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Conflicting needs of improving permeability and also drainage

Jibu_J | Posted in Green Building Techniques on
I am remodeling my new home in San Ramon, CA, and addressing many historical flaws to the site and home. The goal is the make it a better home and hopefully, a greener one as well using a lot of the techniques discussed in this forum and others. 
The issue I am coming to you today is about site drainage to keep water out of crawl spaces and home foundations, and how to counterbalance that with permeability and water infiltration and retention rather than generating run-off.
About the home:
The home was built in 1976 and was among the last homes built here with a raised foundation and crawl space. The perimeter footing and all interior floor supports are sitting on 6-8ft deep piers. The homes in the same development built the following year were build with slab foundations. The issue I have detailed below is common for all the homes in my neighborhood with my foundation system.
The Issue we are trying to fix:
The most significant issue with the home is related to site drainage, site grading, and resultant foundation subsidence. The soil is very clay-rich and so infiltration is very low; this coupled with the fact that the site was poorly graded with most of the flatwork sloping towards the home meant that large quantities of water were pooling in the crawlspace. This poor grading and drainage have resulted in the metal jacks and wood posts that support the floor beams/girders corroding and rotting.
The Proposed Fix:
The fix for the structural issues is straightforward – replace all the existing jacks and posts with new ones; this was proposed and performed in consultation with a structural engineer. This fix, however, only addresses the resulting damage, not the cause. I now need to install perimeter drainage and should also fix the grading issues. 
Here is where my dilemma lies. In consultation with my contractor, we are planning to install a  french drain system on 3 sides of the home that have properties upslope of mine. The trench and pipe will be placed at least 6-8 ft from the home on all 3 sides and the grade from the home will be cut towards these drain trenches to redirect the building adjacent flow towards the trench as well. Subsequent to this rough grading and drainage work, the contractor recommends installing either concrete walkways or other impermeable solid surfaces as the finished surface – again to redirect run-off to the trench. The contractor wants to line this trench with geo-fabric and set a 4″ perforated pipe for in a 12-18″ deep trench all around the property; this trench will be placed 6-10ft away from the home and all the grading will be sloping towards this trench at a 2% or greater slope. The trench will have also have a second 4″ solid pipe to carry the roof run-off away from the home’s footings as well. The lot is relatively flat but we can generate sufficient fall for a 1.25 to 1.5% slope for both pipes redirecting the water to 2 infiltration facilities (dry wells) on either side of the property that will be found in the front yard (and hopefully integrated into the landscaping. He alternatively suggests doing a basin of some kind to hold the water (basically small “ponds” or “water features”. Should either the basin or dry well exceed capacity, the excess would sheet flow off the property into the street side gutter and the storm system.
The Dilemma I need help with:
I however would like to do a permeable paver system that has the french drain system integrated into it so we can retain more of the bulk water on site. The contractor is concerned that even with sufficient grading and the system to take excess water away from the home, the risk to the home for future water issues by using permeable surfaces is not a good idea.
We also face a technical issue on-site – per ICPI details and manufacturer instructions, the necessary depth up against the home to accommodate the standard permeable paver assembly without going below the grade beam for the home is between 13.5 and 15″ including the paver.  Per my structural engineer, I should not go below the grade beam against the home and so I have a maximum of 9-10 inches available for the entire setup. Is there anything we can do to reinforce the bedding assemble to thus reduce the assembly thickness from 15+ inches down to about 9 inches? I know there are geo-grids and geo-fabrics that are used to stabilize and reinforce slopes for retaining walls  – could they be an option here?
Also, even with the subgrade being sloped away from the home between 2-4%, should I avoid having the permeable paving system up against the home’s foundation as suggested by the contractor? Should I just follow his guidance on doing concrete or other non-permeable material, or can I do a waterproof membrane against the home that then tucks under the geo-fabric to keep water away from the footing and hopefully the crawl spaces?
i have attached details and info from ICPI (industry body for pavers) that helped me understand the assembly needed. 

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  1. maine_tyler | | #1

    It's unclear to me what your goal is with permeable pavers. You are trying to increase water percolation rates to keep it out of drain systems? If so (and that's commendable) it's not a good idea to use the area immediately adjacent to the structure as the perc bed. You want to get it away THEN do what you can to encourage percolation.

  2. gusfhb | | #2

    One would think that if your roof drains directly to the french drain system, that the amount of water added between the house and the drain system would be somewhat unimportant in the grand scheme of things, so permeable would seem to me ok. 15 inch build up for pavers? In California? Sounds a bit like frost prevention. If you can answer the 'why' of so much base, then you will know if you can do it.

  3. Jon_R | | #3

    > the risk to the home for future water issues by using permeable surfaces is not a good idea

    I agree - retaining more water in the soil near the house increases risk. You could safely retain water in cisterns.

    I'm curious why a crawlspace sump pump and management of crawlspace humidity isn't part of the solution.

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