4' X 17' Gravel pathway at 20% slope

[M59]

I maintain a longer driveway ( 3/4 mile ) consisting of stone and many sections have a steeper rise than what I see in your picture. This driveway stands up even in torrential rain because it both porous and construction material is too heavy and too well locked together to be moved by water.
The base is always silt fabric ( I've used both non woven and woven ) to keep the mud from pumping up into the ballast layer then a layer of railroad ballast ( 4-6" ). This will pack down to a level but fairly porous surface and is too heavy for even torrential rain to move. If you want a smoother surface then add a bit of crushed ( 1-2" ) that will then filter into voids in ballast and tends to lock the stuff even tighter.

I have run heavy equipment of this buildup, including loaded cement mixers, without leaving ruts. Short of dropping your backhoe teeth into this build it will stay in place.

 

[nyone]

A section of our camp road is about 20% for roughly 20ft. I built the road 3yrs ago and used 2in stone packed into the native soil. Keeping water off the road is key. I have towed a 26ft camper and 4000lb 18ft trailer up it many times. If you plan on driving on this dont use a super fine material as it's easy to wash out and dont put it very thick as you want the native dirt to push up through the stone just a bit. Loose material on a steep slope acts like marbles