Materials Used in Underpinning Foundations

Underpinning foundations is a critical process in construction and civil engineering that involves strengthening and stabilizing the existing foundation of a building or structure. This technique is often necessary when the original foundation is no longer capable of supporting the building loads due to soil subsidence, changes in load distribution, structural modifications, or damage caused by environmental factors. The choice of materials used in underpinning is vital to ensure durability, stability, and safety.

This article explores the various materials used in underpinning foundations, their properties, applications, and considerations for selecting the right materials for underpinning projects.

Understanding Underpinning

Before delving into materials, it’s important to understand what underpinning entails. Underpinning is the process of extending the foundation depth or breadth so that it rests on a more stable soil stratum or distributes loads over a wider area. This can involve:

• Increasing foundation depth.
• Expanding the foundation base.
• Transferring load to stronger soil layers.
• Repairing or strengthening damaged foundations.

Common underpinning methods include mass concrete underpinning, beam and base underpinning, mini-piled underpinning, and pile and beam underpinning. Each method may require different materials depending on site conditions, load requirements, and structural considerations.

Criteria for Selecting Underpinning Materials

When selecting materials for underpinning, engineers consider:

• Strength: The material must have adequate compressive and tensile strength to support additional loads.
• Durability: It should resist degradation from moisture, chemicals, and freeze-thaw cycles.
• Workability: The ease with which the material can be applied or installed on-site.
• Compatibility: Must be compatible with existing foundation materials to avoid adverse chemical reactions.
• Cost-effectiveness: Balancing performance with budget constraints.
• Environmental impact: Preference for sustainable or low-impact materials where possible.

With these criteria in mind, we examine the key materials commonly used in underpinning foundations.

Concrete

Ordinary Portland Cement Concrete (OPC)

Concrete is by far the most common material used in underpinning due to its high compressive strength, versatility, and availability. Ordinary Portland Cement concrete is typically used to create mass concrete underpinning bases or new pads beneath existing footings.

Properties:

• Compressive strength ranges from 20 MPa (megapascals) to over 40 MPa depending on mix design.
• High durability when properly cured.
• Good workability; can be poured into excavated pits or formworks.
• Compatible with most existing masonry or concrete foundations.

Applications:

Mass concrete underpinning involves excavating small sections beneath the existing footing in stages and filling them with concrete to form new support bases. These are usually poured in segments to maintain stability during construction.

Considerations:

• Proper curing is essential to prevent shrinkage cracks.
• Mix design should be adjusted for site conditions (e.g., sulfate-resistant cement if soils are aggressive).
• Reinforcement may be added in some designs for tensile strength.

High Strength Concrete

For underpinning situations involving heavy structures or challenging soil conditions, high strength concrete mixes (exceeding 50 MPa) may be employed. These offer improved durability and reduced section sizes.

Controlled Low Strength Material (CLSM)

CLSM or flowable fill is a self-compacting concrete with lower strength but excellent flow properties. It can be used for void filling under foundations but is less common as primary underpinning material due to lower structural capacity.

Steel

Steel plays multiple roles in underpinning foundations:

Reinforcement Bars (Rebar)

Concrete underpinning bases are often reinforced with steel bars to enhance tensile strength. Rebar ensures that concrete elements can withstand bending stresses induced by soil movements or shifting loads.

Properties:

• High tensile strength (typically 400–600 MPa).
• Ductile behavior prevents sudden failure.
• Good bonding with concrete.

Steel Beams and Props

In beam and base underpinning methods, steel beams like I-beams or H-beams are installed beneath existing walls. These beams transfer loads across weak soil areas to stronger bearing points.

Temporary steel props may also be used during excavation phases to support structures before permanent underpinning is installed.

Applications:

Steel components are particularly important when constructing beam-and-base foundations where beams carry wall loads across extended underpin sections supported by new bases or piles.

Considerations:

Steel must be protected against corrosion through coatings or galvanization since it will be embedded in soil or exposed to moisture.

Timber

Timber was historically used in underpinning but has largely been replaced by modern materials due to durability issues. However, it may still be utilized temporarily as shoring support during excavation works.

Properties:

• Easily available and workable.
• Lower cost compared to steel initially.
• Susceptible to rot, insect attack, and decay if exposed to moisture long-term.

Applications:

Used mainly as temporary formwork or bracing rather than permanent support due to its limited lifespan underground.

Masonry Materials

In some traditional underpinning approaches, masonry blocks or bricks may be used for constructing new supporting walls beneath existing structures. This method requires careful bonding between old and new masonry elements and often includes lime-based mortars compatible with older buildings.

Considerations:

Masonry underpinning is labor-intensive and less common today but may still be suitable for heritage buildings where matching aesthetics is important.

Grouts and Injection Materials

Grouting techniques complement underpinning by filling voids under foundations and enhancing soil stability.

Cementitious Grouts

These are mixtures of water, cement, sand (optional), and additives pumped under pressure into ground voids. Cementitious grouts improve load-bearing capacity by densifying soils around foundations.

Chemical Grouts

Polyurethane-based or acrylate grouts are injected into soil to stabilize loose ground strata rapidly. These materials expand after injection, filling gaps effectively without disturbing existing foundations much.

Applications:

Grouting is often combined with traditional underpinning where soil improvement is required before bearing new foundation loads.

Piling Materials

For deep underpinning solutions such as mini-piling:

Steel Piles

Steel tubular piles driven deep into stable strata provide strong load transfer paths. They are often filled with concrete after installation for added rigidity.

Concrete Piles

Precast concrete piles may also be installed vertically adjacent to existing foundations to carry load downwards.

Composite Piles

Combinations of steel cores encased in concrete optimize both strength and corrosion resistance for difficult environments.

Specialty Materials and Innovations

Modern engineering has introduced specialty materials enhancing traditional underpinning methods:

Fiber-Reinforced Concrete (FRC)

Incorporating synthetic fibers into concrete mixes improves toughness and crack resistance under dynamic loads prevalent in some sites requiring underpinning.

Geopolymers

As an eco-friendly alternative to Portland cement-based concretes, geopolymers offer excellent mechanical properties with reduced carbon footprint though their use in underpinning remains experimental.

Carbon Fiber Reinforced Polymers (CFRP)

Used primarily for strengthening above-ground structural elements but increasingly researched for reinforcing foundation components where minimal intervention is desired.

Environmental Considerations

Sustainable building practices encourage minimizing environmental impact during underpinning:

• Use of recycled aggregates in concrete mixes when possible.
• Selection of low-carbon cement alternatives.
• Minimizing waste generation during excavation processes.
• Ensuring proper disposal of contaminated soils excavated during underpin works.

Conclusion

The success of any underpinning project depends heavily on choosing appropriate materials tailored to site conditions, structural requirements, budget constraints, and longevity goals. Concrete remains the cornerstone material owing to its proven performance under compressive loads while steel provides essential reinforcement and flexibility for more complex foundation systems. Supplementary materials such as grouts enhance soil stabilization efforts critical to effective underpinning.

Advances in material science continue expanding options available for specialists engaged in foundation repair and strengthening. By understanding the properties and applications of various materials—ordinary concrete mixes, high-strength variants, steel reinforcements and beams, grouting compounds, piles, and innovative composites—engineers can design safe, durable solutions that restore building stability efficiently.

Ultimately, proper material selection combined with skilled workmanship ensures that underlying structures remain sound long-term even when faced with challenging subsurface conditions demanding underpin interventions.