French Drain Under My SOG

[BillBey]

While renovating a 23 year-old building, I found significant voids under the first floor slab on grade along a basement foundation wall(the first floor is larger then the basement). The building drawings show a french drain following the basement walls, which is directly under the portions of my sog with the deepest voids(as much as 30"). Is it possible the french drain is undermining the slab on grade? And if so, how do I stop the french drain from further undermining my SOG?

 

[dik]

French drains are generally used where there are water problems. I don't know what the soil is, but it is possible that the voids in the French drain are being filled with fine material from the surrounding soil, and creating the voids you have observed. I generally use a non-woven geotextile to prevent this. Is the void created consistent with the voids in the French drain? Wondering if the voids have been caused by the above scenario...

Is it possible to excavate and remove the French drains or construct them with a geotextile? Why are they there? It would then be a matter of reconstructing the floor. You have to make sure the drains are the cause of the problem.

Dik

 

[civilperson]

Drilling small holes and pumping grout will cured the void problem but will negate the drain usfulness at the grout contact location and if the drain is filled with grout may stop it from functioning altogether. If there is no water problem, then the drain is not needed.

 

[BillBey]

The french drain according to the original drawings is a 4" collector pipe with 1/4" holes, encapsulated all around with 6" of course concrete aggregate and another layer of 6" fine concrete aggregate. I find it difficult to fathom the french drain would wick that much soil down the pipe, if it was constructed as designed.

The average void depth along the wall ranges from 6" to more than 24" deep and is sloped toward the deepest void. Approximately 20 CY of subgrade has been lost over the past 23 years.

A water hose was inserted into a french's furthest upstream clean out and 20 minutes later clean water was seen flowing into the sump tank...which is approximatley 200 LF down stream. All of this makes me think that maybe the French drain is not the source of the problem and maybe we should looking elsewhere...maybe poor compaction of fill at the basement foundation coupled with yet undiscovered old utilty line.

Has anyone else seen 24" of undermining created by a properly constructed french drain over 23 years? It would be helpful if I could eliminate the French drain from consideration as the source of the soil loss problem.

 

[dik]

Properly constructed French drains work well in relatively free draining soils and draw down the water table effectively.

 

[BillBey]

The building is located in Houston, 50' above msl.

 

[dik]

msl (mean sea level?)... how far distant? I usually use a 2% slope for hydraulic gradient as a rough guess for water table in relatively free draining soils.

Dik

 

[civilperson]

The location of the voids, exterior of the basement wall and under the first floor slab, is notorious for not getting the proper compaction. Some specifications even limit the energy or size of machines for fear of over stress at the basement wall. Exterior stair landings and sidewalks adjacent to exterior walls have been known to settle 4' due to construction debris in the fill and lack of compaction. Maybe this is the source of the voids.

 

[jike]

Check the bottom of the sump pit to see if there is any soil in the bottom. If not, then it is likely [poor compaction.

 

[oldestguy]

In answer to your question, "can you get much erosion?", YES.

Look at Thread 158-171016 for some discussion of proper filter for these drain systems. This subject also is in many other threads.

I suspect that your job was not put is as you describe, or if so, the water comes from the side, not passing down thru the sand over the stone fill.

That coarse stone fill is not a filter and, with sufficient water flow, plenty of fines can get carried out from under the building.
I saw one case with one foot voids under the basement and 6 inch footing settlements in a year's time. At the outlet of the drain system there was a big delta. That's erosion.

The solution to doing it right is have the proper filter between all weeping soil and the drain pipe. More often than not, typical workmen will think that gravel is the best pipe backfill to use and your system will have much gravel in contact with weeping soil. I have fought this dumb work for years, since architects just don't know the mechanics of these systems.

Back to "what is a proper French Drain"? I don't know of a "Proper French Drain". because in my definieion it is a trench filled with coarse rock. That is destined to be a failure, since it is not filtering the fines from the soil. You are lucky it has worked this long.

The systems I am describing are "sub-drains", "footing drains", etc. but I never use the words "French drains, because, in my definition, that means coarse rock backfill, which means it will fail some day.

I find the easiest solution and relatively trouble free installation with well meaning workmen, is to say "no gravel allowed on the job". That way they can't do it their way. Use ASTM C-33 concrete fine aggregate (Concrete sand) for all backfill that will carry seeping water. If you use the black currugated plastic pipe, slotted, very little sand will enter the pipe. No clean outs are needed, since this will work forever. No filter sock needed either. They have been known to plug up and seal off those openings.

 

[dik]

I've had no problem with properly constructed French drains. The manner of construction is as follows and a standard CAD detail shows a X-Section.

A small trench is constructed with the bottom sloped towards the middle of the trench. The trench itself slopes to free drainage.

A non-woven polyester geotextile is placed in the trench and a perforated drainage tile is placed in the bottom, middle. Coarse free-draining clean drainage aggregate is placed around the drain tile to the sides of the trence. The trench is filled with the same material to within a foot of the top. The geotextile follows the side of the trench and is wrapped and overlapped at the top. 12" of topsoil is placed on top of the geotextile for seeding or sodding.

I've never had a problem with them silting in and they work as intended. Even with a foot of topsoil the drains sometimes cause the lawn to 'burn' from desication. The drawings show the width and depth of the trench with tile inverts as well as a x-section. They also show the free drainage and riverstone riprap as required. I have not had one fail in 30 years.

Dik