黄土地区地铁车站深基坑变形监测与分析.doc

黄土地区地铁车站
深基坑变形监测与分析

黄土地区地铁车站深基坑变形监测与分析

Liu Jun red: xi 'an university of science and technology institute of architecture and civil engineering, assistant engineer, xi 'an, 710054
Chen xian: xi 'an university of science and technology institute of architecture and civil engineering, master's graduate students, xi 'an, 710054
摘要：针对黄土地区地铁车站深基坑的工程环境和施工要求，制定深基坑围护和变形的监测方案，对变形规律进行现场监测，确保地铁车站的安全施工。重点分析围护桩体的水平变形钢支撑轴力的变化规律、基坑周围地表沉降以及地下水位变化情况，为以后类似工程的信息化施工提供参考。
关键词：地铁车站；深基坑；围护结构；变形规律；现场监测

城市地铁建设的全面铺开带来了深基坑工程的飞速发展。地铁车站深基坑工程是一个复杂的综合性岩土工程，在施工过程中基坑内外土体应力状态的改变不可避免地引起土体的变形。深基坑工程监测不仅可以保证基坑支护和相邻建筑物的安全，还可以验证支护结构设计，指导基坑开挖和围护结构的信息化施工，为完善设计分析提供必要的依据。下面结合陕西省西安市地铁南门车站深基坑工程具体情况，制定监测方案，并研究深基坑的变形规律。
1工程概况及工程环境
1．1工程概况
地铁车站位于南门古城墙外绿化广场与南关正街正下方，沿南北向路中布置，北端跨南门外绿化广场，临近护城河，南端跨南关正街，处于市交通要道，为地下二层岛式站台车站。标准段结构宽20．50m，高1 2．86m，顶部覆土3．60m左右，有效站台的中心里程为YDKl4+611．962，中心轨面高程为390．22m，长度为188．00m。车站主体围护结构范围总长190．20m，宽22．50m，基坑的开挖深度为轨排段17．50～18．50 m，其余部分16．50～17．00m。按设计要求，车站轨排段及标准段围护结构安全等级为特级，加宽段为一级。
1．2水文地质条件
地铁车站位于黄土梁洼的黄土梁区，根据场地土的地层岩性、时代成因及工程特性，地表分布着厚薄不均的全新统人工填土(Q4m1)，其下为上更新统风积(Q3e01)新黄土(局部为饱和软黄土)及残积(Q3e1)古土壤，再下为中更新统风积(Q2e01)老黄土、残积(Q2e1)古土壤、冲积(Q2a1)粉质黏土、粉土、细砂、中砂及粗砂等。车站内区域对建设有直接影响的是地下潜水，埋深8．80～1 3．30m，勘察阶段建立的长期观测孔水位高程为398．29m，地下水年变化幅度为1．50～2．00m，抗浮设计水位为404．00m。
2深基坑围护结构方案
2．1围护结构形式
该车站采用明挖顺做法+局部(军用梁下部分)盖挖顺做法施工，围护结构采用钻孔灌注桩+旋喷止水帷幕+内支撑方案，支撑系统采用钢管内支撑。
2．2围护结构设计
车站标准段钻孔灌注桩直径1000mm，桩心距1 200mm，轨排段钻孔灌注桩直径1 200mm，桩,C,IE1 400mm，局部略有调整。主体部分标准段钻孔灌注桩嵌固深度为7m，加深段分别为8m和10m。车站主体标准断面及加宽断面自上而下设置三道钢支撑，轨排段设四道钢支撑。标准段第一道支撑水平间距为8m，其余为4m；加宽段第一道支撑水平间距为6m，其余为3m；轨排段支撑水平间距为2．5m，局部略有调整。第一道支撑均采用驴600，16mm厚钢管内支撑，钢管支撑设活动端头，以便施加预应力，预加应力不大干支撑设计轴力的40％～60％。车站盖挖顺筑范围内施作临时路面，临时路面采用“加强型六四铁路军用梁”+“预应力钢筋混凝土路面板”的形式，为控制基坑变形，在军用梁下弦杆下设置第一道钢支撑，第一道钢支撑在平面内与军用梁错位布置，以避让军用梁下弦杆。
3监测方案
3．1监测内容与监测周期
根据基坑周边环境特点和基本监测方法、原理，确定基坑监测内容与监测周期，如表1所示。图1中，7—7断面的围护桩体变形、钢支撑轴力变化规律、基坑东侧周围地表沉降和地下水位变化情况的数据较为完整，下面以其为例，对监测情况进行说明。
3．2测点布置
(1)为确保安装成活率不低于80％，围护桩体变形监测共设31个测斜孔，布置见图1。
(2)轴力监测分别在车站主体指定截面的每道钢支撑及军用梁上进行，共布置90个轴力计(应变计)。
(3)地表沉降观测按要求在基坑东、西两侧各布设8个截面，每个截面布点6个。从围护桩外侧起算各点间距分别为3m，5m，5m，5m，10m。在基坑南北两侧，各设一个截面，从围护桩外侧起算各点间距分别为3m，5m，5m，5m，10m，共计108个观测点。
(4)水位观测井布设在基坑的两长边外及北侧的土体中，距围护墙体1～2m。井深度设计为20m(从自然地面起计)，共计7个观测井。
3．3监测仪器
(1)围护桩体变形监测的仪器设备采用CX系列钻孔测斜仪：
(2)支撑轴力变化的测试采用JXG一1型钢弦式应力传感器，SS一¨型频率计数器；
(3)基坑周围地表沉降观测采用DSZ2／FSl型精密水准仪：
(4)地下水位观测采用HBHV一10水位计。
4监测结果分析
4．1桩身水平位移变化规律分析
现分析CX8测点的监测数据。2008年3月3日，CX8测点初始量测，基坑开挖至冠梁底以下0．5m：3月12日基坑开挖至冠梁底以下1．0m，施作第一道钢支撑；4月4日，基坑开挖至冠梁底6．5m，在距冠梁底6m处施作第二道钢支撑：4月30日基坑开挖至冠梁底11．5 m，在距冠梁底11 m处施作第三道钢支撑；5月28日土方开挖完毕，开始进行底板施工；到8月5日陆续拆除了第三、二道钢支撑。各关键阶段的水平变化规律如图3所示。分析可知，基坑水平位移基本向坑内方向发展，但钢支撑的施作对水平位移的发展起到一定限制作用，且使其稍向基坑外回复。基坑开挖过程中，围护桩的最大水平位移与开挖深度和时间关系密切，在开挖到一定深度而未架设钢支撑时，围护桩呈向坑内变形的前倾型曲线，桩项水平位移最大。随着基坑的开挖和支撑的施加，围护桩变形曲线由前倾型逐渐向弓形变化，最大水平位移发生的部位也随之下降，基坑中部的水平位移发4．2钢支撑轴力变化规律分析以7—7断面第二道钢支撑J一02—05为例，对该基坑的钢支撑受力变化规律进行分析，如图4所示。基坑开挖阶段，钢支撑的轴力随着开挖深度的增加而增加。待到开挖完成后，各支撑点的增长速率下降，并趋于稳定。原因是随着土方的开挖，土体卸载，被动土压力减小，导致桩身水平位移有向基坑内侧发展的趋势，钢支撑轴力逐渐增大；在第80天之后，钢支撑的轴力变化曲线波动较为明显是因为拆除第三道钢支撑的缘故。结果表明：基坑施工过程中每层土开挖完毕到施加该层钢支撑这段时间以及钢支撑拆除过程是最不利时期，为保证基坑稳定，应尽量减少基坑无支撑暴露的时间。
4．3基坑周围地表沉降分析
基坑东侧的地表土体的竖向沉降变化曲线如图5所示。基坑开挖过程中，最大沉降点始终发生在距基坑边8m的位置上，整体沉降形状类似于勺形。随着开挖的进行，该处竖向沉降逐渐增大，最大的值达到1O．5mm。当监测点与基坑边的距离不断地增加时，沉降逐渐趋于稳定，并且随着基坑开挖到设计标高，桩顶的沉降量也逐渐增大，最大达到5．5mm。
4．4地下水位监测结果分析
各观测井水位随时间变化曲线如图6所示。均不盖量茎蓍挈淼姜麓淼。鬈排90d蠹5结束语 均不同程度的下降很快，随后逐渐趋于稳定。在第65～ 期间，水位上升明显，这是因为遇到了该地区的雨季，持续时间25d左右。雨季结束，水位回复。另外由于观测井SWl位于基坑北侧，较其他井距离护城河近，所以SWl的水位高于其他观测井1m左右。
结束语
均不同程度的下降很快，随后逐渐趋于稳定。在第65～ 期间，水位上升明显，这是因为遇到了该地区的雨季，持续时间 (1)基坑开挖初期围护桩体变形、钢支撑25d左右。雨季结束，水位回复。另外由于观测井SWl位于基 轴力以及地下水位变化速率都较为明显，随着开挖深度的增加，这一趋势开始减弱，当到达一定程度时趋于稳定。影响深基坑稳定性的因素包括工程环境、围护结构方案和施工组织设计等。
(2)钢支撑对基坑变形有明显的限制作用，减缓基坑变形向内侧发展的速率，保证了基
坑的稳定。同时受围护桩变形影响，钢支撑的轴力随着围护桩变形的增大而增大。
(3)整个开挖过程周围地表竖向沉降沿坑边水平方向呈曲线分布，距坑边一定距离的范围
内沉降最大，随后沿远离坑壁方向逐渐减小，距离坑壁越远变化幅度越小，最终逐渐稳定，每开挖一步，坑后地表都有一定量沉降的增加，每步形成的沉降分布曲线形状相似。
(4)监控量测是保证深基坑施工安全的关键。合理的监测方案、准确的监测数据以及及时
的信息反馈是施工决策和信息化施工的重要保障。

参考文献
【 1 】 LiuJun red, Zhang Baoyuan and feng. Subway station deep foundation pit dewatering design [J]. Railway survey, 2009,
55 (I) : 85-85
【 2 】 Pan Hong, leaf model. The field monitoring of deep foundation pit construction [J]. Sichuan building science, 2001, 27 (2) : 30
【 3 】 edge of. 【 J 】 deep foundation pit engineering monitoring and control. In geotechnical engineering, 2001, 4 (7) : 45
【 4 】 Shi Yong is high, the party wins bo, liu to, etc. The monitoring analysis of deep foundation pit engineering method [J]. Journal of xi 'an university of technology,
2007, 27 (1) : 88-2007
【 5 】 Xia Caichu, Li Yong. Underground engineering test theory and monitoring technology [M]. Shanghai: tongji university press, 1999

The subway station in loess areaDeep foundation pit deformation monitoring and analysis

LiuJun red: xi 'an university of science and technology institute of architecture and civil engineering, assistant engineer, xi' an, 710054
Chen xian: xi 'an university of science and technology institute of architecture and civil engineering, master' s graduate students, xi 'an, 710054
Abstract: in view of the loess area, the subway station deep foundation pit engineering environment and the construction requirements, work out schemes for deep foundation pit retaining and deformation monitoring, deformation law for field monitoring, ensure the safety of the subway station construction. Mainly analyzes the level of retaining pile deformation of steel support axial forces, the change rule of surface settlement around the foundation pit and the underground water level changes, provide a reference for the informatization construction of similar projects in the future.
Key words: subway station; Deep foundation pit; Retaining structure; Deformation law; Field monitoring

Urban subway construction of full-scale brought about the rapid development of deep foundation pit engineering. Subway station deep foundation pit engineering is a complex comprehensive geotechnical engineering, both inside and outside the construction process of foundation pit soil mass stress state change inevitably causes the soil deformation. Deep foundation pit engineering monitoring can not only ensure the safety of foundation pit and adjacent building can also verify supporting structure design, guide the informatization construction of foundation pit excavation and retaining structure, provides the necessary basis for the perfect design and analysis. South gate in shaanxi xi 'an subway station deep foundation pit engineering under the specific conditions, formulate monitoring solution, and studied the deformation of deep foundation pit.
1 project profiles and engineering environment
1.1 project summary
Subway station is located at the south gate outside the ancient city wall greening plaza and sales main street, along the north and south road in the arrangement, north across the south door green square, near the moat, southern cross sales, main street, in the city roads, underground island platform station on the second floor. Standard paragraph structure width 20.50 m, 1, 2.86 m high, the top covering 3.60 m or so, effective at the center of the mileage for YDKl4 + 611.962 platform, center rail surface elevation of 390.22 m, length is 188.00 m. The station main body scope of palisade structure overall length 190.20 m, 22.50 m wide, foundation pit excavation depth of section of track section of 17.50 ~ 18.50 m, the rest of the 16.50 ~ 17.00 m. According to the design requirements, the station section of track segments and the standard section of the enclosure structure safety for premium grade, widened section for level 1.
1.2 hydrogeological conditions
Subway station is located in the loess Liang Wa beam loess area, according to the field of soil strata lithology, age and engineering properties, the surface distribution of the thickness uneven of holocene artificial filled soil (Q4m1), which for the pleistocene series wind (Q3e01) new (local) saturated soft loess and loess eluvial Q3e1 paleosol, is next to pleistocene series in the wind-blown (Q2e01) old loess, residual (Q2e1) ancient soil, alluvial (Q2a1) silty clay, silt and fine sand, medium sand and coarse sand, etc. Station within the area has a direct influence on construction of underground diving, buried depth of 8.80 3.30 m ~ 1, investigation phase of the long-term observation of pore water level height is 398.29 m, variations in groundwater in 1.50 ~ 2.00 m, anti-floating design water level is 404.00 m.
2 deep foundation pit retaining structure scheme
2.1 palisade structure form
Approach the station adopts Ming shun + local (military beam section) under cover dug along the construction, palisade structure using bored piles + rotary spray water stop curtain + inside the support, support system adopts steel pipe support.
2.2 retaining structure design
A diameter of 1000 mm, pile station standard period of 1 200 mm pitch, section of track section of 1 200 mm diameter bored piles, pile, C, 400 mm IE1, local adjust slightly. Main part of the standard section of the bored piles embedded in depth 7 m, deepening period of 8 m and 10 m, respectively. Station main body standard section and widened section from top to bottom set three steel support, set four section of track section steel support. Standard horizontal spacing of the first line of support for 8 m, the rest is 4 m; Widen the horizontal spacing of the first line of support for the 6 m, for the rest of 3 m; Section of track section support the horizontal spacing of 2.5 m, slightly adjusted. First line support all use donkeys 600 mm thick steel pipe support, pipe support set mobile end, so that the prestress, prestressing not gets support design of the axial force of 40% ~ 60%. Station built range applied for temporary cover dug along the roads, temporary road USES the "enhanced military railway beams June" + "prestressed concrete pavement plate" form, to control the deformation, under military beam bottom chord set the first line of the steel support, the first line of the steel supports in the plane and military beam dislocation arrangement, in order to avoid military beam bottom chord.
3 monitoring programme
3.1 monitoring content and monitoring cycle
According to the characteristics of foundation pit surrounding environment and the basic methods, principle, determine the foundation pit monitoring content and monitoring cycle, as shown in table 1. In figure 1, 7, 7 section of retaining pile deformation, steel support axial force variation law, on the east side the ground settlement around the foundation pit and the underground water level changes of data is relatively complete, below it, for example, the monitoring of the situation.
3.2 measuring point layout
(1) to ensure that the installation of no less than 80% survival rate, retaining pile deformation monitoring have 31 survey holes, arranged as shown in figure 1.
(2) axial force monitoring section are specified in the station main body respectively of each steel support and military beam, decorate 90 axial force meter (strain gauge).
(3) the surface settlement observation in the east and west on both sides of the foundation pit layout according to the requirement of eight sections, each section stationing 6. Calculate each point since retaining pile lateral spacing respectively for 3 m, 5 m, 5 m, 5 m, 10

REFERENCES
【1】 LiuJun red, Zhang Baoyuan and feng. Subway station deep foundation pit dewatering design [J]. Railway survey, 2009,
55 (I) : 85-85
【2】 Pan Hong, leaf model. The field monitoring of deep foundation pit construction [J]. Sichuan building science, 2001, 27 (2) : 30
【3】 edge of. 【J】 deep foundation pit engineering monitoring and control. In geotechnical engineering, 2001, 4 (7) : 45
【4】 Shi Yong is high, the party wins bo, liu to, etc. The monitoring analysis of deep foundation pit engineering method [J]. Journal of xi 'an university of technology,
2007, 27 (1) : 88-2007
【5】 Xia Caichu, Li Yong. Underground engineering test theory and monitoring technology [M]. Shanghai: tongji university press, 1999