超高性能混凝土拉压滞回本构模型

为合理进行超高性能混凝土(ultra-high performance concrete, UHPC)结构抗震非线性计算,提出了一种可模拟循环往复荷载作用下UHPC构件力学性能的拉压滞回本构模型。基于UHPC材料单调及重复轴压试验数据,在单调轴压本构模型的基础上,引入线性加卸载路径,建立了考虑钢纤维影响的UHPC重复受压本构模型。基于UHPC材料单调及重复轴拉试验数据,对应力软化型和应力硬化型的UHPC分别提出了双折线和三折线型的受拉应力-应变曲线,并建立了UHPC重复受拉本构模型。在重复受压和重复受拉本构模型的基础上,引入拉压传递机理,建立UHPC拉压滞回本构模型。基于OpenSees二次开发平台,采用文中所提拉压滞回本构模型对循环往复荷载下UHPC柱的力学性能进行计算,并与试验数据进行对比。结果表明：构建的UHPC拉压滞回本构模型可较好地模拟不同钢纤维掺量及不同轴压比下UHPC柱在循环往复荷载作用下的受力性能,可应用于UHPC结构抗震非线性分析。     

城市群花粉过敏网络关注度及影响因素研究

研究花粉过敏网络关注度时空特征及影响因素有利于掌握相关信息,服务民生。本文结合百度指数、气象、遥感数据,分析8个城市群2017—2021年花粉过敏网络关注度时空特征与影响因素,并采用随机森林、反向传播神经网络模型进行模拟。结果表明：（1）时空特征：关注度每年高峰期在春季（4—5月）;人口聚集的城市最高,在空间上聚集,京津冀、山东半岛、长三角、珠三角城市群为热点区。（2）与影响因素关系：有效范围内,温度升高、短时雷暴雨、空气质量差、光合有效辐射吸收分量升高、夜间灯光强,关注度高;高湿、高风速、大雨、久雨,关注度低;夜间灯光相关性最高。（3）城市群方面：北方城市群的温度、沿海城市群的湿度、地形起伏大城市群的风速、南方城市群的降水、珠三角和长三角城市群的空气质量指数、珠三角和京津冀城市群的光合有效辐射吸收分量重要性大,夜间灯光都不可替代。（4）随机森林和反向传播神经网络适用于模拟关注度,各城市群R²均在0.64～0.92之间,RMSE、MAE均在1以下,反向传播神经网络比随机森林模拟效果更好。成渝城市群2个模型拟合度均优,其次是京津冀、珠三角和长江中游城市群。本文的方法和结果可为花粉过敏相关工作提供参考。     

昆明市花粉过敏症预报试验

通过对昆明城区和郊区2000-2001年花粉观测资料的分析,发现一年中花粉浓度有明显的周期性和季节性。分析了气象要素与空气中飘移花粉的相关关系。在此基础上,利用统计方法,找出了与花粉过敏症相关好的气象因子,建立了花粉过敏症的预报方程;应用医院花粉过敏症的发病率进行验证,同时利用气温、风速、相对湿度对预报值加以订正,使预报结果更接近实际。     

始新世以来亚洲内陆干旱区的古气候定量重建

亚洲内陆干旱化的形成与发展是新生代最引人瞩目的事件之一,与全球变冷、青藏高原隆升、东亚环境格局演化等一系列问题密切相关.目前,干旱化的起始时间、演化过程、驱动机制等方面均存在较大的争议.古气候的定量化研究有助于解答以上问题.本研究收集中国西北地区已经发表的、具有古地磁定年的3条孢粉序列,并且将孢粉百分比转换为植物功能型得分;之后,建立植物功能型-气候数据集的现代类比法模型,进而定量重建始新世以来古气候要素(年均降水、年均温度)的变化历史.留一法的交叉验证结果显示,该模型对于重建年均降水和年均温度均具有较高的可靠性.气候要素的定量重建结果显示,始新世以来年均降水从约948 mm下降至约84mm,年均温度介于1～10 ℃之间.具体而言,40～23 Ma阶段,气候温暖湿润,年均降水和年均温度分别介于386～948 mm和2.00～9.44℃之间;23～8Ma阶段,年均降水略有下降,介于187～953 mm之间,年均温度介于-1.11～8.52℃之间,其中17～12 Ma和10～9Ma阶段年均降水明显增加,整体上超过400 mm;约8Ma之后,气候逐渐冷干,年均降水介于84～434 mm之间,年均温度介于-2.84～8.85℃之间.与已有的古环境记录对比,本研究认为西北地区约23 Ma之后的干旱化过程主要受控于青藏高原的隆升和全球变冷的影响.     

Dynamic failure process of expanded polystyrene particle lightweight soil under cyclic loading using discrete element method

Expanded polystyrene(EPS)particle-based lightweight soil,which is a type of lightweight filler,is mainly used in road engineering.The stability of subgrades under dynamic loading is attracting increased research attention.The traditional method for studying the dynamic strength characteristics of soils is dynamic triaxial testing,and the discrete element simulation of lightweight soils under cyclic load has rarely been considered.To study the meso-mechanisms of the dynamic failure processes of EPS particle lightweight soils,a discrete element numerical model is established using the particle flow code(PFC)software.The contact force,displacement field,and velocity field of lightweight soil under different cumulative compressive strains are studied.The results show that the hysteresis curves of lightweight soil present characteristics of strain accumulation,which reflect the cyclic effects of the dynamic load.When the confining pressure increases,the contact force of the particles also increases.The confining pressure can restrain the motion of the particle system and increase the dynamic strength of the sample.When the confining pressure is held constant,an increase in compressive strain causes minimal change in the contact force between soil particles.However,the contact force between the EPS particles decreases,and their displacement direction points vertically toward the center of the sample.Under an increase in compressive strain,the velocity direction of the particle system changes from a random distribution and points vertically toward the center of the sample.When the compressive strain is 5%,the number of particles deflected in the particle velocity direction increases significantly,and the cumulative rate of deformation in the lightweight soil accelerates.Therefore,it is feasible to use 5%compressive strain as the dynamic strength standard for lightweight soil.Discrete element methods provide a new approach toward the dynamic performance evaluation of lightweight soil subgrades.     

Ocean waves propagating over a porous seabed of finite thickness

In the past few decades, considerable efforts have been devoted to the phenomenon of wave-seabed interaction. However, conventional investigations for determining wave characteristics have been focused on the wave nonlinearity. On the other hand, most previous works have been only concerned with the seabed response under the wave pressure, which was obtained from the assumption of a rigid seabed. In this paper, the inertia forces and employing a complex wave number are considered in the whole problem. Based on Biot’s poro-elastic theory, the problem of wave-seabed interaction is first treated analytically for a homogeneous bed of finite thickness and a new wave dispersion relationship is also obtained, in which the soil characteristics are included. The numerical results indicate that the effects of soil parameters significantly affect the wave characteristics (such as the damping of water wave, wave length and wave pressure). Furthermore, the effects of inertia forces on the wave-induced seabed response cannot always be ignored under certain combination of wave and soil conditions.     

Recent sedimentary histories of shallow lakes in the guatemalan savannas

Shallow basins in the savannas of Peten, Guatemala filled with water after 305±55 BP (calibrated age+1430–1660 AD). Aguadas Chimaj and Chilonche possess dilute waters and iron-rich, clayey sediments that are poor in Ca and Mg, reflecting the highly weathered nature of riparian soils. Low ²¹⁰Pb flux rates to Chimaj (0.085 pCi cm^-2 yr^-1) and Chilonche (0.134 pCi cm^-2 yr^-1) are attributed to low ²²²Rn emission rates from the nearby Caribbean Sea. Mean sediment accumulation rates in Chimaj and Chilonche for the past 150 years are 0.015 g cm^-2 yr^-1 and 0.047 g cm^-2 yr^-1 respectively. Forest expansion after 305 BP is documented in pollen profiles from the small aguadas and larger Lake Oquevix. Regional reforestation postdates the 9th century Classic Maya collapse and coincides with indigenous depopulation that was a consequence of European intrusion that began in the early 1500s. The timing of forest regrowth indicates the importance of historical anthropogenic factors in controlling Peten's vegetation. Nevertheless, other sedimentological lines of evidence (e.g. lithology, algal remains and charcoal particles) suggest that changing climate and/or local hydrology may have played a role in the reforestation process.     

Bifurcation and generalized mixed loading conditions in geomaterials

The concept of bifurcation has been widely discussed, based, for example, on homogeneous tests in soil mechanics. Essentially two‐dimensional (axisymmetric) conditions have been considered. This paper aims at deriving a bifurcation criterion in geomechanics, independently of the problem's dimension. This criterion is related to the vanishing of the determinant of the symmetric part of the material's (or the discrete system's) constitutive (or stiffness) matrix. The derivation is essentially based on the notion of loading parameters (controlling the loading). Basically, a bifurcation occurs when the existence of a unique solution to the quasistatic problem, involving a given set of loading parameters, is lost. Interestingly, the criterion is shown to be independent of the choice of loading parameters. As a possible extension, the context of structure mechanics is considered, and the close connection between both soil and structure analysis is discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Development of Winkler model for static and dynamic response of caisson foundations with soil and interface nonlinearities

As an extension of the elastic multi-spring model developed by the authors in a companion paper [Gerolymos N, Gazetas G. Winkler model for lateral response of rigid caisson foundations in linear soil. Soil Dyn Earthq Eng; 2005 (submitted companion paper).], this paper develops a nonlinear Winkler-spring method for the static, cyclic, and dynamic response of caisson foundations. The nonlinear soil reactions along the circumference and on the base of the caisson are modeled realistically by using suitable couple translational and rotational nonlinear interaction springs and dashpots, which can realistically (even if approximately) model such effects as separation and slippage at the caisson–soil interface, uplift of the caisson base, radiation damping, stiffness and strength degradation with large number of cycles. The method is implemented in a new finite difference time-domain code, NL-CAISSON. An efficient numerical methodology is also developed for calibrating the model parameters using a variety of experimental and analytical data. The necessity for the proposed model arises from the difficulty to predict the large-amplitude dynamic response of caissons up to failure, statically or dynamically. In a subsequent companion paper [Gerolymos N, Gazetas G. Static and dynamic response of massive caisson foundations with soil and interface nonlinearities—validation and results. Soil Dyn Earthq Eng; 2005 (submitted companion paper).], the model is validated against in situ medium-scale static load tests and results of 3D finite element analysis. It is then used to analyse the dynamic response of a laterally loaded caisson considering soil and interface nonlinearities.