江阴河塘地裂缝勘察研究

Investigation on Hetang earth fissure in south Jiangyin

江阴河塘地裂缝最早为发生于1995年的局部小裂纹,持续活动时间已有12年,目前已趋缓。其平面展布特征为走向NE20°、线状延伸长达500m的地裂缝带,由主裂缝及主裂缝NW侧的宽约20～60m的凹陷带构成,主裂缝处形成高约20～30cm的陡坎,在凹陷带内一般发育有规模较小的地面裂纹。浅地震勘探及钻探证实,在地裂缝发育处存在隐伏基岩山脊,其走向与地裂缝一致,地裂缝的空间分布与隐伏基岩山脊线基本一致,山脊最浅埋深约64m,山坡坡度约12°～15°,隐伏山体西侧松散盖层一般厚度为140～150m。隐伏基岩面隆起及其所控制的第四系松散层厚度及结构差异是地裂缝发育的空间乃至时间特征的基础,是控制因素;深层地下水的长期超量开采所导致的地层压缩或固结是地裂缝形成的主导因素,该地裂缝正是基岩隆起带地层差异压缩所形成的差异地面沉降的结果。据估算,其发育深度可达基岩隆起的最浅处,实际上已发展成为小断层。

The Hetang earth fissure was originally a small local ground crack in concrete pavement, started in 1995, ,shose continuous activity time had been over 12 years, and now is slow down. It is actually an earth fissure zone now composing of main fissure and its northwest associated depression 20 to 60m wide generally, its linear extension length is about 500m, and strike direction is northeast 20 degree. Due to the differential subsidence, the main fissure shaped as a 20 - 30cm high scarp, and there are a lot of small fissures in depression zone. It is verified by shallow seismic prospecting and drilling that there is bedrock ridge underlying porous Quaternary sediments directly below the main earth fissure, the depth of the uppermost ridge is about 64m, generally 140 ~ 150m deep in piedmont and underlying plain area, with slope angle of 12 to 15 degree. It is the dominant factor of earth fissure forming that long term deep groundwater excessive withdrawal result in confined aquifer sandy layer compaction and its under- and over-lying aquitard clayey layer consolidation, and controlling/ background factor that underlying bedrock ridge and depth and structure of sediments layer which controlled the spatial and time characteristic of the earth fissure zone because of its leading to differential land subsidence. It is estimated that the earth fissure depth may reach to the bedrock ridge, or maybe the groundwater piezometric table, about 64m, and a small fault is actually developed from the main fissure.