Size the trench so there's enough room for the retaining wall 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 ...

Dear James: I want to build a curved glass block wall for a shower stall and a straight one in my kitchen. How do lay a glass block wall so that it will not leak when showering? -- Darlene G.