Drain tiles

[coneboy]

Greetings

I had a rather heated debate with a home owner the other day regarding the performance of his perimeter drain tile system for his house. He was most upset with the performance and wanted to know why the drain tile system only seemed to work for about 7 to 9 years. I told him that typically the gravel and drain tile typically become clogged with fines after approx 8 years and a new system is required. I also told him that you could possible flush the system from the clean out points which might give the system a few more years of life. The house has a standard basement and the surrounding grade generally sloped away from the home. I know some drain age systems use a geotech fabric, but I have found that this does not really help that much.(just adds costs) The soil is a typical variety of topsoil/stiff clay overlying sandy silt and clay. The rain fall is quite heavy in this part of the country. The drain line has a downward slope away from the house to the city connection.

Have I missed something here?
Any comments and usefull insight would be greatly appreciated.
Coneboy

 

[Ron]

If it is truly a "drain tile" system, I would agree that it has probably become obstructed with infiltrated fines. I would disagree that a geotextile only adds cost and not performance to a drainage system.

A proper drain for something such as you have described would include a socked, perforated pipe, installed inside a trench lined with a filter fabric with properties matched to the surrounding in situ soil conditions. The trench should be backfilled with gravel (No. 89 or No. 57 stone), and the filter fabric should then be lapped over on the top of the gravel. This will provide adequate protection from infiltration of fines, and allow water to get into the perforated pipe and be carried to a positive outfall.

Make sure the system is at an elevation somewhat below the bottom of the basement. Otherwise, as the water table rises, even temporarily, water will get into the basement without regard to the performance of the underdrain.

 

[coneboy]

Thanks for the comments Ron.

As I didn't spec the drain tile system I don't know if it is the old style clay tiles or perf PVC pipe.
I was wondering, based on your experience, do you find the geotech fabric clogs up quickly after about 8 years?

I agree that it is common practice to place the system in a manner so that it drains the under-slab area. Again, since I didn't spec this project I don't know the exact placement.

I don't typically require sand/gravel to be placed above the system all the way to the surface. The native soil is usually dumped back in to re-establish grade. Do you follow this procedure?

Thanks for help
Coneboy

 

[Ron]

CB...except in very clayey soils, the geotextile performs quite well. The time to failure is usually more than 8-10 years, unless they had construction deficiencies.

I would agree not necessary to backfill over protected system with clean soil or gravel. Native soil OK. Minimum trench size should be 24"x24" (cross section).

 

[emmgjld]

I have found that geotextile filter fabrics clog due to improper specification of the geotextile 'permittivity and the total area of the actual filter contact with the soil to be drained. Clogging indicates soil particle movement, a result of water seepage velocity being too high. Try to decrease the water seepage velocity by using a 'tighter' fabric and increased drain size to account for the lower rate of seepage from the soil to the drain.

I am usually disturbed if projected performance is less than 20 years, even in clayey or silty clay soils..

 

[coneboy]

Thanks Emmgjld
A few interesting points that I had not thought about.
Appreciate the help.
Coneboy