French drain around basement perimiter from inside 3

[mdjailani]

Hi -

I had a french drain with 2 sumps installed about 3 years ago to replace what was existing when i bought the house. I have been having the following problems:

1. Summ pump discharge pipe (1.5 inch) keeps clogging with clay
2. Water comes up the concret basement floor through cracks
3. Pump is always working rain or not.

I don't have a problem redoing the french drain. What is the proper way of doing it to keep the system from clogging up given the high content of clay that I have.

Thanks for all your help

 

[MiketheEngineer]

Something might be wrong with your float switch.

Usually a "sock" is placed around the French drain piping. This keeps out any soils.

Is the piping low enough - should be down along the footing or lower.


Or maybe you are setting on spring and if so - good luck....

 

[oldestguy]

Hi: This is exactly what one can expect when installing the "french" drain per the usual architectural details. I'm surprised it took as long as 3 years to fail. For some reason these dumb details continue to stick around in design manuals and many current engineers still do the wrong thing..

What was wrong?

Very likely you used gravel as backfill around the pipes. The void space diameter in gravel is roughly 1/5 the particle size. What is the size of the soil particles near that drain? Very likely very much smaller. Gravel is not a filter. You may even had a sock on the pipe. Sometimes they clog over and very little water flows.

The answer to the situation is completely surround the pipe with a filter system consisting of fine aggregate used in concrete, usually called "concrete sand". You hardly can make a mistake using that coarse sand material around the pipe. You won't even need that sock if the slots in the pipe are around 1/16 inch or so. Get the sand at your local ready mix concrete plant.

To simplify the job, dig the trench, place the pipe and back-fill around the pipe and to a foot or more above the pipe with concrete sand. Of course you need a little slope in the pipe if you can and place it as low as possible. If that does not fully do the job, trench through the basement floor and place these sand backfilled pipes inside also, spaced no more that 15 ft.

Other things to do, fill above the trench sand backfill outside with earth and slope top away from the building, extend downspouts out also. In a real bad situation of infiltration of the surface water into the ground, that surface may need waterproofing and we can talk about that later.

There are numerous threads on this subject and I seem to have been preaching this very thing since I joined here, and of course some 50 years prior to that to architects and engineers. We can blame it all on someone starting the drain idea using gravel, maybe in the dark ages, and probably in France. Curiously how many of us in a soils engineering course ever had the prof bring up the principles of filtering? I never heard it once in the many courses I took.

 

[mdjailani]

Thanks so much for the valuable informatiom oldestguy. I thought of using the concreate sand but I am worried that the voids in the concrete sand itself will be clogged by the fine clay. Is this a ligitimate concern? I have a freind of mine -a civil engineer- telling me that the concrete sand filter needs cleaning every once on a while. Your thoughts are greatly appreciated.

 

[oldestguy]

Hi again:

Well I must disagree about cleaning. Installing of openings for cleanout of drains used to be something we would spec, but it never was required (to keep things flowing), so we than left out the clean-out requirement as more jobs were done. Once a few fine grains get in the pipe, the coarse ones bridge over the openings.

If you wish to research this use of concrete sand, see if you can find a US Corps of Engineers paper published about 1938 out of Vicksburg Research Station dealing with design of filters for sub-drains. That's where it was determined that concrete sand will function properly in all soils. Those fat clays, composed of very fine grains, are so well "glued" together that they don't erode enough to worry about.

US Navy NAFAC manuals have design guidelines once you know the grain size distributions, slot sizes in pipes, etc. However, take my word for it, once installed, when concrete sand is used, you can practically never have to fix it again. Of the many systems I've been involved with, not one has failed to my knowledge.

You have to keep in mind which way the water comes from and how it flows through soils. Use of drains to "draw-down" the water table, such as placing one drain out in an athletic field, will barely have any effect. By placing the drain where it can cut off the flow, as for a moat around a castle, is the way to go, if you can.
Sometimes you do a step at a time, seeing how it works, before going whole hog and installing a bunch.

You may find some touting the benefits of surrounding a gravel backfill with a geotech filter fabric. Theoretically it sounds fine and may work fine. However, it is labor intensive. The actual opening size of the fabric has to be very small and what you buy locally may not suit.

I have the theory in construction "If it can be done wrong, it will be done wrong". Using concrete sand is so simple that one really has to nearly intentionally goof things to have it not work. I can't say the same about a gravel backfill surrounded with filter fabric job.

 

[Ron]

My standard detail for a "french drain" is to excavate a 24"x24" trench. Line the trench with filter fabric to keep sand and fines from migrating into the trench. Bed the bottom with No. 89 stone, to depth of 3 or 4 inches. Lay in perforated or slotted drain pipe with sock. Fill to at least 18 inches of trench depth with No. 89 stone. Fold filter fabric over stone and backfill with native soil or sand.

Little or no migration of fines will occur to fill the void space in the stone and all will result in free flowing drainage.

 

[oldestguy]

Ron:
It probably works, if done that way. However, in a trench that has flowing water, some caving present, or is quite deep (cave-in possible?), I prefer to use the concrete sand. Then, you can follow closely to the excavator and do not have to leave much trench open.

As I said, on construction if it can be done wrong, it very likely will be done wrong, especially when it takes some thinking to do the job right.

A common backfilling procedure is done with a front end loader bucket type machine and some operators think that if a little is good, them maybe a lot is better, leaving very uneven filling and laborers not on hand to fix things.

With concrete sand this can happen also, but no problem with how it affects the job.

Never had one fail yet the way I have done it. Washed concrete sand is the same price as gravel here.

 

[mdjailani]

Thank you all for the valuable information. Here is what I am planning to do to fix my wet basement problem:

1. Dig a 2ft. x 2ft. trench around the inside basement perimiter.

2. Lay 6 in. of concrete sand at the bottom of the trench while creating a pitch towards the sump.

3. Lay 4 in. perforated pipe in the trench and back fill trench with concrete sand

4. Cover the trench with concrete.

A few questions:

1. Is 4 in. of concrete sand between soil and bottom of the pipe enough? I have a lot of that super fine grained clay coming in the sump which clogs the discharge pipe and burns the pump. Also, most of the water I get is from the ground and not the surface.

2. Should I make holes in the sump pit to let water in? My existing sump pit has holes letting water in and I think this causing the pump to work all the time because underground water keep coming into the sump pit through these holes.

3. Is there any chance water flow through concrete sand be slowed over time as fine grained clay is filtered. Can fine grained clay clog the voids in the concrete sand?

4. Should I compact the concrete sand in the trench before I pour concrete to cover the trench?

Again, thanks alot for all your help. I really appreciate it.

 

[Ron]

If you are excavating in clayey soil, be sure you line the trench with a filter fabric so that you don't get fines migrating into the concrete sand. I would use material coarser than sand, but that will work.

 

[cvg]

concrete is generally a good "filter match" against fine grained soils. Use of a secondary geotextile filter fabric is probably not necessary. In addition, filter fabric clogs much easier than a granular filter such as sand.

If you are making holes in your sump pit, than make sure it is completely surrounded by a layer of filter sand.

you should make sure your sand is near the optimum moisture content when you place it and it should be compacted.

 

[oldestguy]

CVG would get my vote. However, unless you are below and very close to footings, compaction of the sand hardly seem necessary to me. I'd just walk on the backfill and stomp it down by foot or you may squash the pipe with heavy compaction. The main need is to keep good support for the nearby footings.

On that score, check the pipe before you load it up at the lumber yard. When this stuff sits in the sun it gets brittle. Just step on it and see what it does. If it breaks, go elsewhere.

As to how thick a layer you need for using concrete sand as a filter, I have never seen where too little, other than none, is the case. In my graduate work when I had a research project to see how sub-drains do under highways (1954-6) I would dig into some of the backfill where water has brought along silt and clay and there was a sharp demarcation line at the junction of sand and clay, with no noticeable clay in the immediate voids of the sand. On that basis, I'd have to say "Use at least some concrete sand". Practically a few inches should take care of uneven excavation, etc. Amazing how this system can be done without being too fussy about it.

Using Ron's liking of fabric, at the tub, can you dig down to those holes, line the tub with fabric on the outside and then use the concrete sand as backfill?

If you feel the tub always draining nearby soil is of no use or a continuing problem, before you cover up your job with concrete, do some trials and see if you are causing problems by sealing the holes. Remember that you may have to use a few layers of fabric if what fabric you have may rupture from the forces on it at the hole.

As to the size of your trench, 2 ft. wide seems like a lot wider than I would do it, as long as I could get the drain pipe in proper location with suitable thickness of concrete sand around it. Again, I'd not close off things until I was sure what I did was working. You may have to add some drain lines farther inside the basement, since it sounds like you may have some water by-passing underneath the perimeter drain area and rising up farther inside.
Keep in mind, the best drainage systems cut off the water before it gets to cause problems, rather than trying to "draw-down" the water from a downstream position.

While you are at it, before you leave it as "suitable" drill some holes in the floor and see where the water table is. Do this before you do much of your digging and pipe laying, so you can judge how well your job is working. Seeing those results may help you to plan on any secondary lines.

Finally in your digging, remember to be careful of not undermining the footings. You might decide to partly backfill the job as you go along and them return and lay the pipe, taking only a short time to dig down, lay the pipe and then re fill. It would not be out of the question to ram a mix of concrete sand and gravel (a typical mix of concrete, but no cement)sideways under any footing appearing to have lost some soil support. The usual mix of concrete is 1-2-4. Cement-sand-Gravel. That means twice the amount of gravel than sand.

I hate to allow any gravel on these types of jobs or well meaning workmen will use that instead of the concrete sand. It shows how well indoctrinated people can be on an error prone procedure.

 

[oldestguy]

Another point.

We engineers have been concentrating on drainage and filtering. We missed an important point that CVG touched on.

That basement wall is retaining a lot of earth backfill. It needs support from all edges, top sides and bottom, to resist that lateral force. If saturated, that force can be three times the non-saturated condition.

Digging a 2 ft. deep trench inside the wall opens the door for a significant problem. That footing needs not only vertical support, but lateral resistance. A significant portion of the resistance is likely supplied by the floor slab. Of course the earth below and beside it also helps.

As you dig out that support, the wall may move inwards.

Obviously there are two things in the plan that need attention. The earth lateral support and the slab lateral support.

So, as you remove that support, it would be a good idea to immediately replace it with some temporary measures. For the earth, immediate backfill with sand will help and CVG is right that compaction will help. However, I'd put more faith in using the slab for lateral resistance.

I'd substitute some struts jammed in the cavity where the slab was. That could be 4x4's as struts between the wall and the remaining slab, say every 6 feet. You might consider galvanized steel pipes (1-1/4 I/D.) placed low enough to be left in all the time, even placing the concrete over them. Wet concrete placed when you close things up won't do much until it sets up. Be careful not to do too good an effort on this bracing score such that you move the slab.

Then, maybe digging 2 feet down is not needed either.

Digging to base of footings may be sufficient. Many a drain has been placed at that depth and has done the job. The deeper you dig,the more risk you create for developing wall problems. Conversely, the deeper you dig the better the drainage.

Can you work in sections, say 10 ft. at a time, completing the whole system, part at a time?

Lots to think about.

Finally, how long will this job last before it needs replacement? I've never had to replace one, never had one reported as failed if done as recommended.

Age here is 81.

 

[northengr]

oldestguy restored my faith. I was reading this thread and was worrying about loss of support for the retaining loads on the wall and the footings. Having been paid to assess failed and failing foundations this worried me. Also if the wall is block instead of poured, then I have also seen decades of wet walls freezing and therefore gradually deteriorating towrd failure. As to digging near the walls, as Oldestguy says if you can stay even or close to the height of the footing that can prevent undermining and problems there. As to the loss of the horizontal support at the base of the wall, this too concerns me, but sounds like you already have this opened and un-supported. The slab is the usual componenet for resisting the horizontal load, but some masons used rebar into the footings or keys which would lessen this concern. I hope you are doing this in a dry period and non-frost period so the loads on the utside of the wall are minimized.