The drain rock sounds like a single size filter material, that will allow migration of fines. I would replace it a well graded filter material.

Sounds like you need a typical catch basin design total system as for roadways. The catch basin (entry place) has a deep area well below the outlet elevation so that sediment can collect there and not continue down the drain pipe system. Periodically (every few years), those deep basins are cleaned out by a jetting and suction method or even by hand. You can see when it is needed by visual inspection. For the final infiltration end of the system I have seen them as a rock filled excavation with a graded filter on top, such as a layer of concrete fine aggregate about a foot thick. The water is discharged on to that sand layer and soaks in (ponding). Cleaning is done by removing a few inches of that sand when it no longer lets water soak in. Your system is down out of sight and when totally plugged will be impossible to do anything other than replace the whole thing. That design (rock enclosed in fabric) is usually done for collecting ground water, not the reverse. What fun later working with that fabric enclosed rock zone!!!

Thanks for the reply that makes sense. So, I have a follow-up question. Why is rock placed around perforated dry wells? This rock will inevitably get silted up just like any other unmaintainable part of the system. It's extremely labor intensive to move, why not just have the dry well drain through the bottom or wrap the well in geotextile and backfill with sand or some other fill used to backfill around the dry well?

I understand it will help with infiltration in the short-term but the long term result will be as if the rock had never been installed in the first place?

Usually when one is trying to drain soil water or infiltrate into soil you want as large an area as possible in case you have some plugging of the system and still have some parts functioning.. For draining down out of the bottom,fine,but into what? Suppose if is a tight clay and not much water is taken, so using the side area discharge gives more area. The open graded rock with large voids will carry a lot of water, hopefully then absorbed by nearby ground. However, even that system can get plugged if a lot of soil comes along with the storm water. There is some benefit with the perforated crock in that you can at least clean out some sediment that drops off there. That sediment there negates much use of perforations in the bottom by the way. It isn't perfect and can fail to function in time. Some municipalities would want the overall system designed. For instance at the dry well would want to see a double ring infiltration test so as to size the area of infiltration(rock zone). Rainfall calculations and pipe sizing would be in there also dealing with the quantities of water involved.

Thanks again for the reply. I understand about the surface area. I thought in the event of tight clay you would simply not rely on soil infiltration, and instead you would need to store the entire quantity of water in a large maintainable tank/ dry well until it very slowly infiltrates the soil. Nearly all run off water is going to contain some sediments and any unopen system will become clogged with with sediments given enough time.

If the rock is a required part of the design - It seams like these systems are designed to fail and be replaced?

The bottom of a dry well can be maintains by jetting and pumping to expose good soils once it has been clogged. I don't think you can jet and clean rock effectively though?

I'm just struggling to understand why such a labor intensive investment ( the rock ) is done when it's impossible fix/ maintain? It seams so commonly used my assumption is that it is designed to just last a fixed period of time and then be replaced? Do municipalities/engineers have a period of time these systems are designed to last?

It looks like most drainfields are designed to last 20 years or so? is this considered normal?

I guess we can blame the use of retention basins, etc. on modern "progress" reducing the flood flows, etc. Unfortunately sometime the use of dry wells and open graded rock for drains is done since parts of the properties only are considered. The plugging, etc. is not in the planning picture. Let someone else struggle with it down the road seems to be a common attitude, especially by architects. So, assuming this was about 1850 year and you didn't use catch basins, but let the water run as it did before developing the site, then no need for dry wells.