Abstract: Because of the existence of confined water at the bottom of the deep foundation pit soil， when the weight of impervious soil above cannot balance with hydraulic head，improper construction method can easily lead to piping， the instability of foundation pit enclosures and many other problems. In this paper, many risk sources related confined water are analyzed and many specific requirements are put forward for survey, design and construction. Because the foundation pit is near the river and there are penetration channels between confined water and river water, so the change of river water level will cause the rise or fall of confined water head. In view of the design head was reference the change of groundwater level in nearly one year, not considering the head changes throughout the years, which become the hidden danger caused accident. In the end, the river water level rising during construction, confined water pressure exceeded expectations and caused the piping. After the accident, water channel was soon clogged. And then concrete was poured into soil at the bottom of foundation pit. The soil property improved and thickness of impervious soil increased significantly. Piping problem was resolved and the project was completed safety. Grouting method is an effective way to ensure foundation pit excavation safety when confined water head is larger. Research in this paper has a strong guiding role in controlling risk of the pressure water during design and construction, especially engineering near river or ocean.

Key words: excavation of foundation pit, confined water, risk source, piping, reinforcement with grouting method