Numerical modelling of the long-term cyclic response of laterally loaded piles driven in sands using the high-cycle accumulation framework

The loading frequencies in cyclic tests and the rate of strain or stress in monotonic tests have a significant effect on the soil behaviour, particularly of clayey soil. This study aimed to determine the common characteristics of the behaviour of clay under cyclic loading at different frequencies and the relationship between cyclic and monotonic behaviours. Monotonic triaxial tests and cyclic triaxial tests at frequencies of 0.1 and 0.001 Hz were conducted on undrained saturated reconstituted soft clay. The results showed that the frequency of 0.001 Hz is sufficiently low to make the soil response comparable to that in monotonic tests. Second, there exists a unique “feature line” for cyclic tests in the plot of allowable stress versus strain, independent of the frequency. Third, the feature lines for the cyclic and monotonic tests are close and parallel to each other. Therefore, although the cyclic and monotonic loadings are quite different from each other, there are connections between these different loadings in terms of the relationship between the allowable stress and strain.

Lateral response of monopile reinforced by cement-improved soil in clay to monotonic and cyclic loadings: Laboratory model test and theoretical investigation

This paper describes the results of a pile-testing campaign on open-ended tubular steel instrumented piles driven into the Chalk formation in the UK. The testing campaign comprised the performance of both monotonic and one-way cyclic lateral load tests, performed at different times after pile installation. The tests were performed on five piles with uniform outer diameters of 762 mm and embedded lengths of 4 m and 10 m to investigate the difference in response between short and long piles. Lateral pile head load–displacement behaviour to failure has been analysed. The tangent stiffness evolution during monotonic loading has been evaluated at different times after pile installation and the chalk set-up has been found to have no effect on pile behaviour under lateral loading. The pile secant stiffness during cyclic lateral loading is also investigated. Accumulated pile head lateral displacements are discussed and their pattern is described by a logarithmic function that varies with number of cycles. The creation of a gap between the Chalk and the pile during cyclic lateral loading was observed, which influenced the shape of the load–displacement loops. The influence of the instrument protection system was taken into account in analysing the results.

The intermittent nature of renewable energy sources unavoidably limits their application as primary power supplies. In this sense, balancing the production/demand cycles over seasonal and annual timescales is essential for decarbonizing our economy. An effective long-term solution is represented by hydrogen underground storage in depleted hydrocarbon fields [1] . During overproduction periods, extra power is stored in the form of hydrogen in a porous reservoir formation, surrounded by a low-permeability caprock.

The present study focusses on bucket foundation behaviour under long-term cyclic loading. The paper analyses testing results of a bucket foundation model exposed to cyclic tensile loading. The model, with dimensions of 1 m in diameter and 0.5 m in skirt length, was, installed in dense water-saturated sand. Slow monotonic loading tests and cyclic tensile loading tests were performed (up to 40 000 load cycles) including tests with mean cyclic loading in tension, which is a unique testing condition. High-quality data were documented for load, displacement, and pore pressure response. Conclusions have been drawn regarding static and cyclic loading stiffness and displacement development during long-term cyclic loading. Four cyclic loading tests induced partially drained soil conditions and showed that pore pressure can accumulate during the long-term loading. Post-cyclic monotonic tensile loading tests showed up to 25% reduction in load capacity of the foundation. The research results provide valuable information for the design of an upwind bucket foundation under a jacket structure.

Cyclic loading–induced deformation of soil is a common problem in engineering practice. In current practice, however, monotonic triaxial tests are more commonly used due to the availability of apparatus and ease of operation. Thus, it would be very useful and practical if monotonic triaxial tests could be used to evaluate the behavior of soil under cyclic loading. This study aims to find an explicit relationship between monotonic and cyclic behavior of saturated soft clay. Six monotonic and nine cyclic triaxial compression tests were conducted on undisturbed saturated soft clay under an undrained condition. The test results showed that the monotonic and cyclic tests shared the same stress–strain surface in a three-dimensional p′–q–εa space (p′, effective mean principal stress; q, deviatoric stress; εa, axial strain). It was also found that it is possible to evaluate the effective stress states of cyclic tests at two specific numbers of cycles, using corresponding monotonic tests. Based on these two findings, a simple procedure is proposed to predict the peak axial strain for the saturated soft clay under different cyclic loadings based on the monotonic tests and only one cyclic test, which was further verified against additional test data from the previous literature.

Centrifuge testing on ordinary and hybrid suction caissons subjected to cyclic lateral loading

Lateral cyclic behavior of OWT tripod suction bucket foundation in clays

A practical framework for assessing the effect of cyclic softening of clays on the lateral response of single piles

The behavior of laterally cyclic loaded piles is affected by the magnitude and number of cycles of cyclic lateral loads as well as loading method (1-way or 2-way loading). In this study, calibration chamber tests were carried out to investigate the effects of loading method of cyclic lateral loads on the behavior of piles driven into sand. Results of the chamber tests show that the permanent lateral displacement of 1-way cyclic loaded piles is developed in the same direction as the first loading, whereas that of 2-way cyclic loaded piles is developed in the reverse direction of the first loading. 1-way cyclic lateral loads cause a decrease of the ultimate lateral load capacity of piles, and 2-way cyclic lateral loads cause an increase of the ultimate lateral load capacity of piles. The change of ultimate lateral load capacity with loading method of cyclic lateral loads increases with increasing number of cycles. It is also observed that the 1-way cyclic loads generate greater maximum bending moment than 2-way cyclic loads for piles in cyclic loading step and generates smaller maximum bending moment for piles in the ultimate state. It can be attributed to the difference in compaction degree of the soil around the piles with loading method of cyclic lateral loads. In addition, it is founded that 1-way and 2-way cyclic lateral loads cause a decrease in the maximum bending moment of piles in the ultimate state compared with that of piles subjected to only monotonic loads.

Mono-pile foundations are widely used for wind turbines at present. They are always subjected to significant cyclic lateral loads due to wind and wave actions. In order to better understand the performance of mono-piles under cyclic lateral loads, a series of cyclic lateral load tests was conducted on a stainless steel mono-pile in the centrifuge. The axial and lateral responses of a large diameter mono-pile under one-way force-controlled cyclic lateral loads are described. Accumulated permanent pile shaft lateral displacements caused by cyclic lateral loads are discussed, and a function is utilized to estimate these permanent displacements. Also the influence of cyclic lateral loads on the pile lateral secant stiffness is investigated. These results offer an insight to further research to optimise designs of mono-pile foundations to resist live loads in service. (1994) improved the p-y approach to consider the effect of the number of cycles. Nevertheless, only 50 or less cycles of lateral loads are executed in most of the tests considered. Moreover, the use of p-y curves often fails to account for the permanent deformation that accumulates with increasing cycles (Moss et al. 1998). Therefore, additional cyclic load tests need to be conducted with many more cycles than previously to better understand the influence of cycle numbers. Not only the largest pile deflection in each cycle, but also the accumulated permanent displacement and secant stiffness of the pile should be investigated. In field tests, it is difficult to exert cyclic loading on mono-piles with large diameters due to the limitations of test facilities and high costs. However, centrifuge modelling offers an effective way to understand the influence of cyclic loads on piled foundations. Compared with field tests, centrifuge tests are more convenient, efficient and cheaper. In this research, a series of cyclic lateral load tests were conducted in the centrifuge. The axial and lateral response of a large diameter mono-pile under one-way force-controlled cyclic lateral loads is described. The accumulated permanent pile shaft lateral displacements caused by cyclic lateral loads are discussed, and a function is utilized to estimate these permanent lateral displacements. Also, the influence of cyclic lateral loads on the pile lateral secant stiffness is investigated. 2 EXPERIMENTAL METHODOLOGY 2.1 Acceleration and scaling in centrifuge tests A centrifuge test on a 1/100 scale mono-pile is proposed to be conducted in saturated sand at 100g to model the behaviour of mono-pile foundations for offshore wind turbines. In saturated sand, the effective vertical stress of soil is expressed in the following equation 1: z Gsat v ρ σ ′ = ′ (1) where Gsat is the acceleration in a centrifuge test in saturated sand, i.e. Gsat = 100 g; ρ’ = the buoyant density of saturated soil (1137 kg/m); and z = the depth of soil in a 1/100 scale model. For the purpose of an initial study, the fully drained cyclic response was required. Since excess pore pressures were to be avoided, it was realised that dry sand could conveniently be used. In order to achieve the equivalent soil conditions, the vertical stress of dry sand expressed in equation 2 should be equal to the effective vertical stress of saturated sand in equation 1: z Gd d v ρ σ = (2) where Gd is the acceleration to be imposed in a centrifuge test on dry sand; ρd = the dry density of the sand (1826.5 kg/m), and z = the depth of soil in a 1/100 scale model. Thus the d G value is 62.3 g, obtained by equation 3:

Offshore and other structures often rely on driven piles to carry lateral loads. However, there is currently no established design method to cover lateral loading at chalk sites, which are widespread across Northwest Europe. This paper reports monotonic and cyclic lateral load tests on highly instrumented 508 mm and 1220 mm diameter. open steel piles driven at a well-characterised chalk test site in Kent, UK, for a recent joint industry project that developed new benchmark datasets and analyses, supported by high-quality testing. The ultimate lateral pressures mobilised in the chalk are shown to be relatively low compared to its uniaxial compressive strengths (UCS) due to pile-driving damage, natural fracturing, local yielding and brittleness. Significant gaps opened between the piles and chalk during loading, that led to a substantially softer response on unloading and subsequent reloading, as well as marked axial capacity losses. Reaction curves extracted from the field measurements and applied in a one-dimensional numerical model perform well in reproducing the monotonic lateral tests. As with piles driven in other materials, one-way cyclic lateral loading led to permanent displacement accumulation and stiffness changes that were linked to the cyclic loading parameters. Both effects were more marked under biaxial cyclic lateral loading.

Cyclic lateral response of pile foundations in offshore platforms

Triaxial samples of railway ballast have been modelled using clumps of spheres. The bulk of the clump is formed from ten balls in a tetrahedral shape. The interlocking and breaking of very small asperities which find their way into the voids is modelled using weak parallel bonds between clumps. The interlocking and fracture of larger asperities has been modelled by bonding eight small balls to the clump. Monotonic tests have been performed on triaxial samples under a range of confining pressures from 15 kPa to 240 kPa and the results compared with existing experimental data. Tests were also simulated using uncrushable clumps to highlight the important role of asperity abrasion. Cyclic triaxial tests were then simulated on the same aggregates using the same microparameters as the monotonic tests under a range of stress conditions and the results were compared to existing experimental data for the same simulated ballast. The results show that the clumps are able to capture the behaviour of ballast under both monotonic and cyclic conditions and asperity abrasion is shown to play an important role in governing strength and volumetric strain under monotonic loading, and on permanent strains under cyclic loading. The new contribution is therefore to show that it is possible to model a real granular material under static and cyclic conditions, thus providing much micromechanical insight.

Helical foundations used in transmission towers, wind turbines, solar panel systems and other similar structures must exhibit adequate performance under cyclic loading. Although a number of studies on the cyclic behaviour of helical anchors has increased, the available observations are still incomplete in terms of providing conclusive recommendations for practical applicability. To address this need, cyclic and monotonic axial tensile load tests were conducted on a single-helix model anchor in very dense dry sand in a centrifuge. The cyclic tests were carried out with different load amplitudes and with the number of cycles varying between 1000 and 3000. After the cyclic tests, the post-cyclic tensile capacity was evaluated by way of monotonic loading tests. Two different displacement regimes were observed during cycling, with a noticeable development of cumulative permanent displacements in the first approximately 100 cycles. In most of the tests, a stable cyclic response and a slight reduction in the post-cyclic capacity were observed. Additionally, the effect of the cyclic loading sequence was evaluated by way of two additional tests in which the cyclic loading was applied with different amplitudes in different orders. The results indicated that the order of the cyclic sequences influences the displacement response and the losses in the post-cyclic capacity.