Allan Block retaining walls can be built using a single sized block, or a combination up to 4 different sizes.
Most common is the single size large block, but I personally love the Ashlar patter which uses all 4 sizes.
Allan Block pattern walls have their own unique challenges when building as they are built 2 or 3 rows at a time.
In theory they are built the same, but they do require a lot more preplanning to have enough of all the sizes onsite, as well as a careful eye to not break the patter. They require a little more “fitting” to ensure the seams don’t line up with previous rows. This is especially an issue on curves and when ending a row. Account for at least 30% more time when building pattern walls well.
Pattern walls are just as strong as non-pattern walls. The first row of all walls should be made with the large 17.75” block. Then on subsequent rows you can built whatever pattern you like. There is standard 2-row patterns, and 3 row patterns.
This combination allows you to build whatever number of rows wall you need to.
Yes, the 2-row and 3-row patters can be mixed. If you are building a 6-row wall there is no preference if the 3-row pattern is on the top or bottom.
I.E
base row, 2-row pattern, then 3-row pattern, caps to finish,
or
base row, 3-row pattern, then 2-row pattern, caps to finish