Codes and partial safety factor for infrastructure geotechnical designs updated

29th November 2019 By: Schalk Burger - Creamer Media Senior Contributing Editor

Eurocode 7, which provides detailed guidance for geotechnical designs, includes a key difference to the global safety concept, as it is known in South Africa, which is to consider partial safety factors, says German Federal Defence University Munich’s Professor Conrad Boley.

The codes are being revised and updated, and will be released next year.

Partial safety factors are applied to all parameters, allowing for individual uncertainties to be addressed. The approach for pile design, as described in Eurocode 7, was formally accepted by the Geotechnical Division of the South African Institution of Civil Engineering (Saice) in 2018. The accepted piling recommendation by the German Geotechnical Society was launched as a special Southern African edition, which is available locally from the Geotechnical Division of Saice, explains geotechnical engineering expert Professor Florian Hörtkorn.

“The new Eurocode 7 contains design approaches and informative annexes, and can be adapted to any country’s safety regulations, which would typically seek to balance the higher costs of greater safety levels against the desired level of safety,” he explains.

The inherent challenge of geotechnical designs relates to the difficulty in accurately characterising geological features over an area and to required depths, and how these features will impact on design parameters of infrastructure, such as bridges, tunnels and mines, as well as the design of piles and foundations.

The guidelines for the revision are provided to make the codes more user friendly. The new codes provide guidance for design parameters and optional proof methods. This will provide engineers with a rigorous, yet flexible, range of tools they can adopt to meet the requirements and safety standards of their projects.

Hazard Classes

The Eurocode 7 will also introduce reliability classes, hazard classes and consequence classes to further improve the effectiveness of the partial safety factor concept. It will be up to the different countries to apply specific rules, such as the modification of the partial safety factors depending on the consequence classes.

“The partial safety factor concept is already in place in structural engineering, and now in geotechnical engineering as well. The safety level depends on the consequences and reliability classes whereby consulting engineers can judge and estimate how much they can rely on soil property tests and models and how deep their understanding is of the soil properties, which would mean they can use a lower partial safety factor, and vice versa, if the level of understanding of the soil properties is low.”

The probabilistic safety theory is the basis for applying the partial safety factor after engineering calculations have been completed. It involves the design forces and how they are affected by the partial safety factor.

“Owing to many nonlinearities in geotechnical engineering, the partial safety factor is applied afterwards because of the inability to determine whether earth pressures are actions or resistances,” explains Boley.

Engineering calculations would be less accurate if the partial safety factor were applied at the beginning of the calculations because it would prevent the engineer from controlling for errors and would limit visibility of the impact of the partial safety factor on calculations.

“The basic principles to maintain safety remain the same as for other structural engineering designs, and the same factors for materials are relevant, albeit in this case, for example, ground-bearing failure, transient and constant actions,” he says.

What characterises the value of geotechnical parameters is the careful estimation of the soil parameter or the cautiously selected characteristic value in accordance with the code. The Eurocode 7 provides a definition of such a careful estimation and how these factors would act in a limit state, which is important for designs, adds Boley.

Soil Parameters

“An engineer cannot take an average value to determine the characteristics of soil parameters, since there can be local and global problems with such assumptions. Laboratory tests that provide information on the soil characteristics and causes of failures can be different and not uniformly applicable for all design calculations.”

The ground model, soil layers and parameters in the model, as well as the geotechnical calculation model, are necessary to meet minimum safety standards.

The differences between the design and the safety concepts mean that there should be proof of safety, which would consider some probabilistic aspects, but these are not easy to take into consideration and must be carefully controlled for and mitigated. These are handled in the optional proof methods of the codes, he concludes.

The introduction of the partial safety concept should not to be seen as a threat to national interests or safety, as the current level of safety and local experience can easily be introduced into the Eurocode 7 using national annexes to specify these values, adds Hörtkorn.