Deterministic and probabilistic analysis of great-depth braced excavations:A 32 m excavation case study in Paris

The Fort d’Issy-Vanves-Clamart(FIVC)braced excavation in France is analyzed to provide insights into the geotechnical serviceability assessment of excavations at great depth within deterministic and probabilistic frameworks.The FIVC excavation is excavated at 32 m below the ground surface in Parisian sedimentary basin and a plane-strain finite element analysis is implemented to examine the wall deflections and ground surface settlements.A stochastic finite element method based on the polynomial chaos Kriging metamodel(MSFEM)is then proposed for the probabilistic analyses.Comparisons with field measurements and former studies are carried out.Several academic cases are then conducted to investigate the great-depth excavation stability regarding the maximum horizontal wall deflection and maximum ground surface settlement.The results indicate that the proposed MSFEM is effective for probabilistic analyses and can provide useful insights for the excavation design and construction.A sensitivity analysis for seven considered random parameters is then implemented.The soil friction angle at the excavation bottom layer is the most significant one for design.The soil-wall interaction effects on the excavation stability are also given.

Differential uplift and settlement between inner column and diaphragm wall in top-down excavation

Top structure and basement will confront the risk of being damaged on account of large stress and strain fields incurred by differential uplift and settlement between inner column and diaphragm wall in top-down method. Top-down excavation of the Metro Line 10 in Shanghai was modeled with finite element analysis software ABAQUS and parameters of subsoil were obtained by inverse analysis. Based on the finite element model and parameters, changes in the following factors were made to find more effective methods to restrain differential uplift and settlement: length of diaphragm wall, thickness of jet-grouting reinforcement layer, ways of subsoil reinforcement, sequence of pit excavation, connection between slabs and diaphragm wall or column and width of pit. Several significant results are acquired. The longer the diaphragm wall is, the greater the differential uplift between column and diaphragm wall is. Rigidity of roof slab is in general not strong enough to keep diaphragm wall and column undergoing the same uplift during excavation; Uplift at head of column and differential uplift between column and diaphragm wall decrease when subsoil from-16.6 to-43 m in pit is reinforced through jet-grouting. But, as excavation proceeds to a lower level, benefit from soil reinforcement diminishes. During the process applying vertical load, the larger the depth of diaphragm wall is, the smaller the settlement is at head of column and diaphragm wall, and the greater the differential settlement is between column and diaphragm wall. When friction connection is implemented between column, diaphragm wall and floor slabs, uplifts at head of column and diaphragm wall are larger than those of the case when tie connection is implemented, and so does differential uplift between column and diaphragm wall. The maximum deflection of diaphragm wall decreases by 58% on account of soil reinforcement in pit. The maximum deflection of diaphragm wall decreases by 61.2% when friction connection is implemented instead of tie connection.

Three-dimensional lattice Boltzmann method for simulating blood flow in aortic arch

The three-dimensional （3D） lattice Boltzmann models, 3DQ15, 3DQ19 and 3DQ27, under different wall boundary conditions and lattice resolutions have been investigated by simulating Poiseuille flow in a circular cylinder for a wide range of Reynolds numbers. The 3DQ19 model with improved Fillippova and Hanel （FH） curved boundary condition represents a good compromise between computational efficiency and reliability. Blood flow in an aortic arch is then simulated as a typical haemodynamic application. Axial and secondary fluid velocity and effective wall shear stress profiles in a 180° bend are obtained, and the results also demonstrate that the lattice Boltzmann method is suitable for simulating the flow in 3D large-curved vessels.

Frozen curtain characteristics during excavation of submerged shallow tunnel using Freeze-Sealing Pipe-Roof method

The Freeze-Sealing Pipe-Roof(FSPR)method,which has been applied for the first time in the Gongbei Tunnel of the Hong Kong-Zhuhai-Macao Bridge,is a new approach of tunnel pre-support that allows flexible adjustment of freeze tube arrangement and can be adapted to different environmental conditions.When the FSPR method is used to construct shallow burial submerged tunnels,the frozen wall to hold back groundwater during excavation will be weakened by air and water flows inside and outside the tunnel,and its waterproof performance needs to be further investigated.In this paper,a two-dimensional numerical model of the temperature field considering excavation and moving water boundary is established based on the preliminary design scheme and in-situ conditions and is used to analyze the variation in frozen curtain properties with various active freezing times during excavation.The results show that excavation has a weakening effect on both sides of the frozen wall,with a greater effect on the inner side,and a positive temperature appears in the local area inside the jacked pipe.The concrete fill in the jacked pipe obviously improves the freezing efficiency,and the tunnel excavation after 60 days of active freezing in the interval filling mode can ensure that the frozen soil thickness at the thinnest segment exceeds 2 m,i.e.,the design requirement.In practice,the active freezing time can be extended appropriately to reduce the influence of river water flow above the tunnel.The study serves as a technical reference for the design and implementation of similar projects.

饱和黏性土地基中邻近圆形基坑相互影响分析

间距很小的圆形基坑后方土压力分布复杂,很难判断考虑圆拱效应的竖向弹性地基梁法和三维弹性地基板法的适用性。采用三维连续介质有限元法对位于饱和黏性土中2个间距很小的由地下连续墙组成的圆形事故应急池基坑进行整体建模分析。结果表明,对于1号池开挖到底后和2号池开挖到底后2种工况,地下连续墙的最大水平位移和最大弯矩的差异都不大。基坑位移和竖向弯矩的实测结果与计算值也较为接近。2个相邻圆形基坑在间距很小的情况下,其相互影响也是非常有限的。圆形地下连续墙支护结构对周围土体刚度参数的变化不敏感。采用三维连续介质有限元法对圆形地下连续墙支护结构进行计算分析,得到的结构内力远小于考虑圆拱效应的竖向弹性地基梁法和三维弹性地基板法,在经济性上有明显优势。

基于位移等代法的深部直墙半圆拱形隧洞松动圈厚度预测

【目的】位移等代法是在工程实践中被广泛应用的一种等效圆法。为进一步探索该方法在深部非均匀应力场下预测松动圈厚度的工程效果,并建立具有工程适用性的等效圆半径计算公式,【方法】选取深部隧洞常采用的直墙半圆拱形断面为研究对象,根据位移等代法基本原理,利用FLAC~(3D)数值模拟和理论计算相结合的方式,建立了侧压系数λ为1.1~1.5,跨高比b_0/h为1.3~1.5条件下的等效圆半径函数关系式,并应用到江西曲江某煤矿水仓围岩的松动圈厚度预测。【结果】结果显示:根据等效圆公式计算出的水仓围岩松动圈的纵轴厚度为1.41 m,横轴为1.20 m,呈椭圆形;现场实测的松动圈也呈椭圆形,厚度平均值在1.15~1.45 m,其中最大均值在纵轴1.45 m,最小均值在横轴为1.15 m。理论预测值与实测值相吻合。【结论】结果表明:位移等代法适用于深部软岩松动圈厚度预测;所建等效圆半径公式具有良好的工程适用性,可为评价和预测类似隧洞工程松动圈厚度提供一种简化的、可借鉴的方法和思路。

基于改进修正动员强度设计方法的软土基坑围护结构侧移预测

为了进一步优化软土地区基坑设计方法,提升基坑支护体系的安全性与经济性,在已有研究的基础上,根据工程实践经验,修正了MSD法和其改进方法 (MMSD法).基于圆弧滑动模式提出更符合实际的塑性变形机制,引入基坑开挖主要影响深度来量化位移场尺寸.理论计算方法适用于分析软黏土地区多支点式柔性支护结构的基坑开挖全过程的变形预测.将该方法的计算结果与6个上海地区的基坑工程实测数据、弹性支点法的计算结果及MMSD法的计算结果进行对比分析.对不同开挖阶段的围护结构最大侧移和最大侧移深度进行参数分析.结果表明,利用该方法得到的预测值与实测值吻合效果更佳,预测精度高于弹性支点法和MMSD法,证明了该方法的实用性.

冻结法在基坑地下连续墙缺陷处治中的应用分析

地下连续墙出现缺陷时,存在失稳及渗漏水风险,可采用冻结法对缺陷区域进行加固。基于某地铁基坑工程,开展现场监测及数值模拟分析,对比HS模型与MC模型在冻结工法中的适用性,发现HS模型能更好地反映实际冻结效果,适用于研究冻胀问题。基于HS模型,分别讨论冻结区厚度、深度、宽度对基坑变形及支撑内力的影响,建议冻结区采用0.4~0.6倍地下连续墙幅宽、1.5~2倍基坑深度,同时控制冻结区宽度,可保证冻结效果,控制工程规模。

一次扣拱暗挖逆作法车站结构设计要点分析

随着城市地铁的快速发展,地铁线路通常会穿越人口密集、地面交通繁忙、地面建筑物林立、地下管线密布的市中心繁华地段,全暗挖法施工地铁车站在避免管线迁改、减少交通影响、改善施工环境等方面具有显著优势,但也具有施工条件受限、结构受力转换复杂、变形控制不利等特点。基于北京地铁14号线大望路站工程,分析了由于条件受限而采用全暗挖法施工的必要条件,针对一次扣拱暗挖法施工采用理论分析、数值计算、现场监测等手段对其结构受力体系、施工过程中地层变形控制等关键技术进行了系统分析和总结,揭示了该类型车站施工的受力和变形规律。

盖挖法基坑开挖对临近桥梁桩基的影响研究

城市交通建设往往临近既有建筑结构,为了掌握盖挖法深基坑开挖对临近桥梁桩基的影响,依托之江路输水管廊及道路提升工程,建立数值模型并对工程现场展开监测,分析对比了基坑开挖导致的围护结构变形及桥桩变形规律。结果表明:围护结构最大位移出现在基坑北侧坑中坑开挖后,为6.14mm,最大位移深度随基坑开挖而下移,且坑中坑开挖完成后,其对侧围护结构变形反而减小,坑中坑开挖布置在敏感结构对侧时对临近结构具有一定控制效果;桥梁桩基最大沉降出现在首道混凝土支撑安装后,但随基坑开挖而逐渐减小,盖挖造成的沉降主要水平影响范围距围护结构约为12.71m,对邻近桥桩的影响主要发生在基坑开挖的早期阶段,各项变形均在安全范围内,且不均匀沉降控制效果明显。

双侧壁导坑法在瓦斯隧道开挖中的应用研究

为保证瓦斯隧道的安全开挖以及施工质量,以渝黔高速公路扩能项目大兴隧道为例,研究双侧壁导坑法在瓦斯隧道开挖中的应用。结合隧道探煤洞群以及薄层炭质页岩和少量煤矸石的情况,设计双侧壁导坑法施工方案以及施工流程;同时确定隧道开挖时排放瓦斯所需空气量,以此保证开挖安全。采用ANSYS有限元分析软件构建隧道三维模型,分析该施工技术的施工效果,确定隧道在竖向和横向上的最大位移别为12mm和6mm,满足工程标准。

超深圆形基坑地下连续墙的实用计算方法

若忽略圆形基坑地下连续墙的环向刚度,仅将其当作竖向受弯构件,会导致竖向钢筋配置过多,造成经济浪费且起重吊装困难等问题。因此,提出考虑圆形基坑地下连续墙环向刚度的实用二维增量法弹性地基梁法,将折减后的地下连续墙环向刚度当作内支撑刚度,施加于基坑开挖面以上,模拟土方逐层开挖、支撑逐层架设的工况。与工程实测数据进行对比,验证了该方法的计算结果更接近实测值,能够在保证安全的基础上,有效降低地下连续墙竖向配筋率。

隧道进口穿越断层破碎带洞身开挖施工分析

为探讨隧道进口穿越断层破碎带洞身开挖施工,以立洲水电站工程实例为背景,分析了三台阶设置临时仰拱法在穿越断层破碎带洞身开挖中的应用。分析表明,断层破碎带洞身地质条件复杂,围岩稳定性比较差,存在塌方的概率比较大,利用三台阶设置临时仰拱法配合系统锚杆、钢筋网、钢架及湿喷混凝土联合支护的施工方法,既能保证施工效率,也可以很好地控制塌方问题,促使施工任务高效、安全、有序完成,可为类似工程施工提供参考和借鉴。

复杂环境下地铁纪念堂站暗挖施工风险控制技术——以广州地铁十三号线二期工程纪念堂站为例

广州地铁工程拱盖法暗挖车站较多,由于暗挖车站顶部存在砂层、淤泥质黏土、粉质黏土等软弱地层,其普遍具有高含水量性、高压缩性、低透水性和低强度等特点,施工阶段易造成较大变形,将会引起隧道坍塌和周边建构筑物设施破坏,造成严重的生命与财产损失,继而造成重大的社会影响。以复杂环境下的纪念堂站为施工风险论述对象,采用合理设计、施工方案,降低施工风险,同时增加地质补勘、深层注浆以及施工监测等方法取得很好效果。对软弱地层暗挖车站进行分析和总结,为暗挖车站施工安全提供依据。

地铁浅埋暗挖法超近距穿越邻近建筑物施工技术分析

地铁浅埋暗挖法是一项超近距穿越邻近建筑物的施工技术,应用该项技术能实现地铁线路的顺利延伸,不会影响周边建筑物的安全。文中对地铁浅埋暗挖法超近距穿越邻近建筑物施工技术进行研究,在深入分析技术原理、工艺流程和施工实践基础上,给城市地铁建设提供更加科学的施工方案。

平顶直墙隧道CRD法下穿市政箱涵安全控制研究

随着城镇建设的推进,地铁暗挖隧道相关工程也越来越多。CRD法是适用于软弱地层的隧道施工方法,主要应用于城市地下铁道建设中。以呼和浩特市大学西街站1号出入口暗挖隧道下穿市政箱涵工程为背景,对其施工方案进行介绍,并对平顶直墙隧道CRD法施工下穿市政箱涵工程的安全控制进行研究。该工程应用“平直墙+CRD法”进行土体开挖,并采用深孔注浆并加以小导管注浆对土层进行加固。通过对现场实测数据的分析,得出暗挖隧道的拱顶沉降的最大值为7.3 mm,暗挖隧道水平收敛值最大为2.5 mm,满足变形控制要求,说明背后注浆以及支护方案合理。通过定义控制偏离系数,对既有结构的分析、评估,得到箱涵的结构状态为二级。

某输变电工程深基坑降水止水措施及开挖变形研究

【目的】为了降低某输变电工程地下水和软弱土体带来的施工风险,保证深基坑安全施工,通过分析其工程和水文地质条件,设计了井管降水和高压旋喷桩止水方案。【方法】构建基坑剖面网格模型,采用有限差分法模拟该深基坑开挖导致的变形。【结果】计算结果表明,第一步放坡开挖后,最大水平位移为26.7 mm,最大坑底隆起为10 mm;第二步开挖后,基坑水平位移最大为24.6 mm,地连墙水平位移最大约为15 mm,最大地表沉降为6.5 mm,开挖变形均在安全范围内。【结论】模拟结果表明,在该工程的水文地质条件下,所设计的降水止水及开挖方案安全合理,对类似条件下的深基坑工程施工具有一定参考意义。

复杂环境下地铁车站导洞开挖施工方法对比研究

为了探究不同隧道开挖方法在地铁车站结构受力和沉降方面的影响差异,本研究以长春地铁5号线工农大路车站为实际案例,运用MIDAS FEA软件对两种施工方法(一次扣拱暗挖逆作法和PBA工法)进行模拟分析。比较这两种方法在施工过程中的差异,分析结果显示,在导洞开挖支护施工阶段,一次扣拱暗挖逆作法导致的地表沉降量最大,占总沉降量的59.14%。而PBA工法在二衬扣拱施工阶段起到较为关键的作用,该阶段所引起的地表沉降量占总沉降量的42.65%。对于拱顶沉降,一次扣拱暗挖逆作法的拱顶沉降较为稳定,而PBA工法的拱顶沉降在不同的施工时间都有大幅度的变动。在其他方面,一次扣拱暗挖逆作法相较于PBA工法具有较小的影响,结构更为稳定,可进一步降低施工风险。

隧道开挖及仰拱初期支护成环施工工艺研究

文章以朗晃Ⅱ号隧道为研究对象,分析了隧道开挖及仰拱初期支护成环施工工艺。该工艺对提高隧道施工效率、保证施工安全、降低成本和提高工程质量具有重要意义。其中,在隧道开挖环节,阐述了隧道开挖方法选择和开挖面稳定措施;在仰拱初期支护成环施工中,提出了仰拱初期支护的重要性和仰拱初期支护成环施工工艺,为实际施工提供了科学依据和参考。研究结果表明:隧道开挖及仰拱初期支护成环施工工艺为隧道施工工艺的改进和发展提供了新思路和方法。

砂泥岩地层浅埋大跨暗挖地铁车站转序落底关键技术

大断面暗挖地铁车站工程采用双侧壁导坑法施工,该方法是处理软岩地层、浅埋地段等不利条件下的常用施工技术手段。结合重庆轨道交通18号线北延工程黄沙溪站暗挖车站开挖工艺及施工步序,利用midas GTS建立的计算模型,分析车站中洞开挖对车站围岩和初期支护结构的应力、位移特征及稳定性的影响,通过现场监控量测及自动化应力监测,进一步验证了提前解除车站两端优化中洞的安全性、可靠性、经济性,有力保障了车站隧道的安全施工及毗邻既有建(构)筑物的安全,可为同类工程提供借鉴和参考。