This is a dry stone wall that has been built without mortar or a poured concrete foundation. The site has several challenges:
- the area above the wall slopes so any rain or runoff water will go down the slope into the wall
- there are trees adjacent to the wall whose roots will grow
- the stone is irregular
- the frost line in your area is around three to four feet deep
From the picture below you will notice that a good compacted base is where you have to start. This can be 6 to 8 inches deep but more is better. Use a compactor to set the stone and then rebuild on top. Notice the 4 inch drain pipe with sleeve behind the wall at the base. This is another method to prevent water getting in.
With water and root infiltration and frost heave the only way to restack the wall so you don't have to do it again in a few years is to provide a good base and drainage.
Another thing you can do now is:
- dig a trench about 6 inches deep on the top slope which is parallel the wall and runs the length of the wall.
- This trench should be 6 to 12 inches back from the wall.
- Lay one inch of crushed 5/8" stone without fines in the bottom of the trench
- Place 4' drain pipe with sleeve along the entire trench.
- Back fill with more gravel, top with soil. This will prevent runoff water getting into the wall, freezing and pushing the rocks.
The filter cloth referred to is sold as landscape fabric or geotextile. As this product goes a long way towards preventing soil from washing into the wall try and get the thickest fabric possible that is UV stabilized.