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French Drain/Sub drain with varying soil types

French Drain/Sub drain with varying soil types

French Drain/Sub drain with varying soil types

I have read with great interest thread193-290956: french drain and surface drain in one and thread158-162089: French Drains where the design of combined french sub-drains/surface drains are discussed.

Past Solution for context:

Over a dozen years ago I had to replace an old clay tile drain system that had still been functioning decades later (albeit I suspect at a diminished capacity) until my sewer contractor dug into it despite my providing the drainage as-built I received from the previous owner to the contractor and warning the contractor to stay away from it while digging...  The contractor attempted to fix the resulting problem they created but when it became clear to me they weren't going to adequately address the mess I ended up chasing them off the job.  I designed and instlalled the following solution:  A network of 6" SDR35 Sewer pipe main truck lines with wyes every 20' where a 20' foot french sub-drain connects and drains into the main trunk line.  The french sub-drain(s) consist of 6" ADS smooth interior wall perf pipe (for maintainability) surrounded by 3' wide by 1-2' high of 1 1/2" drain rock wrapped and sealed completely with silver filter fabric which is stapled and folded at all seams with a cleanout that comes to the surface at the end.  Each run of trunk line has a cleanout at each end too.  The main trunk lines also connects to runoff from an artesian well, foundation drains on the slab of the daylight basement and downspouts. The previous onwer told me the clay tile system needed to be cleaned out regularly to remove roots from a nearby redwood tree that had infiltrated the system so eventually the previous owner chose to chop down this enourmous tree to alleviate the problem - but it had since grown back 3 fold - three 12 inch suckers from the stump.  I chose to have 20' "short" runs sub-drains with a 6" trunk line with numerous cleanouts to isolated failures in the system, get the water out quick and provide frequent inspection/access for maintenance so I could keep these trees.  I put cast Iron cleanouts at grade so they are easy to find with a rebar finder after the grass grows over the cleanout.  So far this design has functioned so well without maintenance that I haven't even looked for roots yet.

Current problem:

Another Previous installation - I am now addressing another area of the property where there was no as-built and I only had 4 different pipe outlets that daylighted to start looking from - very difficult to know what is down there with that information. Additionally I knew one of the drainage outlets had roots from the nearby willow trees growing up the outlet and had become too plugged internally to be snaked out. So I had to do a forensic dig with exploratory trenches.

I discovered numerous past drainage installations that used a variety of materials: 6" concrete pipe, 4" concrete pipe, 4" clay tile, 4" PVC perf, 4" flexible perf with holes, 4" flexible perf with slits and 6" perf with slits.  Some of the drain pipe had very little pea gravel surrounding it, some had cobbles, some had 1 1/2" drain rock completely surrounded by filter fabric. The whole thing is like a museum of schools of thought on how to design a french sub-drain.
I also discovered the soils varied a lot along the trenches I dug.  Generally from surface to the bottom of the trench there is a topsoil layer, a hard clay layer, a soft clay layer with fine sand that dissolves and turns into soup in water, various colored sand layers that convey water, a gravel layer (think sandy small gravel "beach mix") that conveys water and then a layer of glacial till - hard compacted gravel sand mixed with larger cobbles that settles into any excavated hole and sets back up like concrete rather quickly.
I discovered the french sub-drains for the most part had filled with silt and were sitting in the clay layer on top of the sand layer.
The roots from the nearby willows had stayed out of filter fabric that completely wrapped the 1 1/2" drain rock ~1' wide by ~1' high surrounding the 6" perf pipe with slits but had actually penetrated the fabric at a few points and then completely enveloped and plugged the perforated drain pipe (the roots had travled up the outlet as well). There was also clay on the outside of the filter fabric.  Not sure if this had plugged it up since there was water draining so maybe it just plugged it at this location.

After I understood the soil layers better I dug the trenches deep enough to reach the water carrying level and the gravel sand mix layer wherever possible (about 3-+4') with a minimum of 1" fall per 10' along the toe of the slope and greater elsewhere to follow the topography.  It all drains now with what are basically open ditches but I want my lawn back. smile

My proposed solution:

This design is again a combined sub-drain/surface drain that builds upon on the previous success of the first sub-drain I installed over a dozen years ago.  I have broken up the trenches into 21' self contained french sub-drain sections.  Each section has 18" catch basins for inspection/maintenance anchoring each end which also provides surface water drainage where necessary in the lowest spots through the top of the basins via a grate.  In each section I planned 2 <20' runs of 4" perf pipe: One perf pipe at the lowest possible elevation that wyes off the main 6" trunk line on the uphill side with a tee to the surface for an inspection port and a second perf pipe on the downhill side fo the trunk line positioned slightly higher and terminating at both basins.  The second perf pipe is redundant and lets me know how much water is being pooled and pulled from the ground.  The overall goal is to drain the water out quickly and frequently in a fashion that can be monitored so there will be no mystery as to what is going on underground and it can be easily maintained going forward.  If a section diminishes in effectiveness or fails from roots etc. then an individual section can be escavated to replace only the filter material of the french sub-drain while reusing the underlying trunk line and basins at each end in the section being repaired.

At this point I am set on the design of the main trunk line and basins and they are installed now so after months of work I am poised to finally put filter material in but I don't want to get this right the first time so I don't have to revisit this again.

I want to make sure I have the right filter material and pipe combination for the soils and water conditions present in each of these sections:

There are a total of 8 21' sections where the water carrying layer at the bottom of the trench varies:
1 Section is mostly clay interspersed with sand seams carrying water
2 Sections are mostly gravel with a lot of water - like springs - coming out of it
3 Sections are in a large seam of sand that carries a lot of water topped with fine clay that oozes
2 Sections are a combination of Sand Gravel small amounts of water

I had planned to use 1 1/2" drain rock wrapped completely in silver filter fabric and cut the roots back from the willows and install root guards.

Then I talked to a geotech engineer who specified Mirafi 140 (thicker black filter fabric with the consistency of felt) on my neighbors french sub-drain. The engineer said this fabric is far superior for keeping the fines out and not clogging so I decided to use Mirafi 140 instead.  Technical Specs: http://www.tencate.com/TenCate/Geosynthetics/documents/N%20Series/TDS_140NC.pdf
I talked to the vender who sells both the silver silt fabric and the black fabric who said they difference between the two is that water flows throught the Mirafi 140 black fabric more slowly (which is a positive according to the engineer I talked to).

Then I talked to the drainage expert at the city who suggested not using filter fabric at all...


What filter materials should I use for the french sub-drains in each section? Is there a solution that works for all of the soil types or should this vary section to section?

Which drain Filter fabric should I use: Encase entire French drain vs. Only on top vs. drain sock around pipe vs. None.
If used: What Type/brand of Filter fabric?
Drain Rock vs. Sand?

There does not seem to be universal agreement on what is best.

Advice from the other threads: oldestguy's recommendation to use sand only is appealing since it would be easy to install and easy to dig out to fix if necessary (access to this location for heavy machinery is difficult) - which he says won't be necessary since it sand is so effective.

RE: French Drain/Sub drain with varying soil types

Step 1: Determine whether you are battling infiltration of surface water OR rising ground water (which may be related to the former).

Step 2:  If a surface water problem, provide surface drainage to get the water off your property (or into a pond where it won't bother you).  (Bear in mind also for thick sod and such the "initial abstration value" is about 1/2 inch - i.e., the first 1/2 in of rainfall doesn't contribute to runoff.)

Step 3:  If it's rising ground water, install interceptors like french drains or such.  Such horizontal spacing is determined by the soil type (horizontal permeability), how successful you want to be and how deep you dig.

One other scenario:  Are you dealing with perched water?  If so, it's a bit more difficult to throw around calculations.  Perched water is governed by the vertical hydraulic gradient and the vertical coefficient of saturated permeability (typically 4 times slower than the horizontal permeability).

I haven't read your long post.  I couldn't visualize all the work you've done, but understand that you have a bunch of trenches.

I'd call an engineer and get this all figured out.


¡papá gordo ain't no madre flaca!

RE: French Drain/Sub drain with varying soil types

Thanks for the reply.  Answers to you questions:

Step 1: Battling rising ground water from underground springs.  The water bubbles up to the surface through sand when it encounters a large seam of clay and other less permeable soils.
Step 2: Providing limited surface water drainage because I am expecting this to diminish once the ground water is addressed.

Step 3:  Agreed.  Now trying to determine the best filter media - sand vs. rock and filter fabric vs. none - for the french drains and soils I have.

One other scenario: It seems the water is coming to the surface from "springs" when it hits a layer of clay.  I am intercepting the water by placing the french drains along the toe of the slope perpendicular to the flow of water before it reaches the clay seam and sand where it is seeping out.

I attached another diagram showing the triangular drainage system.  May help to visualize it as a baseball field.  The water flows towards the lowest point at the vertex of the right angle "Home Plate" and there is a minimum fall sub drain to intercept the water along the hypotenuse "Outfield fence".  

RE: French Drain/Sub drain with varying soil types

Nice to see your thoughts.  I think if you are fearful of more roots, then quite obviously the heavy felt is the best, but of course labor intensive.  Certainly some filter is needed and the city comments are typical of those that install them that way and never are faced with fixing things.

Is there any chance of your springs feeding the system backwards causing problems upstream?   Sometimes systems can be causing more problems than they fix, such as having ice all over the place in winter at the outlet.

One thing I have used for drawdown type systems, where cut-off of flows is not possible, is to calculate the  surface of GW gradient that results from horizontal flow to the pipe.  That, combined with a limit as to depth of system, sets the spacing required.  For instance with a house basement, you may need more than perimeter drains, depending on your permeability of a drainage blanket under the floor.

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