Uplift in Cofferdams

[ONENGINEER]

I doubt about the case, shown in the attachment, that if coarse sand id underlying fine sand, it can increase the uplift. One thinks the uplift could decrease, not increase, because water has freedom to spread laterally instead of upwards. Could someone shed a light on this.
Thank you.

 

[Ron]

The sketch is correct. The pressure of the water on the outside of the cofferdam is greater than on the inside. The free lateral flow of the coarse sand allows greater water pressure upward on the fine sand as the pressure tries to equalize inside and outside the cofferdam.

 

[SlideRuleEra]

If the question relates to an actual project, the sheeting shown in the sketches does not reach the impervious material. A cofferdam in either of the cases shown will need a concrete seal before being dewatered. Without a seal, the pressure differential, that Ron mentioned, will "blow out" the bottom and quickly flood the cofferdam.

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[oldestguy]

If you would draw a flow net showing the lines of equal potential, say every foot of head loss, it would answer the question. Without the seal situation, you should size the depth of sheeting based upon preventing the gradient within the cofferdam as being well under 1.0, which is one foot of head loss per foot of soil path length to prevent a quick condition.