Thankful For Your Family…. Business?
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I don't use "landscape timbers". What I call landscape timbers are about 3 1/2" thick and have two opposite sides that are rounded.
What I do use are treated 6x6's. Get a base prepped (excavated, compacted, compacted crushed limestone, leveled), set your first course so it will be below grade, level it, then use 1/2" rebar to pin it in place. Preferrably 24" rebar, but 18" will suffice. All other courses are secured with 8" timber screws. They cost a little more than 60d spikes (12"), but they more than make up for it in the amount of time spent driving them. If you use spikes, you need to drill a 3/8" hole through both timbers before you drive the spike, or else the spike will tend to wander out the front or back of the timber following the grain...
FWIW, I try not to use timbers at all. Compared to pre-cast block, they have a very short life-span....
Installing a deadman in a wall can seem like a lot of extra work but it is vital to the strength of a timber wall. A deadman is basically a horizontal anchor that holds the wall to the soil it is supporting. Place a deadman about every 4-8' feet apart depending on the height and width of the wall. The anchor itself is made of two 3' long lengths of timber connected in a 'T'. See picture. Place deadmen every third row up a wall. I use to even put "fake" deadman wherever 2 pieces of timber meet up.
on edit -never install timbers so that end joints align. You will most likely need to cut several timbers so that you can stagger them. Secondly, you will need to set the rows with a 1/2" batter. That is, each row should sit back 1/2" from the row below it. This is perhaps a little bit of overkill when you said your only going 3-4 high but something to keep in mind for the future ... we haven't done a timber wall in years but a well built wall can last 20 years.