Resin Injection Ground Engineering – Good or Bad?
Resin injection is a process used to stabilise buildings and pavements that have shifted due to differential settlement. This method does not require excavation or the installation of additional structural elements (i.e. underpins), making it a popular choice among property owners. However, there has been much controversy surrounding this method, particularly among engineers. While resin injection has proven to be quite effective for mitigating liquefaction beneath existing structures caused by an earthquake, its long-term effectiveness and performance for shallow-founded residential houses experiencing differential settlement on reactive soils are still questionable.
Resin injection has been widely used in Australia since the 1990s as a quick and easy solution to address foundation issues on reactive soil, often without consulting a structural engineer. However, this method doesn’t always produce the desired results, and there has been much debate in recent years over whether this is a viable solution or whether it is just a short-term fix to a bigger problem.
This blog post will explore whether resin injection ground engineering is a viable option for re-levelling a residential house. We’ll look at the pros and cons of this method to help you decide if it is the right choice for your project.
What Is Resin Injection Ground Engineering?
In civil engineering, grouting is the process of injecting pumpable materials into a soil or rock formation to change its physical characteristics. This technique has been used since 1800, when it was first employed to repair a sluice in Dieppe, France. Since then, it has been successfully used to fix dams and other structures. In 1950, the process of compaction grouting was introduced. This involved using a zero slump grout made of cement, sand, and water to compress any weak materials in a foundation. After remedying many buildings damaged by the Newcastle earthquakes in 1989, the idea arose to use this solution to re-level sunken concrete slabs and footings.
Although there are many different types of grout for lifting slabs and footings, the most popular one is polyurethane (PU) resin. Polyurethane (PU) resin is a common material used for grouting due to its science-backed ability to quickly and effectively expand when mixed with water and fill in gaps underneath the slab or footing with a foam-like substance. The expansion pressure from the PU resin also compacts the surrounding soil, increasing its bearing resistance. Injecting PU resin beneath a structure can also help to lift it and remediate settlement without having to excavate.
The ground engineering companies that specialise in resin injection have developed their own solutions, such as Uretek or Teretek.
What is Resin Injection Underpinning?
The soil beneath our feet is constantly shifting and changing. This can cause problems for buildings, especially those with shallow foundations. Reactive clay soils are particularly prone to expanding or contracting depending on the moisture content. If there’s more moisture present – like when there’s leakage next to the footing – the clay swells and causes the footing to heave. Less moisture – like when there are trees close to the footing– results in soil shrinkage, which then causes the footing to settle. This can lead to significant building distortions, deflections, deformations, and uneven floors which can be unacceptable. Several methods have been around for fixing the problem, one of which is resin injection ground engineering method.
Resin Injection Underpinning Process
While resin injection can improve structural integrity in certain situations, it is not generally suited for addressing waterproofing issues or water infiltration, which often require specialised techniques. Careful assessment is essential, as resin injection is not reversible and may not be ideal for all foundation concerns.
Resin injection process involves the following:
- Assessment: A thorough assessment of the affected area to determine the extent of the damage and the best approach for repair. Right from the start, it’s a good idea to consult with an experienced Forensic Structural Engineer to make sure resin injection underpinning is the right solution.
- Drilling: Small holes 10-20mm in diameter are drilled into the affected area. Holes can be drilled through internal concrete or timber flooring if necessary.
- Injection: The specialised resin material is injected into the holes under pressure, filling voids and stabilising the soil.
- Expansion: The resin material expands and lifts the structure back to its original position.
- Curing: The resin material cures in minutes, and the footing is restored to its original state.
Advantages of Resin Injection Ground Engineering
When it comes to raising and re-levelling a footing, there are a few different options to explore. Structural engineers often recommend conventional underpinning, which involves excavating around the foundation and installing support piers. However, this can be disruptive and destructive, taking days or even weeks to complete. With resin injection, however, no excavation or heavy machinery is required. As a result, the process is much faster, typically taking less than two days, and causes far less disruption.
Also, underpinning historical and heritage houses built on significantly shallow founded bluestone footings is difficult and sometimes even impossible. In some cases, the only viable option may be to inject resin into the foundation and structure. The resin injection ground engineering method can provide the stability needed to preserve these important historic buildings.
Disadvantages of Resin Injection Ground Engineering
Resin injection ground engineering is not always the best solution for remediation and improving the foundation soil below footings and can actually make the problem worse. In some cases, there has been little to no difference in floor levels and differential movements of the house.
It is important to note that injecting resin below the footings will not prevent further movement if the foundation soil is continually exposed to moisture fluctuations. Therefore, it is crucial to first address the underlying issues causing the building movements/settlement before proceeding with any ground engineering works. Attempting ground engineering works without addressing the root cause of the problem is likely to be ineffective. It may even further complicate and reduce the likelihood of success for future foundation/footing remediation works.
Furthermore, differential movements of the footings and subsequent damage to the house can become significantly pronounced if the foundation soil has become more stabilised due to resin injection compared to the areas of the house where the footing is still founded on reactive soil.
Finally, resin injection may be less intrusive than traditional underpinning, but it is not always as non-destructive as you may believe. For example, if subfloor access is insufficient and the injection must be done from inside the house, holes must be drilled into the internal flooring – which can leave permanent marks or, worse yet, break older tiles. This can significantly add to the initial quoted cost that most homeowners do not consider before committing to resin injections.
How Long Does Resin Injection Last?
The lifespan of resin injection underpinning varies widely, generally ranging from 8 to 15 years, but in some cases, it can last longer under ideal conditions. The following factors play a crucial role in its durability:
- Soil Stability: Reactive or unstable soils can cause shifting over time, which may impact the injected resin.
- Environmental Conditions: Fluctuations in moisture levels, groundwater movement, and temperature can affect the resin’s long-term stability.
- Resin Quality: High-density polyurethane resins that meet Australian standards are more durable. Certified resins designed for foundation repair tend to last longer.
- Installation Quality: Proper application by experienced professionals is essential. Poor installation can lead to premature failure.
- Load Conditions: Heavy structural loads or frequent traffic can reduce the lifespan of the repair.
Is Resin Injection Underpinning Permanent?
Resin injection is not typically considered a permanent solution. While in some cases, it can stabilise foundations and address immediate issues, its susceptibility to soil and moisture changes make it more suitable as a medium-term solution. Other foundation repair methods may offer more permanent results.
Tips for Maximising Resin Injection Longevity
To help extend the lifespan of resin injection underpinning, consider these essential maintenance practices:
- Routine Inspections: Schedule professional inspections every 2–3 years to monitor foundation stability and spot potential issues early.
- Footing Drainage Management: Ensure proper drainage around your footings to prevent excessive moisture exposure, which can compromise the durability of resin injection repairs.
- Soil Moisture Control: Stabilise soil moisture levels near your footings. Reactive soils that dry out or swell significantly can impact resin injection effectiveness over time.
Following these practices can help you get the most out of your foundation resin injection repair, potentially extending its longevity and durability.
Important Note: Given its non-reversible nature and potential issues in certain soil types, resin injection should be considered very carefully, with risks thoroughly evaluated, as it may not be a suitable solution in all cases.
Should you consider ground engineering to fix the foundation of your house?
Every site presents its own unique challenges. Reactive soils are particularly troublesome, as they can cause cosmetic damage to houses even though the underlying structure may be sound. A forensic structural engineering assessment is required to investigate the cause of the damage and identify an appropriate treatment plan. A competent Forensic engineer must do an in-depth risk vs benefit analysis of underpinning and resin injection options before recommending the best course of action. In many cases, the best course of action may be as simple as stabilising moisture conditions or replenishing moisture in dry foundations rather than undertaking major structural repairs.
Important Note: Before choosing resin injection ground engineering, consult with a reputable Forensic Structural Engineer. Remember, once the work is done, there’s no going back, and resin injection companies rarely guarantee results. Professional input is crucial before making this significant decision.
Evaluating Resin Injection Underpinning
- Quick & Clean: Resin injection offers a non-invasive alternative to concrete underpinning, completing in about two days without major excavation.
- Reactive Soil Challenges: injecting resin below the footings will not prevent further movement if the foundation soil is continually exposed to moisture fluctuations.
- Process: Involves injecting expanding resin through small holes, stabilising and lifting foundations with minimal disruption.
- Durability: Typically lasts 8-15 years, making it a medium-term rather than permanent solution.
- Cost: Be mindful of hidden costs, especially if internal floor drilling is required, which might damage tiles or flooring.
- Professional Advice: Consult a Forensic Structural Engineer to avoid further cracking or structural issues, considering the non-reversible nature of the process.
Remember: It’s essential to address the root cause of foundation settlement or structural issues before undertaking any ground engineering solutions.
Read this case study of a failed attempt at resin injection ground engineering works.
