Environmental Effects Induced by Deep Subway Foundation Pit Excavation in Yellow River Alluvial Landforms

Zhengzhou is a second-tier cities in P. R. China and is just located at Yellow River alluvial landforms where the soils are divided into four strata combinations of flood alluvial deposit based on the lithology, sedimentary environment and geomorphic units. Zijingshan Station is one of the most important stations of the total subway lines in Zhengzhou, the first and second subway lines join together at Zijingshan Station, the transfer station foundation pit has the maximum depth of 31.2 m under the ground surface and is now the deepest one in Henan Province that Zhengzhou is belonged to, and so to monitor and analyze the environmental effects induced by this type of deep foundation pit excavation is necessary and important for the foundation pits of similar deep subway stations and buildings in future. During Zijingshan Station foundation pit excavation and main station structure construction, water tables, settlements and lateral deformations around the transfer section and standard section were all measured and analyzed in time, and some conclusions are drawn out, which are that (1) the water tables are controlled to be stable when artificial ground freezing was adopted and the stable changing ratios are all <0.7 mm/day, (2) the maximum ground settlement is not adjacent to the underground continuous walls but at the overpass bridge pier with a certain distance away from the foundation pit edge, which is seriously affected by both large traffic flows and foundation pit excavation, (3) the maximum lateral deformations of the standard section are focused at the position of about 3.0 m under the ground surface, while those of the transfer section are at about 11.0 m under the ground surface; (4) the measured and monitored items such as stable water table decreasing ratio, maximum settlements and lateral deformations all meet with the local building foundation specifications. The first subway line of Zhengzhou that contains Zijingshan Station has been put to use, the environmental effects caused by foundation pit excavation are all controlled to be stable until now, and so underground continuous walls and horizontal supports are the prior selection for the deep foundation pit in Yellow River alluvial landforms.