Ground surface settlements due to construction of closely-spaced twin tunnels with different geometric arrangements

This paper examines the ground surface settlement profiles due to the construction of closely-spaced twin tunnels using the shallow tunnelling method. In the concerned zone, where the twin-tunnelling was performed in stacked and offset arrangements, the ground surface settlements of in total 18 cross-sections were continually recorded during construction. To cater for different conditions of the twin tunnels in the concerned zone, partial face, full face and forepoling reinforcement schemes were adopted. The recorded surface settlements and settlement troughs of three typical sections are reported and illustrated. The surface settlement troughs induced by each of the twin tunnels are fitted by the Gaussian function. The parameters that characterize the surface settlement troughs induced by each of the twin tunnels, such as the maximum settlement, percentage of ground loss, trough width and empirical trough width parameter are presented and compared.     

Stability of long unsupported twin circular tunnels in soils

The stability of two long unsupported circular parallel tunnels aligned horizontally in fully cohesive and cohesive–frictional soils has been determined. An upper bound limit analysis in combination with finite elements and linear programming is employed to perform the analysis. For different clear spacing (S) between the tunnels, the stability of tunnels is expressed in terms of a non-dimensional stability number (γ_maxH/c); where H is tunnel cover, c refers to soil cohesion, and γ_max is maximum unit weight of soil mass which the tunnels can bear without any collapse. The variation of the stability number with tunnels’ spacing has been established for different combinations of H/D, m and ϕ; where D refers to diameter of each tunnel, ϕ is the internal friction angle of soil and m accounts for the rate at which the cohesion increases linearly with depth. The stability number reduces continuously with a decrease in the spacing between the tunnels. The optimum spacing (S_opt) between the two tunnels required to eliminate the interference effect increases with (i) an increase in H/D and (ii) a decrease in the values of both m and ϕ. The value of S_opt lies approximately in a range of 1.5D–3.5D with H/D = 1 and 7D–12D with H/D = 7. The results from the analysis compare reasonably well with the different solutions reported in literature.     

Investigation of response of metro tunnels due to adjacent large excavation and protective measures in soft soils

The inevitable influence of large excavation in soft soils on nearby tunnels is of great concern in practice. In this paper, the influence of a nearby large excavation on existing metro tunnels of the Ningbo Metro Line 1 in sensitive soft soils is investigated and presented. Considerable displacement in the left tunnel closer to the excavation induced by the nearby excavation was revealed by field monitoring. Visible cracks and leakages were observed in left tunnel linings. Three dimensional numerical simulations are conducted to investigate the responses of the ground and left tunnel due to the adjacent excavation. The development of bending moment and displacement of the left tunnel during different construction stages of the nearby excavation is obtained. Then the interaction mechanism between the nearby excavation, surrounding soils and existing twin tunnels is investigated, which is of significance to the interpretation of the influence of the nearby excavation on the existing twin tunnels. Several protective measures for alleviating the influence of adjacent excavation on left tunnel are studied, including divided excavation, soil improvement and a cut-off wall. It is found that the left tunnel is influenced to varying degrees during different construction stages and the time effect is distinct for this large excavation in soft soils, which would be suggestive to engineers to pay more attention to the protection of adjacent tunnel during the crucial construction stages. The bending moment and displacement of the left tunnel is strongly related to the unloading effects and displacement of surrounding soils, which can be alleviated by means of proper improvement of excavation sequence. Comparatively, longitudinally divided excavation is more effective in protecting the left tunnel than soil improvement or a cut-off wall. This study is of certain reference value for protecting metro tunnels adjacent excavation in soft soils.     

Structural design and mechanical responses of closely spaced super-span double tunnels in strongly weathered tuff strata

This paper presents a study of closely spaced double tunnels in Taizhou, China. One is Xiabei Mountain No. 2 four-line super-span high-speed railway tunnel (HRT), and the other is Xiabei Mountain double-line large-span subway tunnel (ST). The excavation spans of HRT and ST are 26.3 and 14 m, respectively. The two tunnels are located at different levels, and their separating distance is 17.2 m. Due to the short construction period, the HRT excavation was completed earlier than ST. The structural design of the HRT, taking account of the disturbance by the ST construction, was analyzed by a numerical simulation. It was found that the “yielding principle” design was more feasible than the “resistance principle” design when considering the safety and durability of the HRT secondary lining. The mechanical responses of the HRT during ST construction were comprehensively monitored and analyzed, including the vault settlement, horizontal convergence, surrounding rock pressure, and the internal stress in shotcrete and steel arch. Results show that the longitudinal influence range of the ST construction on the HRT was approximately 0.6–1.1 times the ST outer diameter; the disturbance was mainly generated in the ST upper bench excavation; and the final axial force of the HRT shotcrete was approximately 9–16 times that of the steel arch, which indicated that the shotcrete was the main bearing structure. The safety status of the HRT was assessed based on the monitoring data, and the minimum safety factors of the HRT shotcrete and steel arch were 1.61 and 1.89, respectively. Parametric studies were performed to show how the lining stress of HRT was affected by the relative angle, pillar width, ST excavation method and excavation footage. Finally, the design and construction optimization were proposed according to the monitoring data and parameter analysis results. This study might provide practical reference for similar projects.     

下穿隧道-土-地表邻近框架结构相互作用体系

在中国城市轨道交通快速发展的推动下,隧道等地下结构近距离下穿地表建筑物的案例越来越多。对于相互穿越工程,较多的关注在于隧道等地下结构动态施工对地表建筑物安全的影响,而忽视了隧道建成后对被穿越的地表临近建筑物抗震性能的影响。拟结合某实际工程,建立盾构隧道-土体-地表邻近框架结构相互作用体系二维数值计算模型,对比分析隧道与邻近框架结构相互之间地震响应影响规律。计算分析表明：①隧道的存在使隧道两侧土体表面出现一个地震响应放大区;②下穿隧道对地表框架结构的地震响应有部分影响,但影响程度有限;③地表结构的存在对隧道结构地震响应的影响较小,可忽略不计。研究成果可为地下结构的规划以及地表结构与地下结构的抗震设计提供参考。     

盾构隧道在可液化场地中的地震响应分析

盾构隧道的装配式管片是其显著的结构特点,目前的抗震研究主要采用简化方法,少有能有效反映管片和接头细部特征的动力反应分析方法,对其在可液化场地中的地震响应规律也需要进一步研究。本文建立了一种精细化装配式管片结构计算模型,并基于砂土液化大变形统一本构模型,采用弹塑性有限元动力时程分析,分析了盾构隧道在可液化场地中的地震响应特征及规律。结果表明接头处响应是盾构隧道抗震的重要考虑因素。装配式管片结构相比于整体式结构柔度更大,受力较小,变形较大。在可液化场地中盾构隧道由于水平向作用力显著增加,在水平向被挤压,受力分布和抗震不利位置相比非液化场地有明显区别。     

黄土隧道围岩的结构性变化特征分析

在分析和修正已有结构性参数的基础上,首次提出了将结构性参数直接应用于工程实践的思路。研究了黄土隧道开挖后围岩结构性的变化规律。研究得出:围岩的结构性参数与径深和洞径密切相关;随着径深的增大,拱顶方向上围岩的结构性参数均呈现出先减小后增大的规律,存在一个最小值。与开挖前原状土的结构性参数相比,围岩的结构性损失在距拱顶约1.7～2.5 m时达到最大。随着洞径的增大,开挖对围岩黄土的扰动增强,拱顶方向上围岩的结构性减弱,结构性参数的最小值变化不大,但最小值发生的位置逐渐远离拱顶。最后,提出了以围岩结构性损失最大为依据判断围岩松动圈的新方法。分析表明,黄土隧道围岩的松动圈在1.7～2.5 m变化。     

顺层隧道地震响应的波动方法研究

 顺层岩体是隧道工程中常见的地质情况,震害突出,但尚无有效的地震响应分析方法,因而难以深入研究其响应机制和规律。为此,建立一种地震波动输入方法及相应公式,该方法以有限元法与时域波动分析法相结合,采用黏弹性人工边界,基于顺层岩体的横观各向同性本构关系,考虑岩体中地震波的偏振特性和相速度变化特点,适用于地震波斜入射方向与隧道纵断面平行的情况。同时,为验证所提出方法的实用性和可行性,应用其分析某顺层岩体中高速铁路隧道结构的地震响应规律。结果显示：当岩层水平或竖直时,则qP波引起的衬砌内力包络图对称;当岩层倾斜时,则qP波引起的衬砌内力包络图不对称,不对称性的幅度与岩层倾角相关;岩层倾角不同,则弯矩值相差较大;弯矩包络图无论对称与否,环向弯矩极值均位于拱脚处,而纵向弯矩极值却位于拱顶和仰拱处。此外,作为对比分析,又将岩层简化为各向同性介质,采用该方法进行计算,结果发现：P波引起的衬砌内力包络图对称,且内力包络图与将岩层简化为横观各向同性介质时相接近。这进一步验证了提出方法的合理性和可靠性。     

Reliability analysis of ground–support interaction in circular tunnels using the response surface method

The first-order and the second-order reliability method (FORM/SORM) are used to evaluate the failure probability of three performance functions of the ground–support interaction in circular tunnels subjected to hydrostatic stresses. The response surface method (RSM) is used to enable reliability analysis of the implicit convergence-confinement method. The friction angle, cohesion and elastic modulus of the rock mass are considered as basic random variables and are first assumed to obey normal distributions. The quadratic polynomial with cross terms is employed as response surface function to approximate the limit state surface (LSS) at the design point. The strategies for the RSM are presented. The failure probability with respect to different criteria are obtained from FORM/SORM and compared to those generated from Monte Carlo simulations. The results show that the support installation position has great influence on the probability of the three failure modes under consideration. Comparison between analysis using correlated and uncorrelated friction angle and cohesion indicates that the influence of the correlation on the reliability analysis depends on the support installation position and the orientation of the LSS. The reliability analysis involving correlated non-normal distributions and the reliability-based design of the support are also investigated.     

层状地基中隧道开挖对临近既有隧道的影响分析

首次采用弹性层状半空间地基模型,建立了多层地基中隧道开挖对临近既有隧道影响的连续弹性分析方法,改变了过去采用简化分析方法仅能在均质地基中求解此类工程问题的状况。首先,采用Laplace积分变换得到了直角坐标系下单层地基应力和位移的初始函数,在此基础上,运用矩阵递推技术,给出了竖向荷载作用下层状地基中应力和位移的解析表达式并将其作为分析该问题的基本解。然后,采用弹性层状半空间地基模型将既有隧道视为Euler-Bernoulli梁,地基土体连续位移采用弹性层状半空间体系的基本解进行计算,并引入临近隧道开挖引起的土体自由位移场影响建立该问题的连续弹性求解方程,从而可以求得隧道纵向位移和内力。最后,结合离心模型试验和有限元数值模拟算例进行分析,验证了本文方法的有效性。此外,为考察地基土成层性对既有隧道性状的影响,还对几种典型层状地基中的隧道进行了参数分析。成果可为合理制定地下工程施工对临近既有隧道的保护措施提供一定的依据。     

弹性准饱和土中球空腔的动力响应

采用工程上通用的土力学模型,考虑饱和度对球空腔动力响应的影响,利用Laplace变换求解了弹性准饱和土中球空腔的动力响应问题,得到了变换域内的解析解。借助数值Laplace反变换,数值分析了弹性准饱和土中球空腔动力响应的位移、应力及孔压的变化规律。为分析地下结构动力响应提供了一种有效的方法,模型符合工程实际,有一定的工程应用价值。     

Kinematic behaviour of a Tunnel Boring Machine in soft soil: Theory and observations

State-of-the-art shield tunnelling in difficult ground conditions still involves a trial-and-error approach as a complete understanding of the physics governing the interaction between the Tunnel Boring Machine (TBM)-shield and the surrounding soil is still lacking. This is particularly concerning as it was demonstrated that the shield–soil interaction, and especially the ground displacement around the shield periphery, gives a significant contribution to the overall soil deformation. This paper quantifies the radial soil displacements induced by a TBM-shield driving in soft ground. The displacements have been obtained by means of a model that captures several aspects of the kinematic behaviour of a TBM, based on theoretical and geometrical considerations. The theoretical model has been verified against TBM monitoring data obtained during the construction of the Hubertus Tunnel, a double-tube road tunnel located in The Hague, The Netherlands. Results show the amplitude and spatial distribution of the ground displacement around the shield periphery as they occurred in practice.     

Dynamic modelling of the response of UK forest soils to changes in acid deposition using the SAFE model

Both observed and modelled data have been examined from the ten UN-ECE Level II forest intensive monitoring sites in the UK to determine the changes and potential impact on soil solution chemistry resulting from changes in acid deposition inputs. The sites represent a range of forest tree types, soil sensitivities and pollutant deposition inputs found in the UK. The dynamic biogeochemical SAFE model was used to explore temporal changes in soil and soil solution chemical parameters that have been used as indicators for potential forest ecosystem and tree damage in national and international assessments of critical loads. The observed data and model results show that there is significant inter-site variation. The model indicates that the historical pollutant inputs have resulted in significant soil acidification at most of the sites. Model predictions generally match current day observations. Recently declining pollutant inputs have reduced and in some cases reversed the trend of increasing soil acidification. A discussion of the results in terms of critical loads, recovery, their wider implications and uncertainty is presented.     

平面Rayleigh波入射下饱和土中浅埋隧道复合式衬砌的动力响应

基于Biot波动理论,采用Fourier-Bessel级数展开法,建立了平面Rayleigh波入射下,饱和土中浅埋隧道复合式衬砌的散射力学模型,求解了频域内复合式衬砌的动应力集中系数、孔压集中系数的解析解,通过参数化分析,研究了Rayleigh波在不同入射频率作用下,内衬和外衬的刚度比、厚度比、隧道埋深等因素对复合式衬...