I am planning to build raised vegetable garden beds. One option is to
use manufactured hollow concrete blocks. The beds will be 1.4 m (4 ft)
by 10 m (30 ft). the standard blocks are nominally 200mm (8 ") high so
I need two courses high. This will be built on a site that has a gentle
slope, the area for the beds will be levelled beforehand. Drains will
be built to prevent ground water from coursing through the vege garden.
We don't have earthquakes, snow, floods, tornados or hurricanes.

The soil is dark and fertile but rather heavy with quite a high
proportion of clay. The inside of the beds will be filled with this
soil amended with compost etc. The blocks come in nominal 100mm (4")
150mm (6") and 200mm (8") widths. The price of the blocks is roughly
proportionate to the width.

My question is what kind of construction do I need to make these beds
stable while not spending time and money over-engineering them? I don't
mind the odd small crack here or there but the walls must not fall over.

As I see it the walls would be stretcher bond but could be:
1) laid dry
2) mortared
3) reinforced
4) mortared and reinforced

Obviously the cheapest will be 100mm blocks laid dry but I wonder if
they will be stable. Mortared and reinforced 200mm will be very strong
but also rather expensive. What is by best compromise that is stable
under these conditions for the least cost?

David