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Chapter 12 — In-Situ Testing & Parameter Selection

12.1 Why test in situ

Laboratory tests are run on small, disturbed samples. In-situ tests measure the soil in its natural state, over a larger volume, avoiding sampling disturbance. They are often the primary source of design parameters, with laboratory tests used to confirm and refine.

12.2 The main in-situ tests

  • Standard Penetration Test (SPT) — a split-spoon sampler driven by a 63.5 kg hammer; the blow count \(N\) indexes density and strength. Cheap, universal, but crude and operator-sensitive.
  • Cone Penetration Test (CPT / CPTu) — a cone pushed continuously, measuring tip resistance, sleeve friction, and (with a piezocone) pore pressure. Excellent for profiling and for soft soils.
  • Vane shear test (VST) — measures undrained strength \(c_u\) in soft clays.
  • Pressuremeter (PMT) — expands a cylindrical probe to give in-situ modulus and strength.
  • Dilatometer (DMT) — a flat blade pushed into the soil gives soil type and parameters.
  • Plate load test — a direct measure of the load–settlement response.

12.3 Correlations

Many parameters are estimated from SPT or CPT through empirical correlations:

  • friction angle \(\phi'\) and relative density from \(N\) or \(q_c\);
  • undrained strength \(c_u\) from \(N\) or cone resistance;
  • modulus \(E\) from \(N\) or \(q_c\);
  • liquefaction resistance from \(N\) or \(q_c\) and the cyclic stress ratio.

Correlations are region- and soil-specific; they must be used with judgment and verified locally.

12.4 Selecting design parameters

Parameter selection is an engineering judgment, not a lookup:

  • Bound the uncertainty — use the range of measured values, not a single number.
  • Match the parameter to the analysis — drained vs. undrained, peak vs. residual, small-strain vs. working-strain stiffness.
  • Prefer direct measurement over correlation where the parameter is critical.
  • Consider variability and scale — a small sample may not represent the mass.
  • Document the basis — record how each parameter was chosen.

12.5 Key takeaways

  • In-situ tests measure undisturbed soil over a large volume — often the primary source of parameters.
  • SPT is universal and crude; CPT is precise and continuous; VST, PMT, DMT target specific parameters.
  • Correlations are useful but region-specific — verify locally.
  • Select parameters by judgment: bound uncertainty, match the analysis, prefer measurement, and document the basis.