Large-Scale Monotonic and Cyclic Tests of Interface Between Geotextile and Gravelly Soil

A series of monotonic and cyclic shear tests, as well as pullout tests, were conducted on gravel-geotextile interfaces using a large-scale apparatus, with development of a new special pullout test element. The macroscopic response of stress and displacement, as well as the movement and crushing process of soil particles, were observed and measured. The interface exhibited evident strain-softening and aeolotropic normal displacement, which were significantly influenced by normal stress. Shear strength decreased and normal displacement increased with increasing number of shear cycles. Shear deformation was composed of slippage at the contact surface and deformation of the soil constrained by the geotextile; and the thickness was estimated at 5-6 times the average soil grain size. There was significant evolution of physical state due to shear application, including soil particle crushing and soil compression, as well as damage to the geotextile. The pullout test underestimated shear stiffness of the interface due to significant deformation of the geotextile itself. Shear strength increased with increasing normal stress, described by a logarithmic equation, according to the pullout tests, rather than the linear relationship obtained using direct shear tests. Therefore, an appropriate test method should be selected with careful consideration of the site conditions.     

Monotonic and Cyclic Behavior of Unsaturated Sandy Soil Under Drained and Fully Undrained Conditions

Changes in air pressure during monotonic and cyclic loading are in some cases important for the behavior of unsaturated soil. For example, in order to investigate the stability of embankments and slope failure during earthquakes, it is necessary to consider the effect of the pore air or the pore gas pressure as well as the pore water pressure and the interaction between the soil and the pore fluids. In the present study, we carried out a series of monotonic and cyclic loading tests on sandy soil used for the improvement of river embankments. The effects of the initial suction, the confining pressure, and the degree of compaction under fully undrained conditions, namely, constant water and constant air shearing tests, as well as under drained conditions for both air and water, were studied. For the stress variables of the unsaturated soil, the skeleton stress was used to describe the experimental results and was defined as the difference between the total stress tensor and the average pore pressure of water and gas (Oka et al., 2010). From the monotonic and cyclic test results, we found that the stress-strain behavior of unsaturated sandy soil strongly depends on the initial suction, especially under fully undrained conditions, due to the difference in pore pressures. In the cyclic loading tests under fully undrained conditions, the mean skeleton stress decreased due to the increase in air pressure and led to the failure of the specimen in the case of a lower level of initial suction. In addition, the test results exhibited the strain rate effect on the stress-strain behavior during cyclic loading under fully undrained conditions.     

土与结构接触面模型的对比研究

对土与结构相互作用的四种常用模型的特点进行了总结,结合地铁车站基坑施工实测数据,采用有限元对基坑开挖主要施工步进行模拟计算,地连墙侧移计算结果表明采用薄层单元和摩擦接触模型计算值与实测值吻合较好,直接接触和弹簧单元模型效果较差.对比研究结果可为土与结构相互作用的数值计算提供参考.     

Monotonic and Cyclic Pushover Tests of Wood Beam-Column Connections: Ancient Chinese Tenon joint vs Typical and Simplified US Connections

Wood beam-column frame is a popular structural system in United States and in ancient China. Chinese wood beam-column frame structures showed better seismic resistance properties than the US ones.The tenon joint is one of the reasons.This study performed monotonic and cyclic pushover tests to understand the behavior of Chinese tenon joints versus the behavior of the commonly used US wood beam-column connections. The test results indicate that the typical US wood beam-column connection is very strong unde...     

降雨作用下碎石土滑坡变形演化过程及孕灾模式研究

在总结碎石土滑坡地质模型的基础上,采用有限元数值方法研究不同降雨强度下碎石土滑坡的变形演化过程,并对降雨过程中及降雨后期1～7 d的应力应变情况进行分析,得出典型碎石土滑坡的变形破坏过程及其孕灾模式.研究表明,降雨过程中,滑坡中前部以水平剪切变形为主,中后部以沉降拉剪变形为主;碎石土滑坡在降雨作用下的孕灾模式可划分为三个阶段:后缘拉裂孕灾阶段、中部破裂扩展孕灾阶段、滑面贯穿孕灾阶段.     

Permeability Characteristics of High-Quality Undisturbed Gravelly Soils Measured in Laboratory Tests

A series of permeability tests on both high-quality undisturbed samples and reconstituted samples was performed using a large-scale triaxial cell. Based on the test results and discussion, the following conclusions were noted.

• (1)The coefficient of permeability in the horizontal direction is larger than that in the vertical direction. However, its difference is between 10% and 70% and not so large.
• (2)The coefficient of permeability decreases with increasing confining stress. The effect of the confining stress can be understood as the effect of the void ratio. And the small change of the void ratio due to consolidation only leads to a small change of the coefficient of permeability.
• (3)There is no clear correlation between the physical properties and the coefficient of permeability.
• (4)The coefficient of permeability of gravelly soils is almost the same as that of sandy soils, even though the 50% diameter of gravelly soils is about ten to a hundred times that of sandy soils. This result implies that the large size particles of gravelly soils have no significant effect on permeability characteristics of gravelly soils.
• (5)A new definition for determination of D₁₀, D₂₀ and fines content only from those soil particles with a diameter smaller than 2 mm was introduced. A correlation similar to sandy soils can be seen between the coefficient of permeability and D₁₀, D₂₀, and fines content based on the new definitions.
• (6)The effect of the small size particles of the gravelly soils on the coefficient of permeability was found significant based on the test results using samples with a special blend of particle size and density reconstituted from undisturbed samples.
• (7)The coefficient of permeability of multi-layer gravelly soils, in the direction perpendicular to the sedimentation, was significantly affected by the lowest permeability. And the coefficient of permeability of the multi-layer in total agrees well with the theoretical estimation.
• (8)Although the data is limited, there was no significant difference of coefficient of permeability between undisturbed and reconstituted samples. This result accords with that reported for sandy soils by Hatanaka et al. (1997). This result also means that the coefficient of permeability of gravelly soil is not affected by the soil fabric. As a result, the in-situ coefficient of permeability of gravelly soils can be well estimated for practical purposes from the reconstituted samples with the same gradation properties.

     

带外覆材料的冷弯型钢墙体框架的单调剪切试验研究

研究带外覆墙体材料的冷弯型钢墙体框架在单调剪力作用下的结构强度。墙体试件采用1.0和2.0两种长宽比。试件分别采用石膏板、硅酸钙板、定向刨花板3种不同外覆材料和9,12mm两种不同厚度。给出了每个试件的极限强度、刚度、耗能能力和延性比。最后提出与整体墙体外形相似的冷弯型钢墙体框架的延性比。     

某加筋碎石土边坡变形破坏模式分析

通过某加筋碎石土边坡的工程实例,现场踏勘初步分析其变形破坏的主要原因,判断其可能的几种破坏模式;采用野外钻探、原位试验及室内土工试验等手段获取边坡各岩土层的物理力学指标,利用Geo-Studio软件建立二维地质模型,并进行边坡渗流和边坡稳定性计算。从正常工况和最不利工况两个角度来验证该边坡产生变形破坏的原因及模式,最后得出结论,并预测未来边坡变性破坏的发展趋势,以期为同类型加筋碎石土边坡的设计和施工提供一些参考。     

圆锥动力触探试验在德阳砂砾卵石层中的应用

由于地基土的复杂性,及影响动力触探试验因素的多样性,使得不同地区的N120和N63.5击数的关系具有区域性,所以确定N120和N63.5的地区经验关系具有重要的意义,现结合工程实例讨论分析了动力触探试验N63.5和N120在德阳地区砂砾卵石层中的关系。     

Modeling of Gravelly Soil with Multiple Lithologic Components and its Application

The behavior of a gravelly soil with multiple lithologic components was investigated using large-scale compression and triaxial tests based on the research scheme “separating before mixing”. A series of test results showed that the behavior of a gravelly soil with multiple lithologic components was significantly dependent on the identity and proportions of the lithologic components. A new constitutive model was established to capture the main behavior of a gravelly soil with arbitrary lithologic proportions using few parameters based on the presentation of an elasto-plasticity model and the deformation mechanism of a complex soil. The parameters of this model can be determined using triaxial tests of a simple soil with a single lithologic component. The numerical efficiency of the model was confirmed by successful application to several high rockfill dams. Three-dimensional finite element (FEM) stress-strain analysis results for the Jishixia concrete-faced rockfill dam were obtained as important references for the selection of rockfill and optimizing the design of the dam.     

Small Strain Stiffness and Non-Linear Stress-Strain Behaviour of Cement-Mixed Gravelly Soil

The stiffness at small strains and non-linear stress-strain relation of compacted cement-mixed well-graded gravelly soil as well as the ageing effects were evaluated by drained triaxial compression tests on compacted moist specimens cured for different periods at isotropic and different anisotropic stress states. In all the tests, the initial stress-strain relation at small strains less than about 0.001% was essentially elastic and the initial Young's modulus, E₀, was essentially the same as the E_eq value evaluated by applying unload/reload cycles under otherwise the same conditions. The E_eq values were rather independent of strain rate. The E_eq value from the first unload/reload cycle applied during otherwise continuous ML became noticeably lower than the elastic modulus evaluated at the same stress state, more as approaching the peak stress state. After a number of small unload/reload cycles and long sustained loading, the E_eq value became closer to the elastic modulus due to a decrease in the viscous effects. The ratio of E₀ to the compressive strength (q_max) was similar to that of concrete but noticeably larger than those of uncompacted cement-mixed soil, sedimentary softrock and unbound gravelly soil. Both E₀ and q_max increased with time by ageing, while the E₀/q_max ratio decreased with time. When ML was restarted at a constant strain rate after ageing with a shear stress, the tangent stiffness became very high for a large stress range with a substantial change in the non-linearity of stress-strain relation.     

土与混凝土接触面大型直剪试验方法研究

土与结构接触面直剪试验是研究接触面力学特性的有效途径,在常规直剪仪的基础上改进了混凝土制备方法,即分别制作混凝土垫块和面块,预埋吊装钢筋,并以三峡库区碎石土为研究对象,开展了一系列不同法向应力和不同初始含水率条件下的碎石土与混凝土室内大型直剪试验,得到了其接触面力学特性的变化规律,试验结果表明该试验方法方便可行,可为土与结构相互作用大型直剪试验研究提供一定的参考。     

Undrained Monotonic and Cyclic Shear Behaviour of Sand Under Low and High Confining Stresses

Monotonic and cyclic loading tests have been carried out on a silica sand over a wide range of stresses in order to compare non-crushing and crushing behaviour. Samples were sieved at several stages of testing to theoretically determine the increase in particle surface area and hence degree of crushing. Undrained shear behaviour was compared for crushing and non-crushing regions above 3 MPa. Samples consolidated to 0.1 MPa demonstrated strong dilative behaviour, while above the yield stress of 3 MPa dilation was suppressed and considerable particle crushing occurred. Shearing caused a marked increase in particle crushing particularly after the phase transformation point. Crushing at the steady state was similar for isotropic and anisotropically consolidated sands. At high confining pressures the cyclic strength curves were similar to those for loose sands except for confining pressure dependency due to particle crushing. For low confining stresses cyclic strength increased with initial stress ratio, while for high confining stresses it decreased with initial shear stress ratio. In the cyclic tests there was no significant crushing for 0.1 MPa. Crushing was seen to increase rapidly after the phase transformation point, where high strains developed and where particle rotation and translation contributed to the crushing process.     

土工隔栅与土界面作用特性的试验研究

通过室内模型试验和现场测试的方法对路基工程中土工格栅与填土界面之间的相互作用特性进行了研究,分析了上覆荷载、层间厚度、压实度、埋设层数、埋设位置等因素对于格栅与土界面作用特性的影响规律.试验结果表明,随着上覆荷载的增大,界面拉应力基本呈线性增长,量值大约为竖向荷载的1/1000;在路堤中心线处的拉应力最大,往两侧逐渐减小;随层厚及压实度的增加,界面摩阻力逐渐增大,格栅界面作用逐渐增强.为保证土工格栅抗变形能力的充分发挥,实际工程中格栅的埋设应遵循"上疏下密"的原则.现场试验也表明,土工格栅有调整荷载分布的作用,对于减小路堤的不均匀沉降有明显的效果,从而对提高路基的稳定性能有较大作用.     

由压缩试验分析砾质花岗岩残积土的结构特性

通过引入应力综合结构势来分析砾质花岗岩残积土的结构性.以深圳南山区太子花园施工工地的砾质花岗岩残积土为例,对土样在不同含水量及干密度条件下进行原状、饱和、重塑状态的压缩试验,由压缩曲线得到应力综合结构势,并由此做出结构性参数曲线,据此考察含水量和干密度的变化对砾质花岗岩残积土结构性的影响.得出含水量与干密度的变化对砾质花岗岩残积土的结构性有显著影响的结论.     

单调、循环及冲击荷载作用下单板抗剪连接件的力学性能

给出力学模型描述钢单板抗剪连接件的结构特性,模型旨在预测连接件在单调、循环及冲击荷载作用下的性能。基于对称分量法、运动硬化理论和能量守恒定律推导出理论公式。该模型和计算式能够根据连接件的几何特性和材料特性预测其在各种荷载作用下的力学性能。根据试验和有限元分析结果验证模型的可靠性和精确性。使用该力学模型进行框架分析时效果显著。     

筋土界面变形破坏模式细观试验研究

利用自制的模型试验设备,在平潭标准砂中对玻纤网布和土工格栅进行了一系列拉拔试验,应用数字照相变形量测技术,从细观角度研究土工合成材料(片状的和格栅状的)接触界面的变形模式,得出接触面的形式和厚度,还研究了土工格栅横肋作用下土体的破坏模式。接触界面区域在传统意义上并不被认为是剪切带,本文模型试验的结果表明二者在本质上是一致的。在土工格栅拉拔试验中,土工格栅的位移逐步从前部向后部发挥,出现上下两条接触面区域,其厚度在密砂和松砂拉拔试验中相当于5倍和7.5倍平均颗粒直径;在玻纤网布拉拔试验中,只出现一条接触面,其厚度为7.5倍平均颗粒直径;随着位移的增加,格栅横肋在土中的最大剪应变集中区域呈“x”形,但并不对称。本文揭示了加筋土的宏细观力学机理,研究内容、方法和主要结论可为类似的研究提供参考,并从细观机理上为接触界面的研究提供新的认识和理解。