Perimeter French Drain with ASTM C33 Concrete Sand as per oldestguy 1

[ausee]

Hi,

I have been lurking on this forum for a while, reading oldestguy's posts advocating the use of ASTM C33 concrete sand for French Drains.

I live in Australia, in a single brick house on stumps, no basement. During a particularly wet winter (ground doesn't freeze here), we were getting a mold smell from the carpet. The building inspector had advised installing French drains around the perimeter, as occasionally there had been water pooling under the house. On the side of the house with the problem, there were two heat pumps draining condensate into a local depression, and also a leaking washing machine with tray drain going under the house I think in that area (since fixed). I figure that the moisture was wicking up through the brickwork. Other than from that local depression, the block gently slopes towards the storm water connection. So this local depression on that side of the house had a locally high water table, as the ground was pretty much permanently (year round) squelchy and shaded. The soil is quite clayey, with clay between 20-40cm below the ground. In fact, this used to be the case not just in this area but on the top (highest elevation) side.

I have dug out a drain around the perimeter of the house, with a connection to the stormwater access. I have dug to a depth of about 45-60cm on the side with the damp problem. Since this has been in place, the water initially was welling out of the side of the drain, and now the ground is quite hard. In all cases, I have dug to about 5-20cm below the clay layer. In the rain, it definitely works to channel the water through this trench to the stormwater access point.

My plan had been to fill the entire trench with ASTM C33 spec sand, after placing 100mm PVC sewer pipe with hand-drilled 5mm holes in the trench. Then I would add some grass seed mix to this on top. I figure that it would collect both the surface water and lower the water table in the problem area. Since I don't have a basement, I figure that the main goal is to collect both the surface water and the water that would be assumed to run above the clay layer in the sod. The main water collection points where it would pass under the house are the depression on one side of the house, and the higher elevation side of the house. So if I have encircled the house with a drain that is under the clay/sod junction, and flows along the whole length to the storm water access, then this will prevent the water from passing under the house, and lower the water table by 30cm at least.

I have also tilted the slab under the problem heat pump to drain directly into the french drain.

Are there any problems with this setup? I figure it is already an improvement on the similar gravel/geotextile sock arrangement proposed by a local contractor. I realize that oldestguy might recommend some sort of surface swale to deal with the surface water, but considering I don't have a basement and it seems like the water issue is already solved in the problem area, I figure that what I've got is sufficient to deal with both surface and sub-surface water. My gut feeling is that now the surface water will be removed promptly, and whatever remains will find its way into the French drain and then leave. On the top side, considering that there is basically no slope, I'd be served just as well by digging the french drain without needing to drastically re-contour the landscape.

I have been browsing these threads for reference:

http://www.eng-tips.com/viewthread.cfm?qid=395513

http://www.eng-tips.com/viewthread.cfm?qid=401636

http://www.eng-tips.com/viewthread.cfm?qid=386193

http://www.eng-tips.com/viewthread.cfm?qid=277245

 

[oldestguy]

OG here: I haven't looked at all he referenced threads, but it seems you are on the right track. Maybe a photo of the house will help.I once lived in a place with water ponding (actually the ground water level) under the house in a crawl space. Venting the area made it all livable. Have you looked at venting? How about covering the exposed moist soil with a plastic sheet? As to moisture wicking up through brickwork, is there any way to seal off that moisture before it gets to the brickwork? I am assuming your "stumps" are made of brick. There are good "paints" that can be applied to masonry to prevent water entry. Sometimes moisture is attracted to features such as: colder than the air, saltier, etc. blamed on vapor pressure reduction there. Look at these possibilities. One thing I have learned with respect to drainage by gravity from soil is that soil can act like a sponge and holds it by both capillarity action and attraction to some clays. Drawdown of water table by a trench nearby doesn't work well in fine grained soil. It works best to intercept the flow of ground water.

 

[oldestguy]

OG once more. I live in a house faced with brick. Moved in in 2010. House built in '99 fully handicap acceptable. Result, no steps coming in. This means the exterior grade near front entry is about 12" above the basement concrete walls. That means there is soil against the brick for a height of 12 inches. Brick "veneer" sits on that concrete supposedly on a metal barrier extending up inside the brick veneer. That is meant to keep moisture from the wood interior there. Maybe yes and maybe no. Checking of the nearby wood framing I have found no effect of moisture. However I didn't like the idea. So I dug down to the concrete wall and coated the brick with a moisture barrier "paint". I also added a plastic sheet, visible in the photo, painted tan. Who knows if I really have solved it, but lowering the grade at the entry would be OK, if it didn't leave a depression that needed surface drainage. By the way that enclosed are was also treated with bentonite mixed to "waterproof" it. I didn't like adding water to basement perimeter drain since the ground water table is only a foot or so below basement.

 

[ausee]

The highest elevation side.

The problem side (upper). Not that the lower border of the local depression is at the fence line approximately.

The problem heat pump: tilted now.

Problem heat pump: rear. I think I will wait for good weather, turn the heat pump off, pressure spray the moss off, and apply silicone to make a higher level and so prevent the water going behind the unit. Mostly solved by the tilting though.

Close up of what I have done to drain the HWC and Sanden hot water heat pump (the best btw).

This is the lower problem side.

This is the the non-problem side, upper side.

Here you can see the house. The left side is the problem side. The right side is the non-problem side. The site slopes down to where the camera is facing, and is basicaally in the direction of the house, parallel to the fences. Under the deck was difficult, as the land slopes towards the house and alongside the house. When raining, there is a flow down towards the house and out, so I have snaked a drain in there to catch the runoff. I dug the drain as close as I could to the deck. Ideally, the drain would have been dug under the deck. If I have motivation I might sprinkle bentonite under there. This is not a problem side of the house though.

Here is a close up of the above. You can see the drain dug to catch the under-deck surface water. Note that water in a heavy rain will pool around the base of the stairs, right where the drain comes through.

Drain going to the storm water access.

Under the house, in the crawlspace. Maybe it's the placebo effect, but it feels like some of the higher elevation dirt is dry. Well, it is dry, just a question of whether the french drain has accomplished it or not.

You can see that even in the non-problem areas, the water advances up the brickwork a ways, but not enough to get to the house.

 

[ausee]

BTW I measured the drains. Mostly 21-22cm wide, 60cm deep in deepest parts, 45-50cm in most places, 30cm on the non-problem side, but penetrating the clay layer by maybe 5cm.

I have considered buying a dehumidifier and sticking it down there, with a permanent drain going outside. It may be necessary, it may not. My gut feeling is that I've already solved the issue, as even in the wettest months I can guarantee that the water table should be about 50cm below ground level, draining practically instantly, never getting a chance to build up. Since the heat pump is right outside the worst of the mold, I think just solving that draining issue will likely do the trick. The drains are practically under the gutters, so minimal rainfall will fall inside the perimeter, at least on the problem side. On the non-problem side, most of the water will run down into the drain or further down to the neighbor's property.

Ironically, there is gravel surrounding the stormwater and sewer pipes leaving the house on the lower side, and I was limited in how deep I could dig without going below the pipes. I suppose I could have dug under them, but it would have been a hassle and probably not required. It is interesting to see because they are surrounded by gravel, and gravel is not a filter. There is some dirt in the bottom part, but that acts like an unintended French drain itself in channeling water through it. I definitely see the logic to using concrete sand. We have a trench at work that is fed by a lot of water, and the water goes through that trench, under a road, and into sand, which "blocks" another pipe that goes to daylight. A lot of water passes through that sand, so evidently sand is an adequately porous medium. A slow sand filter is also something used to filter water. Not to say that it is the same thing exactly, but sand can be used to filter. I was reading your posts and saw the logic very quickly. There are quite a few things in life that work better, and are cheaper, but are not as widely used because someone makes a lot of money from selling a more expensive solution. No one is making money from concrete sand and would see the sense in promoting it, but companies do make money from geotextile fabric.

https://en.wikipedia.org/wiki/Slow_sand_filter

Thanks for your help btw. Note that if anyone is doing this in Australia, ask for an analysis of the sand from the companies in terms of mm sieves, or ask for "drainage sand" (what the company I talked to insisted I needed, not wanting to listen to what I was actually saying, and it turned out that their sand was within ASTM C-33 for artificial/manufactured sand (and very close for natural sand) or AS 2758.1, as it is very similar to ASTM C33 fine aggregate.

 

[ausee]

One more thing: a picture of the area just down from the problem heat pump unit. Note the moss going up several bricks. It seems reasonable to think that the source of the problem is water wicking up the bricks. Previously, the water was going from the heat pump, and landing between the support slab and the house. Not a good situation!

 

[oldestguy]

Great photos showing details. The white stuff on the bricks we call efflorescence, the dissolved chemicals in the brick being left there when the water evaporates from that location.. Is the ground surface outside the house the same as the ground surface elevation in the crawl space? If not which is lower and by how much? Do I see a vent in the wall in one of the shots on the lower problems side with walk next to house?

Am I presuming right, the brick walls basically are the whole wall thickness, not any veneer as I have? So wicking up via the masonry can be a path for moisture getting up and affecting the inside of the walls up there? Those interior "stumps" also appear to be able to do the same.

Once one follows the brick walls down (below what is visible), is there a "footing" upon which the brick wall rests, spreading out the loading? What I am wondering is could one practically coat that wall below any earth cover elevation to prevent water from soaking in? (As I have done).

If you were to really open up the area under the house to vent out any moisture would you have to insulate the floor above so as to keep the living area above from having a temperature change not wanted?

Your outside drainage system appears to be as good as one can do without being significantly more aggressive, such as creating horizontal barriers with flashing within all brick walls and stumps.. Flashing here is usually sheet metal, such as copper sheets or galvanized steel sheet metal. More venting might be needed also.

Hey, my neighbor has a 3 year old boy with his low height basketball hoop also. I think dad is pushing it though having just now installed a full height basketball hoop with some height adjustment capability. So far the kid seems uninterested in both hoops. Dad was a University of Wisconsin football celebrity a few years ago.

 

[ausee]

Hi Oldestguy, thanks for your comments! And of course thanks for your input in the other threads you have given input to, which were the foundation to part to my decision to use the concrete sand.

To answer your questions:
1) Is the ground surface outside the house the same as the ground surface elevation in the crawl space?

In most places yes. I did crawl around inside a couple months ago, and that was the conclusion I came to, within maybe 10cm. This is the area near the walls specifically. I counted bricks below the vents and they seemed similar. There is a gentle slope that matches what is outside.

2) Do I see a vent in the wall in one of the shots on the lower problems side with walk next to house?

Yes. It is a part of the building code that the houses should have such vents. Not that the hole are very big, but they are there. The building inspector suggested enlarging these. In his original report (the house is 12 years old, he also inspected it about a decade ago), he also suggested a French drain around the perimeter. I think now the drains are dug, I should see what effect it has.

3) Am I presuming right, the brick walls basically are the whole wall thickness, not any veneer as I have?

Yes, the bricks are a full brick in thickness, but not a double layer. In Australia this is called "single brick" or "brick veneer". The outside layer is a full brick in width, then there is R 3.5 or something like that in fibreglass insulation batts, I forget which, and then plaster board on the inside layer.

4) So wicking up via the masonry can be a path for moisture getting up and affecting the inside of the walls up there? Those interior "stumps" also appear to be able to do the same.

Yes, that is the case. The building inspector wasn't concerned about it when I was about to purchase the property, due to the fact it is basically dry long before the wooden floor starts. In the area that had been constantly wet for maybe 8 months of the year excluding summer due to the condensate from the heat pump unit, the wicking appears evident from the moss that has grown there.

Once one follows the brick walls down (below what is visible), is there a "footing" upon which the brick wall rests, spreading out the loading? What I am wondering is could one practically coat that wall below any earth cover elevation to prevent water from soaking in? (As I have done).

I will need to check. Maybe it's possible, but I am not sure how deep it goes, and I would think that unless you can coat the whole thing right around, it's just going to find a way in from the uncovered place.

5) If you were to really open up the area under the house to vent out any moisture would you have to insulate the floor above so as to keep the living area above from having a temperature change not wanted?

Actually, I have bought insulation for under the floor, that I was going to put in after the French drain went in. The thinking was that it would be less clammy and damp while I do that job, which will be not every enjoyable. I did actually set up an 80mm computer fan to blow through a hole into the crawl space, which would change the air maybe once every 12 hours or so. However, I am concerned that the dew point has to be low enough such that under the house, which is likely around 13 deg C (annual average temp), will not cause the warmer air to condense. The fan didn't seem to do much. I was thinking that with at least a dehumidifier, you guarantee that the air put out is less humid than the air going in. Maybe blocking the vents temporarily to get rid of the water in the dirt present now, after which it would be at a steady state situation hopefully, would be the way to go.

Hey, my neighbor has a 3 year old boy with his low height basketball hoop also. I think dad is pushing it though having just now installed a full height basketball hoop with some height adjustment capability. So far the kid seems uninterested in both hoops. Dad was a University of Wisconsin football celebrity a few years ago.

That's funny... my son is 2.5 now, and he plays with the hoop occasionally. The sandpit is more popular. Believe it or not, I graduated EE at the University of Wisconsin - Madison and my father has a degree in Mining Engineering and another in Naval Science from the same. It was a good school, I had a great time there.

I probably need to get under the house again to check the issues you have raised and confirm. Thanks for your help. BTW... any idea for the spacing for the 3/16" holes? (~5mm). I assume just two lines of them, at 1/4 or 1/3 marks. In Australia, there appears to be some sort of standard for pipe made for drainage with two lines ground into it with a grinder, alternated spacing. But it's expensive, so I figure hand drilling will be easy enough.

 

[oldestguy]

All sounds good. What seems odd however, it appears that your house type of construction is "standard". It may be that with the shallow depth to full saturation (groundwater) might be a little unusual. As to hles in the pipe Armco Steel (now another name) used to sell underdrain pipe with holes about 16 per foot, but on 8" diameter. Actually almost any spacing is fine, since over a considerable length the area of holes will exceed pipe open area.

The 3/8" seemed a little large to me after seeing them installed.
But on bottom they were less likely to allow sand tog et in.

 

[oldestguy]

OG again. On the subject of using bentonite for waterproofing, one has to be very careful. Given plenty of water bentonite will swell to 16 times its original volume and along the way it is as slick as warm grease. It has to be mixed thoroughly and a content of 10 percent is plenty. Too much and you have a mess. Use only powdered bentonite to get a good blend. In a confined place it should be mixed outside and then placed in a layer of roughly 3 inches thick. Not mixed would be bad.

Reason for knowing the ground surface elevations was if there is a shallow basement lower than the outside drains. Apparently not a problem.

 

[ausee]

Ok, it seems reasonable that the holes are 3/8, if the pipe is 8", based on the picture. I will use a 9mm drill to drill the holes. If that is sufficiently small to allow bridging, then it should work for the pipe. 100mm = 3.94" ~= 4". A 4" pipe carries 1/4 the flow at the same speed, so 1/4 the hole area per length should be adequate. Thus a row of holes each side, 9mm diameter, 6cm apart (looks like 3cm apart in the original).

Thanks for the pic.