Negative skin friction (NSF) is a common problem to be considered in designing pile foundations located in consolidating ground. Failures of foundations due to either NSF induced compressive axial force (Dragload) or excessive pile foundation settlement (Downdrag) has been widely reported in the literature. The results of eleven centrifuge model tests, numerical simulations and simple analytical solutions were reported which were undertaken to achieve four objectives: first, to investigate response of piles subjected to NSF with different pile tip location with respect to end-bearing stratum layer, and secondly, to study the effect of pile shapes on development of negative shear stress on pile shafts, and thirdly, to investigate and compare behaviour of floating piles subjected to NSF with shielding by either a sacrificing ring or a group of sacrificing piles, and finally, to look at the effect of axial live loads on load transfer mechanism along piles experiencing locked-in NSF..
In this study, larger downdrag is developed on floating piles and neutral plane (NP) is located lower towards pile tip with stiffer end-bearing layer. Neutral plane locations on a single pile can be estimated by a simple analytical approach considering displacement compatibility, cavity expansion at the pile tip and vertical force equilibrium of piles. It is found that depth of NP increases with increasing end bearing stiffness of pile, interface friction angle and surcharge.
Axial load effects on load transfer mechanism of piles with different pile tip location with locked-in dragload are investigated. For single floating piles with Y=0.25D and Y=1.00D, the amount of measured axial load requires to completely overcome the dragload was about 1.75 P
max and 1.44 P
max, respectively. Unlike single pile, amount of axial load required to eliminate NSF for pile group can be as large as more than 4 times P
max for the floating pile group.
To study the effect of openness of pile sections on load transfer mechanism of NSF, centrifuge and three-dimensional numerical modeling of conventional pile shapes, circular, rectangular with aspect ratio = 2 and H-pile were carried out. Vertical shear mechanism predicts that external shaft friction induced on piles with close sections is insignificant. Only a little reduction in σ
v̕ result. On the other hand, large reduction in vertical effective stress is experienced by the soil element located at the pan area of an H-pile (open section). Fairly good agreement with centrifuge tests results is made with the analytical prediction by vertical shear mchanism and numerical simulations.
The shielding effects on dragload and downdrag are represented quantitatively using two dimensionless terms, P
r and W
r, respectively. The measured P
r of the center pile surrounded by a sacrificing ring with 0.50L, 0.75L and 1.00L length was 42%, 54% and 67%. Correspondingly, measured W
r of the center pile surrounded by a sacrificing ring with 0.50L, 0.75L and 1.00L length was 15%, 25% and 50%. This shows that shielding effect on dragload increase gently with an increase in sacrificing ring length, while the shielding effect on downdrag increase exponentially with an increase in length of the ring. For center pile shielded by means of a group of 3x3 sacrificing piles in full length measured P
r and W
r are 47% and 43%, respectively. Moreover, the sacrificing ring only occupy one-forth of the material weight of a group of sacrificing piles for the same spacing and provide better shielding effect. Numerical simulations of the centrifuge model tests on center piles revealed that the observed shielding effects on a center pile are attributed to the stress transfer from consolidating soft soil to sacrificing structures. As consolidation proceeds, the relatively stiff sacrificing structure "hang-up" soils, leading to a significant reduction in the σ
v̕, σ
h̕ in the soil and thus NSF on center piles. Sacrificing ring provides a better distribution of material to give a larger surface area for hanging up soils. Soils neighboring the center pile are enclosed or isolated by the relatively stiff ring so that stress transfer from far field to the enclosed soils is negligible. This leads to the enhanced reduction in NSF along lengths of the sacrificing ring.
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