Most engineers have no idea what the strength of a soil or tailings implies. Let me write a little about the physicality of soil and tailings of a given strength.
Tailings with a strength of 1 kPa is but a fluid. Kind of like warm honey. You cannot stand on it. Put a geotextile over it and maybe you can stand on it for a short while. It will be like standing on a water bed: you will bounce up and down as the fluid undulates to your touch.
Then someday the tailings will get to a strength of 5 kPa. It is still a fluid. Kind of like peanut butter that has not been in the fridge. You still cannot stand on it. Your feet will sink in to the gooey mass. The 5 kPa material will still flow or at least ooze under pressure. It will not stand at any slope. In the event of a dam break, most will flow out, although not too far.
That incredibly silly Alberta Directive 74 says that 5 kPa is the desired strength of oil sands tailings after one year. Where they got his number from is a mystery shrouded in their ignorance of the reality of soil and tailings strengths. I met a professor from a prestigious university who told me he was working on the issue of measuring accurately a 5 kPa strength. I, drunk, told him he was frittering away his time on a hopeless & futile endeavour. Who cares? 5 kPa is still a fluid and not a solid.
Fact is that you cannot stand on 10 kPa tailings. You still sink in or get stuck in a messy goo. No equipment I have ever seen can traffic this strength material unless it is made to float, as some equipment is indeed made.
Directive 74 says 10 kPa is the needed strength of oil sand tailings after five years. This is based on the mistake assumption that tailings of 10 kPa strength will be traffickable.
This is patent nonsense. I have spoken to the academic PhD who set this limit. He admitted he took it from a paper by Andy Robertson and Gord McKenna. I have read the paper. The 10kPa is mentioned in an ungrammatical parenthesis. I am convinced that somebody inserted this sentence and that it was not edited by the authors. I have spoken to both authors and pleaded with them to retract the statement. They decline. Pride is a terrible thing!
The authors confused bearing capacity and traficability. For when you traffic over a soft material, you shear the surface. At 10 kPa most equipment readily shears the top surface and digs it way into the mass of soft soil or tailings.
I have seen equipment sink fast into 50 kPa tailings. The large, low pressure tracks simply scraped the upper surface of the tailings and dug the equipment deep into the mass, We had to pull it out with a bulldozer founded on adjacent sand.
Now once the tailings gets to a strength of 15 kPa, it begins to act as a solid. Not a solid solid, but at least not a flowable material. From lots of observations I can assure you that once the tailings is 15 kPa and has a 50 cm upper crust of dry and cracked material you can begin to move on it. The plate effect of the upper crust of dried tailings helps distribute the load from your feet, so if you walk slowly and carefully you may traverse the mass. But be careful: if you break through the crust, you will sink into the 15 kPa tailings. It may be a soil, but it is still not walk-on-able.
If you go on to 15 kPa material and churn it up with a plough or other equipment, you can at least create incipient clods. Do this before the material has a strength of 15 kPa and it simply congeals back to a fluid-like mass. Once over 15 kPa and disturbed, the tailings forms an incipient clog-like structure and does not flow back to a mass. The voids between the individual incipient clods allow wind to blow through the mass and make it dry faster. The voids allow precipitation to flow down and not rewet the clods. You are on your way to a solid. A material you could use to construct perimeter embankments!
But as I noted already, you need to get above 50 kPa before you can traffic the tailings with conventional low pressure equipment.
Directive 74 strength requirement were based on an erroneous reading of a badly written paper. The Directive has given rise to more nonsence and bad engineering than any other regulation I know of. What I write here is based on lots of observation and measurement. Surely it is time for the oil sands people to see the truth, to stop hiding behind a silly, mistaken regulation, and to move on to the horizon of fact and reality.
Stop this wallowing in mistakes and reliance on ignorant regulators. Surely there are people of talent and integrity who can say that the oil sands tailings profession is on the wrong track. Surely there are others besides a mere blogger who can speak up on an issue that is costing millions in misdirected activity and sidetracking us from productive solutions.