Retaining wall with no subdrain 1

[ckissick]

I have a landslide repair job with a proposed stitch-pier wall. There will be a leach field right behind the wall, so the permit agency won't allow a subdrain system behind the wall. The active soil pressure would be 50 PCF if it were drained. If pore pressures can build up behind the wall, what total lateral pressure should be applied for design if soil and water pressures are both mobilized?

 

[MTNClimber]

Is a stitch pier wall another name for a soldier pile and lagging wall? How close is the leach field?

Assuming that the liquid will build up to the top of the wall, active conditions, no surcharge load, and the retained soil is horizontal, Effective Unit Weight of Soil x Ka + Unit weight of Liquid i.e. for regular water: (125 pcf - 62.4 pcf) x (tan(45-phi/2))^2 + 62.4 pcf.

If the hydrostatic pressure won't build up to the top of the wall then you can't just use a normal triangular distributed load anymore. You'll have to resolve the forces... or just use a design software.

 

[ckissick]

That makes sense. Thank you.

 

[GeoPaveTraffic]

If you are designing a wall to stabilize a landslide, then the pressure on the wall will likely be much greater than active.

Mike Lambert

 

[oldestguy]

So you don't place a pipe back there. What's to stop you from backfilling at least the lower zone behind the wall with a sand and gravel mix? Then place weep holes in the wall every 5 feet. Any seepage build up back there will find its way out below the wall regardless. You might warn the authorities that build up of ground water behind the wall likely can cause a land slide also taking the leach field. Seepage drained via a sand and gravel zone will filter out suspended solids. So what comes out the weep holes will be clean water.

Mike: That 125 #/cf liquid is active. How does it get any greater?

 

[BridgeSmith]

OG, landslides can produce pressures up to passive soil pressure conditions.

 

[oldestguy]

OK guys. However, as with many posts,a complete picture is not present. At least we need a profile view of the situation.

 

[ckissick]

The complete picture: The ground above the retaining wall will be excavated, benched, and replaced as compacted fill. The slip plane will be gone. The uphill slope will be inclined at 4:1. The County will not allow weep holes or sand or any sub-drainage at all. There's a creek downhill from the wall and they're very concerned about that. For filtration to be effective they would want a distance of at least 100 feet to the creek, but we don't have that. They still don't like it, but there was a leach field there before and the alternative would be to tell the home owner to demo the house and walk away. And this is in California, so that's a lot of money.

As an aside, if the creek were "impacted", or contaminated, they would require a septic setback of 600 feet!

 

[PEinc]

As OG mentioned, there is not enough information provided to determine what type of wall is needed. Nothing has been said about the soil types or conditions or about ground water levels. If there will be a built-up water head behind the wall, what will keep it from seeping under the wall or around the ends of the wall and then heading toward the creek? It is possible to design a (tiedback?) "stitch" wall to retain the soil, its landslide earth pressures, and full water pressure. However, the water will find its way around or under the wall. The County is kidding themselves if they think that no ground water will get to the creek. It's probably getting there now. A soldier beam and lagging wall would be a porous wall if it does not have a solid, waterproof permanent facing. Precast lagging and wood lagging will not retain water. You would need a CIP or shotcrete final facing or need a sheet pile wall (with joint sealant) if you want to retain water.

http://www.PeirceEngineering.com

 

[oldestguy]

What will the "compacted fill" be? Why not all of that as a porous fill, such as sand? Otherwise you will be blocking off any infiltration and the result will be sewage on the surface. If the county wants to help, ask them to run in a waste water collection pipe to carry that water to a treatment plant. I suppose a holding tank isn't in the picture either. Then I hear about toilets that somehow burn up the waste or showers that re-use the water. On site waste water treatment. Maybe that's the goal of the county. Look into modern submarines that stay submerged for weeks.

 

[BridgeSmith]

It looks to me that you'll have to design the retaining wall for the pressure of saturated backfill + water pressure, similar to what MTNClimber proposed. However, I believe you'd use the actual unit weight of the backfill (125pcf may or may not be correct) and calculate the at-rest soil pressure for backfill under saturated conditions. I'm not sure the "(tan(45-phi/2))^2" is appropriate for all types of soil.

I can understand why they wouldn't allow a drain or weep holes, though. The leech water filtered through the soil all the way to the creek is somewhat better than having it collected and dumped or running on the surface in front of the weep holes. I'm somewhat surprised they haven't forced the homeowner to connect to a sewage system or abandon the house altogether, especially with how heavily regulated most things are in California.

 

[PEinc]

If this is a landslide site, a slope stability analysis will need to be run to get the required stabilizing force needed to provide the required global safety factor. You just can't design a regular old retaining wall to support a landslide.

Sometimes a retaining wall is not needed to stabilize a landslide. See the following links for examples of landslide stabilization using tieback anchors.

https://www.schnabel.com/landslide-walls/

https://www.schnabel.com/projects/whispering-hills/

https://www.schnabel.com/projects/pepperdine-graduate-campus/

http://www.PeirceEngineering.com

 

[BridgeSmith]

I reread the OP, and it does say "landslide repair job". If that's accurate, then PEinc is correct, the pressure on a wall could be considerably higher than even the at-rest pressure, and tiebacks, such as ground anchors or soils nails, may be a better solution than a retaining wall.

 

[ckissick]

As I said above, the landslide mass will be removed and replaced with compacted fill on a benched cut. There's no sewer treatment plant in the area. It's rural, but a small lot. There's no room to relocate the leach field. Anyway, everything has been worked out with the County. It's just a matter of applying the correct lateral earth pressure.

 

[PEinc]

ckissick, are you saying that the unstable sliding soil is only above the top of the proposed retaining wall? How do you know? What makes you think the landslide problem won't extend deeper when you remove and replace the upper unstable soil or the water table rises behind your wall? IMHO, based on the small amount of information and no sketch, you need to look at more than just a wall design. You have home owners who rarely have enough money to fix a problem; and you have a fussy, uncooperative County. Tread lightly! Don't skimp on your analyses.

http://www.PeirceEngineering.com

 

[oldestguy]

I second what PEinc says. If you miss calculate what has to be fixed, you maybe in for one helluva claim in court. Test borings, observation wells and lab tests plus analysis are needed. From what is given, it seems much more evaluation work is needed before any job work is started.

 

[ckissick]

We drilled three soil borings and did a few lab tests. There is stable bedrock at a depth of 7 to 10 feet. The landslide is not very big. 50 CY. The retaining wall will be a maximum of 4 feet tall, however we are neglecting passive resistance above the slip plan of the slide, since we can't touch the soil near the creek. It will probably creep downslope, so it's being designed as a 12-foot tall wall.

 

[oldestguy]

Again a case where we finally get some detail of what is going on. OK,so you clean off the material above rock and replace it with something compacted. Sounds like an ideal for a sealed piece of ground and no outlet. Thus the wall will have some pretty big loads. This then looks like a need for anchoring that wall to the rock. Once this basin is fully saturated, then what? I'd pose that to regulators. Have them show you another job like this that works. My bet is there are none. Time to bring the owner into these things, since they will have to face things when if fails. Here comes the holding tank.

 

[fattdad]

Even accounting for hydrostatic pressure and K-naught conditions, you are only dealing with 730 plf of earth pressure. Build a wall to withstand that and you are good to go!

f-d

ípapß gordo ainÆt no madre flaca!