What a soil investigation actually tells you
Most clients book one because a planning officer asked for it. The report answers a more useful question than the one on the form.
A soil investigation is usually commissioned to satisfy someone else — a planning condition, a lender's checklist, an architect who will not draw a foundation without one. That framing is why so many of them are filed and never read. The report is treated as a permission slip rather than as what it is, which is the only description you will ever get of the material your building has to stand on.
Three questions it answers
Strip away the tables and a ground investigation is answering three things, in order of how much money each one moves.
- What is down there, and at what depth does it change. Layers matter more than any single reading — a competent stratum under two metres of fill is a different building from the same stratum at surface.
- How much load it will take before it moves. Bearing capacity is the number everyone quotes, but the useful figure is usually settlement: not whether it will fail, but how far it will sink and whether the movement will be even.
- What water is doing. Groundwater level, and whether it varies seasonally, decides half the buildability of a plot and most of the temporary works cost.
Why the depth of the boreholes is the whole report
The commonest way a ground investigation misleads is not a wrong reading. It is a borehole that stopped above the layer that mattered. Depth is specified against the load the structure will impose and the depth of influence beneath the foundation — not against a standard figure, and certainly not against budget. A shallow investigation on a heavily loaded plot produces a report that is accurate about the wrong soil.
An investigation that stops short does not produce less information. It produces confident information about the wrong stratum.
Reading it before the architect does
The best time to read a ground report is before anyone has committed to a foundation type, because that is the one decision it can still change cheaply. Once a design assumes piles, an investigation showing competent ground at two metres saves money. Once construction has started, the same finding saves nothing at all — it just documents what was already paid for.
