Civil Engineering

Filter Drain Recycling
Sustainable In-Situ Filter Drain Refurbishment

A strong commitment to innovation

A market-first, recycling rather than removing the aggregate contained in roadside filter drains on-site. Evolving over two decades, our suite of innovative services enable targeted interventions which deliver significant safety, customer, cost and carbon benefits.

Filter Drain Surveys

SmartScan delivers fast and accurate filter drain condition surveys to support targeted interventions.

Developed by Carnell as a ‘risk-based approach’ to the condition assessment of roadside filter drains. SmartScan uses innovative data capture technology combined with bespoke reporting software, this quickly provides an accurate analysis of the filter drain asset’s performance.

SmartScan is a cost-effective way of proactively identifying
at-risk areas along the highways network. This enables the prioritisation of remediation activity and the development of a targeted and long-term maintenance strategy.

on site recycling

StoneMaster returns roadside filter drains to their optimum condition by removing the sediment that accumulates between the filter media. The in-situ recycling process improves road safety and pavement integrity by reducing flooding events and occasions of standing water on the carriageway.

The StoneMaster units recycle the filter drain media on-site removing only material not conforming to Type B specification, returning a ‘better than new’ product to the drain.

We have six custom-built, self-contained StoneMaster vehicles which can be used in tandem for even greater efficiencies. The differing designs mean we can select a particular machine to best suit the specific road and filter drain conditions.

Filter Drain Stabilisation

The application of StableDrain reinforces roadside filter drains so that they can withstand vehicle encroachments. This eliminates vehicle embedment and reduces stone scatter when vehicles stray on to roadside filter drains.

Tests by TRL showed that vehicle embedment is eliminated, while there is a dramatic reduction of stone scatter. Carnell’s patented StableDrain technology improves road safety and can be installed in conjunction with StoneMaster or as a stand-alone operation.

The specific geogrid size and structure ensures that the permeability and attenuation performance is unaffected, and the filter drain continues to quickly remove large volumes of surface water from the carriageway.

Dig out & Replace

It’s not as quick, you get worse outputs and there are way more carbon emissions due to increased haulage, as well as the vehicle movements in and out of the traffic management closure. Sometimes though, conditions dictate that a filter drain requires ‘dig out & replace’. We can do that too.

Some drains were built badly, or have deteriorated so much, that there is only a small proportion of aggregate in them that conforms to Type B specification. In that case, it’s just not worth recycling, or at least not out on the carriageway.

Whether it’s a small section of a StoneMaster scheme, or a large stretch, our teams can use their expertise to manage the dig out & replace operation. 

Testing & waste management

When a length of filter drain has been identified for remediation we undertake trial holes to provide samples of filter drain media either by excavation or vacuum-extraction. This gives us an insight of the filter drain’s properties, both to give certainty of cost and so that we understand what will be removed from site.

Chemical and physical tests carried out by Carnell or independent labs prove suitability for recycling ahead of refurbishment operations, as well as conformity to Waste Acceptance Criteria (WAC).


The combined surface and groundwater filter drain is a largely hidden asset, but using SmartScan you can prove its condition. 

We can carry out post-remediation surveys in accordance with highways drainage standard CS 551 Drainage surveys (Section 5:

Filter drain condition survey by GPR). The data is captured to CD 535 Drainage asset data and risk management specification ready for upload to the Drainage Data Management System (DDMS).

As well as demonstrating the improvement through round-tripping the data, future scans will identify areas which are deteriorating quickest.