Replacing 30 Year Old French Drain 3

[bobrob1892]

I am in the process of replacing the original french drains around the North and East walls of a 30 year old home just purchased in Southern Ontario. There is a serious water seepage problem through the concrete block walls in the basement but not through the floor. A 7' deep, 3' wide and 30' total length trench (10 foot sections) has been dug to reach just below the foundation footing, and the walls have been shored. The old tiles are in fair shape but I plan to replace them with 6" geo-fabric covered perforated pipe. I have several question which require your expertise.

1. What material and what amounts of sand, 3/4" rocks, fabric, etc. should be used to construct the bedding for the pipe and then to cover the pipe? The pipe connects to a working section of the drain that takes the water to the sump pit inside the house.

2. Originally the concrete bock walls were parged with cement and sealed with a very thin layer of tar. The walls have been washed and metal brushed. There are horizontal cracks at the mortar joints in 4' lengths no larger than 1/8". Thera are a couple of vertical cracks along the mortar joints and right through the blocks about 6' long and 3/16" wide. What is the best method of sealing these cracks? My thought is to use hydraulic cement on the vertical cracks, then parge the wall with cement, cover the wall with "Blue tar", place an uninterrupted sheet of waterproofing dimpled membrane (

http://www.superseal.ca/dimpledmembrane.html)

over everything, then backfill.

3. Where the cement blocks meet the foundation, there seems to be gaps allowing water to seep in. The parging cement partially covers these areas but it is fractured mostly. How would I go about sealing this area without taking out the old cement work?

4. The soil in this area is clay like and holds on to water, shoud I discard the earth removed and backfill with clean/engineered soil or mix the current soil with gravel or sand and then backfill?


Any suggestions will be greatly appreciated.

Bob

 

[oldestguy]

First off you did not have a French Drain, since you say clay tile was used. Your experience also points out the fallicy of using tiles without any sort of filter. I suspect this so called perimeter drain has nt functioned for yeara, causing the seepage you have experienced. Nice to see you want to do it right.

Is there any way you can be sure that at least half of the pipe diameter will be below the floor elevation? It is likely that a well functioning drain system will not have any standing water above that line, usually that is. The object here is to have this buried "moat" lower than the floor by as much as possible. Sloping the pipe usually is not practical, but would be desirable.

Next, why worry about the pipe bedding? Do you want water running in the bedding or in the pipe? Lowering the system as low as possible, without out seriously undermining the foundartions. Clean sand backfil is all you need, maybe an inch or two so pipe does not lay on clay..

Next, what about filtering out any silt from the system, including the backfill (the porous section)? This filter is what your original system didn't have. As you view these threads on this subject you will generally see that recommended backfill, at least near the pipe, and hopefully most of it near the wall, is clean coarse sand meeting the requirements of ASTM C-33 Concrete Fine aggregate (Concrete Sand). Get it from a concrete plant. This is sufficiently porous to let water pass better than the natural soil and it has gradations to keep silt out. The larger you can make this sand-to-soil area, per foot of pipe, the better. Thus, backfill up at least a few feet above the floor elevation with this sand and, if possible, extend the zone of sand backfill up next to the wall with it, except for some seal of clay at the top ground surface.

It sounds as though you are using the proper pipe, but with concrete sand filter material next to the pipe, the fabric on the pipe is not needed.

Outside the concrete sand backfill you could use a hauled in bank-run sand that is more porous than the stuff you dug out. Reusing the excavated soil should be OK if you remember to keep sand next to the wall for say a foot or two out and old stuff farther away. Remember to slope the top and use some sort of clay seal there also. No point in adding surface water into this backfill.

Crack filling is another point. If you use this clean sand fill next to the wall, you normally will not have standing water there and thus, no seepage of significance.

Any sort of effective crack sealing has to be done keeping in mind two things. It must bond to the block. It must be stretchible as the dimensions change with time. Routing out cracks is tough and unlikely to be done right, so your thought of some continuous membrane sounds as fancy as you can get, but any effort to cover over those crecks should help. Again, if the drain is functioning as one would want, fancy crack sealing is not generally needed. I'd just tar that whole wall surface with a thick layer, something like roofing tar, not a thin "black paint", with most effort down deep

I just viewed your planned dimpled sheet stuff web site. It is great likely that if you do this, as they show in their detail and do not use sand next to the wall, that this will feed water into your cracks. Using it you do need better drainage outside of it, but do not do like their diagram shows with only ordinary earth next to it. You need all efforts, combined, to keep water from the wall.

 

[concretemasonry]

Most leakage of walls can be eliminated by proper control of the surface water. - Gutters, downspout extensions and slope of the surface.

With that done, you have to deal with the "swimming pool" created when the contractor excavated for the basement/home he built in the hole.

As pointed out earlier, effective drain tile with proper backfill eliminates the need for any specialized treatment on the walls since the source of the water in eliminated.

If you have any cracks, they can be effectively sealed with the time-proven application of hydraulic cement forced into a routed crack opening. the ever=present joint between the interior slab and the wall can also be filled with this material.

Since concrete will always hold a certain amount of moisture, wall coatings are effective fot moisture and humidity control. Avoid the use of paint type "waterproofing" materials on the interior of the walls. Thoroseal (around longer than most practicing engineers) is a proven cement based coating that is compatible with all types of concrete walls (cast-in-place and concrete block). It should be applied to a moistened concrete surface for proper bonding and curing.

 

[eriq]

where can one get technical details(drawings) for french drains?

 

[pfisterscott]

Oldestguy, sounds like a French Drain to me. Henry Flagg French Published a book in 1859, "Farm Drainage
The Principles, Processes, and Effects of Draining Land with Stones, Wood, Plows, and Open Ditches and Espcially with Tiles< Including Tables of Rain-Fall, Evaporation, Filtration, Evcavation, Capacity of Pipes; Cost and Number to the Acre, of Tiles, Etc,". This I believe is where the French in French Drain comes from. Before plastic pipe, drain tile pipe was used with a two inch gap between pipes that were then covered to keep the dirt out.

 

[oldestguy]

OK to pfisterscott: Nice to hear at least someone tried to define it. My experience has been that drain systems made with a trench filled with stone, was a French Drain.

However, in summary all of those you mentioned did not have a filter specifically included. As a result practically all the older ag drain systems failed in time. An attempt was included with some systems to cover the tile with the sod from the surface, before dumping in the soil, but still not perfect.

My father came from Sweden about 1900 and over there in the marshy sugar beet fields they did a mole-hole type drain, that of course had to be re-done frequently. They hitched horses to a form of plow that formed a round hole in the subsoil about a foot down. It helped, but not permanent.

In my many years preaching filters for these systems, I continue to run into architects with specs for backfill to "tile drains", or what ever pipe you use, that is gravel. Even our state plumbing specs call for gravel. I've given up trying to change that, since the committee that sets up the specs is a bunch of plumbers. They don't get called back to fix the failed systems, so they continue to use gravel backfill. Interestingly though, the state code reviewers will OK concrete sand for the backill, providing a PE calls for it, but OK only grudgingly.

 

[pfisterscott]

Oldestguy, I appreciate your insite. I am new to the ins and outs of building perimeter drains. In my journeys to gather information I came across a paper that I would like to get some feedback on.CHANGES IN ENTRANCE RESISTANCE OF
SUBSURFACE DRAINS By Walter J. Bentley ~ and R. W. Skaggs 2. This paper seams to indicate that drainage pipe with fabric around it, (sock) inhibits the flow of water entering the pipe. The following is from the final conclusion section.......... The effective radii of the tubes in the (experimental) soil tanks were calculated. The effective radius for the tube without a fabric envelope increased dramatically
over the test period. For a large portion of the test, Re was greater than the actual radius of the tube. This was probably due to a loss of soil around the perforations. The two drains equipped with fabric envelopes had fairly constant Re-values throughout the test period, and the values are similar to those reported by Mohammad and Skaggs (1984). The resistance factor, R, was calculated for the three tubes throughout the test period. The tube with no envelope showed an initial increase in R,
followed by a decrease. Again, this probably resulted from soil loss around the tube perforations. R increased throughout the test period for the tubes with fabric envelopes as a result of the decrease in K.
The entrance constant, %, was predicted using theory presented by Dierickx (1980) and compared to values calculated from experimental measurements.
Although the values determined from measurements were initially higher than the theoretical value, they decreased significantly with time.
These low values probably resulted from the soil loss immediately around the drain perforations. Experimentally %-values for the tubes with envelopes
were greater than the values predicted due to additional convergence as the water entered the tube perforations.
APPENDIX I. REFERENCES
Bodman, G. B., and Harradine, E. F. (1938). "Mean effective pore size and clay

 

[oldestguy]

Pfisterscott: You quote interesting stuff. However, the crux of any test like that is in how well it filters out the silts and how long it continues to operate.

I have not personally seen failures due to a fabric "sock" covering the openings, but I have heard that silt can clog that fabric over the slots and seriously cut down the flow capacity. All failures I have seen are due to no filter (use of gravel)and then nothing works.

One thing we should keep in mind, any use of concrete sand as a filter material very probably presents a higher permeability (hydraulic conductivity to you younger folks) than the soil from which the water comes. So, the probable controlling factor in how much water the drain collects is the permeability of the nearby weeping soil, not the filter or the pipe characteristics. Currently the practice seems to favor 4 inch diameter pipes and I have yet to see where that is insufficient diameter for perimeter drains.

I did once see a 6 inch diameter perforated metal pipe draining a highway slope that was flowing half full, from a large area of gravel and sand; most unusual.

Back to the use of a sock or none. I don't use a sock and some sand gets in the pipe, but coarser particles then form a brige over the slots. No clean outs are needed, since this inflow of sand is minimal and does not continue.

As a side light, I started examining sub-drains in 1954 as a master's thesis project and that's where the backfill immediately poped up as the main factor that has to be done right for the drains to work. Been "preaching" ever since.

 

[drtdigger]

a common practice in my area is a minimum of four inches of 3/4 drainage stone over the pipe and use of a filter fabric blanket over the top then minimum four feet of clean sharp sand