Sample problem need help - Retaining wall pore pressure

[asmyth95]

Ive been trying to do a sample problem and I'm not sure if I've got it right. Could anyone tell me if my values are right.
A=0kN/m², B=19.62kN/m², C=25.51kN/m², D=29.43kN/m², E=33.35kN/m², F=37.28kN/m². Or is there something I'm missing? Would appreciate any help. Thanks

 

[oldestguy]

Why waste your time with an example like this? With a drainage situation properly designed I can't imagine how such a flow net diagram would ever happen with complete saturation from the surface down to the pipe.

 

[GeoPaveTraffic]

I agree with oldestguy that this is a really bad example. That said, I'll try and point to you to what I think the textbook answer would be:

A would be zero assuming the water is just even with the ground surface, in reality to set this situation up the water would need to be between the top of the ground surface and the top of the wall.
F would also be zero assuming the pipe daylights and there is no tailwater condition.
The other points would reflect the change in water pressures between these two points as represented by the flow net with the max occurring somewhere near point D.

Again, this is a really bad made up problem.

Mike Lambert

 

[CarlB]

I don't think the example is that bad. This case could happen if the pipe was blocked, then opened. Or if the backfill was flooded at a rate beyond the ability of the pipe (or soil) to transmit the water.

I get different answers, but I'm not used to working in metric.
At 2 meters depth (without flow)39.15 kn/m2.

But this pressure difference drops through the soil as indicated by flow net lines perpendicular to the flow, about 20 intervals. So each line indicates a flow pressure drop of about 2 kn/m2.

Calculate the pore pressure at each point by summing the pressure due to submergence (measured from top surface) minus the pressure drop due to flow resistance.

I get that Points C & D are the max, at 8.8 kn/m2.