I'm only in the process of designing my first raised beds myself, but it occurs to me you may be over-engineering the project. A few thoughts:

Having the top of the beds at table height will mean that someone in a wheelchair will have access primarily to the base of the plants. I'd think lower beds, say 20", would give better access to the good stuff - tomatoes on the vine, flowers, etc. - and a better view of the garden as a whole.

Lowering the beds will also reduce or eliminate the need to taper the sides of the beds, making for better access to the beds and more efficient use of space (ie less area lost to the taper of the sides).

You may not need the elaborate drain system unless the beds are on hard pack clay or other impermeable ground. With a drip system you can reduce the watering to the point where pretty much all of the water is taken up by the plants. It was a little unclear, but if you were also thinking of putting french drains under the walkways, I expect that the water that falls on the paths may well get used by the plants, either by their roots finding the water or by capillary action drawing the water into the beds (as water moves from wetter to drier areas). Finally, french drains are built with the holes on the bottom of the pipe, to collect water where the ground is saturated, and to move it elsewhere. You'd need a separate container for water storage.

I must say, it's great that you're going to such efforts to make the garden accessible to your family!

Ian

I've reread your post, and I think I have a better idea of what you're contemplating. You're thinking of having the sides slope inwards, so that you gain clearance for the feet of your grandchild in the chair. So that the cross-section of the bed would be an underlined vee shape, made of solid concrete. Hence the need for the drainage pipe at the bottom of the vee. I think I get it now.

Much as the idea appeals, again I think you'd be much better off with simple, lower, vertical-sided beds. You'd have to have one heck of an inward slope to allow more access from the front of the chair than one would have by reaching to the side (assuming your grandchild has that mobility). The kind of solid concrete bed you're contemplating will definitely require reinforcing steel and probably will need an engineer's input to make sure it will hold together and not collapse outwards or onto its side at some point. And I would think it would cost an order of magnitude more than a simple bed with wood or trex walls, filled with dirt, directly on top of the dirt that is already in place.

Ian

Thanks Ian,
You got it in the second one.the drainage pipe however is another aspect I was hoping to use as a self water, especially if we are gone and the drip system gets knocked out again,hence only holes in the top and caps on each end.
The french drain was a debate as we were looking at the possibility of placing it under the walkways to collect excess water which would run off through the stone wall to water fruit trees.
At table height the grandson can actually roll his chair up with his legs under the beds far enough to be able to acess similarly to the dinner table.He has enough mobility to reach out and grab. I have tried one run with the vertical beds at 2 feet high but he cannot get close enough to them, unless he parallel parks his chair.Problem with that so far means when he tries to reach out for the cherry tomatoes the whole plant gets hooked on his arm and the strawberries he cannot reach at all.
Another aspect to the height is for my mother who if it is at table height can hang onto the edge much like a handrail allowing her to acess the garden goodies with out her walker or chair.
We do concrete,construction,and landscaping.Gotta a kid whos an engineer so no biggy there.I figured we would use a number 4 rebar, we have alot of scraps from our masonry work in short lengths.Right now concrete is at an all time low in our area.
gads, I know the verticals are so much easier to put in.Trex is such a great product for them as well.
Thanks you have me thinking a few other options now as well
Mary

I hate to throw a total curve ball, but given your desire to extend the planting season, you may want to think about building a greenhouse. That would allow you to build lightweight raised beds and control temperature throughout the season. A french drain could easily be installed under the floor to recycle water. Just a thought!

Ian

I used to have a large greenhouse pre-divorce days it was great.One is planned for this place as well however it will be built on the south wall of another building we are in the process of designing.We build green so its taking a while to preplan,design and gather materials from close sources.When we do this building it will also have to accomadate the wheel chair as well as open out into the garden for access.
Right now I have a vegetable garden on the growing in the planters on our green roof.The main purpose of the garden besides food production is to allow the grandkids the opertunity to come help and harvest.
I have used alot of differnt methods to increase our growing time here.The cold frames work pretty well for cool weather crops, these never grew very well in the green house unless I used hydroponics set up in my cooler area.
I actually was figureing that we could lay in strawbales along these for added insulation, add in the heat tapes and the covers fixed with bubble insulation like I used in the greenhouse to extend ripening season.Used these in my organic garden over melons, tomatoes, and peppers

I have raised beds but only about a foot high so not what you need for wheelchair access but wanted to share a couple things. Raised beds do dry out faster so need more water. But they also heat up faster. When planting next to new concrete it is helpful for a couple years to plant flowers up to 9 inches in next to the edge to help leach some of the chemicals. This could hamper access from a wheelchair, however for a couple years. Things like pansies, allisum, lobelia, nastursiums, marigolds that don't get tall would work.

Thanks Margaret,
For watering we were looking at trying to set up type of self water,the pipe in the bottom of the beds.Similar to the reserves kept in self watering planters.The main watering would be the drip irrigation system.We were hoping to advoid the drying out typical in raised beds this way.
I was planning on leaching the concrete before planting but never even thought about some low growers.The grandson would enjoy the flowers as well.
After chatting with Ian as well as several others, we are looking at building one or two as a trial to see if they will work as we hope.While testing them we figure we will do some other raised beds out of recycled wood so we still have the garden.The raised stone beds on the roof are better for shallow rooted crops which need heat.
Thanks for sharing the info
Mary