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Assessment of bearing capacity of axially loaded monopiles based on centrifuge tests
Abstract
The monopile has been widely used to support offshore and coastal structures. A series of centrifuge tests has been performed to investigate the bearing capacity of large diameter monopiles in sandy soil. Both static tests and cyclic tests have been conducted for open-ended and close-ended model piles, and the effects of influence factors, such as loading rates, embedment depths, and loading histories are considered. The piles are then loaded by a sequence of compressive-tensile loadings to estimate the tension capacity, from which the shaft friction is derived. The cyclic load tests are performed with five varying load intensities, and the accumulated settlement is assessed. The centrifuge tests indicate that the pile bearing capacity tends to increase with the initial penetration depth, and the stress state of soil greatly influences the pile behavior. The tensile shaft friction is smaller compared to the compression test. The capacities of the piles reduce significantly under the axial cyclic load, and the maximum cyclic load intensity should be limited to 75% of the ultimate bearing capacity. The API design method is used to calibrate with the centrifuge test results. The method overestimates the bearing capacity at larger depths, and a conservative reduction factor is required.
... Before preparing the soil, the silica sand is oven-dried for 24 h. The dry sand is air pluviated into the rigid container with a constant height of 0.8 m layer by layer, and slight compactions are applied to obtain a desired relative density (Wang et al., 2018a). The de-air water is instilled slowly from the bottom of the rigid container to saturate the dry sand, and vacuum is applied for 24 h for the saturation process. ...
... The compressive and tensile loadings are applied slowly with a duration of 40 min to avoid piping or cavitation (Eid, 2012). Influences of the loading rate have been investigated previously in the centrifuge; it is demonstrated that when the loading duration exceeds 40mins, differences in the bearing capacity is minimized (Wang et al., 2018a). Meanwhile, the record of the PPT does not show significant pore water pressure changes in the static tests, which is consistent with our previous tests (Wang et al., 2017b). ...
... Although the bucket does not run through the soil, large soil deformation is observed in the tests, which is not allowed in practical engineering. The ultimate bearing capacity of the suction bucket foundation is extracted as the vertical load corresponded to the distinct peaks of the load-settlement curves (Eid, 2012;Wang et al., 2018a) as listed in Table 5. ...
The suction bucket foundation is turning to be an increasingly prevalent foundation for offshore wind turbines (OWTs). In this study, a series of centrifuge tests are performed to investigate the vertical bearing capacity of the suction bucket foundation in sandy soil under drained condition. Four bucket models are tested by considering the influence of aspect ratios (APs). The foundations are tested under compressive and tensile vertical loadings. The bucket foundation is in the general shear failure mode, and the ultimate bearing capacity increases with the AP. The tensile bearing capacity is smaller than the compressive capacity. The sharing factor of the base capacity and the skirt friction of the suction bucket foundation is determined with the assistance of the tensile test. The bucket lid provides the majority of the bearing capacity. The internal skirt friction is larger than the external skirt friction due to the soil constraint effect and the increased vertical stress. Several analytical methods are proposed to estimate the bearing capacity, and the results are compared with the test data. The bearing capacity equation is more applicable to the “wide-shallow” bucket foundations. The recommended design method is regarded as the upper bound when calculating the ultimate bearing capacity.
... Silica sand has been widely used to study pile behavior, so it is used in centrifuge tests, which (Wang et al., 2018a). The particle size of silica sand is small, minimizing the size effect of centrifuge tests according to the centrifuge scaling law . ...
... The soil parameters are carefully measured in the laboratory, and the results obtained from laboratory testing, are listed in Table 1. (Wang et al., 2018a). The pile models are installed at the center of the container, and the distance to the pile tip of the container is set at 3.5 m to avoid the influence of boundary effects (Randolph et al., 1992). ...
The increasing demand for offshore wind turbines with larger capacity brings challenges concerning their original foundation design. This study introduces a new type of monopile foundation with internal restriction plates that aim to enhance the bearing capacity of the entire structure. Two types of improved monopiles are proposed through the addition of one-hole and four-hole restriction plates. A series of centrifuge tests are performed to study the response of the improved piles under lateral loading conditions in saturated sand. The pile-soil interactions are characterized using finite element analysis. A sensitivity study and a parametric study are performed using numerical tests. In offshore applications, the improved pile can provide larger lateral resistances than the open-ended pile foundation. This improvement is proportional to the pile diameter. The four-hole restriction plate is more effective than the one-hole plate for larger diameter piles. The rotational axis is located at 80 % of the embedment depth, and the distribution of earth pressure is determined. A theoretical method is proposed to calculate the ultimate lateral capacity of the novel monopile containing restriction plates.
... Meyer and Żarkiewicz (2018) used a new method to determine the mathematical method of pile side pressure and pile tip resistance. In order to study the bearing capacity of large-diameter single pile in sand, Wang et al. (2018) carried out a series of centrifugal tests. Cui et al. (2021) analyzed the influence of post grouting technology on super long bored cast-in-place pile and its internal mechanism using the field static load test method. ...
The Yinxi Railway is located in the western region of China. As a high speed railway line connecting Yinchuan-Xi'an, it is of great significance for the construction of transportation facilities in western China. For cement fly-ash gravel piles and cement-soil compacted piles are used for the subgrade treatment method and reinforced cushions are set inside the embankment fill so that the upper load of the pile top plane acts as a flexible load the best cement mixing ratio of embankment fill is determined by laboratory test, and the suitable water content range of cement-soil is determined, which provides a basis for selecting reasonable construction technology for subsequent construction. Field static load test is used to test the treatment effect of composite subgrade, which proves that this kind of subgrade treatment method is an economical, feasible and effective method to enhance the bearing capacity of subgrade and treat the collapsibility of subgrade.
... Ko et al. (2016) simulated the driving process of open-ended piles in sand using the Coupled Eulerian-Lagrangian (CEL) approach; parametric analysis revealed that the plugging effect was most influenced by driving energy, followed by pile diameter and pile embedment depth. Based on centrifuge tests, Wang et al. (2018) found that during the loading process of open-ended pile, the resistance components would take effect in the order of shaft resistance, annular resistance and plug resistance; also, the ultimate bearing capacity was reached synchronously with the mobilization of the maximum plug resistance. Li et al. (2019) proposed an innovative pile foundation design by setting up restriction plates inside the pipe pile; centrifuge tests showed that the restriction plates could promote plug formation, thus leading to higher bearing capacity compared to that of the conventional open-ended pile; however, the installation resistance (hammer blow counts) was not provided. ...
Mechanisms governing the sand plug behavior inside an open-ended pile are examined using the discrete element method. A series of numerical pile penetration tests have been conducted by considering the influence of soil density, pile geometry and installation method. A novel sample generation method, based on the replication of unit cell, is applied to produce a large and homogeneous sample efficiently. According to the soil deformation pattern, a “nose cone”, with the length about one pile diameter, has been observed beneath the small diameter jacked pile at the end of penetration. Plugging effect is shown to be more prevalent for jacked piles than dynamic installed piles. Also, larger penetration, smaller diameter and higher soil density all seem to promote plug formation, while the influence of wall thickness is not that obvious. This conclusion is later verified by the development of Incremental Filling Ratio (IFR) and porosity distribution. Furthermore, remarkable stress concentration has been observed at the lower part of the soil plug. The development of installation resistance indicates that jacking produces the largest resistance while dynamic installation methods ease pile penetration. Further analysis based on particle movements, contact force chain distribution and stress orientation provides a micromechanical perspective of the plug behavior. Finally, the plug resistance mobilization process at different plugging conditions and the formation process for soil plug are illustrated.
... A circular friction wheel is added at the mudline to strength the monopile, and additional moment resistances are provided at the pile head. The behavior of OWTs with the hybrid monopile foundation under lateral loadings has been studied in our laboratory [43,44]. The innovative design is effective in improving the ultimate and cyclic bearing behavior of OWTs, and preliminary analytical methods have been put forward to predict their in-situ responses. ...
Some offshore wind farms are built in seismically active areas. The offshore wind turbine (OWTs) are high-rise structures and sensitive to lateral failures during the earthquake. In this study, an innovative hybrid monopile foundation is proposed for OWTs. A series of centrifuge shake table tests is conducted to investigate the seismic response and liquefaction characteristics of the hybrid monopile foundation. Mechanisms of the seismic behavior of soil, lateral displacements of the wind turbine, and structural settlements are evaluated. Centrifuge test results indicate that the liquefaction around the hybrid monopile foundation is weakened compared to the traditional single pile. The soil partially keeps its strength and stiffness during the shaking due to the higher confining stress. The lateral stability of the system is enhanced. OWTs with the hybrid monopile foundation tends to settle more during the earthquake due to the soil-structure interaction and the static bearing induced soil shearing. Two types of hybrid monopile foundations are constructed with different weights and materials, which are predominant influence factors for their seismic response. This study aims to investigate the seismic behavior of the hybrid monopile foundation for OWTs during earthquake events and provide references for practical designs.
... It is normally acknowledged that an open-ended pile requires less installation effort than a close-ended pile under the same soil properties . Accordingly, open-ended pipe piles usually display more settlement compared to the close-ended pipe piles during static load tests (Szechy, 1961;Wang et al., 2018a). This is due to the capacity of open-ended piles is typically lower than that of close-ended piles. ...
Pile foundation is one of the most commonly used foundations for offshore and coastal structures. This paper describes an innovative design for the pile foundation, which institutes an innovative strategy over traditional pile foundation to achieve higher axial bearing capacity. This is achieved by adding restriction plates inside the pile to help form the soil plug. A series of geotechnical centrifuge tests are carried out to evaluate the load bearing behaviors of traditional pile foundation and innovative pile foundation with different types of restriction plates. The pile diameter and shapes of the restriction plates on the soil plug behaviors and pile load carrying capacity are analyzed. The results show that the use of restriction plates could significantly increase the axial bearing capacity of large diameter pile foundations. For different pile diameters, the restriction plates with four smaller holes achieve better performance than those with one large hole of the same effective opening areas. A bearing capacity equation is obtained by normalizing the ultimate bearing capacity with the pile diameters for different restriction plates. This study demonstrates the promise of an innovative pile foundation with a restriction plate as an economic and technically feasible way to produce higher load-bearing capacity to support offshore and coastal structures.
... This concept is initiated by the concept of pile caps and embedded retaining wall with stabilizing platforms [18,19]. The ultimate bearing behavior of the hybrid monopile foundation has been investigated in our laboratory through a set of centrifuge tests, and three analytical methods are put forward in estimating its bearing capacity, including the "add-up method", the "equivalent moment method", and "direct calculation method" [20,21]; the response in service conditions have been studied afterward, and the accumulated lateral displacement is predicted [22]. A similar concept, piled footing, has been investigated through tests and numerical analysis; it is demonstrated that the footing effectively reduces the moment applied on the monopile [23]. ...
Some large capacity offshore wind turbines are constructed in seismically active areas. The occurrence of soil liquefaction during an earthquake can result in severe failures of the offshore wind turbine. The seismic response of the structure and the failure mechanism of the soil-structure interactions are necessary to investigate. In this study, the seismic response of an innovative hybrid monopile foundation is investigated through a series of centrifuge tests. The seismic performance of the combined system of the superstructure, foundation, and soil are demonstrated. Five hybrid foundation models are tested by considering the influence of the foundation thicknesses and diameters, and a monopile foundation is tested for comparison. Centrifuge test results reveal that the hybrid monopile foundation is effective in reducing the lateral displacement during the shaking. In the saturated condition, soil keeps its strength and stiffness beneath and adjacent to the foundation. The hybrid foundation system tends to settle more due to the larger shear stress caused by the soil structure interactions. Influences of the wheel specifications are illustrated. The foundations with larger thicknesses lead to smaller lateral displacements and lower tendencies of liquefaction, but the settlements are intensified. The larger diameter foundation provides a longer drainage path for the excess pore water pressure. With a similar weight, the structure settles less during the earthquake.
Single point mooring system anchored by piles has been widely used to support floating offshore platforms. However, the failure mechanism and soil displacement of the anchor pile subjected to oblique pullout loads are still unclear. Based on transparent sand and PIV technique, a visual centrifuge test system for anchor piles of single point moorings was established. A series of centrifuge model tests were performed on the pile under oblique pullout loads, and the measured soil displacement field results were compared with the finite element simulation, indicating that the centrifuge modeling using transparent sand are suitable for studying the anchor pile behaviors and soil deformation. A modified semi-empirical method was also proposed, which can predict the bearing capacity of piles under difference loading angle and loading position in sand. It shows that with the increasing loading angle and pad eye depth, the pile horizontal translation and soil horizontal deformation become more obvious, and the pullout bearing capacity increases. The proportion of the soil horizontal displacement for θ = 75° is nearly 4.1 times of that for θ = 37°, and the horizontal displacement proportion for z = 0.75L increases approximately 25% of that for z = 0.67L.
A semi-analytical 1D model with the eight types of soil reactions is derived from the fishbone frame model for pile foundations of wind turbines, in which the coupling between axial and lateral resistances that matters for large pile diameters and small pile aspect ratios is modeled as the rotational soil reactions. The only correction factor for the calculation of the ultimate bearing capacities of the lateral and rotational soil reactions on the pile shaft is then identified by a curve-fitting approach and validated using a series of experiments and 3D FE analyses for both sand and clay. The proposed model shows favorable agreements with the experiments and 3D FE analyses in terms of the load-displacement curves at the ground level and the rotation and moment responses for the pile foundations with various pile diameters and aspect ratios. Finally, the proposed model is further verified by monopiles in layered soils and applied to investigate the effect of the coupling between axial and lateral resistances on the modal damping of monopile supported wind turbines. Without considering the coupling between axial and lateral resistances, the modal damping ratios for the monopile supported wind turbine are underestimated significantly.
Field testing was performed to examine vertical performances of a deep large-diameter driven pile used to support wind turbines in multilayered soils. The large-diameter pile was semiopen-ended and had an external diameter and length of 2.4 m and 113 m. The vertical load-displacement relationships were determined, and the shaft and base resistances were derived by analyzing segmented axial forces. The ultimate compressive capacity, determined using a modified Davisson's criterion, was 89.48 MN, which was nearly 20% and 100% larger than that estimated using the CABR JGG106-2014 (2014) criterion and dynamic load test data, respectively. The β values obtained using the modified Davisson's criterion were slightly larger than those obtained using the CABR JGG106-2014 (2014) criterion, and the MOT JTS167-2018 (2018) results were the most conservative. Although empirical methods incorporating parameters directly interpreted from standard penetration test results provided the most consistent estimates, they are limited to some geological conditions. Appropriate coefficients were suggested for empirical methods to calculate the base resistance. The shaft resistance contributed approximately 86% of the compressive axial force. In comparison, the ultimate tensile capacity was approximately 54.58 MN when using the CABR JGG106-2014 (2014) criterion, and the shaft resistance contributed approximately 93% of the tensile force.
Larger-diameter open-ended pipe piles are widely used as foundation for offshore structures. The lack of high-quality inner wall axial stress measurement results in difficulties in loading analysis of open-ended pipe piles during penetration and service period due to the effect of soil plugging. In this paper, two groups of model tests to assess the feasibility of bare fiber Bragg grating (FBG) sensing technology in monitoring the inner wall axial stress profile of open-ended aluminous pipe piles during penetration and horizontal loading are presented. One open-ended aluminous pipe pile was instrumented with bare FBG sensors and then penetrated into the sand bed with the rate of 20 and 30 mm/min, respectively. Two horizontal static loading tests were also conducted after 24-h penetration. The installation process of bare FBG sensors along open-ended aluminous pipe pile is introduced. The distribution profiles of axial stress and unit shaft resistance on inner wall and outer wall along model pile is discussed in detail. Model test results indicated that the bare FBG sensing technology was feasible to measure the axial stress of inner wall along open-ended pipe piles during penetration and horizontal loading in sand. During horizontal loading, the average values of qso/qsi are 0.717 and 0.727, respectively, for model pile with penetration rate of 20 and 30 mm/min. The axial stress difference of inner wall and outer wall along model pile in penetration and horizontal loading is the result of a combination of the inside and outside soil friction as well as of the pipe in-wall shear stiffness.
Wind energy is a promising source of renewable energy and is projected to shift to offshore areas increasingly. Monopile foundation is one of the most commonly used foundations for offshore wind applications with the priority in load bearing capability and initial cost. This study describes an innovative monopile foundation, which institutes a creative strategy over the traditional large diameter monopile foundation to achieve higher axially load bearing capacity. This is achieved by adding a restriction plate inside the pile to intensify the soil plug effect. This design is based on the soil plug mechanism, and the arching effects and plug resistance mobilizations are considered. In this study, an extensive amount of geotechnical centrifuge experiments was conducted to analyze the bearing behaviors of the innovative monopile with restriction plates. The pile with 1-hole restriction plate and the pile with 4-hole restriction plate are considered to discuss effects of the plate shape. Twelve models with different diameters and restriction plate types are investigated. The traditional open-ended and close-ended piles are included for comparisons. The static tests are conducted in saturated silica sand first to determine the ultimate bearing capacity of the innovative pile, after which the cyclic tests are performed. The innovative pile is proved to provide a larger bearing capacity than the pipe pile. An analytical method is proposed to estimate the capacity of the innovative pile. The study aims to develop the design code for innovative piles and provide design reference to large-scale offshore wind turbine projects.
Large open-ended pipe piles have been widely used in offshore and transportation engineering. The design formula for the pipe piles in the current AASHTO specification is based on the load-bearing database of pipe piles with diameters less than 24 inches. Therefore, the load/resistance factors may be inaccurate for the design of large open-ended pipe piles. Centrifuge testing is one of the most commonly used geotechnical test methods to study complex geotechnical problems. In this study, an extensive amount of geotechnical centrifuge experiments was conducted to analyze the load bearing behavior of pipe piles and piles with an innovative restriction plate. A customized loading frame was built to apply static loads. By using the scaling law, centrifuge experiments can scale up the model scale experiment to resemble the field scales. The pile diameters are scaled up by applying the centrifugal accelerations. With the validated experimental setup, the study conducted a program to systematically evaluate the influence of restriction plate on the loading bearing behaviors of large diameter open-end pipe piles. The results showed that by using restriction plates, the ultimate bearing capacity increased significantly compared with the corresponding open-ended pipe piles.
Pile foundation is the most popular option for the foundation of offshore wind turbines. The degradation of stiffness and bearing capacity of pile foundation induced by cyclic loading will be harmful for structure safety. In this article, a modified undrained elastic–plastic model considering the cyclic degradation of clay soil is proposed, and a simplified calculation method (SCM) based on shear displacement method is presented to calculate the axial degradated capacity of a single pile foundation for offshore wind turbines resisting cyclic loadings. The conception of plastic zone thickness Rp is introduced to obtain the function between accumulated plastic strain and displacement of soil around pile side. The axial ultimate capacity of single piles under axial cyclic loading calculated by this simplified analysis have a good consistency with the results from the finite element analysis, which verifies the accuracy and reliability of this method. As an instance, the behavior of pile foundation of an offshore wind farm under cyclic load is studied using the proposed numerical method and SCM. This simplified method may provide valuable reference for engineering design.
In order to better understand the response of driven, closed-ended, pipe piles subjected to axial loads and examine the performance of existing pile design methods, static and dynamic load tests were performed on a closed-ended steel pipe pile driven in a multilayered soil profile. The test pile was fully instrumented with electrical-resistance and vibrating-wire strain gauges, which enabled the determination of residual loads before sensors were reset and load-transfer curves for all loads applied at the pile head during the static load test. The ultimate base and limit shaft resistances of the test pile measured in the static load test and estimated from dynamic load tests are compared with estimates made using several cone penetration test-based (CPT-based) and property-based pile design methods. Two additional case histories are used to verify these design methods, most of which produce satisfactory estimates of pile resistance.
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 book presents a review of four CPT-based methods for driven piles in sand as well as a review of the existing non CPT-based API design method.
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.
Comprehensive measurements of the effective stresses developed during the installation, equalization, and load testing of displacement piles in a loose to medium dense quartz sand are presented. The results shed new light on the mechanisms that control shaft friction in sand. First, it is demonstrated directly that the Stresses developed al any given soil horizon depend strongly on both the distance of that horizon from the pile tip and the soil's initial state. Second, pile loading is shown to induce radial effective stress changes associated with the soil fabric set up by installation and dilation phenomena at pile-soil interface. Thirdly, the operational angles of interface friction arc found to be constant volume values that correlate well with the results from laboratory interface shear tests.
The bearing capacity of open-ended piles is affected by the degree of soil plugging, which is quantified by the incremental filling ratio (IFR). There is not at present a design criterion for open-ended piles that explicitly considers the effect of IFR on pile load capacity. In order to investigate this effect, model pile load tests were conducted on instrumented open-ended piles using a calibration chamber. The results of these tests show that the IFR increases with increasing relative density and increasing horizontal stress. It can also be seen that the IFR increases linearly with the plug length ratio (PLR) and can be estimated from the PLR. The unit base and shaft resistances increase with decreasing IFR. Based on the results of the model pile tests, new empirical relations for plug load capacity, annulus load capacity, and shaft load capacity of open-ended piles are proposed. The proposed relations are applied to a full-scale pile load test performed by the authors. In this load test, the pile was fully instrumented, and the IFR was continuously measured during pile driving. A comparison between predicted and measured load capacities shows that the recommended relations produce satisfactory predictions.
Most foundation solutions for transportation structures rely on deep foundations, often on pile foundations configured in a way most suitable to the problem at hand. Design of pile foundation solutions can best be pursued by clearly defining limit states and then configuring the piles in such a way as to prevent the attainment of these limit states. The present report develops methods for load and resistance factor design (LRFD) of piles, both nondisplacement and displacement piles, in sand and clay. With the exception of the method for design of displacement piles in sand, all the methods are based on rigorous theoretical mechanics solutions of the pile loading problem. In all cases, the uncertainty of the variables appearing in the problem and of the relationships linking these variables to the resistance calculated using these relationships are carefully assessed. Monte Carlo simulations using these relationships and the associated variabilities allow simulation of resistance minus load distributions and therefore probability of failure. The mean (or nominal) values of the variables can be adjusted so that the probability of failure can be made to match a target probability of failure. Since an infinite number of combinations of these means can be made to lead to the same target probability of failure, we have developed a way to determine the most likely ultimate limit state for a given probability of failure. Once the most likely ultimate limit state is determined, the values of loads and resistances for this limit state can be used, together with the values of the mean (or nominal) loads and resistances to calculate load and resistance factors. The last step in the process involves adjusting the resistance factors so that they are consistent with the load factors specified by AASHTO. Recommended resistance factors are then given together with the design methods for which they were developed.
The influence of soil plug effect on the dynamic response of large diameter pipe piles during low-strain integrity testing is investigated based on the additional mass model, considering the soil plug weight and displacement phase difference between soil plug and pipe pile. An analytical solution that can consider the stress wave propagating both in the vertical and circumferential directions is derived in the frequency domain through the transfer function method. The corresponding quasi-analytical solution in the time domain for the pipe pile subjected to a vertical semi-sinusoidal impact force is then obtained by utilizing the Fourier transform technique. The validity and accuracy of the solution presented in this paper is compared against a series of model tests and field experiments of pipe pile. Utilizing the developed solution, a parametric study is performed to illustrate the influence of soil plug properties, pipe pile properties and the impulse width of impact force on the composite velocity of the longitudinal stress wave propagating in pipe pile, and the applicability of the plane-section assumption adopting in pipe pile.
The support structure of an offshore wind turbine (OWT) accounts for up to 25% of the capital cost; therefore, investigations into reliable and efficient foundations are critical for the offshore wind turbine industry. This paper describes an innovative hybrid monopile foundation for OWTs, which is an optimization of the original monopile foundation with broader applications. The behavior of OWTs with the hybrid monopile foundation in service conditions are investigated under lateral cyclic loadings, by considering the effects of wind, waves, and ice. A series of centrifuge tests are conducted in order to analyze these behaviors in detail, and OWT models with the original single-pile as well as wheel-only foundations are tested for comparison. Based on these tests, the accumulated lateral displacement and stiffness during cyclic loadings are presented, and the results indicate that the hybrid foundation exhibits a larger cyclic capacity than the other foundations. The influence of the cycle numbers, cyclic loading characteristics, and soil properties is examined during the tests; furthermore, the effects of these factors on the model deformation responses are illustrated. This study proposes the first analytical method for quantitatively estimating the cyclic lateral displacement of the new hybrid foundation in service conditions, and a degradation coefficient is recommended based on the test results. This method aims to provide a simple approach to predicting responses of OWTs with hybrid monopile foundations in service conditions.
The hybrid monopile-friction wheel foundation is an innovative alternative for offshore wind turbines. The concept has wider adaptability and can be used as reinforcement method for existing monopiles. A series of centrifuge tests was performed to investigate the lateral bearing capacities of the hybrid foundation under monotonic loads. Five foundation models and two soil types were considered. According to the recorded responses, the hybrid foundation demonstrated better lateral behaviors that both lateral bearing capacity and stiffness are enhanced. Two analytical methods were proposed and compared with the centrifuge test results. The bearing capacity of the hybrid foundation is smaller than the sum of individual pile and friction wheel, and a reduction factor is suggested for both friction wheels. The friction wheel restrains rotations of monopile and provides extra restoring moments; their effects are idealized as equivalent moments acting on the pile head. The analytical results provide possible solutions in estimating the lateral bearing capacity of the innovative hybrid foundation system for offshore wind turbines by using traditional theories.
Wind Energy is the one of the most promising renewable energy. Suction bucket foundation is considered to be a viable type of wind turbine foundation. Soil liquefaction caused by earthquakes at offshore seismic active area may lead to a significant degradation of soil strength and stiffness. In this study, nine centrifuge tests were carried out to investigate the seismic response of suction bucket foundation under earthquake loading. Both dry and saturate soil conditions were considered in tests. The geometric design of five suction bucket models considered the bucket diameter, penetration depth, and modified buckets with inside compartments. It was found that soil underlying and near the bucket foundation shown a better ability to resist liquefaction in saturated tests comparing to free field while no significant differences were observed in dry tests. The five bucket models performed quite differently, which demonstrated the aspect ratio effects and inside-bucket compartment effects. The results provide insight into optimized design of suction bucket foundation for wind turbine.
Suction bucket foundation is a promising alternative for offshore wind turbine foundations. The lateral bearing behavior and failure mechanism are significant topics for optimizing its design criteria. In this study, a group of centrifuge tests were performed to investigate the lateral bearing capacity of suction bucket foundation with three aspect ratios. Force-controlled lateral static and cyclic tests were performed at a centrifuge acceleration of 50 g. Four soil conditions were considered in centrifuge tests with a combination of loose/dense and dry/saturated sand. The load-displacement and the stiffness-displacement relationships are presented in the paper, and it is concluded that the lateral bearing capacity can be reached when the normalized lateral displacement (displacement divided by the bucket diameter) reached 3% as the first method. Displacement rates are plotted against the lateral load to give the second method for defining the ultimate lateral capacity. In cyclic tests, the lateral displacement and stiffness are correlated to the cycle numbers in the first 5 cycles, but the change is less apparent in the rest. The results demonstrated the behavior of bucket foundation in service conditions. Finally, a simplified calculation method is used as the third method and checked with tests results.
Offshore wind industry is having a great development. It requires progress in many aspects to achieve the sustainable progress of this technology. One of those aspects is the design of foundation, sub-structures and support structures. The most used at present, with more than 80%, is the monopile. Typical piles used in quays in maritime engineering have a maximum diameter about 2 or 3 m. In offshore wind, the diameter can be more than double. There is a risk associated with the difference in scale. Some formulas used for the design of typical piles with diameter less than 2 m can be unsuitable for larger diameter piles. This paper is focused on giving a first estimate of length and weight of piles for knowing its diameter. There are formulas for that for piles with diameters up to 2 m, but there are doubts about whether they can be used for piles with larger diameters. To achieve it, a database gathering offshore wind farms in operation with monopiles is prepared in order to obtain simple formulas relating those parameters. Furthermore, the results of that formula are compared with traditional formula used in maritime engineering for piles with diameters less than 2 m.
This paper presents the results of static compression load tests performed on long rock-socked bored piles installed in stratified soils. Three bored piles with 34 m in embedded length were instrumented in order to separate the shaft and base resistance, and to allow the determination of the distribution of shaft resistance along the pile shaft. Conventional methods of estimating shaft resistance were assessed. It was found that more than 78% of the shaft resistance was provided by shaft friction at the end of tests. The base resistance has not been fully mobilized in the test. The recommendations of Chinese technical code (JG94-2008) to estimate the shaft resistance were more conservative for long rock-socketed bored piles. Empirical correlations to estimate the shaft resistance are limited due to different geological conditions. The methods which incorporated parameters directly interpreted from standard penetration test (SPT) results provided the most consistent estimates.
Nondynamic repeated load tests were conducted on single friction piles embedded in a medium dense sand. The 19-mm (0. 75-in. ) diam pile was instrumented to measure the distribution of axial loads along the shaft. It was found that the response of the pile to repeated loads is governed by the: (1)Type of load; (2)amplitude of load; anf (3)number of cycles of load applied. The effect of repeated loading was to bring about not only large changes in the net movement per cycle, but also a redistribution of loads between the shaft and the base. In general, the pile behavior is marked by two or three stages of relative stability or instability in terms of the net movement per cycle. The response of the pile to a small repeated load is characterized by a long and highly stable stage which, in some cases, is followed by an unstable stage.
Jetting is a technique commonly used to install well conductors, which is faster than the conventional drill and cement method. Conductors are designed to support the buoyed weight of the first string of casings (typically, 3-5 days after jetting) and later to provide the lateral stability for the well system against cyclic loading from environmental elements. The axial bearing capacity of a jetted conductor increases with time due to consolidation and thixotropy effects; however, the field data for set-up only extend to about 10 days. Installation of jetted conductors as piles and anchors may be an economic solution for offshore developments. The long-term axial capacity of jetted conductors was investigated through a series of centrifuge modelling and laboratory thixotropy testing. The centrifuge tests simulated jetting installation of four model conductors and measured capacities at set-up times ranging between 10 and 1000 days in a kaolin clay seabed with an undrained shear strength profile similar to that encountered in deepwater Gulf of Mexico. The centrifuge test results were compared to the field data in the Gulf of Mexico. The implications of the centrifuge test results for deepwater Gulf of Mexico conditions are discussed. A method is presented for estimating the long-term axial capacity of jetted conductors in Gulf of Mexico seabed conditions with recommendations for additional studies.
The ultimate bearing capacity of instrumented vertical single rigid model piles in homogeneous loose sand and soft clay under vertical eccentric and central inclined loads has been investigated. The results of these load tests provide a more realistic lateral soil pressure distribution on the pile shaft and better theoretical estimates of pile capacity under pure moment and under horizontal load. For intermediate eccentricities and inclinations of the load, the bearing capacity can be obtained from simple interaction relationships between the axial load and moment capacities and between the axial and horizontal load capacities, respectively. The influence of lateral soil pressures due to installation of displacement piles in clay is examined in relation to the ultimate load of the pile.
This paper presents a new method for estimating the base capacity of open-ended steel pipe piles in sand, a difficult problem involving great uncertainty in pile foundation design. The method, referred to as the Hong Kong University (HKU) method, is based on the cone penetration test (CPT), and takes into consideration the mechanisms of annulus and plug resistance mobilization. In this method the annulus resistance is properly linked to the ratio of the pile length to the diameter-a key factor reflecting the influence of pile embedment-whereas the plug resistance is related to the plug length ratio, which reflects the degree of soil plugging in a practical yet rational way. The cone tip resistance is averaged over a zone in the vicinity of the pile base by taking into account the failure mechanism of the piles in sand, the condition of pile embedment (i.e., full or partial embedment), and the effect of soil compressibility. The predictive performance of the new method is assessed against a number of well-documented field tests including two fully instrumented large-diameter offshore piles, and through comparisons with major CPT-based methods in current engineering practice. The assessment indicates that the HKU method has attractive capabilities and advantages that render it a promising option. DOI: 10.1061/(ASCE)GT.1943-5606.0000667. (C) 2012 American Society of Civil Engineers.
The load rapacity of driven piles has been reported to sometimes increase or decrease with time after pile installation. An increase in pile capacity with time is known as setup, whereas a decrease in capacity is referred to as relaxation. In this paper, we review the current understanding of pile setup and discuss how to account for it in design. A total of forty-three dynamic tests were conducted over a period of five months on four H piles and four closed-ended pipe piles driven into layered soil. Test results show that the amount and rate of pile setup are quite different from those observed in other studies. Empirical formulas for predicting setup proposed by several researchers were compared with observations. Of the empirical formulas considered, the Svinkin (1996) lower-bound method predicted the rate of setup on these piles driven in layered soil relatively well. Additionally, some theoretical methods for prediction of evolution of static pile bearing capacity with time were tested against the dynamic pile test results. The bearing capacity from these theoretical methods was found to correspond to approximately 2 times the capacity measured at the end of initial driving by a dynamic test.
The analysis and computation of the ultimate lateral capacity of rigid piles is usually based on a simplified soil pressure distribution along the pile length. Actual soil pressure distributions were measured in a rigid model pile along its length and across the diameter and it was found that the simplified theoretical assumptions of pressure distribution needs a modification. A method is suggested in this paper to predict soil pressure distribution and ultimate lateral capacity for rigid piles in cohesionless soils. Field and laboratory data from published literature are used to validate the proposed method.
The response of the soil plug within driven, open-ended pipe piles is very different under the dynamic conditions of installation and the static conditions of loading during service. This paper addresses the latter aspect and describes a combined experimental and numerical study of the response of soil plugs in open-ended pipe piles. The work focuses on the partially drained (static) loading relevant to offshore applications, and the experimental work is conducted using calcareous sand from Bass Strait, Australia. Model tests are conducted in pipe piles of 25-mm and 100-mm internal diameters, with loading rates as great as 6 MPa/s, using a downward hydraulic gradient to achieve appropriate effective stress profiles in the soil plug. The experimental results are assessed within the framework of analytical solutions of the drained and undrained response of the soil plug and numerical studies of the partially drained problem that allow the results to be extrapolated to prototype conditions. Example applications are given for pile geometries and loading rates typical for Bass Strait.
This paper summarizes the parameters that influence the axial capacity of pipe piles driven into sands. The parameters that affect pile behavior are for convenience divided into four categories: (1) Soil characteristics; (2) pile characteristics; (3) method of pile installation; and (4) type of loading. When possible, the influence that these parameters have on the capacity is quantified. In addition, the paper describes supporting evidence from pile load test data, small-scale laboratory tests, and in situ cone probings that has been used to justify limiting values for unit shaft and toe resistances. The use of limiting values has significant economic impact on long piles, especially those used for offshore applications. This information is, therefore, evaluated and critiqued to place the use of limiting values in proper perspective. The results of this paper are based on a synthesis of literature on (1) pile load test data on piles in sand, (2) data from cone penetrometer tests in sands, and (3) information on the stress-strain and strength behavior of sand.
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.
A series of model pile tests have been performed in the geotechnical centrifuge at the University of Western Australia to study the behaviour of driven piles in homogeneous sand. Open, sleeved and closed-ended piles have been driven by a miniature pile driving actuator into silica flour of varying densities. The model pile was fully instrumented, allowing strain gauge data to be monitored during dynamic and static testing. Analysis of the load tests revealed that the shaft friction increased approximately linearly with depth at a low rate, but with a marked increase close to the pile tip. The average shaft friction for the closed-ended tests was shown to be greater than for the open-ended tests. For all tests, the average values of shaft friction were greater than those suggested by current guidelines for design of offshore piles (API RP2A), and the ratio of tensile to compressive shaft capacity was always below unity. The end-bearing resistance normalized by the cone tip resistance was shown to reduce with depth for a given base displacement. Furthermore, tests conducted at similar depths in samples of different densities revealed that this form of normalization allows the end-bearing response to be reduced to a single curve. On the basis of these results, hyperbolic end-bearing mobilization curves have been developed for design and are compared with similar curves proposed by other researchers.
Most of the current design methods for driven piles were developed for closed-ended pipe piles driven in either pure clay or clean sand. These methods are sometimes used for H piles as well, even though the axial load response of H piles is different from that of pipe piles. Furthermore, in reality, soil profiles often consist of multiple layers of soils that may contain sand, clay, silt or a mixture of these three particle sizes. Therefore, accurate prediction of the ultimate bearing capacity of H piles driven in a mixed soil is very challenging. In addition, although results of well documented load tests on pipe piles are available, the literature contains limited information on the design of H piles. Most of the current design methods for driven piles do not provide specific recommendations for H piles. In order to evaluate the static load response of an H pile, fully instrumented axial load tests were performed on an H pile (HP 310x110) driven into a multilayered soil profile consisting of soils composed of various amounts of clay, silt and sand. The base of the H pile was embedded in a very dense nonplastic silt layer overlying a clay layer. This paper presents the results of the laboratory tests performed to characterize the soil profile and of the pile load tests. It also compares the measured pile resistances with those predicted with soil property- and in situ test-based methods.
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.
In most soil types, it is generally assumed that the shaft capacity of a pile is identical under both tensile and compressive loading. However, there is widespread experimental evidence that in sand the shaft capacity is significantly lower for tensile loading, or uplift, than for compressive loading. This paper describes the results of a parametric study that was conducted to explore the theoretical basis for such differences. It is shown that the primary cause of lower tensile capacity is due to a Poisson's ratio effect, and that the ratio of tensile to compressive shaft capacity may be expressed as a function of the relative compressibility of the pile and the slenderness ratio. Design recommendations are proposed, and corroborated with results from high-quality field tests.
This paper analyzes the results of a driven pile side shear setup (SSS) test program performed at the University of Florida and described in a companion paper. All pile segments showed SSS, with similar magnitudes in both cohesionless and cohesive soils, and irrespective of depth. Horizontal effective stresses increased after driving, but then stabilized as induced pore pressures dissipated. SSS continued long after pore pressure dissipation due to other aging effects. Pile load tests in all soil types, and standard penetration tests with torque measurement (SPT-T) in cohesive soils, confirmed the approximate semilog-linear time versus SSS behavior and the usefulness of the setup factor A. Staged (repeated) tests significantly increased SPT-T measurements, suggesting a reduction factor of 0.4 for SSS results measured by staged testing. We believe that engineers can now include SSS in routine design, using a minimum setup factor A=0.1 for soils similar to those tested, or higher if supported by dynamic or static pile tests, or by SPT-T predictor tests. Examples demonstrate a proposed SSS design method.
During installation of open-pipe piles, soil enters the pile until the inner-soil cylinder develops sufficient resistance to prevent further soil intrusion and the pile becomes "plugged." In spite of its frequent occurrence, only limited attention has thus far been given to this phenomenon and its consequences. The effects of plugging on pile performance and design are examined in reference to the following aspects: ultimate static capacity, time-dependent pile capacity, and dynamic behavior. Pile plugging is shown to have the following effects: marked contribution to the capacity of piles driven in sand; delay in capacity gain with time for piles driven in clay; and change in behavior of piles during installation, causing it to differ from that described by the models commonly used to predict and analyze pile driving. Key words: pipe piles, pile plugging, open-ended piles, static capacity, time-dependent capacity, dynamic analysis, pile driving, pile performance.
Summary Design of piled foundations for bridge structures for the realignment of US95 in Sandpoint, Idaho, required a pre-design static loading test on an instrumented, 406 mm diameter closed-toe pipe pile driven to 45 m depth in soft, compressible soil. The soil conditions at the site consist of a 9 m thick sand layer on normally consolidated, compressible post-glacial alluvial deposits to depths estimated to exceed 200 m. Field explorations included soil borings and CPTU soundings advanced to a depth of 80 m. The clay at the site is brittle and strain-softening, requiring special attention and consideration in geotechnical design of structures in the area. Effective stress parameters back-calculated from the static loading test performed 48 days after driving correspond to beta-coefficients of about 0.8 in the surficial 9 m thick sand layer and 0.15 at the upper boundary of the clay layer below, reducing to 0.07 in the clay layer at the pile toe, and a pile toe-bearing coefficient of 6. The beta-coefficients are low, which is probably due to pore pressures developing during the small shear movements during the test before the ultimate resistance of the clay was reached. The analyses of the results of the static loading test have included correction for residual load caused by fully mobilized negative skin friction down to 10 m depth and fully mobilized positive shaft resistance below 30 m depth with approximately no transfer of load between the pile and the clay from 10 m depth through 30 m depth.
Simulation of earthquake geotechnical problems in centrifuge has grown significantly in the past decade and a variety of challenging problems are now being tackled in various centrifuge establishments all over the world. Considerable experience has been gained in simulating successfully earthquake effects in the centrifuge. Simulation earthquake conditions in the geotechnical centrifuge requires careful consideration of a number of factors. These include modelling of base motion, selection of model container with non-reflecting boundaries and use appropriate fluid in the soil. These aspects are discussed in this paper. A brief summary of the current activities in this field are also presented. Finally, details of the geotechnical centrifuge facility established at IIT Bombay and the preliminary details of the proposed earthquake simulator are given.
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