Generally the verification of resistance to hydraulic failure, particularly to failure by hydraulic heave – ultimate limit state named HYD, is considered as the most difficult task for designers of many civil and hydraulic works. Moreover, this limit state is a kind of failure with very high hazard potential because of initiating failure by piping. The hydraulic heave typically occurs in the construction of deep excavation pits demands to lowering the groundwater level and in the construction of hydraulic structures (e.g. weirs) to storage of water in reservoirs. The verification of resistance within the HYD can be done according to the Eurocode 7 [PN-EN 1997-1:2008] using two different ways assuming the total pore pressure and the total vertical stress (eq. 2.9a) or the seepage force and submerged weight of soil prism (eq. 2.9b). In order to check the HYD state the seepage pressure, hydraulic gradients and flow forces should be determined. However, the way how to determine the seepage forces is not specified in PN-EN 1997-1:2008. This paper presents the results of verification against hydraulic heave for weir Jaktorów located in km 11+760 of Pisia Tuczna river. The characteristic values of the required actions have been determined using flow net by the use of numerical simulations with taken into account possible unfavorable conditions as variations in water levels above the soil prism and pore pressure in time as well as downstream excavation. The verification of HYD state shows an enormous influence of assumed condition and applied procedure on weir safety.

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