注水与水下开挖是一种新型的基坑开挖方法，用来解决基坑地下水与江河水有稳定联系时传统的坑外降水与坑内排水方法无法实施的问题。提出了深基坑注水与水下开挖的设计计算方法，结合钢板桩围堰深基坑工程，制定了注水与水下开挖方案，建立了相应的数值模型来研究深基坑先降水形成旱地工作条件开挖、再注水进行水下开挖、水下浇筑封底混凝土后再降水所对应的支护结构受力与变形规律，对注水高度进行了优化。结论如下：先降水形成旱地工作条件时最大降水深度取决于坑底土体抗流砂、管涌的稳定性，开挖深度从2m增加至7m时，支护结构水平位移增加了53.6%，坑底塑性隆起量的最大值逐渐增大至59.0mm；再注水进行水下开挖时，增加注水高度可有效减少坑底土体的隆起变形及支护桩的水平位移，临界注水高度为9m，当注水高度超过该临界值后其抑制坑底和支护桩变形的效果会减弱。封底混凝土厚度取决于其抗浮稳定性和强度，钢板桩嵌固深度取决于水下浇筑混凝土后再降水工况下的支护结构整体稳定性。研究成果对类似深基坑的设计与施工具有参考价值。

Water injection and underwater excavation is a new method of foundation excavation. It is used to solve the problem that traditional methods of off-pit precipitation and in-pit drainage cannot be implemented when there is a stable connection between groundwater in the foundation pit and river water. The design calculation method of deep foundation pit injection and underwater excavation is proposed. Combined with the deep foundation pit project of steel sheet pile cofferdam, a water injection and underwater excavation scheme was developed, and a corresponding numerical model was established to study the force and deformation law of the support structure corresponding to the deep foundation pit firstly precipitated to form dryland working conditions for excavation, then injected water for underwater excavation, and then precipitated after underwater pouring of sealing concrete. The water injection height has also been optimized. The conclusions are as follows: First precipitation to form dryland working conditions when the maximum depth of precipitation depends on the stability of the soil at the bottom of the pit against flowing sand and pipe surges. When the excavation depth increases from 2m to 7m, horizontal displacement of support structure increased by 53.6%, the maximum value of plastic uplift at the bottom of the pit gradually increases to 59.0mm. When re-injecting water for underwater excavation, increasing the height of water injection can effectively reduce the uplift deformation of the soil at the bottom of the pit and the horizontal displacement of the support pile, the critical height of water injection is 9m. When the water injection height exceeds the critical value, the effect of inhibiting the deformation of pit bottom and support piles will be weakened. The thickness of the sealing concrete depends on its floating stability and strength. The embedded depth of steel sheet piles depends on the overall stability of the support structure under the precipitation conditions after the concrete is placed underwater. The research results are of reference value for the design and construction of similar deep foundation pits.

国家自然科学基金(51978540)；