Thanks for the detailed response!
This is mountainous western Montana, about half loam half clay with a mild-moderate number of rocks, most 6" or less on the long side. Topsoil ranges from 2-6" thick. Yes to snow, 50lbs/square ft snow load roof ratings and 48" frost line.
So we need to accommodate snow and rain. All soil with organic material mixed in needs removed from the roadbed area. This is considerable work. Unless you have a means to pick it up and carry it, you need to push it far enough from the roadbed and the "borrow" area to allow access to enough clay to build the roadbed. With the climate you describe, this means the roadbed needs elevated. Probably 12" above existing ground on the uphill side. Easiest to get the elevation material from the high side. Imagine how far back you have to push the topsoil..... And you need to move enough fill material to create a usable shoulder slope on the low side.
Road runs across the gentle slope north/south with east side slightly higher than west throughout. Road elevation change are all gentle slopes up ~4 feet first 500 feet along, then down ~6' next 500 foot, then up maybe ~8' last 500ft.
So I envision north/south travel is uphill? With high side to the left/east? The first 500' needs a drainage ditch on the left to drain the hill. At the breakover before starting down the next 500' we still need that drainage ditch, even when the road is not gaining/losing altitude. Once we start back downhill we maintain this drainage ditch to the first location of a roadbed drainage culvert.
There is one existing drainage canal about 4' deep x 12' wide that it will cross over perpendicularly (runs downhill to west) at the 500' mark. I'm planning on digging out the bottom maybe 6 ", laying gravel, placing a 15" 20' long galvanized culvert at a 5 degree angle downhill, adding more gravel, then compacted fill with the road running over.
From the description of this drainage canal I imagine it has to handle considerable flow at times??? 4' x 12' canal can handle a LOT of runoff. A dozen more times as much as a 15'culvert. Remember, a culvert doesn't reach capacity until it over ran with water. So to get the maximum amount of flow as would be indicated in a flowchart, the culvert must be submerged. Luckily you already have a 4' elevation change so merely filling in the canal will create 2.5' of freeboard above the culvert. To increase capacity you can lay multiple small culverts, 15" for example, or lay a single larger culvert. Understanding that a large culvert multiplies capacity faster than multiple small culverts. A 30" culvert will outflow 4 or 5 15" culverts. Simply because of less culvert wall resistance for the water to pass thru. You'll have to diagnose this and make a logical decision. If it were me I'd go to the Soil Conversation District office and have a surveyor/engineer look at it. They will already know how many acres that drainage ditch is controlling. They'll also know the slope of the surrounding area. This information will give them a calculation as to how big of culvert you need. I use this service often when altering my Township roads and adding culverts.
My goal is to make a driveway that is going to hold up to the snow and rain with min-moderate maintenance for years to come
I agree most heartedly with this!!!! When I'm elevating a roadbed I push dirt until I think I've got it. Then I come back the next day for a fresh look and move more dirt. I usually repeat this 3 times before I get close to enough. Another thing I do is drive on the elevated roadbed in my truck before making the final decision that I'm done
So we are talking about building a 12-14ft wide roadbed, elevated at least 12" above the landscape, 1500ft long. Dealing with side slope which increases the amount of material to move. Dealing with fore/aft slope and controlling runoff.
13ft wide roadbed, 3 to 1 shoulder slope, 12" high requires 18 cu ft of material p/foot of roadbed considering no sideslope. Ponder that for a moment. That's 27,000 cu ft for 1500ft of roadbed. That's 1000cu yds of compacted material to build the roadbed alone on a flat surface with no side slope. Since I don't know the amount of slope I won't try to calculate that added work.
What you want to end up with is a roadbed that water never runs across. Preferably a roadbed that sweeps itself clean if the snow comes with wind. Now you've got a roadbed that lasts for decades with minimal maintenance and infrequent addition of rock/gravel thru those years.
Remember my quote at the beginning of this discussion. "a road without a drainage ditch is a drainage ditch".
