areas susceptible to ‘deep’ soil moisture variations, which is common in drier regions.

Types of Building Foundations and Footings

The foundation of a building is critical to its stability and integrity, serving as the base that transfers its load to the ground. Foundations and footings come in various types, each suited to different soil conditions, building sizes, and architectural designs.

Here’s an overview of the common types used in construction:

  • Slab-on-Grade Foundation: A single layer of concrete, several inches thick, laid directly on the ground. Often reinforced with steel rods, this type of foundation is straightforward and cost-effective, suitable for stable soils.
  • Crawl Space Foundation: Elevates the home off the ground by about two feet, supported by footings and walls. This type allows for easy access to electrical and plumbing installations under the building.
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  • Basement Foundation: Involves digging into the ground and constructing a subterranean room with concrete walls. Basements provide extra living or storage space and are common in cold climates where the foundation must be buried below the frost line for thermal insulation.
  • Pier and Beam Foundation: Consists of piers or pillars sunk deep into the ground to support beams that, in turn, support the load of the building. This type is suitable for areas with shifting soils as it allows adjustments for movements.
  • T-Shaped Foundation: Ideal for areas where the ground freezes. A footing is placed below the frost line, and then walls are erected on top, with the slab poured last. This design provides deep support, countering the effects of freezing and thawing cycles.
  • Raft or Mat Foundation: A large slab supporting the entire structure, ideal for weak soil that cannot support traditional footings. Raft foundations distribute weight evenly, reducing the risk of differential settling.

Each foundation and footing type is designed to address specific structural needs, soil types, and environmental conditions. Selecting the right foundation is crucial for long-term stability and requires careful consideration of the site’s characteristics and the building’s requirements.

Types of Underpinning

Underpinning strengthens and stabilizes the foundation of a structure through several methods, tailored to the specific challenges at hand. Here’s a brief overview of the main underpinning techniques:

  1. Concrete Slab (Mass Pour): A traditional method where concrete is poured in stages under the existing foundation, extending its depth and strength. Ideal for shallow enhancements needing more stable soil.
  2. Screw Pile: Involves driving steel piles deep into stable soil layers and attaching them to the foundation with brackets. This method offers quick stabilization and potential correction of foundation subsidence with minimal disruption.
  3. Grout or Resin Injection: Targets localized foundation issues by injecting a mix that expands to fill voids and stabilize the soil. It offers precision treatment for areas needing specific adjustment.
  4. Piled Raft: Combines deep piling with a new concrete raft foundation to distribute the structural load more effectively. Used for buildings requiring both depth and breadth in foundation reinforcement.
  5. Beam and Base: A reinforced concrete beam is constructed below the foundation, transferring the building’s weight across a broader area or to stable soil via mini-piles. Adaptable to various structural and soil conditions.


Selecting the right underpinning method depends on factors like foundation damage, soil conditions, and budget. Consultation with a structural engineer or foundation specialist can help determine the best approach for ensuring the building’s stability.

The Underpinning Process

Underpinning is a critical process used in the construction and renovation industry to strengthen and stabilize a building’s foundation. Here’s a brief overview of the underpinning process:

  1. Assessment and Planning: The first step involves a detailed assessment of the existing foundation to determine the best underpinning method. This phase includes evaluating the soil characteristics, foundation condition, and the overall structural requirements. Based on this assessment, a comprehensive plan is developed.
  2. Permit Acquisition: Before any work begins, the necessary permits must be obtained from local authorities. This step ensures the project complies with all relevant building codes and regulations.
  3. Excavation: The underpinning process requires excavation around the existing foundation. Work is done in phases (or sections) to maintain the structure’s stability throughout the process.
  4. Underpinning the Foundation: After excavation, underpinning involves extending the foundation to a deeper level. This is usually achieved by pouring concrete under the existing foundation to create a new, lower foundation layer. The method used can vary, including mass concrete underpinning, beam and base underpinning, or more modern techniques like resin injection.
  5. Verification and Finishing: Once the underpinning is complete, the work is inspected to ensure it meets all safety and quality standards. The final step involves backfilling the excavated areas and restoring the site, followed by any necessary cosmetic fixes to the building and landscaping.

Trust VMB Group for Expert Underpinning Solutions


Facing foundation issues? Look no further than VMB Group for professional and dependable underpinning services. Our experienced team provides top-notch solutions using advanced techniques to ensure your property’s foundation is secure and stable.
With VMB Group, you get not just expert service but also the reassurance that your home’s foundation integrity is restored. Choose VMB Group, where we prioritize your property’s safety and your peace of mind!

FAQs About Basement Underpinning

Yes, when done correctly by experienced professionals, underpinning is a long-term solution that can significantly extend the life of a building by ensuring its foundation is stable and strong. It's important to choose the right underpinning method for your specific situation to achieve lasting results.

Typically, smaller underpinning jobs typically take a few weeks, while larger projects involving deep foundations or complex sites can take several months. A detailed assessment by a structural engineer can provide a more accurate timeline.

Properly executed underpinning, done to address and rectify foundation issues, can positively impact the value of your property. It not only assures potential buyers that the home is structurally sound but also can improve its marketability by increasing building stability.

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