Rule of thumb over 1200mm in height requires an engineers design. You don’t want to see a failed retaining wall. Engineers can design gravity walls or concrete/concrete block and steel walls to retain.
Can be messy and expensive to fix. Underpinning of this retaining wall will be necessary before wall can be reinstated. Below is one approach.
Developments in soil nail technology have made this system of earth retention popular for excavation shorings, slope stabilization, and retaining walls. It is a cost-effective method of ground reinforcement for earth retention without excavating and hence spoil.
The helical soil nail consists of helices attached at regular intervals to the entire shaft,including extensions. The result is a helical device with helices spread along the entire length of shaft.The common helical soil nails are 2.1 m long lead or extension with 200 mm diameter helices spaced at 700 mm intervals along the shaft.
An engineer will be needed to draw up details for any structural retaining wall as failure is not an option.
Strong Log Retaining Wall
Structural Details concepts for typical applications
Help manual by Boral: http://www.boral.com.au/masonry-design-guide/pdf/BOR11999_SA_Manual_Bk4.pdf