After the success of the Wee Bridge Build last year, I wanted to tackle a sore spot at the start of Ridgeback that had been bugging us.  Opportunity presented itself in the form of a donation of decking from an anonymous source [thanks guys!].

Step 1: site assessment

I knew this stretch of trail pretty well, as we had been messing with it since early 2009, and it had seen a fair bit of work. It began as a rake ‘n’ ride that got wet and muddy in the spring. We benched it and managed to make it worse, turning the section into a swamp for 2-3 months! We had looked at a number of options, including gravel fill [no], rock armouring [ideal] and harvesting deadfall from the area [meh] in addition to using treated lumber.  A re-route was a poor option, as it would require a long, steep climb or force people through even more mud that this trail was designed to avoid.  Overall, the site was a pain in the buttocks, with running water from a seep working its way through the middle and around a big pine at the start. The area by the tree dries out relatively quickly in the spring, but the middle doesn’t. There were quite a few big roots from the pine that I didn’t want to disturb.

Step 2: design + materials

I didn’t want to drive posts into the ground because of the depth of organic soil [several feet] and how wet it is.  I didn’t think any wood would survive long in that environment.  Also, there was no reason for the structure to be elevated. This was not a stunt/TTF.  Low rise meant no guardrail, further simplifying it.  Since running water would need to pass under the structure, I couldn’t lay the stringers directly on the ground [they would block the flow], and by keeping them away from water I’d extend their lifespan. Also, keeping the stringers off the ground meant that I would have to dig out fewer roots, if any.  A footing would be needed.  I settled on treated 4×6 lumber. Our forest consists of pine, aspen, spruce and poplar. None of these is particularly rot-resistant, and I am not particularly skilled with a chainsaw, so store-bought lumber was a better solution.
The footings are 4×6 lumber, all 32″ in length [3 footings from a 8′ long piece of lumber] and pinned to the ground with 2′ long pieces of galvanized rebar. Holes for the rebar were pre-drilled at opposing angles to make it more difficult to pull out of the ground. All the footings were leveled before being pinned in place, in turn making the stringers [mostly] level.
The stringers are 4×6 treated lumber in 8′ lengths. They are oriented with the long axis vertical to support more load [something like a 1200# point load in the middle of an 8′ span will produce 0.25″ of deflection]. The stringers are fastened to the footings with 10″ ardox spiral spikes, the stringers pre-drilled to prevent splitting.
Decking was donated, but I had to promise not to say from where.  It’s 3″ thick rough cut slabs of spruce, 12″ wide and variable length, from 2’8″ to almost 4′.  Kept in place by 6″ ardox spiral spikes.[wife’s hand for scale]

Step 3: construction
Getting the materials to the site was a complete ******* ******.  Next time I’m renting a helicopter.  Huge thanks to Don and Ryan for helping me carry that lumber all the way in.  Props to Dave for [over]loading his 1-ton to get it out there in one shot.  Bravo to Arthur and Rick, who took pity on us and hauled some stringers after their ride.
Onsite construction required a bit of improv, but went off largely as expected. A few small roots needed to be removed near the pine tree, but for the most part I just had to scrape away a flat spot for the footings, get them level and I was off to the races. The footings are around 88″ apart [OC], allowing for overlap with the 96″ long stringers. Gaps between the decking are generally 0.5″-1.0″.

The big question is, “Will this boardwalk last?” Tough call. The decking is crazy thick, essentially a 4×14. It’s out of the water, it’s away from the dirt, so 10+ years is likely. We could put some organic weather sealant on it to keep the rain out, but I don’t know that it will help that much. The stringers should last even longer. The footings won’t last forever, but they’re on the ground instead of in it, and in compression instead of tension or shear.  Fence posts of the same material regularly see 20 years, so I’m hoping for at least 10.  The average load on this thing is a human, typically under 250#.  It does get some cows, but not that many. The small bridge we built using similar techniques has survived for 10 months without so much as a whimper, so I’m feeling reasonably confident that this will survive too.

A few pics:

Start

Coming back

Happy trails