The Truth About Infiltration Testing

If you're planning a development in England or Scotland, you've likely encountered the requirement for infiltration testing. But what does that mysterious threshold of 1×10⁻⁶ metres per second actually mean, and why is it so critical to your drainage design? Let's cut through the technical jargon and explain what you really need to know.

Why We Test Infiltration Rates On-Site

Infiltration testing isn't just a box-ticking exercise. It's fundamental to understanding how your site will handle surface water. Every piece of ground is different. Two sites just streets apart can have dramatically different soil compositions, compaction levels, and drainage characteristics. Without testing the actual infiltration rate at your specific location, you're essentially designing in the dark.

On-site testing reveals the ground's capacity to absorb rainfall naturally. This information directly influences whether you can use soakaways, infiltration basins, permeable paving, or other sustainable drainage features. Get it wrong, and you could face flooded developments, failed drainage systems, and costly redesigns after construction has begun.

The testing must be conducted at the precise location where you plan to install drainage features, and at the depth where infiltration will occur. Surface conditions can be misleading. What appears to be free-draining topsoil might sit above impermeable clay just a metre down.

The Drainage Hierarchy and National SuDS Standards

The requirement for infiltration testing stems directly from the National SuDS Standards and the drainage hierarchy, which establishes a clear order of preference for managing surface water runoff. This hierarchy prioritises solutions that work with nature rather than against it.

At the top of this hierarchy sits rainwater harvesting, which captures and reuses rainwater for non-potable purposes such as toilet flushing, irrigation, or washing. This represents the most sustainable approach by reducing both surface water runoff and mains water consumption.

Second in the hierarchy comes infiltration, allowing rainwater to soak naturally into the ground where it falls. This is a critical option because it:

• Recharges groundwater supplies

• Reduces flood risk downstream

• Filters pollutants naturally through soil layers

• Minimises the burden on existing drainage infrastructure

• Reduces the need for costly engineered solutions

Only when infiltration is proven unfeasible should you move down the hierarchy to discharge into watercourses, and finally (as a last resort) into the public sewer system. This approach reflects a fundamental shift in how we manage surface water, moving away from the outdated "pipe it away quickly" mentality that has contributed to widespread flooding issues.

The National SuDS Standards make it clear: you must demonstrate that infiltration has been properly investigated and ruled out before proposing alternative drainage methods. Local planning authorities and water companies will expect to see infiltration test results as part of your drainage strategy. Without them, your application is likely to face delays or rejection.

Understanding the 1×10⁻⁶m/s Threshold

Now for the technical bit, but we'll keep it practical. The threshold of 1×10⁻⁶ metres per second represents the minimum infiltration rate at which soakaway systems are considered viable. In more relatable terms, this equates to roughly 0.0036 metres per hour, or about 86 millimetres per day.

This might seem like an arbitrary number, but it's been carefully established based on decades of drainage engineering experience and hydrological research. Here's why this specific limit matters:

The Science Behind the Number

Soakaways and infiltration systems need to empty relatively quickly after rainfall events to have capacity for the next storm. If water infiltrates too slowly, the system remains saturated and cannot accommodate subsequent rainfall, leading to surface flooding and system failure.

The 1×10⁻⁶m/s threshold represents the point below which infiltration becomes impractically slow for most development scenarios. Below this rate, you're typically dealing with clay-rich soils or other low-permeability materials that simply cannot absorb water quickly enough to function as a reliable drainage solution.

Consider this: a typical soakaway must handle not just average rainfall, but design storm events. These are intense downpours that might occur once every 30 or 100 years. If your infiltration rate is below the threshold, even moderate rainfall could overwhelm the system, leaving standing water for days or weeks.

Factor of Safety

Responsible drainage design doesn't just use the raw infiltration test results. We apply a factor of safety (typically dividing the measured rate by two or even three) to account for:

• Long-term clogging from sediment and organic matter

• Compaction during construction

• Seasonal variations in groundwater levels

• Natural variability across the site

• The gradual reduction in permeability over the system's lifetime

This means that if your infiltration rate is hovering just above 1×10⁻⁶m/s, once you apply the safety factor, you may still fall below the viable threshold. This is why borderline results often lead to recommendations for alternative drainage strategies.

What Happens When You're Below the Threshold

If your site tests below 1×10⁻⁶m/s, don't panic. You're not alone. Many sites, particularly those with clay soils common across large parts of England, fall into this category. It simply means infiltration isn't viable, and you'll need to explore other options in the drainage hierarchy.

Alternative strategies might include:

• Attenuation tanks or ponds that store water temporarily before controlled discharge

• Discharge to nearby watercourses with flow control

• Connection to public sewers with attenuation to limit flow rates

• Hybrid systems combining limited infiltration with storage and discharge

The key is demonstrating to the local authority that you've properly investigated infiltration first. Your drainage designer should provide infiltration test results, explain why the rates are insufficient, and justify the proposed alternative approach.

The Practical Implications for Your Project

Understanding the 1×10⁻⁶m/s threshold has real consequences for your development timeline and budget. Infiltration-based systems like soakaways are typically the most cost-effective drainage solution. They're relatively simple to construct and require minimal ongoing maintenance.

When infiltration isn't viable, you'll likely face higher costs for engineered solutions: larger attenuation tanks, pumping stations, or extensive pipe networks. You may also need additional land take for storage features, and ongoing maintenance responsibilities increase.

This is precisely why infiltration testing should happen early in your project planning (ideally during the feasibility stage). Discovering poor infiltration rates after you've finalised your site layout can force expensive redesigns.

Getting It Right From the Start

The 1×10⁻⁶m/s threshold isn't just a technical detail buried in drainage standards. It's a fundamental determinant of how your site will manage surface water, what your drainage system will cost, and whether your planning application will succeed.

Professional infiltration testing conducted by qualified specialists provides the evidence base for your entire drainage strategy. It demonstrates compliance with the National SuDS Standards, satisfies planning requirements, and ensures your drainage system will actually work for the lifetime of your development.

Whether your site tests above or below the threshold, the important thing is having accurate, defensible data that allows your drainage designer to create an appropriate, compliant solution. That's the truth about infiltration testing. It's not about passing or failing, but about understanding your site's characteristics and designing accordingly.

If you're at the early stages of a development project, make infiltration testing a priority. The small investment in proper site investigation will pay dividends throughout the planning and construction process, and help ensure your drainage system performs reliably for decades to come.