Actions of water and ground water
Dear colleagues,
how do you treat actions of water and ground water in verifications using limit state design with partial factors? Do you regard them as variable actions applying a higher partial factor or as a permanent actions applying a lower partial factor? Here are the provisions of a number of countries and how they apply EN 1997-1, Eurocode 7: Geotechnical design - part 1: general rules. The link http://geosnet.geoengineer.org/TG/TG4/T... presents the provisions a table.
Best regards,
Bernd Schuppener
Ireland: We consider that water should be considered to be a permanent action because if it is considered to be variable it would introduce conceptual difficulties when considering effective stresses in, for example, earth pressure calculations. The pressures should be based on the worst credible water level/water pressure. There is a distinction between the method of selection of the worst credible ground water level for cases with tidal or cyclical variations and those cases where the upper water level is known, e. g. in a water tank.
Sweden: As variable part of the load is considered the difference between actual water level and the permanent pressure. If the actual water pressure is based on HHW or LLW they shall be reduced to characteristic values which means the value at which a probability of 98% is not exceeded any time during one year. However, normally the characteristic value is taken as corresponding to HHW or LLW and the normal values corresponding to MHW and MLW. There is also a limit if there are some physical limits for the water pressure e.g. top of a barrier.
South Africa: In South Africa, we regard the groundwater pressure and earth pressure as a single permanent action. Free water above the ground surface including the additional pressure within the ground caused by such water is classified as a variable action. Water pressure arising from temporary flooding is regarded either as a variable action or an accidental action.
Finland: In Finland the partial safety factor for permanent loads is 1,15KFI or 1,35KFI depending on the percentage of permanent and variable loads from the total load (See the formulae 6.10a&b of EN 1990). KFI is a factor ranging from 0,9...1,1 depending of the reliability classes RC3...RC1. Additionally there is a limit if there are some physical limits for the water pressure e.g. top of a barrier. Eero Slunga on an alternative method, which has been used before Eurocodes: According to this method the permanent load (water pressure) corresponds to the mean level of water pressure (MW). The variable part of water pressure corresponds to the difference h between the observed water level and MW. The design water level depends on the length of observation time as follows: Design water level = MW + kh or MW - kh where k = 1.2 , if the observation time > or = 3 years, k = 1.4 , if the observation time > or = 1 year and k = 1.6 , if the observation time < 1 year.
United Kingdom: This problem has been considered occasionally for buoyancy problems, for which the variable part of the water pressure has been treated separately, with higher factors as required by the code. However, more generally, we think that applying prescribed factors to water pressures is fraught with difficulties, so we try to encourage intelligent assessment of the design water levels and pressures. In the National Annex to EN1997-1 (NA) it is stated that partial factor values for actions in STR and GEO verifications should generally be taken from EN1990, but: "The partial factors specified in the National Annex to BS EN 1990:2002 might not be appropriate for self-weight of water, ground-water pressure and other ac-tions dependent on the level of water, see 2.4.7.3.2(2). The design value of such ac-tions may be directly assessed in accordance with 2.4.6.1(2)P and 2.4.6.1(6)P of BS EN 1997-1:2004. Alternatively, a safety margin may be applied to the characteristic water level, see 2.4.6.1(8) of BS EN 1997-1:2004." For the UPL limit state, the NA says: "The partial factor specified for permanent unfavourable actions does not account for uncertainty in the level of ground water or free water. In cases where the verification of the UPL limit state is sensitive to the level of ground water or free water, the design value of uplift due to water pressure may be directly assessed in accordance with 2.4.6.1(2)P and 2.4.6.1(6)P of BS EN 1997-1:2004. Alternatively, a safety margin may be applied to the characteristic water level, see 2.4.6.1(8) of BS EN 1997-1:2004." Brian Simpson:
