UNCERTAINTY AND RELIABILITY - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 117

C URO C ODE 7 AND THE OBSERVATIONAL METHOD

Eurocode
The Eurocode started in 1980 as an initiative of the European Commission and
were handed over in 1990 to CEN (Commission for European Standardization).
From 2010 the Eurocodes are mandatory. The Eurocodes consist of a general
document (Basis of Structural Design) and 9 specific codes. Eurocode 7 (EC7) is
for geotechnical design (part 1) and ground investigation and testing (part 2).
Specifications are defined in the National Annex. In EC7, three reliability classes
(RC) or design approaches (DA) are recognised when using partial safety factors.
Seismic effects are covered in EC8.
EC7 design requires a geotechnical structure be verified for the Ultimate Limit
States (ULS) in the ground and the structure itself also for the Serviceability Limit
States (SLS). ULS requirements comprise overall stability, adequate bearing
resistance, adequate sliding resistance, adequate structural capacity, and no
combined failure of ground and structure. SLS requirements include no excessive
settlement (immediate and delayed), design against heave (immediate and
delayed), and design for vibrating loads. The designer should use commonly
accepted methods. EC7 subdivides ULS requirements into 5 broad categories and
each category has a particular set of partial factor values associated, which should
be applied in corresponding design calculations:
- EQU: loss of equilibrium of the structure or the ground, considered as a rigid
body, in which the strengths of structural materials and the ground are
insignificant in providing resistance;
- STR: internal failure or excessive deformation of the structure or structural
elements (footings, piles, basement walls, etc) in which the structural material is
significant in providing resistance;
- GEO: failure or excessive deformation of the ground, in which the strength of
soil or rock is significant in providing resistance;
- UPL: loss of equilibrium of the structure or the ground due to uplift by water
pressure (buoyancy) or other vertical actions;
- HYD: hydraulic heave, internal erosion and piping in the ground caused by
hydraulic gradients.
The design approaches differ in combinations of partial safety factors related to
actions S , to materials M , and to resistance R . DA1/1 is essentially a STR Limit
State approach, DA1/2 essentially a GEO Limit State approach, DA2 an Action
and Resistance Factor approach, and DA3 an Action and Material Factor approach.
ULS situations are verified by applying partial factors to the main variables of
the calculation model, and by verifying that the fundamental inequality S R is
fulfilled, where S denotes the design value of the action effects and R denotes the
design value of the resistance. Due to the novelty of Limit State Design in most of
the European countries and the wide variety of soil conditions, soil testing and
design methods, EC7 allows for one of three different Design Approaches (DA1,
DA2 or DA3) when assessing R and S for permanent and variable situations. Table
8.2 summarises the sets of values of the partial factors recommended by EC7. The
selection of the mandatory design approach and of the values of the partial factors