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Old 02-06-2010, 08:28 PM   #1
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Drain Tile Repair


I live in Vancouver and have had problems with water coming into my basement. I decided to have a contractor dig out about 20 feet along the back of the house and remove the clay soil and replace wth 3/4" crush together with sealing the foundation and installing PVC drain pipe. Work has gone well expect that the 3/4" plywood used as shoring has been left in. For me this is a problem as the shoring runs from the bottom of the foundation to about 2 feet below grade and needs be removed as it will be interfere with water flow. The contractor thinks this is not a problem and was left in because the ground is unstable and would have caved in. I don't buy this. Does this plywood need to be removed?

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Old 02-07-2010, 07:37 AM   #2
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Drain Tile Repair


as best i understand you, the ply was used for shoring to prevent caveing in ? if that's the case, i'd have no problem leaving it IF its already backfill'd,,, if NOT, grab it - plywood's expensive & its easier to save a $ rather'n go earn it.

hopefully your pvc drain runs to daylight - otherwise all you've done is create a big pond at the btom of your fnd walls which'll find its way into your very fine bsmt some other place.

acceptable foundation sealing mtls can be found at any apron store or roofing supply,,, paintable emulsions're NOT acceptable to me - we trowel OR spray it on then protect w/miradrain-type mtl.

ps - any contractor who does this work professionally will not ' think ' - he'll KNOW !

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Old 02-07-2010, 10:16 AM   #3
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Drain Tile Repair


Thanks for the feedback. Yes the weeping tile is connected to a sump which is in turn connected to city lines. So you have no concern that plywood will impact water flowing intot he drain tile. The back fill is already in so that is why they cannot remove it. I agree contractor should KNOW, that is why I am asking the questions as he is not giving me a lot of confidence!
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Old 02-07-2010, 10:26 AM   #4
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Drain Tile Repair


If the crushed stone extends to below the level of the plywood, and the PVC is perforated pipe (I assume it is), then groundwater will enter the pipe through the crushed stone from below. If you post a sketch of the setup, that would help. From your description, the plywood will act as a partial barrier to groundwater entering the perforated PVC pipe from the side, which will have the effect of reducing the flow into the pipe from the flow you would get IF the plywood were removed. This is probably OK, since the flow into the pipe will likely be adequate to maintain groundwater below the foundation level even if the only entrance point into the pipe is from below. By the way, the perforation holes should face down. And the pipe should be surrounded by filter fabric to prevent clogging, assuming you used perforated pipe. If you installed solid pipe, then that is a completely different discussion, and we would need to know how the groundwater is getting into the pipe.

Long term, the plywood is going to rot,however this may take many years.
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Old 02-07-2010, 10:27 AM   #5
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never had any concern as its not MY house we can't connect to sanitary sewer or the waste wtr treatment plants would be o'loaded.

' not giving me a lot of confidence ' - why'd you hire him ?
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Old 02-07-2010, 10:27 AM   #6
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Drain Tile Repair


never heard of anywhere that would allow tie in to city systems, is that legal there, does city know about it as usually they are involved with tap at their system even with storm water. this is storm water to storm water not storm to city sewer?
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Old 02-07-2010, 10:36 AM   #7
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Drain Tile Repair


I don't see any reason to be concerned about the plywood left in place. It may actually serve a little benefit until it rots away, as stated above.

Is it actually PVC or black perf ABS?

Around here, we have both sanitary & storm sewer. Most munis let you hook into the storm without any issues.

Sanitary is a big no-no.
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Old 02-07-2010, 01:31 PM   #8
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In vancouver it is ok to hook into storm water system to drain weeping tile and down spouts from gutters.

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never heard of anywhere that would allow tie in to city systems, is that legal there, does city know about it as usually they are involved with tap at their system even with storm water. this is storm water to storm water not storm to city sewer?
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Old 02-07-2010, 01:53 PM   #9
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I have attached two picturs showing the configuration of the shoring. They have used 4'x8' pieces of plywood for shoring with 2' pieces at the bottom of the trench. To give some background, work had been previously been done; including new pvc weeping tile with some stone around the pipe. But leaking reoccured - reasons being on sloped property, high clay content, high rainfall and blockages in tile - soil becoming waterlogged and water not flowing into weeping tile. Resolution has been to flush weeping tile and then dig out 20 feet (total backwall is 40' long and I did middle section) and fill with 3/4" crush to grade. So this system is challanged and I am concerned that water will move to the ends of the 20' section repaired and overload the system in those areas. I get your point that the water will go under the plywood and into the perforated PVC pipe - to clarify the shoring goes to the bottom of the hole and conseqently to the bottom of the gravel - the PVC drain pipe is about 6 to 12" above the bottom of the shoring.

Thanks!
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Originally Posted by Daniel Holzman View Post
If the crushed stone extends to below the level of the plywood, and the PVC is perforated pipe (I assume it is), then groundwater will enter the pipe through the crushed stone from below. If you post a sketch of the setup, that would help. From your description, the plywood will act as a partial barrier to groundwater entering the perforated PVC pipe from the side, which will have the effect of reducing the flow into the pipe from the flow you would get IF the plywood were removed. This is probably OK, since the flow into the pipe will likely be adequate to maintain groundwater below the foundation level even if the only entrance point into the pipe is from below. By the way, the perforation holes should face down. And the pipe should be surrounded by filter fabric to prevent clogging, assuming you used perforated pipe. If you installed solid pipe, then that is a completely different discussion, and we would need to know how the groundwater is getting into the pipe.

Long term, the plywood is going to rot,however this may take many years.
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Drain Tile Repair-view-1.jpg   Drain Tile Repair-view-2.jpg  
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Old 02-07-2010, 02:07 PM   #10
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Drain Tile Repair


I cannot tell from the photos if that is perforated pipe, I assumed it was from your post. It is critical that the crushed stone that was presumably placed be surrounded by filter cloth, else you are going to get fines into the soil and into the pipe, and it will stop working correctly. As for the plywood, I would have pulled it after filling of the trench with crushed stone surrounded by filter fabric, as this would allow the side of the trench to drain into the crushed stone and ultimately into the pipe. However, it seems that the crushed stone extends down to the bottom of the plywood, in which case you are going to get flow underneath the plywood and up into the pipe, as I previously noted this is not ideal, but probably OK.
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Old 02-07-2010, 03:03 PM   #11
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can't tell either so the holes must be on the btm which's fine,,, we ALWAYS lay out pipe on bedding stone w/filter cloth under the stone - did it in nj, ny, ct, de, pa, &, now, ga,,, however, the ' sock'd ' pipe isn't satisfactory as the amt of sq feet avail to filter fines is too small to be effective for a long time [ ' long ' isn't defineable ' ] fold the filter cloth over the top of the stone fill.

dan's right - pull the ply as you backfll w/stone,,, you don't need to completely fill w/stone - we like 12' depth x 18' width of stone.
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Old 02-07-2010, 03:21 PM   #12
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Yes the pipe is perforated with the holes facing down. A filter cloth was placed at the bottom of the trench (below the drain tile) and then filled with stone. If I understand correctly you said that filter fabric should have been placed up the side walls of the trench - this makes sense but was not done.

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Originally Posted by Daniel Holzman View Post
I cannot tell from the photos if that is perforated pipe, I assumed it was from your post. It is critical that the crushed stone that was presumably placed be surrounded by filter cloth, else you are going to get fines into the soil and into the pipe, and it will stop working correctly. As for the plywood, I would have pulled it after filling of the trench with crushed stone surrounded by filter fabric, as this would allow the side of the trench to drain into the crushed stone and ultimately into the pipe. However, it seems that the crushed stone extends down to the bottom of the plywood, in which case you are going to get flow underneath the plywood and up into the pipe, as I previously noted this is not ideal, but probably OK.
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Old 02-07-2010, 04:16 PM   #13
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Drain Tile Repair


The purpose of the filter fabric is to prevent fines from migrating into the crushed stone and ultimately into the pipe. Fines will reduce the permeability of the stone, and could eventually fill the pipe. Proper procedure is to place the filter fabric on the bottom of the trench (as was done), and up the sides to approximately a foot above the pipe, then across the top of the stone. This creates a square "tube" of filter fabric encapsulating stone around the pipe, and minimizes the chances of fines entering the pipe and stone. If the natural soil surrounding the trench has little or no fines, you can relax the design of the filter fabric.
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Old 02-07-2010, 05:40 PM   #14
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Thanks, I appreciate you answering my questions

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Originally Posted by Daniel Holzman View Post
The purpose of the filter fabric is to prevent fines from migrating into the crushed stone and ultimately into the pipe. Fines will reduce the permeability of the stone, and could eventually fill the pipe. Proper procedure is to place the filter fabric on the bottom of the trench (as was done), and up the sides to approximately a foot above the pipe, then across the top of the stone. This creates a square "tube" of filter fabric encapsulating stone around the pipe, and minimizes the chances of fines entering the pipe and stone. If the natural soil surrounding the trench has little or no fines, you can relax the design of the filter fabric.

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