I wrote a big long winded response to this at 6:30 this morning. Obviously, I was not awake enough to post it correctly because it is not there. It pretty much said the same thing that GG and PLx said (but in way more rambling words). The biggest thing that people don't anticipate when attempting switchbacks is that you intensify the grade between the walks (the zig and the zag). If you level out a cross slope, you either have to cut the slope on the up side, fill the slope on the down side, or a little of both. This really comes into play at the bends because you have two surfaces at very different elevations which are essentially right next to each other. I had to dig out some old cd's to find this relic. I did this as a side job for a professor's neighbor while I was in school. It had a very low budget. Guests whould have to park on this side of the house and would often try to climb the hill to the front door rather than to walk down the sidewalk, up the side street to the driveway and along the side of the garage to that same door (hidden between the poplar and pine). This walk is 3' wide, and rises about 8 and a half feet. You can see that there is a pretty steep slope in the walk as well as a lot of steps. The actual poured concrete walk is right in front of the house (garage) running from the steps to the right end of the building. You can see that I had to use a lot of rocks to stabilize the slopes both above and below the main part of the walk. If I lowered the elevation, it would be steeper above. If I raised it, it would be steeper below. This actually was a good solution for this particular situation simply because several people actually fell because there was no walk. The concept is much simpler than the application. You really have to think it through.

Doesn't do you any good to be terrified, be pro-active instead. Get a handle on the weather forecast and see what you're in for - keep updating as obviously it can change. Is your driveway (even part of it) in shade at any time of the day? If so, push for work to be done at that time and don't allow the pour at the hottest, most sunny time. If that means tear out on one day and pour the next day to get some shade, then so be it. Get specific on the contractors intended start time, number of workmen and how late he can cancel the truck if the weather turns - where I live you can cancel within the hour of delivery but it depends on your distance from the materials yard. Be upfront with him and ask him how he's going to do this differently to get a proper finish. I have to say, he's acting honorably in his willingness to do this yet again and that counts for a lot but sadly may not impact the final result - hope it's a better finish this time.

Steve, I've solved this problem many times and I hope this solution works for you. 1..purchase the rubber or plastic garden edging that is about 4 inches deep. Install it along the entire length of the driveway, snug against the concrete. It will come in sections and can be fitted together. 2...Now on the outer edge, add decorative stone, crusher run, river rock...which ever you like best but DON'T use P-gravel. Come out at least 6 inches and 2 inches deep and lightly tamp it in.. This will serve to break up the stream of water and minimize the erosion effect. The thin bead on top of the edging will nicely define the edge of the driveway and guide the water nicely down the edges of the driveway, that falls onto the concrete itself. The other side of the edging will guide the water down without letting any to wash out under the driveway. I like this because you can ride over it with a mower to go from one side of the driveway to the other without hitting the rocks or edging with the blade. If the look of stone doesn't appeal to you, you can plant a low growing plant, such as Periwinkle (stays green all winter too).

Each manufacture has options to fill voids,i would follow those. My experience with perneable parking and driveway surfaces has been the need to continusly fill depressions. Ground softens when wet and surface material is pushed down by vehicle weight. I would opt for decorative concrete.