State Key Laboratory of Hydraulic Engineering Simulation and Safety

This paper presents an experimental investigation of the reinforcement effects of two vacuum-preloading methods by large-scale laboratory model tests: traditional vacuum preloading and multistaged vacuum preloading. The fill samples obtained from Tianjin, China were subjected to tests and the judge method, and formation time of clogging of these two vacuum-preloading methods was determined. The results show that clogging problems during vacuum preloading have a prodigious influence on ground treatment effect. Different vacuum-preloading methods have different clog-forming processes. The formation of clogging outside of prefabricated vertical drains (PVDs) can be determined from the amount of water discharge, rate of water discharge, amount of settlement, and pore-water pressure. The model test indicated that the multistage vacuum-preloading method is more effective than the traditional vacuum-preloading method under the tested conditions in the coastal area of Tianjin. In the scaled model, it was observed that a clogging mud layer began forming at approximately 4,500–5,500 min and was fully formed between 35,000–40,000 min when using the traditional vacuum-preloading method. With the multistaged preloading method, the forming time and full formed time changed to 1,000–3,000 and 15,000 min. The observations on the average degree of consolidation demonstrate that the multiloading method is beneficial for foundation reinforcement as well. These test and discussion results are useful for overcoming the challenges of clogging that occurs during vacuum preloading in coastal areas.

Flow past three identical circular cylinders is numerically investigated using the immersed boundary method. The cylinders are arranged in an equilateral-triangle configuration with one cylinder placed upstream and the other two side-by-side downstream. The focus is on the effect of the spacing ratio space, is given, providing physical insights into the complex interactions of the three cylinders.

Sodium aluminosilicate hydrate (NASH) gel is the primary adhesive constituent in environmentally friendly geopolymer. In this study, to understand the thermal behavior of the material, molecular dynamics was utilized to investigate the molecular structure, dynamic property, and mechanical behavior of NASH gel subjected to temperature elevation from 300 K to 1500 K. The aluminosilicate skeleton in NASH gel provides plenty of oxygen sites to accept H-bond from the invading water molecules. Upon heating, around 18.2% of water molecules are decomposed and produce silicate and aluminate hydroxyls. About 87% of hydroxyls are associated with the aluminate skeleton, which weakens the Al-O bonds and disturbs the O-Al-O angle and the local structure, transforming it from an aluminate tetrahedron to a pentahedron and octahedron. With increasing temperature, both Al-O-Si and Si-O-Si bonds are stretched to be broken and the network structure of the NASH gel is gradually transformed into a branch and chain structure. Furthermore, the self-diffusivity of water molecules and sodium dramatically increases with the elevation of temperature, because the decrease in connectivity of the aluminosilicate network reduces the chemical and geometric restriction on the water and ions in NASH gel under higher temperatures. The high temperature also contributes to around 63% of the water molecules further dissociating and hydroxyl groups forming; meanwhile proton exchange between the water molecules and aluminosilicate network frequently takes place. In addition, a uniaxial tensile test was utilized to study the mechanical behavior of the NASH gel at different temperatures. During the tensile test, the aluminosilicate network was found to depolymerize into a branch or chain structure which plays a critical role in resisting the tensile loading. In this process, the breakage of the aluminosilicate skeleton is accompanied with hydrolytic reactions that further deteriorate the structure. Due to the reduction of the chemical bond stability at elevated temperature, both the tensile strength and stiffness of the NASH gel are weakened significantly. However, the ductility of the NASH gel is improved because of the higher extent of structural arrangement at the yield stage and partly due to the lower water attack. Hopefully, the present study can provide valuable molecular insights on the design of alkali-activated materials with high sustainability and durability.

In this paper, the entrainment and movement of coarse particles on the bed of an open channel is numerically investigated. Rather than model the sediment transport using a concentration concept, this study treats the sediment as individual particles and investigates the interaction between turbulent coherent structures and particle entrainment. The applied methodology is a combination of the direct numerical simulation of turbulent flow, the combined finite-discrete element modeling of particle motion and collision, and the immersed boundary method for the fluid-solid interaction. In this study, flow over a water-worked rough-bed consisting of 2-3 layers of densely packed spheres is adopted and the Shields function is 0.065 which is just above the entrainment threshold to give a bed-load regime. Numerical results for turbulent flow, sediment entrainment statistics, hydrodynamic forces acting on the particles, and the interaction between turbulence coherent structures and particle entrainment are presented. It is shown that the presence of entrained particles significantly modifies the mean velocity and turbulence quantity profiles in the vicinity of a rough-bed and that the instantaneous lift force can be larger than a particle's submerged weight in a narrow region above the effective bed location, although the mean lift force is always smaller than the submerged weight. This, from a hydrodynamic point of view, presents strong evidence for a close cause-and-effect relationship between coherent structures and sediment entrainment. Furthermore, instantaneous numerical results on particle entrainment and the surrounding turbulent flow are reported which show a strong correlation between sediment entrainment and sweep events and the underlying mechanisms are discussed.

A composite bucket shallow foundation (CBSF) has been proposed by Tianjin University to adapt the offshore soft geological conditions of China for wind turbines. Vertical bearing capacity modelling and observation tests regarding the CBSF are performed. The test results are accompanied by numerical simulations to provide a better understanding of the failure mechanism of the CBSF. The upper limit of the vertical bearing capacity of the bucket foundation is derived through the upper bound theorem of classical plasticity theory according to the failure mechanism. The soil damage rate is specified as a new empirical parameter in the formula and is defined as the rate between the thickness of the soil that is broken inside the foundation and the radius of the foundation, which indicates the range of soil failure. Furthermore, the range of the soil damage rate is obtained through vertical bearing capacity model tests. The relationship of the bearing capacity factor Nq and Nγ with the friction angle is also discussed under the specific soil damage rates.

Lath martensite transformation in low carbon steel relates to the discontinuity of the transformation rate, which exhibits a series of martensite transformation rate maxima (peaks), especially in low cooling rates. In the present work, we put an emphasis on the influence of annealing twins in parent austenite grains (PAGs) and cooling rates on the discontinuity of lath martensite transformation rate in a low carbon reduced activation ferritic/martensitic (RAFM) steel. Lath martensite microstructure is hierarchical and the lath martensite transformation rate peaks are hierarchical during the cooling process. If the cooling rate was not high enough, the lath martensite transformation rate peaks will be divided into two groups by annealing twins in parent austenite. These transformation rate peaks in the first group were mainly caused by lath martensite transformation in the non–twin zones in the PAGs. Then, the second group mainly consisted of a single transformation rate peak, which was primarily caused by the martensite transformation in the twin zones in the PAGs. In the present work, a new “localized stress field” theory is proposed and successfully used for interpreting the classification mechanism of lath martensite transformation rate peaks and the evolution of these peaks with changing cooling rate.

Dynamic failure of brittle materials is a fundamental physical problem that has significantly impacts to many science and engineering disciplines. As the first and the most important step towards the full understanding of this problem, one has to observe dynamic damage accumulation in brittle solids. In this work, we proposed a methodology to do that and demonstrated it by studying the dynamic compressive damage evolution of a granitic rock loaded with a modified split Hopkinson pressure bar system. To ensure consistency of the experimental results, we used cylindrical rock samples fabricated from the same rock core and subjected them to identical incident loading pulse. Using a special soft recovery technique, we stopped the dynamic loading on the samples at different strain levels, ranging from 0.3% to 1.4%. Therefore, we were able to recover intact samples loaded all the way to the post-peak deformation stage. The recovered samples were subsequently examined with X-ray micro-CT scanning machine. Three dimensional microcrack network induced by the dynamic loading was observed and the evolution of microcracks as a function of the dynamic loading strain was obtained.

The shear stress–shear displacement relationship and shear strength parameters of the interface between the pipe and the surrounding soil are important for designing the jacking force. One most commonly used method to measure the shear strength of the pipe–soil interface is the direct shear test. This paper presents the results of a series of direct shear tests conducted in the laboratory on the pipe–soil interfaces for different moisture contents. Simultaneously, digital image correlation is used to supplement the research of the horizontal displacement field of the interface. The results show that the increase of normal stress will makes the interface more prone to strain hardening during the shearing process, and the influence of the change of specimen moisture content on the interface strain characteristics gradually weakens. The shearing process of the specimen mainly involved shearing contraction, and slight shear dilation occurs only when the moisture content is small. We proposed a unified model for describing the shear volumetric change caused by the combined action of moisture content and normal stress. The shear displacement field presents a layered distribution, which is related to the shear displacement corresponding to the shear stress.

Occurrence of liquefaction in saturated sandy deposits under structure foundation can cause a wide range of structural damages from minor settlement to general failure because of bearing capacity loss. By comparing traditional foundations for offshore wind turbines, the soil inside and underneath the composite bucket foundation is subjected to the overburden pressure from the foundation self-weight and constrained by a half-closed bucket skirt. The objective of this paper is to clarify the effects of the soil-foundation interaction on the soil liquefaction resistance around the skirt and under the foundation. The dynamic response of the composite bucket foundation during earthquake, including coupled soil mode of porous media, is calculated using the ADINA finite-element program. A typical configuration of composite bucket foundation is used for the analysis, and two earthquake waves (peak ground accelerations of 0.035 g and 0.22 g) are applied as the base acceleration. The results show that the composite bucket foundation exhibited good resistance to seismic action by improving the anti-liquefaction capacity of the soil inside and under the foundation because of the overburden pressure of the self-weight and the constraint effect of the skirt.

connections with droplets, which further weakens the spreading ability of water molecules on the CSH surface.

The structural modal parameter information can be generally obtained from different vibration responses triggered by unknown excitations using the classic modal identification methods which are based on the assumption that the input to the structure is exactly the same or close to the stationary random white noise. For the actual projects such as the offshore wind turbine structure, however, the imposed loads cannot be considered as the pure white noise excitation because the harmonic components emerge obviously in vibration responses due to periodic rotation excitations of the rotor. When the created harmonic components have greater energy and their frequencies are close to any natural modal frequency of the structure, the classic methods can no longer separate harmonic modes from actual structural operational modes under this strong harmonic disturbance, thus false modes may be generated and the identification accuracy may be badly affected. To identify the actual structural operational modes accurately under strong harmonic excitation, a modified stochastic subspace identification (SSI) method considering harmonic interference called the harmonic modification SSI (HM-SSI) method was proposed in this paper assuming the input harmonic frequencies are known and time-invariant. Then the effectiveness and accuracy of the HM-SSI method were verified through a simple numerical model of cantilever beam excited by various harmonic inputs superimposed on random loads. Besides, the superior robustness and better accuracy of identification results affected by the level of noise and the proportion of harmonic energy were reflected. Finally, the modal parameter information under different operational conditions was obtained and the safety assessment of an operational wind turbine was made based on the measured data from one offshore wind turbine test prototype at high running speed.

With the rapid development of electronic business, Web services have attracted much attention in recent years. Enterprises can combine individual Web services to provide new value-added services. An emerging challenge is the timely discovery of close matches to service requests among large service pools. In this study, we first define a new semantic similarity measure combining functional similarity and process similarity. We then present a service discovery mechanism that utilises the new semantic similarity measure for service matching. All the published Web services are pre-grouped into functional clusters prior to the matching process. For a user’s service request, the discovery mechanism first identifies matching services clusters and then identifies the best matching Web services within these matching clusters. Experimental results show that the proposed semantic discovery mechanism performs better than a conventional lexical similarity-based mechanism.

This paper presents a series of uniaxial compressive experiments on natural lake ice under moderate strain-rate in the range of 10−1 to 102 s−1 at −0.1 °C. Natural lake ice samples of 8 cm by 8 cm in cross section and 20 cm high were used to investigate strain-rate dependence of uniaxial compressive strength and flaw effects on ice strength under moderate strain rates. The fracture modes of ice at moderate strain rates were also systematically investigated by using high-speed camera. It is found uniaxial compressive strength of natural lake ice increases with increasing strain-rate in the employed moderate strain-rate range. And natural flaws such as air bubble have a significant effect on uniaxial compressive strength of ice under moderate strain-rate, higher air content ice possesses lower compressive strength. Ice fracture mode depends on strain-rate (or compressive velocity) of ice specimen, varying from splitting at strain rates lower than 10 s−1 to crushing at strain rates higher than 10 s−1. Ice specimen crushes into fine fragments may due to insufficient time for micro cracks to propagate, thus results in higher strength. In addition, dependence of compressive strength on strain-rate in a wide strain-rate range is also discussed.

A superhydrophobic film with high stability, self-cleaning function and robust corrosion inhibition on a magnesium alloy.

Tapered beams constitute functionally graded materials that are widely used in various engineering fields. The vibrational characteristics of tapered beams made of axially varying functionally graded materials are investigated via variational iteration method. Natural frequencies and corresponding mode shapes of axially functionally graded tapered beams are examined, and calculated frequencies are compared with results provided by previous researchers using other approximate methods. The efficiency and accuracy of this technique are demonstrated. The effect of nonhomogeneous material distribution, taper ratio as well as boundary conditions on the dynamical behavior of axially functionally graded tapered beams is thoroughly investigated as well.

In this study, a submerged tension leg platform wind turbine (STLPWT) which can be constructed near the quayside and then wet-towed to the installation site as a unit was proposed. This transportation method will consequently reduce the use of heavy offshore cranes. However, as a high-rise structure, the floating wind turbine may sustain large overturning moments induced by wind, wave, current, and towing force in the transport phase. In order to study the stability of the floating wind turbine and the towline force in wet tows, a numerical model of the towing system including a towboat, towline, and STLPWT was established based on multi-body dynamics. Then, the environmental load effects on the towing stability of the floating wind turbine were investigated. In addition, the comparison of the bollard pull and the height of towing points was performed. The results show that the STLPWT was stable under the rough sea towing condition: a significant wave height of 5 m and a wind speed of 17 m/s. An appropriate bollard pull should be chosen, in practice, as it involves the time and costs of the towing process. In addition, it may be a better choice to set the height of the towing points near the mean sea level to lower the pitch motions of the STLPWT as well as the towline force.

This paper investigates probabilistic failure envelopes of strip foundations on spatially variable soils with profiles of undrained shear strength, s u , linearly increasing with depth using lower-bound random finite-element limit analysis. The spatially variable s u is characterised by a non-stationary random field with linearly increasing mean and constant coefficient of variation (COV) with depth. The deterministic uniaxial capacities and failure envelopes are first derived to validate the numerical models and provide a reference for the subsequent probabilistic analysis. Results indicate that the random field parameters COV su (the COV of s u ) and Δ (dimensionless autocorrelation distance) have a considerable effect on the probabilistic normalised uniaxial capacities, which alters the size of the probabilistic failure envelopes. However, an insignificant effect of COV su and Δ on the shape of probabilistic failure envelopes is observed in the V–H, V–M and H–M loading spaces, such that failure envelopes for different soil variabilities can be simply scaled by the uniaxial capacities. In contrast to COV su and Δ, the soil strength heterogeneity index κ = μ k B/μ su0 has the lowest effect on the probabilistic normalised uniaxial capacity factors, but the highest effect on the shape of the probabilistic failure envelopes. A series of expressions is proposed to describe the shape of deterministic and probabilistic failure envelopes for strip foundations under combined vertical, horizontal and moment (V–H–M) loading.

The nonlinear motion of the Spar platform hull is studied by numerical simulation and model experiment in this paper. The nonlinear differential equation for a coupled heave-pitch of a platform hull is established in a regular wave. The bifurcation pictures of response amplitude with wave frequency variation, Poincare’ maps and time histories are calculated to study the nonlinear dynamical behaviors of the platform hull. The parameters domain for unstable pitch motion is calculated numerically. It is found that the platform motion is sensitive to wave frequency. With the changing of wave frequency the platform exhibits harmonic motion, twice super-harmonic motion, 1/2 sub-harmonic motion, quasi-periodic motion and chaotic motion. The nonlinear 1/2 sub-harmonic motion and the parameter domain for unstable pitch motion are qualitatively verified by the model experiment.

The composite bucket foundation (CBF) is a cost-competitive foundation for offshore wind turbines, which can be adapted to the loading characteristics and development needs of offshore wind farms due to its special structural form. There are seven sections divided inside the CBF by steel bulkheads, which are arranged in a honeycomb structure. The six peripheral sections with the skirt have the same proportions while the middle orthohexagonal one is a little larger. With the seven-section structure, the CBF has reasonable motion characteristics and towing reliability during the wet-tow construction process. Moreover, the pressure inside the compartments can control the levelness of the CBF during suction installation. Several large-scale model tests on suction installation of CBF have been performed in order to explore the feasibility of the tilt adjusting technique in saturated silty sand off the coast of Jiangsu in China. The composite bucket foundation in the tests has an outer diameter of 3.5 m and a clear wall height of 0.9 m. During the suction-assisted penetration process, the pressures in all the compartments were controlled to level the foundation in a timely operation. A convenient method is to improve the CBF inclination by controlling the inside differential pressure among the compartments. It can be commonly carried out by applying suction/positive pressure with intermittent pumping among the seven compartments. Another adjusting technique for a big tilt with deeper penetration is operated with decreasing the penetration depth achieved by suction-assisted lowering the relatively high compartments and positive pressures raising the relatively low compartments. Test results show that the reciprocating adjustment process can be repeated until the CBF is completely penetrated into a designed depth.

The composite bucket foundation (CBF) is a new type of for offshore wind turbines, which can be adapted to the loading characteristics and development needs of offshore wind farms due its special structural form. The composite bucket foundation in the large-scale tests has an outer diameter of 3.5 m and a clear wall height of 0.9 m. There are seven rooms divided inside the CBF by steel bulkheads, which are arranged in a honeycomb structure. The six peripheral rooms with the skirt have the same proportions while the middle orthohexagonal one is a little larger. With the seven-room structure, the CBF has reasonable motion characteristics and towing reliability during the wet-tow construction process. Moreover, the pressure inside the compartments can control the levelness of the CBF during suction installation. Several large-scale model tests on suction installation and adjusting levelness of CBF were carried out in saturated silty clay off the coast of Jiangsu in China. During the sinking process under negative pressure, the tubes valves of all the compartments were controlled to level the foundation in a timely operation. When the foundation started to tilt, the feasibility of the bucket tilt adjusting technique was explored by applying suction/positive pressure and intermittent pumping among the seven rooms. When the required levelness of the foundation was achieved by suction-assisted lowering the high compartments and/or positive pressures raising the low compartments, all of the valves were opened so that the entire foundation could sink. Test results show that the reciprocating adjustment process can be repeated until the CBF is completely penetrated into a designed depth.