Dynamic behaviour of suction caissons

2.6 Dynamic stiffness for vertical vibrations 25
|SV V |/K 0 V V
100
80
60
40
Surface footing
H/D = 1/4
H/D = 1
H/D = 2
20
0
0 2 4 6 8 10 12
Dimensionless frequency a 0
3π
4
φV V (rad)
π
2
π
4
0
0 2 4 6 8 10 12
Dimensionless frequency a 0
Figure 2.7: Vertical dynamic stiffness: high frequency behaviour. G s = 1.0 MPa, ν s =
1/3 and η s = 5%.
of Lysmer’s analog ‘wave velocity’ c La =3.4c S /π(1 − ν s ). Wolf (1994) suggests another
approach where c P for ν s ∈ [1/3;0.5] is constant, and equal to c P at ν s = 1/3.
At high frequencies the wavelengths are small compared with the dimensions of the
source (or the vibrating surface). Thus, the soil immediately below the vibrating surface
of a smooth surface footing is only exposed to P-waves. However, the skirts of the
suction caisson generate additional S-waves due to a vertical high-frequency excitation.
For that reason, the limiting damping parameter CV ∞ V of the suction caisson consists
of two contributions: one from the vibration of the lid and one originating from the
vibration of the skirt. CV ∞ V of the suction caisson is then given by
C ∞ V V = ρ s c P A lid + 2ρ s c S A skirt , (2.17)
where A lid and A skirt are the vibrating surface areas of the lid and the skirt, respectively.
Note that S-waves are generated both inside and outside the skirt, hence the factor ‘2’
December 4, 2006