Underpinning Process:
Step-by-Step Guide

Underpinning is a critical construction technique used to strengthen and stabilize the foundation of an existing building or structure. It is commonly required when the original foundation is no longer strong or stable enough to support the building due to various reasons such as soil movement, increased load, structural damage, or changes in building use. This guide provides a comprehensive step-by-step overview of the underpinning process, helping homeowners, builders, and engineers understand how underpinning works and what it entails.

What is Underpinning?

Underpinning involves extending the foundation depth or distributing the load across a wider area to increase its bearing capacity. This process essentially strengthens the foundation to prevent further settlement or structural failure. Commonly applied in older buildings, underpinning allows modifications without demolishing the structure.

There are several types of underpinning methods, including:

• Mass concrete underpinning (traditional pit underpinning).
• Beam and base underpinning.
• Mini-piled underpinning.
• Jet grouting.

Each method suits different types of soil conditions and structural requirements. The most widely used method is mass concrete underpinning, which this article will focus on.

Why Is Underpinning Necessary?

Several scenarios may prompt the need for underpinning:

• Foundation settlement causing cracks and instability.
• Increase in building load after renovation or extension.
• Change in soil conditions, such as subsidence due to drought or flooding.
• Nearby excavation work causing movement.
• Design flaws or poor original foundation construction.
• To convert basements into habitable spaces requiring deeper foundations.

Ignoring foundation issues can lead to serious structural damage, costly repairs, and safety hazards. Professional assessment is essential before proceeding with underpinning.

Step-by-Step Guide to the Underpinning Process

Step 1: Site Investigation and Assessment

Before any physical work begins, a thorough investigation is necessary:

• Structural survey: Inspect cracks, settlement signs, and foundation condition.
• Soil testing: Determine soil type and bearing capacity through boreholes or trial pits.
• Load analysis: Calculate current and future structural loads.
• Environmental factors: Check groundwater levels and nearby construction activity.

This data helps engineers choose the appropriate underpinning method and design an effective solution.

Step 2: Planning and Design

After assessment, engineers draft a detailed underpinning plan considering:

• Location and size of underpin sections.
• Depth to which new foundations will extend (usually below existing foundations).
• Type of underpinning technique to apply.
• Safety measures for neighboring structures.
• Schedule to minimize disruption.

Regulatory approvals and permits may be required at this stage. Informing neighbors about potential noise or vibrations caused by underpin works is also advisable.

Step 3: Preparing the Site

Preparation involves:

• Clearing the work area around the foundation perimeter.
• Protecting existing structures and utilities (for example, water pipes or electrical cables).
• Setting up temporary supporting structures if necessary.
• Marking out areas where excavation will take place.

Ensuring safe access for machinery and personnel is vital.

Step 4: Excavation in Sections

Underpinning requires careful excavation beneath the existing foundation. It is done in small sections called “pins” or “pins pits” to maintain stability.

Process:

1. A small pit, typically 1m wide by 1.2m long and deep enough to reach the new foundation depth, is excavated adjacent to or beneath the existing footing.
2. Excavation is performed manually or with small machinery to avoid vibrations that could destabilize the structure.
3. The size and spacing of pins depend on the load distribution and soil conditions; typically spaced at 1m intervals.

This staged approach ensures parts of the foundation remain supported throughout excavation.

Step 5: Installing Temporary Supports

Before pouring concrete for each pin:

• Temporary supports such as acrow props or needles are inserted through holes drilled into walls above the excavation pit.
• These props bear part of the building weight while foundation material below is removed.

This precaution prevents any collapse during underpin installation.

Step 6: Pouring Concrete for Each Pin

Once excavation for a pin is complete:

1. Reinforcement bars (rebar) may be installed within the pit if specified by design.
2. High-quality concrete mix is poured into the pin excavation.
3. The concrete fills up and forms a new footing beneath the existing one, transferring loads safely downwards.
4. The concrete is allowed to cure fully—usually several days depending on mix type—before moving to adjacent pins.

Proper curing ensures strength development essential for stability.

Step 7: Repeat Excavation and Pouring Cycle

The excavation, temporary support installation, reinforcement placement (if needed), concrete pouring, and curing steps are repeated sequentially for each pin section along the foundation length until all necessary areas are underpinned.

This incremental approach guarantees continuous support throughout works without risking damage or collapse.

Step 8: Finalizing Underpin Works

After all pins have been poured and cured:

• Temporary supports can be carefully removed.
• Backfilling with selected soil or gravel around new concrete footings is conducted to restore ground level.
• Any damaged finishes like plaster or paving around the site are repaired.
• Structural surveys are repeated to confirm effectiveness — no further settlement signs should appear after completion.

Step 9: Post-underpin Monitoring And Maintenance

Even after completion, regular monitoring of the building’s stability is recommended:

• Check for new cracks or movement periodically over months following underpinning.
• Address drainage issues that may affect soil moisture content around foundations.
• Maintain proper landscaping near footing areas to reduce water pooling that can cause erosion or swelling clays to move.

Long-term maintenance helps safeguard investment in underpin works.

Safety Considerations During Underpinning

Underpinning involves several risks such as accidental collapse during excavation or injury from heavy materials. Adhering to safety protocols is essential:

• Only employ skilled professionals experienced with structural works.
• Use protective equipment including helmets, gloves, steel-toe boots.
• Secure site with barriers to prevent unauthorized entry.
• Monitor vibrations especially near neighboring buildings sensitive to ground movement.

Risk assessments should be part of planning stages with contingency plans ready if unexpected problems arise onsite.

Costs Involved in Underpinning

Underpinning can be costly depending on factors like:

• Size of area needing stabilization.
• Depth of new footings required.
• Soil conditions complexity.
• Accessibility for machinery.

On average, mass concrete underpinning might range from $100–$300 per linear foot (or equivalent metric). Mini-piles or advanced methods often cost more but suit challenging conditions better. Although expensive upfront, underpinning prevents far more costly repairs later on by securing structural integrity.

Conclusion

The underpinning process plays a vital role in preserving aging buildings or adapting existing structures for new uses with greater load demands. By following a systematic step-by-step approach—starting from site investigation through staged excavation and pouring concrete pins—engineers can effectively stabilize foundations safely.

Understanding these steps enables property owners and builders to appreciate complexities involved while facilitating informed decision-making regarding foundation strengthening options. Properly executed underpinning not only prevents structural failures but also extends building lifespan significantly, safeguarding investment for years ahead.