Geotechnical properties of 3D-printed transparent granular soil

This paper proposes a 3D-printed transparent granular soil technique based on the contour rotation interpolation method, 3D printing and transparent soil technologies. Laboratory tests, including one-dimensional compression, direct shear and triaxial compression tests, are assessed for mechanical properties of 3D-printed transparent granular soil. The results show that 3D-printed transparent granular soil can be used to consider the effect of shape on macromechanical soil properties, which is an advantage that cannot be achieved by the previously presented transparent soil techniques. Subsequently, a simple model test of rigid flat plate penetration into transparent granular soil is performed. The obtained soil displacements using the PIV technique are compared with the classical shallow strain path method solution. This comparison indicates that the proposed 3D-printed transparent granular soil can capture the soil deformation pattern, although the accuracy needs to be further improved. The proposed 3D-printed transparent granular soil technique could be used for model test that needs to consider the shape of sand particle.

     

冻土-混凝土接触面动剪强度研究

为了研究冻土-混凝土接触面的力学特性,开发了一种低温动荷载直剪仪,它是在普通直剪仪框架的基础上,安装了动力加载系统和温度控制系统。该装置能够保证在负温条件下对冻土进行动荷载直剪试验,从而测定冻土的动力学参数。为了研究寒区冻土与混凝土构筑物的相互作用,使用该仪器进行了一系列的多因素接触面动直剪试验,试验结果表明了冻土-混凝土接触面在动荷载作用下的行为特征,多因素对比试验则揭示了接触面动剪强度的影响因素和变化规律。     

透明砂土与天然砂土动力特性对比

人工合成透明土材料在支撑岩土工程可视化模型试验技术领域得到了良好的应用。然而,关于该新型材料的动力特性研究仍相对缺少,一定程度上影响了其在动力相关模型试验中的推广应用。基于共振柱、动扭剪试验仪器,开展人工合成透明砂土的动变形与动强度特性试验研究,并与天然砂土及福建标准砂的相关动变形与动强度特性进行对比分析;透明砂土由折射率一致的熔融石英砂和混合油配制而成,混合油由15号白矿物油和正12烷按比例混合而成。试验测定并分析了透明砂土的动剪切模量-应变、阻尼比-应变、动剪切模量-阻尼比等关系曲线,以及孔压、动强度特征。试验结果表明,由熔融石英砂与混合油配制而成的透明砂土的动变形和动强度特性与天然砂土相似,可以模拟天然砂土材料开展动力相关模型试验研究。     

超重力振动台柔性叠层箱剪切效能评价方法

针对超重力振动台试验场地边界效应和新研制的一套柔性叠层箱,设计与开展干砂、纯水离心模型对比试验,引入加速度峰值误差EPGA、归一化均方误差NMSE、峰值频率误差EF等指标,评价了该叠层箱的剪切效能和边界效应。结果表明：与土体中心加速度相比,干砂模型叠层梁的EPGA、NMSE和EF,最大分别为21.50%、21.36%和3.1%,表明该叠层箱具有较好剪切效能。相同动荷载条件下,干砂模型各叠层梁的加速度峰值和峰值频率,最高是纯水模型的2.37倍和2.23倍,说明干砂试验中叠层梁的响应被土体响应控制,进一步验证了叠层箱具有良好剪切效能。干砂模型叠层梁与土体中心的剪切模量和阻尼比基本一致,误差分别为3%和18%;纯水模型叠层梁的剪切模量为干砂模型的1/3,仅为9.22 MPa;但阻尼比较大,为34.8%。以加速度峰值误差5%为边界效应临界距离,该叠层箱中心处约90 cm×24 cm范围内的测试结果是有效和可靠的。提出的方法与结论,对评价超重力振动台叠层箱边界效应具有重要指导意义和应用价值。     

Interface creep behavior of grouted anchors in clayey soils: effect of soil moisture condition

This study aims at investigating the influence of moisture conditions on interface shear behavior of element-grouted anchor specimens embedded in clayey soils. The tests involved comparatively short embedment lengths and a device that was specially designed to facilitate moisture conditioning. Rapidly loaded pullout tests as well as pullout tests under sustained (creep) loading were conducted to characterize both the short-term and long-term ultimate shear strength of anchor–soil interfaces. Both values of the interface shear strength were found to decrease exponentially with increasing moisture content values, although their ratio was found to show a linearly decreasing trend with increasing moisture content. The interface shear creep response under pullout conditions was characterized by a rheological hybrid model that could be calibrated using experimental measurements obtained under increasing stress levels. The accuracy of the hybrid model was examined by evaluating the stress-dependent prediction model as well as its governing parameters. This investigation uncovers the coupled impact of soil moisture condition and external stress state on the time-dependent performance of grouted anchors embedded in clayey soils by correlating the interface shear strength with soil moisture content and associating the creep model with stress levels applied to the grout–soil interface.

     

Seismic response analysis of geosynthetic reinforced soil segmental retaining walls by finite element method

The paper presents the results of a finite element analysis of the dynamic response of a geosynthetic reinforced soil retaining wall that is constructed with dry-stacked modular concrete blocks as the facia system. In the finite element model, the cyclic shear behavior of the backfill soil is described by a hyperbolic stress-strain relationship with Masing hysteretic unload-reload behavior. The reinforcement material is modelled using a similar hysteretic model which takes into account the measured response of cyclic load-extension tests performed on unconfined geogrid specimens in the laboratory. Interface shear between wall components is simulated using slip elements. The results of finite element analyses giving the seismic response of a typical geogrid reinforced segmental retaining wall subjected to prescribed acceleration records are presented. The results of analyses highlight the influence of dynamic loading on: (1) wall displacement; (2) cumulative interface shear force and displacement between facing units; (3) tensile forces developed in the reinforcement and; (4) acceleration response over the height of the wall. A number of implications to the design of these structures are identified based on the results of these simulations.     

基于应力修正的土体抗剪强度影响因素分析

为探究地基土体在附加应力作用下抗剪强度的变化规律,对传统剪切试验方法进行了改进。通过气压式全自动固结仪预先对试样施加多组精确的附加应力（超出原状土体上覆土重的荷载）后进行快剪试验,以淤泥质粉质黏土与粉土夹粉砂两类土体为研究对象,分析结构与状态的变化对抗剪强度指标值的影响;从受剪土体与剪切盒各层面受力与沉降关系出发,融入剪切面积动态表达式,推导出剪切正应力的修正公式,代入试验计算抗剪强度参数值。结果表明:两类土体受附加应力的影响存在明显的差异,淤泥质粉质黏土以黏聚力变化为主,粉土夹粉砂则体现在内摩擦角上;含水量及孔隙比的改变与土体抗剪强度参数值的变化之间存在反相关关系。     

Variations in shear wave velocity and soil site class in Kolkata city using regression and sensitivity analysis

The detrimental effects of an earthquake are strongly influenced by the response of soils subjected to dynamic loading. The behavior of soils under dynamic loading is governed by the dynamic soil properties such as shear wave velocity, damping characteristics and shear modulus. Worldwide, it is a common practice to obtain shear wave velocity (V _s in m/s) using the correlation with field standard penetration test (SPT) N values in the absence of sophisticated dynamic field test data. In this paper, a similar but modified advanced approach has been proposed for a major metro city of eastern India, i.e., Kolkata city (latitudes 22°20′N–23°00′N and longitudes 88°04′E–88°33′E), to obtain shear wave velocity profile and soil site classification using regression and sensitivity analyses. Extensive geotechnical borehole data from 434 boreholes located across 75 sites in the city area of 185 km² and laboratory test data providing information on the thickness of subsoil strata, SPT N values, consistency indices and percentage of fines are collected and analyzed thoroughly. A correlation between shear wave velocity (V _s) and SPT N value for various soil profiles of Kolkata city has been established by using power model of nonlinear regression analysis and compared with existing correlations for other Indian cities. The present correlations, having regression coefficients (R ²) in excess of 0.96, indicated good prediction capability. Sensitivity analysis predicts that significant influence of soil type exists in determining V _s values, for example, typical silty sand shows 30.4 % increase in magnitude of V _s as compared to silt of Kolkata city. Moreover, the soil site classification shows Class D and Class E category of soil that exists typically in Kolkata city as per NEHRP (Recommended provisions for seismic regulations for new buildings and other structures—Part 1: Provisions. Prepared by the Building Seismic Safety Council for the Federal Emergency Management Agency (Report FEMA 450), Washington, DC, 2003) guidelines and thereby highlighting the seismic vulnerability of the city. The results presented in this study can be utilized for seismic microzonation, ground response analysis and hazard assessment for Kolkata city.

     

Analysis of mobilized stress ratio of gap-graded granular materials in direct shear state considering coarse fraction effect

Weathered rockfill materials, characterized by a mixture of soil matrix and rock aggregates, are widely distributed in mountainous areas. These soils are frequently used for subgrade or riprap in engineering practice, and the mobilized shear strength is crucial for analyzing the displacement and stability of these geo-structures. A series of direct shear tests are performed on a gap-graded soil with a full range of coarse fraction. The behavior of gap-graded soils is analyzed, and a simple model is proposed for the evolution of mobilized stress ratio during direct shearing process based on mixture theory. The change of inter-aggregate configuration is incorporated by introducing a structure variable which increases with coarse fraction and decreases approximately linearly with the overall horizontal shear strain in double logarithmic plot. It reasonably reflects a gradually transformation from a matrix-sustained structure into an aggregate-sustained one with the increase of coarse fraction. The model has four parameters, and at least two direct shear tests need to be done for the calibration. Validation of the model is done by using the test data in this work and those from the literature.

     

Pile Head Cyclic Lateral Loading of Single Pile

This paper presents an elastic continuum model using an extended nonlinear Davies and Budhu equations, which enables the nonlinear behavior of the soil around the long elastic pile to be modeled using a simple expression of pile-head stiffness method. The calculated results were validated with the measured full-scale dynamic field tests data conducted in Auckland residual clay. An idealized soil profile and soil stiffness under small strain (i.e. shear modulus, G _s and shear wave velocity, V _s of the soil) determined from in situ testing was used to model the single pile tests results. The predictions of these extended equations are also confirmed by using the three-dimensional finite-element OpenSeesPL (Lu et al. in OpenSeesPL 3D lateral pile-ground interaction: user manual, University of California, San Diego, 2010). A soil stiffness reduction factor, G _s /G _s,max of 0.36 was introduced to the proposed method and model. It was found to give a reasonable prediction for a single pile subjected to dynamic lateral loading. The reduction in soil stiffness found from the experiment arises from the cumulative effects of pile–soil separation as well as a change in the soil properties subjected to cyclic load. In summary, if the proposed method and model are accurately verified and properly used, then they are capable of producing realistic predictions. Both models provide good modelling tools to replicate the full-scale dynamic test results.     

CFG桩加固饱和粉土地基的动力特性试验研究

以京沪高速铁路液化土地基加固为背景,利用大型堆叠式剪切变形模型箱,进行了模型比例1:10的CFG桩桩网结构地基加固饱和粉土地基的振动台模型试验,得出各级加载情况下地基路基响应加速度幅值放大系数的分布以及响应加速度幅值放大系数与各级加载加速度幅值的关系,分析了CFG桩桩网结构地基加固饱和粉土地基在地震作用下的加速度传递规律,得到了以下结论：（1）在加载加速度小于0.101g时,地基路基响应加速度幅值基本不放大,且加速度放大系数云图呈现水平状分布;（2）当加载加速度为0.161g时,桩间土的液化引起地基路基的响应加速度幅值放大系数的较大增加;（3）当加载加速度为0.210g时,桩间土液化的同时部分CFG桩发生剪断破坏,从而引起了地基路基的各响应加速度幅值放大系数的增大;（4）当加载加速度为0.324g和0.363g时,地基整体刚度的大幅度减小,在一定程度上降低了桩网结构地基传递振动的能力,从而导致加速度幅值放大系数相对前一级加载时减小;（5）整个加载过程中,粉土层的放大效应显著,而黏土层对加载加速度几乎没有放大作用。     

利用人工合成透明土的岩土物理模拟试验

结合粒子图像测速（PIV）和近景摄影技术,建立了一种利用人工合成透明土的岩土物理模拟试验方法,非侵入观测和记录了浅基础沉降（SFS）引起的土体变形,并与利用天然土的传统模型试验进行了对比研究。透明土是由与熔融石英的折射率相匹配的孔隙溶液组成。采用激光面照射透明土模型并拍摄下SFS引起变形的数字图像,利用基于MATLAB的Geo-PIV软件获得了所产生的位移场。试验结果对比分析显示：与天然砂土中的相比,在透明土中SFS引起的扰动区域较大,最大剪应变扩大了约1.5倍,表层隆起范围约增加了30～50%;然而透明土中土体单元的运动趋势总体上与天然砂土中的相似,表明所建立的试验方法在机制研究上具有一定程度的适用性。     

Micro-mechanical analysis of soil–structure interface behavior under constant normal stiffness condition with DEM

The mechanical behavior at soil–structure interface (SSI) has a crucial influence on the safety and stability of geotechnical structures. However, the behavior of SSI under constant normal stiffness condition from micro- to macro-scale receives little attention. In this study, the frictional characteristics of SSI and the associated displacement localization under constant normal stiffness condition are investigated at both macro- and microscales by simulating a series of interface shear tests with discrete element method. The algorithm to achieve a constant normal stiffness is first developed. The macroscopic mechanical response of the interface shear tests with both loose and dense specimens at various normal stiffness is discussed in terms of shear stress, normal stress, vertical displacement, horizontal displacement and stress ratio. Then, the microscopic behaviors and properties, including shear zone formation, localized void ratio, coordination number, force chains and soil fabric, are investigated. The effect of normal stiffness is thus clarified at both macro- and microscales.

     

FLAC Based Numerical Studies on Dynamic Interference of Two Nearby Embedded Machine Foundations

This paper emphasizes on the dynamic interaction of two closely spaced embedded square or rectangular foundations under the action of machine vibration. One of the foundations is excited with a known vibration source placed on the top of it, called the active foundation. The objective is to study the effect of dynamic motion of the active foundation on the nearby passive foundation through a layered soil medium. The analysis is performed numerically by using the explicit finite difference code FLAC ^3D . The soil profile is assumed to obey the Mohr–Coulomb yield criteria with non-linear failure envelope. The analysis is performed under sinusoidal dynamic loading with varying amplitude. Under the dynamic excitation, the settlement behavior of the interacting foundations is studied by varying the spacing between the foundations. In addition, the variation of vertical normal and shear stress developed beneath the interacting foundations is also explored. The present theoretical investigation indicates that the settlement and vertical normal stress below the active foundation is generally found to be higher than that obtained for the passive foundation, whereas the shear stress response below the foundations follows the reverse trend.     

Implementation of a hydromechanical elastoplastic constitutive model for fully coupled dynamic analysis of unsaturated soils and its validation using centrifuge test results

Prediction of unsaturated soil behavior during earthquake loading has received increasing attention in geotechnical engineering research and practice in recent years. Development of a fully coupled analysis procedure incorporating a coupled hydromechanical elastoplastic constitutive model for dynamic analysis of unsaturated soils has, however, been limited. This paper presents the implementation of a coupled hydromechanical elastoplastic constitutive model into a fully coupled dynamic analysis procedure and its validation using a centrifuge test. First, the fully coupled finite element equations governing the dynamic behavior of unsaturated soils with the solid skeleton displacement, pore water pressure, and pore air pressure as nodal unknowns are briefly presented. The closest point projection method is then utilized to implement the coupled hydromechanical elastoplastic constitutive model into the finite element equations. The constitutive model includes hysteresis in soil–water characteristic curves, cyclic elastoplasticity of the solid skeleton, and the coupling mechanisms between the SWCCs and the solid skeleton. Finally, the analysis procedure is validated using the results from a dynamic centrifuge test on an embankment constructed of compacted unsaturated silt subjected to base shaking. Reasonable comparisons between the predicted and measured accelerations, settlements, and deformed shapes are obtained.

     

跨越走滑断层黏土地层动力响应离心机试验研究

地震中基岩走滑断层活动引起跨断层地下结构物破坏,上覆土层的非一致响应机制尚不明确,隐伏裂缝影响规律认识不足。基于可模拟走滑断层活动的层状剪切箱,针对黏土地层展开两个离心机振动台试验,对比上覆土体中隐伏裂缝的影响。试验结果表明,当地层中不存在隐伏裂缝时,基岩走滑断层错动对两盘地层的非一致振动的影响不显著。当地层中存在隐伏裂缝,错动盘一侧土体的振动加速度幅值随振动循环次数的增加逐渐减小,固定盘和错动盘两侧土体出现非一致振动响应,固定盘一侧土体产生的超静孔压大于错动盘一侧。隐伏裂缝对土体非一致振动响应的影响范围受限于其在地层中的分布范围,探明隐伏裂缝上断点的埋深具有重要工程意义。     

Physical and numerical studies of stability of soil blocks reinforced by brittle shear pins

In this paper, the stability and failure mechanism of soil blocks that were reinforced by brittle shear pins and rested on a low-interface friction plane were studied by means of physical and numerical models. The humid silica sand no. 6 was employed to build the physical models of soil blocks, while the Teflon sheet was employed as the low-interface friction plane. To study the effect of stabilizing piles on slopes, the soil blocks were reinforced by brittle shear pins using pencil leads with 2 mm in diameter. Three-dimensional finite element analyses were employed to analyze the stability of this problem. The effects of numbers and patterns of shear pins on the stability and failure mechanisms of physical and numerical models were compared and discussed.

     

Long-term dynamic bearing capacity of shallow foundations on a contractive cohesive soil

The calculation of the long-term dynamic bearing capacity arises from the need to consider the generation of maximum pore-water pressure developed from a cyclic load. Under suitable conditions, a long-term equilibrium situation would be reached, when pore-water pressures stabilized. However, excess pore-water pressure generation can lead to cyclic softening. Consequently, it is necessary to define both the cohesion and the internal friction angle to calculate the dynamic bearing capacity of a foundation in the long term, being necessary to incorporate the influence of the self-weight of soil and therefore the width of the foundation. The present work is based on an analysis of the results of cyclic simple shear tests on soil samples from the port of El Prat in Barcelona. From these experimental data, a pore-water pressure generation formulation was obtained that was implemented in FLAC2D finite difference software. A methodology was developed for the calculation of the maximum cyclic load that a footing can resist before the occurrence of the cyclic softening. The type of soil studied is a contractive cohesive soil, which generates positive pore-water pressures. As a numerical result, design charts have been developed for long-term dynamic bearing capacity calculation and the charts were validated with the application of a real case study.

     

不同应力路径下饱和黏土耦合循环剪切特性

地基土单元体在波浪荷载的作用下将发生主应力轴连续旋转,这对土的强度和变形特性会产生显著的影响。针对饱和黏土,利用土工静力－动力液压三轴－扭转多功能扭剪仪进行了均等固结下的耦合循环剪切试验,着重研究了不同循环应力路径对应力-应变关系和强度特性的影响。试验结果表明,耦合循环荷载下黏土扭转剪切分量和正向偏差分量的应力-应变关系曲线与应力路径有关,且随着振动次数的增加逐渐疏松,割线模量 和 不断衰减,这与不固结不排水条件下的应力-应变关系曲线的变化规律明显不同;扭转剪切分量大的椭圆应力路径的动强度略小于正向偏差分量大的动强度。饱和黏土在不同应力路径下的力学特性的试验研究可以为建立复杂应力路径下的本构模型提供试验依据。     

Peaty Soil Improvement by Using Cationic Reagent Grout and Electrokintic Method

Peat is known as soft soil with low shear strength and high compressibility. Electrokinetic injection technique is being used by applying a direct electrical potential across the soil specimens to improve physicochemical characteristics of the peat. Such applications cause electrochemical effects on the soil, leading to changes in the soil’s chemical, physical, and mechanical properties. This paper presents the results of the undrained shear strength, pH, water content across the electrokinetic box after injecting the cationic grouts. Four cationic grouts namely; calcium chloride, calcium oxide, Aluminum hydroxide, and sodium silicates were selected as grout. The microstructures of the stabilized peats were investigated by scanning electron microscopy and energy dispersive X-ray spectrometer analysis. The result showed that the cationic stabilizer injected by the electrokinetic technique could significantly increase the peat soil’s shear strength. Furthermore, the result showed that the effect of calcium oxide was the highest on the shear strength of peat due to its physico-chemical properties. The shear strength, pH and moisture content of peats across the electrokinetic box also altered depending on the used electrolytes and time.