The ground investigation report

What it contains

A ground investigation report typically gives:

• the stratigraphy: which layers follow one another and to what depth,
• the physical and mechanical properties of the layers,
• the water table and its likely fluctuation,
• findings on bearing capacity and settlement,
• a recommendation on the foundation type and depth,
• a warning about particular risks: swelling clay, made ground, organic layers, voids.

Why it cannot be skipped

The ground is not visible

The surface of a plot tells you nothing about what is beneath. A layer of made ground or the remains of an earlier building does not show at the surface.

The neighbour’s experience is not enough

Ground conditions can change within a few metres.

The mistake is expensive

A foundation mistake is hard to repair afterwards: settlement, cracking, structural damage. The cost of an investigation is negligible against that.

What to view differently for a light structure

Lower load, but not zero

Because of the low pressure, more ground types are suitable than for a heavy building. That holds even where the ground is otherwise weak — which is why a simpler foundation can often be designed.

Uplift is a new question

The ground also has to take tension where the anchorage goes into it. That matters especially with ground screws.

Differential settlement is more of a problem

An accurate, stiff frame tolerates differential movement less well than a masonry structure, which “absorbs” it in cracks. So limiting differential settlement matters.

The geotechnical category

The depth of investigation is not always the same: it follows the complexity of the task. A simple, small building on known ground calls for less investigation than a complex structure on difficult ground. Geotechnical categories.

When to commission it

Before buying the plot, if possible — or at least at the start of the design. The foundation type is one of the largest cost items, and the ground makes that decision, not the designer.

The point

A ground investigation is not a cost but risk reduction. Either you get to know the ground in advance or you get to know it during construction — and that is more expensive.