Statistical Description of Characteristic Soil Properties
Authors: Lacasse Suzanne , Nadim Farrokh , Rahim Amir , Guttormsen Tom R. 
Published: October 2008

The paper discusses expressions such as "characteristic value", "best estimate", "upper bound" and "lower bound" used to describe a design soil profile in practice through the reanalysis of case studies. Characteristic (design) values of undrained shear strength were compared with the results of unbiased statistical analyses. It would seem that when one relies on laboratory test results to establish the characteristic (design) strength, the experienced engineers tend to lie much lower than the average, perhaps one standard deviation below the mean. On the other hand, when one relies on the results of in situ piezocone tests, the characteristic strength can be much closer to the mean of the interpreted measurements in situ. The authors suggest that the profession adopts a univocal definition for the term "Best Estimate" and for parameter variability. This suggestion is proposed as a subject for debate in the coming months so that agreement can be established for use in practice. The authors also suggest that the geotechnical profession should use more extensively than before statistical analysis when establishing design values, look beyond using exclusively statistical analyses and start addressing the variability and uncertainty in soil parameters explicitly by implementing probability theory and reliability analyses.

Introduction

A detailed geohazards assessment at an offshore site often involves the assessment of the stability of submarine slopes under static and dynamic (ocean wave and seismic) loading. Even the simplest geotechnical calculation models for slope stability and seismic response require mechanical soil properties. These can never be established with complete certainty. Soils are naturally variable because of the way they are formed and the continuous processes of the environment that alter them. The uncertainty in the mechanical properties of offshore soils is due to both the natural variability from point to point within a soil volume, and imperfect interpretation models, measurement errors and other sources.

The selection of soil properties for use in geotechnical assessment is often based on subjective judgment and accumulated experience. The uncertainties in the soil properties are only indirectly accounted for when the characteristic (design) value(s) are chosen.

Statistics and probability are useful tools for the quantification of the mean (most probable, expected) value and the possible range of values of a parameter. Statistical and probabilistic methods can quantify the uncertainties and make it possible to account for them in a rational and consistent manner. They are however rarely used in practice to establish the design soil parameters. The reason for this is unclear, but perhaps it has become a habit that no one questions, or the restricted use of statistical methods may be a reflection that often there are not enough data available to actually implement statistical methods with confidence. DNV (2006) prepared a guidance note on the statistical representation of soil data. The tools are explained in detail. The profession now needs to make a recommendation of which values to use in design.

In this paper, examples of design soil parameters recommended by NGI for offshore sites over the past two decades were reevaluated using statistical methods, and comparisons of characteristic value with best estimates and variance are made.