Retaining Wall Drainage for Soil Types

[RMechEng75]

I'm an ME with a background in HVAC & light residential construction. I'm in the process of planning a retaining wall with a height of approximately 26 inches above finished grade in northern NJ (morris county). The soil type is listed as "RkgBc" Ridgebury Stony Loam (extremely stony) sloped. My questions are regarding the base material below the first course of wall stone, drainage pipe height & Geo-filter fabric or pipe sleeve.

The foundation base trench will be approximately 10-12 inches deep & 36 inches wide. The subsoil will then be leveled & compacted (plate compactor). Approximately 6 inches (depth) of foundation base material (3/4 inch angular crushed clean gravel) will be then be added to trench, then leveled & compacted. The first course of wall stone will sit on this gravel base below grade. Behind the wall will be filled with 3/4 inch clean gravel (same as base) for the entire length with a minimum depth of 12 inches from the rear wall. The trench does have an overall pitch since property is sloped. The finished area behind the wall will be back filled with the native soil & 3/4 inch gravel, then graded level for a pea gravel at finish. Fabric will be installed beneath the finshed pea-gravel layer. The drain pipe behind the wall will be perforated with holes down.

OBSERVATION: The soil is definitely very rocky with many various size angular rocks from golf ball size to that of a shoe box. The trench is partially dug to a depth of 8 inches. After a heavy rain the night before, the lowest point in the trench was full of water the following day. I had to dig/cut a slit to drain it out the side.

My questions are;
1. Shall the drain pipe behind the wall be embedded within the gravel base below the 1st course of wall stone or shall it sit on top of the base? Refer to sketch.

2. Shall the drain pipe be covered with a pipe sock/sleeve or shall the entire gravel foundation base & drainage gravel area behind the wall be enclosed in a Geo-filter fabric similar to a burrito style to keep debris out?

3. Shall the compacted base material beneath the wall stone be something other than the 3/4inch clean angular crushed gravel?

My research has revealed conflicting opinions regarding wrapping the drain pipe in a sleeve/sock or enclosing the entire gravel & pipe area with filter fabric similar to a burrito. I'm sure the site and soil conditions would dictate which course of action to pursue since different soils perform differently regarding clogging up the pipe or fabric holes.

Any guidance is highly appreciated. thank you

 

[RMechEng75]

See Sketch

 

[ACtrafficengr]

What arguments do they give for just wrapping the drain pipe? The more surface area your filter has, the longer it will take to clog.

Given the pointy rocks, you'll want an nonwoven geotextile. The salesman may try to sell you a woven one quoting the tensile strength, but it's bursting strength and tear propagation resistance you want here.

Where will your pipe outlet be?

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[RMechEng75]

ACtrafficenger- The pipe outlet for both scenarios will be to daylight beyond front face of the wall

 

[RMechEng75]

Refer to sketch that was posted after the original post

 

[LOTE]

Hi RMechEng75,
I have a few links to my blog that I hope will be helpful:
1. If you have a toe slope, the preference would be to place the pipe down in the base course, but behind the stones/blocks. If the ground is relatively flat, then the pipe should exist the wall at grade. If the pipe outlets at grade, the material below it should be a lower permeable material, something with 15-35% fines, to force water out the drain pipe and the wall face before it gets to the foundation.
2. If you use fabric, it should wrap the entire extents of the drainage stone, not just a sock around the pipe. It does not sound like you have clay, though loam has the potential to clog the fabric. If you have more than 35% fines, you may consider a sand filter behind the fabric. Check out this article for additional details.
3. A fine aggregate or screenings will be easier to level, but if you are bringing your drain pipe down to the base material, then it should be 3/4" clean gravel.

I hope this helps. Let me know if you have any additional questions.

 

[RMechEng75]

LOTE-
thank you...

 

[RMechEng75]

LOTE-
thank you for thorough response. I had follow on questions however the detailed drawing in the link you provided was extremely helpful. Thank you again....

 

[RMechEng75]

LOTE- what weight do you recommend on the non-woven Geo-filter fabric?

 

[dik]

with geotech fabric enclosure to keep things from 'silting' up.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[LOTE]

I recommend a 6oz nonwoven fabric.

 

[RMechEng75]

LOTE-
thanks again. What type of sand would you recommend between the fabric & native loam soil?

 

[LOTE]

RMechEng75, sorry for the delay. A concrete sand or similar with less than 15% fines.