Lateral Groundwater Pressure in Competent Rock

[GeoTerry]

Should lateral pressure from groundwater be considered when designing foundation walls in competent rock? Let's say rock is immediately below existing ground surface and there will be a 30-foot-deep excavation and the measured groundwater is 10 feet below ground surface. So the competent rock 20 feet above the bottom of excavation is submersed in groundwater. And the groundwater is only a few molecule thick between competent rock and new foundation wall. Any input would be greatly appreciated. Thanks.

 

[aeoliantexan]

Yes, unless there is drainage, the full hydrostatic pressure will soon act against the wall.

 

[GeoTerry]

Thanks aeoliantexan. Correct. the common practice is to consider the lateral hydrostatic pressure in foundation wall design.
What I don't understand is that if the lateral pressure from the rock mass is considered and the rock mass will be like a gravity wall resisting the groundwater pressure. How a layer of water molecule (thickness may be 0.1 mm between the foundation wall and rock masas) can create a lateral load of say 1/2 x 20 feet x 20 feet x 62.4 pcf = 12.5 kips/ft?
Put it simply, if I'm to lay my back against the excavation face of the rock mass with water drizzling down, I will not feel this 12.5 kips behind my back. Could you please explain further for me to understand? thanks.