地铁车站与隧道连接处地震响应分析

结构截面刚度突变处是地下结构抗震的薄弱部位,为研究地铁车站与隧道连接处的地震响应,本文建立有限差分数值模型,分析地震作用下车站与隧道连接处的薄弱部位、连接处附近的侧墙变形分布特征以及地表沉降分布特征,重点探究埋深、地震动特征以及周围土体刚度对连接处隧道应力的影响。结果表明:连接处端墙底部跨中、端墙洞口的顶部和底部是抗震的薄弱部位;连接处端墙存在对侧墙变形分布、地表沉降分布有一定影响;结构埋深,地震动频谱、幅值对连接处隧道应力响应有较大影响,结构周围土体的刚度在一定范围内对连接处隧道应力有较大影响。     

Realistic Modeling of Seismic Input in Urban Areas: A UNESCO-IUGS-IGCP Project

—?The estimation of realistic seismic input can be obtained from the computation of a wide set of time histories and spectral information, corresponding to possible seismotectonic scenarios for different source and structural models. Such a data set can be very constructively used by civil engineers in the design of new seismo-resistant structures and in the reinforcement of the existing built environment, therefore supplying a particularly powerful tool to the prevention efforts of Civil Defense. The availability of realistic numerical simulations enables us to estimate the amplification effects in complex geological structures exploiting the available geotechnical, lithological, geophysical parameters, topography of the medium, tectonic, historical, paleoseismological data, and seismotectonic models. The realistic modeling of the ground motion is a very important source of knowledge for the preparation of groundshaking scenarios which represent a valid and economical tool in seismic microzonation.     

Seismic microzonation of Dehradun City using geophysical and geotechnical characteristics in the upper 30 m of soil column

The understanding of geotechnical characteristics of near-surface material is of fundamental interest in seismic microzonation. Shear wave velocity (Vs), one of the most important soil properties for soil response modeling, has been evaluated through seismic profiling using the multichannel analysis of surface waves in the city of Dehradun situated along the foothills of northwest Himalaya. Fifty sites in the city have been investigated with survey lines between 72 and 96 m in length. Multiple 1-D and interpolated 2-D profiles have been generated up to a depth of 30–40 m. The Vs were used in the SHAKE2000 software in combination with seismic input motion of the recent Chamoli earthquake to obtain site response and amplification spectra. The estimated Vs are higher in the northern part of the study area (i.e., 200–700 m/s from the surface to a depth of about 30 m) as compared to the south and southwestern parts of the city (i.e., 180–400 m/s for the same depth range). The response spectra suggest that spectral acceleration values for two-story structures are three to eight times higher than peak ground acceleration at bedrock. The analysis also suggests peak amplification at 3–4, 2–2.5, and 1–1.5 Hz in the northern, central, and south-southwestern parts of the city, respectively. The spatial distributions of Vs and spectral accelerations provide valuable information for the seismic microzonation in different parts of the urban area of Dehradun.     

Seismic behaviour of circular tunnels accounting for above ground structures interaction effects

Tunnels are commonly designed under seismic loading assuming “free field conditions”. However, in urban areas these structures pass beneath buildings, often high-rise ones, or are located close to them. During seismic excitation, above ground structures may cause complex interaction effects with the tunnel, altering its seismic response compared to the “free field conditions” case. The paper summarizes an attempt to identify and understand these interaction effects, focusing on the tunnel response. The problem is investigated in the transversal direction, by means of full dynamic time history analyses. Two structural configurations are studied and compared to the free field conditions case, consisting of one or two above ground structures, located over a circular tunnel. Above ground structures are modeled in a simplified way as equivalent single-degree of freedom oscillators, with proper mechanical properties. Several parameters that are significantly affecting the phenomenon are accounted for in this parametric study, namely the soil to tunnel relative flexibility, the tunnel dimensions, the tunnel burial depth and the soil properties and nonlinearities during shaking. Tunnels response characteristics are compared and discussed, in terms of acceleration, deformations and lining dynamic internal forces. Internal forces are also evaluated with analytical closed form solutions, commonly used in preliminary stages of design, and compared with the numerical predictions. The results indicate that the presence of the above ground structures may have a significant effect on the seismic response of the tunnel, especially when the latter is stiff and located in shallow depths.     

Preliminary seismic microzonation of Sivas city (Turkey) using microtremor and refraction microtremor (ReMi) measurements

Sivas city, located in the inner east part of Anatolia (Turkey), is far from seismic sources. However, the city is under risk owing to strong earthquakes occurring around the area, and different soil conditions that can produce variation in the ground motion amplification. Microzonation of cities provides a basis for site-specific hazard analysis in urban settlements. In particular, seismic microzonation can be achieved by means of detailed seismic assessment of the area, including earthquake recordings and geological studies. In this paper, we propose a preliminary microzonation map for the city of Sivas, based on the variation in the dominant periods of the sediments covering the area. The periods are retrieved from microtremor measurements conducted at 114 sites, using the horizontal-to-vertical spectral ratio technique. The results of microtremor analysis were compared with those obtained from refraction microtremor measurements at two profiles crossing the studied area. According to the classification of dominant periods, Sivas area can be divided into four zones, probably prone to different levels of seismic hazard. However, specific studies including analysis of weak earthquakes are required in the future to validate our microzonation map.     

Site response studies and seismic microzoning in the Middle Aterno valley (L��aquila, Central Italy)

Following the April 6th, 2009 Abruzzo mainshock, the Italian Civil Protection Department promoted a multidisciplinary study aimed at developing seismic microzonation maps for post-earthquake reconstruction planning. In the framework of this project, a Working Group, including the authors, was assembled to carry out a microzonation study on six villages located in the Middle Aterno valley. This paper focuses on the villages of Castelnuovo and Poggio Picenze, which experienced MCS intensity values of IX–X and VIII–IX, respectively. 1D and 2D linear equivalent site response analyses were carried out on representative geological cross-sections through the damaged centres and the expansion zones. The subsoil models resulting from geological, geotechnical and geophysical investigations were calibrated by comparing numerical amplification functions, in the linear range, with horizontal-to-vertical spectral ratio derived from both aftershocks and noise recordings. The input motions adopted for the analyses were five artificial accelerograms compatible with three response spectra obtained from the Italian seismic code, as well as from ad hoc probabilistic and deterministic studies. The results were expressed in the form of horizontal profiles of amplification factors in terms of peak ground acceleration, F_PGA, as well as of the Housner intensity, FH, in two different range of periods; this latter parameter was shown to be almost independent of the input motion and allowed to express the dependency of site amplification on the frequency range. The amplification factors computed along the representative geological sections were finally extended with a rational procedure to the surrounding areas to draw Grade-3 microzonation maps.     

Seismic microzonation of Chania,Crete (Greece) based on SASW measurements and non-linear site response analyses

Following a brief overview of past applications of, and more recent advances on seismic microzonation, the results of a seismic microzonation study for the city of Chania, Greece, are presented. The study was based on V_s vs. depth profiles obtained at 19 sites of the urban area by performing SASW measurements. The spatial distribution of V_s values was utilized in estimating V_s30 values, depth to bedrock and the fundamental ground period variation across the area of the city as well as for conducting 1-D finite element non-linear inelastic site response analyses. The input earthquake excitations employed in the response analyses were based on the results of an available seismic hazard study for the Chania Area. The results of analyses were utilized for establishing the spatial distribution of rock motion amplification, the expected ground motions and spectral values in the area of the city. Contour maps providing values of the expected ground motion in the urban area are given which may become a practical tool in assessing the seismic risk and expected damage in the Chania area. The maps can also be used in the design of new earthquake resistant structures or the seismic retrofitting of existing ones. Finally, the results were utilized to demonstrate the inadequacy of using V_s,30 values for classifying the soil conditions in the Chania area.     

Seismic ground amplification induced by box-shaped tunnels

The growing use of underground structures, specifically to facilitate urban transportation, highlights the need to scrutinize the effects of such spaces on the seismic ground response as well as the surrounding buildings. In this regard, the seismic ground amplification variations in the vicinity of single and twin box-shaped tunnels subjected to SV waves have been investigated by the finite difference method. To evaluate the effects, generalizable dimensionless diagrams based on the results of parametric numerical analysis considering factors such as variations in the tunnels’ depth, the distances between the tunnels, tunnel lining flexibility, and input wave frequency, have been presented. In addition, to assess the effects of underground box-shaped tunnels on the response spectrum of the ground surface, seven selected accelerograms have been matched based on a specific design spectrum for the stiff soil condition of Eurocode 8 (CEN, 2006). The results underline the significant amplification effect of the box-shaped tunnels on the ground motions, specifically in the case of horizontal twin tunnels, which should be given more attention in current seismic design practices for surface structures.

     

An earthquake scenario for the microzonation of Sofia and the vulnerability of structures designed by use of the Eurocodes

The study of the site effects and the microzonation of a part of the metropolitan Sofia, based on the modelling of seismic ground motion along three cross-sections are performed. Realistic synthetic strong motion waveforms are computed for scenario earthquakes (M=7) applying a hybrid modelling method, based on the modal summation technique and finite differences scheme. The synthesized ground motion time histories are source and site specific. The site amplification is determined in terms of response spectra ratio (RSR). A suite of time histories and quantities of earthquake engineering interest are provided. The results of this study constitute a “database” that describes the ground shaking of the urban area. A case study of experiment-based assessment of vulnerability of a cast-in-situ single storey, industrial, reinforced concrete frame, designed according to Eurocodes 2 and 8 is presented. The main characteristics of damage index and storey drift are discussed for the purposes of microzonation.     

Tunnel flexibility effect on the ground surface acceleration response

Flexibility of underground structures relative to the surrounding medium, referred to as the flexibility ratio, is an important factor that influences their dynamic interaction. This study investigates the flexibility effect of a box-shaped subway tunnel, resting directly on bedrock, on the ground surface acceleration response using a numerical model verified against dynamic centrifuge test results. A comparison of the ground surface acceleration response for tunnel models with different flexibility ratios revealed that the tunnels with different flexibility ratios influence the acceleration response at the ground surface in different ways. Tunnels with lower flexibility ratios have higher acceleration responses at short periods, whereas tunnels with higher flexibility ratios have higher acceleration responses at longer periods. The effect of the flexibility ratio on ground surface acceleration is more prominent in the high range of frequencies. Furthermore, as the flexibility ratio of the tunnel system increases, the acceleration response moves away from the free field response and shifts towards the longer periods. Therefore, the flexibility ratio of the underground tunnels influences the peak ground acceleration (PGA) at the ground surface, and may need to be considered in the seismic zonation of urban areas.     

Seismic ground motion amplification pattern induced by a subway tunnel: Shaking table testing and numerical simulation

A series of 1 g shaking table tests, followed by the numerical simulations, is performed to investigate the effect of a circular subway tunnel on the ground motion amplification pattern. Effects of various parameters, including shear wave velocity of soil, frequency content of input motion, flexibility ratio and tunnel depth on the amplification pattern is investigated. Experimental study revealed that the tunnel did not affect free field response at dimensionless period greater than 10. Subsequent parametric study demonstrated that the amount of amplifications were mainly controlled by dimensionless period, dimensionless depth and flexibility ratio. Tunnel effect on the amplification pattern is more prominent for dimensionless period between 3 to 10, flexibility ratio greater than 1 and dimensionless depth less than 3. The study revealed that subway tunnel influences the seismic response of low period buildings, constructed above the tunnel.     

饱和砂土中箱型隧道的地震反应分析

饱和砂土地基在地震作用下存在液化的潜在危险,液化引起的地基失效可能会导致地下结构的严重震害。以Opensees作为计算平台,对饱和砂土中带中柱箱型隧道的地震反应进行输入不同幅值地震动时的动力数值计算,研究场地和结构的加速度反应及其频谱特性、场地的永久变形、隧道的震后位移以及隧道的内力分布。计算结果表明,饱和砂土中箱型隧道的地震附加内力仍受周围土体的相对位移控制,此外在震后隧道可能会产生侧移和上浮的永久位移,并且可能存在残余内力。     

考虑进洞高程效应的黄土隧道洞口段地震动力响应特征研究

通过大型振动台模型试验并采用Midas-GTS有限元软件进行模拟计算,研究黄土隧道洞口段在地震作用下的动力响应特征、破坏过程和地震波在模型中的传递规律,分析影响黄土隧道洞口段地震动力响应的主要因素。结果表明:边坡沿弧形开裂面的垮塌受坡脚剪切和坡顶拉裂的共同作用;边坡会对其卓越频率内的地震波产生明显放大效应,且在1/2坡高以上放大效应出现饱和现象;隧道临空面是影响隧道洞口段地震动力响应的主要因素。考虑进洞高程效应时隧道洞口段抗震设防长度可取距洞口5倍洞径范围。振动台模型试验与数值计算在位移、加速度、应力三个响应特征上吻合较好,证明二者结果合理可靠。研究成果可为隧道工程设计和地下结构抗震理论研究提供有益参考。     

Earthquake induced ground strains in the presence of strong lateral soil heterogeneities

Transient ground strains are recognized to govern the response of buried elongated structures, such as pipelines and tunnels, under seismic wave propagation. Since a direct measure of ground strains is not generally available, simplified formulas relating peak ground strain to peak ground velocity, and based on 1D wave propagation theory in homogeneous media, are typically used for seismic design. Although they are adopted by most of the available technical guidelines, the use of these formulas may be questionable in complex realistic situations as either in the presence of strong lateral discontinuities, or in the epicentral area of large earthquakes, or in sites where relevant site amplification effects and spatial incoherency of ground motion are expected. To provide a contribution to overcome the previous limitations, a simplified formula relating peak ground longitudinal strain to peak ground velocity is proposed in this paper, as a function of the geometrical and dynamic parameters which have the major influence on strain evaluation. The formula has been obtained under small-strain assumptions, so that it can reasonably be applied under linear or moderately non-linear soil behaviour. The adequacy of this formula in the most common case of vertically propagating S-waves has been checked against 2D numerical solutions by Spectral Elements (SE) for representative geological cross-sections in Parkway Valley (New Zealand) and in the cities of Catania (Italy) and Thessaloniki (Greece). The shear strain and the longitudinal strain variability with depth is also investigated, through some qualitative examples and comparisons with analytical formulas.     

Intensity measures for the seismic response evaluation of buried steel pipelines under near-field pulse-like ground motions

Ground-motion Intensity Measures (IMs) are used to quantify the strength of ground motions and evaluate the response of structures. IMs act as a link between seismic demand and seismic hazard analysis and therefore, have a key role in performance-based earthquake engineering. Many studies have been carried out on the determination of suitable IMs in terms of efficiency, sufficiency and scaling robustness. The majority of these investigations focused on ordinary structures such as buildings and bridges, and only a few were about buried pipelines. In the current study, the optimal IMs for predicting the seismic demand of continuous buried steel pipelines under near-field pulse-like ground motion records is investigated. Incremental dynamic analysis is performed using twenty ground motion records. Using the results of the regression analysis, the optimality of 23 potential IMs are studied. It is concluded that specific energy density (SED) followed by \(\sqrt {VSI[{\omega _1}(PGD + RM{S_d})]} \) are the optimal IMs based on efficiency, sufficiency and scaling robustness for seismic response evaluation of buried pipelines under near-field ground motions.

     

Seismic microzonation and earthquake damage scenarios for urban areas

A methodology for seismic microzonation and earthquake damage scenarios may be considered as composed of two stages. In the first stage, microzonation maps with respect to estimated earthquake characteristics on the ground surface are generated for an investigated urban area. The effects of local geological and geotechnical site conditions are taken into account based on site characterization with respect to representative soil profiles extending down to the engineering bedrock. 1D site response analyses are performed to calculate earthquake characteristics on the ground surface using as many as possible, hazard compatible real acceleration time histories. In the second stage, vulnerability of buildings and pipeline systems are estimated based on site-specific ground motion parameters. A pilot study is carried out to evaluate seismic damage in a district in Istanbul, Turkey. The results demonstrate the significance of site characterization and site response analysis in calculating the earthquake characteristics on the ground surface in comparison to simplified empirical procedures.     

Seismic site effects for shallow and deep alluvial basins: in-depth motion and focusing effect

The main purpose of the paper is the analysis of seismic site effects in various alluvial basins. The analysis is performed considering a numerical approach (boundary element method). Two main cases are considered: a shallow deposit in the centre of Nice (France) [Soil Dyn. Earthquake Engng 19 (2000) 345] and a deep irregular basin in Caracas (Venezuela) [Comput. Geotech. 29 (2002) 573].

The amplification of seismic motion is analysed in terms of level, occuring frequency and location. For both sites, the amplification factor is found to reach maximum values of 20 (weak motion). Site effects nevertheless have very different features concerning the frequency dependence and the location of maximum amplification. For the shallow deposit in Nice, the amplification factor is very small for low frequencies and fastly increases above 1.0 Hz. The irregular Caracas basin gives a much different frequency dependence with many different peaks at various frequencies. The model for Caracas deep alluvial basin also includes a part of the local topography such as the nearest mountain. One can estimate seismic site effects due to both velocity contrast (between the basin and the bedrock) and local topography of the site.

Furthermore, the maximum amplification is located on the surface for Nice, whereas some strong amplification areas also appear inside the basin itself in the case of Caracas. One investigates the influence of this focusing effect on the motion versus depth dependence. This is of great interest for the analysis of seismic response of underground structures. The form and the depth of alluvial deposits are then found to have a great influence on the location of maximum amplification on the surface but also inside the deposit for deep irregular basins. It is essential for the analysis of the seismic response of both surface and underground structures.     

Seismic microzonation of the greater Bangkok area using microtremor observations

Bangkok, the capital city of Thailand, is located at a remote distance from seismic sources. However, it has a substantial risk from these distant earthquakes due to the ability of the underlying soft clay to amplify ground motions. It is therefore imperative to conduct a detailed seismic hazard assessment of the area. Seismic microzonation of big cities, like Bangkok, provides a basis for site‐specific hazard analysis, which can assist in systematic earthquake mitigation programs. In this study, a seismic microzonation map for the greater Bangkok area is constructed using microtremor observations. Microtremor observations were carried out at more than 150 sites in the greater Bangkok area. The predominant periods of the ground were determined from the horizontal‐to‐vertical (H/V) spectral ratio technique. A microzonation map was then developed for the greater Bangkok area based on the observations. Moreover, the transfer functions were calculated for the soil profile at eight sites, using the computer program SHAKE91, to validate the results from the microtremor analysis. The areas near the Gulf of Thailand, underlaid by a thick soft clay layer, were found to have long natural periods ranging from 0.8s to 1.2s. However, the areas outside the lower central plain have shorter predominant periods of less than 0.4s. The study shows that there is a great possibility of long‐period ground vibration in Bangkok, especially in the areas near the Gulf of Thailand. This may have severe effects on long‐period structures, such as high‐rise buildings and long‐span bridges. Copyright © 2003 John Wiley & Sons, Ltd.     

Deterministic Earthquake Scenarios for the City of Sofia

— The city of Sofia is exposed to a high seismic risk. Macroseismic intensities in the range of VIII – X (MSK) can be expected in the city. The earthquakes that can influence the hazard in Sofia originate either beneath the city or are caused by seismic sources located within a radius of 40 km. The city of Sofia is also prone to the remote Vrancea seismic zone in Romania, and particularly vulnerable are the long-period elements of the built environment. The high seismic risk and the lack of instrumental recordings of the regional seismicity make the use of appropriate credible earthquake scenarios and ground-motion modelling approaches for defining the seismic input for the city of Sofia necessary. Complete synthetic seismic signals, due to several earthquake scenarios, were computed along chosen geological profiles crossing the city, applying a hybrid technique, which combines the modal summation technique and finite differences. The modelling takes into account simultaneously the geotechnical properties of the site, the position and geometry of the seismic source and the mechanical properties of the propagation medium. Acceleration, velocity and displacement time histories and related quantities of earthquake engineering interest (e.g., response spectra, ground-motion amplification along the profiles) have been supplied. The approach applied in this study allows us to obtain the definition of the seismic input at low cost, exploiting large quantities of existing data (e.g. geotechnical, geological, seismological). It may be efficiently used to estimate the ground motion for the purposes of microzonation, urban planning, retrofitting or insurance of the built environment, etc.     

考虑地震波激振方向的浅埋偏压双联拱黄土隧道动力响应规律研究

以实际工程为背景,在模型试验结果和数值模拟结果验证合理的基础上,通过建立三维数值模型,研究兰州人工波在不同激振方向下坡-隧体系动力响应规律,通过小波包变换从能量和频域角度对衬砌结构动力响应规律进行分析。研究结果表明：水平、竖直面内垂直隧道轴向(X、Z)的地震波在隧道最大埋深处引起较大响应,水平面平行于隧道轴向的地震波(Y)对埋深较浅的洞口处的结构最为不利。频率在0～12.5 Hz范围内的低频波是引起隧道结构响应的主要波段,该频段中竖直向地震波(Z)能量相较于其他方向地震波能量占比最高。地震作用中衬砌结构的存在对坡体内的围岩变形有一定的抑制效应。X、Y向地震波容易引起坡脚附近的围岩发生剪切破坏,Y向地震波对隧道洞口段仰坡的稳定性影响最大;Z向地震波容易造成坡顶附近区域围岩的拉伸破坏,且对隧道拱顶附近产生最不利响应。研究成果对浅埋偏压双联拱隧道的抗震优化设计具有借鉴意义。