Control and Prevent Land Subsidence Caused by Foundation Pit Dewatering in a Coastal Lowland Mega City: Indicator Definition, Numerical Simulation and Regression Analysis

Coastal mega cities are often commercial centers because of convenient traffic. Safe elevation above sea level is vital for their sustainable development. Global climate change and sea level rising increase flood risk especially in the lowland subsidence area. Shanghai of China was selected as research background. Although groundwater exploitation had been strictly restrained to control land subsidence and reserve safe elevation, lowering groundwater level during underground excavation cannot be avoided. Foundation pit dewatering (FPD) was intensively performed in underground exploitation during urbanization and city renewal. The FPD settlement accelerated land subsidence. Controlling FPD subsidence was urgent. Normally, the maximum horizontal influence radius of foundation pit excavation was less than three times excavation depth (H), and the 3H settlement was only caused by the FPD. The 3H maximum settlement was defined as the evaluating indicator of FPD land subsidence, and the corresponding 3H drawdown was defined as the control indicator of land subsidence. The FPD conceptual models were established on the basis of estimation and investigation of foundation pit information, including pit area, pit shape, pit depth, and curtain depth. Numerical models were established and a total of 5650 FPD numerical simulations were performed to investigate the land subsidence and FPD drawdown. Multi-factor regression analysis was conducted to obtain relations between land subsidence and FPD drawdown. Regression models were established between the 3H drawdown and the shape, area, depth, and curtain depth of foundation pit on the basis of the numerical simulations. A typical example introduced to verify the regression models. The regression models were used to manage the FPD land subsidence by controlling the 3H FPD drawdown. The results can provide reference for the land subsidence control in a coastal lowland city.

Two-phased Mass Rarity and Extinction in Land Plants During the End-Triassic Climate Crisis

Greenhouse gas emissions from large-scale volcanism in the Central Atlantic Magmatic Province is considered to have caused the end-Triassic mass extinction (201.5 million years ago), but the impact on land plants has been debated. Here, abundance changes in spores and pollen record the devastating effects this volcanic induced climate crisis had on coastal and near-coastal lowland mire vegetation around the European epicontinental sea and the European Tethys margin. Combined stress from rising air temperatures and changing climate at the onset of the crisis was exacerbated by a rapidly rising sea-level resulting in fragmentation and destruction of coastal and near-coastal lowland mire habitats, causing mass rarity and extinctions primarily in gymnosperm trees and shrubs adapted to these environments. The devastation of these habitats was further amplified by a subsequent sea-level fall leaving pioneering opportunists and herbaceous survivors to colonize disturbed areas in an environment stressed by increased wildfire activity and enhanced soil erosion. The pioneering flora was severely decimated in a second mass rarity phase and ultimately extirpated. The second mass rarity phase occurred just prior to and at the onset of a prominent negative excursion in δ13Corg. A subsequent sea-level rise appears to have restored some of the near-coastal mire habitats allowing some of plants to recover. The supraregional mass rarity during the end-Triassic crisis affected both previously dominant as well as rare plants and this resonates with ongoing and future climate change and attests to the vulnerability of coastal and lowland vegetation, especially rare plant species, to climatic and environmental disturbances, where rising sea-level threatens entire ecosystems.

ON THE CHAUN PALAEOFLORA FROM NON-MARINE CRERACEOUS OF CHUKOTKA

The Cretaceous fossil floras from the formations of the Okhotsk-Chukotka volcanogenic belt differ significantly by its systematic composition from the same-age fossil floras from the coastal lowland sedimentations. The peculiarities of fossil floras from the volcanogenic disposals are the most obvious in Chaun Flora firstly found in the volcanogenic formations of Pegtymelski Arch in Central Chukotka. L.B. Golovneva presented the results of the long-term study of this flora in the monograph published in 2018. On the base of these data and using our own experience in the stratigraphy of the Okhotsk-Chukotka volcanogenic belt and the study of the vegetation cover dynamics in the areas of modern volcanism of Kamchatka we showed that most likely Chaun Flora formed on juvenile substrata in the central parts of the vast volcanic fields being isolated form the sources of diasporas. The Chaun Flora has not clear affinity in neither lateral nor stratigraphic localization, because the fossil remains of characteristic plant species occur in different-age formations on all extent of the volcanogenic belt. The species characteristic for Chaun Flora are indicators of the certain environmental conditions and reflect the peculiarities of the processes of fossilization in the areas of terrestrial volcanism from Turon until Campanian inclusive.