2.4.5 Characteristic values

2.4.5.1 Characteristic and representative values of actions

(1)P Characteristic and representative values of actions shall be derived in accordance with EN 1990:2002 and the various parts of EN 1991.

2.4.5.2 Characteristic values of geotechnical parameters

(1)P The selection of characteristic values for geotechnical parameters shall be based on derived values resulting from laboratory and field tests, complemented by well-established experience.

(2)P The characteristic value of a geotechnical parameter shall be selected as a cautious estimate of the value affecting the occurrence of the limit state.

(3)P The greater variance of c' compared to that of tanφ' shall be considered when their characteristic values are determined.

(4)P The selection of characteristic values for geotechnical parameters shall take account of the following:

  • geological and other background information, such as data from previous projects;
  • the variability of the measured property values and other relevant information, e.g. from existing knowledge;
  • the extent of the field and laboratory investigation;
  • the type and number of samples;
  • the extent of the zone of ground governing the behaviour of the geotechnical structure at the limit state being considered;
  • the ability of the geotechnical structure to transfer loads from weak to strong zones in the ground.

(5) Characteristic values can be lower values, which are less than the most probable values, or upper values, which are greater.

(6)P For each calculation, the most unfavourable combination of lower and upper values of independent parameters shall be used.

(7) The zone of ground governing the behaviour of a geotechnical structure at a limit state is usually much larger than a test sample or the zone of ground affected in an in situ test. Consequently the value of the governing parameter is often the mean of a range of values covering a large surface or volume of the ground. The characteristic value should be a cautious estimate of this mean value.

(8) If the behaviour of the geotechnical structure at the limit state considered is governed by the lowest or highest value of the ground property, the characteristic value should be a cautious estimate of the lowest or highest value occurring in the zone governing the behaviour.

(9) When selecting the zone of ground governing the behaviour of a geotechnical structure at a limit state, it should be considered that this limit state may depend on the behaviour of the supported structure. For instance, when considering a bearing resistance ultimate limit state for a building resting on several footings, the governing parameter should be the mean strength over each individual zone of ground under a footing, if the building is unable to resist a local failure. If, however, the building is stiff and strong enough, the governing parameter should be the mean of these mean values over the entire zone or part of the zone of ground under the building.

(10) If statistical methods are employed in the selection of characteristic values for ground properties, such methods should differentiate between local and regional sampling and should allow the use of a priori knowledge of comparable ground properties.

(11) If statistical methods are used, the characteristic value should be derived such that the calculated probability of a worse value governing the occurrence of the limit state under consideration is not greater than 5%.

NOTE In this respect, a cautious estimate of the mean value is a selection of the mean value of the limited set of geotechnical parameter values, with a confidence level of 95%; where local failure is concerned, a cautious estimate of the low value is a 5% fractile.

(12)P When using standard tables of characteristic values related to soil investigation parameters, the characteristic value shall be selected as a very cautious value.

2.4.5.3 Characteristic values of geometrical data

(1)P Characteristic values of the levels of ground and ground-water or free water shall be measured, nominal or estimated upper or lower levels.

(2) Characteristic values of levels of ground and dimensions of geotechnical structures or elements should usually be nominal values.

Eurocode 7 Geotechnical design Part 1 : General rules