Soil Mechanics / Ground investigation and ground profiles
Building and using the ground profile
From borehole logs to a ground model, and living with residual uncertainty.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Checks needed for project use
- A geotechnical reviewer must check laboratory interpretation, classification conventions and the limits of drained, undrained and one-dimensional idealisations.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
A ground profile brings together the strata, groundwater and soil properties across the site. The design team needs both the interpreted conditions and a clear account of the uncertainty between investigation points.
Each borehole produces a log: depths of each stratum, descriptions, sample and test positions, and groundwater observations. Individually a log is a line of information; the skill is correlating logs across the site into cross-sections — joining like strata, spotting where a layer thickens, pinches out or steps across a buried channel or fault.
From the sections and the test results comes the ground model: idealised strata with representative properties for design (Modules 4–11), and a design groundwater regime. Selecting a characteristic value for a property means cautiously estimating the value that governs the limit state — not simply averaging, but allowing for scatter, for the volume of ground involved, and for the quality of the data.
The observational method may be appropriate where behaviour can be monitored and effective changes can be implemented in time. Establish acceptable behaviour, the credible range of ground response, monitoring and action thresholds, and practicable contingency measures before construction. Monitoring must give enough warning for those measures to take effect.
Check yourself
Two boreholes 40 m apart both show firm clay to depth. What does this prove about the ground midway between them?
Summary
- Borehole logs are correlated into cross-sections and then an idealised ground model with design properties.
- A characteristic value is a cautious estimate of the value governing the limit state, not a simple average.
- Boreholes sample a tiny fraction of the ground; everything between is interpolation.
- Where uncertainty is high, the observational method manages it by monitoring against pre-set triggers.