It depends on how level your ground is. If it is level or you do some minimal levelling where you place the blocks they should be reasonably stable. Of course frost may heave them somewhat out of line over the winter, but they can always be readjusted. One advantage against setting them in is that they wont tend to get buried by debris (fallen catkins leaves etc) or ground covers. On the negative side, it may look somewhat untidy if weeds or grass pop up around them and you can't mow over them because they are above ground level. This would require hand trimming. An edging of mulch along the path and between the stones might help alleviate this problem.

Sure you can do it. It will be difficult to level and some slabs will break during the levelling process. Clay soil cracks when it dries and expands when wet so if you ever get it level to begin with it won't stay that way for long. So, yes you are asking for trouble.

At the risk of sounding like my mother (not a comparison I relish), here's my take - if it's worth doing, it's worth doing right. IME, taking short cuts is not the way to a successful outcome and often winds up creating more problems and expending more time down the road. While I do believe you can create a very satisfactory and attractive walkway by simply tracing around pavers and cutting out the sod, it doesn't work nearly as well when you are attempting to cover a large area. And 16'X50' is a pretty good sized chunk of real estate.

Doing this in the typically recommended fashion is not nearly as difficult or expensive a task as you imagine - a rather simple DIY project. Get a few friends or family members together and you can whip it out in a weekend. Sand and crushed rock are not pricey - borrow a pickup and pick them up yourself to save $$. Rent a compactor from a local tool rental place and other than shovels, a level and elbow grease, that's pretty much all you need. You won't have to worry about shifting or frost heave much, the underlying weeds/grass will be addressed as will drainage and the finishing with brushed sand will ensure things stay in place.

There are things one probably really shouldn't admit in public, but so that others might learn from the experience....

About 5 years ago the local garden center had 1 ft square cement patio blocks on sale. I picked up enough of them to form pads for the heavy cast iron furniture and the grill. Since there wasn't a better place to put them, I put them in place on top of the grass. They are still there. Every now and then I trim the grass that grows between the blocks, but that's really it. They haven't moved, they haven't broken, they haven't misbehaved in any fashion that I can make out. They have also failed to settle themselves in deeper so that I can run the mower over the edge. The area is visibly level, and fairly heavy clay. It's not one of the horribly compacted areas, though.

The previous owners did a similar thing with bluestone stepping stones.

Here's the thing that worries me, even though some shortcuts on grass or compacted earth can work out okay, as with a few stepstones or a small landing.

The area you describe is pretty large. Differences in grade or level will be more apparent so the whole thing could look kind of heaving and weaving--especially prominent with square pavers as opposed to flagstones. You'll find that with smooth level paver surface and all of the "joints" where 4 paver corners meet, one corner will be going a little up, one down etc, in fact in an idividual paver one corner will be up and one more down, and you can go crazy trying to decide which is the pace-setter paver and how to match the setting of all the ones thereafter. (I am having trouble describing this, but I have experienced it!)You have to space them some way, whether it is an inch, or more, and put SOMETHING between them--will it be sand, or what, and will you grow weeds between (because thats a large area to try to do steppables in all the cracks, and over time sand on clay will make... dirt! If clay, the drainage will be poor, which can actually be okay if a slight slope all in one direction is there somehow, but if you have some relative low spots among the whole you will have puddles, maybe. Finally, in concrete squares alone you will have a substantial investment ( ? 1000 or more, maybe, depending on spacing) and a LOT of carrying for 16 x 50 area, so you could do a lot of work and end up not happy. I would not be as concerned if you were doing a small very casual area but of course that also would be relatively easier to "do right" as well.

I am actually preparing to try to put in a pathway directly on compacted soil but 1) using irregular flagstone with also some surface irregularity 2) taking advantage of a very slight unidirectional slope for drainage 3) informal, secondary garden path to be more forgiving 4) small total area, at least to start, 5) interplanting with tough small groundcover plants to help stabliize and reduce washout and 5) being prepared it might not work out!

Having done every possible permutation of concrete slab placement due to a collection of slabs I sentimentally (and stupidly) saved from my parents' old house, I would suggest that what you have there is basically a workable plan. However, all of the cautions you've received above are valid and even likely. But they aren't deal-breakers. The job that GardenGal suggests is a whiz has been, for us at least, anything but. Between figuring out where to pile the dirt you dig up, co-ordinating and handling the influx of sand and gravel, and getting the compacter to and from the rental place, this is a major undertaking and if I can avoid doing it again for the rest of my life, I will. Given the option, I would put slabs right on dirt, I would indeed.

Three things matter most I'd say. First: 18x18 is HEAVY. Even two people together are going to suffer. I "walk" them everywhere when I have to move mine, unless I have enough space for the handtruck. Have a plan for transporting them to the point of use that won't kill you, and only use this size if you never want to take them out. Otherwise, consider 12x12.

Second, the spaces in between as Frankie mentions. Don't put them cheek by jowl because if you have weed growth, you need to be able to wedge a shovel in between to get them out. I'd make the gaps wider, and actually we now have grass growing in between ours and just mow over it. Do make the dirt between the rocks even with the surface of the rocks, or you have ankle-turning territory.

Third, drainage. Not an issue here so much as the soil is absorbent. What you probably want to do in your area is ensure that you have a bit of slope in the ground - the same slope you would have for an installed patio or for that matter for dirt or a lawn - and that it runs away from the house if the house is nearby.

Within two years your patio will look like waves in the ocean.

What, from frost heave?

Maybe it will. I still reserve the right to accept that maybe someone would rather have that, and a bit of work flattening it out every year, than the work of proper installation from the outset. And maybe, notwithstanding my preference not to move the blocks once they're installed, it is possible to try it, and then back up and do a proper installation if it doesn't work out.

It always depends on something, and in this case whether you can do it properly later as a second choice depends to some extent on how difficult it is to work on the site (my cramped urban yard vs a big suburban tract) and how hefty the homeowner and his friends are.

But I should qualify all my blathering with the point that it is not a bad idea by any means to install properly from the outset. But by the way, it is possible to get "proper" installation wrong too, if the slope is not gotten right for example. That part is NOT easy.

If the elevation is not taken into account then it is not proper instalation in the first place.

When ever I design I make sure I'm not looking at the area with blinders on. What is draining where, etc.

Shoot the grade in the planning stages, and you'll save your valuable time & money with a proper installation from the outset.