I was hoping someone with more experience would answer, but I can tell you a few things. I think an elevated box is the way to go. But I do not think I would use pressure treated wood. There are chemicals in the wood that can leach into the soil and from there into the plants. It would be OK for flowers, I guess, but not for veggies. I would mix the soil you use to fill it. Add compost and something to lighten the soil, like vermiculite or perlite. There is a post just down from yours titled Nursing Home Raised Bed Advice Needed that might tell you more.

You can fill the bottom part of the raised bed with rocks to minimize the amount of soil required.

OR: cheaper ... look into "straw bale gardens" where the stack of straw bales makes up most of the height. You have to keep replacing the straw, but it turns into compost for the next bed.

The table type beds don't work for much except lettuce.

Oh ... don't put rocks in the raised bed. There is absolutely no benefit to be had from doing that.

Yes, that will take a lot of soil to fill. However you can get it cheaply (compared to bagged prices) by the cubic yard, delivered. A 4x10 bed that is "thigh high" - what is thigh high? About 24"? Or more like 30"?

Assume that it is 24". So that is 4x10x2, which is about 3 cu yards, for one bed. Of course you don't want to fill it all the way to the top but that's a ballpark figure. For 2 beds, obviously, that's about 6 cu yards needed.

You want clean screened topsoil. You should have someone knowledgable about garden soils inspect it before you buy and have it delivered to make sure it is satisfactory. It should cost in the area of $15 (give or take a few bucks) per cubic YARD (not cubic foot). There is usually a delivery charge, but they usually won't charge that if you are getting 10 cu yards or more delivered. Maybe your scout leader could negotiate a remittance or reduction of the delivery fee since its for a senior center. Or maybe you could build an extra bed! And use some of the dirt to beef up their ornamentals or flower beds!

I doubt there is anyway to back a dump truck up to the beds themselves so you will need a spot where it can be dumped, and you young strong Eagle Scouts should start trucking the dirt over to the raised beds in wheelbarrows. If the raised bed is 2' or more high, a small moveable ramp (can be made from wood scraps) might be helpful in getting the front of the wheelbarrow high enough up to dump over the lip. 6 eagles scouts armed with one shovel apiece and 3 wheelbarrows should make short work of the process. One eagle scout on either side of the wheelbarrow to dump it.

Keep moving the wheelbarrow down the line to dump the soil. There should be room to dump from either side. So, Dump on one side, then Dump on the other, then move down the line. This way you won't have to spread it as much.

As to the raised beds. Although 4' widths are typical for raised beds, older folks and shorter folks (such as myself, who woke up one morning to discover I was now a member of both categories) have difficulty reaching that far in. Consider making them 3' wide instead. And then consider making 3 beds to make up for the loss of growing space. It depends on how much space they have.

Be sure to leave PLENTY of space in pathways between the beds and anything else nearby to allow for the passage of wheel chairs, walkers, and rolling garden scooters and carts. I suggest 4' minimum clearance per side if possible so people can always get around each other. 5' or even 6' would probably be better. Not to be indelicate, but ... human beings who have reached an advanced age sometimes find themselves in a situation where any impedance in getting to a bathroom could result in a humiliating experience. Among other things.

There are a LOT of reasons to make sure there is plenty of space to move around. Safety considerations, for example. The less chance of someone taking a tumble because access pathways are crowded, the better. At your age, tripping and falling down results in maybe a skinned knee and possibly some embarrassment at your own clumsiness; at my age, it could result in a broken hip and a long slow (or not so slow) decline to the end.

You absolutely CAN use modern pressure treated wood to construct raised beds. The new stuff does not leach like the old stuff, and the old stuff didn't leach that much until it was itself fairly advanced in years. The old stuff was treated with arsenic and that is mostly what could leach into the soil. However, unless your plants are significantly deficient in (I think) potassium? or phosphorus?, they don't take up arsenic, so the risk was always pretty minimal.

With the new stuff, there is no arsenic. The only thing that leaches into the soil is a very small amount of copper. Copper is actually a dietary requirement (in small amounts). And again, plants take up little or no copper and it takes decades for this leaching process to take place.

Organic standards STILL don't allow for the use of any pressure treated woods, but outside of knee-jerk reactions to PERCEIVED risk (as opposed to ACTUAL risk) there is no reason for concern over the use of modern pressure treated woods in your veggie garden. You can read more about it here and decide for yourself.

You could use cement blocks to build the beds instead, which has the added advantage of having a space to sit while working on the bed. Top it with flat pavers or wood for a comfortable ledge upon which the gardener may perch while he or she is at work. It has the advantage of being permanent and sturdy, but the disadvantage of expense. Building up walls that high takes a lotta concrete blocks. It also may winter-heave over time. Depends on your climate.

But honestly the easiest thing to do is just build them out of pressure treated wood. The modern stuff is safe for this use.

You could build some elevated beds as well. Those would need to be a little higher off the ground - "waist" height (whatever that means) but because they are so high off the ground, crops such as tomatoes could not easily be grown in them. But shorter crops such as peppers and eggplants, cabbage, lettuce, etc. The beds would be more susceptible to drying out than a bed at ground level.

If there is time and funding enough, consider installing a drip irrigation system for the beds. Not the expensive underground computer controlled type, but the affordable home gardener type that uses rubber hoses and emitters. Look for Orbit or Rainbird systems. Again some help from a knowledgable gardener would be useful.

Contact your local extension agency and ask for help from one of their Master Gardeners or an extension agent. Using quick disconnects (get solid brass fittings so the oldsters don't have to deal with equipment breakdown in a year or two) you can easily set up a permanently installed system that doesn't have exposed hoses for people to trip over 24/7. Just haul a hose out and plug it in when you're watering and then disconnect, coil up and store the main supply hose when its not in use. Everything else will be mounted in the beds where nobody will ever walk.

Encourage the use of mulch to cut down on evaporative loss and discourage weeds. In my raised beds, I actually lay down wetted flattened brown corrugated carbon boxes and plant through that, then cover over all with bark mulch. You want undyed 100% wood with no demolition content. This may also be purchased for less than bagged prices if you buy it by the cu yard. Again there are delivery charges etc etc. But a lot of municipalities collect yard waste and grind it up and make it available for little or no cost to city residents. Contact your city and perhaps your leader can negotiate a free or low cost delivery of last year's ground up tree limbs to be used as mulch in these raised beds. Or get some dads with trucks.

Remember to leave room for the mulch layer when filling your beds. Also keep in mind that the soil you add to the beds WILL SETTLE. So its a balancing act.

Don't order the dirt until the builds are completed and you have volunteers who will definitely show up to move it. Make sure the senior center will have a spot to dump the fill dirt, and that you have sufficient manpower to get it moved in a timely fashion. Be prepared to clean up any construction mess.

It's more complicated to talk about it than to actually do it.

Here is a link that might be useful: calculator for cubic yards needed

I've thought about this situation a little more, and I've decided that thigh-high garden beds are actually too tall - because they basically force the gardener to either stand or crouch uncomfortably between a standing or kneeling position for a long time. It will also put the taller plants such as tomatoes mostly out of reach.

For a raised bed that is on the ground, I think 2' is really the max height, and that is really perhaps just a tad high. The top of the bed should be just about at standard chair height. I think my desk chair is somewhere between 16 and 18".

Gardeners should have access to garden stools instead, or you could build a seat ledge all the way around the garden. Built in hand rails to help getting back up wouldn't be a terrible idea either. They should be attached to the actual garden bed and partially inserted into the soil for stability. Put them at the corners so the whole bed isn't blocked by hand rails! Don't block access to the beds - put them at a 90degree angle to the bed walls, not parallel.

Look at the pictures at the link below for other ideas about how to design raised beds for elderly/disabled. An elevated bed set to wheelchair height would be a great idea. Talk it over with the folks at the senior center and see what you can work out.

Here is a link that might be useful: Enabled Gardening