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Determining critical gradient

Determining critical gradient

(OP)
Hello,

I am working on a project where we are looking to construct a dam with rockfill shell over normally consolidated silty clay material.
Preliminary seepage modeling indicates locations have hydraulic gradients up to 0.4-0.45, however we were looking to have all values below 0.3.

Is there any sort of standardized lab testing for this?

My first thought was to complete a series of constant head test and just adjust distance between the reservoir and sample, and observe and record if/when erosion/heave is observed.

Any advise?

RE: Determining critical gradient

No lab testing is normally performed. The critical gradient is 1.0, i.e. the force of water flow equals the weight of the material above it. That is why gradients are typically kept to below 0.5 with lower being better.

Dams are typically designed with cutoff trenches and/or other cutoff elements which can lower the gradients.

Mike Lambert

RE: Determining critical gradient

Remember that figure of 1.0 is for upward flow. Horizontal flow erodes at a much lowered number.

RE: Determining critical gradient

(OP)
Thanks for the responses.

GeoPaveTraffic - Agreed about no lab testing normally being performed, however for this scenario we are potentially dealing with a higher consequence structure and are aiming to have all gradients at or below 0.3. I was just wondering if there was lab testing to provide data at what gradient mobilization of material or heave would begin.

BigH - Thanks for the paper, will read through. For our scenario we do not have any issues with the exit gradient, it is just between the silty clay and the rockfill... I am thinking it might be partly due to the large difference in hydraulic conductivity (1e-8 m/s for the clay vs 1e-2 m/s)

oldestguy - Yup. The area of interest for me is vertical flow.

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