How to Interpret a Geotechnical Boring Log?
A boring log is the vertical record of what the drill encountered: soil descriptions, SPT blow counts, groundwater depths, and laboratory results by depth. Reading it means correlating the layers across borings, understanding what the blow counts say about soil strength, and noting the groundwater — the three inputs that drive foundation recommendations. Owners, contractors, and engineers all read the same log for different purposes.
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Soil descriptions
Each layer gets a USCS classification (CL, SM, GW, etc.), color, moisture, consistency (soft/stiff/hard for clays) or density (loose/dense for sands), and notes (fill, organics, debris). 'Fill' in the top layers is the critical flag — undocumented fill cannot support foundations. Organics (OH, PT) mean compressible, problematic soils. Read the descriptions first; they tell the story the numbers quantify.
SPT blow counts
The Standard Penetration Test drives a sampler 18 inches; the blow count for the last 12 inches (the N-value) measures soil resistance. Rough guide: N<4 very loose/soft, 4-10 loose/medium, 10-30 medium dense/stiff, 30-50 dense/very stiff, 50+ very dense/hard (often refusal on rock). N-values correlate to bearing capacity and are the basis for many foundation recommendations. Refusal (50 blows for <6 inches) usually means rock or very dense material.
Groundwater and lab data
Water levels noted during drilling and after stabilization — the design groundwater elevation affects foundations, basements, and dewatering. Lab results (moisture content, Atterberg limits, consolidation, shear strength, corrosivity, sulfate content) quantify what the field described. The boring location plan shows where each log applies — soil conditions between borings are interpolated, which is why boring spacing matters.
The bottom line
Read boring logs for soil descriptions (watch for fill and organics), SPT N-values (strength indicator), and groundwater depth. Correlate across borings; the recommendations section translates the data into design.
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