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Uplift (UPL)

Uplift (UPL)

Heave by seepage of water (HYD)

The basic variables are entered into calculation models, which may include simplifications but are required to 'err on the side of safety'. These models may be analytical (e.g. bearing capacity theory), semi-empirical (e.g. the alpha method of pile design), or numerical (e.g. finite element analysis).

The calculation models are used to verify that limit states are not exceeded. For serviceability limit states, these models must demonstrate that predicted displacements do not exceed limiting values of movement, which are usually project-specific. For ultimate limit states, they must demonstrate that effects of actions do not exceed the available resistance. Ultimate limit states include rupture or excessive deformation of the structure or ground (limit states STR and GEO — discussed further in Chapter 7) and loss of equilibrium, uplift, and hydraulic failure (EQU, UPL, and HYD — discussed further in Chapter 8). Design Approaches provide choice in the way STR and GEO are verified.

3.6.2 Design by prescriptive measures

'Prescriptive measures' are a combination of conservative design rules and strict control of execution that, if adopted, avoid the occurrence of limit states.

The design rules, which often follow local convention, are commonly set by local or national authorities, via building regulations, government design manuals, and other such documents. These design rules may be given in a country's national annex to EN 1997-1.

Design by prescriptive measures may be more appropriate than design by calculation, especially when there is 'comparable experience', i.e. documented (or other clearly established information) in similar ground conditions, involving similar structures - suggesting similar geotechnical behaviour. [EN 1997-1 §1.5.2.2 and 2.5(2)]

Annex G of EN 1997-1 provides a sample method for deriving presumed bearing resistance for spread foundations on rock, which originally appeared in BS 8004.10

3.6.3 Design by testing

Eurocode 7 acknowledges the role of large- and small-scale model tests in justifying the design of geotechnical structures by calculation, prescriptive measures, or observation. However, apart from requiring time and scale effects to be considered and differences between the test and real construction to be allowed for, EN 1997-1 provides very little guidance on design by testing.

3.6.4 Design by observation

In a similar way to design by testing, Eurocode 7 acknowledges the role of the Observational Method in the design and construction of geotechnical structures, but provides little guidance on how to implement it.

Certain actions must be taken: establish limits of behaviour, assess the range of possible behaviour, devise a plan of monitoring, devise a contingency plan, and adopt it if behaviour goes outside acceptable limits. More detailed guidance on implementing the Observational Method may be found in documents such as CIRIA report R185.11

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