Capacity degradation method for piles under cyclic axial loads

Abstract

Piles used for jacket type foundation of offshore wind turbine are subjected to highly cyclic tension and compressive loading. The pile capacity under cyclic tension loading decreases with increased number of loading cycles due to reduction of the pile shaft resistance. A numerical simulation scheme is presented, which allows the calculation of the pile capacity degradation (CDM) due to cyclic loading for driven steel piles. The volume compaction of soil near the pile surface during the cyclic loading is determined from the cyclic simple shear test results and then applied to the pile-soil system.

From the limited number of tests available, interaction diagrams have been developed, which give the number of load cycles leading to failure dependent on the mean load and the amplitude of the cyclic load portion, which are both related to the static pile capacity. However, such diagrams cannot account for different soil conditions or pile geometry and pile stiffness. The calculation results for different piles in sandy soil under cyclic axial loading are presented and compared with existing interaction diagrams. Finally, recommendations regarding further investigations and improvements of the method are given.

Introduction

Piles are often subjected not only to monotonic, but also to cyclic axial loads. For offshore applications in particular, cyclic loads induced by wind and wave acting on a structure are of significant magnitude. These loads are highly variable, but can be considered as quasi-static, since the time period for each loading cycle is typically about 5–10 s.

Piles under cyclic loads are known to experience increase deformation and a reduction of capacity (see e.g. Lehane and White, 2005, Jardine and Standing, 2012). This is mainly induced by changes of the normal contact stress between the pile and soil (Poulos, 1989). Under cyclic shearing, a soil tends to compaction and under constant volume conditions, to stress relaxation. Therefore, the tendency towards compaction in the soil around the pile leads to a reduction in horizontal stress and eventually to a reduced ultimate skin friction. It is generally assumed that the base resistance of a compression pile stays almost unaffected, which means that the capacity degradation only comprises the skin friction portion of the capacity.

Fig. 1 elucidates the cyclic loading parameters. Cyclic load is characterized by a mean value F_mean, cyclic amplitude by F_cyc, and the number of load cycles by N. To ease the comparison of the different pile-soil systems, these values are usually related to the monotonic or static pile capacity F_ult,stat, which is either the compressive or the tensile (pull-out) capacity, depending on which of these ultimate states is decisive:

$$X_{\mathit{cyc}}=\frac{F_{\mathit{cyc}}}{F_{\mathit{ult},stat}}$$

$$X_{\mathit{mean}}=\frac{F_{\mathit{mean}}}{F_{\mathit{ult},stat}}$$

In practice, cyclic loads with varying mean values and cyclic amplitudes occur. However, it is usual to idealize such irregular loads by a number of load packages with constant values of mean and amplitude. Moreover, by applying a hypothesis regarding the effect of the loading parameters on the sought result (for instance capacity degradation), an equivalent number of load cycles for a chosen reference load may be derived.

The relevant regulations for the design of offshore structures (e.g. API, 2014, DNVGL, 2017) demand the consideration of cyclic load effects. In pile design, a common task is to assess whether the pile can withstand the equivalent load cycles derived for a certain cyclic reference load, i.e. whether the number of load cycles is sufficiently below the number of load cycles leading to failure N_f. It must also be determined how much the pile capacity is reduced due to the actual number of load cycles and which accumulated deformation occurs. However, generally applicable calculation methods for these tasks do not yet exist. In particular, little experience exists for piles under very high load cycle numbers. These occur particularly for piles supporting foundation structures for offshore wind energy converters. It is not yet fully understood how pile geometry, soil parameters, and loading parameters affect the pile behaviour under cyclic axial loading.

This study presents a method to determine the number of load cycles to failure as well as capacity reduction and deformation increase due to a given cyclic load package. The method combines numerical simulations of the pile behaviour with the results of cyclic direct simple shear (DSS) tests and is termed ″Capacity Degradation Method″ (CDM). It is principally to deal with arbitrary pile geometries, loading, and soil conditions. However, since piles in sand are particularly sensitive to cyclic loads, and the cyclic loading only affects skin friction, the focus here lies on the bearing behaviour of the piles in sand soils with mean load values only in the tension range.