粗粒土卸载－再加载力学特性试验研究

进行了3种不同粗粒土的常规三轴加载－卸载－再加载的剪切试验,对粗粒土的卸载－再加载的力学特性进行了分析。结果表明：粗粒土存在卸载体缩现象,且随着应力水平的增加,卸载体缩量增加;随着围压的增加,卸载体缩量减小。根据试验数据总结了卸载体缩量与应力水平、围压之间关系式。回弹模量随应力水平的增加呈驼峰型曲线,即应力水平在0.7左右时最大,小于或大于0.7时的弹性模量均较小。平均回弹模量与单调加载时的初始弹性模量的比值随围压呈幂函数降低,母岩硬度大的土料,该比值较大,相反,母岩较软的土料,该比值较小。由单调加载得出的邓肯模型参数k、n和卸载回弹确定的kur、nur的大致关系为kur=(2.64～4.60) 、nur =(0.540～0.885) n。     

The effect of normal stress-friction angle relationship on the stability analysis of a rockfill dam

Summary Accurate stability analysis must consider the variation of the angle of friction of rockfill with the confining pressure. In reality, with increasing depth of a rockfill dam, the apparent friction angle decreases, whereas near the surface it tends to be higher. Conventional methods which employ a constant friction angle throughout the depth of a rockfill shell often yield a lower factor of safety (conservative) for shallow slip planes. On the contrary, they produce a higher factor of safety for deepseated slips subjected to increased normal (confining) stresses. This paper compares the constant friction angle approach with the variable friction angle method based on the stability analysis of a large rockfill dam, and the associated practical implications are discussed. In the latter analysis, the effect of normal stress on the friction angle of rockfill is incorporated through experimental observations.     

地铁振动荷载对下穿越盾构开挖面孔隙水压力的影响

盾构穿越饱和砂土地层中的运营地铁时,在地铁列车振动荷载的作用下开挖面前方土体中会产生超孔隙水压力,影响开挖面稳定,通过动三轴试验对不同类型砂土在地铁列车振动荷载的作用下的超孔隙水压力增长规律进行了研究。研究表明,随着地铁列车振动荷载振幅的增大,超孔隙水压力逐渐增大,当振幅不大于10 kPa时,超孔隙水压力的增加并不明显;开挖面支护力减小,会使振动荷载对超孔隙水压力的影响增强,而且这种影响对土体竖向应力卸荷比土体水平应力卸荷更为敏感;在饱和松散细砂地层中,地铁列车振动荷载对超孔隙水压力的影响最为显著;地铁列车振动荷载引起的开挖面超孔隙水压力绝对数值不大,但能使泥水盾构泥膜承受的压力差至少减小约33%。     

On implementation of bounding surface plasticity models with no overshooting effect in solving boundary value problems

This paper presents a new scheme that can be used to overcome the overshooting effect, one of the well known problems occurs during application of bounding surface plasticity models in numerical analysis of boundary value problems. The scheme is based on definition of clouds of loading surfaces with a specific margin of strains within which unloading does not accompanied kinematic hardening. The basic concept of the scheme is introduced and the methodology and the relevant step by step algorithm to implement this scheme are presented. This scheme has been incorporated in the UNSW bounding surface model and implemented in a finite difference code and used to simulate cyclic triaxial tests as well as complicated monotonic and dynamic boundary value problems. The satisfactory performance of the scheme is demonstrated and its efficiency is discussed thorough these simulations.     

三轴试验抗剪强度指标线性回归方法的讨论

利用线性回归统计抗剪强度指标是岩土工程中的常用手段。对于常规三轴试验,可以通过线性拟合p-q曲线或者拟合大小主应力的方法来求解抗剪强度参数,并给出了这两种方法的完整计算公式,同时应用矩法和线性回归方法统计了小浪底64组320个固结排水三轴试验和多个水利工程大坝填筑料的试验资料,发现对于常规三轴试验,拟合p-q曲线求解抗剪强度参数存在高估摩擦系数、低估粘聚力的问题;利用概率理论对两种方法进行了对比分析也发现了同样的趋势。因此,根据常规三轴试验统计抗剪强度参数时采用直接拟合大小主应力更为合理。     

灰土的浸水强度及残余强度的试验研究

灰土的浸水强度和残余强度在续建工程、改建工程及工程事故分析中经常遇到,故对其进行研究有着重要的工程应用价值。笔者对40余件2：8灰土（兰州黄土与石灰配制）试样进行静三轴试验,探讨其残余强度和浸水条件下强度的变化规律。试验结果发现,灰土的残余强度? R与其围压? 3呈线性关系：? R =A? 3+B。其规律类似于岩石,原因是由于残余强度阶段灰土碎裂成块,破裂面上的摩擦作用类似于岩石碎裂时的作用;灰土首次浸水后强度有所下降,但随着浸水次数的增加其强度有所回升,最后稳定于一个不高于其原始强度的值;另外还观察到,灰土不可简单地判断为一种脆断性的材料,其脆断性受围压和含水率等因素的制约,围压越大,脆性越不明显,当含水率降低到某一值时灰土表现为脆断性。     

软岩隧道围岩加固方法优化对比试验研究与分析

为解决软岩隧道工程的支护优化设计问题,研制出力学曲线与原岩相似的模型材料,形成软岩工程常用的支护设计方法对应的物理模型,即锚杆支护、锚喷支护、锚杆加长的锚喷支护,加上毛洞模型作为参考模型,4种平面模型形成三维模型后,置入大型真三轴模型试验机,在模型边界相同的条件下对软岩隧道的支护方法进行研究。试验结果表明：不同的支护方式具有不同的超载能力和不同的破坏特点;该相似模型材料对软岩大变形特点具有良好的适用性。试验还为软岩隧道支护设计提供了可靠的研究基础。     

冻结粉土三轴压缩变形破坏与能量特征分析

对不同温度和不同围压下的青藏冻结粉土进行了三轴压缩与加卸载试验,得到冻结粉土应力-应变关系曲线、抗压强度等力学参数随温度与围压变化的关系。结果表明,冻结粉土典型应力-应变曲线在低围压下大致可以分为线弹性、峰前塑性变形与峰后软化3个阶段。当? 3 < 3.0 MPa时,应力-应变曲线具有明显的峰后软化现象,随围压的增大,软化现象逐渐减弱,当? 3 达到14 MPa,应变软化现象重新变得明显;冻结粉土的强度与变形模量均随围压的增加先升后降;低围压作用下冻结粉土体积随轴向应变的增加先缩后胀,而高围压下体积变形只有体缩;低围压下冻结粉土体积塑性变形耗散能先是随着体积塑性变形增大而增大,之后由于剪胀而减少,高围压下体积塑性变形耗散能始终增加;剪切塑性变形耗散能与塑性剪应变之间近似成抛物线的关系。     

土工格室加筋土的大尺寸直剪试验研究

采用自行研制的500 mm?500 mm?400 mm（长?宽?高）大尺寸直剪仪,对土工格室加筋土以及土工格室加筋水泥稳定土的剪切性能进行了试验研究。通过大尺寸直剪试验模拟土工格室加筋土的剪切作用过程,得出加筋土剪切应力与剪切应变关系为非线性确定了土工格室加筋土的抗剪强度指标以及土工格室对土的抗剪强度增强机理,土工格室加筋土的黏聚力提高较大,内摩擦角变化相对较小。通过对素土和掺入量为5 %的水泥稳定土进行常规直剪试验、大尺寸直剪试验和三轴压缩试验对比分析,探讨不同试验方法对抗剪强度指标的影响,得出3种试验方法对应的抗剪强度指标及其相对大小;即素土的摩擦角大小依次为：三轴试验小于大尺寸直剪试验小于常规直剪试验,素土的黏聚力大小依次为：大尺寸直剪试验小于三轴试验小于常规直剪试验;水泥稳定土的摩擦角大小为：三轴试验小于大尺寸直剪试验,黏聚力结果比较大小依次为：三轴试验小于大尺寸直剪试验。     

Extended Barcelona Basic Model for unsaturated soils under cyclic loadings

The Barcelona Basic Model (BBM) is an extension of the Cam clay model that has become popular in applications involving unsaturated soils and, in particular, in simulations using the finite element method. Partially saturated soils can be loaded in different ways, for instance, mechanically and/or hydraulically. In addition, cycles of loading and unloading can be applied. The present work introduces a modification of the BBM in order to simplify its computer implementation and also to allow the simulation of elastoplastic behaviour during cycles of both mechanical and hydraulic loading. A unique smooth yield surface is introduced and a two-yield surfaces concept is applied in order to represent the cyclic behaviour. The influence of the intermediate principal stress on the strength is also accounted for. Finally, the numerical integration (stress update) of the extended BBM is briefly discussed.