What a Ground Investigation Report Can and Cannot Tell You

A ground investigation exists to turn the subgrade from an unknown into a described body. The subgrade is not placed by the works; it is found by them, so its composition, water content and variability are properties to be established rather than chosen. The report is the record of that establishing.

It is also the evidence trail. Every later decision, founding level, whether treatment is worth carrying out, whether a shallow foundation is even in the conversation, is made against what the report says, and a design that cannot name its ground information is designing against an assumption.

A trial pit exposes ground in the round at shallow depth: you can see the material, the way it changes, obstructions and any fill, and you can see it in the position it actually occupies. A borehole reaches much further down but describes the ground only along a narrow line.

Neither is a substitute for the other, and the mix chosen is part of the investigation design. What a report can say about the depth of a soft layer and what it can say about how that layer varies across a garden are two different confidences drawn from two different methods.

The common misreading is to worry about whether the holes went deep enough. The harder question is how far across the plot the investigation actually extends, because the point is made directly in the reference entries: ground can vary sharply over short distances, which is why investigation is site-specific.

A piling entry puts the same point as a question for professionals, asking how confident the design is in the ground between the investigation points. Every hole is a sample, and everything between the holes is inference. Knowing where the samples sat relative to your building line is part of reading the report.

Investigations commonly produce a factual account of what was found and, separately, an interpretation of what it means for a design. The first records observation; the second applies judgement, and the judgement is made for a particular structure on a particular layout.

That distinction becomes important the moment the scheme changes. Facts survive a redesign; an interpretation written for a different building may not, which is why an engineer will ask to see both rather than a summary of either.

The subgrade entry states plainly that neighbouring sites do not indicate what will be found. A report from next door can be useful background, and it can shape the scope of your own investigation, but a foundation designed against it has been designed against ground nobody has tested where you intend to build.

The same applies to a report prepared for an earlier scheme on your own plot. It described the ground at a set of positions for a set of loads, and whether it still answers the question depends on where the new building sits and what it weighs.

It does not choose a foundation. It does not state a founding level for your building. It does not say a site is suitable or unsuitable in itself. Those are design determinations made by qualified professionals who read the report alongside the structure being proposed.

The strip foundation entry notes that once a trench is backfilled nothing can be seen again, so what was recorded is what is known. The investigation report is the first half of that record and the inspection of the open excavation is the second. Together they are what any future extension or claim is judged against.