A Wall That Was Fine Until the Weep Holes Plugged
A 5.0 m tall cantilever retaining wall retains level, clean silty sand backfill (γ = 19 kN/m³, φ = 32°, Rankine Ka = 0.31). It has a gravel chimney drain and 100 mm weep holes at 1.0 m above the base. After the second wet season, the owner reports the wall has tilted outward about 25 mm and a horizontal crack has opened along the front face near mid-height. A site visit finds no water exiting the weep holes and fines visible in the gravel behind the wall. Groundwater in a nearby boring sits near the wall base, but the backfill reads saturated to within 0.5 m of the top - the drain appears to have blinded off.
1. Compare the lateral force and overturning moment per meter of wall for the drained condition versus the current saturated backfill, and quantify the increase.
Read the worked answer
Drained condition (design intent): Pa = ½ Ka γ H² = 0.5 × 0.31 × 19 × 5.0² = 73.6 kN/m, acting at H/3 = 1.67 m above the base Mo = 73.6 × 1.67 = 122.7 kN·m/m
Saturated backfill (drain failed, water at the top): Below the water table the soil pushes with effective unit weight γ′ = 19 − 9.81 = 9.19 kN/m³, and the water pushes separately.
- Soil: 0.5 × 0.31 × 9.19 × 5.0² = 35.6 kN/m (at H/3)
- Water: 0.5 × 9.81 × 5.0² = 122.6 kN/m (at H/3)
- Total = 158.2 kN/m, Mo = 158.2 × 1.67 = 263.7 kN·m/m
Increase: 2.15× the design force and 2.15× the design overturning moment. A wall with a typical FS of 1.5 to 2.0 against overturning under the drained case has essentially used up its reserve.
Common error to avoid: computing the soil term as 0.5 × 0.31 × 19 × 25 = 73.6 and then adding the water pressure on top of it. That double-counts the water in the soil term; below the water table the soil must be taken at γ′.
2. The owner's engineer proposes widening the footing. Is that the right first move? Justify your answer with the mechanics of the failure.
Read the worked answer
No - not as the first move. The wall was adequate when drained; the load increase is water pressure, and a bigger footing treats the symptom while the drain continues to blind off. Widening also cannot be built under a wall that has already tilted and cracked without unloading it.
Sequence that is defensible:
- Relieve the water. Drill through the wall and the gravel (or excavate to the drain) at low level so the backfill drains before anything else is touched. This reduces the load immediately.
- Shore and stabilize. The 25 mm tilt plus a mid-height horizontal crack is consistent with flexure from a much larger base moment; the crack is in the tension zone and indicates steel stress. Temporarily brace the wall and restrict backfill surcharge and traffic behind it.
- Investigate. Excavate test pits to see the filter gradation and whether the drain was built with a proper filter versus a wrapped drain board; verify the as-built stem thickness and reinforcement against the design.
- Repair the drainage with a graded filter or geocomposite drain board, weep holes at the base, and a surface seal on the backfill.
- Only then evaluate whether the as-built section can carry the residual loading; if the tilt is structural rather than cosmetic, underpinning or a counterfort/anchor retrofit may be required.
A widening scheme designed for the saturated case would be a permanent, expensive fix for a maintenance failure - and would still leave the wall undrained if the new drain also clogs.