Strengthening Existing Foundations: Purposes and Methods

Strengthening Existing Foundations: Purposes and Methods

In this month’s article, we will explore the purpose of strengthening existing foundations and its available methods. As buildings age, foundation problems develop gradually. When structural loadings change or ground conditions evolve, strengthening existing foundations is a cost-effective way to extend a building’s life.

Typical reasons for foundation strengthening

• Increased Structural Loads: This often arises when a building undergoes changes in use or configuration—such as adding additional stories, constructing a new chimney, or introducing new structural elements that impose extra weight. These modifications can exceed the capacity of the existing foundations, reducing their ability to safely support the structure or prevent further settlements.
• Updates to standards: Structural loadings may change due to updates in Eurocodes standards.
• Differential Settlement: This is a normal occurrence in older buildings when one part of a foundation settles at a different rate than another. Differential settlement, unlike uniform settlement, can cause significant problems, ranging from serious cracking to, in extreme cases, partial or complete structural failures.
• Water-Related Problems: Persistent moisture in the ground can lead to soil swelling, erosion, or loss of compaction, all of which reduce the soil’s ability to support the foundation. This instability often results in subsidence, cracking, and shifting of the structure.
• Deterioration Over Time: Concrete aging, reinforcement corrosion, and chemical attacks can gradually weaken the foundation, reducing its strength and overall performance.

Investigation and assessment

Prior to any foundation strengthening an assessment of the existing situation is crucial. This involves the following:

1. Preliminary Study: Reviewing previous documents on the foundation.
2. Geotechnical Investigation: Conduction subsoil investigation such as taking soil samples, laboratory tests and boreholes.
3. Structural Assessment: Digging trial pits to figure out existing foundation dimensions.  Additionally, ferro scanning is conducted to locate and verify reinforcement details inside the foundation.
4. Performing condition assessment: This assessment determines the urgency of intervention and the most suitable strengthening method.

Common Strengthening techniques

• Underpinning: A traditional strengthening method that involves placing new concrete beneath the existing footing to increase its depth and bearing area. This increases the foundation’s load‑bearing capability.
• Foundation widening and strengthening: Reinforced concrete is added around the existing footing. This method is commonly used when bearing capacity needs only a moderate increase.
• Micropiles: Installed by drilling directly through existing foundations, with steel reinforcement inserted and grouted, connecting the new pile directly to the old structures. This enables load transfer where micropiles bypass weak, compressible soil, transferring structural loads to deeper, more stable strata.

Key Design Criteria

The choice of strengthening method and tools depends on various design conditions:

• Member thickness and reinforcement ratio
• Ground conditions
• Permitted vibrations and noise emissions
• Accessibility
• Time frame, weather and protection periods

Limitations of strengthening methods

There are significant risks when incomplete information is obtained on the existing structure. This includes unforeseen ground conditions, and insufficiently planned construction stages.

 To mitigate these risks, the following precautions need to be undertaken:

• Robust investigations
• A step-by-step approach
• Monitoring deformations
• Selection of low-vibration methods suited for the structure

Conclusion

At LSY, we frequently access existing foundations as part of refurbishment and change-of-use projects across the UK. Early investigation and the right strengthening method can often avoid more invasive rebuilding works. Foundation strengthening should therefore be viewed not as a simple repair, but as a comprehensive engineering process aimed at ensuring the long-term safety, durability, and performance of the structure.