DIN 18134: Ev1, Ev2 and the compaction ratio

Updated: September 30, 2026 · ISKRIT

DIN 18134 does not report an elastic modulus in the usual sense. It loads the plate twice, fits a parabola to each cycle, takes the tangent at maximum stress, and uses the ratio of the two deformation moduli as the acceptance criterion for compaction.

The fit

Each loading cycle gives a settlement curve, fitted as a second-order polynomial in the applied stress:

s = a0 + a1·σ + a2·σ²

The deformation modulus is taken from the tangent at the maximum stress reached in that cycle:

Ev = 1.5 · r / (a1 + a2·σmax)

where r is the plate radius. Because it is a tangent and not a secant, Ev depends on the peak stress applied, which is why the standard fixes the loading schedule rather than leaving it to the operator.

Why two cycles

The first cycle contains both recoverable and permanent deformation. The second, run on ground already compacted by the first, is mostly recoverable. Their ratio therefore measures how much irrecoverable settlement was left in the layer, which is exactly what compaction acceptance is about. Well compacted material barely deforms plastically on the first cycle, so the two moduli come out close and the ratio is low.

Worked

A 300 mm plate, r = 150 mm. The first cycle fit gives a1 = 2.5 and a2 = 1.0 in mm per MN/m², with a maximum stress of 0.5 MN/m²:

Ev1 = 1.5 · 150 / (2.5 + 1.0 · 0.5) = 75 MN/m²

Second cycle, a1 = 1.2 and a2 = 0.4:

Ev2 = 1.5 · 150 / (1.2 + 0.4 · 0.5) = 161 MN/m²

Ev2 / Ev1 = 2.14

Specifications commonly cap the ratio at 2.2 for subbase layers, so this layer passes, and it passes on the ratio rather than on Ev2 alone.

What the ratio does not tell you

A layer can show a low ratio and still be under-compacted if it is uniformly soft: both cycles are then equally poor and their ratio looks respectable. That is why specifications set a minimum Ev2 as well as a maximum ratio, and why both criteria must be quoted when results are reported.

Questions

How is Ev calculated in DIN 18134? Each loading cycle is fitted with s = a0 + a1·σ + a2·σ², and the modulus is the tangent at the maximum stress: Ev = 1.5 · r / (a1 + a2·σmax), where r is the plate radius.

What does the Ev2 / Ev1 ratio show? How much irrecoverable settlement the first cycle left in the layer. Well compacted material deforms little on the first cycle, so the two moduli are close and the ratio is low; specifications commonly cap it at 2.2 for subbase layers.

Can a low ratio still mean poor compaction? Yes. A uniformly soft layer gives low moduli on both cycles and a low ratio, so the ratio is read together with Ev2.

The plate load test calculator fits both cycles, returns Ev1, Ev2 and the ratio, and keeps the polynomial coefficients so the fit can be checked.