Why Your Drainage Designer Is Costing You More Money Than You Think
On most residential schemes, the drainage strategy is treated as a box-ticking exercise. You need it for planning, the LLFA needs to sign it off, and the cheapest quote usually wins. But the design choices in that strategy set the size of your attenuation, the depth of your excavations and the cost of your below-ground works for the rest of the project.
Get it wrong at the start and you pay for it in concrete, crates and muck away.
The one-test problem
Here’s a scenario we see all the time. A site investigation includes a single infiltration test, dug in whichever corner was easiest to reach. The test fails, or returns a rate too slow to be useful. The drainage designer writes “infiltration not viable” in the strategy and designs the entire site on full attenuation with a restricted discharge.
The result is oversized storage tanks, deeper pipe runs to reach them, more excavation, more disposal and potentially a pumped outfall. All of it rests on one hole in the ground.
The problem is that one test tells you about one location. It tells you very little about the rest of the site.
Ground conditions are rarely uniform
This is especially true across Greater Manchester, Cheshire and much of the North West. Glacial till (diamicton) is notoriously variable. A site that is predominantly stiff clay can contain sand and gravel lenses, pockets of glaciofluvial deposits, or areas of made ground with very different drainage characteristics. Conditions can change over a few tens of metres.
A single failed test in a clay-rich corner says nothing about the granular deposits 60 metres away. Yet the whole site gets designed as if it’s impermeable.
What a robust SuDS designer does differently
A designer who takes infiltration seriously treats the whole site as the design problem, not just one test result:
• Desk study first. They review BGS geological mapping, historical borehole records, existing SI data and the site’s history to identify where permeable strata are likely to be.
• Targeted testing. Infiltration testing to BRE 365 is located where the geology suggests it could succeed and where SuDS features would sit, rather than wherever the excavator happened to be parked.
• Zoned design. The site is split into drainage zones based on the ground conditions actually found. Infiltration is used where it works, and attenuation where it doesn’t.
• Partial infiltration counts. Even a modest infiltration rate, applied with the appropriate CIRIA C753 factor of safety, reduces the volume you have to store and discharge. It doesn’t have to be all or nothing.
• Hybrid systems. Permeable paving with partial infiltration, soakaways in favourable zones, and lined attenuation only where it’s genuinely needed.
Where the savings come from
Every cubic metre of water that soaks into the ground is a cubic metre you don’t need to store. That flows through the whole scheme:
• Smaller attenuation. Less tank, crate or oversized pipe volume.
• Shallower excavations. Smaller storage often means shallower inverts and less dig depth.
• Less muck away. On sites with made ground or contamination, disposal costs can be significant.
• Fewer pumped solutions. Shallower networks are more likely to drain by gravity.
• Better LLFA reception. Infiltration sits at the top of the drainage hierarchy. A strategy that shows it has properly investigated infiltration is on firmer ground than one that dismisses it after a single test.
The extra testing and design time is usually a small fraction of what it saves in construction.
Infiltration isn’t always the answer
Being robust also means knowing when to say no. High groundwater, contaminated land, source protection zones, slope stability concerns and proximity to foundations can all rule infiltration out, however good the test results are. The point isn’t to force infiltration onto every site. It’s to make that decision based on the whole site, not one data point.
The bottom line
A cheap drainage strategy that defaults to full attenuation can end up being the most expensive document in your planning pack. Before you accept “infiltration not viable,” ask how many tests that conclusion is based on and where they were dug.
At Moss Drainage, we design SuDS around the ground you actually have. We combine desk study, targeted BRE 365 testing and zoned drainage design to find every opportunity for infiltration, cutting storage volumes and construction costs while keeping the LLFA on side.
If you’ve got a site where infiltration has been written off, or you want a second opinion before committing to a large attenuation tank, get in touch.