海底粉土动力特性的试验研究

为了对杭州湾海底粉质土在波浪荷载作用下的动力特性进行研究,选用杭州湾地区分布较广的粉土,通过一系列模拟不同振动频率的波浪荷载的动三轴试验,探讨了振动频率的变化对粉土动应变、动孔压和动强度的影响。试验结果表明:波浪荷载作用下振动频率的大小对土体的累积轴向应变和动强度有较大影响;振动频率的变化对动孔压的发展趋势的影响不明显,同时围压对动孔压比的影响很小,动孔压比与累积轴向应变的关系曲线具有良好的归一性。     

动荷载作用下杭州粉土动强度试验研究

通过对杭州粉土进行固结不排水动三轴试验,研究围压和固结比在动荷载作用下对杭州粉土动强度的影响,总结了不同固结比、不同围压的条件下,杭州粉土动强度的变化规律。     

粉土液化特性的试验研究

利用TYD-20动三轴实验机对杭州湾附近的近海粉土，模拟不同的波浪荷载作用下相同固结比的不排水动三轴的动强度和液化试验，对所得的试验结果进行比较，研究频率改变对液化特性的影响，从而总结不同的振动频率和不同的振幅对粉土液化强度的影响程度，为判断粉土液化的可能性和附加变形提供科学有效的分析方法。     

低围压下埕北海域重塑粉土动强度特性

为了探讨低围压下粉土的动强度特性,利用低围压条件下固结不排水振动三轴试验,对埕北海域3种不同黏粒质量分数的重塑粉土进行研究。结果表明:低围压条件下埕北海域重塑粉土的破坏准则采用以往的破坏准则不合适,将应变达到2%～3%作为低围压条件下粉土的破坏准则;低围压条件下粉土动应力曲线具有幂函数曲线特征;粉土动强度受固结比的影响,动强度的最大值出现在固结比约为1.5;低围压条件下埕北海域重塑粉土内摩擦角介于8.6°～15.26°,内聚力介于1.20～4.86 kPa,临界循环应力比为1.45。     

高速列车荷载作用的动三轴试验模拟

动三轴试验是目前最常见的获取高速列车荷载作用下土体变形、强度特性的室内试验手段。为探讨模拟高速列车荷载时加荷模式、排水条件、加荷次数等因素对试验结果的影响,利用GDS动三轴仪,对高速铁路沿线典型黏性土进行高振次循环动三轴试验,研究不同试验条件下试样变形、孔压发展规律和临界循环应力比的变化。研究建议可采用半正弦波在排水条件下进行动力试验模拟高速列车荷载,获得地基土体的最大可能变形和临界动应力比,从而简化试验过程,大大缩短试验时间。     

黄河三角洲粉质土初始干密度和黏粒含量对稳态强度的影响研究

通过固结不排水三轴试验,对黄河三角洲粉质土稳态强度进行研究:首先设计了3组干密度分别是1.55,1.65,1.75g/cm3的粉土样,分析了初始干密度对稳态线的影响,论证了稳态线的唯一性;然后又设计了5组不同黏粒含量分别是10%,14%,17%,24%,30%的粉质土,定量分析了黄河三角洲粉质土中的黏粒含量对稳态强度线的影响,得出了黏粒含量为17%的粉质土的稳态强度最大的结论,并分析了产生这种现象的原因,最终建立了稳态强度随黏粒含量变化的关系函数。     

中国尾矿坝地震安全度（9）──大石河尾矿粘性土的动力变形和强度特征

本文采用动三轴仪对大石河尾矿粘性土的动力不排水特性进行了试验研究。研究分析中，考虑了不同应力状态下大石河尾矿粘性土的动力变形、动孔隙水压力的发展和强度的变化，并且对其动强度应力修正系数Ｋα和Ｋσ进行了探讨。本文还系统地研究了尾矿粘性土振后应力－应变关系和残余强度特征。     

尾矿料的动强度特性试验研究

通过对某尾矿坝料进行固结不排水动力三轴试验。对两种尾矿料在不同围压和固结比应力条件下之动强度、孔隙水压力及轴向应变的变化进行了研究。试验结果表明,在循环振动荷载下,对同一固结比,动剪应力随围压的增大而增大,同一围压下,动剪应力随固结比的增大而增大。孔隙水压力的增长主要发生在振动前期,固结比对孔隙水压力的增长影响明显,同时试验结果可用双曲线较好地进行拟合。在低固结比条件下,轴向应变1%附近存在应变突增点。试验研究成果为该坝的动力稳定性分析提供了参数,也为后续尾矿料的动力特性研究提供参考。     

冻融循环后路基粉质黏土动力特性研究

冻融循环作用对路基土的动力特性有着较大的影响。本文以张家口地区路基粉质黏土为例,通过进行冻融循环作用下的室内动三轴试验,得到了路基土动强度、动内摩擦角、动粘聚力等参数随冻融循环次数的变化规律,探究了冻融循环作用对路基土动力特性的影响。试验结果表明,在冻融循环条件下,土体的动强度、动粘聚力呈现明显下降趋势,经历7次冻融循环后,其值趋于稳定;动内摩擦角在某一范围内波动,虽具有一定的下降趋势,但规律性并不明显。     

洞庭湖砂纹淤泥质土动强度试验研究

砂纹淤泥质土是夹有微薄层粉细砂洞庭湖湖相沉积软土,具有独特薄砂层构造。为了研究砂纹淤泥质土动强度特性,通过确定洞庭湖砂纹淤泥质土动强度的破坏标准,以DDS-70动三轴试验系统为试验手段,探讨了围压、振动频率对洞庭湖砂纹淤泥质土动强度特性的影响。研究表明:(1)动粘聚力和动内摩擦角随循环次数的增加总体上呈下降趋势;(2)循环次数较小时动强度下降较快,而当循环周次超过一定次数时,动强度进入缓慢衰减阶段;(3)相同的循环次数下频率和围压越高,动粘聚力和动内摩擦角越大。     

分阶段循环加载条件下温州饱和软黏土孔压和应变发展规律

土体在经过一定次数的循环荷载作用后,其动力特性会发生改变,对后续振动的响应会变得大不相同。通过对温州原状饱和软黏土分别在不排水和排水条件下进行连续循环加载和5个阶段的分阶段循环加载试验,分析了循环荷载作用及一定时间的停振对土体动力特性的影响。试验结果表明,一定时间的振动和停振后,由于土体动力特性发生了改变,后续阶段的残余应变将变小;不排水条件下,停振期孔压稳定在循环加载产生的残余孔压值附近,而振动期产生的变形一部分得到恢复;排水条件下,在振动期孔压呈现出先增大后减小的趋势,在停振期残余应变基本保持不变。这些结论对于深入分析交通荷载下土体动力响应和沉降发展具有一定的指导意义。     

软土循环扭剪应力应变响应的弹塑性分析

为了分析笔者提出的增量弹塑性模型对描述一般应力状态土单元循环应力应变响应的适用性,进行了软土不排水循环三轴压缩试验与无初始轴向偏应力的循环扭剪试验,得到了两种应力状态土单元的循环应力应变响应。进而依据循环三轴压缩试验结果确定模型参数,再利用模型预测软黏土循环扭剪试验土单元的应力应变响应,并与试验结果进行对比分析。结果表明,预测出的无轴向静偏应力作用的循环扭剪土单元环向剪应力应变滞回曲线与试验结果相比偏小,但是总变化趋势与试验结果较为一致;预测出的循环扭剪土单元环向累积剪应变随循环次数的变化与试验结果基本一致。研究工作表明,该增量弹塑性模型能够较好描述不同应力状态土单元的不排水循环应力应变响应,特别是循环累积应变响应。     

初始静孔隙水压力对砂土静动力剪切特性影响的试验研究

选取福建标准砂和滹沱河细砂,利用空心圆柱扭剪仪开展了一系列不同初始静孔隙水压力条件下的不排水循环扭剪试验和单调扭剪试验,着重探讨初始静孔隙水压力对超静孔隙水压力发展及其不排水抗剪强度的影响。试验结果表明:初始静孔隙水压力对超静孔隙水压力的发展产生显著的影响,从而影响砂土的静动力剪切特性。具体地,在不排水循环剪切过程中,初始静孔隙水压力越大,其超静孔隙水压力发展和变形发展越快;在不排水单调剪切过程中,初始静孔隙水压力越大,在砂土剪胀阶段产生负超静孔隙水压力越大,从而使砂土的强度显著提高。基于试验结果,初步探讨了初始静孔隙水压力对超静孔隙水压力及静动力剪切特性的影响机理。研究表明,研究地下水位以下土体(准饱和土)静动力剪切特性尤其是研究液化问题时,应充分考虑初始静孔隙水压力对砂土抗液化强度的影响,室内试验应根据砂土所处的地下水位深度来决定初始静孔隙水压力(反压)的大小。     

Effects of System Compliance on Liquefaction Behavior of Thin Sandy Layer in Undrained Cyclic Triaxial Tests

In a case history of dip slope failure caused by the 2004 Niigata-ken Chuetsu Earthquake, it has been reported that the slope contained a thin sandy soil layer. The layer may have liquefied if it had been saturated at the time of the earthquake. Undrained cyclic triaxial tests comprise one of the available experimental approaches for evaluating the liquefaction behavior of such thin sandy soil layers. In this study, a series of undrained cyclic triaxial tests was performed on specimens containing an artificial sand layer in order to investigate the effects of system compliance that are possibly induced by partial drainage caused by the changes in volume of the filter paper used to saturate the sand layer and the local drainage at the interface among the specimen, the top cap and pedestal, and the rubber membrane. The observed liquefaction behavior depended on the thickness of the sand layer under otherwise similar conditions, suggesting the significant effect of system compliance. By conducting special tests, while correcting the partial drainage, it was possible to reduce its effect on the liquefaction behavior. Cylindrical specimens with a top cap and pedestal, all having the same diameter, exhibited a smaller effect from partial drainage than prismatic specimens with an oversized top cap and pedestal.     

排水条件对不同固结度软黏土动力特性影响试验研究

地铁隧道下卧土层由于施工扰动通常存在不同固结度,列车荷载下土体应变规律也有所差异,进而产生不均匀沉降。通过室内动三轴试验系统对杭州饱和软黏土进行动力测试,研究了排水条件、固结度、振动次数对软黏土动孔压和应变的影响,在试验和现有研究成果的基础上建立了考虑初始固结度、振动次数、排水条件的应变软化模型。结果表明:固结度愈高的土体,孔压发展愈缓慢,并且随振动次数增加,在较低的孔压水平就可达到稳定,排水条件下的孔压发展规律可由不排水试验获取。土体应变随循环次数的增加而增加,随固结度的增加而降低;提出的应变模型考虑了荷载循环过程中土体的排水,可以较好地模拟不同固结度下饱和软黏土的应变发展规律。     

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.     

波浪循环荷载作用下盾构穿越海堤过程中下卧软土的弱化响应研究#br#

 During the sub-sea tunnelling through a seawall, the soil adjacent to the tunnel may simultaneously bear tunneling-induced stress relief and wave-induced cyclic shearing. To investigate the coupling effects of stress relief and repeated loading on the mechanical behaviors of soft clay, a series of experiments were carried out using a hollow cylinder apparatus, which imposed undrained cyclic shearing to the soil samples under a reducing confining stress, followed by undrained shearing. The cyclic stresses simulated in the tests were estimated using elastic solution for a seabed subjected to the wave loadings. The test results show that the variation of each stress component under the coupled loading follows a sine or cosine relationship. The coupled loading plays a more significant role in affecting the radial strain (εr) and hoop strain (εθ) than the vertical strain (εr) and the torsional shear strain (εzθ) of the soft clay. The shear modulus of the soil experiences more degradation under the coupled loading than that under the single action of cyclic shearing. The pore pressure of the soil reduces under the disturbance of shield construction and begins to increase slowly under the wave loading after construction. At a given cyclic magnitude, the reduction of soil strength significantly depends on the magnitude of stress relief. A larger stress relief leads to a more reduction in the post-cyclic shear strength of the soil. Based on the experimental findings presented above, it is suggested that coupling effect between the cyclic shearing and the stress relief should be taken into account while designing sub-sea tunneling through a seawall.     

Cyclic resistance of two unsaturated silty sands against soil liquefaction

The changes of the cyclic resistance of two silty sands under unsaturated, partially saturated and fully saturated conditions are examined based on a series of undrained cyclic tests conducted using triaxial test apparatus specially equipped for testing unsaturated soils as well as ordinary triaxial test apparatus for testing partially saturated and fully saturated soils. Based on the observations of volumetric strain, pore air and pore water pressures of unsaturated soil specimens, the possibility of soil liquefaction triggering under different degrees of saturation is examined and discussed. The changes in the cyclic resistance under different degrees of saturation are then examined. Those two unsaturated silty sands with different grain size compositions are found to give rise to different responses on the volumetric strain as well as pore air and pore water pressure developments during undrained cyclic loading, leading to different relations between cyclic resistance and degree of saturation, covering unsaturated, partially saturated and fully saturated conditions.     

饱和软粘土的循环蠕变特性

本文介绍对上海淤泥质饱和粘土在长期循环荷载作用下的变形特性的试验研究结果。考虑到土的各向异性性质,在三轴试验中,在K0固结条件下把土试样恢复到天然的K0应力状态,然后进行排水及不排水单向循环加载蠕变试验。从试验结果可以观察到一些重要的现象:(1)当循环轴向应力小于初始固结压力的50%时,饱和粘土的循环蠕变可分成三个阶段;(2)循环应变可分成不可逆的累积应变与可逆应变两部分,可逆应变的大小与循环应力幅值近似成线性关系;(3)孔压增长较为滞后,不排水试样孔压增长稳定时其值约为应力幅值50%,而排水试样的残余孔压约为应力幅值的20%。     

循环围压对超固结黏土变形特性影响试验研究

 利用GDS动三轴系统对温州地区典型饱和软黏土进行一系列恒定围压和变围压不排水循环加载试验,重点分析循环偏应力和循环围压耦合作用对超固结饱和软黏土不排水循环累积变形特性的影响。试验结果表明：不排水条件下,与围压保持恒定,偏应力单独循环剪切相比,循环围压的存在,一定程度上限制了超固结饱和软黏土永久轴向应变的发展。在平均主应力–广义偏应力(p-q)空间,循环偏应力和循环围压耦合应力路径斜率p/q每增加1,永久轴向应变将降低约20.18%。进一步在试验基础上提出考虑超固结比和循环围压影响的永久轴向应变经验公式。