Mainmark Ground Engineering – Guide to Underpinning

Look for these signs that indicate a foundation may need reinforcement:

• Cracks in walls, floors, or ceilings (especially diagonal or stair-step cracks).
• Doors and windows that stick or fail to close properly.
• Uneven or sloping floors.
• Separation between walls and ceilings or floors.
• Evidence of subsidence or soil erosion near the foundation.

1. Mass Concrete Underpinning (Traditional Method)

This is the oldest and most straightforward method. It involves excavating sections of soil beneath the existing foundation and replacing it with concrete to create a stable support.

Steps:

• Dig small sections (or “pins”) beneath the foundation, one at a time.
• Fill the void with concrete.
• Allow the concrete to cure before proceeding to the next section.
• Repeat until the entire foundation is underpinned.

Pros:

• Simple and cost-effective.
• Suitable for shallow foundations.

Cons:

• Time-consuming.
• Not ideal for deep foundations or limited space.

2. Beam and Base Underpinning

In this method, a reinforced concrete beam is installed to transfer the load of the structure to a new base.

Steps:

• Excavate sections beneath the foundation.
• Construct a reinforced concrete beam beneath the foundation.
• Support the beam with concrete bases at strategic intervals.

Pros:

• Provides greater strength and flexibility.
• Suitable for heavier loads.

Cons:

• More complex and costly than mass concrete underpinning.

3. Mini-Piled Underpinning

This method uses small-diameter piles (150-300mm) to transfer the load to deeper, more stable strata.

Steps:

• Drill piles around or beneath the foundation.
• Connect the piles to the foundation using a concrete or steel beam.

Pros:

• Ideal for deep foundations or areas with weak soil.
• Minimizes disruption to the structure.

Cons:

• Requires specialized equipment and expertise.
• Higher cost compared to traditional methods.

4. Resin Injection Underpinning

This modern technique involves injecting expanding resin or grout into the ground to fill voids and stabilise the foundation.

Steps:

• Drill small holes in the foundation or ground.
• Inject resin or grout to fill gaps and compact the soil.

Pros:

• Non-invasive and quick to implement.
• Suitable for light loads and minor settlement issues.

Cons:

• Limited to specific soil types and smaller projects.
• Less effective for severe structural issues.

5. Mass Poured Grouting

This technique involves pumping cementitious grout beneath the foundation to stabilise loose or unstable soil.

Steps:

• Drill injection holes around the foundation.
• Pump grout into the soil until the foundation stabilizes.

Pros:

• Addresses voids and soil compaction issues.
• Minimally disruptive.

Cons:

• Not a standalone solution for severe foundation issues.

Planning and Preparation

Before starting an underpinning project, follow these essential steps:

1. Structural Assessment:
   • Hire a structural engineer to assess the building’s condition and determine the cause of foundation issues.
2. Soil Investigation:
   • Conduct a geotechnical survey to analyse soil conditions and identify the depth of stable strata.
3. Building Permits:
   • Obtain necessary permits and adhere to local building regulations.
4. Project Design:
   • Work with engineers to select the appropriate underpinning method and develop detailed plans.
5. Risk Assessment:
   • Evaluate potential risks, such as water ingress, neighbouring structures, and utility lines.

Underpinning is a high-risk activity. Follow these safety measures:

• Shoring: Support surrounding soil and structures during excavation.
• Monitoring: Continuously monitor the building for signs of movement or distress.
• Professional Expertise: Hire experienced contractors and engineers.
• Utility Checks: Locate and protect underground utilities before excavation.
• Protective Equipment: Ensure workers have appropriate PPE (helmets, gloves, etc.).

The cost of underpinning varies based on factors like the method used, soil conditions, and project size. Here’s a general breakdown:

• Mass Concrete Underpinning: $1,000 – $3,000 per meter.
• Beam and Base Underpinning: $1,500 – $3,500 per meter.
• Mini-Piled Underpinning: $10,000 – $20,000 per pile (depending on depth).
• Resin Injection:The cost of underpinning a house using resin injection varies based on the size of the property and the extent of foundation damage.

  However, resin underpinning is generally more cost-effective compared to traditional methods due to its efficiency and lower labour requirements.

Advantages:

• Increases structural stability and safety.
• Allows for building expansions or load increases.
• Prevents future settlement and damage.

Disadvantages:

• Expensive and time-consuming.
• Requires professional expertise and permits.
• Disruptive to occupants and surrounding areas.

Underpinning Alternatives

In some cases, underpinning may not be the best solution. Alternatives include:

1. Soil Stabilisation:
   • Use lime or cement to stabilise weak soils without altering the foundation.
2. Helical Piers:
   • Install screw-like piers to provide support without major excavation.
3. Slab Jacking:

Lift and level sunken concrete slabs by injecting grout beneath them.