Photos might help but I'd suspect the landscape timbers are rotting and the whole stairway structure is being undermined. 15 years - or more - is a long lifespan for these and I'd consider their replacement with something a bit more permanent.

This isn’t a problem that can be solved without seeing it…and finding another method or material may be called for…is it possible to just build steps with the pressure treated material?..is there room for large wide steps…that are not tall so you can use for wheelbarrow,etc. If needed

Describe in exact detail how you built the base that you previously excavated and re-tampered.

How many inches down does that base go, from the level where it starts to bear weight?

When you excavated down to the clay soil, did you put any kind of groundcover on the clay soil before you started to pour in rock?

What is the dimension and style of the rock that was used in the base? Can you take a photo of that rock?

How many inches of that material were placed into the base before you started with a sand layer?

Does the current base measure as level - using an instrument not your eyesight - at all places and orientations around where it is sinking?

Visually, does it look like the base itself is sinking, or is the base perfectly level and the timbers themselves are giving in? It's hard to visualize the timber support system without photos.

The last time I rebuilt the stair/walkway I removed the paving bricks, about an inch of sand and then varying amounts of crushed stone until I came to what seemed to be undisturbed clay soil. I live in central Massachusetts. The entire yard has a varying slope. Depending on where along the way I was working I dug down from 4 inches to around a foot to reach the soil.

I did not put any type of fabric on the clay although I tamped it aggressively with a 30# tamper. I did not remove the PT timbers, but worked on each horizontal section at a time.

On the tamped soil, I threw in 2" of asphalted crushed stone that a local landscape supply yard recommended. I tamped each layer of this mix thoroughly before adding another, to fill up to about 3" below the tops of the timbers. I screeded the leftover sand to the level of the bottom of the pavers, laid them tight and swept fine sand into the cracks.

The surrounding timbers appeared to be in good shape with little rot so I felt comfortable working to them.

@James Eddy Woodworks The timbers appear to be just framing around the edges. Those are pure decoration, not a support system for the pavers inside each frame?

I think the problem is your base is failing. You have issues with base depth and base lining preparation.

Let's talk about base depth. At a high level, the common wisdom is that a paver patio should use a base that is four to six inches deep. A driveway is typically done with a base that is eight to 12 inches. I had another thread here recently where I was trying to cheat on those rules down to two inches, and your case is a great example of why my question was misguided. I think a properly prepared base that is a *minimum* of six inches will solve your problem.

Let's talk about base lining. I know this is hard to believe, but rocks sink into clay over time. If I were to put some rocks on top of clay soil that is exposed to water, and if I were able to do time-lapse photography where each frame was one month of time, what you would see in those videos is rocks slowly sinking into the clay, almost creating the illusion that the clay is a soft mud when of course it is rock hard. What happens when you have a thin base and no liner on the bottom is that parts of your base sink into the mud. This asymmetry creates sections of the base that collapse. Eventually, the pavers collapse in or distort away from level.

Let's assume you want six inches of base material. On top of that six inches, there will be one inch of sand. The pavers go on top of that. So plan the excavation depth using those numbers, with your goal being to get the tops of the pavers where you want them. It will be a lot of digging, and you might want to hire someone to do that. Excavate one ditch and implement it and install and test that before investing in excavating the rest. Or hire a pro to do it all for you.

Once you have excavated the ditch - let's say it is eight to nine inches deep - you need to line the base with landscape fabric. NEVER use plastic. That will block water drainage out of the base. You want to use a landscape fabric that will pass water but block the clay and stone sides from mixing. The cloth helps to spread out the load as well, so any "sinking" into the clay will take place across a broader area and be less likely to cause paver failure.

Assuming you want a six-inch base, you put no more than four inches of base material and then tamper that completely Then two more inches and tamper again. Then you can place one inch of bedding sand, and your pavers go on that. The final tamper of the pavers is important because it forces the bedding sand between the pavers.

95% of contractors use as a base material 3/4 crushed rock that includes small sizes down to fines. It compacts well but does not drain very fast. I prefer the "open-graded base" method that Unilock uses. You can research that on Youtube and the Unilock website, then post a separate question about it here.

Thank you for a very detailed answer.

I had thought that the stone and tar mixture would compact into a fairly dense material that wouldn't sink into the clay.

I've done some preliminary research on Unilock and open graded bases. I'll do some more as I wait for warmer weather before I excavate a section to see what has happened. I may have more questions then.

As I was reading up on open graded bases, it occured to me that since the walkway is on a mild to moderate grade, that an open base would function much like a french drain and allow water to pass downhill under the pavers.

Jim

@James Eddy Woodworks I have never heard of stone and tar being used for a base. How could that possibly drain water well? Tar if anything is used to waterproof things and might impede water flow.

The normal way you see drainage done with open-graded base is to place a perforated pipe running downslope at the bottom of the base. Your point that you have effectively a kind of waterfall configuration, where water could dump from one level to the next, is interesting. I doubt that is the "professional" way to do drainage, but it might work.

This video shows both improved drainage on open-graded base, as well as the use of perforated drainage pipe.

https://www.youtube.com/watch?v=Ld4VCyHEHt8

@James Eddy Woodworks By the way, in a pure "open-graded base" solution, the top layer is not sand, but instead 3/8" crushed stone with irregular surfaces. Technically, you should put a geogrid between the 3/4" crushed stone layer and the 3/8" crushed stone, to prevent the 3/8" stone from moving down into the 3/4" layer. Some videos show ways to use that extra geogrid layer to lay down cement at the edges of the paver patio and prevent movement of the pavers outward.

It doesn’t look like the brick method will ever look nice..or stay level enough to not be a hazard….why not use the framework by adding pressure treated 2x6’s loosely spaced on top of the existing frame ..the bricks removed to be used elsewhere