离心场中土体图像采集与位移测量系统的研制与应用

以存储前移为主要思路,开发了离心场中高质量图像高速采集与位移非接触测量系统。该系统由图像采集、数据存储、通讯控制、分析处理和辅助制样5个子系统组成,能够克服高速旋转的离心场的干扰,在离心模型试验过程中高速采集高质量图像。该系统能够对多点同时进行测量,从而得到土体表面任意时刻、任意区域的位移场,并具有对土体无干扰、测量精度高、前期准备工作量小、自动化程度高等优点。使用该系统对土坡破坏离心模型试验中的土坡变形过程进行了测量,结果表明,土坡的破坏过程可分为均匀变形、应变局部化和破坏后变形3个阶段,加载过程中首先在坡顶和坡脚产生应变局部化。     

海洋平台吸力式基础的土工离心模拟研究与分析

综述了国内外已开展的海洋平台吸力式基础的离心模型试验研究,重点讨论了应用离心机模拟吸力式基础承载特性的研究现状。     

Centrifuge modeling of single pile response due to lateral cyclic loading in kaolin clay

In offshore engineering, pile foundations are commonly constructed in marine deposits to support various structures such as offshore platforms. These piles are subjected to lateral cyclic loading due to wind, wave action, and drag load from ships. In this paper, centrifuge model tests are conducted to investigate the response of the existing single piles due to lateral cyclic loading. The cyclic loading was simulated by a hydraulic actuator. It is found that the residual lateral movement and bending strain are induced in the existing pile after each loading–unloading cycle. This is because plastic deformation is induced in the soil surrounding the existing pile during each loading–unloading cycle. By increasing the applied loads during cyclic loading–unloading process, the lateral movements and bending strains induced in the pile head increase simultaneously. As the cyclic loading varies from 10 to 50 kN, the residual pile head movement increases from 40 to 154?mm, and the residual bending strain of the existing pile varies from 100 to 260 με. The ratio of residual to the maximum pile head movements varies from 0.17 to 0.22, while the ratio of residual to the maximum bending strains is in a range of 0.12–0.55.     

Uplift mechanism for a shallow-buried structure in liquefiable sand subjected to seismic load: centrifuge model test and DEM modeling

Based on a centrifuge model test and distinct element method(DEM), this study provides new insights into the uplift response of a shallow-buried structure and the liquefaction mechanism for saturated sand around the structure under seismic action. In the centrifuge test, a high-speed microscopic camera was installed in the structure model, by which the movements of particles around the structure were monitored. Then, a two-dimensional digital image processing technology was used to analyze the microstructure of saturated sand during the shaking event. Herein, a numerical simulation of the centrifuge experiment was conducted using a two-phase(solid and fl uid) fully coupled distinct element code. This code incorporates a particle-fl uid coupling model by means of a "fi xed coarse-grid" fl uid scheme in PFC3D(Particle Flow Code in Three Dimensions), with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate the uplifts of the shallow-buried structure and the sharp rise in excess pore pressure. The corresponding micro-scale responses and explanations are provided. Overall, the uplift response of an underground structure and the occurrence of liquefaction in saturated sand are predicted successfully by DEM modeling. However, the dynamic responses during the shaking cannot be modeled accurately due to the restricted computer power.     

Mechanical response of a jointed rock beam—numerical study of centrifuge models

In this paper we present a comparison between a set of benchmark centrifuge models of a jointed beam and the predictions of two numerical models: fast Lagrangian analysis of continua (FLAC) and discontinuous deformation analysis (DDA). The primary objective of this paper is a comparison between the measured deformation profiles and thrust evolution to predictions of the numerical methods employed. A secondary objective is an attempt to clarify the issue of compressive arch geometry which is still in controversy among researchers. It is found that both FLAC and DDA result in insufficiently accurate predictions to the measured displacements. The mode of deformation is only partially captured and is dependent on the aspect ratio of the individual blocks which made up the beam. It is shown that the accuracy of the predicted displacements is a function of the assigned interface stiffness. The thrust predicted by both methods is found to be considerably lower than that measured in the model; however, the linear evolution of thrust and equilibrium conditions are correctly captured. The geometry of the compressive arch as predicted by FLAC compares extremely well with the data measured in the physical model. Based on the FLAC analysis it is found that for a beam composed of equidimensional blocks the thickness of the compressive arch varies from 0.8t at the abutment interface to the entire beam thickness (1t) at a distance of a half block width from the abutment face, extending across the interface separating the block and its neighbour, and attains a value of 0.5t at the beam mid span. Copyright © 2006 John Wiley & Sons, Ltd.     

General and local estimation of local average and their application in geotechnical parameter estimations

Two kinds of estimation variance functions for estimating a local average (LA) of a stationary (homogeneous) random field (RF) are derived. One is local estimation (LE) and the other is general estimation (GE) of LA. The former is for estimating LA at the observation location, and the latter is for obtaining LA at any arbitrary location within the RF. The geotechnical implications of these two estimations are that LE is for estimating LA of geotechnical parameters at the spot where the investigations are made, whereas GE is for estimating LA at any arbitrary location within the same layer. The behavior of the two estimation variance functions differs greatly, controlled by the number of observations (i.e. sample size) and the normalized layer thickness (layer thickness divided by autocorrelation distance of RF). Based on the derived estimation variance functions, methods for determining reliable characteristic values of geotechnical parameters and necessary sample size are proposed. The methods are based on the same framework as that of the traditional statistical theory, i.e. confidence interval of estimated parameters. However, the assumption of independently and identically distributed (i.i.d.) samples in the traditional statistical theory is replaced by the assumption of correlated samples from a stationary RF. The results obtained from the proposed methods for LE and GE differ from each other as well as from the traditional results, which has significant implications for geotechnical parameter estimation in geotechnical engineering practice.     

降雨条件下土坡变形机制的离心模型试验研究

自行研制了离心场降雨模拟设备,进行了降雨条件下边坡的离心模型试验。试验模型在离心机中加载到50g,然后开始降雨。采用非接触位移测量系统测量了试验过程中边坡的位移场变化,通过T5张力计测量边坡中典型点的吸力变化。试验结果表明,边坡的位移随降雨量的增大逐渐发展,发生明显变形的区域也逐渐变大,主要集中在边坡表面。边坡某处应变迅速变化的时刻与土体含水率迅速增大的时刻相一致。边坡某点的应变随降雨量的增加不断增大,并存在2个拐点,形成湿润锋和稳定锋2个锋面,并把边坡分成3个区域,通过锋面的变化反映降雨条件下边坡的变形过程。     

Use of a single-bowl continuous-flow centrifuge for dewatering suspended sediments: Effect on sediment physical and chemical characteristics

The use of a single-bowl continuous-flow centrifuge (CFC, Sharples-Pennwalt Model AS-12) for dewatering suspended sediment from large volumes of river water is evaluated. Sediment-recovery efficiency of 86-91 per cent is comparable to that of other types of CFC units. The recovery efficiency is limited by the particle-size distribution of the feed water and by the limiting particle diameter that is retained in the centrifuge bowl. The limiting particle diameter, using the parameters for this study (bowl radius = 10.5cm; bowl length = 71.1 cm; rotational velocity = 16000 r min^?1; flow rate = 2 L min^?1, and an assumed hydrated particle density = 1.7 gm cm^?3), is 370 nm. There seems to be no particle-size fractionation within the centrifuge bowl—the median particle size was the same at the top as at the bottom. Particle electrophoretic mobility plays some role in fractionation of particles within the centrifuge. The mobility ranged from ?1.19 to ?2.01 × 10^?8 m² V^?1 s^?1, which is typical of clays coated with organic matter, the charge of which is partially neutralized by divalent cations and iron. Contamination by trace metals and organics is minimized by coating all surfaces that come in contact with the sample with either FEP or PFA Teflon and using a removable FEP Teflon liner in the centrifuge bowl. Because of the physical and chemical factors affecting particle fractionation within the centrifuge, care must be exercised in interpreting the environmental consequences of particles collected by continuous-flow centrifugation.     

Dynamic centrifuge tests of concrete dam

Dynamic tests of a concrete gravity dam are, for the first time, performed inside a centrifuge. Details of the experimental procedure, data interpretation, and results are presented. It is shown (in conjunction with a parallel paper) that these tests cannot only provide a direct assessment of certain aspects of dam safety, but more importantly provide a data base for possible non‐linear finite element code validation. Copyright © 2005 John Wiley & Sons, Ltd.     

The 2001 Adana landslide and its destructive effects,Turkey

The current study deals with a catastrophic landslide that occurred due to heavy rainfall in Adana, Southern Turkey in 2001. The research area, which is one of the largest populated districts in Adana, and one of the most tropical places between the Middle East and southern Europe, has about 25,000 people. On the basis of geological and geotechnical surveys, the landslide phenomena and the effects of landslide are examined. To understand the slide mechanism of the landslide, ground reconnaissance, laboratory and in situ tests are conducted. It is observed that the clayey and silty layers of the site are heavily saturated due to extreme rainfall in winter, and consequently sheared. The landslide consists of a block sliding in the upper portions and a debris flow/soil flow component around the margins of the sliding blocks in the middle parts and at the toe. Although there was no loss of human life, it has caused significant economic losses. Besides, it is determined that if precautions are not taken around the landslide area, larger landslides could definitely occur in the future.