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Isotropic compression test

In an isotropic compression test the specimen is loaded by an equal all-around (cell) pressure with no deviatoric stress. The measured response is the relationship between mean effective stress and volumetric strain, which characterizes the soil's volumetric stiffness and its compression law.

What it constrains

Isotropic compression tests constrain the volumetric / compression behaviour independently of shearing. They are particularly useful for fixing compression parameters and limiting void ratios in models that separate the volumetric response (e.g. the SANISAND family's critical-state / compression law, or the limiting void ratios of hypoplastic models).

How numgeo-ACT simulates it

numgeo reproduces the isotropic compression test as a single axisymmetric finite element (U4-solid-ax) — the same geometry as the triaxial tests (\(r = h/2\), directions \(x_1\) radial and \(x_2\) axial).

The conditions under which the test is simulated are:

  • Symmetry: u₁ = 0 on the axis (nleft) and u₂ = 0 at the base (nbottom).
  • Loading: an equal all-around stress is applied to the lateral and top faces and increased stress-controlled from the initial to the recorded levels (the radial and axial face loads are ramped together), so the specimen is loaded purely isotropically with no deviatoric stress.
  • Drainage: drained — a solid element carrying effective stresses, with the volumetric strain free to develop. The simulated volumetric response is compared with your data.

Data mapping (IC-# sheet)

Cell / column Meaning Units
B1 initial void ratio \(e_0\) –
row 2 column headers (informational) –
column 0 (from row 3) mean effective stress \(p\) kPa
column 1 (from row 3) volumetric strain (as a fraction, not %) –
column 2 (optional) initial-state string(s) –

Strain is a fraction here

As with the oedometer sheet, the strain column is a fraction (enter 0.0128 for 1.28 %), and stresses below 1 kPa are ignored.

The full specification is on the Excel sheet reference, and an IC-1 example sheet is provided in the template.