Capacity of vertically loaded piles in low density sands.

Abstract

The reduction of pile capacity associated with volume contraction of the soil close to the pile

surface has been reported in carbonate deposits offshore North West Australia and in residual

deposits of Southern Africa.

Knowledge of the load carried by the shaft and the pile tip is critical for the determination of the

load settlement behavior of piles in structurally unstable and highly variable sand deposits. While

the Static and Dynamic formulas and Pile load tests are used for the determination of pile

carrying capacity, they are limited in terms of site coverage, cost and adequacy of load settlement

data. Since the mode of shearing around a pile shaft is very similar to that observed in

the direct shear tests, it is thus cost effective to develop analytical methods based on controlled

laboratory model tests in order to predict load settlement behavior and bearing capacity of piles.

A simple shear apparatus was developed to investigate whether or not significant contractile

strains are induced in low density residual sands subject to simple shear strain and to study the

effect of such contractile strain of a soil close to the pile shaft on pile load settlement behavior.

The design and development of the simple shear apparatus was based on a new simple shear

stress equation. Series of constant normal stresses, constant normal stiffness and constant volume

tests were conducted on samples of Berea Sands compacted to low density in the new apparatus,

supported by moisture induced collapse settlement and matric suction tests. The tests revealed

significant volume contraction of Berea Sands due to imposed simple shear strain.

The tests data were fed into a new Winkler - type load transfer model and were used to determine

the load - transfer curves of vertically loaded piles. The curves revealed that both the loadsettlement

behavior and pile capacity in low density sands are dependent on the volume

contraction of the soil in the plastic zone close to the pile surface, horizontal stress normal to the

pile shaft and stiffness of the soil outside the plastic zone.