循环作用下粘土应变软化研究

循环荷载作用下,饱和软粘土将发生应变软化现象。通过对杭州饱和软黏土进行应力控制的循环三轴试验,研究循环次数、循环应力比、固结比、频率、超固结比对土体应变软化的影响。试验结果表明,循环次数的增加,循环应力比的提高,都将加快土体软化;循环荷载作用下,软黏土存在临界循环应力比,当循环应力比较小或较大时,软化指数与lgN近似为线性关系,而当循环应力比在临界循环应力比左右时,两者表现为明显的曲线关系。     

Settlement Prediction of Surcharge Preloaded Low Embankment on Soft Ground Subjected to Cyclic Loading

This article studies the effect of dynamic cyclic loading and surcharge preloading method on the post-construction settlement of low embankments. Soil samples obtained from the soft ground under an embankment were consolidated by surcharge preloading followed by static and dynamic cyclic loading in the odometer. The results show that the consolidation deformation of the soil samples is independent of the frequency of the dynamic cyclic loading, which was simulated to follow the half-sine wave, and this is consistent with the energy concept. The post-construction settlement increases with increasing amplitude of cyclic load and the effectiveness of surcharge preloading depends on the difference between magnitude of surcharge and amplitude of the cyclic load. Based on the consolidation theory combined with the test results, a formula has been proposed to compute the post-construction settlement of a low embankment under cyclic loading.     

北京平原南口—孙河断裂南段第四纪活动性及其构造意义

通过对北京平原南口—孙河断裂南段上、下两盘钻孔ZK17和ZK18的磁化率和自然伽马测井曲线进行频谱分析,识别出显著的米兰柯维奇沉积旋回,揭示出北京平原区第四纪古气候及沉积过程受到地球轨道力的控制。通过对初始年代格架下的数据曲线进行100 ka的短偏心率信号滤波处理并调谐至天文理论偏心率曲线,建立了精度为01 Ma的连续天文年代标尺,在此年代学研究成果的基础上实现了对目标断裂活动性精细探讨的目的。通过对比断裂两盘的沉积速率变化,可以识别出南口—孙河断裂第四纪以来6个相对较强的活动幕。其中早更新世存在4个明显的活动幕,活动周期具有04 Ma准周期的特点,第Ⅰ幕和第Ⅲ幕分别对应了青藏高原隆升期的青藏运动的B幕(26 Ma)和C幕(17 Ma),中更新世的活动特征则对应了“昆黄运动”,北京地区新构造运动特征与之有较好的呼应关系。     

分级循环动荷载下水泥土动力特性试验研究

随着车辆载重的变化,其荷载幅值也会随之变化。然而,已有的研究大多关注恒定动荷载下水泥土动力特性,对分级循环荷载作用下水泥土的动态特性研究较少。利用GDS动三轴仪,开展一系列水泥土动三轴试验,探讨分级荷载条件下静偏应力和围压等因素对水泥土动力特性的影响。试验结果表明:水泥土的轴向塑性应变随着围压和静偏应力比的增加而增加。水泥土轴向应变在0.1%～0.5%范围,其应变发展属于稳定型,分级加卸载对应变发展过程影响较大,对最终应变影响较小。建立了考虑水泥土加卸载条件、围压和静偏应力比的塑性应变的经验模型,分级卸载的相关系数大于0.95,而分级加载的相关系数在0.8～0.9之间。分级卸载时,第一级荷载对应变发展起主导作用,而分级加载时,最大动应力比无法主导前几级荷载较小时的应变发展。在分级加卸载条件下,水泥土的骨干曲线表现为2种模式:上升型和下降型,可将其简化为多段直线,利用建立的累积塑性应变计算方法,提出了确定多段直线分界点的方法,并验证了其可行性。     

法向应力对接触面循环单剪力学特性的影响研究

法向应力对土与结构接触面循环单剪力学特性的影响研究具有重要意义。运用80 t大型三维接触面试验机,进行了不同法向应力下粗粒土与结构接触面单剪试验,分析了法向应力对接触面循环单剪力学特性的影响规律。试验结果表明,法向应力对接触面变形位移、滑动位移、切向应力、可逆性剪切体变和不可逆性剪切体变以及抗剪强度等接触面性能参量的数值有显著影响,但对参量间关系形式影响较小。法向应力越大,接触面变形位移在初始几个循环内越大,随循环剪切的进行,其减小速率越快,最终稳定值越小;切向刚度越大,切向应力稳定值也越大,初始几个循环内切向刚度系数越小;不可逆性剪切体变越大,可逆性剪切体变峰值和稳定值越小,相变位移和相变应力比越小;抗剪强度越大,其异向程度则先增大后减小。不同法向应力下,接触面循环抗剪强度与法向应力符合摩尔-库仑准则;切向应力比-切向位移关系、不可逆性剪切体变和剪切功密度关系均具有良好一致性,基本不受法向应力影响,可用双曲线模型描述,这将大大简化接触面本构描述。     

Simulation of earthquake‐induced slope deformation using SPH method

This paper presents the development, calibration, and validation of a smoothed particle hydrodynamics (SPH) model for the simulation of seismically induced slope deformation under undrained condition. A constitutive model that combines the isotropic strain softening viscoplasticity and the modified Kondner and Zelasko rule is developed and implemented into SPH formulations. The developed SPH model accounts for the effects of wave propagation in the sliding mass, cyclic nonlinear behavior of soil, and progressive reduction in shear strength during sliding, which are not explicitly considered in various Newmark‐type analyses widely used in the current research and practice in geotechnical earthquake engineering. Soil parameters needed for the developed model can be calibrated using typical laboratory shear strength tests, and experimental or empirical shear modulus reduction curve and damping curve. The strain‐rate effects on soil strength are considered. The developed SPH model is validated against a readily available and well‐documented model slope test on a shaking table. The model simulated slope failure mode, acceleration response spectra, and slope deformations are in excellent agreement with the experimental data. It is thus suggested that the developed SPH model may be utilized to reliably simulate earthquake‐induced slope deformations. This paper also indicates that if implemented with appropriate constitutive models, SPH method can be used to model large‐deformation problems with high fidelity. Copyright © 2013 John Wiley & Sons, Ltd.     

A numerical model for foundation settlements due to deformation accumulation in granular soils under repeated small amplitude dynamic loading

Repeated small amplitude dynamic loading of the soil in the vicinity of buildings, as arising from traffic or construction activities, may cause differential foundation settlements and structural damage. In this paper, a numerical model for soils under repeated dynamic loading is formulated. It is assumed that the dynamic part of the loading is small with respect to the static part, reflecting the stress conditions in the soil underneath buildings. As the plastic deformation in the soil is only observed after a considerable amount of dynamic loading cycles, only the accumulation of the average plastic deformation is considered. The model accounts for the dependency of the deformation on the stress conditions and the dynamic loading amplitude. The accumulation model is implemented in a finite element framework, using a consistent tangent approach in combination with a backward Euler integration scheme. A triaxial test is considered in a first numerical example. The available analytical solution for this problem allows to validate the numerical implementation. Second, the differential settlement of a two‐storey building founded on loose sandy soil under repeated vehicle passages is considered. The differential foundation settlement causes the stresses to increase at the bottom of the wall, which may result in damage. Copyright © 2009 John Wiley & Sons, Ltd.     

On the determination of a set of material constants for a high‐cycle accumulation model for non‐cohesive soils

The paper discusses the determination of a set of constants for the high‐cycle accumulation model (HCA) proposed by the authors. The HCA model predicts permanent strains or excess pore water pressures in non‐cohesive soils due to a cyclic loading with a large number of cycles and with small to intermediate strain amplitudes. The laboratory tests necessary for the determination of the material constants and their analysis are explained in detail in this paper. Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of Frequency on the Behaviour of Plate Anchors Subjected to Cyclic Loading

This article reports the response of embedded circular plate anchors to varying frequencies of cyclic loading. The effects of time period of loading cycles and pre-loading on movement of anchors and post-cyclic monotonic pullout behavior are studied using a model circular (80 mm diameter) plate anchor, buried at embedment ratio of six in a soft saturated clay. The frequencies of loading cycles have showed considerable effect on movement of anchors. For given duration of loading, higher frequency cycles cause more movement of anchor than lower frequency cycles. Pre-loading reduces the movement of anchors in subsequent loading stages. When anchors are recycled at a load ratio level less than the pre-cycling load, the movement of anchor in recycling phase are very much reduced, but if the recycling is done at a higher load ratio level, the effect is not that much pronounced and the anchors behave as if they were not subjected to any cycling load in the past. Anchor subjected to cyclic loading and then monotonic pullout shows an increase in initial stiffness, whereas the peak pullout load was found to decrease marginally over that of an anchor not subjected to any cyclic loading. For the present test conditions, the relative post-cyclic stiffness of anchors is found to vary from 1.169 to 1.327.     

Degradation of lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading

Abstract

In this article, the degradation of the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading is studied numerically. A modified kinematic hardening constitutive model is employed to simulate the degradation of soft clay after cyclic loading. The modified model is verified by comparing the numerical simulation results with the results of centrifuge model tests. Furthermore, the modified model is applied to numerical simulations for evaluating the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading. The degradation of the lateral bearing capacity of piles in soft clay after different cyclic displacement levels and different numbers of cycles is investigated. The study reveals that the modified kinematic hardening constitutive model can effectively estimate the cyclic degradation behavior of piles in soft clay subjected to cyclic lateral loading. The degradation of the ultimate lateral bearing capacity progresses slowly with increasing cyclic displacement level for fewer cycles, and the degradation develops significantly at higher levels of cyclic displacement after applying a larger number of cycles.