A Study On Stability Of Deeply-Embedded Large-Diameter Cylindrical Structure In Soft Ground

With the development and utilization of marine source and space as well as rapiddevelopment of deep-water harbor and channel engineering construction, a lot of thick softand weak ground is inevitably encountered with in engineering practice. On such weakground, traditional types of gravity structure and pile foundation are not applicable in designbecause that these types of foundation need a relative harder soil layer with higher strengthand stiffness, so large amount of soft soils is required to be removed and then filled withstronger counterparts or the embedment depth of pile foundation is necessary to increase,therefore leads to extremely high engineering cost. However, the deeply-embeddedlarge-diameter cylindrical structure is a novel type of structure which has been increasinglyused in deep-water breakwater and dock structures or port and harbour facilities in China inrecent years. It is mainly a steel or reinforced concrete thin-wall cylindrical shell structure,penetrated into the soil up to a given depth by static loading or vibration procedure. Such astructure, similar to a cylindrical caisson without bottom and inner linkage walls, is especiallysuitable for complex condition such as marine soft soil ground and worse sea environments.To compare with other traditional types of structures, it takes advantages in lowerconstruction cost and Shorter construction period. Such type of structure will be utilizedwidely, especially in thick soft ground, with the rapid development of harbor and oceanengineering. However, because the working mechanism of the deeply-embeddedlarge-diameter cylindrical structure are different with that of traditional gravity foundationand pile foundation, the especial loading conditions, complex interaction mechanism betweenstructure and soil, failure mechanism under combined static and dynamic loading, andcomputational methods for the bearing capacity of such type of structure have been not wellclarified. Therefore it will be theoretically important and practically significant to examine theworking mechanism and failure pattern and to work out effective methods for evaluatinglateral bearing capacity of deeply-embedded large-diameter cylindrical structure in softground under complex conditions. In this dissertation, the studies are emphasized onnumerical methods for evaluating the lateral bearing capacity and failure mechanism ofdeeply-embedded large-diameter cylindrical structure in soft ground under wave loading. Themain investigations consist of the following parts.1. Based on general-purpose finite element software ABAQUS, the finite elementcomputational model for deeply-embedded large-diameter cylindrical structure in soft groundunder monotonic loading is established.In order to examine the effects of interface behavior between soil and cylindrical structureon the failure mechanism and bearing capacity of cylindrical structure in soft ground, theadvanced contact pair algorithm is utilized. Two finite element models, one considering thepotential crack between outside of cylinder wall and soil and another one withour consideration of the potential crack between outside of cylinder wall and soil, are constructed.At the same time, friction contact between the inside of cylinder wall as well as the bottom ofcylinder wall and neighbouring soil is considered. Therefore the drawbacks of conventionalfinite element analyses which either do not consider the potential crack between the walloutside and soil or do not take account of the friction contact between the wall inside and soilare overcome and the detailed soil-structure interaction mechanism can be simulated in amore rational way. On the basis of finite element analyses, it is concluded that the structuretends to overturn around a point which is located below the ground surface and above thecylinder bottom. This failure mechanism is remarkably different from that of traditionalgravity structure and pile foundation. Depending on separation or bonding state on theinterface between the cylindrical structure and neighboring soil in the active wedge, thefailure mechanism of deeply-embedded large-diameter cylindrical structure in soft grounddisplays two failure patterns including single-sided failure mode and double-sided failuremode. in the double-sided failure mechanism, two wedges exist respectively in both activeand passive zones near the mudline while only an individual wedge exists in passive zone inthe single-sided failure mechanism.Moreover, the perfect elasto-pastic constitutive model based on Hill's anisotropic yieldcriterion is employed to examine the effect of anisotropy of undrained shear strength of softground on bearing capacity of cylindrical structure. Parametric computations and comparativeanalyses indicate that the single-sided failure mechanism may be more critical thandouble-sided failure mechanism and will be most likely instability pattern. This fact has beenverfies by the experimental results of centrifuge model tests. For both failure mechanisms, thelateral bearing capacity predicted without considering anisotropy of the soft soil with lowertriaxial extension strength will be on unconservative side.The failure mechanism of large-diameter cylindrical structure in inhomogeneous soft clayof strength and modulus increasing with depth is also studied. The computational resultsindicate that the crack between the wall outside and soil is more easily to form in heavilyover-consolidated soil stratum while in the normally consolidated or lightly over-consolidatedsoil, no crack formed in general and the effect of crack on the bearing capacity is relativelylow.The failure mechanism of instability and distribution type of earthpressure from finiteelement computation and some model tests is compared qualitatively, and the validity of finiteelement analyses is verified2. A modified three dimensional upper bound plastic limit analysis method for bearingcapacity of the deeply-embedded large-diameter cylindrical structure in soft ground isproposed based on the failure mechanism obtailed from finite element analyses.On the base of the failure mechanism obtailed from finite element analyses, the most likelyfailure mechanism is assumed to be of a composite rupture surface which is composed of anindividual wedge in passive zone or two wedges in both active and passive zones near themudline, depending on separation or bonding state at the interface between the cylindrical structure and neighboring soils in the active wedge, and a truncated spherical slip surface atthe base of the cylinder, when the structure tends to overturn around a point which is locatedon the symmetry axis of the structure. Therefore the drawback of incompatibility betWeenhorizontally moving mode of soil in depth and rotation mode of the whole structure in failuremechanism proposed by Murff and Hamilton is modified. According to the well-known pasticlimit analysis theory, at the same time considering the anisotropy property of undrained shearstrength of soft soil, the modified three dimensional upper bound limit analysis method isproposed. Both the depth of rotation center of the structure and lateral bearing capacitypredicted by the plastic limit analysis method are validated by finite element numericalcomputations and limit equilibrium method. For the K0-consolidated ground of clays typicallywith anisotropic undrained strength property, it is indicated through a parametric study thatlimit analysis with no consideration of anisotropy of soil used to overestimate the lateralultimate bearing capacity of deeply-embedded cylindrical structure in soft ground with lowtriaxial extension shear strength. Given the same soil condition, the normalized bearingcapacity increases when aspect ratio L/D increases and decreases when nondimensional heightof load action point Lp/D increases.3. Based on the concept of cyclic shear strength proposed by Andersen et.al, to considercyclic softening behavior of soft clay seabed, which is characterized by strength weakeningand stiffness degradation, induced possibly by wave cyclic loading, a nonlinearelasto-plastic-cyclic strength model is suggested and embedded into the finite elementsoftware ABAQUS through second-phase development, then a quasi-static finite elementmethod for assessing cyclic bearing capacity of offshore foundations or structures isdeveloped. The proposed method is numerically implemented in the framework of thegeneral-purpose finite element software ABAQUS. In the method, nonlinear stress-strainrelationship proposed by Duncan-Chang and Mises yield criterion are combined together toestablish a nonlinear elasto-plastic-cyclic strength model of soft clay. The computationalresults indicate that the failure mode of cyclic instability and distribution of equivalent plasticstrain in soft ground is somewhat different from those of ultimate failure state. In the failuremode of cyclic instability, the rotation center deviates obviously from the symmetry axis ofthe cylindrical structure and near to the seaward side of the structure, while the rupturesurface at the cylinder bottom is a more complex spacial surface instead of a spherical surfacewhich center is located on the rotation center of the structure. Parametric studies show that thebearing capacity is reduced considerably due to the cyclic softening effect induced by waveloading with respect to ultimate bearing capacity and is influenced by some factors such asthe embedment, height of load action point and cyclic loading number to failure etc.4. Utilizing the interface of UMAT in finite element software ABAQUS, the improveddynamic cam-clay model proposed by Carter et.al, is embedded into ABAQUS/standardmodule through implicit integration algorithm.The implicit integration algorithm for improved dynamic cam-clay model is investigated indetail and numerically implemented into the ABAQUS platform. Numerical simulation is made on triaxial soil tests under monotonic and cyclic loading respectively. For triaxial testunder monotonic loading, the results computed by the model developed in the thesis cancompare well with those computed by modified cam-clay model integarated by an algorithmused by ABAQUS itself and automatic substepping explicit integration algorithm with errorcontrol. For triaxial test under cyclic loading, the relation curve between mean effective stressand cyclic loading number is higher than that of from experiments, however the error iswithin the range permitted in soil dynamic computation in engineering practice.5. Utilizing the improved cam-clay dynamic model developed in the thesis, theelasto-plastic effective stress analysis model for deeply-embedded large-diameter cylindricalstructure in soft ground under wave cyclic loading is set up on the platform ofABAQUS.Based on simplified form of general Biot's consolidation theory neglecting inertia effect ofsoil skeleton and pore fluid, the effective stress finite element computational model isconstructed and then effect of cyclic degradation of soil strength and accumulation of excessporepressure on cyclic bearing capacity is investigated. Results indicate that when waveheight is higher, the cyclic displacement of soil-structure coupled system is higher andresidual displacement is lower, and the instability of cylindrical structure is induced by highercyclic displacement. When wave height is lower, the residual displacement of soil-structurecoupled system is higher and leads to the instability of cylindrical structure in soft ground.