Interlocking Retaining Wall Blocks Form a Complete Soil-Retention System

Interlocking retaining wall blocks are dry-stacked concrete units made for segmental retaining walls (SRWs). A lip, shear key, pin, clip or aggregate-filled core develops shear resistance between courses and may establish a backward batter. Unlike a mortared concrete blockwork wall, the block face is only one part of the structure: the leveling pad, foundation soil, drainage aggregate, backfill and—when required—geogrid work together.

That distinction controls purchasing and construction. Do not choose a block from face texture and price alone, and do not mix units, connectors or geogrids from unrelated systems without design approval.

Choose the wall system before the block

There are two basic configurations:

• Gravity wall: Unit weight, depth, inter-course shear capacity and batter resist the retained soil. This is generally the simpler option for a low wall under favorable conditions.
• Geogrid-reinforced wall: Horizontal geogrid layers connect the facing to a compacted reinforced-soil mass. The wider composite mass resists sliding, overturning and pullout, making this configuration suitable for taller walls, surcharges and more difficult geometry.

The Concrete Masonry & Hardscapes Association (CMHA) explains that achievable gravity-wall height changes with unit weight and width, soil properties, batter, backslope and surcharge. Its generic height tables illustrate those effects; they do not authorize a particular wall using any available block.

Before selecting a system, establish:

1. Total height from the bottom of the leveling pad to the wall top—not just exposed height.
2. Toe and top grades, including slopes above or below the wall.
3. Foundation, reinforced-fill and retained-soil conditions.
4. Water sources, groundwater and the drainage outlet.
5. Loads behind the wall, including vehicles, buildings, stockpiles, fences and upper wall tiers.
6. Available excavation width for reinforcement.
7. Utilities and nearby foundations.

Where local requirements do not set another threshold, CMHA recommends project-specific engineering when total design height exceeds 4 ft (1.21 m). A lower wall can also require design when it carries surcharge, stands on weak or expansive soil, retains a slope, forms part of a tiered arrangement or lacks room for the specified geogrid. Local permit and design rules govern; 4 ft is not a universal exemption.

The 2024 International Building Code requires retaining-wall stability against overturning, sliding, excessive foundation pressure and water uplift. It also requires dry-cast SRW units to comply with ASTM C1372. The locally adopted code may be a different edition or include amendments (2024 IBC Section 1807.2).

Compare blocks as complete systems

For each candidate, obtain the manufacturer’s current technical manual and check:

• ASTM C1372 compliance and freeze-thaw documentation appropriate to the exposure
• unit face dimensions, depth and installed weight, including required core fill
• setback and resulting wall batter
• connection method and tested unit-to-unit shear capacity
• compatible geogrids and connection data
• inside and outside corner, curve, cap and step details
• gravity-wall charts for conditions matching the site
• pallet quantity, pallet weight and handling method

ASTM C1372 addresses dimensional tolerances, finish and appearance, compressive strength, absorption and applicable freeze-thaw durability. It does not make block systems structurally interchangeable. CMHA’s SRW inspection guide also notes that wall systems can develop inter-course shear resistance through lips, keys, pins, clips or aggregate-filled cores.

Calculate block quantity from the total installed face area, including buried base courses, stepped bases and units concealed at corners—not exposed area alone. Add a system- and layout-appropriate allowance for cuts and breakage. Count caps, corner units, pins and approved adhesive as separate line items. For reinforced walls, take geogrid type, strength direction, elevations and lengths from the design rather than a generic roll label or online height chart.

Jobsite access can change the best choice. Compare pallet weight with the loader, forks and delivery route. Plan staging so pallets, spoil and equipment do not impose an unplanned surcharge near the excavation or behind a partly completed wall. OSHA identifies spoil, vehicles and equipment as surcharges that can affect excavation stability (OSHA excavation guidance). Leave room for aggregate, backfill, compaction equipment and excavation extending to the full geogrid length.

Build from a level base in controlled lifts

Follow the project drawings and instructions for the selected system. A typical sequence is:

1. Lay out and excavate. Remove organics and unsuitable material. Excavate the reinforced zone to its design limits rather than shortening the geogrid to fit the hole. Locate utilities and protect adjacent foundations before digging.
2. Prepare the foundation. Verify that exposed soil agrees with the design assumptions. Remove soft, wet or disturbed areas as directed and replace them with approved compacted material.
3. Place the leveling pad. Install and compact the specified crushed aggregate or concrete pad. Start at the lowest elevation and step the base in full-block increments.
4. Set the first course. Check every block front-to-back, side-to-side and along the alignment. Errors in the first course grow as the face rises.
5. Place core and drainage aggregate. Fill cores where required and maintain the specified free-draining zone behind the face.
6. Backfill and compact in lifts. Use approved fill at suitable moisture. As one system-specific example, Allan Block limits lifts to 8 in. (200 mm) and calls for at least two plate-compactor passes; the project specifications and selected system control (Allan Block installation guide).
7. Install geogrid as drawn. Use the specified product, elevation, length and strength direction. Pull it taut over compacted fill before covering it. Do not splice it in the load direction unless an approved detail permits that splice.
8. Complete drainage and grading. Route surface water away from the wall and reinforced fill, protect the toe from erosion and discharge collected water to an approved outlet.

Heavy compaction close to the face can move blocks out of alignment. For example, Allan Block’s commercial specification allows only hand-operated compaction equipment within 3 ft (0.9 m) of its wall face and requires at least 6 in. (150 mm) of fill before tracked equipment crosses geogrid (reinforced-wall specification). Use the limits for the actual wall system and drawings.

Drainage is more than a pipe behind the blocks

Water increases soil pressure, carries fines and can erode the toe or foundation. Drainage aggregate and perforated pipe behind the face collect incidental water; they do not replace site drainage. CMHA recommends directing water away from the wall and designing surface and subsurface drainage for site conditions. Its general detail shows at least 12 in. (305 mm) of free-draining aggregate behind the units, but the approved design controls (CMHA design note).

Stop and obtain design direction if excavation exposes weak soil, uncontrolled fill, seepage, utilities in the reinforced zone or conditions that differ from the plans. Stop as well if there is insufficient room for the specified geogrid. Shortening reinforcement, substituting backfill or adding drainage stone does not preserve the original design; each changes part of the wall system and is not a field fix.