Experimental application of FRF-based model updating approach to estimate soil mass and stiffness mobilised under pile impact tests

Abstract

The dynamic response of structures in contact with soil is receiving increasing interest and there is a growing need for more accurate models capable of simulating the behaviour of these systems. This is particularly important in the field of offshore wind turbines, where accurate estimates of system frequency are needed to avoid resonance, and in the structural health monitoring fields, where accurate reference damage models are used. Previous work has shown that there is significant uncertainty in how to specify mobilised soil stiffness for dynamic soil-pile interaction modelling. Moreover, the contribution of soil mass in dynamic motion is often ignored. This paper applies a finite-element iterative model updating approach previously developed by the authors to two experimental piles to ascertain the mobilised soil stiffness and mass profiles from impact test data. The method works by obtaining a frequency response function (FRF) from an impact test performed on a test pile, developing a numerical model of this system, applying initial estimates of soil mass and stiffness, and updating these properties to match the experimental FRF with that generated in the numerical model. A range of elements are investigated including multiple runs of the approach to test repeatability, the influence of different starting estimates for stiffness, the effect of variability in experimental test data, and the influence of the pile length over which masses are distributed. Moreover, potential sources of error are discussed. The method provides reasonably consistent estimates of the soil stiffness and mass acting in the lateral dynamic motion of a given pile tested in this paper. The approach may be useful in the continued improvement of Soil-Structure Interaction (SSI) modelling for dynamic applications.

Impact of Geotechnical Uncertainty on the Preliminary Design of Monopiles Supporting Offshore Wind Turbines

“…The SSI is incorporated using a beam-Winkler framework [10,11,23] whereby the soil-pile interaction is considered as a distributed system of linear springs in contact with the pile beam elements, as described previously. The spring stiffnesses are obtained by application of subgrade reaction formulae, which couple soil and pile properties together [24,45,46]. The coupling formulae used in this paper are discussed previously in section 2.3.…”

“…The design of foundations under these conditions is at the operating limit of engineering experience to date, and confidence is lessening. There is growing concern over issues such as the accuracy of soilstructure interaction (SSI) coupling stiffness [23,24], the governing operating parameters [25],…”

“…geotechnical parameter transformation [26], measurement uncertainty [27], influence of added mass [24,28], and the effects of model uncertainty [29,30]. These issues have the potential to undermine the pace of developments if not addressed.…”

Apparent wave velocity inverse analysis method and its application in dynamic pile testing

“…The first one is that the existing soil–pile models mainly focus on the radiation damping (or inertia damping) of the soil, while the viscous damping at the soil–pile interface is often neglected (plane strain model and continuum model) or not activated until the dynamic soil force reaches its maximum static resistance and slippage between the pile and soil occurs. The second limitation of the aforementioned models is that the contribution of the inertia effect of the soil during dynamic motion is usually not taken into consideration, which in fact has important influence on the dynamic characteristics of the pile 29,30 …”

Factors Influencing the Prediction of Pile Driveability Using CPT-Based Approaches

“…Arguably the most challenging aspect of performing driveability studies lies with the accurate modelling of soil-structure interaction [1,3,4]. Several models have been put forward to derive SRD profiles, see for example [2,[5][6][7][8].…”