Highway flooding on a narrow tree-lined road; solved with Hydrorock

Tilford Street is a narrow, but busy country lane connecting Tilford with the B3001 to Farnham, Surrey. The width of the road, with minimal space in the verges due to tree roots and services, meant that there would be insufficient space for a conventional solution without extensive excavation of the roadway itself.

Surrey County Council wanted to install non-traditional, sustainable solutions to not only reduce the risk of flooding and future maintenance, but also increase biodiversity through creation of habitat. Any highway drainage solution that could increase the amount of surface water kept on site and infiltrated into the surrounding land would therefore contribute to those objectives. Learning of the capability and versatility of Hydrorock, the team also wanted to pursue a minimal-design approach; effectively creating a solution on site in real time.

Stefan Jankowski of Surrey County Council explains the issues with the project site.

Key Characteristics – select for more information

Stefan explains why conventional systems were not appropriate solutions for this project.

System design brief

The verges alongside the road are minimal and there are tree roots and other services underground, that any excavation and drainage installation had to safely work around.

In terms of sustainability, the Council wished to minimise the use of plastic in favour of natural materials and, whilst needing to ensure that the roadway stayed clear of standing water, they wanted to infiltrate as much of the surface water on site without discharging into existing drainage or natural water courses.

The solution

The versatility of Hydrorock’s “blind” blocks (without plastic channels and connectors) empowered a more innovative, improvised approach to reach a solution; where a high-level design was set but execution was a collaboration between engineers and contractors on site; developing the solution block by block. They understood the characteristics and capabilities of Hydrorock and conceived different ways in which it could be deployed.

Standard density blocks were used in both verges, varying the number of blocks side-by-side to work around obstacles like tree roots, and linked them together using high density blocks stacked three high under the highway. As every block was in contact with at least one other, a large volume unitary aquifer could be created; sufficient to buffer and infiltrate the required volume of run off.

In addition to its flexibility, the speed of installation of Hydrorock meant that the drainage elements of the project completed within the four week timeline.

How important was speed of installation?

The solution in numbers Capacity per block L Quantity Total Volume m3
D170 Standard density blocks 172 147 25.3
HD170 High density blocks 169 147 24.8
TOTALS 294 50.1

To give you a sense of scale, 50 cubic metres is about the same carrying capacity as a large domestic removals lorry.

Hydrorock benefits

Working collaboratively with Surrey County Council and Hydrorock Solutions, main contractor Kier and delivery contractor R&W Civil Engineering were able to create a sustainable drainage solution that not only met the system brief and dealt with the challenges of the site, but also became a beacon flood-mitigation scheme for other project teams and local authorities.

  • Fits in a narrow site

  • Doesn’t require an extensive engineering design

  • Is flexible to adapt to underground obstacles

  • Works in harmony with nature

  • Minimises the use of plastic

  • Provides a sustainable, long term solution

System used

To provide a Sustainable Drainage System (SuDS) solution that absorbs the rainwater and runoff, before infiltrating within the confines of the site, our Hydrorock Aquifer Water Infiltration System was installed.

Using blocks without the integrated perforated plastic channels, a unitary aquifer can be created simply by ensuring that each block is in physical contact with at least one other for water to flow by capillary action and with regard to gradient. The shape and volume of the aquifer can be flexed by using different sized blocks, varying the number of abutting units and by stacking blocks to a greater or lesser depth.

Although standard density blocks have sufficient load-bearing capability for use in a verge, high density units can withstand greater forces and are therefore recommended for installations underneath a road surface. The two densities can be used in combination with no compromise to the creation of a single aquifer system.

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