One-Dimensional Consolidation

[dontchange]

I have been given a problem which is as follows:

"A tailings disposal facility is provided with an underdrain to enhance consolidation and setttlement. To reclaim the land it is proposed to place free draining material on top of the tailings at a rate of 1m/yr (15kPa/yr) for a period of 4 years. Use the spreadsheet [an excel program which calculates settlement and pore pressure over time] to explore the consequences on the pore pressure distribution and settlement of the under-drain clogging up at various times after the land reclamation starts. The tailings deposit is 10m thick and has c_v [coefficient of consolidation] = 2m^2/year and m_v [coefficient of volume decrease] = 0.0002m^2/kN."

What I am finding difficult to grasp is what are the underlying issues with this problem? I don't quite understand what is said by the clogging up of the underdrain.

My grasp on the problem is we have tailings which are to be consolidated, whose height is 10 metres. Every year after that 1 metre of free-to-drain soil is placed on top of that. The result of the free to drain material is a stress over the tailings which allows consolidation and drainage to occur.

Am I right in saying that the tailings are two-way drainage or is it impermeable at the top.

I'm really quite confused! Any help would be greatly appreciated!

 

[iandig]

Well, in very simple terms, as water is squeezed out, it is likley that very fine material will be carried along with it. This can build up over time so that your 'free-draining' materials performance is reduced (void ratio/content reduces which in turn reduces permeability of drainage layer). Also, as the material consolidates, it becomes less permeable. There is a relationship between k(permeability) and cv which you should be aware of, or if not read up on it as it is basic geotechnical theory. You can assess over time how the performance/consolidation of the material changes and you can calculate what degree of consolidation has occured based upon time, mv and cv, for each loading.
Importantly, when you place the surcharge, the pore pressure will increase, until over time the pore pressure dissipates and water is 'squeezed out' [think of the difference between total and effective stress and where pore pressure fits into the equation]. Therefore you can plot the predicted pore pressure over time, as it decreases as s function of permebaility expressed as cv.