To address the question of slippage or settlement, generally slippage results in cracks parallel to the alignment as well as semi circular cracks for a local slip failure. You also may see the bulging of the toe of the fill, trees, if any, will lean down hill.
Slippage down hill will usually result in a syrface scarp or a series of them, along with settlement. Cracks open perpendicular to the alignment and of course will let surface water in, but that is almost impossible to remove. So you prevent water getting to the fill to begin with.
Settlement due to no compaction or similar may show cracks not necessarily showing a pattern. A simple way to check on poor compaction is to drive rods or pipes into the earth and evaluate how difficult it is to do that. Lacking any experience in that, try driving them in undisturbed earth up hill from the cut to serve as a guide as to a standard.
A farmer's 4 ton 36" jack and a chain wrapped around the rod will get them out.
Bentonite is a name for a volcanic clay, the mineral montmorillonite. Most plumbing supply houses sell it to well drillers. It consists of very small platelet particles that are held together by electrical forces (a rough way to describe it) and there is great affinity for water into these platelet arrangements. This affinity for water, combined with the small size makes for a rather water tight material. It works great for sealing ponds, etc.
When you mix it with ordinary earth (usually particles shaped like rough balls, not plates), it fills the voids between them and seals the mix. It does not take a lot to fill these voids for a well graded material.
To mix with earth, almost any earth, but usually a sandy type, you can use any of many ways, such as s concrete mixer, rototillers in place and even hand mixing. For this job you might do mixing at an open area placing down a layer of silty sand, sand, etc. adding the bentonite on the top, and running roto tillers through the area until the mix is uniform Then haul to the job and spread. Compaction can be done, but not needed unless you are fearful of immediate wash away with storm water flow in the ditch. I'd tamp it manually at least. A 4 inch layer shuld help a lot. This also holds water so vegetation grows well there. Don't add water to this mix since it will gather it from nearby.
Like the posts say, you might want to check out the cause or type of failure before fixing it.
If the job was done by digging into the slope and just dumping down hill, then both settlement as well as slippage can be in the picture. More than likely settlement is present due to the loose stuff consolidating (although that word is not correct usage here). That's where I'd over load it rather than waiting a few years for things to stabilize.
Is there any way you can get the observations of neighbors, etc. to hear if there was any compaction equipment there? If all they saw was a dozer, a grader or maybe a back-hoe, then you know.
Of course you could hire a firm that tests these things.
