How to Plan a Site Investigation

Every structure stands on assumptions about the ground, and a site investigation is how those assumptions become knowledge. This guide walks the whole workflow, from deciding how many holes to drill through to the report a reviewer signs off, with a free browser tool linked at every step.

Step 1: Decide how many holes, and how deep

Spacing follows structure risk: a settlement sensitive building earns a 15 m grid, a light warehouse can live with 40 m, and roads take holes every few hundred meters. Depth follows stress: boreholes should reach 1.5 to 2 footing widths below foundation level and never less than 6 m, because the layer that fails is rarely the one you build on. The Site Investigation Planner turns your footprint and footing sizes into a borehole count, depths, and a sampling schedule you can price.

Step 2: Test as you drill

The SPT every 1.5 m is the workhorse: crude, standardized, and priceless when corrected properly with the SPT N Value Corrections Calculator. Where ground conditions suit it, a CPT gives a continuous profile in hours; the CPT Interpretation Tool converts the raw readings into Robertson soil behavior types, strength in clays, and density in sands.

Step 3: Classify in the lab

Grading and Atterberg limits turn samples into named soils. Run the numbers through the USCS Classification Tool for the full ASTM decision tree, and lean on the Property Library when you need typical values before your own results arrive.

Step 4: Draw what you found

The Borehole Log Creator renders each hole as a scaled, colored log with SPT values and the water table. The Cross Section Maker then correlates the holes into the section every report is built around, and the 3D Viewer shows the site in space for the meeting where drawings die.

Step 5: Report it

Each tool carries a Project bar: press Save inputs as you work and Report Studio assembles the whole investigation, cover page to appendix, into one PDF. The honest caveat belongs in every report: screening numbers deserve verification, and the investigation exists to say where.

The mistakes that cost the most

Stopping holes at foundation level instead of below the stressed zone.
Testing dry when the ground will serve wet.
Treating fill as soil instead of as a question.
Writing the report from memory instead of from the logs.

Related Calculators and Reference

Sieve Analysis and Gradation Test, for the laboratory step the borehole schedule feeds.
USCS Soil Classification Tool, to classify what comes back from each hole.
Soil Description Generator, to write consistent descriptions across the whole job.
Cross Section Maker for Boreholes, to turn the completed logs into a ground model.
ASTM D2487, the classification standard the descriptions follow.