permanent road
consider the corduroy road option, as others have mentioned. or other options, which others have mentioned first. we have built roads over
70 year old corduroy roads, where the excavated cedar logs we excavated looking just like fresh cut-down timber. the logs are laid down side by side and do a good job of stabilizing the soil.
if it were me, i would still strip the 18 inches of peat before using this method. this is a good way to bridge over unstable soil.
that said,
this will undoubtably be the most costly option of road building. but if you want permanent, and if you have the bucks. to build a permanent no bull#### road over unstable soil:
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now the fun begins.
probably the hardest part is to get a track hoe, dozer, and dump trucks to get in there to do the job without getting stuck. a working platform, a pad, most likely will need to be built up to get the track hoe stable enough to dig and load trucks and to get the loaded trucks in and out. if this cannot be done, it's game over. i have seen CAT 345C track hoes stuck, CAT D8's dozers stuck, various makes of end dumps stuck (off road dumps) stuck.
to add to this, i have seen all three of these get stuck trying to pull just one of them out. those are highway building size machines. even experienced folks who have operated hoes for 25 years can not walk them out in all instances.
on road 10-wheel dumps get stuck very easily. dumping or loading them on unstable soil is dangerous.
during excavation, even the hardest soil that is dry, can turn into a 4 foot deep mudpit after a storm fills the hole.
if,
the soil under the peat is stable (could potentially hold a load), you could potentially build on that, only if it is not clay/silt. but, since it is under the water/table already, at minimum, it will need to be dried out first, before it can support a load. until the underlying (oversturated, waterlogged) soil is dried out, consider this soil to be unstable.
this step is difficult.
to dry it out, construct retention ponds, dig swales, ditches or lay pipe to move the water as far away from the road as possible. it could literally take months before things dry out. exactly how you will dry out an area which holds runoff for 160 acres with natural springs in itself is could most likely be an impossible task. overflow from the pond(s) will need to be diverted away as well.
but in case you are able to accomplish this,
you have have an 18 inch depth of peat muck. this is considered unsuitable material. by definition organic.
this will never hold any form of compressible load, even if dry.
this will need to be stripped off.
so,
250 x 12 x 1.5 feet = 4500 ft3 = 166 CY (dug and moved and/or trucked away)
next,
the road will have to be higher than existing water table (-) the 18" of peat you just removed.
so, you will need, to fill the future road with some kind of "stable" soil/stone/gravel. this is called embankment.
get the elevation of the road as high as money allows.
roll the embankment with a dirt roller in static mode, until you are well above the water table. also consider that the weight of the embankment will settle over time. do not use a plate tamper! the vibes will cause more problems than help. consider the water table height change at spring thaw and/or rainstorm conditions.
if the embankment is high (thick enough) enough, the following steps other than the test may not be necessary.
build up the embankment in layers, and find the thinnest embankment layer that can pass this test.
roll a filled dump truck over the embankment, watch for "pumping". if the tires of the loaded truck, look like they are riding on a pillow, the embankment is unstable. this can be addressed by undercutting the unstable soil. this unstable soil is dug out and replaced with stable soil. if digging deeper to find more stable soil does not work, (and it rarely will in a swamp)... an undercut combined with geo fabric will be necessary. if a layer of geo fabric combined with an undercut will not do it, then a layer or geo fabric, then geo grid with a deeper undercut is used.
at the point where.... the added thicker layer of gravel becomes more costly than the fabric, use the fabric. also vice versa.
some geo-fabrics claim to be equivalent to one additional foot of gravel undercut.
another way to avoid this entirely, is to keep adding embankment lift layers, until this whole problem is solved.
at this point, you are ready for the next step.
once the embankment lift can pass the truck test, you can begin compaction using vibration. do this in equal lifts if possible. if you have a source on your land for this, great, if not trucking can be expensive.
next,
some quick math:
i do not know, what material you will be using, or the height of embankment material you will be using.....
the following math could be used for embankment as well. any non-organic, stable when dry soil coild potentially be used.
for the 2' depth gravel you will need to place over the embankment material:
driveway length = 250 ft
guessing width = 12ft
depth (fill) = 2 ft
250 x 12 x 2 = 6000 ft3 (cubic feet) = 222.22 CY (fill needed)
weight of gravel approx. = 110 #/ft3 (this will vary, but it's a conservative side value that we use)
so,
6000 ft3 x 110#/ft3 = 660,000 pounds = 330 tons
so,
330 tons divided by 22 tons (max. legal load on a 10-wheel dump in my state) = 15 loads
keep in mind, more than this amount will be nessessary. for potential undercuts, and the fact that the x-section of a road is a trapezoid shape, where the base is wider than the top surface, and also working platform. also much of this material will be wasted, lost sinking into the existing ground.
consider at least 2 feet of gravel minimum. if there are pipes under the road, take into consideration the recommended cover necessary over the pipe. this will vary with type of pipe used (corrogated steel, plastic, etc.) if not enough cover over the pipe is used, it will eventually collapse.
pipe sizing diameter, is based on runoff area, total volume, and existing slope. this is beyond the scope of my reply.
seek advice for this.
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as others have said, here, be careful in a designated wetland. use approved methods of construction (erosion control) or hefty and i mean hefty fines can result. i could not be doing what you want to do where i live. even ponds require a permit here.
also, ponds can fill up over time and will need to be mucked out. consider that when digging the initial depth
and use the clay/silt to line the bottom of pond.
good luck
-OMB