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Numerical derivation of CPT-based p–y curves for piles in sand

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Numerical derivation of CPT-based p–y curves for piles in sand

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The formulations for the lateral load-displacement (p-y) springs conventionally used for the analysis of laterally loaded piles have been based largely on the back-analysis of the performance of small-scale instrumented piles subjected to lateral load. Although such formulations have been employed with much success in industry, their applicability to large-diameter piles, such as those used to support offshore wind turbines, is uncertain and has necessitated further research in this area. Moreover, with the growth in popularity of in-situ cone penetration tests (CPTs), there are demands for a theoretically supported direct method that can enable the derivation of p-y curves from the CPT end resistance (q c). In this paper, a numerical derivation of CPT-based p-y curves applicable to both small- and large-diameter laterally loaded single piles in sand is presented. Three-dimensional finite-element analyses are performed using a non-linear elasto-plastic soil model to predict the response of single piles in sand subjected to lateral loads. The corresponding CPT q c profile is derived using the same soil constitutive model by way of the cavity expansion analogue. An extensive series of computations of the lateral pile response and CPT q c values is then employed to formulate a direct method of constructing p-y curves from CPT q c values. The proposed method is shown to be generally consistent with existing empirical correlations and to provide good predictions in relation to the measurements obtained during lateral load tests on instrumented piles in an independent case study.
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... CPT-based p-y curves have been developed based on taking the cone tip end resistance (qc) and empirically deriving exponential functions for the p-y relationship (Dyson and Randolph, 2001;Li et al., 2014;Suryasentana and Lehane, 2014). However, it has been demonstrated that their effectiveness at encapsulating lateral soil resistance decreases as L/D reduces, and their suitability for the problem relies on the range at which they were empirically calibrated (Li et al., 2014). ...
... This informs evenly discretized points along the monopile, thereby characterizing the p-y and m-θ springs (Fig. 1). The p-y springs used in this paper are informed through Equation 1 which were empirically derived by Suryasentana and Lehane (2014) for low L⁄D monopiles in sand. ...
Conference Paper
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Offshore Wind Turbines (OWT) are a successful renewable energy solution; however, emerging turbine sizes require pile geometries beyond the calibration range of existing design standards. This necessitates soil sampling and rigorous finite element modelling, which is problematic for quick design estimates. Cone Penetration Test (CPT)-based p-y methods can provide preliminary deflection estimates, although their applicability becomes increasingly uncertain as pile slenderness ratios, length normalized by diameter (L/D), reduce. This is due to the increase in diameter incurring additional resistances that p-y models alone cannot account for. To incorporate the additional resistance, this paper defines a CPT-based moment-rotation (m-θ) model by rescaling empirically derived axial capacity functions (known as τ-w curves). Various monopile dimensions are simulated and pile-head displacements are compared for CPT-based p-y models with and without m-θ springs. The net effect of incorporating m-θ springs increases as monopile diameters (and rigidity) increase and diminishes as piles increase in slenderness,
... CPT-based p-y curves have been developed based on taking the cone tip end resistance (qc) and empirically deriving exponential functions for the p-y relationship (Dyson and Randolph, 2001;Li et al., 2014;Suryasentana and Lehane, 2014). However, it has been demonstrated that their effectiveness at encapsulating lateral soil resistance decreases as L/D reduces, and their suitability for the problem relies on the range at which they were empirically calibrated (Li et al., 2014). ...
... This informs evenly discretized points along the monopile, thereby characterizing the p-y and m-θ springs (Fig. 1). The p-y springs used in this paper are informed through Equation 1 which were empirically derived by Suryasentana and Lehane (2014) for low L⁄D monopiles in sand. ...
... The most popular method employed for the analysis of laterally loaded piles is the p-y method [5,12,30,34]. In this method, the pile is modelled as a series of beam elements and the soil is represented by non-interacting, nonlinear springs distributed along the pile length [30]. ...
... The first term of this equation corresponds to the normalized ultimate soil pressure and incorporates the same dependence on q c raised to the power of 0.7 deduced by Suryasentana and Lehane [34]. Both the experimentally derived p-y curves and those calculated by Eq. 3 were used to input into the program LAP [11], which is web-based software employing the traditional 1D beam element to model the pile and nonlinear, non-interacting spring springs to model the soil [11,13]. ...
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Large diameter, short monopiles are the preferred foundation type for offshore wind turbines. These piles demonstrate a rigid response with significant rotation of the pile base under ultimate lateral load. As traditional empirical p-y curves used in lateral loaded pile analysis have been derived from tests on small diameter onshore piles, there is some doubt about their applicability to rigid monopiles, particularly in view of the significant difference in the response of the pile base compared with a typical onshore pile. To address this issue, this paper reports on a unique series of field tests using instrumented driven pipe piles, complimented by numerical analysis, which was performed to examine explicitly the contribution of the pile base to the response under lateral load. The field tests, which included tests on pipe piles that had the sand plug removed to below pile tip level prior to testing, confirmed that the influence of the base on the lateral response for the tested 273- and 457-mm-diameter piles was negligible. Numerical analyses that were calibrated using the field test data showed that the contribution of the base to the lateral capacity of a monopile with a diameter as large as 10 m is negligible. Results also indicated that p-y curves are not affected by the length to diameter ratio and can be used to predict the response of monopiles in sand.
... Since such properties are typically inferred from in-situ CPT results, an intermediate step is necessary to translate the measured resistance to cone penetration ( ) into or values -which adds a further layer of uncertainty to the overall calibration procedure. An interesting alternative is offered by so-called CPT-based − methods, where model parameters are directly correlated to the values measured in-situ (Novello, 1999;Dyson and Randolph, 2001;Suryasentana and Lehane, 2014b;Li et al., 2014). Particularly worth mentioning is the work of Lehane (2014b, 2016), who proposed a CPT-based monotonic − method that is applicable to piles of different cross-section shape and aspect ratio in (in)homogeneous sand profiles. ...
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The analysis of cyclically loaded piles is acquiring ever greater relevance in the field of geotechnical engineering, most recently in relation to the design of offshore monopiles. In this area, predicting the gradual accumulation of pile deflection under prolonged cycling is key to performing relevant serviceability assessments, for which simplified pile–soil interaction models that can be calibrated against common geotechnical data are strongly needed. This study proposes a new cyclic p−y model for piles in sand that takes a step further towards meeting the mentioned requirements. The model is formulated in the framework of memory-enhanced bounding surface plasticity, and extends to cyclic loading conditions the previous monotonic, CPT-based p−y formulation by Suryasentana and Lehane (2016); additionally, detailed modelling of pile–soil gapping is introduced to cope with the presence of unsaturated sand layers or, more generally, of cohesive soil behaviour. After detailed description of all model capabilities, field data from an onshore cyclic pile loading test are simulated using the proposed p−y model, with the most relevant parameters calibrated against available CPT data. Satisfactory agreement is shown between experimental and numerical results, which supports the practical applicability of the model and the need for further studies on a fully CPT-based calibration.
... Soil properties vary in space both vertically and horizontally as a result of the complex stratum formation by a combination of various geological, environmental, and physical-chemical processes (Phoon and Kulhawy 1999b). The spatial variability is often identified by random field theory, where it can be characterized by the mean value μ, coefficient of variation (COV), and the correlation distance δ (Vanmarcke 1977 (Suryasentana and Lehane 2014). Typical values of COV for soil modulus lie in the range of 10%-70% (Depina et al. 2015;Phoon and Kulhawy 1999a;Hu and Randolph 1998). ...
... For the analysis of cavity expansion theory, a 2D axisymmetric FEM formulation was used in Plaxis 2D, in which the hypoplastic soil model with intergranular strain (Niemunis and Herle 1997;von Wolffersdorff 1996) has been added as user defined soil model. The procedures and appropriate model and mesh sizes were discussed in several studies (Suryasentana and Lehane 2014;Xu and Lehane 2008). Based on these studies, the cavity was simulated as a linear elastic sphere with a radius of = 0.1 m in the centre of an axisymmetric model with a size of 12 times 28 m (Fig. 5). ...
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The simulation of granular soil using advanced soil models relies on the precise determination of model parameters from laboratory or field experiments, or both. Although it is well known that sample reconstitution influences the stress-strain response of granular soil, the effect of different reconstitution methods on the calibration of advanced soil models has not been comprehensively addressed before. In the present study, a hypoplastic model was calibrated from geotechnical laboratory tests on Cuxhaven Sand samples. Triaxial tests were performed on samples having been reconstituted by three different methods: moist tamping, vacuum pluviation, and horizontal vibration. The influence of different reconstitution methods on the simulation performance of the hypoplastic model was quantified through three scenarios of increasing complexity: (1) stress-strain behavior in representative element volume (REV), i.e., triaxial tests; (2) in situ plate load test (PLT); and (3) laboratory cone penetration test (CPT). The latter two being typical examples of boundary value problems with different strains and stiffnesses. The adopted reconstitution methods significantly affected the REV simulation at high relative density of both peak friction angles and peak dilation angles. The reconstitution methods have a limited effect on boundary value problem simulations, being moderate for PLTs and small for CPTs. The influence of stiffness (intergranular strain) parameters on simulation results increased from REVs (no influence detected), over boundary value problems with low strain and stiffness (PLTs) to boundary value problems with high strain and stiffness (CPTs).
... and Suryasentana & Lehane (2014) performed FE analyses and suggest that the variation of the net lateral pressure (P=p/D) with normalized displacement (y/D) is independent of the pile diameter. This independence as well as the profile of P with depth are investigated here using the validated FE approach described above for drained lateral loading in Toyoura sand. ...
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