下伏基岩堆积体边坡抗滑桩加固前后地震响应特征离心模型试验

 为研究抗滑桩加固上覆堆积体--下伏基岩二元结构边坡的抗震机制,开展2组1∶50比尺的离心振动台模型试验,以对比分析下伏基岩堆积体边坡在抗滑排桩加固前后的地震响应特征与抗滑桩的桩身弯矩分布规律。试验时,输入4级加速度峰值连续增大的El Centro波,监测边坡模型坡面与坡体内的加速度响应、坡顶沉降变形以及抗滑桩上静、动弯矩的分布。试验结果显示由于抗滑桩抑制了上覆堆积体的下滑,坡顶的加速度峰值(PGA)放大系数、加速度反应谱以及竖向沉降变形均有不同程度的降低。抗滑桩一方面加固了上覆堆积滑体另一方面在坡体内产生了地震波的反射叠加效应,使得边坡水平响应加速度放大系数出现了桩前增大桩后减小的现象。下伏基岩堆积体边坡坡顶沉降与Arias烈度在抗滑排桩加固前后均具有良好的正相关线性关系。地震荷载作用过程中抗滑桩动力响应弯矩变化幅值明显大于地震作用后的静弯矩增量,且静弯矩与动弯矩变化幅值的分布均在基岩面附近达到峰值,易在基岩面附近造成抗滑桩的破坏,类似工况下抗滑桩的抗震配筋设计应充分考虑这一特点。     

风干堆积体边坡地震响应的动力离心模型试验

 堆积体边坡在我国西南地区广泛分布,为深入研究其地震响应规律,设计完成了1∶50比尺的概化边坡离心振动台模型试验,分析4级不同强度地震连续作用下,风干堆积体边坡的加速度响应、边坡变形及其失稳模式。试验结果表明,堆积体边坡水平向PGA放大系数表现出了典型的高程放大效应与趋表放大效应。沿堆积体边坡高程方向,输入地震波频谱特性发生了明显改变,各测点加速度傅里叶谱的卓越频率随PGA增大而降低。考虑竖直向加速度放大效应的影响后,发现合放大系数与水平向夹角随高程有减小的趋势,反映了坡面处发生的波场分裂与波型转换现象。随地震波幅值的增大,水平向与竖直向PGA放大系数均先减小后增大。试验过程中观察发现在地震波加速度峰值达到0.216 g时堆积体边坡开始失稳,坡顶沉降明显,失稳模式以浅层崩滑为主。     

Shaking table tests on gravel slopes reinforced by concrete canvas

The behaviour and performance of different reinforced slopes during earthquake loading were investigated through a series of shaking table tests. Concrete-canvas and composite reinforcement (geogrid attached to concrete-canvas) were proposed for reinforcing slopes. By considering the effects of different reinforcement methods, the seismic responses of the reinforced slopes were analysed, along with the accelerations, crest settlements, and lateral displacements. The failure patterns of different model slopes were compared using white coral sand marks placed at designated elevations to monitor the internal slide of the reinforced slopes. Both the concrete-canvas and composite reinforcement could increase the safety distance, which ranged from the slide-out point to the back of the model box. The composite reinforcement decreased the volume of the landslide and increased the failure surface angle as a result of the larger global stiffness in the reinforced zone. These results indicate that the recently developed concrete canvas has a better effect on restricting the slope deformation during seismic loading than the nonwoven geotextile reinforcement, and that the use of composite reinforcement could improve the seismic resistance of slopes.     

Simplified method for analyzing soil slope deformation under cyclic loading

Reasonable assessment of slope deformation under cyclic loading is of great significance for securing the safety of slopes. The observations of centrifuge model tests are analyzed on the slope deformation behavior under cyclic loading conditions. The potential slip surface is the key for slope failure and follows two rules: (i) the relative horizontal displacement along the potential slip surface is invariable at an elevation, and (ii) the soil along the slip surface exhibits the same degradation pattern. These rules are effective regardless of the location of the potential slip surface throughout the entire deformation process of a homogeneous slope, ranging from the initial deformation stage to the failure process and to the post-failure stage. A new, simplified method is proposed by deriving the displacement compatibility equation and unified degradation equation according to the fundamental rules. The method has few parameters that can be determined through traditional element tests. The predictions from the proposed method agree with the centrifuge test results with vertical loading and shaking table loading. This result confirms that the proposed method is effective in predicting the full deformation process of slopes under different cyclic loading conditions.     

Behavior of Sheet Pile Quay Wall Stabilized by Sea-Side Ground Improvement in Dynamic Centrifuge Tests

Sea-side ground improvement is one of the techniques used for increasing seismic stability of sheet pile quay wall. A series of centrifuge shaking table test is conducted to investigate the seismic behavior of sheet pile quay wall stabilized with sea-side cement deep mixing (CDM). At 50 g centrifugal acceleration, the clay ground is consolidated and four to five stages of shaking are applied to the quay wall. The recorded input-output accelerations, deflection of the quay wall, earth pressures along depth and pore pressures were used to evaluate the behavior of the stabilized sheet pile quay wall. The results indicated that an improved area provides significant resistance against seismic loading. The ground improvement can reduce 30% to 60% bending moment during the seismic loading. The horizontal deflection of the quay wall decreases rapidly with increase in area of the CDM until it reaches a certain limit. Prediction by the numerical model agrees fairly well with the results of centrifuge model tests.     

抗滑桩加固黏性土坡变形规律的离心模型试验研究

采用土工离心机及专用振动台,进行了静动力加载条件下抗滑桩加固黏性土坡的离心模型试验。测量了试验过程中边坡的位移场和加速度响应的变化过程以及抗滑桩的位移和应变分布。试验结果表明地震过程中土坡的加速度响应自下而上逐渐增大,震后残余变形的水平分量最值相比震前向坡上部移动。静动力加载过程中,抗滑桩内侧土体存在一个面,其内外两侧的土体位移表现出不同的水平位移变化规律,从而可以将抗滑桩加固土坡划分成4个分别具有不同变形特性的区域;同一高程处桩的水平位移大于内侧土体而小于外侧土体;抗滑桩与土发生较复杂的相互作用,并使得土坡的变形趋于均匀。     

基于离心振动台的堆积型滑坡加速度响应特征

 加速度响应规律是解释滑坡震害、合理确定地震影响系数的基础。为此,设计完成了50倍重力加速度条件下的堆积型滑坡离心振动台模型试验,用来研究堆积型滑坡的加速度响应特征及规律。模型滑坡放置于600 mm×400 mm×500 mm(L×W×H)的刚性模型箱内,采用汶川地震清溪台站反演的基岩波作为基底输入,调整其幅值,研究不同强度地震动作用下堆积型滑坡的加速度响应特征及规律。试验结果表明：坡面水平向和竖直向峰值加速度(PGA)放大系数随滑坡的高程增加而增大,趋于坡顶时,增速明显变大,具有明显的高程放大效应;地震动作用下坡面与坡体内部的加速度响应特征明显不同,具有坡面浅表放大效应;滑床岩体自下向上水平向加速度有放大趋势,但与滑坡浅表土体相比,放大倍数则明显减弱;坡顶附近存在显著的波型转换现象;随着输入地震波强度的增大,PGA放大系数总体上表现为递减趋势。所得到的成果初步揭示了堆积型滑坡的加速度响应特征,为解释滑坡震害、确定地震影响系数等提供较可靠的依据。     

地震作用下堆积体边坡振动台模型试验及抛出现象分析

国道213线都江堰-汶川段普遍发育着堆积体边坡。在“5.12汶川地震”中,此类边坡更易发生滑坡灾害,并伴有独特的山石抛出现象。结合堆积体边坡实例进行振动台模型试验,研究堆积体边坡在多次地震作用下结构损伤累积并最终发生失稳的特征及边坡破坏模式。试验结果表明：堆积体边坡中上覆堆积体与下部基岩存在动力响应差异,边坡滑坡形式不同于静力失稳,表现为裂缝一旦出现便迅速扩展,边坡坡肩处部分小块体在水平和垂直向动力耦合作用下与原结构脱离抛出,这与汶川地震后边坡调研中发现的抛出现象一致。分析了模型边坡块体抛出现象与裂纹形成角度的关系,只有当裂纹角度和竖向加速度组合满足一定的触发条件时,才可能出现抛出现象。     

抗滑桩加固边坡地震响应离心模型试验

为研究抗滑桩加固边坡的地震响应和桩土相互作用规律,利用土工离心机及专用振动台进行了砂土边坡的动力离心模型试验。在50g离心加速度条件下,输入El Centro地震波,记录了边坡不同位置的加速度时程并作频谱分析,采集了桩前动土压力和抗滑桩应变等。结果表明:边坡的地震动力响应自下而上逐渐放大;抗滑桩对周围土体响应有一定阻滞作用;桩前动土压力随着地震输入而迅速增大至峰值,此后保持稳定直至地震结束;抗滑桩各截面弯矩的变化规律与动土压力类似,弯矩最大值出现在抗滑桩下部。     

黄土斜坡坡面位移和加速度响应特性的振动台试验研究

以平凉崆峒区黄土塬斜坡为原型,开展含裂隙黄土斜坡和不含裂隙黄土斜坡的对比振动台模型试验,研究地震荷载作用下黄土斜坡坡面位移和加速度响应规律。通过三维光学测量分析系统,高精度、实时获得斜坡表面的变形量。试验表明：随着输入地震荷载的增加,含裂隙黄土斜坡和不含裂隙黄土斜坡坡面PGD均呈现出随着坡高的增加逐渐增大的变化趋势;在相同地震波输入下,水平向加载作用下坡面峰值位移略大于垂直向加载,有裂隙斜坡坡肩处位移值约为无裂隙斜坡的1.5倍。通过对比坡面不同高度处的PGA放大系数,表明两类斜坡水平向的加速度放大效应大于垂直向放大效应,并在坡肩达到最大值,呈现出高程效应与趋表效应;有裂隙斜坡坡肩处的PGA放大系数明显大于无裂隙斜坡。     

Earthquake-Induced Flow Slides of Fills and Infinite Slopes

Earthquake induced flow slides of saturated or partially saturated sloping ground have taken place in man-made earth structures during past earthquakes. Two series of dynamic centrifuge model tests (1:50 scale) were conducted to simulate the behavior of embankment fills and infinite slopes resting on a stiff sloping base. Conditions that yield earthquake-induced flow slide were investigated. The test parameters included soil density, slope angle, level of phreatic water and shaking intensity. The test results showed that downward deformation increased with decreasing soil density, increasing phreatic water elevation and shaking intensity, and that there existed a threshold soil density below which flow slides occurred. Undrained monotonic loading torsional simple shear tests were also conducted. Conditions inducing very large deformation or flow slide that were obtained by the undrained shear tests were consistent with those observed in the model tests.     

循环荷载下节理岩体边坡动力响应的试验研究

 在振动荷载的长期循环作用下,岩质边坡中的原生结构面可能扩大并产生新的破裂面,直接影响到岩质边坡的稳定性。以某节理岩体边坡为原型,根据相似理论制作模型框架并在其内建造室内节理岩体边坡模型,利用振动台对边坡模型进行振动加载,研究在振幅分别为1.0,1.25,1.5 mm的振动荷载作用下节理岩体边坡的动力响应规律,对比水平方向和竖直方向振幅都为1.5 mm时边坡模型的变形差异。循环加载试验表明,在振动荷载作用下,边坡变形随振动荷载振幅、振动时间的增大而增大;边坡模型对于竖向的振动荷载更加敏感;边坡模型在受到振动荷载作用时,顶部的变形大于底部的变形;长期的振动荷载作用会导致原生裂隙延伸、扩张、贯通,引起边坡变形增大;根据变形情况提出边坡在振动荷载作用下的变形过程及变形导致的整体性滑坡和局部滑塌2种可能的破坏模式。研究结果对节理岩体边坡在循环荷载作用下的动力响应、疲劳劣化机制和边坡工程的长期安全性等理论研究和工程应用具有参考意义。     

含软弱夹层斜坡地震动力响应特性的振动台试验研究

 斜坡中的软弱夹层往往控制着其变形破坏模式。在地震作用下,软弱夹层与地震波的复杂作用机制使得该类斜坡的地震响应特征很难被充分了解。在已有认识的基础上,开展4个含软弱夹层斜坡模型的振动台试验,旨在通过与均质斜坡模型响应进行比较,揭示含软弱夹层斜坡的地震动力响应特性,并观察软弱夹层在这一响应过程中所起的作用。主要研究结果为：(1) 4个含软弱夹层模型的水平分量(PHA)和竖直分量(PVA)加速度响应均呈现出高程放大效应,且主要体现在夹层以上部位。在夹层以下部位,水平分量加速度放大系数始终保持在1.5以内。(2) 与均质模型响应的比值表明,在夹层以上部位,夹层的存在对2个加速度分量的影响与夹层特征和激振强度密切相关。当激振强度较弱(≤0.3 g)时,厚夹层和薄夹层的存在都能对PHA和PVA响应起到增强作用。当激振作用增强时,厚夹层表现出了隔震作用,在坡顶,PHA和PVA响应相对均质模型分别减弱50%和70%。(3) 与加速度响应一致,坡表水平向位移响应也呈现出了高程放大特征,且在坡顶最大。然而,与均质斜坡模型的初始变形出现在坡顶这一现象不同的是,含夹层模型的变形最早出现在夹层以上、坡顶以下的部位。分析产生这一差异的可能原因在于,当激振强度较大时,含夹层模型相对于均质模型在该部位的水平向响应强度显著增强,而夹层的挤压变形也可能造成初始变形部位更靠近夹层。(4) 含软弱夹层模型的最终破坏部位和破坏程度,与夹层的厚度和倾角密切相关,表现为含水平夹层模型的破坏部位高于含反倾夹层模型,而含薄夹层模型的破坏程度高于含厚夹层模型。     

强震过程滑带超间隙水压力效应研究:大光包滑坡启动机制

大光包滑坡是汶川地震触发的最大规模滑坡,其滑带地质背景为埋深400 m的饱水层间构造破碎带。为研究强震过程该带间隙水压力(孔隙和裂隙水压力)响应特征,设计夹饱水软弱层地质体单元模型,并开展系列振动台试验,结果表明:振动过程软弱层超间隙水压力显著大于上下硬层,且具有瞬态生成和逐渐累积特性,进一步揭示依赖模型非协调变形的"振动冲压-拉张"和"振动剪切"是瞬态超间隙水压力成因、依赖软弱层非协调变形的动力损伤是超间隙水压力累积成因,从而建立了垂直和水平振动耦合作用下包含间隙水压力系数r_w和间隙水压力作用的有效面积系数h的地震滑坡滑带有效应力模型。对于大光包滑坡,滑带h估测范围为0.7~0.9,当r_w为0.73~1.04时滑带抗剪强度可降为0;振动台试验表明,汶川强震过程大光包滑坡区地震动参数可使r_w达到这一水平。最后,提出大光包滑坡启动的"强震作用-层间错动带控滑-动力非协调变形致损、扩容-超间隙水压力激发-滑坡启动"演化过程机制。     

Experimental investigation on the performance of multi-tiered geogrid mechanically stabilized earth (MSE) walls with wrap-around facing subjected to earthquake loading

Construction of mechanically stabilized earth (MSE) walls in multi-tiered configurations is a promising solution for increasing the height of such walls. The good performance of this type of walls after recent major earthquakes was reported in a number of technical studies. In the present study, an experimental approach was adopted to compare the seismic performance of single-tiered and multi-tiered MSE walls using physical modeling and through conducting a series of uniaxial shaking table tests. To do so, several geogrid-reinforced soil walls with wrap-around facing (i.e., three-, two-, and single-tiered) with a total height of 10 m were designed in the form of prototypes of 1-m-height wall models. The step-wise intensified sinusoidal waves were applied to the models in 14 typical forms. Comparing the shaking table test results confirmed the post-earthquake advantages of multi-tiered MSE walls. The results revealed that tiered walls exhibited better behaviors under earthquake loading in terms of the seismic stability of the wall, displacement of the wall crest, horizontal displacement of the wall facing, deformation mode and failure mechanism of the wall, settlement of backfill surface, and seismic acceleration responses.     

Centrifuge model study on low permeable slope reinforced by hybrid geosynthetics

The objective of this paper is to study the performance of hybrid geosynthetic reinforced slopes, with permeable geosynthetic as one of its components, for low permeable backfill slopes subjected to seepage. Four centrifuge tests have been performed to study the behavior of hybrid geosynthetic reinforced slopes subjected to seepage, keeping the model slope height and vertical spacing of geosynthetic reinforcement layers constant. Centrifuge model tests were performed on 2V:1H slopes at 30 gravities. One unreinforced, one model geogrid reinforced and two hybrid geosynthetic reinforced slope models with varying number of hybrid geosynthetic layers were tested. The effect of raising ground water table was simulated by using a seepage flow simulator during the flight. Surface movements and pore water pressure profiles for the slope models were monitored using displacement transducers and pore pressure transducers during centrifuge tests. Markers glued on to geosynthetic layers were digitized to arrive at displacement vectors at the onset of raising ground water table. Further, strain distribution along the geosynthetic reinforcement layers and reinforcement peak strain distribution have been determined using digital image analysis technique. The discharge for the performed model tests is determined by performing seepage analysis. It was confirmed by the centrifuge tests that the hybrid geosynthetics increases the stability of low permeable slope subjected to water table rise. The hybrid geosynthetic layers in the bottom half of the slope height play a major role in the dissipation of pore water pressure.     

地基自由场离心机振动台模型试验研究

在清华大学土工离心机实验室50g 离心场环境中进行了一系列地基自由场离心机振动台试验。试验包括1个砂土地基、1个黏土地基和2个成层土地基振动台试验。试验中模型箱采用叠环式模型箱,其平面尺寸为0.5 m和0.2 m,试验中输入地震波采用Kobe波和Parkfield波。给出了部分试验结果分析,包括加速度时程及频谱反应、加速度峰值放大系数和土层中剪应力-剪应变变化曲线。试验结果发现,不同类型地基中加速度反应存在差异,在低频部分加速度峰值放大系数从底部到顶部逐渐增大。土体中剪应力-剪应变变化规律在砂土地基和黏土地基中存在差异。     

土体含水率对边坡动力破坏模式及动力响应影响的振动台试验研究

正确认识地震作用下边坡的破坏模式及动力响应特征可以为边坡抗震设计提供理论指导。为了探讨边坡土体含水率的变化对边坡破坏模式及动力响应的影响,针对该问题展了室内振动台模型试验,分析了不同含水率边坡失稳破坏的物理过程,以及在地震荷载作用下边坡动力响应规律。试验结果表明土体含水率对边坡破坏模式有较大影响,含水率6.8%和10%的砂土边坡分别表现为震裂–溃滑型和震裂–溃散型滑坡破坏形式,含水率18.1%和24.6%的黏土边坡分别表现为震裂–溃散性和震裂–蠕滑型滑坡破坏形式;在参数考虑的范围内含水率较大的边坡比含水率较小的边坡更稳定;在水平动力载荷作用下,边坡表面的土体位移变化规律能够反应出边坡失稳破坏特征;对于该文讨论的砂土和黏土边坡而言,含水率大的边坡加速度放大效应弱于含水率小的边坡;从土体阻尼角度解释了含水率变化对边坡失稳影响的机理。     

地震作用下黄土滑坡加速度深度放大效应及震后变形模式研究

基于相似比理论,设计并完成了典型黄土滑坡物理模型试验,采用先进的离心机振动台技术,实现水平+垂直振动,研究黄土滑坡的地震动放大效应及变形模式,并配合有限差分数值模拟方法相互验证。结果表明：沿滑坡体浅表层加速度放大作用具有明显的趋表效应,水平向和垂向加速度放大效应呈非线性增加,且水平向大于垂向;在滑坡体的滑动面附近加速度放大作用呈现出岩性结构效应;随高程增加加速度响应逐渐增大,表现出高程效应,滑坡后壁放大作用明显。随入射地震波强度的增加,滑坡体内部关键部位加速度放大作用基本是先增大后减小的趋势。强震作用下黄土滑坡的破坏形式为：滑坡后壁形成拉裂隙并逐渐扩展,滑坡后壁发生崩塌,滑体略有下挫,形成拉槽,坡体中部鼓胀,坡脚有大量崩积物。研究结果为探讨地震作用下黄土滑坡的加速度放大效应和变形破坏情况,以期为天水地区黄土滑坡的地震稳定性评价和抗震设计提供参考。     

锚杆格构支护边坡振动台模型试验研究

 以锚杆格构支护的均值土坡为研究对象,设计并完成了几何相似常数为15的硅胶边坡模型振动台试验,研究了支护结构的动力地震响应特征。通过试验,得出如下结论：(1) 在同一正弦波激励下,格构同一测点的动应变值的变化范围是基本保持不变的,与正弦波激励的变化规律一致。但在破坏阶段,测点的动应变值表现为无规律的变化且应变值均较大。(2) 同一激励下,横、纵格构梁的受力水平是基本相平的;对于同一行格构,中间应变值较大;同一列格构,由上到下呈“大→小→大→小”的变化特征,说明边坡竖向变形变化很大且受锚杆约束的影响;格构梁的不同部位受力不同,需要对薄弱部位进行补强。(3) 相同频率下,格构测点的动应变值随着加速度的增大,整体上表现为递增趋势。(4) 低频正弦波的频率较接近边坡的自振频率,坡体加速的动力响应就明显,边坡的相对位移则越大,因此,格构的动应变相应增大。试验结果表明,试验采用的硅胶边坡模型可用于边坡支护结构的动力响应分析,且模型可以重复使用,极大地降低了试验成本。