Retaining Wall - Sheet Drain Versus Granular Fill Material 1

[JoelTXCive]

I have some larger retaining walls under construction now.

The wall heights vary from 2ft cantilevered sections to 40ft. counterfort walls.

Currently we have 3ft of granular fill material directly behind the wall.

The contractor has submitted an RFI wanting to delete the granular fill material; and then add plastic drainage sheets.

Does anyone have an opinion on these sheet drains? I don't doubt that once water hits the sheet drains; it is able to flow down to the weep holes.

I'm concerned that I am losing my 'buffer zone' around the walls though. I have a question out to the geotech, but wanted to ask others too.

Thank you!

 

[KootK]

I am nowhere close to being an expert but I feel that I'd actually prefer the drainage mat. Less chance of getting gunked up I would think.

 

[jayrod12]

Does the backfill the contractor wants to use change the pressure diagram against your wall?

 

[JoelTXCive]

Jayrod - No, the contractor just wants to install the drainage mat in lieu of the granular fill material. The wall was designed for the in-situ levee fills though; and not the drain rock. I was maxed out on my design for overturning and concrete capacity; so I don't want to do anything that could increase the lateral earth pressure delivered to the walls.

I have had this come up on other jobs and we have responded no.

This a a large job and it's with a contractor we have a good relationship with; so I want to give it due consideration.

I reached out to the manufacturer of the sheet drain product today and asked them if they have any research literature comparing their product to the normal blanket of granular material.

I'd be happiest if the geotech would make the decision, but I sealed the walls; so I think ultimately will be my call.

 

[HTURKAK]

Sheet drain is also common in my zone for the last three decades.. I agree that replaces traditional sand and gravel drains.
I have two objections for the use of sheet drain;

- Although the catalogs say 'excellent durability and long-term performance' we do not have 100 years performance data .IMO, this product is not fully buried and so ,subject to oxidation..

- If the contract is LS contract and signed for the granular fill material, this will also be contractual matter ,and should be agreed with client for negative change order and equitable price reduction.

 

[ATSE]

Both sheet drain and gravel wrapped in a drainage geotextile for all retaining walls over 6 ft tall. Not kidding. After 30 years, it might still work. I deal with 40 and 50 year old walls, and the drainage has failed half the time.
Not your question, but here an uninvited opinion: 40ft tall counterfort wall seems to be the most uneconomical construction I can imagine.

 

[Enable]

Around these parts we use TREMDrain or MiraDRAIN without granular backfill all the time. Backfilling two types of material side by side is a PIA to do correctly, and is rather expensive. Most contractors do what KootK suggested and kinda...well...just throw shit in the hole and so it gets done quick and no one sees the lack of separation.

That said, I cant say I've ever seen it for a 40ft high counterfort wall. Never done one. I'd ask the manufacturer about it.

BTW at 40ft the lateral earth pressure will not be trivial and so make-sure that the matt you accept (if you do) and the one installed are load rated for that kind of application. There are many drainage matts, almost all looking identical, but with vastly different compressive strengths and ability to hold form under load. If you put the wrong one in place it might crush with that kind of lateral pressure! We use high-load drainage matts on roof decks in fire truck paths for example.

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[JoelTXCive]

Yes, the counterforts are not as efficient as an MSE or a soil nail wall, but concrete structures similar to this are how all the large drainage structures in the region are constructed.

There are half a dozen little municipalities south of Houston; and they all have large concrete drainage structures similar to what we designed.

The older structures built in the 1980s and 1990s are also 40ft tall; and they are designed as pure cantilevered wall sections, which is really crazy.

 

[ATSE]

One more thing: the red clouded comment about less than 10% fines passing #200. I suggest this note be edited.
Should be more like 2% or less. Heck, you might be able to run a PL test with 10% fines (you need washed drain rock, not dirty clayey rock).

 

[JedClampett]

Similar to others, our standard approach is to use the drain board. I just think that the gravel, drain rock, etc. is a pain to install and to inspect its installation. I suspect that the gravel serves the purpose (removing groundwater from the face), but we don't trust it.
As far as sizing the board, I think you're never talking about high flows. Has anyone ever seen a retaining wall weep hole with full flows? Normally they just dribble a little. I go to the Mirafi website (a challenging one) and pick a drain board with a filter attached.

 

[wrxsti]

JoelTXCive what is the wall bottom thickness for the purely cantilever 40ft?

 

[JoelTXCive]

On the old ~1990 plans.... the walls are 48" thick at the bottom; and the footing is also 48" thick. For the main tension bars, they have #11 bars at 7" spacing.

When I first started the project, I attempted to recycle the design; but I couldn't get the numbers to work. I couldn't get the design to work unless I dropped the lateral earth pressures down to something in the 40 or 50 pcf region. My geotech report has equivalent lateral earth values approaching 80pcf if fully saturated.

I abandoned their design and came up with my own counterfort design. I still have a 48" thick footing, but my wall thickness is cut down to 30" (the shear is the killer).

Here's 2 photos from last week......the large blocks are the counterfort bases (4 per side). This is an outfall structure that penetrates through a levee. For constructability, I use the same counterfort base at the bottom, then I start to make geometric changes in the upper regions. The shorter counterforts don't need as much steel.

I also transition to normal battered cantilevered walls when the wall height gets down around ~22ft or so.

The huge grade beams you see tie the two sides together to keep the walls from sliding (the grade beams didn't seem that large on the plans - I might have overkilled them a tad bit.)

 

[JoelTXCive]

 

[wrxsti]

thanks for sharing joeltxcive

what can be done if you were to do that height around a corner though?
do people counterfort around a corner?