Shaking table study on liquefaction behaviour of different saturated sands reinforced by stone columns

Abstract

Liquefaction is a phenomenon developed in loose and saturated layers of sands subjected to dynamic or seismic loading, and often leads to excessive settlement and subsequent failures in structures. Several methods have been proposed to improve soil resistance against liquefaction, among which use of stone columns is one of the most applicable methods. In this research, the effect of stone columns with different geometries and arrangements on the liquefaction behaviour of loose and very loose saturated sands subjected to vibration is investigated using shaking table. Results of the experiments show that when using stone columns in sand layers, the level of maximum settlement is significantly reduced. Further, the presence of stone columns significantly reduces pore water pressure ratio. This further indicates that stone columns have a positive effect and reasonable performance, even in relatively strong earthquakes, provided that the number and cross-section of the columns are sufficient. In addition, stone columns reduce the pore water pressure dissipation time. Moreover, by increasing cross-sectional area and the number of columns, both pore water pressure and settlement decrease. Stone columns in loose sand have a greater effect on the reduction of pore water pressure compared to that of very loose sand.     

Monotonic and cyclic behaviors of loose anisotropically consolidated calcareous sand in torsional shear tests

ABSTRACT

The behavior of loose anisotropically consolidated calcareous sand obtained from an island in the South China Sea was investigated under undrained monotonic and cyclic loading in a hollow cylinder torsional apparatus. The tests were conducted on specimens which consolidated under various initial effective confining pressures and consolidation stress ratios. The monotonic test results show that the failure and phase transformation line are essentially independent of the consolidation conditions, while the initial contractive tendency of the specimens decreases with an increasing consolidation stress ratio. During monotonic loading of the anisotropically consolidated specimens, a same major principal stress direction is observed at the constant stress ratio lines up to the phase transformation line, irrespective of initial effective confining pressure. The cyclic strength of the sand increases with an increasing consolidation stress ratio. Moreover, a pronounced stress dependence is observed in the sand with higher consolidation stress ratio. During cyclic loading, the generated excess pore water pressure presents considerable fluctuations. The normalized terminal excess pore water pressure is described as a function of consolidation stress ratio. The tests show that the particle shape, rather than particle crushing, plays an important role in the monotonic and cyclic behaviors of the calcareous sand.     

Discussion of “formation mechanism of large pockmarks in the subaqueous Yellow River Delta”

Abstract

The excess pore pressure accumulation is a key factor when estimating the formation mechanism of large pockmarks, as it determines the liquefaction potential of marine sediments due to water waves. The governing equations for excess pore pressure may have different forms for various types of sediments and then shall reflect the cyclic plasticity of the soil. For water waves propagating over a porous seabed, the liquefaction area induced by waves is generally progressive, which indicates that the liquefaction area will move forward following the wave train. Therefore, the excess pore pressure accumulation can be used to explain the occurrence of the large pockmarks, but the dimension of the pockmark may be related to the heterogeneity of sediment or the wave properties affected by the topography in the subaqueous Yellow River Delta.     

Characteristics of Pore Water Pressure of Saturated Silt Under Wave Loading

The characteristics of dynamic stress in the seabed under wave loading are constant principal stress and continuous rotation of the principal stress direction.Cyclic triaxial-torsional coupling shear tests were performed on saturated silt by the hollow cylinder apparatus under different relative densities,deviator stress ratios and vibration frequencies to study the development of pore water pressure of the saturated silt under wave loading.It was found that the development of pore water pressure follows the trend of "fast～steady～drastic".The turning point from fast to steady stage is not affected by relative density and deviator stress ratio.However,the turning point from steady to drastic stage relies on relative density and deviator stress ratio.The vibration cycle for the liquefaction of saturated silt decreases with increasing deviator stress ratio and increases with relative density.The vibration cycle for the liquefaction of the saturated silt increases with vibration frequency and reaches a peak value,after which it decreases with increasing vibration frequency for the relative density of 70%.But the vibration cycle for the liquefaction of saturated silt increases with vibration frequency for the relative density of 30%.The development of pore water pressure of the saturated silt is influenced by relative density and vibration frequency.     

Geotechnical aspects of earthquake‐induced settlement of clay layer

Abstract

It has been observed that earthquake‐induced settlement depends on the excess pore water pressure accumulated during an earthquake. In particular, in the case where a clay layer is overconsolidated, excess pore water pressure is produced and settlement occurs by dissipation of the excess pore water pressure, which is very large in comparison with the coefficient of secondary compression. Therefore, if the settlement of clay ground induced by secondary compression becomes a serious problem, careful consideration of the earthquake‐induced settlement is needed. In this article, the settlement characteristics of a clay layer induced by cyclic shear are discussed, including the effects of loading period, the threshold shear strain below which no excess pore pressure or no settlement takes place, and the relationships between uniform shear strain cycles and irregular strain‐time histories. Then a calculation procedure for estimating the earthquake‐induced settlement is developed and applied to three soil profile cases, including the clay layers in Mexico City and Osaka Bay in Japan. In the case of a soil profile in Mexico City, settlements of about 0–3 cm are estimated and these values agree reasonably with the leveling results for the Mexico City earthquake of 1985. Furthermore, it is pointed out that the settlement induced by earthquakes is considerably affected by differences in the accelerograms.     

Experimental study on the cyclic behavior of loose calcareous sand under linear stress paths

Abstract

Hollow cylinder torsional shear tests on loose isotropically and anisotropically consolidated calcareous sand were conducted to investigate the cyclic behavior under three different linear stress paths, including horizontal line, oblique line, and vertical line stress paths, in a coordinate system of the normal stress difference and the horizontal shear stress. The dominant strain components of the isotropically consolidated specimens are affected by the stress paths. With increasing consolidation stress ratio, axial strain gradually becomes the dominant strain component under the three different stress paths. The cyclic strength of the isotropically consolidated specimens under the three different stress paths are almost the same, while for the anisotropically consolidated specimens, the cyclic strengths are strongly affected by the stress paths. These results indicate that conventional cyclic triaxial tests may overestimate cyclic strength in some cases. Irrespective of the stress paths and cyclic stress ratios, the terminal residual excess pore pressure ratio decreases with increasing consolidation stress ratio. Moreover, an empirical equation is proposed to describe the relationship between the normalized shear work and the normalized residual excess pore pressure ratio. The comparative study reveals that the relationship proposed for silica sand is not suitable for the dynamic analyses of calcareous sand.     

Cyclic behavior of natural organic clay under variable confining pressure that match traffic loading conditions

ABSTRACT

One-way cyclic loading is more typical for traffic loading and cyclic triaxial test has been recognized as a useful method for solving many engineering problems. Under traffic loading, the influence of variable confining pressure on cyclic behavior of natural organic clay subjected to cyclic traffic loading is rarely reported in the literature. In this study, a laboratory investigation on undrained cyclic behavior of natural organic clay is presented and conducted by cyclic triaxial apparatus. Tests are conducted by both constant confining pressure and variable confining pressure, to simulate the loading conditions induced by passing vehicles in actual engineering. Different stress levels are also considered in this study. By comparing between the results of constant confining pressure tests and variable confining pressure tests, it shows that the one-way cyclic behavior of organic clay is influenced significantly by variation of confining pressure, in terms of pore water pressure, permanent axial strain and stress–strain hysteretic loops.     

Experimental study of interactions between multi-directional focused wave and vertical circular cylinder,part II: Wave force

We describe experiments with multi-directional focused waves interacted with a vertical circular cylinder in a 3D wave basin. The focus of this study is on the run-up of multi-directional focused waves, wave forces, and wave pressures on the cylinder. Part I, the study on wave run-up, has already been presented by Li et al. (2012). In this paper, the analysis of the wave force on the vertical cylinder is presented.In this experiment, a cylinder with 0.25 m in diameter was adopted and different wave parameters, such as focused wave amplitude, peak frequency, frequency bandwidth and directional spreading index, are considered. The model scale k_pa (k_p is the wave number corresponding to peak frequency, a is the radium of the cylinder) varies from 0.32 to 0.65. The maximum forces of multi-directional focused wave on cylinder were measured and investigated. The results showed that the wave parameters have a significant influence on the wave force, and that the spatial profile of the surface of multi-directional focused wave can also affect its force on the cylinder, which is different from two-dimensional wave. In addition, the ‘secondary loading cycle’ phenomenon was also observed and discussed. In our experiments, the ‘secondary loading cycles’ occur when kA > 0.36 for all cases. While in some referred small scale experiments, the secondary load cycles are observed even for kA = 0.2, when the waves are longer enough. To larger model scale, the pronounced secondary load cycle occurs with larger wave steepness waves.     

The effect of cyclic loading on the undrained shear strength of a silty clay

Abstract

The strength of saturated clays subjected to low‐frequency cyclic loading is of considerable importance in the design of offshore structures. A series of postcyclic undrained triaxial strength tests were carried out on samples of a silty clay (Keuper Marl), which had not failed under cyclic loading, to investigate the effect of this loading on the postcyclic monotonie effective stress path and strength of the soil. Undrained cyclic loading of normally consolidated and lightly overconsolidated samples by generating positive pore pressures caused the apparent degree of overconsolidation to increase. Samples subsequently subjected to monotonie testing followed effective stress paths commensurate with this apparent overconsolidation and failed on the “Hvorslev” surface on the “dry” side of the stable state boundary surface rather than on the critical state as would be expected of samples of this initial stress history. Samples which were initially heavily overconsolidated did not generate large pore pressures under cyclic loading and did not therefore exhibit large changes in postcyclic monotonie strength. The reduction in strength after cyclic loading therefore is greater for normally consolidated and lightly overconsolidated samples. The pore pressures generated under postcyclic monotonie loading are related to the degree of apparent overconsolidation achieved during cyclic loading.     

An experimental investigation of the characteristics of a cylinder interacting with waves and flow in the dominant-load regime

We describe in this paper the experimental investigations of the interaction of a bottom-pivoted vertical cylinder with water waves and flow, to determine the dominant-load-regime map by application of response step functions and response RAO. A rigid circular cylindrical mass-damper-spring oscillator system is investigated in regular waves and uniform flow to determine the response characteristics in the frequency domain. Interaction with waves dominates in the high frequency range f* = f_osc/ω_v = 0.862–1.547, with magnitude in the range of 0.1 rad. On the other hand, interaction with flow dominates at lower frequency range, f* = 0.442–0.862, with magnitude in the range of 0.01 rad. These are caused by the non-overlap peak positions of the magnitude response in waves and flow due to the change in added mass of the cylinder moving in different types of fluid loads. The frequency f* = 0.862 is the point where the dominant factors are transferred. The location of separation points determines the pressure distribution to induce the added mass changed. Separation positions determine the magnitude response, but do not determine the configuration of response RAO. That allows to enhance or reduce the magnitude response of the cylinder by taking advantage of the dominant-load-regime map in the frequency domain.     

Shaft capacity of pre-bored grouted planted nodular pile under various overburden pressures in dense sand

Abstract

This paper presents the results of a series of model tests performed to study the shaft capacity of pre-bored grouted planted nodular (PGPN) pile in dense sand. The influence of the vertical overburden pressure on the shaft capacity of the PGPN pile is also investigated based on the test results. The test piles were equipped with strain gauges to measure the axial loads during the loading process, moreover, a foam plate was buried beneath pile tip to eliminate the influence of tip resistance on the shaft capacity. Some conclusions can be drawn based on the test results: the peak skin friction of PGPN pile increases with the increase of vertical overburden pressure applied on the foundation soil, while the rate of increase decreases with the increasing overburden pressure; the surface of the pile–soil interface of PGPN pile is relatively rough, and significant dilatant increase in lateral stress occurs during the loading process.     

随机波作用下埋管海床动态响应及液化研究

基于广义Biot动力理论和Longuet-Higgins线性叠加模型,构建波浪-海床-管线动态响应的有限元计算模型,求解随机波作用下,多层砂质海床中管线周围土体孔隙水压力和竖向有效应力的分布。采用基于超静孔隙水压力的液化判断准则,得出液化区的最大深度及横向范围,从而判断海床土体液化情况。考虑海洋波浪的随机性,将海床视为多孔介质,海床动态响应计算模型采用u-p模式,孔隙水压力和位移视为场变量。并考虑孔隙水的可压缩性、海床弹性变形、土体速度、土体加速度以及流体速度的影响,忽略孔隙流体惯性作用。参数研究表明:土体渗透系数、饱和度以及有效波高等参数对海床土体孔隙水压力、竖向有效应力和液化区域分布有显著影响。     

Cyclic behaviour and liquefaction resistance of Izumio sands in Osaka,Japan

Abstract

This article presents a study of liquefaction resistance behaviour of sand using a cyclic triaxial test. The site investigation was performed, and frozen undisturbed specimens were taken from the Izumio site in Osaka, Japan. According to the evidence in 1995 Kobe Earthquake, the first two sand layers are vulnerable to undergo liquefaction. The effect of deviatoric stress on liquefaction resistance was focused on in this study. The excess pore pressure ratio, hysteresis loop, and effective stress path from the cyclic triaxial tests were reported. A multispring element model was employed to simulate the soil behaviour under cyclic loading. The results showed that applied deviatoric stress could influence the liquefaction resistance of sandy soil samples. The plots of the cyclic stress ratio versus the number of cycles to generate liquefaction known as a liquefaction resistance curve can be then constructed and compared with other sands.     

Numerical calculation of forces induced by short-crested waves on a vertical cylinder of arbitrary cross-section

Most off-shore oil platforms are supported by vertical cylinders extending to the ocean floor. An important problem in off-shore engineering is the calculation of the wave loading exerted on these vertical cylinders. Analytical solutions have been found for the case of plane incident waves incident on a circular cylinder by MacCamy and Fuchs [(1954), Wave forces on piles: a diffraction theory. U.S. Army Corps of Engineering, Beach Erosion Board, Technical Memorandum No. 69] and also for short-crested waves incident on a circular cylinder by Zhu [(1993), Diffraction of short-crested waves around a circular cylinder. Ocean Engng 20, 389–407]. However, for a cylinder of arbitrary cross-section, no analytic solutions currently exist. Au and Brebbia [(1983), Diffraction of water waves for vertical cylinders using boundary elements. Appl. Math. Modelling 7, 106–114] proposed an efficient numerical approach to calculate the wave loads induced by plane waves on vertical cylinders by using the boundary element method. However, wind-generated waves are better modelled by short-crested waves. Whether or not these short-crested waves can induce larger wave forces on a structure is of great concern to ocean engineers. In this paper wave loads, induced by short-crested incident waves, on a vertical cylinder of arbitrary cross-section are discussed. For a cylinder of certain cross-section, the wave loads induced by short-crested waves can be larger than those induced by plane waves with the same total wave number.     

Dynamic response of a large-diameter pile with variable section in layered saturated soil considering construction disturbance effect

ABSTRACT

An analytical solution is developed in this paper to investigate the vertical time-harmonic response of a large-diameter variable-section pile, and it considers the radial inhomogeneity of the surrounding soil caused by construction disturbance. First, the saturated soil surrounding the pile is described by Biot’s poroelastic theory and a series of infinitesimally thin independent layers along the shaft of the pile, and the pile is represented by a variable-section Rayleigh–Love rod. Then, the dynamic equilibrium equations of the soil and pile are solved to obtain an analytical solution for the impedance function at the pile top using the complex stiffness transfer method and impedance function transfer method. Finally, the proposed solution is compared with previous solutions to verify its reliability, and a parameter study is conducted to provide insights into the sensitivity of the vertical dynamic impedance of the pile and velocity response in low-strain integrity testing on defective piles.     

Effects of frequency and confining pressure on consolidation drainage behavior of soft marine clays

Abstract

This article presents an experimental investigation on the dynamic consolidation (DC) drainage behavior of soft marine clays. A sinusoidal harmonic load with different frequencies was applied to simulate the DC method in which the conventional impact load was replaced by the cyclic load. Four geotextile-filter strips were used to form the side drainage channels simulating the wick drain method. A series of loading tests were conducted on soft soil specimens at different confining pressures (i.e., 20, 40, 70, and 100?kPa) and different vibration frequencies (i.e., 0, 0.5, 1, 1.5, 2, and 5?Hz). Test results showed that both confining pressure and frequency have significant influences on the drainage behavior of soft marine clay specimens. The magnitude of drainage volume consistently decreases linearly with increasing confining pressure. Compared to static loading condition, specimens under cyclic loading condition at different frequencies show a better drainage performance. Specimen at applied frequency of 1?Hz exhibits the maximum cumulative drainage volume due to the resonant effect.     

Effect of Cyclic Loading Frequency on Undrained Behaviors of Undisturbed Marine Clay

Based on a series of cyclic triaxial tests, the effect of cyclic frequency on the undrained behaviors of undisturbed marine clay is investigated. For a given dynamic stress ratio, the accumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for beth the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results demonstrate that the reasons for the frequency effect on the undrained soil behaviors are beth the creep effect induced by the loading rate and the decrease of sample effective confining pressure caused by the accumulated pore water pressure.     

Marine geotechnology—state of the art

Abstract

The state of the art in marine geotechnology can best be defined with reference to what is known about soils on land. Differences between these two states of knowledge are the significant problems for the marine environment. Among the major problems addressed in this paper are (a) Sampling of soils, which involves much more serious disturbance than is considered acceptable on land. Disturbance results from several uniquely marine factors including total stress release and drilling mud overpressure, (b) Underconsolidation, or excess in situ pore pressure, caused by rapid rates of sedimentation, gas, leaks from an artesian pressure source, or cyclic loading. (c) Gas in sediments, which can cause an increase in the in situ pore pressure, hinders subsurface investigation, and is a major cause of sample disturbance. (d) The difficulty and necessity of in situ measurements. (e) The predominance of dynamic loading effects which can cause significant changes in soil behavior. A major difference between geotechnical engineering on land and in marine areas is the use of effective stress methods. Significant improvement in geotechnical engineering offshore can be achieved through the increased use of effective stress methods. Illustrations of these improvements are presented in this paper with particular reference to the problems of submarine slope stability.     

Superpile system: A feasible alternate foundation for tension leg platforms in deep water

Abstract

This article discusses a possible alternate foundation system for a tension leg platform (TLP) in deep water. It was developed through Tecnomare S.p.A. for Agip S.p.A. during a feasibility study of a site located in the southern Adriatic Sea having a water depth of 827 m. The system is a combination of a pile‐gravity foundation, with relatively short (?20 m) steel piles of very large diameter (6 to 12 m) called “superpiles,”; which are closed at the top and open at the bottom. The superpiles are installed in soft soil under the effect of self‐weight and active suction. Permanent tension of the TLP tendons is equilibrated by self‐weight only; tension due to wave action is equilibrated by the weight of the soil inside the superpiles. In fact, a pulsating tension applied to the superpiles generates a suction in the pore water that tends to keep the soil plug inside the cylinder and prevents the cylinder as a whole from being extracted from the (impervious) foundation soil.

The results of analyses of the installation and in service behavior of superpiles are described on conventional principles of soil mechanics and related to a typical profile at the Adriatic site. Installation problems are briefly discussed; then the stability in service is examined to evaluate the capacity of superpiles in compression and tension and the associated factors of safety. Finally, considering the pore pressure gradient generated by the applied tension inside the soil plug, the flow rate and related deformations during storms and during the entire expected life of the TLP are evaluated.

The proposed superpile system appears to be feasible, simple to install, and economical. Further study and experimental research are justified to optimize the system.     

Characteristics of Excess Pore Water Pressure of Nanjing Saturated Fine Sand Under the Interaction of Wave and Earthquake Loading

A stress path with continuous rotation of the principal stress direction and continuous alteration of amplitude of deviatoric stress difference under the interaction of wave and earthquake loading was proposed based on the characteristics of the stress path under wave and earthquake loading, respectively. Using a GDS dynamic hollow cylinder apparatus, a series of cyclic triaxial-torsional coupling shear tests were performed on Nanjing saturated fine sand via the stress path mentioned previously under different relative densities, effective initial confining pressures, plastic fines contents, and loading frequencies to study the development of excess pore water pressure (EPWP) of saturated sand under the interaction of wave and earthquake loading. It was found that the development of EPWP follows the trend of fast-steady-mutative-drastic, which is different from that under the cyclic triaxial test or wave loading. The number of cycles causing initial liquefaction (N_L) of saturated sand increases remarkably with relative densities. However, the relationships between N_L and effective initial confining pressures, plastic fines content, or loading frequencies are more complex.