Whindust path

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What is it?

A 1.5m wide whin dust surface on top of a recycled Type 1 granular sub base with the ground beneath the sub base strengthened with pulverised fuel ash (PFA) and cement to form a hard formation level for building the path on.

whin dust path with stabilised soil oatridge college

How does it work?

Depending on the strength of soil where the path will be built, a path may include a separation and reinforcement layer between the sub base and prepared ground. The separation material would be a geotextile sheet to prevent the sub base sinking in to the soil and soil pumping up into the sub base, creating soft spots. The reinforcement material is a geogrid which binds and strengthens the sub base on top of the soil. Here at Oatridge College the prepared ground beneath the sub base has no geotextile sheet or geogrid. Instead, PFA and cement materials were mixed together in the soil. The PFA and cement start to set as they absorb water in the soil, and harden when dry to form a hard formation level like a 'concrete slab' – so geogrid or geotextile sheet are not required. This soil stabilisation technique can make a weak soil stronger, allowing a structure like a path to be built on top.

What are the benefits?

Whin dust surfacing can provide a number of benefits as a path surface:

  • A reasonably smooth, durable unbound surface on level ground with gentle path gradients that can be used by most path users.

  • A cheaper surface to install than a bound surface like bitmac or asphalt.

  • A natural-looking, formal surface that blends in well with most landscape settings.

Using the soil stabilisation technique can also provide some benefits:

Is it suitable?

If you are thinking about using whin dust as a path surface, here are a few points that you need to consider.

  • Whin dust surfaces can be liable to water scouring if water is not allowed to run off the path. Good path drainage is essential, for example using camber or cross fall, but careful path edge landscaping is equally important.

  • Whin dust surfaces are not very durable on steep gradients – damage caused by water flowing down the surface is highly likely.

  • Whin dust surfaces are very susceptible to frost heave. When first laid, insufficient compaction of whin dust surface may hold water that can freeze, then thaw, loosening the surface which is washed off by running water. Plenty of good compaction of the surface with a heavy vibrating roller can reduce damage from frost heave.

  • Whin dust surfaces are very susceptible to wear and tear from path users. When first laid, whin dust surfaces can be easily marked by most users, particularly horses during or after wet weather, or cyclists breaking hard on corners, bends or steep gradients.

  • Whin dust surfaces require frequent maintenance, particularly if the level of use is high or where there are steep gradients.

How much will it cost?

Whin dust path with stabilised PFA / cement formation may cost between £20 - £35/ linear metre.

How do you install it?

Here at Oatridge College, the method of construction was:

  • A machine operator, using a tracked excavator, stripped turfs from the existing grass surface to a width of 2000mm and depth of 50mm along the central line of the route to form a shallow formation tray. Turfs and topsoil were set aside along the length of the route for path edge and verge landscaping work.

  • Path workers spread the pulverised fuel ash (PFA) and cement to a depth of 15mm over the soil in the shallow tray. Then they rotavated the PFA and cement thoroughly in to the soil to a depth of 100mm with a pedestrian-operated power harrow. The rotavated soil containing the PFA and cement was allowed to set and harden, and then with a walk-behind double drum vibrating roller, the surface was compacted to refusal ready for building the path on.

  • At the recycled Type 1 heap, path workers used a tracked excavator to load tracked power barrows. The path workers transported their loads to the far end of the compacted hard surface. Working backwards, the recycled Type 1 was tipped on to the compacted hard surface along the central line of the route. Path workers spread the tipped materials to levels and falls to form a 2000mm wide, 100mm thick sub base layer with 2% (1:50) cross fall, and then compacted the material to refusal with a walk-behind double drum vibrating roller, maintaining the cross fall.

  • Any part of the compacted sub base surface deviating from the required level, or cross fall, was raked off or topped up with additional recycled Type 1 to form the correct level and cross fall, and then re-compacted to refusal. The compacted sub base surface was now ready for surfacing.

  • Path workers loaded the tracked power barrows with 6mm whin dust, and tracked their barrow loads to the far end of the newly laid compacted sub base surface. Working backwards, the path workers tipped the whin dust onto the compacted sub base surface. The path workers spread the tipped whin dust over the surface to a depth of 25mm, and to levels and falls to form a 2% (1:50) cross fall, then the whin dust was compacted to refusal with a walk-behind double drum vibrating roller, maintaining the cross fall. That completed the surfacing work.

  • Path workers then formed the path edges and verges with the set-aside excavated topsoil's and turfs to ensure the compacted recycled Type 1 sub base layer and whin dust surface was contained. The line of the path was defined and its appearance 'softened'. All path and landscaping works were now finished!

Detailed drawing - Whin dust path

If you like the look of the whin dust path at Oatridge College, as a potential path surface for your project, download the detail drawing here icon Whin Dust Path (PFA and Cement) - Detail Drawing

For a generic standard detail drawing and specification details, download them here icon Whin Dust Path - Standard Detail Drawing & Specification Details

What variations are available?

Granite dust can be used instead of whin dust for path surfacing. Granite dust is free draining, less susceptible to frost heave and has good binding properties.

Detailed drawing - Granite dust path

For a generic standard detail drawing and specification details, download them here icon Granite Dust Path - Standard Detail Drawing & Specification Details

© 2014 Paths for All - Registered Scottish Charity No: SC025535, Company Limited by Guarantee No: 168554 inc. 19 Sept 1996 at Companies House, Edinburgh