Summary
The intent of this project is to examine the feasibility of conducting a detailed analysis of pile driving taking into account the nonlinear behavior of the soil and tracing the penetration of the pile into the soil. The study is limited to clayey soils. A full-scale, three-dimensional model is used for this purpose, which reduces to a two-dimensional model is used for this purpose, which reduces to a two-dimensional analysis due to the axisymmetric nature of the problem. Using the finite element technique, a rigorous and realistic discretization of the hammer system, pile, and surrounding soil is obtained. A nonlinear time domain dynamic analysis is then performed, using the constant-average-acceleration method, in which the hammer blows on the pile are represented by a periodic impact loading; this, in turn, forces the driving and therefore penetration of the pile into the soil occurs following each blow, until a desired pile capacity is obtained.
The nonlinear nature of soil behavior requires the use of an inelastic constitutive model; the bounding surface model in this case as applied to clays relating effective stress to strain. While the bounding-surface model predicts the effective state of stress, the undrained nature of clays requires the calculation of the pore-water pressure, leading to the total stress in the soil. The undrained analysis is treated using a penalty formulation in which the clay is assumed to be fully saturated and therefore nearly incompressible. It is also assumed that no dissipation of pore-water pressure occurs during driving and the flow problem will not be addressed. This would constitute a fair representation of offshore soil conditions underneath water immediately after driving. Because of the large deformations that are expected in the soil in the vicinity of the pile during penetration, geometrically nonlinear analysis is incorporated in the finite element model.
The pile material is expected to remain in the elastic range. The study is limited to closed-end piles, either round concrete piles, or steel pipe piles to which a shoe is attached at the tip.
The frictional contact between the pile and surrounding soil is handled by means of interface elements along the pile shaft using a penalty formulation. The friction law is of the Coulomb type in which the frictional (shear) stress between pile and soil is proportional to the normal stress.
The objective of this research is, firstly, to determine whether the driveability of the pile can take place, given a particular hammer-pile-soil system and its specifications, and, if necessary, modify that system usually by varying the hammer or its dropping characteristics. The integrity of the pile can also be checked to determine any excessive stresses that might have occurred during driving. Finally, and most importantly, the actual state of stress in the soil is traced during and at the end of driving, including the development of excess pore-water pressure.
Related Publications: Mabsout, M.E. and Tassoulas, J.L., “Analysis of Pile Driving: A Finite Element Approach,” Proceedings, Eurodyn ’93, Second European Conference on Structural Dynamics, The Norwegian Institute of Technology, Trondheim, Norway, June 21-23, 1993.
Mabsout, M.E. and Tassoulas, J.L., “A Contact-Friction Slideline Application in the Analysis of Piles,” Proceedings, Contact Mechanics 95, Second International Conference on Contact Mechanics, Ferrara, Italy, July 11-13, 1995.
