So, if you were to import material to fix a real rocky uphill section of trail, would deco granite be the best product? Decent soil is scarce nearby & rather than bring in standard soil, am I right in thinking that deco granite would make for more of an all weather trail?
The deco wouldn't look as natural, but could be a worthwhile compromise.
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I think I know this type of "soil". If similar to Nerang, it is lots of loose floaters on a layer of broken rock with roots through it and mostly on a clay layer. Regardless, what I see is a stony tread that has good outslope and excavated rock on the outslope. Pics make everything blend and I bet it is a rough surface.
Whilst most people say you should not line the outslope with anything as it impedes drainage, in fact here we have found it can work to accumulate silt over this sort of stony tread and smooth it over time. However, you have to have limited outslope, like the classic flattish or cupped, fall line trail where water runs along the trail and scours out soil exposing the stones. In this pic an old log has maintained a bit of tread and kept it silted where all around it has scoured
Limited trail sculpting slows water flow and deposits sediment along the barrier. Logs work better than stone barriers, being more water-tight (wash my mouth out with IMBA Holy Water).
However, because your trail looks well built, I'm afraid the answer is not adding anything yet, but waiting for rain. When this late season ends and the ground is soaked, get out with sledge hammers and bash every stone into the tread. Some will break up and provide a layer of fill. Then tamp it all down flat. Next time it softens you could add 2cm gravel. Blue metal may look right on your trail. Smash as much into the flat tread as you can. It may be better to just repeat the sledge hammering at intervals. Hard yakka, but very effective as the tread becomes more hardened each time you do it.
There is also the option to dig rather large grade reversals. It may remain a rough trail, but the grade changes make climbers feel stronger as they get the odd rest and can pick up speed to flow over stones (29'er style :mullet
. Using a bit of eye of faith in this pic you can see smaller stones over roots where larger rock (like in the background) was removed and you can see the deeper clay layer in the grade dip. This is really rocky and stony terrain and it took a lot of work to get it this flat
I hope you get riders happy. It is hard to do so in some places, so do it bit by bit. Best of luck