Effect of abutment modeling on the seismic response of bridge structures

Abutment behavior significantly influences the seismic response of certain bridge structures. Specifically in the case of short bridges with relatively stiff superstructures typical of highway overpasses, embankment mobilization and inelastic behavior of the soil material under high shear deformation levels dominate the response of the bridge and its column bents. This paper investigates the sensitivity of bridge seismic response with respect to three different abutment modeling approaches. The abutment modeling approaches are based on three increasing levels of complexity that attempt to capture the critical components and modes of abutment response without the need to generate continuum models of the embankment, approach, and abutment foundations. Six existing reinforced concrete bridge structures, typical of Ordinary Bridges in California, are selected for the analysis. Nonlinear models of the bridges are developed in OpenSees. Three abutment model types of increasing complexity are developed for each bridge, denoted as roller, simplified, and spring abutments. The roller model contains only single-point constraints. The spring model contains discrete representations of backfill, bearing pad, shear key, and back wall behavior. The simplified model is a compromise between the efficient roller model and the comprehensive spring model. Modal, pushover, and nonlinear dynamic time history analyses are conducted for the six bridges using the three abutment models for each bridge. Comparisons of the analysis results show major differences in mode shapes and periods, ultimate base shear strength, as well as peak displacements of the column top obtained due to dynamic excitation. The adequacy of the three abutment models used in the study to realistically represent all major resistance mechanisms and components of the abutments, including an accurate estimation of their mass, stiffness, and nonlinear hysteretic behavior, is evaluated. Recommendations for abutment modeling are made.     

水沙调节后荆江典型河道横向调整过程的响应——Ⅱ.上、下荆江调整差异初探

三峡水库2003年6月蓄水运用后,下游河道水沙过程已发生大幅变化,对于岸滩抗冲性较弱的荆江河道来说,河道的横向调整过程势必受到影响。采用考虑岸滩崩塌的河势研究数值模型,针对三峡水利枢纽工程运行所引发的水沙变异过程,初步探讨了荆江典型河道横向调整及河势变化对水沙条件变化的响应。对于下荆江石首河段来说,来沙减少后,冲刷加剧,局部河岸坍塌及平面变形加剧,主要发生在受弯道水流顶冲的位置,但河势演变趋势及平面变形总体上基本一致,并未发生较大变化。对于上荆江沙市—新厂河段来说,来沙量减小后,河道平面变形幅度总体上减小,局部最大减幅可达50%左右。     

列车动荷载下桩网结构路基土拱效应试验研究

通过三维模型试验研究了列车动荷载作用下的土拱效应。试验结果表明,动荷载作用下土拱效应依然有效,随着填土的增高,土拱效应在增强,土工格栅的存在可以增强土拱效应的作用。但动荷载条件下动应力的分布与自重条件下的应力分布截然不同。动荷载作用下土拱效应会发生退化,表现为动荷载施加过程中先前由桩所承担的部分动应力会转移至桩间土;完成动荷载施加后,最初由桩承担的部分自重荷载也会转移至桩间土。填土高度和土工格栅对于动荷载作用下土拱的稳定性有很大的影响,但当高度达到一定值之后,填土高度的影响就可忽略,土工格栅的存在使得土拱效应的削弱程度减小了一半。     

降雨入渗条件下粗粒土路堤暂态饱和区发展规律及稳定性研究

为了分析降雨入渗条件下粗粒土路堤暂态饱和区发展规律及稳定性,结合工程实际及气象资料,通过拟定不同的降雨计算方案,采用饱和-非饱和渗流理论对粗粒土路堤边坡在不同降雨条件下雨水入渗过程进行数值模拟.研究结果表明:降雨过程中路堤边坡表面会形成一定范围的暂态饱和区,暂态饱和区的形成及发展过程与降雨强度及降雨时间密切相关;降雨入渗仅会引起路堤边坡坡面以下水位线的大幅上升,对路堤中部地下水位线影响较小;降雨停止后,路堤上部暂态饱和区迅速消散,而路堤中下部暂态饱和区消散过程则表现出明显的滞后特征,但总体而言暂态饱和区的面积呈下降趋势;降雨入渗将导致入渗区域内基质吸力的丧失与含水率的增加,是造成路堤边坡稳定性降低的主要因素,路堤的失稳形式表现为整体滑移与表面局部坍塌相结合.     

灰关联神经网络法及防洪堤边坡稳定性分析

灰关联分析神经网络方法首先是对防洪堤边坡稳定性影响因素进行灰关联分析,得出各因素影响程度的大小排序,然后建立该工程的神经网络模型,选择灰关联分析得到的优势因子作为样本输入以及关联度作为网络权值确定的一个因素,利用大量南宁市防洪堤边坡工程实例对网络进行训练学习,最后用训练成功的网络对工程进行预测。预测的结果与实际情况进行对比表明,这个方法能够满足南宁市防洪堤边坡稳定性预测的要求。     

三峡水库蓄水后上荆江不同河床组成江心洲的演变过程及其机制

三峡及上游梯级水库群运用后,大坝下游江心洲以冲刷为主,直接影响分汊河段河势条件及航道边界的稳定性。本文采用卫星遥感影像、实测水沙及固定断面床沙级配等资料,分析上荆江枝江、沙市河段中不同河床组成江心洲的演变过程及其机理。结果表明:(1)三峡水库蓄水后(2003—2019年),沙市段沙质江心洲较枝江段卵石夹沙质江心洲萎缩更为显著,出露面积的减幅分别达31%和24%。(2)以关洲和金城洲分别代表卵石夹沙质和沙质江心洲,三峡工程运用后关洲洲头形态较为稳定,受无序采砂的影响其沙质组成的洲尾面积显著减小,而金城洲面积萎缩程度更大。(3)床沙组成对江心洲冲刷程度差异具有重要影响,关洲洲头较金城洲抗冲性更强,与其床沙在年内达到起动条件的数量更少、时长更短有关;建立了江心洲面积与水流冲刷强度及相对水深的定量关系,该关系能综合考虑水沙变化与床沙组成调整的影响,能更好地反演近期江心洲的面积变化特点。     

三峡水库175 m试验性蓄水期调度运行对洞庭湖蓄水量变化的影响

长江干流与洞庭湖存在复杂的并联型分汇关系,当三峡水库调度改变长江径流过程时,会引起洞庭湖年内槽蓄量的变化,对于洞庭湖地区防洪、水资源配置和水环境保护产生显著的影响.本文建立了枝城至螺山站的荆江-洞庭湖水流模型,利用2008-2017年的三峡水库实际调度日数据,分析有、无三峡水库调度两种情况下洞庭湖槽蓄量的变化过程,同时利用建库前和近期的水位流量关系反映河道过流能力,分析了河道调整的影响.结果表明:由河道调整引起的槽蓄量变化在汛前消落期、汛期、汛末蓄水期和枯水期分别为-3.06%、0.12%、-0.01%和-13.31%.有三峡水库影响情况下,汛前消落期由于荆江"三口"进入洞庭湖的多年平均总径流增加23.94%,洞庭湖出口处城陵矶多年平均水位升高0.53 m,阻碍了洞庭湖出流,洞庭湖多年平均槽蓄量增长13.30%;汛期由于荆江"三口"分流量减少3.54%,城陵矶水位降低0.02 m导致出湖流量增多,因此洞庭湖多年平均槽蓄量减少0.20%;在汛末蓄水期,荆江"三口"分入洞庭湖的多年平均总径流量减少37.18%,城陵矶多年平均水位降低1.33 m,导致出湖流量增多,因而洞庭湖多年平均槽蓄量减少27.74%;在枯水期,荆江"三口"多年平均总径流量增加5.61%,城陵矶多年平均水位上升0.07 m,最终洞庭湖多年平均枯期槽蓄量增加2.96%.     

长江中游沙质河段床沙级配调整过程计算

三峡工程运用后,长江中游荆江河段持续冲刷,床沙与推移质、悬移质泥沙不断交换,从而造成该河段床沙发生不同程度的调整,对长江中下游河床演变及非平衡输沙机理的研究具有重要影响.在新水沙条件下,总结分析了沙波运动特性及床沙交换方式,引入Markov三态转移概率及非均匀沙隐暴系数,得到基于状态转移概率的沙质河段床沙级配调整的计算模型.研究结果表明:(1)20092014年,沙市站年内床沙中值粒径有先增大后减小的趋势,而监利站年内床沙中值粒径则先减小后增大,且荆江河段年际床沙中值粒径总体呈上升趋势,粗化程度约为6.9%~9.3%;(2)20092014年,沙市站床沙组成中粒径d<0.062 mm的泥沙所占比重不变,0.062 mm≤d<0.25 mm的泥沙所占比重逐年减少(累计减少11.4%),d≥0.25 mm的泥沙所占比重逐年增加(累计增加11.4%),而监利站床沙组成均存在波动性变化;(3)荆江河段床沙转换为推移质的概率随着泥沙粒径的增大而增大,床沙转换为悬移质的概率随着泥沙粒径的增大而减小,而推移质和悬移质转换为床沙的概率均随着泥沙粒径的增大而增大,河床发生冲刷粗化时泥沙输移的主要形式为悬移质(概率为81%~87%),而淤积细化时床沙补给主要来源于推移质(概率为8%~12%).通过验证,本文概率模型的计算结果与实测资料符合较好,能够应用于长江中游沙质河段年际床沙粗化及年内床沙级配调整过程预报,为进一步开展三峡工程下游非均匀悬移质泥沙沿程恢复机理的研究提供理论基础.     

Characteristics of thawed interlayer and its effect on settlement beneath embankment in permafrost regions—A case study for the Qinghai-Tibet Highway

Based on ground temperatures and deformations monitored at the Xieshuihe site along the Qinghai-Tibet Highway(QTH)in permafrost regions from 2004 to 2015,variation of artificial permafrost table(APT),maximum frozen depth(MFD),thawed interlayer thickness(TIT)and ground temperature beneath embankment is analyzed,respectively.The results indicate that under the embankment,the change of APT occurred from October to December of that year and presented a deepening trend.The change of MFD occurred from April to June of that year with no obvious change trend,and TIT had an increasing trend year by year,which mainly resulted from the deepening artificial permafrost table.Mean annual ground temperature at 0.5 m depth was 3.91°C higher beneath the embankment center than that under the natural field.The rising ground temperature at shallow layer of embankment resulted in the development of thawed interlayer beneath the embankment and warming of underlying permafrost.Embankment settlement is closely associated with TIT.Greater settlement easily occurs when permafrost with higher ice content exists under the thawed interlayer,and in turn the settlement is smaller when permafrost with lower ice content exists under the thawed interlayer.     

Numerical analysis on the thermal regimes of thermosyphon embankment in snowy permafrost area

Snow covers the road embankments in winter in high latitude permafrost zones. The effect of snow cover on embankments was simulated based on field measurements of boundary conditions and initial ground temperature profile in Mohe, China. The effect of thermosyphons on the embankment warmed by snow cover was evaluated by numerical simulations as well. The results indicate that the difference of thermal regimes between non-thermosyphon and thermosyphon embankments reaches to 22 m in depth below the ground surface. It is much warmer in the non-thermosyphon embankment body in winter. Affected by the snow cover, heat flux gradually spreads into the deep ground of the subgrade over time. The permafrost table under the slope toe of a thermosyphon embankment is 1.2 m higher than that of a non-thermosyphon embankment in the 20th year. In addition, the permafrost table at the slope toe of a thermosyphon embankment is 26 cm deeper over 20 years. These results indicate that thermosyphons can greatly weaken the warm effect of snow cover. However, thermosyphons cannot avoid the degradation of permafrost under the scenarios of snow cover. Therefore, composite measures need to be adopted to keep embankment stability in snowy permafrost zones.