Long-term consolidation behavior interpreted with isotache concept for worldwide clays

The consolidation characteristics interpreted with the isotache concept have been studied by many researchers, including the authors. The aim of most of these studies has been to calculate secondary consolidation with high accuracy in order to evaluate the long-term settlement of large-scale structures. In a previous study, the long-term consolidation characteristics of Osaka Bay clay, collected from the construction site of the Kansai International Airport, were examined, and a simplified method based on the isotache concept, using a compression curve and the relationship between the consolidation yield stress (preconsolidation pressure) and the strain rate, was proposed. The former and the latter were obtained from constant rate of strain consolidation (CRS) tests and long-term consolidation (LT) tests, respectively. The latter is expressed by an equation with three isotache parameters. In addition, it is noteworthy that the isotache parameters can be commonly determined for the Osaka Bay clays retrieved from various depths up to 300 m below the seabed. In the present study, the proposed method was applied to worldwide clays with various characteristics using the common values for the isotache parameters determined for the Osaka Bay clays. It was found that the long-term consolidation behavior of those worldwide clays can be well characterized by the proposed method, along with the common values for the isotache parameters.     

Compressive Behavior During the Transition of Strain Rates

The application of a strain-rate-dependent model, for example, an isotache model, is very useful and highly effective for predicting the settlement due to consolidation, including secondary consolidation. In the isotache model, compression curves are not only determined by pressure, but also by strain rate. The validity of this model has been experimentally confirmed by many researchers using different types of oedometer tests, such as constant rate of strain (CRS) tests, incremental loading (IL) tests, etc. However, considerable scatter has been observed in the test results, which show the effects of the strain rate, and questions arise as to whether such scatter is caused by the heterogeneity of the soil samples or by the incompletion of the model. To avoid the heterogeneity of the tested samples, special CRS tests, in which the strain rate is not kept constant but is varied during the tests, were carried out on intact and reconstituted Osaka clay samples. The effects of the strain rate on the compressive behavior of these clays were carefully evaluated in terms of visco-plastic strain, assuming that the total strain consists of visco-plastic strain and elastic strain. It was confirmed that the stress and the visco-plastic strain relation of clay samples strongly depends on the visco-plastic strain rate. However, the effects of the strain rate, under a given constant visco-plastic strain rate, do not become constant when the visco-plastic strain rate becomes very small. The reason is assumed to be due to the development of structures under a constant small visco-plastic strain rate. The development of structures may restrict the applicability of the isotache model to the compressive behavior of clay.     

The Second Phase Construction of Kansai International Airport Considering the Large and Long-Term Settlement of the Clay Deposits

Kansai International Airport was planned to provide a fundamental solution to the aircraft noise pollution problem in the area surrounding Osaka International Airport (Itami Airport) and to the increasing demand for air transportation This man-made island was constructed 5 km offshore in Osaka bay to minimize noise pollution in residential areas. The airport commenced operations in September 1994. The second phase of construction work involved building an island further offshore than the island built in the first phase. Since the sea water is deep at the Kansai International Airport construction site and the layers below the seabed consist of a very soft layer of Holocene clay (immediately below the seabed surface) followed by alternate layers of Pleistocene clay and sand/gravel, the construction of an airport island was expected to produce a considerable amount of ground settlement. The amount of settlement during and after construction needed to be predicted in the design of the airport islands, and the results needed to be considered in the details of the land development work. This report outlines the second phase construction work at Kansai International Airport and describes the related geotechnical issues, with a particular emphasis on settlement.     

Numerical Assessment for the Behavior of the Pleistocene Marine Foundations due to Construction of the 1st Phase Island of Kansai International Airport

A series of elasto-viscoplastic finite element analyses is performed to assess the stress and deformation of the Pleistocene marine foundation due to construction of the 1st phase island of Kansai International Airport in Osaka Bay. Attention is paid to the modeling of permeability for Pleistocene sand gravel layers considering the sedimentation environment because the performance of excess pore water pressure is strongly dependent on the extent of distribution as well as the change of thickness of those permeable sand gravel layers. The concept of “mass permeability” is introduced to model the actual process of dissipation of excess pore water pressure in the field. The mechanism for the propagation of excess pore water pressure is also discussed. Special attention is given to the modeling of the compressibility of the highly structured Pleistocene clay layers, exhibiting already significant visco-plastic deformations even in the quasi-overconsolidated effective stress range. The present numerical analyses are found to describe the large and long-term settlement together with the slow dissipation of excess pore water pressure in the Pleistocene clay and sand gravel layers that actually has taken place in the field. A detailed comparison of the distribution of excess pore water pressure and the differential compression for the individual Pleistocene clay layers shows that the calculated performance can well describe the actual behavior of the Pleistocene deposits due to the construction of the 1st phase island of Kansai International Airport.     

Interactive behavior of Pleistocene marine foundation of existing 1st phase island due to construction of 2nd phase island of Kansai International Airport

A series of elasto-viscoplastic finite element analyses is performed to assess the interactive behavior of the Pleistocene marine foundation of the existing 1st phase island due to the construction of the adjacent 2nd phase island of the Kansai International Airport in Osaka Bay. Marine foundations modeled independently for the 1st and 2nd phase islands, considering the increase in thickness of the individual layers toward the offshore area, are proposed for the finite element analyses. The mechanism for the propagation of excess pore water pressure, due to the construction of the adjacent reclamation, is discussed through a numerical procedure using the concept of “mass permeability” introduced to model the actual process of the dissipation of excess pore water pressure in the field. A modeling of the compressibility of the quasi-overconsolidated Pleistocene clay is also adopted to analyze the long-term settlement behavior. It is found that the present numerical analyses are capable of describing the large and long-term settlement together with the insufficient dissipation and propagation of excess pore water pressure due to the construction of the adjacent reclamation in the Pleistocene clay and sand gravel layers that has actually taken place in the field. The calculated performance is validated by comparing it with the measured results obtained for over 20 years from the start of the construction.     

基于等时曲线的软黏土弹黏塑性模型

软黏土的变形与时间密切相关,包括固结效应与蠕变效应两部分,两者相互作用共同影响着土体的变形及长期变形。在Bjerrum等时曲线理论的基础上,推导了一个能描述软黏土变形时效特性的弹黏塑性本构模型,将此模型代替一维Terzaghi固结理论中的线弹性应力–应变关系,并进一步考虑非线性渗流的影响,实现了一维条件下蠕变–固结的非线性耦合作用。本模型中共有8个模型参数,物理意义明确,均可通过室内试验简单获取,利用Crank-Nicolson有限差分格式在一定的边界条件下可对控制方程进行求解。为验证此模型的有效性,对室内一维K0侧限固结试验及Leroueil和Kabbaj的等应变率试验成果进行了模拟,理论计算值与试验结果能够较好吻合。     

Combined effects of strain rate and temperature on consolidation behavior of clayey soils

It is considered that the long term settlement of clay deposits, known as secondary consolidation, is caused by clay viscosity. In this paper, the viscous property of clayey soils is examined from two viewpoints: one is temperature and the other is the effect of the strain rate. To investigate these effects, a special constant rate of strain (CRS) loading test, in which the strain rate is changed during the test, was carried out at temperatures of 10 and 50 °C on reconstituted clay samples. Under the normal strain rate, such as the order of 10⁻⁶ s⁻¹, well-known temperature effects on the consolidation behavior were confirmed. That is, the high temperature condition leads to increased hydraulic conductivity due to the reduction in the viscosity of pore water at higher temperatures. It is also observed that the yield consolidation stress decreases with increasing temperature due to the viscous properties of soil skeletons. However, it is found that with higher temperature and smaller strain rates, the clay specimen does not follow conventional viscous behavior, like the Isotache model, but the gradient of stress–strain curve considerably decreases. The reason for different behavior from the Isotache model may be attributed to the creation of a new structure to resist the external deformation, under high temperature and a slow strain rate.     

“剖面”的新思维和设计方法

本文通过对Renzo Piano的两个作品和Eyemeam艺术和技术博物馆竞赛的两个方案（分别由LEESER Architecture和diller ＆ scofidio设计）以及瑞士建筑师Kerez的新作“五户公寓”的分析,试图阐明剖面在他们的设计中的意义和对建筑设计理念的的挖掘。     

Method for calculating horizontal drain induced non-linear and large strain degree of consolidation

Consolidating dredged clay slurries using a combination of vacuum pressure and prefabricated horizontal drains (PHDs) is widely used in practice. This is a large strain problem, but there is no existing large strain theory for PHD induced consolidation. A method with explicit equations has been proposed to consider both mechanical and geometrical non-linearities in analyzing PHD induced consolidation. The method considers stepwise variation of properties using imaginary time concept. An imaginary time is determined by the condition of continuity of degree of consolidation before and after changing the properties. Then the method was applied to analyze a large scale model test of vacuum consolidation with PHD. Two analyses were conducted. One considered both the mechanical and geometrical non-linearity (large strain), and another only considered mechanical non-linearity (small strain). The results of large strain analysis agree with the measured settlement curve and excess pore pressures well. While the small strain analysis under-estimated the rate of consolidation significantly. The results from this study indicate that for analyzing PHD induced consolidation of clay slurries considering the effect of large strain is important, and the proposed method can be a useful design tool.     

推算最终沉降中工后次固结沉降比例研究

为了更准确地预测工后沉降,利用室内常规压缩试验、长期压缩试验和现场试验研究了三点法、双曲线法、星野法等最终沉降推算方法推算的黏性土地基最终沉降与工后次固结沉降的关系。研究表明,采用三点法推算的黏性土地基最终沉降包含的工后次固结沉降较少,采用双曲线法推算的黏性土地基最终沉降包括约23%～52%的工后次固结沉降,采用星野法推算的黏性土地基最终沉降基本包含了绝大部分工后次固结沉降。故建议采用三点法和双曲线法推算黏性土地基最终沉降并预测工后沉降时还应考虑工后次固结沉降。     

高含水率疏浚泥轴对称大应变固结模型

高含水率疏浚泥在外加荷载作用下通常产生大应变固结变形,不适用于传统的Barron轴对称小应变固结理论。为此,基于Gibson一维大应变固结理论和Hansbo径向固结理论,摒弃小应变假定,考虑高含水率疏浚泥的材料和几何非线性、径竖向渗流等因素,建立了等应变条件下以孔隙比为变量的轴对称大应变固结模型ALSC,Gibson、Hansbo、Kjellman等建立的固结方程是该模型的特例。基于有限差分法,编制了计算程序,进行了ALSC模型与小应变模型的数值模拟,验证了ALSC的有效性。研究结果表明:土体变形较小时,ALSC模型与Barron模型计算的固结度和超静孔压数值基本吻合;土体变形较大时,ALSC模型与"Barron+Terzaghi"理论计算的最终沉降量和固结速率取决于土体的固结参数;当C_c/C_k=1时,ALSC模型的最终沉降量小于"Barron+Terzaghi"理论,但二者固结速率相当;当压缩系数av保持不变,ALSC模型(C_k=1)与"Barron+Terzaghi"相比,最终沉降量大,固结速率慢。     

车振荷载引起的软土越江隧道长期沉降分析

随着越江隧道建设的高速发展,车振荷载引起的隧道长期沉降已成为亟待解决的问题。针对一些由于交通车辆运行引起的饱和软土地基长期沉降问题,比较实用的预测方法是基于循环累积变形的经验拟合公式。考虑到土体的真实K0固结应力状态对土体强度和变形的影响,对笔者以前提出的经验拟合公式进行了改进,建立了K0固结条件下软黏土的不排水累积应变与累积孔压实用计算公式;并对上海地区典型饱和软土所进行的非等向固结不排水循环三轴试验结果进行了经验拟合,得到了相关参数。同时结合工程实用的分层总和法,建立了长期沉降的简化计算方法,最后对某越江隧道工程的长期沉降进行预测。更多还原 AbstractFilter('ChDivSummary','ChDivSummaryMore','ChDivSummaryReset');     

饱和软粘土的再固结性状研究

围绕“动静结合排水固结法”课题,通过室内试验研究饱和软粘土在冲击荷载作用下的再固结性状,重点分析排水条件对土体固结和再固结的影响,指出孔隙压力升高与再固结体应变之间的唯一性关系。提出再固结体积压缩系数和再固结压缩指数的概念并给出其确定方法。对冲击荷载作用后软土地基沉降计算方法进行讨论,包括多遍冲击和部分排水条件的情形,并给出试验验证。     

淤填黏土铺盖裂缝大变形固结试验研究

Gibson的一维大变形固结理论存在很大的局限性,为符合特定的需求,不断对Gibson理论进行修正。本文针对黏土铺盖裂缝的淤填处理问题,对渗压作用下的淤填黏土的固结机理进行了研究。首先,推导出有限应变渗压固结理论模型,再基于有限应变固结理论分别推导出以超孔隙水压力和孔隙比表示的有限应变渗压固结偏微分方程,并通过设计模型试验模拟了裂缝处淤填固结的全过程,同时对理论模型和固结偏微分方程进行了验证,最后以模型试验结果为依据指导了现场试验。     

天津滨海重塑软黏土一维固结蠕变特性研究

软黏土蠕变产生的长期变形严重影响建筑物及地基的安全性和稳定性,故通过设置不同预压条件模拟软黏土的初始受力状态,对重塑软黏土进行一维固结蠕变试验。研究表明:不同预压条件下,重塑软黏土一维固结蠕变历时曲线的瞬时变形较大,随着加载时间的增长,土体产生缓慢的蠕变,变形量逐渐减小,最终趋于稳定;此外,软黏土一维固结蠕变等时曲线中应变随应力水平的增大而增大,曲线延伸方向逐渐向应变轴靠拢,非线性愈加明显。基于上述研究,引入时间参量t,建立了软黏土一维固结蠕变预测公式,并对公式的准确性和适用性进行验证。该公式可对软黏土地基因蠕变产生的长期变形进行预测分析,为实际工程的设计、施工提供理论指导。     

砂土液化后再固结体变规律表征与离心模型试验验证

针对一定相对密度的饱和砂土,首先开展单元体K₀固结试验和振动液化试验研究,发现饱和砂土液化后体变规律受再沉积和再固结两种机制制约：其中再沉积部分与所受振动历史密切相关,尤其是液化触发后的应变历史,土骨架累积剪应变比越大、再沉积体变越大;而再固结部分受先期固结历史和循环振动历史影响显著,再固结曲线会沿原有正常固结曲线趋势发展,其稳定段再压缩指数比相同条件下的正常固结曲线的压缩指数稍大。据此提出了考虑先期固结和振动历史的砂土液化后体变模型和简化算法,将再沉积和再固结两者统一表达成再固结体变,并建议了再固结压缩指数和假设起始应力的确定方法。进一步开展了水平场地地震液化离心机模型试验,监测模型固结和振动液化过程的沉降,从模型尺度进一步揭示砂土液化后体变规律,并初步验证了本文模型与简化算法的有效性。     

A Simple Pneumatic Loading System Controlling Stress and Strain Rates for One-Dimensional Compression of Clay

A simple but automated pneumatic loading system that can control the stress and strain rates for one-dimensional (1D) compression of clay was developed. The rate-dependency of stress-strain behaviour due to the viscous property of clay was investigated by 1D compression tests on standard-size specimens by various loading methods: 1) Standard Consolidation Tests (SCTs), stepwise increasing the axial stress two times every one day; 2) ordinary Constant-Rate-of-Strain (CRS) tests at different strain rates; 3) special CRS tests including unloading and reloading cycles with different stress amplitudes at strain rates of which the absolute value was either kept constant throughout respective tests or changed at the start of reloading; and 4) special CRS tests including a number of sustained loading (SL) during otherwise primary loading or unloading or reloading at constant strain rate. Sufficiently low strain rates were employed to ensure essentially fully drained condition. The followings were found. Despite that the newly developed pneumatic loading system is rather simple, 1D compression tests following such various loading histories as above can be performed on four types of clay rather accurately. The stress-strain behaviour of clay is significantly rate-dependent, exhibiting significant creep strains at SL stages. The creep strain rate is significantly different whether SL starts during otherwise primary loading or unloading or reloading, controlled by the magnitude and sign of the initial strain rate at the start of SL. The whole observed trends of rate-dependent stress-strain behaviour can be qualitatively explained by the non-linear three-component elasto-viscoplastic model extended to cyclic loading conditions.     

Long-term consolidation analysis of a large-scale embankment construction on soft clay deposits using an elasto-viscoplastic model

In this paper, the consolidation analysis of a large-scale embankment construction in Osaka City is presented, where a conventional levee with a height of about 8 m has been extended to a super levee with a total width of 215 m. The ground consists of alluvial sandy layers and soft clay deposits, which have been locally improved by several methods, including deep mixing beneath the conventional levee and the combination of sand drains and sand compaction piles under the extended back slope. A long-term consolidation analysis of this super levee construction is carried out using an elasto-viscoplastic constitutive model. The layered construction procedure is applied to properly simulate the construction sequence of the super levee. The effects of the structural degradation and the strain dependency of the shear modulus, as two aspects of destructuration in clay materials, are studied in terms of the consolidation behavior for the unimproved case. For the improved case, the analysis is implemented by including the ground-improved zones in the finite element simulation. The field observation data obtained during the preloading process, before the construction of the super levee, are employed to verify the assumptions and to calibrate the material properties of the improved layers. The effects of destructuration in the natural ground cases are observed as excess pore pressure build-up after construction and strain localization. The effects of ground improvement techniques are studied through a comparison of the deformation results and the excess pore water pressure responses with the natural ground cases. The numerical results show that it is important to carefully estimate the unequal long-term settlement for the construction of large-scale embankments.     

VERIFICATION OF THE ELASTO-VISCOPLASTIC APPROACH ASSESSING THE LONG-TERM DEFORMATION OF THE QUASI-OVERCONSOLIDATED PLEISTOCENE CLAY DEPOSITS

A series of elasto-viscoplastic finite element analyses is performed to describe the long-term settlement measured at the reclaimed islands in Osaka Port. The compression modeling for the quasi-overconsolidated Pleistocene clays proposed by the authors has been incorporated to the procedure in which an elasto-viscoplastic deformation is assumed to occur even in the quasi-overconsolidated region (P₀≤P≤P_c). One-dimensional analysis is adopted for assessing the settlement of Sakishima and Yumeshima Reclaimed islands, whereas the plane-strain finite element analysis is carried out for investigating the deformation of the Pleistocene deposit beneath the sheet-pile composite revetment at Maishima Reclaimed Island. The proposed procedure is found to provide larger settlement for all cases compared to the conventional approach because of the consideration of viscoplastic deformation in the quasi-overconsolidated region. As for the case of the sheet-pile composite revetment, the effect of stress concentration due to ground improvement with SCP for the Holocene clay layer is remarkable on the underlying Pleistocene deposit. The measured settlement of the individual Pleistocene clay layers as well as the total settlement of the Pleistocene deposits are precisely described by the proposed procedure, whereas the conventional approach in which the instantaneous elastic deformation is assumed to occur in the quasi-overconsolidated region exhibits serious underestimation. From these results, the predictive accuracy of the proposed procedure is verified for assessing the long-term settlement at the reclaimed islands in Osaka Port.     

Geotechnical Hybrid Simulation System for One-Dimensional Consolidation Analysis

A system for a hybrid consolidation simulation was developed to predict more realistically the nonlinear behavior of the consolidation settlement of soft clay, including a large strain consolidation problem. The hybrid simulation method can introduce the mechanical behavior of the soils directly by combining numerical analysis and the soil element test in an online state. In this paper, a fundamental study to apply the hybrid simulation method to the consolidation settlement problem, a basic concept, and the corresponding algorithm and components are presented. Furthermore, using preconsolidated Kaolinite clays, verification studies were also conducted in the laboratory to verify the operational performance and data reliability of the system. On the basis of the test results, we concluded that the new consolidation analysis system using the hybrid simulation concept provided an adequate performance in the reproduction of the consolidation behavior of preconsolidated Kaolinite clays in the aspect of the dissipation of excess pore water pressure and corresponding consolidation settlement.