Georisk launched the “Spotlight” series in 2013. The purpose of this series is to invite distinguished scholars and practitioners to review an important topic, to highlight research gaps, and to suggest fruitful research directions. Volume 16, Issue 2 (2022) of Georisk is pleased to present the fifth Spotlight paper on “What is a characteristic value for soils?” by three prominent researchers in the field, Tim Länsivaara, Kok Kwang Phoon, and Jianye Ching.
Prof. Länsivaara is Vice Dean of Research, and Head of TerraGeo Research Group in the Faculty of Built Environment of Tampere University, Finland. He focuses on developing field and laboratory investigation methods for determining the strength and stiffness properties of soils aiming to identify more reliable and faster investigation methods, improving calculation methods, and merging them together in a design framework to reduce risks and minimise overdesign for a safe and sustainable design outcome.
Prof. Phoon is Provost of the Singapore University of Technology and Design. Previously, He also served as Chair of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE)’s TC304 (Engineering Practice of Risk Assessment and Management) and ASCE Geo-Institute’s Risk Assessment and Management Committee. He is a recipient of numerous awards, including garnering the Norman Medal twice (2005, 2020) – the highest honour granted by the ASCE, the NUS Outstanding Researcher Award (2010), the NUS Annual Teaching Excellence Award (2011 and 2012), as well as the Humboldt Research Award from the Alexander von Humboldt-Stiftung Foundation (2017). Prof Phoon is a founding editor of the Georisk journal and Managing Editor of the ASCE-ASME Journal of Risk and Uncertainty in Engineering Systems, Part A: Civil Engineering.
Prof. Ching is a Distinguished Professor in the Department of Civil Engineering, National Taiwan University. Currently he is Chair of ISSMGE TC304 (Engineering Practice of Risk Assessment and Management) and Chairman of the Executive Board of GEOSNet. He is the recipient of the Outstanding Research Award and the Wu-Da-Yu Memorial Award from the Ministry of Science and Technology of Taiwan.
The choice of the characteristic value of a material property is fundamental in all civil engineering design, particularly in geotechnical engineering in which rock and soil exhibit complex and strong heterogeneous behaviour. This Spotlight paper presents a critical review of the geotechnical characteristic value. It starts by reviewing the total uncertainty of a geotechnical property and how such uncertainty is considered in several versions of Eurocodes. The authors then present several practical considerations in the selection of a characteristic value to clarify the limitations of existing practice from statistical and mechanical perspectives. They are of the opinion that simple equations consistent with the qualitative guidelines provided in Eurocode 7 can be developed to account for spatial variability, failure mechanism, and their interactions more appropriately. In a recent draft for new EN 1997-1, the choice of characteristic follows closely the present standard, leaving some important considerations unanswered. A modified equation has been recommended to correctly account for the reduction in the point mean due to propensity to seek the weakest path and the reduction in the point variance due to spatial averaging. The selection of characteristic values for ULS and SLS analyses is discussed. For ULS, the use of a characteristic value with a single partial factor of safety is contrasted with the application of multiple resistance factors to cater for different degrees of site understanding in the current Canadian Highway Bridge Design Code. For SLS, a classical 1D settlement example is analysed to illustrate the problem of selecting multiple characteristic values in the presence of cross-correlations between different soil parameters. The authors expanded on the range of complications encountered in the selection of characteristic values for a routine finite element analysis. They opined that a reliability-based design method is actually simpler to apply if the practitioner were to recognise the full range of uncertainties (spatial variability, cross-correlations, model uncertainties, etc.) and appreciate their complex interactions with mechanics.
The authors have recommended a framework allowing for different approaches with different levels of sophistication and accuracy. A three-level framework is proposed as an example: variable partial factor approach, semi reliability-based design, and full reliability-based design. The authors believe that any design standard adopted for future needs should include the following two principles; (a) all parties in a project should be engaged to reduce uncertainties to an acceptable level, and (b) to achieve (a) all parties should be engaged to share and actively use all data available for decision making. In this way, data will be viewed as an asset that can lead to more alignment with digital transformation occurring in other industries.
This Spotlight paper is an important reference for engineering design code developments and offers directions to improve the present codes of practice.