A critical step in foundation design is determining strata and reasonable strengths to be assigned to each stratum. Selecting design parameters that exceed those existing at the project site will result in increased risk of unacceptable performance. Selecting design parameters that are substantially less (or overly conservative) than those that exist will lead to increased costs from excessively conservative design or construction issues.

Divide the subsurface materials into strata based on material description and test values. Review all tests within each stratum to evaluate the variability of the data. If a single, unusually high strength test is present among a group of distinctly lower test values, disregard the anomalous test value. An average strength may be assigned for an entire layer(s) broken down into Engineering Stratigraphic Units (ESUs) when the test values are reasonably similar.

Avoid defining very thick strata with widely variable test values. Subdivide thick strata with test values varying from soft near the top to distinctly harder toward the bottom into two or more strata with compatible values. Failure to subdivide may result in an unconservative average strength being applied to foundations that terminate in the upper zone of that stratum.