An Eight-Foot Segmentally Block Wall That Is Moving

Three distinct mechanisms are present, and they compound.

1. Drainage failure (the trigger). No underdrain, no drain gravel, crusher run with fines as a leveling pad, and silty clay backfill (PI 22). That combination is essentially impermeable. Rainwater enters the terrace and drive, saturates the backfill, and cannot leave except by seeping out the face at the base - which is exactly the continuous seepage you see. Saturated clay backfill also raises unit weight and adds hydrostatic pressure the wall was never designed for.

2. Internal stability failure (the reinforcement). Geogrid in every third course was installed only in the top three courses. The lower two-thirds of the wall - where earth pressure is highest - has no reinforcement at all. The 40-ft bulge is the face moving outward relative to the retained soil.

3. External/surcharge effects. The drive crowns toward the wall and carries vehicle surcharge within H/2 (4 ft) of the face. The design must include that surcharge (typically on the order of 250 psf or a traffic-load equivalent). If the original design omitted it, the wall is under-designed regardless of construction quality.

Read the pavement cracks carefully. Cracks 12-18 in. behind the face sit inside the reinforced soil mass. That is settlement or lateral movement of the backfill, not just cosmetic face distress. Treat it as a stability question, not a paving repair.

What the efflorescence does and does not mean. It confirms long-term water movement through the units - corroborating the drainage diagnosis. It is not a material defect in the block.

Before repair:

• Safety and monitoring first. Survey points on the wall face and the drive, read monthly (weekly if movement continues). A wall moving 2 in. in two years may be moving faster now. If there is any question of a global slip surface, close the drive.
• Investigate. Test pits behind the affected section to confirm what reinforcement is actually present, backfill gradation and moisture content, and where the water table sits. A global stability analysis with the actual backfill properties and the drive surcharge - a bulge can be the toe of a deep slip surface, not a local face problem.
• Notify. Advise the owner and the carrier in writing. Do not begin to dismantle the evidence before it is documented with photos, elevations, and test results.

Repair approach - rebuilding the face is not a repair.

1. Remove and stockpile the block units for the affected section.
2. Excavate backfill to the full geogrid extent at each affected level. Do not undercut the remaining wall without shoring.
3. Install a free-draining gravel trench and perforated underdrain at the base, daylithed to a positive outfall, with filter fabric or a graded filter at the soil interface; outlets at 50-ft maximum spacing.
4. Replace the backfill within the reinforced zone with specified granular fill, placed and compacted in lifts.
5. Install geogrid per a design that includes the drive surcharge and the retained height, at the designed vertical spacing to the full embedment length.
6. Rebuild the facing, then restore the drive pavement only after backfill movement is settled - paving over an unstable mass hides the movement for a season and then reappears.

Specify the system, not just the block.

• Reinforcement: geogrid size, type, and vertical spacing shown on a wall section for each height condition; state the design surcharge explicitly (traffic surcharge where any vehicle comes within H/2 of the face) and require stamped design calculations from the wall supplier for each condition. Make the calculations a required submittal, not an option.
• Backfill: specify gradation and plasticity limits for the reinforced zone (drainage aggregate, non-plastic, with a maximum fines content), not "select backfill." Site silty clay is not backfill, no matter how convenient the stockpile is.
• Drainage: underdrain, gravel drain trench, filter fabric or graded filter, weep points, and a positive outfall. Show it on the section so it cannot be value-engineered out of the field.
• Leveling pad: specify concrete, or a well-graded free-draining aggregate without fines; crusher run with high fines under a retaining wall is a water trap.
• Submittals and observation: require reinforcement placement inspection before backfill, and hold points for each lift. The failure here was undetectable after backfill - which is precisely why the inspection point has to be in the specification.
• Warranty language: an installed-system warranty tied to the stamped design, with a defined performance standard (no movement beyond stated tolerances).