2 Basis of geotechnical design
2.1 Design requirements
(1)P For each geotechnical design situation it shall be verified that no relevant limit state, as defined in EN 1990:2002, is exceeded.
(2) When defining the design situations and the limit states, the following factors should be considered:
- site conditions with respect to overall stability and ground movements;
- nature and size of the structure and its elements, including any special requirements such as the design life;
- conditions with regard to its surroundings (e.g.: neighbouring structures, traffic, utilities, vegetation, hazardous chemicals);
- ground conditions;
- ground-water conditions;
- regional seismicity;
- influence of the environment (hydrology, surface water, subsidence, seasonal changes of temperature and moisture).
(3) Limit states can occur either in the ground or in the structure or by combined failure in the structure and the ground.
(4) Limit states should be verified by one or a combination of the following:
- use of calculations as described in 2.4;
- adoption of prescriptive measures, as described in 2.5;
- experimental models and load tests, as described in 2.6;
- an observational method, as described in 2.7.
(5) In practice, experience will often show which type of limit state will govern the design and the avoidance of other limit states may be verified by a control check.
(6) Buildings should normally be protected against the penetration of ground-water or the transmission of vapour or gases to their interiors.
(7) If practicable, the design results should be checked against comparable experience.
(8)P In order to establish minimum requirements for the extent and content of geotechnical investigations, calculations and construction control checks, the complexity of each geotechnical design shall be identified together with the associated risks. In particular, a distinction shall be made between:
- light and simple structures and small earthworks for which it is possible to ensure that the minimum requirements will be satisfied by experience and qualitative geotechnical investigations, with negligible risk;
- other geotechnical structures.
NOTE The manner in which these minimum requirements are satisfied may be given in the National annex.
(9) For structures and earthworks of low geotechnical complexity and risk, such as defined above, simplified design procedures may be applied.
(10) To establish geotechnical design requirements, three Geotechnical Categories, 1, 2 and 3, may be introduced.
(11) A preliminary classification of a structure according to Geotechnical Category should normally be performed prior to the geotechnical investigations. The category should be checked and changed, if necessary, at each stage of the design and construction process.
(12) The procedures of higher categories may be used to justify more economic designs, or if the designer considers them to be appropriate.
(13) The various design aspects of a project can require treatment in different Geotechnical Categories. It is not required to treat the whole of the project according to the highest of these categories.
(14) Geotechnical Category 1 should only include small and relatively simple structures:
- for which it is possible to ensure that the fundamental requirements will be satisfied on the basis of experience and qualitative geotechnical investigations;
- with negligible risk.
(15) Geotechnical Category 1 procedures should be used only where there is negligible risk in terms of overall stability or ground movements and in ground conditions, which are known from comparable local experience to be sufficiently straightforward. In these cases the procedures may consist of routine methods for foundation design and construction.
(16) Geotechnical Category 1 procedures should be used only if there is no excavation below the water table or if comparable local experience indicates that a proposed excavation below the water table will be straightforward.
(17) Geotechnical Category 2 should include conventional types of structure and foundation with no exceptional risk or difficult soil or loading conditions
(18) Designs for structures in Geotechnical Category 2 should normally include quantitative geotechnical data and analysis to ensure that the fundamental requirements are satisfied.
(19) Routine procedures for field and laboratory testing and for design and execution may be used for Geotechnical Category 2 designs.
NOTE The following are examples of conventional structures or parts of structures complying with Geotechnical Category 2:
- spread foundations;
- raft foundations;
- pile foundations;
- walls and other structures retaining or supporting soil or water;
- bridge piers and abutments;
- embankments and earthworks;
- ground anchors and other tie-back systems;
- tunnels in hard, non-fractured rock and not subjected to special water tightness or other requirements.
(20) Geotechnical Category 3 should include structures or parts of structures, which fall outside the limits of Geotechnical Categories 1 and 2.
(21) Geotechnical Category 3 should normally include alternative provisions and rules to those in this standard.
NOTE Geotechnical Category 3 includes the following examples:
- very large or unusual structures;
- structures involving abnormal risks, or unusual or exceptionally difficult ground or loading conditions;
- structures in highly seismic areas;
- structures in areas of probable site instability or persistent ground movements that require separate investigation or special measures.