Consolidation testing measures the compression behaviour of saturated clay soils under load. Settlement analysis uses these results to predict how much and how fast a structure will settle over time—critical information for foundation design, embankment construction, and preloading programs.
What Is Consolidation?
Consolidation is the gradual process by which saturated clay soils compress when subjected to an applied load. Water is squeezed out of the soil pores over time, transferring load from the pore water to the soil skeleton.
Primary Consolidation
The main phase of settlement where pore water is expelled and effective stress increases.
Secondary Consolidation (Creep)
Long-term compression continuing after primary consolidation is complete, caused by rearrangement of soil particles.
The Oedometer Test (1-D Consolidation)
The standard laboratory test for consolidation behaviour using an oedometer (consolidometer).
Test Procedure (AS 1289.6.2.1)
| Step | Description |
|---|---|
| 1 | An undisturbed soil sample is trimmed into a ring (typically 50–75 mm diameter, 20 mm height) |
| 2 | The sample is placed in the oedometer with porous stones on top and bottom |
| 3 | The sample is submerged in water to simulate saturated conditions |
| 4 | A load is applied (typically 12.5, 25, 50, 100, 200, 400, 800, 1600 kPa — doubling each stage) |
| 5 | Settlement is recorded at time intervals (0.1, 0.25, 0.5, 1, 2, 4, 8, 15, 30 min, 1, 2, 4, 8, 24 hours per load stage) |
| 6 | Unloading stages are also measured for rebound characteristics |
Parameters Obtained
| Parameter | Symbol | Description | Typical Range |
|---|---|---|---|
| Compression Index | Cc | Slope of e-log σ' curve (virgin compression) | 0.1–0.5 (low) to 0.5–2.0 (high) |
| Recompression Index | Cr | Slope of e-log σ' curve (unload/reload) | 0.01–0.1 |
| Preconsolidation Pressure | σ'p | Maximum past effective stress | 50–1,000+ kPa |
| Coefficient of Consolidation | cv | Rate of consolidation | 0.1–10 m²/year |
| Coefficient of Volume Compressibility | mv | Volume change per unit stress increase | 0.01–1.0 m²/MN |
| Overconsolidation Ratio | OCR | σ'p / σ'v₀ | 1 (NC) to 10+ (highly OC) |
Settlement Analysis
Immediate (Elastic) Settlement
Occurs instantly upon loading in all soil types.
$$ S_i = \frac{qB(1-\nu^2)I}{E} $$Where:
- q = applied load
- B = footing width
- ν = Poisson's ratio
- E = elastic modulus
- I = influence factor
Primary Consolidation Settlement
$$ S_c = \frac{C_c}{1+e_0} \times H \times \log_{10}\left(\frac{\sigma'_{v0} + \Delta\sigma}{\sigma'_{v0}}\right) $$For overconsolidated soils:
$$ S_c = \frac{C_r}{1+e_0} \times H \times \log_{10}\left(\frac{\sigma'_p}{\sigma'_{v0}}\right) + \frac{C_c}{1+e_0} \times H \times \log_{10}\left(\frac{\sigma'_{v0} + \Delta\sigma}{\sigma'_p}\right) $$Time Rate of Consolidation
$$ t = \frac{T_v \times H_{dr}^2}{c_v} $$Where:
- t = time to reach given degree of consolidation
- Tv = time factor (from degree of consolidation U)
- H_dr = longest drainage path
- c_v = coefficient of consolidation
| U (%) | Tv |
|---|---|
| 50 | 0.197 |
| 90 | 0.848 |
| 95 | 1.129 |
| 99 | 1.781 |
Applications
Foundation Settlement
Predict total and differential settlement beneath footings and slabs.
Acceptable Limits:
| Structure Type | Total Settlement | Differential Settlement |
|---|---|---|
| Isolated footing | 25 mm | 15 mm |
| Raft foundation | 50 mm | 25 mm |
| Bridge abutment | 50 mm | 20 mm |
| Warehouse floor | 100 mm | 30 mm |
| Steel portal frame | 50 mm | 25 mm |
Embankment Settlement
Predict settlement during and after embankment construction.
- Stage construction design
- Preloading time estimates
- Vertical drain spacing optimisation
Preloading and Vertical Drains
Preloading applies a surcharge load to consolidate soft ground before construction:
Preloading without drains:
- Suitable for thin clay layers (< 3 m)
- Time: months to years
Preloading with PVDs (Prefabricated Vertical Drains):
- Drain spacing: 1–3 m triangular or square grid
- Reduction in consolidation time: 70–90%
- Time: weeks to months
Creep (Secondary Consolidation)
$$ C_{\alpha} = \frac{\Delta e}{\Delta \log(t)} $$Where Cα = coefficient of secondary compression.
| Soil Type | Cα/Cc |
|---|---|
| Clays (low plasticity) | 0.03–0.06 |
| Clays (high plasticity) | 0.03–0.10 |
| Organic soils | 0.04–0.08 |
| Peat | 0.06–0.15 |
Australian Standards
| Standard | Title |
|---|---|
| AS 1289.6.2.1 | Determination of the consolidation properties of a soil — Oedometer test |
| AS 1289.6.2.2 | Determination of the consolidation properties of a soil — Determination of the compression index and coefficient of consolidation |
| AS 1726-2017 | Geotechnical site investigations |
| AS 2159 | Piling — settlement of pile groups |
| AS 2870 | Residential slabs — settlement criteria |
Frequently Asked Questions
How many consolidation tests are needed?
Typically 1 test per clay layer encountered in the boreholes. For large sites or variable conditions, more tests are required.
How long does a consolidation test take?
A standard multi-stage oedometer test takes 7–14 days. Longer tests may be needed for soils with low permeability.
What is the difference between normally consolidated and overconsolidated clay?
Normally consolidated (NC) clay has never been subjected to stress greater than its current overburden. Overconsolidated (OC) clay has been preloaded in its geological past (by glaciers, erosion of overlying strata, or desiccation). OC clays are stiffer and settle less.
Can consolidation be accelerated on site?
Yes. Preloading with vertical drains is the most common method. Vacuum consolidation and electro-osmosis are also used in specialist applications.