Before focusing on design by calculation, some of the basic and novel concepts of limit state design in the Eurocode require brief explanation.
3.6.1
Limit state design
EC7-1 is a “limit state” design code which focuses on the avoidance of limiting conditions. In effect, limit states should not be exceeded. Limit states are defined as states beyond which the structure no longer fulfils the relevant design criteria. For example, a limit state could be:
an unsafe situation damage to the structure economic loss.
In EC7-1, the avoidance of such limit states is achieved by requiring the design to deliver a particular result, for example that:
a foundation does not fail, causing the structure to collapse
a foundation does not settle to such a degree that the structure distorts and/or cracks.
Ultimate limit states (ULS) of full collapse in or failure of geotechnical structures are fortunately quite rare. However, an ultimate state may develop in the supported structure because of large displacement of a foundation, which has itself not failed. This means, for example, that a foundation may be stationary, after initially settling, but part of the supported structure may have failed (for example, a beam has lost its bearing and collapsed owing to substantial deformation in the structure). EC7-1 requires that both possible states are avoided.
In marked contrast to current British codes, five distinct ultimate limit states are identified in EC7-1. For simplicity, they are given the acronyms shown in parenthesis and bold italics have been used by the authors for emphasis:
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
[EQU] (an example would be the tilting of a rigid retaining structure about the edge of its foundation which bears on rock)
internal failure or excessive deformation of the structure or structural elements, including footings, piles, basement walls, etc, in which the strength of structural materials is significant in providing resistance [STR] (an example would be the resistance to cracking of a pile in tension)
failure or excessive deformation of the ground, in which the strength of soil or rock is significant in providing resistance [GEO] (examples would be overall stability of a slope and bearing resistance of spread or pile foundations)
loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy) or other vertical actions [UPL]
hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients
[HYD].
Of these five ultimate limit states, STR and GEO represent the critical issues normally faced by designers of simple geotechnical structures such as shallow spread footings, conventional piled foundations and low-height retaining walls, while UPL and HYD will be important for deep excavations and cuttings below the water table43. EQU will apply only on infrequent occasions, such as when checking the rotational stability of a rigid structure resting upon a rigid foundation44. Accordingly, the more detailed explanation of the design calculation framework given in Section 3.7.5 concentrates on calculations to check that STR and GEO limit states are not exceeded, with a brief discussion of the other limit states.
A particularly important and novel feature of EC7-1 is the clear separation it makes between the ultimate limit state and the serviceability limit state. While making many statements about the need to ensure that an SLS is not exceeded, the code says very little about how to calculate settlements and deformations. This is important because the code proposes a combination of values of geotechnical parameters (such as shear strength) and partial factors in a design calculation that is specifically intended to avoid failure, rather than to limit deformations. This is in marked contrast to much “routine” geotechnical design practice in the UK where design values of parameters in effect
arise from the use of global factors of, say, 2-3 on bearing capacity so as to prevent excessive settlement, a condition that is usually far removed from “collapse”. This distinction becomes especially important when avoiding large settlements (but not necessarily a ULS condition in the ground) that might cause the collapse of a structural element in the supported superstructure (for example, the bearing of a beam is lost by deformation of the structure when a large foundation settlement occurs).
3.6.2
Design requirements
Limit states are generally checked by considering design situations in which adverse conditions apply. Design values, which are deliberately pessimistic, are used for the destabilising actions (eg loads), for the material strengths that provide the reactions and for the resistances of geotechnical elements. Design values are used in calculations for both ULS and SLS45. Since the aim of limit state design is (a) generally to avoid limit states and (b) specifically to make any possibility of exceeding a ULS very remote, EC7-1 may require the adoption in calculations of design values for parameters that appear to be remote from reality.
3.6.3
Design situations
EC7-1 presents a list of items for consideration when identifying design situations. The geotechnical design must be checked for all relevant design situations. These should be selected to encompass all conditions which are reasonably foreseeable as
likely to occur during the construction and use of the structure. EC7-1 deals with
persistent, transient and accidental situations, for ultimate and serviceability limit states46. These situations are largely distinguished from each other by time47.
The safety requirements may be different in specific design situations. For example, in an accidental situation, a structure may be required merely not to collapse, with the serviceability condition being of limited concern.
Seismic design situations are rarely of general concern in the UK. Though not for this reason, since seismicity is a problem for several EU member states, EC7-1 does not deal with seismic design and refers the reader to EN 1998-5.
3.6.4
Durability
Durability is the ability of a structure to remain fit for use during its design life, taking into account any required maintenance. For geotechnical structures, maintenance is often difficult or impossible. The design should take into account any potential degradation of materials with time due to any aggressiveness of the environment (ground, groundwater chemistry) by providing adequately resistant materials or protection for them48.