Questions and answers from the GMA Techline
Landfill stability analysis
We are involved in a landfill that is beginning to shows signs of distress on one 3:1 (H:V) slope. I am attempting to back-calculate the waste strength in the distressed area using 2-D limit equilibrium method and insipient failure (FS=1); however, the liquid level in the waste, along with the waste strength, are my most sensitive unknown variables. I only know where pockets of liquid are located from gas extraction well borings.
Do you have any advice on juggling these
two unknowns to get the most accurate
(Tyler | South Carolina)
Thanks for your question. We have analyzed more than 20 landfill failures and, indeed, liquids were involved in each of them. When the liquids were in the waste mass and above the liner system, it was generally the triggering mechanism. We first analyzed the situation without liquid head (the FS was always above 1.0) and then used an unknown leachate head to drop it to FS = 1.0. The values ranged from a few meters to many meters.
The point in saying this is that without piezometers in the waste, one is really guessing as to head on the liner. Perched head is quite another and, if discontinuous, only results in a heavier waste mass. I’ll send a paper in which we conceptually describe the different liquids situations. After you read it, get back if you care to discuss.