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dw8
November 15th, 2007, 08:32 AM
Webmaster, if this is wrong forum please move and notify.

Just completed a mortared rock retaining wall 100 feet long by 5 1/2 feet high from grade. Wall wraps back to bank on one end but is open at low end.

Drainage next issue, before backfill.

Wall has weep holes every 4 to 5 feet, but are not on a straight horizontal plane because of the rock. Also, because rock, the wall is not perfectly straight in back (juts in and out depending of rock size).

Have looked at some of the newer products (e.g. "Mirafti") which is a plastic waffle mat coated with a nonwoven polypropolene filter fabric. Rejected because of inability to contour product along rocks and connect to weep hole pipe. Question also exists regarding life of product -- both plastic mat and filter fabric.

Wall curves so laying rigid perforated PVC presents a problem along the wall at its base. Local supplier of flexible perforated PVC indicated max burial depth is 4 feet.

Looking for a alternate drainage solution. Considering 6 inches crushed stone along back of wall with filter fabric (still have question re: life of product), then ABC backfill. No pipe will be used.

Questions:

1) Any experiences burying flexible perforated PVC greater than 4 feet?
2) Best way to filter solids to get the maximum flow life from the gravel?
3) Would pee gravel in between crushed stone and ABC work better to help filter solids?
4) Will the above provide adequate -- and long lived -- drainage?
4) Any help / suggestions greatly appreciated.

David

Wgoodrich
November 15th, 2007, 11:14 AM
Drainage is not required for a retainer wall. If you wish to install a drainage system then a 4" perferrated flexible hose laying along the bottom of the wall not compromising any footer with a 12" x 12" stone bed of number 4 or larger washed rock or stone bed with a filter cloth on the top of the stone bed would serve you well.

Life expectency of a filter cloth is only a concern until the dirt settles back to normal conditions. The dirt fines then is filtered by the settled dirt no longer needing the filter cloth. The filter cloth will have served its purpose once the disturbed dirt has equallized say about 5 years back to normal conditions. Then the normal subsurface soil will filter the dirt fines from getting to the stone bed to plug it up. That is the purpose of a filter cloth.

Pea gravel is so symetric that it is a pour rip rap bed to increase water flow. Test this yourself. Go to a gravel company in july and have the end loader pick up a scoop of 5L washed rock and no water will pour out. Then have him dump that load and pick up a load of pea gravel and the trapped water within that pea gravel water will pour out even in July.

You have a 100 foot long wall but have said nothing about installing ground anchors or a cantelever footer or Supporting columns buried into the bank to support that wall. Suspect you are inviting this wall to move being pushed by the dirt behind it over a few years without some type of supports to the bank.

Good Luck

Wg

dw8
November 19th, 2007, 08:04 AM
WGoodrich thanks for response


Drainage is not required for a retainer wall. If you wish to install a drainage system then a 4" perferrated flexible hose laying along the bottom of the wall not compromising any footer with a 12" x 12" stone bed of number 4 or larger washed rock or stone bed with a filter cloth on the top of the stone bed would serve you well.


I have weep holes and am contemplating also a 4 inch pipe. My intent was to put a 6 - 8 inch wide strip of rock from the footing all the way to within 1 foot of the top of the wall to get good drainage. Based on your comment if appears rock other than to bury the pipe is not needed. Is this correct?


Life expectency of a filter cloth is only a concern until the dirt settles back to normal conditions. The dirt fines then is filtered by the settled dirt no longer needing the filter cloth. The filter cloth will have served its purpose once the disturbed dirt has equallized say about 5 years back to normal conditions. Then the normal subsurface soil will filter the dirt fines from getting to the stone bed to plug it up. That is the purpose of a filter cloth.


Would not the filter cloth become clogged (with sand/organics) over the five year period limiting the flow of water to the rock bed and pipe?

Any suggestions for other long-lived media, beside filter cloth, that would work as a good filter while things are settling -- someone suggested 15lb felt paper but don't see how water will flow through the paper to the rock bed?


You have a 100 foot long wall but have said nothing about installing ground anchors or a cantelever footer or Supporting columns buried into the bank to support that wall. Suspect you are inviting this wall to move being pushed by the dirt behind it over a few years without some type of supports to the bank.


The wall is battered 1 1/2 inch per foot of height. No tie in back to bank -- deadmen. Not sure how you would tie a rock wall back to the bank.

What's the best way to tie in a rock wall to a bank (details appreciated)?

Thanks
David

Wgoodrich
November 19th, 2007, 08:46 AM
The filter cloth catches the dirt fines but does not plug up. This method of the filter cloth has been used for both subsurface drainage and septic fields alike for decades without a problem of plugging that filter cloth. However many stone beds have plugged up with dirt fines if the filter cloth was not installed on top of the stone bed. I would not use felt paper because you would be starting out with a plugged filter. Felt paper is not designed for water to flow through at all. The filter cloth is the same cloth as use over the fingers of a septic system also having used this filter cloth method for decades successfully.

If your footer for your retainer wall is not L shaped with the flat part of the L behind the wall with the fill dirt sitting on it then you need another type of support. Many retainer walls have an L of the wall placed at the ends using the angled retaineer wall to keep the wall from being pushed out be the fill dirt behind the wall. A third method is to drill a hold in the mortar installing all thread or rebar through the mortar a ways into the dirt behind the wall then welding a flat plate at the end about 3' behind the wall to be buried in the dirt holding the wall from being pushed out. This is done about every 12' along the wall. A fourth method used for existing retainer walls would be drill a larger hole through the mortar removing a rock from the wall then drilling an auger type anchor into the earth with a long rod attached to the auger that has threads on the end of that rod then installing a bolt and washer to be buried inside hte wall in the mortar when reinstalling that removed rock by mortaring it back in place after the anchor has been installed. This option is often done to stop an existing retainer wall from being puhed out over the years by freeze and thaw of the earth behind that existing retainer wall has already done the pushing out of the wall. This is a common fix for existing retainer walls that were not supported as we are discussing doing on your new wall before you back fill.

Good Luck

Wg

dw8
November 19th, 2007, 05:47 PM
Wgoodrich.

Thanks again for your time and input.

Will definitely take your comments into consideration.

Happy Thanksgiving to you and your family.

David