Safe to dig beside foundation for drain tile placement 2

[Lawrence P]

Helping a relative install drain tile beside a leaking basement (and partial crawl). Intention is to dig directly beside the wall, cover in a waterproof membrane then place a French drain beside the footer. I'm a bit concerned (for them) about soil compaction and shifting once we disturb the soil near the footer. Looking for any insight.

 

[Lawrence P]

OG, It looks like all your applications were commercial-municipal grade. Do you still recommend the cement sand for residential use ? and please clarify if you recommend the sand and a filter or does the cement sand do the job. Thanks

 

[dik]

We have a product called Pea Gravel that is often used.

Dik

 

[oldestguy]

Wow,

Dik: I'd like to know how pea gravel can filter out silt when the void spaces there are much large than silt particle sizes? IT AIN'T A FILTER! Go ahead and don't change, it's not for me. Pea gravel, or other materials with no filtering ability are allowed on my jobs because "well meaning workers" see those large voids and figure water flows faster. Fast rate of flow for seepage collection is not needed.

Lawrence: There is no distinction between highway subgrades and house basements when it comes to keeping water out. Hell, I use concrete sand and perforated pipes for stabilizing side hill earth slippage caused by excessive water. Same purpose. I showed the photo to illustrate how you can have holes on the bottom and leaves a smoother path down the center. It is not critical if the bottom also has holes. However, if the collecting water has to flow up into the pipe, it is more difficult for fine particles of sand to migrate in.

No point in making a simple job complicated.

 

[dik]

It doesn't filter our silt... it provides relatively free drainage. I cannot imagine cement sand to behave any better. With time, the cement sand will simply 'clog' with the silt particles...

Dik

 

[oldestguy]

Dik:

Let's not confuse things. Here is your statement from way above.

use a nonwoven geotextile between existing soil and drainage layer

That's correct. But, without the fabric covering the open graded stuff, you can have plugging of tghe open graded as well as the pipe.

There is one job that I came across that says you need a large area of filter between soil and the collecting place. Using fabric covered slotted pipe and an open graded collecting zone between earth and it. Or, no open graded material at all. With spaced slots in the pipe silt and clay can build up on the fabric just there and plug everything. That's where fabric over the gravel or just concrete sand can do the job.

by the way it's not cement sand.

 

[dik]

The drainage stone is totally encapsulated by protective.

Dik

 

[oldestguy]

Now that that is settled, what about settled footings? I'd recommend an experienced geotech, since some form of soil examination may be needed.

 

[Lawrence P]

Gentlemen, final dilemma hopefully. The floor of the crawl is anywhere from 28 inches to 45 inches below grade. We dug down at lowest level and found that the stone wall foundation actually has no true footer per say and the bottom of wall is around 48 inches below grade.

I think I'm going to pour a slab after I level it but now I'm concerned with an old stone wall and no footer how close I can get to the bottom of the wall with the french drain without running the risk of drawing water from under it and causing erosion under the wall.

 

[oldestguy]

Another subject here. I've run into very few erosion problems as related to water flow from under footings and slabs. Usually that is when there is significant water flow and the stuff that moves is fine sand. First off is that stone foundation just a bunch of loose stones, no mortar to amount to? That is not good, but not terrible. If so, I'd keep my excavation away some, leaving say a triangular shaped zone against the lower zone.

 

[oldestguy]

And here is a case where you don't want to erode any of that zone. So, in no way use an open graded stone against that without a filter. Of course using concrete sand you do have that as a filter.

Otherwise ,another main problem I have seen is vibrations in the are causing the sand, usually damp, to actually flow out from under. That can be running a vibrating roller nearby or a large industrial compressor operating about 40 feet away.

 

[oldestguy]

Another one. Around the old prison at Waupun, WI there is a high stone wall made many years ago. It is about 3 feet wide and down about 4 feet for "foundation. Quite obviously it was built by prisoners. It is well cemented together above grade, but not so below grade. Through the years the wall has tilted a little this way and that. A grouting firm was hired to straighten it by pumping grout into zones below it as "jacks". However first any voids in the "foundation" need filling to keep it intact. I was able to take a folding ruler and insert it easily far into the voids in that "foundation". The explanation is, the guards apparently didn't watch closely because that loose foundation would be no sweat to dig through by hand to escape some day. However, the weights imposed on that supporting soil were darn high from that stone wall, yet these spaced stones carried it pretty well. So maybe the subject house is in pretty good shape even if not the most solid foundation.

 

[Lawrence P]

I've heard of this 45 degree rule, looks like thats what your speaking of. I found the soil at 48 inches down to be cold and clumpy but not truly wet to the touch. Not being an expert in this field I'm not sure if this is expected at this elevation (degree of moisture in the dirt). But I didnt have visible water per say.

I'm leabing toward keeping french drain about 1 ft out from the wall.

 

[oldestguy]

Sounds great, since fine sand would not hold together. Since experience there says water sometimes inside, that likely is from rain and not permanent ground water. Where there is a slope in the area, I'd suppose the most likely entry comes from that slope side, thus not coming out of the foundation. Sometimes the water gets by that outside drain and comes up a ways inside the basement. That can be collected with a drain there that then leads to the outside drains. Hopefully you can run your collection system to a down hill surface discharge by gravity. In some cases we need a manhole near the building (made from 2' diameter culvert pipes) in which a sump pump sits that discharges away in a safe place.. If you have cold winters and these discharges may freeze, it takes some doing to work with that (next time here).

 

[Lawrence P]

Thanks very much OG, everything u described matches (down slope etc). I was just worried as that its been 3 days since rain and the soil is still showing signs of moisture

 

[concretemasonry]

The 45 degree concept is recognized as acceptable for avoiding the loaded soil profile. It makes sense and is easy to apply. For drainage pipe along a strip footings, it is the most commonly accepted load distribution for most soils.

It also is easy to implement for new construction IF the builder has the desire to avoid foundation problems and install perforated drain tile.

I had a friend that was a builder and also a member of the state building code committee that made recommendations to the use and implementation to the building officials.

He built over 200 homes per year and vowed to never have wet basement complaint. He had very practical, efficient method that required some special block for the walls, drainage pipe and strict construction methods that were easy to implement when you are big enough control design, construction and sell/guarantee a home. This method was based on keeping the drain tile outside of the 45 degree spreading out from the bottom of the footing toes.

We had a 48" frost depth.

Dick

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

 

[Lawrence P]

Thanks all, if my math is right though were only talking about just a few inches away from the footer. I was thinking of putting the drain tile out about 12 inches and just slightly (2-3 inches) above the base of the footer to be on the safe side. Does anyone see a major issue with this in theory

 

[oldestguy]

I'd not worry about undermining the foundation due to this excavation to footing grade if there is no major flow of water encountered. A reasonably intact wall and "foundation" likely will not displace soil. That is what is called a soil rupture or shear failure. Such a condition is only possible if the soil is near a soup condition, about never happening. The main settlement problems come from the soil just becoming more compact with the footing pushing down.

I'd set the pipe at footing grade. I'd fill the trench up to what might be called "highest water coming in". That fill would be concrete sand, the simplest technique, most rapid, fool proof and cheapest. Following the comments on pea gravel and fabric is labor intensive and easily can be done wrong, takes longer and can invite cave in due to time taken to lay fabric, then pipe, gravel, and fabric. Suppose you are partly done and get cave in or big storm before the gravel is in or covered? A holy mess. Dig out and start over. With concrete sand, you can cover with sand as soon as each section is laid. Cave-in on that sand likely might not hurt, or can be removed later.

Final fill to grade can be what was dug out.

In summary, the interior footing settled examination by geotech might also involve him for these walls, but don't go for something without roper filters. That part of geotgech education usually is missed. The detail I'm going into here is much more than what the operators of this web site like to have us do. About all I'd like to leave is "don't forget filters".

 

[Lawrence P]

Going to try to attach a few mark ups I've done. Wondering now if the French
, or curtain drain proposed will be overkill

 

[Lawrence P]

My picture is only showing up as a link, not sure what I've done wrong

 

[oldestguy]

Picture is fine. which way is the camera pointing and what do those words mean? Perhaps a sketch of what is proposed will help. You can also insert a picture as a JPG file using the icon above with a circle at the lower right corner of the screen , about in the center of the Post Options icons above. Picture won't show as you insert it, but thereafter will show along with the text.