The Chartership Forum

Foundation Engineering / Bearing capacity from field tests

CPT and plate tests

Continuous data, and a test at the wrong scale.

Lesson review: expert verification remains open

This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.

Corrections checked in this pass

  • Bounded the SPT correlation to its stated range and clarified CPT correction and calibration assumptions.

Checks needed for project use

  • A geotechnical reviewer must confirm drainage and stress basis, bearing-factor formulation, correlations and settlement influence factors. Partial factors and project-specific ground conditions are outside these teaching checks.
  • Confirm test-energy corrections and local sand applicability; CPT Nkt needs local calibration and qt needs cone calibration/pore pressure.

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

Why this matters

SPT measurements are usually taken at selected depths; CPT provides a much more continuous penetration profile. The appropriate test depends on the ground and purpose. Both require interpretation and suitable calibration.

For clays, undrained shear strength comes from the net cone resistance divided by a cone factor:

su = (qt − σv 0) / Nkt

Everything on the right except Nkt is measured. Nkt is chosen. It commonly falls between about 10 and 20, it depends on the clay and on how the correlation was calibrated, and the answer is inversely proportional to it.

A CPT giving qt = 1.8 MPa at a depth where σv 0 = 150 kPa yields su = 110 kPa with Nkt = 15. Choose 10 instead and it becomes 165 kPa; choose 20 and it becomes 83 kPa. Same data, same soil, a factor of two between the extremes — decided entirely by a number the engineer supplied.

Plate loading tests measure the ground's response directly, which is appealing — no correlation, no cone factor, just load and settlement. The difficulty is scale.

A 300 mm plate stresses roughly 300–600 mm of ground. A 3 m footing stresses 4.5 to 6 m. They are interrogating different soil, and on a layered site they can give entirely different answers: the plate may sit happily on a firm crust that the real footing will punch straight through.

A plate test tells you about the ground near the surface. It becomes a foundation prediction only with a scaling rule, and the scaling rules are themselves empirical.

Check yourself

A 300 mm plate test on a site with 1 m of firm crust over soft clay gives an excellent result. What should be concluded about a proposed 3 m wide footing?

Practice

A CPT gives a corrected tip resistance of 1.8 MPa at a depth where the total vertical stress is 150 kPa. Using Nkt = 15, what is the undrained shear strength, in kPa?

Try it

From a blow count to a bearing factor

Follow the SPT corrections and a bounded sand correlation. Bearing factors are shown only within the adopted correlation range.

18
75 %
3.0 m
3.0 °

What each correction removes

  • N₆₀ removes the rig — the energy the hammer actually delivered.
  • (N₁)₆₀ removes the depth, so a uniform sand does not appear to stiffen with depth.
  • The adopted sand correlation uses (N₁)₆₀ = 4.55–39. The original reported N₁ range is 3.5–30 at 78% energy (Hatanaka and Uchida, 1996); this is converted to 60% energy. Local applicability still needs assessment.
  1. 1MeasuredN = 18
  2. 2Energy correctedN₆₀ = 22.5
  3. 3Effective stress at depthσ′v = 57.0 kPa
  4. 4Overburden corrected(N₁)₆₀ = 29.8
  5. 5Density statemedium dense
  6. 6Friction angleφ′ = 41.4°
Nq at lower angle
51.8
Nq at estimated angle
78.5
Nq at upper angle
123.1
Ratio across sensitivity band
2.38×

The ±3.0° band is a chosen sensitivity study. It is not a measured confidence interval or a foundation design check.

Summary

  • N₆₀ removes the rig; (N₁)₆₀ removes the depth
  • φ′ from SPT carries about ±3°, which moved Nq by a factor of 2.4 in the worked example
  • su from CPT is inversely proportional to Nkt, a number chosen rather than measured
  • Plate tests measure the ground near the surface, not the ground a footing will load
  • With correlated parameters, a range is a more honest output than a single value