城市轨道交通车站基坑围护结构渗漏点对其周边地下水渗流场变化规律的影响

Seepage Point Impact in Urban Rail Transit Station Foundation Pit Enclosure Structure on the Variation Law of Surrounding Groundwater Seepage Field

[目的]在地下工程施工事故诱因中,大约20%的基坑事故是由围护结构渗漏水引起的,围护结构渗漏水会造成基坑发生大变形或失稳。为提高基坑工程施工的安全性,有必要在基坑开挖前对渗漏点进行检测,并针对渗漏点对于基坑围护结构周边渗流场的影响规律进行研究。[方法]以青岛地铁4号线某车站基坑为背景,施工前采用分布式光纤检测方法对基坑围护结构渗漏点开展了检测,并找到了疑似渗漏点。对基坑疑似渗漏点附近的地下水位进行了监测。利用SEEP/W软件模拟了不同尺寸、不同数量的渗漏点对基坑围护结构周边地下水渗流场的影响,并在渗漏点附近布设地下水位监测点以获得现场实测数据。[结果及结论]在基坑开挖降水过程中,由于坑内外地下水压力差逐渐增大,坑内地下水位逐渐下降,在止水帷幕周围形成降水漏斗。在地下水靠近止水帷幕且无渗漏点时,基坑坑外地下水位下降深度约为坑内开挖降水深度的20%;渗漏点宽度为10 mm时,基坑坑外地下水位下降深度约为坑内开挖降水深度的30%。止水帷幕出现渗漏点,导致基坑附近地下水渗入基坑内,水通量、水传导率及水压在渗漏点位置急剧增大,且其数值随渗漏点尺寸的增大而增大。基坑地下水渗流数值模拟结果与工程实测结果基本吻合,说明采用SEEP/W软件模拟地下水渗流场合理可行。

[Objective]In terms of underground engineering construction accident causes,approximately 20%of foundation pit accidents are caused by water seepage through enclosure structures.Such seepage can lead to significant deformation or instability of foundation pit.To enhance the safety of foundation pit project construction,it is necessary to detect seepage points before foundation pit excavation and study the impact of these seepage points on the groundwater seepage field around the foundation pit enclosure structure.[Method]Taking the foundation pit of a station on Qingdao Metro Line 4 as the engineering background,a distributed optical fiber detection method is employed to identify seepage points in the enclosure structure prior to construction,locating the potential seepage points.The groundwater levels near these suspected seepage points are monitored.The SEEP/W software is utilized to simulate the effects of different sizes and quantities of seepage points on the groundwater seepage field around the enclosure structure.Groundwater level monitoring points are set up near the seepage points to obtain field data.[Result&Conclusion]During the dewatering process of foundation pit excavation,the difference in groundwater pressure inside and outside the pit gradually increases,causing the groundwater level inside the pit to decrease,thus forming a dewatering funnel around the waterproof curtain.When the groundwater is close to the waterproof curtain without seepage points,the groundwater level outside the pit decreases to about 20%of the excavation dewatering depth inside the pit.With a seepage point width of 10 mm,the groundwater level outside the pit decreases to about 30%of the excavation dewatering depth inside pit.The presence of seepage points in the waterproof curtain leads to groundwater entering the pit near these points,sharply increasing water flux,hydraulic conductivity,and water pressure at the seepage points,with these values rising as the size of the seepage points increases.The numerical simulation results of foundation pit groundwater seepage using SEEP/W software prove to be in good agreement with the field measurements,demonstrating that using SEEP/W for simulating groundwater seepage fields is reasonable and feasible.