The geotechnical report is the most important document nobody reads. Here's the guided tour.
The boring logs: what the ground actually is
Each boring log is a vertical slice: the soil descriptions (the USCS classifications — CL, SM, SP — the engineer's shorthand for clay, silty sand, poorly-graded sand), the SPT N-values (the blow counts — the number correlating to density/strength: N<4 very loose, 4-10 loose, 10-30 medium, 30-50 dense, >50 very dense), and the groundwater observations (the water table depth — the single most consequential line on the log).
Read the logs for: the fill (undocumented fill = cannot support foundations), the organics (compress forever), and the consistency across borings (the uniform site vs. the variable one — variability is risk).
The recommendations: the money section
Bearing pressures (the allowable psf — the foundation sizing number), the foundation type recommendation (shallow vs. deep — the report's core advice), and the settlement estimates (total and differential — the numbers that actually govern; a soil strong enough can still settle too much).
The earthwork section (the compaction specs — 95% modified Proctor typical; the fill materials; the subgrade preparation) governs the site work. The pavement section (the section thicknesses — the CBR or R-value basis) governs the parking lots.
The caveats: what the report doesn't promise
The limitations section (the report applies to the design as understood — changed footprints, deeper cuts, or heavier loads need the geotechnical engineer's review), the differing-conditions clause (the ground between borings is interpolated — surprises belong to the differing-site-conditions process), and the observation requirement (the geotechnical engineer observing the earthwork — the report's recommendations assume the verification).
How owners should use it
Get the report early (the $10,000-$50,000 investigation informing the million-dollar foundation decision), share it fully (the entire report in the bid documents — hiding it creates claims), and bring the geotechnical engineer to the foundation decision (the alternatives analysis — shallow vs. improved vs. deep, with costs — is the deliverable worth paying for).
The bottom line
The report's numbers — bearing pressure, settlement, groundwater — are the project's ground truth. Read the logs, heed the recommendations, respect the limitations, and never value-engineer the investigation. The cheapest geotechnical report is the one that prevents the expensive surprise.