Start with the geotechnical report
Every serious wall starts with the soil. Reactive clay, fill, sandstone and wet ground each rule systems in or out before height or finish come into it. A crib or gabion wall flexes with clay that moves; a rigid cantilever on the same ground can crack. If there is no geotechnical report yet, that is the first thing to commission.
Height and what sits above the wall
Height on its own is only half the question. A two-metre wall with a car park on top is a different structure from a two-metre wall with a garden above it. Surcharge drives geogrid lengths on a segmental wall, section on a crib wall and steel on a cantilever.
Curves, corners and space behind
Crib block and Allan Block follow curves without special units. Sleeper walls want straight runs. Segmental and crib systems need room behind the face for backfill and geogrid; a cantilever needs almost none, which is why it wins on tight boundaries.
Finish and programme
If the wall is the landscape, a Keystone or Allan Block face is usually the answer. If it is a hardstand or a batter nobody will look at, Keymass or crib block will be faster and cheaper. Programme matters too: machine-placed systems go up in days.
