Statistical charts for determining sample size at various levels of accuracy and confidence in geotechnical site investigation

Site investigation is indispensable in geotechnical practice, but only a small portion of the soil affected by the intended geotechnical construction is examined and/or tested in the laboratory or in situ, leading to significant uncertainties in site investigation results. The uncertainty in the site investigation results depends greatly on the number of specimens tested (i.e. sample size) during site investigation. In many other areas that involve sampling, such as polls, surveys and quality control, sample sizes are often determined using statistical charts, based on the target levels of accuracy and confidence. However, conventional statistical charts, which only deal with independent data, are seldom used in geotechnical site investigation, because geotechnical properties are usually spatially correlated. No statistical chart with consideration of spatial correlation is available for determining sample sizes in geotechnical site investigation. Existing geotechnical design codes around the world (e.g. Eurocode 7) only provide conceptual principles on sample size determination. This study develops statistical charts for determination of sample size at different levels of both accuracy and confidence for geotechnical site investigation. Real cone penetration test data and real laboratory test data are used to illustrate application of the proposed statistical charts, and the charts are shown to perform well.