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.     

液化场地浅埋钢筋混凝土结构物变形及 动土压力分析

 基于多重剪切机构塑性模型和液化前缘面的有效应力分析方法,分析不同地震强度下液化场地中浅埋大断面矩形钢筋混凝土结构物变形与地震动土压力分布特征,进而探索0.85 g输入地震波条件下结构物与液化土间的相对位移差、结构物侧壁和顶底板土体的动土压力、剪切应力、有效应力和超孔隙水压力的变化规律。研究得出结构物的最大变形、弯矩和曲率值随着地震强度的加大而增大,结构物最先发生屈服变形部位位于拐角处,并逐步向周围扩展;场地发生液化模型中的结构物–液化土相互作用系数数值小于场地未发生液化模型,结构物与土体间的相对位移差值随着场地液化而剧增到一定值;作用于结构物侧壁的动土压力最大值和震后值随地震强度加大而增加,但不是简单的线性增长;结构物侧壁动土压力随着振动持续而增长,而作用于顶底板土层的剪切应力和侧壁有效应力随着土体液化而剧减。研究结论可为液化场地浅埋结构物的抗震设计提供可靠的依据和参考。     

可液化地层中地铁隧道地震响应数值模拟及其试验验证

饱和砂土地层中的地下结构在地震作用下可能因地基液化而发生破坏。采用动力固结两相体有限元程序DIANA SWANDYNE-II对可液化地层中地铁隧道结构的地震响应进行了模拟,并与动力离心模型试验结果对比以验证其效果。选用广义塑性模型Pastor-Zienkiewicz III模拟可液化土的动力特性,基于Biot方程的u–p形式建立有限元方程,进行饱和土动力固结的耦合计算。计算表明,该数值模型可较合理地模拟地下结构的地震反应特性,计算结果与试验现象基本相符。地基液化引起的结构附加内力及隧道上浮主要受地基液化时土水压力变化的影响,截断墙的设置可有效减轻隧道结构的上浮。     

软土地层中的圆形隧道载荷模式研究

对圆形隧道作用荷载的影响因素，如隧道埋深、施工工艺以及计算方法等作了系统分析。通过对作用在上海软土地层中的3条隧道上的土压力实测值的分析表明，作用在隧道上的土压力值及其分布规律不仅由于隧道的施工方法（挤压盾构或者EPB盾构）有较大差异，而且在隧道的施工阶段和正常使用阶段也不同。在隧道施工阶段，挤压盾构法隧道衬砌呈现“竖鸭蛋”变形，而EPB盾构隧道衬砌则呈现“横鸭蛋”变形。根据经典的主动土压力理论和静止土压力理论计算出的隧道侧压力小于现场的实测值，且二者的偏差较大。根据本文提出的圆形隧道土压力的余弦变化规律理论对3条隧道的荷载进行计算，计算结果与实测值吻合得很好。     

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.     

基坑开挖对旁侧隧道影响及隔断墙作用离心模型试验研究

开展了干砂地层中基坑开挖对旁侧隧道影响及隔断墙保护作用的三维离心模型试验和数值分析,获得了隧道上浮、隧道内力、隧道周围土压力、地表沉降等变化规律以及隧道空间位置和基坑开挖深度的影响。试验结果表明,基坑回弹量与采用Boussinesq解计算的回弹量比较接近;地表沉降量与文献报道的试验结果相近,而明显小于现场实测沉降;靠近基坑一侧的隧道周围土压力有所减小,而远离基坑一侧的隧道周围土压力则有所增加。隔断墙的设置可以一定程度上减小地表沉降、隧道外土压力变化、围护墙水平位移以及隧道上浮和弯矩。数值计算结果表明,隧道上浮量和水平位移随着隧道埋深及其与围护墙距离的增大而减小,而随着基坑开挖深度的增大而增大。     

The uplifting behavior of shallow tunnels within the liquefiable soils under cyclic loadings

The liquefaction of soils under earthquake loadings has always been a main concern for geotechnical engineering practices. As an earthquake causes the ground to liquefy, the effective stress and hence the shear strength of the soil decreases sharply, and large deformations happen in the area. This phenomenon occurs only rarely when the liquefaction occurs at a large depth. However, deformations increase extensively when this layer is located in shallow depths near the ground level. In this case super structures and also underground structures may be severely damaged. The tunnels which are constructed in this layer may be affected by the liquefaction as well. In this condition the liquefaction may cause settlement in the ground, deformation in the tunnel shield, buoyancy in the underground buildings, reduction in bearing capacity and increases in lateral spread and pore pressures. In this paper the FLAC software has been used to model the pore pressure changes during earthquakes that lead to soil liquefaction. The studies thus far have been focused on the impact of the soil liquefaction on the tunnels constructed in this area. According to the studies, the buoyancy and uplifting forces due to liquefaction have major effects on the behavior of underground structures. Increasing the soil parameters such as friction angle and damping ratio causes the liquefaction effects on the tunnel to decrease, and increasing the geometric parameters such as tunnel diameter and location depth causes these effects to increase.     

相对刚度对圆形隧道结构地震反应影响规律的研究

隧道结构与周围地层的相对刚度是两者之间相互作用的根本,本文针对圆形隧道,采用动力有限元方法分别讨论了土质条件(剪切波速)、衬砌厚度、衬砌直径等参数对结构地震反应的影响规律。然后引入柔度比的概念来综合考虑上述因素的影响。计算结果显示:柔度比越大,隧道结构的地震内力反应越小,并且对计算结果进行拟合得到衬砌内力与柔度比的函数关系,可以作为隧道结构抗震分析、设计的参考。     

Interaction of tunnel linings and soft ground

Although soil-lining interaction is highly dependent on the tunnelling technology used, most of the available design methods for tunnel linings fail to take into consideration this important factor. During tunnel excavation, the in-situ stresses are significantly altered, depending on the tunnelling technique as well as the configuration of the tunnel and the characteristics of the soil deposits. The reduced radial stresses are the starting point of the soil-lining interaction at lining activation. This paper presents a method of lining design that considers the details of the excavation procedure and lining installation. Interaction between the tunnel lining and the ground is analysed in two stages—excavation and interaction. The excavation stage is responsible for determining the pre-lining soil deformations and the reduced in-situ stresses. The interaction stage models the soil-lining system together. Soil continuum, tunnel lining, and the interface between them are idealized in the whole system using nonlinear finite-element techniques. The deformations of the soil-lining system, as well as the lining internal forces, and equilibrium soil pressures are determined. Finally, results of the proposed analytical method as well as commonly used procedures are compared with field measurements compiled during the construction of two tunnels in which a precast segmental lining and rib and lagging lining were used.     

Assessment of damage in mountain tunnels due to the Taiwan Chi-Chi Earthquake

Tunnels, being underground structures, have long been assumed to have the ability to sustain earthquakes with little damage. However, investigations of mountain tunnels after the Chi-Chi Earthquake in central Taiwan revealed that many tunnels suffered significant damage to various extents. This work describes the findings of a systematic assessment of damage in the mountain tunnels in Taiwan after the earthquake. It was found that among the 57 tunnels investigated 49 of them were damaged. The damage patterns are summarized based on the characteristics and the distribution of the lining cracks. This systematic investigation, involving geological conditions, design documents, construction and maintenance records of these tunnels, has been conducted to assess the potential factors that may have influence on the various damage patterns and the earthquake loading for tunnels. The results show that the degree of damage is associated with the geological condition and structural arrangement of the tunnel. A tunnel passing through a displaced fault zone will definitely suffer damage. The extent of geological weak zones, distance from the epicenter, and the existence of a slope face are also significant influencing factors. The seismic capacity of the tunnel is influenced by its structural arrangement, type of lining, invert setup, lining reinforcement, and other parameters.     

Shaking table tests on seismic measures of a model mountain tunnel

A series of three dimensional (3D) shaking table tests were carried out to investigate the mechanism and effect of seismic measures of mountain tunnel using a scaled model based on a real tunnel. Key technical details of the experiment, including similarity relations, seismic measures simulation, boundary conditions, sensor layout, modeling methods, and ground motion input were presented. Main seismic measures, including reinforcing surrounding rock with anchors, increasing lining flexibility with steel wire mesh, and installing seismic isolation layer between reinforced surrounding rock and tunnel lining, were investigated in this study. Experiment results show that: (1) adding a layer of steel wire mesh in the tunnel lining can improve the flexibility and seismic performance and also may effectively prevent radial cracks from crossing the lining; (2) installing a geofoam isolation layer between the reinforced surrounding rock and the tunnel lining reduces dynamic earth pressure by 70–90% for the lining without a seismic isolation layer; (3) the flexible joints can effectively avoid global failures of tunnel lining for they reduce dynamic strain and bending force in the tunnel lining and decrease the seismic energy transmission along the lining in axial direction; (4) reinforcing surrounding rock with anchors significantly reduces dynamic earth pressure and strain of the lining by about 50%. In addition, the length of seismic reinforcement for general mountain tunnel portal is recommended to be 50 m from the tunnel portal along the axial direction.     

隧道上覆土柱水平地震力传递机制及动力模型试验研究

为改变隧道横断面抗震计算静力法中上覆土柱水平地震力传递机制不清的现状,以Ⅴ级围岩为例,利用动力有限元法对隧道上覆土柱水平地震力传递机制进行研究,对静力法进行了修正,并通过动力模型试验予以验证。研究结果表明：隧道上覆土柱水平地震力以均布剪切摩擦力形式作用于二衬上半拱圈|埋深25m条件下,隧道上覆土柱水平地震力传至二衬结构上的摩擦力最大|埋深小于50m时,地震烈度越大,摩擦力越大|埋深大于50m时,摩擦力受地震烈度的影响很小,隧道进入深埋状态,结构受地震影响很小|以均布剪切摩擦力代替水平集中力修正静力法,修正后静力法的抗震计算结果偏于安全,比原静力法计算结果更接近动力计算结果,更能反映实际地震中隧道结构的动力性能。研究成果可为隧道抗震设防设计提供参考。     

Contact interface in seismic analysis of circular tunnels

The seismic analysis of underground structures requires a careful consideration of the important effect of shear strains in the soil due to vertically propagating horizontal shear waves. These strains result in ovaling deformations of circular tunnels or racking deformations of rectangular tunnels. Closed-form solutions as well as numerical analyses are used to characterize this soil-structure interaction problem. Many of these solutions assume full normal contact at the interface between the soil and tunnel lining. This work describes a numerical finite element study of soil-circular tunnel lining interaction with contact conditions that allow both limited slippage and separation to prevent development of potentially unrealistic normal tensile and tangential forces at the interface. The analyses highlight the significant limitations of widely used closed-form solutions in engineering practice. The finite element solutions demonstrate the need for realistic representation of the soil-tunnel interaction using numerical modeling approaches.     

Centrifuge experiments for shallow tunnels at active reverse fault intersection

Tunnels extend in large stretches with continuous lengths of up to hundreds of kilometers which are vulnerable to faulting in earthquake-prone areas. Assessing the interaction of soil and tunnel at an intersection with an active fault during an earthquake can be a beneficial guideline for tunnel design engineers. Here, a series of 4 centrifuge tests are planned and tested on continuous tunnels. Dip-slip surface faulting in reverse mechanism of 60-degree is modeled by a fault simulator box in a quasi-static manner. Failure mechanism, progression and locations of damages to the tunnels are assessed through a gradual increase in Permanent Ground Displacement (PGD). The ground surface deformations and strains, fault surface trace, fault scarp and the sinkhole caused by fault movement are observed here. These ground surface deformations are major threats to stability, safety and serviceability of the structures. According to the observations, the modeled tunnels are vulnerable to reverse fault rupture and but the functionality loss is not abrupt, and the tunnel will be able to tolerate some fault displacements. By monitoring the progress of damage states by increasing PGD, the fragility curves corresponding to each damage state were plotted and interpreted in related figures.     

Observed Behaviour of a Tunnel in Sand Subjected to Shear Deformation in a Centrifuge

The paper describes observed behaviour of a model tunnel embedded in dry sand subjected to cyclic ground shear deformation in a centrifuge, as well as the behaviour of the model ground during shear deformation. Detailed data on earth pressures acting on the tunnel lining and the sectional forces of the lining are presented during ground shear deformation. The data suggest that the earth pressure at tunnel crown before ground shear deformation is smaller than the full overburden pressure probably due to the formation of arch action and the arch action may deteriorate with the cyclic ground shear deformation, resulting in an increase in the earth pressure at crown and changing the distribution of the sectional forces, which are largely influenced by conditions between tunnel lining and invert.     

逐级加载下浅埋软岩隧道动力破坏振动台试验

浅埋隧道围岩的质量普遍较低,整体稳定性差,隧道震害表明强震作用下浅埋隧道极易发生震动破坏。通过开展V级围岩条件下浅埋隧道在小震下的震动响应和逐级加载下的震动垮塌振动台试验,研究了小震作用下围岩加速度沿地层的分布、衬砌结构的内力变化和围岩内部的水平位移变化规律,强震作用下衬砌结构裂缝开展和围岩震动垮塌。结果表明:围岩加速度随距地表距离的减小而增加,地表加速度约为拱顶处加速度的1.63倍,相同高度平面内靠近隧道的围岩振动具有一定的加强;隧道拱顶围岩内部的水平位移大约是拱腰围岩内部的1.23倍,围岩内部位移随着远离隧道而逐渐减小,随着震动烈度的增加而不断增加;隧道拱顶上方垮塌区形状近似漏斗,震动引起隧道衬砌结构拱脚处的轴力和弯矩变化最大,且拱肩和拱脚处裂缝分布最多,应加强拱肩和拱脚处结构的抗震性能。     

北京地铁运营隧道病害状态分析

基于北京地铁隧道病害检测结果,分析结构形式、配筋和运营时间对隧道病害状态的影响。研究结果表明,管片接缝变形是盾构隧道病害的根源,并由此引发了盾构断面的椭圆化变形、管片的压溃与错台以及盾构隧道的渗漏水。衬砌开裂是矿山法隧道的主要病害,裂缝宽度与深度受运营时间影响大且具有离散性强、随机性大的特点,配筋对隧道结构安全有积极影响。渗漏水受降水的影响较大,多出现于隧道衬砌结构的缝隙,如盾构隧道的接缝、螺栓孔或矿山法隧道的变形缝、施工缝和衬砌裂缝等。衬砌空洞多位于拱顶,形状接近于长条形、正方形和椭圆形,且多伴随着邻近衬砌的开裂。混凝土碳化深度与运营时间成正比,碳化深度和速率在隧道道床位置最大、边墙次之、拱顶最小。裂缝是整体式道床的常见病害,道床在剥离的同时还伴随沿其纵向的扭转。     

层状围岩中管片衬砌受力特征的模型试验研究

深部层状围岩结构强度具有各向异性特点,此类地层中修建盾构隧道,管片衬砌易受偏压作用,对结构安全构成挑战。开展层状围岩与盾构管片衬砌相互作用关系的相似模型试验研究,研究不同层理倾角下管片衬砌壁后围岩压力、管片衬砌内力和变形分布规律。研究结构表明:管片衬砌受力和变形特征受层理面控制明显,管片衬砌受力极不均匀,弯矩、轴力和变形呈现非对称分布;管片衬砌壁后围岩压力最大值集中在强度最弱的层理面法线方向,该方向上管片衬砌的弯矩最大,轴力最小,变形最大;层理倾角对管片衬砌的受力和变形影响显著,层理倾角不仅影响管片衬砌壁后围岩压力分布形状还影响其量值大小;均质地层中,管片衬砌裂缝主要出在封顶块接头处和其他环向接头处,层状地层中管片衬砌裂缝出现位置受接头位置影响减弱,而受层理倾角影响明显,管片衬砌裂缝出现位置主要集中在层理面法向。研究结果对层状围岩中修建盾构隧道的支护结构型式设计具有一定参考价值。     

Seismic design and analysis of underground structures

Underground facilities are an integral part of the infrastructure of modern society and are used for a wide range of applications, including subways and railways, highways, material storage, and sewage and water transport. Underground facilities built in areas subject to earthquake activity must withstand both seismic and static loading. Historically, underground facilities have experienced a lower rate of damage than surface structures. Nevertheless, some underground structures have experienced significant damage in recent large earthquakes, including the 1995 Kobe, Japan earthquake, the 1999 Chi-Chi, Taiwan earthquake and the 1999 Kocaeli, Turkey earthquake. This report presents a summary of the current state of seismic analysis and design for underground structures. This report describes approaches used by engineers in quantifying the seismic effect on an underground structure. Deterministic and probabilistic seismic hazard analysis approaches are reviewed. The development of appropriate ground motion parameters, including peak accelerations and velocities, target response spectra, and ground motion time histories, is briefly described. In general, seismic design loads for underground structures are characterized in terms of the deformations and strains imposed on the structure by the surrounding ground, often due to the interaction between the two. In contrast, surface structures are designed for the inertial forces caused by ground accelerations. The simplest approach is to ignore the interaction of the underground structure with the surrounding ground. The free-field ground deformations due to a seismic event are estimated, and the underground structure is designed to accommodate these deformations. This approach is satisfactory when low levels of shaking are anticipated or the underground facility is in a stiff medium such as rock. Other approaches that account for the interaction between the structural supports and the surrounding ground are then described. In the pseudo-static analysis approach, the ground deformations are imposed as a static load and the soil-structure interaction does not include dynamic or wave propagation effects. In the dynamic analysis approach, a dynamic soil structure interaction is conducted using numerical analysis tools such as finite element or finite difference methods. The report discusses special design issues, including the design of tunnel segment joints and joints between tunnels and portal structures. Examples of seismic design used for underground structures are included in an appendix at the end of the report.     

穿越活动地裂缝地铁隧道震害机制研究

 通过振动台模型试验对穿越活动地裂缝的地铁隧道的动力响应进行了研究,研究内容主要包括地铁隧道的加速度反应,土压力及应变的变化规律。分析结果表明,穿越活动地裂缝的地铁隧道在地震荷载作用中,活动地裂缝场地产生不均匀沉降,上盘沉降大于下盘区,预设地裂缝部位沉降值最大,不均匀沉降导致次生裂缝及沉降陡坎产生,地铁隧道上方场地土体产生细小裂缝;地铁隧道与活动地裂缝的加速度时程曲线均与地震动荷载加速度时程具有一致性,地铁隧道各部位加速度时程保持一致,说明在地震中地铁运动保持整体性,上盘场地的加速度峰值较大,表明在活动地裂缝中上盘区对地震动力有一定的放大效应;活动地裂缝场地中土压力呈现出动土压力曲线变化,地震加载结束后隧道结构侧向的土压力受力状态及大小均产生变化,隧道结构顶部的土压力有较大增加;应变曲线表明在扩大断面的马蹄形隧道结构中拱腰部位的应变增值最大,拱顶部位次之,底板的应变增值相比最小。以上成果对于合理认识跨越地裂缝的地铁隧道的地震响应特征具有重要意义,可为地铁隧道实际工程设计和施工的抗震设防提供宝贵的基础资料。