综合地球物理方法在成都地铁站异常沉降勘察中的应用

Application of Integrated Geophysical Method in Investigation of Abnormal Settlement of Chengdu Metro Station

一些地面沉降的发生可能是由于非抽水因素和非地质因素的影响,这些因素引起地面沉降的机理研究较少,也难被查明。本研究以成都轨道交通18号线火车南站站沉降为例,采用综合地球物理测井,井下摄像以及井间地震层析成像方法,并结合井间地震波衰减分析,查明了异常沉降原因。成都地铁火车南站站基坑东侧泥岩声波速度比西侧低5%左右,东西两侧差异较小。但是,基坑东侧穿透泥岩的地震直达波低频能量强,高频弱,高频成份衰减大,其平均峰值频率约340 Hz,而西侧穿透泥岩的地震直达波平均峰值频率约450 Hz,东西两侧相差约30%。直达波衰减程度的显著差异说明,基坑东侧的泥岩层裂隙较发育,泥岩强度较低,西侧泥岩层则较完整。结合基坑东侧井温异常,本研究得到废弃地源热泵使得泥岩在长期热交换作用下产生物理风化,促进裂隙发育,地下水进一步软化泥岩,降低岩体承载力,导致地面沉陷的结论。本研究表明,为探测岩体完整性,使用传统的地震波层析成像,当波速差异不明显时,难以判断岩体裂隙发育情况或是岩体软化情况。地震波衰减分析额外利用了频谱信息,提高了岩体力学性质刻画的准确度和可靠性,为沉降成因机理分析提供了科学依据。

Some land subsidence may be due to the influence of non-pumping factors and non-geological factors,and the mechanism of land subsidence caused by these factors is less studied and challenging to be identified.Taking the subsidence of the south railway station of Chengdu rail transit line 18 as an example,this study uses comprehensive geophysical logging downhole imaging and cross-well seismic tomography methods,combined with cross-well seismic wave attenuation analysis,to find out the cause of abnormal subsidence.The acoustic velocity of mudstone on the east side of the foundation pit of Chengdu metro south railway station is about 5%lower than that on the west side,and the difference between the east and west sides is small.However,the average peak frequency of direct seismic waves penetrating mudstone on the east side of the foundation pit is about 340 Hz,while the average peak frequency of direct seismic waves penetrating mudstone on the west side is about 450 Hz,with a difference of about 30%between the east and west sides.The significant difference in the attenuation degree of direct wave indicates that the mudstone layer on the east side of the foundation pit is more developed and the mudstone strength is lower.In contrast,the mudstone layer on the west side is more complete.Combined with the abnormal well temperature on the east side of the foundation pit,the research concludes that the abandoned land source heat pump causes physical weathering of mudstone under the action of long-term heat exchange,promotes the development of cracks,further softens mudstone by groundwater,reduces the bearing capacity of rock mass,and leads to ground subsidence.This study shows that in order to detect the integrity of rock mass,it is difficult to judge the development of rock fracture or the softening of rock mass when the difference in wave velocity is not obvious by using traditional seismic wave tomography.The spectral information is used in seismic wave attenuation analysis,which improves the accuracy and reliability of rock mass mechanical characterization and provides a scientific basis for the analysis of the subsidence mechanism.