How to read a borehole log.

What each column means.

• Depth. Metres below ground level, usually with the level of the ground surface given at the top.
• Strata description. Each layer described in a standard way: strength or density, colour, then the soil or rock type, for example "firm brown sandy clay". Descriptions follow BS 5930 and the BS EN ISO standards.
• Legend. A pattern showing each layer at a glance.
• Samples. Where each sample was taken and what type: disturbed, bulk or undisturbed (U100).
• In-situ tests. Usually SPT results.
• Water. Where groundwater was struck and where it rose to.

SPT N values.

The standard penetration test drives a sampler into the bottom of the borehole with a standard 63.5kg hammer and drop. After a 150mm seating drive, the blows for the next 300mm are counted. That count is the N value: the higher it is, the denser or stiffer the ground. Engineers use it in foundation and pile design.

From log to design.

The logs and laboratory results go to your engineer, who uses them to design foundations, drainage and retaining walls. If you would like the data explained, just ask.