钱塘江冲海积粉土剪胀性三轴试验研究

钱塘江两岸广泛分布着冲海积粉土.通过大量的三轴固结不排水CU和固结排水CD试验,探讨饱和粉土的应力应变性状和剪胀特性。试验表明,钱塘江冲海积粉土显著的特性之一是在剪切中膨胀,剪胀主要与粉土的密度和围压有关。CU试验中密(e=0.739～0.866)的原状样在围压600kPa范围内剪切膨胀,稍密(e=0.890～0.906)的击实样在围压低于400kPa剪切膨胀。由于粉土膨胀的本质不排水强度在剪切中保持增加,对于这种情况就没有惟一的"不排水强度";CD试验稍密(e=0.889～0.945)的击实样在围压低于400kPa时剪切膨胀。钱塘江冲海积粉土的剪切破坏要经历一个大的变形过程,固结排水剪切过程的应力应变性状可分为4个阶段。取应变继续而没有进一步体积变化的临界状态做破坏标准,确定粉土排水剪切强度指标较合理。     

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。     

压实粉土非线性应力-应变关系的试验研究

对京九线路基粉土用GDS三轴仪进行固结不排水剪试验,研究不同围压、含水量、压实度下压实粉土样的应力-应变关系。结果表明:压实粉土在低围压下为应变软化,高围压下为应变硬化,存在一个屈服应力;含水量和压实度对粉土强度和应力-应变关系影响显著,随着含水量的减小、压实度的增大,压实粉土样强度增大,应力-应变曲线上升。对围压小于屈服应力的应变软化曲线,用常规的土的软化关系式进行拟和;对围压大于屈服应力的应变硬化曲线,用邓肯-张模型进行拟和;这二者的结合能描述压实粉土的应力-应变关系曲线,效果良好。     

饱和粉土静态液化性能试验研究

通过固结不排水三轴压缩试验,分析了围压、固结比和干密度等因素对饱和粉土静态液化特性的影响。试验结果表明,在干密度较小时,饱和粉土的偏应力-应变曲线呈现明显的硬变软化型,随轴向应变增大超静孔隙水压力增加、有效应力减小而发生静态液化,当干密度达到1.58 g· cm-3时,饱和粉土的偏应力-应变曲线表现出硬变硬化现象,超静孔隙水压力为负值或接近0,饱和粉土不再发生静态液化,即饱和粉土存在静态液化的干密度临界值;其他条件不变,随着围压、固结比或干密度的增大,偏应力峰值和残余强度均增大,静态液化势降低;根据有效应力路径建立了流滑面以作为饱和粉土稳定区与非稳定区的分界面。     

石灰改良膨胀土的应力-应变-强度特征与本构描述

为探讨石灰改良膨胀土的变形特征与破坏机制,以压实度为95 %的荆门石灰改良膨胀土为研究对象,开展了单轴、侧限、三轴压缩应力状态下的力学性质试验。试验结果表明：即使湿化饱和后石灰土也具有较高的刚度与强度;单轴压缩状态下,无论饱和还是非饱和状态,石灰土的破坏都为典型的脆性破坏;三轴压缩状态下石灰土破坏前剪缩,同时伴随应变强化,即将破坏时开始剪胀,随后表现为应变软化;围压对石灰土的脆性破坏与破坏后的剪胀有一定的抑制作用,但即使在200 kPa围压下,试样仍发生脆性破坏。在辨明石灰土应力、变形机制的基础上,选用Duncan模型描述其脆性破坏前表现出的压硬性、剪缩性与应变强化特性,标定了相应模型参数,通过数值模拟与平行试验的对比验证了模型的适用性与参数的可靠性。     

基于状态参数的粗粒土应变软化和剪胀性模型研究

基于状态参数的概念,将粗粒土峰值应力比和剪胀比视为状态参数的函数,同时考虑初始模量随围压的变化,提出了一个弹塑性模型和模型参数的确定方法。该模型形式较简单,概念清晰,可以描述较大的密度和压力范围内粗粒土的应变软化和剪胀性,以及它们对物理状态和有效围压的依赖性。通过比较模型预测与粗粒土的三轴试验结果,证明了模型的合理性。     

堆石料应力-应变特性大型三轴试验研究

通过对水布垭筑坝堆石料进行0.2~3.5 MPa围压下的大型三轴固结排水剪(CD)试验,分析了不同围压下堆石料的偏应力-轴应变关系及体应变-轴应变关系的变化规律;引入破坏强度和体变与围压的关系,采用临界偏应力σ_(cre)和临界体应变ε_(vcre)作为归一化因子对不同围压下的应力-应变关系试验结果进行归一化处理,建立了不同围压下的应力-应变关系表达式,并将计算预测与试验结果进行对比验证。研究表明:对于颗粒级配确定的密实堆石料,初始变形期存在一定的弹性增长阶段,且该直线段的斜率受围压的影响较大;低围压时表现为应变软化,先剪缩后剪胀为主的力学特性,且围压越低,剪胀效应越明显;高围压(0.8 MPa≤σ_3≤3.0 MPa)时表现为应变硬化、剪缩为主的特性;剪胀消失的临界围压处于0.45~0.8 MPa之间;超高围压(σ_33.0 MPa)条件下,虽表现为应变软化、剪缩的特性,但其应变软化的机制与低围压下不同;分析认为,超高围压下剪切过程中颗粒的相对破碎率达15%以上,颗粒破碎引起的堆石料强度降低大于围压的增大对堆石料强度的提高。峰值主应力比随围压的增大而降低,且随轴应变的增大而趋于一致。堆石料的破坏强度与围压呈线性相关。最后,通过计算预测结果证明了该应力-应变关系表达式的正确性。     

基坑土体侧向卸荷真三轴试验研究

对武汉地区具有代表性的粉质粘土,用基坑开挖过程中坑周土体的应力路径在真三轴上进行模拟试验,得到平面应变条件下固结不排水卸荷试验结果,将其整理成应力-应变关系曲线、孔隙水压力-竖向应变关系曲线、中主应力系数-竖向应变关系曲线和卸荷试验应力路径图。试验结果表明,竖向( )压力不变,侧向( )卸荷时,其应力-应变关系随固结压力的增加,由应变硬化型向应变软化型转化;固结压力较低时,表现为剪胀,固结压力较高时,表现为先剪缩,后剪胀;当中主应力( )始终大于但接近于侧向压力( ),按所定义的中主应力系数bd值较稳定;有效应力路径随固结压力增大产生偏转。这些结论为进一步研究粘性土中主应力的作用奠定了基础。     

不排水条件下砾石土的应变率效应

为研究应变率对砾石土应力、应变、强度特性的影响和应变率效应产生机制,对砾石和黏土混合备制的饱和砾石土心墙料,在应变率0.028～0.690 %/min范围内进行了不同围压下的固结不排水三轴压缩试验,定量地分析了三轴不排水抗剪强度的应变率效应。试验结果表明,不同的应变率下砾石土应力-应变曲线具有相似性;应变率增大10倍,不排水抗剪强度增大7.4%～29.9%;砾石土的应变率效应是剪切产生的孔隙水压力和土料本身黏滞性共同作用的结果,在不同的围压下二者发挥的程度不同     

高应力卸荷条件下砂岩扩容特征及其剪胀角函数

为研究高应力水平条件下岩石卸荷扩容特性及其剪胀角变化规律,开展了若干不同初始围压水平的三轴峰前卸围压试验,同时进行了相应围压水平的常规三轴压缩试验。基于试验成果分析了卸荷应力路径对砂岩扩容的影响效应,总结了卸荷应力路径下剪胀角变化规律,进而提出了基于卸荷应力路径的剪胀角函数,并给出了其数值实现方法,最后通过对三轴峰前卸围压试验的数值模拟,验证了所提出的剪涨角函数的可行性与合理性。研究表明：（1）加载路径时,不同围压水平下峰前扩容体积应变值相差不大,而卸荷应力路径时,随着围压水平的增加,峰前扩容体积应变从最小值3.15×10?3增加到最大值9.65×10?3,卸荷路径下的峰前扩容体积应变约为加载路径的1.1～4.0倍;（2）卸荷应力路径下偏应力峰值对应的体积应变基本为0或接近于0,而加载应力路径下偏应力峰值对应的体积应变均为负值;（3）卸荷应力路径条件下峰前扩容体积应变在总扩容体积应变中所占比例显著大于加载路径;（4）两种应力路径下,剪胀角随塑性剪切应变增加均经历了先增加后减小的过程,并且低围压工况下偏应力峰值对应的剪胀角和最大剪胀角均大于高围压工况;（5）与加载路径相比,卸荷路径下的剪胀角更快达到剪胀角最大值,并且其偏应力峰值对应的剪胀角和最大剪胀角更大;（6）基于砂岩三轴卸荷试验成果,采用线性拟合方法得出了以围压和峰后塑性剪切应变增量为自变量的剪胀角函数,基于该函数与应变软化本构模型的砂岩三轴峰前卸围压试验的数值模拟结果与试验结果吻合较好,表明该函数能较好地描述三轴峰前卸围压条件下砂岩的扩容特性。以上结论可为深部地下工程变形预测、稳定性分析与支护设计提供理论基础。     

粉土及其稳定土的三轴试验研究

以粉土和4 %石灰、2 %水泥+2 %石灰、4 %SEU-2型固化剂、8 %SEU-2型固化剂处理的粉土为研究对象,通过不固结不排水三轴剪切试验(UU)和固结不排水三轴剪切试验(CU)对变掺量、变龄期条件下粉土及稳定土的强度和变形特性进行了研究。试验结果表明：粉土及其稳定土的应力-应变曲线主要为软化型。SEU-2型固化剂在改善粉土的凝聚力方面起了很好的作用,综合考虑了不同稳定方法的强度指标,表明掺SEU-2型固化剂是稳定粉土的最有效的方法。     

Holocene Saharan dust deposition on the Cape Verde Islands: sedimentological and Nd-Sr isotopic evidence

Holocene aeolian silts deposited on the Cape Verde Islands provide information about the origin of African palaeodusts that have fallen on the north-eastern Atlantic ocean over the last 10 000 years. Sedimentological composition indicates that most of these aeolian silts are unquestionably of continental origin. Their Sr and Nd isotopic composition identifies a Saharan origin-suggesting transport by Harmattan winds. We estimate that Saharan dust comprises 75–95% of material in these Holocene silts, the rest coming from the weathering of local basaltic bedrock.     

On the use of effective stress in three-dimensional hydro-mechanical coupled model

In the last decades, a number of hydro-mechanical elastoplastic constitutive models for unsaturated soils have been proposed. Those models couple the hydraulic and mechanical behaviour of unsaturated soils, and take into account the effects of the degree of saturation on the stress–strain behaviour and the effects of deformation on the soil–water characteristic response with a simple reversible part for the hysteresis. In addition, the influence of the suction on the stress–strain behaviour is considered. However, until now, few models predict the stress–strain and soil–water characteristic responses of unsaturated soils in a fully three-dimensional Finite Element code. This paper presents the predictions of an unsaturated soil model in a Three-dimensional Framework, and develops a study on the effect of partial saturation on the stability of shallow foundation resting on unsaturated silty soil. Qualitative predictions of the constitutive model show that incorporating a special formulation for the effective stress into an elastoplastic coupled hydro-mechanical model opens a full range of possibilities in modelling unsaturated soil behaviour.     

土-水特征曲线预测非饱和土的抗剪强度对比研究

目前,非饱和土的工程性质已成为岩土工程界研究的热点问题,而非饱和土的抗剪强度的研究对土的工程性质尤为重要。笔者总结了4种用土-水特征曲线预测非饱和土的抗剪强度的公式,用5个Bar的压力板仪测得了马家沟Ⅰ号滑坡体上土体的土-水特征曲线,并采用MATLAB软件,利用Van Genuchten模型对试验数据进行拟合;用国际先进的GDS非饱和反压剪切仪对控制基质吸力为50kPa和100kPa的非饱和土进行直剪试验,得到了相应的抗剪强度。用土-水特征曲线对非饱和土抗剪强度的4种预测值与试验结果进行对比分析,验证各预测公式的准确性,总结其优缺点。最后得出Fredlund预测公式的准确性最高,黄润秋预测公式最为简单易行。     

Water characteristic curve of soil with bimodal grain-size distribution

Soil–water characteristic curve (SWCC) is the most fundamental and important soil property in unsaturated soil mechanics. It has been used for analyzing slope stability due to the infiltration of rainfall into slopes and water flow in unsaturated embankments. Generally, SWCC is obtained by laboratory tests. However high cost, long duration and difficulty of the tests impede the application of unsaturated soil mechanics to practical design or analysis. Therefore, several equations have been developed to predict the SWCC using grain-size distribution (GSD) curve. However, most of the equations were limited to soils with unimodal characteristics and the parameters of the equations are not related to the physical properties of the soil. In this paper, an equation to predict SWCC for soils with bimodal characteristics is proposed. The parameters of the proposed equation are related to the physical properties of soil and the variables of SWCC closely. The proposed equation is evaluated with data from the literature and laboratory tests carried out in this study. In addition, the computer codes for the computation of the predicted bimodal SWCC are presented.     

Two hundred million years of karst history, Dachstein Limestone, Hungary

Platform carbonates of the Upper Triassic Dachstein Limestone in Naszály Hill have been karstified extensively over the past 200 million years. They provide an excellent example of polyphase karstic diagenesis that is probably typical of many subaerially exposed carbonate sequences. Seven karstic phases are recognized in the area, each of which include polyphase karstic events. The first karst phase was associated with the Löfer cycles. Meteoric waters caused dissolution; enlarged fractures and cavities, were filled by marine and/or vadose silts and cement. The second karst phase was caused by local tectonic movements. Bedding-plane-controlled phreatic caves were formed, and filled by silts. The third karst phase lasted from the end of the Triassic to the Eocene. This was a regional, multiphase karstic event related to younger composite unconformities. Bauxitic fill is the most characteristic product of this phase. The karst terrain reached its mature or senile stage with very little porosity. Narrow veins and floating rafts of white calcite marks karst phase 4, which resulted from hydrothermal activity associated with Palaeogene magmatism. The early Rupelian phase of Alpine uplift caused large-scale rejuvenation of the former karst terrain (karst phase 5). Subsequently Naszály Hill was buried as an area of juvenile karst with significant porosity. A second period of hydrothermal activity in the area (karst phase 6) was induced by Miocene volcanism, which resulted in wide, pale green calcite veins. Finally karst phase 7 was of tectonic origin. Following the most recent, Miocene uplift of the Naszály Hill, the carbonates have again become the site of vadose karst development.     

鄂尔多斯盆地古隆起东北侧马五41储层膏模孔类型及充填过程分析

利用岩芯和薄片资料,通过薄片孔隙描述、铸体薄片图像分析等方法,在对研究区马五₄1储层孔隙类型划分的基础上,详细划分了膏模孔充填类型,并对其充填过程进行了阐述。研究结果表明,研究区马五₄1储层主要发育溶蚀孔（膏模孔和其他溶孔）、晶间孔、晶间溶孔、微裂缝4种类型的孔隙,且以膏模孔为主。通过镜下薄片对膏模孔内不同充填物观察发现,充填物主要为白云石、方解石、硬石膏,其中半充填的膏模孔中主要充填物为白云岩粉砂、白云岩粉砂+石英、白云岩粉砂+铁白云石3种类型,而全充填的膏模孔中主要充填物为白云岩粉砂+方解石、白云岩粉砂+石英+方解石、白云岩粉砂+硬石膏3种类型。综合分析认为膏模孔是含膏云岩中石膏等易溶晶体溶蚀后,晶体轮廓保留而形成的,首先裸露风化壳期先充填白云岩粉砂和深灰色方解石,其次深埋藏期充填自生石英、白色亮晶方解石、铁白云石及硬石膏。     

Experimental evaluation of shear strength behaviour of plastic and non-plastic silts

Silt is available in many parts of the world in combination with sands and clays. However, due to lack of clear understanding of its engineering behaviour, most of the time it is interpreted in terms of either sands or clays. Structures that are usually built on silty soils are designed to take into account design procedures developed for sandy or clayey soils. Presence of silts in combination with varying amount of sand and clays produces silt that is either plastic or non-plastic in nature. Silt is available in and around the Delhi region, in a majority mixture along with fine sands, which is non-plastic in nature. On the other hand silty deposits found in offshore Bombay High region are found in abundance along with significant amounts of clays and are termed as plastic silts. In this paper a comparison of the stress-strain behaviour of plastic and non-plastic silts is carried out under triaxial compression loading during both drained and undrained conditions. Two representative samples each from Delhi and Bombay High regions were considered for this comparison and results of stress-strain under four sets of confining pressure are discussed in detail. It is observed from this study that behaviour of silts is mainly dependent on the composition and structure of the resultant soil matrix. It is concluded from the results that shear strength parameters as well as volume change/pore pressure response of silty soils is dominated by the constituent soil present along with the silt. It is seen from the comparative behaviour of non-plastic and plastic silts that the presence of sand and clays has a governing effect on pore pressure development and the resultant friction angle. The study also corroborated that the nature of silt is transitional both in the case of plastic and non-plastic forms.     

盐溶液法测定非饱和膨润土的水分特征曲线

对于高放废物的最终处置，深地质处置被国内外公认为最合适的方法，而膨润土也被认为是高放废物处置库缓冲回填材料的良好基质。鉴于膨润土各项性质研究的重要性。本文介绍了利用盐溶液法进行非饱和膨润土水分特征曲线的测定工作，发现盐溶液法适合测定吸力极高的非饱和土，是一种值得推广的方法。