基于位移控制边界单元法盾构隧道开挖引起分层土体变形分析

 考虑地基土体非均质性的影响,基于弹性层状半空间地基模型,提出分析多层地基中盾构隧道开挖引起周围土体不排水变形的位移控制边界单元法,改变了过去采用简化分析方法仅能在均质地基中进行求解的状况。针对盾构隧道开挖边界引入椭圆化非等量径向土体位移移动模式,建立层状地基中洞周边值问题的边界积分求解方程,并采用高阶等参单元代替低阶常分布单元得到边界离散方程,同时以弹性层状半空间地基模型的基本解代替常规均匀介质体的Kelvin或Mindlin基本解,最终求得隧道洞周给定位移条件下的土体位移场。算例分析表明：位移控制边界单元法在计算均质地基和非均质层状地基中都具有较好的精度;对于非均质层状地基,如果采用以往的将不同土体参数近似折算成平均值进而按照弹性均质地基进行求解会带来较大的计算误差。研究成果可为合理评估盾构隧道施工对周围环境的影响提供一定的理论依据。     

层状地基中盾构隧道开挖非均匀收敛引起临近管道变形预测

采用弹性层状地基模型来考虑地基土体的非均质性,并针对隧道开挖边界引入椭圆化非等量径向土体位移模式;采用基于位移控制技术的边界单元方法来求解土体自由位移场,提出层状地基中盾构隧道开挖非均匀收敛引起临近管道变形影响的位移控制分析方法。结合位移控制有限元数值模拟和既有离心模型试验结果进行对比分析,验证该方法的有效性。算例分析结果表明,土体自由位移场计算对管道纵向变形性能的评估具有较大影响;对于非均质层状地基,如果采用以往的将不同土体参数近似折算成平均值,进而按照弹性均质地基进行求解会带来较大的计算误差。研究成果可为合理制定城市地铁隧道施工对周围环境影响的保护措施提供一定理论依据。     

软土基坑引起下卧隧道隆起的非线性流变

软土地区深基坑开挖改变了周边土体的初始应力,引起周边土体的位移,对周边构筑物造成不均匀沉降、混凝土开裂等不利的影响,并且可能危及临近地铁隧道的安全。对基坑开挖引起的下卧隧道的隆起变形进行了研究并提出了实用的预测方法。基坑开挖土体卸载引起的土体变形采用了Boussinesq应力解进行求解,隧道反力引起的土体变形采用了弹性半空间Mindlin应力解进行分析。隧道本身变形采用了弹性的地下连续梁进行分析,并且考虑隧道与土体的相互作用。通过引入软土的非线性流变模型,考虑了软土变形的时间效应,因此可以对复杂开挖过程进行模拟分析。还对基坑开挖对隧道隆起的效应进行了讨论,通过上海市某重点工程实例的隧道隆起量的预测结果与实测值对比分析,隧道隆起的监测结果证实了该方法的有效性。     

隧道开挖对层状地基中邻近管道影响的 DCBEM-FEM 耦合方法

目前,在城市地铁隧道施工中常常遇到邻近市政管线的影响。而较多的研究集中在隧道施工引起的周围地层变形上,考虑邻近管线 作用的隧道开挖理论分析方法并不多见。为此,针对隧道洞周引入椭圆化非等量径向土体位移控制模式,将邻近管线视为 Euler-Bernoulli 梁,同时将层状地基土体视为弹性层状地基模型,提出了 层状地基中隧道开挖引起邻近管道纵向变形的 DCBEM-FEM （位移控制边界元与有限元） 耦合分析方法。最后结合现场实测数据和位移控制有限元数值模拟进行分析,验证了本文方法的有效性。研究表明： DCBEM-FEM 耦合 方法可以较好的体现隧道开挖所引起的地层损失问题,同时本文方法在考虑邻近管线作用时具有较好的计算精度。研究成果可为合理制定城市地铁隧道施工对邻近管线的保护措施提供一定的理论依据。     

分层地基中隧道开挖对邻近刚性桩筏基础竖向影响分析

基于两阶段分析方法：第一阶段采用Loganathan方法计算隧道开挖引起的土体自由场位移,第二阶段采用层状地基中弹性理论法,计算桩和土,桩和桩之间的相互作用,并考虑刚性筏板对桩基的约束作用,提出了一套能够分析层状地基中隧道开挖对刚性筏板群桩基础竖向影响的解析方法。将计算结果与离心试验结果及现有方法计算结果进行对比,得到了较好的一致性,验证了其正确性。并首次对层状地基中桩筏基础承载特性进行了分析,讨论了桩基变形对遮拦效应的削减作用,结果表明此方法意义明确,层状地基模型更符合工程实际情况,适于推广使用。     

堆载作用下土体分层特性对地铁隧道纵向变形的影响研究

随着城市化进程的不断推进,地铁隧道附近可能会出现不同形式的地表堆载,将对地铁隧道的健康状态产生不利影响。现有的方法多是基于Boussinesq解求解地表堆载引起的附加应力,无法考虑土体的分层特性。首先采用Fourier积分变换得到了直角坐标系中竖向荷载作用下层状地基附加应力的基本解;然后将下卧隧道视为Timoshenko梁,基于Winkler地基模型和两阶段法求得隧道纵向位移和内力。为考察地基土成层性对既有隧道性状的影响,对上软下硬、上硬下软两种典型层状地基中的隧道进行了参数分析。结果表明：地基土成层特性对隧道形状影响比较大,相比而言,隧道下卧土层控制沉降的效果要优于隧道上覆土层。     

开挖卸荷引起下卧已建盾构隧道的纵向变形研究

开挖卸荷引起的下卧已建盾构隧道纵向变形是城市基础设施建设面临的热点和难点问题。将已建盾构隧道视为三参数Kerr弹性地基上的无限长梁,采用两阶段法研究基坑开挖卸荷引起下卧盾构隧道的纵向变形。对简化弹性空间法获得的地基参数进行讨论,通过与有限元模拟结果对比确定了合适的Kerr地基模型参数选取,并对Winkler、Pasternak与Kerr 3种地基模型的模拟结果进行了对比。为获得更合理的上方卸荷引起的隧道响应,推导并求解了考虑隧道侧向土体对隧道作用的平衡微分方程,解析结果与有限元结果基本吻合,与工程实例监测数据对比,验证了方法的有效性。研究表明：相对于Winkler与Pasternak模型,Kerr地基模型更具优越性;对通过简化弹性空间法得到的地基参数进行适当修正可以使弹性地基模型的模拟结果更理想;隧道侧向土体对隧道的作用比较显著,应在计算中考虑。     

堆载作用下土体分层特性对地铁隧道纵向变形的影响研究

随着城市化进程的不断推进,地铁隧道附近可能会出现不同形式的地表堆载,将对地铁隧道的健康状态产生不利影响。现有的方法多是基于Boussinesq解求解地表堆载引起的附加应力,无法考虑土体的分层特性。首先采用Fourier积分变换得到了直角坐标系中竖向荷载作用下层状地基附加应力的基本解;然后将下卧隧道视为Timoshenko梁,基于Winkler地基模型和两阶段法求得隧道纵向位移和内力。为考察地基土成层性对既有隧道性状的影响,对上软下硬、上硬下软两种典型层状地基中的隧道进行了参数分析。结果表明:地基土成层特性对隧道形状影响比较大,相比而言,隧道下卧土层控制沉降的效果要优于隧道上覆土层。     

被动状态下地埋管线的地基模量

 地下工程基坑、隧道开挖产生的自由土体位移场会引起周边地埋管线(如市政给排水管线、煤气管道等)产生附加变形并受力。基于Winkler地基模型的位移控制分析方法,将自由土体位移作为外部荷载即被动位移施加于管线上,管线与土体的相互作用采用Winkler地基弹簧模拟,弹簧常数一般根据Vesic公式得到。在基于弹性理论的推导中,Vesic公式的前提条件是弹性地基梁置于弹性半空间地表,竖向集中力或弯矩(主动荷载)置于梁的中心位置,这与地埋管线的一定埋深以及位移的施加情况是不一样的。为考虑上述2种因素对Winkler地基模量的影响,进行相应的理论探讨,提出考虑埋深影响的位移作用下地基模量的理论公式,并基于该地基模量进行隧道开挖对邻近地埋管线影响的位移控制理论分析。通过与弹性理论法、离心机试验以及实际工程观测结果的对比,验证该方法的合理性和正确性。     

基于虚拟力法的盾构隧道施工扰动地层附加应力分析

目前针对盾构隧道掘进引起周围地层附加应力的研究较少,且既有成果一般均假定施工场地为均质土体。基于层状地基基本解,结合虚拟力法,建立了考虑分层效应的盾构隧道施工扰动地层附加应力的分析方法,重点研究了盾构施工正面附加推力、盾构与土体间掘进摩擦力引起的地层附加应力。结合上海典型分层地基工程实例,分析了盾构施工引起的附加应力分布规律,且与均质土体工况下的计算结果进行了对比研究,考察了地基分层效应对扰动地层应力场的影响。计算结果表明：盾构与土体间掘进摩擦力与正面附加推力引起的土体附加应力分布规律较为相似,靠近开挖面时附加应力衰减较快且影响范围一般较小,而远离开挖面时应力衰减明显变慢,在实际工程中要重视掘进摩擦力和正面附加推力造成的施工风险;此外,考虑分层效应与常规不考虑分层效应时的附加应力场有较大差别,在设计中应重视地基非均质性带来的影响。成果可为合理制定层状地基中盾构隧道施工对临近管道以及建筑物的保护措施提供一定的理论依据。     

基坑开挖对邻近隧道纵向位移影响的计算方法

基坑开挖卸荷会对邻近隧道产生影响,因此有必要对隧道的变形进行预测,确保隧道正常运行.针对目前计算模型的分析方法未考虑基坑壁应力卸荷对隧道位移的影响,以及有限元分析过程较为复杂繁琐,提出采用Mindlin解计算基坑壁与坑底卸荷的附加应力.然后将隧道结构视为弹性地基无限长梁,将开挖引起的附加应力施加于隧道结构上,建立隧道结构纵向变形方程,从而得到隧道位移及内力的计算公式.最后,将计算方法与数值模拟算例、工程实测进行对比分析,计算结果与其较为吻合.     

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.     

Prediction of tunnel displacement induced by adjacent excavation in soft soil

Deep excavation may have impact on the adjacent tunnels. The interaction between new excavations and existing tunnels has been increasingly serious with the rapid development of underground space and metro system in urban area. It hence creates a high necessity to predict tunnel displacement induced by nearby excavation to ensure the safety of tunnel. In this paper, a semi-analytical method to evaluate the heave of underlying tunnel induced by adjacent excavation is presented and verified by field measurement results. The influence of excavation and the resistance of tunnel are obtained based on Boussinesq’s and Mindlin’s solutions, respectively. Then the soil–tunnel interaction behavior is analyzed based on the displacement coupling condition by assuming the tunnel as an elastic beam. A visco-elastoplastic model (VEP model) is employed to simulate the rheologic deformation of soil. The behavior of the tunnel underneath excavation is studied by the new method to discuss the influence of different factors, including excavation area, relative distance and construction procedure. Results of case studies show a good agreement between prediction and measurements.     

Theoretical prediction of ground movements induced by tunnelling in multi-layered soils

The theoretical predictions of ground movements induced by tunnelling are usually based on the assumptions that the ground is homogeneous. Actually, layered formations with different soil properties are usually encountered in situ and effects of soil stratification should be taken into account. This paper presents the displacement controlled boundary integral method aimed specially at investigating the effect of soil stratification on the tunnelling-induced ground movements. The modified elastic layered half-space model is combined with the method to consider the soil non-homogeneous characters, whereas the Kelvin and Mindlin solution based on homogeneous materials are employed previously. The applicability of the proposed method is verified with other available published results as well as the displacement controlled finite element analysis. Surface settlements and lateral displacements induced by tunnelling on the layered soil condition are compared with those based on the homogeneous soil. The results discussed in this paper show that the soil stratification, neglected in previous solutions, have a significant influence on the tunnelling-induced ground movements in multi-layered soils.     

考虑上浮效应的盾构下穿对既有隧道影响研究

新建盾构隧道下穿会对上部既有隧道产生扰动,并可能使隧道产生结构病害。由于盾构管片重量远小于相应开挖土体重量,新建隧道的上浮效应会对既有隧道产生影响。基于两阶段分析法提出考虑新建盾构隧道上浮影响的隧道下穿引发既有隧道纵向变形计算模型,并利用现场实测数据进行验证。结果表明：考虑上浮效应的理论计算结果与实测数据较为吻合;控制新建隧道上浮量或减小新建隧道直径可有效减小由于上浮效应造成的既有隧道隆起变形。研究成果可为全面合理评估隧道下穿对既有隧道的影响提供依据。     

双线隧道不同布置方式下相互作用影响的地层位移解析

双线隧道开挖引起地层变形的既有理论研究,一般基于单线隧道计算结果直接采用线性叠加进行分析,而忽略了双线隧道之间的相互作用影响。为研究近距离地下工程开挖的相互影响,基于位移控制Schwarz交替法和复变函数理论,结合隧道洞周椭圆化收敛变形边界条件,提出了任意布置方式下双线盾构隧道开挖引起周围地层变形的计算方法。该方法以位移控制边界条件进行求解,体现了双线隧道开挖间的相互作用影响。通过离心试验和工程实例,将基于位移控制Schwarz交替法的复变函数理论解与线性叠加法解答以及实测数据进行了对比验证;针对双线隧道水平和斜交两种不同布置方式进行相关参数分析,获取了双线隧道间距变化引起的地层变形影响规律。结果表明:基于位移控制Schwarz交替法的复变函数方法求解得到的地表沉降曲线与实测值吻合较好,而直接线性叠加法解答存在较大误差。在地层等值线图规律分析中,水平与斜交布置下,两个隧道周围土体等值线图产生较为明显的相互"吸引"现象,隧道上方的土体沉降峰值线均向双线隧道中间倾斜,但随着双线隧道间距增加,"吸引"现象逐渐减弱。研究成果对于近距离双线盾构隧道开挖施工的现场保护措施制定提供一定的理论依据。     

Mechanical effects of tunneling on adjacent pipelines based on Galerkin solution and layered transfer matrix solution

The mechanical analysis of undercrossing tunneling on adjacent existing pipelines is an important challenge that geotechnical engineers may need to face when designing new excavation projects. A Galerkin solution and a layered transfer matrix solution for the tunnel–soil–pipeline interaction are given in order to compare the effects of soil stratifications on the pipeline behavior subjected to tunnel-induced soil movements. For the Galerkin solution, the soil is modeled by the modulus of subgrade reaction and the governing differential equations are converted to finite element equations using the Galerkin method. To take full consideration for non-homogeneous soil characteristics, a layered soil model is employed in the layered transfer matrix solution by applying the double Laplace transform and transfer matrix method. The differences between the two proposed solutions are verified with several examples including centrifuge modeling tests, finite difference numerical analysis and measured data in situ. Furthermore, the parametric analysis to existing pipelines in several representative layered soils in Shanghai is also carried out. The results discussed in this paper indicate that the Galerkin solution can estimate the pipeline mechanical behavior affected by tunneling in homogeneous soil with good precision. The layered transfer matrix solution is more suitable to simulate the soil stratifications on the pipeline behavior than the Galerkin solution.