Experimental Investigations of Response of Suction Caissons to Transient Vertical Loading

Suction caissons are a relatively new design being considered for use as foundations in a wide variety of offshore applications. They are unusual in that, in contrast with the development of offshore piling, there is no onshore experience that can be used as the basis for the development of designs. It is therefore essential to identify key behavioral patterns and mechanisms that govern capacity under a wide variety of loading regimes, particularly those derived from the cyclic wave loading. The object here is to establish a broad framework of response. More detailed work specific to a site or project would be required for detailed design. This paper describes experimental investigations into the vertical loading response of a suction caisson foundation. The experiments are carried out on the laboratory floor using a sophisticated three-degree-of-freedom loading rig. The caissons are embedded in sand saturated with viscous silicon oil so that modeled drainage times are representative of typical offshore conditions. The experiments involve cyclic loading about different mean loads, including cycling into tension and monotonic loading at different rates. The relationship between the cyclic loading and monotonic loading is explored. One of the key conclusions from the results is that serviceability requirements will dictate design rather than capacity. Perhaps surprisingly, for the experiments undertaken, the rate of loading had little effect on the response.     

Effects of Moment-to-Shear Ratio on Combined Cyclic Load-Displacement Behavior of Shallow Foundations from Centrifuge Experiments

Current design guidelines for shallow foundations supporting building and bridge structures discourage footing rocking or sliding during seismic loading. Recent research indicates that footing rocking has the potential to reduce ductility demands on structures by dissipating earthquake energy at the footing-soil interface. Concerns over cyclic and permanent displacements of the foundation during rocking and sliding along with the dependence of foundation capacity on uncertain soil properties hinder the use of footing rocking in practice. This paper presents the findings of a series of centrifuge experiments conducted on shear wall-footing structures supported by dry dense to medium dense sand foundations that are subjected to lateral cyclic loading. Two key parameters, static vertical factor of safety (FSV), and the applied normalized moment-to-shear ratio (M/(H?L)) at the footing-soil interface, along with other parameters, were varied systematically and the effects of these parameters on footing-soil system behavior are presented. As expected, the ratio of moment to the horizontal load affects the relative magnitude of rotational and sliding displacement of the footing. Results also show that, for a particular FSV, footings with a large moment to shear ratio dissipate considerably more energy through rocking and suffer less permanent settlement than footings with a low moment to shear ratio. The ratio of actual footing area (A) to the area required to support the vertical and shear loads (Ac), called the critical contact area ratio (A/Ac), is used to correlate results from tests with different moment to shear ratio. It is found that footings with similar A/Ac display similar relationships between cyclic moment-rotation and cumulative settlement, irrespective of the moment-to-shear ratio. It is suggested that shallow foundations with a sufficiently large A/Ac suffer small permanent settlements and have a well defined moment capacity; hence they may be used as effective energy dissipation devices that limit loads transmitted to the superstructure.     

Experimental Investigation of the Reinstallation of Spudcan Footings Close to Existing Footprints

The large footprints that remain on the seabed after offshore mobile jack-up platforms have completed operations provide hazardous conditions for any future jack-up installation at that site. The slope of the footprint and varying soil strengths below the surface cause detrimental horizontal and moment loads to be induced on the spudcan during the preloading process where only vertical loads are expected. Experimental data from 12 tests investigating the reinstallation of a spudcan footing close to an existing footprint is presented in this paper. The experiments were carried out using a geotechnical drum centrifuge at a radial acceleration level equivalent to 250 times that of Earth’s gravity. The stiffness of the loading leg and model spudcan shape were scaled to ensure conditions of stress similitude between the model and prototype. In all of the experiments, an initial footprint was created. The spudcan was then offset and reinstalled with the combined vertical, horizontal, and moment loads on the spudcan recorded. The effects of reinstallation location, preloading levels, and change in leg stiffness were investigated. The worst location for reinstallation was found to be at an offset half a spudcan diameter from the initial spudcan installation. The horizontal and moment loads were also greater when a more extensive footprint was created by the initial spudcan being embedded deeper and with a higher preload. For the range of conditions tested, changing the leg stiffness did not affect the results.     

海上风电复合基础承载性能对比研究

受到上部结构自重以及海洋环境荷载的影响,海上风电基础设计时应考虑竖向荷载、水平荷载以及弯矩荷载作用下基础的承载性能。本文通过有限元软件ABAQUS,对比研究了饱和黏土场地中大直径单桩基础、桩?平台复合基础以及桩?筒复合基础在竖向荷载V、水平荷载H、弯矩荷载M作用下的承载性能。研究结果表明两种复合基础较单桩基础呈现出显著的承载性能优势。桩?平台复合基础的竖向承载力、水平承载力以及抗弯承载力随着附加平台直径的增大呈指数型增加;桩?筒复合基础的竖向承载力以及抗弯承载力随着筒结构入土深度的增加先增大然后趋于稳定,桩?筒复合基础的水平承载力与筒直径以及筒入土深度为双参数线性增加关系。V?H以及V?M复合荷载加载条件下,两种复合基础比单桩基础的破坏包络线空间大,两种复合基础的稳定性相对单桩基础有显著提升。在一定承载范围内,附加平台结构或筒型结构可以减小桩的直径或入土深度。      

非均质土中海上风电单桩基础动力响应特性

采用有限元分析软件ABAQUS建立了非均质土中海上风电单桩基础数值计算模型,将桩基础受到的波浪、洋流及风荷载等效成双向对称循环荷载,对水平循环荷载作用下桩身水平位移、桩身剪力、桩身弯矩和桩侧土抗力进行了研究,并对不同循环次数下桩身水平位移进行了对比分析。研究表明,桩身水平位移随时间变化逐渐累积,随着循环次数的增加,泥面处桩身最大位移发生的时间点滞后;桩身剪力出现负值;桩身弯矩最大值发生在浅层土体;桩身外壁土抗力曲线随时间的变化在埋深约2/3处出现分界点,分界点上下范围内土抗力变化规律正好相反,在淤泥土和粉砂土分界面处增加显著;不同时间点桩身内壁沿埋深承担的荷载基本不变。      

Contact Interface Model for Shallow Foundations Subjected to Combined Cyclic Loading

It has been recognized that the ductility demands on a superstructure might be reduced by allowing rocking behavior and mobilization of the ultimate capacity of shallow foundations during seismic loading. However, the absence of practical reliable foundation modeling techniques to accurately design foundations with the desired capacity and energy dissipation characteristics and concerns about permanent deformations have hindered the use of nonlinear soil–foundation–structure interaction as a designed mechanism for improving performance of structural systems. This paper presents a new “contact interface model” that has been developed to provide nonlinear relations between cyclic loads and displacements of the footing–soil system during combined cyclic loading (vertical, shear, and moment). The rigid footing and the soil beneath the footing in the zone of influence, considered as a macroelement, are modeled by keeping track of the geometry of the soil surface beneath the footing, along with the kinematics of the footing–soil system, interaction diagrams in vertical, shear, and moment space, and the introduction of a parameter, critical contact area ratio (A/Ac); the ratio of footing area (A) to the footing contact area required to support vertical and shear loads (Ac). Several contact interface model simulations were carried out and the model simulations are compared with centrifuge model test results. Using only six user-defined model input parameters, the contact interface model is capable of capturing the essential features (load capacities, stiffness degradation, energy dissipation, and deformations) of shallow foundations subjected to combined cyclic loading.     

Behavior of Piled Raft Foundations Under Lateral and Vertical Loading

This article presents a new method of analysis of piled raft foundations in contact with the soil surface. The soil is divided into multiple horizontal layers depending on the accuracy of solution required and each layer may have different material properties. The raft is modeled as a thin plate and the piles as elastic beams. Finite layer theory is employed to analyze the layered soil while finite element theory is used to analyze the raft and piles. The piled raft can be subjected to both loads and moments in any direction. Comparisons show that the results from the present method agree closely with those from the finite element method. A parametric study for piled raft foundations subjected to either vertical or horizontal loading is also presented.     

海上风电桩–筒复合基础承载性能研究

基于有限元软件ABAQUS平台,建立了非匀质饱和黏土场地的海上风电桩–筒复合基础数值计算模型,对比研究竖向荷载V、水平荷载H和弯矩荷载M作用下不同筒结构尺寸的桩–筒复合基础的承载力系数,并采用正交试验法开展桩–筒复合基础的各向承载性能的影响因素研究.结果表明,饱和黏土的非匀质特性系数K对竖向承载力系数N_cV影响较小;K对水平承载力系数N_cH和抗弯承载力系数N_cM的影响呈指数型递减.筒结构直径D和入土深度L对各向承载力系数的影响存在交互作用.D对桩–筒复合基础承载力系数的影响最大,可以通过增加筒结构直径从而有效地提高桩–筒复合基础的承载性能.研究结果为海上风电桩–筒复合基础的设计提供了依据.     

Capacity, Settlement, and Energy Dissipation of Shallow Footings Subjected to Rocking

The effectiveness of structural fuse mechanisms used to improve the performance of buildings during seismic loading depends on their capacity, ductility, energy dissipation, isolation, and self-centering characteristics. Although rocking shallow footings could also be designed to possess many of these desirable characteristics, current civil engineering practice often avoids nonlinear behavior of soil in design, due to the lack of confidence and knowledge about cyclic rocking. Several centrifuge experiments were conducted to study the rocking behavior of shallow footings, supported by sand and clay soil stratums, during slow lateral cyclic loading and dynamic shaking. The ratio of the footing area to the footing contact area required to support the applied vertical loads (A/Ac), related to the factor of safety with respect to vertical loading, is correlated with moment capacity, energy dissipation, and permanent settlement measured in centrifuge and 1 g model tests. Results show that a footing with large A/Ac ratio (about 10) possesses a moment capacity that is insensitive to soil properties, does not suffer large permanent settlements, has a self-centering characteristic associated with uplift and gap closure, and dissipates seismic energy that corresponds to about 20% damping ratio. Thus, there is promise to use rocking footings in place of, or in combination with, structural base isolation and energy dissipation devices to improve the performance of the structure during seismic loading.     

Design of Dynamically Wind-Loaded Helical Piers for Small Wind Turbines

The expansion of alternative energy has created a demand for sustainable alternatives for wind turbine foundation design. This study investigates the proposed application of helical piers as foundations for guyed cables of small (1–10-kW) wind towers. Before the foundation system can be implemented, pier response to typical working loads and extreme environmental conditions must be determined. Field conditions were determined by equipping an existing wind tower with accelerometers and load cells while monitoring wind speed to determine tower response. A full-scale testing program was conducted, which simulated dynamic loading conditions based on the tower response. The testing program varied between typical working conditions and extreme load events to determine the critical loading case and creep potential from long-term loading. This paper discusses the effects of dynamic loading on helical pier performance and compares the results to that of uplift prediction methods typically used in helical pier design.     

Experimental Studies on Seismic Behavior of Steel Pile-to-Pile-Cap Connections

To investigate the seismic behavior of bridge pile-to-pile-cap connections, five full-scale H-shaped steel pile-to-pile-cap connection subassemblies representing a portion of a typical bridge pile foundation were tested. Two of the full-scale subassembly specimens were subjected to a vertical cyclic load simulating axial forces in a pile caused by footing overturning during an earthquake attack. Two others were loaded with cyclic lateral force and constant vertical load. One specimen was tested under proportionally varied vertical and horizontal forces. It was found that, although it was designed as a pinned connection following the current design standard, the pile-to-pile-cap connection can sustain a significant amount of moment. Localized brittle failure was observed in the vicinity of the pile-to-pile-cap connection, as the results of unexpected moment resistance. Test results also showed that the anchorage details using two V-shape bars could not develop the full-design ultimate tensile capacity. Analytical methods are developed and found adequate to evaluate the strength of the pile-to-pile-cap connection.     

Lateral Force Induced by Rectangular Surcharge Loads on a Cross-Anisotropic Backfill

This article presents approximate but analytical-based solutions for computing the lateral force (force per unit length) and centroid location induced by horizontal and vertical surcharge surface loads resting on a cross-anisotropic backfill. The surcharge loading types include: point load, finite line load, and uniform rectangular area load. The planes of cross-anisotropy are assumed to be parallel to the ground surface of the backfill. Although the presented solutions have never been proposed in existing literature, they can be derived by integrating the lateral stress solutions recently addressed by the author. It is clear that the type and degree of geomaterial anisotropy, loading distances from the retaining wall, and loading types significantly influence the derived solutions. An example is given for practical applications to illustrate the type and degree of soil anisotropy, as well as the loading types on the lateral force and centroid location in the isotropic/cross-anisotropic backfills caused by the horizontal and vertical uniform rectangular area loads. The results show that both the lateral force and centroid location in a cross-anisotropic backfill are quite different from those in an isotropic one. The derived solutions can be added to other lateral pressures, such as earth or water pressure, which are necessary in the stability and structural analysis of a retaining wall. In addition, they can be utilized to simulate more realistic conditions than the surcharge strip loading in geotechnical engineering for the backfill geomaterials are cross-anisotropic.     

Modeling of Shallowly Embedded Offshore Pipelines in Calcareous Sand

The results of centrifuge modeling of pipe–soil interaction for shallowly embedded offshore pipelines are presented. A non-associated bounding surface model is constructed in vertical–horizontal (V–H) load space on the basis of test data and the theory of plasticity to simulate the response of a pipeline embedded in sandy soil under combined (vertical and horizontal) monotonic loading, taking into account possible pre-loading effects. The model needs nine parameters that can be back calculated from model or field tests and in some cases estimated theoretically. It provides a suitable basis for modeling the load-displacement response of shallowly embedded offshore pipelines. The model reproduces the key features of the load-displacement response of pipelines observed in centrifuge model tests. In particular, the adoption of bounding surface plasticity allows a gradual transition from elastic to plastic response to be simulated and the introduction of a non-associated flow rule allows the model to predict the strain-softening behavior of pipes under horizontal loading. The lateral breakout resistance predicted by the model agrees very well with experimental data.     

Ice Tank Tests on Ice Rubble Loads Against a Cable-Moored Conical Platform

Presented herein are ice-tank data concerning ice-rubble loads against a moored conical platform. Of prime interest is how rubble-ice loads are influenced by horizontal stiffness of the platform’s mooring system. An important, and heretofore undocumented, feature of rubble loading and clearing around the platform is the accumulation of ice rubble as a false bow, or prow, at the platform’s leading perimeter. The ice-tank data show that, under certain conditions of ice-rubble loading, the lowest stiffness of mooring tested resulted in prow instability and sloughing, which in turn lead to a cyclic pattern of loading, and to rubble congestion around the platform. An outcome of such rubble congestion was a marked increase in the overall rubble-ice load exerted against the platform.     

Mechanical evaluation of craniofacial osseointegration retention systems

This study evaluates mechanical behavior of retention systems that have been used in craniofacial osseointegration. A loading/measuring apparatus was custom designed and constructed. Test bases that represented a typical auricular situation were constructed. These bases allowed for three points of retention. Jigs that could be reproducibly positioned carried the reciprocal portion of the retentive components. The test apparatus provided vertical and horizontal loads in five locations. The system was used to test two ball-and-socket attachments (Dalla Bona; Nobelpharma), cast and preformed bar and clips (Nobelpharma), and three magnet systems (Dynamag; Neomag; Technovent). The loading/measuring apparatus was also used to evaluate the performance of two facial prosthetic adhesives. Retention systems employed in craniofacial osseointegration offer more predictable retention than the facial prosthetic adhesives. The mechanical retention systems are best suited to situations where tensile and shear forces will operate. Magnet systems are best used where only tensile forces are anticipated or where horizontal forces on the implants are to be avoided.     

Nonlinear Finite-Element Modeling of Batter Piles under Lateral Load

In this paper, a finite-element model is developed in which the nonlinear soil behavior is represented by a hyperbolic relation for static load condition and modified hyperbolic relation, which includes both degradation and gap for a cyclic load condition. Although batter piles are subjected to lateral load, the soil resistance is also governed by axial load, which is incorporated by considering the P-Δ moment and geometric stiffness matrix. By adopting the developed numerical model, static and cyclic load analyses are performed adopting an incremental-iterative procedure where the pile is idealized as beam elements and the soil as elastoplastic spring elements. The proposed numerical model is validated with published laboratory and field pile test results under both static and cyclic load conditions. This paper highlights the importance of the degradation factor and its influence on the soil resistance-displacement (p-y) curve, number of cycles of loading, and cyclic load response.     

Lateral Performance of Full-Scale Bridge Abutment Wall with Granular Backfill

Bridge abutments typically contain a backwall element that is designed to break free of its base support when struck by a bridge deck during an earthquake event and push into the abutment backfill soils. Results are presented for a full-scale cyclic lateral load test of an abutment backwall configured to represent the dimensions (1.7?m height), boundary conditions, and backfill materials (compacted silty sand) that are typical of California bridge design practice. An innovative loading system was utilized that operates under displacement control and that assures horizontal wall displacement with minimal vertical displacement. The applied horizontal displacement ranged from null to approximately 11% of the wall height (0.11H). The maximum earth pressure occurred at a wall displacement of 0.03H and corresponded to a passive earth pressure coefficient of Kp = 16.3. The measured force distribution applied to the wall from hydraulic actuators allowed the soil pressure distribution to be inferred as triangular in shape and the mobilized wall-soil interface friction to be evaluated as approximately one-third to one-half of the soil friction angle. Post-test trenching of the backfill showed a log-spiral principal failure surface at depth with several relatively minor shear surfaces further up in the passive wedge. The ultimate passive resistance is well estimated by the log-spiral method and a method of slices approach. The shape of the load-deflection relationship is well estimated by models that produce a hyperbolic curve shape.     

Undrained Limit States of Shallow Foundations Acting in Consort

There is no established procedure for the calculation of bearing capacity of a shallow foundation system comprising cojoined footings. Ad hoc approaches are relied on and may simply involve summing the ultimate limit states of the individual footings as if they acted independently; neglecting additional capacity of the system available from the kinematic constraint provided by the structural connection between the footings. In this study, the undrained capacity under general loading of rigidly connected two-footing systems at various separations has been investigated with finite-element analyses. Results are presented in terms of ultimate limit states under pure vertical (V), horizontal (H), and moment (M) loading, and failure envelopes defining limiting load states under combined VH, VM, HM, and VHM loads. Kinematic failure mechanisms observed in the finite-element analyses are presented and in cases used to provide the basis for upper bound solutions.     

Liquefaction-Induced Softening of Load Transfer between Pile Groups and Laterally Spreading Crusts

Laterally spreading nonliquefied crusts can exert large loads on pile foundations causing major damage to structures. While monotonic load tests of pile caps indicate that full passive resistance may be mobilized by displacements on the order of 1–7% of the pile cap height, dynamic centrifuge model tests show that much larger relative displacements may be required to mobilize the full passive load from a laterally spreading crust onto a pile group. The centrifuge models contained six-pile groups embedded in a gently sloping soil profile with a nonliquefied crust over liquefiable loose sand over dense sand. The nonliquefied crust layer spread downslope on top of the liquefied sand layer, and failed in the passive mode against the pile foundations. The dynamic trace of lateral load versus relative displacement between the “free-field” crust and pile cap is nonlinear and hysteretic, and depends on the cyclic mobility of the underlying liquefiable sand, ground motion characteristics, and cyclic degradation and cracking of the nonliquefied crust. Analytical models are derived to explain a mechanism by which liquefaction of the underlying sand layer causes the soil-to-pile-cap interaction stresses to be distributed through a larger zone of influence in the crust, thereby contributing to the softer load transfer behavior. The analytical models distinguish between structural loading and lateral spreading conditions. Load transfer relations obtained from the two analytical models reasonably envelope the responses observed in the centrifuge tests.     

Undrained Capacity of Plate Anchors under General Loading

Under general conditions of loading, a plate anchor is subjected to six degrees of freedom of loading, three force components and three moment components. Prediction of the anchor performance under general conditions of loading requires realistic estimates of the anchor pullout capacity for each individual load component as well as the interaction effects when these loads are applied in combination. This paper presents an analysis of plate anchor capacity under these general conditions of loading. The study considers a range of plate width-to-length ratios ranging from 1:1 to 2:1. The anchor capacity estimates and interaction relationships were developed based on finite-element studies and upper bound plastic limit analyses. Interaction relationships developed from the numerical and analytical studies were fitted to a simple six degrees-of-freedom yield locus equation.