Here is a figure that shows how fill placed over the crust that may form on the top of a soft tailings deposit may respond to loading by the wheel of a vehicle trying to spread a new reclamation cover. This figure is from Stabilization of Tailings Deposits: International Experience by Jakubick et al.

None of these is an exciting prospect if you have to place a cover over very soft tailings. One way to avoid this is to place a geogrid and/or a geotextile and/or a geodrain before placing the soil.

If the tailings are very wet, you may have to precede this with a floating cover.

If you can succeed in putting a single and/or a series of geosynthetics over the soft tailings, you then have to get the tailings to consolidate and gain a bit of strength before you put the final cover. Tailings consolidation may be expedited in such a case by one or more of the following technical approaches: Electrokinetic stabilization; Strip drains; preloading; etc.

More information can be obtained from geosynthetic manufacturers including Maccaferri, Nilex, Huesker, or any one of the 33 listed in the InfoMine Suppliers Database.

Here are links to a number of other related and/or relevant papers I came across researching this topic: