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I need to dig a 3 foot trench in soft clay to put in a new water pipe.
If I use I drain spade to dig it about 16cm ( 6 inches) wide will I have enough space to do the digger?
Is there a better type of spade to use?
About what length of trench can fit people did in a day?
(I hope I will only need to dig about 12 feet of length, but may need to dig about 30 feet if I am forced to connect to the other road.)
It's physically very difficult to dig a very narrow but deep trench. You may have better luck just renting a ditch digger/trencher, which is kind of like a big chainsaw for dirt. You can dig trenches that are very narrow, and the only thing that really slows you down with it is large rocks.
You should have utilities marked before you dig (whether using a trencher or not). In some areas, this is enforced by law (eg: in Ontario, Canada, it is law under the Ontario Occupational Health And Safety Act).
Speaking as one who has done this (or close - my clay was rather hard once you got into it, and it was a deeper and longer trench) I'd strongly suggest one of these; a mattock.
Peel off your turf if any, put it aside, use the mattock to break out the clay, use your shovel to lift it over the side. Clear your feet a nice space in the bottom of the trench, and work in it (not DO NOT do this in deeper trenches where you could suffocate if the trench collapsed - in a 3 foot deep trench, you would at worst be inconvenienced for a while until you dug your legs back out.) Keep your standing area cleaned out and clear. You'll need to be somewhat wider than 6 inches to have a hope in heck of digging to 3 feet of depth, IME. You may be able to reach down that far with a long narrow shovel (which I'm assuming your drain spade is) but it's hard to actually DIG when you are that far away from the edge - the leverage is not good.
A small excavator would be much faster, but costly (I've done them that way too) - it's a perfectly reasonable job to do by hand if you'd like to do it that way, and tool rental does not have what you'd like at a price to make it favorable. To keep to the 6" width you'd pretty much have to find a trencher, IME - or you could use a post-hole digger, but that would be more inefficient than digging a wider trench you can work in, IME.
What do you want to bury?
In my area, 4' deep is necessary to prevent freezing. 3' deep is required by code for power; but low-voltage lines (e.g. ethernet, fiber optic, telco, television) only have to be deep enough to avoid accidental destruction as the owner sees fit.
Clay can be a bitch. We live in glacial till; so we have layers of sand/clay. When you hit one of our clay layers, you need a pick to bust out little pieces. "Digging" is not an option. The conventional wisdom is to use a "ditch-witch" as described above. They can come equipped with a spool of conduit and a blade to dispense the pipe right behind the digger. After the conduit is in place, you just "blow" a string thru it with a vacuum cleaner; and use that to pull whatever.
There is a service here (actually required by law) called: "dig safe." One calls them and within a very few days, power, phone, cable, etc. show up and mark your designated area with chalk lines illustrating the path of their lines. It makes it fairly easy to avoid with a ditch-witch.
I just finished digging a thirty foot three foot deep trench. It took me four and a half hours and I was working in rocky clay. A pick and good shovel are nessesary. You are going to have to dig it 12 to 16 inches wide to have enough room to stand in it as you dig. Watch out for buried pipes and wires. I broke a water line this morning digging mine which was not fun. It's hard work but it is manageable.
How big is your pipe? Trying to dig a 12 to 30 foot long trench manually will be a nightmare. My recommendation is to bite the bullet and rent a trencher to do the work for you. Even at 12 inches, your work is cut out for you, but 36 inches is really deep. Regardless of what you decide, you may want to wet the area with a sprinkler before hand. You don't want the ground wet, but you want it moist. I can't speak for your area (I'm in Portland, OR), but of it's hot and dry then the clay in the ground can harden and make penetration difficult.
