Drainage tile outside of foundation wall 1

[PostFrameSE]

A co-worker and I were discussing the best location for a perimeter drain tile around a foundation. Is it best to place it on top of the footing immediately next to the wall or is it better to hold it away, perhaps beside of and within the same elevation as the footing itself?

Thanks.

 

[concretemasonry]

The drain tile should have the bottom at the elevation of the bottom of the footing and to surround it (top, bottom and sides) with a proper backfill to increase the water gathering area. This minimizes the effects of moisture on the soil under the foundation.

For residential, this applies to either interior or exterior drain tile since it can reduce/eliminate any upward pressure on an interior slab. - Common and standard on better homes.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[JAE]

I would never place a drain tile below a footing or lower than the footing while adjacent to it (or even set the invert level with the footing bottom).

If you have silty soils - like loess, you run the risk of transferring the soil directly under the footing into the pipe and thus undermining your support.

You also ask a contractor to dig a trench next to and slightly lower than the footing bearing and thus possibly disturb the soil in the area of influence.

If you are trying to minimize lateral soil pressure on the wall, the exact placement of the drain isn't all that critical since placing it near the bottom of the wall is going to minimize almost all of the additional lateral pressure from excessive moisture. So near the bottom, at the side, or over the top of the lip doesn't ultimately change the overall design.

If you are trying to keep the exterior moisture below the interior floor slab - then placement might be more critical. Some high water table conditions would use an exterior drain "near" the footing and a series of separate interior drains under the floor slab - all going to a sump.

 

[TDAA]

My standard is the elevation of the bottom of the footing, and sloping to deeper. I have also put deeper drains in.

As the drain gets deeper, I recommend it be a minimum of 1:1 from the corner of the footing.

 

[concretemasonry]

I agree with TDDA.

If you look at the classic buried drain tile installation, the drain tile is surrounded (by about 6") by proper aggregate under AND ALONG the drain tile. This puts the drain tile out of the influence of the load on the soil from the footing since the invert is at the same elevation as the bottom of the footing is about 6" plus 1/2 the tile diameter. This puts the excavation outside the classic 1:1 ratio of the line defining the slope of influence.

Nothing new and used for years. Sorry if my original description was a little sloppy.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[JAE]

I agree with you in general - but in SOME areas you have loess type soils that are very sensitive to moisture and also have a propensity to erode into adjacent "open" granular materials. You do not want to put a drain tile with drain gravel next to soils like that. Just sayin.

By putting the drain tile very low, you aren't doing anything significant to change the lateral load pressures on the basement wall vs. a little higher - say level with the top of the continuous wall footing.

 

[oldestguy]

One can argue these things all day, but it should depend on the soil you are in, the likelihood of ground water versus surface water only and the building itself.

In many locations I also recommend interior drains situated in a sand bed (not gravel by the way)and spaced no further apart than 15 feet, assuming a 1 to 7 slope of a water table flowing towards the drain.

Generally speaking, the deeper the better, assuming you don't undermine footings, but practically speaking that is not always possible.

On sand versus gravel backfill, I can tell you a story about how to cause major problems with the building support using gravel backfill.

 

[TDAA]

JAE, point taken, however, that is just one purpose for the drain. In areas with expansive soils, the intent is to have the drain low, so you are not holding water in the relatively permeable backfill adjacent to the foundation. This condition would allow the water below the drain to slowly, but steadily, seep into the ground and increase the depth of wetting. Generally, you want it to be the lowest part of the excavation to reduce this effect.

 

[emmgjld]

I agree with oldestguy, TDAA & concretemasonry and recognize the valid concerns of JAE.
If you are trying to keep water out of something, the drain should be lower than the thing to be protected and placed between the water source and the thing to be protected.

The use of screened rock is a poor choice for most soils conditions. Sands are almost always better, both from minimizing soil piping or transport of soil into the drainage media and from the standpoint of promoting efficient seepage from the soil into the drain pipe. Open gravel is terribly inefficient in getting the water out of the soil and into the drainpipe.

I tend to place the drain 1 to 2 feet away from the footings. Potential loss of footing support is one reason however, I have never seen a case of foundation compromise due to drain excavation encroachment in the 'area of influence'. The problem of soil migration (piping or 'squeezing') into the gravel has been my experience.

 

[fattdad]

What's the purpose of the foundation drain? If it's to prevent moisture from getting to the foundation bearing soils to prevent "softening," then I say it's not needed in the first place. Properly prepared soils don't go soft when they get wet! Heck if this were true, all the embankment dams would fail when the water got to them - doesn't happen. Also, look at CBR testing - you soak the soil and then measure its resistance.

If you are trying to build against a hillside and you want to collect moisture so your basement or crawl space doesn't get damp, then that's a different story.

I just hate foundation drains installed out of convention. It's unnecessary expense.

Rant over. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[apsix]

f-d
Elimination of hydrostatic pressure to reduce lateral loads on a basement wall could also be the reason (as the question is located in the 'earth retention' forum).

 

[oldestguy]

apsix has part of it right. In my experience it also is to keep the basement dry.

FD, unfortunately in some areas (mine included) the state uniform dwelling code calls for those drains and a sump pump on EVERY JOB. Plumber unions may have had a hand there.

 

[concretemasonry]

I never suggested screened rock since a properly graded (by size) can be specified. If the surrounding soil is easily eroded, a filter cloth can be used. This also defines a larger perimeter of the drained area, which will reduce the infiltration rate of water and help prevent clogging of the filter and drain tile.

There is nothing wrong with reducing the moisture under a foundation and there is a long term benefit of continuously being able to maintain a more uniform moisture content.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[VAD]

PostFrameSE

Intersting topic that seems to have a variety of answers. I tend to agree with fattdad's opinion except that if engineered fills are used there can be an issue if drainage left out. This can be debated as well. We should first ask what drain is used and how it is installed. In many instances there is not always positive gradient to discharge locations especially in how houses are constructed today. Many instances the weeping tile allows water to be transmitted to the foundations rather than removing same.

I agree with JAE, soil type has alot to do with decisions of this type. Unfortunately, the codes of some jurisdictions have allowed any but anything to be used as engineered fill etc as this is often governed by environmental evaluations for chemical analysis and geotechnical engineering has taken the back seat. Further more truckers seem to decide on materials as their concern is to remove and replace materials and make money. Once it is dirt and clean it is good. Construction forces in residential and commercial as well do not seem to want inspection of anything beyond the footing base. Hardly is the there inspection of the weeping tile placement and backfill. I have seen enough new building propped up by helical piles with plastic weeping tile exhumed looking like sphagetti. Granular material is now replaced by drainage board and anytype of backfill or onsite material which can hardly be controlled.

We seem to have walked away from an important part of engineering which as engineers we dwell in and own as the other guy next door and we also are amazed when problems occur since once things are buried we are the same as everyone.

We need to engineer the use of drainage systems against foundations rather than just throwing in pipe as we seem to do today. How can we prevent surface water getting below the ground in foundation areas need to be addressed. There is lack of discussion of ground water position or likely position soil type, whether fill or native ground, likely after construction performance that seems to be left out of reports. Of course at the cost of doing a report these days what else can be expected. The helical screw pile industry savours our inabilitues and perhaps rightly so. It is probably good practice to prop all our structures with these piles.

Enough said. Unless we can be represented on the code committes then we will continue to debate this topic for years to came. of course we also feel that this is not the real engineering despite we spend our time in it for the major part of our day to day life.

Excuse the rants.

One question that I have is that there is a decison often made with the use or non-use of weeping tile in buildings basementless foundations. The saying and recommendation is that if the floor slab is about 300 mm above the exterior grade than weping tile is not necessary. The height can be as small as 200 mm. This is based on the premise that if the exterior is sloped away from the building water does not get into the ground, and if it does than there is not sufficient head to move it to the underside of the floor slab as it should not rise above the level of the exterior grade. Seems okay reasoning but do we want water getting in at the foundation level. What about external water from rainfall entering the foundation adjacent to the exterior walls and outside landscaping. There is a frictionless would created at this location that creates a weak zone. Is this taken ino consideration. Is the watering of plants that are often placed adjacent to some buildings taken into the equation. Is the performance of engineered fills which are often compacted without too much recognition of moisture content during compaction taken into consideration. Most of us recognize that while density may be good moisture is often low as compactive efforts are higher. How is the moisture content rationalized for control of compaction. Unfortunately we are often the creators of this problem as we tend to give a half hour training to new hires on how to use a nule gauge and off they go to make some money. What are your thoughts. There is considerable amount of money used in housing and this is only going to increase. Housing inspectors and companies offering solutions to these and other problems are on the rise. Is there a geotechnical issue here or have we moved out of town. Do we need to spend some more time on this topic.