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Finite Element Analysis of Soil Plug Behaviour Within Open-Ended Piles
Abstract
Current design standards for offshore open-ended piles vary in their consideration of the soil plug’s contribution to the pile’s axial capacity and, indirectly, to its stiffness. API RP2 GEO (2011) and UWA-05 (Lehane et al., 2005) consider the contribution of the plug, but ICP-05 (Jardine et al., 2005) combines the plug capacity with the end bearing from the pile annulus. The current study critically examines the behaviour of the plug using a specifically-designed 1D finite element method, developed to model each component of the plug-pile-soil interaction. The load-transfer into the surrounding soil and base is modelled using non-linear axial soil reaction curves. Using this methodology, a theoretical assessment of the contribution of the soil plug to the pile axial performance can be made. The findings of this theoretical assessment are compared to the current treatment of soil plug response in the design standards.
... Experimentally, the impact of plug removal on pile load capacity has been studied by Joseph et al. (Joseph et al. 2017). A new apparatus was designed to replicate the pile load test in the field. ...
Herein, an experimental investigation of the plugging impact on the capacity of open-ended piles installed in clayey soil is presented. Model tests involving static axial compression load tests are carried out on four open-ended modeled piles with different diameters (12.5, 19, 25, and 50 mm). In the same manner, four closed-ended modeled piles are tested to make a comparison. A load cell system is used to determine the resistance acting on the piles, with an instrumented transducer on the outside walls of the pile. The eight steel model piles are tested in the circular steel tank with a diameter of 350 mm and 400 mm in length. Three undrained shear strengths of the clay sample, 5 KPa, 10 kPa, and 18 kPa, are used in model tests. The outcomes of the tested models uncover that the pipe pile capacity is mainly mobilized at the low shear strength. In addition, it is realized that the soil plugging in the small diameter has an impact on pipe pile capacity. Based on experimental results, five statistical models are established to achieve relations between the maximum load for the closed-ended pipe. It is realized that the value of Qexp is highly correlated with the diameter and Qcalc..
A Comprehensive Database of Tests on Axially Loaded Driven Piles in Sands reviews the critical need to develop better load-test databases for piles driven in sands. The key quality parameters, population of current entries and reporting formats are described before offering preliminary results obtained from comparisons between axial capacities calculated by various predictive approaches and site measurements. This book also shows that the "simplified" and "offshore" ICP and UWA variants proposed by some practitioners are over-conservative and that their use could be discontinued. The new pile capacity and stiffness database offers a broad scope for evaluating potential prediction biases relating to a wide range of soil and pile parameters. Submission of further high quality tests for inclusion in regularly updated versions is encouraged. Presents a comprehensive and updated database for piles driven in predominantly silica sands. Features reviews of the design procedures for driven piles in sand. Assesses the performance of various mainstreams design procedures applied for piles driven in sand. Provides comprehensive information of case histories of pile load tests. © 2016 Zhejiang University Press Co., Ltd. Published by Elsevier Inc. All rights reserved.
Based upon comparisons between calculated and measured axial capacity of driven piles in sand, it is concluded that the present API RP2A recommendations should be revised to better reflect the measured capacities. The authors propose a new empirical calculation method called NGI-99. This method was calibrated against well documented pile tests with results from CPTs in a database established by NGI. Based upon this method a best-fit conversion between SPT and CPT was established. Results of comparisons between calculated and measured pile capacities for the NGI-99 and two other methods are presented. The proposed new method gives a good agreement between calculated and measured capacities for the most relevant tests in NGI's database.
This paper presents an experimental study of the plugging effect on the capacity of open-ended piles installed in sandy soil. Full-scale tests, including dynamic and static axial compression load tests, were carried out on three instrumented piles with different diameters (508.0, 711.2, and 914.4 mm). To measure the outer and inner shaft resistances acting on the piles, a double-walled system was utilized, with instrumented strain gauges on the outside and inside walls of the pile. The results of field tests show that the inner shaft resistance was mostly mobilized at the location between the pile tip and 18%–34% of the total plug length. It was found that the soil plugging in the lower portion has influence on the inner shaft resistance. In addition, it can be also demonstrated that the ratio of inner shaft resistance plus annulus load resistance to total resistance was decreased with increasing pile diameters. The results of these tests show that the relationship between the degree of plugging and pile diameter is clearly established. Direct observations of the soil plugs were made and used to quantify both the plug length ratio (PLR) and the incremental filling ratio (IFR). Based on this result, it was realized that the N value of the standard penetration test (SPT) is highly correlated with the IFR.
Numerous cone penetration test (CPT)-based methods exist for calculation of the axial pile capacity in sands, but no clear guidance is presently available to assist designers in the selection of the most appropriate method. To assist in this regard, this paper examines the predictive performance of a range of pile design methods against a newly compiled database of static load tests on driven piles in siliceous sands with adjacent CPT profiles. Seven driven pile design methods are considered, including the conventional American Petroleum Institute (API) approach, simplified CPT alpha methods, and four new CPT-based methods, which are now presented in the commentary of the 22nd edition of the API recommendations. Mean and standard deviation database statistics for the design methods are presented for the entire 77 pile database, as well as for smaller subset databases separated by pile material (steel and concrete), end condition (open versus closed), and direction of loading (tension versus compression). Certain methods are seen to exhibit bias toward length, relative density, cone tip resistance, and pile end condition. Other methods do not exhibit any apparent bias (even though their formulations differ significantly) due to the limited size of the database subsets and the large number of factors known to influence pile capacity in sand. The database statistics for the best performing methods are substantially better than those for the API approach and the simplified alpha methods. Improved predictive reliability will emerge with an extension of the database and the inclusion of additional important controlling factors affecting capacity.
Both the driving response and static bearing capacity of open-ended piles are affected by the soil plug that forms inside the pile during pile driving. In order to investigate the effect of the soil plug on the static and dynamic response of an open-ended pile and the load capacity of pipe piles in general, field pile load tests were performed on instrumented open- and closed-ended piles driven into sand. For the open-ended pile, the soil plug length was continuously measured during pile driving, allowing calculation of the incremental filling ratio for the pile. The cumulative hammer blow count for the open-ended pile was 16% lower than for the closed-ended pile. The limit unit shaft resistance and the limit unit base resistance of the open-ended pile were 51 and 32% lower than the corresponding values for the closed-ended pile. It was also observed, for the open-ended pile, that the unit soil plug resistance was only about 28% of the unit annulus resistance, and that the average unit frictional resistance between the soil plug and the inner surface of the open-ended pile was 36% higher than its unit outside shaft resistance.
The cone penetration test (CPT) has been used as a means of assessing the end bearing of driven piles in sand for many decades. This paper examines the predictive performance of four new such CPT-based methods recently included in the commentary of the 22nd edition of the American Petroleum Institutes recommended practice for fixed offshore structures. It is demonstrated that the formulations given by one of these methods, referred to as UWA-05, provides better predictions than the three other CPT methods when tested against (i) an existing database of base-capacity measurements, (ii) results from a new series of load tests on small-diameter piles, and (iii) base capacities measured in two recently conducted load tests on 1.5m diameter pipe piles. It is shown that the UWA-05 has better predictive performance (and hence reliability) as it accounts explicitly for the effects of partial plugging during pipepile installation and for variations in CPT resistance in the vicinity of the pile tip.
Under static loading in compression, open-ended piles may fail in a plugged mode, with the soil plug moving with the pile, or in an unplugged mode, with shear failure occurring between the soil plug and the pile shaft. It may be shown that, under drained loading conditions, the former mode of failure will generally occur, because arching action within the pipe pile leads to high frictional capacity of the plug. However, under faster rates of loading relevant to the offshore environment, the increase in effective stresses within the soil plug is limited and the plug capacity is significantly lower. The Paper presents a simple one-dimensional analysis of the soil plug under partially drained conditions. The analysis has been implemented numerically, and the resulting program used to derive design charts which give the plug capacity as a function of the soil plug parameters and the rate of loading. These design charts are presented in appropriate non-dimensional form, with example calculations included for typical offshore piles in calcareous soil.
This paper presents a new design approach for predicting the degree of soil plugging and the inner skin friction of axial-loaded open-ended piles. The main objective of this study was to propose the SPT-based design method considering the plugging effect, since the SPT test is commonly used to identify the subsoil condition in sandy soils. The plugging effect for open-ended piles was quantified using field plugging measurements and the results of three full-scale field pile load tests. Based on the plugging measurements, the relationship of the plug length ratio (PLR) with the soil properties, pile geometry and pile driving condition was established. Additionally, a linear relationship between the PLR and incremental filling ratio (IFR) was proposed. Full-scale tests were performed on three instrumented piles with different diameters (508.0, 711.2 and 914.4 mm). An instrumented double-walled pile system was used to measure the outer and inner skin friction along the pile shaft. Based on the results of the full-scale field pile load tests, the inner skin friction of the open-ended piles was proposed as a function of the IFR and pile diameter. The predicted values were consistent with the measured values, such as the IFR and inner skin friction. The proposed method can predict the degree of soil plugging and the inner skin friction of open-ended piles and be selected as convenient option in engineering field.
Stress wave theory is applied to open-ended pipe piles to clarify the effects of soil plug on the behaviour of piles during driving and static loading. Measured field data and various numerical models are reviewed; methods are presented to calculate wave propagation in both the pile and the soil plug; modelling is presented which takes into account the interaction between the soil plug and the pile; also presented is simplified method to estimate the load-settlement relation of the pipe pile in static loading. By correlating observed and calculated values in two analytical cases, the authors demonstrate that incorporation of the soil plug (modelled as a series of masses and springs) is required to correctly predict pile behaviour during driving and static loading.
Calibration chamber tests were conducted on open‐ended model piles driven into dried siliceous sands with different soil conditions in order to clarify the effect of soil conditions on load transfer mechanism in the soil plug. The model pile used in the test series was devised so that the bearing capacity of an open‐ended pile could be measured as three components: outside shaft resistance, plug resistance, and tip resistance. Under the assumption that the unit shaft resistance due to pile‐soil plug interaction varies linearly near the pile tip, the plug resistance was estimated. The plug capacity, which was defined as the plug resistance at ultimate condition, is mainly dependent on the ambient lateral pressure and relative density. The length of wedged plug that transfers the load decreases with the decrease of relative density, but it is independent of the ambient pressure and penetration depth. Under several assumptions, the value of earth pressure coefficient in the soil plug can be calculated. It gradually reduces with increase in the longitudinal distance from the pile tip. At the bottom of the soil plug, it tends to decrease with increase in the penetration depth and relative density, and to increase with the increase of ambient pressure. This may be attributed to (1) the decrease of friction angle as a result of increase in the effective vertical stress, (2) the difference in the dilation degree of the soil plug during driving with ambient pressures, and (3) the difference in compaction degree of soil plug during driving with relative densities. Based on the test results, an empirical equation was suggested to compute the earth pressure coefficient to be used in the calculation of plug capacity using one‐dimensional analysis, and it produces proper plug capacities for all soil conditions.
A series of model pile tests have been performed in the geotechnical centrifuge at the University of Western Australia to study the plugging behaviour of piles in sand. Open and sleeveended piles have been driven and jacked by a miniature pile driving actuator into silica flour of varying densities. The progression of the soil column has been measured during installation and static loading. It was found that the plug length increased with increasing relative density during driving and decreased with increasing relative density during jacking. During installation, the jacked piles exhibited a greateer tendency to plug than the driven piles. Piles fitted with an internal driving shoe provided significant stress relief within the soil plug durling jacking, leading to longer plug lengths compared with internally flush piles. All static load tests at high embedments behaved in a completely plugged manner. An annlysis of the plug capacity is conducted and a model for the variacapacity is conducted and a model for the variation of the earth pressure coefficient inside the pile is proposed. Differential base pressure on the pile annulus and the internal soil plug is postulated and validated by means of the experimental data.
This paper presents the results of a series of field tests performed to study the effect of soil plugging during installation on the base resistance developed by an open-ended (pipe) pile in clay. Three instrumented pipe piles were installed in soft clay, and the installation resistance and soil plug development were recorded. The tests revealed that the annular base resistance was equal to the cone penetration test q(c) resistance and was independent of the soil plug development. In contrast, the soil plug resistance increased from a minimum when the pile was fully coring to a maximum when the pile was fully plugged. A simple expression is proposed that links the plug resistance to the cone penetration test q(c) value at the specified depth and the incremental filling ratio value developed during installation. This expression is shown to provide reasonable estimates of the plug resistance developed by a large-diameter pipe pile driven into stiff to hard clay in the North Sea. The proposed relationship will be particularly useful for modelling the installation resistance of monopile foundations.
The paper presents the results from an experimental program carried out at Trinity College Dublin, in which instrumented model piles were jacked into loose dry sand in a large testing chamber. A number of pile installations were carried out to study the effects of in situ stress, diameter, and wall thickness on the behavior of open-ended piles in sand. These indicated that plug stiffness and capacity may be expressed as simple functions of the cone penetration test end resistance and the incremental filling ratio prior to loading. The magnitude and distribution of shear stresses measured on the inner wall are shown to be compatible with existing experimental data and can be related directly to the stress level, interface friction angle, and dilation of the sand at the pile wall. The data are shown to facilitate a better understanding of the factors controlling plug resistance.
This paper provides a rational method for evaluating a realistic lower bound for the base resistance of pipe piles in siliceous sand. Separate expressions are developed to represent the response to load of the pile plug, the sand below the pile base, and the sand below the pile annulus. These expressions are combined to give the overall base response of a pipe pile. Predicted responses are compared with databases compiled on the ultimate capacities of pipe piles and with base pressure-displacement characteristics observed in static load tests. The estimations are shown to match observed base resistances of large diameter piles for which the coring mode of penetration during driving dominates.
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