Module 3 · Lesson 3.3
Building and using the ground profile
From borehole logs to a ground model, and living with residual uncertainty.
Why this matters
The point of investigation is not a stack of borehole logs — it is a ground profile the whole design team can use: the strata, the groundwater and the properties, joined up across the site. Building it well, and being honest about where it is uncertain, is what separates a safe design from a lucky one.
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.
Where the ground is variable or the model uncertain, one response is the observational method: design for the most probable conditions, define in advance what will be monitored and the trigger levels and contingency actions, and adjust as construction reveals the real ground. It turns residual uncertainty into a managed risk rather than a hidden one.
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.
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint