Hey Northy! I'm not a big track builder, but I've raced on many, many tracks here around the states and would be glad to give some ideas/suggestions.
I'll start with the kind of dirt we race on: of course the dirt type matters - it affects available traction, how much maintenance is needed, car breakage, etc - but most of the tracks over just bring in whatever is available locally, as few have the resources to be very picky. So there's a huge variety of "dirt" surfaces, but they all can make great racetracks.
One of the big variables about the surface is how much sand vs clay the dirt has:
Sand doesn't like to clump together, but it dries out very quickly. So "sandy" tracks usually have less overall traction and have a loose, or "loamy" top layer. The advantages of this track are that it drains well - often tracks can be run just a few hours after even large storms - and they break cars less, as the surface is softer. The disadvantages are lower overall traction, increased car maintenance due to the sand getting everywhere, and increased track maintenance needed, as ruts and bumps form very easily.
Clay holds moisture, so it clumps together and "packs" very nicely. So you get most of the opposite: higher traction, smoother, cleaner cars, more "sculpted" jumps that hold up better, but when it rains, it can take a while to dry out.
An important thing, no matter what the surface, is to try and get rid of as many rocks (and sticks) as you can. If the track can afford to buy sifted material, that's the best way to go.
Then, of course, there's how the surface is prepared. Generally we try and pack the track as much as possible, regardless of the surface. Probably the best tool for this is a hand-pushed tamper like this
. Tracks will either rent one or buy an old one they can find. Alternatively you can find small rollers to push around or even drive on the track with like an ATV/quad bike thing to pack it down.
As for dealing with water: most tracks over here just to just build the track up some and on high ground so water runs off it. Don't build any features down into the ground, obviously. When there is a puddle area on the track, dig a little trench leading it off the track. Probably the best way to deal with rain is to put drainage tile underneath it - I don't know if I've ever seen a track fully do that, but I think it would help dry it out immensely.
The biggest difference between the UK and US is not just the number and size of the jumps, I think, but primarily in how they're faced and the landings provided for them. Creating a good jump here in the states is almost an art from, and the people that can do it well are sought-after. Good jumps are not straight boards, but they have a parabolic shape to them. Think about the ramps and half-pipes you see at skate parks or on TV; obviously the jump face doesn't go all the way vertical like that but you get the idea. Even if we have a "roller" jump that gives the car very little, if any, air, those will be rounded rather than triangular. The lip of the jump is very important, as it determines the attitude of the car in the air. It helps to have a charged-up car ready when building the track, as you can build a decent launch ramp using trial and error.
Then, nearly every jump in the US has some sort of provided landing - rarely do we have to "flat-land" the car like I see in many of Jimmy's reports. This downside ramp usually has the same sort of parabolic shape that the lift-off did, just reversed. This makes US jumps much more a rhythmic part of the track than obstacles, I think. Good track builders can put in series of jumps, perhaps through a corner or two even, that really flow together and are lots of fun to drive. I referenced it before on a thread in Oople, but a great example of this was in the 2004 mid-winter track in CRCRC. Check the lower left-hand portion of the track in this pic
; you'll see a single - double - double - tabletop with a 180 on it - down into the pit - up for double - down again - roller, corner. Just an awesome combination of features.
Wow, I've said more than enough already. I hope some of that was helpful; let me know if you want me to go on and on about anything else!