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Second International Seminar, Pile Foundations, Nanyang Technological Institute, Singapore, November 28 - 30, P. Eng. University of Ottawa, Canada 1984. 12 p.
A relative movement between a pile and a soil produces shear stress along the interface of the pile and the soil. Such movement can be induced by a push-load on the pile pressing it down into the soil, or by a pull-load moving it upward. ` A relative movement can also be induced when the soil settles in relation to the pile, or, in swelling soils, when the soil moves upward in relation to the pile.
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Fellenius, B. H., 1984. Negative skin friction and settlement of piles. Second International Seminar, Pile Foundations, Nanyang Technological Institute, Singapore, November 28 - 30, 12 p.
NEGATIVE SKIN FRICTION AND SETTLEMENT OF PILES Dr. Bengt H. Fellenius, P. Eng. University of Ottawa, Canada Introduction A relative movement between a pile and a soil produces shear stress along the interface of the pile and the soil. Such movement can be induced by a push-load on the pile pressing it down into the soil, or by a pull-load moving it upward. ` A relative movement can also be induced when the soil settles in relation to the pile, or, in swelling soils, when the soil moves upward in relation to the pile. By definition, if the movement of the pile is downward, i.e., the shear stress induced in the pile is upward, the direction of the shear is positive. If the movement of the pile is upward, the shear stress direction is negative; accordingly, the induced shear stress is called positive or negative. In older terminology, the induced shear along a pile was called 'skin friction'. In modern terminology, the term 'shaft resistance' is used and a distinction is made between on the one hand, positive and negative shaft resistance by which is meant shear stress induced by load on the pile in the form of push-load and pull-load, respectively, and, on the other hand, negative and positive skin friction, which is shear stress induced by settling or swelling soil, respectively. Negative skin friction produces (accumulates to) a dragload which can be very large for long piles. Johannessen and Bjerrum (1965), Bjerrum et al., (1969), and Bozozuk (1972) reported measurements of dragloads that exceed the allowable loads that ordinarily would have been applied to the piles. Bjerrum et al. (1969) also demonstrated the efficiency of coating the piles with bitumen to reduce the negative skin friction. Fellenius and Broms (1969) and Fellenius (1969) presented measurement showing that a dragload can develop alone from the reconsolidation following the disturbance caused by the pile driving. Walker and Darvall (1973) presented a comparison between bitumen coated and uncoated steel piles, and Clemente (1981) reported measurement of dragloads on coated and uncoated concrete piles. Fellenius (1975; 1979) discussed some practical aspects of bitumen coating of piles to reduce negative skin friction. In all the papers referenced above, the emphasis is on the dragload. When the authors report observations of deformation and settlement, the main use of these is to calculate the loads in the pile. When the consequence of the negative skin friction for design are included, these are discussed in terms of reduction of pile bearing capacity of allowable load. In contrast, this paper suggests that the problem of negative skin friction is one of settlement and not of bearing capacity, i.e., the magnitude of the dragload is of no direct relevance to the geotechnical capacity of the pile, nor to the allowable load of the pile. Consequently, as recommended below, in the design for a downdrag condition, the calculation of the distribution of settlement is emphasized.
Movement Necessary for Negative Skin Friction to Develop The magnitude of the movement nec