Geotechnical testing and the soils report: borings, SPT, perc test

One-line orientation

Geotechnical testing is how the soil’s bearing capacity and groundwater are actually established before design — and the exam tests which test answers which question (foundations vs septic).

Key points

• Purpose: determine soil conditions and bearing capacity so the right foundation can be designed.
• Test boring: drills down to take a direct sample of the soil layers — gives a depth profile of soil type and strength. The primary tool for bearing-capacity information.
• In-situ tests (run within/at the boring):
  • Standard Penetration Test (SPT): drive a sampler and count blows to advance it (N-value / blow counts) — more blows = denser, stronger soil.
  • Cone Penetration Test (CPT): pushes a cone to read soil density and strength continuously.
• Percolation (“perc”) test: measures the rate water drains through the soil — used for septic system / drain-field feasibility, a different question from bearing capacity.
• Test pit: a shallow excavation to visually inspect near-surface soil and groundwater directly (useful at shallow depths where a boring sample is less informative).
• The geotechnical (soils) report — typically commissioned by the owner and prepared by a geotechnical engineer — reports:
  • Geologic findings: rock types, fault lines, overall stability.
  • Seismic concerns: especially in earthquake-prone areas.
  • Groundwater conditions: depth and movement (drives basement levels, waterproofing, foundation type).
  • Soil analysis: lab results on composition and strength for foundation design.

Confusions / comparison

Test	Measures	Answers the question
Test boring	Soil layers/profile (direct sample)	What soils are at depth?
SPT (blow counts)	Density / strength in-situ	How strong/dense is it? (bearing)
CPT	Continuous density/strength	How strong/dense is it? (bearing)
Percolation test	Water drainage rate	Will a septic drain field work?
Test pit	Visual near-surface inspection	What’s just below grade / groundwater?

Related

→ Soil properties (this module): bearing capacity and shear strength the tests quantify · Soil types (this module): the soils the boring identifies · Water table (this module): groundwater depth from the report drives foundation and waterproofing choices.