Simplified Calculation Method for the Natural Frequencies of the Offshore Wind Turbine Structures with Monopile Foundations Based on Euler–Bernoulli Beam Theory

Table 2

Parameters for hardened soil with small-strain stiffness.

Soil	Thickness/m	c/kPa	/	/MPa				/MPa	/MPa	/MPa	/MPa
Clay	3.6	15	8.4	2.74	0.0001	1.23	0.80	4.56	3.04	24.32	44.80
Clay	8.6	15	8.4	2.68	0.0001	1.26	0.80	4.47	2.98	23.84	42.70
Silt	8.1	18	26.4	10.59	0.0001	0.72	0.60	11.80	11.80	50.03	96.84
Sand	9.2	7	33.4	16.34	0.0001	0.63	0.55	18.20	18.20	54.60	111.00
Silt	9.2	22	12.1	4.03	0.0001	0.97	0.80	5.60	4.48	29.21	67.00
Clay	15	37	17.8	5.48	0.0001	0.77	0.80	6.09	6.09	30.45	90.24
Sand	56.3	7	33.6	14.84	0.0001	0.66	0.55	16.50	16.50	54.45	106.08

γ_0.7 is generally 0.0001 as the threshold shear strain. e is the void ratio. m is the power for stress-level dependency of stiffness, with a value between 0.5 and 1. Sand and silt are near 0.5, and soft clay is near 1. is the secant stiffness in standard drained triaxial test. is the tangent stiffness for primary oedometer loading. is the unloading/reloading stiffness. is the shear modulus at the small strain. The un-/reloading Poisson’s ratio is 0.2.