Free Online Site Investigation Planner: Borehole Spacing and Depth

Geotechnical
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Spacing follows structure risk: 15 m grids for heavy or settlement-sensitive structures, up to 40 m for light sheds on uniform ground, and holes every few hundred meters along roads. Depth follows stress: boreholes should reach 1.5 to 2 times the footing width below foundation level, and at least 6 m, because the layer that fails is rarely the one you build on.

How many holes, and how deep

Every investigation budget starts with the same two questions. The tool above answers them with the standard rules of thumb: a borehole grid across the footprint at a spacing set by structure risk, each hole carried 1.5 to 2 footing widths below foundation level and never less than 6 m, with SPT sampling every 1.5 m and roughly one laboratory test set per three samples. Pick a structure type and the spacing adjusts; roads switch to a linear layout with shallow pavement-depth holes. The output is the skeleton of a drilling contract: hole count, meters, samples, and lab sets.

Plan view of a building footprint with a grid of boreholes at 15 to 30 meter spacing, with notes on choosing borehole spacing and depth
A borehole grid across the footprint, each hole deep enough to catch the stressed zone below the foundation.

This completes the Geotechnical Site Investigation Suite: plan the campaign here, interpret the cone and lab data with the suite's other tools, and draw the result in the borehole log creator.

The logic behind the rules

  • Spacing follows variability and consequence: a settlement-sensitive high-rise earns a 15 m grid, a light warehouse on uniform ground can live with 40 m.
  • Depth follows stress, not convenience: the pressure bulb under a footing reaches about two widths down, so that is where the holes must reach; rafts push the rule to 1.5 times the building width.
  • The 6 m minimum exists because the most expensive surprises, buried fill and soft lenses, live shallow but below trial-pit depth.
  • These are starting rules: the first boreholes routinely change the plan, which is exactly what they are for.

What this tool is for

Scoping and pricing an investigation before a driller is called, sanity-checking a consultant's proposed program, and teaching why investigation scope follows structure risk. Codes and local practice adjust the constants; the logic of spacing by risk and depth by stress is universal.

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