Negative skin friction and geotechnical capacity
Negative skin friction and geotechnical capacity
(OP)
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 !
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 !





RE: Negative skin friction and geotechnical capacity
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.
RE: Negative skin friction and geotechnical capacity
RE: Negative skin friction and geotechnical capacity
RE: Negative skin friction and geotechnical capacity
RE: Negative skin friction and geotechnical capacity
For the similar situation, B.Fellenius has some very good theory.You could make a search on the internet.
RE: Negative skin friction and geotechnical capacity
RE: Negative skin friction and geotechnical capacity
RE: Negative skin friction and geotechnical capacity
I normally check equilibrium in vertical direction taking into account the negative skin friction, and adopt safety factor of 1.5 against plunging
RE: Negative skin friction and geotechnical capacity
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.