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VoyageofDiscovery (Structural)
3 Mar 06 22:11
I have been trying to figure out how to calculate uplift force using flownets and permeability but I am still at a loss.

VOD
Helpful Member!  PEinc (Geotechnical)
4 Mar 06 13:20
From Design of Gravity Dams, US Dept. of Interior, 1976, Design Manual for Concrete Gravity Dams, Page 27, Section 3-9:

"Hydrostatic pressures from reservoir water and tail water act on the dam and occur within the dam and foundation as internal pressures in the pores, cracks, joints, and seams.  The distribution of pressure through a horizontal section of the dam is assumed to vary linearly from full hydrostatic head at the upstream face to zero or tailwater pressure at the downstream face, provided the dam has no drains or unlined water passages.  When formed drains are constructed, the internal pressure should be modified in accordance with the size, location, and spacing of the drains...............  For preliminary design purposes, uplift pressure distribution in a gravity dam is assumed to have an intensity at the line of drains that exceeds the tailwater pressure by one-third the differential between headwater and tailwater levels.  The pressure gradient is then extended to headwater and tailwater levels, respectively, in straight lines.  If there is no tailwater, the downstream end of a similar pressure diagram is zero at the downstream face.  The pressure is assumed to act over 100 percent of the area.  In the final dsign for a dam and its foundation, the internal pressures within the foundation rock and at contact with the dam will depend on the location, depth, and spacing of drains as well as on the joints, shears, and other geologic structures in the rock.  Internal pressures within the dam depend on the location and spacing of the drains.  These internal hydrostatic pressures should be determined from flow nets computed by electric analogy analysis, or other comparable means."
VoyageofDiscovery (Structural)
20 Mar 06 21:22
Thanks PEinc,

It was the last sentence I was trying to figure out.  I have been having a hard time finding examples of such computations.

VOD
BigH (Geotechnical)
20 Mar 06 21:46
VOD - if my memory serves me right - get a copy of Terzaghi and Peck (1967) - they have a good section on flow nets, under dams, etc.  It should be able to point you in the right direction.
cheers

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