摘要
润扬长江公路大桥南汊北锚碇超深基坑长69m,宽50m,深约50m。深基坑支护结构为外部钢筋混凝土地下连续墙,内部设多道钢筋混凝土水平框架内支撑。为了探讨超深基坑支护结构与土体相互作用,本文系统分析了该超深基坑的施工监测资料,结合三维数值模拟,对围护墙体水平位移、坑外地面沉降、坑壁土压力及其相互关系进行了深入研究,获得了如下主要认识和结论。
(1)随着开挖深度增大,墙体最大水平位移不断增大,但在不同阶段墙体变形增大幅度不同。在开挖深度22.3m之前,有加速发展趋势,之后便不断收敛。同水平面上,基坑长边中点、短边中点变形大于角部。墙体水平位移沿深度方向总体呈中部大、上部和底部小的“凸”形形状,随开挖进行最大位移位置不断下移。通过深入分析得出:开挖深度在22.3~26.3m之前,最大位移位置与相应的位移量变化基本成正相关关系。而在开挖深度达26.3m之后,最大位移位置的下移和最大位移的增长均较小。地面沉降在开挖深度22.3m以前较小,之后受坑外降水影响沉降显著加大。
(2)对于墙体水平位移,通过Peck法估算值和实测值的对比分析,提出了针对多支撑体系,系统整体刚度较大,抗隆起安全系数为1.4时的估算墙体水平位移(变形)的修正公式。
(3)针对润扬基坑的实际地表沉降提出了沉降变形估算包络线。
(4)开挖初期土压力表现出静止土压力特性,随着开挖进行,土压力曲线出现挠曲,并且大部分测点的土压力值有所减小。
(5)通过研究将本基坑开挖过程中土压力沿深度变化形式从概念上归为斜直线模式、波状递增模式、阶状递增模式、附加应力作用叠加模式四种模式。每种模式给出了相应的分布图式。
(6)研究正常施工情况(无坑外降水)墙体水平位移(变形)与坑壁土压力的关系得出:开挖初期(挖深约小于6m时)土压力和墙体变形均较小;随后在墙体变形仍然较小时而土压力突然增大。随后随变形不断发展土压力虽有波动,但总体平稳,曲线的相对斜率趋于0或小于0。特定的开挖深度对曲线的斜率影响较小,不同深度曲线的斜率变化较大。
(7)坑外降水开始后,基坑浅部(约18m)以上变形向相反方向发展,在基坑的较深部位(约18m)以下变形仍在增加,但增加的幅度已大幅减小,即随着测点深度增加,墙体变形与土压力的关系在坑外降水影响下可由:变形和土压力均明显减小(近y轴的斜直线)→变形相对稳定土压力减小(竖直线)→变形仍有所发展土压力减小(远y轴的斜直线)进行转化。
(8)通过分析得出土压力与墙体变形关系系数与坑壁深度的关系,利用此关系可计算考虑基坑变形的土压力问题。
(9)通过不同土压力分布模式的结构验算,得出了地连墙设计中分段配筋应慎用,以及考虑波状递增土压力形式作用下,由于墙体挠曲复杂,为保证结构安全,应采用双面对称配筋,并且配筋率应较计算结果有一定提高,本文建议增大20%的配筋率以应对可能出现的非常情况。
(10)通过支护结构与土相互作用采用四因素五水平正交数值模拟结果分析得出:
①支护结构的安全决定于支护结构体系刚度和土体强度(刚度)的匹配情况。一般土体强度较高、刚度较大,支护结构体系刚度可相对较小,而软弱土体则要求较大的支护结构体系刚度才能保证支护结构安全。
②墙体变形随墙体厚度增加而不断减小。表现出墙体厚度小于1.2m时,增加墙体厚度可以使墙体变形迅速减小,而当墙体厚度超过1.2m时,增加墙体厚度,墙体变形减小不太显著;可能存在一临界墙体厚度,当超过此厚度,增加墙体厚度的效益变得不太显著,而若小于此厚度一定值,结构便会向不稳定状态发展。
③墙体变形随基坑土体强度(刚度)增强而不断减小。
④支撑间距、支撑截面尺寸对墙体变形的影响没有土体强度、墙体厚度的影响显著。
本文的研究成果从润扬长江公路大桥南汊北锚碇特定的基坑规模和支护结构体系获得,对类似工程具有一定的借鉴和指导作用,对超深基坑的支护结构与土相互作用的理论研究具有一定意义。
The north anchor foundation of the south cable bridge across Changjiang River which connected Yangzhou to Zhenjiang (named Runyang) had deep excavation with 69m long, 50m wide and about 50m deep. The retaining and protection structure for this deep excavation was concrete diaphragm wall supported by plane concrete frame inside. In order to study retaining structure and soil interaction in deep excavation, monitoring data (especially deformation of the wall, settlement of ground surface and earth pressure etc.) in the process of excavating and supporting was analyzed. And three-dimensional numerical method was used simulating construction with four factor (wall thickness, support space, support section size, soil strength) changing in five lever. Some conclusions come as follows.
(1)Lateral movements of the diaphragm wall increased with excavating, but had different behavior in every stage. Before excavating at depth of 22.3m, the wall deformation increased at a accelerate state. After that point it would be slowdown. The wall deformation at the midpoint of rectangular side was great than that of corner in the same horizontal plane. In vertical direction, the deformation curve of the wall was entasis, that is great in middle, and mini upside or at the bottom. The max deformation position descended with excavating. Before excavating to depth of 22.3m~26.3m, the depth of maximum deformation position was correlativity to deformation value. After that, both the decline of the maximum deformation position and the deformation increase were relatively small. Settlement of the ground surface was small before excavating to depth of 22.3m. After that, it augmented remarkably because of dewatering outside the pit.
(2) A formula was gained to estimate the maximum deformation of the wall under the condition of multi-bracing system, great system stiffness, factor of safety against basal heave was about 1.4.
(3) Envelope line was drawn out for estimating the maximum settlement of ground surface.
(4) At the beginning of excavation, Earth pressure on the wall was linear increased with depth add, and generally still earth pressure distribution. As excavating went on deeper and deeper, the curve of earth pressure distributing was more flexure at the primary turning point. When excavating to the bottom of pit, earth pressure curve was not only flexure strongly but earth pressure value minified than that of beginning.
(5) Four earth pressure patterns in the process of foundation pit excavation were sum up. those were liner increase mode, undulance increase mode, step-shape increase mode and extra load action mode.
(6) Under normal construction (no dewatering outside the pit), the deformation of the wall and the earth pressure on the wall both very small before 6m excavating depth. At the following shortly period, the deformation increase a little, but earth pressure gained a sharp increase. Latterly, in the main excavating process, earth pressure presented a relatively stable state with the wall deformation increase (the slope of the curve could be equal or less than 0) except a little fluctuant. The slope of the curve could be hardly influenced by excavating depth, but changing with the depth of the wall.
(7) After dewatering, above 18m of the pit, earth pressure reduced and wall deformation reversed. Below that point, earth pressure reduced and wall deformation increased. The curve of the earth pressure and wall deformation could be changed from pattern of earth pressure reducing with wall deformation reversed (close to y axis), earth pressure quickly down but deformation unchanging (parallel to y axis), earth pressure decreasing with deformation developing (away from y axis) with depth increase.
(8) A formula expressing the relationship between coefficient of earth pressure & wall deformation and wall depth was proposed to calculate earth pressure induced by wall deformation.
(9) Internal force of diaphragm under above four earth pressure mode was calculated and gained some points, such as: Be careful of subsection setting steel in design, steel percentage setting in the wall should augmented because of complicated wall deformation under undulance increase mode earth pressure. At the author’s point it should be increased 20% for coping with the abnormity. (10) Numerical method was used to simulate interaction between retaining structure and soil considering four factors and five levels in the process of excavating.
The facts were found as follow:
①The safety of supporting system relied on the corresponding of retaining structure to soil strength. The more strength and stiffness of the soil, the less stiffness of retaining structure could be. But if the soil was soft, there should be great stiffness of the retaining structure for soil mass stabilization in deep excavation.
②Wall deformation tended to small with the thickness of wall increase. Wall deformation could decreased rapidly when wall thickness less than 1.2m. But Wall deformation decreased indistinctively when wall thickness great or equal 1.2m. Their might be a critical wall thickness value. When wall thickness large than that value, the increae of the wall thickness did little effect to wall deformation.
③Wall deformation decreased with the strengthening of soil.
④Support space, support section size influenced the wall deformation not very clearly.
The conclusions of this thesis were gained from specifically deep excavation condition of Runyang great bridge foundation and would have important guidance for similar project, and have academic value in retaining structure and soil interaction study.
引文
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