The gravel is the "floor" of the box. All pipes coming in/out of the box come in on the sides, and all the valves sit above the gravel floor. Nothing is below the gravel, so there shouldn't ever be a need to dig it up.
This is good in theory. However, experience has taught me that in time the ground moves and that seemingly empty valve box ends up filled to the top with dirt and whatever else is in the box - if you put in gravel, it will eventually push the gravel into the box - been there, done that.
5. Zone wiring, make sure you have at least 3 to 4 "free" wires in the cable that is run so if you are running say 10 zones, I would suggest running at least 12 conductors and 16 would be better in case you want to expand the system in the future. The reason for having "extras" is if one wire gets shorted/cut you can switch to another line.
I agree with running spares in with the wires. If you are going to have many valves you may want to compare the cost difference of running multi-conductor wire or running several runs of single strand wire.
Multi-conductor will have several colors, one for each zone and white for common. If you run single-strand, they can all be the same color as long as you also run a white as a common. (if you try to run all single-strand runs in different colors it will cost more and they will eventually all look black in a few years)
Multi-strand comes inside a jacket that helps protect the wire. When you hook up a valve you will need to either cut the outer jacket and separate the color you want for that zone and the white for the common or cut and splice all the colors. Also, once you use a color there is no need to keep splicing or continuing that color to additional locations.
...normal installations have a "feeder line" from which the zones feed off. It sounds like you want to do a central manifold and feed from that? How many zones?
You can have a central manifold or "in-line" valves or a combination. Just be sure to bury your wires under the mainline (feeder line) and put any zone lines that share the same ditch above the mainline. By doing this, you will always be able to locate the mainline with a locating tool since the wire and mainline run together.
...Heads per zone and balancing the pump output. Typical is 5-7 heads per zone and I would suggest you map the output of the pump to keep it running at its optimum and not turning on/off when irrigating - this will save you $ and increase pump life.
Rather then counting "heads-per-zone" it is better to use "gallons-per-zone" to design the system. The final layout for zone purposes should be done after you have hooked up the pump and have done a flow test to determine how many gallons-per-minute your pump is delivering. Then you can lay out the zones and calculate the gallons for each zone.
It is correct that you do not want a pump to cycle while watering, but generally speaking, pumps that pull from a pond/lake do not use a pressure switch system. They use pump start and run each time a zone turns on. Since they run this way it is important that all the water they deliver is used for the system so that the excess water does not cause pressure to build. In a pump start system you should have a pressure relief valve that will allow the system to dump any un-used water.
A flow test is fairly simple and you can do one yourself - let me know if you need info on how to do this.
Your mainline and system size will vary depending on the flow. If you pump is delivering 15-20 gallons-per-minute a 1" mainline will be fine. However, if you system is producing 23-26 gallons-per-minute you will want to run 1 1/4" mainline.
The size of the mainline and flow will determine the size and number of zones. For instance, if you have a large grassy area you are watering with rotors and you have a flow for 1" mainline you could put 6 rotors with 3 gal nozzles on a zone (they come standard with a 3 gal nozzle) If you have a 1 1/4" mainline that same zone could have 8-9 rotors.
Spray heads also use various amounts of water depending on the nozzle installed - for instance a 15' half uses 2 gpm while a 12' half only uses 1 gpm.
One thing I learned is that a pipe puller is a far better way to go than trenching. Some places you really need a trench - like if you have a main trunk run where there are several pipes running together, but for single pipes - PULL them, don't trench! When I did mine 15 yrs ago a friend had his pipes pulled and I trenched mine myself. What a mess mine was. If you can't rent a vibratory pipe puller, people have reported good luck by using a subsoiler on the 3pt here, though I haven't tried it myself.
I have not used a subsoiler or bottom plow to bury line yet, however, I have used the trencher, pipe-puller and ground saw.
The pipe-puller (vibratory puller) does leave a nice look when done. However, you have to dig a hole everywhere there needs to be a connection and they do not work well on new grass, in heavy roots or on lose dirt. Each pipe must be pulled separately - multiple pipes cannot be in the same ditch.
A trencher will go deeper then the other two methods but they are hard to control and heavy. They also do not do well in heavy roots or rocks.
A ground saw makes a nice ditch and will go through fairly heavy roots as well as rocks. However, they max out at 13" deep (great for here in Fl - but I am not sure about your area)
Also, when you make your zone runs, you can reduce pipe size as you use gallons. Say you have a 1" mainline and you have a zone with 6 rotors. If they are laid out so that 3 of them are along one ditch and the other 3 are in another ditch - each ditch only needs to be piped with 3/4" pipe.
Pipe capacity for irrigation (based on water running at 5' per second) is approximately as follows - at least these are the values we use - keep in mind that irrigation is usually run in "thin-wall" pvc not schedule 40. Schedule 40 runs less gpm since the inside hole is smaller.
1/2" - 6 gpm
3/4" - 9-11 gpm
1" - 18 gpm
1 1/4" - 28 gpm
1 1/2" - 36 gpm
Also, you can generally use a vale one size down from the mainline size. For a 1 1/4" mainline you can use 1" valves - they even make male adapters that are 1 1/4" slip by 1" thread. Same goes for 1 1/2" mainline.
For 1" mainline I would stick with the 1" valves as 3/4" valves (yes, they do make them) seem to cost more then the 1" ones.
Also, you should never combine rotors and sprays on the same zone. They use water at different rates.
A spray delivers water to the entire patterned area the whole time it is running. Say a 15' quarter-circle. This means that the entire area covered by the head is getting watered the entire time it is spraying.
A rotor, on the other hand, only waters the area it is pointing at while it is rotating. So, to cover the same quarter circle with the same precipitation rate as a spray it will need to run longer (we use a 3/1 ratio - if a sprays run 15 minutes we run the rotors for 45 minutes.
You might ask why one would use rotors if they need to run longer - 15' is the largest fixed spray usually used while a rotor can cover 45' with proper water pressure. Sprays are used for plated beds and small area while rotors are used for larger open areas.
