Consolidation Testing and Settlement Analysis

Consolidation is the time-dependent compression of saturated soil as an applied stress change generates and then dissipates excess pore-water pressure. It is especially important for soft or compressible fine-grained deposits beneath buildings, embankments, tanks and fills.

The equations and worked example are provided on the separate Consolidation Settlement Calculation page.

What consolidation testing measures

An oedometer test applies controlled vertical stress increments to a laterally confined specimen and records deformation with time. Interpretation may provide:

• Compression and recompression behaviour
• Preconsolidation stress and stress history
• Coefficient of volume compressibility or constrained modulus
• Coefficient of consolidation
• Secondary-compression behaviour

Results depend on sample disturbance, specimen preparation, load schedule, drainage, measurement resolution and the selected interpretation procedure.

Normally consolidated and overconsolidated behaviour

The preconsolidation stress separates recompression from virgin compression in simplified settlement models. A future stress path that crosses this boundary must be calculated in appropriate stages. The overconsolidation ratio and stress history should be based on engineering interpretation, not inferred from one number without context.

Settlement by sublayer

Compressible deposits are commonly divided into sublayers because initial effective stress, stress increase and material properties vary with depth. Settlement is calculated for each sublayer and then summed. The stress-distribution method and representative depth should be recorded.

Time rate and drainage path

The rate of primary consolidation depends on the coefficient of consolidation and the drainage-path length. Single and double drainage produce different path lengths. Field drainage may also be influenced by stratification, seams, vertical drains and three-dimensional flow.

Laboratory time-rate behaviour may not translate directly to the field without accounting for scale, sample disturbance, anisotropy and construction sequence.

Construction and observational controls

Preloading, staged filling, surcharge, wick drains, excavation and groundwater changes can materially affect both magnitude and timing. Settlement plates, piezometers and survey monitoring may be used to compare actual behaviour with predictions.

Calculation resources

• Consolidation Settlement and Time-Rate Calculations
• Soil Compressibility and Foundation Settlement
• Effective Stress in Soil

Engineering note

Consolidation parameters and drainage assumptions must be tied to the tested material, stress range and field profile. A calculated time or settlement is not a universal prediction for the site.