What Your Soil Investigation Report Is Actually Telling You
A soil investigation report typically runs ten to twenty pages of borehole logs, standard penetration test (SPT) values and lab results — dense enough that most clients skip straight to the consultant's recommendation. That's understandable, but a basic understanding of two numbers helps you ask better questions of your structural engineer.
The first is the Safe Bearing Capacity (SBC), usually expressed in tonnes per square metre. This single number, more than any other, determines whether your foundation can be a simple isolated footing system or needs to be a more expensive raft or pile foundation. Low SBC doesn't mean a site is unbuildable — it means the foundation design needs to work harder, which has a direct cost implication worth understanding upfront.
The second is the SPT N-value trend across depth. A report showing N-values increasing steadily with depth suggests a foundation can potentially be shallower and more economical; erratic or consistently low values across depth are a signal that deeper or reinforced foundation systems will likely be required regardless of what the surface soil looks like.
Our advice to clients: always request the soil investigation before finalizing structural design, not after. A foundation designed against actual data — rather than an assumed 'standard' bearing capacity — is both safer and, in the majority of cases, less expensive than an over-conservative design based on guesswork.
