A buried rectangular tank wall has to be checked for two opposite load cases that each size a different face of the same wall — tank full with no backfill bends the wall one way, tank empty with full backfill bends it the other — and missing either case under-designs one face entirely.
Every check above runs automatically inside the Structyze Underground Tank module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- Internal 6 × 4 m, height 3 m
- Wall 300 mm, base slab 350 mm, cover 40 mm
- Backfill γ = 18 kN/m³, φ′ = 30° (Ka = 0.33); water table at base
- Allowable bearing 120 kN/m², minimum flotation FS 1.1
- Concrete C30/37, steel B500
Working Through It
Case 1 (tank full, no backfill): hydrostatic pressure pmax = γliqH = 29.4 kN/m² bends the wall outward, factored by γF = 1.2, giving MEd = 52.9 kN·m/m and sizing the interior vertical steel (Ast ≈ 508 mm²/m).
Case 2 (tank empty, full backfill): active earth pressure KaγsoilHwall bends the wall inward instead, factored by γG = 1.35, giving MEd = 48.0 kN·m/m and sizing the exterior steel (Ast ≈ 462 mm²/m).
Both cases end up covered by identical 12 mm bars at 150 mm on each face — Case 1 slightly governs at utilisation 0.67 against Case 2's 0.61. With the water table sitting right at the base, there's zero uplift and flotation is automatically satisfied.
What Governs, and Why
Case 1 (tank full) governs the wall design at utilisation 0.67, narrowly ahead of Case 2. The real sensitivity here is the water table: with it at the base there's no uplift at all, but if the design water table were allowed to rise, the flotation margin would fall quickly and could require toe extensions or tension piles to restore it.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Underground Tank module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For Underground Tank and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why does the long-term condition (both cases present) not need checking?
With both the liquid inside and the backfill outside present simultaneously, their pressures act on opposite faces of the wall and partially offset each other — less severe than either Case 1 or Case 2 alone, so it's never the governing combination.
What happens if the water table can rise above the base?
Flotation (EN 1997's uplift limit state) becomes a truly governing check rather than an automatic pass — the structure's self-weight must exceed the uplift force with an adequate safety factor, which may require a wider base, ballast, or tension piles.
Final Thoughts
The two-opposite-load-cases structure of an underground tank is easy to under-think by hand — it's tempting to design for whichever case seems obviously worse and skip verifying the other face entirely.