The logic of EN 1997
What is different about the ground
Section titled “What is different about the ground”It is not homogeneous
Section titled “It is not homogeneous”The properties of a steel section are known and uniform. The ground is layered and variable, and sampling only gives information at points.
It is not a manufactured material
Section titled “It is not a manufactured material”There is no manufacturer’s declaration of performance: the properties are established by testing and experience.
It changes over time
Section titled “It changes over time”The water table fluctuates, consolidation takes years, swelling soil moves with the moisture.
So geotechnical design is not only calculation but engineering judgement.
The design tools under EN 1997
Section titled “The design tools under EN 1997”Calculation
Section titled “Calculation”The classical route: characteristic values, partial factors, verification of the limit states.
Empirical procedures
Section titled “Empirical procedures”Applying proven, documented relationships where the situation is simple and the experience is reliable.
Testing
Section titled “Testing”Load testing — for piles and screwed elements especially, one of the strongest forms of evidence.
The observational method
Section titled “The observational method”Measuring during construction and intervening on the basis of the measurement. Used for complex, high-risk tasks.
The limit states apply here too
Section titled “The limit states apply here too”Ultimate
Section titled “Ultimate”Bearing failure, sliding, flotation, slipping — the ground or the structure loses its stability.
Serviceability
Section titled “Serviceability”Settlement, rotation, differential movement. For most buildings this governs: the ground could take it, the structure cannot tolerate the movement.
For light gauge steel that is particularly true: an accurate, stiff frame tolerates differential movement less well.
The generation change
Section titled “The generation change”Eurocode 7 is also being renewed as part of the second-generation standards. The change will affect foundation practice too.
The point
Section titled “The point”Geotechnical design is not less rigorous but rigorous differently: it handles uncertainty with testing and observation as well as with factors.