基于有效固结应力法确定结构性黏性土不排水抗剪强度

由于结构性的存在使得很多天然黏性土的强度和变形特性不同于重塑土和非结构性土。首先回顾了适用于确定重塑土和非结构性土不排水抗剪强度的有效固结应力法,并推导了相应的方程。在此基础上,对于结构性黏性土,采用两段不同斜率（内摩擦角正切）和截距（黏聚力）的直线模拟其抗剪强度包络线,建立了确定其不排水抗剪强度的有效固结应力法方程;当有效应力小于结构屈服应力时,有效固结应力方程中有效应力部分需乘以0.8的修正系数;只要已知剪切前的有效应力,利用相应的有效固结应力法公式,可确定结构性土体的不排水抗剪强度。利用连云港结构性软黏土的等压固结三轴试验数据,验证了有效固结应力法的适用性。分析表明：对于连云港软黏土的不排水抗剪强度,有效固结应力法的计算结果与试验结果吻合较好;对于结构屈服应力,有效固结应力法的预测结果与试验数据点的拟合结果有一定偏差,但并不明显。     

连云港软黏土的不排水强度试验研究

对连云港软黏土不扰动样和重塑样进行三轴CU对比试验研究。结果表明,不扰动样的不排水强度包线是由以屈服压力为界的双折线组成,而重塑样的不排水强度包线可以用通过圆点的直线表示。屈服前阶段,不扰动样由于受结构性影响产生的抵抗作用,其不排水强度包线高于重塑样;屈服后阶段,不扰动样的不排水强度包线介于具有不同初始含水率的重塑样的包线之间。通过对比分析,发现不扰动样与重塑样的不排水强度包线存在差异的主要原因是由于试样的不同初始含水率引起的。不扰动样在屈服后阶段的含水率变化规律与不排水强度增长规律与重塑土相一致,采用Burland提出的孔隙指数Iv,可将不同初始含水率重塑样及屈服后阶段不扰动样的压缩性状较好地归一化。     

含水率对重塑淤泥不排水强度性质的影响

通过室内调配不同含水率的重塑淤泥,利用研制的室内微型高精度十字板剪切仪,研究了含水率对重塑淤泥重塑不排水强度的影响。该仪器的剪切强度分辨率为1 Pa,经试验验证,仪器测量结果的稳定性较好,但对强度越低的重塑淤泥试样其测量值的相对平均偏差越大。淤泥含水率对淤泥的不排水强度有显著影响,不排水强度均随含水率/液限的增大而减小,含水率/液限越大,不排水强度下降趋势越小。相同的含水率/液限时,不同种类的重塑淤泥的不排水强度比较接近。在双对数坐标中,不排水强度均随含水率/液限的增大而线性减小。由试验数据得出了不同种类重塑淤泥的不排水强度与含水率/液限之间的关系式,利用该关系式可以求得每一种重塑淤泥任意含水率下的不排水强度。文中的试验值大于洪振舜提出的关系式的计算值。当IL >2时,试验值与Locat和Leroueil提出的关系式的计算值基本吻合,当IL <2时,试验值大于Locat和Leroueil提出的关系式的计算值。     

强结构性天然沉积土的强度变形特性

广泛分布在日本大分县的天然硅藻土是一种令人惊奇的具有正常固结历史的天然沉积结构土,其含水量如豆腐,却硬如石头。由于具有强结构性,其土样不会受到取样及成形过程的扰动影响,能够在室内对其天然状态进行力学性状研究,是一种难得的天然沉积结构性土。采用最大压力达10 MPa的高压三轴仪对天然沉积硅藻土进行试验,以探讨强结构性天然沉积土的力学性状。结果表明,其固结屈服压力达到上覆压力的57倍以上,固结屈服前的压缩曲线是水平的,固结压力达到固结屈服压力时,压缩性突增。强度试验结果显示,处于固结屈服前状态的强度与应力水平无关,而处于固结屈服后状态时其强度包络线与重塑粘土同样为通过原点的直线。     

原状和重塑海洋粘土经历动载后的静强度衰减

根据海洋粉质粘土原状和重塑土样的动三轴试验结果, 比较和分析了两种土样应力、应变、孔隙水压力和不排水抗剪强度行为, 得到了土样的静不排水抗剪强度衰减与波浪荷载作用下土样产生的动应变以及平均累积孔压之间的关系。 并将波浪荷载作用使土样内孔压升高, 有效应力降低, 形成拟超固结现象的理论, 应用到土样不排水抗剪强度衰减与平均累积孔压之间关系的分析;同时与重塑土样的超固结静态剪切试验结果进行比较, 得到了土样在波浪荷载作用后的归一化不排水抗剪强度与拟超固结比之间的关系式。 建议以少量原状土样, 配合大量重塑土样的动三轴试验结果, 实现对实际海洋粘土地基在波浪荷载作用后的静不排水抗剪强度衰化规律的评估。     

两种粉质黏土的动、静强度特性对比研究

以京津高铁和杭州地铁沿线的两种典型粉质黏土为研究对象,并采用英国GDS多功能三轴仪完成了静、动力强度测试,研究了两种土的静力不排水抗剪强度和在交通荷载高振次循环下的动强度、临界动应力比、应变发展模式以及振后抗剪强度等方面的差异。对比发现,长期循环荷载作用下两种土的应变发展和振后强度均有很大不同。与京津地区粉质黏土相比,杭州地铁沿线土含水率高、孔隙比大、密度低、灵敏度高、强度低。两种原状土在小幅振动后土体强度略有增大,而随振幅增大,振后强度将低于未经历振动时的不排水强度值;而重塑土振后强度均低于静剪强度。     

Observed Behaviour of Laterally Expanded Stone Column in Soft Soil

The paper describes the behavior of remolded kaolin clay reinforced by stone column as investigated in laboratory. The installation of stone column was simulated by performing lateral expansion at different rates within hollow cylindrical remolded kaolin specimens initially subjected to K0 consolidation path. These specimens were, then, subjected to classical consolidated undrained triaxial tests while recording excess pore pressure values. The objective of the experimental programme was to quantify the effects of consolidation stress and stone column on the undrained Young’s modulus and shear strength of kaolin clay. Obtained results showed a significant improvement in Young’s modulus when the cavity expansion ratio and the consolidation stress increase. It was also found that the increase in undrained shear strength of improved kaolin clay mainly occurred at lower consolidation stress. Another important finding of this study is that the ratio of undrained Young’s modulus to undrained shear stress increases when the consolidation stress decreases. Finally, the paper presents a model developed for the design of stone columns.     

饱和重塑黄土抗剪强度影响因素的试验研究

强降雨和灌溉导致黄土强度劣化并发生饱和破坏现象,进而诱发黄土滑坡,给当地带来严重的灾难。饱和黄土抗剪强度的影响因素较多,本文根据水头饱和与反压饱和相结合的原理,利用改装后的TFB-1型非饱和土应力-应变控制式三轴仪对党川地区马兰黄土重塑土样进行饱和试验和CU试验。分别采用50 kPa、100 kPa、200 kPa、400 kPa围压对饱和重塑黄土进行剪切速率分别为0.02 mm·min-1、0.06 mm·min-1、0.2 mm·min-1、0.4 mm·min-1的CU试验,探讨了围压和剪切速率对饱和重塑黄土抗剪强度的影响。研究发现:同一围压下,饱和重塑黄土的抗剪强度随着剪切速率的增大呈先增大后减小的趋势;同一剪切速率下,饱和重塑黄土的抗剪强度随着围压的增大逐渐增大;在干密度相同的条件下,饱和重塑黄土的总黏聚力及有效黏聚力随着剪切速率的增大先减小后增大,总内摩擦角及有效内摩擦角随着剪切速率的增大先增大后减小。该研究可为黄土滑坡预报和范围预测提供一定依据。     

不同应力路径下饱和重塑黄土的力学特性

土体的力学特性往往因应力状态和应力路径而异。为了探讨垂直加载和等剪路径下饱和土的力学特性,制备饱和重塑黄土试样,通过固结不排水（CU）和常剪应力排水剪（CSD）三轴试验,分别测定并绘制其应力-应变曲线、孔隙水压力变化曲线和应力路径曲线。试验结果表明,饱和重塑黄土在2种路径下有明显不同的变形特点:CU路径下的应力-应变曲线皆呈弱软化型,孔隙水压力先快速上升后逐渐趋于稳定;CSD路径下维持偏应力为一常量,施加孔隙水压力后的很长时间内试样变形很小,当孔隙水压力增大至试验围压的60%~75%时,试样迅速破坏。CSD路径无偏应力峰值,文中根据轴应变随平均有效应力变化曲线定义了等效峰值破坏线。通过对比发现,2种路径下饱和重塑黄土的有效峰值强度指标差异明显,而有效残余强度指标相近,表明有效残余强度指标是重塑黄土内在属性,受应力路径的影响不大。该研究结果可为实际工程选取正确的应力路径试验提供参考。     

Static Liquefaction Analysis Considering Principal Stress Directions and Anisotropy

Static liquefaction failure of a sloping ground occurs when the shear stress applied by a monotonic triggering load exceeds the undrained yield (peak) shear strength of the saturated liquefiable cohesionless soil. Current practices for determining the in-situ undrained yield strength for ground subject to static shear stress either rely on a suite of costly laboratory tests on undisturbed field samples or empirical correlations based on in-situ penetration tests which do not account for the effects of anisotropic consolidation, intermediate principal stress, and mode of shear on the degree of strain-softening and brittleness of cohesionless soils. This study investigates the effects of variations in the direction and relative magnitudes of principal stresses associated with different modes of shear and ground slopes on static liquefaction failure of cohesionless soils. Empirical relationships are developed between soil brittleness index and maximum excess pore water pressure ratio to characterize soil shearing behavior observed in a database of 271 undrained laboratory shear tests collected from the past literature. The application of these relationships for estimating the static liquefaction triggering strength of cohesionless soils under sloping grounds is described for plane-strain boundary conditions and the results are compared with those back-calculated for several cases of static liquefaction flow failures. The proposed procedure incorporates variations in mode of shear and initial stress anisotropy in an empirical formulation based on in-situ penetration tests.     

考虑应力历史的饱和土抗剪强度测试方法探讨

通过直剪试验和三轴试验,研究了超固结状态和正常固结状态土强度指标的差异性。抗剪强度指标在大于和小于前期固结压力的压力段明显不同,试验资料整理时应分别确定抗剪强度指标。剪切前,试样在其自重固结压力（重塑土为预固结压力）下作预处理,比较了在垂直压力或围压小于前期固结压力和大于前期固结压力两个压力段的强度指标,前者不固结不排水剪黏聚力c小于后者,内摩擦角φ大于后者;固结不排水剪黏聚力c大于后者,内摩擦角φ小于后者。重塑土强度试验模拟的应力历史很难真实反映实际工况土体的强度特性,室内试验应尽量使用原状土进行抗剪强度测试。室内试验确定土的抗剪强度指标时,应先确定地基土的前期固结压力、K0状态参数,按工程实际应力状态确定。     

用快剪试验推求先期固结压力

土体先期固结压力是判断土体应力历史的关键性指标。依据Wroth在一维固结条件下提出的不排水强度与超固结比（OCR）的关系式,利用快速直剪的剪切强度计算临界状态孔隙水压力系数Λ₀,从而计算出OCR以及先期固结压力。对南京某黏土的重塑饱和黏土的试验分析结果表明,快剪试验确定的超固结比与设计超固结比基本一致,因此可作为求解土体先期固结压力的一个途径。      

Strength behavior of slip-zone soils of landslide subject to the change of water content

Landslides are the major natural hazards in many countries all over the world and are usually caused by heavy rainfall, water level change of reservoir, excavation, earthquake, etc. Whether the landslide occurs or not in rainfall season, the strength variation of slip-zone soils of landslide is regarded as the vital control factor. Thus, strength behavior for slip-zone soils of landslide subject to the change of water content is required to be evaluated in a potential landslide area. In this paper, the shear strength of typical slip-zone soil, six groups of 25 specimens of remolded clay samples from Daxishan reservoir landslide, was systemically investigated using the improved direct shear test apparatus in order to fully understand its physical and mechanical properties, and also the shear and failure behavior. Furthermore, the fitting equations for expressing the relationship between the shear strength (effective cohesion and internal friction angle) and vertical loadings, initial water contents of slip-zone clay were established based on the experimental results. In particular, a series of shear stress–shear strain curves under various vertical loadings and different water contents were observed. The results show that a “softening” stress–strain behavior is achieved for unsaturated slip-zone soil, while a “hardening” curve is found for saturated slip-zone soil.     

膨胀土临界状态强度特性及其模型拟合

针对蒙自地区膨胀土,进行了饱和重塑条件下的固结排水(CD)与固结不排水(CU)试验,系统研究了饱和土在不同固结围压3、初始干密度d、初始含水量0、剪切速率、排水条件下的应力应变特性。通过对临界状态试验结果的归一化处理,揭示了不同试验方法对同一土样抗剪强度量测差异的内在统一性,得到了重塑饱和膨胀土在固结排水与不排水条件下p'-q-空间内的破坏线。利用双曲线模型较好地拟合了弱硬化型应力应变曲线,用指数曲线模型拟合了弱软化型应力应变曲线,为模拟边坡在降雨条件下的抗剪强度变化及构筑物土体处于正常水位时的强度状态等工况提供了技术参数与理论支持。     

Undrained behavior of silty sand: effect of the overconsolidation ratio

This experimental study deals with the effect of the overconsolidation ratio on the monotonic undrained shear behavior of silty sand. The study is based on the undrained monotonic triaxial tests for the overconsolidation ratios (OCR?=?1, 2, 4, and 8), with different silt contents ranging from 0% to 40%. The laboratory tests were carried out at an initial relative density of Dr?=?50%. The paper is composed of two parts. The first one presents the tested soils; the second one gives an analysis of the test results and discusses the influence of the overconsolidation ratio on the shear strength of the soil. The test results indicate that the shear strength of the soil increases with the increase of the overconsolidation ratio resulting in an increase of soil dilatancy. The increase in the amount of fines from 0% to 40% increases the phase of the contractancy and consequently reducing the phase of dilatancy of the tested material     

往返荷载下粘性土的强度及取值标准试验研究

通过对粘性土进行一系列动三轴试验，测定并分析了动荷载作用下为粘性土的动剪应力，轴向应变及超孔隙水压力随时间的变化规律，分析了破坏时不同固结比的粘性土对静，动剪强度和孔隙水压力影响规律，得出了粘性土的动剪强度随固结比变化的关系式，并对粘性土的动剪强度判别方法的标准进行探讨，得出了有益的结果。     

Effect of inherent anisotropy on the strength of natural granite residual soil under generalized stress paths

It is universally known that residual soils behave very differently from sedimentary soils. While the latter is widely known as cross-anisotropic, little is known regarding the strength anisotropy of residual soils. This study presents how the inherent anisotropy affects the strength of natural granite residual soils under generalized conditions, where intact specimens were carefully prepared and sheared under triaxial compression, extension, simple shear, and hollow cylinder torsional shear tests. The strength of natural residual soil, in terms of ultimate stress ratio M and undrained shear strength S_u, is found to be significantly anisotropic in a different way from normally consolidated clays with the maximum strength obtained under triaxial compression and the minimum under simple shear or at intermediate principal stress direction. As a result, the existing method failed to measure the anisotropy degree of the studied soil. Two parameters were proposed accordingly to quantify the anisotropic strength under general conditions, taking the special strength anisotropy pattern and cohesive-frictional nature of GRS into account. The proposed parameters enable the direct comparison of strength anisotropy among soils. This study serves as a data set to better understand residual soils regarding their anisotropic behaviors under generalized conditions. Although specific to granite residual soils in China, this study is expected to be more widely applicable to other weathered geomaterials.

     

Laboratory Investigation into the Effects of Silty Fines on Liquefaction Susceptibility of Chlef (Algeria) Sandy Soils

In a number of recent case studies, the liquefaction of silty sands has been reported. To investigate the undrained shear and deformation behaviour of Chlef sand–silt mixtures, a series of monotonic and stress-controlled cyclic triaxial tests were conducted on sand encountered at the site. The aim of this laboratory investigation is to study the influence of silt contents, expressed by means of the equivalent void ratio on undrained residual shear strength of loose, medium dense and dense sand–silt mixtures under monotonic loading and liquefaction potential under cyclic loading. After an earthquake event, the prediction of the post-liquefaction strength is becoming a challenging task in order to ensure the stability of different types of earth structures. Thus, the choice of the appropriate undrained residual shear strength of silty sandy soils that are prone to liquefaction to be used in engineering practice design should be established. To achieve this, a series of undrained triaxial tests were conducted on reconstituted saturated silty sand samples with different fines contents ranging from 0 to 40 %. In all tests, the confining pressure was held constant at 100 kPa. From the experimental results obtained, it is clear that the global void ratio cannot be used as a state parameter and may not characterize the actual behaviour of the soil as well. The equivalent void ratio expressing the fine particles participation in soil strength is then introduced. A linear relationship between the undrained shear residual shear strength and the equivalent void ratio has been obtained for the studied range of the fines contents. Cyclic test results confirm that the increase in the equivalent void ratio and the fines content accelerates the liquefaction phenomenon for the studied stress ratio and the liquefaction resistance decreases with the increase in either the equivalent void ratio or the loading amplitude level. These cyclic tests results confirm the obtained monotonic tests results.     

原状膨胀土剪切力学特性的应变速率效应

为研究应变速率对原状膨胀土力学性状的影响,通过GDS三轴试验系统进行了不同速率和围压下的固结不排水三轴剪切试验,分析了应力−应变曲线、孔隙水压力、剪切强度以及破坏模式随应变速率的变化规律。结果表明：不同应变速率下,膨胀土应力−应变曲线均呈应变硬化型。随着应变速率的增加,不排水剪切强度单调递增,引入应变速率参数ρ_0.9后发现,不排水强度增长率为14.3%～23.2%,平均值为18.4%。低围压下,应变速率对孔隙水压力影响较小,随着围压的增大,孔隙水压力的发展趋势由软化型转变为硬化型,孔隙水压力峰值随应变速率的增大而减小。原状膨胀土应变速率效应与其多裂隙性密切相关,破坏形式表现为小应变速率下主剪切带与次剪切带共存,大应变速率下仅有主剪切带,裂隙或多剪切带的出现强化了膨胀土强度的应变速率效应。     

非饱和紫色土三轴试验颗粒流宏细观参数关系研究

为确定非饱和紫色土颗粒流宏细观力学参数间关系,采用三维颗粒流软件(PFC3D)并结合控制变量法与莫尔-库仑破坏准则,对不同含水率(8%、10%、12%、14%、16%、18%)及围压(100kPa、200kPa、300kPa、400 kPa)条件下的非饱和紫色土三轴固结不排水试验进行了数值模拟,并与室内结果进行对比。结果表明:重塑紫色土黏聚力随含水率增加呈先增后减的趋势,且存在临界含水率12%;内摩擦角与含水率呈一阶线性负相关。紫色土的宏观强度参数黏聚力与细观参数切向黏结强度呈线性正相关,而内摩擦角值由颗粒切向黏结强度及摩擦系数共同决定。建立了紫色土含水率与颗粒切向黏结强度、摩擦系数之间的定量关系,通过细观参数切向黏结强度及摩擦系数的改变来模拟不同含水率下紫色土的黏聚力与内摩擦角的变化,将数值模拟的应力-应变曲线与室内试验曲线进行对比,验证了该关系的正确性。利用PFC3D内置的切片工具对含水率为12%,围压为200kPa的数值试件进行剖切,可以较好地观察到紫色土三轴微观颗粒的运动情况及颗粒间接触力的分布特征,为深入探究紫色土的抗剪强度特性及应力-应变性状提供参考。