Retaining wall movement can be due to a variety of reasons. The most common is a failure to provide an escape route for ground water that may collect behind it. Here in the Northeast, with the normal freeze/thaw cycles of winter, knowing that water expands as it freezes, that can mean substantial movement as the years pass and water pressure behind the wall isn't taken care of. Usually that happens when the wall creates an impermeable barrier...like concrete or mortared stone without free draining gravel backfill and weeps or drain pipe behind it. It can still happen with dry set stone, but that pretty much will only happen when the backfill is a loamy clayey soil through which water can't flow easily. Other reasons can be a change in the load above the wall, or just plain failure of the material with which the wall was constructed. Chances are that your wall moved because of the former...water not being able to drain though it...but that's only a guess, as internet diagnosises are subject to being. In any case, it sounds like you're faced with dismantling the wall, removing any impermeable backfill within a foot of the back of the wall, and then reconstructing the wall with an appropriate batter (tilt towards the back - not a lot, only about a half to an inch of tilt per foot of wall height), and backfilling with clean gravel. If the stone is to be mortared in place, you'll need to install weeps about every 8 to 10 feet about 6 inches up from the bottom of the wall on the low side. Weeps are really just pieces of small diameter pipe (1-2 inches) with screening covering the buried end that project through the wall (front to back) allowing any groundwater that collects there to drain away. If the wall is to be dryset, some folks might still recommend the weeps, but the spaces between the stones should provide ample area through which the water can flow.

The answer to your problem is quite simple. Use a commercial grade weed fabric to line the backside of your wall to prevent dirt seepage. This is the same method used in block retaining walls also. Just to note there are 2 types of commercial weed fabric out there, good stuff and JUNK!! The GOOD commercial grade weed fabric is a plastic MESH with fuzzy black material on one side, I highly recommend using this stuff. The other "commercial weed fabrics" out there look like a big sheet of perforated plastic and they are flat out JUNK, it doesnt hold up for long and it tears very easily so stay away from this stuff. The good stuff is available at Davis Seed Co. and its a 3ft x 250ft roll for $101. Smaller rolls of the same stuff may be available elsewhere. If you dont mind me saying so, I see a problem with your current plan. That problem is that clay retains alot of water, if you live in a area that drops below freezing during winter months the clay is going to freeze and heave(expand) during the winter. Which will in turn push you wall over eventually. To stop this from happening what you need to do is build your wall, line it with the weed fabric, than instead of back-filling with clay, use "Pea stone" as a back-fill. Pea stone is very small rounded stone. It has good drainage which helps prevent water build up directly behind your wall, and also has the ability to shift. When the clay behind it heaves rather than pushing your wall out, the pea stone will shift alleviating some of the pressure put against the back of the wall. This prevents ground heaving from pushing your wall over. Pea stone is very inexpensive per yard and will make your wall last alot longer. Stone drip edges are also a good solution for what your trying to do. All you need for that is plastic or metal edging, roll of plastic(for use as a weed barrier)and a decorative stone. If there are not plants inside your drip edge using plastic is the way to go, if there are plants inside your drip edge you will need to use the weed fabric I discussed earlier that way the plants will still recieve water. Hope this helps. Good luck with the project. If you need any other pointers on the wall or drip edge drop me a email at dlarose@nycap.rr.com

That root you see is most likely seeking the moisture, which the mulch is holding. Three years ago, I planted three single trunked Heritage river birch trees in an area which sometimes gets flooded to an about 4 inch depth whenever the area experiences frequent rain periods. In the hot humid summers the soil dries out well. The Soil is a silty loam, and when I planted these trees I found that the soil had been hard packed by the frequent yearly flooding. The planting turned out to be a lot of work because of the effort it took to dig that hard of soil. This was especially hard since at the same time I installed 5 "T" stakes for keeping the wind from whipping the young trees and tearing their roots. This is on a vacant, slightly larger than half acre residential lot. That means any watering I give any of the trees I plant on that lot has to be trucked there. For this reason, I have not supplimental watered these riverbirch trees. Still, during the last three years, I have never seen any surface roots, growing from these birch trees, along the surface of the soil. Three years ago I could not find any clumped Heritage birch trees at any of my local nurseries. And many of my local nurseries did not have any birch trees for sale. The nursery owners, at that time, told me that the demand for birch trees had drastically decreased, so it was no longer affordable to stock birch trees. The reason for this decreased customer demand? Several nursery owners told me that customers found out that roots from birch trees they had planted did too much damage to their sidewalks and other concrete structures. Even though no nursery would sell me a 10-15 gallon sized clumped Heritage River birch, I still wanted that tree for this area on my lot where no concrete or asphalt stuctures would ever need to be installed near enough for the roots to damage. That is why, once I found a nursery who had 3 gallon sized potted single trunked Heritage birches for sale, I purchased three of them and planted them in a triangle about 2 feet apart. Since planting, our area has encountered a couple very dry and hot years with drastic sudden drop and rising of temps during those year's winters. We even experienced one winter event where light rain fell off and on for three days in 28-32 degree temps. This created heavy layers of damaging ice on the local trees' branches and even trunks. Many of these trees suffered massive branch failure, and some on some of the trees in the area, their main leader trunks even snapped off. Last winter when I checked my property for ice damage, I found that my Heritage birch trees needed to have a few broken branches pruned away. After pruning they looked quite bare. This years' growing season has turned out to be one of the wettest years I can remember occuring locally. So now those well established birch trees have recovered and look the strongest I have have ever seen them. Even with all the stress my trees have encountered, they still are not showing any signs of growing surface roots. I do not know why. Maybe it could be due to fact that, after the breakdown of the first application of mulch I gave them, I never reapplied any mulch under these birch trees. The only reason I have been able to get away with not continuing to mulch them might be because, growing nearby my birch trees on the neighboring property, there is a multi-trunked pecan tree, which provides afternoon shade for them.

Yes, I have seen the three tree method that some growers use to produce multi trunk ornamental trees. I even planted a paperbark maple tree that had been handled like that. One of the paperbark trunks dominated and the other two died off each at different times. It did not seem to matter what I did to try and save those, they still died while the dominant one thrived as if no stressors of any kind existed for it. Also planting three young trees together is the technique I myself had tried to get a multi trunked River Birch tree. Five years later, one of those trunks thrived and now has a 3 inch wide trunk with beautifull pealing bark, and which has grown to about 10 feet tall. The other two trunks are currently about 4 feet shorter, also with some bark pealing and with only about 1 inch in trunk diameter. This drastic imbalanced look is why I dug the two shorter trunks, and now have them in bare root form. I have seen other reports of people having success at growing a new single main leader by the cutting back nearly to the ground of the damaged main leader. Of course those damaged trees are usually fairly well established. So I do not know why I could not do the same thing with my single trunked birch trees. That is, as long as I do it at the right re-established root development point. Is this truely a bad idea? I do think that my trees are nearly twice as mature than what past growers generally used for clumping out multi trunk birches, but if I do not begin the process to eventually prune out the current one inch wide trunks for these two trees; they too will likely dominate too much over any new sprouts which I might get to pop up out of each of their own root balls. I have also seen what you suggested concerning the trunks of clumps spreading to far; that is on a neighbor's birch that has only two leader trunks, and which is almost 30 years old. Is my only other option to keep them as single trunked trees. Is this delima, also why I have had such a difficult time finding young multi-trunked Heritage birch trees at my local nurseries. Also am I wrong in thinking a three trunked rootball would tend to grow more slowly and end up with less height potential if all three trunks emerging from one rootball grew at a relatively well balanced rate, and no trunk is allowed to remain, which seemes to so drastically dominate over the other two.