10 Hydraulic failure
(1)P The provisions of this Section apply to four modes of ground failure induced by pore-water pressure or pore-water seepage, which shall be checked, as relevant:
- failure by uplift (buoyancy);
- failure by heave;
- failure by internal erosion;
- failure by piping.
NOTE 1 Buoyancy occurs when pore-water pressure under a structure or a low permeability ground layer becomes larger than the mean overburden pressure (due to the structure and/or the overlying ground layer).
NOTE 2 Failure by heave occurs when upwards seepage forces act against the weight of the soil, reducing the vertical effective stress to zero. Soil particles are then lifted away by the vertical water flow and failure occurs (boiling).
NOTE 3 Failure by internal erosion is produced by the transport of soil particles within a soil stratum, at the interface of soil strata, or at the interface between the soil and a structure. This may finally result in regressive erosion, leading to collapse of the soil structure.
NOTE 4 Failure by piping is a particular form of failure, for example of a reservoir, by internal erosion, where erosion begins at the surface, then regresses until a pipe-shaped discharge tunnel is formed in the soil mass or between the soil and a foundation or at the interface between cohesive and non-cohesive soil strata. Failure occurs as soon as the upstream end of the eroded tunnel reaches the bottom of the reservoir.
NOTE 5 The conditions for hydraulic failure of the ground can be expressed in terms of total stress and pore-water pressure or in terms of effective stresses and hydraulic gradient. Total stress analysis is applied to failure by uplift. For failure by heave, both total and effective stresses are applied. Conditions are put on hydraulic gradients in order to control internal erosion and piping.
(2) In situations where the pore-water pressure is hydrostatic (negligible hydraulic gradient) it is not required to check other than failure by uplift.
(3)P The determination of hydraulic gradients, pore-water pressures or seepage forces shall take account of:
- the variation of soil permeability in time and space;
- variations in water levels and pore-water pressure in time;
- any modification of the boundary conditions (e.g. downstream excavation).
(4) It should be considered that the relevant soil stratification may be different for different failure mechanisms.
(5)P When hydraulic heave, piping or internal erosion are significant dangers to the integrity of a geotechnical structure, measures shall be taken to decrease the hydraulic gradient.
(6) The measures most commonly adopted to reduce erosion or to avoid hydraulic failure are:
- lengthening the seepage path by screens or shoulders;
- modifications of the project in order to resist the pressures or gradients;
- seepage control;
- protective filters;
- avoidance of dispersive clays without adequate filters;
- slope revetments;
- inverted filters;
- relief wells;
- reduction of hydraulic gradient.