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Steep/switchback access road question

Steep/switchback access road question

Steep/switchback access road question

I'm looking at a couple of options for getting access to build and then use a recreational cabin on a steep site.  

Scenario 1 would be a "full service" cabin with well, septic, etc., so it would require access for a well drilling rig, cement mixer truck, deliveries on a flatbed, etc. during construction, and then access via a 4x4 passenger vehicle for regular use.

Scenario 2 would be a "primitive" cabin - no services, and the materials all arriving in the bed of a 4x4 pickup truck.  

Either option would be graded to have a minimal (say 2%) cross slope and a compacted gravel surface.  

Because of the nature of the terrain, a switchback access drive seems to be the only option, so I have concerns with both max slope and turning radius.  

I've checked some equipment manufacturer websites, and it looks like some of these cement trucks and well rigs can climb pretty steep slopes (30% to even 40%) but that's probably under ideal conditions.  Turning radius of about 80' on a 16' wide roadway seems to meet or beat the published specs, but I'm dubious of that too.  

Can anybody offer any input on some real-world figures or guidelines for either of these scenarios?

RE: Steep/switchback access road question

  Based on one minesite I worked at,  which had 17 separate switchbacks to access the upper access, I offer the following.

Provided a D7 dozer is available to assist, concrete trucks can negotiate almost any terrain.  However depending on the number of switchbacks required, you can have some interesting stability issues at the upper elevations. Essentially the lower switch backs undercut the rest of the road and the entire side of the hill moves downwards.  I had to relevel the upper structures about 8 feet every two years or so.

RE: Steep/switchback access road question

Using Google earth, search for L'Alpe D'Huez. Check out the switchback road leading to it. Tough climb for the Tour de France.

For truck use I'd say 20% max slope (15% if possible), 10% max through the turn, 80' radius should be plenty.  

RE: Steep/switchback access road question

  For what its worth again , theres a difference of opinion developing here as to what constitutes a switch back.  Underground we frequently drive our -15% ramps with a radius of 50 feet which is easily traversed by equipment including 4x4  trucks. On the  mountain I previously alluded to, some of the turns were so tight that even a conventional 3/4 ton truck could not drive thru without backing up at least once. I would guess these had a radius equal to the road width.. say 12-15 feet.

2 Band ,  what general specs do you have in mind for your road??


RE: Steep/switchback access road question

Thanks for the info everybody

I'm thinking of a max slope of 20% over most of the road, but there's one short (about 100') area where I'll probably have to go to 25%.  As far as the switchbacks are concerned, I may have to go as high as 15% through the curve, but only a minimal cross slope and a nice wide (80') radius.

I'm pretty confident that a 4x4 passenger vehicle could navigate those slopes, at least in fair weather, I'm just concerned about the construction equipment.  But then again, most of that will be a one-time use, so if winches, or a tow, etc. is needed, then that's a possibility.

Just not sure if I'll be able to find a contractor who thinks so....uh....."creatively"

RE: Steep/switchback access road question

I would explore a smaller radius at the switchback(s).  Most construction trucks should be able to traverse a 40'-45' radius, particularly if you widen the road and lessen the grade (~15%)
through the switchback.  Using a smaller radius through the switchback will also allow you to better match the existing terrain and more flexibility to minimize cut/fills.  

You should anticipate some high turning forces on the surface and subgrade in these switchbacks, so any cut/fills will require proper compaction, and gravel surfacing should be installed as early as possible.  

Keeping the gravel driving surface dry will be important so incorporate an uphill drainage ditch into road section.
Understanding stability of existing materials and maximum allowable sideslopes will also be important - I would ask some contractors for their thoughts, or even better a geotechnical engineer.

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