Drainage for Tabgent Caisson Wall

[curvbridger]

Has any one seen an effective way to establish positive drainage behind a tangent caisson wall in a cut situation with an exposed height of about 30 feet?

Could vertical holes be drilled behind the shafts and filled with a free draining material and then outlet at the base of wall somehow?

Thanks,

Curvbridger

 

[ishvaaag]

I think it depends mainly on the nature of the soil behind the wall, for you want to build more or less a filter that won't become clogged midterm or longterm. You can build the shafts behind the wall (even many won't look these things with sympathy, since it can interfere with your wall) using any proper technique and then build the filter. But depending upon the case it may prove too costly to be worth the effort.

 

[PEinc]

curvbridger,

I did something very similar to this a few years ago. It wasn't a tangent caisson wall but I think our situations are similar.

A reinforced concrete cantilevered wall was leaning out about 7 inches in about 14 feet vertical. The wall's weep holes were clogged. The wall had no permeable backfill. The owner also piled his plowed snow right behind the back of the wall.

I designed tieback anchors for the wall to stop the leaning. Then I had the contractor clean the weep holes and drill a few more at the base of the wall. Then I had him auger vertical holes about 24 inches in diameter from OG to the top of footing, behind the wall, at each new and old weep hole. We made and installed a 24 inch diameter geosynthetic filter fabric tube vertically down each auger hole. We filled the fabric tubes with pea gravel which pushed against the back of the weep holes and filled the auger holes. Behind the top of the wall, we excavated a 2 feet wide trench about 2 feet deep which connected the tops of the stone filled auger holes. We lined and covered the trench with the same filter fabric and filled the trench with pea gravel. Then the contractor mulched the area and restored the plantings behind the wall. A facade was built in front of the concrete wall to hide the tieback anchor heads. When the work was finished, no repairs were visible except the new facade. The weep holes work fine.

 

[Focht3]

Hmmm,

I'm not fond of drilling sand filters behind a tangent wall - I see lots of performance issues. But it might work...

I prefer to excavate between shafts with a 150 mm (6 inch) side-to-side spacing as the excavation advances in front of the drilled shafts. (The shafts are offset slightly, not overlapping.) A small area between each pair of adjacent shafts can be excavated, and burlap bags containing concrete (filter) sand stuffed into the excavated areas. This approach has a minimal impact on the shaft performance while providing a high quality drain. A face wall can be gunited over the shafts, sand bags and drain piping after the excavation is complete.

Yes, it's labor intensive. But it's very effective.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[clayR]

Hello,
Depending on the soil type, horizontal drilled in drains are common, say spaced 6' horizontal by 20'-30'long, say 1-1/2" perforated pvc in a sock.
Also along Focht3 lines, going from tangent to closely spaced pile could be easier and cheaper. A slightly different twist could be drain board vs the sandbags with a shotcrete facing. This method is commonly used in closely spaced auger cast pile wall construction. Also, you may see economy from pile spacing, but to do this you must design a shotcrete facing for the span, but if water or unraveling is a problem then you could be asking for trouble.

 

[Focht3]

Drainage has to be controlled i.e. no seepage through the face of the wall. But raveling should not be a problem if groundwater isn't an issue - used this approach in a heavily slickensided OC clay in Houston. It works beautifully.

Indeed, the gunite face will be needed with this approach -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[curvbridger]

Thanks to all for your input. At this point, I intend to design for hydrostatic pressure, while tying to drain it away anyway. I will be using closely spaced rather than truly tangent shafts, although controlling this with temporary casing feels iffy. All things considered, horizontal drains seem most attractive to me, since the drainage system can have extension both in height and in depth behind the wall. Now, I have to decide about how to handle the weep flow as it falls on the toe of slope. Hmmm...

 

[Focht3]

A final comment...

Be aware that failure to put the drains between each pair of shafts and continuous vertically can result in real problems in performance from a usage standpoint. Gerald Hines' group in Houston decided to omit every other drain in the Galleria III garage in Houston - wall protects the shallow foundations for the Marshall Field's store (Alabama Rd. side.) The store is fine; but Hines has a maintenance headache: there's a vertical crack in the gunite face at every other gap between piers, and seepage through the finished wall. It has created a mold/mildew problem in the below grade parking area -

Wall drains have more than one function...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.