Segmental wall systems and foundations

Segmental Retaining Wall Mechanics: Gravity vs. MSE Systems

Segmental Retaining Walls (SRWs) are modular, dry-stacked retaining structures composed of manufactured high-density concrete units. Unlike traditional mortared masonry or cast-in-place concrete walls, SRWs do not utilize wet mortar between courses. Instead, the individual units interlock mechanically and accommodate slight ground movements and seasonal freeze-thaw cycles without cracking.

Shear Interlock Mechanisms

To resist the lateral earth pressure exerted by retained soils, modular SRW units rely on engineered shear interlock systems across adjacent courses:

1. Integrated Rear Lips or Flanges: Molded concrete lips projecting downward along the rear bottom edge of the block that hook over the rear top edge of the course below. This lip automatically establishes a consistent backward setback batter (typically 3/4 inch to 1 inch per vertical course, or 3° to 12° from vertical).
2. Shear Pins and Dowels: High-strength fiberglass, nylon, or composite pins inserted into pre-drilled vertical core holes in the blocks. The pins fit into receiving slots in the units above, locking adjacent courses together against shear forces while setting the course setback.
3. Integral Concrete Shear Lugs and Keys: Top-face or bottom-face tongue-and-groove concrete lugs that nest into corresponding channels in mating blocks, providing high shear capacity without loose hardware.

Gravity SRW vs. Mechanically Stabilized Earth (MSE) Walls

SRW installations are categorized structurally into two distinct operational systems based on wall height and loading:

• Gravity SRW Systems: Rely entirely on the combined mass (self-weight) of the modular dry-stack concrete units, their depth from front to back, and their built-in setback batter to counterbalance lateral soil thrust. Unreinforced gravity walls are strictly limited in height by manufacturer specifications—typically 3 to 4 feet maximum height under level backfill conditions with stable soils.
• Geosynthetic-Reinforced / Mechanically Stabilized Earth (MSE) Walls: When wall height exceeds the manufacturer's unreinforced gravity threshold, or when the wall supports a surcharge (such as a sloped backfill, driveway, parking lot, or structure), horizontal layers of geosynthetic reinforcement (geogrid) must be integrated between block courses. The geogrid embeds deep into the backfill mass behind the wall, interlocking with the compacted aggregate. This transforms the retained soil and geogrid into a unified, coherent composite gravity mass capable of safely retaining slopes exceeding 20 feet in height.

Leveling Pad Preparation and Subgrade Standards

The long-term structural integrity of an SRW depends completely on the stability of its foundation leveling pad. Because SRWs are flexible dry-stacked systems, any settlement, dip, or unlevel spot in the base pad will telegraph upward through every successive course, causing misaligned joints, gaping faces, and potential wall failure.

Foundation excavation

Use the wall-system and engineered detail for leveling-pad width, thickness, embedment, and drainage. The foundation must bear on suitable soil, not organic debris or unresolved soft fill. Verify elevation and step the base according to the detail where terrain changes.

Do not substitute one generic six-inch pad or trench width for every wall. A wall carrying a surcharge or founded on weak soil may require a different design. Keep excavation safe and protect adjacent roots, utilities, and structures. Confirm acceptance before placing units over concealed conditions.

Leveling materials and compaction

Use the specified aggregate or concrete leveling pad. Dense-graded crushed aggregate and clean open-graded stone have different properties; an ODOT three-quarter-inch-minus product is not the same gradation as AASHTO No. 57. The system detail determines material, thickness, lift, density, and any concrete alternative.

Compact aggregate in controlled lifts at suitable moisture and verify levels and bearing. A stated 95% Standard Proctor requirement compares field dry density with the ASTM D698 laboratory maximum; 95% Modified Proctor uses a different basis. For an assumed Standard Proctor maximum of 125 pounds per cubic foot, 95% is 118.75. A field result of 117 would fail that stated criterion, while 120 would meet it, subject to the full acceptance requirements.

Check the first course

Check front-to-back and along-wall levels using appropriate instruments and the system's tolerance. Small foundation errors can accumulate through successive courses. Do not import one eighth-inch-over-ten-feet tolerance into every product.

Dry-set and check base units, clean contact surfaces, maintain the specified setback, and confirm corners and curves with the actual unit geometry. Record base elevation and embedment before concealing them. If a unit rocks, correct its support rather than shim it with unsuitable loose material.

Base Unit Embedment (Burial) Rules

The lowest course of SRW modular blocks must never be set directly on surface grade. It must be permanently embedded (buried) beneath the finished grade at the front of the wall toe.

Purpose of Embedment

1. Prevents Toe Kickout: Lateral earth pressure exerts maximum shear force along the wall base. Passive soil resistance from the buried front soil locks the base block in place.
2. Protects the Leveling Pad: Burial prevents surface runoff from washing out, eroding, or undermining the compacted aggregate base pad.
3. Frost Heave Protection: In cold interior regions of Oregon (such as Central and Eastern Oregon), embedment helps place the base pad beneath the regional frost penetration depth.

Embedment is a design detail

Buried base units protect against sliding, scour, frost effects, and loss of toe support. Required embedment depends on wall height, grade at the toe, foundation conditions, erosion exposure, and the particular wall system. Use the engineered or manufacturer's detail; a rule such as one inch per foot of height is not a statewide statutory requirement.

Distinguish exposed height from total structural height, which includes embedded courses. A six-inch and an eight-inch block produce different course counts. Establish the bottom and top elevations on the drawing and calculate courses from the actual unit dimensions. Do not reduce embedment to save excavation without approval.