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Undrained to Drained stress

Undrained to Drained stress

Undrained to Drained stress

i want to explain in a simple way the behaviour of a soil subject to a loading such as an embankment. as per design, undrained and drained checks need to be done if the soil is a clay.

the question that i have is how do you check when a clay moves to undrained to drained conditions?

i have an answer but i am not sure if i am right:

undrained stress happened instantaneosly due to the build up of pore water pressure. therefore the Cu governs the mechanism. pore water pressure needs to be dissipated to achieve a effective stress, therefore the permeability of the soil comes into the system. however how can I calculate the time and what is the border (in terms of Ru?) between undrained and drained stress?

thank you for your assistance.


RE: Undrained to Drained stress

There is no simple way to analize the properties of a soil as it transitions from undrained to drained behavior. While the time for the transition is dependent on the permeability of the soil, it is the bulk or mass permeability that matters most. It is very difficult to determine the bulk permeability in most cohesive soil deposits and almost always cost prohibitive.

As for analysis, typically they are preformed for the undrained and the drained condition and the design constructed around the most critical condition.

Hope this helps.

Mike Lambert

RE: Undrained to Drained stress

thank you Mike.

i agree with you, i hope there was a simple answer to that. However if you analyze more in details, i might use the pore water pressure (Ru) instead.

when a loading is applied to a clay soil. Ru=1 which means that the full load is taken by the water and the soil does consolidate.
considering a consolidation test, it is possible to calculate the consolidation time correlated with the void index.
the void index varies the density of the soil as well as the moisture content, therefore if i check what is the optimum moisture content of the clay i can assume that it would be the consolidate value and therefore i can calculate the void index and get back to the time.

therefore i can consider a linear relationship between Ru and consolidation which will give me a gradient of how fast Ru reduce up until 10-20% where a soil is considered to be working in effective stress.

what do you think?

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