Using a sonotube (a cardboard tube) to prevent drag down during a surcharge period 1

[dwathugala]

Hi
I have a project to construct an embankment fill in between two retaining walls. The foundation type of these walls is driven steel pipe piles.
The soil is expected to have a long term settlement due to embankment fill (the soil at the site is soft to medium stiff clay). I recommended to have a surcharge (over the embankment fill) and a settlement period before driving piles. The contractor suggests the procedure given below.
1. Excavate to bottom of footing
2. Drill 30” hole 5’ deep and place 30” x 5’ Sonotube (it is a cardboard tube)
3. Drive the piles to tip elevation
4. Fill the annular ring between the pipe pile and Sonotube with either pea gravel or sand
5. Fill any gap between the Sonotube and dirt with slurry
6. Place dirt for surcharge

The idea of the Sonotube is that it will prevent drag down during the surcharge period.
Does anybody know about such a procedure?
(I do not know why contractor suggests the depth of the hole to be only 5’. I think it should be more than that, extending to cover the compressible soil layer).
Thank you.

 

[oldestguy]

I'd be more concerned about the surcharge moving the walls sideways due to the unbalanced horizontal pressures on the piles down in the soft layer.
Assuming a good end bearing for piles, theoe downward loadings usually can be tolerated, such as for bridge backfill at abutments, as a road over a stream.

 

[PEinc]

Ideally, the proposed foundation piles should be sleeved from the bottom of the compressible soil zone to the top of the surcharge embankment. However, it could be impossible to sleeve through the compressible soil zone. Usually, the bearing piles are sleeved through the embankment that will be placed above the compressible zone. The bearing piles are then designed to resist the downdrag load that results when the compressible soil settles while adhering to the piles. Details for this can be found on the web sites for state DOT's where they show their standard details drawings for bridges built above MSE walls. The sleeve, filled with cohesionless soil, prevents or at least minimizes the settling soils from dragging down the bearing piles.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

Sonotube is a poor choice to use as a sleeve. They are designed to contain the internal pressure from fresh concrete, not external pressure (that they could experience until backfilled). Also their structural strength is limited - that's probably the reason the Contractor proposed a depth of only 5'. The tube would probably crush (longitudinally) during installation of a longer length. Sonotube's relatively low cost is likely another reason for the Contractor including it in his proposal.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[PEinc]

SlideRuleEra is correct. These sleeves are usually made of corrugated metal or corrugated plastic pipe.

http://www.PeirceEngineering.com

 

[ATSE]

Before you do anything silly like coating or sleeving piles, ask: How much additional settlement do you expect from the new embankment fill? Evaluate settlement of the pile tops. If you piles are primarily end-bearing and the soil strata below the neutral plane is nearly imcompressible and the pile has sufficient structural strength, leave it alone and drive / drill conventional piles without the coating or sleeving. Yes, negative skin friction adds load to the pile. Negative skin friction and drag load are okay; downdrag is not.
Negative skin friction is almost always present (long term), and only requires 1mm to 2mm of relative movement between the adjacent soil and pile to engage. So don't de-rate your pile geotechnical capacity because of negative skin friction without understanding the settlement behavior, or you will end up de-rating every pile.
A good reference for this topic is B. Fellenius, who discusses negative skin friction and drag load and downdrag at great length in several papers and are core ideas in the Unified (Pile) Design Method.

 

[Ron]

Agree with SRE. In the past, we used PVC pipe for settlement plates. Haven't surcharged anything in quite a while! Also, I'm not so sure I'd be that concerned with a 5-foot exposure of the pile for downdrag.