(1999). Ge  ote ch ni qu e 4 9, No. 6, 7 77 ± 7 99 A. E. Holeyman, Universite  Catholique de Louvain The discusser commends the authors for their re-analysis of ®eld data: it demonstrates that ®ne-tuning efforts of pile±soil dynamic interaction models still provide for interesting developments. The discusser fully supports the authors' quest for a more physical modelling of the dynamic interaction between the driven pile and the surrounding soil. In that regard, the sophistication of models should be tolerated only inasmuch as they lead to a more truthful representation of the physical reality, accompanied by a deeper insight into the mechanisms at play, and in particular intòdamping'. However, the discusser questions that rate-dependent effects be pegged into only two categories, namely radiation damping and viscous damping. Although Simons' (1985) suggestion to lump those two effects into two distinct components helps one to differentiate between near-®eld and far-®eld effects, that model cannot properly account for: (a) hysteretic damping (energy loss during unloading), (b) intrinsic viscous damping at low strains, and (c) non-linear rate dependence of the ultimate soil strength. A more comprehensive model was developed earlier by the discusser for both friction and end bearing (Holeyman, 1984; also described in Holeyman, 1985, and Holeyman, 1988). The one-dimensional radial model to represent skin friction and its simpli®ed lumped parameter system, along with their mobilisa-tion curves, are shown in Fig. 24. The simpli®ed system was suggested after checking its equivalence against numerous simulations of the more complete model under several kinetic input signatures, typical of pile driving (see e.g. Fig. 24(c) and 24(d)). The model shown in Fig. 24(a) consists of a succession of concentric cylinders with a linearly increasing depth. The equations of movement are integrated for each cylinder based on their dynamic shear equilibrium in the vertical direction, in a manner similar to that used by Smith (1960) in the longitudinal direction. The model allows the shear strain/shear stress 70 Pile shaft S k i M i R o R i R m C τ (τ/τ r) k(τ/τ′, v) (τ/τ′) r = f (v) τ + τ′: MPa τ′ τ 0·2 0·1 0-0·1-0·2 τ + τ′: MPa 0·2 0·1 0-0·1-0·2 0 2 5 7 5 50 100 Time: ms 0 5 10 20 15 Displacement: mm 1-D radial model Simplified model (a) (b) (c) (d) Fig. 24. Pile friction models: (a) one-dimensional radial model; (b) simpli®ed model; (c) time response; (d) shear stressÐ displacement mobilisation Downloaded by [] on [10/05/18].
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