Seismic response of box-type tunnels in soft soil: Experimental and numerical investigation

A series of dynamic centrifuge tests were carried out at the geotechnical centrifuge facility of IFSTTAR in Nantes, to investigate the response of box-type tunnels embedded in dry sand under sinusoidal and seismic excitation, as affected by soil-tunnel relative flexibility and soil-structure interface rugosity. The system under investigation was analyzed by means of full dynamic time history analyses, implementing rigorous finite element models. The numerical models were calibrated on the basis of back analysis of tests, while the numerical predictions were compared with experimental data, in terms of soil and tunnel horizontal acceleration, soil shear strains and tunnel deformations. The validated numerical models were then employed to further investigate several aspects of the system seismic response. Results indicate a rocking deformation mode coupled with the well-known racking distortion of box-type tunnels under seismic shaking. The effect of the soil-tunnel interface characteristics and soil yielding on the racking deformation of the tunnel, the dynamic earth pressures and shear stresses around the tunnel, as well as on dynamic lining forces is also reported. Soil yielding leads to post-shaking, residual, dynamic earth pressures, shear stresses and lining forces, especially in the case of flexible tunnels, while interface characteristics affect the distributions of these response parameters around the perimeter of the tunnel section. The ability of simplified seismic design methods for tunnels to predict the response is finally discussed, by comparing their predictions with the recorded data and the numerical results.     

1-g shaking table tests on seismic enhancement of existing box culvert with partial ground-improvement method and its 2D dynamic simulation

An underground structure buried in a soft ground located in a seismic-active region may experience intolerable deformation due to earthquake loading. The collapse of Daikai Station of the Kobe subway line during the 1995 Hyogoken-Nambu Earthquake demonstrated that underground structures may also sometimes be at high risk for failure in a huge earthquake. In particular, box culverts constructed in a soft ground with a thin overburden are vulnerable to earthquakes. Some previously constructed box culverts do not meet the present requirements of seismic design standards. In this paper, numerical analyses and 1-g shaking table tests are conducted to evaluate the effect of partial ground improvement (PGI) as a seismic countermeasure for the above-mentioned existing box culverts. Particular attention is paid to the optimum pattern of PGI that can most efficiently reduce the impact of an earthquake on a box culvert. The underground structure of Daikai Station is taken as an example and its physical and geometric conditions are used to model the box culvert in the analyses and the tests. In order to accurately consider the influence of the soil-structure interaction (SSI) on the underground structures, both the soil and the box culvert are considered in the analyses and the shaking table tests. Four different patterns of PGI are considered. Based on the analyses and the tests, an optimum pattern for the PGI of existing box culverts is proposed.     

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。     

Dynamic centrifuge tests on isolation mechanism of tunnels subjected to seismic shaking

Isolation layer is one of the countermeasures to enhance seismic safety of tunnels. Its behavior under earthquake is affected by many factors such as shape of the tunnel, stiffness of the isolation layer and the characteristics of the input motion. However, current knowledge on the effects of these parameters on the seismic behavior of isolation layer is limited to lack of experimental data. This paper focuses on the mechanism of isolation layer, especially the efficacy of input motion frequencies on the seismic behavior of a square tunnel with isolation layer around its outer surface. Dynamic centrifuge tests were carried out on model tunnels which took isolation layer as seismic countermeasure using input motion of sinusoidal waves of different frequencies. Actual records of ground motions, magnified to approximate 15 g peak acceleration, formed the basis of the excitations to verify the actual efficacy. Due to the difference between model material (aluminum alloy) and prototype material (concrete), the similar flexural deformation law and the similar axial deformation law could not be satisfied simultaneously. Given the fact that cross-sectional moments were one of the main factors that influenced the safety of tunnels under dynamic loadings, the similar flexural deformation law was accepted in model preparation. The results show that the bending strains of tunnel with isolation layer around its outer surface are lower than those of tunnel without isolation layer, which indicates that isolation layer has positive effect on moment reduction, especially at corners. Increasing of the input motion frequency decreases the dynamic cross-sectional bending moments. In addition, isolation layer has little influence on frequency contents of acceleration response of tunnel. This study has clarified the mechanism of isolation layer on shock absorption, which is proved to be an effective method to improve the safety of tunnel against earthquake.     

Centrifuge and numerical model studies on the behaviour of geogrid reinforced soil walls with marginal backfills with and without geocomposite layers

The aim of this paper is to study the effect of geocomposite layers as internal drainage system on the behaviour of geogrid reinforced soil walls with marginal backfills using centrifuge and numerical modelling. A series of centrifuge model tests were carried out using a 4.5 m radius beam centrifuge facility available at IIT Bombay. A seepage condition was imposed to all models to simulate rising ground water condition. Displacement and pore water pressure transducers were used to monitor the performance of all centrifuge models. A geogrid reinforced soil wall without any geocomposite layer experienced catastrophic failure soon after applying seepage due to the development of excess pore water pressure within the reinforced soil zone of the wall. In comparison, reinforced soil wall with two geocomposite layers at the bottom portion of the wall was found to have a good performance at the onset of seepage and by embedding four geocomposite layers up to the mid-height of the wall from bottom as a result of lowering phreatic surface much more effectively. For analysing further the observed behaviour of centrifuge model tests, stability and seepage analysis were conducted using SLOPE/W and SEEP/W software packages. A good agreement was found between the results of numerical analysis and observation made in centrifuge tests. The effect of number of geocomposite layers as well as its transmissivity was further analysed using parametric study. The results of parametric study revealed that the number of geocomposite layers plays a main role on the good performance of the geogrid reinforced soil walls with marginal backfill.     

可液化地基单桩基础离心机模型试验的三维数值分析

文章通过三维水土耦合动-静一体有限元程序DBLEAVES对饱和砂土地基（Dr=40%）单桩基础的离心机模型试验进行模型试验相对应的原型三维有限元数值模拟和分析。对比分析小震（峰值加速度0.08g）和大震（峰值加速度0.47g）情况下的土体加速度、超静孔隙水压、沉降位移以及桩身弯矩等变化规律。其中,地基土的性质采用应力诱导各向异性的交变移动弹塑性模型模拟,基桩采用弹性梁单元模型模拟。结果表明：①超静孔隙水压会“隔断”振动波的传递,当土体接近完全液化时,土表面峰值加速度会明显小于输入波峰值加速度,而当超静孔隙水压比较小时,土表面加速度相对于输入波则可能会放大;②地震时所达到的最大超静孔隙水压比是地基土沉降量的主要因素之一,且一大部分沉降发生在震后的孔压消散期;③数值模拟与模型试验结果的对比分析表明,交变移动模型可以较好地反映土体在交变荷载下的动力响应特性,验证了所采用的DBLEAVES程序和有限元方法的有效性。     

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.     

黄土地区地铁车站振动台试验研究

在黄土地区地铁车站振动台试验中,测得模型体系加速度反应与位移反应,模型结构应变反应,地表沉降及土与结构间动土压力。分析了模型地基的边界效应、模型地基与结构的加速度反应规律及模型结构对地基地震反应的影响特征;归纳了不同峰值加速度及频谱特性地震动作用下,模型地基水平位移反应规律;并对模型体系地表沉降进行了分析。最后,根据现场观察及影像资料,对试验中模型地震破坏特点进行描述。结果表明：地震动向上传播时,模型地基加速度傅里叶谱低频部分增大,高频部分减小;在较大地震动下,土与结构动力作用较强,模型结构动力反应受周围土体控制;模型结构对地震动在地基中传播特性无显著影响;地震动具有一定方向性,且随着输入峰值加速度增大其方向性更加显著。研究结论可为黄土地区地铁车站、区间隧道及地下商业街等地下结构的抗震设计及相关理论研究提供可靠资料。     

考虑地震动空间非一致性的岩体地下洞室群 地震反应分析

 阐述人工合成考虑空间非一致性的多点地震动加速度时程步骤,提供合理的空间非一致地震动输入。在此基础上,建立一大型地下洞室群的三维数值计算模型,分析一致与非一致两种地震动输入情况下该地下洞室群的地震响应特征。研究结果表明,洞室群地震响应与激振源的位移时程曲线形式接近,表明围岩的惯性效应可以忽略;地震波幅值对于地下洞室地震响应具有决定性作用,而地震动的空间非一致特性对于讨论的轴向长度水平(轴线长度尺寸为300～400 m)的地下洞室的破坏未产生不利影响。因此,在进行类似的岩体地下洞室地震动安全评价时可以不考虑地震动空间非一致性的影响。     

The effect of weak interlayer on the failure pattern of rock mass around tunnel – Scaled model tests and numerical analysis

Weak interlayer is one of unfavorable geological discontinuities often encountered in underground engineering. Many failures of underground openings were reported to be closely related to the existence of weak interlayer nearby. For the purpose of exploring the effect of weak interlayer on failure pattern of rock mass around tunnel, both physical model tests and numerical analysis were carried out to simulate tunnel excavation near an interlayer. In the model tests, by comparison of the failure patterns between homogenous ground and ground with a weak interlayer, it was found that the weak interlayer affected the stability of tunnel by increasing the failure zones and causing asymmetrical stress distribution. The results of model tests were then verified by numerical analysis. Furthermore, based on the numerical analysis results, the location, dip and thickness of the interlayer as well as the distance from the interlayer to the tunnel were proved to be important factors influencing tunnel stability, and the relationships of the induced damage zones with these parameters of the weak interlayer were established. These results can provide a useful guidance for support design and safe excavation of tunnel near or crossing through a weak interlayer.     

群洞效应对紧邻交叠盾构隧道及场地土地震响应影响的初步分析

随着地下空间开发利用的深度和广度的进一步发展,近距离、复杂、相互穿越的大型地下空间工程必将大量涌现,各地下工程必将影响到其他相邻地下工程的抗震性能。以武汉地铁二号线和四号线工程在洪（洪山广场站）中（中南路站）区间的紧邻四孔交叠盾构隧道为依托,建立不同隧道数目的计算模型,分析了00093;群洞效应00094;对近距离多孔盾构隧道地震响应以及对场地土动力特性的影响规律。计算分析结果表明：①下卧隧道能在一定程度上阻断地震波的传播,起到减震的作用;②上层隧道的存在降低了周围土层对邻近隧道的约束作用,即降低了邻近隧道的抗震性能;③00093;群洞效应00094;对场地土基本周期以及地表加速度放大效应的影响均不显著;④浅层上覆土层对隧道抗震性能及场地土加速度放大效应影响显著。     

Seismic performance of group pile foundation with ground improvement during liquefaction

A pile foundation with ground improvement under the footing is a composite foundation with the objectives of enhancing the seismic performance and rationalizing the substructure by combining the pile foundation with ground improvement. Although the effectiveness of this method has been confirmed in previous studies for application to soft grounds, the applicability of this method to liquefiable grounds has yet to be fully investigated. In this study, therefore, centrifuge model tests and finite element analyses were conducted to clarify the effectiveness of this method and to ascertain the improvement in strength (stiffness) when the method is applied to a liquefiable ground. Firstly, in order to investigate the effect of an improved ground on the behavior of the pile foundation during liquefaction, dynamic centrifuge model tests were conducted for three cases with different strengths of the improved ground. Then, three-dimensional soil–water coupled finite element analyses of the centrifuge model experiments were performed to validate the applicability of the analytical method. After that, parametric studies, in which the strength of the improved ground and the input ground motion were changed, were conducted using the same analytical model. The results confirmed that the horizontal displacement of the pile heads was reduced by the improved ground even in the liquefiable ground, and that the effect of this reduction was more remarkable in cases of high stiffness of the improved ground. Furthermore, it was possible to reduce the bending moments at the pile heads by applying the ground improvement. However, since the bending moment at the boundary between the improved ground and the natural ground became the local maximum, there was an optimum stiffness of the ground improvement at which the maximum bending moment of the piles was reduced. This is because improving the ground around the pile heads has the same effect as extending the footing. It was thus concluded that the behavior of the pile foundation is similar to that of a composite foundation comprised of a caisson and group piles.     

盾构双隧道不同开挖顺序及不同布置形式对管线的影响研究

在离心模型试验中同时考虑隧道开挖所致地层损失效应和质量损失效应,研究不同开挖顺序及不同布置形式下双隧道开挖对管线的影响规律。同时采用基于地层损失比的位移控制有限单元法对离心模型试验及其他4组拓展工况进行分析,其中土体本构模型采用考虑土体小应变特性的HP(Hypoplasticity model)模型,并将试验结果与已有的解析方法进行对比。研究结果表明,双隧道不同开挖顺序及不同布置形式对地表沉降、管线沉降、管线弯曲应变的影响显著;管线存在所产生的"遮拦"效应对管线正上方地表沉降的影响程度随着自由场最大地表沉降的增加而逐渐加剧;双隧道开挖所致管线沉降的主要影响区域为-1.2D_T~1.2D_T;实际工程中应加强浅埋后继隧道开挖时管线工作性状的监测工作,且不应简单采用叠加原理对不同施工工序及不同布置形式的双隧道开挖所致地表沉降、管线沉降及管线弯曲应变进行预测,应合理考虑后继隧道开挖所致土体的累计剪切应变及上覆隧道的遮拦效应对管–土相对刚度的影响。     

地震波三维斜入射作用下隧道#br# 对场地地表地震动的影响

利用2.5维有限元-边界元耦合方法,以饱和土场地中圆形隧道为例研究了地下隧道对场地地表地震动加速度的放大效应,主要讨论了地震波入射角的影响。假定地震波在基岩面三维斜入射,用水平入射角θh和竖向入射角θv两个角度来表示地震波的入射方向。研究表明,地震波斜入射时的场地地表地震动加速度放大系数显著大于地震波垂直入射情况,且地震波入射角对场地地表加速度反应谱的幅值、形状以及卓越频率也均有一定程度的影响,尤其对竖向加速度反应谱卓越周期的影响显著。此外,针对本文算例,θh< 30°且θv> 30°时,场地地表地震动竖向加速度与水平加速度的比值大于规范推荐的0.65,且此时竖向加速度反应谱的幅值可能会大于水平加速度反应谱的幅值。     

Seismic performances of dyke on liquefiable soils

Various field investigations of earthquake disaster cases have confirmed that earthquake-induced liquefaction is a main factor causing significant damage to dyke, research on seismic performances of dyke is thus of great importance. In this paper, seismic responses of dyke on liquefiable soils were investigated by means of dynamic centrifuge model tests and three-dimensional (3D) effective stress analysis method which is based on a multiple shear mechanism model and a liquefaction front. For the prototype scale centrifuge tests, sine wave input motions with peak accelerations 0.806 m/s², 1.790 m/s² and 3.133 m/s² of varied amplitudes were adopted to study the seismic performances of dyke on the saturated soil layer foundation with relative density of approximately 30%. Then, corresponding numerical simulations were conducted to investigate the distribution and variations of deformation, acceleration, excess pore-water pressure (EPWP), and behaviors of shear dilatancy in the dyke and the liquefiable soil foundation. Moreover, detailed discussions and comparisons between numerical simulations and centrifuge tests were also presented. It is concluded that the computed results have a good agreement with the measured results by centrifuge tests. The physical and numerical models both indicate that the dyke hosted on liquefiable soils subjected to earthquake motions has exhibited larger settlement and lateral spread: the stronger the motion is, the larger the dyke deformation is. Compared to soils in the deep ground under the dyke and the free field, the EPWP ratio is much smaller in the shallow liquefiable soil beneath the dyke in spite of large deformation produced. For the same overburden depth soil from free site and the liquefiable foundation beneath dyke, the characteristics of effective stress path and stress–strain relations are different. All these results may be of theoretical and practical significance for seismic design of the dyke on liquefiable soils.     

双仓综合管廊抗震性能模型试验研究

利用振动台试验系统,基于模型试验相似理论开展了双仓地下管廊的抗震性能试验研究。试验中,采用1952年Taft地震波作为输入地震波,并将其输入加速度峰值调整为0.2g,0.4g,0.8g和1.2g,以考虑不同PGA（peak ground acceleration）的影响。通过分析试验数据得知,管廊侧壁最大动土压力响应沿深度呈倒立“W”形分布,振动后管廊结构与周围土体之间的土压力场发生了改变;管廊侧壁土体中最大加速度响应随输入PGA增大而增大,沿深度整体呈减小趋势,加速放大系数为0.5~1.5;从加速度时程曲线和傅里叶谱来看,管廊侧壁结构与其周边土体的加速度响应基本一致,在15~30 Hz的频段土体中的振幅稍大于结构;地震过程中,结构拐角处产生较大的弯矩响应,且随输入PGA的增大而增大。同时结合ABAQUS数值软件开展了数值模拟研究,以和模型试验结果开展对比比较,结果表明试验结果具有较高的可靠性。     

黏性土爆炸成坑和地冲击传播的离心模型试验研究

 以往研究结果表明化爆是一种体积效应引起的热力学现象,根据这一现象可以确定爆炸离心模型试验的能量比尺为1/N3(N为离心机加速度)。采用清华大学50 g•t土工离心机进行黏性土爆炸成坑和地冲击传播规律的离心模型试验,给出试验过程中实测加速度波形,对测试结果进行分析,得出黏性土中爆炸成坑和地冲击传播的一些认识：弹坑形成过程基本不受科氏加速度影响,但弹坑抛出物以及分布形态受科氏加速度影响;弹坑体积大小明显与重力加速度有关,小比尺模拟必须增加重力加速度来模拟。结果证明对于研究爆炸相关问题,土工离心机是一个非常有效的试验工具。     

Shaking table tests of tunnel linings in progressive states of damage

Tunnels have long been assumed to be able to withstand earthquakes and suffer little damage. However, investigations of tunnels after the Wenchuan earthquake in China revealed that over 30 tunnel linings were cracked. Different types of cracks were found in almost all the tunnel linings. In this study, a series of shaking table tests were conducted on scaled tunnel models under seismic excitations with increasing intensities. White noise sweep tests were interlaced with the seismic excitations to identify the damage in the tunnel linings. The accelerations and strains of the tunnel lining were measured. The test results are discussed based on the dominant frequencies, amplification factor, and lining strains. Furthermore, this paper presents the results of damage identification for various states of damage using the white noise sweep method. The dominant frequencies decreased with an increase of the input peak ground acceleration (PGA), which also reflected the damage of the tunnel lining. The dominant frequencies decreased by approximately 32% for an input PGA intensity of approximately 0.6 g, but the first cracks could still be visually detected. The cracks gradually propagate with the PGA increasing from 0.6 to 0.8 g. The effect of lining damage on the seismic performance of tunnel linings should be considered in the seismic design of tunnels.     

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。     

大型地下洞室群地震响应分析的动力子模型法

 水电站地下洞室群埋深大,在动力分析时若将模型建至地表将使网格规模庞大,不利于分析效率的提高。提出适合于大型地下洞室群地震响应分析的动力子模型法。该方法将大范围、粗网格的地下地震波动场计算与小范围、细网格结构动力计算分开,分别建立粗网格和细网格模型进行分析。首先,根据深部基岩的地震波传播特性,采用幅值线性折减的方法对地表加速度时程向基岩深处推算,得到在模型底边界的加速度时程,再输入模型可算得大范围的地震波动场。其次,把结构计算模型边界节点放入波动场计算模型中插值,可获得计算模型边界条件,最后,完成地震荷载的输入。算例表明,推算所得模型底部加速度时程考虑工程具体条件,比规范规定的单一折减系数更能反映实际工程的地下地震动特性。同时,在地下洞室的地震响应分析中,地震波过早被地表反射或不考虑地震波的反射,都会对计算结果造成误差,动力子模型方法能够在保证计算结果精确性的同时,使细网格的计算模型不建至地表自由面,可显著缩减网格规模,提升动力计算效率。