A high-cycle accumulation model for clay and its application to monopile foundations

Abstract

A high-cycle accumulation (HCA) model predicting the accumulation of permanent strain or excess pore water pressure in clay under a large number of load cycles is presented. Data from an extensive laboratory testing program on kaolin under undrained cyclic loading has been analysed for that purpose. The influence of strain amplitude, void ratio, stress ratio, overconsolidation ratio and loading frequency on the accumulation rates is considered in the constitutive equations of the HCA model. The proposed model is validated first by the simulation of element tests. Subsequently, its application to offshore wind turbine foundations under long-term lateral cyclic loading is presented by the back-analysis of a centrifuge test on a monopile in soft clay. The results are in good accordance with the measurements in terms of pile displacement and bending moment versus number of applied cycles. It is concluded that the proposed model is feasible to describe the long-term behaviour of clay subjected to high-cyclic loading.

Novel approach to apply existing constitutive soil models to the modelling of interfaces

“…Equation (39) and Equation (40) have to be solved simultaneously in an iterative scheme to secure consistency. Experience shows that using Equation (37) alone leads to fast convergence rates despite the negligence of the change of state variables with respect to displacement and stress.…”

“…Experience shows that using Equation (37) alone leads to fast convergence rates despite the negligence of the change of state variables with respect to displacement and stress. Equation ( 37) is computational advantageous compared to Equation (39), since it does not require any additional iterations and has to be evaluated only 2 ⋅ 𝑛𝑑𝑖𝑚+1 times, with 𝑛𝑑𝑖𝑚 being the number of dimensions of the boundary value problem. Therefore, Equation ( 37) is used for the implementation of the numerical differentiation scheme.…”

“…2 numgeo (Machaček & Staubach, see Ref. [35][36][37][38][39][40][41][42]75 and http://www.numgeo.de) is a stand-alone finite element program developed by the first two authors for the solution of non-linear, coupled (dynamic) geotechnical BVPs. 3 The stress tensor of the interface 𝝈 interface is not to be confused with the contact stress vector 𝒕 and its components.…”

“…74) with respect to the number of iterations is given in Figure B1 for the simulation of the monotonic simple interface shear test (see Section 5). In case of the numerical differentiation scheme both approaches, the simplified Equation (37) where only the displacement increment is perturbed, and Equations (39,40), for which in addition all state variables are also perturbed, are used. The first increment of the monotonic shearing phase of the simulation is considered.…”

“…22numgeo (Machaček & Staubach, see Ref. 35–42,75 and http://www.numgeo.de) is a stand‐alone finite element program developed by the first two authors for the solution of non‐linear, coupled (dynamic) geotechnical BVPs.…”