Base layer depths and construction methods

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Base layer depths

It is not necessary to carry out precise calculations for providing different base layer depths for paths that are not expected to carry vehicles. The following table provides a guide to base layer depths and whether to or not to use a geotextile.

Formation description Test for soil strength Geotextile Base layer depth
Disused railway, with ballast still in place Visual No 50mm
Disused railway, ballast removed, ash present Visual No 100mm
Glacial till/sand and gravel No heel marks produced, no standing water, free draining shallow topsoil No 100mm
Firm well drained granular soil, parkland, agricultural fields Small or no heel marks, no standing water, topsoil may be deep (100-150mm) No 100-150mm depending on topsoil depth
Granular soil with high clay content, hard when dry, soft when wet Medium to large heel marks dependant on wetness Yes 150-200mm depending on users
Clay with minimal granular content Large marks unless very dry – test in wet to avoid false reading Yes 200-250mm
Peat with minimal granular content, wetland Visual, no support of any load Yes 300-500mm rock causeway with 100mm base on top

 

An extra 50mm or more of base layer depth is recommended on paths where equestrian use is expected to be heavy.

Construction methods

The approach to constructing the base layer, generally depends on hardness of the exposed formation level in the tray. If the formation level is reasonable hard and well drained, and the depth of granular sub base is to be less than 150mm, the base layer can be laid and compacted as one single layer. However, if the formation level will not be able to support the weight of dumpers travelling on it, because of its soil properties (soft when wet, clay or peat with minimal granular content), and the depth of granular sub base is to be more than 150mm, the base layer is best laid and compacted as two single layers - up to 150mm at a time and compacted before adding the next layer of aggregate - this helps to prevent damage to the formation tray level, reduce irregularities, and improves the compaction of the aggregate.

The best approach to undertaking the double layer construction method is to lay the first layer of granular sub base from the start of the formation tray (the end nearest to the material heap) and continue to lay materials along the tray to opposite end (the end furtherest from the material heap). That first layer is then compacted with a heavy vibrating roller. The second (final) layer of granular sub base material is then laid on top of the compacted first layer. A dumper, which can now easily travel along the formation tray without damaging it, will lay the second layer of granular sub base material on top of the compacted first layer working from the opposite end and back to the start of the path. The final layer of laid granular sub base is compacted to refusal with the roller again.

drag box

Drag box...

A ‘drag box’ is a useful method of making sure each layer of granular sub base material is laid relatively evenly, either as a single layer or double layer. A 'drag box' is a sledge which is filled with aggregate and dragged (by excavator or dumper) along and inside the excavated formation tray. Aggregate is spread rapidly and evenly, and some Contractors have drag boxes that can form a camber, making it easier to roll the camber to specification.

Specification - Base Layer Construction
Using handrake or drag box, lay dry granular sub base material to the specified width and depth in the formation tray forming a camber or cross fall as required
The base layer should be laid and compacted in layers not exceeding 150mm. For example, a 200mm thick base should be built as two separate layers - 150mm layer and then 50mm layer
Compact the granular sub base material to refusal by multiple passes with a heavy smooth wheeled vibrating roller (either ‘double drum’ or ‘single drum’ type, depending on the width of path)
There should be no high spots or dips along the base layer surface. Check the level of the surface at regular intervals along the base layer for a consistent even surface, which should have a maximum gap of 10mm under a 3m straight edge laid along the compacted surface. Any area of the base layer surface deviating from the required level should be raked off or topped up with extra material and re-compacted to the correct levels
The final compacted base layer surface should be ‘closed tight’ with no exposed surface voids.  If necessary, fill any openness with fines. The compacted surface should also be free of ruts, dips, potholes and roller marks, before the surface layer is laid

 

Where the lower half of the base layer is built with lower quality granular sub base material such as demolition waste, asphalt road planings or as dug material, a specified higher quality granular sub base material, such as Type 1 can be laid on top to bind together the lower layer to make it stronger. This practice also removes all irregularities (low points, high points or hollows) providing an even surface ready for laying the path's surface layer.

Specification - Mixed Base Layer Construction
Using a drag box lay lower quality granular sub base material (screened ‘as dug’ stone / RSA) to 150mm depth in the formation tray forming a camber or cross fall
Compact to refusal with heavy vibrating roller
Re-run the drag box over the compacted base layer laying maximum 50mm depth of higher quality granular sub base (recycled or quarry Type 1)
Re-compact to refusal with heavy vibrating roller

 

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