Filter drain recycling: what happens to the stone
Short answer
A filter drain stops working because the stone clogs with silt and fines, not because the stone fails. Recycling lifts that material, screens the fines out and returns the clean stone to the trench. It avoids carting away good aggregate and buying replacement, which cuts lorry movements, cost and disruption.
The short version
- Filter drain stone clogs with fines; the stone itself is usually still sound.
- Recycling screens the fines out and returns the clean aggregate to the trench.
- The saving is mostly in avoided haulage and avoided replacement aggregate.
- Not all material passes specification, so some removal is normally still needed.
- It is called a filter drain in the highways specification; French drain is the informal term.
A filter drain stops draining. The instinct is that it is worn out.
It usually is not. The stone is fine. The gaps between the stone are full.
That distinction decides what you should do about it, and it is worth understanding before anyone prices a repair.
Two ways to fix it
You can dig it out and replace it. Cart the lot away, buy new aggregate, fill it back up.
Or you can lift the material, take the fines out of it, and put the clean stone back.
| Dig out and replace | Recycle on site | |
|---|---|---|
| Existing stone | All carted away | Screened, sound material returned |
| New aggregate | Full replacement volume | Only tops up what failed spec |
| Lorry movements | Out and back in | Substantially fewer |
| Waste | Sound aggregate to tip | Only non-compliant material |
| Best when | Material is genuinely failed | Stone is sound but clogged |
Both are legitimate. The second is better when the stone is sound, which it very often is.
What the process involves
The material comes out of the trench. It goes through screening, which separates the fines from the aggregate.
What comes back is assessed against the specification. Material that meets it goes back in the trench. Material that does not is removed properly.
That last point matters and gets glossed over in marketing. Recycling does not mean everything goes back. It means the sound material goes back instead of being thrown away with the rest.
Where the saving actually comes from
Not from the screening. From what you avoid.
You avoid carting sound aggregate to a tip. You avoid buying its replacement. Both of those are lorry movements, and lorry movements are cost, carbon and disruption in one.
On a long highway run that adds up quickly. It is also the part that reads well in a sustainability return, which increasingly matters on public work.
You are not paying to clean stone. You are avoiding paying to throw it away and buy it back.
Traffic management is often the biggest line
Here is the thing people outside highways work underestimate.
On a live carriageway, the cost of being there can exceed the cost of the work itself. Lane closures, signing, convoy working, night shifts. All of it priced by the day you are occupying the road.
Which changes the economics completely. A method that is slightly more expensive per metre but gets you off the road sooner can be the cheaper scheme overall.
| Driver | Why it matters | Who controls it |
|---|---|---|
| Days on the road | Traffic management is priced per day | Method and sequencing |
| Proportion reusable | Decides how much aggregate you buy | The existing material |
| Haulage distance | Every load out and back in | Site location and tip |
| Working window | Night or convoy working costs more per hour | The authority |
How this sits with the rest of the work
Filter drains rarely sit on their own. They are usually part of a wider package with carriageway work around them.
Because we also do the highway works and surfacing, the drain and the road surface are the same contractor and the same programme. That keeps traffic management to one occupation rather than two.
That single point is worth more than it sounds. Two contractors means two mobilisations, two sets of traffic management and two lots of road occupation, and the authority charges you for the road either way.
You can see the range of that in our core services, our full groundworks capability and our recent work.
Got a filter drain that has stopped performing? Tell us where it is and we will tell you honestly whether it is a recycling job or a replacement one.
Common questions
Why does a filter drain stop working?
Silt and fines migrate into the voids between the stones. Water can no longer pass through at the rate the design assumed. The stone is usually still perfectly sound; the gaps between it are the problem.
Is recycled stone as good as new stone?
Where it passes the specification, yes. That is the key qualifier. Material is screened and assessed, and anything not meeting spec is removed rather than put back.
What is the actual saving?
Mostly haulage and materials. You are not carting sound aggregate to a tip and you are not buying its replacement. On a long run that is a meaningful number of lorry movements removed from the network.
Is a French drain the same thing?
Informally people use the terms interchangeably. In the highways specification they are not the same: filter drain is the formal term for the trench and pipe arrangement, while French drain is defined elsewhere as a combined surface and sub-surface drain.
How long have you been doing this?
Since 2002. It was unusual when we started and it is more common now, which is a good thing. Less material going to landfill is better for everybody.