The Roads in our area, here on FM 1125 in Bowie, TX, are prone to washout during heavy rain. If this is your issue, it is important to look at the grade around your road to see where runoff is hitting it, especially in roads that have a heavy slope like the one pictured.
On this road, it had four washout points, all of which I used large boulders to slow the speed of the water before hitting the road.
I scraped all the gravel off and regraded it, then placed a heavy underlayment and pinned it to the ground, brought the old rock back up, then added more rock with a decomposing granite mix to build it up and create a proper runoff channel on both sides of the road. The customer was extremely satisfied, as it had been two years and there was no deterioration. I regraded it after three months, as it had given it time to settle, and I told them in advance that it would happen, but I did not charge for the regrade, as it is part of the initial job.