Diamond Pro Retaining Wall: Install Guide

The Diamond Pro retaining wall system, made by Anchor and sold under the Diamond Pro Stone Cut and related lines, is one of the most widely installed segmental block walls in the country, and for good reason. Each unit is a hefty concrete block, roughly 18 inches wide, 12 inches deep, and 8 inches tall, weighing about 80 pounds, with a built-in setback and a rear-lip locking system that stacks fast and true. It handles everything from a 2-foot garden terrace to engineered walls well over head height. Get the base and the drainage right and one of these walls outlasts the house behind it. Skip those steps and it bulges and fails within a few seasons. Here is how to build it correctly.

Know the Block and Plan the Wall

Diamond Pro blocks lock together with a rear lip that automatically sets each course back about three-quarters of an inch from the one below, creating the batter that leans the wall into the slope for stability. Some versions use a pin system instead; either way, the setback is what resists the soil pushing from behind. At roughly 1.5 blocks per square foot of wall face, a wall 20 feet long and 3 feet tall, 60 square feet of face, needs about 90 blocks plus caps.

Before you buy anything, mark the wall line and check the height. Walls up to about 3 to 4 feet are typical do-it-yourself territory. Anything over 4 feet of exposed height, or a wall with a slope, driveway, or structure surcharge on top, legally requires an engineered design and often a permit in most jurisdictions. Do not guess on a tall wall; the failure mode is dangerous and expensive.

Excavate and Build the Base

The base is 90 percent of the job. Dig a level trench along the wall line wide enough for the block plus 6 to 8 inches behind it, and deep enough to bury the entire first course plus your base material. A good rule is to bury one course, so for an 8-inch block plan on the trench bottom sitting about 14 inches below finished grade to allow for base and buried block.

1. Excavate the trench to a consistent depth, removing all sod, roots, and soft soil.
2. Compact the native soil at the bottom with a plate compactor.
3. Add 6 inches of crushed stone base (3/4-inch minus or crusher run), spread evenly.
4. Compact the base in 2-to-3-inch lifts until it is rock solid.
5. Screed the top smooth and dead level in both directions.

Do not use pea gravel or sand for the base; they will not compact into a stable footing. A well-compacted crushed-stone base that does not move is what keeps the wall from settling unevenly.

Set the First Course (The Most Important Step)

Everything above depends on the first course being perfectly level, because every error here multiplies as the wall rises. Lay the first block, then use a 4-foot level to check it front-to-back and side-to-side. Tap it down with a rubber mallet, add or remove a pinch of base under low corners, and do not move on until it is exact.

Set each subsequent block tight against the last, checking level as you go and running a string line to keep the course straight. Take your time; an hour spent perfecting the base course saves days of frustration and prevents a leaning wall later. Sweep the block tops clean before stacking the next course so no grit throws off the seating.

Stack, Backfill, and Compact

With the first course dialed in, the wall goes up quickly. Stagger the vertical joints like brickwork so no seam lines up, which ties the courses together. Pull each block forward until the rear lip catches the course below; that engages the automatic setback. Sweep the tops clean before each course.

Backfill as you build, not all at once at the end. Place 12 inches of 3/4-inch clean crushed stone directly behind the blocks as drainage aggregate, and use compactable soil beyond that. Compact the backfill with a plate compactor after every one or two courses, working from the block outward. Never run heavy compaction equipment right up against the blocks; compact the first foot behind the wall by hand or with a light plate to avoid pushing the wall out of line.

Drainage Is What Saves the Wall

Water pressure behind a wall, called hydrostatic pressure, is the number one cause of retaining wall failure. Diamond Pro walls are built to let water escape, but only if you install the drainage. At the base of the wall, behind the first course, lay a 4-inch perforated drain pipe, holes down, wrapped in filter fabric or bedded in the crushed stone, and slope it at least a quarter inch per foot to daylight or a drain outlet at the wall end.

Keep that 12-inch column of clean crushed stone behind the blocks running up the full height so water moves down to the pipe instead of building up. On taller or wetter sites, add a layer of geotextile filter fabric between the drainage stone and the backfill soil to keep fines from clogging it. Cap the top with a few inches of low-permeability soil graded to shed surface water away from the wall.

Reinforce Taller Walls With Geogrid

Once a wall passes roughly 3 to 4 feet, the block setback alone is not enough to resist the soil load, and you need geogrid soil reinforcement. Geogrid is a strong plastic mesh laid horizontally between courses at intervals specified by an engineer, extending back into the compacted backfill, typically 4 to 6 feet or more depending on wall height and soil. It ties the block facing to a large mass of reinforced soil so the whole thing acts as a unit.

The grid placement, length, and spacing are not something to eyeball, which is exactly why walls over about 4 feet require an engineered design. If your wall crosses that threshold or carries any surcharge, get a stamped design and follow the geogrid schedule to the letter.

Finish With Cap Blocks

The top course of caps gives the wall a clean, finished edge and protects the cores below. Dry-lay the caps first to check the layout and mark any cuts. Then glue them down with a bead of exterior-grade concrete adhesive on the block below, which keeps them from being knocked loose. For a curved wall, you may need to trim caps with a masonry saw for a tight fit, and a slight overhang of about an inch past the face looks best and sheds water.

Building Curves, Corners, and Steps

One reason Diamond Pro is so popular is how easily it forms curves and corners. Because the blocks are trapezoidal, with the back slightly narrower than the front, they naturally fan around gentle curves without cutting. For tighter inside curves you may need to trim the occasional block with a masonry saw or splitter. Outside corners are handled with corner blocks or by scoring and splitting units so the finished faces show on both sides.

Terracing and steps let you handle tall grade changes without one imposing wall. Rather than build a single 6-foot wall that demands heavy engineering, two 3-foot walls set back from each other, separated by a planted bench at least twice the height of the lower wall, spread the load and look far softer. Integrated steps are built by pouring a compacted base for each tread and stacking blocks as stringers, keeping every tread level and consistent in rise.

Common Installation Mistakes to Avoid

Almost every failed segmental wall I have been called to fix traces back to the same handful of shortcuts. Knowing them up front is half the battle.

• A thin or uneven base: Skimping on the 6-inch compacted crushed-stone footing guarantees settling and a wavy wall.
• An out-of-level first course: Every error here is magnified as the wall rises, so it is worth an extra hour.
• No drainage stone or pipe: Water builds up behind the wall and pushes it over; this is the top cause of failure.
• Backfilling with the wrong material: Solid clay against the blocks traps water instead of letting it drain.
• Over-compacting against the face: Running a heavy plate compactor right against the blocks shoves them out of alignment.
• Skipping geogrid on tall walls: Anything over about 4 feet needs engineered reinforcement, not hope.

Avoid those six and you avoid the vast majority of retaining wall problems. None of them cost much to do right; they only cost money when you do them wrong and have to rebuild.

What a Diamond Pro Wall Costs

Installed cost for a segmental block wall like Diamond Pro typically runs $20 to $40 per square foot of wall face for a straightforward do-it-yourself or basic contractor job, and more for tall engineered walls with extensive geogrid and drainage. Blocks themselves run a few dollars each, and you will also budget for crushed-stone base and backfill, drain pipe, filter fabric, geogrid on tall walls, adhesive, and caps.

For a homeowner, the material savings of doing it yourself are real, but the labor is serious; the blocks are heavy and the base work is exacting. Whether you build it or hire it, spend your money and attention on the base, the drainage, and, on tall walls, the engineering. Those three things separate a Diamond Pro wall that stands straight for 40 years from one that fails in five.