上海软土单层地下室大面积换撑基坑变形特性实测分析

Deformation Characteristics Analysis of Single-Story Basement with Large-Area Bracing Replacement Method in Shanghai Soft Soil

上海某医院项目东大型地下室单层层高达8.5 m,其不规则基坑共设置两道混凝土支撑,在拆除第一道混凝土支撑,对地下室顶板进行施工之前,需要实施大面积换撑施工作业,避免出现基坑围护结构悬臂过长的情况,为此需在施工期内开展全面监测。监测项目包括基坑围护结构侧移、围护结构外侧土体侧移、围护结构顶部位移、立柱回弹、支撑轴力以及坑外承压水水位等内容,重点分析了基坑开挖换撑过程中的围护结构变形、内力发展趋势。结果表明:高大单层地下室基坑的换撑作业有助于基坑围护在失去第一道支撑后的变形收敛,但过程中基坑围护变形较大,仍需密切监测;基坑坑外弃土可能引起土体固结,围护桩行为由回弹转换至沉降,在坑中立柱持续回弹过程中,混凝土支撑不均匀变形加剧,次应力增加,进而影响基坑稳定性。

The underground basement of a hospital in Shanghai is a single-story large structure with a height of up to 8.5 meters.Due to the irregular foundation pit,two layers of concrete bracings are set.Before the removal of the first layer of concrete bracings and the construction of the basement ceiling,large-area bracing replacement operations are required to prevent the cantilevered deformation of the excavation pit retaining structure.For assessment,comprehensive monitoring is conducted during the excavation phase,including retaining structure deflection,lateral displacement of soil behind retaining structure,the vertical displacment of retaining structure top,uplifts of interior steel columns,axial forces of propping structs and water level of artesian water.Based on the analysis of field data,the deformation and internal force development trendency of retaining structure during the bracing replacement process is analyzed.The results reveal that the bracing replacement procedure inhibits the excessive deformation of the super-high single-story basement foundation pit after losing the first layer of bracings.However,the retaining structures still deform considerably during the process and needs to be closely monitored.Furthermore,placement of abandoned soil outside the pit may induce soil consolidation,causing the behavior of the retaining piles to transform from rebound to settlement.During the continuous rebound process of the column pillars,the uneven deformation of the concrete bracings will intensify and the secondary stress will be aggravated,thereby affecting the stability of the foundation pit.