This configuration would also be good for folks who have problems bending over to reach the weeds.

Looks good but a lot of digging

I think the idea of an underground greenhouse is fantastic. Mike Oehler has some good designs, but one of my favorites is the one featured on Treehugger a while back

Not sure how well that design would work in a place with high groundwater, though...

Given your relatively high groundwater levels, the trench might become a pool.

Maybe a path at ground level with raised beds on both sides would work better at your location?

I bet it would be more efficient to build the brick wall above ground, facing south, and the translucent walls built as a lean-to. With the sun low in the sky during the cool months, most of the bricks in the trenched version are in the shade the way it's pictured. I doubt if such a shallow, small trench provides much of a geo-thermal effect to provide additional heat inside that structure.

My simple hoop house (no digging & no brick work) allows me to keep harvesting greens, radishes & tomatoes well into November, located 42*N.

The heat capacity of water is around 4 J·g⁻¹·K⁻¹. Concrete is only 0.88 J·g⁻¹·K⁻¹. So a kg of water can store 4x the amount of heat of a kg of concrete.

Even if you factor in the density (1 g/cm³ for water, 2.4 g/cm³ for concrete), the heat capacity of water is 4 J·cm⁻³·K⁻¹ against 2.1 J·cm⁻³·K⁻¹ for concrete. So a volume of water can store twice the amount of energy of the same volume of concrete.

We actively pursued this option in southern Indiana. The water table is too high and aside from a lot of water in our holes, we got a lot of mold. I advise against the underground version in places with high water tables. A version backed into a hill might work well, if you have good drainage around the back and sides.

This could still work with a high water table as long as you build up rather than dig down. It would take quite a bit of soil to do this, however, or to build it against another structure. I have been intrigued with using solar radiant heating for our home but this could extend to a greenhouse like this too. Radiantec is a company that does this commercially and they have a lot of good diagrams that show systems where you can build a heat sink to store excess heat from your water filled solar array (with antifreeze in colder areas). http://www.radiantcompany.com/system/solar.shtml

This is a great greenhouse idea for someone who can't bend down. Thing is, the plants are growing at ground level=cold in winter. We're exploring the idea of a walipini - plants groww at least one metre below ground level where temp is more stable.