In the presence of water and in conditions of initiation of a filtration motion, in the ground is generated a force of direct filtration upwards which can cancel the weight of the soil and, if this is lacking cohesion, can drag the particles and produce a collapse of the ground. This collapse mechanism is known as siphoning; it manifests itself in the outlet point of the first flow line, namely that in adherence to the bulkhead. Another phenomenon to check in flow conditions is the lift of the dredge.
Vertical effective stress in the presence of hydraulic gradient
In the presence of a hydraulic gradient i, the effective vertical stress is calculated with the following formula:
in the above formula the meaning of the symbols is the following:
•γ' is the unit weight of effective ground
•i is the hydraulic gradient
•γw is the unit weight of water
The vertical stress in annulled when:
The safety factor for siphoning is the ratio between the critical gradient ic and the efflux gradient iE
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