Basement wall — preliminary sizing

What this page computes

The page preliminarily sizes a basement wall: it gives the earth pressure, the maximum moment, the reactions at the two supports, and from there the indicative thickness and reinforcement.

The wall is treated as a vertical strip 1 m wide, simply supported between the slab over the basement (top) and the raft or footing (bottom).

The pressure is at rest, not active

This is the decision that changes the result by about 50%, and it is the classic mistake on basement walls.

Active pressure does not exist by itself: it only mobilises if the wall can rotate, by something on the order of 0.1…0.5% of its height. The soil needs somewhere to expand into before it can shed pressure.

A basement wall is held between the slab and the raft. It cannot rotate. So the soil stays in the state it was placed in — the at-rest state, with Jáky's coefficient:

\(K_0 = 1 - \sin\varphi'\)

The difference is not a subtlety:

The page uses K₀ by default. Active remains available, because there are legitimate cases — a wall that moves at the top before the slab is cast, for instance — but choosing it produces an explicit note. It is not a prohibition; it is a requirement that the choice be a deliberate one.

Overconsolidated soil

If the ground was once loaded more than it is now — by a demolished building, an eroded layer, ice — the "remembered" horizontal pressure is higher. The relationship is corrected by the overconsolidation ratio:

\(K_0 = (1 - \sin\varphi') \cdot OCR^{\sin\varphi'}\)

At OCR = 4 and φ = 30°, the factor is exactly 2. The OCR value must come from the geotechnical report, not be assumed — it is the kind of parameter that doubles the result.

Water is not multiplied by K

Below the water table two things change:

1. the soil acts with its submerged unit weight γ', so the solid skeleton pushes less;
2. the water pressure is added in full, not multiplied by K.

The second effect dominates the first. Water has no friction angle — it cannot "lean on itself" the way soil does — so the entire hydrostatic column reaches the wall. At φ = 30°:

\(\gamma_w = 9.81 \quad > \quad K_0 (\gamma - \gamma') = 0.5 \cdot 8 = 4\)

So water increases the net pressure, even though the soil beneath it pushes more weakly. A wall designed without water and built in ground with a water table is seriously undersized.

It is worth checking whether perimeter drainage changes the situation: drainage that actually works can remove the hydrostatic component entirely. But then the calculation depends on its maintenance over the whole life of the building, which is an assumption about operation, not about structure.

Structural scheme and load factors

The strip is simply supported, so the reactions follow from equilibrium: the moment of the resultant about the base, divided by H, gives the top reaction; the rest goes into the raft. Because the diagram grows with depth, the resultant sits below mid-height and the lower support takes the larger share.

The maximum moment occurs where the shear force vanishes; the page locates it numerically and reports its height.

The load factor is not a single number. Pressure from the self-weight of the soil is a permanent action (γ_G = 1.35), pressure from surcharge is variable (γ_Q = 1.5). The two are factored separately, in proportion to how much each contributes to the resultant.

The sizing

The required thickness follows from the condition that the reduced moment does not exceed μ = 0.167 — that is x/d = 0.45, the usual ductility limit without compression reinforcement. The value is rounded up to 25 mm and never falls below 200 mm, the practical minimum for a cast basement wall.

Reinforcement is computed at the proposed thickness, and A_s,min of §9.6.1 is shown alongside: when it governs, the page says so rather than leaving you to compare two numbers.

What it does not cover

• The shear check — rarely governing for basement walls, but not to be skipped.
• Cracking — for a member in contact with soil, the exposure class often imposes a crack-width limit that dictates the reinforcement more than strength does.
• Anchorage of the reinforcement into the slab and the raft, which is precisely where the structural scheme becomes real or does not.
• Passive pressure in front of the wall, neglected in favour of safety.
• Multi-level basements, where the wall has intermediate supports and becomes a continuous beam.
• Vertical loading from the superstructure, which adds compression and changes the section check.

The result is a starting point for the full design, not a substitute for it. The pressure underneath comes from the earth-pressure service, which has an external benchmark; the structural scheme above it is simple and verified through equilibrium, but not against a published source.