天津滨海地区晚新生代地层自然固结与地面沉降研究

Study of natural consolidation of Late Cenozoic era clay and land subsidence in Tianjin coastal area

天津滨海地区地处渤海湾西岸,晚新生代沉积了巨厚的松散沉积物。地下水位下降、地层自然固结、地表载荷的加速增长等复合因素造成了严重的地面沉降。利用在天津滨海新区塘沽地区施工的一眼1 226 m全取芯钻孔,通过原状样品测试分析,系统研究了晚新生代土层的物理力学性质、黏性土固结特征,并结合欠固结黏性土层沉降量计算等方法阐述了土层固结状态空间特征,探讨了土层固结特征与地面沉降的相关关系。结果表明：该地区0～100 m深度土层具有低天然密度、高孔隙比、高含水率、高压缩性等特点,表现出软土的性质,在地表荷载增大的情况下,易发生地面沉降;100～550 m的黏性土大都处于超固结和微超固结状态,主要是由于过去地下水的大量开采造成的;550 m以下的黏性土多为正常固结,局部存在欠固结黏性土夹层。钻孔中存在合计约218 m的欠固结黏性土夹层,这些欠固结黏性土夹层在自重应力下的最终沉降量为1 985 mm,沉降量最大的土层对应于第1、6含水组,分别达614 mm和665 mm,这一沉降过程完成所需时间为数十年甚至上百年。

Tianjin coastal area （TCA）, located at the coastal region on the west of the Bohai Gulf, has suffered severe land subsidence due to compaction of the huge thick unconsolidated sediments deposited since Late Cenozoic era. Several factors, including groundwater level decline, clayey soils natural consolidation and surface loading rapid increase are the primary causes of land subsidence in this region. A borehole with depth of 1 226 m is drilled at Tanggu in the TCA;and core samples are extracted at various depths. Through laboratory test of these undisturbed soil samples, the physical and mechanicanl properties and the engineering geological properties of all kinds of clayey soils are analyzed. Integrating estimation of total compaction of the under-consolidated clayey soils, the spatial characteristics of natural consolidation of the unconsolidated sediment layers are delineated;and the relation between consolidation characteristics and land subsidence is discussed. The results indicate distinct compaction characteristics of soil layers at different depths： clayey soils in depth less than 100 m show the states of low natural density, high porosity ratio, high moisture content and high compressibility of under-consolidated soft soil;clayey soils in depth of 100-550 m are in the states of over-consolidation or slightly over-consolidation, which is caused by long-term over-exploitation of groundwater in the past;clayey soils with depth greater than 550 m show the state of normal consolidation and include under-consolidated clayey soils interlayers. The total thickness of under-consolidated clayey soils in the borehoe core is about 218 m. These under-consolidated clayey layers are predicted to bring about cumulative compaction of 1 985 mm in a period of several decades to a century. The first and sixth aquifer groups are the primary contributors to this compaction, with predicted compaction of 614 mm and 665 mm respectively.