Junior engineer here. Everyone says 'drain the wall' but I want to understand WHY the hydrostatic case dominates so heavily, not just apply the detail. Can someone walk through the first principles?
Undrained, water pressure (≈9.81 kPa/m) stacks on the active earth pressure AND removes the effective stress that gives the soil its strength. Drain the wall and you design for roughly a third of the load.
Auto-drafted from the question to help visual learners; a chartered engineer signs it off before it shows.
Great question to ask the 'why' of. Water pressure is the killer because it acts as a full triangular distribution with no shear resistance — γ_w·h, ~9.81 kPa per metre of head — and it doesn't care about the soil's friction angle. Active earth pressure on a granular backfill might be K_a·γ·h with K_a ≈ 0.3, so ~5-6 kPa/m. Add an undrained water table behind the wall and you've roughly tripled the destabilising pressure AND removed the effective stress that gives the soil its strength. So drainage isn't a detail — it's the single biggest lever on the design load. Drain it and you design for ~⅓ of the undrained case.
Post an answer
Verified professionals only (chartered, incorporated, or Master Trade). Your credential badge appears on every answer.
This thread is a candidate for a permanent Knowledge Entry
A Curator can promote the accepted answer into a verified, attributed Entry — and the author earns a share of subscription revenue under the APAR Pathway. This is how Roots feeds the corpus.