Sunday, June 14

Making mtb tracks at Kaiteriteri

The base soil at Kaiteriteri is Separation Pt. Granite. This presents both our greatest challenge and greatest asset in track making.
When the surface is loose, due to being disturbed or simply drying out over summer, it is extremely vulnerable to being washed away by heavy rain. On some of the older firebreaks that descend steep spurs, there are ruts that can swallow bikes whole.
The over-riding focus for all of our track design is, therefore, to avoid the damaging effects of erosion. The IMBA handbook Trail Solutions (one of two trail building guides donated to us by Ground Effect) has been invaluable in this regard.

It emphasises that the key to building 'sustainable trail' is to ensure that water is continually being shed from the track before it has a chance to gather either volume or momentum.
The best ways of achieving this are with an outslope and by the frequent use of grade reversals.
An outslope (i.e. a slope towards the outer edge of the track) will disperse water where that slope is greater than the gradient of the track. Even on steeper sections, the water will continually flow towards the outer edge, though not as quickly.
To ensure it doesn't gain sufficient volume to start causing a rut, grade (or gradient) reversals are built into the trail design. This is a dip, or 'valley', that arrests the flow of water and sheds it from the track. Done well, these small ups & downs give a nice undulating feel to the track and make it more interesting to ride.
On steeper sections where a gradient reversal may not be possible, water bars (diagonal channels across the track) achieve the same effect but, to maintain track flow, need to be well-rounded.

By applying these principles, we are able to take advantage of the more positive aspect of the granite soil here. Once it is compacted, it becomes very hard.
We've just had a week of consistent rain, often heavy. Those tracks that have now compacted through time & use showed no effects of erosion whatsoever. In several places water could be seen draining on to the track from cut benches but was being harmlessly shed by the outslope.
There was almost no evidence of recent wet-weather bike traffic except for on a couple of berms where the soil is not yet fully compacted.
So we've had the best confirmation we could get that the mtb tracks we're making for the Park are sustainable and suitable for all-weather, all-year-round riding.

On the other hand, biking tracks that are new and/or soft when they're wet is not a good idea. The narrow ruts made by tyres simply create more work for those involved in their maintenance. As Swamp Monster is still a track 'under construction', please don't ride it if it's wet as it just delays the compaction process. Until then, enjoy the others!

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