The role of water is significant but very varied. In the first instance it acts on the state or consistency of materials, for example, for clay and for an increase in water holding capacity, passage from a solid to a plastic and then to a liquid state. Secondly, the water transports material. Finally and most important, it acts on the underground water by means of the Archimedes thrust (Terzaghi’s law) which lifts or lightens the soil (Terzaghi, 1950). In the soil, the “Archimede thrust” is characterized by a decrease of effective normal stress (transmitted by the solid soil skeleton). The soil is lightened and the bulk unit weight is like: g – gw = g’
The consequence for the slopes is a decrease in the friction forces along the surfaces of the rupture, which promotes movement. Everyone can observe this phenomenon in the bath or in the swimming pool!
Effective normal stress (s’) Due to the decrease of the effective normal stress at the surface of the bathroom with the body, the man could easily move and fall down in the water!! The role of water may be appreciated by studying previous climatic conditions, both close and distant, which will contribute to feeding the water table (see case study: Villerville-Cricqueboeuf landslide IS MISSING).