常剪应力路径下含气砂土的三轴试验

天然气水合物完全分解时,产生的气体使得能源土孔隙压力急速增加,有效应力减小,进而引起土体液化破坏。此时深海能源土斜坡的应力状态与静力液化失稳过程可简化为含气土在常剪应力排水(或不排水)应力路径下的破坏问题。以此为背景,提出了制备含气砂土试样的改进充气管法,并开展了含气砂土的常剪应力路径三轴试验。22组试验结果表明:同一孔隙比的含气密砂在不同围压与常剪应力下具有相同的失稳线;含气砂土试样失稳时的应力比和体变均随初始相对密实度的增大而增大;含气密砂在常剪应力路径下饱和度对失稳特征影响的规律性在排水与不排水条件下均不明显,但在不排水条件下含气砂土的孔压(或体变)对变形的敏感性降低;含气密砂在常剪应力路径到达失稳点之后,排水条件下是瞬变的液化鼓胀破坏,不排水条件下是渐变的剪切破坏。     

饱和悬浮塑料砂流动变形可视环剪试验研究

为了分析剪切条件下零有效应力状态砂土的流动变形规律,对传统环剪仪进行了试样可视化改造,研制了透明环形剪切盒,通过对剪切盒膨胀性能分析及与标准砂的剪切试验对比,验证了环剪装置改造的合理性。通过分析不排水条件下饱和悬浮塑料砂的剪应力-应变曲线,发现其剪切强度具有应变软化的特性。通过分析环剪仪中饱和悬浮塑料砂试样的有效应力,证实了可视环剪试验中的土体基本处于零有效应力状态。开展了饱和悬浮塑料砂的流动变形可视环剪试验研究,结果表明：饱和悬浮塑料砂在固结不排水条件下剪切变形不连续,直接在剪切面发生断裂;在不固结不排水的条件下饱和悬浮塑料砂的剪切变形表现出流动形态,且与剪切速率有关：在低剪切速率下,剪切变形仅在剪切面处形成具有曲线轨迹的流动变形而在其他区域不发生变形;而在高剪切速率下剪切变形为整体的倾斜变形,符合黏性流体的流动变形特征。     

Effects of boundary conditions and partial drainage on cyclic simple shear test results—a numerical study

Owing to imperfect boundary conditions in laboratory soil tests and the possibility of water diffusion inside the soil specimen in undrained tests, the assumption of uniform stress/strain over the sample is not valid. This study presents a qualitative assessment of the effects of non‐uniformities in stresses and strains, as well as effects of water diffusion within the soil sample on the global results of undrained cyclic simple shear tests. The possible implications of those phenomena on the results of liquefaction strength assessment are also discussed. A state‐of‐the‐art finite element code for transient analysis of multi‐phase systems is used to compare results of the so‐called ‘element tests’ (numerical constitutive experiments assuming uniform stress/strain/pore pressure distribution throughout the sample) with results of actual simulations of undrained cyclic simple shear tests using a finite element mesh and realistic boundary conditions. The finite element simulations are performed under various conditions, covering the entire range of practical situations: (1) perfectly drained soil specimen with constant volume, (2) perfectly undrained specimen, and (3) undrained test with possibility of water diffusion within the sample. The results presented here are restricted to strain‐driven tests performed for a loose uniform fine sand with relative density D_r=40%. Effects of system compliance in undrained laboratory simple shear tests are not investigated here. Copyright © 2004 John Wiley & Sons, Ltd.     

中高龄期模型固体废弃物三轴试验与强度特性分析

为揭示中、高龄期城市固体废弃物（MSW）的力学特性,通过向石英砂、高岭土混合物中加入不同质量比的草炭人工配制模型固废试样,开展了三轴固结排水剪切和固结不排水剪切试验研究。试验结果表明,中高龄期模型固废均表现出持续硬化的应力应变特性,固结排水试验中中龄期模型固废的体变略大于高龄期,固结不排水试验中高龄期模型固废的孔压略高于中龄期。采用15%轴向应变对应的剪应力描述其抗剪强度,并根据Skempton有效应力原理,通过对比固结排水试验和固结不排水试验结果,得出了固废的孔隙水压力折减系数和抗剪强度参数,孔隙水压力折减系数随龄期增加而增大,高龄期模型固废得到的黏聚力低于中龄期,高龄期固废得到的内摩擦角则高于中龄期。     

Mathematical identification of diffuse and localized instabilities in fluid‐saturated sands

The mathematical properties of diffuse and localized failure modes in fluid‐saturated sands are investigated. The granular medium is modeled as an elastoplastic solid, and a recently proposed set of scalar indices, here referred to as moduli of instability, is used to identify the onset of potential bifurcations of the incremental response. First, the analytical properties of these moduli are discussed, stressing their dependence on the kinematic constraints associated with the imposed deformation modes. Then, by using an elastoplastic model for sands, drained and undrained loading paths are simulated under axisymmetric, plane‐strain and simple shear conditions. For each deformation mode, the instability moduli are computed and monitored throughout the simulations, with the purpose of elucidating the consequences of changes in control conditions. In addition, it is illustrated that suitable linear transformations allow the same strategy to be used to perform drained or undrained shear band analyses and predict the interval of possible band inclinations. The final comparison against literature experiments on loose Hostun sand shows that the instability moduli are indicators of the loss of resistance against specific modes of deformation. As a result, they can be used to identify and explain a number of failure mechanisms that can be commonly observed in experiments. Copyright © 2013 John Wiley & Sons, Ltd.     

Mechanism of a long-runout landslide triggered by the August 1998 heavy rainfall in Fukushima Prefecture,Japan

Heavy rainfall from 26 to 31 August 1998 triggered many landslides in Nishigo Village of southern Fukushima Prefecture, Japan. The Hiegaesi landslide, a long-runout landslide with travel angle of 11°, which occurred in loamy volcanic-ash/pumice layer and was deposited in a nearby rice paddy, was investigated. In an observation pit dug in the middle part of the landslide deposit, the sliding zone just above the deflected rice plants was observed, and it was confirmed that grain crushing occurred in the sliding zone. The triggering and sliding mechanisms of this landslide then were investigated by ring-shear tests in laboratory. For the triggering mechanism, one saturated naturally drained test (test A: torque-controlled test) and one saturated undrained test (test B: speed-controlled test) were conducted on the samples taken from the source area of the landslide. Even in the naturally drained test opening the upper drain valve of the shear box, a temporary liquefaction occurred. In the undrained test, excess pore-pressure was generated along with shearing, and “sliding-surface liquefaction” phenomenon was observed. The effective stress and shear resistance finally decreased to near zero. These results can explain the observed phenomenon of small friction resistance like a flow of liquid when the sliding mass slid out of the source area. For the sliding mechanism of the landslide in the rice paddy, saturated undrained test (test C: speed-controlled test) was performed on soil sample above the deflected rice plants. The apparent friction angle obtained in this test was 8°. In addition, the residual friction angle measured after test B and test C was the same value of 41°. Combining with the observation on the shear zone in the ring-shear box after test C, it is concluded that, during the sliding in rice paddy, the undrained shear strength of the soil layer itself mainly influenced the high mobility of the landslide, probably because the friction between rice plants and soils is greater than the undrained shear strength inside the soil mass.     

Influence of shear speed and normal stress on the shear behavior and shear zone structure of granular materials in naturally drained ring shear tests

“Sliding Surface Liquefaction” is a process causing strength loss and consequent rapid motion and long runout of certain landslides. Using a new ring shear apparatus with a transparent shear-box and digital video camera system, shear-speed-controlled tests were conducted on mixed grains (mixture of three different sizes of sand and gravel) and mixed beads to study shear behavior and shear zone development process under the naturally drained condition in which pore pressure is allowed to dissipate through the opened upper drainage valve during shearing. Higher excess pore water pressure and lower minimum apparent friction were observed in the tests where grain crushing was more extensive under higher normal stress and higher shear speed. Along with the diffusion of silty water generated by grain crushing, smaller particles were transported upward and downward from the shear zone. Concentration of larger grains to the central and upper part of the shear zone was confirmed by means of visual observation together with grain size analysis of sliced samples from several layers after the test. On the other hand, smaller particles were accumulated mostly below the layer where larger grains were accumulated. The reason why larger grains were accumulated into the shear zone may be interpreted as follows: grains under shearing are also subjected to vertical movement, the penetration resistance of larger grains into a layer of moving particles is smaller than that into the static layer. Therefore, larger grains tend to move into the layer of moving grains. At the same time, smaller particles can drop into the pores of underlying larger grains downward due to gravity.     

珠江口海洋软土不排水抗剪强度及循环弱化特性试验研究

为了获得广东珠江口海洋软土的不排水抗剪强度,分别开展了不固结不排水剪切和固结不排水剪切三轴试验、无侧限抗压试验、现场取土离心机正常固结模型地基的T-bar静力贯入试验以及现场十字板剪切试验和静力触探试验（CPT）,并采用土力学经验公式进行计算,确定了珠江口海洋软土不排水抗剪强度沿深度的分布规律,并综合评价了各种方法的有效性。为了获得该区域软土的循环弱化规律,分别开展了循环动三轴试验、现场取土离心机正常固结模型地基的T-bar循环贯入试验和现场单桩水平循环加载试验,揭示了软土循环动模量比与循环次数之间的双对数线性关系,获得了3种试验条件下珠江口海洋软土的循环弱化因子。其研究结果可为该区域海洋建筑物设计提供直接依据,也可为其他类型工程提供借鉴。     

An AI-based model for describing cyclic characteristics of granular materials

Modelling cyclic behaviour of granular soils under both drained and undrained conditions with a good performance is still a challenge. This study presents a new way of modelling the cyclic behaviour of granular materials using deep learning. To capture the continuous cyclic behaviour in time dimension, the long short-term memory (LSTM) neural network is adopted, which is characterised by the prediction of sequential data, meaning that it provides a novel means of predicting the continuous behaviour of soils under various loading paths. Synthetic datasets of cyclic loading under drained and undrained conditions generated by an advanced soil constitutive model are first employed to explore an appropriate framework for the LSTM-based model. Then the LSTM-based model is used to estimate the cyclic behaviour of real sands, ie, the Toyoura sand under the undrained condition and the Fontainebleau sand under both undrained and drained conditions. The estimates are compared with actual experimental results, which indicates that the LSTM-based model can simultaneously simulate the cyclic behaviour of sand under both drained and undrained conditions, ie, (a) the cyclic mobility mechanism, the degradation of effective stress and large deformation under the undrained condition, and (b) shear strain accumulation and densification under the drained condition.     

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。     

Dynamic stability analysis of undrained simple shear on water-saturated granular soils

Dynamic stability of undrained, simple-shear deformations on water-saturated granular soil specimens is discussed. The soil is described by a 2D-flow theory of plasticity for frictional and dilatant material. Contractant material becomes unstable (liquefies) at the state of maximum shear stress, whereas dilatant material becomes unstable after the state of maximum effective stress obliquity is reached in the softening regime of the background drained behaviour. In both cases the correct evolution equations for the growth coefficient of the instability are derived and compared with the results of the inertia-free formulation of the problem.     

花岗岩风化料弃土快速堆填过程中不排水抗剪强度评估

沿海沿江城市地下开挖产生的工程渣土含泥量大、含水率高且较松散,主要运往渣土场进行堆填处置。由于产量巨大而处置场地有限,许多渣土场在运营过程中存在堆填速度快、缺乏排水设施、超高超库容堆填等问题,容易引发堆填体失稳事故。目前对于非饱和工程渣土堆体在快速堆填过程中的失稳机制认识尚不清晰,尤其是对这一过程中的高饱和度工程渣土强度增长规律缺乏足够的认识。以深圳红坳渣土场填料——花岗岩风化料(CDG)填土为研究对象,对不同初始饱和度土样进行三轴不排水不排气等向压缩和剪切试验,结果表明:非饱和CDG填土不排水抗剪强度随围压增大呈非线性增长,增长速率与试样初始饱和度密切相关;当土样压缩后的饱和度超过0.7,不排水强度随围压的增长速率显著降低。基于有效固结应力法的原理,结合Hilf孔压公式和修正剑桥模型,提出了一种工程渣土不排水抗剪强度估算方法,并通过与试验结果对比验证了该方法对初始饱和度高于0.6的CDG填土的适用性。利用该方法确定的不排水强度cu与正应力σn的关系可应用于高饱和度工程渣土快速堆填中的稳定性分析。     

复杂应力条件下饱和砂土单调剪切特性的试验研究

利用土工静力-动力液压-三轴扭转多功能剪切仪,针对相对密度为30 %的福建标准砂,进行了复杂初始固结条件下应力路径变化的应力控制式单调排水与不排水剪切试验。控制试验过程的平均主应力保持不变,变化中主应力系数和主应力方向,分别探讨在不同排水条件下中主应力系数和主应力方向对饱和砂土剪切特性的影响。通过对比表明：与排水条件无关,中主应力系数对归一化的应力-应变关系具有影响,但对体变或孔压的影响并不明显。初始条件相同,偏应力比随中主应力系数的增大而降低。主应力方向的影响同样显著,排水试验的主应力方向角不同时应力-应变关系所表现出的变化规律取决于水平面与竖直面上受到的剪应力作用。不排水试验的峰值有效偏应力比随着主应力方向角的增大而减小。     

A unified method to predict diffuse and localized instabilities in sands

A simplified method to analyse diffuse and localized bifurcations of sand under drained and undrained conditions is presented in this paper. This method utilizes results from bifurcation analysis and critical state plasticity theory to detect the onset of pure and dilatant shear band formation, static liquefaction and drained shear failures systematically. To capture the soil collapse observed in experiments, the instability state line concept originated by Chu, Lo and Lee in 1993 is adopted. Emphasis is given to examine how the presence of pore-fluid may facilitate or delay instability after yielding occurs. The predictions of instabilities are compared with experimental data from triaxial compression tests on Toyoura and Changi sands.     

Strain softening of K0-consolidated Changi sand under plane-strain conditions

Experimental results are presented in this paper to study the strain softening behaviour of a marine dredged sand under plane-strain conditions. K₀ consolidated drained and undrained tests were conducted using a new plane-strain apparatus to characterize the strain softening behaviour of the sand under plane-strain conditions. For medium dense specimens, strain softening and shear bands were observed to occur under both drained and undrained conditions. For very loose specimens, no shear bands were observed and critical states were reached within the homogeneous deformation region in both drained and undrained tests. Strain softening was observed to occur at small strain for very loose specimens under undrained conditions. Two types of strain softening, the homogenous softening and banding softening, were identified and the conditions for strain softening were established. The results obtained from this study were compared with the studies by Han and Vardoulakis (Géotechnique 41(1):49–78, 1991), Finno et al. (J Geotech Eng ASCE 122(6):462–473, 1996, Géotechnique 47(1):149–165, 1997) and Mokni and Desrues (Mech Cohes-Frict Mat 4:419–441, 1998).     

NGI‐ADP: Anisotropic shear strength model for clay

Many geotechnical problems involve undrained behavior of clay and the capacity in undrained loading. Most constitutive models used today are effective stress based and only indirectly obtain values for the undrained shear strength. To match the design profiles of undrained shear strengths, in active (A), direct simple shear (D) and passive (P) modes of loading are complicated. This paper presents the elastoplastic constitutive model NGI‐ADP which is based on the undrained shear strength approach with direct input of shear strengths. Consequently, exact match with design undrained shear strengths profiles is obtained and the well‐known anisotropy of undrained shear strength and stiffness is accounted for in the constitutive model. A non‐linear stress path‐dependent hardening relationship is used, defined from direct input of failure strains in the three directions of shearing represented by triaxial compression, direct simple shear and triaxial extension. With its clear input parameters the model has significant advantages for design analysis of undrained problems. The constitutive model is implemented, into finite element codes, with an implicit integration scheme. Its performance is demonstrated by a finite element analysis of a bearing capacity problem. Copyright © 2011 John Wiley & Sons, Ltd.     

颗粒级配对粒状材料不排水力学特性的影响

为了研究颗粒级配对粒状材料不排水力学特性的影响,采用两种形状自相似度很高的材料（玻璃球和Hostun砂）进行了一系列传统三轴不排水压缩试验。对于每种材料,保持相近的相对密实度制作了6个不同颗粒级配（不均匀系数 1.1～20）的试样,然后进行剪切。试验结果表明,颗粒级配对粒状材料的不排水力学性能存在明显影响：两种材料试样的偏应力水平随着不均匀系数的增加而降低,抗剪强度随着材料的不均匀系数 的增加而降低,并在 5后呈现出趋稳势态。基于不排水剪切试验中颗粒集合体二阶功的演化规律,对材料受荷载过程中出现不稳定性的条件进行分析。结果表明,在相同密实度下,粒状材料的静态液化潜能随着材料不均匀系数 的提高而增强,高不均匀系数的材料变得不稳定。     

Drained and undrained pullout capacity of a stiff inclusion in a saturated poroelastic matrix

Shear‐lag analysis is used to obtain closed‐form solutions for the problem of a stiff inclusion embedded in a poroelastic soil matrix. The following assumptions are made: the soil matrix and the inclusion are elastic; plane strain conditions apply; and shear stresses at the soil‐inclusion interface follow Coulomb's friction law. Two solutions are obtained, the first one for drained conditions where no excess pore pressures are generated, and the second one for undrained conditions where excess pore pressures are produced and the soil does not change volume during pullout. The solutions are verified by comparing analytical predictions with numerical results obtained using a finite element method. Predictions from the analytical solutions are also compared with results from experiments conducted in a large‐scale pullout box. Both comparisons show good agreement. The analytical solution shows that the pullout capacity in drained and undrained conditions is overall independent of the relative stiffness of the soil and the inclusion. The most important factor controlling the pullout capacity is the coefficient of friction between the soil and the inclusion. Both drained and undrained pullout capacities increase with the coefficient of friction; although the drained capacity shows a proportional increase, it is not so for the undrained capacity. The ratio of undrained to drained pullout capacity is about 0.9 for friction coefficients smaller than 0.2, but can be as small as 0.6 for a coefficient of friction of 1.0. Copyright © 2006 John Wiley & Sons, Ltd.     

天津市区第一海相层粉质黏土卸荷路径下强度特性的试验研究

土的强度特性与土体中应力变化过程、固结状态有关,常规的三轴试验并不能真实地反映土体经历的应力路径对其强度特性的影响。采用自制的应力-应变联合控制式三轴试验仪,对天津市区地层中第一海相层的粉质黏土原状饱和试样进行一系列特殊三轴试验,研究了排水卸荷条件对其强度特性的影响。试验结果表明：经历排水卸荷路径的土体的加荷剪切总应力强度指标大于常规试验结果,也大于前人在不排水卸荷路径下的试验结果,但几种应力路径下土体的有效应力强度指标基本相等。研究成果对天津市区基坑开挖及支护结构设计计算、软土隧道施工研究及其他相关岩土工程问题的研究均有较大的实用价值。