Negative skin friction and geotechnical capacity 1

[Flexibl]

During the checking of a bridge abutment on piles, the question to use or not to use the Downdrag load to check conformity against geotechnical capacity arised.

HP310x94 metric batter piles (1 hor./4 vert.slope) are driven to the end bearing rock through 20 m of compressible clay and a thin till layer from 0.1 to 0.6 m.

- Structural capacity of piles was checked at neutral plane near rock level for DL + Downdrag (1000 + 1200 = 2200 kN factored)
- Geotechnical capacity is planified to be tested on site with PDA method at 1750 kN (3500 / SF of 2). This will verify only that DL + LL (1000 + 300 = 1300 < 1750 kN)
The combination DL + Downdrag is not accounted for against geotechnical capacity (2200 kN > 1750 kN.)

Since neutral plane is at rock level and in this case geotechnical capacity equals structural capacity, I beleive that geotechnical capacity should be tested for the same load than the structural capacity (2200 kN).
The question will be irrelevant for vertical piles if they are well seated on rock because the real geotechnical capacity will be much higher than 1750 or 2200 kN. For batter piles otherwise, the horizontal capacity at tip level is part of the geotechnical capacity.
This is a concern because the real load that will experience the tip, in the future, is not tested.

Expected settlement given by geotechnical engineer is 25 mm maximum because light fill is used behind abutment. Secondary effects on piles due to this settlement where checked.
It is likely that piles with conventionnal shoes will not penetrate the extremely hard roc.

Since testing piles at 2200 x 2 = 4400 kN is impossible, I beleive we should reduce the combination DL+Downdrag to the level of 1750 kN and add more piles.

Help will be appreciated !

 

[Jalthi]

I assume the project site is in Canada. If so, my understanding of CFEM is that the two loading cases to be analyzed for structural capacity are:
1. DL + drag load but no LL
2. DL + LL but no drag load

However, the geotechnical axial capacity is selected to resist DL + LL (Case 2) because down-drag load does not affect the geotechnical axial capacity (?).

If my interpretation is correct, you are in the safe side. You need to verify only for 1300x2 kN.

 

[Melzar]

What about bending induced by the downdrag? Is this included in the "secondary effects" described in the OP? The AASHTO LRFD manual states simply to avoid battered piles in cases of downdrag, or something to that effect.

 

[BigH]

Is downdrag really an issue if the settlement is only 25 mm?

 

[Ron]

Bingo, BigH....you're at end bearing. Unless you're concerned about buckling the pile (which will take the pile length to develop for buckling...so not likely), who cares about the downdrag? It's one of the least accurate parameters you can predict.

 

[paganel]

For such problem,it is not so relevent for the geotechnical bearing capacity as it goes into the rock. With larger settlement/displacement, obviously the pile will have a larger bearing capacity. So only the settlement/displacement matters other the bearing capacity.

For the similar situation, B.Fellenius has some very good theory.You could make a search on the internet.

 

[BigH]

Suggest that colleagues interested read some of the papers from I think the 1965 ISSMFE conference - one of the conferences back then had a number of papers with horrific results of huge downdrags on piles to rock - with large compressible soils. More load as the tip is restrained. But for the 25 mm suggested here . . .

 

[molerat2210]

Downdrag is a load (not sure if considered a live load or not) that should be included in the load test program. 25 mm seems just at the lower limit to induce downdrag. I do not about the depth of the compressible layer. But, it may be possible to consider less downdrag by only counting the for a portion of the compressible layer (as opposed to the entire layer). This is likely justified if your calculations show that the expected settlment is below the threshold to induce downdrage at the bottom of the layer.

 

[irawanfirmansyah]

Negative skin friction will develop if the compressible soil settling more than the pile settlement. Neutral plane will be located slightly below the rock surface, but for practical purpose, it cab be assumed at the rock surface.

I normally check equilibrium in vertical direction taking into account the negative skin friction, and adopt safety factor of 1.5 against plunging

 

[emmgjld]

I agree with BigH & Ron.
Look at the numbers, 25mm against 20meters.
The amount of settlement is less than what I would consider practical to compute, considering the amount of movement required to develop loads on the pile, even considering the battered pile geometry. The soil settlement should probably be considered trivial.