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

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.EI092579-25863