Make the trench wide, deep and level - Size the trench so there’s enough room for the block and at least 8 in. of space behind it. Excavate deep enough to completely bury at least one full course, including space for 6 to 8 in. of base material.Establish a level trench to ensure an even layer of base material. That will help prevent the wall from tipping after freeze/thaw cycles. Our experts use a laser level and a story pole to determine the depth of the trench.