CFG桩基坑内施工对基坑周围环境稳定性的影响

在地下工程中,由于天然地基承载力不足,带有地下室的主体结构采用CFG桩复合地基。因为CFG桩长螺旋钻施工设备限制,地下室底板下的CFG桩必须在深基坑开挖一部分后进行施工。在某深基坑工程中,随着CFG桩的施工,基坑周围地表出现明显开裂现象。为探究其原因,结合该基坑工程实例,利用FLAC3D软件,通过数值模拟分析考虑渗流作用下CFG桩基坑内施工对基坑周围地表变形的影响规律,并把计算结果同实际监测数据进行对比分析。研究结果表明:CFG桩在部分开挖基坑内施工的快速取土作用对基坑内被动土压力区产生扰动,削弱原有的被动土压力,导致基坑周围土体变形。基坑周围地表变形的影响范围超出2倍基坑深度的监测范围,因此,部分开挖基坑内施工CFG桩的基坑工程周围环境的监测范围应在满足国家规范要求的基础上适当增大。根据计算结果建议类似基坑工程监测范围距基坑边缘的距离采用基坑开挖深度与基坑底面以下CFG桩长之和。类似基坑工程设计应加大支护结构和止水帷幕深度,施工时从基坑内部向外部隔桩跳打,并适当增加工期,将有利于降低由于CFG桩基坑内施工对基坑周围土体的影响。

Influence of CFG Piles Construction in Foundation Piton Surrounding Environmental Stability

In underground engineering,the main structure with the basement is built with CFG pile composite foundation because the natural ground bearing capacity is insufficient.Because the construction length of the CFG pile is limited by the spiral drilling machine,CFG piles under the floor of the basement must be constructed after a part of the deep foundation pit is excavated.In certain deep foundation pit engineering with CFG pile construction,there is an obvious crack phenomenon on the ground surface around the foundation pit.In order to explore its causes,the deformation characteristics of the foundation pit are analyzed considering the action seepage when CFG piles are constructed in the foundation pit after part excavation.The analysis uses FLAC^3D as the numerical simulation tool. The calculated results are compared with actual monitoring data of the foundation pit.Finally,the causes of cracks in the ground around the foundation pit during construction are studied.The results show that the rapid sampling effect in the foundation pit decreases the earth pressure in the passive zone of the foundation pit when CFG piles are constructed in the foundation pit after part excavation.The effect scope of earth deformation around a foundation pit is beyond the scope of monitoring which is twice the depth of the foundation pit.Therefore,the scope of monitoring of the ground deformation around similar foundation pits should increase appropriately on the basis of meeting the requirements of the National Standard.According to the calculation results,it is suggested that the scope of monitoring in similar foundation pit engineering should adopt the sum of the foundation pit excavation depth and the CFG pile length at the foundation pit bottom.Similar foundation pit engineering design should increase the length of the support structure and the depth of the waterproof curtain.Construction of CFG piles in foundation pits should be from the inside to the outside edge of the foundation pit with piling every a few piles.The construction time should increase appropriately.These measures will help to reduce the deformation of surrounding soil due to the construction of CFG piles in foundation pits after part excavation.