Retaining Wall Drainage and Overturning
A 4.0 m high cantilever gravity retaining wall retains a granular backfill (γ = 19 kN/m3, φ' = 32°). The wall stem is 0.4 m thick at the base, and the base slab is 3.0 m wide and 0.5 m thick. During construction the contractor omitted the specified weep holes and filter drain, arguing that the granular fill is free-draining. Two years later, the wall shows a visible forward lean and a horizontal crack at the stem-base junction. Design groundwater is at 3.0 m depth behind the wall. The designer's original calculation used Rankine active pressure with no water pressure.
1. Explain how the omission of drainage changes the pressures acting on the wall and why the observed distress is consistent with this.
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The original design used dry Rankine active pressure: Ka = (1 - sin32°)/(1 + sin32°) ≈ 0.307, giving a triangular pressure peaking at 0.307 × 19 × 4 ≈ 23 kPa at the base, or about 47 kN/m total.
Without drainage, water accumulates in the backfill. Once the water table rises to near the surface, the wall experiences:
- Hydrostatic water pressure in addition to the soil pressure. Water pressure at 4 m depth is 4 × 9.81 ≈ 39 kPa, adding about 78 kN/m - more than the entire original soil thrust.
- Reduced effective stress, so the soil's own contribution is computed on submerged unit weight below the water table, but the total lateral load rises substantially because water pressure is not attenuated by Ka.
- Loss of suction/apparent cohesion that may have been relied on informally.
The total lateral force roughly doubles. The forward lean indicates the overturning and sliding margins have been exceeded, and the crack at the stem-base junction indicates the stem's moment capacity is exceeded - consistent with a much larger base moment from the added water thrust. The granular fill being free-draining does not help if water cannot escape; weep holes and a filter drain are the escape path.
2. Outline the remedial measures and the verification checks needed to confirm the repaired wall is adequate.
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Immediate: relieve water pressure by coring weep holes through the stem at low level with a granular filter behind, and install a continuous toe drain. Monitor movement until stable. Structural repair of the cracked junction (e.g. dowels and encasement) follows once pressures are relieved.
Verification checks:
- Overturning: compute restoring moment from wall and base self-weight about the toe and compare with the overturning moment from the combined soil plus water thrust, with a factor of at least 1.5 (or 1.0 on destabilising per the chosen code).
- Sliding: check base friction (tan δ on the base) against the total horizontal thrust, including water.
- Bearing pressure: ensure the resultant stays within the middle third and the maximum toe pressure is below allowable.
- Structural capacity of the stem and base: recompute bending moments under the full drained-but-wet condition and check reinforcement.
The design assumption must change: even with drainage restored, a blocked drain scenario should be checked, since drains clog. Many codes require the wall to be checked for a water table at a specified height regardless of drainage provision. The repair should not simply restore the original no-water assumption.