What's beneath your site? Checking geology and ground conditions for a soakaway

It is a screening tool, not a ground investigation. But it tells you early whether a soakaway is worth pursuing and what a drainage strategy will later need to prove.

What the ground beneath a site tells you

The ground under a development is layered, and each layer behaves differently for both drainage and foundations. At the base is bedrock, the solid rock that underlies everything. Above it sit superficial deposits, the younger, looser materials left by ice, rivers and the sea. At the surface there may be made ground, where the land has been raised or filled by people. Knowing which materials are present, and in what order, is the starting point for any judgement about whether water will drain into the ground or has to be managed another way.

Bedrock, superficial deposits and made ground explained

Bedrock is the consolidated rock beneath the site. Its drainage behaviour depends on the rock type: sandstone, chalk and many limestones are permeable and can accept water, whereas mudstone, clay and shale shed it.

Superficial deposits, sometimes called drift, are the unconsolidated materials lying over the bedrock. Sands and gravels drain freely; clays, silts and glacial tills do not. Where no superficial deposits are mapped, the bedrock is at or near the surface, which is what a shallow soakaway would sit in.

Made ground, or artificial ground, is material placed by human activity, from old quarry backfill to general development fill. It is variable by nature and often unsuitable for infiltration without intrusive investigation, because its composition and compaction are rarely known until the ground is opened up.

What does the geology mean for a soakaway?

Granular ground, dominated by sand, gravel, sandstone or chalk, is potentially permeable, so a soakaway may be feasible. Cohesive ground, dominated by clay, silt or mudstone, is low-permeability, and infiltration is unlikely to work. Mixed ground can drain in places and fail in others, so it cannot be assumed either way.

One trap matters here. Glacial till, often called boulder clay, is mapped as “clay, sand and gravel” but behaves as low-permeability ground, so the sand and gravel in the description should not be read as a green light. As a rough guide, infiltration rates in the order of 10⁻⁴ to 10⁻⁵ metres per second support a sensibly sized soakaway, while below about 10⁻⁶ m/s infiltration becomes impractical and an alternative discharge route is usually needed. Our explainer on BRE365 infiltration testing sets out how that rate is measured and used.

Made ground, landslip and ground stability

Two further factors the checker flags are worth weighing early. Mapped made ground signals fill of unknown quality, which complicates both infiltration and foundation design. Mapped landslip or mass-movement deposits point to slope-stability questions that matter for where you site a soakaway and how you route any discharge, since concentrating water into unstable ground is rarely wise. Neither is a reason to stop, but both are reasons to investigate before committing to a drainage approach.

How to check the geology beneath your site

Enter a postcode and the checker queries the BGS 1:50,000 digital geology, returning the mapped bedrock, superficial deposits and made ground at that location, an indicative infiltration read, the geological age and depositional setting, any landslip nearby, and an interactive map you can interrogate layer by layer. You can also download a one-page screening report.

For the underlying data you can also consult the BGS Geology of Britain viewer and the BGS Infiltration SuDS Map, which is the standard national screening source for whether a site may suit infiltration. One caveat applies to all of these: 1:50,000 mapping describes the geology around a postcode, not the exact plot, so it screens feasibility rather than confirming it.

From a desk check to a site investigation

A geology screening answers “is this worth pursuing?”, not “will it pass?”. UK drainage policy under the 2025 National Standards for Sustainable Drainage Systems sends surface water into the ground first, so a Lead Local Flood Authority will expect evidence before accepting an infiltration-led strategy. That evidence comes from an in-situ BRE 365 soakaway test, and, where the water table is a concern, groundwater monitoring across the wet season. Where a desk check shows clearly unsuitable ground, that screening can itself help justify an alternative approach and whether testing is needed at all.

What happens when infiltration won't work?

Low-permeability ground does not stop a site being developed; it changes the drainage strategy. The usual route is attenuation, storing surface water on site and releasing it at a controlled rate to a sewer, watercourse or surface-water system. Permeable paving can still be used as a tanked (lined) system that stores and treats water without relying on infiltration. One quirk to note: even where ground is plainly unsuitable, an authority may still require a test to prove the site has failed before consenting to an alternative discharge.

Turn a geology screening into a drainage strategy that gets approved