How to Read an SPT Boring Log: A Guide for Engineers and Clients

The boring log is the document foundation decisions rest on. Knowing how to read it prevents surprises on site and helps you spot a weak investigation before it costs money.

What to check in each column

• Elevation and depth: confirm the datum and the boring location.

• Blows per 6 in (150 mm): three counts per test; the N-value is the sum of the last two.

• Soil description: classification (USCS), color, density or consistency, moisture.

• Groundwater: date, time and whether the reading is after stabilization.

• Termination criteria: depth reached and why drilling stopped (refusal, target depth).

• Hammer type and energy ratio: needed to compute N60.

Five red flags

1. Identical or suspiciously regular blow counts across layers.

2. Soil descriptions copied from one boring to the next.

3. Missing or undated groundwater readings.

4. Early termination with no stated reason.

5. No reference to the standard, the rig or the hammer system.

Reliable data starts in the field

A calibrated rig and a controlled-drop hammer produce more consistent logs. Explore Diamond Drill equipment and talk to our technical team about your investigation program.