I've often found dirt deep within a leek, under several layers. Going through it all can be a very tiresome process. Is there any trick you can use to clean a leek, without cutting it up and ruining it's natural shape?
Most dishes require a chopped leek in any case, but for an uncut leek you will have to separate by hand and wash each leaf separately.
If you want the leek in an untouched form I think you're out of luck. You could try reassembling it after separating and washing :)
Depending on the form you want your leaves in there are different ways.
If you want them in strips you just cut lengthwise from above the root - keep the root on for now. Then you can rinse the leaves as they should be easy to separate. After rinsing remove the root.
For 'chopped' leaves cut the leek lengthwise and chop crosswise, removing the root in the process. The leaves will separate, and you can then wash the dirt from them in a colander. Skipping the lengthwise cut will usually make the leaves stay together, trapping the grit you want to get at.
If you want to make doubly sure, you could first place the cut leeks in a bowl and swirl it around. The dirt will separate to the bottom. You can then take out the leeks and finish rinsing in the colander. Don't pour the bowl into the colander as this just pours the grit over the leaves.
Trim the tops and discard the out leaves. Then cut from 2 or 3 inches above the root to the top, turn 180 degrees and cut again (they should look like the picture).
You can then get the water down inside and get all the dirt out.
Not that I'm aware of. My Mum has spent decades cleaning leeks by slicing the bottom of them to the core and working the layers under the tap to get the water in there and wash the dirt out, and I've never been able to figure out a better alternative which doesn't involve chopping several inches off the bottom of the thing and throwing it on the compost heap.
The tips I've always used are to always wash them root end up under running water, and then to rinse them in a colander again, under running water, once you've chopped them.